Inorganic and Organic Thin Films: Fundamentals, Fabrication, and Applications [1 ed.] 3527344977, 9783527344970

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Inorganic and Organic Thin Films

Inorganic and Organic Thin Films Fundamentals, Fabrication, and Applications

Edited by Yujun Song

Volume 1

Inorganic and Organic Thin Films Fundamentals, Fabrication, and Applications

Edited by Yujun Song

Volume 2

Editor Yujun Song

University of Science and Technology Beijing School of Mathematics and Physics 30 Xueyuan Road Haidian District 100083 Beijing China

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The Deutsche Nationalbibliothek lists this publication in the Deutsche Nationalbibliografie; detailed bibliographic data are available on the Internet at . © 2021 WILEY-VCH GmbH, Boschstr. 12, 69469 Weinheim, Germany All rights reserved (including those of translation into other languages). No part of this book may be reproduced in any form – by photoprinting, microfilm, or any other means – nor transmitted or translated into a machine language without written permission from the publishers. Registered names, trademarks, etc. used in this book, even when not specifically marked as such, are not to be considered unprotected by law. Print ISBN: 978-3-527-34497-0 ePDF ISBN: 978-3-527-34501-4 ePub ISBN: 978-3-527-34499-4 oBook ISBN: 978-3-527-34498-7 Typesetting SPi Global, Chennai, India Printing and Binding

Printed on acid-free paper 10 9 8 7 6 5 4 3 2 1

v

Contents

Volume 1 Biography xv Preface xvii Acknowledgments 1 1.1 1.2 1.3 1.4 1.5 1.6 1.7

2 2.1 2.2

xxi

Introduction: Progress of Thin Films and Coatings 1 Yujun Song Introduction 1 Thin Films for the Innovation of Information Technology 2 Thin Films for Ultrasensitive Sensing Devices 7 Thin Films for Sustainable Energy Application 9 Thin Films and Coatings for Key Sources and Ecological Environment of Earth 28 Thin Films and Coatings for Biomedical Engineering and Life Science 32 Thin Films and Coatings for National Defense and Homeland Security 38 Acknowledgments 41 List of Abbreviations 42 References 44 Fundamental in Functional Thin Films and Coatings 59 Weiwei Zhang and Yujun Song Introduction 59 Theory of Magneto-electric Coupling in Magnetic Thin Films 59

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Contents

2.3 2.4 2.5 2.6 2.7 2.8 2.9 2.9.1 2.9.2 2.10 2.11 2.12 2.13 2.14 2.15

3

3.1 3.2

3.2.1 3.2.2 3.2.2.1 3.2.2.2 3.2.3

Theory of Electronic Thin Films: Electronic Percolation and Spintronic Theory on the Semiconductor Thin Film 60 Theory of Metal Structural Thin Films: Metamaterials and the Negative Permeability Theory and Maxwell Theory 62 Theory of Surface Plasmon Resonance and Magnetoplasmonic Thin Films 66 Heterojunction Theory 73 Topological Insulator, Topological Semi-metal, and Perovskite 74 Acoustic Theory 77 Theory of Magnetoacoustic and Photoacoustic Coupling 79 The Mechanism of the Sound Pressure in the Presence of the Pulse Magnetic Field 80 The Mechanism of the Sound Pressure in the Presence of the Pulsed Magnetic Field and Static Magnetic Field 80 Theory of Acoustooptic Effect 82 Magnetothermal Thin Films: Phonon Thermal Theory 83 Theory of Thermoelectric Effect 84 Thermal Barrier Insulation Theory for TBC Coating 86 Permeability Theory: Fick First Diffusion Theory and Fick Second Diffusion Theory 87 Multi-physical Field Coupling Theory and Simulation Software Introduction 88 Acknowledgments 90 List of Abbreviation 91 References 91

Multilayered Magnetic Thin Films for Electron Transport Control and Signal Sensing: From GMR, CMR, TMR to Quantum Anomalous Holzer Effect 95 Weiwei Zhang and Yujun Song Introduction 95 Multilayered Magnetic Thin Film for the Electron Transport Control and Sensing Based on Magnetoresistance (GMR) Effect 96 Introduction of GMR 96 Fabrication of GMR Multilayered Thin Films 97 MBE Method for the Fabrication of the GMR Devices 99 Magnetron Sputtering Method for the Fabrication of GMR Devices 99 GMR Applications for Sensors 100

Contents

3.3

3.3.1 3.3.2 3.3.3 3.4

3.4.1 3.4.2 3.4.3 3.5 3.6

4

4.1 4.2 4.2.1 4.2.2 4.2.3 4.3 4.3.1 4.3.2 4.3.3

Multilayered Magnetic Thin Film for the Electron Transport Control and Sensing Based on Colossal Magnetoresistance (CMR) Effect 102 Introduction of CMR 102 Fabrication of Multilayered Thin Films Based on CMR Effect 103 CMR Applications 105 Multilayered Magnetic Thin Film for the Electron Transport Control and Sensing Based on Colossal Tunneling Magnetoresistance (TMR) Effect 106 Introduction of TMR 106 Fabrication of Multilayered Thin Films of the TMR Effect 107 TMR Applications 110 The Multilayered Magnetic Thin Film Based on Quantum Anomalous Holzer Effect (QAHE) 111 Summary and Perspectives 112 Acknowledgments 113 List of Abbreviation and Symbol 114 References 114

Bismuth-Substituted Iron Garnet Films for Magnetophotonics: Part A – Fabrication Methods and Microstructure Property Study 125 Andrey A. Voronov, T. Mikhailova, Olga V. Borovkova, Alexander N. Shaposhnikov, Vladimir N. Berzhansky, and Vladimir I. Belotelov Introduction 125 Fabrication Methods 126 Synthesis Technology and Conditions of Bismuth-substituted Iron Garnet Films 126 Fabrication of Fabry–Perot 1D-MPC with BiIG Bilayer 135 Fabrication of Tamm 1D-MPC with BiIG Bilayer 136 Properties of the Structures 139 Magneto-optical Properties of FP-1D-MPCs 139 Magneto-optical Properties of T-1D-MPCs with BiIG Bilayer 143 An increase of the Magneto-optical Response in the Ultrathin Films 145 Acknowledgment 155 List of Abbreviations and Symbols 156 References 156

vii

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Contents

5

5.1 5.2 5.3 5.3.1 5.3.2 5.3.3 5.3.4 5.3.5 5.3.5.1 5.3.5.2 5.3.5.3 5.3.5.4 5.3.6 5.3.7 5.3.7.1 5.3.7.2 5.3.8 5.4

6

6.1 6.2 6.2.1 6.2.2 6.2.3

Bismuth-substituted Iron Garnet Films for Magnetophotonics: Part B – Devices and Applications 161 Andrey A. Voronov, Daria O. Ignatyeva, Nikolay A. Gusev, Petr M. Vetoshko, Nazar V. Lugovskoy, Yujun Song, Vladimir N. Berzhansky, and Vladimir I. Belotelov Device Assemble and Application of Iron Garnet Films for Ultrasensitive Magnetometry 161 Devices Assemble and Application of BiIG Films for Biosensing 174 Devices Assemble and Application of Iron Garnet Films for Magneto-optical Eddy Current Flaw Detection 178 Introduction 178 Experimental Part 179 Introscope 180 Physical Properties of MO Sensors 181 The Sensory Properties of the EA Films 184 The Effect of Alternating Field Amplitude 184 The Effect of Alternating Field Frequency 184 The Effect of Bias Magnetic Field 185 Dynamic Domains in the Garnet Film Sensor Element 186 The Sensory Properties of the EP Films 188 Applications of MOEC: Imaging of Welds 188 Nondefective Welds 188 Defective Welds 190 Simulation of EC Magnetic Fields in Samples with Defects 191 Conclusions and Perspectives 193 Acknowledgments 193 List of Abbreviation and Symbol 194 References 194

MEMS, NEMS, AEMS, and Quantum Films for the Next Generation of Computing and Information Technology 199 Haishuai Chai, Junmei Wang, and Yujun Song Introduction 199 Typical Fabrication Methods for MEMS, NEMS, and AEMS 200 Fabrication of Microstructures 200 Fabrication Process of Complementary Metal Oxide Semiconductor (CMOS) 203 Fabrication Process of Field Emission Transistors (FET) 203

Contents

6.2.4 6.3 6.3.1 6.3.2 6.3.3 6.4 6.4.1 6.4.2 6.5 6.6

7

7.1 7.2 7.2.1 7.2.2 7.2.3 7.2.4 7.3 7.3.1 7.3.2 7.3.3 7.3.4 7.4 7.4.1 7.4.2 7.4.2.1 7.4.2.2 7.4.3

Giant Magnetoresistance (GMR) Sensor and Its Fabrication Method 204 From MEMS to NEMS and then to Quantum Films and AEMS for the Next Generation of Information Technology 205 The Trend of Microsystem Integration Technology 205 The Development Trend of Microsystem Packaging Technology 207 Challenges in the Development of Microsystems Technology 207 NEMS and AEM 210 NEMS 210 AEMS 211 Quantum Films for Information Technology 212 Summary and Perspectives 214 Acknowledgments 214 List of Abbreviations 214 References 215 Metamaterial or Metastructural Thin Films for EM Wave Control 221 Menglin L.N. Chen, Luzhou Chen, Xunwang Dang, Maokun Li, Li Jun Jiang, and Wei E.I. Sha Introduction 221 Modeling and Synthesis Methods of Metasurfaces 222 Jones Vector and Jones Matrix 223 Polarizability Model 224 Susceptibility Model 225 Equivalent Impedance Model 226 Simulation Algorithms of Quasi-periodic Electromagnetic Surfaces 227 Introduction to EM Surfaces 227 Design of Quasi-periodic EM Surfaces 228 Simulation Algorithms of Quasi-periodic EM Surfaces 229 Review of Simulation Algorithms of Quasi-periodic EM Surfaces 230 Orbital Angular Momentum of Electromagnetic Waves: Generation and Detection 233 Introduction 233 Generation of Orbital Angular Momentum 234 Geometric-phase Metasurfaces 234 Photonic Crystals 237 Detection of Orbital Angular Momentum 238

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Contents

7.5 7.5.1 7.5.2 7.5.3 7.5.4 7.5.5 7.6

8

8.1 8.2 8.2.1 8.2.2 8.2.3 8.3 8.3.1 8.3.2 8.3.3 8.3.4 8.4 8.4.1 8.4.2 8.4.3 8.4.4 8.4.5 8.5

9

9.1

Application in Spontaneous Emission Modification 241 Spontaneous Emission in Inhomogeneous Electromagnetic Environment 241 Calculation of Spontaneous Emission Rate 242 Metamaterials Enhance Spontaneous Emission 242 Metasurfaces Enhance Spontaneous Emission 243 Other Potential Application in Quantum Optics 245 Conclusion and Perspectives 245 Acknowledgments 245 List of Abbreviations 246 References 247 Semiconductor Thin Films for Information Technology 257 Na Chen Introduction 257 Fabrication of Semiconductor Thin Films 258 Molecular Beam Epitaxy (MBE) 259 Magnetron Sputtering 259 Metal–Organic Chemical Vapor Deposition (MOCVD) 260 Nonmagnetic Semiconductor Thin Films and Typical Applications 261 Semiconductor Thin Films for Light-emitting Devices 261 Thin Film Transistors for Displays 263 Phase-change Semiconductor Thin Films 264 Semiconductor Thin Films for Sensors 268 Magnetic Semiconductor Thin Films 269 Diluted Magnetic Semiconductors 270 Amorphous Magnetic Semiconductors 272 Phase-change Amorphous Magnetic Semiconductor Thin Films 275 Magnetic Semiconductor Thin Film-based Spintronic Devices 277 Prospective for Magnetic Semiconductors 279 Conclusion and Outlook 280 List of Abbreviations 280 References 280 Glass Transition in Organic Semiconductor Thin Films 285 Han-Nan Yang and Zheng-Hong Lu Introduction 285

Contents

9.2 9.3 9.4 9.5

10

10.1 10.2 10.3 10.3.1 10.3.2 10.3.3 10.4 10.4.1 10.4.1.1 10.4.1.2 10.4.1.3 10.4.1.4 10.4.2 10.4.2.1 10.4.2.2 10.4.2.3 10.4.2.4 10.4.3 10.4.3.1 10.4.3.2 10.5 10.5.1 10.5.2 10.5.3

Determination of Glass Transition Temperature in Organic Thin Films 287 Model for Predicting Glass Transition Temperature of Organic–Organic Composites 291 Model for Predicting Glass Transition Temperature of Nano-organic Composites 292 Summary 295 Acknowledgments 296 List of Abbreviations 296 References 296

Thermoelectric Films for Electricity Generation 299 Metin Yurddaskal, Melis Yurddaskal, Ozan Yilmaz, and Serdar Gultekin Introduction 299 Thermoelectricity 300 Overview of Inorganic and Organic Thermoelectrics for Thin Films 301 The Seebeck Effect 301 The Peltier Effect 307 The Thomson Effect 309 Classification of Thin Film Thermoelectric (TE) Materials 311 Inorganic Thermoelectric Thin Films 311 Bi–Te-Based Superlattices 311 Cobalt Oxide-Based Thin Films 311 Zn-Based Thin Films 312 Cu-Based Thin Films 312 Organic-based Thin Film TE Materials 313 Polyacetylene and Polyaniline 313 Poly(3,4-ethylenedioxythiophene) 313 Polypyrrole and Polythiophene 314 Other n-Type Polymers 314 Inorganic–Organic Composite Thermoelectric Thin Film Materials 314 Metal–Organic Frameworks 315 Carbon Nanotube–Polymer Composites 315 Applications of Thermoelectric Materials 315 Thermoelectric Cooling 316 Thermoelectric Power Generation 316 Organic Inverter Circuit 316

xi

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Contents

10.5.4 10.5.5 10.5.6 10.5.7 10.6 10.6.1 10.6.2 10.6.3 10.6.4 10.6.5 10.6.6 10.6.7 10.7

Organic Light-Emitting Diode (OLED) 318 Organic Radio Frequency Identification Tags 319 Organic DNA Sensors 319 Limitations 320 Techniques of Thin Film Deposition for Thermoelectric Device 320 Sputtering 320 Molecular Beam Epitaxy (MBE) 321 Metal–Organic Chemical Vapor Deposition (MOCVD) 321 Electrochemical Deposition (ECD) 323 Flash Evaporation (FE) 323 Thermal Evaporation 324 Pulsed Laser Deposition (PLD) 324 Conclusion and Future Trends 326 List of Abbreviations and Symbols 327 References 328 Volume 2 Biography xv Preface xvii Acknowledgments

xxi

11

Template-assisted Fabrication of Nanostructure Thin Films for Ultrasensitive Detection of Chemicals and Biomolecules: Part A – Template-assisted Nanoimprinting Technology for Functional Thin Films 339 Xiaomin Zhu, Xinhua Chen, Andrey A. Voronov, Vladimir I. Belotelov, and Yujun Song

12

Template-assisted Fabrication of Nanostructured Thin Films for Ultrasensitive Detection of Chemicals and Biomolecules: Part B – Detection of Chemicals and Biomolecules Based on Nanostructured Thin Films 381 Xiaomin Zhu, Xinhua Chen, and Yujun Song

13

Polymer-based Films for Artificial Intelligence 411 Ran Liu, Junmei Wang, and Yujun Song

14

Selective Permeable Thin Films and Membranes 447 Qiong Wu, Xiaoxiong Zhao, Lifan Peng, and Yujun Song

Contents

15

Biomass-Derived Functional Films and Coatings 489 Gao Xiao

16

Polymer Composite Coating for Anti-marine and Related Organism Corrosion 511 Kaifeng Chen, Zhipeng Xie, Yu Liang, Jingjing Wang, and Haiyan Zhuang

17

Anechoic Coating for Underwater Vehicles 549 Weiwei Zhang and Yujun Song

18

Thin Films and/or Coating for Electromagnetic Interference and Stealth 587 Junmei Wang and Yujun Song

19

Thermal Barrier Coating for Aerial and Aerospace Engine 615 Zaidao Li and Yujun Song

20

Perspectives for Thin Films and Coatings 647 Yujun Song Index

681

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v

Contents

Volume 1 Biography xv Preface xvii Acknowledgments

xxi 1

1

Introduction: Progress of Thin Films and Coatings Yujun Song

2

Fundamental in Functional Thin Films and Coatings 59 Weiwei Zhang and Yujun Song

3

Multilayered Magnetic Thin Films for Electron Transport Control and Signal Sensing: From GMR, CMR, TMR to Quantum Anomalous Holzer Effect 95 Weiwei Zhang and Yujun Song

4

Bismuth-Substituted Iron Garnet Films for Magnetophotonics: Part A – Fabrication Methods and Microstructure Property Study 125 Andrey A. Voronov, T. Mikhailova, Olga V. Borovkova, Alexander N. Shaposhnikov, Vladimir N. Berzhansky, and Vladimir I. Belotelov

5

Bismuth-substituted Iron Garnet Films for Magnetophotonics: Part B – Devices and Applications 161 Andrey A. Voronov, Daria O. Ignatyeva, Nikolay A. Gusev, Petr M. Vetoshko, Nazar V. Lugovskoy, Yujun Song, Vladimir N. Berzhansky, and Vladimir I. Belotelov

vi

Contents

6

MEMS, NEMS, AEMS, and Quantum Films for the Next Generation of Computing and Information Technology 199 Haishuai Chai, Junmei Wang, and Yujun Song

7

Metamaterial or Metastructural Thin Films for EM Wave Control 221 Menglin L.N. Chen, Luzhou Chen, Xunwang Dang, Maokun Li, Li Jun Jiang, and Wei E.I. Sha

8

Semiconductor Thin Films for Information Technology 257 Na Chen

9

Glass Transition in Organic Semiconductor Thin Films 285 Han-Nan Yang and Zheng-Hong Lu

10

Thermoelectric Films for Electricity Generation 299 Metin Yurddaskal, Melis Yurddaskal, Ozan Yilmaz, and Serdar Gultekin Volume 2 Biography xv Preface xvii Acknowledgments

11

11.1 11.2 11.2.1 11.2.2

xxi

Template-assisted Fabrication of Nanostructure Thin Films for Ultrasensitive Detection of Chemicals and Biomolecules: Part A – Template-assisted Nanoimprinting Technology for Functional Thin Films 339 Xiaomin Zhu, Xinhua Chen, Andrey A. Voronov, Vladimir I. Belotelov, and Yujun Song Development of Template-assisted Nanoimprinting Technology 339 Nanosphere Lithography (NSL) 340 Size and Shape Controlled Fabrication of Nanomaterials via NSL 340 Multi-hierarchy Micro Windows (MHMW) for Single Nanostructure and/or Array Identification 344

Contents

11.2.3

11.2.4 11.2.4.1 11.2.4.2 11.2.4.3 11.3 11.3.1 11.3.2 11.3.2.1 11.3.2.2 11.3.3 11.3.3.1 11.3.3.2 11.3.3.3 11.3.4 11.3.5 11.4

12

12.1 12.2

Identification of Localized Surface Plasmon Resonance (LSPR) of Single Noble Metal Nanoparticles and/or Nanoarrays by MHMW-assisted NSL 346 Development of NSL for Nanomaterials Synthesis 351 Aqueous Phase Ag Nanoparticles with Controlled Shapes Fabricated by NSL 351 Ultrathin Nanopore Arrays with Uniform Size 356 Fabrication of Periodic Uniform Nanoporous Films with Controlled Layers 357 Anodic Aluminumoxide (AAO) Template-assisted Nanoimprinting 359 Introduce of AAO Template 359 Preparation Methods 361 Preparation of Anodic Alumina Template 361 Preparation of Thin Film Substrates for Surface-enhanced Raman Scattering 362 Preparation and Characterization of Nanoporous Gold Thin Films Based on Anodic Alumina Template (AAO) 363 Substrate Transfer and Characterization of Nanoporous Gold Thin Films 364 Preparation and Characterization of Aqueous Spherical Gold Nanoparticles 367 Composite of Nanoporous Gold Thin Films and Aqueous Gold Nanoparticles 367 AAO Template-assisted Fabrication of Ultra-dense Nanoparticle Arrayed Thin Films 367 AAO Template-assisted Fabrication of Semiconductive Nanowire Thin Films 371 Summary and Perspective 376 Acknowledgments 377 List of Abbreviations and Symbols 378 References 378

Template-assisted Fabrication of Nanostructured Thin Films for Ultrasensitive Detection of Chemicals and Biomolecules: Part B – Detection of Chemicals and Biomolecules Based on Nanostructured Thin Films 381 Xiaomin Zhu, Xinhua Chen, and Yujun Song Introduction 381 Nanostructured Thin Films with Enhanced Magneto-optical Kerr Effect 381

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Contents

12.3 12.4 12.4.1 12.4.2

12.4.3

12.5 12.5.1 12.5.2 12.5.3 12.5.4 12.5.4.1 12.5.4.2

12.5.5 12.5.5.1 12.5.5.2 12.5.5.3 12.5.5.4 12.6

13 13.1 13.2 13.2.1 13.2.2 13.3 13.3.1

Surface Plasmon-enhanced Magneto-optical Kerr Effect for Chemical Analysis 386 The Application of AAO in the Detection of Biological Cells 388 Difficulties and Bottlenecks in Cancer Treatment 388 Label-free Reflectometric Interference Microchip Biosensor Based on Nanoporous Alumina for Detection of Circulating Tumor Cells 389 Surface and Interface Engineering Multilayered Nanopore Films for Enhanced Fabry−Pérot Interferences for Biological Application 393 Nanostructured Thin Films for SERS 394 Introduction of SERS 394 Development Nanostructured Thin Films for SERS 396 Preparation of SERS Thin Film Substrates 397 Characterizations of SERS 398 Properties of Porous Gold Nanofilms with Composite Aqueous Gold Nanoparticles in SERS Based on PMMA Substrate 398 Surface-enhanced Raman Properties of Porous Gold Nanofilms and Their Composite Aqueous Gold Nanoparticles Based on PDMS Substrate 399 Chemical and Biological Application of SERS Based on Nanostructured Thin Films 401 Application of SERS in Ion Detection 401 Application of SERS in Environmental Pollution 401 SERS Detection of R6G on Magneto-optical Thin Films with Nanoholes 402 Application of SERS in Cancer Cell Detection 402 Summary and Perspective 405 Acknowledgments 406 List of Abbreviations and Symbols 406 References 407 Polymer-based Films for Artificial Intelligence 411 Ran Liu, Junmei Wang, and Yujun Song Introduction 411 Preparation and Integration Methods of Smart Polymer Thin Films and Coatings for AI 412 Preparation of Multifunctional Polymer Thin Films and Coatings 412 Integration of Smart Devices for AI 413 Thin Films or Coatings for AI+ Biological Application 415 Tactile Sensor (for Artificial Skin) 415

Contents

13.3.2 13.3.3 13.3.4 13.3.5 13.4 13.4.1 13.4.1.1 13.4.1.2 13.4.1.3 13.4.2 13.4.2.1

Thermal Sensor (for Temperature Monitoring) 417 Human Thermoregulation 418 Biosensor Film Substrate 421 Bio-Integrated Wearable Sensors 422 Thin Films or Coatings for AI+ Environmental Protection 425 Thin Films or Coatings for AI+ Environmental Monitoring 425 Ultraviolet Radiation Monitoring 425 Explosives Detection 426 Ammonia Sensor 428 Thin Films or Coatings for AI+ Wastewater Treatment 428 Efficient Degradation of Complex Phthalocyanine Dye Wastewater 428 13.4.2.2 Efficient Adsorption of Patulin 429 13.4.3 Thin Films or Coatings for AI+ Seawater Desalination 430 13.5 Thin Films or Coatings for AI+ Energy 431 13.5.1 Thin Films for AI+ Solar Energy 431 13.5.2 Thin Films for AI+ Capacitor 432 13.5.3 Thin-Film Thermoelectric Devices 434 13.6 Artificial Intelligence for Information Technology 435 13.6.1 Artificial Intelligence for Optical Polymer-based Films 435 13.6.2 AI+ Smart Display (Plastic Liquid Crystal Display) 436 13.6.3 High-efficiency THz-Wave Modulators 438 13.6.4 Thermal Management for Electronic Equipment 439 13.7 Summary and Perspectives of Thin Films and Coatings for AI+ 439 Acknowledgments 440 List of Abbreviations 441 References 442 Selective Permeable Thin Films and Membranes 447 Qiong Wu, Xiaoxiong Zhao, Lifan Peng, and Yujun Song 14.1 Introduction 447 14.2 The Principle of Membrane Separation 449 14.3 Types of Selective Permeable Membranes 452 14.3.1 Types and Characteristics of Membranes 452 14.3.2 The Rise of New Membranes 453 14.3.2.1 Polymer Separation Membrane 453 14.3.2.2 Polymer Functional Membrane 454 14.3.2.3 The Plasma Polymer Film 454 14.4 Preparation Methods of Varieties of Selective Permeable Membranes 456 14.4.1 Phase Inversion 456

14

ix

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Contents

14.4.2 14.4.3 14.5 14.5.1 14.5.2 14.5.2.1 14.5.2.2 14.5.3 14.5.3.1 14.5.3.2 14.5.3.3 14.5.3.4 14.5.3.5 14.6

15 15.1 15.2 15.2.1 15.2.2 15.2.3 15.2.4 15.2.5 15.2.6 15.2.7 15.3 15.3.1 15.3.2 15.3.3 15.4 15.4.1 15.4.2 15.4.3 15.4.4

Interfacial Polymerization 456 Chemical Modification 456 Application of the Selective Permeable Membranes 457 Purification of Water 457 Application in Gas Separation 469 Oxygen Separation 470 CO2 Separation 472 Application in the Separation and Purification of Low Molecular Substances 476 Isolation and Purification of Oligosaccharides 476 Fructooligosaccharides 477 Galactooligosaccharides 477 Isolation and Purification of Amino Acids 477 Application in the Isolation and Purification of Antibiotics 478 Current Status and Recent Progress and Perspectives 479 Acknowledgments 481 List of Abbreviations and Symbols 482 References 483

Biomass-Derived Functional Films and Coatings 489 Gao Xiao Introduction 489 Biomass-derived Polymers 490 Collagen 491 Cellulose Nanofibrils 492 Pectin 494 Starch 495 Chitosan 496 Xylan 497 Lignin 498 Coating Technologies of Biomass Thin Films 500 Sol–Gel Coating Method 500 Atomic Layer Deposition (ALD) 501 Multilayers Coating Method 502 Degradable Biomass-derived Functionalized Films 502 Polysaccharide-based Films 502 Protein-, Pig Skin Gelatin-, Lipid-based Films 503 Biomass Plastics Films 503 Cellulose-based Films 504 Acknowledgments 505 References 505

Contents

16

16.1 16.2 16.2.1 16.2.2 16.3 16.3.1 16.3.2 16.3.3 16.3.4 16.3.5 16.4 16.4.1 16.4.2 16.4.2.1 16.4.2.2 16.4.2.3 16.4.3 16.4.3.1 16.4.3.2 16.4.3.3 16.4.3.4 16.5

17 17.1 17.2 17.2.1 17.2.2 17.2.3 17.3 17.4 17.4.1

Polymer Composite Coating for Anti-marine and Related Organism Corrosion 511 Kaifeng Chen, Zhipeng Xie, Yu Liang, Jingjing Wang, and Haiyan Zhuang Introduction 511 Microbial Corrosion and Protection 512 Primary Mucosa-forming Organisms and Their Attachment 512 Overview of Fouling Biological Control 513 Anti-mold Coating 516 Silicone Anti-mold Coating 517 Acrylic Mold-proof Coating 517 Polyurethane Anti-mold Coating 517 Epoxy-based Mold-proof Coating 517 Development Trend of Anti-mold Coatings 518 Antifouling Coatings 518 Main Types and Selection of Antifouling Agents 518 Main Types and Selection of Antifouling Coatings 520 Abrasion-Resistant Antifouling Coating 520 Self-polishing Antifouling 523 Fouling Release Type Antifouling Paint 531 Development Trend of Antifouling Technology 535 Antifouling Agent Design 535 Gel Mixed Fouling Release Type Antifouling Paint 538 Surface Microstructure Bionic Antifouling Paint 539 UV Antifouling Technology 540 Summary and Prospects 540 List of Abbreviations 541 References 541 Anechoic Coating for Underwater Vehicles 549 Weiwei Zhang and Yujun Song Introduction 549 Fundamental Physics for Sonar System: Echolocation and Doppler Effect 552 Types and Construction of Sonar Systems 553 Introduction of Some Typical Sonar Systems 555 Threat from Advanced Hostile Sonar System 565 Basic Physics of Anechoic Coatings 573 Development of Structure and Materials Design for Anechoic Coatings 574 Perspective for the Sonar System and Anechoic Coating Development 582

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Acknowledgments 582 List of Abbreviations 583 References 583 18

18.1 18.2 18.2.1 18.2.2 18.2.2.1 18.2.2.2 18.2.2.3 18.2.2.4 18.3 18.3.1 18.3.1.1 18.3.1.2 18.3.1.3 18.3.2 18.3.2.1 18.3.2.2 18.3.2.3 18.4

19

19.1 19.2 19.3 19.4 19.5 19.5.1 19.5.2 19.5.3 19.5.4

Thin Films and/or Coating for Electromagnetic Interference and Stealth 587 Junmei Wang and Yujun Song Introduction 587 EMI Shielding Materials 589 Basic Theory of Electromagnetic Shielding 589 EMI Shielding Materials 591 Surface Coating Film Shielding Material 591 Ferromagnetic Material and Good Metallic Conductor Material 591 Conductive Coating Shielding Materials 592 Composite Shielding Materials 592 Stealth Coatings 599 Radar Stealth Coatings 600 Radar-Absorbing Materials 601 Radioisotope RAM 604 Carbonaceous-Based RAM 604 The Development of Stealth Coating 606 Negative Refractive Stealth 606 Multiband Absorbing Material 607 Plasmon Active Stealth 607 Summary and Prospects 608 Acknowledgments 608 List of Abbreviations and Symbols 608 References 609 Thermal Barrier Coating for Aerial and Aerospace Engine 615 Zaidao Li and Yujun Song Introduction 615 Superalloy Substrates for TBCs 618 TBC System Compositions 619 Applications of TBCs 620 Processing Techniques for TBCs 621 Electron Beam-Physical Vapor Deposition (EB-PVD) 621 Plasma Spraying (PS) Process 624 Solution Precursor Plasma Spraying (SPPS) Process 627 Suspension Plasma Spraying (SPS) 629

Contents

19.6 19.7

Thermal Transport in TBCs 633 Summary and Perspectives 635 Acknowledgments 637 List of Abbreviations 637 References 637

20

Perspectives for Thin Films and Coatings 647 Yujun Song Introduction 647 Development of the Subversive Novel Concepts for the Theoretical and Technological Breakthrough and New Findings of Thin Films and Coatings 647 Development of Highly Precise Fabrication Techniques for Thin Films and Coatings 650 Perspective in the Development of High-Spatiotemporal-Resolution Characterization Methods for Microstructures and Properties 655 Perspectives in the Further Study of Relationship Between Microstructure and Property 661 Perspectives of Thin Films and Coatings Promoting the Modern Technological Innovation and the Society Progress 668 Acknowledgments 671 List of Abbreviations 671 References 672

20.1 20.2

20.3 20.4

20.5 20.6

Index

681

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Biography Dr. Yujun Song is currently a full-time Professor in Physics and Applied Physics at University of Science and Technology Beijing (USTB), Deputy Director of the Center for Modern Physics Research of USTB, and Director of the Zhejiang Key Laboratory for Pulsed Power Technology Translational Medicine. After obtaining his PhD degree in Materials Science and Engineering in 2000, Dr. Song has successively studied/worked at the Center for Advanced Microstructures and Devices of Louisiana State University, Old Dominion University, Beihang University, University of Toronto, and Harvard University, etc. His research areas are focusing in the integrated innovation of modern physics fusing into biomedicine, information technology, and energy/catalysis mediated by novel nanomaterial and high precise fabrication technology, particularly the nanomedicines mediating ultra-strong pulsed field activating immunogenicity for novel therapy of intractable diseases. Dr. Song has been responsible for more than 30 funds as PI or co-PI, such as the National S&T Major Project, the NSFC–BRICS STI Framework Program, the National Natural Science Foundation of China. He also took part in many special funds as one key scientist, such as the major project on bio-nanoscience of NIH: the study of biomolecular transport mechanism by nanobiomolecular probes funded, the major project on nanoscience of NSF: The design and preparation of nano optical biomolecule probe for biomolecular transport study, the major project on biosensor of DARPA: design and fabrication of GMR biosensor, the NSF – EPSCoR project Microfluidic synthesis of magnetic nano particles and their application in cancer diagnosis and therapy.

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Preface During the past decades, great progress has been achieved in promoting thin films and coatings toward new technological innovation and the development of basic theories such as the magnetoplasmon technology and the quantum anomalous Holzer effect. As told by Confucius, “if a man keeps cherishing his old knowledge, so as continually to be acquiring new, he may be a teacher of others.” With engineers and scientists serving as both thinkers and practitioners, it is only fitting that the recent progress in the fundamental physics and chemistry of the microstructure-dependent property of thin films and coatings be summarized, including their successful applications that will feedback to promote the deeper fundamental study on the microstructure-property relationship of this vivid field toward greater achievements in their applications. Thin films are microscopically thin layers of materials that are deposited onto a metal, ceramic, semiconductor, polymer substrate or a layer on a supporting liquid. These have been developed with thickness ranging from one atom/molecule layer to millions of atoms/molecule layers, i.e. from hundreds of micrometers (μm: 10−6 m) to nanometers (nm: 10−9 m) or even atomic layer thin (10−10 m; e.g. atomic electronic mechanics). Thin films can be conductive or dielectric (non-conductive), which are used for advanced electronic and/or optoelectronic components (e.g. capacitor, resistor, coil, cryotron, transistors), devices, or circuits for various applications (e.g. computing, sensing, signal/energy/mass transmitting or transporting or exchanging). On the other hand, coatings on the surface of substrates create or improve corrosion protection, heat and radiation resistance, thermal management, electromagnetic responses (e.g. stealth or detecting), acoustic responses (sensing or anechoic), water and ice protection, friction reduction, antifouling and antibacterial properties, and self-cleaning and other specific physical or chemical functions. Their thickness usually ranges from micrometer (10−6 m) to millimeters (10−3 m) or thicker, which can be expanded to centimeters (cm) or above (e.g. EM stealth coating or anechoic coating) and even thinner (e.g. nanocoating). Thin films and coatings are sometimes combined to achieve multifunctionality (e.g. magneto-optical, thermoelectric). Thin films and coatings are intrinsically related to surface and interface science and technology, but becoming more and more multidisciplinary. Research and application both involve basic physics, chemistry, materials science, biology and life, and engineering in materials, aerospace, marine, energy, and information technology.

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Great academic and industrial achievements in information technology, computer science, new energy, aeronautics, astronautics, and ocean engineering have been made in the past decades owing to the fundamental breakthrough in surface and interface science and the corresponding precise fabrication technology. These fundamental quantum theory in thin films, construction of thin film devices precisely in atomic scale, orientation growth and microstructure control of coating, and bonding theory according to their unique applications. At present, thin films and coatings have been upgraded as a brand-new technology, not only for their own functions but also for their role as an indispensable bridge between fields of multidisciplinary science and technology leading to innovations. Quoting Herbert Kroemer, “The interface is the device.” However, a surface is only a kind of special interfaces between the gas phase and the solid phase or the liquid phase. Therefore, the fundamentals of the unique surface/interface effects on the thin films and coatings will be firstly summarized and primarily described: interfacial magneto-electric coupling, electronic percolation, metasurface and metastructure-related complex permittivity and Maxwell equation, surface plasmon resonance, magnetoacoustic and photoacoustic coupling, interfacial heterojunction, thermal barrier insulation, and the interface diffusion theory. In addition, the recently developed multiphysical field coupling theory and the related simulation software will be briefly introduced for the readers’ further study. Subsequently, the key fabrication technology, the structure–property relationship, and typical applications of some typical thin films and coatings of special functions will be summarized and discussed in detail, chapter by chapter, based on these surface/interface effects. This book can be a fundamental tool for the current researchers and commercial users of thin films and coatings who investigate the underlying physics and chemistry theory and the manual of their current and future fantastic applications. In addition, this would be a useful and powerful reference for the newcomers to enrich their knowledge and enlighten their own strategies in the development of new theories and extended applications of thin films and coatings. There is an abundance of available literature related to thin films and coatings. We have compiled them into two volumes for the readers’ convenience. Volume 1 covers topics closely related to inorganic (e.g. metallic, dielectric, semiconductor) thin films with unique magnetic, optical, electronic, and/or thermal properties will be mainly summarized together. We will summarize and discuss the recent progress on thin films and coatings in Chapter 1 and then their unique functions in detail in Chapter 2. The key thin film technologies will be presented in Chapters 3–8 mainly for the current and next generation of computing and information technology (5G and 6G). Specifically, we will discuss in Chapter 3 multilayered magnetic thin films for electron transport control and signal sensing, from GMR to CMR, then to TMR, and finally to quantum anomalous Holzer effect. We will tap on the recently developed magnetophotonic thin films including their basic physics and typical fabrication methods in Chapter 4 and their structure–property relationship and applications in Chapter 5, for example, using bismuth-substituted iron garnet thin films. The recent progress on the semiconductor thin films and related MEMs, NEMs,

Preface

and AEMs will be summarized and discussed in Chapter 6. We will then review comprehensively in Chapter 7 the recently developed metamaterials and metastructural thin films for the electromagnetic wave control, including their theoretical foundations, design routes, numerical methods, and engineering applications of the metastructural films (metasurfaces). The semiconductor thin films for information technology and one recent key issue on the organic semiconductors (or their glass transition) will be discussed in Chapters 8 and 9, respectively. In the final chapter of Volume 1, by comparing with conventional technologies, we will focus on one emerging energy technology, a thermoelectric thin film, whose main principle for the electricity generation and advantages. In Volume 2, the first three chapters are about the organic–inorganic composite thin films: the recent progress in the magnetoplasmonic thin films via template-assisted nanoimprinting lithography and their surface modification for some chemical or biological applications (e.g. surface-enhanced Raman scattering, ultrasensitive biosensor) in Chapters 1 and 2 and the organic–inorganic composite films for new energy technology in Chapter 3. Following this, the basic physicochemical theory and the recent progress of some key organic thin films will summarized and discussed: polymer functional thin films for artificial intelligence in Chapter 4; the selective permeable thin films or membranes for the mass separation, concentration and purification in Chapter 5; and biomass-derived functional films and coatings in Chapter 6. Finally, some advanced coatings for marine engineering and aero-engineering will be summarized and discussed: polymer composite coatings for anti-marine microbial coating in Chapter 7, anechoic coatings for the underwater vehicles in Chapter 8, thin films and coatings of electromagnetic compatibility and/or stealth in in Chapter 9, and thermal barrier coatings in Chapter 10, which are of important applications for national key industrial sectors and the national long-term scientific development strategies. Finally, we will share some prospects for the development of inorganic and organic thin films and coatings in the breakthrough in basic theory and concepts, design principle, experimental testing methods, the in-depth study on the relationship between microstructures and properties and their future impact on the society and economy. “Actions are done after thorough consideration rather than casual decision,” as quoted from the book “Quan Xue Jie (Learning Persuasion)” by Yu Han in Tang Dynasty. We hope that this book could be an effective tool or reference that would benefit the readers’ critical thinking to master the reasonable direction of this field. This book shall contribute to the research and teaching in this field as a valuable addition to the literature and also encourage more readers to pay attention to this rapidly developing field. It is impossible to include all the reported progress and principles with regard to thin films and coatings because both are inherently related that they significantly cover several interdisciplinary topics that bind almost all academics and industrials. Therefore, we will feel honored if only this monograph can serve as a useful handbook that can provide readers support on learning and critical thinking in order to thrust new scientific and technological developments.

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Finally, I am much grateful to all authors of each chapter, my students who took part in writing the chapters, and the editors from Wiley during editing. I dedicated this book to my family for their utmost support and encouragement (particularly from my father, Mr Sigan Song, and my mother, Ms Xiuyun Meng) while I was editing this book during the COVID-19 pandemic. Much interestingly, my lovely daughter, Xinran Song, influenced by the endless writing and reading for this book, starts writing her own fiction novel. 2 February 2020 Beijing, China

Yujun Song

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Acknowledgments This book received support from the NSFC-BRICS STI Framework Program (No. 51861145309), the National S&T Major Project (No. 2018ZX10301201), the National Natural Science Foundation of China (No. 51971029), the all-English Teaching Demonstration Course Construction Project of University of Science and Technology Beijing (No. KC2015QYW06, 2016), the Joint Research Project of University of Science and Technology Beijing and the University of Science and Technology Taipei (Grant No. TW2018007), the “100 Talent Plan” fund of Fujian province (No. 39080067), and the “1125” Zhihui Zhengzhou Talent Project of Henan province (No. 39080070 in USTB).

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1 Introduction: Progress of Thin Films and Coatings Yujun Song 1,2 1 University of Science and Technology Beijing, Center for Modern Physics Technology, Applied Physics Department, School of Mathematics and Physics, 30 Xueyuan Road, Beijing 100083, China 2 Zhejiang Key Laboratory for Pulsed Power Technology Translational Medicine, Hangzhou Ruidi Biotechnology Company, Hangzhou 310000, China

1.1 Introduction As one cornerstone of advanced modern technologies, thin films and coatings have been expanding its scope into varieties of emerging research areas and developing rapidly to match the requirement from academics and industrials in the past decades. As elucidated in this book, thin films and coatings that are never dispensable for the development of modern science and technology will continuously play the key innovation driving force in the next generation of computing and information technology, new energy, biology and life science, new medicines, astronautics and aeronautics, geology and ocean engineering, military science, etc. Eventually pushed by the requirement of industrial sectors of countries, the successful application of this field depends intrinsically on the fundamental progress of the surface/interface science and the precision of the related mass fabrication technology. This may be the main reason for “the interface is the device” stated by Herbert Kroemer, the Nobel Prize winner in Physics in 2000 [1, 2] and for so many Nobel Prize winners in the fields relating to surface/interface science (e.g. 1930 Nobel Prize winner in Physics Chandrasekhara Venkata Raman; 1981 Nobel Prize winner in Physics Kai Siegbahn; 2007 Nobel Prize winners in Physics Peter Greenberger and Albert Fert; 2007 Nobel Prize winner in Chemistry Gerhard Eitel; 2010 Nobel Prize winners in Physics Konstantin Novoselov and Andre Geim; 2016 Nobel Prize winners in Physics David Thouless, Duncan Haldane, and Michael Kosterlitz; 2018 Nobel Prize winners in Physics Arthur Ashkin, Gerard Mourou, and Donna Strickland; from www.nobelprize.org). Thus, it is no doubt that each innovation in this field generates much profit mainly from the theoretical breakthrough and the subversive novel concepts in basic physics and chemistry, particularly the condensed matter physics and interface chemistry, and the related measurement and characterization technology, besides the invention of the advanced fabrication and synthesis technology. These innovations will enable Inorganic and Organic Thin Films: Fundamentals, Fabrication, and Applications, First Edition. Edited by Yujun Song. © 2021 WILEY-VCH GmbH. Published 2021 by WILEY-VCH GmbH.

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the discovery of several singular physical phenomena and materials (e.g. topological semimetals: Dirac semimetal, Weyl semimetal, nodal-line semimetals, and triple degenerate semimetals [3]; one-dimensional van der Waals [4]). Sequentially, these innovations promote the application of the finally constructed devices, equipment, and instruments using thin films and coatings as building blocks extending into novel fields (e.g. 2D transistor [5], extremely huge magnetoresistance [3]), and/or upgrading the conventional fields (e.g. rollable optoelectronic devices, such as flexible and foldable organic light-emitting diode [OLED], and graphene enforced rubber composites for industrials [6–9]). (https://china.Huanqiu.com/ article/9CaKrnJY80x; http://en.tireworld.com.cn/cnews/201937/6540.html; http:// www.gianroitire.cn/gianroitire/vip_doc/12760379.html). These will lead to the revolution of modern science and technology and then to societal reform toward a high-level civilization. Thus, thin films and coatings have been instrumental in bridging the academic studies and their industrial applications and the society. Sources, energy, environment, and sustainable development are four fundamental themes of the society that we use to provide the basic necessities of life: food, clothing, shelter, and transportation. These themes depend on the continuous technology innovation in information, transportation, communication, and public health and safety supported by the basic research in physics, chemistry, biology, and life science. Now, several issues and crises arise related to source-, energy-, and ecology-related sustainable development as well as ultrafast and high-throughput communication related to security, public health, national defense and homeland security, etc. Technologies derived from thin films and coatings have been used to develop tools.

1.2 Thin Films for the Innovation of Information Technology In the development of information technology, in order to satisfy the precise navigation for the exploration of outer space and the highly efficient nanosatellites, rapid processing of huge data and long-distance communication are needed, and the related devices should be as small as possible. With the progress of data processing and information technology, this requirement is desired not only in astronautics and aeronautics but also in many other military and society fields, leading to the formation of subversive concepts in miniaturized devices and the planar fabrication technology. Finally, silicon-chip-based thin-film fabrication via photolithography and the subsequent e-beam lithography appears formatting a brand-new industrial field: microelectronics. This requirement also intensively promoted the basic research progress in condensed matter physics (e.g. Si-based semiconductors; complementary metal-oxide semiconductors [CMOS]), chip processing technology for chip-based thin-film devices (e.g. field emission transistor [FET]; vertical transistors [10]; laser emission diodes [LED]), and microelectronic device assembly (e.g. integrated circuit), and various data and imaging processing technologies (e.g. graphics processing unit [GPU] derived software) for information technology. Particularly, several novel physical phenomena or findings related to the surface/interface effects

1.2 Thin Films for the Innovation of Information Technology

are discovered in the thin films constructed by multilayers of magnetic, electronic, and optical materials, such as magneto-optical effects, giant magnetoresistance (GMR) effects, colossal magnetoresistance (CMR) effects, and tunneling magnetoresistance (TMR) effects. A more intensive study was carried out to investigate the effects of the new physics of these thin films (see Chapters 1, 2, 5, 9, and 10), which further boosted the fundamental physics progress and simultaneously widened the range of applications of computer science and communication technology in industrials. As the related large-scale economical fabrication/processing technology was developed along with the progress and interconnection of the physical technology, chemistry synthesis, and materials science and engineering, these concepts and technologies were realized commercially, thus enabling profit generation by the primarily funded enterprises, which can invest more in basic research and advanced technology, forming a benign circling of this field. With the information technology development, more application concepts have been advanced (e.g. 3D vision and artificial intelligence [AI] technology), and ultrafast high-throughput data and image processing technology and the related information security are needed. The current Si-based chip information technology can no longer satisfy this demand since the integration limited by the feaure size of the Si-based chip will reach the ceiling against Moore’s law in the near future. Therefore, more and more novel technical concepts or terms emerge with the rapid technology evolution, such as quantum computer, quantum regulation, quantum information, magnetron light quantum, quantum entanglement communication, and long-distance low-loss optical communication. These concepts have initiated new findings, technologies, and related theoretical studies, such as spintronics, spin-orbital coupling, single photo control, the quantum anomalous hall effect (QAHE), quantum key, room-temperature superconductivity, and high k-space lasing in a dual-wavelength quantum cascade laser technology. In the literature, thin films have been reported as important media that can be used in technologies that need new materials to be further designed. As a result, many new materials for thin-film devices have been explored, such as two-dimensional (2D) materials (e.g. graphene [Gp]; black phosphorus: P-Hg; hexagonal boron nitride [hBN]; transition metal dichalcogenides [TMDs]: WSe2 , MoS2 ), topological insulators (e.g. (Hg, Cd)Te 2D materials [11] Bi2 Se3 nanoribbon [12]), magnetically doped topological insulators enable the QAHE [13], van der Waals heterojunctions (more than one single-atom layer of 2D materials organized using van der Waals force, such as magic-angle graphene [14], TMD/Gp [15], graphene/hBN [16, 17]), nanostructural hybrid multilayered thin films of high magneto-optical effects [18–22], metasurface [23–27] and metastructures [28], etc. Clearly, the development of large-scale thin-film fabrication methods at low cost is primarily required for high-performance devices fulfilling their industrial applications. Besides the conventional fabrication methods (e.g. physical vapor deposition, ion implanting, chemical vapor deposition [CVD], epitaxial growth, nanoimprinting, mechanical exfoliation from bulk crystals), large-scale high-resolution reliable synthesis and fabrication technology have been developed in the past decades. The fabrication limit has reached sub-10 nm line precision and then atomic resolution

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fabrication or even subatomic resolution, particularly in the two-dimensional (2D) materials based thin films [10, 13, 29] or sensing devices based on single molecular assembing thin films [30, 31] or single molecular sensor-assembled thin-film devices. Two-dimensional materials contain electrons that can only move freely in non-nanoscale (1–100 nm) objects, such as nanoscale films, superlattices, and quantum wells. Two-dimensional heterojunctions can combine individual layers of different properties in the atom resolution, which can be used to study to study and understand the exhibited novel physical phenomena that can be further employed for potential applications. The availability of numerous different 2D materials – with different band structures, from semimetals to semiconductors to insulators – also makes it possible to assemble unique materials with well-designed band alignments [32]. Particularly, 2D materials based on thin films, consisting of 2D heterojunctions/heterostructures formed by atomically thin crystal layers bound by the van der Waals force, have attracted much interest because of their potential in diverse technologies, including electronics, optoelectronics, and catalysis. Therefore, 2D thin films are used as an example to exhibit this prerequisite and their development. The self-assembly approach to materials synthesis is generally related to the strong chemical bond to hold different components together (e.g. epitaxial growth via biaxially textured technology, molecular beam epitaxial and metal organic CVD), which is usually limited in those materials of highly matched crystal structures and compatible process. Now, this strategy can be extended to the weak van der Waals interaction to pre-assemble the chemical building blocks physically to realize the fabrication without chemical bonds (e.g. van der Waals heterojunctions), which can avoid the limitations of crystal lattices and manufacturing process [33]. Thus, there are two important kinds of 2D heterostructural materials that can be developed conveniently for the fabrication of microstructures for more tunable physicochemical properties of thin films and coatings. One is the 2D heterostructures stacked by strong bonding (e.g. GaN/NbN epitaxial semiconductor/superconductor heterostructures [34]), and the other is the very popular 2D heterojunctions stacked by van der Waals force (e.g. magic-angle graphene). The function of the 2D heterojunction or heterostructures can be flexibly modified according to the “embedded” design theory. They can be used to assess and reduce the difficulty encountered in the bandgap regulation of a single component [16] and the low on/off current ratio caused by the semimetal nature of single component graphene-based devices [10]. The “embedded” design theory for 2D heterojunction/heterostructures can be conveniently used to fabricate novel highly efficient electronic devices (e.g. vertical transistors formed by graphene/molybdenum ditelluride [MoTe2 ]) [10] and optoelectronic devices [13]. Therefore, the “embedded” design theory for 2D heterojunctions can have fewer components to realize the high-performance analog signal modulation, avoiding the limit of miniaturization with increasing demand on energy in the conventional metal oxide semiconductor field-effect transistor (MOSFET). A wide perspective in the telecommunication field would be developed due to their vast application in the high-performance analog circuits. There are currently two kinds of methods that have to be developed here for the fabrication of the emerging research field of thin films, such as the 2D

1.2 Thin Films for the Innovation of Information Technology

heterojunctions, the topological insulators [35] and the topological semimetals [10]: One type is the top-down physical methods, such as the intercalation/stripping method [36] and the liquid-phase [37] or liquid–air interface assemble [38], usually relating to the progress in physical fabrication. Another type is the “bottom-up” methods, usually relating to chemistry synthesis, such as the gradually modified on-surface synthesis technology. These approaches may require the conventional epitaxial growth (e.g. molecular beam epitaxy [MBE]) process [34] or doping steps [13] to prepare some of the 2D layers. Of course, the first prerequisite for the two types of fabrication methods is the 2D materials, many of which have to use chemistry methods firstly. Among 2D materials, the 2D semiconductors, such as MoS2 or WSe2 , have great applications in the large-scale thin-film electronics. However, the synthesis of high-quality soluble-processing 2D semiconductor nanosheets, nanoribbons, or nanoflakes remains challenging. A general intercalation/stripping approach was recently developed by Duan and Huang group [36] for the preparation of highly uniform, solution-processable, phase-pure semiconducting nanosheets. This process involves the electrochemical intercalation of quaternary ammonium molecules (such as tetraheptylammonium bromide) into 2D crystals, followed by a mild sonication and exfoliation process. Phase-pure, semiconducting 2H-MoS2 nanosheets with a narrow thickness distribution can be obtained, which can be further processed into high-performance thin-film transistors. The structure and composition of the exfoliated MoS2 nanosheets obtained was characterized by this approach [36]. The scalable fabrication of large-area arrays of thin film transistors can be further fabricated, enabling the construction of functional logic gates and computational circuits, including inverter gates, NAND (neither agree nor disagree [surveys]) gates, NOR gate, AND (agree nor disagree) gates, and XOR (exclusive-OR) gates, and a logic half-adder [36]. This kind of solution-processable method can be extended to fabricate many other high-quality 2D nanosheets for large-scale electronics, such as WSe2 , Bi2 Se3 , NbSe2 , In2 Se3 , Sb2 Te3 , and black phosphorus. In addition, even the traditional method can be further developed into sophisticated technologies by combining them with some advanced instruments in thin-film fabrication. For example, the “tear and stack” technique for graphene fabrication [39] can be further developed by combination with layer exfoliation, electron-beam lithography, and reactive ion etching (RIE) to a modified “tear and stack” technique for the precise angle control in the fabrication of the stacking 2D superlattices [14]. The success of this method has made it possible to synthesize two different layers with controlled stacking angles, such as the magic-angle graphene superlattices, which provide the essential materials to study their correlated insulator behavior at half-filling in magic-angle graphene superlattices for exotic quantum phenomena [14] and to reveal their unconventional superconductivity [35]. Even though the epitaxial growth (particularly molecular beam epitaxial growth) approach is still the main “bottom-up” technology for the 2D films including semiconductors and their heterojunction and van der Waals heterojunctions [34], a new bottom-up method, or the on-surface synthesis technology, has been gradually

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developed for large-scale synthesis of 2D heterojunction thin films at atomic precision (e.g. heterostructural nanoribbon assembled thin films) [29, 40]. This method entails using the precursors to synthesize ribbons with controlled width and edge microstructures on the oriented crystal surfaces of the metal substrates. The basic fabrication steps of the on-surface synthesis process using graphene nanoribbons (GNRs) as models to produce the prototypical armchair graphene ribbon of width N = 7 obtained using 10,10′ -dibromo-9,9′ -bianthryl as precursor are schemed [40]. The metal surfaces are usually very clean Au(111) and Ag(111) single crystals as well as 200 nm Au(111) thin films that are epitaxially grown on mica for GNR growth. Thermal sublimation of the monomers onto a solid substrate surface is firstly conducted to remove their halogen substituents, yielding the molecular building blocks of the targeted graphene ribbon in the form of surface-stabilized biradical species. During the first thermal activation step, the biradical species diffuse across the surface and undergo radical addition reactions to form linear polymer chains as imprinted by the specific chemical functionality pattern of the monomers. Then, a surface-assisted cyclodehydrogenation is performed in the second thermal activation step establishing an extended fully aromatic system. For the fabrication of straight N = 7 armchair GNRs, the substrate is maintained at 200 ∘ C during monomer deposition to induce dehalogenation and radical addition. After deposition, the sample is post-annealed at 400 ∘ C for 10 minutes to cyclodehydrogenate the polymers and form GNRs. For the chevron-type GNRs, the preparation is identical using another precursor monomer 2: 6,11-dibromo-1,2,3,4-tetraphenyltriphenylene monomer, except for the fact that slightly higher substrate temperatures of 250 and 440 ∘ C are used during monomer 2 deposition and for post-annealing, respectively. In both cases, essentially all deposited monomers are attached to the GNRs, spontaneously transforming into the desired GNR structures. This approach is versatile for the on-surface synthesis of GNRs of different widths and edges, controlled by the monomers and reaction conditions. Ultrasmall transistors – and thus the next step in the further miniaturization of electronic circuits – are the obvious application possibilities. Although they are technically challenging, electronics based on nano-transistors actually work fundamentally the same as today’s microelectronics. The Empa researchers would produce transistors with a channel cross section 1000 times smaller than those typically manufactured today from semiconducting nanoribbons. However, further possibilities are also feasible, for example, in the fields of spintronics or quantum informatics [41]. These nanoribbons can self-assemble to dense monolayered thin films via the well-established self-assembly for the further precise study of their microstructure dependent properties that some of them may be only theoretically studied years ago. Thus the on-surface synthesis technology makes it possible to fabricate the nanostructured thin films at the atomic resolution to realize the experimental study for some theoretically predicted physical effects or phenomena in atomic-scale thin films. The typical examples can refer to the different origins of energy gaps for GNRs with armchair-shaped edges and with zig-zag-shaped edges calculated by ab initio calculations according to the first-principles approach [42], and the existence of symmetry-protected topological phases, junction states, and

1.3 Thin Films for Ultrasensitive Sensing Devices

spin centers in armchair-edge graphene ribbon systems, and the interesting topological phases protected by the spatial symmetries in the chevron GNRs and cove edged GNRs [43]. The topological nontrivial and trivial study of GNRs [29] can be further extended due to the success in the precise synthesis of the graphene nanoribbons of different edge structures and controlled widths (e.g. zig-zag, armchair). Steven G. Louie advanced the armchair topological band engineering of GNRs, which classify them as topological nontrivial and topological trivial according to the difference in the width and the edge microstructure [29]. As the two GNRs of different types (e.g. nontrivial and trivial) are intersected, the interface state of topology protection (the zero-energy state) will be formed. The GNR with armchair edge preserves the width-dependent semiconducting properties, and that with zig-zag edge preserves the magnetic side. The simulated result suggests that the edge state at the same side has the same spin states and preserves the ferromagnetic property, while the spin state of the other edge state is contrary to this edge, leading to the antiferromagnetic property in the whole ribbon [42]. Besides the abovementioned progress in the fabrication of the 2D heterojunctions or heterojunctions, the liquid–air self-assemble process has been developed by Christopher B. Murray group 10 years ago [38]. The liquid–air self-assemble approach can be treated as a cost-efficient and convenient approach by arranging varieties of monodisperse nanomaterials to form heterogeneous thin films by adjusting the interface interaction between the nanomaterial solution and the air. This method straightly expanded the precision of the nanosphere Lithographie, Galvanoformung, Abformung (LIGA) process [44, 45] from several hundred nanometers to 10 nm or higher. Clearly, this approach significantly depends on the synthesis of monodisperse nanoparticles (NPs) and the matched interface between the liquid solution and air, which can possibly reach sub-10 nm resolution to assemble different nanoparticles to varieties of heterogeneous nanoparticle arrayed thin films.

1.3 Thin Films for Ultrasensitive Sensing Devices One of the main goals to develop large-scale thin-film fabrication methods and study their structure–property relationship is to manufacture ultrasensitive sensing devices. Besides the integrated circuit design and manufacture process for their application in high-performance computers, the assembling and integrating methods (e.g. microelectronic mechanics [MEMs] approach) are needed to realize their application in other fields, which can process thin films into active devices and couple with necessary outer instruments (e.g. display, data processing devices) even though “interfaces themselves are devices.” With the progress of thin-film fabrication, processing thin-film based devices into ultrasensitive sensors have evolved from the traditional MEMs to the advanced nanoelectronic mechanics (NEMs) and then to atom-scale electronic mechanics (AEMs) or single molecule electronic mechanics (details can be referred to Chapter 6). In addition, the continuously developing template-assisted nanoimprinting methods, template transfer nanoimprinting molding process [18, 20, 22, 46–49], can be conveniently

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coupled with other advanced nanoimprinting approaches (e.g. e-beam LIGA, super-resolution lithography) due to their flexibility in some special nanostructure (e.g. nanopore, nanolace) fabrication at large area economically, which can be used in the sensing film fabrication economically for MEMs, NEMs, and AEMs for ultrasensitive electronics, optoelectronics, magnetic detectors, and other kinds of flexible sensors. One of the important applications in thin films is the flat screen for video information display. OLED are still the fastest growing technology in the display industry even though only one chapter in this book discusses one topic related to this field in this book (referring to Chapter 11) [50]. OLED is an emerging display technology that enables beautiful and efficient displays and lighting panels. OLEDs are already being used in many mobile devices and TVs, and the next generation of these panels will be flexible and bendable. One of the many benefits of OLEDs includes its manufacturability on rigid (glass) or flexible substrates. A number of technologies are required for the fabrication of OLEDs on flexible substrates or foldable OLEDs (FOLEDs). FOLEDs are OLEDs built on nonrigid substrates such as plastic or metal foil. This enhances the durability and enables conformation to certain shapes and even repeated bending, rolling, or flexing. FOLEDs, still in their infancy, will usher in a range of new design possibilities for the display and lighting industries. Imagine having a mobile phone that looks like a pen but has a bright, full-color display that rolls in and out for use, which is called the Universal Communication DeviceTM (UCD). In the future, it may be very common to open a foldable smartphone from your pocket into a tablet display, to roll up a television into your pocket, and to use conformable transparent interior lighting panels that could be unbreakable. These ideas offer, you should believe, a mere glimpse into the wonders and possibilities that FOLEDs pffer. The market can reach US$ 63 billion in 2030 predicted by IDTechEx company (https://www.idtechex.com/en/research-report/ flexible-printed-oled-displays-2020-2030-forecasts-markets-technologies/693). Simultaneously, the flexible and wearable devices [51] have also been developed rapidly together with the progress of flexible OLED technology [50, 52, 53], providing much convenience in dreaming light and acoustic tasting of new technology making new life. If we can fabricate these abovementioned 2D materials under the progress of these thin-film fabrication and device assemble technology, the invention of ultrathin flexible and foldable sensing devices and more efficient OLED displays will be of interest. In addition, thin films are always associated with nanotechnology and precise atom manipulation based on interface physics and chemistry. Introducing nanostructures or atomic structures into thin films can even sprout an old academic field. For example, nanoporous magnetoplasmonic thin films developed by Song’s group can enhance the amplitude of the Fabry–Perrot interference and tune their optical oscillating into infrared (IR) range with wavelength at least from optical range to 2600 nm due to the surface plasmon resonance-enhanced Fabry–Perrot interference in the nanoporous structures [18, 47]. They also discovered the optical cavity-enhanced magneto-optical Kerr effect with a distinct reversed magneto-optical Kerr signal [22] and further improve the Raman scattering signals

1.4 Thin Films for Sustainable Energy Application

for biosensors by careful design of pore size, depth, interspacing, and components of the multilayered nanoporous thin films [54]. Acousto-optical devices, such as modulators, filters, or deflectors, implement a simple and effective way of light modulation and signal processing techniques. However, their operation wavelengths are restricted to visible and near-infrared (NIR) frequency region due to a quadratic decrease of the efficiency of acousto-optical interaction with the wavelength increase. At the same time, almost all materials with a high value of acousto-optical figure of merit are nontransparent at wavelengths larger than 5 μm, while the transparent materials possess significantly lower acousto-optical figure of merit. Daria O. Ignatyeva’s group designed a hybrid nanostructural Otto structural thin film to overcome this limit [21]. Their results indicated that the acousto-optical light can be modulated to the mid-infrared spectral range (more than 5 μm) by the planar semiconductor structures supporting guided modes at low loss using this kind of Otto-type multilayered nanostructures (i.e. semiconductor prism/noble metal/acoustic piezoelectric). Belotelov et al. also demonstrated that magnetic field sensors based on magnetoplasmonic effects can be theoretically up to a detecting resolution of fT/Hz−1/2 if designing nanostructural thin films with a high-quality Q factor [19]. We can expect that the detecting resolution for a weak magnetic field can be further increased, possibly up to aT/Hz−1/2 if other physics fields (Fabry–Perrot interference, electric field, localized surface plasmon resonance induced near field) or effects (inversed Hall effect, spin-orbital coupling effect), can be coupled into the magneto-optical effects. Clearly, success of these devices is the result of the strength to leverage multiphysics coupling by designing suitable multilayered nanostructural thin films. In the future, the combination of nanotechnology with the multiphysics coupling may address many other limitations in the ultrasensitive sensor development by precisely constructing suitable nanostructured thin films.

1.4 Thin Films for Sustainable Energy Application The rapid development of human society enforces consumption of more and more energy and resources. It is difficult for our current science and technology to achieve zero pollution and perfect recycling, leading to eco-environment deterioration. Now, people still face resource crisis, energy crisis and ecocrisis. Thin-film technologies have been impinging into these fields to address some issues in these crises. Facing the energy crisis, the sustainable and/or zero pollution energy and the related technology including energy-saving and recovery technology are desired. Solar energy, geothermal energy, and ocean energy are promising sustainable primary energy, and hydrogen energy is one of the real zero-pollution potable energy. These technologies can simultaneously address the eco-crisis because less wastes and environmentally harmful gases will be discharged into our ecosystem. The escalating energy demand for efficiently running the modern society requires the exploration of clean alternative or sustainable energy and the development of new and effective materials for energy conversion, storage, recovery, and

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transfer. In the past decades, new kinds of solar cells based on thin films and coatings (e.g. multilayer heterostructure semiconductor coating cells, thin-film cells, and perovskite coating cells), thermoelectric (TE) thin films, and the related proton-exchange membranes (PEMs) for hydrogen fuel cells (FCs) have been developed intensively. In addition, lots of 2D materials (e.g. graphene) have been used in the fabrication of high-performance film-based electrodes for new batteries, such as the scaffold or interfacial layer for lithium-metal anodes and the cathode for Li/Na–O2 batteries in the past decades [55]. The thermoelectric (TE) process is fundamentally a microscopic one involving the transport and exchange of energy by and between electrons and lattice vibrations or phonons, in a solid (i.e. TE materials). The TE process is generally based on the oldest basic thermoelectric phenomena of the Peltier and Seebeck effects [56, 57]. TE generators are fabricated based on the Seebeck effect or an electrical potential being generated when there is a temperature difference across a piece of TE materials (e.g. Bi2 Te3 [58], PbTe [59], and SiGe [60]). Thermoelectric power generation has been significantly pioneered by NASA to overcome the energy issue faced by some special missions, such as the interstellar mission of Voyager-1 launched in 1977. Solar cells and other energy sources are infeasible in this kind of interstellar mission because of the extended distance from the sun or operating on densely shielded planets. Thus, the radioisotope thermoelectric power generator (RTG) was invented, by which the heat generated by the radioactive decay of a 238 Pu source can be converted to the electrical power by a thermoelectric generator (http://voyager .jpl.nasa.gov/index.html). Since then, many other NASA missions have been powered by RTGs. The most famous example is the Mars Science Laboratory, “Curiosity” rover (http://mars.nasa.gov/msl). Thermoelectric generators have been currently much actively being pursued and used as the main power supply by the National Aeronautics and Space Administration of many countries in the aeronautic project and the astronautics exploration. Due to the capability of directly converting heat into electricity by the utilization of small temperature difference, TE materials have also shown great potential to generate electricity particularly from the industrial waste heat, geothermal source, ocean thermal gradient, and body thermal in the past decades. These heat sources are previously much difficult to be utilized by traditional energy transfer technology. They are promising materials for power generation from environmentally friendly sources, thus reducing our dependence on fossil fuels and the associated risk of a future energy crisis. Inversely, the Peltier effect can provide all-solid-state heat pumping under electrical activation for climate control, such as Peltier coolers and portable refrigerators. Peltier coolers are often implemented in a charge-coupled device (CCD) detectors or in the heat sinks of microprocessors for reliability and precise temperature control. Due to small size and the absence of moving parts and hence no vibrations, TE niches and thin films can be used to make portable picnic refrigerators and flexible thermal or electric devices in cooling microelectronics (e.g. CPU chips) and optoelectronics (e.g. infrared detectors, laser diodes) [61]. Most microelectronics and optoelectronics devices require responsive small-scale or

1.4 Thin Films for Sustainable Energy Application

localized spot cooling that does not impose a large heat load, which is best satisfied by the thermoelectric refrigeration or coolers [62]. Terrestrial applications of thermoelectric power generation and all-solid-state heat pumpers have become more pertinent. Their unique features of TE materials have led to surging interest in the basic research in TE materials and structure design of high efficiency in the past decade, such as the band structure engineering and the nanoscale effect in the electron crystal and phonon glass [63–67]. The study has promoted the mature of the TE materials design and found lots of novel TE materials, such as varieties of nanocomposites (e.g. lead-antimony-silver-telluride family: PbTe–AgSbTe2 ; Pb-chalcogenide nanocomposites), PbTe1−x Sex , YB66 , CeFe4 Sb12 , tetrahedrites (Cu3 SbSe4 ), MgSb2 , La, and Fe- or Co-doped Ca–Co–O misfit-layered cobaltites, and filled skutterudite antimonides [67–70]. Recent progress in new thermoelectric materials has paved the thin-film power generators of TE materials, which can be referred to in Chapter 8 of this book for details. Up to now, TE applications cover a wide spectrum of heat sources, from utilizing environmentally friendly heat sources to generate electricity, to using body heat for portable electronics, and to using thin-film cooler/generator. The stringent requirements on the Seebeck coefficient, electrical conductivity, and thermal conductivity that are necessary to produce good thermoelectric behavior continue to offer challenges and inspiration to myriad researchers. Thermoelectricity has become a proving ground for numerous new and innovative concepts in physics, chemistry, and materials science, including quantum size effects, nanostructuring, phonon-glass–electron crystal behavior, and large anharmonicity. New knowledge uncovered in these and other areas is driving improvements in material performance, and the future holds the promise that highly efficient thermoelectric devices for both power generation from waste heat and solid-state climate control will become a reality [71]. Given the progress in the understanding of TE, we still want to mention that no single technology can meet the world’s energy needs in the twenty-first century. One needs a combination of many technologies and novel materials into thin films, such as the solar-thermoelectric hybrid power generator of full band range by using TE materials and semiconductors together [72]. Solar energy may be the most popular sustainable clean energy on the Earth. Photovoltaic (PV) technology, such as solar cells harvesting solar energy directly into electricity, has been developed rapidly in the past decades. Besides the semiconductor multilayered thin-film solar cells, the perovskite thin-film solar cells are another viable competitor to the commercially available silicon-based solar cells. Apart from low-cost, simple device processing and manufacturability combined compatibility with roll-to-roll processing and fabrication on flexible substrates add to the merits of the perovskite PV technology by comparing with III–V semiconductor [73–79]. The term “perovskite” was attributed to the crystal structure of calcium titanate (CaTiO3 ), which was discovered by the German mineralogist Gustav Rose in 1839 and named in honor of the Russian mineralogist Lev Perovski [80]. The perovskite solar cells (PSCs) present numerous advantages include unique electronic structure, bandgap tunability, superior charge transport properties, facile processing, and low cost [73, 79]. PSCs have demonstrated unprecedented progress in efficiency and its

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architecture evolved over the period of the last decade since 2009, leading to the advent of new low-cost PV technology. They can be easily fabricated on the flexible substrates (conductive polymers) by the traditional ink-coating technology and conveniently coupled with other technologies and materials (e.g. graphene, semiconductors, ferroelectrics, Si chip) for enhanced performance. Their power conversion efficiency (PCE) has achieved a high PCE of about 22% in 2016 [79], serving as a promising candidate with the potential to replace the existing commercial PV technologies. This breakthrough led to the so-called “perovskite fever” [81], attracting much research interest in the following years, eventually increasing the efficiency to a record 22.1% (National Renewable Energy Laboratory [NREL]) in early 2016 [79]. PV technology is a multidisciplinary and versatile field in which lots of novel advanced technologies can be coupled with varieties of the related targeting materials, the progress of fundamental research, and even the traditional technologies very well, finding their own strong points and/or creating novel pinpoints to address the issues in solar cells. Much more advanced design methods (e.g. tandem solar cells [TSCs], multi-junction cells such as that formed by semiconductors and perovskites) and fabrication technologies have been invented for the development of more and more types of perovskites (e.g. metal halide perovskite, perovskite–copper indium gallium selenide [CIGS]) of high photoelectric effects after 2016. In March 2018, Jinsong Huang group [82] developed a molecular (i.e. 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquino dimethane: F4TCNQ) additive-assisted strategy for p-type molecular doping of solution-bladed perovskite films (i.e. methylammonium lead iodide: MAPbI3 ) to address the issue of the mismatched energy band between the indium tin oxide (ITO) and the perovskite (Figure 1.1a). Their final fabricated F4TCNQ-doped films exhibiting increased electrical conductivity, especially at grain boundary regions, and increased charge carrier concentrations (Figure 1.1b–j). Molecular doping of perovskite film by F4TCNQ led to the considerable enhancement of PV performance from 11.0% to 20.2% [82]. The simple but effective approach enabled the scalable fabrication of HTL-free PSC devices with a simplified device geometry using the convenient and economical Doctor blading and doping fabrication technique (Figure 1.1). As one emerging PV technology rooted from the Earth-rich element compounds and the economical and simple solution casting fabrication techniques, PSCs are expected to realize the economic electricity generation. It plays one of the main roles to settle the energy crisis and the ecological deterioration once and for all by fully utilizing solar energy. However, their stability and large-scale fabrication are two obstacles to commercialization. In September 2018, Hongwei Han’s and Edward H. Sargent’s group summarized the progress of PSCs again and pinpointed the challenges for their commercializing [83]. The power transfer efficiency and stability have been gradually increased after developing various device configurations (including mesoscopic, planar, triple mesoscopic, and tandem structures) and lots of highly efficient materials systems in the past few years [83]. The highest laboratory photoelectric conversion efficiency of PSCs notarized by a third party reached 23.3% in Sept. 2018, which has exceeded the commercial polycrystalline silicon solar cells, the CdTe and CIGS thin-film solar cells, exhibiting the commercialization

1.4 Thin Films for Sustainable Energy Application

F4TCNQ

Blade

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Perovskite ink ITO

N

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2000 –5.2

1500 1000

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500 0 –0.10

(i)

MAPbl3

Number of points

3000

–0.05 0.00 Potential (V)

0.05

(j)

–5.5

–8.3

Figure 1.1 Doctor blading and doping of perovskite films by F4TCNQ. (a) Schematic illustration of a doctor-bladed perovskite film and the chemical structure of the F4TCNQ dopant. (b) Cross-sectional SEM image of the MAPbI3 film deposited directly onto ITO glass via bladed coating at 150 ∘ C, showing the film thickness of around 500 nm. Topography KPFM images of (c, f) MAPbI3 , (d, g) F4TCNQ-doped MAPbI3 , and (e, h) F4TCNQ solid-diffused MAPbI3 films. CPD represents the contact potential difference between the tip and the sample’s surface. (i) Surface potential profiles of different perovskite films as indicated. (j) Schematic illustration of the energy diagram and electron transfer process for MAPbI3 :F4TCNQ blends. F4TCNQ: 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquino dimethane; MAPbI3 : methylammonium lead iodide; ITO: indium tin oxide. Source: Wu et al. [82]. © 2018, Springer Nature Limited.

perspective and great potential market value. As for the stability of devices, there are varieties of PSCs showing no distinct efficiency damping after the 10 000 hours’ standard testing under the required testing conditions (e.g. high temperature, continuous illumination, high humidity) using the simulated sunlight. A 110 m2 perovskite PV system with printable triple mesoscopic PSC modules (3600 cm2 for each) has been launched by Wonder Solar in China. According to their configurations and coating components for light harvesting, these solar cells can be classified as single-junction GaAs cells, multi-junction cells

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1 Introduction: Progress of Thin Films and Coatings

(two-terminal, monolithic), crystalline silicon cells, thin-film PVs technologies, and emerging PVs as reported from the National Center for Photovoltaics (NCPV) at NREL of the USA (https://www.nrel.gov/pv/cell-efficiency.html). In addition, there are two key top layers in the multi-junction cells: III–V semiconductor top coatings and perovskite top coatings. Particularly, the marriage to other advanced materials (e.g. 2D materials) and the optimized structure design theory have greatly improved the solar energy transfer efficiency and long lifetime after hastening varieties of thin-film solar cells, such as the double-junction (noncondensing) thin-film solar cells and perovskite–silicon stack cells, perovskite-based TSCs, and organic–inorganic solar cells [73, 74, 79]. After three years’ accumulation in the theoretical and experimental study and the application transformation since 2016, the PV technology experienced a blowout development after 2019 that is indeed a great year for solar cells. There are so many achievements that deserve our consideration, which can encourage us to have more interest in this vivid field. Some of them will be briefly described as following. Wei Huang group highlighted the current status and recent advances in perovskite-based TSCs, including perovskite–silicon, perovskite–perovskite, and perovskite–CIGS integrations. Figure 1.2 describes the solar efficiency growth of the three kinds of perovskite-based solar cells in the past 4 years [73], among which the perovskite–silicon can reach up to 27.1% near to the theoretical limit or the Shockley–Quiesser limit of 33.16%. Thereby, more attention has to be paid to TSCs that are suggested as an alternative to beat the efficiency limit since a maximum efficiency of 42% can be reached if using two subcells with bandgaps of 1.9 eV/1.0 eV, opening up a great potential to

33

Perovskite single-junction theoretical limit 33.16% 27.1% Solliance

28 26 23.9% ANU

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12

4T Perovskite–CIGS

10

2015

2016

2017 Year

2018

2019

Figure 1.2 Progress in the efficiency of the perovskite single-junction solar cells in the past five years. CIGS: copper indium gallium selenide. Source: Hu et al. [73].

1.4 Thin Films for Sustainable Energy Application

develop perovskite-based TSCs for commercialization [73]. This kind of TSCs can serve as a promising candidate with the potential to replace the existing commercial PV technologies in the near future [73], as shown in the progress in the early of the following year (2020) (https://www.nrel.gov/pv/cell-efficiency.html). Dion Jacobson (DJ) two-dimensional halide perovskites have been attracting much attention. However, the barrier thickness of the related quantum well (QW) is not well-known even though it is key in the design of highly efficient DJ PSCs. Wei Huang and Yonghua Chen’s group designed the efficient DJ thickness by regulating the orientation and uniform dispersion of DJ perovskites via the chain length of large-volume organic ammonium spacers [84]. It was found that the DJ perovskite could have the suitable QW barrier thickness, excellent orientation, and more uniformly dispersed QW if using 1.3-propanediamine (PDA) and 1.4-butanediamine (BDA), leading to smooth bandgap transition, longer carrier diffusion length, higher charger mobility, and lower defect density [84]. Finally, they successfully fabricated DJ-type PV cells with efficiency up to 16.38% using BDA [84]. It is a substantial challenge to prevent the degradation of metal PSCs by humid air to improve their stability and lifetime for their future commercialization. Michael Grätzel’s group invented an intercalation method to improve their stability in humid air by forming the 3D/2D bilayer perovskite. This method entails introducing a 2D A2 PbI4 perovskite layer by employing pentafluorophenylethyl ammonium (FEA) as a fluoroarene cation inserted between the three-dimensional (3D) light-harvesting perovskite film and the hole-transporting material (HTM) [85]. The perfluorinated benzene moiety confers an ultrahydrophobic character to the spacer layer, protecting the perovskite light-harvesting material from ambient moisture and reducing the mitigating ionic diffusion in the device. The 2D layer simultaneously enhances interfacial hole extraction to suppress nonradiative carrier recombination, enabling a PCE up to 22.09%. Surprisingly, their unsealed 3D/2D PSCs can retain 90% of their efficiency during PV operation for 1000 hours in humid air under simulated sunlight. Rosei and coworkers [86] reported the integrated effects of carbon quantum dots (C-dots) in the fabrication of high-efficiency inverted plane heterojunctions (PHJ) PSCs by using C-dots to modify the hole-transporting layer in the plane PSCs. The PSC efficiency can be up to 16.2% as introducing C-dots onto oxide graphene layers as the hole-transporting layer, whose efficiency can reach 16.8% under the UV range if using C-dots as the downshift layer. The introduction of C-dots can also extract the hole and transfer it to the conductive substrate and delay the charge recombination, leading to the enhanced stability of PSCs. Energy loss in the hybrid lead halide perovskite cells is interrelated with the non-radiation combination in the interface and the perovskite layer. Liao and coworkers [87] developed a simple but efficient strategy to reduce this loss via the coupling of the external electric field with the intrinsic doping of ferroelectric polymer as the interface polar layer. This strategy entails doping a series of polar ferroelectric (PFE) polymers into the methylammonium lead iodide (MAPbI3 ) layer and/or inserting them between the perovskite and the hole-transporting layers to modify and/or enhance the build-in field (BIF), improve the crystallization

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1 Introduction: Progress of Thin Films and Coatings

of MAPbI3 , and regulate the non-radiative recombination in PSCs. Doped PFE polymers can enable MA+ orderly arranged to reduce trap states and optimize the oriented growth of the perovskite layer. At the same time, inserting PFE into the gap between the perovskite layer and the hole-transporting layer can enhance the BIF via the widened depletion region in the cell. The consequently assembled cells exhibit an open-circuit voltage of 1.14 V and a PCE of 21.38% [87]. Cost is always one main issue for the commercialization of PSCs. For reducing the materials cost, Mohammad Khaja Nazeeruddin’s group [88] developed the synthesis of three enamine HTMs based on TrçgerQs base scaffold. The best performing material of HTM3 demonstrated 18.62% PCE in PSCs and presented a markedly superior long-term stability in nonencapsulated devices [88]. Moreover, the high glass-transition temperature (176 ∘ C) of HTM3 also suggests promising perspectives in high-temperature device applications of PSCs. An organic solar cell (OSC) is another promising PV device, particularly for the flexible potable power providers. Ternary OSCs also show great potential to enhance the PV property of single-junction OSCs. Liu et al. [89] have prepared a series of ternary OSCs using the previously developed PM7: ITC-2Cl [89–91] as the main system, the ultralow-bandgap receptor of IXIC-4Cl as the ternary component. The active absorbance layer can be up to 1000 nm and the PCE of 15.37% with only 0.42 eV energy loss [89]. It is an efficient method to improve the power transfer efficiency for OSCs by intensifying the light absorbance through enhancing the intramolecular push–pull effect of PV materials. However, as for the electron acceptors, the design strategy for halide molecular usually decreases the molecule energy level, leading to the reduced open-circuit voltage. Yao et al. [92] designed and synthesized a kind of chloride non-fullerene acceptor, showing extended optical absorbance and higher voltage than the fluoride compounds. The chloride non-fullerene acceptor can modify the short-circuit photocurrent density and the open-circuit voltage, realizing the PCE of 16.5%. This result indicated that reducing the band gad voltage offset can dramatically improve the power transfer efficiency by precisely tuning the organic PV materials, suggesting the promise of fullerene-free OSCs in practical applications. The III–V semiconductor solar cells are also one promising type of PV devices because of their high photoelectric efficiency, power density, and stability. However, their expensive manufacturing cost severely impedes their commercialization, partially due to the expensive Ge-based substrates for the epitaxial growth. Oh and coworkers [93] proposed a germanium-on-nothing (GON) technology to fabricate ultrathin Ge films for lightweight and thin GaAs solar cells. As shown in Figure 1.3, the ultrathin epitaxial single crystalline Ge membrane can be formed as the reusable substrate by utilizing the reorganization of cylindrical pores of porous Ge films during hydrogen annealing enable the growth and transfer of GaAs cells. Compared with previous porous Ge studies, the surface quality of reformed Ge can be significantly improved by engineering the initial pore morphology and surface passivation before annealing. The GaAs cells growing on the reformed Ge can have an efficiency of 14.44%, much near to GaAs cells growing on the bulk Ge substrates (16.53%). Their open-circuit voltage is almost the same as those GaAs cells growing

1.4 Thin Films for Sustainable Energy Application

Growth and liftoff of a GaAs cell

Ge film Void Ge bulk 4𝞵m Germanium on nothing

Current densuty (mA/cm–2)

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Figure 1.3 The germanium-on-nothing (GON) technology to fabricate ultrathin Ge films and the GaAs cell, and the voltage-dependent current density curve. Source: Park et al. [93]. Copyright from Elsevier.

on the bulk Ge substrates. This GON technology can reduce the materials cost distinctly of III–V solar cells. The requirement for flexible electronic and optoelectronic devices provokes the development of flexible energy sources. The flexible composite solar cells have been paid more and more attention recently. With the application of graphene, CVD graphene and the sponge-like 3D reduced graphene oxide (rGO) have been used as a sole HTM in the PSCs, exhibiting excellent performance due to their good deformation features. Cai and Yu in their article on the graphene application summarized this kind of graphene-based flexible PSCs [94]. Figure 1.4 gives the layered structure of this kind of graphene-based flexible PSCs (a,b), V–C curves at different bending radius, and their PCE% and short-circuit currents change with the bending times, showing good cell performance under various bending angles and cycles. These Gp-FPSCs shall have great applications in the flexible folded-potable or wearable electronic and optoelectronic devices [95]. This exciting rapid development of thin-film PSCs continues with the coming of 2020; more and more amazing signs of progress have been achieved. Sargent’s group precursor complexes to prepare the metal halide perovskite [96] designed PbI2−x x nanoflakes (PNPLs) with more uniform multi-quantum well distribution based on the idea that iodine-based PNPL permitted wide-band absorbance of sunlight, leading to large Stokes displacement to overcome the bandgap limited absorbance range of the conventional Br based perovskites. Using these I-based nanoflakes,

17

1 Introduction: Progress of Thin Films and Coatings

Ag PCBM ite Perovsk

r = 0.670 cm

r = 0.365 cm

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Figure 1.4 (a) Scheme of layered structures of graphene-based flexible PSCs; (b) pictures of the flexible solar cell units at different bending radius; (c) the C–V curves of the flexible solar cells at different bending radius; (d) the power conversion efficiency (PCE%) change with the bending times (black line) and the short-circuit current (red line). Source: Cai and Yu [94].

they fabricated the luminous solar concentrator (LSC) to realize the highly efficient luminescence. The photoluminescence quantum yield (PLQY) of this kind of thin films can reach up to 56%, and their light scattering efficiency is only 2.0%, which is 1.3 times of the best 10 × 10 cm LSC fabricated under room temperature. Cost is forever one of the key issues for large-scale application of PSCs. Efficient electron transport layer-free perovskite solar cells (ETL-free PSCs) due to their cost-effective and simplified design, highe efficiency, and potential compatibility greatly demonstrates the large-area flexible application of PSCs. However, the absence of ETL usually results in the mismatched interface energy level between the ITO layer and perovskite layer, limiting the charge transfer and collection, which leads to significant energy loss and low device performance [97]. Ge’s group invented a method to lower the work function of ITO and optimize the interface energy level alignment by virtue of an inherent dipole by introducing a polar nonconjugated small-molecule modifier to address this issue. The formed barrier-free ITO/perovskite interface is a benefit for the efficient charge transfer and restraining nonradiative recombination, which endows the device with enhanced open-circuit voltage, short-circuit current density, and fill factor. Consequently, the PCE of the modified device can reach 20.55%, much higher than 12.81% of

1.4 Thin Films for Sustainable Energy Application

the ITO-based device, and comparable to state-of-the-art PSCs with an ETL [97]. Moreover, the stability is enhanced with decreased hysteresis effect due to interface defect passivation and inhibited interface charge accumulation. The key advantages of the present device design are the high conversion efficiency potential with simple device structure and the fact that the whole device production process can be carried out at economical and energy-efficient temperatures. This work facilitates the further development of highly efficient, flexible, and recyclable ETL-free PSCs with simplified design and low cost by interface electronic structure engineering through facile electrode modification. After 60 years of research, the PCE of Si solar cells is approaching the Auger recombination-constrained Shockley−Queisser limit of 29.8% [98, 99]. To further increase the PCE while simultaneously reducing the cost per kWh, new strategies such as tandem configurations have been developed in the past decades. Organometal-halide perovskite/Si TSCs were proposed as one promising candidate to surpass Si efficiency records. A TSC consists of two or more cells that are optically coupled by absorbing different parts of the incident spectrum. This allows for a more efficient conversion of the broad-band solar spectrum into electric power. In a two-cell configuration, the high-energy region of the spectrum is absorbed by the top cell, whereas the transmitted low-energy light is further absorbed by the bottom cell. Hybrid organic–inorganic perovskite-based cells are especially well suited as a top cell for Si-based TSCs due to their high charge carrier mobility, high quantum yield, long diffusion length, sharp absorption edge, and large tunable bandgap covering almost the entire solar spectrum. Several analyses on the limiting efficiency of TSCs have been performed using detailed-balance calculations showing efficiencies up to 69.9% for an infinite number of subcells under 1 sun illumination [100–102]. However, the efficiency of perovskite/Si TSCs is strongly affected by spectral and temperature changes. Consequently, weather conditions at the specific site of deployment should be taken into account when designing perovskite/Si TSCs. Futscher and Ehrler have theoretically suggested in 2016 that perovskite/Si TSCs with PCE limits above 41% are possible for all three tandem configuration even at nonideal climate conditions by using a perovskite top cell with the ideal bandgap for the respective tandem configuration [102]. In addition, it is challenging to monolithically process PSCs directly onto the micrometer-sized texturing on the front surface of record-high-efficiency amorphous/crystalline silicon heterojunction (SC) cells, which limits both high temperature and solution processing of the top cells. The challenge for solar cell design is that both perovskite and Si are finicky. It is well-known that graphene is a 2D material with unique and powerful electronic properties. Graphene is notoriously difficult to work with, and perovskites have a durability issue that needs to be factored into the design. Nevertheless, researchers have been tinkering around with the graphene–perovskite combo to address these issues by utilization of their advantages and suppressing their shortcoming by all in one strategy to fabricate graphene/perovskite/silicon heterojunctions (SCs). To tackle these hurdles, Di Carlo and coworkers developed a mechanically stacked two-terminal perovskite/silicon TSC, with the subcells independently fabricated, optimized, and subsequently coupled by contacting the back electrode of the

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Glass/FTO cTiO2 mTiO2 Perovskite Spiro-OMeTAD or PTAA ITO

Mechanically stacked perovskite/silicon tandemsolar cell

Perovskite top cell Silicon bottom cell

Front contact +metal grid HJT Si cell Stabilized PCE = 25.9%

Figure 1.5 Scheme of the microstructure and the graphene-doped mesoporous electron selective layer mechanically stacked into the perovskite top cell and the silicon bottom solar cell forming the perovskite/graphene/silicon heterojunction tandem SCs. Source: Lamana et al. [103].

mesoscopic perovskite top cell with the texturized and metalized front contact of the silicon bottom cell. The structure and the graphene layer bonding to the heterojunction are schemed in Figure 1.5 [103]. By minimizing optical losses, as achieved by engineering the hole selective layer/rear contact structure, and using a graphene-doped mesoporous electron selective layer (the middle layer between the two layers as shown in the right image of Figure 1.5), the perovskite top cell reaches better electrical performance by graphene doping of the electron selective layer [103]. This heterojunction microstructure design endows a solar cell around those twin challenges and achieved an impressive solar conversion efficiency of 26.3% (25.9% stabilized) over an active area of 1.43 cm2 , the best solar cell ever of great potential industrialization. More amazing progress in solar cells is coming in early 2020. On Jan 27, 2020, NREL of the USA announced that the PCE% of single-junction perovskite–silicon stack solar cell invented by Berlin Institute of materials, at Helmholtz (HZB, Germany), has reached 29.15%, exceeding the record of 28% previously reported by Oxford PV company (https://www.nrel.gov/pv/cell-efficiency.html). At the same time, groups from Stanford University and Arizona State University declared that the PCE% of their single-junction PSCs was 25.3%, up to the world record kept by Massachusetts Institute of Technology and Korea Institute of Chemical Technology. Finally, and soon, NREL obtained a PCE% of 32.9% in their Double junction (noncondensing) thin-film solar cells. The PCE% of the single-junction perovskite–silicon stack solar cells by HZB was further confirmed and authorized by Fraunhofer Institute of Solar Systems (Germany), updated the record in the PCE% chart of varieties of solar cells (Figure 1.6) by NREL (https://www.nrel.gov/ pv/cell-efficiency.html; https://www.nrel.gov/pv/assets/pdfs/best-research-cellefficiencies.20200128.pdf; https://www.helmholtz-berlin.de/pubbin/news_seite? nid=21020;sprache=en;seitenid=1). In this design, a special electrode contact layer

1.4 Thin Films for Sustainable Energy Application

52

Best research-cell efficiencies

48 44 40

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36 32 28 24 20 16 12 8 4 0

1975

1980

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Figure 1.6 Updated solar cell efficiency by NREL in February 2020 for multi-junction cells, single-junction GaAs cells, crystalline Si cells, cells by thin-film technologies, and cells by emerging photovoltaic technologies. Safe and stable energy storage devices of high energy density at low cost are also essential for our routine transportation and other industrial applications. Varieties of rechargeable supercapacitors and high-performance and high-volume batteries have been developed, such as Li-S batteries, AIBs, and graphene-based supercapacitors [55, 104–107]. The recently developed Li/Na-S batteries and supercapacitors may be the most promising energy storages satisfying these requirements for the next-generation rechargeable energy storage with the rapid coupling of 2D thin-film materials into this field [55, 104]. Metal–sulfur batteries hold practical promise for next-generation batteries because of high energy density and low cost. Development is impeded at present, however, because of unsatisfied discharge capacity and stability in long cycling. Combination of experimental and theoretical approaches can be used to develop insight into the relationship between electrochemical behavior of sulfur redox and metal stripping-plating and the structural properties of electrode materials. With metal–sulfur batteries, two-dimensional (2D) thin-film nanomaterials are a suitable model with which to connect and test experimental results with theoretical predictions and to explore structure–property relationships. Through the view of combining experimental and theoretical approaches, sulfur redox conversion on 2D nanomaterials in various reaction stages was explored, and crucial factors affecting 2D nanomaterials as artificial solid electrolyte interfaces (SEIs) and host materials in protecting Li and Na metal anodes were critically unveiled by Shi-Zhang Qiao [55]. It is indicated that Li/Na-sulfur batteries hold practical promise for next-generation batteries because of high energy density and low cost. Significant progress has been made in understanding mechanisms of sulfur redox and metal stripping/plating with a judicious combination of experimental and theoretical approaches. Two-dimensional (2D) nanomaterials offer a suitable model to correlate experimental results with theoretical predictions and, importantly, with which to explore structure–property relationships. Future research effort should focus on the establishment of correlations between macroscopic conversion kinetics and the electronic structure of the electrode materials with agreed standards and advanced combined experiments and theory. In addition, fabrication of three-dimensional (3D) electrodes from 2D materials might be a promising approach to promote the energy and power densities of the Li/Na-sulfur batteries and other metal–sulfur batteries. Source: https://www.nrel.gov/pv/cell-efficiency .html; https://www.nrel.gov/pv/assets/pdfs/ best-research-cell-efficiencies.20200128.pdf.

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was developed for the TSCs, and then, the interface layer was modified. Thus, the top perovskite layer can utilize the optical range of sunlight, and the bottom Si layer of a special SiO2 interlayer can transfer most of the IR-NIR light into electric energy, leading to the enhanced PCE% comparing to each single cells. This kind of perovskite/graphene/silicon heterojunction tandem SCs have been fabricated into solar cell modules using the industrial scale fabrication and sealing process for the three kinds of pre-commercialization life testing. They have successfully survived the light resistance test, the damp heat resistance test, and the thermal cycling test on 23 January 2020, which is the first time for the PSCs to realize this goal for commercialization. The PSC modules can be either rigid or flexible and either transparent or semitransparent, which makes them suitable for varieties of application scenarios, such as assembling in windows, roof tiles, external wall faces, roads, sound baffles, and automobile roofs. They can also be colorful via imitation coloring by varieties of structure colors made from the nano/microfabrication. As it is well-known that PSCs and their modules preserve high-efficiency, low-cost processing ability and cheap and rich sources. These marvelous and exciting achievements make PSCs full of perspective for sooner commercialization and serving renewable energy for our routine life. Rechargeable aluminum-ion batteries (AIBs) are regarded as one promising candidate for post-lithium energy storage systems (ESSs) [107]. For addressing the critical issues in the current liquid AIB systems, here, a flexible solid-state AIB is established using a gel-polymer electrolyte (a kind of electric thin films) for achieving robust electrode–electrolyte interfaces. As shown in Figure 1.7, employment of polymeric electrolytes mainly focuses on addressing the essential problems in the liquid AIBs to transfer into the solid AIBs, including unstable internal interfaces induced by mechanical deformation and production of gases as well as unfavorable separators, which is much different from the utilization of solid-state systems for alleviating the safety issues and enhancing energy density in lithium-ion batteries [107]. Particularly, such gel electrolyte enables the solid-state AIBs to present an ultrafast charge capability within 10 seconds at a current density of 600 mA/g. Meanwhile, an impressive specific capacity ≈120 mA/h/g is obtained at a current density of 60 mA/g, approaching the theoretical limit of graphite-based AIBs. In addition to the well-retained electrochemical performance below the ice point, the solid-state AIBs also hold great stability and safety under various critical conditions. The results suggest that such a new prototype of solid-state AIBs with robust electrode–electrolyte interfaces promises a novel strategy for fabricating stable and safe flexible ESSs. Li storage is one key issue for the fabrication of high-performance Li batteries with high volumetric energy density. The carbon allotropes are mainly used as the host materials for reversible lithium uptake in Li-ion batteries [108, 109], thereby laying the foundations for existing and future electrochemical energy storage. Recently, the Li storage in layered 2D materials, such as graphene, has been developed for enhanced Li storage density. However, insight into how lithium is arranged within these hosts is difficult to obtain from a working system because

1.4 Thin Films for Sustainable Energy Application

Figure 1.7 Overall comparison of liquid-state aluminum-ion batteries (AIBs) and solid-state AIBs: (a) Schemes for the configurations of these two AIBs. (b) Schemes for demonstrating the electrode–electrolyte interfaces: an unstable interface based on porous separator in the liquid-state AIBs and robust interface based on the GPE electrolyte in the solid-state AIBs. (c) Schemes for illustrating the production of gases in the two prototypes of batteries. Source: Yu et al. [107].

(a)

–

v

+

– AlCl4-

v

+ GPE

Al2Cl7Cation Anion

Liquid-state AlBs

(b) Unstable interface

Porous textile separator

(c) Pronounced bubbles

Solid-state AlBs Robust interface

Compacted gel electrolyte Suppressed bubbles

the in situ high-resolution transmission electron microscopy (HR-TEM) [110–112] is not suitable to probe those light element, especially lithium (Li) with atomic number less than carbon due to their low scattering cross section for impinging electrons and their susceptibility to knock-on damage [113–115]. This unknown structure–property relation hinders the design and fabrication of more dense Li storage for high-performance Li batteries and the related supercapacitors. Jurgen H. Smet’s and Ute Kaiser’s groups [105] realize the study of the reversible intercalation of lithium into bilayer graphene by in situ low-voltage transmission electron microscopy, using both spherical and chromatic aberration correction to enhance contrast and resolution to the required levels [116]. It is found that lithium atoms can form a multilayered close-packed order between the two carbon sheets [105]. Consequently, the lithium storage capacity associated with this superdense phase far exceeds that expected from the formation of LiC6 , which is the densest configuration known under normal conditions for lithium intercalation within bulk graphitic carbon [117]. This finding shall insight the design and fabrication of more dense Li storage for future high-performance Li batteries. Supercapacitors are another energy storage mode besides the batteries. Thanks to their ultrafast rechargeable ability, high power, long-term lifetime, wide working temperature, and safety, supercapacitors have shown extraordinary promise for potable miniaturized electronics, hybrid electric vehicles, standby power, and field power. However, the power density (i.e. power in unit volume) of the electrochemical supercapacitors is relatively lower than the batteries, which restricts their broader applications, particularly in the potable smart equipment. The power density of supercapacitors needs to be further improved but is usually limited by

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electrodes with rather low volumetric performance, which is largely due to the inefficient utilization of pores in charge storage. Benefited from the progress of the magic materials: graphene and some graphene-based technologies have been invented to address this issue. Recently, a freestanding graphene laminate film electrode with highly efficient pore utilization was developed for the compact capacitive energy storage by precisely tuning the interlayer spacing of graphene laminate films for optimized porosity [118]. The preparation process is as follows [118]: The mixture made from graphene oxide (GO) and thermal expansion reduced graphene (EG) of different ratios were firstly prepared and then vacuum filtrated to form the composite graphene thin film of tunable interlayer spacing, whose layered microstructures are characterized by scanning electronic microscope (SEM). The porosity of the electrode materials can be optimized by regulating the interlayer spacing. By systematically tailoring the pore size matching for the electrolyte ions, utilization of pores and their interspacing can be optimized, and thereby, the volumetric capacitance will be maximized. Consequently, flexible all-solid-state supercapacitors can be fabricated, which can deliver delivers a stack volumetric energy density of 88.1 Wh/l in an ionic liquid electrolyte, representing a critical breakthrough for optimizing the porosity toward compact energy storage [118]. Moreover, the optimized film electrodes exhibit excellent bending ability due to the intrinsic flexibility of graphene [118]. They can further be assembled into ionogel-based, all-solid-state, flexible smart devices with multiple optional outputs and superior stability, demonstrating enormous potential as a portable assembled to smart devices, realizing varieties of output results by designing the suitable circuits [118]. Hierarchically ordered structures with low tortuosity, excellent mechanical flexibility, high optical transparency, and outstanding electrical conductivity are critically important in developing flexible transparent supercapacitor electrodes for innovative applications in electronics and displays. Bionic technology has been recently immersed in the design and fabrication of supercapacitors with the abovementioned microstructures and properties [106]. The leaf-skeleton inspired electrodes have been successfully fabricated by a CVD process, which are reticulated monolithic networks consisting of carbon nanostructures serving as a 3D spongy core and graphene-based films as a protective/conductive shell [106]. The network electrodes show optical transmittance of 85–88%, an electrical sheet resistance of ∼1.8 Ω/sq, and an areal capacitance of 7.06 mF/cm2 (at 0.78 mA/cm2 in a three-electrode cell) in Na2 SO4 aqueous electrolyte. Flexible transparent and symmetric supercapacitors, based on poly(vinyl alcohol) (PVA)/H3 PO4 gel and the network electrodes, possess a stable working voltage of 1.6 V, energy and power density of 0.068 μWh/cm2 and 47.08 μW/cm2 at an optical transparency of ∼80%, and no capacitance loss over 30 000 flat-bend-release cycles. Similar to energy storage and utilization of renewable energy source, energy conversion technology is also intimately dependent on thin films and coatings. Energy conversion using electrochemical reactions for fuel cells has attracted increasing attention because of its advantages over traditional fossil energy sources, such as renewability, eco-friendliness, and high efficiency, one of whose key components is

1.4 Thin Films for Sustainable Energy Application

catalytic thin-film based electrodes or catalytic functional PEMs [119, 120]. Among the fuel cell technology, the PEM fuel cells (FCs) may be one of the promising models to determine the factors that influence the commercialization for common transportation, which is based on the highly efficient catalyst filling PEM (i.e. Pt catalyst filling porous Nafion thin films) as the cathodes [121]. PEMs of high electrochemical catalytic performance are particularly desired for the transportation of routine life using zero-waste-releasing potable fuels with high energy density (i.e. hydrogen) if considering the ecological issues as burning fuels of different kinds [122, 123]. Hydrogen fuel cell automobiles have become one popular routine transporter in Japan as Japan has started the FH2R (Fukushima Hydrogen Energy Research Field) model project since 2018, which is just completed at the end of February 2020. The FH2R intends to produce 1200 Nm3 /hr via a 10 MW water electrolysis system powered by a 20 MW PV device equipped on the 180 000 m2 field reported from the New Energy and Industrial Technology Development Organization of Japan in March 2020. This model project realizes the perfect coupling between the potable hydrogen fuel cell technology and renewable solar cell technology. The oxygen reduction reaction (ORR) is a critical factor associated with electrochemical energy conversion in fuel cells. It is an important cathode reaction in many electrochemical energy conversion devices, including hydrogen fuel cells and direct methanol fuel cells [122, 123]. The main difficulty associated with the ORR is the sluggish multiple-electron transfer process, which has to be catalyzed typically by those precious metals such as Pt, Pd, Ru, or their alloys [124–127]. This issue impedes the future common worldwide application. Therefore, the reduction of Pt or precious metal load in the cathode for PEM fuel cells but still keeping high power density is highly needed, particularly reducing the Pt content in the cathode thin films [128–133]. However, the low Pt content in the cathodes will result in the high voltage losses that come from the mass transport resistance of O2 through the platinum–ionomer interface in the PEM, the location of the Pt particle with respect to the carbon support and the supports’ structures. Strasser and Orfanidi group recently proposed a new Pt catalyst/support design that substantially reduces local oxygen-related mass transport resistance [130]. This new design includes the use of chemically modified carbon supports with tailored porosity enabling controlled deposition of Pt nanoparticles on the outer and inner surface of the support particles, resulting in unprecedented uniform coverage of the ionomer over the high-surface-area carbon support thin films, especially under dry operating conditions. Consequently, the present catalyst design exhibits previously unachieved fuel cell power densities in addition to high stability under voltage cycling. Owing to the Coulombic interaction between the ionomer and N groups on the carbon support thin films, homogeneous ionomer distribution and reproducibility during the ink manufacturing process for the catalytic polymer thin films can be ensured. Another alternative is to develop non-precious catalysts, such as some transition metal (TM) complexes, single-atom catalysts (SACs), or single-site catalysts (SSCs) [121, 134]. TM complexes have been widely used in physical and biological science, particularly playing essential roles in catalysis, chemical synthesis, materials science, photophysics, and bioinorganic chemistry [135–137]. Since 1964, N4

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macrocycles of non-noble metals (e.g. cobalt) based on the organometallic complexes and bionics have been developed as fuel cell cathode catalysts [138]. Various non-precious earth-abundant metal catalysts, especially those based on transition metal–nitrogen–carbon (MNC) compounds, have been continuously developed to address the issues related to the cost and earth-abundant resources, and their catalytic performance has been gradually enhanced [139–142]. However, these no precious catalyst-based fuel cells are currently suffering from limited activity, poor stability, poor durability, limited ability for a large-scale yield of high-performance catalysts, impeding their sustainable commercial application by replacing those precious metal catalyst-based fuel cells [128, 143]. Recently, Song’s group presents a simple sequenced ultrasonic atomization microreaction, pyrolysis, acid leaching, and calcination process for the mass synthesis of FeNC SSCs with excellent ORR catalytic performance, as shown in Figure 1.8A [121]. The ultrasonic atomization process provides a huge number of microreactors to produce the highly dispersed iron precursors. The subsequent pyrolysis and calcination processes ensure that the iron atoms are conjugated to nitrogen ligands, anchoring them onto carbon black. This synthesis system is eco-friendly by only using water and ethanol as solvents and does not generate pollutant emissions. A combination of various microstructure and composition characterization of these catalysts and the related ORR performance based on these catalysts filling proton-exchange Nafion membrane suggest that the active centers root in the single-atom Fe sites chelating to the fourfold pyridinic N atoms or calling as SSCs. Their electrochemical catalytic ORR performance outperforms the commercial Pt/C catalysts, having much enhanced halfwave potential and kinetic current density (Figure 1.8B) and the substantially enhanced long-term stability and outstanding tolerance to methanol (Figure 1.8B). This synthetic strategy provides a new general method for the eco-friendly large-scale synthesis of high-performance single-atom catalysts for fuel cells. In addition, even though the large-scale production of hydrogen has recently been realized, the electrocatalysis for water splitting reaction, including hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), is still depending on the catalytic proton-exchange Nafion thin films filled with Pt or other precious-metal-based catalysts. Therefore, similar to those catalysts for the catalytic thin films in ORR, reducing the precious metal loading in catalytic thin films or developing non-precious-metal-based catalysts for HER and OER are desired to further reduce the price of hydrogen as the economic portable fuel [127, 144, 145]. The late strategy is most desired since those precious metals particularly for Pt are nonrecyclable Earth-deficient sources. In particular, for both the OER and HER in electrolysis, it is necessary to develop non-precious, efficient, and durable catalysts. Considering the cost and yield efficiency, it is greatly desired for highly efficient bifunctional catalysts for overall water splitting in an alkaline medium. The transition metal oxides, metal sulfides, metal phosphides, layered bimetallic hydroxide (LDH), and metal alloys have been investigated as the catalysts for OER and HER, whose catalytic performance can be improved through morphology engineering, composite, and doping technology. Particularly, FeNi alloy and their

1.4 Thin Films for Sustainable Energy Application

(B) (b) FeNC FeNC SACs Pt/C

–2

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(A) Ultrasonic nebulizer

30 0

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20 000

30 000

Time (s)

Figure 1.8 (A) Schematic of the preparation of FeNC SACs by ultrasonic atomization in conjunction with pyrolysis and calcination. (B) Evaluation of electrochemical performance on FeNC, FeNC SACs, and 20% Pt/C catalysts. (a) Linear sweep voltammetry (LSV) curves, (b) comparison of halfwave potential (E1/2 ) and kinetic current density (Jk ), (c) Tafel plots, (d) Nyquist plots obtained by electrochemical impedance spectroscopy (EIS), (e) relative current density–time curves of FeNC SACs and 20 wt% Pt/C catalysts at 0.6 V with methanol injection, and (f) relative current density–time curves at 0.6 V for 36 000 seconds (10 hours). Source: Ma et al. [121].

LDH preserve OER and HER bifunctions in the same reaction medium, having been studied intensively. It was also found that CoFe LDH, CoMn LDH, CuCoO nanowires, FeCoOH, Cu@CoFe, and NiMo nanorods have OER and HER bifunction. Recently, Inamdar et al. developed another robust non-precious copper–iron (CuFe) bimetallic composite that can be used as a highly efficient bifunctional catalyst for overall water splitting in an alkaline medium. Their catalyst exhibits outstanding OER and HER activity, and very low OER and HER overpotentials (218 and 158 mV, respectively) are necessary to attain a current density of 10 mA/cm2 [144]. When used in a two-electrode water electrolyzer system for overall water splitting, it not only achieves high durability (even at a very high current density of 100 mA/cm2 ) but also reduces the potential required to split water into oxygen and hydrogen at 10 mA/cm2 to 1.64 V for 100 hours of continuous operation. Many 2D materials, such as MoS2 , have been studied for catalyzing the HER of water, showing great promise as a cost-effective alternative to Pt even though the current catalytic efficiency is still worse than that of Pt. [146, 147] Cao and coworkers recently report a strategy to enable the catalytic activity of monolayer MoS2 films that are even better than that of Pt via engineering the interface interaction of the monolayer with supporting substrates [146]. The monolayer films were grown with CVD processes and controlled to have an optimal density (7–10%) of sulfur vacancies. They found that the catalytic activity of MoS2 could be affected by substrates in two ways: forming an interfacial tunneling barrier with MoS2 and modifying the chemical nature of MoS2 via charge transfer. Thus, excellent catalytic activities at the monolayer MoS2 films can be obtained by using substrates that can provide n-doping to MoS2 and form low interfacial tunneling barriers with MoS2 (e.g. Ti). The catalytic performance may be further boosted to be even better than Pt by crumpling the films on Ti-coated flexible polymer substrates, as the Tafel slope of the film is substantially decreased with the presence of crumpling-induced compressive strain.

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The monolayer MoS2 films show no degradation in catalytic performance after being continuously tested for over two months. Another kind of thin-film electrode for water splitting, which can be a potential candidate to replace the precious metal catalyst-based electrode is based on TM-based nanocatalysts. They have been attracting great attention due to their low cost and abundance as compared with those precious-metal-based catalysts, but their low performance (particularly insufficient activity) still remains a challenge. Many strategies (e.g. size, shape, hierarchical structure, and composition control) have been developed to improve their performance in the past few years. One example of the progress in these TM-based nanocatalysts is the 4 nm Mn3 O4 nanoparticles (NPs) developed recently by Nam and coworkers [148]. It is confirmed that the overall increase in the active surface area is distinctly related to the superb catalytic activity of the 4 nm Mn3 O4 NPs by comparing them with those 8 nm species. To further enhance the OER performance, the interface interaction between the catalysts and the support should be optimized. As expected, introducing of Ni foam substrate can indeed maximize the entire number of the NPs participating in OER. An outstanding electrocatalytic activity for OER was obtained using the 4 nm Mn3 O4 /Ni foam electrode, with an overpotential of 395 mV at a current density of 10 mA/cm2 under neutral conditions (0.5 M phosphate buffer saline (PBS), pH 7).

1.5 Thin Films and Coatings for Key Sources and Ecological Environment of Earth Water is the first key source for all organisms on Earth [149, 150]. Even though 71% of Earth’s surface is covered by ocean, only 2.53% of water is fresh water that is not the uniform distribution in the earth. Many areas are very deficient of fresh water, such as the Sahara and many countries in Africa, the Middle-East areas, California and Nevada of the USA, North and Northwest District of China, and most isolated islands. Water has played a key role in social and national security in the history of human beings [149, 150]. In order to resolve the freshwater shortage, governments have invested lots in many huge projects, such as the partly-finished South-to-North Water Diversion Project in China. While the USA has paid attention to desalination technology, to obtain fresh water from the ocean or brackish water has been on the watch by the USA government to address the water shortage in California since the 1950s [151–153]. The reverse osmosis technology based on highly efficient permeability membranes (e.g. reverse osmosis thin-film composite membrane) emerges as the times require insight from the semipermeability of animal bladders [153]. Dr. Song has systemically summarized the progress of permeable membranes in his dissertation in 2000 [153]. With the rapid development of new separation concept, novel membrane materials science, advanced fabrication technology, and automatic control engineering in the past two decades, great progress has been achieved in membrane science and separation technology [151, 154–162]. Because of varieties of advantages of select osmotic membrane technologies (e.g. the high selectivity [based on molecule weight, geometry, affinity, and configuration] and

1.5 Thin Films and Coatings for Key Sources and Ecological Environment of Earth

permeability, the possibility non-phase transformation or controlled phase change during separation, noncontact and temperature difference between products and feeds, the possibility for nonthermal exchange process, and low operation cost), lots of permeable membranes or related separation technologies have been invented for the separation, purification, and concentration of desired products in almost all industry involving the fluids (gas or liquid), particularly foods, pharmacy, gas separation, chemical engineering, petroleum refining, chemistry synthesis, environment protection, and waste recycling and reutilization [149, 152, 154–165]. When using a membrane to separate materials, the efficiency of the separation is limited by how fast the gas or liquid passes through the membrane and by how selective it is. Thinner membranes usually allow for faster flow rates but are usually less selective and strength. In order to increase the flux without loss of selectivity, many technologies and membrane materials have been invented in the past decade. The most impressive progress may be the incorporation of the magic two-dimensional (2D) materials [166–171] and/or varieties of carbon allotropes (i.e. porous carbon, graphene, carbon nanotube) into the thin-film composite membranes that have greatly addressed the trade-off between selectivity and permeability, one of the main issues in the membrane materials development [149, 154, 156, 164, 172, 173]. Particularly, graphene – with great mechanical strength, chemical stability, and inherent impermeability – offers a unique 2D system with which to realize this membrane and study the mass transport. One sophisticated perforating strategy to maintain the selectivity without losing permeability was successfully developed by Celebi et al. [174], which entails the precisely drilling holes of controlled diameter in a graphene sheet about two layers thick, up to a few million pores with narrowly distributed diameters between less than 10 nm and 1 μm. For such a thin membrane, the primary barriers to separation come from entrance and exit from the holes and not from the motion through the membrane, which can have highly efficient mass transfer across physically perforated double-layer graphene but still maintain a high selectivity. The measured transport rates are in agreement with predictions of 2D transport theories. Attributed to its atomic thicknesses, these porous graphene membranes show higher permeance of gas, liquid, and water vapor far in excess of those shown by finite-thickness membranes, highlighting the ultimate permeation these 2D membranes can provide. Varieties of graphene-based (including GO: partially oxidized and stacked sheets of graphene [175]) thin-film composite membranes have been developed, which can possibly address this issue and has reached the separation resolution of molecule/ion sieving ability in the selectivity [149, 156, 164, 172, 173]. Two-dimensional materials such as graphene and graphene oxide membranes (GOMs) [175], MoS2 , can provide ultrathin, high-flux, and energy-efficient membranes possible with ångström-scale channels with atomically flat walls for precise ionic and/or molecular sieving in aqueous solutions [166, 169–171, 176, 177] and gas-phase (e.g. H2 or He) separation [167, 168, 178]. These materials have been made into varieties of thin films and coating of different microstructures and layers, showing potential in a variety of applications, including water desalination and purification [179–181] and gas and ion separation [160, 167, 168, 178, 182–184]

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However, there are still lots of work to do in the controlled modification of 2D materials for highly efficient permeable membranes to overcome their intrinsic limitation, for example, graphene, unlike the pores of carbon nanotube membranes that have fixed sizes, while the pores of GOMs and the interlayer spacing between GO sheets are of variable size [185], and it is difficult to reduce the interlayer spacing sufficiently to exclude small ions and to maintain this spacing against the tendency of GOMs to swell when immersed into aqueous solution [186]. These challenges hinder the potential ion filtration applications of GOMs. In 2017, Fang and coworkers [160] developed a cationic control strategy to tune the interlayer spacing of GOMs with ångström (Å) precision using K+ , Na+ , Ca2+ , Li+ , or Mg2+ ions. Membrane spacings controlled by one type of cation can efficiently and selectively exclude other cations that have larger hydrated volumes. Using this strategy, they fabricated centimeter-scale GOMs on ceramic supports experimentally, achieving facile and precise control of the interlayer spacing, with a precision of down to 1 Å, and corresponding ion rejection, through the addition of one kind of cation (e.g. K+ ) [160]. This method is based on the understanding of the strong noncovalent hydrated cation–π interactions between hydrated cations and the aromatic ring, indicating that other ions could be used to produce a wider range of interlayer spacings [160]. Generally, this method provokes a step toward graphene-oxide-based thin-film applications, such as water desalination and gas purification, solvent dehydration, lithium-based batteries and supercapacitors, and molecular sieving. Freshwater flux and energy consumption are two important benchmarks for the membrane desalination process. Nanoporous carbon composite membranes have been developed by Sheng’s group in April 2018 [156], which comprise a layer of porous carbon fiber structures grown on a porous ceramic substrate, showing 100% desalination and a freshwater flux of 3–20 times higher than existing polymeric membranes. Thermal accounting experiments demonstrated that the carbon composite membrane could save over 80% of the latent heat consumption. Conventional technology for the purification of organic solvents requires massive energy consumption, as well as to reduce such expending calls for efficient filtration membranes capable of high retention of large molecular solutes and high permeance for solvents. In September 2019, Tang’s group [161] reported a surface-initiated polymerization strategy through C–C coupling reactions for preparing conjugated microporous polymer (CMP) membranes of high resistance to organic solvents due to the all-rigid conjugated systems in the backbone of the membranes. The prepared 42-nm-thick CMP membranes supported on polyacrylonitrile substrates exhibited excellent retention of solutes and broad-spectrum nanofiltration in both nonpolar hexane and polar methanol, the permeance for which reaches 32 and 22 l/m2 /h/bar, respectively. Besides this issue, antifouling may forever be one of the main issues during the long-term operation, particularly biofouling, which will result in the flux reduction, the deterioration of selectivity, and/or the broken of membranes, finally leading to a short lifetime of membranes [163, 187]. Many novel methods have been invented in resolving the membrane fouling problem [163, 187].

1.5 Thin Films and Coatings for Key Sources and Ecological Environment of Earth

Another interesting innovative technology in clean water production using membrane separation technology should be the bionic-plant-leaf-inspired sunlight-driven purifier for high-efficiency clean water production [155]. It is well-known that the transpiration and guttation process of natural vascular plant leaves can produce tons of clean water via osmotic pressure differences powered by sunlight. Inspired by this transpiration and guttation process, a sunlight-driven purifier is designed for high-efficiency water purification and production. This sunlight driven purifier is constructed by a negative temperature response poly(N-isopropylacrylamide) (PNIPAm) (PN) hydrogel anchored onto a superhydrophilic melamine foam skeleton, and a layer of PNIPAm modified graphene (PG) filter membrane coated outside. Molecular dynamics simulation and experimental results show that the superhydrophilicity of the relatively rigid melamine skeleton significantly accelerates the swelling/deswelling rate of the poly (isopropylacrylamide) (PNIPAm) (PN) and PNIPAm modified graphene (PG) -filter (PNPG-F) purifier. Under one sun, this rational engineered structure offers a collection of 4.2 kg/m2 /h and an ionic rejection of >99% for a single poly (N-isopropylacrylamide) (PNIPAm) (PN) and PNIPAm modified graphene (PG) (PNPG) filter from brine feed via the cooperation of transpiration and guttation. It is envisioned that such a high-efficiency sunlight-driven system could have great potential applications in diverse water treatments. Besides the freshwater resource crisis, another great challenge before people may be the deterioration of our ecology and environment, particularly the atmospheric pollution that is mainly resulted from more and more toxic gas releasing and greenhouse gas emissions. One strategy to reduce the pollution of these gases is to resource them, such as reducing CO2 into ethylene or CO into methanol. Lots of resource technologies have been developed in the past decades. The electrochemical reduction reaction of CO2 (CO2 RR) may be the most promising method to fulfill this issue by transferring CO2 into chemical resources (e.g. urea, merlon, ethylene) or synfuels (e.g. methane, methanol) [188–191]. The key to this technology is to find the catalysts of high efficiency and low cost to replace the expensive precious-metal-based species. Carbon-based solid-state catalyst materials containing small amounts of nitrogen and TM have emerged as a selective and cost-efficient alternative to noble metal catalysts for the direct electrochemical reduction of CO2 into CO, formic acid, or methane. Recently, Ana Sofia Varela and Peter Strasser summarized the recent progress of MNC catalysts for the CO2 RR [190]. There is a growing interest in MNC materials as catalysts for the CO2 RR, given their remarkably high activities and selectivity toward CO formation, their ability to form “beyond CO” hydrocarbons, and their affordable synthesis methods. Studies have shown that the metal center plays a crucial role in determining the catalytic performance of the material. For instance, Fe has been shown to produce CO selectively at low overpotentials and to have the ability to reduce CO further to traces of CH4 [192], whereas Ni-containing materials have been reported as highly selective toward CO production. They have mentioned that the density functional theory-guided experimental studies could provide the elucidation of key experimental parameters and molecular descriptors. The catalytic performance in

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the presence of different metals can be rationalized based on the binding energy between the metal center and the reaction intermediates, which might promote the activity and selectivity of the MNC catalysts for future highly efficient nonprecious catalyst development. In this review, it is further pinpointed that there are still several key scientific issues that deserved to be deeply studied in this emerging field. One is the nature of the active sites. Understanding the role of all the possible active sites will be crucial for designing optimal catalysts containing highly active and selective catalytic sites. One needs new instruments for this active site study. The second issue is how to optimize the reaction conditions since they play a major role in the catalytic process besides the catalyst structure and composition. For instance, working in acidic pH favors the competing process of the HER reducing the selectivity toward the CO2 RR. Therefore, it would be desired to work in neutral or alkaline pH values to have a highly selective process. Another important factor to consider is the accessibility of the active sites. When working with a highly active catalyst, the CO2 RR can be limited by the transport of CO2 to the active sites. In this regard, the structure of carbon support plays a crucial role in the transport of CO2 through the catalyst layer. The low CO2 solubility, however, is also a major reason for mass transport limitations. Therefore, in general, we have to determine the reaction conditions that are practical for the catalytic screening based on their industrial application. For an instance, the use of gas diffusion electrodes (GDEs) can help to overcome the pH limitations by having the gas stream and electrolyte stream separated by the electrode, which preserve a more realistic assessment of the catalytic performance of a given material for the CO2 RR at a large scale if the screening is carried out using GDE electrolyzers.

1.6 Thin Films and Coatings for Biomedical Engineering and Life Science Polymers and biomass form an integral part of our existence and everything that surrounds us – from the basic building blocks of life constituting of proteins, nucleic acids, and polysaccharides to the commercial products obtained from automobile, construction and transportation industries, plastic toys and tools, reading glasses, etc. Most of these materials are composed of a combination of one or more materials to form polymer composites. And a film made of a polymer as a matrix or a carrier is referred to as a polymer-based film. Due to its excellent composite properties and processing diversity, polymer films have been increasingly employed in production and are gradually applied to defense, transportation, aerospace, marine engineering, and other fields (e.g. thin films for artificial intelligence). Many functional polymers, particularly biodegradable species, have been fabricated into thin films and coatings of special surface patterns as biosensors or other kinds of functions (antibacterial, antifouling) for biomedical engineering and life science [187, 193–196]. In this book, the polymer-based films for artificial intelligence, selective permeable thin films and their applications for water purification and wastewater treatment, biomass-derived functional films, and anti-marine corrosion coatings

1.6 Thin Films and Coatings for Biomedical Engineering and Life Science

have been summarized in details in Chapters 13, 14, 15, and 19, respectively. Here, we will briefly introduce some other interesting progress of polymeric thin films and coatings for biomedical engineering and life science. One of them is the thin films formed via self-assembly of biocompatible or biodegradable hyperbranched polymers (HBPs), which have lots of cytomimetic applications, summarized by Jin et al. [197] The HBPs have demonstrated great potential to be used as model membranes to mimic cellular behaviors, such as fusion, fission, and cell aggregation via self-assembly into varieties of sizes, shapes, and structures, such as honeycomb structures. Natural honeycomb structures are usually observed in plants or beehives with a columnar and hexagonal array of hollow cells formed between thin vertical walls. Now, many synthetic honeycomb films can be prepared through the self-assembly of homopolymers, linear block copolymers, and star copolymers according to a breath-figure technique. The amphiphilic HBPs could self-assemble into honeycomb-like microporous films by the slow evaporation of a chloroform solution of the precursors in a humid atmosphere. The pore diameter could be controlled easily by adjusting the casting volume, polymer concentration, molecular weight, and so forth. Another interesting progress is the relation between the polymers and nanoparticles for biomedicines, such as the antibacterial dressing [193], the formation the surface patterning of nanoparticles with polymer patches as biosensors [198], and artificial electronic skins [199]. Patterning of colloidal particles with chemically or topographically distinct surface domains (patches) has attracted intense research interest [198]. Surface-patterned particles act as colloidal analogs of atoms and molecules, serve as model systems in studies of phase transitions in liquid systems, behave as “colloidal surfactants,” and function as templates for the synthesis of hybrid particles. Although the fabrication of micrometer- and submicrometer-sized patchy colloids has been matured, it is still difficult to prepare the patched surface patterns from inorganic nanoparticles of tens of nanometers. These inorganic nanoparticles exhibit size- and shape-dependent optical, electronic, and magnetic properties, and their assemblies show new collective properties [200]. Nanoparticle patterning is usually limited to the generation of two-patch nanoparticles [201–203] and nanoparticles with surface ripples [204] or a “raspberry” [205]surface morphology. Choueiri et al. invented a method to precisely prepare nanoparticle surface patterning utilizing thermodynamically driven segregation of polymer ligands from a uniform polymer brush into surface-pinned micelles following a change in solvent quality [198]. Patch formation is reversible but can be permanently preserved using a photocrosslinking step, which would suppress nanoparticle assembly and enable the utilization of solutions with a higher nanoparticle concentration, thereby increasing the yield of patchy nanoparticles. This methodology offers the ability to control the dimensions of patches, their spatial distribution, and the number of patches per nanoparticle. These patchy nanocolloids have potential applications in fundamental research, the self-assembly of nanomaterials, diagnostics, sensing, and colloidal stabilization [198]. Patterning of multicomponent nanoparticles and the self-assembly of patterned nanoparticles into complex, hierarchical structures can be further explored by some other kinds of surface modification methods, such as

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surface grafting. Furthermore, due to the progress in the synthesis of nanoparticles of different sizes, shapes, and structures, the proposed strategy should enable fundamental studies of polymer segregation on surfaces with large curvatures or surfaces with multiple curvatures. Silk protein is one of the promising biodegradable and biocompatible materials as substrates for electronic devices in artificial intelligence, such as on-skin and implantable electronic devices. However, its intrinsic brittleness and poor thermal stability limit its applications. Recently, robust and heat-resistant silk fibroin composite membranes (SFCMs) are synthesized by mesoscopic doping of regenerated silk fibroin via the strong interactions between silk fibroin (SF) and polyurethane invented by Guo and coworkers [199]. The schemed process to fabricate the integrated protein-based electronic skin (PBES) via SFCMs is illustrated in Figure 1.9 [199]. Some traditional micromachining techniques, such as inkjet printing, can be used to print flexible circuits on such protein substrates (Figure 1.9a). The obtained SFCMs can endure the tensile test more than 200% and the thermal resistance up to 160 ∘ C. Based on this substrate, Ag nanofibers (NFs) and Pt NFs networks have been successfully embedded onto both sides of the SFCMs as heaters and

PU

SF solution Silkworm cocoon

Degumming

Dialysis

Modification

Film formation

(a) Ag

Ag NFs networks Pt Network transferring Tearing tap

(b)

Pt NFs networks Temperature sensor (Pt NFs) Printing electrode (Ag) SFCM

PBES

Heater (Ag NFs)

(c)

Figure 1.9 Fabrication of PBES based on SFCM. (a) Schematic illustration of the preparation of flexible transparent SFCM; (b) schematic illustration of the fabrication of PBES realizing heating and temperature detection; (c) photographic images of the PBES attaching to the human neck and hand closely. Source: Huang et al. [199].

1.6 Thin Films and Coatings for Biomedical Engineering and Life Science

temperature sensors, respectively (Figure 1.9b,c). The integrated PBES exhibits high thermal stability and temperature sensitivity (0.205%/∘ C). Heating and temperature distribution detection are realized by array-type PBES, contributing to potential applications in dredging the blood vessel for alleviating arthritis (Figure 1.9c). This PBES is both inflammation-free and air-permeable, which can directly be laminated onto human skin for long-term thermal management. Single-molecule detection is vital for basic research and practical applications in nanobiotechnology and nanomedicine [206]. Fabrication techniques that yield sensors with repeatable performance are critical to the sensitive detection and precise control of biomolecules [194, 207]. One way is to fabricate bio-wells as small as possible to only save one or several biomolecules [194]; another way to exert control on a biomolecule is to use guiding structures to impose a change in shape or behavior on the molecule of interest [194, 206, 207]. Nanopore or nanocavity thin films provide nanoscale channels or femtoliter volume, which are now well established as possible single-biomolecule sensors label-free or not, which hold great promise as sensing elements in diagnostic and gene sequencing applications [194, 206, 207]. Nanopore technology is particularly attractive for DNA sequencing because of its potential advantages, which include long reads and high speed [206]. Single nanopores can be used for highly sensitive detection of DNA by threading the polymer through the pore, blocking some of the electrical current that normally would pass through the pore. Usually, the nanopore or cavity thin films can be fabricated by traditional e-beam and RIE processes [194, 206, 207]. However, this promise has been limited by the expensive, labor-intensive, and low-yield methods used to fabricate low-noise and precisely sized pores or cavities [207]. Nanoscale preconfinement of DNA is shown to reduce the variation of passage times through solid-state nanopores. Preconfinement is previously achieved by forming a femtoliter-sized cavity capped with a highly porous layer of nanoporous silicon nitride (NPN). This cavity is formed by sealing an NPN nanofilter membrane against a substrate chip using water vapor delamination. However, this method of fabrication cannot keep a consistent spacing between the filter and solid-state nanopore due to thermal fluctuations and wrinkles in the membrane, nor can it be fabricated on thousands of individual devices reliably. To overcome these issues, McGrath and coworkers advanced a new process to fabricate the femtoliter cavity monolithically using a selective xenon difluoride (XeF2 ) etch to hollow out a polysilicon (poly-Si) spacer sandwiched between silicon nitride (SiNx ) layers, as schemed in Figure 1.10 [194]. These monolithically fabricated cavities behave identically to their counterparts formed by vapor delamination, exhibiting similar translocation passage time variation reduction and folding suppression of DNA without requiring extensive manual assembly. The ability to form nanocavity sensors with nanometer-scale precision and to reliably manufacture them at scale using batch wafer processing techniques will find numerous applications, including motion control of polymers for single-molecule detection applications, filtering of dirty samples prior to nanopore detection, and simple fabrication of single-molecule nanobioreactors.

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(a)

Lithography (c)

Wet etch (d)

NPN

RIE

Photoresist

SiO2 pnc-Si SiNx polySi Si (e)

(f)

XeF2 (g)

Sensing membrane

EDP

Nanopore drilling

Figure 1.10 Monolithically created femtoliter cavity process flow. (a) pnc-Si is formed as a hard mask over the thin-film stack with RTP and (b) lithographically patterned. (c) The exposed capping SiO2 layer is removed using an ammonium bifluoride wet etch. (d) The nanopore pattern is transferred into the prefilter layer using an RIE process (SF6 + O2 ), forming NPN in the desired cavity geometry. (e) After an EDP etch to create access to the backside of the device, (f) the femtoliter cavity is formed through the use of XeF2 , which enters through the NPN and hollows out the poly-Si layer between the NPN and the sensing SiNx membrane (15 nm). The remaining pnc-Si and SiO2 are also concurrently removed. (g) Finally, the cavity is wetted, and then, a sensing nanopore is formed in the sensing membrane layer via CBD. Source: Madejski et al. [194].

To address the problem for the nanopore membrane fabrication, Goto et al. have been developing a controlled dielectric breakdown process to enable rapid nanopore fabrication since 2016, and now, this process can offer an upgraded method to fabricate nanopore membranes for DNA-sequencing technology [206, 208]. Recently, Waugh et al. proposed another low-cost and scalable solid-state nanopore fabrication method, termed controlled breakdown (CBD), which is rapidly becoming the method of choice for fabricating solid-state nanopores. Since its initial development, nanopore research groups around the world have applied and adapted the CBD method in a variety of ways, with varying levels of success that present their accumulated knowledge of nanopore fabrication by CBD, including a detailed description of the instrumentation, software, and procedures required to reliably fabricate low-noise and precisely sized solid-state nanopores with a yield of >85% in less than one hour. The general platform for this method is illustrated by Waugh et al. [207]. Unlike traditional beam-based nanopore fabrication technologies, the methods presented here are low-cost and low technical barriers for the fabrication of nanoscale pores in thin solid-state membranes, which is accessible for non-experts. In addition, Song and coworkers, also developed an alternative method to fabricate nanopore membranes of controlled sizes, shapes, and substrates, named as template transfer nanoimprinting [18, 20, 22, 47, 209], which will be summarized in Chapters

1.6 Thin Films and Coatings for Biomedical Engineering and Life Science

10 and 11 of this book. We believed that industrial applications that take advantage of this sensing modality include DNA sequencing DNA barcoding and investigating single-molecule capture and transport shall be realized soon with the progress of the marriage of the nanofabrication technique to the biomedical engineering. The third promising progress of thin films or coatings for biomedical and life science exists in some functional thin films formed by 2D materials, such as graphene. As a new nanomaterial, graphene has shown great promise in drug delivery, cancer therapy, and other nano-biomedical techniques due to its unique microstructures and mechanical and electronic properties. Graphene has been used in the fabrication of other kinds of permeable membranes with enhanced permeability [210], such as reverse osmosis membranes that can preserve great potential in the purification of wastewater and soft water produced by desalination of ocean water for life science, which were summarized in Chapter 14 of this book comprehensively with other kinds of permeable membranes. Recently, one of the promising methods based on the electric properties of GO composites is to introduce into the separation membrane to control the water permeability using an electric field, which is very important in the healthcare technologies related to the controlled water permeability in capillaries or membranes (e.g. artificial skins) [211]. Previous attempts to control water permeation through membranes (mainly polymeric ones) have concentrated on modulating the structure of the membrane and the physicochemical properties of its surface by varying the pH, temperature, or ionic strength [212, 213]. Electrical control over water transport is an attractive alternative. Many micrometer-thick GOMs have been invented for ultrafast permeation of water [178, 214]and molecular sieving [169, 215], with the potential for industrial-scale production. To achieve electrical control over water permeation, Nair and coworkers created conductive filaments in the GOMs via controllable electrical breakdown in 2018 [211]. The scheme of how to fabricate the E-controlled GO composite membranes was given as follows [211]. The GO multilayered membranes were first deposited in a well formed by a polymer molding method supported on a porous silver substrate [211]. The metal/GO/metal sandwich structures were then formed by depositing a thin (≈ 10 nm) gold (Au) film on top of the GOM prepared on the porous silver (Ag) substrate. One of the metal–GO–metal sandwich membranes was attached to the Polyethylene Terephthalate (PET) sheet. Such a thin layer of gold is sufficiently porous and shows the discontinuities and voids in a 10-nm gold thin film on a GOM. Therefore, the coated gold layer does not change the permeation properties of the membranes. However, the water permeability depended on the thickness of the gold layer and the concentration of hydrogen ions and hydroxide ions. Then, the electric field can be imposed between the silver layer and the gold layer to fulfill the E-controlled permeability testing. The electric field that concentrates around these current-carrying filaments ionizes water molecules inside graphene capillaries within the GOMs, which impedes water transport. Thus, the water permeation can be precisely controlled from ultrafast permeation to complete blocking. This work opens up an avenue for developing smart membrane technologies for artificial biological systems, tissue engineering, and filtration.

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1.7 Thin Films and Coatings for National Defense and Homeland Security There are many sophisticated thin films and coatings used in varieties of instruments and equipment for the national defense and homeland security. In this book, we will emphasize some distinctive key thin films and coatings for aeronautics, information security, and marine voyages. The armor coatings for tanks or armored cars will not be discussed in this book. As for coatings for aeronautics, the most key coatings are the thermal barrier coatings (TBCs) and radar stealth coatings. TBCs are a kind of ceramic layers grown or deposited on the superalloy substrates constructing the turbine blades in the “heart” of aircraft or the jet engine in the propulsion system. They function as the thermal and corrosion protection layers of the superalloy substrates in the combustion chamber of jet engines free from high corrosion and oxidizing gas spraying at high temperatures (∼2000 ∘ C or higher) and high speed (sometimes several times of sound velocity). Readers can refer to Chapter 19 of this book for detailed microstructures and fabrication processes of varieties of TBCs. In order to evade the detection by the hostile radar system, stealth coatings for electromagnetic (EM) waves have to be equipped on the top surface of air vehicles together with the whole structure stealth design, particularly for fighters and strategic/long-range bombers. These coatings are usually called radar-absorbing materials made of magnetic nanomaterials and/or carbon-based porous materials, which are discussed in detail in Chapter 18 of this book. The modern information communication, particularly for long-distance communication, is mainly based on the encoding-emission-transmission-receiving-decoding technologies of EM waves up to now even though the quantum communication that is still a long time for commercialization has emerged. Therefore, electromagnetic interference (EMI) compatibility coatings are still the key technique for the information security related to the national defense and homeland safety. The microstructure and material design of these coatings is also based on the EM-wave–matter interaction, which will be summarized in Chapter 18 together with the radar stealth coatings. In the marine voyages, the most key coatings are possibly those coatings related to the anti-marine corrosion coatings and the anechoic coating systems in the sonar (sound navigation and ranging) systems for the underwater vehicles and surface ships, which are intimately related to the national coastal defense safety and the maritime trade profit security. The detailed analyses on these coatings are summarized in Chapters 16 and 17, respectively. Here, we just want to mention one interesting progress in the hydrogel microphones for stealthy underwater listening related to the future marine source exploration [216]. Vehicles traveling in oceans, particularly for those underwater vehicles (e.g. submarines, unmanned undersea vehicles), usually navigate based on their sonar systems to monitor flow velocities and sound waves to navigate, to identify hostile objects, to track ocean currents and surface waves, and to communicate with each other [217, 218]. However, the conventional ceramic-piezoelectric (PZT)-based

1.7 Thin Films and Coatings for National Defense and Homeland Security

sonar systems suffer from a large acoustic impedance mismatch with water, causing them to be easily detected by hostile vehicles during the current era of stealthy navigation because they efficiently reflect incoming acoustic signals. [216, 219, 220]. In addition, the detection efficiency of PZT-based acoustic sensors is relatively poor at low frequencies [216]. Alternatively, suspended thin membranes of poly(vinylidene fluoride) [221] or graphene stretched over air cavities [222] have been proposed as microphones to afford a higher sensitivity than PZT [223], but these configurations introduce even larger mismatch in acoustic impedance between the device (air) and water [216]. Considering the advances in acoustic metamaterial cloaking, which greatly attenuates incoming acoustic signals, thereby concealing submarine bodies from sonar detection, a ceramic PZT detector or cavity-based microphone, which is necessarily kept outside of this “invisibility cloak,” remains a strong acoustic reflector [224–226]. In contrast to a rigid solid such as ceramics or a low-density compliant medium such as air, hydrogels have almost perfect acoustic impedance matching with water [216]. Polar functional groups from the backbone or side chains allow hydrogels to absorb a large amount of liquid into three-dimensional polymer networks without leaking. Different from dielectric capacitors, where their capacitance is governed by the distance between two parallel electrodes [227], hydrogel capacitors derive their capacitance from electrical double layers (EDLs) [216, 228]. With the excellent acoustic impedance match to water, a hydrogel capacitor seems to be a promising acoustic transducer. The problem, however, is that the low compressibility of water means that an EDL capacitor would have low sensitivity to pressures. In addressing this limitation, a suitable sensor can be made by incorporating a deformable network of metal nanoparticles (MNPs) into the hydrogel. The MNP network makes the capacitor highly sensitive to mechanical stimuli through a coupling between the deformation of the MNP network and the ion modulation. As a result, this MNP–hydrogel capacitor is able to detect deformation, pressure, and acoustic waves. The key to this hydrophone is the fabrication of the MNP–hydrogel network. Figure 1.11a schemes the synthesis process of the MNP–hydrogel network. In step 1, hydrogel is presoaked in an aqueous bath of AgNO3 (e.g. 10 mM) [216]. Then, hydrogel is sandwiched and biased between an amorphous silicon (a-Si) and an ITO plate (step 2). After that, photoactivated a-Si reduces Ag+ into Ag0 nanoparticles at specific locations (step 3). Finally, MNP–hydrogel is soaked in a copper sulfate bath to prepare a smooth and robust layer of surface electrodes (step 4). Figure 1.11b,c gives the photograph of the patterned Ag nanoparticles in the skin depth of the hydrogel, the high-resolution SEM image, and the schematic of the dendritic MNP network inside the hydrogel. Furthermore, hydrogel microphones can be fabricated by the cavity-free coatings via integrating the easily deformable MNP–hydrogel network in the hydrogel matrix, as highlighted in Figure 1.12a–g [216]. Figure 1.12a shows an example of a 9 mm2 hydrogel microphone fabricated by forming an MNP network consisting of dendritic structures 2–3 mm in size and being buried inside the soft and translucent hydrogel matrix (Figure 1.12b). This MNP–hydrogel microphone was electrically biased at 1 V and submerged in water (Figure 1.12c), where it picked

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(a) –

a-Si

hv

– +

+ Ag+ ITO Step 1

Hydrogel Step 2

Step 3

Cu2+ Step 4

(b)

(c)

Ag + 500 nm

Figure 1.11 Fabrication of hydrogel microphones. (a) Steps to fabricate a deformable network of metal nanoparticles (MNPs) and surface electrode. (b) Photos of patterned Ag nanoparticles in the skin depth of the hydrogel. Scale bar, 3.0 mm. (c) High-resolution SEM image and schematic of a dendritic MNP network inside the hydrogel. Source: Gao et al. [216].

up acoustic waves and produced a signal 30 dB stronger at low frequencies than a commercial hydrophone (Figure 1.12d). Moreover, the hydrogel microphone has a wide frequency response, up to 2 kHz (Figure 1.12e), and has a pronounced directional sensitivity perpendicular to the sensor surface (Figure 1.12f). MNP–hydrogel (Figure 1.12g: solid lines) responds to a static pressure of 5.4 kPa with more than four times in relative capacitance change or seven to eight times in capacitance change than MNP-free device (Figure 1.12g: dashed lines). Since MNPs can be densely implanted as inclusions and can even be arranged in coherent arrays, the general performance testing results suggest that this microphone can detect static loads and air breezes from different angles, as well as underwater acoustic signals from 20 Hz to 3 kHz at amplitudes as low as 4 Pa [216]. Unlike dielectric capacitors or cavity-based microphones that respond to stimuli by deforming the device in thickness directions, this hydrogel device responds with a transient modulation of electric double layers, resulting in an extraordinary sensitivity (217 nF/kPa or 24 μC/N at a bias of 1.0 V) without using any signal amplification tools [216]. Due to their perfect acoustic impedance matching with water, ultrasensitive for low-frequency acoustic waves, the nanomaterial–hydrogel-based hydrophones have currently become an

Acknowledgments

(a)

3m

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Figure 1.12 Highlight of hydrogel microphone. (a) Photos (full view and sliced) and (b) scanning electron microscopy (SEM) image of the hydrogel membrane implanted with a patch (3 × 3 mm2 ) of silver dendrites (highlighted yellow). (c) Setup and circuit using the membrane as a microphone. (d) Better performance of the hydrogel microphone at low frequencies than a commercial device (hydrophone). (e) Hydrogel microphone is capable of detecting underwater sound at 2 kHz and (f) at all angles. Note: the 0_ orientation is for the top surface of the microphone facing toward the loudspeaker. (g) MNP–hydrogel (solid lines) responds to a static pressure of 5.4 kPa with more than four times in relative capacitance change or seven to eight times in capacitance change than MNP-free device (dashed lines). The relative errors of direct current (DC) and DC/C 0 for MNP–hydrogel are, respectively, 21–25% and 3.5–4.5%. For MNP-free hydrogel, the relative errors of DC and DC/C 0 are, respectively, 13.0–13.5% and 3.0–4.0%. Source: Gao et al. [216].

exciting area for developing hydrophones for potential antiscouting sonar [229, 230] or ultrasensitive stretchable pressure sensors [231, 232].

Acknowledgments This chapter was supported by the NSFC-BRICS STI Framework Program (No. 51861145309), the National Natural Science Foundation of China (No. 51971029), the National S&T Major Project (No. 2018ZX10301201), the “All English teaching demonstration course construction project of University of Science and Technology Beijing” (No. KC2015QYW06, 2016), the “1125” Zhihui Zhengzhou Talent project of Henan province (Fund No. in USTB: 39080070), the “100 talent plan” fund of Fujian province (Fund No. in USTB: 39080067), and the development of a highly sensitive magneto-optical biomolecular sensor experimental prototype (Fund No. in USTB: 2019-0649) by Hangzhou Ruidi Biotechnology Co. Ltd.

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List of Abbreviations 2D 3D AEMs AIBs AND BDA BIF CBD CCD C-dots CIGS CMOS CMP CMR CO2 RR CVD DJ EDLs EG EM EMI ESSs ETL-free PSCs F4TCNQ FCs FEA FET FOLEDs GDEs GMR GNRs GO GON Gp GPU h-BN (hBN) HBPs HER HR-TEM HTM ITO LDH LED

two-dimensional three-dimensional atom electronic mechanics aluminum-ion batteries agree nor disagree butanediamine build-in field controlled breakdown charge-coupled device carbon quantum dots copper indium gallium selenide complementary metal-oxide semiconductor conjugated microporous polymer colossal magnetoresistance reduction reaction of CO2 chemical vapor deposition Dion Jacobson electrical double layers expansion reduced graphene electromagnetic electromagnetic interference energy storage systems electron transport layer-free perovskite solar cells 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquino dimethane fuel cells pentafluorophenylethyl ammonium field emission transistor foldable OLEDs gas diffusion electrodes giant magnetoresistance graphene nanoribbons graphene oxide germanium-on-nothing graphene graphics processing unit hexagonal boron nitride hyperbranched polymers hydrogen evolution reaction high-resolution transmission electron microscopy hole-transporting material indium tin oxide layered bimetallic hydroxide laser emission diodes

List of Abbreviations

LSC MAPbI3 MBE MEMs MNC MNPs MO MOSFET MoTe2 NAND NCPV NEMs NFs NPN NPs NREL OER OLED ORR PBES PCE PDA PEM PFE PG P-Hg PHJ PLQY PN PNPL PSCs poly-Si PV PZT QAHE QW rGO RTG SACs SEIs SEM SFCMs SiNx SMEMs SSCs

luminous solar concentrator methylammonium lead iodide molecular beam epitaxy microelectronic mechanics metal–nitrogen–carbon metal nanoparticles magneto-optical metal-oxide-semiconductor field-effect transistor molybdenum ditelluride neither agree nor disagree National Center for Photovoltaics nanoelectronic mechanics nanofibers nanoporous silicon nitride nanoparticles National Renewable Energy Laboratory oxygen evolution reaction organic light emitting diode oxygen reduction reaction protein-based electronic skin power conversion efficiency propanediamine proton-exchange membrane polar ferroelectric PNIPAm modified graphene black phosphorus plane heterojunctions photoluminescence quantum yield poly(N-isopropylacrylamide) (PNIPAm) halide perovskite nanoflake perovskite solar cells polysilicon photovoltaic piezoelectric quantum anomalous hall effect quantum well reduced graphene oxide thermoelectric power generator single-atom catalysts solid electrolyte interfaces scanning electronic microscope silk fibroin composite membranes silicon nitride single-molecule electronic mechanics single-site catalysts

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TBCs TE TM TMDs TMR TSCs UCD XeF2 XOR

thermal barrier coatings thermoelectric transition metal transition metal dichalcogenides tunneling magnetoresistance tandem solar cells Universal Communication DeviceTM xenon difluoride exclusive-OR

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2 Fundamental in Functional Thin Films and Coatings Weiwei Zhang 1 and Yujun Song 1,2 1 University of Science and Technology Beijing, Center for Modern Physics Technology, Applied Physics Department, School of Mathematics and Physics, 30 Xueyuan Road, Beijing 100083, China 2 Zhejiang Key Laboratory for Pulsed Power Technology Translational Medicine, Hangzhou Ruidi Biotechnology Company, Hangzhou 310000, China

2.1 Introduction Films and coatings with multi-functions have novel physical and chemical properties resulting from the combination of their interfacial multi-physical field couplings (i.e. magneto-electric coupling [1–3], magneto-optical (MO) coupling [4, 5], magnetothermal coupling [6–9], and photoacoustic coupling [10, 11]). This field has been developed with new materials, processing, and applications being envisaged. In this chapter, we summarize the underlying mechanisms associated with thin films and coatings.

2.2 Theory of Magneto-electric Coupling in Magnetic Thin Films To date, various magnetoresistances (i.e. giant magnetoresistance (GMR), colossal magnetoresistance (CMR), tunneling magnetoresistance (TMR)) and quantum anomalous Holzer effect (QAHE) combining the electron spin and charge properties with magneto-electric coupling at mesoscopic sales are capable of modulating electron transports. GMR based on the dependence of electron scattering on the spin orientation is a quantum mechanical magnetoresistance effect, which significantly change electrical resistance depending on the magnetization of adjacent ferromagnetic layers is in a parallel or an antiparallel alignment in the presence of an external magnetic field. It is found that the overall resistance is relatively low for parallel alignment and relatively high for antiparallel alignment. Usually, the value of the GMR can be defined as MR =

𝜌(H) − 𝜌(0) Δ𝜌 = 𝜌(0) 𝜌(0)

Inorganic and Organic Thin Films: Fundamentals, Fabrication, and Applications, First Edition. Edited by Yujun Song. © 2021 WILEY-VCH GmbH. Published 2021 by WILEY-VCH GmbH.

(2.1)

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Here, 𝜌(H) represents the electrical resistivity in the presence of the applied magnetic field H, and 𝜌(0) is the electrical resistivity without the applied magnetic field. CMR effect associated with a ferromagnetic to paramagnetic phase transition under an external magnetic field is usually based on manganese-based perovskite oxides, and CMR can reach many orders of magnitude exceeding GMR of multilayered and granulated systems [12, 13]. TMR that depends dramatically on the alignment of the ferromagnets, in parallel or antiparallel, in magnetic tunnel junctions made up of ferromagnets/insulator/ferromagnets system. As shown in Eq. (2.2), the TMR effect is a typical example of the spin-dependent electron transport and can be evaluated by the ratio of conductance: TMR =

Gp − GAp Gp

(2.2)

where Gp and GAp are the conductances for the parallel and antiparallel alignments of the magnetic tunnel junctions, respectively. The value of TMR is mainly dependent on the intensity of two ferromagnets’ spin polarization at the Fermi energy. It is concluded that the conductance will be larger if they have the same sign of the spin polarization and the magnetic layers are aligned parallel, which is considered as the positive sign of the TMR. However, it is experimentally demonstrated that the conductance of the TMR can be widely decided by a broad range of atomic and electronic factors such as an insulator and the ferromagnet/insulator interfaces, materials of electrodes, the height, the shape and even the disorder of the barriers, and the impurities in the barrier. QAHE is a quantized version of the Holzer effect observed in two-dimensional (2D) electron systems under approximately 10 T magnetic field strength that makes the experimental realization challenging and greatly hinders real-world applications. However, QAHE, which may be a consequence of the combined spin–orbit coupling and reduction of the time-reversal symmetry (TRS) due to intrinsic magnetization, can be realized without an external magnetic field that may lead to the development of low-power-consumption electronics [14]. Nowadays, broad investigations have been carried out to seek new platforms for the realization of the QAHE [15–17]. Among the investigations, graphene-like honeycomb materials [18, 19] and magnetically doped topological insulators [14, 20] have mainly attracted increasing attention.

2.3 Theory of Electronic Thin Films: Electronic Percolation and Spintronic Theory on the Semiconductor Thin Film Spintronics refers commonly to phenomena, where the spin of electrons in a solid-state environment plays the determining role. Particularly, semiconductor spintronics as an emerging research discipline and an important advanced field in

2.3 Electronic Percolation and Spintronic Theory on the Semiconductor Thin Film

physics has developed quickly and obtained fruitful results and remarkable success in recent decades [21–24]. An important research topic in semiconductor spintronics is to use semiconductor devices that are based on the long spin coherence time of electron spin and nuclear spin to complete quantum information processing [25, 26]. However, a crucial problem in making spintronic devices is how to inject spinpolarized electrons from magnetic semiconductors into nonmagnetic ones without a strong magnetic field or at room temperature. Up to now, there are five main injection methods: ohmic injection, tunnel junction injection, ballistic electron spin injection, hot electron injection, and dilute magnetic semiconductor injection. Ohmic injection. The most direct spin injection structure for injecting spin-polarized current into a semiconductor is the ohmic contact formed by the ferromagnetic material/semiconductor. Since electrons are spin-polarized in the ferromagnetic material, it is desirable to inject spin-polarized electrons into the semiconductor. However, the typical metal–semiconductor ohmic contact requires heavy doping on the semiconductor surface, which will cause the spin-flip scattering of carriers and result in the loss of spin polarization. Because the semiconductor surface is heavily doped, spin inversion scattering and spin polarizability decrease. Therefore, the spin injection rate of this method is very low. Tunnel junction injection. The tunnel junction in the vacuum can effectively inject the spintronics into the semiconductor. High impedance ferromagnetic/ insulating layer/ferromagnetic structures have also demonstrated that spin polarization can be maintained during tunneling, suggesting that tunneling may be a more efficient spin injection method than diffusion transport. Hot electron injection. This method uses spin-polarized hot electrons to be injected into the ferromagnetic film through a tunnel junction, and the energy injected into the hot electrons is regulated by adjusting the bias of the tunnel junction. When the inelastic mean free path is different, the hot electrons passing through the ferromagnetic metal layer can produce an electron current with polarizability of over 90%. Depending on the transmission probability determined by the band structure of the semiconductor and metal at the interface, a highly polarized current can retain a considerable portion. If there is less spin-flip scattering at the interface, the ballistic current entering the semiconductor is still highly polarized. Spin detection technology. This method is also a crucial concept for spintronic devices. There are optical and electrical methods for spin detection. The optical detection method is relatively mature and has made great progress. The greatest advantage of the optical method is that it avoids other electrical effects. The most direct electrical method for detecting spin polarization is to utilize the spin-dependent transport properties of the semiconductor/ferromagnetic interface. However, the electrode in this method uses ohmic contact, and the conductivity mismatch still exists.

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2.4 Theory of Metal Structural Thin Films: Metamaterials and the Negative Permeability Theory and Maxwell Theory A metamaterial is an artificially structured material that has extraordinary electromagnetic properties involving physics, electrical engineering, materials science, optics, and nanoscience. The properties of the metamaterial can be widely tailored by their internal physical structure design. An attractive interest in metamaterial is its effects on light propagating. Here we introduce some models to deal with the interaction between an electromagnetic wave and metamaterial. Jones vector and Jones matrix [27] compose a simple approach to model scatterers on metasurfaces. We assume a light wave propagating along the z-direction. As shown in Eq. (2.3), the incident electric field is composed of two components, x and y, and can be described by the Jones vector without time-harmonic factor: ( ) i (2.3) Ei (r, t) = x e−ikz iy Here, k is the wavenumber and the complex amplitudes ix and iy are the polarization states of the incident waves. After the incident wave impinging on the metasurface, the transmitted wave is described in the same manner: ( ) t Et (r, t) = x e−ikz (2.4) ty where tx and ty represent the polarization states along the x and y directions of the transmitted wave. Then, the localized scatterer on the metasurface can be expressed by Jones matrix, J. It connects the transmitted field components, tx, and ty to the incident ones: )( ) ( ) ( ) ( J J ix i tx = xx xy =J x (2.5) ty iy iy Jyx Jyy where the first and second subscripts of J denote the polarization states of the transmitted wave and incident wave, respectively. The reflected field can be modeled in the same way. Additionally, the Jones matrix, J can be transformed into a circular basis by coordinate transformation so that circularly polarized fields can be directly manipulated: ) ( J J Jc = ++ +− J−+ J−− ( ) 1 (Jxx + Jyy ) + i(Jxy − Jyx ) (Jxx − Jyy ) − i(Jxy + Jyx ) . (2.6) = 2 (Jxx − Jyy ) + i(Jxy + Jyx ) (Jxx + Jyy ) − i(Jxy − Jyx ) where Jc connects the incident circularly polarized Jones vectors to the transmitted or reflected circularly polarized ones. + and − represent the left circularly polarized and right polarized components, respectively. We should see that only the normal incidence and reflection/transmission are considered by using the Jones matrix. However, the coupling between scatterers is not taken into consideration rigorously. Polarizability model. Incident waves can bring polarization electric current, leading to the discontinuities of field components when crossing the metasurface

2.4 Metamaterials and the Negative Permeability Theory and Maxwell Theory

plane, which results in the conventional boundary conditions unable to describe the system that should be replaced by the generalized sheet transition conditions (GSTCs) [28]. n × (H1∕∕ − H2∕∕ ) = Jtoe = i𝜔P∕∕ − n × ∇∕∕ Mn P n × (E1∕∕ − E2∕∕ ) = −Jtom = −i𝜔𝜇0 M∕∕ − n × ∇∕∕ n 𝜀0 P ∕∕ n ⋅ (E1∕∕ − E2∕∕ ) = −∇ ⋅ 𝜀0 n ⋅ (H1∕∕ − H2∕∕ ) = −∇ ⋅ M∕∕

(2.7) (2.8) (2.9) (2.10)

where P and M are the surface electric and magnetic polarization densities, correspondingly. Subscripts “//” and “n” denote the tangential and normal components, respectively, while superscripts “1” and “2” refer to the fields at the two sides of the metasurface. Jtoe and Jtom denote the effective total electric and magnetic currents. Equations (2.6)–(2.9) provide us the information on the induced polarization density that transforms the impinging field in the desired manner, but still, it cannot give an intuitive insight of the metasurface design. To investigate this problem, models based on different homogenized parameters such as the polarizability [29], susceptibility [30], and equivalent impedance [31] have been proposed and demonstrated. Polarization density can be expressed as a form of the polarizability and the incident field [29]: P=

𝛼⌢ee 𝛼⌢ Ei + em Hi S S

(2.11)

𝛼⌢me 𝛼⌢ Ei + mm Hi (2.12) S S where Ei and Hi are the known incident fields, S is the area of the unit cell of the 𝜇M =

̂ is the effective polarizability dyadic, which represents the collective metasurface, 𝛼 effect of a single scatterer itself, together with the coupling and interaction from the whole metasurface array [31]. To synthesize a metasurface, one should first determine the polarization by substituting the known incident fields (Ei ,Hi ), desired ̂ components transmitted fields (Et ,Ht ), and reflected fields (Er ,Hr ), then find the 𝛼 through the obtained (P, M). For normal incident plane wave and the metasurface with uniaxial symmetry, a simple form of the polarizability and fields can be derived [29]. ) [( i𝜔 1 co co cr cr 𝜂0 𝛼 Er = − ̂ee +𝛼 ̂em +𝛼 ̂me − 𝛼 ̂mm I t 2S 𝜂0 ) ] ( 1 cr cr co co + 𝜂0 𝛼 ̂ee −𝛼 ̂em −𝛼 ̂me − 𝛼 ̂mm J t ⋅ Ei (2.13) 𝜂0 ( )) (( i𝜔 1 co co cr cr 𝜂0 𝛼 Et = 1− ̂ee + 𝛼 ̂em − 𝛼 ̂me + 𝛼 ̂ It 2S 𝜂0 mm ( ) ) i𝜔 1 cr cr co co 𝜂0 𝛼 − ̂ee −𝛼 ̂em +𝛼 ̂me + 𝛼 ̂ J Ei (2.14) 2S 𝜂0 mm t

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where I t = I − z ⋅ z is the tangential unit dyadic, while J t = z × I t is the transverse rotation dyadic. Superscripts “co” and “cr” mean the symmetric and anti-symmetric components of 𝛼 ̂, respectively ⎧ ⌢cr ̂ee = 𝛼⌢co ee I t + 𝛼 ee J t ⎪𝛼 ⎨ ⌢co ⌢cr ⎪𝛼 ⎩̂em = 𝛼 em I t + 𝛼 em J t

(2.15)

⎧ ⌢cr ̂me = 𝛼⌢co me I t + 𝛼 me J t ⎪ 𝛼 (2.16) ⎨ ⌢co ⌢cr ⎪𝛼 ̂ = 𝛼 I + 𝛼 J mm t mm t ⎩ mm Susceptibility model. Alternatively, the polarizability of the metasurface can be homogenized by surface susceptibility. Polarization density is described by [30]. √ (2.17) P = 𝜀𝜒 ee Eav + 𝜇𝜀 𝜒 em Hav √ 𝜀 𝜒 E (2.18) M = 𝜒 mm Hav + 𝜇 me av u = [Eu + (Eu + Eu )]∕2 and H u = [H u + (H u + H u )]∕2 (u = x, y, z) where Eav r av r t t i i Closed-form relation of the fields and the susceptibility tensor can be obtained for the simplified case, assuming only tangential components of the polarizations are induced, so that Pz = Mz = 0. Substitute Eqs. (2.17) and (2.18) into Eqs. (2.7)–(2.10), it leads to ( xx ( ( xx y) xy ) ( x ) xy ) ( x ) √ 𝜒ee 𝜒ee Eav Hav −HΔ 𝜒em 𝜒em = i𝜔𝜀 + i𝜔 𝜀𝜇 (2.19) yx yy yx yy HxΔ 𝜒ee 𝜒ee Eyav 𝜒em 𝜒em Hyav ( xx ( y ) ( xx xy ) ( x ) xy ) ( x ) √ 𝜒mm 𝜒mm Hav Eav EΔ 𝜒me 𝜒me = i𝜔𝜇 + i𝜔 (2.20) 𝜀𝜇 yx yy y yx yy y x −EΔ 𝜒mm 𝜒mm Hav 𝜒me 𝜒me Eav

with EΔu = Etu − (Eiu + Eru ) and HΔu = Htu − (Hiu + Hru ) denoting the differences of the fields at the two sides of the metasurface, correspondingly. To this point, the susceptibility tensor matrix still cannot be completely determined for a specified field transformation. The number of unknown matrix components should be reduced by enforcing some extra conditions. For example, to monoanisotropic and uniaxial xy yx xy yx medium, so that 𝜒 em = 𝜒 me = 0 and 𝜒ee = 𝜒ee = 𝜒mm = 𝜒mm = 0, Eqs. (2.19) and (2.20) degrades to a simple relation. y

xx = 𝜒ee

yy = 𝜒ee

−HΔ x i𝜔𝜀Eav

HΔx y

i𝜔𝜀Eav

(2.21)

(2.22)

y

xx 𝜒mm =

yy = 𝜒mm

EΔ x i𝜔𝜇Hav

−EΔx y i𝜔𝜇Hav

(2.23)

(2.24)

2.4 Metamaterials and the Negative Permeability Theory and Maxwell Theory

Figure 2.1 Equivalent transmission line model (T-circuit) of a metasurface. Source: Asadchy et al. [32].

i2

i1 Z1 V1

Z2 Z3

V2

Therefore, the metasurface can be synthesized according to the desired fields on the two sides. Equivalent Impedance Model. The equivalent impedance model based on the transmission line theory is also a powerful method for the metasurface design [31, 32]. Impinging plane waves to the metasurface is an analogy to a propagating signal in a transmission line with proper equivalent parameters. Metasurface described by Eqs. (2.7)–(2.10) that can be modeled by a T-circuit as shown in Figure 2.1, with the equivalent impedance matrix that connects the voltages and currents by ( 1) ( ) ( 1) v Z11 Z12 i = (2.25) v2 Z21 Z22 i2 where Z 11 = Z 1 + Z 3 , Z 22 = Z 2 + Z 3 , Z 12 = Z 21 = Z 3 , linking the tangential fields at the two sides of the metasurface to the voltages and currents of the transmission line: ( ) ( ) )( E1∕∕ n × H1∕∕ Z11 Z12 = (2.26) E2∕∕ Z21 Z22 −n × H2∕∕ It has been reported that metamaterial with negative-permeability has been widely investigated [33, 34], which means that the solutions of Maxwell magnetostatic equations to include negative permeability values are extended. The understanding of these new solutions allows us to devise a negative-permeability material as a suitably tailored set of currents arranged in space, overcoming the fact that passive materials with negative permeability do not exist in magnetostatics, making the metamaterials particularly attractive. It is known that in the static case, electric and magnetic fields decouple, which means controlling magnetic fields requires only dealing with permeabilities. However, natural materials exist with extreme permeability values, such as 𝜇 → 0 and 𝜇 → ∞. In contrast, it is very difficult to fabricate materials with zero permittivity and only approximate results can be achieved based on resonances. However, some advantages of the full electromagnetic case have not yet a counterpart in magnetostatics. One of these is the possibility of having negative –𝜇 materials, whereas resonances in different kinds of natural and artificial substances can yield negative values of 𝜇 and 𝜀 at nonzero frequencies [35], no such negative –𝜇 materials exist in magneto-statics. Negative values of 𝜇 and 𝜀 have enabled very interesting novel phenomena for electromagnetic waves [36]. Devising ways to create the effective response of negative −𝜇 materials would pave the way toward the realization of these properties and also for static magnetic fields.

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2.5 Theory of Surface Plasmon Resonance and Magnetoplasmonic Thin Films Surface plasmon resonance (SPR) is the resonant oscillation of conduction electrons at the interface between a negative and positive permittivity material stimulated by incident light. The resonance condition is established when the frequency of incident photons matches the natural frequency of surface electrons oscillating against the restoring force of positive nuclei. SPR in subwavelength scale nanostructures can be polaritonic or plasmonic in nature. SPR is the basis of many standard tools for measuring adsorption of material onto planar metal (typically gold or silver) surfaces or onto the surface of metal nanoparticles. It is the fundamental principle behind many color-based biosensor applications and different lab-on-a-chip sensors. Since the wave is on the boundary of the conductor and the external medium (for example, air, water, or vacuum), these oscillations are very sensitive to any change of this boundary, such as the adsorption of molecules to the conducting surface. Localized surface plasmon resonances (LSPRs) are collective electron charge oscillations in metallic nanoparticles that are excited by light. They exhibit enhanced near-field amplitude at the resonance wavelength. This field is highly localized at the nanoparticle and decays rapidly away from the nanoparticle/dielectric interface into the dielectric background, though far-field scattering by the particle is also enhanced by the resonance. Light intensity enhancement is a very important aspect of LSPRs and localization means the LSPR has a very high spatial resolution (subwavelength), limited only by the size of nanoparticles. Because of the enhanced field amplitude, the MO effect that depends on the amplitude is also enhanced by LSPRs. In order to excite surface plasmons in a resonant manner, one can use an electron bombardment or incident light beam (visible and infrared ranges are typical). The incoming beam has to match its momentum to that of the plasmon. [4] In the case of p polarized light (polarization occurs parallel to the plane of incidence), this is possible by passing the light through a block of glass to increase the wavenumber (and the momentum) and achieve the resonance at a given wavelength and angle. An s polarized light (polarization occurs perpendicular to the plane of incidence) cannot excite SPR. In order to excite SPR waves, Otto and Kretschmann setups are two main configurations. As shown in Figure 2.2, in the Otto setup, the light illuminates the wall of a glass block, typically a prism, and is totally internally reflected. A thin metal film is positioned close enough to the prism wall so that an evanescent wave can interact with the plasma waves on the surface to excite the SPR.

Prism

Prism

Kretschmann

Otto (a)

(b)

Figure 2.2 Schematics of (a) Otto configuration and (b) Kretschmann configuration.

2.5 Theory of Surface Plasmon Resonance and Magnetoplasmonic Thin Films

(a)

(b)

(c)

Figure 2.3 (a) One-dimensional strip grating; (b) two-dimensional nanorod grating; and (c) two-dimensional hexagonal nanoarray.

In the Kretschmann configuration, the metal film is evaporated onto the glass block. The light again illuminates the glass block, and an evanescent wave penetrates through the metal film. The SPR is excited at the outer side of the film. This configuration is used in most practical applications. Another method is to use the grating to realize the excitation of SPR. The basic mechanism of grating coupling is to use the increment of wave vector generated by grating diffraction. The wave vector can be given as [37] −−−−−⇀ − ⇀ − ⇀ (2.27) Rel(Kspp ) = |k=incident + mGx + nGy | −−−−−⇀ where k=incident is the horizontal component of the incident wave, m and n are the − ⇀ − ⇀ diffraction series, and Gx and Gy are the base vectors. Here we introduce three main grating nanostructures whose schematic are shown in Figure 2.3. The light is incident to the surface of the grating from the medium with a refractive index n, incident angle 𝜃, and azimuth angle 𝜑. The relative position of the incident wave vector and the grating is shown in Figure 2.4a (𝜃 ≠ 0, 𝜑 = 0). Here the wave vector matching condition is given as | 2π | (2.28) Rel(Kspp ) = |k0 n sin 𝜃 + mG | = ||k0 n sin 𝜃 + m || p| | As shown in Figure 2.4b, when 𝜃 ≠ 0, 𝜑 ≠ 0, the wave vector matching condition can be shown as | 2𝜋 | Rel(Kspp ) = ||k0 n sin 𝜃 cos φ⃑i + k0 n sin 𝜃 sin φ⃑j + m ⃑i|| (2.29) p | | where ⃑i and ⃑j are the unit vectors in the x and y directions respectively, and a and b are the periods in the unit vector directions.

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z

z

y

o

y

o x

x

θ

θ φ

(a)

(b)

Figure 2.4 The relative position of the incident wave vector and the grating. (a) azimuth angle is 0 and (b) azimuth angle is not 0.

Up to now, SPR has been used to enhance the surface sensitivity of several spectroscopic measurements including fluorescence, Raman scattering, second harmonic generation, and MO effects. MO effects can be described by the interaction of incident photons with the electron spins through spin–orbit coupling that has the general form. Hso =

h P ⋅ (𝝈 × ∇V) 8𝜋c2 m20

(2.30)

where m0 is the free electron mass, c is the velocity of light, 𝝈 = 𝝈(x, y, z) is a vector, which components are the Pauli matrices, V is the electric potential, and P represents the canonical momentum. In the presence of an external magnetic field, B = r × A, P should be replaced by the kinetic momentum P = p + eA. But MO effects can be completely explained by the classical magnetic properties of the material. For instance, MO effects in an anisotropic material that can be conveniently expressed in terms of its relative dielectric permittivity, where 𝜀 is a 3 × 3 tensor as follows: ⎡𝜀xx 𝜀xy 𝜀xz ⎤ 𝜀 = ⎢𝜀yx 𝜀yy 𝜀yz ⎥ ⎥ ⎢ ⎣𝜀zx 𝜀zy 𝜀zz ⎦

(2.31)

Here, the subscripts x, y, z represent the three Cartesian coordinate axes. For a linear anisotropic material, it is further reduced to just a diagonal tensor as follows: ⎡𝜀xx 0 0 ⎤ 𝜀 = ⎢ 0 𝜀yy 0 ⎥ ⎥ ⎢ ⎣ 0 0 𝜀zz ⎦

(2.32)

However, in the presence of an internal magnetization M pointing, for instance, in the y-direction, the off-diagonal elements of the permittivity tensor in the x- and z-direction become coupled. Microscopically, this can be explained using classical electron theory. When an incident electromagnetic wave (EM) field illuminates

2.5 Theory of Surface Plasmon Resonance and Magnetoplasmonic Thin Films

a material, a Lorentz force acts on the material’s electrons and consequently, an oscillation in one direction tends to create another oscillation in its transverse direction manifesting itself as nonzero off-diagonal elements. The material’s permittivity gets modified as: 0 ⎤ ⎡𝜀xx 0 𝜀 = ⎢ 0 𝜀yy −ig⎥ ⎥ ⎢ ⎣ 0 ig 𝜀zz ⎦

(2.33)

where the off-diagonal elements represent the magnetically-induced part and are directly proportional to M. g is referred to as the MO constant of a material. Its value varies from material-to-material and is a function of the incident wavelength. g = g(𝜆, M)

(2.34)

The tensor elements of the permittivity tensor are complex in general. For an isotropic material at room temperature, the diagonal elements can be expressed as a square of the complex refractive index of the material. 𝜀xx = 𝜀yy = 𝜀zz = N 2

(2.35)

N = n + ki

(2.36)

Here, n represents the dispersion and k represents the extinction coefficient of the material. Here, we will mainly summarize the mechanism of Kerr and Faraday effects. Magneto-optical Kerr effect (MOKE): MO effects in reflection mode are called MOKE and have the same physical origin as the Faraday effect. They lead to a change of the polarization properties and/or of the light intensity when a linearly polarized light gets reflected from a magnetized medium. When a linearly polarized optical wave travels through or is reflected by an isotropic, non-magnetic medium; it could be in either of its two eigenmodes (waves that preserve their polarization while propagating through a medium). However, when it is incident on a magnetized medium, the off-diagonal elements of the permittivity tensor of this medium induce a coupling between the two polarizations or modify the reflectivity depending on the magnetization orientation. There are three configurations of the MOKE based on the direction of magnetization. We describe the different forms of dielectric tensors of three different MOKE. It is assumed that the sample plane is parallel to the xoy plane, and the incident plane of the light is parallel to the xoz plane. Table 2.1 summarizes the MO Kerr effect configurations with a schematic depiction of each effect. Polar magneto-optical Kerr effect (P-MOKE): The magnetization direction of the material caused by the external magnetic field is perpendicular to the sample surface and parallel to the incident surface. In this case, the dielectric tensor can be expressed as: ⎡𝜀xx −gi 0 ⎤ 𝜀 = ⎢ gi 𝜀yy 0 ⎥ ⎥ ⎢ 0 𝜀zz ⎦ ⎣0

(2.37)

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Table 2.1

The different configurations of MOKE.

MOKE and its diagram z

P-MOKE

M

M

M y o

z

z x

L-MOKE

y o

x

T-MOKE

y o

Tensor

⎡𝜀xx −gi 0 ⎤ ⎢ ⎥ 𝜀 = ⎢ gi 𝜀yy 0 ⎥ ⎢ ⎥ 0 𝜀zz ⎦ ⎣0

⎡𝜀xx 0 0 ⎤ ⎢ ⎥ 𝜀 = ⎢ 0 𝜀yy −gi⎥ ⎢ ⎥ ⎣ 0 gi 𝜀zz ⎦

⎡𝜀xx 0 −gi⎤ ⎢ ⎥ 𝜀 = ⎢ 0 𝜀yy 0 ⎥ ⎢ ⎥ ⎣ gi 0 𝜀zz ⎦

Effects

Rotation and ellipticity

Rotation and ellipticity

Change of intensity

x

For the P-MOKE, both s polarized light and p polarized light have an electric field component in the vertical direction of the magnetic field, so both s polarized light and p polarized light can produce Kerr angle and ellipticity, and their magnitude depends on the angle of incident light and the magnitude of the magnetic field that determines the magnitude of g. The P-MOKE can be expressed as: ] [√ (Nr 2 − tan2 𝜃in ) ∓ sin 𝜃in tan 𝜃in Nr 2 gi 𝛹= (2.38) 𝜀xx (Nr 2 − 1)(Nr 2 − tan2 𝜃in ) where, N r is the ratio of the refractive index of the magnetic medium to the refractive index of the medium. In this article, we assume that in the ∓ and ± symbols, the upper sign represents s polarized light and the lower indicates the p polarized light. The polar Kerr deflection angle and ellipticity can be expressed as: { 𝜃 = Rel(𝛹 ) (2.39) 𝜑 = Im(𝛹 ) In general, when studying the P-MOKE, the incident ray from the air (refractive index 1), the incidence angle is usually close to 0∘ , and for isotropic substances (namely 𝜀xx = 𝜀yy = 𝜀zz = N 2 ), Eq. (2.38) can be simplified to 𝛹= √

gi 𝜀xx (𝜀xx − 1)

(2.40)

Longitudinal magneto-optical Kerr effect (L-MOKE): the direction of magnetization caused by the external magnetic field is in the incident plane and parallel to the surface of the sample. The dielectric tensor of the L-MOKE can be given as: 0 ⎤ ⎡𝜀xx 0 𝜀 = ⎢ 0 𝜀yy −gi⎥ ⎥ ⎢ ⎣ 0 gi 𝜀zz ⎦

(2.41)

Different from the P-MOKE, the incident angle of the L-MOKE is generally not 0, and the change of the Kerr angle is also greatly related to the incident angle. The

2.5 Theory of Surface Plasmon Resonance and Magnetoplasmonic Thin Films

L-MOKE angle and ellipticity can be expressed as: [ ] √ ⎧ ⎛ sin 𝜃in Nr 2 gi sin 𝜃in tan 𝜃in ± (Nr 2 −tan2 𝜃in ) ⎞ ⎪𝜃 = Rel ⎜ ⎟ √ ⎪ ⎜ 𝜀xx (Nr 2 −1)(Nr 2 −tan2 𝜃in ) (Nr 2 −sin2 𝜃in ) ⎟ ⎝ ⎠ ⎪ [ ] ⎨ √ ⎛ sin 𝜃in Nr 2 gi sin 𝜃in tan 𝜃in ± (Nr 2 −tan2 𝜃in ) ⎞ ⎪ ⎟ ⎪ 𝜑 = Im ⎜ √ ⎜ 𝜀xx (Nr 2 −1)(Nr 2 −tan2 𝜃in ) (Nr 2 −sin2 𝜃in ) ⎟ ⎪ ⎝ ⎠ ⎩

(2.42)

Transverse magneto-optical Kerr effect (T-MOKE): the direction of magnetization caused by an external magnetic field is parallel to the surface of the sample but perpendicular to the light incident surface. The primitive tensor can be expressed as: ⎡𝜀xx 0 −gi⎤ 𝜀 = ⎢ 0 𝜀yy 0 ⎥ ⎢ ⎥ ⎣ gi 0 𝜀zz ⎦

(2.43)

From the relative position relationship between the incident plane, the sample surface and the plane where the magnetization direction is located in the T-MOKE system, we can see that when the incident light is an s polarized light, the magnetization direction is parallel to the polarization direction, so the T-MOKE device will not affect the s polarized light wave. The p polarized light doesn’t produce s polarization component that is parallel to the magnetic field, and it just changes the reflectivity of p-polarized light. The following equation is usually used to value the intensity of the T-MOKE. I+ − I− R+ − R− = (2.44) 𝛿= R0 I0 R+ and I + represent the reflectivity and intensity of reflected light when the magnetic field direction is positive. R− and I − represent the reflectivity and reflected light intensity of samples with the magnetic field direction being the opposite. R0 and I 0 represent the reflectivity and reflected light intensity of samples with demagnetization. To reflect the relationship between the T-MOKE and the incident angle of the light wave, the intensity of the T-MOKE can be a function of the incident angle. ) ( 4(tan 𝜃in )Nr 2 gi (2.45) 𝛿 = Rel 𝜀xx (Nr 2 − 1)(Nr 2 − tan2 𝜃in ) Faraday effect: MO effects in transmission mode. Another important MO phenomenon is the Faraday effect or Faraday rotation. The Faraday effect causes a rotation of the plane of polarization that is linearly proportional to the component of the magnetic field in the direction of propagation. An application of an external magnetic field to the sample (Figure 2.5) can lead to magneto-induced anisotropy and to the appearance of additional off-diagonal terms in the electric permittivity. The static magnetic field is perpendicular to the surface and along the z-axis. The direction of the wavevector is along the −z-direction and the polarization direction of the electric field is parallel to the x-axis In this configuration, the external magnetic field is perpendicular to the surface and along the z-axis. The direction of the wavevector is along the −z-direction and the polarization direction of the electric field is parallel to the x-axis. Here,

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θ M

Figure 2.5 Schematic drawing of Faraday configuration.

z X

we assume that the MO medium is isotropic and 𝜀xx = 𝜀yy = 𝜀zz = 𝜀f . Then, the permittivity tensor can be expressed as follows. ⎡𝜀f −gi 0 ⎤ 𝜀 = ⎢ gi 𝜀f 0 ⎥ ⎥ ⎢ ⎣ 0 0 𝜀f ⎦

(2.46)

We are looking for a solution to Maxwell’s equations in the form of a monochromatic plane wave with the frequency 𝜔 propagating along the −z axis, with electric field: ⃑ =E ⃑ 0 e−i(kz z−𝜔t) E Substituting this into Maxwell’s equations we obtain 1 ⃑ ) = i𝜔𝜀 E ⃑ − ∇ × (∇ × E i𝜔𝜇

(2.47)

(2.48)

This gives the following values for the wave vectors kzL and kzR in the transmission. √ √ 2𝜋 𝜀f −g ⎧ L ⎪kz = 𝜔 𝜇(𝜀f − g) = √𝜆 √ (2.49) ⎨ R 2𝜋 𝜀f +g k = 𝜔 𝜇(𝜀 + g) = ⎪ z f 𝜆 ⎩ The phase difference through thickness L can be expressed as √ √ 2𝜋L( 𝜀f + g − 𝜀f − g) R L (2.50) 𝛿 = (kz − kz )L = 𝜆 This complex phase-difference makes the polarization plane rotated by a certain angle. This is known as the Faraday effect. The Faraday angle is expressed as: 𝜃F = Rel(𝛿∕2)

(2.51)

The expression for the ellipticity (the ratio of minor to major axis) 𝜑F can be given as: ( ( )) 𝛿 𝜑F = − tanh Im (2.52) 2 Replacing 𝜀f and g by the refractive elements using equation N± = 𝜀f ± g we get the following expressions for Faraday effect: ) ( 𝜋L(N+ − N− ) 𝜃F = Rel 𝜆 ) ( 𝜋L(N+ − N− ) 𝜑F = − tanh 𝜆

(2.53)

(2.54) (2.55)

2.6 Heterojunction Theory

M

X

θ

z θ 2θ

Figure 2.6

Mechanism of the magneto-optical isolator based on the Faraday effect.

A very useful property of the Faraday effect is the irreversibility of its optical path. Its mechanism schematic is shown in Figure 2.6. When the incident light passes through the MO medium, the polarization direction of the transmitted light deflects by an angle of theta from the original direction. When the transmitted light is reflected back to the original medium, the angle of deflection will not change to 0 but to 2 thetas with respect to the original polarization direction. This special property can be used in MO isolators to prevent reflected light from entering the laser and in one-way communication devices. To quantify the intrinsic Faraday effect activity of a material, it is convenient to define a new parameter called specific Faraday rotation Q ( ∘∕cm) that can be expressed as: Q=

𝜃F L

(2.56)

2.6 Heterojunction Theory Heterojunction with an interface between two different materials consisting of unequal bandgaps is composed of two or more semiconductor materials [38, 39]. Nowadays, heterojunction is a distinctly important component in electronics, transistors, photovoltaic cells, diodes, and sensors, memory devices, photodetectors, and optoelectronic devices that have made remarkable changes in our daily lives. A well-defined heterojunction can be formed using different materials with a tight interface and its physicochemical properties can be tailored to obtain desirable functionalities. Based on the various materials, heterostructures can be classified into five categories [40]. They are metal or semimetal/semiconductor, semimetal/insulator-based, semiconductor/semiconductor, semiconductor/insulator, and metal/semimetal-based heterojunctions. However, according to the arrangement and interface of the different materials, heterostructures can be classified into four categories: (1) spherical zero-dimensional (prominent heterostructures include nanoclusters, nanodispersions, quantum dots, quantum wells, and core–shell structures [41]), (2) cylindrical one dimensional, (3) planar two dimensional, and (4) cubic three dimensional.

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Various heterostructures show different synergistic relations between two or more building blocks that improve functional characteristics. To enhance the physical properties of heterostructures, crucial issues are the design and synthesis of complex heterostructures with the controlled assembly of each section of materials, including the size, shape, and uniformity of the building blocks. Fundamental understanding of the systematic mechanisms is needed to understand the morphological evolution of heterostructures, which could be highly desirable for developing effective heterostructured devices. Among the various heterojunctions, the abrupt and isotype heterojunction is found to be a good approximation for many heterojunctions. The mechanisms for charge carrier transport in abrupt an isotype heterojunctions have been explored by various models including diffusion model, emission model, emission-recombination model, tunneling model, tunneling-recombination model [40]. The schematic of diffusion model with a typical energy band profile of two isolated pieces of p- and n-type semiconductors and an equilibrium energy band profile of an abrupt p–n heterojunction formed by bringing them into intimate contact are shown in Figure 2.7a,b. In this model, the two semiconductors have different energy gaps (Eg ), dielectric constants (𝜀), work functions (𝜑) and electron affinities (𝜒). Neglecting the generation-recombination current, the predicted current–voltage relation is given by )[ ( ) ( )] ( qV2 qV1 qVD2 exp − exp − (2.57) I = A exp − KT KT KT where V D2 and V 2 are the portions of the applied voltage appearing in p- and n-type semiconductors, K is the Boltzmann constant, T is the absolute temperature. Although this model is invariably used to predict the energy band diagrams of heterojunctions, neither the voltage nor the temperature dependences observed by various workers are adequately described. The emission model is the combination of a model for the evaluation of emission currents and a diffusion model. The emission-recombination model is according to two considerations: there is a thin layer at the interface with a strongly disturbed lattice and fast recombination and the electrons and holes reach the interface via thermal emission over their respective barriers; tuning model is according to the consideration that the electrons can go through the potential barrier in the n-type wide-bandgap material; in tunneling-recombination model, either the electrons tunnel from the conduction band of the wide-bandgap material into empty interbond states located in the narrow-band gap material, then recombine with holes, or holes tunnel from p-type material into occupied states in the n-type material, then recombine with electrons.

2.7 Topological Insulator, Topological Semi-metal, and Perovskite Topological insulators (TIs), a class of materials that present unique quantummechanical properties and a new quantum state of matter, which is characterized

2.7 Topological Insulator, Topological Semi-metal, and Perovskite

75

Electron energy

Vacuum level

χ2

ϕ2 χ1

ϕ1

EC2 ΔEC

EC1

Fermi level 2

Eg1

Eg2 Fermi level 1

EV1

ΔEV

EV2

(a)

Electron energy

Vacuum level VD = ϕ1 – ϕ2

χ1 EC1

ϕ1

“NOTCH”

“SPIKE” VD1

ΔEC

VD2

ϕ2

χ2 EC2

EV1

Fermi level ΔEV EV2 X1 X0

X2

(b)

Figure 2.7 Equilibrium energy band diagrams (a) before and (b) after the formation of an abrupt heterojunction. Source: Sharma and Purohit [40].

by peculiar edge or surface states that show up due to a topological character of the bulk wave functions, are particularly important. TIs concern a qualitatively new aspect of quantum mechanics, i.e. the topology of the Hilbert space, they opened a new window for understanding the elaborate workings of nature and wave functions describing their electronic states span a Hilbert space that has a nontrivial topology. A large part of the unique quantum-mechanical properties of TIs come from the peculiar characteristics of the edge/surface states. Currently, the TI research is focused mostly on time-reversal (TR) invariant systems, where the nontrivial topology is protected by TRS. In those systems, the edge/surface states present Dirac dispersions (Figure 2.8), hence the physics of relativistic Dirac fermions becomes relevant. Furthermore, spin degeneracy is lifted in the Dirac fermions residing in the edge/surface states of TR-invariant TIs and their spin is locked to the momentum. Such a spin state is said to have “helical spin polarization”

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E

E k

(a)

Figure 2.8 (a) Surface state ordinary insulator and (b) surface state topological insulator. k

(b)

and it provides an opportunity to realize Majorana fermions in the presence of proximity-induced superconductivity, not to mention its obvious implications for spintronics applications. As shown in Figure 2.8, the electron motion of the topological insulator does not conform to the usual metallic electron dispersion relation E = k2 /2m, but E = v * k. Here, v is the velocity of the electron motion when k0 = 0. Notice that, for the light, E is equal to light velocity c times k. In this regard, the electron does not like non-relativistic particles, but like a light. This property makes the electrons respond sensitively to external electric fields and they can be used as the basis for semiconductor devices such as field-effect tubes. Topological insulators are a very special type of insulators. Due to the spin-orbit interaction (relativistic effect), there is always a massless Dirac-type electronic state on the surface of such insulators, so the surface is always metallic and conductive. Electrons are carriers of electricity, and in addition to having an electric charge, they have a property of spin, just as the earth goes around the sun, and the earth itself spins. In today’s widely used computer chips, the spin state of the electron is uncertain. As the electrons travel from one end of the transistor to the other, it is as if they were going through a disarranged disco. Inevitably, they collide, which heats up and dissipates heat. In topological insulators, there is a definite relation between the direction of electron rotation and the direction of the current. Electrons moving in different directions behave like cars on a highway, each electron does not interfere with the other, resulting in low energy dissipation. The unique properties of topological insulators make them potentially important in the fields of low-energy electronic devices and fault-tolerant quantum computing. Therefore, the discovery of topological insulators quickly aroused the great interest of scientists in physics, material science, and other fields. Topological semi-metal is a new class of topological electronic states, which are different from topological insulators. The band crossing degenerate point, the Weyl node, which happens to sit on the Fermi surface, gives a very special class of topological semi-metals. Topological metals have special energy band structures that contain some singularities of energy band structures. In short, the intersection point with two energy bands can be described by the relativistic Weyl equation with chirality. Completely different from two-dimensional space (e.g. graphene), in three-dimensional momentum space, such band intersections are a very stable topological structure and cannot introduce mass terms, that is, they cannot open the energy gap through perturbation, so they are very stable. Such band crossing degeneracy points are called Weyl nodes. If we examine the Weyl node in detail,

2.8 Acoustic Theory

we will find that there are two completely different types of Weyl nodes, which can be described as ± symbols in Hamiltonian, corresponding to left-handed and right-handed Weyl nodes, so they are topologically different. When a left-handed and a right-handed Weyl node coincide in the momentum space, it needs to be described by the 4 × 4 Dirac equation. Such a 4∘ degeneracy point is called a three-dimensional Dirac node, and its existence requires the protection of crystal symmetry (since mass terms can be introduced in the 4 × 4 equation). In most metal materials, such a Weyl/Dirac node, would be far from the Fermi surface, but if such a Weyl/Dirac node happened to be located on the Fermi surface, it would give rise to a very special class of electronic structures: “topological semi-metals” whose Fermi surface is reduced to Fermi points, with zero energy gap, and linear dispersion. Such topological semi-metallic states will exhibit wonderful physical properties, for example, their surface states have Fermi arcs, their bodies have magnetic monopoles in momentum space, unique transport properties, magnetism, and so on. Perovskite is a calcium titanium oxide mineral composed of calcium titanate (CaTiO3 ). Its name is also applied to the class of compounds, which have the same type of crystal structure as CaTiO3 , known as the perovskite structure. Many different cations can be embedded in this structure, allowing the development of diverse engineered materials. Perovskites have a cubic structure with the general formula of ABO3 . In this structure, an A-site ion, on the corners of the lattice, is usually alkaline earth or rare-earth element. B-site ions, on the center of the lattice, could be 3d, 4d, and 5d transition metal elements. A large number of metallic elements are stable in the perovskite structure if the Goldschmidt’s tolerance factor t is in the range of 0.75–1.0. R + RO (2.58) t= √ A 2(RA + RO ) where RA , RB , and RO are the ionic radii of A and B site elements and oxygen, respectively [42]. Perovskites have sub-metallic to metallic luster, colorless streak, cube-like structure along with imperfect cleavage and brittle tenacity. Colors include black, brown, gray, orange to yellow. Crystals of perovskite appear as cubes, but are pseudocubic and crystallize in the orthorhombic system. Perovskite crystals have been mistaken for galena; however, galena has a better metallic luster, greater density, perfect cleavage, and true cubic symmetry [43]. Perovskites are a class of materials that share a similar structure, which displays a myriad of exciting properties like superconductivity, magnetoresistance, and more. These easily synthesized materials are considered the future of solar cells, as their distinctive structure makes them perfect for enabling low-cost, efficient photovoltaics. They are also predicted to play a role in next-gen electric vehicle batteries, sensors, lasers and much more.

2.8 Acoustic Theory It is assumed that the medium is non-viscous and the propagation of sound waves in the medium is lossless. In the case of silent disturbance, the medium is

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macroscopically stationary and its initial velocity is zero. The static pressure P0 and the static density 𝜌0 are constants and sound waves travel adiabatically. For the multidimensional, the wave equations can be given as: ⎧ ∇2 p = ⎪ ⎪ ⎪ 2 ′ ⎨∇ 𝜌 = ⎪ ⎪ ⎪ ∇2 v = ⎩

1 𝜕2 p c0 𝜕t2 1 𝜕 2 𝜌′ c0 𝜕t2

(2.59)

1 𝜕2 v c0 𝜕t2

Here we focus on the propagation of sound waves in one dimension and the wave equations can be given as: 2 2 ⎧𝜕 p = 1 𝜕 p ⎪ 𝜕x2 c0 2 𝜕t2 ⎪ ⎪ 𝜕 2 𝜌′ 1 𝜕 2 𝜌′ ⎨ 2 = 2 2 c0 𝜕t ⎪ 𝜕x ⎪ 2 2 ⎪ 𝜕 v = 1 𝜕 v 2 ⎩ 𝜕x2 c0 𝜕t2

(2.60)

Here, p is the acoustic pressure, 𝜌′ is the density increment, and 𝜈 is the velocity at the x position. The solution to the wave equation can be given as: p(x, t) = Aej(𝜔t−kx) + Bej(𝜔t+kx)

(2.61)

If there is no barrier, the acoustic pressure p(x, t), velocity v(x, t) and displacement 𝜉 can be given as: p(x, t) = pa ej(𝜔t−kx)

(2.62)

j(𝜔t−kx)

v(x, t) = va e (2.63) ) ( 𝜋 v −j kx0 + j𝜔t 2 e 𝜉 = ae = 𝜉a ej(𝜔t−𝛼) (2.64) 𝜔 The particle at position x0 just moves back and forth near the equilibrium position but does not move a distance. In fact, it is this kind of back and forth the vibration of the media particle near the equilibrium position that affects the surrounding and even further media particles to vibrate back and forth near the equilibrium position, thus spreading out the vibration energy of the sound source. For an ideal gas: ( ) 𝛾P dP 2 c0 = = 0 (2.65) 𝜌0 d𝜌 s,0 The velocity of acoustic in an ideal gas can be shown as follows under the temperature t (∘ C). √ c (0∘ C) 𝛾P0 t (2.66) c0 (t∘ C) = (273 + t) ≈ C0 (0∘ C) + 0 𝜌0 273 × 2 c0 (t∘ C) ≈ 331.6 + 0.6t(m∕s)

(2.67)

2.9 Theory of Magnetoacoustic and Photoacoustic Coupling

The kinetic energy: 1 (𝜌 V )v2 2 0 0 The potential energy:

(2.68)

ΔEk =

p

ΔEp = −

∫0

p dV =

V0 2 p 2𝜌0 c0 2

(2.69)

The total energy of sound: ΔE = ΔEk + ΔEp = = V0

( ) V0 1 𝜌0 v2 + 2 2 p2 2 𝜌0 c0

pa 2 cos2 (𝜔t − kx) 𝜌0 c0 2

(2.70)

The energy density (the acoustic energy contained in a unit volume medium in a sound field): ( ) 1 1 ΔE (2.71) = 𝜌0 v2 + 2 2 p2 𝜀= V0 2 𝜌0 c0 Mean energy density: 𝜀=

pa 2 pe 2 ΔE = = V0 2𝜌0 c0 2 𝜌0 c0 2

(2.72)

where Pe is the effective pressure. Mean acoustic power W = 𝜀c0 S

(2.73)

Acoustic intensity (the mean acoustic power per unit area perpendicular to the direction of acoustic propagation): T

I=

W Re(p)Re(v) dt = pe ve = 𝜀c0 = ∫0 S

(2.74)

2.9 Theory of Magnetoacoustic and Photoacoustic Coupling Magnetoacoustic coupling. The phenomenon of energy exchange or mutual excitation between spin waves (magnetons) and sound waves (phonons) in magnetic materials is called the magnetoacoustic effect. It results from magnetoelastic coupling, the coupling between magnetization and elastic strain, which also results in magnetostriction. Strong magnetic objects subjected to an alternating magnetic field will produce corresponding mechanical vibration. This principle has been used in ultrasonic transducers and the acoustic vibration on magnetization. In a static magnetic field, the magnetization of a strong magnet varies with acoustic vibration. In remanence state, acoustic vibration will cause remanence reduction. The magnetoelastic effect coupled the spin-wave (magneton) to the lattice vibration (phonon). When the frequency and wavelength of the spin-wave and the acoustic wave in the

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strong magnet are equal, it is called crossover. They are strongly coupled and can be converted to each other. The result is a magnetoelastic wave of two coupled waves. The acoustic waves that cause this effect are strongly attenuated. When the stable magnetic field and high-frequency sound field act together and meet certain conditions, some weakly magnetic (diamagnetic or paramagnetic) materials will conduct the resonance transition of electrons between the space moving energy level (landau energy level) or the resonance transition of electron spin energy level, which is called magnetoacoustic resonance.

2.9.1 The Mechanism of the Sound Pressure in the Presence of the Pulse Magnetic Field [44] The sample with the electrical conductivity 𝜎(r ′ ) is put in the pulse magnetic field B(r ′ , t). Here there will be two effects in the sample. One is the magnetoacoustic effect with the induced eddy. The sample in an alternating magnetic field and static magnetic field that can generate eddy current J(r ′ , t) due to the alternating magnetic field. Then the eddy current result in Lorentz force in the presence of the alternating magnetic field and static magnetic field. This causes the organization’s acoustic vibration of the ultrasonic signals, which can be collected by the ultrasonic transducer to reconstruct the distribution of the conductivity of biological tissue. The Lorentz force and can be given as: F𝟏 (r′ , t) = J(r′ , t) × B(r′ , t)

(2.75)

Anther effect is a thermoacoustic effect. Its basic principle is that the alternating magnetic field produces eddy current in the sample and this current introduces the heat generation. The sample absorbs joule heat, which generates transient thermal expansion and emits thermoacoustic signals. The acoustic source is a function of heat, defined as the energy absorbed per unit volume per unit time. And this can be given as: Q(r′ , t) = 𝜎E2 (r′ , t)

(2.76)

E2 (r ′ ,

Here, t) is the model of the electric field. Therefore, the pressure that comes from the heat function and Lorentz force follows the wave equation: 𝛁𝟐 P(r, t) −

𝛽 𝜕Q(r ′ , t) 1 𝜕2 P(r, t) = 𝛁 ⋅ F1 (r ′ , t) − 2 𝜕t2 Cp 𝜕t cs

(2.77)

where P(r, t) is the pressure of the location r and the time t. cs is the speed of sound in a medium, Cp is specific heat capacity, and 𝛽 is the coefficient of cubical expansion. It is can be concluded that the intensity of the ultrasonic signal depends on the value of the pulsed magnetic field, the value of the induced electric field strength, and the distribution of the conductivity.

2.9.2 The Mechanism of the Sound Pressure in the Presence of the Pulsed Magnetic Field and Static Magnetic Field [44] The sample with the electrical conductivity 𝜎(r ′ ) is put in the pulse magnetic field and static magnetic field B0 . The expression of the heat function is not affected, and

2.9 Theory of Magnetoacoustic and Photoacoustic Coupling

the expression of Lorentz is modified: F𝟐 (r′ , t) = J(r′ , t) × (B(r′ , t) + B𝟎 )

(2.78)

The pressure is given as: 𝛁𝟐 P(r, t) −

𝛽 𝜕Q(r′ , t) 1 𝜕2 P(r, t) = 𝛁 ⋅ F𝟐 (r′ , t) − 2 𝜕t2 Cp 𝜕t cs

(2.79)

It can be concluded that the intensity of the ultrasonic signal depends on the value of the pulsed magnetic field and the static magnetic field, the value of the induced electric field strength, and the distribution of the conductivity. Photoacoustic coupling. The interaction of acoustic and optical signals are under intensive studies these years. One of the typical examples is the coupling of optical wave and acoustic wave in the Bragg gratings. Bragg gratings have a periodic variation of the refractive index, a very large group velocity dispersion, and nonlinear effects. Optical waves propagating in the Bragg gratings may form optical solitons. Moreover, the light may drive sound through compressing the medium due to variations of the intensity of light; conversely, acoustic waves react into the optical wave through the dependence of the refractive index on the material density. Considering the electrostriction, optical gap solitons may couple to acoustic waves and generate the optoacoustic solitons. Photoacoustic coupling equations in Bragg gratings can be given as [45]: ik0 ′ 𝜇t + i𝜇z + 𝜅𝜈 +

2𝜋( 𝜔0∕c)2 (𝜒s |𝜇|2 + 𝜒𝜒 |𝜈|2 )𝜇 + 𝜒es 𝜔𝜇 = 0 k0 A

(2.80)

ik0 ′ 𝜈t − i𝜈z + 𝜅 ∗ 𝜇 +

2𝜋( 𝜔0∕c)2 (𝜒s |𝜇|2 + 𝜒𝜒 |𝜈|2 )𝜈 + 𝜒es 𝜔𝜇 = 0 k0 A

(2.81)

𝜔𝜇 − 𝛤 𝜔tzz − 𝛽s2 𝜔zz + 𝜆(|𝜇|2 + |𝜈|2 )zz = 0

(2.82)

Here, 𝜇 = 𝜇(z, t) and 𝜈 = 𝜈(z, t) are the envelope functions of a light wave in opposite directions (i.e. the amplitude envelope function of the light wave propagating forward and backward in the grating). 𝜔 = 𝜔(z, t) is the density function of the medium in the acoustic region, 𝜇 t , 𝜈 t , and 𝜔zz are the derivatives with respect to the time and dk (𝜔 = 𝜔0 ) is the group velocity of a light wave, 𝜅 is the Bragg space variables, k0 ′ = d𝜔 reflectance, and 𝜒 𝜒 and 𝜒 s are cross-phase modulation coefficient and self-phase modulation coefficient, respectively. A is the effective cross-section of a waveguide, ( ) 𝜔 dn 𝜆 is the electrostrictive coefficient associated with energy density, 𝜒es = c0 d𝜔 is the electrostrictive coefficient associated with the variation of the wave vector, 𝛽 s is the speed of sound, and Γ is the acoustic viscosity coefficient that can be neglected in the grating nanostructure. It has been reported that the photoacoustic coupling equations in Bragg gratings can be reduced to the standard nonlinear Schrodinger equation by the multi-scale method, and the approximate photoacoustic soliton analytical solutions in Bragg gratings, such as single-photoacoustic soliton solutions and double-photoacoustic soliton solutions [45].

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2.10 Theory of Acoustooptic Effect When the ultrasonic wave passes through the medium, it will cause local compression and elongation strains in the medium. The strain changes periodically with time and space, so that the medium appears the phenomenon of dense phase, just like a phase grating. Diffraction occurs when light passes through this medium disturbed by ultrasonic waves. This phenomenon is called the acoustooptic effect. Acoustooptic effect is the light scattering or diffraction when it passes through a medium disturbed by sound waves. Due to the elastic-optical effect, when the ultrasonic longitudinal wave propagates in the medium in the form of the traveling wave, the refractive index of the medium will change in sine or cosine law and will propagate with the ultrasonic wave. When the laser passes through the medium, light diffraction, namely acoustooptic diffraction, will occur. The change of refractive index caused by elastic deformation can be written as: 1 𝛥n = − n3 ps (2.83) 2 where n is the refractive index of the medium, s is the degree of deformation of the medium, and p is the acoustooptic (or elastomeric) coefficient. The characteristics of acoustooptic diffraction are related to the length L of acoustooptic interaction. The characteristic length of acoustooptic diffraction is defined as: L0 =

nv2 𝜆0 f 2

(2.84)

where 𝜆0 is the light wavelength in vacuum, v is the speed of sound, and f is the frequency. It can be seen from the above equation that L0 is very small at high frequency. When a beam of monochromatic collimation light is incident vertically on the ultrasonic grating (the direction of the light propagation is within the gate plane of the grating), the outgoing light is diffraction light, as shown in Figure 2.9. In the figure, m is the number of diffraction order, and 𝜃 is the diffraction angle of the diffraction light of the mth order. It can be proved that, like the optical grating, the condition for the formation of diffraction at all levels is: m𝜆 sin 𝜃m = ± (m = 0, ±1, ±2, …) (2.85) 𝜆s where 𝜆 is the wavelength of incident light and 𝜆s is the wavelength of ultrasonic wave. According to the frequency of the ultrasonic wave and the length of the acoustooptic interaction in the medium, there are two common extreme diffractions, Raman–Nath diffraction, and Bragg diffraction. The parameter for evaluating these two types of diffraction can be given as: 𝜆 Q = 2𝜋L 2 (2.86) 𝜆s where L is the length of the acoustooptic interaction, 𝜆s is the ultrasonic wavelength, and 𝜆 is the wavelength of light passing through the acoustooptic medium. Raman–Nath diffraction occurs when Q is less than 1 (accurately, Q ≤ 0.3). Bragg diffraction occurs when the ultrasonic frequency is high, the acoustooptic region

2.11 Magnetothermal Thin Films: Phonon Thermal Theory

Figure 2.9 Diffraction effect of ultrasonic grating on the light beam.

Direction of ultrasonic propagation Acoustic wall +m +2

θ1

Laser Acostooptic medium

+1 0 –1 –2

Electroacous converter

–m d

is long, and the ray and ultrasonic wave surface have a certain angle of oblique incidence. Bragg diffraction occurs when Q is more than 1 (accurately, Q ≥ 4). In the region of 0.3 < Q < 4, the diffraction phenomenon is relatively complex, and the conventional acoustooptic devices do not work in this range.

2.11 Magnetothermal Thin Films: Phonon Thermal Theory The magnetothermal effect refers to the phenomenon that paramagnetic or soft ferromagnetic materials will release heat under the action of the external magnetic field, and absorb heat during demagnetization. In some paramagnets, magnetothermal effects are used to produce extremely low temperatures. The magnetothermal effect is the inherent nature of all magnetic materials, which is determined by the microstructure of such materials [46]. According to Boltzmann statistics, the entropy of a system can be expressed as a function of Boltzmann’s constant k and the number of states of the system W: S = k ln W

(2.87)

If the number of particles in the system is N and the total quantum number is j, when the system is not affected by the external magnetic field, the energy of particles is randomly distributed in 2j + 1 degenerate state, and its angular momentum and magnetic moment orientation are 2j + 1, so the number of states of the system is W = (2j + 1)N . When the external magnetic field is introduced, the probability of the distribution of the particle’s angular momentum ( ) and magnetic moment orien−g𝜇B mi B tation in each energy level state is P ∝ exp , where g is the Landau factor, KT 𝜇 B is the Bohr magnetic moment, mi is the magnetic quantum number written as 2j + 1, B is the magnetic induction intensity of the external magnetic field, and T is

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the temperature of the system. If the external magnetic field intensity is large and the system temperature is relatively low, the magnetic moment orientation will tend to be the same, and the number of states will be nearly 1, leading to the approximately zero entropy. The magnetic entropy change of the system in this process is ΔS ≈ − Nk ln(2j + 1). The system release or absorb heat energy ΔQ ∝ ΔS. So the larger the magnetic field with the lower the temperature, the greater the entropy of the system that leads to the greater magnetic heat effect.

2.12 Theory of Thermoelectric Effect The thermoelectric effect is due to the coupling of heat and electricity flows, which is composed of the Seebeck effect, the Peltier effect, and the Thomson effect. The Seebeck effect was discovered in 1827 by the German physicist Thomas Seebeck. As shown in Figure 2.10, metals A and B were connected into a ring, and the temperature of one joint was at T. When another joint was heated at T + ΔT, it is distinctly found that the compass inside the ring moved due to the generated magnetic field. In this process, the carriers on the hot joint moved fast, causing negative charges to accumulate on the cold side. This creates a potential difference in the metal, creating an electric field. This electric field impedes the carriers’ movement and when the carrier distribution reached an equilibrium state, there is a stable voltage at both ends of the metal, and a stable electric field is formed in the metal. The voltage is proportional to the temperature difference and the proportional coefficient is called the Seebeck coefficient S (T), which is a physical quantity of the material itself and is related to the temperature. The voltage can be given as: T+ΔT

V=

∫T

(2.88)

(SB (T) − SA (T))dT

where SA (T) and SB (T) represent the Seebeck coefficients of metal A and metal B. If SA (T) SB (T) and do not change with the change of temperature, the above equation can be expressed as follows: (2.89)

V = (SB (T) − SA (T))ΔT Figure 2.10 effect.

Cold T

E

N

Metal A W

84

S

Hot T+ΔT

Metal B

e‒

e‒ e‒ e‒

The diagram of the Seebeck

2.12 Theory of Thermoelectric Effect

Figure 2.11 The diagram of the Peltier effect. Metal A

Jq

Jq

Metal B Je Power supply

Another thermoelectric effect is the Peltier effect discovered in 1834, which can be considered as the inverse effect of the Seebeck effect. As shown in Figure 2.11, when an electric current passes through a circuit composed of two different conductors, in addition to producing joule heat, the Peltier heat is absorbed or given off at the two joints. This is due to the fact that the applied voltage on the two joints can drive the carriers that can make the energy transferred from one joint to the other with the heat absorbed and released. It is found that the process is reversible. When the current direction is changed, the endothermic and exothermic joints exchange. Peltier heat Q can be expressed as a function of the Peltier coefficient P. (2.90)

Jq = P × Je

The third one is the Thomson effect. As shown in Figure 2.12, when an electric current passes through a conductor with a certain temperature gradient, in addition to joule heat, another transverse heat flow flows called the Thomson effect into or out of the conductor (i.e. endothermic or exothermic). The temperature gradient (pointing in the direction of temperature rise) is opposite to the direction of the current, that is, the current flows from the hot end to the cold end-exothermic (flowing out laterally).The average velocity of free electrons at the high-temperature end is greater than that at the low-temperature end, so more electrons are diffused from the high-temperature end to the low-temperature end than from the low to the high end, forming a potential difference of temperature difference V(T1, T2), and the direction from the high-temperature end to the low-temperature end. The external current is in the same direction as V(T1, T2), and the electrons are accelerated to obtain energy. Apart from the increase of kinetic energy needed for the electrons to reach the high-temperature end, the remaining energy is transferred to the lattice through the collision between the electrons and the lattice, so that the temperature of the whole metal rises and heat is released. The direction of the temperature gradient is the same as the direction of the current – endothermic (flowing in laterally). The

Je

T QT

Δ

Thomson effect

Δ

Figure 2.12 The diagram of the Thomson effect.

QT = –K × Je × T

K is Thomson coefficient

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electrons are slowed down by a temperature difference electric field, and when they collide with the lattice, they obtain energy from the metal atoms, reducing lattice energy and the whole metal temperature and absorbing heat from the environment. The amount of heat absorbed or released per unit time dQ/dt is proportional to the current I and temperature gradient dT/dx. dQ dT = SI dt dx

(2.91)

where S represents the Seebeck coefficient. When the circuit composed of two conductors is analyzed, the three thermal effects will be generated at the same time when the temperature of the two contact ends is different.

2.13 Thermal Barrier Insulation Theory for TBC Coating Thermal barrier coatings (TBCs) enable the engines to operate at higher temperatures without raising the base metal temperatures using cooling systems inside the hot section components and thus, enhance the operating efficiency of the engines [47]. TBCs with low thermal conductivity, phase stability, and high resistance to sintering have ever-increasing demands [48]. TBCs are primarily a two-layer system, which is consisting of a porous ceramic topcoat layer and an alumina forming bond coat layer. Topcoat layer (thermal insulation), thermally grown oxide (TGO) layer (bonding of TBC to bond coat and slows subsequent oxidation), topcoat layer (providing thermal insulation), TGO layer (bonding of TBC to bond coat and slows subsequent oxidation), bond coat layer (containing the source of elements to create TGO in an oxidizing environment and provides oxidation protection), and superalloy substrate (mechanical load) are four main components with unique functions that influence TBC life [49]. Each of these components has markedly different physical, thermal and mechanical properties that are strongly affected by processing conditions. During fabrication and most notably during use, these components interact chemically and mechanically. Dynamic relationships between these layers control the durability of TBC. Impedance spectroscopy (IS) is used for structure detection and life prediction of the thermal barrier layers. The impedance characteristics of a material or device depend on dry temperature, thickness, microstructure, or damage, and the damage is detected by measuring the impedance changes caused by such microstructure changes. Applying a sinusoidal amplitude voltage V(t) = V m sin(𝜔t) to the thermal barrier coating will measure the steady-state current i(t) = I m sin(𝜔t + 𝜃). Here, 𝜔 is the angular velocity and 𝜃 is the phase difference between voltage and current. The impedance of the coating is the ratio of the applied voltage to current, which can be given as: Z(𝜔) =

V(t) i(t)

(2.92)

2.14 Permeability Theory: Fick First Diffusion Theory and Fick Second Diffusion Theory

2.14 Permeability Theory: Fick First Diffusion Theory and Fick Second Diffusion Theory [50] Fick’s law proposed by Adolf Fick in 1855 describes the relationship between the mass transfer flux and the concentration gradient in the process of molecular diffusion when the mass transfer occurs without macroscopic mixing. Diffusion occurs in response to a concentration gradient expressed as the change in concentration due to a change in position 𝜕C . The local rule for movement or flux J is given by Fick’s 𝜕x first law of diffusion: J = −D

𝜕C 𝜕x

(2.93)

Here, J is the diffusion flux: the mass of a diffuser per unit area in the direction of diffusion per unit time, D is diffusion coefficient, C is the diffusion concentration, and x is the distance in the diffusion direction. This law reflects a steady-state diffusion, which concentrations do not vary with time. The flux is proportional to the diffusivity and the negative gradient of concentration. The negative sign indicates that J is positive when movement is down the gradient. As shown in Figure 2.13 the negative sign cancels the negative gradient along the direction of positive flux. Consider diffusion at the front and rear surfaces of an incremental planar volume. Fick’s second law of diffusion describes the rate of accumulation (or depletion) of concentration within the volume as proportional to the local curvature of the concentration gradient. The local rule for accumulation is given by Fick’s second law of diffusion: ( ) 𝜕C 𝜕C 𝜕C = D (2.94) 𝜕t 𝜕x 𝜕x Figure 2.13 The flux is driven by the negative gradient in the direction of increasing.

C J

J=

X0

–D д C дX

X

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2 Fundamental in Functional Thin Films and Coatings

Figure 2.14 Incremental planar volume accumulates concentration because the front gradient at x1 drives more flux J1 into the volume than the flux J2 driven out of the volume by the rear gradient at x2 . Source: [51].

Volume

C J1 X1

J2 X2

X

If the diffusion coefficient is not a function of concentration, the Fick’s second law of diffusion can be given as: 𝜕2 C 𝜕C =D 2 𝜕t 𝜕x

(2.95)

is proportional to the diffusivity and the second derivawhich the accumulation 𝜕C 𝜕t tive (or curvature) of the concentration. The accumulation is positive when the curvature is positive, i.e. as shown in Figure 2.14, when the concentration gradient is more negative on the front end of the planar volume and less negative on the rear end that more flux is driven into the volume at the front end than is driven out of the volume at the rear end. For the three-dimensional diffusion problem, the Fick’s second law of diffusion is expressed as: 𝜕C = D ⋅ ∇2 C 𝜕t

(2.96)

At steady state, we have 𝜕C = 0, which means no concentration change, where we 𝜕t can get the Fick’s first law that is considered as a specific (simplified) format of the second law.

2.15 Multi-physical Field Coupling Theory and Simulation Software Introduction In engineering, there are various physical fields including the temperature field, the stress field, the humidity field, and so on. It is found that many of the problems we have to solve are the superimposing of these physical fields that affect each other. Multiple physical fields superimposing on each other is called the multi-field coupling. With the rapid development of computer technology, finite element analysis (FEA) is increasingly used in the field of engineering simulation to solve real engineering problems. Over the years, a growing number of engineers applied mathematicians, and physicists have shown that this method of solving partial differential equations, which describe flows, electromagnetic fields, and structural mechanics, can solve many physical phenomena. The finite element method is used to convert these well-known mathematical equations into approximate digital images. Here, we mainly introduce two typical multi-physical field coupling software, i.e. COMSOL Multiphysics and ANSYS Multiphysics. COMSOL Multiphysics is a multiphysical field analysis software based on FEA, which can be used for mechanical, optical, chemical, and other aspects of simulation, has been widely used in scientific research simulation and calculation. This

2.15 Multi-physical Field Coupling Theory and Simulation Software Introduction

software strives to meet all requirements of user simulation and become the preferred simulation tool for users. It is versatile, flexible, easy-to-use, and more powerful than other FEA software due to the fact that it can be easily extended with additional functional modules. Its distinctive features are as follows: (1) Solving multi-field problems. Users only need to choose different professional differential equations for arbitrary combinations. This can easily realize the direct coupling analysis of multi-physical fields. (2) A fully open architecture. Users can easily define the required professional partial differential equations in a graphical interface. (3) Controlled solution parameters, material properties, and boundary conditions. (4) Professional computing model library. (5) Rich embedded CAD modeling tools. (6) Comprehensive CAD import function. It supports the current mainstream CAD software format file import. (7) Powerful grid generation ability. It supports a variety of grid generation and mobile grid functions. (8) Large scale computing power. This software has the 64-bit processing power and parallel computing on Linux, Unix, and Windows systems. (9) Rich post-processing functions. It can output and analyze a variety of data, curves, and pictures. (10) Professional online help documents. Users can easily master the operation and application of the software through the operation manual. (11) Multi-language interface. As shown in Figure 2.15 this software has a convenient and quick interface for load conditions, boundary conditions, and parameter settings. COMSOL Multiphysics is a great innovation in multi-field coupling simulation. It integrates rich algorithms that are functional, flexible, and practical. Moreover, it can be applied and expanded conveniently through additional professional solution modules including AC/DC module, heat transfer module, CFD module, chemical reaction engineering module, RF module, structural mechanics module, microfluidics module, batteries and fuel cells module, MEMS module, geomechanics module, subsurface flow module, electrodeposition module, plasma module, acoustics module, pipe flow module, corrosion module, nonlinear structural materials module, and an optimization module. ANSYS Multiphysics software is a large general FEA software and integrates structure, fluid, electric field, magnetic field, and sound field analysis. It is developed by ANSYS company that is one of the largest FEA software companies in the world. It can be compatible with most CAD software for data sharing and exchange, such as Pro/Engineer, NASTRAN, Alogor, i-deas, AutoCAD, etc. It is one of the most advanced CAE tools in modern product design. This software mainly consists of three parts: pre-processing module, analysis, and calculation module and post-processing module. The preprocessing module provides a powerful entity modeling and mesh generation tool that allows users to easily construct finite element models; the analysis and calculation module includes structural

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2 Fundamental in Functional Thin Films and Coatings Untitled.mph - COMSOL Multiphysics File

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Composite Materials Module Corrosion Module Design Module ECAD Import Module Electrochemistry Module Electrodeposition Module Fatigue Module Geomechanics Module Heat Transfer Module MEMS Module Actuators biased_resonator_2d_basic

An electrostatically actuated MEMS resonator is simulated. The device is driven by an AC + DC bias voltage applied across a parallel plate capacitors. In this example, the normal mode shapes and frequencies are computed, as a function of applied bias.

biased_resonator_2d_freq biased_resonator_2d_modes biased_resonator_2d_pull_in_pull_out biased_resonator_2d_pull_in biased_resonator_3d_basic biased_resonator_3d_freq

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Figure 2.15

biased_resonator_3d_modes COMSOL Multiphysics and MEMS Module Electrostatics Solid Mechanics

Modules window of the COMSOL Multiphysics. Source: COMSOL Multiphysics.

analysis (linear analysis, nonlinear analysis, and highly nonlinear analysis), fluid dynamics analysis, electromagnetic field analysis, sound field analysis, piezoelectric analysis, and coupling analysis of multiple physical fields, which can simulate the interaction of multiple physical media with sensitivity analysis and optimization analysis capabilities; and the post-processing module can display the calculation results by color-contour display, gradient display, vector display, particle flow trace display, three-dimensional section display, transparent and translucent display and other graphics, and can also display or output the calculation results in the form of charts and curves. The software provides more than 100 unit types to simulate various structures and materials in engineering. The software comes in various versions and can run on a variety of computer devices from personal computers to mainframes, such as PCS, SGI, HP, SUN, DEC, IBM, CRAY, etc.

Acknowledgments This chapter is supported by the NSFC–BRICS STI Framework Program (No. 51861145309), the National Natural Science Foundation of China (No. 51971029), the National S&T Major Project (No. 2018ZX10301201), the Postdoctor Research Foundation of Shunde Graduate School of University of Science and Technology Beijing (No. 2020BH005), the Project funded by China Postdoctoral Science Foundation (No. 2020M680336), the “All English teaching demonstration course construction project of University of Science and Technology Beijing” (No. KC2015QYW06, 2016), the “1125” Zhihui Zhengzhou Talent project of Henan province (Fund No. in USTB: 39080070), the “100 talent plan” fund of Fujian province (Fund No. in USTB: 39080067), and the development of a highly sensitive

References

MO biomelecular sensor experimental prototype (Fund No. in USTB: 2019-0649) by Hangzhou Ruidi Biotechnology Co. Ltd.

List of Abbreviation CMR TMR QAHE 2D GSTCs SPR MO MOKE P-MOKE L-MOKE T-MOKE TIs TR TRS IS

colossal magnetoresistance tunneling magnetoresistance quantum anomalous Holzer effect two-dimensional generalized sheet transition conditions surface plasmon resonance magneto-optical magneto-optical Kerr effect polar magneto-optical Kerr effect longitudinal magneto-optical Kerr effect transverse magneto-optical Kerr effect Topological insulators time reversal time-reversal symmetry impedance spectroscopy

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8 Kurita, N., Ronning, F., Tokiwa, Y. et al. (2009). Low-temperature magnetothermal transport investigation of a Ni-based superconductor BaNi2 As2 : evidence for fully gapped superconductivity. Physical Review Letters 102 (14): 147004. 9 Pons, J.A., Miralles, J.A., and Geppert, U. (2009). Magneto-thermal evolution of neutron stars. Astronomy and Astrophysics 496 (1): 207–216. 10 Huang, P., Lin, J., Li, W. et al. (2013). Biodegradable gold nanovesicles with an ultrastrong plasmonic coupling effect for photoacoustic imaging and photothermal therapy. Angewandte Chemie International Edition 52 (52): 13958–13964. 11 Mallidi, S., Larson, T., Tam, J. et al. (2009). Multiwavelength photoacoustic imaging and plasmon resonance coupling of gold nanoparticles for selective detection of cancer. Nano Letters 9 (8): –2825, 2831. 12 Ramirez, A.P. (1997). Colossal magnetoresistance. Journal of Physics: Condensed Matter 9 (39): 8171–8199. 13 Rodriguez-Martinez, L.M. and Attfield, J.P. (1996). Cation disorder and size effects in magnetoresistive manganese oxide perovskites. Physical Review B: Condensed Matter 54 (22): R15622–R15625. 14 Chang, C., Zhang, J., Feng, X. et al. (2013). Experimental observation of the quantum anomalous Hall effect in a magnetic topological insulator. Science 340 (6129): 167–170. 15 Hu, J., Zhu, Z., and Wu, R. (2015). Chern half metals: a new class of topological materials to realize the quantum anomalous Hall effect. Nano Letters 15 (3): 2074–2078. ´ B.K. (2017). Monolayer of the 5d transition 16 Sheng, X.-L. and Nikolic, metal trichloride OsCl3 : a playground for two-dimensional magnetism, room-temperature quantum anomalous Hall effect, and topological phase transitions. Physical Review B 95 (20): 201402.1–201402.5. 17 Qiao, Z., Yang, S.A., Feng, W. et al. (2010). Quantum anomalous Hall effect in graphene from Rashba and exchange effects. Physical Review B 82 (16): 161414. 18 Onoda, M. and Nagaosa, N. (2003). Quantized anomalous Hall effect in two-dimensional ferromagnets: quantum Hall effect in metals. Physical Review Letters 90 (20): 206601. 19 Weng, H., Yu, R., Hu, X. et al. (2015). Quantum anomalous Hall effect and related topological electronic states. Advances in Physics 64 (3): 227–282. 20 Liu, C.-X., Qi, X.-L., Dai, X. et al. (2008). Quantum anomalous Hall effect in Hg1−y Mny Te quantum wells. Physical Review Letters 101 (14): 146802. 21 Rajan, P.I., Mahalakshmi, S., and Chandra, S. (2017). Occurrence of spintronics behaviour (half-metallicity, spin gapless semiconductor and bipolar magnetic semiconductor) depending on the location of oxygen vacancies in BiFe0.83 Ni0.17 O3 . Royal Society Open Science 4 (6): 170273. 22 Schliemann, J. (2017). Colloquium: persistent spin textures in semiconductor nanostructures. Reviews of Modern Physics 89: 011001. 23 Suzuki, R., Wakabayashi, Y.K., Okamoto, K. et al. (2018). Quantum size effect in an Fe quantum well detected by resonant tunneling carriers injected from a p-type Ge semiconductor electrode. Applied Physics Letters 112 (15): 152402.

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24 Yan, W.-W., Li, X.-F., Zhang, X.-H. et al. (2019). Family-dependent magnetism in atomic boron adsorbed armchair graphene nanoribbons. Journal of Materials Chemistry C 7: 6241–6245. 25 Kawakami, E., Jullien, T., Scarlino, P. et al. (2016). Gate fidelity and coherence of an electron spin in an Si/SiGe quantum dot with micromagnet. Proceedings of the National Academy of Sciences of the United States of America 113 (42): 11738–11743. 26 Pershin, V.Y. (2005). Long-lived spin coherence states in semiconductor heterostructures. Physical Review B 71 (15): 155317. 27 Menzel, C., Rockstuhl, C., and Lederer, F. (2010). Advanced Jones calculus for the classification of periodic metamaterials. Physical Review A 82 (5): 053811. 28 Idemen, M.M. (2011). Discontinuities in the Electromagnetic Field. New Jersey: Wiley-IEEE Press. 29 Niemi, T., Karilainen, A.O., and Tretyakov, S.A. (2013). Synthesis of polarization transformers. IEEE Transactions on Antennas and Propagation 61 (6): 3102–3111. 30 Achouri, K., Salem, M.A., and Caloz, C. (2015). General metasurface synthesis based on susceptibility tensors. IEEE Transactions on Antennas and Propagation 63 (7): 2977–2991. 31 Tretyakov, S. (2003). Analytical Modeling in Applied Electromagnetics. Boston: Artech House. 32 Asadchy, V.S., Albooyeh, M., Tcvetkova, S.N. et al. (2016). Perfect control of reflection and refraction using spatially dispersive metasurfaces. Physical Review B 94 (7): 075142. 33 Sakai, O. (2016). Negative-permittivity plasma generation in negative-permeability metamaterial space. Plasma Sources Science and Technology 25 (5): 055019. 34 Machac, J. (2017). Amorphous metamaterial with negative permeability. IEEE Antennas and Wireless Propagation Letters 16: 2138–2141. 35 Sun, K., Fan, R.H., Zhang, Z.D. et al. The tunable negative permittivity and negative permeability of percolative Fe/Al2 O3 composites in radio frequency range. Applied Physics Letters 106 (17): 172902. 36 Zhang, X. and Liu, Z. (2008). Superlenses to overcome the diffraction limit. Nature Materials 7 (6): 435–441. 37 Xia, W.B., Gao, J.L., Zhang, S.Y. et al. (2014). Optical and magneto-optical properties of periodic Co double layer film. Chinese Physics B 23 (10): –103303. 38 Geim, A.K. and Grigorieva, I.V. (2013). Van der Waals heterostructures. Nature 499 (7459): 419–425. 39 Jariwala, D., Sangwan, V.K., Lauhon, L.J. et al. (2014). Emerging device applications for semiconducting two-dimensional transition metal dichalcogenides. ACS Nano 8 (2): 1102–1120. 40 Sharma, B.L. and Purohit, R.K. (1974). Theory of heterojunctions. In: Semiconductor Heterojunctions, vol. 5 (eds. B.L. Sharma and R.K. Purohit), 1–23. Pergamon.

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41 Kumari, A. and Singh, R.R. (2017). Encapsulation of highly confined CdSe quantum dots for defect free luminescence and improved stability. Physica E: Low-dimensional Systems and Nanostructures 89: 77–85. 42 Peña, M.A. and Fierro, J.L. (2001). Chemical structures and performance of perovskite oxides. Chemical Reviews 101 (7): 1981. 43 Luxová, J., Ulcová, P., and Trojan, M. (2008). Study of perovskite compounds. Journal of Thermal Analysis and Calorimetry 93 (3): 823–827. 44 Yang, Y., Liu, G., and Xia, Z. (2017). Simulation research on magnetoacoustic effect and thermoacoustic effect of pulsed magnetic excitation. Journal of biomedical engineering 34 (1): 21–26. 45 Hua-Xing, L. and Ji, L. (2011). The perturbed optoacoustic solitons in Bragg grating. Acta Physica Sinica 60 (12): 124201. 46 Wang, G.F., Song, L., Ou, Z.Q. et al. (2007). Calculation of the magnetization and magnetocaloric effect in the MnFeP0.45 As0.55 compound. Acta Metallurgica Sinica 20 (4): 265–269. 47 Tang, F., Ajdelsztajn, L., and Kim, G.E. (2006). Effects of variations in coating materials and process conditions on the thermal cycle properties of NiCrAlY/YSZ thermal barrier coatings. Materials Science and Engineering A 425 (1): 94–106. 48 Matsumoto, M., Takayama, H., Yokoe, D. et al. (2006). Thermal cycle behavior of plasma sprayed La2 O3 , Y2 O3 stabilized ZrO2 coatings. Scripta Materialia 54 (12): 2035–2039. 49 Darolia, R. Thermal barrier coatings technology: critical review, progress update, remaining challenges and prospects. International Materials Reviews 58 (6): 315–348. 50 Fantini, S., Walker, S.A., Franceschini, M.A. et al. (1998). Optical characterization of breast tumors by frequency-domain optical mammography. Advances in Optical Imaging and Photon Migration. Orlando, FL: Optical Society of America. 51 Diffusion theory. https://omlc.org/classroom/ece532/class5/ficks1.html.

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3 Multilayered Magnetic Thin Films for Electron Transport Control and Signal Sensing: From GMR, CMR, TMR to Quantum Anomalous Holzer Effect Weiwei Zhang 1 and Yujun Song 1,2 1 University of Science and Technology Beijing, Center for Modern Physics Technology, Applied Physics Department, School of Mathematics and Physics, 30 Xueyuan Road, Beijing 100083, China 2 Zhejiang Key Laboratory for Pulsed Power Technology Translational Medicine, Hangzhou Ruidi Biotechnology Company, Hangzhou 310000, China

3.1 Introduction The electron spin and the motion of the electron charge of solids were investigated independently in the last century [1–4]. However, during the past three decades, both the emergence of various fabrication techniques (e.g. magnetron sputtering [5–8], photo etching, microfluidics [9–12], and template transfer imprinting method [6, 13]) that allow the control of materials at the micro- and nanoscale and the discovery of the anisotropic magnetoresistance (AMR), giant magnetoresistance (GMR) that consists of two ferromagnetic layers separated by a nonmagnetic highly conductive metal, colossal magnetoresistance (CMR) whose materials are mostly manganese-based perovskite oxides, tunneling magnetoresistance (TMR) that consists of ferromagnets/insulator/ferromagnets, and quantum anomalous Holzer effect (QAHE) lead to the development and combination of electron spin and charge properties. Figure 3.1 shows their typical structures. This combination is termed as spintronics (magnetoelectronics or spin electronics). Different from the conventional microelectronics, spintronics regulates the spin state at mesoscopic sales through magnetic fields, electric fields, and even electromagnetic wave to modulate electron transport characteristics. Besides its fundamental scientific importance, regulation of the spintronics at the micro- and nanoscale has been a topic of much interest due to its potential exploitation toward electron transport and signal sensing applications. In this chapter, we aim to focus on the currently used magnetic thin films for electron transport control and signal sensing applications based on GMR, CMR, TMR, and QAHE by covering the fabrication technologies, the structure and property characterization, and the spintronic applications.

Inorganic and Organic Thin Films: Fundamentals, Fabrication, and Applications, First Edition. Edited by Yujun Song. © 2021 WILEY-VCH GmbH. Published 2021 by WILEY-VCH GmbH.

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(b) FM NM FM

Spin current

(c) (a) M θ I

Magnetoresistance

Perovskites

AMR Upper contact Ferromagnet

(d)

Insulator Ferromagnet

Figure 3.1 Types of magnetoresistance. (a) AMR is a consequence of spin-polarized scattering in a ferromagnetic metal; (b) GMR exists in heterogeneous magnetic material that consists of ferromagnets separated by conducting spacers; (c) CMR based on mostly perovskites results from interactions predominantly between adjacent atoms; and (d) TMR in magnetic tunnel junctions results from spin filtering as spin-polarized electrons tunnel across an insulating barrier from one ferromagnetic to another.

3.2 Multilayered Magnetic Thin Film for the Electron Transport Control and Sensing Based on Magnetoresistance (GMR) Effect 3.2.1

Introduction of GMR

GMR was first discovered in 1988 [15] and based on the dependence of electron scattering on the spin orientation, which is a quantum mechanical magnetoresistance effect. This nanotechnology makes it possible to miniaturize hard disks so radically in recent years. For instance, sensitive read-out heads based on GMR can read data from the compact hard disks used in laptops and some music players. The 2007 Nobel Prize in Physics was awarded to Albert Fert and Peter Grünberg for the discovery of GMR [16], which was regarded as a global recognition to the rapid development of GMR, in terms of both physics and engineering [17]. This effect is a significant change in the electrical resistance depending on whether the magnetization of adjacent ferromagnetic layers are in a parallel or an antiparallel alignment in the presence of an external magnetic field. It is found that the overall resistance is

3.2 Multilayered Magnetic Thin Film for the Electron Transport Control and Sensing

relatively low for parallel alignment and relatively high for antiparallel alignment. Usually, the value of the GMR can be defined as MR =

𝜌(H) − 𝜌(0) Δ𝜌 = 𝜌(0) 𝜌(0)

(3.1)

Here, 𝜌(H) represents the electrical resistivity in the presence of the applied magnetic field H and 𝜌(0) is the electrical resistivity without the applied magnetic field. The most pioneering work of GMR was the study of the interlayer coupling mechanism of Fe/Gr/Fe multilayered film system [18]. Interestingly, there existed a critical Gr thickness (about 1 nm) for the magnetization of adjacent ferromagnetic layers, which could be changed from ferromagnetic coupling to antiferromagnetic (AF) coupling. This phenomenon was further proved by the spin-polarized low-energy electron diffraction measurements experiment [19]. Subsequently, the GMR effect was first observed in multilayers composed of alternating ferromagnetic and non-magnetic conductive layers system. In this system, huge magnetoresistance of the (Fe(001)/Gr(001))n body-centered cubic (bcc) superlattices on GaAs(001) substrates prepared via a molecular beam epitaxy (MBE) method was achieved in the presence of the applied magnetic field. This was attributed to the spin-dependent transmission of the conduction electrons between Fe layers through Cr layers [15] in AF coupling resulting from the indirect exchange interactions through the Cr layers, which was subsequently demonstrated in the neutron diffraction experiment [20]. Different from the conventional magnetoresistance that results from the increase of the electron-scattering cross section attributed to the functional magnetic field imposing on the electric charge, the magnetoresistance of the GMR is much larger. For instance, in the Fe/Gr superlattice multilayered system, the MR can reach 50% at 4.2 K [15]. Subsequently, it was found that both the transport and magnetic properties of these sputtered polycrystalline Fe/Cr superlattice structures were similar to the reported single-crystal structures [21]. Shortly afterward, Co/Cu multilayers grown on Fe buffer layer displayed the saturation magnetoresistance at room temperature of more than 65% [22]. In a Co90 Fe10 /Cu system, the magnetoresistance reached 63% and the magnetoresistance up to 110% for Fe/Pt/Cu/[Co95 Fe5 /Cu]120/Pt grown by epitaxy on a MgO(110) single crystal substrate was also reported. These related multilayers become the fundamental materials of GMR sensors and storage devices today [14].

3.2.2

Fabrication of GMR Multilayered Thin Films

Generally, various systems such as transition metal magnetic multilayer films (such as spin valve structure), magnetic nanoparticles, oxide films, and magnetic tunnel junction have been fabricated to explore the GMR effect on the electron transport control and signal sensing applications. Among the fabrication methods, the MBE method, magnetron sputtering method, and the electron beam deposition method are three main methods to fabricate the samples. Table 3.1 summarized the typical multilayered magnetic thin film fabricated by these three methods. Besides these three main methods, some combinatorial thin-film magnetic structures can be also obtained by the thermionic vacuum arc method. In this process, the

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Table 3.1 Typical multilayered magnetic thin film fabricated by MBE, magnetron sputtering and electron-beam evaporation methods. Fabrication technology

Multilayered films

MBE

(Fe/Cr)n /GaAs [15], Co/Ag [23] Co/Cu [24–40], Mn/Fe/Mn/Ru [41], Mn/Ru [41], Fe/Ag/CoxFe1−x [42]

Magnetron sputtering

Fe/Gr [43], Co/Cu/Fe (buffer) [44], Ni81 Fe19 /Cu [45], Fe16 Ni66 Co18 /Cu [46], Co–Cu/Si [47], Co/Ag [48], FexAg100−x [49], Co–Cu [50], Fe/Cr [51], Fe/Ag and Co/Ag [52], Ni66 Fe16 Co18 /Cu/Fe (buffer) and Ni66 Fe16 Co18 /Cu/ NiFeCo [53], Co/Cu/glass [54], NiFeCo/Cu [55], (CoFe)x Ag1−x [56], CO/CU [57], Ni–Co/Cu and Co–Fe/Cu [58], Ta(tTa)/(Ni80 Fe20 )60 Cr40 (50 Å)/[Co90 Fe10 (15 Å)/ Cu(tCuÅ)]n /Ta(50 Å) [59], NiFeCr [59], Ta/NiFeCr [59], NiFeCr [59]

Electron-beam evaporation

Cu/Co/Cu/glass [60], Co/Cu/Co sandwiches with Si buffer [61], Co2 MnSi (CMS)/n–Si [62], CoFe/p–Si [63], Ta–NiFe–Cu–Co–FeMn–Ta [64]

Quartz microbalance Sample holder

Co

MgO

Cu Fe

FeCo + ‒ H.V.

Figure 3.2 Schematic view of both GMR and combinatorial GMR + TMR structures. Source: Jepu et al. [65]. ©2014, Elsevier.

accelerated electrons focus on the anode start the evaporation process, which leads to the ignition of a bright plasma discharge in pure anode vapors. All processes take place in high vacuum conditions. Take the fabrication of the Fe–Co magnetic structure as an example [65]. As shown in Figure 3.2, due to the substrate position with respect to each element to be deposited, different elemental concentrations were obtained in the same deposition batch. The entire coating process was performed in a high vacuum chamber, with a base pressure in the deposition time of 5 × 10−6 Torr. This high vacuum pressure assures the obtaining of high purity structures without any other unwanted inclusions.

3.2 Multilayered Magnetic Thin Film for the Electron Transport Control and Sensing

3.2.2.1 MBE Method for the Fabrication of the GMR Devices

MBE method, developed on the basis of vacuum deposition technology that usually reaches more than 6 × 10−9 Pa [66], was mainly used in the early research of GMR films. Usually, MBE system consisting of ultrahigh vacuum (UHV) chambers, substrate preparation part, thin-film growth and analysis parts, and sample exchange load-lock chamber. It is a method of depositing new monocrystalline films with complete crystal structure one by one along the grain axis of substrate materials under appropriate conditions. This method can be used to grow extremely thin monocrystalline films and has the ability to accurately control the film thickness, composition, and dopants. However, it needs high requirements for the substrate and the epitaxial material whose atomic lattice mismatch is not more than 7%. Since the early work on MBE-grown multilayered Fe/Gr/Fe for the GMR investigation, MBE is now a well-established epitaxial process of major importance in the development of electron transport control and signal sensing applications. 3.2.2.2 Magnetron Sputtering Method for the Fabrication of GMR Devices

The magnetron sputtering system uses magnetrons to realize strong electric and magnetic fields to confine charged plasma particles, which will make the charged plasma particles close to the surface of the sputter target. The plasma can be sustained at lower pressure and the sputtered atoms are neutrally charged and unaffected by the magnetic trap. In the magnetron-sputtering setup (Figure 3.3), specially designed sputter guns with unusually strong permanent magnets are often used in compensation. During the sputtering process, electrons move along the magnetic field lines in the presence of an inert gas (such as argon) and the extra argon ions created as a consequence of these collisions lead to a higher deposition rate. It is found that compared with the evaporation of materials in a resistance evaporator or Knudsen cell, for the magnetron sputtering method, even the materials with very high melting points can be easily sputtered and the sputter deposited films have a better adhesion on the substrate with a composition close to that of the source material due to the different elements spreading differently because of their different mass (light

Water

–DC and RF Shutter

Ar gas

Magnet Target Plasma

Ar Ar+ e– Ar

Ar

M M

Ar

e– M

Substrate Platform

(a)

(b)

Figure 3.3 Schematic of the magnetron sputtering system. Source: Zhang and Song (Image permitted from Tianjin Tianzhao Yuhua Science and Technology Company, Tianjin, China).

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elements are deflected more easily by the gas). For the insulating targets, charged process can be avoided due to the introduction of a radiofrequency (RF) sputtering where the sign of the anode–cathode bias is varied at a high rate and RF sputtering works well to produce highly insulating oxide films. Up to date, a broad of multilayered systems based on GMR effect have been fabricated via the magnetron sputtering method (Table 3.1).

3.2.3

GMR Applications for Sensors

Due to the microlevel dimension, excellent sensitivity, and complementary metal– oxide-semiconductor (CMOS) compatibility, GMR sensors have attracted more consideration and interest for conventional applications such as read heads in hard disk drives [14], magnetic memory that brings an improvement to the memory devices, and at the same time, a significant increase in the reading speed of this type of storage device, detection of weak magnetic fields, angular velocity and acceleration, inertia navigation and positioning, and recently emerging applications like eddy current sensing probe [67], ultralow pressure measurement [68], biological magnetic sensors (such as gene expression analysis and influenza virus detection) [69–73], some novel technologies in water pollution detection [74], wireless charging for electric vehicles, and even current monitoring for smart power grids [75]. For the biological sensors, GMR devices have been shown to detect proteins, like cancer biomarkers [69], autoantibodies (Figure 3.4 shows GMR biosensor microarray chips. Each chip has 72 effective sensors as the platform for autoantigen microarrays.) [70], and common allergens (Figure 3.5 shows the process of the multiplexed allergen sensing on GMR sensor array) [71], with high sensitivity and specificity. They have also been utilized for DNA detection [72] and have been well characterized for simultaneous mutation and methylation analysis [73]. (i) Individual GMR sensors were spotted with different capture antibodies, specific to their respective allergens. (ii) Allergens were added and bind to respective capture antibodies. (iii) Biotinylated detection antibodies were added and bind to respective

50μm

500μm

(a)

(b)

Figure 3.4 GMR microarrays for the detection of auto antigen. Optical images of a GMR biosensor chip (a) and a cartridge with a reaction well and their sensors (b). Source: Lee et al. [70].

3.2 Multilayered Magnetic Thin Film for the Electron Transport Control and Sensing

(2)

(1)

GMR sensor

GMR sensor

GMR sensor

(4)

GMR sensor

GMR sensor

(3)

GMR sensor

GMR sensor

GMR sensor

GMR sensor

GMR sensor

GMR sensor

GMR sensor

GMR sensor

(6)

(5)

GMR sensor

GMR sensor

GMR sensor

GMR sensor

GMR sensor

Figure 3.5 Illustration of the GMR-based multiplexed allergen detection assay. Source: Ng et al. [71].

allergens. (iv) The sandwich immunoassay structures were formed on individual sensors. (v) Streptavidin microbeads were added to the reaction well. (vi) As the magnetic particles bind to the biotinylated detection antibodies, local magnetic fields were generated, and changes in the resistance of the GMR sensor produce a signal that could be correlated to each allergen concentration [71]. A real-time detection system for water pollution status and pollutant concentration based on GMR sensor considering water pollution detection has been reported [74]. It can realize multipoint automatic sampling, detection, analysis, data uploading analysis, real-time monitoring, alarm, and other functions and overcome the defects of existing technologies. It has been verified that the real-time detection system for water pollution status and pollutant concentration applied with GMR detector exhibit excellent characteristics. A simple, cost-effective, magnetic pressure sensor consisting of a polymer diaphragm, a permanent magnet, and a GMR-based magnetic field gradient sensor is shown in Figure 3.6. The fabricated prototype was calibrated for ultralow differential pressure range, and it showed sensitivity up to 16.67 μV/(V Pa) and nonlinearity of 1.5% full scale (FS) in the range of 0–300 Pa. The concept was not limited to the low pressure but wide ranges of pressure could be realized depending on the type of the diaphragm [68]. GMR could also be as a sensor to make a magnetic nanoparticle detection scheme without the presence of an external magnetic field. In this device, there is a magnetic sensor in a patterned groove structure and external magnetic field is not needed to magnetize the magnetic nanoparticle. An example is given based on a GMR sensing

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3 Multilayered Magnetic Thin Films for Electron Transport Control and Signal Sensing

P

Diaphragm Magnet GMR sensor PCB Movable stage

Diaphragm Magnet dz

z R1,R3 y

dx d

Aluminium housing

Aluminium spacer R2,R4

GMR sensor PCB Base

x

Stage lock

(a)

Electrical connection

(b)

Figure 3.6 (a) Schematic of magnetic pressure sensing mechanism and (b) Schematic of pressure sensor prototype. Source: Borole et al. [68]. Generation

G GMR Current sensor

Transformation

Transmission

G

GMR sensor networks

G

Distrubution

Consumption

G

Data transmission G Dispatch center

Figure 3.7 Schematic diagram of GMR sensor applications in the smart grid. Source: Ouyang et al. [75].

device with a spin valve structure [76]. In this structure, the detection of magnetic nanoparticles located inside the groove and near the free layer is demonstrated without magnetic field, which will be useful for magnetic biosensing system miniaturization and power consumption control. Since the GMR was first commercialized in 1997 [14], GMR structures have been widely used in the conventionally modern hard drives, magnetoresistive random access memory (MRAM) as cells that store one bit of information and sensor applications in the measurements of current [17, 77], angular displacement, accelerated velocity as well as biological signatures of diseases and smart grid as shown in Figure 3.7.

3.3 Multilayered Magnetic Thin Film for the Electron Transport Control and Sensing Based on Colossal Magnetoresistance (CMR) Effect 3.3.1

Introduction of CMR

CMR effect belongs to the magnetoresistance effect associated with a ferromagnetic to paramagnetic phase transition under the application of a magnetic field.

3.3 Multilayered Magnetic Thin Film for the Electron Transport Control and Sensing

Materials, mostly manganese-based perovskite oxides, featuring CMR may demonstrate many orders of magnitude exceeding GMR of multilayered and granulated systems [78, 79]. CMR phenomena were first discovered in the hole-doped (e.g. Ca or Sr substitution for La) compound LaMnO3 family, in which large magnetoresistance was observed near a high temperature (T) insulating paramagnetic phase to a low-T conducting ferromagnetic phase [80]. Lately, it is found that CMR materials constitute the broad class of doped cubic perovskite manganites termed as R1−x Ax MnO3 . Here R represents a rare earth and A is alkaline earth. They can exhibit huge resistance in the presence of a magnetic field. Recently, it is also reported that CMR can also be realized in a compressed single-valent LaMnO3 manganite compound by generating an inhomogeneous, nonconductive one with a unique structural distortion and a metallic one without distortion using pressure [80]. Besides that, large CMR has also been found in Tl2 Mn2 O7 and some Cr chalcogenide spinels compounds that are different greatly from the manganite perovskites. The phenomenon of CMR is currently of considerable interest because of its value in fundamental physics and potential applications.

3.3.2

Fabrication of Multilayered Thin Films Based on CMR Effect

Much attention has been paid to the fabrication of artificially CMR structures such as p–n junctions, tunnel junctions, and superlattices, using the magnetron sputtering method, pulsed laser deposition method (PLD), MBE, and other methods such as solid-state method. Table 3.2 summarized the typical multilayered magnetic thin film fabricated by these three methods. Magnetron sputtering method is a physical vapor deposition method and can be controlled by many complex parameters including deposition pressure, time, and temperature of the substrates. This method allows for a large degree of control over the growth and microstructure of the film. In the magnetron sputtering method, the off-axis is usually introduced to fabricate thin films for the investigation of the CMR properties. Table 3.2 Typical multilayered magnetic thin film fabricated by magnetron sputtering, PLD, and MBE methods. Fabrication technology

Multilayered films

Magnetron sputtering

La0.7 Ca0.3 MnO3 [81, 82], La0.67 Sr0.33 MnO3 /La0.75 MnO3 / La0.67 Sr0.33 MnO3 [83], Nd0.52 Sr0.48 MnO3 [84], Sm0.53 Sr0.47 MnO3 [85], BiFeO3 [86], La1−x Srx MnO3 [82, 87, 88], La0.67 Ca0.33 MnO3 [89], La0.67 Ca0.33 MnO3 /YBa2 Cu3 O7−δ bilayers [89]

PLD method

YBa2 Cu3 O7−y /LaMnO3 /YBa2 Cu3 O7−y [90], La1−x Pbx MnO3 [91], LaCaMnO/CeO2 /Si(100) [92], La0.9 Sb0.1 MnO3 [93], Pr0.7 Sr0.3 MnO3 /SrTiO3 (001) [94], Pr0.7 Sr0.3 MnO3 /La0.5 Ca0.5 MnO3 [94, 95], Pr0.6 Sr0.4 MnO3 /SrTiO3 , and Pr0.6 Sr0.4 MnO3 /LaAlO3 [96]

MBE method

La1−x Cax MnO3 [97, 98], La1−x Srx MnO3 [99–104], Sr-doped LaMnO3 , and Nb-doped SrTiO3 [105, 106]

103

104

3 Multilayered Magnetic Thin Films for Electron Transport Control and Signal Sensing Sputterubg gun Target

Figure 3.8 Diagram of magnetron sputtering system geometry. Source: Eom et al. [107].

Substrate Heater

In situ grown YBa2 Cu3 O7−d thin film [107] and La0.67 Sr0.33 MnO3 film [108] were grown by 90∘ off-axis single-target from a stoichiometric target using magnetron sputtering method that exhibits novel physical and material properties. Here, stoichiometric targets were prepared from “freeze-dried” powders. The deposition processes were performed in a high oxygen partial pressure environment. Substrates were placed on the side of a planar magnetron gun to avoid back sputtering damage from negative oxygen ions as shown in Figure 3.8. The investigation showed that films less than 4 nm thick on are purely oriented and epitaxially aligned in the substrate plane [107]. PLD method is a highly flexible film deposition technique. This method uses a high-power pulsed laser beam inside a vacuum chamber to vaporize target to form a thin film on a substrate under an UHV or in the presence of a background gas. In this fabrication system, high particle energies in the deposition plume can promote surface diffusion and crystal growth [109]. Nowadays, the PLD method can be widely used in most materials, such as metals, refractory metals, and rare earth mainly due to its very successful application to high-temperature superconducting materials. For example, in order to investigate the effects of biaxial strains and microstructure on the magnetic anisotropy, the La0.7 Sr0.3 MnO3 layer was deposited on La0.7 Ca0.3 MnO3 buffer layer to form the epitaxial CMR heterostructure using a PLD system with a 10 Hz of energy density [110]. It was found that the magnetic anisotropy was determined by both the magnetoelastic effect and the magnetocrystalline effect. MBE method is another epitaxy method for thin-film deposition of single crystals, especially for the semiconductor. MBE system can also be modified according to need. Oxygen sources, for example, can be incorporated for depositing oxide materials for advanced electronic, magnetic and optical applications, as well as for fundamental research. For instance, in contrast to the negative CMR of the LaMnO3 compound family, as shown in Figure 3.9, a positive CMR has been discovered in an epitaxial multilayer p–n heterostructure fabricated with Sr-doped LaMnO3 and Nb-doped SrTiO3 by MBE method [106]. The fabrication of artificial crystalline materials through layer-by-layer growth with good control over the composition and structure at the atomic level has become possible and even routine for some oxides. To obtain better p–n interfaces, a computer-controlled MBE system was used to deposit the multilayered p–n heterostructure. In this heterostructure, the CMR effect and the magnetic field modulation of the heterostructure current are very evident in low magnetic fields, even at room temperature [106]. Besides the above mentioned fabrication methods, we also show another two methods to fabricate the CMR devices using typical examples. LaMnO3 sample was

3.3 Multilayered Magnetic Thin Film for the Electron Transport Control and Sensing

Figure 3.9 Diagram of the sample and measurement scheme. Source: Lu et al. [106].

I

V In

In SrNb0.05Ti0.95O3 La0.9Sr0.1MnO3

SrNb0.05Ti0.95O3 La0.9Sr0.1MnO3

SrNb0.05Ti0.95O3 La0.1Sr0.1MnO3

SrNb0.05Ti0.95O3 La0.1Sr0.1MnO3

SrNb0.01Ti0.99O3

Monolayer G B Graphene Au

Au BP

SiO2 Heavy doped Si

Vg Multilayer BP

Figure 3.10 Schematic drawing and performance of monolayer graphene on multilayer flake graphene black phosphorus device. Source: Liu et al. [111].

synthesized via a solid-state method using La2 O3 and Mn2 O3 [80]. First, pellets were prepared from the thoroughly mixed powders and allowed to react at 1200 ∘ C for a total time of at least 90 hours, during which they were reground and pelletized at least twice. The pellet was then conditioned in argon at 900 ∘ C to assure the oxygen stoichiometry. Here, oxygen stoichiometry was determined by means of thermogravimetric analysis confirming the correct LaMnO3 stoichiometry, and the CMR effect took place at the percolation threshold. Moreover, a large memory effect was observed together with the CMR, suggesting the presence of magnetic clusters. As shown in Figure 3.10, a phonon-mediated CMR device was fabricated to make a highly sensitive sensor. The fabrication process can be described as follows. The black phosphorus was first exfoliated in a glove box with an argon atmosphere onto a silicon substrate. Then, optical microscopy under the argon atmosphere was used to avoid degradation of black phosphorus and identify the desired black phosphorus. Finally, monolayer graphene was aligned onto the black phosphorus flake with the desired rotation angle in the presence of a transfer platform.

3.3.3

CMR Applications

There exists an increasing demand for magnetoresistance sensors with high sensitivity, low energy consumption, spatial resolution at micro-nanoscales and room temperature operation, which results in a broad of investigations of physical phenomena in advanced materials, and fabrication of novel magnetoresistive device [111].

105

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3 Multilayered Magnetic Thin Films for Electron Transport Control and Signal Sensing

CMR can be used as phonon-mediated CMR. As shown in Figure 3.10, a phonon-mediated CMR device with a GMR of 775% at 9 T magnetic field exceeding by far the magnetoresistance effects from devices was made from either monolayer graphene or few-layer black phosphorus alone. It shows that electron–phonon coupling between 2D material and a suitable substrate can be exploited to create giant MR effects in Dirac semimetals [111]. Dirsyte et al. etc. [112], proposed and fabricated a novel magnetic field sensor based on the combination of a single-layer graphene and thin nanostructured manganite La0.8 Sr0.2 MnO3 film-hybrid graphene–manganite structure. This hybrid graphene–manganite sensor results in the enhanced sensitivity to the magnetic field of the hybrid sensor on the macroscopic level and has a lower sensitivity to temperature variations in comparison to the manganite sensor and can be applied for position sensing. Based on of CMR-B-scalar sensor that was formed by a thin manganite film deposited on a substrate with metal contacts deposited on it, a pulsed magnetic field measurement system for railgun investigation was also studied [113]. This system was made up of four personal computer-controlled B-scalar meters and each of them was connected by a twisted pair cable. Optionally, each B-scalar meter can be connected to the PC directly via USB. The measurement at each channel can be triggered separately by an electrical or an optical signal. It was reported that the system was able to measure the magnitude of pulsed magnetic fields independently on their orientation in the range from 0.3 to 25 T.

3.4 Multilayered Magnetic Thin Film for the Electron Transport Control and Sensing Based on Colossal Tunneling Magnetoresistance (TMR) Effect 3.4.1

Introduction of TMR

There exists a quantum mechanical effect called TMR that depends dramatically on whether the ferromagnets are aligned in parallel or antiparallel in magnetic tunnel junctions made up of ferromagnets/insulator/ferromagnets system. The TMR effect that is a typical example of the spin-dependent electron transport can be evaluated by the ratio of conductance. TMR =

Gp − GAp Gp

(3.2)

Here, Gp and GAp represent the conductances for the parallel and antiparallel alignment of the magnetic tunnel junctions respectively. Illustrated in a single model, the value of TMR is mainly dependent on the intensity of two ferromagnets’ spin polarization at the Fermi energy. The conductance will become larger if they have the same sign of the spin polarization and the magnetic layers are aligned parallel, which is considered as the positive sign of the TMR. However, it is experimentally demonstrated that the conductance of the TMR can be widely

3.4 Multilayered Magnetic Thin Film for the Electron Transport Control and Sensing

decided by a broad of atomic and electronic factors including an insulator and the ferromagnet/insulator interfaces, materials of electrodes, the height, the shape and even the disorder of the barriers, and the impurities in the barrier. In the early time, for the barriers, the investigations of tunneling junctions focus on amorphous alumina that makes the TMR ratios of the order of 50%. Later, a new fully crystalline junction with the MgO barrier was proposed in Fe/MgO/Fe system and the ratio exceeded 1000% with just a few atomic layers [114]. The organic single barrier was also investigated and it was found that a large TMR appears in the La2/3 Sr1/3 MnO3 (Co)/quaterthiophene/La2/3 Sr1/3 MnO3 organic magnetic tunnel junction with a large spatial spin polarization [115]. Besides the single insulator layer, metallic spacers were inserted between the insulator and the magnetic film to form the Fe/Au/MgO/Au/Fe(001) system [116]. Interestingly, the ratio was found to oscillate as a function of the metallic spacer thickness, and the epitaxial can endow the system with high TMR ratios. Nowadays, this TMR phenomenon assisted with a broad of fabrication methods has attracted more and more interest due to its broad applications in spin-electronic devices including magnetic sensors and magnetic random access memories.

3.4.2

Fabrication of Multilayered Thin Films of the TMR Effect

In this part, we summarized the main methods to fabricate the TMR devices in the past 10 years. Similar to the fabrication methods of the GMR, CMR sensors and devices, magnetron sputtering, MBE and PLD are still the common tools to be used to fabricate the TMR devices. Furthermore, we will introduce the TMR devices prepared by these three methods and other efficient setups with typical examples respectively. Figure 3.11 presents a magnetic tuning junction consisting of a buffer layer/PtMn/ CoFe/Ru/CoFeB/MgO/CoFeB/cap layer deposited on a SiO2 /Si substrate using magnetron sputtering [117]. In this device, the thickness of every layer was set to be nanoscale. After the deposition process, the multilayer stacks were patterned by milling and annealed in 1 T for 120 minutes at 330 ∘ C to make the CoFeB layers crystallize. It was found the TMR ration in the CoFeB/MgO/CoFeB-based magnetic tunneling junctions can be over 200% at 3 K. Hybrid multilayered system Mo/Ru/Mo/Co20 Fe60 B20 /MgO/Co20 Fe60 B20 /Mo/Ru whose morphology and composition distribution were shown in Figure 3.12, was deposited on the silicon substrate at ambient temperature using a magnetron Figure 3.11 Multilayered magnetic tunnel junction with measurement setup for the resistance. Source: Arakawa et al. [117]. ©2011, AIP Publishing LLC.

Lock-in amplifier

Multimeter

100 kΩ or

PtMn (15)

CoFeB (2) MgO (1.05) CoFeB (3) Ru (0.85) CoFe (2.5) H VTI(2-300 K)

+ – Digitizer + –

107

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3 Multilayered Magnetic Thin Films for Electron Transport Control and Signal Sensing

(a)

EDX composite

HAADF-STEM (b)

10 nm (c)

Mo

Ru

Fe

Co

Mg

Figure 3.12 (a) TEM image of the cross section of a Mo-based perpendicular magnetic tunnel junction annealed at 673 K for two hours. (b) A composite scanning map energy dispersive X-ray (EDX) spectroscopy of the perpendicular magnetic tunnel junction composition and the colors are assigned in (c). Source: Almasi et al. [118].

STEM

sputtering system equipped with a residual gas analyzer to monitor the H2 O partial pressure [118]. During the deposition process, the Mo and Ru targets were sputtered in a 2 mTorr Ar gas atmosphere using a direct current (DC) sputtering apparatus. The MgO and Co20 Fe60 B20 layer was deposited by a RF magnetron sputtering at 1.3 mTorr of argon gas. After the deposition process, the samples were annealed in a rapid thermal annealing setup in an inert atmosphere. The experimental result highlighted the importance of the heavy-metal layers adjacent to CoFeB electrodes for achieving larger TMR and further demonstrated that the TMR can reach as high as 162% injunctions with Mo layers, due to the improved thermal stability. Figure 3.13 shows a schematic of MgO-based spin-valve multilayered structure fabricated using the magnetron sputtering method with a top Co2 Fe6 B2 free layer and incorporated a single SyAF [Co(0.4 nm)/Pt(0.3 nm)]3 layers and a new buffer layer of Co/Pt/Co. After the deposition process, an ex situ annealing of 350 ∘ C for 30 minutes under a vacuum below 10−6 Torr and a perpendicular magnetic field of 3 T was carried out to ensure a memory margin and avoiding read [119]. As shown in Figure 3.13c, it had a TMR ratio of 180% and anisotropy exchange field of 3.44 kOe. This was due to the fact that the decreasing surface roughness in the fcc–MgO tunneling barrier significantly led to a high level of performance. For the PLD setup, the film grown process can occur in an UHV or in the presence of a background gas. This technique can also be used to grow films to form the magnetic tunnel junction. For instance, epitaxial discontinuous Fe/MgO multilayers was grown by PLD on single-crystal substrates. It was observed that the increasing deposition temperature caused an improvement in crystal quality and was accompanied by higher TMR ratios of 9.2% at room temperature and 18 kOe magnetic field trebles that of polycrystalline samples deposited simultaneously on glass substrates. MBE setup is another method to fabricate TMR devices. Multilayered magnetic tunnel junction made up of a fully epitaxial Fe/MgO/Fe/MgO/Fe was fabricated on MgO single-crystal substrate via an MBE equipment. After the measurement of the bias voltage effects on both TMR and conductance. It was found that the TMR ratio

3.4 Multilayered Magnetic Thin Film for the Electron Transport Control and Sensing

trode Ta/Ru elec W cap p MgO ca e upper fre CoFeB er ac sp W e fre er low Fe/CoFeB er nnel barri MgO tu pinned CoFeB ) 48 (0.18~0. W bridge Co t(0.3)] 3 Co/Pt Ru Co

[Co(0.4)/P

t(0.3)] 6

[Co(0.4)/P

Pt seed Ta buffer

ectrode SiO 2 Si sub.

W/TiN el

(b)

(c) Tex = 350 ºC trode Ta/Ru elec W cap p MgO ca per free up eB CoF er W spac e lower fre Fe/CoFeB er nnel barri MgO tu d ne pin CoFeB 48) (0.18~0. W bridge buffer o Co/Pt/C Ru Co t(0.3)] 3 [Co(0.4)/P Pt seed Ta buffer

ectrode SiO 2 Si sub.

W/TiN el

180

TMRMAX: 156% TMRMAX: 180%

MR ratio (%)

(a) Tex = 350 ºC

150

120

30 Double [Co/Pt]n SyAF Single [Co/Pt]n SyAF

0

0.2

0.3 0.4 tw (nm)

0.5

Figure 3.13 Schematics of double MgO based nanostructures with a top Co2 Fe6 B2 free layer using (a) a double SyAF [Co/Pt]n layers, (b) a single SyAF [Co/Pt]n layer, (c) W thickness-dependent TMR ratio. Source: Choi et al. [119]. © Springer Nature Limited. Licensed under creative commons 4.0.

can reach up to 110% with a single MgO barrier at room temperature. The nanostructure also exhibited a large asymmetry in the bias dependence of the TMR ratio. This was ascribed to the asymmetric conductance curves especially for the parallel magnetization configuration. Besides the abovementioned method, the cluster deposition system was also used to fabricate magnetic tunnel junction. For instance, a tunnel junction was deposited at room temperature on thermally SiO2 substrates, with the layer sequence Ta/Ru/Ta/NiFe/IrMn/CoFe/Ru/Co40 Fe40 B20 (CoFeB)3 /MgO/CoFeB/Ta/Ru. After deposition of the bottom electrodes using DC sputtering, both MgO layers were deposited by RF sputtering using two MgO targets in a target-facing-target gun. Subsequently, top electrodes were sputtered. The pressure for the MgO layers’ growth was ranged from 1.3 to 4.0 mTorr. It was demonstrated that frequency 1/f barrier noise as low as 2.5–3.3 E−12 μm2 with a TMR ratio of up to 330% at room temperature was observed [120]. Chemical vapor deposition (CVD) technique is also an efficient method to fabricate the TMR sample. NiFe/Gr–hBN/Co magnetic tunnel junction was fabricated via thermal CVD technique [121]. First, Cu foil was moved into a CVD furnace tube. After the furnace pressure and temperature were set to be 0.1 mTorr and 1010 ∘ C respectively, the CH4 and H2 gases entered into the furnace at a flow rate of 20 and 5 sccm, respectively, to form graphene. Subsequently, the sample was cooled to room temperature. The hBN was grown on the Cu foil in a similar way. Here, the Cu foil temperature was adjusted to 990 ∘ C for 30 minutes with a H2 gas flow rate of 5 sccm. After the cleaning process, borazine and H2 gases entered into the furnace tube and the temperature was kept at 997 ∘ C for 30 minutes to form the hBN layer

109

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3 Multilayered Magnetic Thin Films for Electron Transport Control and Signal Sensing

and then, the temperature was adjusted to 500 ∘ C. Then, the polymethyl methacrylate (PMMA) was coated on the hBN surface. The new sample was then immersed in an aqueous solution of ammonium persulfate to remove the Cu foil. After that, deionized water was used to wash the sample. The new sample then entered into CVD grown graphene on Cu foil. The samples were then dried on a hot plate at 80 ∘ C. Again, the bottom Cu foil was removed through etching by keeping the sample in an ammonium persulfate solution. After this, Gr/hBN/PMMA was washed with deionized water for cleaning. Next, Gr/hBN/PMMA was transferred upon SiO2 /Si substrate where the NiFe electrode had already been deposited through metal masks. The samples were then placed in acetone for the lift-off PMMA. Consequently, the top Co electrode was then patterned on the top surface of hBN using metal masks. The final device configuration was Co/Gr–hBN/NiFe/SiO2 . It was observed that the ratio of TMR monotonically increases with decreasing temperature. The magnitude of the TMR ratio is found to progress from −0.85% at room temperature to −1.88% at 4.2 K.

3.4.3

TMR Applications

The TMR effect in magnetic tunnel junctions [122] is the key to developing magnetoresistive random-access-memory [123, 124], novel programmable logic devices [125], and magnetic sensor that poses a bright perspective in micro- and nano-scale strain sensing technologies [126, 127]. Recently, tunnel magneto-resistance sensors made up of magnetic tunnel junctions have been used to measure weak bio-magnetic fields [128]. For instance, CoFeB/MgO/CoFeB magnetic tunnel junctions with a perpendicularly magnetized synthetic antiferromagnetic reference layer were fabricated for magnetic sensor applications and this sensor showed dynamic ranges more than 62.5 kOe [129]. Meiling Wang et al. [128], measured the triaxial magnetocardiography signal using a high sensitivity tunnel magnetoresistance sensor. All the biomagnetic field measurements were performed in a magnetically shielded room to counteract the influence of external magnetic fields. The screening factor of the magnetically shielded room was about 60 dB@1 Hz and the residual magnetic field in the magnetically shielded room was about 5 nT. The measurement can be carried out at room temperature and this would make magnetocardiography instrumentation cheaper for extensive clinical applications. In contrast to the behavior of conventional TMR sensors under mechanical stress as well as their sensitivity to the applied stress that depends on the magnetization configuration of magnetic tunnel junctions with respect to the stress axis, as shown in Figure 3.14, CoFeB/MgO/CoFeB-based system using an inverse effect on the tunnel resistance by tensile and compressive stresses was proposed [130]. Field and strain loops on this proposed configuration were calculated. It was demonstrated that the junction exhibited big factors of 2150±30 and −260 for tensile and compressive stresses, respectively, under a −3.2 kA/m bias magnetic field. This configuration result provided a design path for high sensitivity and ability to detect both tensile and compressive stresses by a single TMR sensor.

3.5 The Multilayered Magnetic Thin Film Based on Quantum Anomalous Holzer Effect (QAHE)

Figure 3.14 Schematic of the TMR sensor in which applied press makes the black ceramic pieces move, and then the ceramic transfers the stress signal to the TMR junction. Source: Tavassolizadeh et al. [130].

Fixed Al body 18 mm TMR junction

6 mm ΔZ

Pusher block H

3.5 The Multilayered Magnetic Thin Film Based on Quantum Anomalous Holzer Effect (QAHE) The quantum Holzer effect is a quantized version of the Holzer effect observed in two-dimensional (2D) electron systems under approximately 10 T magnetic field strength that makes the experimental realization challenging and greatly hinders real-world applications. However, QAHE that may be a consequence of the combined spin–orbit coupling and reduction of the time-reversal symmetry due to intrinsic magnetization, can be realized without an external magnetic field that may lead to the development of low-power-consumption electronics [131]. Nowadays, broad investigations have been carried out to seek new platforms for the realization of the QAHE [132–134]. Among the investigations, graphene-like honeycomb materials [135, 136] and magnetically doped topological insulators [131, 137] have mainly attracted increasing attention. Efforts on the investigations of the magnetically doped topological insulators system lead to the first experimental realization of the QAHE. In 2013, QAHE was experimentally observed in a magnetic topological insulator, namely, thin films of chromium-doped (Bi,Sb)2 Te3 [131], in which it was found that at zero magnetic fields and under low temperature, the gate-tuned anomalous Holzer resistance reached the quantized value of h/e2 (h is Planck’s constant and e is the elementary charge), accompanied by a considerable drop in the longitudinal resistance. Although the temperature of the observation of the QAHE of the intrinsic magnetic topological insulators has been increased much, QAHE is still a challenge for further electron transport control and signal sensing applications. In this chapter, we will mainly introduce the recent efforts on the experimental investigation of the QAHE of intrinsic magnetic topological insulators, including the fabrication method and its application for the electron transport control and signal sensing applications, although the investigations of the QAHE mainly focus on the simulation [132, 138–140]. Intrinsic magnetic topological insulator Cr0.15 (Bi0.1 Sb0.9 )1.85 Te3 with a thickness of five periodic layers grown on dielectric SrTiO3 (111) substrates by MBE setup was investigated [131]. In this structure, the chemical potential can be adjusted to the electron- or hole-conductive regime by a positive or negative gate voltage

111

112

3 Multilayered Magnetic Thin Films for Electron Transport Control and Signal Sensing

respectively, due to the fact that the film thickness in this system was nearly charged neutral. In order to carry out the transport measurements, the system was manually cut into a hall bar nanoarrays after the growth process. The measurement results demonstrated that the QAHE was first achieved with low mobility without any magnetic field in this intrinsic magnetic topological insulator system and this system that was fabricated via the MBE paved a path for the further development of the low-power-consumption, topological quantum electronic and spintronic devices. (Bi1−y Sby )2 Te3 and Zn1−x Crx Te were chosen as a topological insulator and ferromagnetic insulator respectively to form a Zn1−x Crx Te/(Bi1−y Sby )2 Te3 /Zn1−x Crx Te sandwich heterostructure via MBE method [141]. The detailed process was as following: InP(111)A substrate was first installed to the chamber to experience an annealing process at 350 ∘ C in a vacuum. Then, a 2 nm thick ZnTe buffer layer was grown on semi-insulating InP(111)A substrate. Subsequently, Zn1−x Crx Te (10 nm)/(Bi1−y Sby )2 Te3 (8 nm)/Zn1−x Crx Te (10 nm) sandwich heterostructure was gown on the substrate. Finally, a 2 nm thick ZnTe buffer layer was adopted to improve the crystallinity of the Zn1−x Crx Te layer. After the growth process, and AlOx protecting layer with a thickness of approximately 5 nm was deposited via atomic layer deposition at room temperature to prevent deterioration of the sample. In this sandwich heterostructure, QAHE was driven by the magnetic proximity coupling. Nowadays, the discovery of the QAHE makes a promise for the construction of the spintronic devices with lower dissipation as a consequence of the dissipationless nature of the hall currents and chiral propagation. The spintronic application based on the domain wall is a prototypical example, owing to its non-volatile nature. In this application, racetrack memory has been expected to be the next new generation spintronics memory device [139] due to a prominent advantage that the device does not need electricity to keep information. However, the challenge is that a current-driven device of ferromagnetic metals suffers from Joule heating. Fortunately, the QAHE or Weyl semimetal states of kagome layered materials were proposed to overcome this barrier. It is reported that in these materials, domain walls can be driven by the electric field and a large electric current that causes Joule heating is not necessary [142–144]. A quantum anomalous Holzer insulator with the amagnetic configuration of a domain wall was proposed [142]. In this system, a magnetically doped topological insulator with the configuration of a domain wall was connected by two voltage-controlled electrical leads. The opposite QAHEs in two domains were generated due to the domain-wall configuration and the domain wall can be removed in the presence of an external magnetic field. This device can provide an efficient scheme to reconfigure the domain-wall chiral interconnects for possible memory and can also be used as a magnetoelectric piston when the insulator was contacted by electrical reservoirs.

3.6 Summary and Perspectives Carrying information in both the charge and spin of an electron potentially offers devices with a greater diversity of functionality, and with the development of the

Acknowledgments

spintronic, novel magnetotransport phenomena appear due to the GMR, CMR, TMR, and QAHE. In this chapter, we summarized the main fabrication methods of composition and microstructure defined magnetic thin films for these four quantum effects. Their novel electron transport control were also discussed according to their unique signal sensing applications for the integrated information technology. (1) GMR that exists in multilayered films is a significant change in the electrical resistance depending on whether the magnetization of adjacent ferromagnetic layers is in a parallel or an antiparallel alignment in the presence of an external magnetic field. Among the fabrication methods, the MBE method, magnetron sputtering method, and electron beam deposition method are three main methods to fabricate the samples. GMR has been widely used in the emerging applications including eddy current sensing probe, ultra-low pressure measurement, biological magnetic sensors, some novel technologies in water pollution detection, wireless charging for electric vehicles, and even current monitoring for smart power grids. (2) CMR effect mostly based on manganese-based perovskite oxides demonstrates many orders of magnitude exceeding GMR of multilayered and granulated systems. The devices with CMR effect can be fabricated via magnetron sputtering method, PLD, MBE and other methods such as solid-state method. CMR can be used as a phonon-mediated CMR magnetic field sensor applied for position sensing and pulsed magnetic field measurement. (3) TMR that exists in magnetic tunnel junctions consisting of ferromagnets/insulator/ferromagnets system is the key to developing magnetoresistive random-access-memory, novel programmable logic devices, and magnetic sensor that poses a bright perspective in micro- and nano-scale strain sensing technologies. Magnetron sputtering, MBE, PLD, cluster deposition system, and CVD technique can be used to fabricate TMR devices. (4) QAHE that exists in graphene-like honeycomb materials and magnetically doped topological insulators is a consequence of the interplay between spin–orbit coupling and reduction of the time-reversal symmetry due to intrinsic magnetization. The discovery of QAHE in intrinsic magnetic topological insulator system without an external magnetic field paves a path for the further development of the low-power consumption, topological quantum electronic and spintronic devices. (5) The development and advanced application of controllable preparation methods of quantum effect, structure and composition in thin films, and how to combine with optical communication and sensing technology to break through the bottleneck of molar effect and build the next generation of high-speed and high-throughput quantum communication technology are prospected.

Acknowledgments This chapter is supported by the NSFC–BRICS STI Framework Program (No. 51861145309), the National Natural Science Foundation of China (No. 51971029),

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the National S&T Major Project (No. 2018ZX10301201), the Postdoctor Research Foundation of Shunde Graduate School of University of Science and Technology Beijing (No. 2020BH005), the Project funded by China Postdoctoral Science Foundation (No. 2020M680336), the “All English teaching demonstration course construction project of University of Science and Technology Beijing” (No. KC2015QYW06, 2016), the “1125” Zhihui Zhengzhou Talent project of Henan province (Fund No. in USTB: 39080070), the “100 talent plan” fund of Fujian province (Fund No. in USTB: 39080067), and the development of a high sensitive magneto-optical biomolecular sensor experimental prototype (Fund No. in USTB: 2019-0649) by Hangzhou Ruidi Biotechnology Co. Ltd.

List of Abbreviation and Symbol AMR GMR CMR TMR QAHE MBE MRAM PLD DC RF PMMA 2D

anisotropic magnetoresistance giant magnetoresistance colossal magnetoresistance tunneling magnetoresistance quantum anomalous Holzer effect molecular beam epitaxy magnetoresistive random access memory pulsed laser deposition direct current radio frequency polymethyl methacrylate two-dimensional

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132 Hu, J., Zhu, Z., and Wu, R. (2015). Chern half metals: a new class of topological materials to realize the quantum anomalous hall effect. Nano Letters 15 (3): 2074–2078. ´ B.K. (2017). Monolayer of the 5d transition 133 Sheng, X.-L. and Nikolic, metal trichloride OsCl3 : a playground for two-dimensional magnetism, room-temperature quantum anomalous Hall effect, and topological phase transitions. Physical Review B 95 (20): 201402. 134 Qiao, Z., Yang, S.A., Feng, W. et al. (2010). Quantum anomalous Hall effect in graphene from Rashba and exchange effects. Physical Review B 82 (16): 161414. 135 Onoda, M. and Nagaosa, N. (2003). Quantized anomalous hall effect in two-dimensional ferromagnets: quantum hall effect in metals. Physical Review Letters 90 (20): 06601. 136 Weng, H., Yu, R., Hu, X. et al. (2015). Quantum anomalous Hall effect and related topological electronic states. Advances in Physics 64 (3): 227–282. 137 Liu, C.-X., Qi, X.-L., Dai, X. et al. (2008). Quantum anomalous hall effect in Hg1−y Mny Te quantum wells. Physical Review Letters 101 (14): 146802. 138 Lan, T.-B., Xu, Y., Tan, H. et al. (2019). Quantum anomalous Hall effect with Landau levels in nonuniformly strained silicene. Journal of Applied Physics 126: 104303. 139 Kobayashi, K., Takagaki, M., and Nomura, K. (2019). Robust magnetotransport in disordered ferromagnetic kagome layers with quantum anomalous Hall effect. Physical Review B 100 (16): 161301. 140 Kovalev, V.M. and Savenko, I.G. (2019). Quantum anomalous valley Hall effect for bosons. Physical Review B 100 (12): 121405(R). 141 Watanabe, R., Yoshimi, R., Kawamura, M. et al. (2019). Quantum anomalous Hall effect driven by magnetic proximity coupling in all-telluride based heterostructure. Applied Physics Letters 115 (10): 102403. 142 Upadhyaya, P. and Tserkovnyak, Y. (2016). Domain wall in a quantum anomalous Hall insulator as a magnetoelectric piston. Physical Review B 94 (2): 020411. 143 Araki, Y., Yoshida, A., and Nomura, K. (2016). Universal charge and current on magnetic domain walls in Weyl semimetals. Physical Review B 94 (11): 115312. 144 Kurebayashi, D. and Nomura, K. (2019). Theory for spin torque in Weyl semimetal with magnetic texture. Scientific Reports 9 (1): 5365.

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4 Bismuth-Substituted Iron Garnet Films for Magnetophotonics: Part A – Fabrication Methods and Microstructure Property Study Andrey A. Voronov 1,2 , T. Mikhailova 3 , Olga V. Borovkova 1,2,4 , Alexander N. Shaposhnikov 3 , Vladimir N. Berzhansky 3 , and Vladimir I. Belotelov 1,2,3 1 Lomonosov Moscow State University, Faculty of Physics, Department of Photonics and Microwaves Structures, Leninskie Gory, Moscow 119991, Russia 2 Russian Quantum Center, 45, Skolkovskoye Shosse, Moscow 121353, Russia 3 V.I. Vernadsky Crimean Federal University, Physics and Technology Institute, Department of Experimental Physics, Vernadsky Avenue 4, Simferopol 295007, Russia 4 Lomonosov Moscow State University, Faculty of Physics, Department of Physics of Oscillations, Leninskie Gory, Moscow 119991, Russia

4.1 Introduction Iron garnets, whose properties were comprehensively studied in the middle of the twentieth century [1], can be attributed to ferrites – magnetically ordered substances with low conductivity, in which the magnetic moments of the atoms of different sublattices are oriented antiparallel, but the moments of the sublattices are not equal, which leads to a spontaneous nonzero magnetization. Due to the combination of a sufficiently large Faraday effect [2, 3] with high transparency (compared with metallic ferromagnets), this material has found a wide application in magneto-optics. One should additionally note a great interest in iron garnet due to the inverse Faraday effect and high-quality spin waves excited in this material [4], spintronics [5], and Bose–Einstein condensation of magnons [6]. The general structural formula of iron garnets is {R3 }3+ [Fe2 ]3+ (Fe3 )3+ (O12 )2− where R is a rare-earth element or yttrium (Y, Gd, Tb, Dy, Ho, Er, Sm, or Eu). In general case iron can also be partially replaced by ions of other metals. The iron garnet is a cubic body-centered syngony, and is formed by oxygen ions O2− , in the voids between which there are ions of rare-earth elements and iron. There are three types of voids in the garnet structure: tetrahedral, where the metal cation is surrounded by four oxygen ions; octahedral, by six oxygen ions; and dodecahedral, by eight oxygen ions. As a result, Fe3+ iron cations usually occupy tetrahedral and octahedral positions, and rare-earth cations R3 + are dodecahedral [7]. The iron garnet fabrication as epitaxial films allows one to vary the chemical composition, and the presence of three cationic positions enables to introduce Inorganic and Organic Thin Films: Fundamentals, Fabrication, and Applications, First Edition. Edited by Yujun Song. © 2021 WILEY-VCH GmbH. Published 2021 by WILEY-VCH GmbH.

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more than half of the chemical elements of the periodic table, which contributes to imparting certain physical properties to certain samples and setting magnetic and optical parameters. The presence of three magnetic sublattices connected by a ferromagnetic interaction and induced magnetic anisotropy allows one to vary the size of the domains (from 10−7 to 10−3 m), the saturation magnetization (from 0 to 1.5 × 102 kA/m), the uniaxial anisotropy constant (from −104 to +104 J/m3 ), and the Hilbert attenuation parameter (from 10−5 to >1). Such a wide range of property changes contributes to the suitability of iron garnet for several applications [8]. Iron garnets are characterized by high transparency in the near-infrared (IR) region of the spectrum in the wavelength range of 1300–5500 nm; at 𝜆 > 5500 nm there is an increase in optical absorption associated with the absorption of photons by the crystal lattice, and absorption at 𝜆 < 1500 nm is associated with the edge of the electron band, whose center is approximately 900 nm. The optical absorption coefficient κ in the transparency region is less than 0.1 cm−1 . At the electromagnetic wavelength range from 100 μm and up to the microwaves, iron garnets have high transparency, which is also important for applied problems [7]. However, for the problems of magneto-optics, the visible and near-IR spectral regions are of interest. The maximal (up to 3 deg/μm) Faraday rotation (FR) is characteristic of yttrium iron garnet [3] and its Bi-containing varieties [8]. It is established that the introduction of Bi into the composition of iron garnet contributes to a significant increase in the magneto-optical (MO) qualities of the material. In this chapter we will describe the fabrication methods of the bismuth-substituted iron garnet (BiIG) films and some types of magnetophotonic crystals (MPCs) with transparent BiIG layers, MO properties of such MPCs, and MO response enhancement in the ultrathin BiIG films with plasmonic covering as Part A. The unique MO properties and large values of MO effects in heterostructures based on iron garnet films open up prospects for the use of such materials for novel sensors. In particular, we will further consider some typical applications of these iron garnet films and iron garnet-based metamaterials for magnetic field sensing, biosensing, and eddy current flaw detection as Part B in the following chapter.

4.2 Fabrication Methods 4.2.1 Synthesis Technology and Conditions of Bismuth-substituted Iron Garnet Films BiIG films are often used as basic elements of one-dimensional magneto-photonic crystals (1D-MPCs) as they exhibit high transmittance and large specific Faraday rotation (SFR) in the visible and near-IR optical spectrum regions [7, 9–12]. Depending on the 1D-MPC optical spectrum operating range, the thicknesses of these films can vary from a few tens to several hundred nanometers. The structural quality of the magneto-active layer and characteristics of film−substrate transitional interface can significantly affect the 1D-MPC parameters such as the FR magnitude, transmittance, and, as a result, MO figure of merit (FM). Thus the investigation of the

4.2 Fabrication Methods

Figure 4.1 The scheme of synthesis of ultrathin BiIG films, deposited by RIBS (at the left) and crystallized by annealing (at the right). Source: Okuda et al. [9] Li et al. [20]; Parkin et al. [21]; Baibich et al. [22]; Candid et al. [23]; Grunberg et al. [24]; Alvarado and Carbone [25].

Furnace

Argon Oxygen

Target

Ion gun Substrate

Air sample

Sputtered material

optical and MO properties of ultrathin BiIG films, as a function of the substrate surface and other synthesis conditions, is of great interest. The understanding of the kinetics of film growth is crucial to the development of the new generation of MO devices. To clarify the influence of different grown factors, we discuss the results on the optimization of synthesis conditions of ultrathin BiIG garnet films, deposited by reactive ion-beam sputtering (RIBS) [9, 13–19]. RIBS technology consists in the processes of bombardment of ceramic target of certain composition by ion beam and the subsequent deposition of material onto a substrate (Figure 4.1) [9, 13–19]. To obtain high-quality films, it is necessary to satisfy the optimum synthesis conditions. It is assumed that the chemical composition of the garnet phase is closest to the target composition; specific FR has a maximum value, and optical absorption and surface roughness are minimal at optimum synthesis conditions. At the first stage of the technological process, the seven targets of the following compositions were used for BiIG sputtering: ● ● ● ● ● ● ●

M1: Bi1.0 Y0.5 Gd1.5 Fe4.2 Al0.8 O12 , aM1 = 1.2444 nm. M2: Bi2.8 Y0.2 Fe5 O12 , aM2 = 1.2600 nm. M3: Bi1.5 Gd1.5 Fe4.5 Ga0.5 O12 , aM3 = 1.2535 nm. M4: Bi2.5 Gd0.5 Fe3.8 Al1.2 O12 , aM4 = 1.2502 nm. M5: Bi2.5 Y0.5 Fe5 O12 , aM5 = 1.2576 nm. M6: Bi0.9 Gd1.4 Lu0.7 Fe4.1 Al0.9 O12 , aM6 = 1.2404 nm. M7: Bi1.5 Gd1.5 Fe4.5 Al0.5 O12 , aM7 = 1.2506 nm.

Here, the lattice constants a are shown for BiIG corresponding to the target composition. The constants were calculated using Vegard’s law. The lattice constant of yttrium iron garnet a YIG = 1.2376 nm was taken as the basic parameter. The selection of target chemical compositions was determined by optimization of film characteristics such as FR angle value ΘF , saturation magnetization H S , coercive force H c , compensation T comp , Curie temperatures T c , transmittance K t , and squareness ratio of MO Faraday hysteresis loops (FHLs) K g . The chemical compositions of the synthesized targets and films were studied on an REM-106 scanning electron microscope equipped with an EDS-1 energy-dispersive spectrometer. The lattice parameters of the synthesized films were determined with a DRON-3 diffractometer. The FR ΘF , the coercivity H c , compensation T comp , and Curie temperatures T c were determined from FHLs using Faraday magneto-polarimeter operating at 𝜆 = 655 nm in 20–150 ∘ C temperature range.

127

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4 Bismuth-Substituted Iron Garnet Films for Magnetophotonics

The targets were prepared by conventional ceramic technique [26] in several stages: (i) Cold pressing of homogenized mixtures at the pressure of 300 kg/cm2 . (ii) First solid-phase synthesis in air at atmosphere pressure and temperatures 800–1000 ∘ S for 8 or 12 hours. (iii) Grinding, repeated homogenization, and cold pressing of mixtures. (iv) Second solid-phase synthesis at the same conditions. According to X-ray phase analysis, two phases of Bi2 Fe4 O9 and BiFeO3 are the main constituents of M2 target after second solid-phase synthesis. Mass fractions of the phases were 22% and 78%, respectively. In other targets, a little shift of those phase peaks as compared to the M2 target proved the presence phases comprising Gd, Lu, and Al. The targets were fabricated in the form of disks with a diameter of 120 mm. Single-crystal plates of gadolinium–gallium garnet (GGG), calcium–niobium– gallium garnet (CNGG), and calcium–manganese–zirconium–gadolinium–gallium garnet (CMZGG) of orientation (111) with lattice parameters aGGG = 1.2383 nm, aCNGG = 1.2507 nm, and aCMZGG = 1.2495 nm were used as substrates, respectively. In addition, fused silica KU-1 and heat-resistant optical glass were applied. The synthesis of BiIG films was carried out at a URM 3-279.014 setup in oxygen–argon mixture (Figure 4.1) using an ion-beam Kholodok-1 source. The films were grown by two different methods: (i) During deposition on “hot” substrate (in situ). (ii) Sputtering to a “cold” substrate (radiative heating to 80 ∘ C) and subsequent annealing in a vacuum chamber (post-annealing crystallization). In the second method, the amorphous layer deposited on a “cold” substrate was subjected to annealing in a furnace (Figure 4.1). The main variable parameters for RIBS method are the substrate temperature T S , partial pressure of working gases (argon PAr and oxygen PO2 ), residual pressure Pb , and accelerating voltage U at the anode of ion source, ion-beam current J, and the “target−substrate” distance. These parameters determine the target sputtering r S and deposition r D rates. The rate r D of the layers on a “hot” substrate was relatively low, from 1.5 to 2.5 nm/min. The rate r D on a “cold” substrate varied from 4.8 to 10 nm/min. The rate r S of films with a high Bi content was higher than a rate rS of films with a lower Bi content. The rate r D was determined by the deposition time 𝜏 S and the thickness of already synthesized films h. The film thickness was determined using a Biolar PI polarization-interference microscope and MII-4 Linnik micro-interferometer or was calculated from the transmission spectra of the films. Subsequently, the rate r D was used to calculate the thickness of films of various series obtained at the same conditions and from the same target. At the post-annealing crystallization, the choice of annealing temperature T a , annealing time 𝜏 a , and heating rate during the annealing is decisive. Table 4.1 shows the optimal synthesis conditions of BiIG films on different substrates.

4.2 Fabrication Methods

According to microanalysis data, the sputtering and annealing regimes at low value of T S and T a for films of high bismuth content are optimal, which is associated with the substantial difference in the condensation temperature of bismuth-containing vapor in comparison to other garnet-forming elements. In addition, the bismuth evaporation during annealing is influenced. A detailed description of the technological process and the characteristics of BiIG films of different compositions is given [15–19]. The BiIG films grown by RIBS with post-annealing crystallizations have the best optical and MO performance for multilayer structures [16]. The temperature dependencies of FR and FHLs at the some T a for M1 and M2 films are shown in Figure 4.2a. For M2 films on GGG, increasing of T a from 500 to 680 ∘ S occurs with FR increasing from zero to maximum value −5.5 deg/μm at T a = 660 ∘ C. M2-garnet phase is completely destroyed at T a > 950 ∘ C (Figure 4.2a, curve 1). The maximum value of ΘF = −1.9 deg/μm for M1 films on GGG is achieved at the range of T a from 720 to 820 ∘ C (Figure 4.2a, curve 2). The maximum value of ΘF = −0.9 deg/μm for the M1 films on SiO2 is reached at T a range from 680 to 700 ∘ C. An interesting peculiarity of crystallization processes of the M1 films on SiO2 is the inversion of FR sign at T a > 900 ∘ C (Figure 4.2a, curve 3). This can be attributed to the thermo-activated redistribution of garnet constitutive elements and bismuth reevaporation at high temperature and garnet formation of another composition [15]. Thus, at the temperature region from 660 to 680 ∘ C, FR has maximum values for all the films. Thus, we crystallized M2 and M1 films at optimal for both compositions T a = 680 ∘ C The lattice parameter for M2 films is 1.2620 nm and these films have monocrystalline quality. M1 films on SiO2 have polycrystals with different anisotropies as confirmed by our ferromagnetic resonance (FMR) experiments [27]. Presented in Figure 4.2b,c are resonance fields as functions of azimuthal angle at rotation in plane of M1 layer (b) and bilayer M1/M2 (c) on GGG substrates. These dependencies are evidence of monocrystalline quality of M2 films on GGG, which are typical for cubic symmetry with (111) plane. The high quality of M2 films is proved by the maximum value of ΘF = −5.5 deg/μm. The cause of some resonance characteristic asymmetry in film plane is possible off-orientation of (111) plane relative to the film normal. It is interesting that the signal symmetry of structure GGG/M1/M2 differs from the signal symmetry of M2 film on GGG. Such signal symmetry of M2 film on sub-layer M1 is characteristic for the crystallographic plane (100). The FMR line width ΔH is the structural sensitivity parameter, which for M2 film on GGG is ΔH = 600 Oe and on sublayer M1 is ΔH = 100 Oe. This is because the lattice mismatch of M2 film and GGG is more than one of M2 and M1 films. In the result, the stresses in structure GGG/M2 are more than in the M1/M2 bilayer. Using atomic force microscopy data on the surface topography [19, 28], we also found that high (up to 80%) oxygen content in the gas mixture at sputtering and low heating rate r (∼2 deg/min) during crystallization annealing provided a more low surface roughness 𝜎 rms without changing the value of MO effects.

129

Table 4.1

Optimal synthesis conditions of BiIG films.

Substrate

Target

Sputtering and crystallization conditions U (kV)

J (mA)

P b (Torr)

P Ar (Torr)

PO2 (Torr)

T s (∘ S)

5 × 10−4

T a (∘ S)

𝝉 a (min)

In situ

GGG

0.5 × 10−4

520–540

—

—

M3

0.6 × 10−4

590

—

—

M4

0.5 × 10−4

560

—

—

M5

0.5 × 10−4

560

—

—

M6

0.6 × 10−4

650

—

—

0.5 × 10−4

80

M2

5

100

5 × 10−6

Post-annealing crystallization (air, atmospheric pressure) GGG

M1

5

5 × 10−6

5 × 10−4

680–690

15–20

650–680

15–20

M3

650–680

15–20

770

180

M5

730

180

M6

780

180

680–690

15–20

540–550

15–20

M4

Silica/SiO2 Quartz/SiO2

160

M2 100

M1

—

0.5 × 10−4

160

Post-annealing crystallizations (vacuum) GGG

M2

5

90–100

5 × 10−6

5 × 10−4

0.5–0.8

80

M5

0.7

520

M7

0.7

650

4.2 Fabrication Methods Hres(Oe)

1 ΘF(º/μm)

3680

3 0 –1

500

600

700

800

900

1000 Ta (ºC)

3640

–2

3620

2 (b)

–3

4800 4700 4600 4500 4400 4300

–4 –5 –6 (a)

3660

1 (c)

0 60 120 180 240 300 360 𝜑 (º) Hres(Oe)

0

60 120 180 240 300 360 𝜑 (º)

Figure 4.2 (a) Specific FR ΘF as a function on the annealing temperature T a for the films: M2 on GGG (1), M1 on GGG (2), and M1 on SiO2 (3). FHLs at some T a are shown in the insets. FHL scale for (2) and (3) is multiplied by three in comparison with (1). Azimuthal dependencies of resonance field Hres for M2 layer (b) and bilayer M1/M2 (c) on GGG. Source: Berzhansky et al. [16].

As expected, during optimization of synthesis conditions arise the technological difficulties of deposition and crystallization of garnet films with a high bismuth content (for example, M2 and M3) on the SiO2 layers, silica, or quartz substrate. To effective growth of garnet phases with high bismuth content, a two-step synthesis technology has been proposed [26, 29–31]. The sub-layer with a lower bismuth content (less than 1 at./f.u.) was deposited on SiO2 layer, silica, or quartz substrate. Then, after crystallization annealing of sub-layer, the main magneto-active layer with a higher Bi content (more than 1.5 at./f.u.) was deposited and crystallized by annealing. Due to the proposed method of crystallization, new record values of the FR angle of the microcavity and Tamm 1D-MPCs in optical wavelength range were achieved [31–33] (see also hereinafter). The state of the substrate surface determines the crystallization processes of the films and the surface roughness, structure, and properties of “substrate–film” transition layer. To determine the effect of substrate pretreatment on the properties of synthesized M2 films, the substrates were processed before the film deposition by argon and oxygen ions with varying energies and currents [18, 19]. The duration of ion pretreatment was five minutes. Some of the GGG substrates were also pre-annealed in air at the atmospheric pressure. The sputtered amorphous films were crystallized by annealing at the temperature T = 650 ∘ C for 20 minutes. According to the substrate pretreatment regimes, the films were divided into four types (Table 4.2). An ion source with a cold cathode “Radical” II-4-015 was used for substrate pretreatment and thermal activation. The dependencies of the specific FR on the film thicknesses h for three types of BiIG M2 films are shown in Figure 4.3. The corresponding FHLs are schematically shown in the insets. At room temperature, FR drops to zero for all film types when h is below 6 nm. I-type films demonstrate a negative FR and have an easy-plane or close to an easy-plane magnetic

131

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4 Bismuth-Substituted Iron Garnet Films for Magnetophotonics

Table 4.2

The regimes of the substrate pretreatment.

Film types

Gas

Ion energy

Ion current density (mA/cm2 )

Substrate pre-annealing, T a and 𝝉 a

I

Ar

Low-energy plasma

1.0

—

II

Ar

Ion beam, 1.0 keV

2.5

—

III

Ar

Ion beam, 4.0 keV

5.0

—

IV

O

Low-energy plasma

1.0

In air, 800 ∘ S , 60 min

ΘF (°/μm) 2.0 1.0 0 ‒1.0

h(nm) 10

20

30

40

50 90

120

150 180

‒2.0 ‒3.0 ‒4.0 ‒5.0 ‒6.0

II III I

Figure 4.3 Thickness dependencies of FR and FHLs for M2 films of types I, II, and III. Source: Shaposhnikov et al. [19].

anisotropy for all thicknesses in the studied range, which is typical for the thick BiIG films prepared using conventional techniques. However, the saturation magnetic field of ultrathin I-type films (H S is approximately 4.5 kOe) is much higher than that of the thick ones (H S is approximately 1.0 kOe). The FR angle approaches the bulk material value for h, above 12 nm, which is approximately 10 lattice periods. It reaches the lower limit, ΘF = −5.8 deg/μm, for h ≥ 100 nm. There is no compensation point in this type of films. The measured Curie temperature is T C = 350 ∘ C. A quite different behavior was observed for II- and III-type films. Here, there is an interval of thicknesses where FR is positive. The FR angle becomes negative only above a critical thickness hcr (10.6 nm for II-type films and 8.3 nm for III-type ones). The films display perpendicular anisotropy for all thicknesses in the [hmin , hcr ] interval. The increase of Ar+ ion energy as well as the ion-beam current density leads to the narrowing of the interval while the maximum value of FR becomes larger. Above hcr , the FR angle is negative. The observed FR sign inversion in II- and III-type films is related to the variation of T comp with the film thickness. Our measurements of T comp for II-type films show that it drops from 70 to 28 ∘ C when h increases from 9.7 to 11.2 nm. Thus, for thin films with h < hcr , the compensation point is above the room temperature, and the magnetization of their octahedral sublattices exceed the magnetization of the tetrahedral ones. Above the critical thickness, the situation

II type h = 8.7 nm h = 5.8 nm

5

A, 103 cm–1

A, 103 cm–1

10 I type

Phase diagram

4.2 Fabrication Methods

0

10

FIM I

I type II type

Ma + Mc

PM

FIM II

FIM I Md

Ma + M c

Md External H

I type

5 0

–5

II type

–5 h = 8.7 nm h = 5.8 nm

–10 400

300

500

300 400 λ, nm

(a) I type

A, 103 cm–1

4

–10 500

600

(b)

5

10

15 h, nm

20

25

II type

300 K

2

250 K 150 K 110 K 50 K 8K

0

–2

(c)

300

400

500 λ, nm

600

700

Figure 4.4 (a) MCD spectra of I- and II-type films at 300 K. (b) Thickness dependencies of amplitude of MCD long-wave peak A at 300 K (bottom) and phase diagram of films (at the top). (c) MCD spectra of 2.9 nm I-type film at 300 K and II-type film at different temperatures. The value of MCD signal is twofold increased for film of II-type at temperature 250 K and is halved for film of I-type. Source: Berzhansky et al. [18].

is reversed. A similar change of T comp with thickness is observed in III-type films. Here, the compensation point drops below room temperature at a lower thickness (of about 8 nm). The highest values of ΘF = −7.8 deg/μm at 𝜆 = 655 nm were obtained for 100-nm-thick IV-type film. The substrate pretreatment for these films were practically analogous to I-type films except for the fact that the oxygen plasma was used for the processing instead of argon plasma. As a result, the thickness dependence of FR for IV-type films is similar to I-type one. The increase of FR in these films is most probably related to lower destruction of the substrate surface at oxygen plasma treatment (decrease of breaking bond number on the surface and its amorphization). The results were confirmed by magnetic circular dichroism (MCD) measurements of the films [18, 19]. MCD spectral measurements were performed using a Jobin–Yvon dichrograph at H = 5.5 kOe in 270–850 nm wavelength range with the step 1 nm. All MCD spectra of I- and II-type films in the investigated range of thickness (at least for h > 5.8 nm) have the form that is typical for spectra of BiIG with high content of bismuth: two peaks of opposite signs and the point of intersection with the wavelength axis (Figure 4.4a) [34–36]. The positions and amplitudes of short- and long-wave peaks as well as the value of wavelength for zero point depend on the resonant frequency, half-width, and intensity (the density of active ions and oscillator strength) of MO transitions [7, 35].

133

4 Bismuth-Substituted Iron Garnet Films for Magnetophotonics

Since in the case of II-type films the substrate surface pretreatment leads to the substrate surface destruction and amorphization, the film thickness is reduced (below 15 nm), MCD spectra of II-type films demonstrate features of Ga and Gd dilution: the “blue shift” and change in the intensity of MO transitions. MCD and FR measurements at 300 K show that as II-type film thickness decreases, the series of magnetic phase transition appear (Figure 4.4b). Inversion of signs observed at the critical thickness of hcr = 11 nm suggests a concentrational spin-orientation phase transition from ferrimagnetic of I-type (I-type FIM) to ferrimagnetic of II-type (II-type FIM). According to MCD spectra measurements at different temperature for 2.9 nm II-type films, both long-range magnetic order and MCD spectra typical for iron garnet are formed at temperatures less than 130 K (Figure 4.4c). As a result, when thickness approaches 5 nm, the second phase transition from the ferrimagnetic phase II-type FIM to paramagnetic (PM) occurs. The distribution of Ga is not uniform along the film thickness, and the thickness of film–substrate transition layer of II-type films is 15 nm. The influence of the substrate surface pretreatment on the properties of I-type films is much less pronounced. MO activity at 300 K is present through all thickness ranges, even for the films of thicknesses 1.5 and 2.9 nm. Inverse effects are absent and “blue shift” is negligible. However, decreases of the intensity of MO transitions is still observed at thicknesses less than hcr = 11 nm. To clarify the reasons of inversion, the next experiment was realized [17]. At the beginning, a series of the films of II-type with h = 8.2 nm was synthesized. Then, the nanolayers of the thickness from 0.5 to 3.0 nm were deposited on these films from target of the same composition M2 and separately crystallized under the same conditions as bottom layers. The results of investigation of these bilayer nanostructures are shown in Figure 4.5. Here FHLs of the film with h = 8.2 nm (a) and bilayer nanostructures (8.2 + 0.5) nm (b), (8.2 + 1.5) nm (c), and (8.2 + 3.0) nm (d) are presented. As one can see, the film with h = 8.2 nm is characterized by the positive sign of FR and the so-called “left” FHL. The top BiIG film of thickness 0.5 nm essentially (fourfold) reduces the value of FR without leading to a change in the sign of effect (Figure 4.5b). The top BiIG films of thicknesses 1.5 nm and above considerably change the shapes of FHL (and, consequently, the type of magnetic anisotropy) and the sign of effect (Figure 4.5c,d). The reason for the sign inversion of MO effects (or MO transitions) in the vicinity of hcr is reorientation of sublattice magnetization 0.08

(a) h = 8.2 nm

(b) h = (8.2 + 0) nm

(c) h = (8.2 + 1.5) nm

(d) h = (8.2 + 3.0) nm

0.04

ΘF (°)

134

0.00

–0.04 –0.08 –2

–1

0

1

2 –2

–1

0

1

2 –2

H (kOe)

–1

0

1

2 –2

–1

0

1

Figure 4.5 FHLs of bottom 8.2 nm film (a) and bilayer structures: (8.2 + 0.5) nm (b), (8.2 + 1.5) nm (c), and (8.2 + 3.0) nm (d). Signal of substrate was substracted. Source: Berzhansky et al. [17].

2

4.2 Fabrication Methods

respectively to direction of external field, i.e. so-called spin-orientation phase transition that was discussed hereinabove (Figure 4.4b) [37]. T comp ≈ 40 ∘ S. But at small fields (up to 300 Oe), T comp is much greater. This allows controlling the compensation point by value of external magnetic field, and it is the evidence of possible existence of spin-flop phase in bilayer nanostructure in intermediate state, in which MO effects disappear and its dependencies pass through zero at hcr . The Curie temperature T S of such nanostructure is achieved at 152 ∘ S. High value of N s for nanostructure of the thickness (8.2 + 1.5) nm (Figure 4.5c) may be due to the closeness of its T comp to temperature of measurements. The compensation temperature in the bottom film of composition M2 is absent, and T S ≈ 360 ∘ S. Thus, the top film of thickness just 0.5 nm with negative sign of FR substantially changes temperature and field dependencies of initial film of thickness 8.2 nm with the positive sign of FR. Possible mechanisms of the spin-reorientation phase transition observed in the bilayer structures are as follows: (i) Concentration mechanism. After the top BiIG layer deposition, the integral content of Bi3+ and Fe3+ ions in the structure increases due to the interdiffusion of layers and exceeds threshold value at critical thickness of the top layer ∼1.5 nm and above. This results to appearance of compensation point in the investigated temperature interval and the sign inversion of FR. Stratification of composition in thickness in this case is created artificially and can be controlled. (ii) The mechanism of exchange interaction between layers. During crystallization annealing from the sputtered top film of thicknesses 1.5 nm and above, a monolayer of iron garnet of composition M2 with negative FR is synthesized on a layer of mixed composition (Bi,Gd,Y)3 (Fe,Ga)5 O12 with positive FR. Antiferromagnetic exchange interaction between the layers appears. As a result, the magnetic moment of bottom layer is reoriented, and summary FR of bilayer nanoscale structure became a negative.

4.2.2

Fabrication of Fabry–Perot 1D-MPC with BiIG Bilayer

Fabry–Perot one-dimensional magnetophotonic crystals (FP-1D-MPCs) with transparent BiIG layers are the first structures of MPCs proposed to control optical response by MO effects [38, 39]. According to experimental results obtained by various groups [39–41], all FP-1D-MPCs based on non-garnet Bragg mirrors (BMs) showed the values of MO quality factor Q not higher than 6∘ and the values of figure of merit F not higher than 4% in the range of wavelengths from 550 to 850 nm. Here MO quality factor Q and figure of merit F of 1D-MPC are determined according to [39–42] as Q = −2|ΘF |∕ ln(T) (∘ )

(4.1)

F = T ⋅ sin(2ΘF ) (%)

(4.2)

where ΘF and T are FR and transmittance of 1D-MPC, respectively. These characteristics evaluate the success of practical implementation of 1D-MPCs with MO layers.

135

136

4 Bismuth-Substituted Iron Garnet Films for Magnetophotonics

Figure 4.6 Schematic representation of FP-1D-MPCs on the basis of BiIG with m = 5.

MO layer

SiO2 TiO2

Substrate

The results of numerical calculations for configuration [Ta2 O5 /SiO2 ]m /Bi:YIG/ [SiO2 /Ta2 O5 ]m with different repetition number m potentially showed F = 12.5% [42]. In order to increase the characteristics Q and F of FP-1D-MPCs based on non-garnet BM, we used of a BiIG bilayer that allow to overcome technological difficulties of deposition and crystallization of garnet films with a high bismuth content on SiO2 layers [31]. FP-1D-MPCs with general formula [TiO2 /SiO2 ]m /M/[SiO2 /TiO2 ]m were proposed, where M is a bilayer MO defect and m is the repetition number of layer pairs in BM. The structure is shown schematic in Figure 4.6. The fabrication process of the whole structure included the following three stages [32]: (i) The synthesis of the bottom BM on the substrate by electron beam evaporation with SiO2 and TiO2 layers. (ii) The formation of magnetic films (with different thicknesses in different FP-1D-MPCs) on the bottom BM by RIBS and annealing. (iii) The synthesis of the top BM on crystallized bilayer by the technique used for the growth of the first BM.

4.2.3

Fabrication of Tamm 1D-MPC with BiIG Bilayer

Application of BiIG bilayer allows us to increase the MO effect of Tamm one-dimensional magnetophotonic crystals (T-1D-MPCs) [33, 39]. New original T-1D-MPCs with single layer and bilayer were modeled and studied. Proposed T-1D-MPCs consist of a seven-pair dielectric BM on the surface of which BiIG single layer or bilayer, SiO2 buffer layer, and Au layer are placed successively. The top Au layer has the gradient thickness. Formulas of structures are listed below: ● ●

T-1D-MPC-1: GGG/[TiO2 /SiO2 ]7 /M1/SiO2 /Au. T-1D-MPC-2: GGG/[TiO2 /SiO2 ]7 /M1/M3/SiO2 /Au.

4.2 Fabrication Methods

Au

Au

MO layer M3

MO layer M1

(a)

MO layer M1

SiO2

SiO2

TiO2

TiO2

Substrate

Substrate

(b)

Figure 4.7 Schematic illustration of synthesized T-1D-MPCs with BiIG single-layer (a) and bi-layer (b).

Schematic diagrams of configurations are shown in Figure 4.7. The buffer layer of SiO2 was used to reduce the absorption of light at the magnetic layer–metal interface. The parameters of the structures were previously calculated. The following thicknesses were used in modeling: ● ● ●

hTiO2 = 73 nm and hSiO2 = 115 nm for BM layers. hM1 = 108 nm and hbSiO2 = 140 nm for M1 and SiO2 buffer layers of T-1D-MPC-1. hM1 = 55 nm, hM3 = 178 nm and hbSiO2 = 80 nm for M1, M3, and SiO2 buffer layers of T-1D-MPC-2.

The structures with single layer and bilayer were modeled to form a Tamm plasmon polariton (TPP) mode at the center of photonic band gap (PBG) 𝜆0 = 655 nm. The values of layers component used in simulation are listed in Table 4.3. Synthesis of the structures occurred in several stages. Dielectric BM were synthesized by electron-beam evaporation. The thickness of layers was optically controlled during deposition. Iron garnet layers were fabricated by RIBS of corresponding ceramic targets in argon–oxygen mixture and crystallized in the annealing process at the air. Detailed description of the methods and conditions of garnet synthesis can be found hereinabove or in [13–16]. Au film with a gradient of thickness hAu from 0 to 70 nm along the chosen direction on the sample surface was deposited by thermal evaporation in vacuum using a technique described in [43]. The length of sample along the gradient of Au thickness was 12 mm. Therefore, the gradient of Au thickness was 5.8 nm/mm. Investigation of transmittance was carried out using an automated spectrophotometer KFK-3. Measurements of FR were performed using handmade computer-controlled spectropolarimeter by compensation method in saturation fields. The beam aperture and the gradient of Au thickness at the scale of beam aperture were, respectively, 0.1 and 0.6 nm.

137

Table 4.3

Permittivity tensor components for layers of model 1D-MPCs.

Wavelength 𝝀 (nm)

600 650 700 750

M1𝜺1xx , 𝜺1xy

M2𝜺2xx , 𝜺2xy

7.039 + 0.121 • i,

8.35 + 0.177 • i

−0.014 + 2.656 • 10−3 • i

−0.072 + 0.019 • i

6.591 + 0.066 • i,

7.863 + 0.063 • i

−9.187 • 10−3 + 2.702 • 10−3 • i

−0.037 + 0.01 • i

6.277 + 0.038 • i,

7.518 + 0.044 • i

−7.37 • 10−3 + 2.813 • 10−3 • i

−0.024 + 7.801 • 10−3 • i

6.06 + 0.021 • i,

7.279 + 0.041 • i

−6.619 • 10−3 + 2.95 • 10−3 • i

−0.018 + 7.886 • 10−3 • i

M3𝜺3xx , 𝜺3xy

SiO2 and KU-1𝜺SiO2

TiO2 𝜺TiO2

2.125

5.409

−9.34 + 1.47 • i

2.12

5.282

−12.864 + 1.198 • i

2.117

5.182

−16.519 + 1.113 • i

2.114

5.102

−20.22 + 1.122 • i

Au𝜺Au

−0.050 + 5.305 • 10−4 • i −0.027 + 1.081 • 10−4 • i −0.018 + 5.253 • 10−5 • i −0.015 + 4.191 • 10−5 • i

4.3 Properties of the Structures

4.3 Properties of the Structures 4.3.1

Magneto-optical Properties of FP-1D-MPCs

Before synthesis of structures, to define the effectiveness of FP-1D-MPCs based on a BiIG bilayer, the modeling of different configurations has been carried out. We compared the FP structures with BiIG single layer or bilayer schematically illustrated in Figure 4.8(a, b): ● ● ●

M1 with optical thickness of lM = 𝜆0 /2 (MPC1). M1/M2 of lM = 𝜆0 /2 (MPC2). M1/M2 of lM = 𝜆0 (MPC3).

Simulation results are presented in Figure 4.8c–e. In the figure, amplification factor of Faraday effect of MO layer in Fabry–Perot structure t is also shown. The value t calculated as (4.3)

t = ΘF ∕𝛼F (times)

where 𝛼 F is FR of defect layer of a certain thickness, estimated in our case from specific FR angle spectra of M1 and M2 films. The amplification factor t shows how many times FR of FP-1D-MPC exceeds FR of a BiIG bilayer. The light propagation in proposed FP-1D-MPCs was modeled by computational solution of Maxwell’s equations [44, 45]. It was considered that transverse magnetic (TM) polarized light falls perpendicularly to the structure surface. We used the antisymmetric permittivity tensor with a complex off-diagonal component for MO layers and the tensors with a diagonal component for BM layers [32]. The spectral dependencies of the permittivity components were taken into account. The components of permittivity tensor were calculated using the refractive indexes and extinction coefficients evaluated from the measured transmittance spectra of single BM [TiO2 /SiO2 ]m , M1 and M2 films [28, 32]. To calculate non-diagonal elements, the

15

(c)

(d)

TiO2

Substrate

ΘF, (arbitrary units)

SiO2

1

(e)

80 60

10

40

5

20 0

0

100

t

20

MO layer M2 or M3 MO layer M1

TiO2

15

40 5

1 2 3 4 5 6 7 8 9 10 1 2 3 4 5 6 7 8 9 10 m m

SiO2

20

80 60

10

0

MO layer M1

100

t

Q (°) ; F (%)

20

Q (°) ; F (%)

(b)

(a)

1 2 3 4 5 6 7 8 9 10 m

0

(f)

0

Substrate

–1

2

1 –2

–1

0 1 H (kOe)

2–2 –1

0 1 H (kOe)

2 –2 –1

3 0 1 H (kOe)

2

Figure 4.8 Schematic representation of FP-1D-MPCs on the basis of single layer (a) and bilayer (b) BiIG with m = 5. MO quality factor Q, figure of merit F, and FR amplification factor t of model structures MPC1 (c), MPC2 (d), and MPC3 (e) as a function of repetition number m. (f) FHLs of synthesized FP-1D-MPCs [TiO2 /SiO2 ]5 /M/[SiO2 /TiO2 ]5 with layers M1 (1), M1/M2 (2), and M1/M3 (3). Source: Berzhansky et al. [31].

139

140

4 Bismuth-Substituted Iron Garnet Films for Magnetophotonics

estimated optical parameters and experimental spectra of specific FR and MCD were used. The components are listed in Table 4.3. The design resonant wavelength is 𝜆R = 690 nm, that is, PBG center 𝜆0 , 𝜆0 = 𝜆R . The presence of substrate KU-1 with thickness of 0.5 mm was taken into account. The layer thicknesses in FP-1D-MPCs were determined as hTiO2 = 𝜆R ∕4nTiO2 , hSiO2 = 𝜆R ∕4nSiO2 , nM1 hM1 = 𝜆R ∕2, hM1∕M2 = nM1 hM1 + nM2 hM2 = 𝜆R ∕4 + 𝜆R ∕4 = 𝜆R ∕2 or hM1∕M2 = nM1 hM1 + nM2 hM2 = 𝜆R ∕4 + 3𝜆R ∕4 = 𝜆R . In Figure 4.8c–e, the dependencies of modeled Q and F versus m have analogue i correspond to optimal repetition numtype. The maximum values of Qimax and Fmax i bers of mopt (i is the index of FP-1D-MPC: i = 1 for MPC1, i = 2 for MPC2, i = 3 3 1 2 for MPC3) and satisfy to relations Q1max < Q2max < Q3max and Fmax < Fmax < Fmax . Increasing of optical thickness of MO layer from 𝜆0 /2 to 𝜆0 results in decreasing of miopt without of reduction of values of Qmax or F max . It was also shown [18] that efficiency of implementation of BiIG bilayer depends on MO figure of merit of MO layers f = QM1 /QM2 (or QM1 /QM3 ). Q and F of FP-1D-MPCs with BiIG bilayer are reduced in the case of f < 1, and using bilayer is not profitable. Q and F of configurations with m ≤ mopt are low as the substrate was taken into account. The values of Q and F for FP-1D-MPCs with m ≥ mopt are not influenced by the presence of substrate. To compare the properties of FP-1D-MPCs with different MO layers, the three structures were fabricated with m = 5 at different resonant wavelengths 𝜆R . The resonant wavelengths for FP-1D-MPCs with bilayer M1/M2 and M1/M3 are 680 and 635 nm, respectively. FP-1D-MPC only with M1 layer was made for comparison with 𝜆R = 655 nm. FP-1D-MPC 𝜆R are defined by the total thickness of MO layers: hM1 = 150 nm, hM1/M2 = 305 nm, and hM1/M3 = 260 nm. The thicknesses of dielectric layers are hTiO2 = 71 nm and hSiO2 = 103 nm for all MPCs. All layers of FP-1D-MPCs were synthesized by RIBS of corresponding targets in argon–oxygen mixture on GGG substrates. FHLs are shown in Figure 4.8f. FHLs indicate that FP-1D-MPCs of various types of the magnetic anisotropy can be implemented depending on the composition of the magnetic layer. Synthesized FP-1D-MPC with an M1 layer, characterized by an “easy-axis” anisotropy, has the maximum squareness ratio K S = 0.83 and low saturation field H S = 0.5 kOe. FP-1D-MPC with a M1/M2 layer has “easy-plane” anisotropy, K S = 0.19, H S = 1.6 kOe. FP-1D-MPC with a M1/M3 layer has intermediate values of magnetic characteristics, K S = 0.41, H S = 0.7 kOe. These magnetic properties of FP-1D-MPCs allow the use of such crystals in different devices. The best MO quality demonstrates the structure with M1/M2 layer. The characteristics of FP-1D-MPCs with M1 and M1/M2 layers are Q = 5.3∘ , F = 2.6%, t = 21 and Q = 11∘ , F = 4.3%, t = 13, respectively. The characteristics of FP-1D-MPCs with MO bilayer can be improved using the configurations with MO layer of optical thickness of 𝜆0 /2 < lM < 𝜆0 or 𝜆0 < lM < 3𝜆0 /2

4.3 Properties of the Structures

and with two resonances inside PBG at 𝜆R1 and 𝜆R2 (𝜆0 ≠ 𝜆R ). We optimized of FP-1D-MPCs with bilayer of composition M1/M2 [32]. The structure design was performed by changing the number of layer pairs in BM m and the optical thickness of BiIG bilayer lM to achieve high values of MO characteristics. We used the following parameters of structure during optimization: (i) The fixed wavelength of PBG center 𝜆0 = 690 nm. (ii) The fixed thicknesses of nonmagnetic layers hSiO2 = 116 nm and hTiO2 = 75 nm, which are close to the optical thickness l = 𝜆0 /4. (iii) The thicknesses of BiIG bilayer hM in the range from 97 to 383 nm that corresponds to the optical thickness lM ranging from (0.74⋅𝜆0 /2) to (3⋅𝜆0 /2). The defect thickness was increased by changing the thickness of M2 layer hM2 from 29 nm (0.24⋅𝜆0 /2) to 315 nm (2.5⋅𝜆0 /2). The thickness of M1 layer hM1 was fixed and chosen such that in the experiment, quality deterioration of the garnet bilayer on SiO2 did not occur (lM1 = 𝜆0 /4, hM1 = 68 nm) [27, 31]. The results of calculation are shown in Figure 4.9. From the figures, we notice that for the structures with the optical thickness of magnetic layer in the vicinity of (𝜆0 /2), the largest FR enhancement and specific FR are observed for the first I peak. For the structures with the optical thickness of magnetic layer in the vicinity of (2.5⋅𝜆0 /2), the second II and third III peaks coexist; and high values of its Q and F can be achieved at the same time. Optimum numbers m, at which FP-1D-MPC have high Q and F at resonance wavelength for investigated thickness range, are mopt = 4 for the first I and second II resonance peaks and mopt = 3 for the third III peak. Therefore, the structures with maximum values of F and Q for resonance peaks I and II and the optimum number mopt = 4 and the structures with high t, non-zero value of F, and m = 7 were experimentally implemented. FP-1D-MPCs with bilayer M1/M2 and m = 4 and m = 7 on the quartz substrate were fabricated. Optical thicknesses of the bilayer were (0.8⋅𝜆0 /2), (1.2⋅𝜆0 /2), (1.8⋅𝜆0 /2), and (2.5⋅𝜆0 /2) for m = 4 and (0.8⋅𝜆0 /2), (1.2⋅𝜆0 /2), (1.6⋅𝜆0 /2), and (2.5⋅𝜆0 /2) for m = 7 (𝜆0 = 690 nm). Transmittance and FR spectra of synthesized FP-1D-MPCs with M1/M2 bilayer are presented in Figure 4.10. Measurements were carried out at room temperature by a spectral device based on a KSVU-6 system with a double-diffraction MDR-6 monochromator [32]. The magnetic field applied during FR measurements was significantly higher than a saturation field of FP-1D-MPC with bilayer of composition used (H s = 1.6 kOe). Evidently, as the defect thickness increases from (0.8⋅𝜆0 /2) to (3⋅𝜆0 /2), the three resonance peaks corresponding to the resonance conditions (𝜆R /2), first I peak; (𝜆R ), second II peak; and (3⋅𝜆R /2); third III peak pass the PBG. The following maximum values of MO quality factor 15.1∘ for 624 nm, specific FR −113 deg/μm (that exceeds in 62 times the FR of bilayer film) for 654 nm, and absolute FR −20.6∘ for 626 nm were achieved for three different configurations of the structure with m = 4, lM = (2.5⋅𝜆0 /2); m = 7, lM = (0.8⋅𝜆0 /2); and m = 4, lM = (2.5⋅𝜆0 /2), respectively. According to calculations of electric field intensity inside FP-1D-MPCs with m = 4, lM = (0.8⋅𝜆0 /2) at resonance wavelengths, the asymmetry of distribution

141

-15 -10 -5 0 80

(c)

60

20 0

3

7 4 3 1

I peak

7 5 4 3 1

7 5 4 3 1

5 4 3 1 III peak

7 5 4 3 1

15 12 (d) 9 5 6 6 3 7 0 21 18 (e) 15 12 5 6 9 7 6 0.6 0.8

I peak 4 3 2 1

I peak 4

3 2 1

1.0

1.2

1.4

1.6

1.8

lM *(λ0/2)

1.5 1.8 2.1 2.4 2.7 3.0 lM *(λ0/2)

15 II peak 4 12 (f) 9 5 6 6 3 7 0 21 II peak 4 18 (g) 5 15 6 12 9 7 6 1.4 1.6 1.8 2.0 2.2 2.4

3 2 1

3 2 1 2.6

2.8

lM *(λ0/2)

Q (°)

F (%)

0.6 0.9 1.2

II peak

4 5 7 III peak 7

Q (°)

5 7 II peak 5

7 5 4 3 1

40

3 4

F (%)

T (%) ΘF (°)

-20

1.5 1.8 2.1 2.4 2.7 3.0 II peak III peak 1 1

Q (°)

0.6 0.9 1.2 I peak (a) 80 1 3 60 4 40 5 20 7 0 -30 (b) I peak -20

100

F (%)

4 Bismuth-Substituted Iron Garnet Films for Magnetophotonics

t

142

15 3 III peak 12 (h) 2 9 4 6 1 5 3 6 7 0 21 III peak 3 2 18 (i) 4 1 15 5 12 6 9 7 6 2.2 2.4 2.6 2.8 3.0 3.2 Optical thickness of MA defect layer lM *(λ0/2)

Figure 4.9 Calculated transmittance T (a), FR angle ΘF (b), FR enhancement factor t (c), MO figure of merit F (d, g, i), and MO quality factor Q (e, f, h) of FP-1D-MPCs with M1/M2 bilayer on fused quartz substrate as a function of the repetition numbers m = 1, 3, 4, 5, 7 and the optical thickness of bilayer lM for three resonance peaks corresponding to conditions (𝜆R /2), first I peak; (𝜆R ), second II peak; and (3𝜆R /2), third III peak. The arrows indicate the positions at which the peaks pass the PBG center 𝜆0 . Source: Mikhailova et al. [32].

occurs (Figure 4.11). The effects of light localization in the structure reduce the influence of magnetic layer absorption at resonance. The most influence on the properties of structures is provided by “SiO2 –M1” and “M2–SiO2 ” interfaces. The obtained values of MO quality factor Q and figure of merit F of proposed FP-1D-MPCs exceed approximately in two times the same characteristics of known FP-1D-MPCs based on non-garnet BM in the wavelength range from 550 to 850 nm [39]. New FP-1D-MPCs provide not only high FR angles but also essentially lower MO quality factor compared to those of all-garnet structures [40, 41]. MO efficiency of FP-1D-MPCs can be modified by the utilization of gold coatings, gratings, or embedded nanoparticles, leading to interplay of optical resonances of different nature [39, 47, 48].

4.3 Properties of the Structures -20

100 m=4

(a) 50

-10

0 100

2.5

0 -10 ΘF (°)

0 100

-10

1.2

50

1.8

0

0 100

IM*(l0/2)

IM*(l0/2)

1.8

50 T (%)

m=4

(b)

2.5

1.2

0

50

-10

0.8

0.8 0

0 400

500

600 700 𝜆 (nm)

800

500

600 700 𝜆 (nm)

800

100 50

-20

m=7

(c)

2.5

0 100

m=7 2.5

0 -20

50

1.8

50

1.2

0 100

-10 ΘF (°)

0 100

1.8

0 -20 -10

IM*(l0/2)

IM*(l0/2)

T (%)

(d)

-10

1.2

0 -20

50

0.8

0 400

-10

0.8

0 500

600

𝜆 (nm)

700

800

500

600

𝜆 (nm)

700

800

Figure 4.10 Transmittance (a, c) and FR (b, d) spectra of synthesized FP-1D-MPCs on KU-1 with M1/M2 bilayer. Source: Mikhailova et al. [32].

4.3.2

Magneto-optical Properties of T-1D-MPCs with BiIG Bilayer

A characteristic feature of the spectra of T-1D-MPC-1 and T-1D-MPC-2 presented on Figure 4.7 is the presence of a pronounced resonant peak (Figure 4.12), the position of which depends on the thickness of Au layer. In the calculated spectra with increasing of Au thickness hAu from 6.1 to 65.2 nm, the blue shift of TPP mode takes place. Nevertheless, the red and blue shifts of resonance occur in the experiment at 6.1 nm < hAu < 30 nm and 30 nm < hAu < 65.2 nm, respectively. The red shift of the mode results from the changes of granularity, surface roughness, and continuity of Au coating (structural changes) with increase of its thickness, which leads to a change of optical properties of coating. Blue shift of TPP modes is caused by the change in the coating thickness itself, since in the simulation we assume smooth boundaries, continuity, and non-gradient thickness of Au layer. The surface roughness of layers, discontinuity, and thickness gradient of Au coating in the beam aperture in the experiments lead to increasing of half-width of the resonant peaks. Experimental and calculated spectra of the T-1D-MPC-2 without Au coating are identical and have the peak at 675 nm. The maximum resonant FR value were achieved for T-1D-MPC-2 −12.3∘ at 645 nm for configuration with hAu = 65.2 nm. Calculated spatial distribution of electric field intensity in T-1D-MPC-2 with Au coating of different thickness, T-1D-MPC-2 without Au coating, BM with magnetic

143

4 Bismuth-Substituted Iron Garnet Films for Magnetophotonics

Electric field intensity (relative units) 0 2 4 6 8 10 12 14 16 18 20 1600

TiO2 SiO2

1400 Structure depth (nm)

144

1200 1000

TiO2 SiO2 TiO2 SiO2 TiO2

600

SiO2 M1 M2 SiO2 TiO2

400

SiO2 TiO2

800

SiO2

200

TiO2 SiO2 TiO2

Figure 4.11 Calculated distribution of electric field intensity inside FP-1D-MPC KU-1/[TiO2 /SiO2 ]m /M1/M2/[SiO2 /TiO2 ]m with m = 4 and lM = (0.8⋅𝜆0 /2) at the first I resonance peak. Source: Berzhansky et al. [46].

layers, and single BM is shown in Figure 4.12i. The distribution is classical for TPP mode. Distribution in magnetic bilayer is characterized by asymmetry as in the case of FP-1D-MPCs. The maximum intensity is observed in the layers adjacent to the plasmonic coating and for configuration with thickness of Au layer of 28.3 nm. This is in good agreement with the experimental results and indicates that a coating thickness is optimal for such type of structure. Consequently, the resonances on TPP have the maximum optical quality factor and transmittance at the vicinity of Au thickness of 30 nm for both structures T-1D-MPC-1 and T-1D-MPC-2. MO quality factor Q of T-1D-MPC-1 and T-1D-MPC-2 increases from 0∘ to 0.96∘ and from 0.55∘ to 5.01∘ as the thickness of Au layer increases from 0 to 65.2 nm. Obtained values of Q are higher than Q of the first magnetophotonic Tamm structure [SiO2 /BiIG]5 /Au (Q = 0.58∘ ) [39]. So the optimal synthesis conditions of ultrathin BiIG garnet films, deposited by RIBS, were observed. It was shown that temperature of crystallization annealing and the regimes of substrate pretreatment had substantial effects on the MO properties of films. To effective growth of garnet phases with high bismuth content, two-step synthesis technology has been proposed. The sub-layer with a lower bismuth content (less than 1 at./f.u.) was deposited on SiO2 layer, silica, or quartz substrate. Then, after crystallization annealing of sub-layer, the main magneto-active layer with a higher Bi content (more than 1.5 at./f.u.) was deposited and crystallized by annealing. Due to the proposed method of crystallization, new record values of the FR angle of the Fabry–Perot and Tamm 1D-MPCs in optical wavelength range were achieved. MO effect of synthesized structures exceeds of the same values of known analogue.

4.3 Properties of the Structures 100

hAu = 0 nm

hAu = 65.2 nm

hAu = 8.6 nm

0

Bragg mirrors

–0.5

ΘF (°)

TPP mode

2.0

60 T (%)

1.5

–1.0

0.0

600

650 700 λ (nm)

12.5 times

400

800

1200

1600

0

750

8 4

600

700

800

hAu = 65.2 nm

hAu = 6.1 nm

T (%)

3

0

λ (nm)

8 4

hAu = 65.2 nm hAu = 28.3 nm

–1.0

hAu = 58.6 nm

2

hAu = 6.1 nm

1 0

700

–0.5

TPP mode

4

20

680

Bragg mirrors

5

60

660

and hSiO2 = 0 nm

ΘF (°)

80

640

0.0

hAu = 0 nm

hAu = 8.6 nm

620

(b) hAu = 0 nm

hAu = 28.3 nm

40

600

900

λ (nm)

(a) 100

T (%)

1600

–2.0

500

hAu = 0 nm

–1.5 600

650 700 λ (nm)

15 times

hAu = 0 nm

750

and hSiO2 = 0 nm

–2.0

0 500

600

700

800

600

900

100

hAu = 65.2 nm

hAu = 0 nm

hAu = 28.3 nm

hAu = 0 nm

hAu = 6.1 nm

1.5

650 700 λ (nm)

750

700 λ (nm) hAu = 65.2 nm

0

hAu = 8.6 nm

5

TPP mode

–4

hAu = 65.2 nm hAu = 28.3 nm

–6

hAu = 58.6 nm hAu = 6.1 nm

9 times

2

hAu = 0 nm

–8 600

700

0

650 700 λ (nm)

750

hAu = 0 nm and hSiO2 = 0 nm

–10

0 500

680

660

λ (nm)

–2

3

0

640

hAu = 0 nm

1

20

0

Bragg mirrors

4 T (%)

40

620

(h)

and hSiO2 = 0 nm

hAu = 6.1 nm

60

Air

Structure depth (nm)

600

900

hAu = 0 nm

hAu = 28.3 nm

80

800

ΘF (°)

600

0

4

–12

500

4

hAu = 6.1 nm hAu = 0 nm

–10

0

100

8

8

hAu = 58.6 nm

10 times

(e)

0

0

hAu = 28.3 nm

–6 –8

600

4

4 hAu = 65.2 nm

1.0

0.0

700

8

8

–4

0.5

20

680

0

Bragg mirrors

TPP mode

2.0 T (%)

40

660

λ (nm)

–2

ΘF (°)

2.5

60

640

0

and hSiO2 = 0 nm

hAu = 8.6 nm

80

620

(d)

λ (nm)

(c)

T (%)

hAu = 6.1 nm

–1.5

0

T (%)

1200

hAu = 58.6 nm

0.5

20

800

4

hAu = 28.3 nm

1.0

40

400

(i)

8 hAu = 65.2 nm

Electric field intensity (relative units)

T (%)

and hSiO2 = 0 nm

hAu = 6.1 nm

2.5

0.0

hAu = 0 nm

hAu = 28.3 nm

80

–12 600

700 λ (nm)

800

900

600

620

640

660

680

700

λ (nm)

Figure 4.12 Measured (a, b, e, f) and calculated (c, d, g, h) transmittance and FR spectra of synthesized BM, BM with BiIG layers, and T-1D-MPC-1 (a, b, c, d) and T-1D-MPC-2 (e, f, d, h) as a function of the thickness of Au layer. (i) Calculated spatial distribution of electric field intensity in T-1D-MPC-2 with different Au thickness, T-1D-MPC-2 without Au coating, BM with BiIG layers, and a BM. Source: Mikhailova et al. [33]. ©2018, EDP Sciences.

4.3.3 An increase of the Magneto-optical Response in the Ultrathin Films A study of the MO response in ultrathin films has drawn attention of researchers just recently. In the pioneer work [49] published in 2015, the authors demonstrated the monotonic increase in SFR with the reduction of the iron garnet film thickness. The effect appears when the film thickness is about 100 nm and shows the dramatic growth when it reduces up to 20 nm. As soon as the SFR directly depends on the MO response in the material, the SFR growth in the ultrathin films indicates that the MO response also arises in them. Besides the Faraday effect, one can address another MO effects to explore the dependence of the MO properties of the film on its thickness. For instance, the plasmon-enhanced transverse magneto-optical Kerr

145

146

4 Bismuth-Substituted Iron Garnet Films for Magnetophotonics

effect (TMOKE) [50] is known to be very sensitive to the properties of the material. So this effect allows to measure and analyze the MO response of even a few nanometer-thick iron garnet films. On the contrary, the Faraday effect is cumulative, and in such extremely thin layers, it turns out to be very small and is hardly detectable. However, MO analysis by means of the TMOKE requires a fabrication of the plasmonic grating on top of the film. Here we review the investigation of the MO properties by means of both techniques, plasmon-enhanced TMOKE and Faraday effect. In Ref. [51], the MO response in ultrathin film was studied by means of the plasmon-enhanced TMOKE in bismuth-substituted iron garnet films of thickness less than 60 nm with a one-dimensional gold grating cover. It was shown that the differences of the TMOKE values appearing when the film thickness is decreased can be explained by a variation of the non-diagonal element of the dielectric tensor and by localization of the surface plasmon polaritons (SPPs) at the metal–dielectric interface. For the experimental studies, Bi0.8 Gd0.2 Lu2 Fe5 O12 films of thicknesses 19, 46, and 60 nm were used. These samples were grown by a liquid-phase epitaxy (LPE) on (100) GGG (Gd3 Ga5 O12 ) substrates. The films exhibit planar magnetic anisotropy and saturation magnetization of 4𝜋M s = 1800 G. All the samples were made from the same original magnetic film by a sequential etching in an ortho-phosphoric acid bath with a slow rotation rate to ensure uniform thickness. Thickness uniformity was verified by probing different points of the sample surface via transmission electron microscopy (TEM). To fabricate the magnetoplasmonic crystal (Figure 4.13), an 80-nm-thick gold layer was deposited on the magnetic films by the magnetron sputtering. A resistive mask was then formed on top via electron-beam lithography, and gold-layer patterning was performed by ion etching in an argon-ion plasma single-frequency discharge. The pattern is a periodic array of slits, and its period was determined by preliminary numerical modeling to provide the extrema of the TMOKE resonances at the same frequencies for the different samples. This allows one to avoid the dielectric dispersion impact and to isolate the influence of the magnetic garnet film thickness on the MO response. The fabricated plasmonic gratings had the following periods and widths of the slits, respectively: d = 324 nm, r = 85 nm for the 60-nm-thick film; d = 322 nm, r = 75 nm for the 46-nm-thick film; and d = 347 nm, r = 60 nm for the 19-nm-thick film. All three samples demonstrate pronounced TMOKE resonances in the same wavelength range. The addressed MO effects were measured over a wide range of wavelengths (visible and near-IR ranges) and incidence angles. The sample was placed in a uniform external magnetic field of 2000 Oe along y-axis in Figure 4.13 generated by the electromagnet. The applied magnetic field exceeded the magnetic field required to saturate the magnetization of the iron garnet films under consideration and guarantees the reproducibility of the results. The light was collimated after it exited the sample and detected with the spectrometer. A 2D charge-coupled device (CCD) camera in the spectrometer was used to observe the spectral decomposition along one axis

4.3 Properties of the Structures

Figure 4.13 The scheme of the plasmonic crystal. Ultrathin LPE-grown bismuth-substituted iron garnet film on GGG substrate and covered by sub-wavelength gold grating is illuminated by obliquely incident p-polarized light.

p-Polarized light

z

y x

Gold grating

Ultrathin iron garnet film

SPP

M

GGG substrate

and the incidence angle decomposition along the perpendicular axis. Therefore, the angular and wavelength resolved transmission spectra of all samples for two opposite directions of the magnetic field were measured. Each measurement with alternating opposite directions of the magnetic field was repeated 200 times, and then these results were averaged. This regime provided reproducibility of the measurements, with a signal-to-noise ratio exceeding three orders of magnitude in the spectral range of our interest. Based on these spectra, one can find 𝛿, the value of the TMOKE, as a relative change of the transmitted light intensity T(M) when the structure is re-magnetized [7]: 𝛿=2

T(M) − T(−M) T(M) + T(−M)

(4.4)

The measured wavelength- and angular-resolved transmission (left column) and TMOKE (right column) spectra of the three samples are given in Figure 4.14. Excitation of SPPs at the [gold]/[ferromagnetic dielectric] interface leads to the dips in the transmission spectra from the calculation of the SPP dispersion based on the phase synchronism condition (dashed lines in Figure 4.14) [52]. The corresponding numerical simulations performed by the rigorous coupled-wave analysis (RCWA) [53, 54] yield the same experimental results with high accuracy. In Figure 4.14a, secondand third-band SPPs are observed. The transverse in-plane magnetic field spectrally shifts the transmission dips either toward lower or higher frequencies depending on the direction of the magnetic field with respect to the normal to the sample surface and SPP wavevector. As a result, near the frequencies of SPP, excitation resonances in the TMOKE spectra are noted with an S-shape with positive and negative maxima where the TMOKE reaches 0.04. The TMOKE spectra is antisymmetric with respect to the normal incidence when TMOKE vanishes due to symmetry reasons. Although the thicknesses of the magnetic garnet films in the plasmonic crystals under consideration are different from each other, the resonance positions in all three cases are almost the same due to the proper choice of the grating periods.

147

4 Bismuth-Substituted Iron Garnet Films for Magnetophotonics 950

60-nm-thick film transmission

0.10

60-nm-thick film TMOKE

950

0.040

0.09 900

0.032

900 0.08

0.024 850

0.07 0.06

800

0.05 750

0.04 0.03

700

Wavelength (nm)

Wavelength (nm)

850

0.016 0.008

800

0.000 750

–0.008 –0.016

700

–0.024

0.02 650 600 –5

0.00 0

5

10 15 Angle (°)

20

600

25

–0.032 –5

–0.040 0

5

10 15 Angle (°)

20

25

(b) 46-nm-thick film transmission

46-nm-thick film TMOKE

0.80

900

0.040

900

0.72

850

0.032

0.64

0.024

850

0.56 800 0.48 750

0.40 0.32

700

0.24 650

0.08 5

10 15 Angle (°)

20

0.000

700

–0.008 –0.016 –0.024 –0.032

600 –5

25

–0.040 0

5 10 Angle (°)

15

20

25

(d)

(c) 950

0.008 750

0.00 0

0.016

800

650

0.16

600 –5

Wavelength (nm)

Wavelength (nm)

650

0.01

(a)

19-nm-thick film transmission

0.40

950

19-nm-thick film transmission

0.030

0.36

900

0.024 900

0.32 850

0.018 850

0.28 0.24

800

0.20 750

0.16 0.12

700

Wavelength (nm)

Wavelength (nm)

148

0.012 0.006

800

0.000 750

–0.006 –0.012

700

–0.018

0.08 650

650

0.04 600 –5

(e)

0.00 0

5

10 15 Angle (°)

20

600 –5

25

–0.024 0

5 10 Angle (°)

15

20

25

–0.030

(f)

Figure 4.14 Wavelength and angular resolved transmission (a, c, e) and TMOKE (b, d, f) spectra of the 60-nm-thick (a, b), 46-nm-thick (c, d), and 19-nm-thick bismuth-substituted iron garnet films (e, f). White dashed lines show the SPP dispersion calculated in the simplified model of smooth interfaces. Gold grating periods are 324 nm (60-nm-thick film), 322 nm (46-nm-thick film), and 347 nm (19-nm-thick film). Source: Borovkova et al. [51].

4.3 Properties of the Structures

If the magnetic garnet films are thick enough, then the TMOKE hardly depends on the film thickness (compare the TMOKE spectra for the 60-nm- and 46-nm-thick films in Figure 4.14b,d). However, for thinner films, the TMOKE tends to decrease. Nevertheless, the decrease is not very pronounced so that the TMOKE maximum drops to 0.03 for the 19-nm-thick film (Figure 4.14f). Such behavior in the TMOKE can be due to the changes in the magnetic garnet film permittivity tensor and modifications of the SPP modes caused by the decrease in film thickness. The physical origins of the observed TMOKE were revealed by an electromagnetic modeling based on RCWA. The refractive indices of gold, GGG, and BiIG were taken from [40, 55]. Geometrical parameters of the gold grating were measured by TEM imaging. There remained only one unknown parameter: the off-diagonal components of the magnetic garnet film permittivity tensor, 𝜀1 . Therefore, 𝜀1 were found by matching the calculated and experimentally measured transmission and TMOKE spectra. In Figure 4.15 (dots), the spectral range between 0.7 and 0.9 μm in wavelength is shown in detail where two TMOKE S-shaped resonances are present. These are caused by the SPPs propagating in opposite directions that makes their signs opposite to each other. At 10∘ incidence, the two resonances are relatively close so that a slight overlap between them takes place. For 20∘ incidence, the resonances are sufficiently separated and do not interfere with one another. It should be noted that 𝜀1 found for all three samples have similar dispersion but are different in values. Particularly, at 𝜆 = 0.754 μm, close to one of the TMOKE resonances, 𝜀1 = 0.0084 for the 60-nm-thick film, 𝜀1 = 0.0088 for the 45-nm-thick film, and 𝜀1 = 0.0102 for the 19-nm-thick film. Therefore, TMOKE measurements indicate a slight growth in the MO gyrotropy parameter of the film as the thickness decreases. This agrees with the previous results obtained from Faraday effect measurements [49]. Interestingly, in spite of the growth of 𝜀1 , the TMOKE decreases for thinner films. As TMOKE is related to the plasmonic resonances, this phenomenon can be understood by analyzing the SPP modes in these structures. The spatial distribution of the electromagnetic field at the SPP resonance (Figure 4.16) indicates that the penetration depth of the SPP wave is comparable with the thickness of the magnetic garnet films in the samples. The SPP field amplitude decreases by a factor of e at the depth of about 40 nm. In plasmonic crystals with 60-nm-thick and 46-nm-thick films, SPP field is localized mostly inside the ultrathin magnetic garnet film. However, in the 19-nm-thick film, a significant part of the SPP field penetrates into the nonmagnetic GGG substrate. This diminishes the influence of the magnetic field on the SPP in the 19-nm-thick film sample, the SPP frequency is shifted by a smaller value, and, consequently, the TMOKE decreases even though 𝜀1 is a bit larger. If 𝜀1 is assumed to be independent of the film thickness, then the TMOKE decrease in the thinner films is more pronounced from the calculated curve in Figure 4.16d. Further decrease in film thickness makes the TMOKE smaller, but it has relatively large values even for the nanometer-thick films. For example, for a 2-nm-thick film sample, the TMOKE is 1.8 × 10−3 that can be easily detectable. It should be noted that the Faraday effect for such films is very small (2.7 × 10−6 deg for the 2-nm-thick film)

149

4 Bismuth-Substituted Iron Garnet Films for Magnetophotonics

0.06

60-nm-thick film, incident angle 20°

0 –0.02 –0.04 0.65

Experiment Model

0.02 0 –0.02

0.7

0.75 0.8 0.85 Wavelength (µm)

–0.04 0.65

0.9

(a)

0.7

0.75 0.8 0.85 Wavelength (µm)

0.9

(b)

0.04

46-nm-thick film, incident angle 20°

46-nm-thick film, incident angle 10° 0.04

Experiment Model

0.03 0.02

0.02

0.01

0.01

0 –0.01

0 –0.01

–0.02

–0.02

–0.03

–0.03

–0.04 0.65

0.7

0.75

0.8

0.85

0.9

Experiment Model

0.03

TMOKE

TMOKE

60-nm-thick film, incident angle 10°

0.04 TMOKE

0.02

TMOKE

0.06

Experiment Model

0.04

–0.04 0.65

0.95

0.7

Wavelength (µm)

0.75

0.8

0.85

0.9

0.95

Wavelength (µm)

(c)

(d) 0.03

19-nm-thick film, incident angle 20°

0.03

Experiment Model

0.02

19-nm-thick film, incident angle 10° Experiment Model

0.02 0.01 TMOKE

0.01

TMOKE

150

0

–0.01

–0.01 –0.02 0.65

0

–0.02 0.7

0.75

0.8

0.85

0.9

0.95

–0.03 0.65

(e)

0.7

0.75

0.8

0.85

0.9

0.95

Wavelength (µm)

Wavelength (µm)

(f)

Figure 4.15 Theoretical (solid curves) and experimental (dots) TMOKE spectra for 60-nm-thick films (a, b), 46-nm-thick films (c, d), and 19-nm-thick films (e, f) for 20∘ and 10∘ incident angles. Gold grating periods are 324 nm (60-nm-thick film), 322 nm (46-nm-thick film), and 347 nm (19-nm-thick film). Source: Borovkova et al. [51].

and hardly measurable. However, for the ultrathin films, more than 10-nm-thick Faraday effect provides high-precision tool along with the TMOKE. So far, an increase of the MO response with the decrease of the iron garnet film thickness has been considered in assumption of the constant distribution of the MO effect inside the ultrathin film. To refine the model and find out the most optimal distribution of the MO response inside the ultrathin film, two hypotheses have been proposed. The proposed models were verified by means of the compositional

4.3 Properties of the Structures 60-nm-thick film, field intensity

0

46-nm-thick, field intensity

9

9

0

8

8

7

0.05

7

0.05

5

0.1

4

6 z (μm)

z (μm)

6

5

0.1

4

3 0.15

3 0.15

2

2

1 0.2

0

0.1

0.2 x (μm)

0.3

1 0.2

0

(a) 0

0.2 x (μm)

0.3

0

0.035

9 8

0.03

6 5

0.1

4 3

0.15

max(TMOKE)

7

0.05 z (μm)

0.1

(b) 19-nm-thick film, field intensity

0.2

0.025 0.02 0.015 0.01

2 0.005

1

(c)

0

0

0.1

0.2 x (μm)

0.3

0

0

0

10°. incidence λ = 0.754 μm 10 20 30 50 60 40 Magnetic garnet filim thickness (nm)

(d)

Figure 4.16 The SPP wave field distribution in the plasmonic crystal with (a) 60-nm-thick, (b) 46-nm-thick, and (c) 19-nm-thick bismuth-substituted iron garnet film. White dashed lines mark the contours of the gold grating and the ferromagnetic film. Gold grating periods are 324 nm (60-nm-thick film), 322 nm (46-nm-thick film), and 347 nm (19-nm-thick film). Incidence angle is 20∘ . (d) Calculated dependence of the TMOKE maximum value at 𝜆 = 0.754 μm on the magnetic film thickness. Source: Borovkova et al. [51].

analysis and experimental measurements of Faraday effect and compared with the simplest model of constant gyrotropy in the film. The addressed theoretical models are given in Figure 4.17. In Model 1 (green line), the MO response of the ultrathin film is supposed to be constant as it was assumed above. Model 2 (blue line) takes into account that the dielectric properties do not change instantly at the [Bi-substituted iron garnet]/[GGG substrate] interface. Actually, the compositional analysis (see Figure 4.18) reveals that there is a thin (about 30-nm-thick) transitional layer, where the distribution of the elements typical for the iron garnet changes gradually to the substrate composition. Model 3 (red line) contains two peculiarities, namely, the gradual decrease of the dielectric properties at the [Bi-substituted iron garnet]/[GGG substrate] interface and the sharp increase of the gyrotropy at [air]/[Bi-substituted iron garnet] interface. There were fabricated two sets of the samples. These films with Bi0.8 Gd0.2 Lu2 Fe5 O12 and Bi0.7 Gd0.4 Lu1.9 Fe4.1 Ga0.9 O12 per formula unit (pfu) were grown by LPE on GGG (100)-oriented substrates. The films for any set of measurements were all taken from the same wafer and wet-etch-thinned-down sequentially in ortho-phosphoric acid, thus avoiding possible composition differences due to

151

4 Bismuth-Substituted Iron Garnet Films for Magnetophotonics

g BiREIG

15 nm

hˈ/2

hˈ/2

15 nm

Air

Model 1

GGG

g=const

Model 3

Model 2

0

h (nm)

Figure 4.17 Schematic depiction of the gyrotropy parameter g (vertical axis) distribution across the film (horizontal axis) in the three models. BiREIG stands for Bi-rare-earth-substituted iron garnet, and GGG stands for gadolinium–gallium garnet substrate. Source: Levy et al. [56]. ©2019, The Optical Society.

5.0

Ga Substrate

Film

(pfu)

(a.u.)

152

Bi

(a)

0

20 40 60 Nanometers

Film LU

1.0 0.0

‒40 ‒20

Fe

Ga 4.0 Substrate 3.0 Gd 2.0

80

Bi ‒100

0

100 Nanometers

200

300

(b)

Figure 4.18 (a) Bi and Ga concentrations in Bi0.8 Gd0.2 Lu2.0 Fe4.5 O12 (Sample 1), arbitrary units, determined by SIMS on a 60-nm-thick film. Bi concentration is shown multiplied by factor 10 for the sake of clarity. (b) Elemental concentrations (pfu) for Bi, Lu, Gd, Ga, and Fe in Bi0.7 Gd0.4 Lu1.9 Fe4.1 Ga0.9 O12 (Samples 2 and 3) determined by cross-sectional S-TEM EDX. Both show uniform concentrations as a function of position above (film) and below (substrate) the transient layer. The zero of horizontal coordinates is set at the film–substrate interface. Source: Levy et al. [56]. ©2019, The Optical Society.

growth conditions. The bulk magneto-optic gyrotropy parameter g, obtained from measurements in 2-μm-thick films, is different for the two different composition. For Bi0.7 Gd0.4 Lu1.9 Fe4.1 Ga0.9 O12 pfu films (Samples 2 and 3), the bulk is g = 0.029 + 0.003i, and for Bi0.8 Gd0.2 Lu2 Fe5 O12 (Sample 1), it is g = 0.038 + 0.004i. Compositional secondary-ion mass spectroscopy (SIMS) measurements were performed with O2 + bombardment and positive ion detection mode using an IMS-7f (CAMECA) microanalyzer. The primary beam was rastered over an area of 125 × 125 μm2 , and the secondary ions were collected from the central part of this area (diameter of 33 μm). In order to avoid charging effect, the sample was coated with a layer of gold (50 nm), and electron flooding was employed using a normal incidence e-gun. Electron micrographs and energy dispersive X-ray (EDX) spectroscopy maps were obtained on Bi0.7 Gd0.4 Lu1.9 Fe4.1 Ga0.9 O12 (Samples 2 and 3) using an FEI Titan Themis aberration-corrected S-TEM operated at 200 kV. The point resolution in this aberration-corrected mode is 0.08 nm. The microscope is fitted with a Super-XTM

4.3 Properties of the Structures

X-ray detector, which is a combination of four detectors for fast X-ray mapping in S-TEM mode. For the present experiment, 1-nm resolution EDX maps were taken with an average beam current of 100 pA. The size of the maps was 512 × 512 pixels, and 50 μs/pixel dwell time was used for collecting the signal. All maps are generated by summing over 10 frames. Drift correction during data collection and subsequent analysis were performed using Velox software. These results are shown in Figure 4.18b, with a standard deviation of ±0.05 pfu for Bi and ±0.11 pfu for Fe. In Figure 4.18a, one can see how the concentrations of Bi and Gd depends on the depth of the nanostructure. These elements have been chosen as soon as they are key elements and allow to distinguish the magnetic layer and nonmagnetic substrate. From left to the right, one can observe how the concentration of Gd decreases to 0 and at the same time the concentration of Bi grows until the saturation value. The layer where the atoms of Gd disappear and atoms of Bi appear can be considered as a transitional layer. Its thickness is about 30 nm. Based on these data, Model 2 (Figure 4.17) has been proposed. The Faraday effect has been measured experimentally in the addressed ultrathin films. Light from a continuous-wave (CW) laser source was used to probe the FR at 532 nm. Measurements normal to the film surface were conducted in a rotating-polarizer configuration. Faraday-rotation hysteresis loops were recorded for each sample, and the FR at saturation was chosen to characterize the response of each film. The paramagnetic signal of the GGG substrate was subtracted out from the overall FR signal. The experimental setup is described in [49]. Film thickness was measured via ellipsometry, and FR per unit length is plotted in Figure 4.19 for both sets of samples as a function of thickness. For the ferrimagnetic films thinner than 50 nm, the SFR grows dramatically with a decrease in film thickness. One can see that the SFR reaches 10 ∘ μm−1 in the 20-nm-thick film, almost 2.5 times greater than SFR in the same thick film. For the thin films we observe oscillations due to interference of the emerging waves because of multiple reflections inside the thin film. The obtained experimental results presented in Figure 4.19 have been analyzed by the theoretical model based on classical electrodynamics (see also [56]). Linearly polarized light impinges on the magnetic film at normal incidence. The magnetization is directed along the propagation direction, perpendicular to the film surface. Refractive indices and film thickness are as shown in the inset of Figure 4.19. The magneto-optic gyration and the refractive index in the iron garnet film are initially assumed to be uniform inside the film and parameterized by the gyrotropy parameter g. Multiple reflections are included in the analysis, giving rise to an oscillatory behavior in FR as a function of film thickness h. The FR angle 𝜃 is given by [7] ( ) √ √ √ ⎡ ⎤ n3 ⎢ (1 + n3 ) cos(k0 𝜀2 − gh) − i √𝜀2 −g + 𝜀2 − g sin(k0 𝜀2 − gh) ⎥ 1 ⎥ arg ⎢ ( ) ⎢ ⎥ 2 √ √ √ n3 ⎢ (1 + n3 ) cos(k0 𝜀2 − gh) − i √𝜀 −g + 𝜀2 − g sin(k0 𝜀2 − gh) ⎥ 2 ⎣ ⎦ (4.5)

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4 Bismuth-Substituted Iron Garnet Films for Magnetophotonics 14

11 Air n1=1

12 11

n2

10

n3 Substrate

Sample 1 Sample 2 Sample 3

532 nm

M

10 9 8

h

7

9

6

8

5

7

4

6

3

5

2

4

1

3

0

0.05

0.1

0.15 0.2 0.25 0.3 Magnetic film thickness (μm)

0.35

0.4

Specific Faraday rotation (°/μm)

Input light

13

Specific Faraday rotation (°/μm)

154

0 0.45

Figure 4.19 Specific Faraday rotation at 532 nm versus film thickness for Bi0.8 Gd0.2 Lu2 Fe5 O12 (Sample 1) and Bi0.7 Gd0.4 Lu1.9 Fe4.1 Ga0.9 O12 (Samples 2 and 3), showing a dramatic increase below 50 nm. Inset: Schematic depiction of the samples under consideration. Source: Levy et al. [56]. ©2019, The Optical Society.

where n3 is the refractive index of the substrate. The refractive index of the ferrimag√ netic film can be written as 𝜀2 ± g for positive and negative polarization helicities. k0 = 2π/λ is the wave number in vacuum, and 𝜆 is the wavelength. Equation (4.5) has two asymptotes, for h ≪ 2π/λ ultrathin films and for h ≫ 2π/λ bulk iron garnet material, with SFRs given by { g 2𝜋 − 2n 𝜆 , if h ≫ 2𝜋 𝜃 𝜆 2 (4.6) SFR = = g 2𝜋 − 1+n 2𝜋 , if h ≪ h 𝜆 𝜆 3

From Eq. (4.6), the ratio of the SFR at small h with respect to the SFR at large h is, thus, SFRh→0 2n2 = SFRh→∞ 1 + n3 The refractive indices at wavelength 𝜆 = 532 nm are given by n3 = 1.980 + 0.003i in the GGG substrate and n2 = 2.610 + 0.056i in the Bi-substituted iron garnet films for all samples. Therefore, at 𝜆 = 532 nm this ratio is predicted to be about 1.75, but the experimentally measured ratios are always larger than 2. In particular, SFRh → 0 /SFRh → ∞ = 10.25/5 ≈ 2.05 for Sample 1, and SFRh → 0 /SFRh → ∞ = 9.818/4.226 ≈ 2.32 for Samples 2 and 3. Thus, the analytical expression derived from classical electrodynamics cannot account for all the effects operating in this system. Besides the preceding analysis we also performed an electromagnetic modeling based on an RCWA [53, 54]. The result of the corresponding numerical simulation in the case of constant magneto-optic gyration inside the whole magnetic film is given in Figure 4.20 by the red line (Model 1). One can see that this model does not

Figure 4.20 Theoretical fits to Sample 2 data based on uniform gyrotropy parameter in the film (Model 1), gyrotropy parameter proportional to Bi content in the film (Model 2), and gyrotropy parameter enhancement at the top surface (Model 3). A sevenfold g-value magnification is predicted within 2 nm of the surface (Model 3), and fourfold magnification over the next 2 nm, as compared with bulk g value. Source: Levy et al. [56]. ©2019, The Optical Society.

Specific Faraday rotation (°/μm)

Acknowledgment

10 9

Experiment Model 1 Model 2 Model 3

8 7 6 5 4 3 0

0.1 0.2 0.3 Magnetic film thickness (μm)

0.4

reproduce the pronounced increase in the SFR obtained experimentally (points, squares, and diamonds in Figure 4.19). Therefore, the assumption of constant gyration inside the ferrimagnetic film does not describe the observed abrupt growth in the SFR in ultrathin ferromagnetic films. Assuming a gyration proportional to the Bi content, comprising also its decline in the transient layer (Figure. 4.17, blue line), the numerical simulation produces an even poorer coincidence with the experimental data than Model 1, constant gyration. This is shown by the blue line (Model 2) in Figure 4.20. Notice that Model 1 in effect already assumes a significant increase in magneto-optic gyration in the interfacial region if one considers the decrease in Bi content in the transient layer. Yet the calculated SFR in that case remains well below the experimental data. To achieve coincidence with the experimental data, the gyrotropy parameter g was assumed to increase near the air/film interface. Best fits to the experimental data are produced by a rise in g value taking place over a very thin layer adjacent to the ′ surface, which is h = 4 nm thick. In Model 3, the gyration evinces a near-surface steplike growth, as depicted in Figure 4.5. One can see from Figure 4.20 (Model 3) that this assumption provides a very good agreement with the SFR, including sub-50-nm-thick iron garnet films. Note that this model has a good agreement not only with the experimental data for Sample 2 (Figure 4.20) but also with the measurements of all three samples. All theoretical curves in Figure 4.19 are based on the Model 3 and demonstrate a good agreement with the experiment. These fits predict that the magneto-optic gyrotropy parameter exhibits a sevenfold amplification in magnitude within 2 nm of the surface and a fourfold amplification in the next 2 nm over its bulk value. Please see Chapter 5 for the Devices and Applications based on the thin iron-garnet films.

Acknowledgment This work is financially supported by the Russian Ministry of Education and Science, Megagrant project N 075-15-2019-1934.

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List of Abbreviations and Symbols 1D-MPC BiIG BM CMZGG CNGG CW FHLs FM FP-1D-MPC FR GGG LPE MCD MO MPC PBG RCWA RIBS SFR SIMS SPP T-1D-MPC TMOKE TPP

one-dimensional magneto-photonic crystal bismuth-substituted iron garnet Bragg mirror calcium–manganese–zirconium–gadolinium–gallium garnet calcium–niobium–gallium garnet continuous wave Faraday hysteresis loops figure of merit Fabry–Perot one-dimensional magnetophotonic crystal Faraday rotation gadolinium–gallium garnet liquid-phase epitaxy magnetic circular dichroism magneto-optical magnetophotonic crystal photonic band gap rigorous coupled-wave analysis reactive ion-beam sputtering specific Faraday rotation secondary- ion- mass- spectroscopy surface plasmon polariton Tamm one-dimensional magnetophotonic crystal transverse magneto-optical Kerr effect Tamm plasmon polariton

References 1 Geller, S. and Gilleo, M.A. (1957). Structure and ferrimagnetism of yttrium and rare-earth–iron garnets. Acta Crystallographica 10 (3): 239–239. 2 Vasiliev, M., Alam, M.N.-E., Kotov, V.A. et al. (2009). RF magnetron sputtered (BiDy)3 (FeGa)5 O12 :Bi2 O3 composite garnet-oxide materials possessing record magneto-optic quality in the visible spectral region. Optics Express 17 (22): 19519–19535. 3 Wettling, W. et al. (1973). Optical absorption and Faraday rotation in yttrium iron garnet. Physica Status Solidi (b) 59 (1): 63–70. 4 Kajiwara, Y. et al. (2010). Transmission of electrical signals by spin-wave interconversion in a magnetic insulator. Nature 464 (7286): 262. 5 Uchida, K. et al. (2010). Spin Seebeck insulator. Nature Materials 9 (11): 894–897. 6 Bozhko, D.A., Serga, A.A., Clausen, P. et al. (2016). Supercurrent in a room-temperature Bose–Einstein magnon condensate. Nature Physics 12: 1057–1062.

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7 Zvezdin, A.K. and Kotov, V.A. (1997). Modern Magnetooptics and Magnetooptical Materials. CRC Press. 8 Randoshkin, V.V. and Chervonenkis, A.Ya. (1990). Prikladnaya magnitooptika (Applied Magnetooptics). Moscow: Energoatomizdat. 9 Okuda, T., Koshizuka, N., Hayashi, K. et al. (1988). Synthesis of new magnetooptical material. IEEE Translation Journal on Magnetics in Japan 3: 483–484. 10 Kahl, S. and Grishin, A.M. (2004). Enhanced Faraday rotation in all-garnet magneto-optical photonic crystal. Applied Physics Letters 84: 1438–1440. 11 Fedyanin, A.A., Kobayashi, D., Nishimura, K. et al. (2005). Materials Research Society Symposium Proceedings 834 (J1.5): 1–4. 12 Inoue, M., Uchida, H., Nishimura, K. et al. (2006). Magnetophotonic crystals – a novel magneto-optic material with artificial periodic structures. Journal of Materials Chemistry 16: 678–684. 13 Okuda, T., Katayama, T., Satoh, K. et al. Recent advances in magnetism and magnetic materials. Proceedings of the Fifth Symposium on Magnetism and Magnetic Materials, 61–76. 14 Okuda, T., Koshizuka, N., Hayashi, K. et al. (1987). Faraday rotation in highly Bi-substituted yttrium iron garnet films prepared by ion beam sputtering. IEEE Transactions on Magnetics 23 (5): 3491. 15 Berzhansky, V.N., Karavainikov, A.V., Milyukova, E.T. et al. (2010). Synthesis and properties of substituted ferrite-garnet films for one-dimensional magnetophotonic crystals. Functional Materials 17: 120–126. 16 Berzhansky, V.N., Shaposhnikov, A.N., Prokopov, A.R. et al. (2011). One-dimensional magnetophotonic crystals based on double-layer Bi-substituted iron garnet films. Materialwissenschaft und Werkstofftechnik 42: 19–23. 17 Berzhansky, V.N., Shaposhnikov, A.N., Karavainikov, A.V. et al. (2013). The effect of Faraday rotation enhancement in nanolayered structures of Bi-substituted iron garnets. Solid State Phenomena 200: 233–238. 18 Berzhansky, V., Mikhailova, T., Shaposhnikov, A. et al. (2013). Magneto-optics of nanoscale Bi: YIG films. Applied Optics 52: 6599–6606. 19 Shaposhnikov, A.N., Prokopov, A.R., Karavainikov, A.V. et al. (2014). Modification of Bi: YIG film properties by substrate surface ion pre-treatment. Materials Research Bulletin 55: 19–25. 20 Li, J., Zhang, W., Song, Y. et al. (2016). Template transfer nanoimprint for uniform nanopores and nanopoles. Journal of Nanomaterials 2016: 1–7. 21 Parkin, S., Xin, J., Kaiser, C. et al. (2003). Magnetically engineered spintronic sensors and memory. Proceedings of the IEEE 91 (5): 661–680. 22 Baibich, M.N., Broto, J.M., Fert, A. et al. (1988). Giant magnetoresistance of (001)Fe/(001)Cr magnetic superlattices. Physical Review Letters 61 (21): 2472–2475; Press release. 23 Candid, R., María-Dolores, C.B., and Diego Ramírez, M.O. (2009). Magnetic field sensors based on giant magnetoresistance (GMR) technology: applications in electrical current sensing. Sensors 9 (10): 7919–7942.

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24 Grunberg, P., Schreiber, R., Pang, Y. et al. (1987). Layered magnetic structures: evidence for antiferromagnetic coupling of Fe layers across Cr interlayers. Physical Review Letters 61 (8): 3750–3752. 25 Alvarado, S.F. and Carbone, C. (1988). Magnetism and epitaxy of Fe/Cr(001) multilayers. Physica B & C 149B+C (1–3): 43–48. 26 Okuda, T., Katayama, T., Satoh, K., and Yamamoto, H. (1991). Preparation of polycrystalline Bi3 Fe5 O12 garnet films. Journal of Applied Physics 69 (8): 4580. 27 Shaposhnikov, A.N., Berzhansky, V.N., Prokopov, A.R. et al. (2010). Scientific Notes of Taurida National University, Series: Physics and Mathematics Sciences, vol. 23, 62, No. 1, 146. 28 Berzhansky, V.N., Shaposhnikov, A.N., Prokopov, A.R. et al. (2016). One-dimensional magnetophotonic crystals with magnetooptical double layers. Journal of Experimental and Theoretical Physics 123 (5): 744–751. 29 Toraya, H. and Okuda, T. (1995). Crystal structure analysis of polycrystalline Bi3 Fe5 O12 thin film by using asymmetric and symmetric diffraction techniques. Journal of Physics and Chemistry of Solids 56 (10): 1317. 30 Okuda, T., Kudox, A., Yoshihara, S. et al. (1997). In situ growth of polycrystalline bismuth-iron-garnet films on quartz glass substrate. Journal de Physique IV 7: C1–C707. 31 Berzhansky, V.N., Mikhailova, T.V., Karavainikov, A.V. et al. (2012). Microcavity one-dimensional magnetophotonic crystals with double layer iron garnet. Journal of the Magnetics Society of Japan 36 (1_2): 42–45. 32 Mikhailova, T.V., Berzhansky, V.N., Shaposhnikov, A.N. et al. (2018). Optimization of one-dimensional photonic crystals with double layer magneto-active defect. Optical Materials 78: 521–530. 33 Mikhailova, T., Shaposhnikov, A., Prokopov, A. et al. (2018). Tamm plasmon-polaritons structures with Bi-substituted garnet layers. EPJ Web of Conferences 185: 02016. 34 Deb, M., Popova, E., Fouchet, A., and Keller, N. (2012). Magnetooptical Faraday spectroscopy of completely bismuth-substituted Bi3 Fe5 O12 garnet thin films. Journal of Physics D: Applied Physics 45: 455001. 35 Dionne, G.F. (2010). Magneto-optical properties. In: Magnetic Oxides, 343–384. Springer Science & Business Media. 36 Wittekoek, S., Popma, T.J.A., Robertson, J.M., and Bongers, P.F. (1975). Magneto-optic spectra and the dielectric tensor elements of bismuth-substituted iron garnets at photon energies between 2.2–5.2 eV. Physical Review B 12: 2777–2788. 37 Belov, K.P., Zvezdin, A.K., Kadomseva, A.M., and Levitin, R.Z. (1979). Orientation Transitions in Rare Earth Magnetics. Moscow: Nauka (In Russian). 38 Lyubchanskii, I.L., Dadoenkova, N.N., Lyubchanskii, M.I. et al. (2003). Magnetic photonic crystals. Journal of Physics D: Applied Physics 36: R277–R287. 39 Inoue, M., Baryshev, A.V., Goto, T. et al. (2013). Magnetophotonic crystals: experimental realization and applications. In: Magnetophotonics (eds. M. Inoue, M. Levy and A.V. Baryshev), 163–190. Berlin Heidelberg: Springer-Verlag.

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40 Dzibrou, D.O. and Grishin, A.M. (2009). Fitting transmission and Faraday rotation spectra of [Bi3 Fe5 O12 /Sm3 Ga5 O12 ]m magneto-optical photonic crystals. Journal of Applied Physics 106: 043901. 41 Khartsev, S.I. and Grishin, A.M. (2007). High performance [Bi3 Fe5 O12 /Sm3 Ga5 O12 ]m magneto-optical photonic crystals. Journal of Applied Physics 101: 053906. 42 Takahashi, K., Takagi, H., Shin, K.H. et al. (2007). Figures of merit of magneto-optic spatial light modulators with magnetophotonic crystals. Physica Status Solidi (c) 4: 4536–4539. 43 Tomilin, S.V., Berzhansky, V.N., Shaposhnikov, A.N. et al. (2016). Ultrathin and nanostructured Au films with gradient of effective thickness. Optical and plasmonic properties. Journal of Physics: Conference Series 741: 012113. 44 Yin, C.P., Wang, T.B., and Wang, H.Z. (2012). Magneto-optical properties of onedimensional conjugated magnetophotonic crystals heterojunctions. European Physical Journal B 85: 104. 45 Berzhansky, V.N., Shaposhnikov, A.N., Prokopov, A.R. et al. (2016). One-dimensional magnetophotonic crystals with magneto- optical double layers. Journal of Experimental and Theoretical Physics 123: 744–751. 46 Berzhansky, V.N., Karavainikov, A.V., Mikhailova, T.V. et al. (2017). Nano- and micro-scale Bi-substituted iron garnet films for photonics and magneto-optic eddy current defectoscopy. Journal of Magnetism and Magnetic Materials 440: 175–178. 47 Armelles, G., Cebollada, A., Garcia-Martin, A., and Ujue Gonzalez, M. (2013). Magnetoplasmonics: combining magnetic and plasmonic functionalities. Advanced Optical Materials 1: 10–35. 48 Khokhlov, N.E., Prokopov, A.R., Shaposhnikov, A.N. et al. (2015). Photonic crystals with plasmonic patterns: novel type of the heterostructures for enhanced magneto-optical activity. Journal of Physics D: Applied Physics 48: 095001. 49 Levy, M., Chakravarty, A., Huang, H.-C., and Osgood, R.M. Jr., (2015). Large magneto-optic enhancement in ultra-thin liquid-phase-epitaxy iron garnet films. Applied Physics Letters 107: 011104. 50 Belotelov, V.I., Akimov, I.A., Pohl, M. et al. (2011). Nature Nanotechnology 6: 370–376. 51 Borovkova, O.V., Hashim, H., Kozhaev, M.A. et al. (2018). TMOKE as efficient tool for the magneto-optic analysis of ultra-thin magnetic films. Applied Physics Letters 112: 063101. 52 Maier, S.A. (2007). Plasmonics: Fundamentals and Applications. Springer-Verlag US. 53 Moharam, M.G., Grann, E.B., Pommet, D.A., and Gaylord, T.K. (1995). Journal of the Optical Society of America A 12: 1068. 54 Li, L. (2003). Journal of Optics A: Pure and Applied Optics 5: 345. 55 Palik, E.D. (1998). Handbook of Optical Constants of Solids. Academic Press. 56 Levy, M., Borovkova, O.V., Sheidler, C. et al. (2019). Faraday rotation in iron garnet films beyond elemental substitutions. Optica 6 (5): 642–646.

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5 Bismuth-substituted Iron Garnet Films for Magnetophotonics: Part B – Devices and Applications Andrey A. Voronov 1,2 , Daria O. Ignatyeva 1,2 , Nikolay A. Gusev 2 , Petr M. Vetoshko 2,3 , Nazar V. Lugovskoy 4 , Yujun Song 5,6 , Vladimir N. Berzhansky 4 , and Vladimir I. Belotelov 1,2 1 Lomonosov Moscow State University, Faculty of Physics, Department of Photonics and Microwaves Structures, Leninskie Gory, Moscow 119991, Russia 2 Russian Quantum Center, 45, Skolkovskoye shosse, Moscow, 121353, Russia 3 Kotelnikov Institute of Radioengineering and Electronics, Mokhovaya 11-7, Moscow, 125009, Russia 4 V.I. Vernadsky Crimean Federal University, Physics and Technology Institute, Department of Experimental Physics, Vernadsky Avenue 4, Simferopol, 295007, Russia 5 University of Science and Technology Beijing, Center for Modern Physics Technology, Applied Physics Department, School of Mathematics and Physics, 30 Xueyuan Road, Beijing 100083, China 6 Zhejiang Key Laboratory for Pulsed Power Technology Translational Medicine, Hangzhou Ruidi Biotechnology Company, Hangzhou 310000, China

5.1 Device Assemble and Application of Iron Garnet Films for Ultrasensitive Magnetometry One of the applications of iron garnet films is magnetometry, which is associated with the increase in the sensitivity of flux-gate magnetometers [1], based on the electrodynamics principle of operation, to the level of quantum magnetometers – the SQUIDs [2] and the optically pumped magnetometers (OPMs) [3]. Flux-gate sensors were proposed in 1936 [4] and subsequently proved to be very good in searching the mines during the Second World War. The flux-gate principle for measuring magnetic fields is based on the periodic magnetization and demagnetization of a ferromagnetic core using an excitation coil [1]. When the external (measured) magnetic field effects on such a system, the magnetization reversal frequency is transformed, and additional signals appear at even harmonic frequencies. Usually, a second harmonic is used to record the magnetic field. Flux-gate magnetometers are characterized by maximal interaction energy with the measured magnetic field and have the sensitivity up to 100 pT/Hz1/2 . They are cheap and easy to handle, consume low power, and do not require cooling or heating unlike quantum magnetometers. However, the sensitivity of flux-gate magnetometers is limited due to the magnetic cores magnetic energy fluctuations appearing in the process of magnetization reversal. Such fluctuations are often many orders of magnitude higher than the Inorganic and Organic Thin Films: Fundamentals, Fabrication, and Applications, First Edition. Edited by Yujun Song. © 2021 WILEY-VCH GmbH. Published 2021 by WILEY-VCH GmbH.

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interaction energy of the core with an external magnetic field [5]. This circumstance corresponds the fact that the magnetization reversal is a nonstationary process accompanied by transformation of domain structure and the formation of dynamic instabilities of magnetization under the action of the own demagnetizing field of the core [6]. As a conventional method, the use of magnetic cores with high magnetization and small anisotropy based on iron–nickel alloys allows one to reduce the size and increase the number of domains in the magnetic sample, resulting in smoother averaging of the demagnetizing curve. However, the random character of appearance and disappearance of domains in the process of demagnetization does not change, and the magnetization curve dependence on the applied magnetic field consists of a number of shots related with processes of domain restructuring – Barkhausen’s jumps [5]. Thus, a saturated – single domain – state of the magnetic core element is the basic condition for achieving ultimate sensitivity of the flux-gate magnetic field sensor [7]. A single domain state of the magnet is attainable by applying external saturation field H s = 4𝜋M s . However, a rather high magnetic susceptibility of the core is required in order that the sensor electronics noise will not exceed the intrinsic noise of the core [8]. The magnetic susceptibility, in turn, is inversely proportional to saturation field H s . Therefore, it is necessary to look for ways to reduce the saturation fields of magnetic sensor cores. At a known ratio between thickness and diameter, the minimal saturation field have the magnets in the form of an oblate ellipsoid. However, the use of a special form that provides a minimal saturation field and a single-domain state is a necessary but insufficient method to achieve the ultimate sensitivity of a flux-gate magnetometer. One proposed to use epitaxial iron garnet films as the material for the approximation steps. The epitaxial bismuth iron garnet (BiIG) films with nominal compositions of Re3−x Bix Fe5−y Mey O12 (0 < x < 2, 0.3 < y < 0.7), where Re is a rare-earth element and Me is an iron-substituting element. As shown in [9], films of such composition have an order of magnitude lower constant of the cubic anisotropy than undiluted iron garnet. Moreover, the effective anisotropy of the films in the (111) plane is lower by two orders of magnitude. In addition to the described advantages, the iron garnet films are characterized by low number of defects per unit volume due to large lattice constant and low (compared with ferromagnetic metals) parameter of the Hilbert dissipation. The abovementioned facts provide the necessary material properties for magnetic sensors operating in the homogeneous magnetization rotation regime. In addition, it should be noted that setting up the elliptical thickness profile is a very complex technological problem for iron garnet films. To overcome these difficulties, a special technological approach was developed, which concludes in a stepped profile (Figure 5.1) approximation by multilayer photolithography [10]. The above approaches allowed one to overcome the limitations of the traditional flux-gate technology and obtain a noise level of 100 fT/Hz1/2 , which was experimentally demonstrated [11]. As a sensitive element, a round iron garnet film with a three-step thickness profile was used (Figure 5.2), winded by two orthogonal coils. The coils receive a harmonic signal from the generator and create a rotating control field. Information about the measured field can be obtained by recalculating

5.1 Device Assemble and Application of Iron Garnet Films for Ultrasensitive Magnetometry

Figure 5.1

The thickness profile approximation by the steps.

X-coil Y-coil Iron garnet film (a)

(b)

(c)

Figure 5.2 The sensitive element: (a) the photograph of the three-step round iron garnet film, (b) the winding element, and (c) the winding configuration: X-coil corresponds to the x-component of the measured field and Y-coil corresponds to the y-component. Source: Nikolay A. Gusev, Petr M. Vetoshko.

Figure 5.3 Animal measurement equipment: the sensor, the board, the plate, and the shield. Source: Nikolay A. Gusev, Petr M. Vetoshko.

the amplitude of the response signal arriving form the coils at a double frequency. Another advantage of such device was the synchronous measurements of both magnetic field vector components lying in the plane of the iron garnet film because the electromagnetic filter (EMF) signals from the coils of the film winding, carrying information about the magnetic field, are fed to the measuring system with a certain phase shifts. Each signal at the frequency of the second harmonic carries information about the corresponding vector field component. Operation of the proposed magnetic field sensor is demonstrated by measuring the magnetocardiography (MCG) signals – magnetic field of healthy rat heart. A lab rat was positioned on a textolite plate fixed above the sensor element inserted into the wooden board. In order to suppress external magnetic noise, the board with a rat and sensor was placed inside a magnetic shield comprising four permalloy cylinders characterized by attenuation factor of about 1500 (Figure 5.3). In this configuration, the sensor measures magnetic field components in plane of the rat thoracic cage. The directions were determined by axes of the detecting magnetic coils. The MCG measurements were performed in two periods: real-time monitoring and time averaging for 10 seconds.

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5 Bismuth-substituted Iron Garnet Films for Magnetophotonics: Part B – Devices and Applications

Figure 5.4 Obtained MCG signal of measured rat without averaging.

15 10 H (pT)

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800

900

According to the noise characteristic of the magnetometer, the sensor noise in a frequency interval of 1–100 Hz is about 100 fT/Hz1/2 (Figure 5.2b). The peak observed at 50 Hz is related to the AC mains source frequency and amounts to about 400 fT/Hz1/2 . According to the √ Van der Ziel criterion [12], the minimal level of observable signal is Hmin = k ⟨Hf2 ⟩Δf , (where k ∼ 6 and Δf is the signal frequency interval), one can obtain an estimate of H min = 6 pT for MCG measurements in the 1–100 Hz interval. Figure 5.4 shows the obtained MCG signal of measured rat without averaging. The signal period was about 165 ms. The main R peak with a magnitude of about 15 pT is well recognized on the noise background. It should be emphasized that the analogous peak intensity in MCG patterns of humans is 10–30 times higher [13]. The experimentally measured noise level about 4 pT, in agreement with the Van der Ziel criterion. Combining the flux-gate magnetometry with the magneto-optical scanning leads to new possibilities: vector measurements [14, 15] and imaging and micrometer spatial resolution [16]. In the absence of the reading coils in such magnetometers, the Schottky noise can be avoided. Moreover, magneto-optical measurements are contactless, allowing one to use the magnetic sensors in conjunction with optical fibers, which opens new horizons for invasive and probing magnetometry [17, 18]. The following work [14] describes the development of vector measurements of the magnetic field through reading the magnetization state of an iron garnet film by the Faraday rotation together with the technique of magnetization reversal by homogeneous rotation. The control magnetic field H rotates the film magnetization M in the film plane (Figure 5.5) with the rotation frequency 𝜔. Magnetocrystalline anisotropy of the film deviates the magnetization from the film plane and provides the out-of-plane component. This component can be detected with the linearly polarized light beam due to the magneto-optical Faraday effect. A magnetic film of cubic crystal lattice and crystallographic axis orientation (111) is fully symmetric with respect to the film normal. Since the magnetocrystalline anisotropy leads the magnetization out of the film plane, the Faraday signal becomes sensitive to both the in-plane and out-of plane components of the measured magnetic field. This important property establishes a possibility for the vector magnetometry with the iron garnet films.

5.1 Device Assemble and Application of Iron Garnet Films for Ultrasensitive Magnetometry

S

P Z[111] M

𝜔 F

θ 𝜔t

H Y[011]

X[211] A H sin(𝜔t)

H cos(𝜔t)

D

Figure 5.5 The principle scheme of the vector magneto-optical magnetic sensor. “F” is a magnetic film of cubic magnetic anisotropy, “S” is a light source, “P” is a polarizer, “A” is an analyzer, and “D” is a photodetector. The control rotating magnetic field H is generated by two coils.

If the incident beam impinges upon the sample normally, the observed Faraday angle Ψ(t) senses the out-of-plane magnetization component and Ψ(t) ∼ 𝜃(t), where 𝜃 is the polar angle of the magnetization. Since the azimuthal angle of the control magnetic field is 𝜙H = 𝜔t, one can directly measure the azimuthal dependence of 𝜃, 𝜃(𝜙H ). If there is no external magnetic field, then 𝜃(𝜙H ) is a periodic function with a period of 𝜋/3. In the presence of the monitored magnetic field h (h ≪ H), the symmetry is broken, and even Fourier harmonics occur. The magnetometer measures the Fourier amplitudes An and Bn in the Fourier ( ) ∑∞ series 𝜃 𝜙H = n=0 An cos n𝜙H + Bn sin n𝜙H . It can be shown that 𝜃(𝜙H ) = c1 hx sin 2𝜙H + c2 hy cos 2𝜙H + c3 sin 3𝜙H + c4 hx sin 4𝜙H + c5 hy cos 4𝜙H + c6 hz cos 6𝜙H , where ci is the coefficient depends on the magnetic parameters of the film, including the saturation magnetization, M s , and the uniaxial and cubic anisotropy constants, K U and K 1 , respectively. Therefore, all three components of h can be found from A2 , B2 , and A6 normalized by B3 hx =

c3 B2 cA cA , h = 3 2 , and hz = 3 6 c1 B3 y c2 B3 c6 B3

(5.1)

In general case, expressions for coefficients ci versus M s , K U , and K 1 are quite K H ≪ 1, 4𝜋M1 2 ≪ 1. It cumbersome. However, for iron garnet is usually satisfied 4𝜋M s

allows the significant simplifications: √ ) c3 c3 2 2( 2 c3 2𝜋Ms2 − KU H = = H, = c1 c2 3 c6 3K1

s

(5.2)

165

166

5 Bismuth-substituted Iron Garnet Films for Magnetophotonics: Part B – Devices and Applications

Equation (5.2) shows that

c3 c1

and

c3 c2

are linear in field H and do not depend on c

anisotropy in these assumptions. On the contrary, c3 strongly depends on K 1 , K u , 6 and M s . In any case, the decrease in H increases the sensitivity. However, H should be high enough to saturate the magnetic film. For experimental demonstration of the proposed concept, a series of the rare-earth iron garnet films of nominal composition (BiLuPrTmGd)3 (FeAlGa)5 O12 were used. The results obtained for an 11.7–μm-thick film of composition Bi1.1 Lu1.45 Pr0.2 Tm0.2 Gd0.05 Fe3.5 Al0.8 Ga0.7 O12 are presented below. The film has a low value of the saturation magnetization 4𝜋M s (75 G), and the out-of-plane magnetic field saturation is H ⟂ = 166 Oe. The latter is important to increase the amplitudes of 𝜃. The magnetic film was placed inside three mutually orthogonal Helmholtz coils (Figure 5.5). Two of them are used to generate the rotating control magnetic field H. The amplitude of the control field is chosen as H = 20 Oe to saturate the sample. The external magnetic field to be measured was generated by the Helmholtz coils as well. The sample was illuminated with a linearly polarized light from a laser diode “S” at wavelength 𝜆 = 630 nm. The whole experimental setup was placed in the magnetic shielded room with the attenuation coefficient of 1500 at frequencies below 1 kHz. It allows one to eliminate Earth’s magnetic field along with 50 Hz AC magnetic fields generated by wire. In the absence of the monitored magnetic field (h = 0), 𝜃 (𝜙H ) has only the third harmonic (black curves in Figure 5.6a). The presence of a small additional constant external magnetic field h causes some distortion of the signal (Figure 5.6), and the other harmonics appear (Figure 5.6b–d). Experimental data show that A2 is linear in hy only (Figure 5.6b), while B2 is linear exclusively in hx (Figure 5.6c). At the same time, A6 is linear in hz (Figure 5.6d). This is in full agreement with Eq. (5). Note that in a relatively large field hz > 0.6 Oe, the dependency A6 (hz ) becomes nonlinear (Figure 5.6d). Moreover, each Fourier amplitude depends nonlinearly on two other components of h (A2 depends nonlinearly on hx and hz , A6 – on hx and hy , and B2 – on hy and hz ). This does not contradict to Eq. (5) since only linear in h terms was sustained in them. These nonlinear contributions do not influence the sensor functionality since the corresponding derivatives vanish for h → 0. The linear dependence of A2 , B2 , and A6 on different components of the external magnetic field allows one to use the proposed structure as a vector magnetometer. At first, the magnetometer should be calibrated by applying the external magnetic field in three orthogonal directions. It allows one to find c3 /c1 , c3 /c2 , and c3 /c6 from the slopes of the straight line fits of the corresponding dependencies on h (dashed lines in Figure 5.6b–d) and from the amplitude of the third harmonic. As c1 = c2 , they can be found by averaging the slopes of A2 (hy ) and B2 (hx ) divided by A3 . In our case, c3 /c1 = c3 /c2 = 15.01 and c3 /c6 = 38.34. The magnetometer detection threshold is determined by the total fluctuations arising of the magnetic material during the magnetization reversal and the noise of the optical tract appearing due to fluctuations of the light source intensity and photon shot noise. The last two factors are determined by the optical resolution of the

x[211] 0.012

45+

0.006

x[211]

z[111] 315+

x[211]

45+

315+

45+

315+

Faraday angle, red

0.000 –0.006 –0.012 –0.000

270+ 90+

90+

270+ 90+

270+

–0.006 0.000 0.006

225+

135+

225+

135+

225+

135+

0.012

(a)

0.06 0.05

180+

180+

hx

hy

hz

A2/A3(hx) A2/A3(hy) A2/A3(hz)

0.07 0.06

Linear fit of A2/A3(hy)

0.05

0.03

0.020

Linear fit of B2/A3(hx)

0.03

0.012 0.008

0.01

0.01

0.004

0.00

0.00

–0.01 0.0 (b)

0.2

0.4

0.6 hi (Oe)

0.8

1.0

–0.01 0.0 (c)

Linear fit of A6/A3(hz)

0.016

0.02

0.02

A6/A3(hx) A6/A3(hy) A6/A3(hz)

0.024

0.04 B2/A3

A2/A3

0.04

B2/A3(hx) B2/A3(hy) B2/A3(hz)

A6/A3

0.07

180+

0.2

0.4

0.6 hi (Oe)

0.8

1.0

0.000 –0.01 0.0 (d)

0.2

0.4

0.6

0.8

hi (Oe)

Figure 5.6 (a) The variation of the signal Ψ(𝜙H ) for three orthogonal directions (x, y, z) of the external field h – 0 Oe (black), 3 Oe (red), and 6 Oe (blue) and Fourier amplitudes A2 (b), B2 (c), and A6 (d) versus magnetic fields hx (black), hy (red), and hz (blue) normalized to A3 . The linear in hi harmonics can be used to measure all the three projections of the external magnetic field.

168

5 Bismuth-substituted Iron Garnet Films for Magnetophotonics: Part B – Devices and Applications

√ device. In modern balancing detectors, this value is ⟨ΔI 2 ⟩∕I ∼ 10−9 , in where I and ΔI are incident intensity and its fluctuation, respectively [19]. Taking into account the conversion factor of 1.5 × 10−3 rad/Oe, one obtains the sensitivity of the order of 100 pT in a 1 Hz bandwidth. Since the induction reading circuit of the magnetometer above gives 100 fT/Hz1/2 , one can conclude that the sensitivity of the discussed vector magneto-optical magnetometer is limited by the optical parameters of the detection scheme, which could be improved by utilizing magneto-optical intensity effects instead of Faraday effect. In general, the in-plane magnetization of the transparent magnetic films is not straightforwardly detectable by light. For technical reasons it is preferable to observe magnetic films at normal or slightly oblique incidence; otherwise special correcting optics or prisms are required that make the sensing element bulky and introduce excess noise. Therefore, one should exploit the magneto-optical effects that are sensitive to the magnetization component orthogonal to the light wave vector. Among such effects, the widely used transverse and longitudinal magneto-optical Kerr effects require oblique incidence at large angles, and in addition the transverse effect is negligibly small for transparent media. In addition to the Kerr effects is the Voigt effect, which is usually quite small for transparent media and may be insufficient for employment in magnetometry. It is well known that a plasmonic cover of the transparent films significantly modifies the magneto-optical response, and the Faraday and transverse Kerr effects are resonantly enhanced by several orders of magnitude [20–22]. The transverse Kerr effect might not be an optimal option compared to other magneto-optical effects arising in the magnetoplasmonic crystals. In the case of a magnetic film covered with a one-dimensional gold grating of subwavelength slits, the longitudinal magnetophotonic intensity effect (LMPIE) arises [20]. Importantly, the LMPIE takes place for normal light incidence, which perfectly fits the requirements of the magnetometry with the in-plane magnetized films. Furthermore, in contrast to the Faraday effect and other bulk magneto-optical effects, to obtain high values of the LMPIE does not require thicker magnetic films. In fact, the LMPIE almost saturates for the 1-μm-thick magnetic films, which provides room for miniaturization. In the next work [23], one proposes and demonstrates a novel type of magneto-optical magnetic field sensor based on the LMPIE in the magnetoplasmonic crystal: a structure of an iron garnet film and a thin gold layer pierced with a periodic slit array. One shows that such a structure provides enhancement of magneto-optical magnetometry and allows using the planar component of magnetization for reading low magnetic fields. The proof-of-concept sample of the magnetoplasmonic sensor is demonstrated in the experimental study a sensitivity level of 2 nT/Hz1/2 . Let us consider, similar to the previous case, an iron garnet film of cubic crystal lattice and crystallographic axis orientation (111) placed into a saturating control magnetic field H, which is rotated in the film plane at a frequency 𝜔. The azimuth angle of the control field varies as 𝜙H = 𝜔t, and then the film magnetization M follows it with some delay in azimuth angle, Δ𝜙 = 𝜙 − 𝜙H , where 𝜙 is azimuth angle of M. The magnetocrystalline anisotropy of the film deviates the magnetization

5.1 Device Assemble and Application of Iron Garnet Films for Ultrasensitive Magnetometry

from the film plane by an angle 𝜃 and provides its out-of-plane component. In the presence of the monitored magnetic field h, the time dependences of both angles Δ𝜙(t) and 𝜃(t) are modified, and analyzing their spectra allows to determine the external magnetic field value and direction. The behavior of Δ𝜙(t) and 𝜃(t) depends on the relation between the control magnetic field and the field of the cubic magnetic anisotropy H C ∼K 1 /M s , where K 1 is the cubic anisotropy constant and M s . is the saturation magnetization. We restrict our consideration to the case of relatively weak field of the cubic magnetic anisotropy H C ≪ H. At this the angle 𝜃 is negligibly small and it is more preferable to detect Δ𝜙(t). Therefore, it is necessary to use a magneto-optical effect that is sensitive to the in-plane magnetization. However, in the case of the transparent magnetic films the Voigt and transverse Kerr effects governed by the in-plane magnetization are quite small and are not suitable. Thus, we deposited on the magnetic film a plasmonic one-dimensional grating to make optical transmittance and reflectance strongly sensitive to the in-plane magnetization as made possible due to the LMPIE. The LMPIE is even in M x and is measured by relative change of the detected light intensity 𝛿 for the remagnetization of the structure from the state with M x = M s to M x = 0. For the sake of mechanical stability, it is more advantageous to detect the transmitted signal. In this case the LMPIE magnitude is given by [20]: ( ) T Ms − T0 (5.3) 𝛿= T0 where T(M s ) and T 0 are transmittance for M x = M s and M x = 0, respectively. Due to the LMPIE, the rotating magnetization causes variations in transmittance that are quadratic in M x [20]: [ ) ] ( Mx (t) 2 𝛿 (5.4) T (t) = T0 1 + Ms where 𝛿 is the LMPIE value measured for the fully saturated magnetic film. Consequently, measuring and analyzing light transmittance through the magnetoplasmonic crystal allows determining M x (t) from Eq. (5.4). To establish a magnetometer scheme on this basis, one needs to relate M x (t) and the monitored magnetic field h. To get the main expressions characterizing the proposed magnetic sensor, one can assume that the monitored magnetic field h varies much slower than H(t) and consequently, it can be considered constant. In this case, the total magnetic field influencing the magnetization is [ ] ] [ Htot (t) = H (t) + h = H cos (𝜔t) + hx ex + H sin (𝜔t) + hy ey + hz ez (5.5) where ei are unitary vectors along the coordinate axes. As the sensor is aimed at the measurement of weak magnetic fields (h ≪ H), we use the linear in h approximation. Neglecting the influence of anisotropy, we assume that magnetization is directed along H tot . Then, taking into the account that the magnetization is fully saturated, for the component of magnetization perpendicular to the slits M x , from Eq. (5.5), one can find } { hy hx hx cos (2𝜔t) − sin (2𝜔t) + (5.6) Mx (t) = Ms cos (𝜔t) − 2H 2H 2H

169

170

5 Bismuth-substituted Iron Garnet Films for Magnetophotonics: Part B – Devices and Applications

Substitution of Eq. (5.6) to Eq. (5.4) provides the dependence of the optical transmittance on h, which can be presented as the decomposition into the temporal harmonics: ̃2 (t) + … ̃1 (t) + T ̃0 + T T (t) = T where T k (t) is the combination of terms with cos(k𝜔t) and sin(k𝜔t). In particular, ] [ hy hx ̃3 (t) = −𝛿T0 cos (3𝜔t) + sin (3𝜔t) (5.7) T 2H 2H It means that the third harmonic of T(t) is proportional to the in-plane component h|| = (hx , hy , 0) of the monitored magnetic field. The same is valid also for the first harmonic of T(t). The proposed concept implies restrictions on H and 𝜔. The rotating field H should be strong enough so that its contribution to the free energy of magnetic field is much | | K2 stronger than that of anisotropy, which implies H ≫ || 4M 2𝜋M1 2 −K ||, where K U and | s( s U ) | K 1 are the uniaxial and cubic anisotropy constants, respectively. These constants are material parameters that characterize the energy of the magnetic anisotropy. Therefore, for the measurement of h|| , one can analyze the first or the third harmonic of the modulated transmittance. Since the photodetector current J is equal to the number of electrons knocked out by photons received by the photodetector per unit time: 𝜂e𝜆 IT J= (5.8) 2𝜋cℏ 0 For the amplitude of the third harmonic of the photodetector voltage U 3 , using Eq. (5.8), one can obtain h∥ e R𝜂𝜆I0 T0 𝛿 (5.9) 4𝜋cℏ H where I 0 and 𝜆 are the intensity and wavelength of the incident radiation, R is the photodetector transimpedance amplifier photocurrent-to-voltage conversion ratio, 𝜂 is the quantum yield of the photodetector, e is electron charge, c is speed of light in vacuum, and ℏ is Planck’s constant. One can conclude that the detected signal is proportional to the laser intensity, optical transmittance through the magnetoplasmonic crystal, and the LMPIE magnitude. The plasmic gratings were deposited on two rare-earth iron garnet films of Bi0.9 Gd2.1 Fe4.41 Sc0.59 O12 and Bi0.9 Y1.2 Lu0.9 Fe4.2 Sc0.8 O12 composition. Then, the optical and magneto-optical properties of the magnetoplasmonic crystals were measured, and three magnetoplasmonic samples with the maximal values of LMPIE were selected. The parameters of the samples are shown in Table 5.1. Scheme of the experimental setup for demonstration of the proposed magnetic field sensing method, based on the optical balanced method, is shown in Figure 5.7. Two tunable semiconductor lasers were used: the first with a power of 500 mW (laser-A) and wavelength near 805 nm and the second one with a power of 90 mW and wavelength near 780 nm (laser-B). The optical setup was based on a balanced measurement method. The radiation generated by the laser diode 1 was collected U3 =

5.1 Device Assemble and Application of Iron Garnet Films for Ultrasensitive Magnetometry

Table 5.1

The parameters of the samples used.

Name

Magnetic layer thickness (μm)

Sample 1

Sample 2

Sample 3

1.9

1.9

1.6

Specific Faraday rotation (deg/μm) (at 632 nm)

1.4

1.4

1.3

Grating period (nm)

340

335

350

Grating slit width (nm)

120

200

170

LMPIE

0.28

0.093

0.074

Wavelength 𝜆 (nm)

804.1

781

776.4

U 3 at 10 nT (mV)

0.26

0.04

0.05

I 0 T 0 𝛿 (mW)

16.1

2.7

3.2

Best observed sensitivity (nT)

8.5

2.4

2.9

10

41

42

Sensor noise hn at 𝜈 = 0 (pT/Hz ) 1/2

1

2

3

5

4

6

7

h

h

Figure 5.7 Scheme of the experimental set-up for demonstration of the magnetoplasmonic magnetometer. (1) Diode laser, (2) collecting lens, (3) polarizer, (4) sample with coils, (5) diffuser lens, (6) Wollaston prism, and (7) balanced photodetector. The monitored field is directed along the grating slips (h = {0,hy , 0}).

by an aspherical close-focus lens 2 into a converging beam. The polarization of this beam was set linear by the Glan–Taylor polarizer 3 so that, on the sample 4, which is an iron garnet film with a layer of gold gratings, it was oriented at an angle of 45∘ to the grating lines. Due to the small focal length of the lens 2 equal to 6 mm, the optical waist length reached 1.2 mm, which allowed the sample to be set in the center of the waist and to provide a narrow angular spectrum of optical radiation incident on the plasmonic crystal and high intensity of light at the same time. The transmitted light was modulated by a magnetic field in accordance with the laws described above. Next, the radiation was divided into two beams with TE and TM polarization using a Wollaston 6 prism and was focused by the lens 5 on photo sensors of a balanced photo detector 7 as it shown in Figure 5.7. The photodetector contained the two Si photodiodes and a specially made transimpedance amplifier with its own output noise of about 0.6 μV/Hz1/2 and a current-to-voltage conversion ratio R = 2500 Ω. The photodetector signal was digitized by the data acquisition board and transferred to a PC for further processing. The signal processing is used in obtaining its spectrum by

171

172

5 Bismuth-substituted Iron Garnet Films for Magnetophotonics: Part B – Devices and Applications

a fast Fourier transform and extracting the first three harmonics from this spectrum: the second one to estimate the LMPIE value, and the first and third ones to measure the external field h. The sample was placed in a magnetic field H rapidly rotating in the film plane. The field was formed in a small volume, about 10−3 cm3 , by a system of coils without a core to eliminate the influence of its magnetic noise. The frequency of rotation of the magnetic field in the experiment was 126 kHz. The field amplitude did not exceed 4 Oe, which is sufficient to saturate the sample magnetic film providing a single domain state. Optical radiation passing through the sample was modulated by a magnetic field at a doubled frequency of rotation of the magnetic field due to the LMPIE effect. The inhomogeneity of the rate of rotation of the magnetization of the sample caused by the presence of a constant or oscillating external field h with a low frequency led to the appearance of the first U 1 at 126 kHz and third U 3 at 378 kHz harmonics in the spectrum of the photodetector signal. Thus, the output photodetector signal consists in three harmonics: U 1 , U 2 and U 3 , where U 1 and U 3 are proportional to the measured field h. In the experiment the field h was formed by the reference system of Helmholtz coils with a diameter of 100 mm. The inductance of this coil allowed one to generate a magnetic field at the frequencies below 1 kHz. As a result of the magneto-optical modulation, this spectral range was transferred to frequencies near the first (126 kHz) and third (378 kHz) harmonics of the photodetector signal. Since LMPIE is a resonant effect, the lasers were tuned to wavelengths corresponding to the selected LMPIE values (row “wavelength” in the Table 5.1). A monitored magnetic field h was applied parallel to the slits of the magnetoplasmonic crystal and varied from 1 nT to 10 μT, and the time dependence of the transmitted light intensity at the operating wavelengths corresponding to the maxima of the LMPIE were measured. The noise characteristic of the setup measured for the sample-2 at frequencies less than 1 kHz is shown in the inset of Figure 5.8. In this range the noise level almost does not change with frequency, which is essential for the magnetometry of low-frequency magnetic fields. The photodetector signal of the third harmonics by amplitude U 3 is proportional to h for all three samples (see linear fits for the experimental data for samples 1, 2, and 3 shown with circles, triangles, and squares, respectively, in Figure 5.8). This completely agrees with Eq. (5.9). The largest signal is observed for sample 1 (circles, blue line in Figure 5.8). It exceeds U 3 for the other two samples by six and five times, respectively. Actually, in accordance to Eq. (5.9), for a given monitored magnetic field, the signal of the photodetector is proportional to the product of the incident light intensity, transmittance through the grating and the LMPIE magnitude: U 3 ∼I 0 T 0 𝛿. The relative values of this product are in good agreement with the observations: the product for sample 1 is approximately six times larger than for sample 2 and five times larger than for sample 3. The sensitivity of the magnetometer scheme is also related by the noise level, which is U 3c = 0.225 mV for the laser-A (at 𝜆 = 802.9 nm), U 3c = 0.011 mV for the laser-B (at 𝜆 = 781.0 nm), and U 3c = 0.015 mV for the laser-B (at 𝜆 = 776.6 nm). Therefore, the laser-B is much more suitable for the magnetometer measurements.

5.1 Device Assemble and Application of Iron Garnet Films for Ultrasensitive Magnetometry

Sample 1 102

Sample 2 Sample 3

100

100 U3 (mV)

U3 (mV)

101

10–1

10–2

10–1 10–2 10–3 0

200

10–3 1

10

100

1000

400 600 f (Hz)

10 000

800

1000

100 000

hy (nT)

Figure 5.8 Dependence of the third harmonics amplitude, U3, on the monitored magnetic field h = {0, hy , 0} oscillating at 515 Hz. Inset: The noise characteristic of the magnetometer setup measured for the sample 2.

As a result, the lowest monitored field was detected with the sample-2: hmin = 2.4 nT (red asterisk in Figure 5.8). For the sample-3, it is a bit larger: hmin = 2.9 nT. Potentially, sample-1 would give much better results if laser-B could be tuned to its main resonance at 𝜆 = 804.1 nm; in this case the minimum detectable field would be less than 1 nT: hmin = 0.9 nT. In Figure 5.8, the magnetometer has a large dynamic range. The maximal measured field could be considered about 11 μT for sample 1 and 75 μT for samples 2 and 3: above the given values of the monitored fields, the scheme passes in a nonlinear mode, due to the fact that such values are close to the magnitude of the control field. In conclusion, let us give an estimate of the ultimate sensitivity parameters for a proposed magnetometry scheme. The noise value of the third harmonic of the photodetector current in the frequency band Δf is composed of the shot noise arising from the photodetector and the laser intensity fluctuations that are related to the physical processes of photon generation and are caused by fluctuations in the laser pump current. In view of this circumstance, for the magnetometer noise level hn in the Δf band, one can get (√ ) √ 2H 4𝜋cℏ hn = +𝜈 𝛥f (5.10) δ 𝜂𝜆I0 T0 where 𝜈 is the relative amplitude of the noise modulation of the laser intensity.

173

174

5 Bismuth-substituted Iron Garnet Films for Magnetophotonics: Part B – Devices and Applications

When the photodetector noise prevails over the noise level of the laser (i.e. if 𝜈 ≪ 4𝜋cℏ/(𝜂𝜆I 0 T 0 )), then the sensitivity increases proportionally to the product √ of 𝛿 I0 T0 . However, if the laser noise is larger than the photodetector noise (i.e. 𝜈 ≫ 4𝜋cℏ/(𝜂𝜆I 0 T 0 )), then the sensitivity increases proportionally to 𝛿. In the experimental setup, the noise was mainly due to the laser noise. However, assuming use of an ideal laser, i.e. 𝜈 = 0, so that the only noise source of the measuring scheme is the shot noise, one could find the detection limit of this magnetometer scheme for three samples (see last row in Table 5.1). The highest sensitivity is expected for the sample 1 and might reach 10 pT/Hz1/2 , which is 10 times higher than the estimate for a smooth magnetic film in the previous case. One of the main advantages of the optical reading approach with respect to the induction method is related to a high spatial resolution, which can be obtained. In the lateral direction, it is limited by the diameter of the focused laser beam. To achieve necessary functionality of the plasmonic grating, at least several periods of the grating must be illuminated by the laser beam, increasing the size of the illuminating beam to a few microns. On the other hand, spatial resolution in the orthogonal to the magnetic film direction is determined by the thickness of the magnetic film. The LMPIE magnitude remains at a rather high level even for magnetic films of 100 nm in thickness that provides submicron resolution in this direction. In the end of our discussion of different types magnetometry schemes with iron garnet films and heterostructures, it should be noted that the noise related to the fluctuations of the magnetization, estimated for the square garnet film with 1 × 1 × 0.0001 cm3 with 4𝜋M s = 1750 G and 𝛼 = 0.0001 at room temperature according to [24], is at the level of 1 fT/Hz1/2 . To reach this level of sensitivity, one should overcome the shot noise and other types of noise coming from the detection scheme.

5.2 Devices Assemble and Application of BiIG Films for Biosensing Plasmonic sensors are one of the most sensitivity nowadays [25, 26]. They are based on the detection of resonances resulting from the excitation of surface plasmon polariton waves, whose properties are very sensitive to the optical characteristics of the surrounding medium. By measuring the optical response of a plasmonic structure (the reflection coefficient) and its variation due to the changes in the refractive index of the medium surrounding the structure, one can determine the concentration of various analytes in liquid solutions and gas mixtures very precisely. Selectivity is ensured by the deposition of special adsorbing coatings on the surface of the sensor. Plasmonic sensors are successfully employed in numerous industries, including food quality control and ecological monitoring. However, the sensitivity of such sensors was insufficient for a number of applications. Thus, two approaches that currently seem the most promising for increasing the sensitivity of magnetoplasmonic sensors were proposed. The first one uses the excitation of long-range modes in plasmonic structures, providing for considerably increased Q-factors of the resonances and the enhanced response of the sensor

5.2 Devices Assemble and Application of BiIG Films for Biosensing

to changes in the refractive index of the surrounding medium [27]. Originally, symmetric dielectric/metal/dielectric structures with a thin metal layer were used for this purpose. However, most applications in biology and chemistry deal with liquids and gases, possessing much lower refractive indices than solid dielectrics. For the excitation of long-range modes at a boundary with these media, it was suggested to use photonic crystal structures, whose effective impedance can be matched to the impedance of the medium under study [28]. The second approach was to use magneto-optical rather than optical measurements. Instead of the reflection spectra, the spectra of the transverse MO Kerr effect (TMOKE) in plasmonic structures are measured in this case [29]. Owing to the presence of metal layers, the magneto-optical response of the structure near plasmon resonances is considerably enhanced [30–32]. Magnetoplasmonic resonances feature higher Q-factors than optical resonances, thus allowing to achieve a severalfold increase in the sensitivity of plasmonic sensors. Both approaches were combined in this paper [33] to make a magnetophotonic plasmonic heterostructure for sensor applications. The heterostructure under investigation contains a photonic crystal required for the occurrence of a long-range mode in an asymmetric structure with a gas-phase analyte, a gold film needed for the excitation of surface plasmon polaritons and a magnetic material providing the magneto-optical response. In [33], bismuth-substituted iron garnet, which is a ferrimagnetic dielectric, was used as the magnetic material. Also, it was demonstrated that instead of magnetic dielectric, ferromagnetic metals can be used, for example, thin layers of cobalt [34]. However, a magnetic dielectric instead of a metal to obtain the magneto-optical response increases considerably the Q-factor of the resonance and the sensitivity of the structure in comparison to magnetoplasmonic sensors based on ferromagnetic metals. A basic feature of photonic crystal structures with a magnetic dielectric layer is that they can support both plasmon modes (when additional metal layers are deposited) and surface electromagnetic waves propagating along the boundary of the magnetic layer. Thus, bearing in mind future sensor applications, a direct comparison of the characteristics obtained using photonic crystals both with and without a thin gold layer has been done. Also, magnetoplasmonic structure with cobalt was added to this comparison to show the advantages of the structures with bismuth-substituted iron garnet layer. The structure with iron garnet under investigation is a photonic crystal formed by 16 layers of tantalum pentoxide (Ta2 O5 , 119.3 nm) alternating with 16 layers of silica (SiO2 , 164.7 nm), a 125-nm-thick layer of bismuth-substituted iron garnet and an 8-nm-thick gold film. However, a part of the sample was left without gold coating in order to study properties of surface electromagnetic waves (Figure 5.9). The excitation of a long-range mode in a magnetophotonic plasmonic structure leads to a resonant enhancement of the transverse magneto-optical Kerr effect. The properties of the obtained enhancement determine the advantages of the proposed sensor (Figure 5.10). The width of the optical resonance was 0.05∘ , which corresponds to a Q-factor of 890. The magneto-optical resonance has a still smaller width of 0.02∘ , corresponding

175

5 Bismuth-substituted Iron Garnet Films for Magnetophotonics: Part B – Devices and Applications

Laser

Figure 5.9 Principle scheme of magnetoplasmonic sensor with bismuth-substituted iron garnet magnetic layer.

CMOS matrix Electromagnet

Prism SiO2

Photonic crystal Ta2O5 / SiO2 Garnet layer Au film Gas cell

1.0

0.008 δ 0.004

R (a.u.)

176

0.8 0 0.6

‒0.004

Air

Air Helium

Helium 0.4 44.0

(a)

44.1

44.2 44.3 θ (deg)

44.4

44.5

‒0.008 44.0

(b)

44.1

44.2 44.3 θ (deg)

44.4

44.5

Figure 5.10 (Color online) Angular spectra of the (a) reflectance and (b) transverse Kerr effect for a magnetophotonic plasmonic heterostructure recorded upon the filling of the gas cell with air or helium (black and red curves, respectively). The results of numerical simulations are shown with darker lines.

to a Q-factor of 2200. A variation in the refractive index of the gas under study causes a shift of the resonance by 𝜕𝜃/𝜕n = 39 ∘ /RIU. This yields sensitivities to the refractive-index variation for reflectance and magneto-optical measurements of 𝜕R/𝜕n = 720 RIU−1 for a fixed angle of 𝜃 R = 44.127∘ and 𝜕𝛿/𝜕n = 2.4 × 103 RIU−1 for 𝜃 𝛿 = 44.147∘ , respectively. Calculations indicate that, for an ideal magnetoplasmonic structure with optimum parameters, the width and Q-factor of the reflectance resonance are 0.018∘ and 2.4 × 103 , respectively, while the width and Q-factor of the magneto-optical resonance are 0.005∘ and 8.7 × 103 , respectively. The sensitivity to the refractive-index variation is 𝜕R∕𝜕n = 2.3 × 102 RIU−1 and 𝜕𝛿∕𝜕n = 5 × 103 RIU−1 for reflectance and magneto-optical measurements, respectively. In an ideal structure with no gold

5.2 Devices Assemble and Application of BiIG Films for Biosensing

film, the resonances have smaller widths and higher Q-factors: 𝛥𝜃 SPR = 0.007∘ , 𝛥𝜃 𝛿 = 0.001∘ and QSPR = 6.2 × 103 , Q𝛿 = 4.3 × 104 , respectively. However, the excitation of plasmon polaritons in an ideal sample with a gold film yields a deeper reflectance resonance with a minimum reflectance value of Rmin = 0.17%, in contrast to the structure with no gold film, where Rmin = 1.7%. A greater depth of the optical resonance also gives rise to the enhancement of the transverse Kerr effect in the plasmonic structure: 𝛿 max = 62% and 19% in the structures with and without a gold film, respectively. As a result, in comparison with an ideal magnetoplasmonic structure, the sensitivity of an ideal structure without gold coating appears to be higher for optical measurements (𝜕R𝜕n = 3.7 × 103 RIU−1 ) measured for the fabricated sensor structures along with those calculated for the structures with optimum parameters are listed in the table. Also, the results of theoretical calculations with the ideal structure covered with cobalt and gold layers were added to this table to demonstrate the advantages of applying iron garnet layer. The ideal cobalt structure is one-dimensional photonic crystal consisting of 14 pairs of 118.8-nm-thick Ta2 O5 and 164.7-nm-thick SiO2 . Under the photonic crystal, there is an additional layer of Ta2 O5 with a thickness of 107.5 nm, cobalt and gold film both with a width of 8 nm. Table 5.2 shows that the excitation of long-range plasmon polaritons in a magnetoplasmonic heterostructure leads to a significant increase of magneto-optical sensitivity; as a result one can conclude that samples with a gold layer are appropriate for magneto-optical sensors applications. However, the structure without gold components allows to obtain resonances with higher Q-factor and provides better sensitivity for optical measurements. All previous results were obtained during experiments with air or helium gases. However, all these sensors also can be used for measurements with water chamber. Table 5.2 The excitation of long-range plasmon polaritons in magnetoplasmonic heterostructure. Fabricated structure with a gold film

Fabricated structure without gold film

Ideal structure with a gold film

Ideal structure without a gold film

Ideal structure with cobalt and gold films

Width of the plasmon resonance (deg)

0.13

0.05

0.018

0.007

0.03

Width of the TMOKE resonance (deg)

0.06

0.02

0.005

0.001

0.002

Parameter

Q-Factor

700

2200

8700

43 000

21 000

Sensitivity of the reflectance measurements, RIU−1

250

720

2300

3700

1100

Sensitivity of the TMOKE measurements, RIU−1

18

2400

5000

1500

7500

177

5 Bismuth-substituted Iron Garnet Films for Magnetophotonics: Part B – Devices and Applications

1000

Surface concentration, (mm–2)

178

100

10

1

0.1

0

50

100

150

200

250

Radius of nanoparticles (nm)

Figure 5.11 Surface concentration distribution for three types of ideal structures: red line corresponds to the one with cobalt, black line – iron garnet layer with gold cover, and blue line – iron garnet layer without gold.

For a theoretical analysis of the possible results, a calculation was performed for the available sensors. Spherical nanoparticles are deposited on them. From the parameters of nanoparticles and the studied material, the minimum surface concentration of nanoparticles, which can be measured by sensors of each type, was determined. Figure 5.11 shows the dependence of the minimum surface concentration, which can be measured experimentally, on the radius of the studied nanoparticles. All the previous data confirm the wide spectrum of sensor applications for the structures with iron garnet layers. The excitation of surface waves leads to a significant increase of the structure’s sensitivity. The potential of biosensors is rated quite high. The speed of detection, high detection thresholds, and low cost of components are factors to consider for an effective replacement for existing technologies in the long term.

5.3 Devices Assemble and Application of Iron Garnet Films for Magneto-optical Eddy Current Flaw Detection 5.3.1

Introduction

The generation of the eddy currents (ESs) and the accompanying magnetic fields (MFs) are widely used in the technology, and recently in medicine. Eddy current probes are used for the rapid analysis of cardiac activity, to register venous pulse, hyperthermia, etc. [35]. In some cases, it is necessary to know the magnetic field’s distribution of magnetic nanoparticles [36] of various organs of the human [37]. Perspective materials based on epitaxial ferrite garnet (EPFG) films can be used for

5.3 Devices Assemble and Application of Iron Garnet Films

magnetic field sensors. There are various ways of creating magnetic sensors using EPFG. In [8, 38] a high-sensitivity magnetic modulation sensor, which can compete with the SQUID, is used, for example, in magnetocardiography. On the basis of EPFG can be created magneto-optical sensors. The main advantage of such sensors is the possibility of direct observation of the topography of magnetic fields generated by magnetic objects [39–41] or eddy currents excited in the conductive objects [42–46]. In reviews [47, 48], physical principles and various applications of magneto-optical films are considered, and the properties of uniaxial and planar magneto-optical indicator films are described in detail. The authors note that in recent years, the MO imaging has turned into a very useful technology with numerous applications and, despite a number of limitations, continues to develop. This paper presents the results of studies of the application of eddy current magneto-optical imaging for the determination of various defects in magnetic and nonmagnetic objects of control. The principle of operation of the eddy current magneto-optical (MO) introscope is based on the reaction of the magnetization vector or dynamic domain structure (DDS) of the MO sensor to the distribution of magnetic fields generated by eddy currents (EC) in the test sample of conductive material. An inductor of alternating magnetic field excites eddy currents. Visualization of changes in the magnetic system of the magneto-optical sensor is provided by the Faraday effect. Defects in the test object lead to a change in the trajectory of the eddy currents and to a corresponding change in the configuration of the magnetic fields they generated. Transparent magnetic films based on BiIG with maximum values of the Faraday effect are usually used as MO sensors. The optical contrast, size, and quality of the MO image depend on such parameters as the frequency and amplitude of the EC of the exciting field, the bias field, and also the manifestation in certain cases of nonlinear properties of the dynamic domain structure [49, 50].

5.3.2

Experimental Part

The BiIG films with nominal compositions of Re3−x Bix Fe5−y Mey O12 (0 < x < 2, 0.3 < y < 0.7), where Re–Y, Sm, Tm, Gd, Lu, Ca and Me–Al, Ga, Sc, Ge, Si, and Be have been synthesized by means of liquid phase epitaxy (LPE) method on the monocrystalline gadolinium gallium garnet Gd3 Ga5 O12 substrates with orientation (111) [51–53]. The thickness of the films varied in the range from 2 to 12 μm. Depending on the requirements to the magnetic properties of the samples, the necessary chemical elements have been introduced into the composition, in order to minimize the mismatch of the lattice parameters 𝛿 = (a f − a s )/as , where af and as are the lattice parameters of the film and the substrate. The magnetization and anisotropy in films have been adjusted by replacing Fe ions with Al and Ga ions, as well as with Be–Si, Ca–Ge pairs. Films with different types of magnetic crystallographic anisotropy easy plane (EP) and easy axis (EA) have been investigated for magneto-optic eddy current (MOEC) visualization of different defects in nonmagnetic and magnetic metal material. EA films are characterized by a period of equilibrium domain structure

179

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5 Bismuth-substituted Iron Garnet Films for Magnetophotonics: Part B – Devices and Applications

5–32 μm, effective uniaxial anisotropy field 0.5–3 kOe, saturation field H S 22–77 Oe. Out-of-plane magnetic field saturation in EP films H s was 235–328 Oe. The test samples of linear defects of the “through-slit” type with a width of 20–500 μm and defect-free welds of aluminum and stainless steel, as well as defective welds in steel pipes, made by electric and gas welding, have been investigated. The study has been carried out in the frequency range f from 8 to 80 kHz. The intensity of the alternating field of the inductor H i can vary from 2 to 400 Oe, and the bias field H_ from 0 to 40 Oe.

5.3.3

Introscope

Simplified block scheme of the experimental equipment is presented in Figure 5.12. The device consists of optical part for registration in the real-time video in reflected polarized light, changeable inducers, generator (frequency f range from 1 to 80 kHz), LED matrix, synchronizer, and MO EPFG. An alternating current I is generated in the inductor, and the domain structure (DS) of the EPFG responds to the generated field when it increases or decreases. An important aspect of the operational principle of the stroboscopic MO introscope is the correct choice of the exposition moment (strobe phase 𝜃) and duration. The video system or the illuminator (LED) must provide registration of images only at the certain moments. Pulsing mode of operation (in comparison with harmonic one) is conditioned by the simple electronic scheme realization and high efficiency of power amplifier. Triangular waveform of EC excitation is realized because the active resistance of all changeable inducers is much lower than inductive one. A programmable microcontroller is used. It generates signals for feeding the inducer and delivers a short gating pulse to the LED. The controller firmware selects the frequency of current in the inducer, time and duration of the strobe. Inducer with normal and in-plane field orientation to film surface was used. Figure 5.13 presents the picture of the eddy current excitation by a normal field inductor in the vicinity of a linear defect. It can be seen that the distribution of eddy currents changes in the vicinity of the defect. The alternating field N ∼ generates two 6 11

7

10 5

8

9 2

4

3 1

Figure 5.12 Simplified scheme of the introscope: (1) object for inspection; (2) EC inducer; (3) garnet film (MO sensor); (4) generator; (5) synchronizer unit; (6) LED matrix; (7) glass diffuser; (8) polarizer; (9) analyzer; (10) objective lens; and (11) CCD matrix.

5.3 Devices Assemble and Application of Iron Garnet Films

Figure 5.13 Separation of the eddy current contour by linear defect.

H𝛛n H~ H𝛛t

EC contours (Figure 5.13) and each of them forms EC magnetic field with normal H dn and tangential H dt components.

5.3.4

Physical Properties of MO Sensors

Figure 5.14 demonstrates equilibrium domain structure of easy axis garnet film. It is the binary mazelike domain structure with the magnetic moments in opposite directions. If normal oriented bias magnetic field N_ exists, domain structure is modified and at N_ > H S domain structure disappears. In the resulting field H eff = N_ + (H ∼ − H dn ), domain structure “reflects” topology of stray fields from the defect. The H eff magnitude depends on frequency f , inducer field amplitude and N_. The optical and magneto-optical properties of the films have been investigated in the wavelength range of 500–950 nm. The maximum values of the specific Faraday rotation at a wavelength of 520 nm for EA and EP samples are 1.6 deg/μm and 0.74 deg/μm. An important characteristic of MO sensors is the spectral dependence of the magneto-optical Q factor of the sensors. It determines both the sensitivity of the sensors and the type of the most optimal sources and receivers of optical radiation necessary for the functioning of the MO introscope. The magneto-optical quality factor of the sensors is estimated by the formula: Q = 2θf ∕ ln T

(5.11)

where 𝜃 f is the Faraday rotation value, T is the optical transmission. Figure 5.15 shows the spectra of radiation sources and magneto-optical quality factor of one of the EA MO sensors. One can see that the maximum of the Q factor is at the wavelength of 575 nm and the most optimal radiation source is warm LED, whose spectrum has the greatest overlap with the spectrum of MO of quality factor. Figure 5.14 Equilibrium domain structure in EA MO sensor at 100x optical zoom. Source: Nazar V. Lugovskoy.

181

5 Bismuth-substituted Iron Garnet Films for Magnetophotonics: Part B – Devices and Applications

100

24

Warm LED

22

Netural LED

20

Relative radiant power (%)

80 70

18

Cold LED

16

60

Halogen lamp

14

50

12 40

10

30

8

20

6

10

4

0

2

Magneto-optical quality factor

90

400 450 500 550 600 650 700 750 800 850 900 950 1000 Wavelength (nm)

Figure 5.15 The spectrum of the Q-factor for EA film and the emission spectra of various light sources.

10 3

8

2

Faraday rotation (deg)

Faraday rotation (deg)

182

1 0

–1

6 4 2 0 –2 –4 –6

–2

–8 –3

–10 –60

–40

(a)

Figure 5.16

–20

0 20 H (Oe)

40

–400

60 (b)

–200

0 H (Oe)

200

400

MO hysteresis loops at the wavelength of 640 nm for EA (a) and EP (b) films.

An important characteristic of films for operation as sensors in the MOEC of flaw detection are the field dependences of magnetization, which largely determine their sensitivity. Figure 5.16 presents similar dependences, measured at the wavelength of 640 nm, which for EA and EP films have hysteresis and hysteresis-free character, respectively. The main parameters that determine the sensitivity of the films are the normalization field H n for the EP films, the saturation field H s , and the coercive force H c for EA sensors.

5.3 Devices Assemble and Application of Iron Garnet Films

The coercive force H c and the type of domain structure are important factors in the operation of the MO introscope because they determine the remagnetization process in alternating magnetic fields. The process of visualization of the distribution of magnetic fields of eddy currents depends on the dynamics of the domain structure in EA sensors. In all EA films, a labyrinth domain structure is observed with a period from 5 to 32 μm. When using EPFG as sensors in the MOEC introscopy, certain conditions are imposed to the roughness and defectiveness of film surface. This is necessary for the rapid remagnetization of the film for EC magnetic field visualization, sinc domain walls are very sensitive to defects in the film structure and can linger on them. The average roughness of the studied films has been measured by atomic force microscopy using an NTEGRA scanning probe microscope and it does not exceed 5 nm [54]. The value of H c is determined by the defectiveness of the films and in the studied EA films will vary from 0.5 to 2.7 Oe. The advantage of EP films is the hysteresis-free nature of its magnetization reversal (Figure 5.16b). The normalization and saturation fields are determined by the values of the effective magnetic anisotropy field: a Heff = Hdem + Hu + Hk

(5.12)

where H dem = 4𝜋 ms , demagnetizing field of the sample form, H u = 2K u /M s is the field of uniaxial anisotropy, and H k = −4/3 K 1 /M s is the field of cubic magnetic anisotropy. The uniaxial anisotropy constant to the greatest extent is determined by the magnetoelastic contribution due to the magnitude and sign of the lattice parameters mismatch 𝛿. The cubic anisotropy constant in Bi-garnets is usually small, so that H k < H dem , H u . To determine the effective magnetic anisotropy fields N a eff , as well as to assess the degree of film homogeneity, the ferromagnetic resonance (FMR) method has been used. FMR spectra for the EA (a) and EP (b) films are presented in Figure 5.17. The effective magnetic fields of anisotropy N a eff are determined from FMR data on the 800 6000

EP

0 90

4000

0 90

600 400

2000 I (a.u.)

I (a.u.)

200 0 –2000

0 –200

–4000 –400 –6000 –600 –8000 –800 0

100

200

300

400

500

0

H (mT)

(a)

100

200

300

400

500

H (mT)

(b)

Figure 5.17 FMR spectra for the EA (a) and EP (b) sensors with excitation field in the normal (0) and in the plane (90) to the sample surface.

600

183

5 Bismuth-substituted Iron Garnet Films for Magnetophotonics: Part B – Devices and Applications

base of well-known Kittel formulas for the resonance fields in perpendicular and parallel configurations [55].

5.3.5

The Sensory Properties of the EA Films

5.3.5.1 The Effect of Alternating Field Amplitude

Figure 5.18 demonstrates the MOEC image of linear defects with dimension 20 μm (f = 60 kHz, H ∼ = 0–180 Oe). One can see that in comparatively low fields the defect is presented as a white zone. At fields that are higher than 80 Oe, in the center of this zone, a dark strip appears in accordance with opposite orientation of the normal component of EC field. The dimension of MOEC images increases with increasing of the field magnitude. An accuracy of the white zone determination is lower than of the dark one, contrast of which is higher. The dimension of the 20 μm defect MOEC image can be increased more than 20 times for white zone and 10 times for dark one (Figure 5.19). It means that a MOEC flaw detector can be considered as electromagnetic microscope, in which optical image can be varied by the alternating field tuning. At very high amplitudes of alternating field in the dark zone of image a “thin structure” arises due to the appearance of domains with opposite magnetization directions. This obstacle complicates the determination of real defect configuration. 5.3.5.2 The Effect of Alternating Field Frequency

Figure 5.20 demonstrate the frequency dependence of the pair of linear defects MOEC image at weak H ∼ = 60 Oe and strong H ∼ low value of alternating field. The width of the white and dark zone in MOEC image is decreased when the frequency grows.

(a)

(c)

(b)

(d)

Figure 5.18 Dependence of 20 μm slit MOEC image on the alternating field amplitude at f = 60 kHz, H_ = 0 Oe, H∼ : (a) 20, (b) 60, (c) 100, (d) 180 Oe. Source: Nazar V. Lugovskoy. White Dark

500 400

a (μm)

184

300 200 100 0 20 40 60 80 100 120 140 160 180 200

H~ (Oe)

Figure 5.19 Dependence of the width of white and dark zones of the 20 μm slit MOEC image on the alternating field amplitude H∼ at f = 60 kHz, H_ = 0 Oe.

5.3 Devices Assemble and Application of Iron Garnet Films

(a)

(b)

(c)

(d)

(b)

(c)

(d)

(A)

(a) (B)

Figure 5.20 Dependence of the MOEC image of the line defects on the frequency at weak 60 Oe (A) and strong 120 Oe (B) alternating magnetic field, f : (a) 8, (b) 15, (c) 30, (d) 60 kHz. Source: Nazar V. Lugovskoy. Figure 5.21 Dependence of the width of the white zone of MOEC image on frequency at H∼ = 60 Oe. a (μm)

2400

1800

1200

600

0.1

0.2

0.3

0.4

f –1/2

Figure 5.21 shows the frequency dependence of white zone width in the 20 μm defect MOEC image. One can see that it correlates with frequency dependence of skin-layer depth, which has the same character. The contrast of the MOEC image increases with the amplitude increasing of the alternating field; however, the fine structure of the dark zone (Figure 5.20) is more distorted with decreasing frequency. Chaotic vortex formations appear in the domain structure of the garnet film. In the beginning, they concentrate on the boundaries of the dark zone of the image, and at low frequencies – in the entire field of view. 5.3.5.3 The Effect of Bias Magnetic Field

The effect of an out-off-plane uniform external magnetic field (bias field) H_ on the EC MO image in EA sensors is shown in Figure 5.22. With the increase in the bias field, the width of the dark zone in the MO image of slit increases and the MO image of the defect disappears at large fields (Figure 5.23). The bias field improves the quality of MO visualization of linear defects in the field range H_ = 0–10 Oe. At N _ = 20 Oe, a vortex domain structure appears on the boundaries of the dark zone, which makes it difficult to determine the real width of

185

5 Bismuth-substituted Iron Garnet Films for Magnetophotonics: Part B – Devices and Applications

(a)

(b)

(c)

(d)

(e)

(f)

Figure 5.22 Dependence of MOEC image on the bias field at f = 60 kHz, H∼ = 100 Oe, H_: (a) 0, (b) 8, (c) 20, (d) 32, (e) 36, and (f) 40 Oe. Source: Nazar V. Lugovskoy. Figure 5.23 Dependence of the width of the dark zone of the MOEC image of the slit on the bias field (H_) at f = 60 kHz, H∼ = 100 Oe.

1250 1000 a (μm)

186

750 500 250 0

5

10

15

20

25

30

35

Hbias (Oe)

the MOES image. The disappearance of the MOES image of defects at H = 40 Oe is associated with saturation of the film. Such changes are possible because the bias field H_ on the one hand changes the configuration of the equilibrium domain structure in the garnet film, and, on the other hand, it shifts the working point on the static magnetization curve, relative to which the modulation of the magnetic state of the garnet film occurs. Based on these experiments, the best parameters for MOEC introscopy of linear defects in aluminum alloys EA sensors are an inductor current frequency of 60 kHz, an alternating magnetic field of 100 Oe, and a bias magnetic field of 8 Oe. 5.3.5.4 Dynamic Domains in the Garnet Film Sensor Element

To understand the behavior of dynamic domain structure in EA sensor regime of garnet film, it is necessary to investigate its behavior in such regimes but without contact with object of control. The dynamic domain structure in garnet films at low frequencies has been investigated in [47, 48].Formation of different and mostly axis-symmetrical dynamic structures, such as spiral and ring domains, bubble domain lattices depends on the frequency and amplitude of alternating fields. The processes of self-organization in domain structure take place continuously

5.3 Devices Assemble and Application of Iron Garnet Films

(a)

(b)

(c)

(d)

Figure 5.24 Dynamical domain structure in the EA garnet film in the different alternating magnetic fields at f = 15 kHz, H_ = 0 Oe, H∼ : (a) 60, (b) 100, (c) 120, and (d) 140 Oe. Source: Nazar V. Lugovskoy.

(a)

(b)

(c)

(d)

Figure 5.25 Giant dynamic domain structures in the EA garnet film at different frequencies, H∼ = 120 Oe, N _ = 0 Oe, f : (a) 8, (b) 15, (c) 30, and (d) 60 kHz. Source: Nazar V. Lugovskoy. Figure 5.26 Dependence of giant stripe domains width from the frequency at N ≅ 140 Oe, N _ = 0 Oe.

1200 1050

d (μm)

900 750 600 450 300 150 10

20

30

40

50

60

70

80

f (KHz)

at strong alternating magnetic fields, and they depend on excitation conditions, magnetic parameters of garnet films, and their defectiveness. All the processes can be manifested in the MOEC introscopy and make difficult flaw detection. In fact, at the low amplitude of alternating field, the stochastic domain structure (Figure 5.24) transforms in the spiral domain structure with spiral period about 26 μm (Figure 5.24a). With increasing of alternating field up to 120 Oe, the dynamic chaos appears (Figure 5.24c). In the field H ∼ = 140 Oe, this state is replaced by the giant stripe dynamic structure with dimensions 150 μm (Figure 5.24d). The analogous behavior is observed at all frequencies. With frequency growth, the field of domain structure stochastization increases. The images of giant domain structure in the film at different frequencies are clearly seen in Figure 5.25. The size of giant domains increases at frequency decreasing of an alternating magnetic field (Figure 5.26).

187

188

5 Bismuth-substituted Iron Garnet Films for Magnetophotonics: Part B – Devices and Applications

(a)

(b)

(e)

(c)

(f)

(d)

(g)

Figure 5.27 Effect of the alternating field (a–d) and frequency (e–g) on the detection efficiency of a 40 μm slits in Al sample by the EP MO sensor. f = 30 kHz, H∼ = 12 (a), 16 (b), 20 (c), 32 Oe (d); bottom row H∼ = 8 Oe, f = 15 (e), 30 (f), 60 kHz (g). Source: Nazar V. Lugovskoy.

Thus, the distortion of the magneto-optical eddy current introscopy image of the defect at large amplitudes of the alternating field, which grows with decreasing frequency (Figure 5.20), is associated with non-threshold dynamic structures that are observed in EA garnet films. These features of EA garnet films impose certain restrictions on their use in magneto-optical eddy current detection of defects.

5.3.6

The Sensory Properties of the EP Films

The results of visualization of slits in the Al sample by EP films differ from the results obtained by EA films (Figure 5.27). At any amplitudes of the alternating field, linear defects are displayed only by the light bands. The sensitivity of the EP sensor increases with increasing excitation field (Figure 5.27a–d) and frequency (Figure 5.27e–g), which can be seen from the increase in contrast when changing these parameters. In EP sensors, the bias magnetic field does not affect the visualization of defects, unlike in EA films, where the bias field modifies the domain structure, which is absent in EP films.

5.3.7

Applications of MOEC: Imaging of Welds

We have investigated the possibility of analyzing the quality of welds in products from magnetic and nonmagnetic metal alloys using MOEC introscopy with EA sensors. Thus, we used alternating field inductors of two configurations, which created a magnetic field either in the plane of the sample (parallel inductor) or perpendicular to it (normal inductor). 5.3.7.1

Nondefective Welds

Model nondefective welds made from both magnetic and nonmagnetic materials have been investigated. Figure 5.28 shows optical and magneto-optical images of

5.3 Devices Assemble and Application of Iron Garnet Films

(a)

(b)

(c)

(d)

(e)

(f)

Figure 5.28 Optical (a,d) and magneto-optical (b, c, e, f) images of welds: aluminum (a–c), stainless steel (d–f); orthogonal (b,e) and longitudinal (c,f) excitation of eddy currents; f = 25 kHz, H∼ = 150 Oe, N _ = 3.6 Oe. Source: Nazar V. Lugovskoy.

welds in aluminum and stainless-steel samples measured at f = 25 kHz and alternating magnetic field H ∼ = 150 Oe. It can be seen that the visually hardly visible (Figure 5.28a,e) welds are clearly visible in the MOEC images for both types of welds. In a stainless-steel sample, the sensitivity of the MO sensor is significantly higher than in aluminum samples. In such a sample, in the field of a normal inductor as in Figure 5.28e, a binary MO EC image is observed, while a dynamic vortex-like domain structure fills the entire space outside the seam. The sensor above the seam in the field H ∼ = 120 Oe is in a saturated state, which produces a clear and contrast image of the weld. In a parallel field in the MO images (Figure 5.28c,f), an “analog picture” of the weld structure is observed, especially on stainless steel, where all the inhomogeneities that are formed during the welding process and are invisible become visible. In the same figures, we see clear boundaries of the weld, represented by a narrow black and white frame of the defect. This indicates the opposite direction of the magnetic field of the eddy currents at the weld boundaries during longitudinal excitation of eddy currents. The influence of the frequency of an alternating magnetic field on the MO visualization of welds is investigated. It was found that the effectiveness of MO imaging decreases with increasing frequency (Figure 5.29) and in order to increase

(a)

(b)

(c)

(d)

Figure 5.29 Magnetooptical images of a weld in a stainless-steel sample with orthogonal excitation of eddy currents, f : (a) 8, (b) 15, (c) 30, and (d) 45 kHz. Source: Nazar V. Lugovskoy.

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5 Bismuth-substituted Iron Garnet Films for Magnetophotonics: Part B – Devices and Applications

its efficiency, an increase in the amplitude of an alternating magnetic field is required. Analysis of MO images in magnetic and nonmagnetic samples shows that the magnitude of the EC magnetic fields is much larger in stainless steel samples because these samples have significantly higher values of magnetic permeability. Therefore, saturation of epitaxial ferrite-garnet films at the locations of defect mapping caused by EC magnetic fields occurs in a magnetic sample at lower values of H ∼ = 120 Oe than in a nonmagnetic sample H ∼ = 300 Oe. Such a state leads to a contrasting and clear image of defects. The use of the orthogonal bias field H_ leads to a significant decrease of these values. For example, a bias field, H_ = 24 Oe, reduces the saturation fields up to 80 Oe and 220 Oe for magnetic and nonmagnetic samples, respectively. The effect of the constant bias field and the frequency of the alternating field on the MO imaging of defects in magnetic samples is more pronounced than in the nonmagnetic ones [56].

5.3.7.2

Defective Welds

To test the operation of the MOEC method under real conditions, samples of welds of steel pipes with a diameter of 50 and 70 mm using electric and gas welding were made. MO introscopy was carried out at frequencies from 8 to 60 kHz, in alternating magnetic fields from 40 to 440 Oe. It is shown that the optimal conditions in this case are the following: f = 25 kHz, H ∼ = 80 Oe, the bias magnetic field N_ = 0 Oe. Optical and MO images obtained under these conditions are shown in Figure 5.30. MO images in Figure 5.30 are obtained with equal parameters of alternating and constant fields, the difference is only in the direction of generation of the excitation field. It is clearly seen that the MO image of the defective area contains much more information than a simple optical snapshot. The use of the MOEC method allows detecting defects that lie in the depth of the seam [56]. An interesting result was received at using of ferrite garnet film with an easy-plane anisotropy as a sensor, thus obtaining an “analog picture” of the welds on a stainless-steel sample using an inductor of a normal field.

(a)

(b)

(c)

Figure 5.30 The defects in welds of steel pipes: optical (a) and magneto-optical (b,c) images; orthogonal (b) and longitudinal (c) excitation of eddy currents. Source: Nazar V. Lugovskoy.

5.3 Devices Assemble and Application of Iron Garnet Films

150 120 90 60

2000

Hcr

δ, (MA/M)

30 Dark area

0 –30 –60

H (A/M)

1500

White area

1000

–90 –120 –150

300

0 –1.2

(a)

–4 –1.4 0 0.4 y(mm)

1.2

–2

0

2

4

y (MM) (b)

Figure 5.31 MOEC image of slit and Hz – simulation over a slit (a), the distribution of eddy current in the cross section of the Al sample with a slit (b). Source: Nazar V. Lugovskoy.

5.3.8

Simulation of EC Magnetic Fields in Samples with Defects

The distribution of eddy currents and the magnetic fields induced by them in the vicinity of a linear defect was simulated by the finite element method in the Comsol Multiphysics CAD system [53] and the direct method of integral differential equations relating the density of excited electric charges, eddy currents, and magnetic fields generated by them [56–58]. The magnitude of the magnetic field acting on the magneto-optical sensor is superposition field of the inductor and eddy current of the defect sample. The calculated distribution of the z-component of the resulting alternating magnetic field acting on the EA magneto-optical sensor was analyzed. Black and white images of defect correspond to different signs of alternating magnetic field along normal to defect (y-direction, Figure 5.31). When the amplitude of the alternating field increases, the magneto-optical images are broadened, reflecting a spatial broadening distribution of eddy current magnetic fields [57, 58]. Figure 5.32 shows the model distribution of the normal component of the EC magnetic field in the vicinity of the defect for different frequencies of eddy currents excited by the inductor. It is seen that as the frequency increases, the maximum of Hz -component of the EC field increases, and the width of the distribution decreases. For comparison with the experimental data obtained on easy-plane MO sensors, it is necessary to present the brightness of the MO image of defect (Figure 5.27) as a digital dependence. For processing and analysis of obtained experimental magneto-optical images, an image analysis program written in the MatLab package was compiled. It is based on

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5 Bismuth-substituted Iron Garnet Films for Magnetophotonics: Part B – Devices and Applications

8000 Hz 15000 Hz 30000 Hz 60000 Hz

10 8 6

Bz (mT)

60

MO image width FWHM Bz Maximum Bz

1.6

50

1.4 40

1.2 1.0

30

0.8

20

4 2

0

12.8

25.6

38.4

51.2

64

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0

89.6

10

20

30

40

50

60

Frequency (kHz)

x (mm)

(a)

10

0.6

0

Maximum Bz (mT)

Bz(x)

Width(mm)

192

(b)

Figure 5.32 The frequency dependences of the Bz -component of the EC field distribution near the defect (theory) (a), and the maximum and the width of the Bz (FWHM) distribution (theory) and the width of the EP film MO image (experiment) (b).

the translation of a graphic file into a special matrix, which contains the brightness value of all pixels of the MO image. The electrodynamic simulation of the EC magnetic fields in the sample under study adequately describes the change in the brightness of the MO of the image of the sensor obtained by the EP film with increasing amplitude and frequency of the alternating field. For example, Figure 5.32b shows the dependence of the half-width of the experimental MO image in the frequency range from 8 to 60 kHz and theoretically predicted values of this parameter found by mathematical processing. We can see that they are in good agreement. In accordance with theoretical calculations, an increase in the amplitude of the maximum with a frequency (Figure 5.32b) also leads to an increase in the contrast of the magneto-optical image in the experiment (Figure 5.32) [58]. Figure 5.33 shows the results of modeling a magnetic field above the surface of the welds. When manufacturing a welded seam, a change in the structure of the material inevitably occurs, which leads to a decrease in the conductivity of the conductor in the weld region. In calculations, the conductivity of the weld is assumed to be 1.5 × 107 S/m, which is 40% of the conductivity of pure aluminum. As can be seen, the presence of a weld with a lower conductivity than the conductivity of adjacent regions of the conductor leads to a significant increase in the contrast of the topogram of the magnetic field. The boundaries of the seam are clearly traced. It is natural to expect that the higher the difference in conductivity of the conductors, the greater contrast of the picture of the field, and, consequently, the MO image of variable part of sample will also be more contrast. The presence of ferromagnetic properties in the sample from stainless steel leads to the appearance of magnetization currents. In contrast to eddy currents, the effect of which leads to a decrease in the resulting magnetic field, the magnetization currents tend to increase this field. Therefore, in the case of a non-ferromagnetic sample, the magnetic field is always less than in the case of a ferromagnetic sample, even with a relatively low magnetic permeability, as in stainless steel. The obtained distribution of the normal component of the magnetic field acting on the MO sensor in the vicinity of the welds is in qualitative agreement with the experimental results.

Acknowledgments 1600

420 380

1

340

1200 2 H (A/m)

H (A/m)

300 260 220

800

1 180 140 100

–10

(a)

400 2 0 –6

–2

0 2 y (mm)

6

–10

10

(b)

–6

–2

0 2

6

10

y (mm)

Figure 5.33 Distribution of the normal component of the EC magnetic fields above the sample with a welded seam: (a) non-ferromagnetic sample without (1) and with weld (2); (b) non-ferromagnetic (1) and ferromagnetic (2) samples with welds.

5.4 Conclusions and Perspectives Multicomponent films of Bi garnets with easy plane and easy axis magnetic anisotropy for eddy current magneto-optical flaw detection were synthesized by the LPE method. The spectral dependences of the optical transmittance, the Faraday effects and the magneto-optical Q factor were measured. The magnetic characteristics of the films, the saturation field and the coercive force for films with easy axis and the normalization field for films with easy plane were determined by the magneto-optical method. The effective magnetic anisotropy fields were determined by the FMR method. A comparative analysis was performed regarding the sensory effectiveness of the easy plane and the easy axis films in the detection of various defects by eddy current magneto-optical introscopy in a wide range of frequencies, as well as alternating and bias magnetic fields. Possible research topics and applications can be discussed freely as one unique section.

Acknowledgments This work is financially supported by the Russian Foundation for Basic Research, projects N 18-52-80038 and 18-29-02120, and the NSFC-BRICS STI Framework Program (No. 51861145309). The authors from V.I. Vernadsky Crimean Federal University TVM and ANSh acknowledge support by grant of Russian Science Foundation (project no. 19-72-20154) for modelling of one-dimensional magnetophotonic crystals with bi-layers.

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List of Abbreviation and Symbol SQUID OPM BiIG MCG EMF LMPIE TMOKE ES

MF EPFG MO DDS LPE EP EA MOEC DS FMR

Superconducting quantum interferometer magnetometer optically pumped magnetometer bismuth-substituted iron garnet magnetocardiography electromagnetic filter longitudinal magnetophotonic intensity effect transverse MO Kerr effect eddy current magnetic field epitaxial ferrite garnet films magneto-optical dynamic domain structure liquid phase epitaxy easy plane easy axis magneto-optic eddy current domain structure ferromagnetic resonance

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47 Grechishkin, R., Chigirinsky, S., Gusev, M. et al. (2008). Magnetic imaging films. In: Magnetic Nanostructures in Modern Technology (eds. B. Azzerboni et al.), 195–224. Springer. 48 Grechishkin, R., Kustov, M., Ilyashenko, S. et al. (2016). Magneto-optical imaging and analysis of magnetic field micro-distributions with the aid of biased indicator films. J. Appl. Phys. 120 (174502). 49 Kandaurova, G.S. (2002). New phenomena in the low-frequency dynamics of magnetic domain ensembles. PHYS-USP 45 (10): 1051–1072. 50 Kandaurova, G. and Svidersky, A. (1990). Self-organization processes in multidomain magnetic media and the formation of stable dynamic structures. JETPh. 97: 1218–1229. 51 Berzhansky, V.N., Karavainikov, A.V., Mikhailova, T.V. et al. (2017). Nano- and micro-scale bi-substituted iron garnets films for photonics and magneto-optic eddy current defectoscopy. Journal of Magnetism and Magnetic Materials: 440, 175–178. 52 Prokopov, A.R., Vetoshko, P.M., Shumilov, A.G. et al. (2016). Epitaxial Bi-Gd-Sc iron-garnet films for magnetophotonic applications. Journal of Alloys and Compounds 671: 403–407. 53 Lugovskoy, N., Berzhansky, V., Glechik, D., and Prokopov, A. (2018). Epitaxial film of garnet ferrite with anisotropy “easy plane” for magneto-optical eddy current flaw detection. J. Phys.: Conf. Ser. 1124: 051063, 4 p. 54 Lugovskoy, N.V., Berzhansky, V.N., E. Yu. Semuk et al. (2019). Magneto-optical properties of easy-plane and easy-axis garnet ferrite films for eddy current testing. IOP Journal of Physics: Conference Series in print. 55 A.G. Gurevich, G.A. Melkov Magnetization Oscillations and Waves Magnetization Oscillations and Waves: 1996, CRC-Press P. 464 56 Berzhansky, V.N., Lugovskoy, N.V., Filippov, D.M., et al. (2017). Investigation of welds by the method of magneto-optical eddy current flaw detection. EPJ Web Conference Moscow International Symposium on Magnetism (MISM 2017) V 185. 57 Filippov, D.M., Kozik, G.P., A.V. Fursenko, and V.N. Fedorovsky (2016). The Secondary Sources Method Analysis and Experimental Modeling of the Permanent Magnet Eddy Currents, International Conference on Industrial Engineering, Applications and Manufacturing, ICIEAM, 2 58 Berzhansky, V.N., Filippov, D.M., and Nazar, V. (2016). Lugovskoy/magneto-optical visualization of Eddy current magnetic fields. Physics Procedia C 82: 27–31.

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6 MEMS, NEMS, AEMS, and Quantum Films for the Next Generation of Computing and Information Technology Haishuai Chai 1 , Junmei Wang 1 , and Yujun Song 1,2 1 University of Science and Technology Beijing, Center for Modern Physics Technology, Applied Physics Department, School of Mathematics and Physics, 30 Xueyuan Road, Beijing 100083, China 2 Zhejiang Key Laboratory for Pulsed Power Technology Translational Medicine, Hangzhou Ruidi Biotechnology Company, Hangzhou 310000, China

6.1 Introduction Microelectromechanical systems (MEMS) adopt silicon micromachining technology to manufacture thousands of microelectronic devices (the metal oxide semiconductor field-effect transistor, MOSFET) on a single silicon wafer, which addresses the bottleneck of mass production of traditional processes, thereby reducing costs and improving efficiency in the integrated microelectronic circuits. MEMS conjugate thermal, optical, magnetic, chemical, biological, and other functional structures and devices onto microchips through microelectronics and other micromachining processes and through integration with circuits and constructing complex microsystems. Due to the high degree of cross-linking and permeability of MEMS, research and development efforts are diverse. As great progress has been achieved in microfabrication and microprocessing technologies, the size of electronic devices has been scaled down to submicrometers. This is especially embodied in the Si metal oxide semiconductor (MOS)-based devices, e.g. chips and memories, obeying the well-known Moore’s law. To minimize the short-channel effect, the thickness of dielectric oxide in a MOS device is approaching a few nanometers [1, 2]. Recently, basic research in MEMS based on nanotechnology has been scaled down to the nanoscale or even atom level in one or more dimensions, leading to nanoelectromechanical systems (NEMS, or AEMS). Usually, thin films are modeled by a rectangular deep potential well. However, in recent years, the profile of the wells, for example, parabolic or other forms, has been considered. Due to the technology of film growth, their surfaces may not be the same. In the case of classical films, the difference in surfaces could be considered, most likely, by the introduction of different surface scattering parameters or by different band bends [3, 4]. In the case of quantum films, a solution of the Poisson equation with given surface charges, which is an asymmetric potential well, could determine the difference in surfaces. This will affect the energy spectrum of the carriers [4]. Inorganic and Organic Thin Films: Fundamentals, Fabrication, and Applications, First Edition. Edited by Yujun Song. © 2021 WILEY-VCH GmbH. Published 2021 by WILEY-VCH GmbH.

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6.2 Typical Fabrication Methods for MEMS, NEMS, and AEMS 6.2.1

Fabrication of Microstructures

Microstructures are usually fabricated by the LIGA process. LIGA process was first developed at the Institute for Microstructure Technology (IMT) (at the Research Center Karlsruhe) in the early eighties under the leadership of Dr. W. Ehrfeld [5]. The acronym LIGA comes from the German name for the process (Lithographie, Galvanoformung, Abformung). LIGA uses lithography, electroplating, and molding processes to produce microstructures. It is capable of creating very finely defined microstructures of up to 1000 μm or higher. Using this process, it is possible to mass-produce at low-cost microstructures with high aspect ratio. By making use of the full potential of the three basic steps in the LIGA process, metal, polymeric, and even ceramic microstructures can be produced. Danny Banks has summarized [5] the detailed process of the LIGA process. As briefly described in Figure 6.1, the X-ray or other radiation sources (UV, excimer lasers, synchrotron-based radiation) are represented as an appropriately designed mask onto a thick photoresist layer (sensitive to the type of radiation source), which is coated on a desired conductive substrate. This resist is then developed to form a “negative” feature. In the process, as originally developed, a special kind of photolithography using X-rays (X-ray lithography) is used to produce patterns in very thick layers of photoresist. The X-rays from a synchrotron source are shone through a special mask onto a thick photoresist layer (sensitive to X-rays) that covers a conductive substrate (Figure 6.1a). This resist is then developed (Figure 6.1b). The pattern formed is then electroplated with metal (Figure 6.1c). The metal structures produced can be the final product; however, it is common to produce a metal mold (Figure 6.1d). This X-Rays

(a)

(c)

(e)

(b)

(d)

(f)

Figure 6.1

The typical steps of the LIGA process.

6.2 Typical Fabrication Methods for MEMS, NEMS, and AEMS

mold can then be filled with a suitable material, such as a plastic (Figure 6.1e), to produce the finished product in that material (Figure 6.1f). As the synchrotron source makes LIGA expensive, alternatives are being developed. These include high-voltage electron beam lithography that can be used to produce structures of the order of 100 high and excimer lasers capable of producing structures of up to several hundred microns high. Electroplating is not limited to use with the LIGA process, but may be combined with other processes and more conventional photolithography to produce microstructures. The related fabrication techniques or the MEMS were further developed, initially, for microelectronics applications to construct sensors and actuators for the above hardware for computers and information technology [5, 6]. Based on the LIGA process and the advances in the field of Si-based semiconductor complementary metal oxide semiconductor (CMOS) or field-effect transistor (FET) thin films, design and fabrication techniques for microstructures and nanostructures have been further developed for miniaturized sensing and reaction systems of biological and chemical processes together with those for computers and information technology, as well as those microfluidic devices [6–8]. In the past decades, the development of micro/nanofabrication technologies and the corresponding design strategies have given an enormous impetus also to the progress in the field of fabrication of varieties of structures and micro/nanoelectronic devices [7–10]. Different fabrication processes were developed for the construction of microstructures based on a variety of materials and applications, including most commonly metals, ceramics, polymers, glass, silicon, stainless steel, and their composites [11]. The fabrication methods can be broadly classified as mechanical machining, LIGA (lithography, electroplating, and molding; using either UV or X-ray radiation) (Figure 6.1), micro molding, embossing, chemical etching, electroplating, laser ablation, E-beam lithography in addition to original classical bulk machining, deep reactive ion etching (DRIE), etc. [5, 6]. Microstructural thin films, reported to date, have been fabricated by “standard” microfabrication techniques using borosilicate glass or polymers as substrates that have been used in the fabrication of multilevel microstructures for advanced microelectronics devices. Figure 6.2 gives one typical process to fabricate multilevel microstructures using SU-8 as construction materials on pre-micromachined poly-ether-ether-ketone (PEEK) substrates [11, 12]. Photolithography and wet etching process are usually used to fabricate Si or glass-based microelectronic devices [5]. The etch rates for micromachining using different etchers for various materials are summarized in reference [13]. Typically, a thin layer of metal, such as chromium, is deposited on the surface of a glass substrate [5]. A layer of positive photoresist is then spin coated on top of the chromium to a depth of 0.5–2.0 μm. The pattern of the required network of interconnecting channels is transferred to the photoresist layer using photolithography. After exposure, the photoresist is developed to open the areas of glass to be etched. The plate is then allowed to dry to ensure complete evaporation of volatiles before performing the chemical etching. The channels are then etched using, for example, a mixture of

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6 MEMS, NEMS, AEMS, and Quantum Films Holesϕ 2 mm

PEEK

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(3)

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Threads (1)

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Sealed microfluidics Embedded structure SSPT

Sacrificial PI layer (6)

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Semisolid SU-8 layer

Embedded substrate (7)

Microfluidics with enforced substrate (8)

Binding the second channel with the first Forming the second microstructure layer by solid transfer process after aligning Forming the third microstructure layer with aligning marks layer with aligning marks (9) Sealed microfluidics with 3 layers of 3D structures by aligning (12)

(10) Thousands of layers of microstructures with connectors (13)

(11) Sealed microfluidics with desired layers of 3D structures (e.g. 3 layer) by SSPT (14)

Figure 6.2 Process for the fabrication of multilevel microstructures using UV–LIGA process and SU-8 (an epoxy photonic resin) as construction material on premachined PEEK substrates. (1) Connection point between micromachining and external macro equipment. (2) Seal joint points with su-8 photosensitive resin and mask to form uncleaned openings. (3) Rotate and apply a thin layer of su-8 and mask to form an uncleaned opening. (4) The su-8 layer is rotationally coated, and a mask is used to form the microreactor body. (5) Flush the unexploded su-8 to form an open microchannel. (6) Preparation of semisolid su-8 sealing membrane. (7) The semisolid su-8 sealed membrane seals the microchannel, forming a semi-open microfluidic reactor. (8) The microfluidic reactor is strengthened with an enhancement layer. (9) Prepare the second microchannel layer. (10) Bind the second microchannel to the first. (11) Preparation of the third microchannel layer. (12) Bind the third microchannel to the second microchannel. (13) Multilayer microchannel system required for replication. (14) Use SSPT technology to seal the entire microchannel system [11, 12]. Source: Modified from Song [11].

1% HF and 5% NH4 F in water at 65 ∘ C, resulting in an etch rate of 0.3–0.5 μm/min. A glass block, with predrilled holes to act as reservoirs and if necessary electrode supports, is aligned with the channel geometry and thermally bonded to the glass base plate, producing a glass-based microfluidic reactor. In addition to the specific microfabrication processes described above, hybrid fabrication techniques use a combination of microfabrication processes, such as low-pressure chemical vapor deposition (LPCVD), buffered oxide etch (BOE), chemical etching, dry etching and sputter, producing multilevel microstructures with heating parts, sensing parts, control parts, and thin catalyst membranes for reaction that can be all fabricated in wafers or other high-temperature stable substrates (e.g. Si) for special applications [14–17]. Ye et al. described the fabrication of a typical palladium membrane microreactor using this approach as one MEMS example for materials synthesis [14]. The unique feature of the hybrid fabrication process, where typically microreactors are fabricated using Si wafer, glass, or other ceramics, is that they can be used at high temperatures and are flexible to incorporate different types of thin membranes for catalytic applications. Such

6.2 Typical Fabrication Methods for MEMS, NEMS, and AEMS

microreactors have been successfully developed to carry out gas or liquid-phase oxidation or partial oxidation of organic compounds by depositing a thin catalyst membrane in the channel walls [17–20].

6.2.2 Fabrication Process of Complementary Metal Oxide Semiconductor (CMOS) Mizuki Ono et al. described a complementary metal oxide semiconductor–fieldeffect-transistor (CMOSFET)-compatible tip fabrication process. The process flow for cantilever fabrication is schematically shown in Figure 6.3 [22]. The starting material is a p-type silicon wafer with a diameter of 100 mm. After the company Austrian Mikro Systeme Int. used a traditional CMOS process to manufacture piezoresistive and on-chip circuit components, the film has been released from the back of the wafer with 27 wt% KOH at 90 ∘ C [23]. Anisotropic corrosion is stopped at the p2n junction between the n-well and the p-type substrate [24] using electrochemical corrosion resistance technology. Therefore, the obtained thin film is composed of CMOS n-holes. At the tip of this area, a deeper n-well CMOS process is used. The underside of the cantilever is uneven; however, extra material is consumed at the tip. Silicon dioxide is then deposited on both sides of the wafer. Plasma-enhanced chemical vapor deposition (PECVD) technology is used to mix silane (SiH4 ) and nitrous oxide (N2 O) at 3001 ∘ C. Then, a photolithography process is performed to determine a mask for needle tip manufacturing. The silica on the front is a reactive ion etching (RIE) pattern with CF4 (carbon tetrafluoride) and CHF3 (trifluoromethane). This step can also be completed by a wet etching process, for example, a mixture of NH4 F, CH3 COOH, and HO(CH2 )2 OH. After removing the photoresist with oxygen plasma, the tip is prepared at a temperature of 301 ∘ C with 25% by weight of tetramethylazanium hydroxide (TMAH) without any stirring. The wafers are immersed in diluted hydrofluoric acid (HF) to remove the natural oxide film prior to tip manufacturing. The etching depth in the silicon [1] direction is 3 mm. The total corrosion time is 2.5 hours. After the preparation of the tip is completed, a mixture of NH4 F, CH3 COOH, and HO(CH2 )2 OH is used to remove silicon dioxide on both sides of the wafer. As mentioned earlier, neither silicon nor aluminum is attacked during this process. Photolithography is performed to determine the shape of the cantilever: Using a patterned photoresist layer as a mask, silicon is etched in an RIE process using a mixture of sulfur hexafluoride (SF6 ) and CHF3 to release the cantilever beam. Finally, the photoresist is removed using an oxygen plasma.

6.2.3

Fabrication Process of Field Emission Transistors (FET)

In order to fabricate 100 μm channels (Figure 6.3, [21], aluminum electrodes were deposited on the TiO2 nanofibers with thermal evaporation of a mask after being calcined. After that, dipped a bit of polyvinylpyrrolidone (PVP) aqueous solution onto the two sides of SiO2 substrates, which aims at connecting the top-gate electrode. At last, the top-gate electrode was made of a Si wafer, which was highly doped,

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100 nm

AI

Si SiNx

TiO2 AI

SiO2

Si

AI

AI

SiO2

204

(AIt + A)

Figure 6.3 Schematic diagram of a single TiO2 nanofiber nonvolatile memory device based on a top- gate field effect transistor and SEM images and photos of TiO2 nanofiber. Source: Zhang et al. [21]. © 2019 Elsevier.

coated with a 1.6-μm-thick SiO2 dielectric. The gap between mechanically contacted top-gate electrodes is automatically filled with dry air (5–20% RH), which is used in conjunction with SiO2 as the gate dielectric.

6.2.4 Giant Magnetoresistance (GMR) Sensor and Its Fabrication Method Giant magnetoresistance (GMR) refers to a very large change in resistance in ultra-thin magnetic multilayer films (Figure 6.4). The basic GMR material structure consists of a fixed layer and a free layer; the free layer is affected by an external magnetic field. A suitable external magnetic field has a range larger than the saturated magnetic field of the free layer and smaller than the stable magnetic field of the fixed layer. The free layer is magnetized with the change of the magnetic field when the free layer rotates. In the case of fixed reference layer magnetization and free layer synchronous magnetization, the magnetic resistance is a simple cosine function of the angle of the rotor relative to the fixed sensor. The resistance of a spin value is related to the angle between the free and the pinned layer magnetizations in R∕Rp = 1 + 1∕2GMR(1 − c)

(6.1)

where the Rp is the minimum of the resistance value when the two magnetizations are parallel. At last, the GMR is the biggest percentage magnetoresistance. The finite element modeling method divides the solution domain into smaller regions. The program uses Maxwell’s equations as the basis for electromagnetic field analysis. In magnetostatics, the unknowns (degrees of freedom) are usually magnetic vector potentials that can be approximated by polynomial shape functions. Other magnetic field quantities, such as magnetic flux density, magnetic field strength, current density, energy, force, loss, inductance, and capacitance, all come from degrees of freedom [9]. The size of the element must be small enough to provide sufficient precision [10].

6.3 From MEMS to NEMS and then to Quantum Films and AEMS Spin up

Spin down

Spin up

Spin down

Ta gap Antiterromagnet

FeMn pinning layer Co pinned layer Cu spacer NiFe free layer Ta buffer Si substrate

Ferromagnet

Figure 6.4 The GMR effectively measures the difference in angle between the two magnetizations in the magnetic layers. Small angles (parallel alignment) give a low resistance; large angles (anti-parallel alignment) give a higher resistance.

The manipulator is required to finish many complex works, in order to obtain an accurate angle signal, thus, making the angle sensor important in the sensor system. The GMR sensor is ideal for systems that are highly integrated (Figure 6.5) [25].

6.3 From MEMS to NEMS and then to Quantum Films and AEMS for the Next Generation of Information Technology 6.3.1

The Trend of Microsystem Integration Technology

Microsystem technology that combines microelectronics, microelectromechanical, and light electricity technique, through system architecture and software algorithm, and the micro sensor, micro control, micro actuator, micro energy, and various interfaces form integrated soft hardware such as multifunctional integration, micro/nanomanufacturing and micro-integration technique that was adopted to realize the system structure of micro/nanoscale; it is recognized as one of the revolutionary technology of the twenty-first century. Microsystems have the advantages of high integration, miniaturization, low power consumption, high reliability, and high efficiency. New materials, new methods, new processes, and other technological changes in microsystems will certainly disrupt the development and manufacture of dual-use systems. China’s microsystem technology has made some progress. application-specific integrated circuit (ASIC) devices have been tested in orbit and started to be applied in space, but they need to be improved at the manufacturing process level. At this stage, China will “manufacture its own micro-devices” as the development goal

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0.5 mm Ø6 mm

0.7 mm

Ø3 mm

Live shaft Bearing Ø4.5 mm GMR sensor chip PCB

Figure 6.5 An example of the GMR sensor in 5-finger dexterous. Source: Lan et al. [25]. © 2009 Elsevier.

of China’s microsystems and will self-innovate while using existing resources and learning from foreign advanced technology experience. In recent years, many new micro/nanoprocessing technologies, new packaging technologies, and new materials have been deeply studied in the domestic technology platform. Throughout the 5th International Symposium on Microsystems and Nano-Engineering, the development trends of domestic microsystems are the following: (i) The reliability and stability of microsystems have been greatly improved. (ii) Research and application prospects in physical and medical integration are broad. (iii) The integration of micro/nanodevices and processing technology development has been put on the agenda. In recent years, China has been gradually increasing its support for microsystem technology through establishing policies and various conditions; and new breakthroughs in microelectronics technology has also been made: ultra-deep submicron integration technology has reached the international advanced level – chip design. For example, integrated circuits have grown significantly in the fields of high-end IC cards for digital TVs, media signal processing for mobile phones above 5G, and information security, and we have achieved major breakthroughs in the development and industrialization of core chips designed for chip products with independent intellectual property (the design level of integrated circuits has reached 0.13 μ). With the strong support of domestic policies, China is gradually relying on technological innovation to replace from low end to high end. Water silicon-based microelectronics technology continues to narrow the gap with foreign countries.

6.3 From MEMS to NEMS and then to Quantum Films and AEMS

6.3.2

The Development Trend of Microsystem Packaging Technology

In 2012, China has a research team for electronic packaging technology. The 25 domestic electronics packaging industry chain-related units have established the “Integrated Circuits and Tests Industry Chain Technology Innovation Alliance” to establish a high-density IC packaging technology engineering laboratory, mainly researching foreign developed countries. The monopoly of packaging technology, the progress of the application of packaging and testing, has played an effective role in the electronic packaging industry chain [26]. In recent years, the international competitiveness of China’s electronic packaging technology has gradually improved, mainly in the following aspects: new packaging technologies such as chip-scale package (CSP), multiple-chip package (MCP), and ball grid array (BGA) have been applied in some electronic packaging production lines; TSV silicon channel, SiP RF, and 50 three-dimensional stacking and packaging technologies for ultrathin chips of μm and below have been widely used; and electronic packaging technologies such as supervised primitive fitting network (SPFN), management information system (MIS), and FBP have also obtained independent intellectual property rights. In addition, domestic innovation in MIS flip-chip packaging technology can effectively reduce the cost of packaging and technically achieve good support for the current mainstream packaging technology [26]. For monolithic microwave-integrated circuit (MMIC), foreign MMIC chips are developing toward terahertz, while packaging in the direction of 3D-MMIC. The spectral range of this product is from microwave to millimeter wave. In semiconductor materials, the emergence of wide bandgap semiconductor provides a solid foundation for the development of microsystems. The second generation of semiconductors represented by GaAs has matured in foreign countries, and the third generation of semiconductor MMIC represented by Gan is booming. After decades of development, the three-dimensional MMIC technology has evolved from a single device to an integrated transceiver chip. The multifunctional multiboard multistage amplifier is integrated on one chip. While improving monolithic performance, it reduces the design and manufacture of space structure, only one chip built-in tertiary amplifier, gaining up to 22 dB.

6.3.3

Challenges in the Development of Microsystems Technology

In the past 10 years, as the characteristic size of transistors has decreased, quantum efficiency should become more and more significant, Moore’s law gradually fails, and the growth rate of transistors tends to be linear. With the introduction of new principle device materials and processing technologies, it is still feasible to further reduce the feature size. For many MEMS devices, controlling the level of strain in the deposited film is critical. A film structure made of polysilicon, a common MEMS mechanical material, requires a tensile film strain of less than −0.001; otherwise, the film will break. Many MEMS sensors have been fabricated that use many different methods to convert physical changes into electrical signals. The most common method of

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Glass

Top fixed electrode

Figure 6.6 Schematic illustration of a sensor designed to correlate measured changes in capacitance with pressure changes. Source: Kordi´c [29]; Baltes and Popovic [30].

Movable bottom electrode Silicon Flexible membrane

detecting motion involves piezoresistance. Other common transducer methods for mechanical deflection include piezoelectric effects, capacitance changes, and magnetic effects (such as the Hall effect). The piezoelectric effect [27, 28] is a phenomenon via a force to certain crystalline materials that causes an electrical charge to be generated on the surface of the crystal. The amount of the charge is directly related to the applied force. The application of a stress causes deformation, which created electric dipoles. These electric dipoles induce surface charges on the crystal that in turn create an electric field [26, 29]. The electric field induced by surface charge cancels the stress-induced electric field from dipoles. For example, a sensor based on the Hall effect was fabricated (Figure 6.6) [29–31]. These sensors have been used for the detection of electric current, proximity detection, and position of rotating shafts. Piezoresistivity is a property of materials that describes the change in electrical resistance as a function of mechanical stress applied to the material [32]. The discovery of piezoresistive silicon is critical to the development of silicon pressure sensors. The theoretical interpretation of piezoresistance is based on holes as a function of lattice orientation. For MEMS devices made with (100) silicon wafers, the most important parameter is the doping type. A detailed description of the piezoresistive can be found in the scientific file [33, 34]. Stephen A. Campbell shows a bar of single-crystal silicon subjected to a tensile force F, with a current flowing in a direction perpendicular to F. R denotes the resistance of the silicon bar. The force F generates a stress 𝜎 in the bar and a resistance change due to the piezoresistivity effect. The resistance change ∇R is 𝛻R∕R = 𝜋l 𝜎 where 𝜋 f is defined as the transverse piezoresistance coefficient. For situations in which the current flow is parallel to the stress, the longitudinal piezoresistance coefficient 𝜋 l i used. In general case in which both transverse and longitudinal stresses are present, we have 𝛻R∕R = 𝜋t 𝛿t + 𝜋l 𝛿l

(6.2)

To use Eq. (6.2) to calculate a resistance change, the piezoresistance coefficients need to be calculated. As previously mentioned, the values of 𝜋 t and 𝜋 l depend on several properties of the crystal and on the orientation of the stress 𝜎 relative to the crystal lattice orientation of the single-crystal silicon.

6.3 From MEMS to NEMS and then to Quantum Films and AEMS

Bulk micromachining refers to MEMS fabrication processes that involve the removal of significant amounts of the silicon substrate in order to form the desired structure. Etching is the cornerstone of bulk micromachining [33, 35, 36]. Historically, wet etching with both isotropic and anisotropic etchants has dominated MEMS devices, but more recently, other techniques, such as isotropic vapor-phase etching and high-density plasma-based processes, have been used. Silicon isotropic dry etchants such as hydrofluoric acid–nitric acid–acetic acid are well known, showing an uncontrolled etch profile. The undercut of the features defined by the masking material results in the contours of the circular holes and grooves. Stephen A. Campbell shows etch profiles for isotropic etchants with and without agitation of the etchant. With agitation, the profile generally exhibits the shape expected for etch rates that are nearly the same for all crystal orientations. Without agitation, the profile shows u flatter bottom, which is the result of reduced etching in the vertical direction. This reduction in etch rate is caused by a depletion of the etching species near the etch surface (the etch is diffusion rate limited). The agitation assists the diffusion of fresh etchant to the surface, resulting in the expected isotropic etch profile. For bulk-micromachined structures, etch depths often approach the full wafer thickness. When isotropically etching these deep structures, the considerable undercutting of the etch profile mask means the features must be separated by at least the depth of the etch. The MEMS fabrication process utilizes many of the same process steps as conventional silicon LC processes. However, differences have arisen, such as the nature of electronic circuits of MEMS sensors and actuators. MEMS devices typically include structural components that are movable, implying an independent or unsupported element. Material mechanics include the stress/strain relationships and motion of the membrane and cantilever beam. Two traditional methods of forming MEMS devices are bulk micromachining and surface micromachining. Batch micromachining is a relatively simple processing technique for devices such as piezoresistive pressure sensors. Batch micromechanical devices cannot easily be integrated with standard IC processing of on-chip signal processing circuits for MEMS devices. Surface micromachining is more suited to integration with on-chip electronics. But this integration requires considerable care. Surface micromachining using multiple layers of structural material and sacrificial material enables the fabrication of complex structures not attainable with bulk micromachining, such as small motors, resonators, and optical elements out of the plane of the surface of the wafer. MEMS actuators, which convert an electrical signal to motion using many different transduction methods, are finding application in many fields for small controlled motion. High-aspect-ratio microsystem (HARM) processing combines X-ray lithography with electroplating to form structures that can be hundreds of microns high. These high-aspect-ratio plated structures can then be used as molds for precise formation of plastic parts using molding processes, as well as for mechanical structures such as gears and micromotors.

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6.4 NEMS and AEM 6.4.1

NEMS

Single-crystal diamond has the best properties for NEMS/MEMS among the semiconductors owing to its outstanding mechanical strength, thermal conductivity, electronic properties, and chemical inertness. However, on-chip single-crystal diamond (SCD) NEMS/MEMS with all electrical sensing and actuation integrated into one SCD wafer have the challenges such as arising from the lack of shallow bulk dopants necessary for high electrical conductivity to overcome. A heterodyne frequency downconversion method is utilized to characterize the resonance frequency of the MEMS and NEMS cantilevers. The cantilevers are actuated by the gate electrode, and the motion detected electrically by the S–D electrodes using the “1ω” method [37], as shown in Reference [38]. The on-chip SCD NEMS/MEMS displays its advantages: high self-sensitivity, low-voltage actuation (mV), little energy dissipation, and high-frequency (>MHz) and high-temperature operation (873 K). It shows that the actuation force is enhanced 800 times by a self-sensing readout voltage as low as 1 V compared with those without sensing. The SEA mechanism enables the removal of the direct current (DC) voltage prebiased on the gate, differing from the reported configurations [33, 37, 39, 40]. A radiofrequency (RF) signal with an amplitude of g ac V and a frequency of ω was applied along the bottom gate, while a second RF signal with an amplitude of d ac V and a frequency of 𝜔 + Δ𝜔 was applied to the drain electrode. The readout was performed by a lock-in amplifier at Δ𝜔 = 5 kHz at an impedance of 50 Ω. The finite element method (FEM) revealed that a strong electric field is confined around the edge of the SCD cantilever. Traditional MEMS devices are characterized as either sensors or actuators. Examples of MEMS sensors include acceleration sensors used for automobile airbag deployment control, pressure sensors mounted on the tip of catheters for use in intracardiac (within the heart) monitoring of blood pressure, and chemical sensors that quantitatively detect gaseous compounds. Example of MEMS actuators include video display systems using digital mirror devices consisting of over one million individually controlled micromirrors, ink-dispensing nozzles used in inkjet printers, and valves and pumps used in miniature fluidic systems (fluid volumes in the microliter range). Many processing issues do affect both standard IC and MEMS fabrication, including thin film stress, planarization, and selective wet and dry etching. NEMS switches used for various portable electronic NEMS switches used for various portable electronic NEMS and RF communication systems are the ideal bi-stable switch and the off state can be achieved through the electric switch in a reliable way. In this chapter, the nonideal characteristics of the cantilever NEMS switch, namely, the stability and parameter sensitivity of three representative system models based on Euler–Bernoulli beam theory, the electrostatic interaction between parallel plate capacitor and surface form of Leonard–Jones more physical fields, are discussed. The

6.4 NEMS and AEM

device geometry, material properties, and surface features are compressed in several devices of dimensionless quantities, thus creating the space, the dimensional parameters low enough to provide physical-related balance of all access system model analysis about this kind of bistable system and three steady-state conditions. The multistability region and instability boundary of dimensionless parameter space are determined, and the instability sensitivity of dimensionless parameter space is studied. Under the defined method, cantilever MEMS/NEMS capacitor switches show unity, gender, and triple stability under the condition of physical accessibility, even including the most inherent Lennard–Jones surface interaction model. The results provide a framework for exploring the coupling between multistability and surface interaction, which is easy to expand and is related to the design and performance of such devices.

6.4.2

AEMS

As Si transistors rapidly approach their projected scaling limit of ∼5-nm gate lengths, exploration of new channel materials and device architectures is of utmost interest [41–43]. This scaling limit arises from short-channel effects [44]. Direct source-to-drain tunneling and the loss of gate electrostatic control on the channel severely degrade the off state leakage currents, thus limiting the scaling of Si transistors [45, 46]. Certain semiconductor properties dictate the magnitude of these effects for a given gate length. Heavier carrier effective mass, larger bandgap, and lower in-plane dielectric constant yield lower direct source-to-drain tunneling currents. Uniform and atomically thin semiconductors with low in-plane dielectric constants are desirable for enhanced electrostatic control of the gate. Thus, investigation and introduction of semiconductors that have more ideal properties than Si could lead to further scaling of transistor dimensions with lower off state dissipation power. Transition metal dichalcogenides (TMDs) are layered two-dimensional (2D) semiconductors that have been widely explored as a potential channel material replacement for Si [47–50], and each material exhibits different band structures and properties [51–55]. The layered nature of TMDs allows uniform thickness control with atomic-level precision down to the monolayer limit. This thickness scaling feature of TMDs is highly desirable for well-controlled electrostatics in ultrashort transistors (3). For example, monolayer and few-layer MoS2 have been shown theoretically to be superior to Si at the sub-5-nm scaling limit [56]. As for atom-electromechanical system (AEMS), Sujay B. Desai et al. [57] reported a MoS2 with 1-nm gate lengths. The device is based on 1D-gated, 2D semiconductor field-effect transistors (1D2D-FET). The experimental device structure of the 1D2D-FET consists of a MoS2 channel (number of layers vary), a ZrO2 gate dielectric, and a single-walled carbon nanotube (SWCNT) gate on a 50-nm SiO2 /Si substrate with a physical gate length (LG ∼ d) of ∼1 nm. Long, aligned SWCNTs grown by chemical vapor deposition were transferred onto a n + Si/SiO2 substrate (50-nm-thick SiO2 ) [58], located with a scanning electron microscope (SEM), and contacted with palladium via lithography and metallization. These steps were

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followed by atomic layer deposition (ALD) of ZrO2 and pick-and-place dry transfer of MoS2 onto the SWCNT covered by ZrO2 [53]. Nickel source and drain contacts were made to MoS2 to complete the device. Qiu C. et al. [59] reported a kind of carbon nanotube field-effect transistors (CNT FET), together with three cross-sectional transmission electron microscopy (TEM) images showing a p-type FET, an n-type FET, and the gate stack structure of these devices. Thanh Luan Phan et al. [10] reported a screening-engineered carbon nanotube (CNT) network/MoS2 /metal heterojunction vertical field-effect transistor (CNT–VFET) is fabricated for an efficient gate modulation independent of the drain voltage. Compared with gr-VFFET, m-SWCNT network realizes a new VFET design based on vdWH with high on/off ratio (>104 ) at room temperature. By theoretical calculation and experimental systems, we find that a high gate modulation independence of the CNT network through the empty space between the gate voltage and the drain modulation is the key to achieving high performance of VFET [60]. Compared with the conventional field-effect transistors, their actual current CNT–VFET is 7.3 times and 112.2 times MoS2 barristor and MoS2 planar FETs.

6.5 Quantum Films for Information Technology When the size of molecular devices is reduced to the nanometer scale, Ohm’s law in the macroscopic system no longer applies, but instead appears as the superposition of multiple discrete orbital transmission coefficients when electrons pass through the molecule, that is, the quantum interference effect of single-molecule electrical transport, similar double slit interference. Recently, Prof. J. Fraser Stoddart’s group at Northwestern University, in collaboration with Prof. Wenjing Hong’s group at Xiamen University and Prof. Guo Hong’s group at McGill University, used single-molecule electricity, which has the accuracy of micro/nanoscale electrical measurement and the sensitivity of subnanometer displacement control. The quantum interference effect based on charged macrocyclic molecular system has been studied by the measurement technology, and a new self-gated quantum interference mechanism has been proposed, which can reach more than 50 times of the single-molecule conductance regulation. In this study, the author relies on the high electrical measurement accuracy and the displacement control crack sensitivity of the scanning tunnel junction technique and firstly proposes the quantum interference based on the gated mechanism to make the two-channel conductance of the molecular system more than that of the single-channel system. More than 50 times, it has broken through the traditional rules of quantum superposition theory, restricted the interdisciplinary research results, confirmed the important role of the strong electrostatic interaction between molecular orbitals on the electrical transmission process, and provided an important theoretical basis for the

6.5 Quantum Films for Information Technology

future design and preparation of high-performance single-molecule electronic devices. Nanofilms of the asymmetric profile can also be modeled by well-known asymmetric potential functions such as the Morse potential, the electrostatic potential of the electric field [61], the semi-parabolic potential [62], etc. These potentials are well-known and have applications in various fields of physics, in particular, solid-state physics, for example, the Morse potential as a potential for interaction between atoms in molecules [63], in a solids [64], in metallurgy [65]. The semi-parabolic quantum well also attracts the attention of researchers. While the interband absorption is under the action of intense laser radiation, a linear optical absorption are considered in [62, 66]. The optical properties in electric and magnetic fields are considered in another paper [67]. It should also be noted [68, 69], where the influence of the asymmetry of the quantum well on the mobility of carriers and the electron–phonon interaction is investigated. In this study, we used an asymmetric distribution of Morse potential to account for the difference in the surface of the quantified membrane [61]. According to the solution of the one-dimensional Schrödinger equation, the energy spectrum of the electrons in the film with Morse potential distribution can be obtained. The results show that the density of the electronic states in the asymmetric membrane is sufficiently strong as the energy increases, and it can be several times the density of the electronic states in symmetric holes such as parabolic and rectangular. In addition, the influence of the asymmetry of the potential well on the electron concentration dependence of Fermi energy and the electron thermoelectric potential of the electron gas was investigated. The results show that in the classical strong magnetic field, the electron thermal potential of the strongly degraded electron gas is an oscillation function of the electron concentration as a function of the period. The period of the electron concentration first increases, then passes through the maximum value, and then decreases to a smaller value without becoming zero. For symmetrical, such as parabolic or rectangular deep wells, the corresponding period is even increased or infinitely extended. In addition, Fermi energy was found to be a piecewise linear function of electron concentration. Comparison with the experiment [70, 71] shows a qualitative agreement. A further decrease in transmission was observed when the complex was coupled or mixed into the polymer film with graphene quantum dots (GQD). The photodegradation threshold recorded by PCS embedded in the polymer film is higher than the photodegradation threshold in solution, resulting in higher stability and better NLO response [72]. The transmittance of the representative z-scan section of the dimethyl formamide (DMF) is decreased near the focus, which may be due to the absorption of the reverse saturated excited state, which is manifested as the positive nonlinear absorption of incident light. The optical-limiting properties improve when the complexes are embedded in thin films, and at the same time, the third-order susceptibilities and hyperpolarizabilities increase, and the limiting-threshold values are further reduced.

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6.6 Summary and Perspectives The MEMS/NEMS technique is more developed now, and the technology of AEMs has great advantage compared with the earlier devices, including the improvement of production and life, the development of quantum film, its easy preparation and convenient storage, and the superior performance of quantum thin film material. These technologies will certainly have a profound impact on the next generation of computing technology and bring revolutionary improvement to computing technology. Driven by many scientific and technological innovations, the semiconductor industry has shown a significant trend of miniaturization. However, if this trend continues faster, cheaper computers will become available, and then the size of microelectronic circuit components will soon need to reach the atomic or molecular scale, a goal that will conceptually require new device structures. The idea that several or even one molecule could be embedded between electrodes and perform the basic functions of digital electronics (rectification, amplification, and storage) was proposed in the mid-1970s and it has now been implemented for individual components. However, the economic manufacture of complete circuits at the molecular level remains challenging because of the difficulty of connecting molecules to each other, which could be resolved using “monomolecular” electronics, in which individual molecules would integrate the basic functions and interconnections required for computing.

Acknowledgments This chapter is supported by the NSFC-BRICS STI Framework Program (No. 51861145309), the National Natural Science Foundation of China (No. 51971029), the National S&T Major Project (No. 2018ZX10301201), the Postdoctor Research Foundation of Shunde Graduate School of University of Science and Technology Beijing (No. 2020BH005), the Project funded by China Postdoctoral Science Foundation (No. 2020M680336), the “All English teaching demonstration course construction project of University of Science and Technology Beijing” (No. KC2015QYW06, 2016), the “1125” Zhihui Zhengzhou Talent project of Henan Province (Fund No. in USTB: 39080070), the “100 Talent Plan” fund of Fujian Province (Fund No. in USTB: 39080067), and the development of a high sensitive magneto-optical biomolecular sensor experimental prototype (Fund No. in USTB: 2019-0649) by Hangzhou Ruidi Biotechnology Co. Ltd.

List of Abbreviations AEMS ALD BGA

atom-electromechanical systems atomic layer deposition ball grid array

References

CMOS CNT CNT-VFET CSP FET GMR GQD HARM HF LPCVD MCP MEMS MMIC MOS MOSFET NEMS PECVD SCD SEM SWCNT TMDs

complementary metal oxide semiconductor carbon nanotube vertical field-effect transistor chip-scale package field-effect transistor giant magnetoresistance graphene quantum dots high-aspect-ratio microsystems hydrofluoric acid low-pressure chemical vapor deposition multiple-chip package microelectromechanical systems monolithic microwave integrated circuit metal oxide semiconductor metal oxide semiconductor field-effect transistor nanoelectromechanical systems plasma-enhanced chemical vapor deposition single-crystal diamond scanning electron microscopy single-walled carbon nanotube transition metal dichalcogenides

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40 Verd, J., Uranga, A., Teva, J. et al. (2006). Integrated CMOS–MEMS with on-chip readout electronics for high-frequency applications. IEEE Electron Device Letters 27 (6): 495–497. 41 Theis, T.N. and Solomon, P.M. It’s time to reinvent the transistor. Science 327 (5973): 1600–1601. 42 Chau, R., Doyle, B., Datta, S. et al. Integrated nanoelectronics for the future. Nature Materials 6 (11): 810–812. 43 Franklin, A.D. Nanomaterials in transistors: from high-performance to thin-film applications. Science 349, 6249: aab2750. 44 Lundstrom, M. (2003). Moore’s law forever? Science 299 (5604): 210–211. 45 Luisier, M., Lundstrom, M., Antoniadis, D. A., Bokor, J. (2011). Ultimate device scaling: intrinsic performance comparisons of carbon-based, InGaAs, and Si field-effect transistors for 5 nm gate length. 2011 International Electron Devices Meeting, 5-7 December 2011; pp. 11.2.1–11.2.4. 46 Kawaura, H., Sakamoto, T., and Baba, T. Observation of source-to-drain direct tunneling current in 8 nm gate electrically variable shallow junction metal–oxide–semiconductor field-effect transistors. Applied Physics Letters 76 (25): 3810. 47 Radisavljevic, B., Radenovic, A., Brivio, J. et al. Single-layer MoS2 transistors. Nature Nanotechnology 6 (3): 147–150. 48 Sarkar, D., Xie, X., Liu, W. et al. A subthermionic tunnel field-effect transistor with an atomically thin channel. Nature 526 (7571): 91–95. 49 Liu, H., Neal, A.T., and Ye, P.D. (2012). Channel length scaling of MoS2 MOSFETs. ACS Nano 6 (10): 8563–8569. 50 Wang, H., Yu, L., Lee, Y.-H. et al. Integrated circuits based on bilayer MoS2 transistors. Nano Letters 12 (9): 4674–4680. 51 Mak, K.F., McGill, K.L., Park, J., and McEuen, P.L. (2014). The valley hall effect in MoS2 transistors. Science 344 (6191): 1489–1492. 52 Jariwala, D., Sangwan, V.K., Lauhon, L.J. et al. Emerging device applications for semiconducting two-dimensional transition metal dichalcogenides. ACS Nano 8 (2): 1102–1120. 53 Fang, H., Battaglia, C., Carraro, C. et al. Strong interlayer coupling in van der Waals heterostructures built from single-layer chalcogenides. Proceedings of the National Academy of Sciences 111 (17): 6198–6202. 54 Novoselov, K.S., Jiang, D., Schedin, F. et al. (2005). Two dimensional atomic crystals. Proceedings of the National Academy of Sciences of the United States of America 102 (30): 10451–10453. 55 Lee, C.-H., Lee, G.-H., van der Zande, A.M. et al. Atomically thin p–n junctions with van der Waals heterointerfaces. Nature Nanotechnology 9 (9): 676–681. 56 Yoon, Y., Ganapathi, K., and Salahuddin, S. (2011). How good can monolayer MoS2 transistors be? Nano Letters 11 (9): 3768–3773. 57 Desai, S.B., Madhvapathy, S.R., Sachid, A.B. et al. MoS2 transistors with 1-nanometer gate lengths. Science 354: 97–102.

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58 He, X., Gao, W., Xie, L. et al. (2016). Wafer-scale monodomain films of spontaneously aligned single-walled carbon nanotubes. Nature Nanotechnology 11 (7): 633–638. 59 Qiu, C., Zhang, Z., Xiao, M. et al. (2017). Scaling carbon nanotube complementary transistors to 5-nm gate lengths. Science 355 (6322): 271–276. 60 Yu, W.J., Li, Z., Zhou, H. et al. (2012). Vertically stacked multi-heterostructures of layered materials for logic transistors and complementary inverters. Nature Materials 12 (3): 246–252. 61 Elton, B.L.R. (1958). Quantum Mechanics, Non-Relativistic Theory: Vol. 3 of Course of Theoretical Physics. Physics Bulletin 9 (10): 270–271. 62 Niculescu, E.C. and Eseanu, N. Interband absorption in square and semiparabolic near-surface quantum wells under intense laser field. European Physical Journal B 79 (3): 313–319. 63 Kaplan, I.G. and Rodimova, O.B. (1978). Intermolecular interactions. Uspekhi Fizicheskih Nauk 126 (11): 403–449. 64 Slutsker, A.I. Characteristics of elementary acts in the kinetics of metal fracture. Physics of the Solid State 46 (9): 1658–1666. 65 Girifalco, L. and Weizer, V. (1959). Application of the morse potential function to cubic metals. Physical Review 114 (3): 687–690. 66 Tien, N.T., Hung, N.N.T., Nguyen, T.T., and Thao, P.T.B. (2017). Linear intersubband optical absorption in the semiparabolic quantum wells based on AlN/AlGaN/AlN under a uniform electric field. Physica B: Physics of Condensed Matter 519: 63–68. 67 Yan, R.-Y., Tang, J., Zhang, Z.-H., and Yuan, J.-H. (2018). Optical properties in GaAs/AlGaAs semiparabolic quantum wells by the finite difference method: combined effects of electric field and magnetic field. International Journal of Modern Physics B 32 (13): 1850159. 68 Lima, F.M.S., Fonseca, A.L.A., Nunes, O.A.C., and Fanyao, Q. (2002). Electric field effects on electron mobility in n-AlGaAs/GaAs/AlGaAs single asymmetric quantum wells. Journal of Applied Physics 92 (9): 5296. 69 Stavrou, V.N., Babiker, M., and Bennett, C.R. Influences of asymmetric quantum wells on electron-phonon interactions. Journal of Physics: Condensed Matter 13 (30): –6489, 6498. 70 Martin, J. (2009). Enhanced Seebeck coefficient through energy-barrier scattering in PbTe nanocomposites. Physical Review B: Condensed Matter 79 (11): 5311. 71 Miller, N., Haller, E.E., Koblmüller, G. et al. (2011). Effect of charged dislocation scattering on electrical and electrothermal transport in n -type InN. Physical Review B 84 (7): 2989–2996. 72 Nwaji, N., Mack, J., Britton, J., and Nyokong, T. (2017). Synthesis, photophysical and nonlinear optical properties of a series of ball-type phthalocyanines in solution and thin films. New Journal of Chemistry 41 (5): 2020–2028.

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7 Metamaterial or Metastructural Thin Films for EM Wave Control Menglin L.N. Chen 1 , Luzhou Chen 2 , Xunwang Dang 3 , Maokun Li 4 , Li Jun Jiang 1 , and Wei E.I. Sha 5 1 The University of Hong Kong, Department of Electrical and Electronic Engineering, Pokfulam 999077, Hong Kong 2 École Polytechnique de Montréal, Department of Electrical Engineering, 2500 Chemin de Polytechnique, Montréal, QC H3T 1J4, Canada 3 Science and Technology on Electromagnetic Scattering Laboratory, Beijing 100854, China 4 Tsinghua University, Beijing National Research Center for Information Science and Technology, Department of Electronic Engineering, State Key Laboratory on Microwave and Digital Communications, ShuangQing Road No. 30, Beijing 100084, China 5 Zhejiang University, College of Information Science and Electronic Engineering, Key Laboratory of Micro-Nano Electronic Devices and Smart Systems of Zhejiang Province, Department of Electronic Engineering, 38 Zheda Road, Hangzhou 310027, China

7.1

Introduction

Electromagnetic (EM) response from bulk materials governed by Maxwell’s equations depends on the constitutive relations with bulk permittivities, permeabilities, conductivities, etc. The homogenization of many-body behaviors of microscopic electrons, ions, and protons results in elegant constitutive relations with the bulk EM parameters. By exploring the same principle, the spatially inhomogeneous EM parameters caused by periodic, quasiperiodic, and random scatterers can also be homogenized in the macroscopic level if these scatterers are significantly smaller than the EM wavelength. Consequently, the scatterers can be reshaped, orientated, translated, transformed, and dynamically tuned to flexibly modify the properties of EM waves locally and form a specific EM structure with peculiar and fascinating homogenized EM parameters. Initially, the arrangements of the scatterers spread to three-dimensional (3D) space, and the corresponding structures are called metamaterials in the literature. Metamaterials control EM wave properties in an extraordinary way in comparison with traditional homogeneous bulk materials. Metamaterials with negative, zero, and gradient refraction indices were invented and studied extensively in both microwave and optical frequencies. For example, at microwave frequencies, a metamaterial sample formed by periodically arranged split-ring resonators and

Inorganic and Organic Thin Films: Fundamentals, Fabrication, and Applications, First Edition. Edited by Yujun Song. © 2021 WILEY-VCH GmbH. Published 2021 by WILEY-VCH GmbH.

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wires was experimentally demonstrated to exhibit negative refraction index (NRI) [1, 2]. With the NRI, the metamaterial can amplify evanescent waves so that it can be used as superlens to overcome the diffraction limit for imaging [3, 4]. Besides, zero-index-metamaterials (ZIM) with zero-phase delay nearly independent of their geometries were proposed as a coupler [5] and cloak [6]. Furthermore, by adopting transformation optics, the impedance of metamaterial-based cloaks can be matched to free space for an arbitrary cloaked object [7]. Metamaterials can also be applied to enhance the directivity of antennas, such as gradient-index metalens [8, 9] and metamaterial-based antennas [10, 11]. Additionally, metamaterials with anisotropic EM response can be designed as polarizers [12, 13]. However, the 3D structural thin films on desired substrates are not easy to be fabricated, integrated, and particularly not compatible to modern planar designs. Afterward, the scatterers are spatially arranged in a flat or curved plane like a metastructural film, which is called metasurface in the literature. Unlike metamaterials that change the properties of EM wave gradually along the wave path in the media, metasurfaces alter the EM wave property by the abrupt phase shift at the scatterers [14]. Compared with metamaterials, metasurfaces not only manipulate EM waves to achieve marvelous functionalities but also remain ultrathin configuration. Therefore, at microwave frequencies, metasurfaces can be conveniently fabricated using printed circuit board (PCB) technique, on different substrates based on the design targets. By designing scatterers with different geometries or orientations, phase shifts from 0 to 2𝜋 can be covered. Therefore, a metastructural film formed by those scatterers can realize arbitrary beam forming, such as beam shaping, steering, and focusing [15–17]. In this chapter, we will review the theoretical foundations, design routes, numerical methods, and engineering applications of the metastructural film (metasurface) in a comprehensive manner. Because the response of a metasurface differs locally, the simulation of both the single scatterer and the whole metasurface needs to be conducted. In Sections 7.2 and 7.3, we will introduce several modeling approaches for metasurfaces to facilitate fast design process and accurate simulation. In Section 7.4, metasurfaces designed for orbital angular momentum (OAM) generation will be reviewed. The metasurfaces introduce additional degrees of freedom in the microwave and optical communications and could be applied to the OAM-based multiplexing and demultiplexing. In Section 7.5, we will introduce the potential application of metasurface in modifying spontaneous emission (SE) of quantum emitters.

7.2

Modeling and Synthesis Methods of Metasurfaces

To transform arbitrary incident field into targeted reflected and transmitted fields as shown in Figure 7.1, we need a synthesis method that can link an effective macroscopic parameter, which represents the homogenized property of the metasurface, to the EM fields at the two sides.

7.2 Modeling and Synthesis Methods of Metasurfaces

Figure 7.1 Schematic presentation of a metasurface that transforms an arbitrary incident field into desired reflected and transmitted fields.

Ei, Hi Er, Hr

Et, Ht

7.2.1

Jones Vector and Jones Matrix

A simple approach to model scatterers on metasurfaces is the Jones matrix [18]. We consider a monochromatic plane wave propagating along the z direction. The incident electric field can be decomposed into x and y components and described by Jones vector: ( ) i (7.1) Ei (r, t) = x e−ikz iy where the time harmonic factor is omitted, k is the wave number, and the complex amplitudes ix and iy represent the polarization states of the incident waves. After the incident wave impinging on the metasurface, the transmitted wave is described in the same manner: ( ) t (7.2) Et (r, t) = x e−ikz ty where tx and ty represent the polarization states of the transmitted wave. Then, the local behavior of a scatterer on the metasurface can be modeled by Jones matrix, J. It connects the transmitted field components, tx and ty to the incident ones: )( ) ( ) ( ) ( J J ix i tx = xx xy =J x (7.3) ty iy iy Jyx Jyy where the first and second subscripts of J denote the polarization states of the transmitted wave and incident wave, respectively. The reflected field can be modeled in the same way. When the scatterer is rotated by an angle 𝛼, as shown in Figure 7.2, the new Jones matrix can be easily obtained by using the rotation matrix R(𝛼): ( ) cos(𝛼) sin(𝛼) J(𝛼) = R(−𝛼)JR(𝛼), R(𝛼) = (7.4) − sin(𝛼) cos(𝛼) Additionally, the Jones matrix, J, can be transformed into circular basis by coordinate transformation so that circularly polarized fields can be directly manipulated: ( ) J++ J+− c J = J−+ J−− ( ) 1 (Jxx + Jyy ) + i(Jxy − Jyx ) (Jxx − Jyy ) − i(Jxy + Jyx ) (7.5) = 2 (Jxx − Jyy ) + i(Jxy + Jyx ) (Jxx + Jyy ) − i(Jxy − Jyx )

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Inc.

Inc.

Figure 7.2 Schematic representation of a scatterer that rotates an angle of 𝛼.

α x

y

x

y

where Jc connects the incident circularly polarized Jones vectors to the transmitted or reflected circularly polarized ones. + and − represent the left circularly polarized (LCP) and right polarized components, respectively. We should see that only the normal incidence and reflection/transmission are considered by using the Jones matrix. However, the coupling between scatterers is not taken into consideration rigorously.

7.2.2

Polarizability Model

Incident wave induces polarization electric current and magnetic current, leading to the discontinuities of field components when crossing the metasurface plane. For this situation, the conventional boundary conditions fail to describe the system but should be replaced by the generalized sheet transition conditions (GSTCs) [19] n × (H1∕∕ − H2∕∕ ) = Jtoe = i𝜔P∕∕ − n × ∇∕∕ Mn P n × (E1∕∕ − E2∕∕ ) = −Jtom = −i𝜔𝜇0 M∕∕ − n × ∇∕∕ n 𝜀0 P ∕∕ n ⋅ (E1∕∕ − E2∕∕ ) = −∇ ⋅ 𝜀0 n ⋅ (H1∕∕ − H2∕∕ ) = −∇ ⋅ M∕∕

(7.6) (7.7) (7.8) (7.9)

where P and M are the surface electric and magnetic polarization densities, correspondingly. Subscripts “//” and “n” denote the tangential and normal components, respectively, while superscripts “1” and “2” refer to the fields at the two sides of the metasurface. Jtoe and Jtom denote the effective total electric and magnetic currents. Equations (7.6)–(7.9) provide us the information of the induced polarization density that transforms the impinging field in a desired manner, but still it cannot give an intuitive insight of the metasurface design. To investigate this problem, models based on different homogenized parameters such as the polarizability [20], susceptibility [21], and equivalent impedance [22] have been proposed and demonstrated. Polarization density in Eqs. (7.6)–(7.9) can be expressed as a form of the polarizability and the incident field [20]: P=

μM

𝛼 ̂ee 𝛼 ̂ E + em Hi S i S

=

𝛼 ̂me 𝛼 ̂ E + mm Hi S i S

(7.10)

(7.11)

7.2 Modeling and Synthesis Methods of Metasurfaces

where Ei and Hi are the known incident fields and S is the area of the unit cell of the metasurface. 𝛼 ̂ is the effective polarizability dyadic, which represents the collective effect of a single scatterer (inclusion) itself, together with the coupling and interaction from the whole metasurface array [22]. To synthesize a metasurface, one should first determine the polarization by substituting the known incident fields (Ei , Hi ), desired transmitted fields (Et , Ht ), and reflected fields (Er , Hr ), into ̂ components through the obtained (P, M) and Eqs. (7.6)–(7.9) and then find the 𝛼 Eqs. (7.10) and (7.11). For normal incident plane wave and the metasurface with uniaxial symmetry, a simple form of the polarizability and fields can be derived [20] [( ) i𝜔 1 co co cr cr 𝜂0 𝛼 ̂ee + 𝛼 ̂em + 𝛼 ̂me − 𝛼 ̂ I Er = − 2S 𝜂0 mm t ) ] ( 1 cr cr co co + 𝜂0 𝛼 ̂ee −𝛼 ̂em −𝛼 ̂me − 𝛼 ̂ J ⋅ Ei (7.12) 𝜂0 mm t ( )] [[ i𝜔 1 co co cr cr 𝜂0 𝛼 ̂ee + 𝛼 ̂em − 𝛼 ̂me + 𝛼 ̂ It Et = 1 − 2S 𝜂0 mm ) ] ( i𝜔 1 cr cr co co 𝜂0 𝛼 − ̂ee −𝛼 ̂em +𝛼 ̂me + 𝛼 ̂ J ⋅ Ei (7.13) 2S 𝜂0 mm t where I t = I − z ⋅ z is the tangential unit dyadic, while J t = z × I t is the transverse rotation dyadic. Superscripts “co” and “cr” mean the symmetric and antisymmetric components of 𝛼 ̂, respectively:

7.2.3

co cr 𝛼 ̂ee = 𝛼 ̂ee It + 𝛼 ̂ee Jt

(7.14)

co cr 𝛼 ̂em = 𝛼 ̂em It + 𝛼 ̂em Jt

(7.15)

co cr 𝛼 ̂me = 𝛼 ̂me It + 𝛼 ̂me Jt

(7.16)

co cr 𝛼 ̂mm = 𝛼 ̂mm It + 𝛼 ̂mm Jt

(7.17)

Susceptibility Model

Besides the polarizability, alternatively, metasurface can be homogenized by surface susceptibility. Polarization density is described by [21]. √ P = 𝜀𝜒 ee Eav + 𝜇𝜀 𝜒 em Hav (7.18) √ 𝜀 M = 𝜒 mm Hav + 𝜒 E (7.19) 𝜇 me av u = [Eu + (Eu + Eu )]∕2 and H u = [H u + (H u + H u )]∕2 (u = x, y, z) are the where Eav r av r t t i i average fields at the two sides of the metasurface. Closed-form relation of the fields and the susceptibility tensor can be obtained for simplified case, assuming only tangential components of the polarizations are induced, so that Pz = M z = 0. Substitute Eqs. (7.18) and (7.19) into Eqs. (7.6)–(7.9), it leads to

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( (

y)

−HΔ HΔx y

EΔ −EΔx

(

xy ) ( x ) Eav y Eav

𝜒eexx 𝜒ee = i𝜔𝜀 yx yy 𝜒ee 𝜒ee

)

( = i𝜔𝜇

xy

xx 𝜒mm 𝜒mm yx yy 𝜒mm 𝜒mm

)(

x Hav y Hav

( xx xy ) ( x ) √ Hav 𝜒em 𝜒em + i𝜔 𝜀𝜇 yx yy y 𝜒em 𝜒em Hav )

( xx xy ) ( x ) √ Eav 𝜒me 𝜒me + i𝜔 𝜀𝜇 yx yy y 𝜒me 𝜒me Eav

(7.20)

(7.21)

with EΔu = Etu − (Eiu + Eru ) and HΔu = Htu − (Hiu + Hru ) denote the differences of the fields at the two sides of the metasurface, correspondingly. To this point, susceptibility tensor matrix still cannot be completely determined for a specified field transformation. The number of the unknown matrix components should be reduced by enforcing some extra conditions. For example, to a mono-anisotropic and uniaxxy yx xy yx ial medium, so that 𝜒 em = 𝜒 me = 0 and 𝜒ee = 𝜒ee = 𝜒mm = 𝜒mm = 0, Eqs. (7.20) and (7.21) degrades to a simple relation: 𝜒eexx = 𝜒eeyy = xx 𝜒mm yy 𝜒mm

−HΔy

(7.22)

x i𝜔𝜀Eav x HΔ

(7.23)

y

i𝜔𝜀Eav EΔy = x i𝜔𝜇Hav −EΔx = y i𝜔𝜇Hav

(7.24) (7.25)

Therefore, the metasurface can be synthesized according to the desired fields at the two sides.

7.2.4

Equivalent Impedance Model

Equivalent impedance model based on the transmission line theory is also a powerful method for the metasurface design [22, 23]. Impinging plane wave to the metasurface is analogy to a propagating signal in a transmission line with proper equivalent parameters. Metasurface described by Eqs. (7.6)–(7.9) that can be modeled by a T-circuit as shown in Figure 7.3, with the equivalent impedance matrix that connects the voltages and currents by ) ( 1) ( 1) ( Z11 Z12 i v = (7.26) v2 Z21 Z22 i2 where Z 11 = Z 1 + Z 3 , Z 22 = Z 2 + Z 3 , Z 12 = Z 21 = Z 3 . Linking the tangential fields at the two sides of the metasurface to the voltages and currents of the transmission line, i1

i2 Z1 v1

Z2 Z3

v2

Figure 7.3 Equivalent transmission line model (T-circuit) of a metasurface. Source: Asadchy et al. [23]. © 2016 American Physical Society.

7.3 Simulation Algorithms of Quasi-periodic Electromagnetic Surfaces

(

E1∕∕ E2∕∕

)

) )( ( n × H1∕∕ Z11 Z12 = Z21 Z22 −n × H2∕∕

(7.27)

one can obtain the corresponding impedance matrix of the metasurface.

7.3 Simulation Algorithms of Quasi-periodic Electromagnetic Surfaces 7.3.1

Introduction to EM Surfaces

With the development of EM theory, EM surfaces have attracted more and more attentions due to their flexible design features and rich functions. An EM surface is a planar structure with certain EM properties. In general, its thickness is much smaller than the wavelength, and the aperture area is larger than the wavelength, and it has scattering, transmission, or absorption effects on EM waves. EM surfaces could achieve specific functions through a variety of structures. Most EM surfaces are planar arrays that consist of scatterers arranged in small electrical sizes. Some scatterers in the array are identical, forming a periodic EM surface; some scatterers in the array are not exactly the same, forming a quasi-periodic EM surface. The different scatterers of these EM surfaces respond differently to space EM waves, increasing the degree of freedom for designing the EM surface and enabling many interface characteristics that are not found in nature, such as artificial magnetic conductor (AMC) [24], frequency selective surface (FSS) [25], etc. Figure 7.4 shows a schematic pattern of a general quasi-periodic EM surface. Researchers have proposed many types of quasi-periodic EM surfaces, such as reflect-array [26], transmit-array antenna [27], nano-optical arrays [28], and so on, which can modulate the amplitude and phase of EM waves. The scatterers of quasi-periodic EM surface are generally not the same. Usually, there are some changes such as scaling, rotation, etc., which can effectively adjust the amplitude and phase of the EM waves, and provide flexibility for EM surface design to achieve more functions. For example, a scatterer whose phase changes linearly with the size will achieve a low-profile reflection array with a relative 3 dB bandwidth of more than 20% [29], which is suitable for a wideband mobile communication system. A dual band scatterer with an independent phase adjustment is proposed to realize a circularly polarized low-profile reflectarray [30]. An FSS can combine with a quasi-periodic reflectarray to achieve a reflective EM surface insensitive to incident angles [31]. Quasi-periodic EM surfaces are also flexible in radiation pattern design. A multi-beam reflectarray is designed for wireless communications

1

Figure 7.4

2

3

4

Illustration of a quasi-periodic EM surface.

N−1

N

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7 Metamaterial or Metastructural Thin Films for EM Wave Control

and electronic countermeasure systems [32]. A satellite antenna array with the beam shape covering a European region is designed using beam-forming techniques [33]. A metasurface, which could detect OAM mode for multiplexing communication, is proposed using quasi-periodic EM surface at a microwave band [34]. A nano-metasurface is designed for wideband and wide-angle holographic imaging [35]. In addition, quasi-periodic EM surfaces can also introduce reconfigurable designs to dynamically control their performances. A mechanically controlled reconfigurable quasi-periodic EM surface uses rotating scatterers to control the phase responding to circularly polarized incident waves, thus changing the reflected beam direction accordingly [36]. A reconfigurable quasi-periodic EM surface with PIN diodes is proposed in [37]. The scatterers’ performance in phase response is achieved by the direct current (DC) bias control of a PIN diode, which has a good integration property. A reflectarray with power amplifiers is designed to compensate the incident power [38]. A reconfigurable mid-wavelength optical quasi-periodic EM surface with graphene and gold is designed for beam-forming [39]. The scatterers are controlled by different DC voltages to achieve different phase shifts. These studies have extended the applications of quasi-periodic EM surfaces and proved their advantages over traditional designs in controlling EM waves.

7.3.2

Design of Quasi-periodic EM Surfaces

The design of quasi-periodic EM surfaces mainly contains three steps: scatterer simulation, array computation, and full wave validation. (1) Scatterer simulation. Use the periodic boundary condition (PBC) [40, 41] to simulate and model the scatterer with different parameter values. The PBC is based on Floquet or Bloch theorem. It expands the current distribution on the scatterer into different modes and can model the response of scatterers with respect to different incident waves. Many commercial simulation software integrates such functions. (2) Array computation. The configurations of the scatterers are determined according to the design requirements, and then array computation is performed on the entire quasi-periodic EM surface through the array method or the aperture field method. The array method does not consider the polarization direction of the EM field, and the aperture field method considers it. Consequently, the far field pattern of the co-polarization and the cross-polarization can be calculated. (3) Full-wave validation. After the design is completed, it is often necessary to simulate the entire quasi-periodic EM surface using full-wave methods to verify whether the design achieves the desired goal. Here we take the design of a reflectarray antenna [26] as an example to specifically describe the actual design process of a quasi-periodic EM surface. Reflectarray antennas use feed antennas to generate incident waves. The scatterers are located in the far field of the feed antenna, so the directional pattern of the feed antenna can be used as the amplitude information of the EM wave as an excitation. The phase of incident EM waves are determined by path lengths. The response of scatterers can

7.3 Simulation Algorithms of Quasi-periodic Electromagnetic Surfaces

be calculated using commercial simulation software. As an example, in HFSS, the amplitude and phase responses of the scatterers to plane waves can be described by the S-parameters of different modes of the Floquet ports. The far-field radiation characteristics of the array are computed using either the array method or the cross-polarized aperture field method. In the design of a quasi-periodic EM surface, the PBC is an approximation to the actual situation, and the array calculation process is also not accurate enough. It is pointed out that since the size of each scatterer and the surrounding scatterers are different, the phase of the actual contribution of one scatterer will be different from the phase required [42], and increase the phase error of this scatterer. In order to design a high-performance quasi-periodic EM surface, accurate full-wave simulation is needed and then the array design is adjusted according to the phase error to improve the gain of the array and reduce the side lobes. However, full-wave simulation cannot be widely used in the array optimization process because the time of full-wave simulation of a single array is too long, which usually takes tens of hours. Therefore, the new quasi-periodic EM surface simulation algorithm is urgently needed to reduce the full-wave simulation time to the level of the scatterer simulation time so that a large number of full-wave simulations can be applied during the design process to improve the design accuracy and help to discover more physical phenomena on quasi-periodic EM surfaces.

7.3.3

Simulation Algorithms of Quasi-periodic EM Surfaces

Basic EM simulation methods mainly include method of moments (MoM) [43], finite-element method (FEM) [44], finite-difference time-domain (FDTD) [45], etc. They all discretize the original Maxwell equation into the matrix equation. These full-wave simulation methods can be divided into two categories based on integral equations and partial differential equations. The MoM is based on the integral equation method, and the FEM and FDTD are methods based on partial differential equations. As electrical size of the EM surface is usually large, its simulation is usually based on the surface integral equation using MoM. This is because the MoM can define the unknown on the surface of the geometry to be solved without discretizing the whole 3D space and can reduce the number of unknowns compared to other methods. At the same time, the equations of MoM include the radiation boundary condition of EM waves and do not need to deal with extra radiation boundary conditions, which leads to higher accuracy. Currently, the MoM has been widely applied to the simulation modeling of large scatterers [46], periodic arrays [47], infinite multilayer media [48], and random rough surfaces [49]. In practical applications, the matrix equations corresponding to the MoM contain high-dimensional dense matrices. It is difficult to solve large-scale problems and it needs to be accelerated. To this end, researchers have developed various general acceleration algorithms, such as multilevel fast multipole algorithm (MLFMA) [50], adaptive cross-approximation [51], and domain decomposition [52]. With the development of computers, software developers compile the codes of these algorithms as executable programs and pack

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them into commercial software such as CST (3DS–CST, www.3ds.com), HFSS (ANSYS–HFSS, www.ansys.com), FEKO (Altair–Feko, www.altairhyperworks .com), COMSOL (COMSOL, www.comsol.com), and so on. At the same time, it can be accelerated according to the characteristics of the programming language, hardware platform, and other tools, such as the use of CUDA graphics processing units (GPU) [53], parallel programming [54], and so on. The full-wave simulation of quasi-periodic EM surfaces is a large- and multi-scale problem. The matrix equation with a large condition number has millions of unknowns. It is generally difficult to effectively solve the problem by using existing full-wave simulation algorithms. This limits the flexibility and accuracy of the quasi-periodic EM surface design and is a bottleneck restricting the development of quasi-periodic EM surfaces. If we design specific fast simulation algorithms for quasi-periodic EM surfaces at the algorithm level, we can effectively reduce the time for full-wave simulation, then improve the precisions of the design processes, and help to design quasi-periodic EM surfaces efficiently.

7.3.4

Review of Simulation Algorithms of Quasi-periodic EM Surfaces

When solving MoM equations on quasi-periodic EM surfaces, common acceleration methods can be divided into four types: approximation algorithms, dimension reduction, matrix compression, and fast Fourier transform (FFT), as shown in Figure 7.5. The main idea of the approximate simulation method is to physically approximate the solution area such as PBC, impedance homogenization, and so on. The PBC is based on the Floquet theory and the current distribution of the scatterers is expanded into linear combinations of different modes [55]. This expansion method is only valid for PBC, which corresponds to the infinite periodic surface formed by the same scatterer, and is usually used for the design phase of the quasi-periodic surface as an approximate model for scatterer simulation. Impedance homogeneity means that when the size of the array unit is small, the EM surface is uniformly processed, and the characteristics of the scatterer are described by electrical parameters. For example, by modeling quasi-periodic surfaces with impedance boundary conditions, the MoM matrix equations can be established [56]. The GSTC can also be used to simplify the EM field equation [57]. The equivalent surface impedance is generally expressed in terms of the polarizability of the electric and magnetic fields. It is related to thermoelectric (TE) or transition-metal (TM) polarization and can be obtained by the Fresnel formula [58]. The GSTC can also be combined with the integral equation Quasi-periodic EM surface simulation algorithms

Approximate simulation method

Figure 7.5

Dimension reduction method

Matrix compression method

FFT-based method

Classification of quasi-periodic EM surface simulation algorithms.

7.3 Simulation Algorithms of Quasi-periodic Electromagnetic Surfaces

method. For example, the researchers used the surface integral equation in the spectral domain to build the MoM equation on the EM surface [59]. When the EM surface mathematically satisfies a certain continuous function, its dielectric constant can be approximated as a continuous function, and its surface characteristics can be directly derived from the EM field propagation equation. An approximate analysis of a quasi-periodic surface with the cosine function shaped scatterers is proposed [60]. The solution to this equation is the Mathieu function. From this, we can get the electric field distribution of the scatterer at different frequencies. For the array of rotating scatterers, a quasi-periodic array consisting of rotating scatterers based on Green’s function modeling is proposed [61], where each scatterer of this array is rotated by a certain angle different from the previous one. The Green’s function of its electric field can be expressed by the Fourier expansion, similar to the Green’s function of the periodic array. Researchers use hierarchical dipole approximation (HDA) to align periodic EM surfaces for modeling [62]. It reduces the number of unknowns and does not require a triangular mesh. It is suitable for small-sized scatterers. In general, the approximate methods have clear physical pictures and can effectively reduce the number of unknowns. The disadvantage is that they are only suitable for electrically small scatterers, or a relatively smooth varying EM surface, and the simulation accuracy is insufficient for large and complex scatterers. The dimension reduction method [63] basically adopts new basis functions to reduce the total number of unknowns in the matrix equation. The first method is to employ global basis functions. For example, for a disc-type quasi-periodic surface, a Gaussian ring basis function defined on the entire EM surface is used to effectively reduce the number of unknowns and accelerate the solution [64]. The reference [65] is based on Fourier–Bessel global base function for elliptic quasi-periodic EM surfaces. In addition, the similarities among scatterers in simulating quasi-periodic EM surfaces and linearly combine the original basis functions can be used to obtain a new basis function. The reference [66] used the characteristic basis function (CBF) method to simulate the quasi-periodic EM surface, which is composed of rotating scatterers and generates the EM waves with angular momentum (AM) mode. The CBF is proposed in [67]. It extracts the information of the current distributed on the array scatterers with EM waves at different incident angles and thus obtains a set of CBFs that can be defined on the whole scatterer. The basic idea of this method is that EM waves can be expanded into superpositions of plane waves of different angles. As long as the number of incident angles is enough, it can be used to represent the responses from different excitations on the array’s scatterers. After the set of CBFs is obtained, the equations of the original MoM can be projected onto the set of basis functions, thereby reducing the number of unknowns. Synthetic basis function (SBF) method has been adopted to apply a certain acceleration method to the actual quasi-periodic EM surface simulation and the corresponding software have been developed [68]. The method of SBF uses the scattered current generated by the point sources around the scatterers in the array as the basis function for the current expansion [69]. Compared with CBF, it extracts basis functions from the near-field EM response, which can represent the role of near-field coupling more effectively. Generally, it can use fewer unknowns than the feature basis function [70]. Simulating periodic EM surfaces with the macro basis functions (MBF), which

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extract the main vector components from the current distribution of multiple scattering, has been developed as a new basis function [71]. In general, the dimension reduction method can effectively reduce the number of unknowns and has strong versatility. The disadvantage is that it does not use the characteristics of the scatterer arrangement information. Thereby, the performance of the algorithm still has room for further improvement. The method of matrix compression mainly relies on finding the low-rank representation of the impedance matrix because the interaction of basis functions that are far apart can be represented by fewer multiple items and have mathematically low-rank characteristics. Adaptive Cross Approximation (ACA) is explored to calculate the near-field interaction in quasi-periodic EM surface simulation, which can reduce the computational complexity [72]. The ACA method applies the greedy algorithm to decompose the far-field part of the impedance matrix into the product of two low-rank matrices [51]. The literature [73] uses the H-matrix algorithm to compress the near-field interaction of quasi-periodic EM surfaces. The H-matrix algorithm was proposed as a general framework to hierarchically represent low-rank matrices and to reduce the storage and computational complexity [74]. The matrix compression method is versatile and can effectively reduce the complexity of the representation of the impedance matrix. The disadvantage is lack of the scatterer alignment information, and thus the performance of such algorithms can be further improved based on the periodic alignment of scatterers. FFT-based method generally relays on the translation invariance or periodicity of the basis functions. The matrix vector multiplication should be converted into cyclic convolution operation and then employ FFT to accelerate. The matrix generated by the translation invariant basis functions has the Toeplitz property, that is, the entries on each subdiagonal line are equal. In some simple problems, the basis functions themselves can provide periodicity. However, for the general three-dimensional MoM equations, a surface triangular mesh is generally used, on whichthe basis functions are also defined. They do not exhibit translation invariance. This requires that the basis functions defined on the triangular mesh are represented by those defined on uniform grids to obtain translation invariant basis functions and the Toeplitz property of the corresponding impedance matrix. For example [73], uses integral equation fast Fourier transform (IE–FFT )) [75] to accelerate the simulation of quasi-periodic EM surface, and adopts the H-matrix algorithm [74] to further compress the matrix. The IE–FFT method was proposed by Seo and Lee [75]. It uses the Lagrangian interpolation on uniform grids to represent the Green’s function interaction between basis functions. The adaptive integral method (AIM) is combined with an SBF to simulate a quasi-periodic EM surface by De Vita et al. [68] and improves the sparse matrix multiplication in the AIM [76]. The number of unknowns in the original AIM can be reduced, which approximates the basis function with the superposition of impulse functions on uniform grids [77]. It should be noted that the acceleration based on the FFT method is suitable for matrix vector multiplication operation, which needs to be combined with a matrix iterative solver, such as the conjugate gradient method and the fast Fourier transform (CG–FFT) method [78–80]. In addition, other iterative algorithms, such as BCG (bi-conjugate gradient)–FFT [81, 82], TFQMR (transpose free quasi minimal residual)–FFT [83],

7.4 Orbital Angular Momentum of Electromagnetic Waves: Generation and Detection

and stable BCG and so on [84, 85], can also be used as an extension to the basic iterative algorithm to achieve the O(Nlog N) computational complexity, where N is the number of total unknowns. The FFT based methods can effectively speed up the matrix vector multiplication. The disadvantage is that the number of unknowns will increase when the triangular grid is mapped to the uniform grids. Additionally, this kind of algorithm has not been used to explore the scatterer similarity of the quasi-periodic EM surface. From the above literature, the following is a preliminary summary of the research on periodic EM surface fast simulation algorithms: (1) The core idea of the acceleration algorithms is to compress redundant information in quasi-periodic EM surfaces. (2) Some methods required certain assumptions such as small scatterers, smooth change of the surface, etc., with a narrower scope of application. (3) It is necessary to make full use of the characteristics of the EM surface itself. (4) The advantages of existing fast algorithms can be merged by using hybrid methods.

7.4 Orbital Angular Momentum of Electromagnetic Waves: Generation and Detection 7.4.1

Introduction

EM waves carry two types of AM, namely, the spin angular momentum (SAM) and the OAM [86]. In view of a single photon, its value of SAM is ±ℏ for circularly polarized EM waves (ℏ is the reduced Planck constant). Here, +/− sign is taken for left/right-handed circular polarization (L/RHCP). Different from SAM, OAM manifests the orbital rotation of photons and each photon can have an OAM of value lℏ, where l is known as the OAM index and can be any integer. In 1992, Allen et al. first found that Laguerre–Gaussian (LG) beams carry well defined OAM [87]. For EM waves carrying OAM, different values of l correspond to mutually orthogonal wave states, and therefore, these states can be utilized for multiplexing in communication systems [88–91]. Also, there have been other emerging applications of OAM, such as in super resolution imaging [92], optical tweezers [93], and detectors [94]. With the great potential of OAM in various applications, research on its generation has been undertaken extensively. At optical frequencies, to generate OAM beams, optical devices, such as spiral phase plates (SPPs) [95], computer-generated holograms (CGHs) [96], and q plates [97] are commonly used. Since 2010, antenna arrays that generate OAM waves at radio frequencies have been analyzed [98], followed by the traveling-wave antennas [99] and circularly polarized antennas [100] demonstrated to produce OAM waves in radio. In addution, with the proposed concept of metasurface, various prototypes of metasurfaces are designed to radiate EM waves carrying OAM, at both optical and radio regimes [14]. The OAM detection is also an area worthy of study. Although OAM detection is a reciprocal process of OAM generation, it is much more challenging due to the divergence and spatial-dependence nature of an OAM wave. The approaches for

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OAM detection can be classified into three categories, mode analysis based on field data [101, 102], observation of OAM induced effects such as the rotational Doppler shift [103], and the beam reforming using holographic technology [104].

7.4.2

Generation of Orbital Angular Momentum

Compared with plane waves, the OAM-carrying waves have helical wave fronts, resulting from an additional phase term, eil𝜑 , where 𝜑 is the azimuthal angle. Therefore, by introducing the azimuthal-angle dependent phase delay to the path of plane wave, the plane wave will be modulated to carry an OAM. There are several ways to introduce the phase delay, but generally, they can be categorized as independent of and dependent on the wave polarization. The first scheme employs isotropic materials, such as the SPPs and CGHs. The thickness of an SPP gradually changes along the azimuthal coordinate, proportional to the value of required phase delay, l𝜑, while that for CGHs, they are diffractive optical elements. Different orders of OAM can be observed at the locations according to different diffraction orders [105]. Alternatively, the excitation of an antenna or antenna array can be directly modulated to fulfill the required phase condition so that an OAM wave can be radiated out [106]. Moreover, by utilizing the abrupt phase shift at scatterers or resonators, the desired phase delay can be produced with a reduced device size. For example, by designing the scatterers with different sizes and geometries, a full coverage of 2𝜋 phase shift can be realized [14]. Therefore, by patterning the scatterers according to the required phase distribution, arbitrary beam forming can be achieved, including generating OAM waves [107]. The second scheme is based on the coupling and conversion between SAM and OAM [108] and requires inhomogeneous and anisotropic media, such as q plates. A q plate has a spatially varying optical axis and at the same time, which changes the helicity of circular polarization. Its behavior can be explained by the AM conservation law. The helicity change of the circular polarization indicates a change of ±2ℏ of SAM. When the q plate is cylindrically symmetric, the outcoming wave must carry an OAM of ∓2ℏ to conserve the total AM. When the q plate is not cylindrically symmetric, it introduces extra AM to the system so that different orders of OAM can be generated. q plates are usually implemented using liquid crystals. Recently, metasurfaces have been used to implement the feature of q plates, where anisotropic scatterers are designed to change the helicity of circular polarization and they are placed with different orientations, correlating with the spatially varying optical axis of q plates [109, 110]. Apparently, the phase shift of scatterers depends on their orientations. This phase shift originates from the change in the polarization state along different paths on the Poincare sphere known as geometric phase [111]. Therefore, this type of metasurfaces is also known as geometric-phase metasurface, which is fundamentally different from the metasurfaces adopting the first scheme. 7.4.2.1

Geometric-phase Metasurfaces

In the following, we will introduce several geometric-phase metasurfaces for OAM generation.

7.4 Orbital Angular Momentum of Electromagnetic Waves: Generation and Detection

al

z x

g

as

p

p

y

h (b)

(a) Amplitude

(c)

(d)

Phase

0

1

–π

π

0

1

–π

π

Figure 7.6 Schematic and the response of the ultrathin complementary metasurface. (a) Geometry of the unit cell; (b) Geometry of the metasurface for l = 4; (c, d) Amplitude and phase distributions of the cross-circularly polarized component of the electric field on a transverse plane. The results in (c) are calculated from the equivalent dipole model with the distributed dipole moments shown in (b). The results in (d) is from the full-wave simulation. Source: Chen et al. [112]. © 2017 IEEE.

Figure 7.6 shows an ultrathin complementary metasurface to radiate an OAM wave with high efficiency [112]. The unit cell consists of four complementary split ring resonators (CSRRs) in different sizes and orientations. The fundamental resonance of the larger two CSRRs is excited by y polarization, while that of the smaller two CSRRs is excited by x polarization. By tuning their sizes, the transmitted x and y components can have same amplitude and a 𝜋 phase difference. Therefore, the unit cell transmits the incident circularly polarized wave with a reversed helicity. The double-layer complementary structure guarantees the high transmittance. Then, a whole metasurface is built by arranging the unit cells with varying orientations, as shown in Figure 7.6b. The axial rotation angle for each unit cell, 𝛼 = 2𝜑. So, the phase shift introduced by the unit cell is ±2𝛼 (i.e. ±4𝜑). The OAM order it generates

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should be ±4. The sign depends on the helicity of incident circular polarization. The EM responses of the metasurface are obtained by two approaches, namely, the equivalent dipole approximation and the full-wave simulation using CST MWS. In the equivalent dipole model, each unit cell is considered as two orthogonal magnetic dipoles as shown in Figure 7.6b (blue and yellow arrows). Then, the EM response of the whole metasurface is calculated by adding up the responses of all the equivalent magnetic dipoles. This approach is useful for analyzing metasurfaces consisting of a large number of scatterers, since these scatterers can be decomposed into point sources with different strengths, locations, and orientations. The calculated field distributions are shown in Figure 7.6c. Clearly, we can observe the phase singularity at the center. The phase changes by 8𝜋 along a circular path around the center, which verifies the successful generation of an OAM of order 4. Full-wave simulation results are shown in Figure 7.6d. The divergence between the results from the dipole model and the full-wave simulation comes from the coupling among unit cells. Other than the transmission-type metasurface, a reflection-type metasurface is also shown below [113]. The metasurface contains an anisotropic perfect electric conductor (PEC) layer on top of an isotropic perfect magnetic conductor (PMC) layer. The top views of two metasurfaces are shown in Figure 7.7. The PEC layers are composed of the red strip arrays, which are metals. For simplicity, the isotropic PMC layers are indicated by the blue surfaces beneath the red strips. Due to a 𝜋 reflection phase difference from a PEC and a PMC layers, the composite structure reflects the incident LCP/right circularly polarized (RCP) plane wave to a RCP/LCP y Phase

Amplitude

𝛼 x 0

1

–π

π

0

1

–π

π

(a)

(b)

Figure 7.7 The composite PEC–PMC metasurfaces for OAM generation. (a) l = 1 and (b) l = 2. The left panels show the top views of the PEC layers. The middle panels and the right panels show the amplitude and phase distributions of the reflected waves on a transverse plane away from the metasurfaces.

7.4 Orbital Angular Momentum of Electromagnetic Waves: Generation and Detection

wave. And the locally different inclination angle between the metal-strip tangent and x axis, 𝛼 introduces a locally modulated phase shift, ±2𝛼. Therefore, to generate an OAM of order l, at each azimuthal location, 𝛼 should be equal to l𝜑/2 so that a phase factor e±il𝜑 can be introduced to the reflected wave. In Figure 7.7a, we make 𝛼 = 𝜑/2 and in Figure 7.7b, 𝛼 = 𝜑. Then, the metasurfaces are illuminated by an LCP Gaussian wave and the amplitude and phase distributions of the reflected RCP field components at a transverse plane away from the metasurfaces are plotted. As expected, in both Figure 7.7a,b, clear phase singularities are observed. We see the field distribution of an OAM wave with l = 1 in Figure 7.7a, because the phase encounters a total 2𝜋 change along a closed path enclosing the center. The total phase change around the center is 4𝜋 in Figure 7.7b, so the reflected wave carries an OAM of order 2. The patterning of the metasurface is accomplished with the assistance of grating vectors [114]. It is different from existing design protocols for geometric-phase-based metasurfaces, where complicated optimization process of single scatterer is needed. Since the PEC layer presents a quasi-continuous pattern, the distortions from local-response discontinuity of discrete scatterers are avoided. Moreover, by keeping the local period small enough, no high-order diffraction exists. 7.4.2.2

Photonic Crystals

In the following, a different scheme based on the superposition of two vibrational modes with proper weights and phases is explored for OAM generation. Particularly, in Figure 7.8, the EM energy is transferred from guided wave in a line defect in a photonic crystal (PC) to localized resonant mode in a point defect and then to unbounded OAM state in free space [115]. For a PC, there exists a band gap where no EM state is allowed to propagate. However, a localized state will be supported at the photonic band gap (PBG) if a defect is introduced [116]. In Figure 7.9a, we show the localized quadrupole modes (quadrupole-xy and quadrupole-diag modes) inside the PBG by increasing the

Figure 7.8 A schematic representation of OAM generation in PCs. Source: Chen et al. [115]. © 2018 American Physical Society.

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θ = π/2 BE2zeiθ

AE z1

0

+ 0

(a)

1

= 0

1

–π

π

–π

π

θ = – π/2

1

(b)

0

1

Figure 7.9 The quadrupole states (quadrupole-xy and quadrupole-diag states) and the superposed states (amplitude and phase patterns). Ez1 and Ez2 are the electric fields of the two quadrupole modes. A and B are the weights of two modes and 𝜃 is the relative phase of the mode 2 with respect to the mode 1.

radius of a dielectric rod in the PC. It is well known that combining two vibrational modes with a proper phase delay will generate a rotational mode [117]. Therefore, in Figure 7.9b, we superpose the two quadrupole modes and find that pure OAM states with the spatial phase dependence of e±i2𝜑 can be generated when the optimal weights (A = B = 1) and phase (𝜃 = ±𝜋/2) are chosen. To excite the two quadrupole modes simultaneously with the same weights and a phase difference of 𝜋/2, we did optimization in CST MWS. The simulation model is same as in Figure 7.8. To be specific, there are dielectric rods sandwiched between two metallic plates. A row of dielectric rods is removed to form a waveguiding channel. Right above the defect rod, there is a circular opening on the top metallic plate to allow the modes to radiate. The eigenfrequencies of the quadrupole modes are fixed when the size of the defect rod is fixed. Thus, at any frequency between the eigenfrequencies, the mode can be considered as the superposition of the two quadrupole modes. By sweeping the frequency, the weights and relative phase will change dramatically. Therefore, the operating frequency should be carefully chosen. The other parameter that needs to be optimized is the height of the rods, because it directly determines the modes at the air–dielectric interface. By feeding the structure from the other end of the channel, the helicity of the radiated OAM wave will be changed. OAM states of order ±1 can be generated by exciting two dipole modes simultaneously. Experimental results are shown in Figure 7.10. Higher-order OAM waves may be produced by continuously increasing the radius or permittivity of the defect so that higher-order localized modes will appear and can be manipulated.

7.4.3

Detection of Orbital Angular Momentum

As discussed in the introduction part, detection of OAM is another issue that needs to lay emphasis on. In the following, a holographic metasurface is proposed for the detection of multiple OAM beams [118]. The detection process is summarized in Figure 7.11. The metasurface converts the incident wave to a Gaussian wave and the radiation direction of the Gaussian wave is distinguishable according to the order of

7.4 Orbital Angular Momentum of Electromagnetic Waves: Generation and Detection

Defect rod

Probe

Absorber

Horn antenna

y

y

110 mm

–π

(b)

160 mm x

110 mm

Waveguide channel

(a)

–π

π (c)

π

160 mm x

Figure 7.10 The experimental results of the proposed PC. (a) Experimental setup. The phase distributions of E z at a transverse plane 30 mm above the structure with (b) quadrupole defect, measured at the frequency f = 8.75 GHz and (c) dipole defect, measured at the frequency f = 9.55 GHz. Source: (a) Reproduced with permission from Chen et al. [115]. © 2018 American Physical Society, (b, c) Chen et al. [115]. © 2018 American Physical Society.

Figure 7.11 Schematic representation of multiple OAM-beam detection by using a single metasurface.

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7 Metamaterial or Metastructural Thin Films for EM Wave Control

incident OAM. Consequently, by locating the Gaussian wave, the incident OAM can be conveniently determined. To start with, we model the metasurface by a transmittance function: ∑ Am ej(lm 𝜙+kxm x+kym y) (7.28) t(r, 𝜙) = m

where r is the radial position, 𝜑 is the azimuthal position, Am is the weight of the mth beam, lm is the corresponding OAM index, and kxm , kym are the transverse wave numbers of the mth beam. Then, the far-field response of the metasurface illuminated by an incident wave carrying an OAM of order l0 is calculated by doing the Fourier transform: ∑ E = F{Ein ⋅ t} = Am F{EOAM(lm +l0 ) (kxm , kym )} (7.29) m

Therefore, multiple beams are generated and at the designed k-space position (kxm , kym ), the OAM order is lm + l0 . When lm + l0 = 0, the beam is Gaussian and its beam axis is at (kxm , kym ). It is known that OAM wave has a singularity at its beam axis. Therefore, by examining the field intensity at the positions of (kxm , kym ), we can identify the Gaussian beam, i.e. the one that is not null. Then, the incident OAM order l0 can be determined. As a proof of concept, a five-beam case with lm = 2, 1, 0, −1, −2 at the directions of 𝜃 = 40∘ and 𝜑 = 90∘ , 18∘ , 306∘ , 234∘ , 162∘ is demonstrated. The transmittance t(r, 𝜑) is calculated first. Then, its phase information is extracted and implemented using the unit cell shown in Figure 7.6a. Full-wave simulated radiation patterns are shown in Figure 7.12. It can be seen that the maximum radiation directions (axis of the Gaussian beam) when l0 = −2, −1, 0, 1, 2 are at 𝜃 = 40∘ and 𝜑 = 90∘ , 18∘ , 306∘ , 234∘ , 162∘ , respectively, which is as expected. In summary, in this section, we have demonstrated several types of geometric-phase based metasurfaces for OAM generation based on the local phase manipulation. The local phase variation on the metasurfaces can be in discrete and continuous formats. For discrete scatterers, the patterning of the metasurface is straightforward and more flexible. While when the scatterers are quasi-continuous, the patterning of the metasurface is achieved through grating vectors. Additionally, a PC has been designed for OAM generation by introducing line and point defects. Based on the transmission function, a holographic OAM detection method has been proposed and implemented also by using geometric-phase-based metasurfaces.

ϕ−162° ϕ−234°

ϕ−306° ϕ−90°

l = −2

ϕ−18°

l = −1

l=0

l=1

l=2

Figure 7.12 Full-wave simulated far-field power patterns when the incident wave carries OAM of order −2, −1, 0, 1, and 2. Source: Chen et al. [118]. © 2018 IEEE.

7.5 Application in Spontaneous Emission Modification

7.5

Application in Spontaneous Emission Modification

7.5.1 Spontaneous Emission in Inhomogeneous Electromagnetic Environment SE as a fundamental phenomenon of light-matter interaction, is responsible for most of the photon creation process. From daily lighting and displaying, to optical communication, toward quantum information transportation, manipulation of SE is a vital requirement to build up efficient and controllable photonic devices. Since the pioneering work of Purcell in 1946, people open the gate of using macroscale EM environment to control the SE of an atom or molecule. To describe it quantitatively, the concept of spontaneous emission rate (SER) can be introduced by Fermi’s golden rule that how “fast” an quantum emitter transits from an initial state |i⟩ to a final state |f ⟩, and depends on both the atomic structure and density of EM mode of its environment [119] 𝜋𝜔0 2 𝛾= |p| 𝜌(r, 𝜔0 ) (7.30) 3ℏ𝜀0 where p is the transition dipole moment, ℏ is the reduced Planck’s constant, 𝜀0 is vacuum permittivity, and 𝜌(r, 𝜔0 ) is the local density of state (LDOS) of the EM field of frequency 𝜔0 for a quantum emitter located at r, which can be modified by many nanophotonic structures and therefore tune the SER flexibly. Typically, modification factor of the SER is denoted by the Purcell factor 𝛾 Fp = (7.31) 𝛾0 which compares the SER in the studied structure with free space. Currently, one of the most widely studied structures is the PC as shown in Figure 7.13a. Composing by two alternatively arranged materials with different refractive index (namely, n1 and n2 ), PCs introduce the artificial periodic EM environment that analogous to electron crystals. Due to the overlapped destructive interference, optical mode at certain frequency 𝜔0 doesn’t allow to exist along certain direction inside the PC. This frequency region is the so-called PBG where LDOS equals to zero, and therefore, SE is completely inhibited. If the 𝜔0 locates at where optical modes are dense, for example, near the PBG edge, SE will be highly increased. Besides, PC cavity with a point defect can also realize such enhancement effect when the emission mode is resonant with the PC cavity mode. Using single mode approximation, SER can be enhanced by a factor of Q/V (Q is the quality factor of the PC cavity, V is the mode volume). Another kind of the popularly investigated structures for SER modification is the plasmonic nanoparticle (Figure 7.13b). Emission of an atom or molecule close to a metallic nanoparticle is different from the situation in free space. Forster resonant energy transfer theory explains such phenomenon where the energy transfer between the quantum emitter and the closely located nanoparticle can tailor the emission rate, which is a bit similar to the case of two coupled dipoles. The former is the energy exchange process between photon and excited plasmon of the nanoparticle, while the latter is a nonradiative process.

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7 Metamaterial or Metastructural Thin Films for EM Wave Control

z

(b)

(a)

Tip

y gnp

x

z y x

E Excitation 0

1.0

Spectrometer, CCD,APD

Figure 7.13 Typical nanostructure for spontaneous emission modification. (a) Photonic crystal cavity. Source: Englund et al [120]. © 2005 American Physical Society. (b) Plasmonic nano-antenna. Source: Kühn et al. [121]. © 2006 American Physical Society.

7.5.2

Calculation of Spontaneous Emission Rate

For nanostructures of regular geometrical shape such as some cavities and PCs, LDOS, and therefore, the SER might be obtained analytically according to its definition. A more universal method is widely applied based on the dyadic Green’s function by which LDOS can be calculated from the imaginary part of the Green’s tensor at the location of the quantum emitter [119] 6𝜔0 ⟨p ⋅ Im[G(r, r, 𝜔0 )] ⋅ p⟩ (7.32) 𝜋c2 For quantum emitter has a specified dipole orientation, for example, along the y-axis, SER reduces to 𝜌(r, 𝜔0 ) =

𝛾=

2p2 𝜔20 ℏ𝜀0 c2

Im[Gyy (r, r, 𝜔0 )]

(7.33)

While if it does not have a fixed dipole axis, SER should be considered as the average value of all the directions 𝛾=

2p2 𝜔20 3ℏ𝜀0 c2

Im{Tr[G(r, r, 𝜔0 )]}

(7.34)

It is worth to note that this technology is only available for weak coupling situation when atom-field coupling constant 𝜅 is much smaller than 𝛾. Under such assumption, decay rate of a classical dipole emitter and a quantum emitter come to agreement.

7.5.3

Metamaterials Enhance Spontaneous Emission

Possibility of using metamaterials to change SER draws wide attentions in recent years. One of the most famous structure is the hyperbolic metamaterial (HMM),

7.5 Application in Spontaneous Emission Modification

𝜀

⊥

>

𝜀

0, 𝜀 < 0 ⃦

⃦

> 0, 𝜀⊥< 0

h

MA

R6G/PM

Si

kz

W

Ag

ky

H

kx d

x

(a)

(b)

te

a ubstr

z

ss Glas y

(c)

Figure 7.14 Iso-frequency surfaces for of two kinds of hyperbolic metamaterials: (a) 𝜀zz = 𝜀∥ < 0, 𝜀xx = 𝜀yy = 𝜀⟂ > 0 and (b) 𝜀∥ > 0, 𝜀⟂ < 0. Source: Poddubny et al. [122]. © 2013 Springer Nature. (c) Patterned hyperbolic medium for enhancing spontaneous emission and far field radiation. Source: Lu et al. [123]. © 2014 Springer Nature.

which consists of periodically stacked metal-dielectric layers of subwavelength thickness (1D HMM), or metal nanowires array embedded in the dielectric medium host (2D HMM). Along one direction, HMM behaves like metal, while along the other directions, it is more similar to a dielectric medium. This anisotropic property results in opposite signs of the effective permittivity along different directions 𝜀⟂ • 𝜀// < 0, therefore making this dispersion relation an open hyperbolic curve (Figure 7.14a,b) kx2 + ky2 𝜀⟂

+

kz2 𝜀∕∕

=

𝜔2 c2

(7.35)

which is very different from the isotropic medium for which the above formula is a closed circle. LDOS, which illustrates how “dense” the optical modes accumulate at frequency 𝜔0, can be intuitively represented by the volume between the iso-frequency surfaces at 𝜔0 and 𝜔0 + d𝜔. Theoretically, open hyperbolic shape shell in the k-diagram has infinite large LDOS and is hence significantly higher than ordinary medium with closed spherical or ellipsoid shell. However, this infinite is limited, in practice, by the finite period of the HMM, which prevents the emission to access large k region. Different from cavities where resonating condition is required for obvious SER modification, leading to the drawback of narrow bandwidth, HMM can enhance SER in a wide frequency range due to its hybrid plasmonic mode supported by the multiple-metal-dielectric interfaces and therefore has great potential for emission enhancement (Figure 7.14c).

7.5.4

Metasurfaces Enhance Spontaneous Emission

Compared with 3D bulk metamaterials, practical implementation of metasurface is less demanding and less challenging especially in the optical region. Besides, for some photonic devices such as the light-emitting diodes and solar cells, using a planar metasurface cover instead of changing the whole epitaxy structure can avoid perturbing the inner light emitting/absorption region and thus ease fabrication difficulty.

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7 Metamaterial or Metastructural Thin Films for EM Wave Control

n0 n

z

Et1

y

X=0 Er1

X1

X2

X3

z=0

Ei1

MS1

d1 Emitter d2

Er2

MS2

Ei2

(b)

backreflector

(a)

3000 2500 2000 1500 1000 500 0 –500 –1000 –1500 –2000 –2500 –3000

3000 2500 2000 1500 1000 500 0 –500 –1000 –1500 –2000 –2500 –3000

x (nm)

x

x (nm)

244

–4000

–2000

0 z(nm)

2000

4000

(c)

–4000

–2000

0 z(nm)

2000

4000

Figure 7.15 Metasurface-cavity enhanced single-photon source. (a) Design of the metasurface cavity, where MS1 is a partially reflective metasurface while MS2 is a completely reflective metasurface. Radiation power of (b) a traditional planar singe-photon source and (c) metasurface-cavity enhanced single-photon source. Source: Chen et al. [124]. © 2017 American Physical Society.

Figure 7.15 shows a double metasurface cavity [124], which has great potential to improve single-photon source by enhancing the SE rate, light extraction efficiency and far field directivity, simultaneously. Schematic view of the single-photon source is shown in Figure 7.15a with a quantum emitter embedded in the semiconductor medium. A partially-reflective metasurface (MS1 in Figure 7.15a) is placed at the upper semiconductor-air interface, which turns the upward incident dipole field Ei1 of all the incident angles into two parts: vertically reflected field Er1 and vertically transmitted field Et1 . Another metasurface (MS2 in Figure 7.15a) is responsible for turning all the downward incident field Ei2 of all the incident angles into vertically reflected field Er2 . Both of the metasurfaces can be synthesized using the susceptibility tensor technology as mentioned above by Eqs. (7.22)–(7.25). For MS1, yy 𝜒ee1 =

x x x 2 Ht1 − (Hi1 + Hr1 ) y y y i𝜔𝜀0 Et1 + (Ei1 + Er1 ) y

xx = 𝜒mm1

y

(7.36)

y

2 Et1 − (Ei1 + Er1 ) x x i𝜔𝜇0 Ht1 + (Hi1x + Hr1 )

(7.37)

and for MS2 yy

x x x 2 (Hi2 + Hr2 ) − Ht2 y y y i𝜔𝜀0 (Ei2 + Er2 ) + Et2

(7.38)

y y y 2 (Ei2 + Er2 ) − Et2 = x x i𝜔𝜇0 (Hi2x + Hr2 ) + Ht2

(7.39)

𝜒ee2 = xx 𝜒mm2

yy

yy

y

xx xx x = 𝜒mm1 = 𝜒ee2 = 𝜒mm2 = 0 and Et2 = Ht2 = 0. Since now all the emitted with 𝜒ee1 wave become vertically propagating inside the semiconductor region, leading to much stronger resonance and field confinement at the dipole location compared with no metasurface situation. SER is, therefore, obviously enhanced by a factor of 1.9 times. Besides, light trapping issue due to the sudden change of refractive index between the semiconductor/air interface is almost resolved due to the vertical emission (Figure 7.15b,c). Far field directivity is also naturally improved by the increased radiation aperture. Given these advantages, metasurface cavity has great potential to develop highly efficient single-photon source with high-single photon purity.

Acknowledgments

7.5.5

Other Potential Application in Quantum Optics

Beyond the application in SE modification of the quantum emitter, metasurface can realize anisotropic response, which could induce quantum interference between two spontaneous decay channels [125]. This is accessible by synthesizing an anisotropic metasurface cavity, which has different response to the dipole emitters polarized along different orientations. For example, enhancing the SER of a y-polarized dipole while suppressing the SER of an x-polarized dipole. The degree of quantum interference typically defined as QI = (𝛾 x − 𝛾 y )/(𝛾 x + 𝛾 y ) can be flexibly controlled by the metasurface. This offers more degrees of freedom to improve and control many quantum-interference-based phenomena such as EM induced transparency and lasing without inversion [126].

7.6

Conclusion and Perspectives

Metastructural film or metasurface, which exploits scatterers to realize the abruptly local change of EM response, plays more and more important roles in engineering control of propagation, scattering, and radiation of EM waves. The capabilities of flexible beam shaping make metastructural films useful in various applications, such as antenna engineering, flat optics, etc. Using metasurfaces improves the performance of antennas in many ways. For example, the Huygens’ metasurface antennas have high directivity and efficiency [127]. The bandwidth of circular polarized antennas can be largely increased using metasurfaces [128], and the size can be miniaturized [129, 130]. Metasurfaces are also employed for reconfigurable design of antennas [131]. The utilization of metasurfaces for antenna designs has achieved specified and advanced functions and becomes one of the most important applications of metasurface. At optical frequencies, metasurfaces can lead to a new class of flat optical components, such as planar lens [132, 133], planar wave plates [134–136], beam transformers, and splitters [137]. By exploring the spin degree of freedom, metasurfaces are also applied in spin-controlled photonics [138]. Furthermore, metasurfaces can replace the complex cascades of optical devices by providing different optical functions simultaneously, which cannot be achieved by conventional optical components [139]. Research on metasurfaces will be extended to nonlinear and quantum regimes, where multiscale and multiphysics modeling and designs should be developed. EM field-matter interaction should be controlled at both microscopic and macroscopic levels by using the metastructural films or metasurfaces.

Acknowledgments This work was supported in part by the Research Grants Council of Hong Kong (GRF 17209918) and National Natural Science Foundation of China (Nos. 61975177, 61701424, and 61571264)

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7 Metamaterial or Metastructural Thin Films for EM Wave Control

List of Abbreviations 3D ACA AIM AM AMC BCG CBF CG–FFT CGHs CSRRs EM FDTD FEM FFT FSS GPU GSTC HDA HMM IE–FFT LCP LDOS LG MBF MLFMA MoM NRI OAM PBC PBG PC PCB PEC PMC RCP SBF SE SER SPPs TFQMR ZIM

three-dimensional Adaptive Cross Approximation adaptive integral method angular momentum artificial magnetic conductor bi-conjugate gradient characteristic basis function conjugate gradient method and the fast Fourier transform computer generated holograms complementary split ring resonators electromagnetic finite-difference time-domain finite-element method Fast Fourier Transform frequency selective surface graphics processing units generalized sheet transition condition hierarchical dipole approximation hyperbolic metamaterial integral equation FFT left circularly polarized local density of state Laguerre–Gaussian macro basis functions multi-level fast multipole algorithm method of moments negative refraction index orbital angular momentum periodic boundary condition photonic band gap photonic crystal printed circuit board perfect electric conductor perfect magnetic conductor right circularly polarized synthetic basis function spontaneous emission spontaneous emission rate spiral phase plates transpose free quasi-minimal residual zero-index metamaterials

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99 Zheng, S.L. et al. (2015). Transmission characteristics of a twisted radio wave based on circular traveling-wave antenna. IEEE Transactions on Antennas and Propagation 63 (4): 1530–1536. 100 Barbuto, M. et al. (2014). Circular polarized patch antenna generating orbital angular momentum. Progress in Electromagnetics Research-Pier 148: 23–30. 101 Mohammadi, S.M. et al. (2010). Orbital angular momentum in radio: measurement methods. Radio Science 45: RS4007. 102 Schulze, C. et al. (2013). Measurement of the orbital angular momentum density of light by modal decomposition. New Journal of Physics 15: 073025. 103 Courtial, J. et al. (1998). Measurement of the rotational frequency shift imparted to a rotating light beam possessing orbital angular momentum. Physical Review Letters 80 (15): 3217–3219. 104 Genevet, P. et al. (2012). Holographic detection of the orbital angular momentum of light with plasmonic photodiodes. Nature Communications 3: 1278. 105 Moreno, I. et al. (2009). Vortex sensing diffraction gratings. Optics Letters 34 (19): 2927–2929. 106 Liu, K. et al. (2016). Generation of OAM beams using phased Array in the microwave band. IEEE Transactions on Antennas and Propagation 64 (9): 3850–3857. 107 Yu, S.X. et al. (2016). Generating multiple orbital angular momentum vortex beams using a metasurface in radio frequency domain. Applied Physics Letters 108 (24): 241901. 108 Bliokh, K.Y. et al. (2015). Spin-orbit interactions of light. Nature Photonics 9 (12): 796–808. 109 Kang, M. et al. (2012). Wave front engineering from an array of thin aperture antennas. Optics Express 20 (14): 15882–15890. 110 Karimi, E. et al. (2014). Generating optical orbital angular momentum at visible wavelengths using a plasmonic metasurface. Light-Science & Applications 3: e167. 111 Berry, M.V. (1987). The adiabatic phase and pancharatnam phase for polarized-light. Journal of Modern Optics 34 (11): 1401–1407. 112 Chen, M.L.L.N., Jiang, L.J., and Sha, W.E.I. (2017). Ultrathin complementary metasurface for orbital angular momentum generation at microwave frequencies. IEEE Transactions on Antennas and Propagation 65 (1): 396–400. 113 Chen, M.L.N., Jiang, L.J., and Sha, W.E.I. (2016). Artificial perfect electric conductor-perfect magnetic conductor anisotropic metasurface for generating orbital angular momentum of microwave with nearly perfect conversion efficiency. Journal of Applied Physics 119 (6): 064506. 114 Chen, M.L.L.N., Jiang, L.J., and Sha, W.E.I. (2019). Quasi-continuous metasurfaces for orbital angular momentum generation. IEEE Antennas and Wireless Propagation Letters 18 (3): 477–481. 115 Chen, M.L.L.N., Jiang, L.J., and Sha, W.E.I. (2018). Generation of orbital angular momentum by a point defect in photonic crystals. Physical Review Applied 10 (1): 014034.

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8 Semiconductor Thin Films for Information Technology Na Chen Tsinghua University, School of Materials Science and Engineering, Key Laboratory for Advanced Materials Processing Technology (MOE), Haidian district, Beijing 100084, China

8.1 Introduction Information technology (IT) has changed the world by revolutionizing the manner in which people live [1–3]. The core of IT is the use of computers to perceive, transmit, process, and store information. Semiconductors enable the control of charge carriers for information detection, logical operation, and data storage, which lay foundation of the current IT. In particular, Si-based semiconductors are the key materials for fabricating microprocessor and random-access memory in computers. Except for the single-crystalline Si chips applied in integrated circuits, semiconductors in the form of thin film have played an important role in optics and electronics for IT. For example, thin film transistors (TFTs) based on the hydrogenated amorphous Si (a-Si:H) enable the development of large-area flat display screens for laptop computers and mobile phones [4], which are now indispensable in our daily life. Besides a-Si:H, new amorphous semiconductor thin films from an In–Ga–Zn–O system have been developed and exhibited high carrier mobility that can be used for transparent electronics [5]. Although Si-based semiconductors can be used in random-access memory for data store, the memory is volatile. Once power is turned off, the stored information is lost. New device concepts have been proposed to develop nonvolatile memory. Among them, there are two leading candidates. One is the nonvolatile phase-change random-access memory based on chalcogenide semiconductors [6, 7]. These chalcogenide phase-change semiconductor thin films show significant resistance difference between their amorphous and crystalline states, which can be utilized to encode digital information. The other is the magnetic semiconductor (MS)-based spintronic device with nonvolatility, low power dissipation, and rapid response [8–10]. These MS thin films are capable of manipulating both the charge and spin of electrons, thereby realizing two functionalities of data processing and data storage at the same time. Inorganic and Organic Thin Films: Fundamentals, Fabrication, and Applications, First Edition. Edited by Yujun Song. © 2021 WILEY-VCH GmbH. Published 2021 by WILEY-VCH GmbH.

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With ever-growing necessity for miniaturization of electronic devices for low power consumption and high speed, the search for new materials with a combination of desirable functionalities has spurred the development of advanced semiconductor thin films. These newly developed semiconductor thin films are revolutionizing the current IT, which may partly replace traditional Si-based semiconductors by holding promises for applications in next-generation devices of IT. In this chapter, we aim to focus on the currently used semiconductor thin films for IT by covering the preparation techniques, structure and property characterization, and the electronic devices based on them. In addition, we will introduce the progress made on the newly developed semiconductors including the chalcogenide phase-change materials and MS thin films.

8.2 Fabrication of Semiconductor Thin Films Semiconductor thin films including Si and III–V semiconductors have been widely used in optics and electronics. To prepare semiconductor thin films with reproducible, controlled, and well-defined properties, a variety of deposition processes have been developed as listed in Figure 8.1. Among them, the most common and important ones comprise physical vapor deposition (PVD) including vacuum evaporation and sputtering and chemical vapor deposition (CVD) including metal–organic chemical vapor deposition (MOCVD) and plasma-enhanced chemical vapor deposition (PECVD) [11]. This section focuses on several representative

Semicenductor thin film deposition

Physical process

Chemical process

Vacuum evaporation

Sputter deposition

Resistive heating

Direct current

MOCVD

Electron beam

Radio frequency

PECVD

Laser ablation

Magnetron

MBE

Ion beam

Figure 8.1

Thermal growth

Sol–gel

CVD

Classification of different processes for semiconductor thin film deposition.

8.2 Fabrication of Semiconductor Thin Films

UHV chamber

In situ analysis systems including REED, AES, etc. Sensors

Substrate preparation

Substrate stage Shutter

Single-crystal GaAs substrate

Sn

n M

Al Ga

Figure 8.2

As

Schematic of the MBE system.

deposition techniques in use today. Topics discussed cover molecular beam epitaxy (MBE), magnetron sputtering, and MOCVD.

8.2.1

Molecular Beam Epitaxy (MBE)

MBE is a special and sophisticated epitaxial thin film growth technique, which requires an ultrahigh vacuum (UHV) environment usually greater than 6 × 10−9 Pa (5 × 10−11 Torr) [11]. Figure 8.2 shows schematic of the MBE system, consisting of UHV chambers, substrate preparation, thin film growth and analysis, and sample exchange load-lock chamber. MBE is basically a thermal evaporation process performed under UHV. The evaporated beams of atoms or molecules of the source materials react with the heated crystalline substrate to form the epitaxial layer. Since the early work on MBE-grown thin films of GaAs and related III–V compounds [12, 13], MBE is now a well-established epitaxial process of major importance in the development of optoelectronic and microelectronic devices [14]. The real-time and in situ monitoring of the MBE system allows the deposition of high-quality single-crystal semiconductor thin films with precise control of composition, growth condition, and doping. Owing to its technological advantages, MBE was used to develop new functional materials, leading to a breakthrough in creating III–V-based diluted magnetic semiconductors (DMSs) such as (In,Mn)As and (Ga,Mn)As [15, 16]. The details regarding the DMSs are discussed in a later section.

8.2.2

Magnetron Sputtering

Magnetron sputtering technology is a kind of PVD. Since its development, this technique has made significant progress for deposition of various semiconductor thin

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t

Si substrate t t

t t

t

t

t

E Plasma e-

Ar

+

t

t

Ar

+

e-

Target

N

Figure 8.3

S

N

Schematic of magnetron sputtering process.

films and the related microelectronic devices. A typical magnetron sputter deposition system consists of a vacuum chamber, a sputter target, a substrate holder, and a pumping system. Figure 8.3 shows schematic of the sputtering process. In the vacuum chamber, an inert gas such as Ar is filled in the chamber to create a neutral gas condition at low pressure. The target is mounted on a cathode, while the substrate is connected to an anode. A glow discharge is initiated by applying a high voltage on the order of 103 V between them. The positive Ar ions are accelerated by the electric field before they bombard on the surface of the target. Owing to the bombardment of these high velocity ions, the surface atoms of the target are ejected and condense on the substrate by forming a thin film. The magnetron effect means the use of a closed drift path of crossed magnetic and electric fields to trap electrons in. In the regions of the electron trapping, the efficiency of the ionization process is enhanced, which allows the plasma to be generated at lower pressures. As a result, the energy loss of the ejected atoms due to the gas collisions is reduced, thereby enabling the deposition of high-quality thin films at high deposition rates.

8.2.3

Metal–Organic Chemical Vapor Deposition (MOCVD)

Since the first demonstration of single-crystal films of GaAs grown by MOCVD, MOCVD has been widely used for preparing high-quality semiconductor epitaxial thin films applied in optoelectronics and microwave devices [14, 17]. In particular, MOCVD is the main technique for growing light-emitting diode (LED) epitaxial wafers, which become the cornerstone of the current semiconductor lighting technologies. Figure 8.4 shows a schematic of the deposition process. MOCVD provides an efficient way for preparing single-crystal semiconductor thin films with precise control of stoichiometric composition, doping level, and purity.

8.3 Nonmagnetic Semiconductor Thin Films and Typical Applications

Deposition chamber Gas sources

Ga(CH3)3 + AsH3

GaAs + 3CH4

Ga(CH3)3 + NH3

GaN + 3CH4

GaAs Heating stage

Figure 8.4

To pump

Schematic of chemical vapor deposition process.

8.3 Nonmagnetic Semiconductor Thin Films and Typical Applications Semiconductor thin films including Si in amorphous and crystalline states and compound semiconductors play essential roles in optical and electronic devices currently used for IT. In particular, further advances of light-emitting semiconductor devices, displays, and memories have evolved in important research fields with a focus on creating new semiconductor materials and developing innovation technologies. These traditional semiconductors are usually nonmagnetic. The unique optical and electrical properties of these traditional semiconductor thin films are utilized in applications of optic-electrical devices. This section discusses nonmagnetic semiconductor thin films such as GaAs, amorphous Si:H, IGZO, and chalcogenide semiconductors toward the uses in light-emitting devices, TFTs, and nonvolatile memories.

8.3.1

Semiconductor Thin Films for Light-emitting Devices

Semiconductors lay foundation of the current IT, mainly attributed to their capability of being doped into p-type or n-type semiconductors with tunable electrical and optical properties. The semiconductors of p-type and n-type are integrated to form a p–n junction, which is the basic functional component in light-emitting devices including LEDs and laser diodes (LDs). These devices are the key for information display and information storage. A LED consists of multiple layers of semiconductors. When current flows through the LED in the forward bias, the electrons and holes combine to release energy in the active layer, accompanied by the emission of photons. The color of the emitted light can be red, orange, yellow, green, or blue, depending on the bandgap of the semiconductors used in the LEDs (see Figure 8.5). Such a semiconductor electroluminescence effect was firstly discovered by an Englishman, Captain Henry Joseph Round, in 1907. Later in 1927, a Russian scientist, Oleg Vladimirovich Losev, also observed the light emission from the silicon carbide-based diode. This experiment

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Electron energy

262

e–

Conduction band e– e– e–

e– hv

Eg h+

h+

h+

h+

Valence band

h+

Color of light

Wavelenght (nm)

Energy hv(eV)

Blue

480

2.6

Green

530

2.3

Yellow

580

2.1

Orange

610

2.0

Red

680

1.8

Figure 8.5 The emitted light with colors depending on the bandgap energy of the used semiconductors.

can be recognized as the creation of the first LED. Till the 1970s, LEDs have become commercially successful products. Now LEDs are widely used in our daily life, which is largely attributed to the discovery of high-brightness blue diodes in the 1990s. The invention of blue diodes has enabled the development of white LEDs, which are rapidly replacing conventional lighting sources in the 2000s. Compared with the incandescent bulb-based lights, these white LEDs have improved visibility, lower electric power consumption, and longer lifespan. Meanwhile, the discovery of the blue diodes leads to the development of blue LDs, which enabled the development of the large-capacity optical storage Blu-ray disks. The breakthrough made in the development of blue diodes is based on the preparation of high-quality p-type GaN-based semiconductor thin films by using MOCVD technique [18, 19]. Although the electroluminescence effect was firstly observed in silicon carbide, silicon carbide has an indirect transition band structure. The energy efficiency of the silicon carbide-based LEDs is thus very low, posing significant hurdle for the development of blue diodes. Another potential candidate is zinc selenide, which has a suitable bandgap for the development of blue LEDs. Unfortunately, zinc selenide-based LEDs and LDs could not be commercialized due to their short lifespan. Research on GaN started in the 1970s. Despite this, there existed two main challenges that required to be overcome before a possible blue diode using GaN could be fabricated. One was the preparation of high-quality GaN thin films with good crystal quality and uniform thickness, and the other was the preparation of p-type GaN thin films. Three Japanese scientists Isamu Akasaki, Hiroshi Amano, and Shuji Nakamura have made the major contribution to the development and commercialization of blue LEDs and blue LDs by overcoming the challenges through technological innovations. Figure 8.6 shows a schematic of the first blue LED based on a p-type Mg-doped GaN thin film [18], which exhibited the typical I–V characteristics for p–n junctions. The p-type GaN thin film doped with Mg was produced by low-energy electron-beam irradiation treatment. The hole concentration and the resistivity of this p-type GaN:Mg were not sufficient for further developing blue LDs and high-brightness blue LEDs. Later, low resistivity p-type GaN films were obtained by N2 -ambient thermal annealing at temperatures above 700 ∘ C. The invention of the blue diodes enabled the completion of the light spectrum for semiconductor devices, resulting in the development of large-area, high-brightness,

8.3 Nonmagnetic Semiconductor Thin Films and Typical Applications

Al electrode GaN:Mg

P-type GaN:Mg prepared by low-energy electron-beam irradiation treatment Al electrode Undoped n-type GaN AIN buffer layer Sapphire substrate

Figure 8.6 et al. [18].

Schematic of the first GaN-based blue LED. Source: Modified from Amao

and full-color displays, the white LEDs and the LDs. These light-emitting devices enable many applications including information display and information storage in our daily life, which are important for people to live more conveniently and happily.

8.3.2

Thin Film Transistors for Displays

TFTs are fundamental components in displays of smartphones, large flat-panel TVs, computers, and flexible electronics. Despite the dominant role of single-crystal Si in the semiconductor technology, the commercial application of TFTs was driven by the use of hydrogenated amorphous silicon (a-Si:H) [20]. In addition to a-Si:H, amorphous oxide semiconductors including indium–gallium–zinc oxide (a-IGZO) have gained much attention due to their potential application in transparent TFTs for electronic paper displays. Certainly polycrystalline silicon and organic semiconductors can also be used for TFT materials. Amorphous semiconductors have several advantages over polycrystalline semiconductors. First, large-area amorphous semiconductor thin films can be easily prepared by conventional deposition techniques, which enables the development of large-area flat-panel displays with high performance and low cost. Second, the thin film properties are uniform down to sub-nanometer scale because of the homogeneous amorphous structure without crystalline defects such as crystal boundaries and dislocations. Third, the amorphous structure can accommodate local strains by atom rearrangements in terms of structural relaxation. This character makes amorphous materials be suitable for flexible electronics. Fourth, amorphous materials, unlike crystalline materials, have no solubility limitation for foreign elements to dissolve in. Therefore, a large amount of foreign elements such as hydrogen can be included in them without altering the amorphous nature. It is known that amorphous Si shows high resistivity and high dangling bond density. These dangling bonds behave as the electron trapping centers to degrade the electrical transport properties, thereby hindering the amorphous Si from the use in TTFs. As shown in Figure 8.7, the hydrogen incorporation in amorphous Si resulted in the reduction of the dangling bond density by several orders of magnitude [4]. The improvement in the electrical properties of the a-Si:H semiconductor led to the development of useful TFTs, which dominate the flat-panel display

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Figure 8.7 Schematic of the fourfold-coordinated amorphous silicon random network with dangling bonds indicated by dashed line and a dangling bond passivated by a hydrogen atom. Source: Modified from Street [4].

Si

H

industry, particularly for active matrix LCDs and for active matrix electrophoretic displays. The most common architecture of the a-Si:H-based TFTs is an inverted staggered structure primarily consisting of four fabrication steps [20]. The first step is to prepare a bottom metal gate by sputtering deposition and photolithographic definition. On the top of the metal gate electrode, a gate dielectric layer a-SiNx :H, an intrinsic a-Si:H layer, and n-type a-Si:H thin films are sequentially deposited by a plasma-enhanced CVD technique. Then the metal source and drain contacts are deposited, followed by the etching of the exposed region of the n-type a-Si:H. The a-Si:H-based TFTs are characteristics of uniformity, reproducibility, and reliability, which fulfill the requirements for the commercial application of the displays. Despite the dominant role of a-Si:H in the TFTs used for the displays, amorphous oxide semiconductor thin films such as a-IGZO are becoming competitive TFT materials for driving high-resolution active matrix displays. The post-transition-metal-oxide semiconductors are characterized by ionic bonding, whereas Si is characterized by covalent bonding (Figure 8.8). Direct overlap between the neighboring metal s orbitals is rather large and is not significantly affected even in an amorphous structure (Figure 8.8b). Compared with a-Si:H, the a-IGZO thin films showed the Hall effect mobility exceeding 10 cm2 /(V−1 s−1 ), about one or two orders of magnitude higher than a-Si:H thin films [21]. In particular, the a-IGZO thin films are wide bandgap semiconductors with high optical transparency. This is advantageous for the development of TFTs that are transparent and flexible for future transparent electronics (Figure 8.9). The a-IGZO thin films are n-type conduction. To form the basic building blocks of p–n junctions in electronics, a p-type amorphous oxide semiconductor ZnO⋅Rh2 O3 has been discovered [22]. The successful fabrication of amorphous oxide p–n heterojunction diodes would enable the transparent oxide electronics to grow rapidly (Figure 8.10).

8.3.3

Phase-change Semiconductor Thin Films

Phase-change materials can be reversibly switched between the crystalline and amorphous states. The most thoroughly investigated phase-change materials are

8.3 Nonmagnetic Semiconductor Thin Films and Typical Applications

Covalent semiconductors, for example,Si Crystalline Si

Post-transition-metal oxide semiconductors Crystalline Oxygen 2p-orbital

sp3-orbital

Metal ns-orbital

Amorphous Si

Amorphous

(a)

(b)

Figure 8.8 Schematic orbital drawings for the carrier transport paths (that is, conduction band bottoms) in crystalline and amorphous semiconductors. (a) Covalent semiconductors have carrier transport paths composed of strongly directive sp 3 orbitals, so structural randomness greatly degrades the magnitude of bond overlap, that is, carrier mobility. Note that the orbitals shown are illustrative and do not show exact wave functions. (b) Amorphous oxide semiconductors composed of post-transition-metal cations. Spheres denote metal s orbitals. The contribution of oxygen 2p orbitals is small. Source: Reprinted from Nomura et al. [21]. © 2004 Springer Nature.

ITO

ITO

Y2O3

a-IG ZO

ITO

PET

Film thicknesses a-IGZO active layer : 30 nm Y2O3 gate : 140 nm ITO electrode : 40 nm

(a)

(b)

(c)

Figure 8.9 Flexible TTFTs. (a) Structure of TTFT fabricated on a plastic sheet. (b) A photograph of the flexible TTFT sheet bent at R = 30 mm. The TTFT sheet is fully transparent in the visible light region. (c) A photograph of the flexible TTFT sheet. The transparent TFT devices are made visible by adjusting the angle of the illumination. Source: Reprinted from Nomura et al. [21]. © 2004 Springer Nature [21].

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Figure 8.10 (a) Schematic of the p–n heterojunction structure. Source: Modified from Narushima et al. [22].

Au P-type a-ZnO • Rh2O4 N-type a-InGaZnO4 ITO Glass or plastic substrates

Temperature Melting temperature Crystallization temperature

High/short current or laser pulse Crystal to amorphous Lower/longer current or laser pulse Amorphous to crystal

Time

Figure 8.11 Schematic of the phase transformation processes in chalcogenide-based phase-change materials.

chalcogenide-based semiconductor thin films because of their extremely rapid crystallization process. These chalcogenides are fast transformers, switching between the amorphous and crystalline states in responses to an external stimulus of a laser pulse or a current pulse. Figure 8.11 shows schematic of the phase transformation processes. The crystal thin film transfers to an amorphous phase when heated to the temperatures above its melting temperature by a high-power current or laser pulse within a very short time period. The amorphous thin films recrystallize when heated to the temperature above its crystallization temperature by a lower-power current or laser pulse within a longer time period. The different atomic and electronic structures of these two states result in their significantly different optical and electrical properties. Therefore, the chalcogenide-based phase-change materials offer an opportunity to use the pronounced property changes in their amorphous and crystalline states for optical data storage and phase-change memory technologies, respectively [23–25]. This has revolutionized the exiting data storage industries by developing low-cost, high-speed, portable, and nonvolatile devices to store high-density data. Utilizing the optical or electrical property difference in the amorphous and crystalline states leads to the development of two main data storage technologies. One is the mature rewritable optical storage technology that has been commercialized in products of rewritable compact disks (CDs), digital versatile disks (DVDs), and

8.3 Nonmagnetic Semiconductor Thin Films and Typical Applications

Figure 8.12 The fast switching alloys on the pseudobinary line between GeTe and Sb2 Te3 . Sources: Yamada et al. [29], Raoux et al. [30].

1 5/6 2/3

Sb 0 1/6 Sb2Te 1/3

1/2 1/3

Ge1Sb4Te7 Ge2Sb2Te5

1/2 Sb2Te3 2/3

1/6 5/6 0 1 Ge 1 5/6 2/3 1/2 1/3 1/6 0 Te GeTe

Blu-ray disks. The other is the phase-change random-access memory (PCRAM) technology that is thought to be a leading candidate for next-generation digital memory hierarchy [26, 27]. The first optical switching of a Te81 Ge15 Sb2 S2 thin film was demonstrated on the microsecond timescale in 1971 [28]. The crystallization of these early phase-change materials in the range of microsecond to millisecond became the main barrier for them to realize the commercialization. It was found that the alloys on the pseudobinary line between GeTe and Sb2 Te3 showed fast phase transformations (Figure 8.12), reducing the crystallization time down to tens of nanoseconds [29, 30]. Based on these fast transformers, the phase-change optical storage has evolved into the most prevalent technology. Rewritable optical storage media use the significant difference in the optical reflectivity between the amorphous and crystalline states to store information. In addition to the optical properties, the electrical properties change dramatically after the crystallization of the chalcogenide semiconductor phase-change materials. The rapid progress in the phase-change optical storage technology triggered the development of PCRAM technology, which uses the pronounced resistance difference between the amorphous and crystalline states. The commercialized PCRAM products based on Ge2 Sb2 Te5 thin films presented the writing speed of the tens of nanoseconds determined by the stochastic crystal nucleation and growth velocity. To achieve sub-nanosecond high-speed cache-type PCRAM memory, new phase-change materials with faster SET (writing) speed than Ge2 Sb2 Te5 thin films are needed. Rao and his collaborators proposed a new design principle for finding phase-change materials with enhanced thermodynamic driving force to stabilize crystalline precursors [27]. Meanwhile, the geometric conformability between the crystalline precursor and the crystalline phase should be high enough to decrease the energy barrier for crystal nucleation. It was reported that alloying transition metal Ti into Sb2 Te3 caused superior crystallization speed as compared with GeSbTe [31]. Taken into account the above considerations, two essential criteria were used to select the transition metals for further accelerating the crystallization process [27]. First, the crystal-like motifs in the amorphous state should be in a highly geometrically match to the rock-salt structure of Sb2 Te3 .

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Second, the introduction of the transition metal should bring in chemical bonds of high strength, which stabilize the crystal-like motifs in the amorphous state. Based on this strategy, a new phase-change material of Sc0.2 Sb2 Te3 was developed, which allowed a writing speed of only 700 ps [27]. Compared with Ge2 Sb2 Te5 thin films, the newly developed Sc0.2 Sb2 Te3 thin films enabled the SET speed of the PCRAM devices based on them to be one order of magnitude faster at all voltages. In addition, the ScSbTe-based device showed a cyclability of ∼105 under the sub-nanosecond switching condition [27]. Chalcogenide-based semiconductor thin films are the most important phase-change materials, which are explored for the rewritable optical storage technology and the emerging nonvolatile PCRAM technology. To further improve the performance and realize the miniaturization of commercial products, the material design and optimization are of great importance for the existing and new applications. The currently used phase-change materials are characterized by high-speed phase transition, high thermal stability of the amorphous phase, pronounced optical and electrical difference between the amorphous and crystalline states, large cycle number of reversible phase transition, and high chemical stability. In addition to these crucial properties, additional functionality of ferromagnetism is useful for achieving ultrahigh-density polymorphic data storage. In addition to the data storage, phase-change semiconductor thin films can be used for an optoelectronic framework that has many applications, such as ultrafast, solid-state displays with nanometer-scale pixels, semitransparent glasses, and artificial retina devices [32].

8.3.4

Semiconductor Thin Films for Sensors

A sensor is a detector that perceives a physical or chemical stimulus and converts it into a readable or processable signal. The detected variables consist of force, heat, light, gas pressure, sound, and so on (Figure 8.13). Accordingly, sensors can be categorized as mechanical, thermal, optical, chemical, and acoustic sensors. Semiconductor thin film sensors are devices that use semiconductor thin films as sensor operation. With the development of IT, the demand for intelligent, miniaturized, and programmable sensors is rapidly increasing. To meet this trend, developing semiconductor thin film sensors integrated with semiconductor-based chips is of significant importance. The integrated semiconductor thin film sensors are usually small in size, highly sensitive, and reliable and at low cost, resulting in high performance-to-cost ratio.

Force Heat Light Input Pressure Sound etc.

Figure 8.13

Semiconductor sensor device Signal detedtion

Signal conversion

Output

Signal conditioning

The main functions of a semiconductor device system.

Electrical signal

8.4 Magnetic Semiconductor Thin Films

Table 8.1

Different types of sensors fabricated with the most widely used semiconductors.

Sensor type

Representative semiconductor used

Representative fabrication technique

Acoustic sensor

ZnO, AlN, PZT

Sputtering [34–36]

Mechanical sensor

Polycrystalline Si

Sputtering, surface micromachining [37]

Magnetic Sensor

Si, III–V compound semiconductors

Sputtering, MBE [38]

Radiation sensor

HgCdTe, CdTe, GaAs, InP, InSb

Liquid-phase epitaxy (LPE), MBE, MOCVD [39]

Thermal sensor

Polysilicon

Sputtering, surface micromachining [40]

Chemical sensor

SnO2 , ZnO, Fe2 O3 , TiO2

PVD, CVD, particulate film assembly by aerosol synthesis and deposition of nanoparticles [41]

Surface micromachining is the main processing technique for mass production of thin film sensors, which permits the fabrication of structurally complex sensors by stacking and patterning layers [33]. Table 8.1 lists different type of sensors with the currently used semiconductors including Si, GaAs, ZnO, SnO2 , and CdS. The selected semiconductors are characterized by several essential properties to meet the requirements for industrial sensor applications. First, the deposited thin films are strongly adhesive to the substrate. Second, the fabrication techniques including the deposition and etching of the thin films are compatible with the very-large-scale integration (VLSI) process. Third, the semiconductor thin films exhibit high sensitivity toward the specific sensor applications. Fourth, the cost is as low as possible. Fifth, the semiconductor thin film sensors have good durability/long service lifespan.

8.4 Magnetic Semiconductor Thin Films IT is the use of computers, networking, and other physical devices to create, process, store, secure, and exchange all forms of electronic data. These operations are mainly realized by two fundamental components of microprocessors and hard disk in computers. Si-based semiconductors are the key materials in microprocessors to enable the control of charges for transmitting and processing data, while ferromagnets provide the spin of electrons to be utilized for storing data in hard disk. Generally, semiconductivity and ferromagnetism do not coexist in a material (Figure 8.14a). However, it was found that europium chalcogenides showed anomalous optical, magnetic, and transport phenomena [42, 43]. The physics behind this anomaly result from the strong d–f exchange interaction between the magnetic-exciton electrons of 5d band and the conduction 4f electrons [43]. As a result, there existed a relationship between the Curie temperature and the conduction electron concentration of

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8 Semiconductor Thin Films for Information Technology Nonmagnetic elements

(a)

(b)

Magnetic elements

(c)

Nonmetallic elements

(d)

Figure 8.14 Four types of semiconductors: (a) a nonmagnetic semiconductor without magnetic ions, (b) an europium chalcogenide magnetic semiconductor with magnetic elements as the main constituents, (c) a diluted magnetic semiconductor with few magnetic elements dissolving in the host nonmagnetic semiconductor, and (d) an amorphous magnetic semiconductor with nonmagnetic elements dissolving in the host ferromagnetic alloy.

these semiconducting europium chalcogenides. Based on these phenomena, Methfessel first proposed the concept of ferromagnetic semiconductor, whose magnetic properties could be modified by carrier injection, electrostatic fields, or other means that changed the free carrier concentration in semiconductors [44]; that is, both the charge and spin of electrons can be manipulated simultaneously in these ferromagnetic semiconductors. The europium chalcogenides are therefore regarded as the first-generation MSs (Figure 8.14b). Despite their scientific importance, these magnetic rare earth compounds usually have complex crystalline structure, compared with the widely used semiconductor materials such as Si and GaAs. It is difficult to obtain high-quality interface structure when integrating them with Si or GaAs. In addition, most of them show Curie temperatures far below room temperature. Therefore, the utilization of these europium chalcogenides for practical devices is a challenging task. Nevertheless, MSs hold promise for uses in next-generation spintronic devices, in which they function as both ferromagnets and semiconductors to realize information processing, communications, and storage. The search for new materials with both ferromagnetism and semiconductivity has spurred the development of the MS field. Basically, there are two approaches that can be used for preparing MS thin films. One is to make nonmagnetic semiconductors ferromagnetic (Figure 8.14c), and the other is to make ferromagnetic metals/alloys semiconducting (Figure 8.14d).

8.4.1

Diluted Magnetic Semiconductors

To maintain the most attractive semiconducting properties used in electronic devices, an approach was proposed to introduce magnetic elements into nonmagnetic semiconductors for creating new type of MSs named as diluted magnetic semiconductors [8, 45, 46]. With ever-growing necessity of miniaturization of future electronic devices, DMSs have stimulated great interest due to their potential for realizing new functionalities and revolutionizing device concepts. III–V compound semiconductors including GaAs and InAs are one type of the most important semiconductors used in electronic devices. Thanks to the development

8.4 Magnetic Semiconductor Thin Films VG > 0

VG < 0

VG = 0

Metal gate insulator (In,Mn)As InAs (AI,Ga)Sb AlSb GaAs substrate 50

50 25 0

25

–25

22.5 K

RHall (Ω)

(a)

RHall(Ω)

–50 –8

–4

0

4

B (mT)

8

0 0V +125 V –125 V 0V

–25

–50 –1.0 (b)

–0.5

0

0.5

1.0

B(mT)

Figure 8.15 (a) Field-effect control of the hole-induced ferromagnetism in magnetic semiconductor (In,Mn)As field-effect transistors. Shown are the cross sections of a metal–insulator–semiconductor structure under gate biases V G . This controls the hole concentration in the magnetic semiconductor channel (filled circles). Negative V G increases hole concentration, resulting in enhancement of the ferromagnetic interaction among magnetic Mn ions, whereas positive V G has an opposite effect. The arrow schematically shows the magnitude of the Mn magnetization. The InAs/(Al,Ga)Sb/AlSb structure under the (In,Mn)As layer serves as a buffer relaxing the lattice mismatch between the structure and the GaAs substrate to produce a smooth surface on which the magnetic layer is grown. (b) RHall versus field curves under three different gate biases. Application of V G = 0, +125 and −125 V results in qualitatively different field dependence of RHall measured at 22.5 K. When holes are partially depleted from the channel (V G = +125 V), a paramagnetic response is observed (blue dash-dotted line), whereas a clear hysteresis at low fields ( 0

VG < 0 EDL

M(emu/cm3)

40 EDL

20 0 +2 V, 30 min As–prepared

–20

–2 V, 30 min

–40

–2 V, 60 min

–60 –30 5 nm

(a)

(b)

–20

–10

10 0 H(kOe)

20

30

(c)

Figure 8.22 (a) HRTEM image of the newly developed CFTBO AMS. (b) Schematic diagrams for the electric-field control of carrier concentrations at different V G . (c) Ms increasing with both positive and negative V G . Source: (a) Reprinted from Chen et al. [61], (b, c) Reprinted from Chen et al. [61]. © 2019 Chinese Institute of Electronics.

sample could be an intrinsic MS. Based on this intrinsic MS, we may use V G to control its electrical conduction type of the different parts within the material, which would form a p–n junction at the interface between the different parts. To our best of our knowledge, this should be the first demonstration of an intrinsic MS. Meanwhile, the conduction type of this intrinsic MS can be tuned to be n-type or p-type by using an external electric field.

8.4.5

Prospective for Magnetic Semiconductors

The reverse thinking enabled the development of a new family of MSs by oxidizing originally ferromagnetic AAs. These amorphous MSs showed much higher Curie temperatures than room temperature. Based on them, prototype magnetic semiconductor-based spintronic devices such as spin-light emitting diodes and spin-field effect transistors could be fabricated. One of the most concern issues is the carrier mobility of these amorphous MSs. Generally, the structural disorder would enhance the scattering strength of the charger carriers and hinder them from moving fast in amorphous materials. However, it is possible to discover unprecedented interface effects that could be used to overcome the existing difficulties in the applications of this new family of MSs. For instance, the electron gas may emerge at the interfaces between two MSs. To test the universality of the design concept, various ferromagnetic amorphous alloy systems can be used as hosts for the development of new amorphous MSs. Among them, both p-type and n-type MSs may be prepared and integrated together to form the basic component p–n heterojunctions. These magnetic p–n heterojunctions would provide a platform for exploring the new physics at the interfaces of two MSs and furthering our understanding of the interplay between the electricity and ferromagnetism. In addition, the crystallization induced semiconductor-metal transition is also interesting and worth to be investigated systematically. In these ferromagnetic phase change materials, the coexistence of phase change induced drastic difference in both the resistance and ferromagnetism may open a new avenue for designing high density multi-state memory.

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8.5 Conclusion and Outlook This chapter outlines the currently used techniques for fabrication of semiconductor thin films, an overview of the representative semiconductor thin films developed toward the specific applications, and the materials being developed for next-generation electronic/spintronic devices. With the matured manufacturing process of integrated circuits, the traditional semiconductor thin films including Si, Ge, GaAs, and wide bandgap ones have been playing important roles as the key materials in micro/nanoelectronics. Their unique properties and combined functionalities render them to work in a variety of digital applicants including computers and cell phones, which are now essential items of our modern information society. In addition to these conventional semiconductor thin films, amorphous materials such as phase-change chalcogenides, transparent oxides, and MSs have received great attention due to their unique properties. They are being developed or will partly replace the currently used semiconductors for realizing new devices with highly desirable functionalities. The continuous development of IT poses new challenges for further extending Moore’s law that requests miniaturized, high-integration, low power consumption, and rapid response ICs. To meet this demand, the two-dimensional (2D) crystals including graphene, black phosphorus, MoS2 , MoSe2 , and WSe2 exhibit high performance when integrated in nanoelectronic devices. Moreover, new physical phenomena emerge at their surfaces or the interfaces between these 2D thin films. Utilizing the intriguing surface or interface effects may lead to the breakthrough discoveries and innovations that help these materials to work as “post-silicon” materials for further developing IT.

List of Abbreviations ICs IT MSs PCMs TFTs

integrated circuits information technology magnetic semiconductors phase-change materials thin film transistors

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9 Glass Transition in Organic Semiconductor Thin Films Han-Nan Yang 1 and Zheng-Hong Lu 1,2 1 Yunnan University, Center for Optoelectronics Engineering Research, Department of Physics, No. 2, Cuihu North Road, Kunming, Yunan Province 650031, China 2 University of Toronto, Department of Materials Science and Engineering, 27 King’s College Circle, Toronto, ON M5S 1A1, Canada

9.1 Introduction Organic semiconductor thin-films are key building blocks for fabricating future generation of organic optoelectronic devices such as organic light-emitting diodes (OLED). Already OLED technology has been industrialized in making vivid flat-panel displays for a variety of electronic gadgets such as smart phones and televisions. The structure of OLED comprises a stacked of multiple organic molecular thin-films deposited either by physical vapor deposition (PVD) or by solution-casting. Each of these layers plays a different role and collectively they dictate the performance of organic optoelectronic devices [1–5]. In stark contrast to inorganic semiconductors, each organic molecular is topologically flawless and therefore amorphous packed organic films do not need lattice matching with substrates. This unique amorphous property enables fabrication of OLED on any type of substrate including flexible plastics [6, 7]. Amorphous materials, also frequently referred to as glasses, are a special class of materials characterized by their liquid-like disorder structure. The most common route to form a glass is by quenching a molten liquid directly into a non-crystalline solid without crystallization. In this case, before forming a glass, the super-cooled state is formed, that is, a system in metastable equilibrium [8–10]. In general, a glass transition temperature (T g ) in a non-crystalline solid is defined by a distinct inflection point in the specific volume change as the temperature is varied. Because of the complexity in defining molecular packing in an amorphous network, the glass transition physics remains largely elusive. It is known that glass transition is not a typical phase transition, as it does not involve abrupt changes in any observable physical properties. Up to now, numerous theories have been proposed to describe the glass transition processes and the characteristic glass transition temperature T g , which is frequently defined by changes in viscosity, or in free volume or in other observable material properties [11]. Because the glass transition Inorganic and Organic Thin Films: Fundamentals, Fabrication, and Applications, First Edition. Edited by Yujun Song. © 2021 WILEY-VCH GmbH. Published 2021 by WILEY-VCH GmbH.

9 Glass Transition in Organic Semiconductor Thin Films

is somewhat a convoluted process involving dynamics and thermodynamics, some second-order physical properties of amorphous materials at around T g will change discontinuously. These physical properties include viscosity, coefficient of thermal expansion (CTE), heat capacity, optical refractive index, etc. Thus, the T g of an amorphous material is generally determined by a temperature at which some of these physical properties show a significant deviation from below to above this temperature. One of the key challenges in OLED technology is thermal stability. For example, for applications requiring very high brightness in harsh working environments such as outdoor displays and automobile displays, where the working temperature could be higher than 400 K in summer, thermal stability of organic materials become a major issue [12]. It is generally known that T g is a critical factor dictating a device’s thermal stability, device degradations, molecular orientation in thin-films, and thin-film interlayer diffusions [13–16]. In particular, the material having the lowest T g in an OLED device determines the thermal-failure temperature of the device [17, 18]. Figure 9.1a shows abrupt operation failures, defined by a sudden drop in luminance, of OLEDs as operating temperatures are increased to the catastrophic failure temperature (T cf ). The hole transport material in these devices have the lowest T g of all organic thin-films used. Figure 9.1b shows a strong linear correlation between T g and T cf . Engineering organic thin-films for higher glass transition temperature is thus important for developing future generation of robust organic devices. Now let’s discuss briefly methodology in measuring glass transition temperature. For bulk materials, T g is typically measured by differential scanning calorimetry (DSC) method. For thin-film materials, ellipsometer has been used to measure T g by monitoring changes in film thicknesses, i.e. volume change. As mentioned above, there are two types of organic thin-film semiconductors: polymeric thin films that are normally deposited by spin coating and small molecular thin films that Tg (K) 390

Tcf

1.0

380

0.8

400

410

TPD: C60 (0 wt%) TPD: C60 (10 wt%) TPD: C60 (30 wt%) TPD: C60 (50 wt%)

410

400 Tcf (K)

370

0.6 0.4

360 390

0.2 0.0

350

NPB: C60 (0 wt%) NPB: C60 (10 wt%) NPB: C60 (30 wt%)

Pure TPD TPD:C60 in 50 wt%

300

320

340

360

380

400

340 340

Temperature

(a)

390

380

Tcf (K)

Normalized luminence (a.u.)

286

(b)

350

360

370

380

380 390

Tg (K)

Figure 9.1 (a) Electroluminance versus temperature of OLEDs having pure TPD and TPD:C60 as hole transport materials, respectively. (b) T cf versus T g plots of OLEDs materials made with various organic:C60 composite hole transport materials and glass transition temperatures. Source: Yang et al. [18]. © 2018 AIP Publishing.

9.2 Determination of Glass Transition Temperature in Organic Thin Films

are deposited by PVD. First, the T g of polymeric thin-films has been successfully determined by measuring an inflection point at their thickness versus temperature plot, i.e. a disruptive change of CTE. Using this method, the relationship between variables such as molecular weight and glass transition temperatures of polymeric thin-films have been studied [19–21]. The work on ellipsometry study of glass transition in PVD small molecular thin-film organic semiconductors and their composite thin films has recently become available. Organic composite semiconductor thin-films is a material system in which two organic constituents are mixed to achieve a specific functionality, and composite thin-film is a common practice in making organic devices [22, 23]. For example, two component host-dopant systems with doping concentration ∼10–20 wt% are widely used in fabricating OLEDs and solar cells. In this chapter, the T g of pure organic thin films and organic composite thin films will be discussed. The composite organic thin films will include: (i) organic–organic composite thin films [24]; (ii) fullerene nano-organic composite thin films [18].

9.2 Determination of Glass Transition Temperature in Organic Thin Films Several archetypical organic molecules used in OLEDs and solar cells are selected for variable temperature spectroscopic ellipsometry studies. These organics are N,N ′ bis(1-naphthalenyl)-N,N ′ -bis-phenyl-(1,1′ -biphenyl)-4,4′ -diamine (NPB), N,N ′ -bis (3-methylphenyl)-N,N ′ -bis(phenyl)benzidine (TPD), 1,3,5-tris(1-phenyl-1Hbenzimidazol-2-yl)benzene (TPBi), 4,4′ ,4′′ -tris(N-3-methylphenyl-N-phenylamino)triphenylamine (m-MTDATA), 1,3,5-tri(m-pyrid-3-yl-phenyl)benzene (TmPyPB), 4,4′ , 4′′ -tris(carbazol-9-yl)-triphenylamine (TCTA), and C60 . The chemical structures of these molecules are shown in Figure 9.2. Thin films of these molecules are formed by condensation of these molecular vapors on silicon substrates under ultra-high vacuum, i.e. PVD. Each type of molecular vapor is generated by thermal sublimation of the molecule from solid powder placed in a vacuum furnace or referred to as Knudsen cell. For a composite thin film, two types of vapors are simultaneously generated and condensed on silicon substrates. The weight percentage in the films is controlled by controlling the Knudsen cell temperature. To ensure accuracy and reproducibility, vapor flux is monitored in real-time by quartz crystal microbalance inside the PVD chamber. The thickness of organic thin-film is kept at 100 nm for all samples. The thin film deposition rate was carried out at 0.1 nm/s under ∼10−7 Torr vacuum. The spectroscopic ellipsometry measurements were performed at a 75∘ incident angle using a polarized light at wavelengths varying from 400 to 800 nm. The ellipsometry data acquisition time for each temperature point is 15 minutes. The substrate temperature was calibrated by a thermocouple placed directly on the silicon. The temperature ramping rate is 2 K/min. The temperature is scanned from room temperature to a temperature slightly above the T g . The schematic diagram of the variable temperature ellipsometry measurement set-up is shown in Figure 9.3. The thin-film

287

288

9 Glass Transition in Organic Semiconductor Thin Films N

N

N

N

N N N

TPD

NPB

N

N

N

N

N

N

TmPyPB

N

N

N N

N N N

TPBi

Figure 9.2

N

m-MTDATA

TCTA

C60

The chemical structures of various organic molecules.

Light source

Detector Polarization generator

Polarization analyzer 75°

Thin-flim

Thermal tape Substrate Thermal stage

Figure 9.3

Schematic set-up for variable temperature ellipsometry measurement.

thickness at a given temperature is obtained by modeling analysis of the ellipsometry parameters (Ψ and Δ). Figure 9.4a shows a percentage change in thickness of pure NPB thin film as a function of temperature. Here, a disruptive change in thickness is observed near its T g obtained by DSC measurement of NPB in powder form. This disruptive change in thickness indicates that the glass transition of the NPB film behaves in a manner similar to that of a crystal melting. This abrupt change in NPB film thickness is significantly different from that of traditional polymeric thin-films. As a comparison, the green dashed line in Figure 9.4a shows thickness variation of a polymeric polymethyl methacrylate (PMMA) thin-film at around its glass transition temperature using data reported in Ref. [21]. For all small molecular thin-films formed by PVD method, an abrupt change in thickness occurs within a narrow temperature range (∼3 K, in the case of NPB thin film). To determine the glass transition temperature, three linear regression fittings of three distinct thickness-temperature plot regions can be used. As shown in

9.2 Determination of Glass Transition Temperature in Organic Thin Films

0.08

0.06

(d–d0)/d0

(d–d0)/d0

0.06 0.04

0.02

0.04 0.02 0.00

0.00 310

330

350

370

390

410

300

430

Temperature (K)

330

360

390

420

450

480

Temperature (K)

(b)

(a) 0.06

Before

After

(d–d0)/d0

0.05 0.04 0.03 0.02 0.01 0.00

500 μm 230 300 320 340 360 380 400 420 440

(c)

Temperature (K)

(d)

Figure 9.4 (a) NPB thickness versus temperature plot. The solid red solid lines are the linear regression of the experimental data in three distinct temperature regions. The dashed line is for polymeric PMMA thin film replotted using data from Ref. [21]; (b) NPB:TPD composite thickness versus temperature plot. The solid red lines are the linear regression of the experimental data in three distinct temperature regions. The dashed line is for polymer blend PS:TMPC film replotted using data taken from Ref. 19; (c) NPB:C60 composite thickness versus temperature plot. The solid red lines are the linear regression of the experimental data in three distinct temperature regions; (d) NPB film surface images taken before and after the ellipsometry measurement shown in (a). Source: (a, b) Yang et al. [24]. © 2018 Elsevier, (c, d) Modified from Yang et al. [24]. © 2018 Elsevier.

Figure 9.4a, three linear lines provide a fairly good fit to the experimental data. The highest temperature used for fitting the data is a temperature above which there is a significant decrease in thickness (Figure 9.4a). This sudden decrease in thickness is caused by the delamination in organic thin film, as shown in Figure 9.4d. The intersections of these three lines are used to determine the onset and end of glass transition process in PVD thin films. The mid temperature between onset and end transition temperature is then used to define the glass transition temperature of PVD thin-films. The T g of the NPB thin films is determined to be 379.6 K, which is slightly higher than 373.2 K of NPB in solid powder measured by DSC. The T g of PVD-deposited NPB thin film is expected to be slightly higher than that of NPB in the powder form because molecular packing in a thin film is not completely random. This type of material is referred to as ultra-stable glass that typically requires a normal glass to age over thousands of years [8, 25]. The origin of this ultra-stable glass forming, i.e. higher than bulk transition temperature, is related to the formation process

289

290

9 Glass Transition in Organic Semiconductor Thin Films

Table 9.1 T g of various organic molecules in thin-film forms and in powder forms [27–31].

Organic molecule

T g (K) (in thin-film form)

T g (K) (in powder form)

TPD

342.4 ± 1.1

338.2 [27]

m-MTDATA

356.5 ± 1.2

348.2 [28]

TmPyPB

354.7 ± 1.2

352.2 [29]

NPB

379.5 ± 1.2

373.2 [27]

TPBi

397.9 ± 1.2

395.2 [30]

TCTA

428.4 ± 1.3

424.2 [31]

Source: Yang et al. [24]. © 2018 Elsevier.

of these organic thin-films. During PVD deposition, lateral diffusion of molecules on a film surface combined with molecular conformational packing may lead to the formation of somewhat ordered packing microstructure within the film. This results in enhanced kinetic stability and lower enthalpy [26]. This ordered molecular packing may also explain that thin-film organic semiconductors exhibit a disruptive change in thickness or volume at T g , similar to the melting process of a crystal solid. Table 9.1 compares the T g of various pure PVD organic thin films measured by ellipsometer (T g PVD ) to the T g of these organic molecules in powder forms reported literatures (T g powder ) [27–30, 32]. It is found that the T g of these organic thin-films are all slightly higher than their corresponding materials in powder forms. The relationship between T g powder and T g PVD is plotted in Figure 9.5. A linear regression fit to the experimental data yields the following equation: powder

TgPVD = Tg

+ 4.7

(9.1)

This equation indicates that the glass transition temperature of a one-component PVD thin-film can be simply calculated by the glass transition temperature of its starting source material in powder form. Now let’s examine the glass transition in organic–organic composite PVD thin films, Figure 9.4b shows a variation of NPB:TPD composite thickness as a function of temperature. The solid red lines are the linear regression of the experimental data in three distinct temperature regions. The dashed line is for polymer blend PS:TMPC film re-plotted using data taken from Ref. [19]. Again, the glass transition process in small molecular organic–organic composite thin film shows a disruptive change in thickness, in sharp contrast to the polymeric composite films. For organic nano-composite thin-film system. The fullerene C60 is used as a nano filler for organic matrix. Figure 9.4c plots the NPB:C60 composite thickness versus temperature plot [18]. Interestingly, the nano-composite thin film also shows a disruptive change in thickness at around glass transition temperature. The solid red

9.3 Model for Predicting Glass Transition Temperature of Organic–Organic Composites

440

420

TPD TmPyPb m-MTDATA NPB TPBi TCTA

Tg PVD

400

380

360

340

340

360

380

400

420

440

Tg powder

Figure 9.5 T g powder versus T g PVD plot based on data from Table 9.1. The red dashed line is a linear regression fit to the experimental data.

lines are the linear regression of the experimental data in three distinct temperature regions. In summary, all three types of PVD organic thin-films show a unique disruptive change in film thicknesses as a function of temperature. This disruptive thickness change can then be used to define the glass transition temperatures of the organic semiconductor thin films.

9.3 Model for Predicting Glass Transition Temperature of Organic–Organic Composites Two component host-dopant systems with doping concentration ∼10–20 wt% are widely used in fabricating OLEDs and solar cells. To develop a model for predicting the T g as a function of constituent concentration, we selected three different groups of Host:Dopant combinations: NPB:TPD (x wt%), NPB:TPBi (x wt%), and NPB:TCTA (x wt%). The doping concentration x wt% in each group is varied with x = 0, 5, 25, 50, 75, 90, 95, and 100. Using the PVD T g definition, we have successfully determined the T g of each sample. As shown in Figure 9.6, these data exhibit a similar linear relationship from 0 to 100 wt% doping concentration. This indicates

291

9 Glass Transition in Organic Semiconductor Thin Films NPB:TPBi

NPB:TPD

Tg (K)

292

380

400

370

395

360

390

NPB:TCTA 430 420 410 400

385

350

390

380

340 0

20

40

60

80

100

380 0

Concentration (wt%)

(a)

20

40

60

80

100

0

Concentration (wt%)

(b)

20

40

60

80

100

Concentration (wt%)

(c)

Figure 9.6 The measured T g (shown in open symbols) of composite mixture films are plotted against dopant concentration. The theoretical data computed using the Fox equation are shown as dashed lines. Source: Yang et al. [24]. © 2018 Elsevier.

that there is no or little interaction between two constituent organic molecules in the composite thin-films. To quantify the result, we try to fit the experimental data using the Gordon–Taylor equation developed for polymers [31]. Tgm =

w1 ∗ Tg1 + k ∗ w2 ∗ Tg2 w1 + k ∗ w2

(9.2)

𝜌 ∗𝛥𝛼

where k = 𝜌1 ∗𝛥𝛼2 , T gm is the T g of the mixture material. w1 , w2 , T g1 , T g2 , 𝜌1 , 𝜌2 are 2 1 the weight fraction of doping concentration, glass-transition temperature and density of component 1 and 2, respectively. As these organic components have similar densities, Eq. (9.2) will be simplified as: w w 1 = 1 + 2 (9.3) Tgm Tg1 Tg2 This equation is also known as the Fox equation. It is widely used to predict the T g of polymeric mixtures. The small molecular organic molecules used in organic semiconductors are relatively rigid and have similar molecular weight [33, 34]. We now apply Eq. (9.3) to experimental T g of composite organic films. The dashed lines shown in Figure 9.6 are the computed lines using Eq. (9.3). As shown in Figure 9.6, each theoretical line shows a good agreement with the experimental data. In conclusion, the Fox equation is proven valid in predicting T g of organic–organic composite semiconductor thin films.

9.4 Model for Predicting Glass Transition Temperature of Nano-organic Composites Another type of composite thin-film system is the interacting composite thin-films. As a model system, we chose the hole transporting molecule doped with nano-carbon C60 to establish a rule for predicting T g . In this system, there is an interaction between the hole transporting molecules and C60 [35]. As we have discussed in the previous section, for organic–organic composite systems, the FOX equation is valid to predict the T gm of the non-interaction composite thin

9.4 Model for Predicting Glass Transition Temperature of Nano-organic Composites

films. However, the T g of C60 composite (T gc ) thin films with various C60 doping concentration clearly deviate from the Fox equation. This indicates that the C60 composite thin film is different from the classical two-component organic thin-film system. The Fox equation can effectively predict the T g of the organic–organic small molecular thin-films is because there is little molecule–molecule interaction in that system. In contrast, there is an interaction between the hole transporting molecules and C60 . Thus, we have to develop a new equation to quantify the T g of these C60 composite thin-films. Figure 9.7 shows that T g increases linearly with increasing C60 doping concentration. Thus the linear regression analysis is used and the results are shown as the (a)

40

TPD:C60

ΔTg (K)

30 20 10 0 (b) 50

NPB:C60

ΔTg (K)

40 30 20 10 0 (c) 60

ΔTg (K)

50

m-MTDATA:C60

40 30 20 10 0 0

10

20

30

40

50

Concentration (wt%)

Figure 9.7 (a) TPD:C60 , (b) NPB:C60 , and (c) m-MTDATA:C60 thin films as a function of weight concentration. The dashed lines are the linear regression fits to the experimental data. Error bars represent measurement error. Source: Yang et al. [18]. © 2018 AIP Publishing.

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9 Glass Transition in Organic Semiconductor Thin Films

Table 9.2 The 𝜅, 𝛼, T g , and M of organic–C60 composite thin-films.

C60 composite thin films

M (host molecule) (g/mol)

T g (host molecule) (K)

𝜿

𝜶 (10−3 )

TPD:C60

516.7

342.4

0.77

1.49

NPB:C60

588.7

379.5

0.92

1.56

m-MTDATA:C60

789.0

356.5

1.19

1.51

Source: Yang et al. [18]. © 2018 AIP Publishing.

dashed lines in the figure. From this linear-fit data analysis, the T g of these C60 composite thin films is found to follow a simple linear sum equation: Tgc = 𝜅 ⋅ wD + Tg0

(9.4)

where T gc is the glass transition temperature of the C60 doped organic thin-films, 𝜅 is a fitting parameter, wD is the weight fraction of C60 doping concentration, and T g0 is the glass transition temperature of the pure organic thin film. The 𝜅 values of these C60 composite thin films are listed in Table 9.2. This linear relationship suggests that there is an interaction between organic molecules and C60 molecules. As C60 molecule is a very strong electron acceptor, there is a strong π–π interaction between the benzene ring in the organic molecules and C60 . As a result of this interaction, a permanent dipole between organic–C60 molecules may tangle the motion of the constituent organic molecules and thus increases T gc . Equation (9.4) shows that T g depends only on κ for different C60 composite thin films. Therefore, κ is an indicator of the strength of the interaction between C60 and organic molecules. Based on this argument, we speculate that 𝜅 may be somewhat related to molecular weight. Based on experimental data, we discover that κ and M follows a simple relationship 𝜅 =𝛼⋅M

(9.5)

where M is the molecular weight, 𝛼 = 1.5 × 10−3 is a constant determined experimentally. The calculated 𝛼, 𝜅, and M of these three C60 composite thin films are given in Table 9.2. Then, a universal formula for calculating T g of C60 –organic composite thin films can now be given by a simple mathematical equation with only one variable, wD Tgc = 𝛼 ⋅ M ⋅ wD + Tg0

(9.6)

Therefore, equipped with this equation, the T gc of all organic–C60 composite thin-films can now be quantitatively computed by the C60 concentration. To illustrate the validity of Eq. (9.6), the ΔT gc (T gc − T g0 ) now can be calculated for various composite thin films. Figure 9.8 plots the computed theoretical ΔT gc versus the measured experimental ΔT g . The excellent fit indicates Eq. (9.6) can be used effectively to predict all T g of C60 composite thin films.

9.5 Summary

70

60

Experimental ΔTg (K)

50

40

30

20 TPD:C60

10

NPB:C60 m-MTDATA:C60

0 0

10

20

30

40

50

60

70

Theoretical ΔTgc (K)

Figure 9.8 The plot of ΔT g versus ΔT gc . ΔT gc is the theoretical value calculated by Eq. (9.6), and ΔT g is the experimental value. Source: Yang et al. [18]. © 2018 AIP Publishing.

9.5 Summary Thermal stability of amorphous organic thin-film plays an important role in determining the performance of organic optoelectronic devices. In a device, the organic film having the lowest glass transition temperature T g dictates the maximum operating temperature of an OLED product. Synthesis of high T g organic molecules without disrupting other optoelectronic properties and manufacturability is quite difficult. Thus, organic–organic and nano-organic composites provide practical thin-film engineering methods to boost glass transition temperatures of existing compounds. In this chapter, glass transition temperatures of pure and composite organic PVD thin-films are discussed. Experimentally, variable temperature spectroscopic ellipsometer is shown to be a reliable method for measuring T g of organic thin-films. All types of PVD thin-film systems exhibit disruptive changes in thicknesses at their glass transition temperatures. Thus, T g can be defined as the mid-point temperature of this disruptive change regime. For one component PVD organics, the thin-film glass transition temperature can be calculated by adding 4.7 K to the glass transition temperature of its raw organic powder. For non-interaction organic–organic composite thin-films, the inverse sum Fox equation is shown to provide an excellent prediction of the composite T gm . For interacting nano-organic composite thin-films,

295

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9 Glass Transition in Organic Semiconductor Thin Films

a simple universal linear sum equation is shown to provide an accurate prediction of the glass transition temperature at any given composition.

Acknowledgments Financial support for this work is provided by the National Natural Science Foundation of China (Grant Nos. 11774304 and 11804294) and by the Natural Science and Engineering Research Council of Canada.

List of Abbreviations CTE DSC OLED PVD

coefficient of thermal expansion differential scanning calorimetry organic light emitting diode physical vapor deposition

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10 Boucher, V.M., Cangialosi, D., Alegría, A., and Colmenero, J. (2017). Reaching the ideal glass transition by aging polymer films. Physical Chemistry Chemical Physics 19: 961–965. 11 Debenedetti, P.G. and Stillinger, F.H. (2001). Supercooled liquids and the glass transition. Nature 410: 259–267. 12 Scholz, S., Kondakov, D., Lüssem, B., and Leo, K. (2015). Degradation mechanisms and reactions in organic light-emitting devices. Chemical Reviews 115: 8449–8503. 13 Shibata, M., Sakai, Y., and Yokoyama, D. (2015). Advantages and disadvantages of vacuum-deposited and spin-coated amorphous organic semiconductor films for organic light-emitting diodes. Journal of Materials Chemistry C 3: 11178–11191. 14 Ohisa, S., Pu, Y.J., Yamada, N.L. et al. (2015). Molecular interdiffusion between stacked layers by solution and thermal annealing processes in organic light emitting devices. ACS Applied Materials and Interfaces 7: 20779. 15 Smith, A.R., Lee, K.H., Nelson, A. et al. (2012). Diffusion – the hidden menace in organic optoelectronic devices. Advanced Materials 24: 822–826. 16 Mcewan, J.A., Clulow, A.J., Nelson, A. et al. (2017). Dependence of organic interlayer diffusion on glass-transition temperature in OLEDs. ACS Applied Materials and Interfaces 9 (16): 14153–14161. 17 Tokito, S., Tanaka, H., Noda, K. et al. (1997). Temperature dependences of electroluminescent characteristics in the devices fabricated with novel triphenylamine derivatives. IEEE Transactions on Electron Devices 44: 1239–1244. 18 Yang, H.N., He, S.J., Zhang, T. et al. (2018). Nano-composites for enhanced catastrophic failure temperature of organic light-emitting diodes. Applied Physics Letters 113: 163301. 19 Pham, J.Q. and Green, P.F. (2002). The glass transition of thin film polymer/polymer blends: interfacial interactions and confinement. The Journal of Chemical Physics 116: 5801–5806. 20 Keddie, J.L., Jones, R.A., and Cory, R.A. (1994). Size-dependent depression of the glass transition temperature in polymer films. EPL (Europhysics Letters) 27: 59. 21 Richard, A. (1994). Interface and surface effects on the glass-transition temperature in thin polymer films. Faraday Discussions 98: 219–230. 22 Su, S.J., Sasabe, H., Takeda, T., and Kido, J. (2008). Pyridine-containing bipolar host materials for highly efficient blue phosphorescent OLEDs. Chemistry of Materials 20: 1691–1693. 23 Holmes, R., Forrest, S., Tung, Y.-J. et al. (2003). Blue organic electrophosphorescence using exothermic host–guest energy transfer. Applied Physics Letters 82: 2422–2424. 24 Yang, H.N., He, S.J., Zhang, T. et al. (2018). Glass transition temperatures in pure and composite organic thin-films. Organic Electronics 60: 45–50. 25 Singh, S., Ediger, M.D., and de Pablo, J.J. (2013). Ultrastable glasses from in silico vapour deposition. Nature Materials 12: 139.

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26 Dalal, S.S., Walters, D.M., Lyubimov, I. et al. (2015). Tunable molecular orientation and elevated thermal stability of vapor-deposited organic semiconductors. Proceedings of the National Academy of Sciences 112: 4227–4232. 27 Inada, H., Yonemoto, Y., Wakimoto, T. et al. (1996). Organic electroluminescent devices using novel starburst molecules, 1,3,5-tris[4-(3-methylphenyl-phenylamino)phenyl] benzene and 4,4′ ,4′′ -tris(3-methyl-phenylphenylamino) triphenylamine, as hole-transport materials. Molecular Crystals and Liquid Crystals 280: 331–336. 28 Su, S.J., Chiba, T., Takeda, T., and Kido, J. (2008). Pyridine-containing triphenylbenzene derivatives with high electron mobility for highly efficient phosphorescent OLEDs. Advanced Materials 20: 2125–2130. 29 Mayr, C. and Brütting, W. (2015). Control of molecular dye orientation in organic luminescent films by the glass transition temperature of the host material. Chemistry of Materials 27: 2759–2762. 30 Kuwabara, Y., Ogawa, H., Inada, H. et al. (1994). Thermally stable multilared organic electroluminescent devices using novel starburst molecules, 4,4′ ,4′′ -tri(N-carbazolyl) triphenylamine (TCTA) and 4,4′ ,4′′ -tris(3-methylphenylphenylamino) triphenylamine (m-MTDATA), as hole-transport materials. Advanced Materials 6: 677–679. 31 Hancock, B.C. and Zografi, G. (1994). The relationship between the glass transition temperature and the water content of amorphous pharmaceutical solids. Pharmaceutical Research 11: 471. 32 Usluer, O., Demic, S., Egbe, D.A. et al. (2010). Fluorene–carbazole dendrimers: synthesis, thermal, photophysical and electroluminescent device properties. Advanced Functional Materials 20: 4152–4161. 33 Fox, T.G. and Flory, P.J. (1950). Second-order transition temperatures and related properties of polystyrene. I. Influence of molecular weight. Journal of Applied Physics 21: 581–591. 34 Fox, T.G. and Loshaek, S. (1955). Influence of molecular weight and degree of crosslinking on the specific volume and glass temperature of polymers. Journal of Polymer Science 15: 371–390. 35 Yuan, Y., Grozea, D., and Lu, Z. (2005). Fullerene-doped hole transport molecular films for organic light-emitting diodes. Applied Physics Letters 86: 143509.

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10 Thermoelectric Films for Electricity Generation Metin Yurddaskal 1,2 , Melis Yurddaskal 3 , Ozan Yilmaz 1 , and Serdar Gultekin 1 1 Dokuz Eylul University, Engineering Faculty, Department of Metallurgical and Materials Engineering, Adatepe District Dogus Street No: 207/I, Buca, Izmir 35390, Turkey 2 Dokuz Eylul University, Center for Fabrication and Application of Electronic Materials, Adatepe District Dogus Street No: 207/I, Buca, Izmir 35390, Turkey 3 Manisa Celal Bayar University, Engineering Faculty, Department of Mechanical Engineering, Yunusemre District, Muradiye, Manisa 45140, Turkey

10.1 Introduction Most of the human activities require a power source. Fossil energy sources are limited and will disappear. Electricity is one of the essential energies of modern life. We need electricity to manage our daily routine such as transportation, communication, heating, food storing, etc. The need of electricity has increased since its discovery. Our modern life activities inevitably depend on electricity; therefore, solutions to generate electricity lies in renewable sources. Until today there have been several kinds of renewable energy sources that we used like wind, water, sunlight, etc. There is also one more relatively new renewable energy source – global warming. It is possible to use global warming in a positive way. It gives us a good opportunity to generate electricity, thanks to thermoelectricity. Heat that we radiated to do other task may be used to generate electricity. The solution lies in devices capable of recovering energy from the environment surrounding the device or the user. Even human body radiates heat due to metabolic activities. In addition, many other devices that could be used as heat sources to generate electricity include air conditioners, exhaust systems in vehicles, and industrial processes. Thermoelectric generators are able to use the waste heat radiated from these devices, which are not used, to produce heat to generate electricity. Recently, one of the most promising solutions has been considered as direct conversion of heat into electricity via advanced thermoelectric materials (TE). Over the last couple of years, much achievement has been maintained in their dimensionless figure of merit (ZT), which therefore has a word in calculating conversion efficiency of TE devices. It can be said that ZT is associated with different interlocked factors such as the Seebeck coefficient, electrical conductivity, and thermal conductivity, in which it is very odd that these factors are interrelated in bulk TE materials, Inorganic and Organic Thin Films: Fundamentals, Fabrication, and Applications, First Edition. Edited by Yujun Song. © 2021 WILEY-VCH GmbH. Published 2021 by WILEY-VCH GmbH.

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which means if you modify one, it has effects on the others. In this chapter, some TE fundamentals will be introduced, and a thorough examination will be made in inorganic and organic TE materials in order to see if there are any improvements and to see is there is a future for them. It seems inevitable to be sustainable technology called thermoelectric effect since there is a high demand for energy all over the world and much concern regarding climate and diminishing fossil fuels, showing effort for using, converting, and recovering sources. The main principle of TE conversions can be seen described elsewhere. Among the advantages of this technology are as follows: (i) when compared with others, TE is more silent and reliable since no movement is involved in the process; (ii) they are simple, compact, and safe; (iii) it is a green technology since no heat, no gas, or no chemical disposals are produced during the process; and (iv) it is convenient when it comes to working in remote outer areas.

10.2 Thermoelectricity The origin of thermoelectric effect dates back to 1822. Thomas Seebeck observed that when a temperature gradient was applied to the junction of two different materials (metal or semiconductor), the mobile charge carriers at the hot end tend to diffuse to the cold end, resulting in electric current [1]. This mechanism is just like the temperature sensors in Figure 10.1. Seebeck effect “α” can be described by voltage differences caused by difference of temperature: ΔV , (𝜇V∕K) (10.1) ΔK Unaware of Thomas Seebeck’s invention, Jean Peltier discovered in 1834 that heat could be generated if the electric current was applied to the junction of two materials. He also stated that it was possible to absorb heat by changing current direction [2]. Even if the inventions of Thomas Seebeck and Jean Peltier were related to each other, it would be injustice to say that these two inventions are not independent. In 1838, Heinrich Lenz produced a setup made out of bismuth (Bi) and antimony (Sb) shown in Figure 10.2. In order to observe temperature changing at the junction of Bi and Sb, he used a drop of water. He was able to freeze the water droplet by applying electric current. When he reversed the direction of the current, the ice melted. In this respect Heinrich Lenz created the first thermoelectric device. In 1848, William Thomson (Lord Kelvin) came up with new formula that defined the Peltier coefficient (𝜋) using Seebeck coefficient (𝛼): 𝛼=

𝜋 = 𝛼T, (V)

(10.2)

where T represents the temperature at the junction of the materials [3]. Thomson took his work a step further and revealed that there might be a third effect. This effect states that an electrical conductor cools or heats with electric current if it is exposed to temperature gradient. Thermoelectric studies were continued by other scientists John William Strutt and Edmund Altenkrich [4–6]. Especially, Altenkrich revealed considerable studies. One of the most important properties of

10.3 Overview of Inorganic and Organic Thermoelectrics for Thin Films

Electric energy generated

Generated voltage

i

i

V

Material A

Load

Material A

Material B

Heart source (a)

Material B

Heart source (b)

Figure 10.1 An experimental design of Seebeck experiment: (a) temperature sensor and (b) thermoelectric generator.

thermoelectric materials is its figure of merit (Z), which defines the efficiency of thermoelectric materials. According to Altenkrich, good thermoelectric materials should have high Seebeck coefficient and low thermal conductivity (λ). Low thermal conductivity retains heat at the junction of two materials. In addition, another characteristic of good thermoelectric material is low electrical resistivity (ρ). Electrical resistivity needs to be as low as possible to obtain low Joule heating. The figure of merit of a thermoelectric material can be assigned by the following formulas: σ Z = α2 , [K−1 ] (10.3) λ where “σ” is electrical conductivity that is the inverse of resistivity “ρ ” : l (10.4) ρ = R , [Ω − cm−1 ] A Because the temperature can affect the figure of merit, “T” should be considered and the formulation modified as below: T T or ZT = α2 σ (10.5) ZT = α2 𝜌𝜆 λ

10.3 Overview of Inorganic and Organic Thermoelectrics for Thin Films 10.3.1 The Seebeck Effect As we mentioned before, low thermal conductivity and high electrical conductivity are the most important parameters for high Seebeck coefficient. However, these

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Drop of water ice

Drop of meltwater

H2O H2O Bismuth (Bi)

Antimony (Sb)

Bismuth (Bi)

Heart absorbed

Heart released

Electric current (a)

Figure 10.2

Antimony (Sb)

Electric current (b)

Experimental setup of the Peltier effect: (a) cooling and (b) heating.

three parameters are difficult to be all best, because they are function of carrier concentration, which cannot be varied independently. As the carrier concentration increases, the Seebeck coefficient decreases, and thermal and electrical conductivities increase [7]. In this respect carrier concentration should be at optimal value to maximize the power factor α2 σ (Figure 10.3). The sophisticated relationships between characteristic parameters of the materials complicate the approach of adjusting carrier concentration; alone it is difficult to improve the figure of merit. In the last few decades, to increase power factor and reduce thermal conductivity, great developments have been carried out. It is possible to divide thermoelectric materials into three sections depending on their optimal working temperature: low temperature, middle temperature, and high temperature. These temperatures are typically below 400 K, between 600 K and 900 K, and above 900 K, respectively. Bi2 Te3 -based thermoelectric materials belong to low temperature. PbTe-based materials belong to middle temperature. SiGe-based materials belong to high-temperature thermoelectric materials. Since the emergence of the Seebeck effect in 1821, thermoelectric materials have been divided into three generations according to ZT values [8]: ZT values 0 for p-type semiconductors and 𝛼 n < 0 for n-type semiconductors. The Seebeck coefficient, also known as entropy per charge carrier, is generally correlated with the density of energy levels. While undoped semiconductors have the Seebeck coefficient values on the order of mV/K, the Seebeck coefficient values of doped semiconductors decrease typically, and it even reaches the order of a few μV/K for highly doped materials [37]. The unit of the Seebeck coefficient is usually expressed as V/K (μV/K or μV/∘ C). It has been found that the Seebeck effect shows only in a material called a thermocouple consisting of a combination of two different materials. Although both ends of the same material actually have intrinsically Seebeck coefficient, they do not possess a Seebeck effect as a result of symmetry. The Seebeck effect, however, is a bulk property and does not depend on the specific arrangement of the tip or material, nor the specific method of joining them [38]. Conductive materials have different Seebeck coefficients or thermoelectric sensitivities. This Seebeck coefficient can be positive or negative. For example, the Seebeck coefficient of iron is 19 μV/∘ C at 0 ∘ C, which means that a positive Seebeck voltage (or thermoelectric emf) of 19 μV is induced in iron for every 1 ∘ C difference in temperatures near 0 ∘ C. As an example with a different Seebeck coefficient, constantan (a copper–nickel alloy) has a thermoelectric emf of −35 μV/∘ C at 0 ∘ C. The semiconductors are more promising for the construction of thermocouples since the Seebeck coefficient is generally greater than 100 μV/K, while that of metals are 10 μV/K or less. Looking the relationship between thermoelectric voltage and temperature, the relationship is linear only for small changes in temperature, and it becomes nonlinear if the temperature difference is increased [7]. It is therefore necessary to specify the temperature at which the thermoelectric coefficient is being used [39]. A load must be connected to the thermoelectric material (or series of materials) in order to convert the voltage produced by the Seebeck effect into electricity, which will result in an electrical current. In order to maximize the electrical energy generated, it is necessary to minimize both the electrical losses due to Joule heating and the thermal losses caused by the heat flow between the hot and cold junctions. Therefore, the thermoelectric material should exhibit high electrical conductivity (𝜎) and low thermal conductivity (k). The high electrical conductivity and the low thermal conductivity required by the high Seebeck coefficient, when combined with a dimensionless number called thermoelectric figure of merit (ZT), allow comparison of the yields of different thermoelectric materials as given in Eq. (10.6).

10.3 Overview of Inorganic and Organic Thermoelectrics for Thin Films

10.3.2 The Peltier Effect Thirteen years after the discovery of the Seebeck effect, Jean Charles Athanase Peltier, a French watchmaker, invented the second of thermoelectric effects in 1834. Peltier has found that an electric current creates a minor heating or cooling effect, depending on the direction of its passage through the thermocouple. Since the Joule heating effect is also present, it is difficult to see the Peltier effect in metallic thermocouples. When the current is passed in one direction, it is sometimes difficult to determine whether heating or cooling is greater. If an arrangement as in Figure 10.6 is used, the Peltier effect can be demonstrated, theoretically, by connecting the galvanometer or with a direct current source and placing a small thermometer on the thermocouple junction [40]. In this effect, cooling effect is seen in one junction, and the temperature increases in the other junction, when the electric current passes through a circuit consisting of two different conductive materials. This makes the Peltier effect inverse of the Seebeck effect. Let us take the example that copper conductor wires are connected to both ends of the battery as in Figure 10.7 and the circuit is completed with bismuth between these copper wires. It was observed that when the circuit was closed, a temperature gradient occurred as predicted by the Peltier effect phenomenon. At the point where the current passes from copper to bismuth, the temperature increases, while at the point where current passes from bismuth to copper, the temperature decreases. Also, symbolic representation of the Peltier effect is shown in Figure 10.8. Figure 10.6 An experimental design to demonstrate the Peltier effects.

Heat source or thermometer Conductor A

Conductor B

Galvanometer or electric current source

Bismuth

Cold Junction

Hot junction

Copper

+– Current

Figure 10.7

Voltage

Schematic representation of the Peltier effect.

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Cold side Isolator (ceramics)

p

n

Conductor (copper)

p

p

n n p p Semiconductor of p-type Semiconductor Hot side of n-type –

Figure 10.8

n +

Symbolic representation of the Peltier effect. I

Figure 10.9 A schematic illustration explaining the Peltier Effect.

Material A

Qp = αIT

Reject Material B Absrorb

I

In other words, the Peltier effect can explain the temperature difference that is generated by the voltage applied between two electrodes connected to a semiconductor material. In this effect, if an electrical current is passed through the junction of two different materials, heat is generated or absorbed, as can be seen in Figure 10.9, depending on the direction of the current. In this figure, 𝛼 shows the Seebeck coefficient. In fact, this effect is due to the difference between the Fermi energy of the two materials. When a current (I) is passed through a circuit as in Figure 10.10, heat is generated at the upper junction (at T 2 ) and is absorbed at the lower junction (at T 1 ). The amount of Peltier heat (Q) absorbed per unit time at the lower junction can be given by Q = ΠAB I = (ΠB − ΠA ) I

(10.8)

where ΠA and ΠB are the Peltier coefficients of the each material and ΠAB is the coefficient of the entire thermocouple. As the names imply, the Peltier coefficient of

10.3 Overview of Inorganic and Organic Thermoelectrics for Thin Films

Figure 10.10 Demonstration of the application of the Peltier effect for cooling experiment.

Active cooling p

n

Heat rejection

–

+

I

the p-type semiconductors is typically positive (though not above ∼550 K), whereas that of the n-type semiconductors is negative. The major advantage of the Peltier effect is that it can create effectively heating or cooling without moving parts; thus, the probability of malfunction is much lower than conventional heating or cooling systems. They are also virtually maintenance-free. The devices operating using the Peltier effect are very quiet and can operate at temperatures as low as −80 ∘ C (−176 ∘ F). The Peltier effect can work successfully even at microscopic levels where conventional cooling systems would not work. In applications, Peltier devices comprising n-type and p-type materials are connected electrically to each other in series and thermally in parallel. Figure 10.10 demonstrates a Peltier device operating in cooling mode.

10.3.3 The Thomson Effect At first, the dependence of the Seebeck and Peltier phenomena on one another was not immediately realized. Later, the Thomson effect, called the third thermoelectric effect, discovered by William Thomson (later known as Lord Kelvin) in 1854 is a combination of Seebeck and Peltier effects [21]. Thomson found that the relationship between the two effects should have an additional effect. The theoretical definition of this effect is that the homogeneous conductor carrying an electric current between two points with different temperatures either emits or absorbs the heat depending on the direction of the current and material, which is additively the Peltier heating. For example, if a copper wire carrying a constant electric current is exposed to external heating in a short portion while the remaining portion remains colder, the heat is absorbed from the copper, as the conventional current approaches the hot spot, and the heat is carried away from the hot spot. The heat absorbed or emitted is called the Thomson heat (Q) and is given by the equation Q = 𝜌J 2 − 𝜇 ∗ J

dT dx

(10.9)

where 𝜌 is the resistivity of the material, J is the current density, 𝜇* is the Thomson coefficient, and dT/dx is the temperature gradient through the conductor. The term 𝜌J 2 is called irreversible Joule heating always produced as a current in a wire, and

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QThomson.A Wire A

QPeltier.AB

TL

I

–

+

TH

QPeltier.AB

QThomson.B

Wire B

Figure 10.11

Schematic demonstration of the Peltier effect and the Thomson effect.

the second term is Thomson heating whose sign changes according to the direction of the current [41]. Thomson also examined the relationship between these three effects thermodynamically, demonstrating that the electrical Seebeck effect is due to a combination of thermal Peltier and Thomson effects. Despite the Thomson effect being small, it leads to a very useful and important relationship called the Kelvin relationship, with Seebeck and Peltier effects. When the current flows as shown in Figure 10.11, the heat is absorbed in wire A because of the negative temperature gradient and released in wire B due to the positive temperature gradient. The Thomson heat is proportional to the electric current and the temperature gradient shown schematically in Figure 10.11 and Eq. (10.9). The Thomson coefficient is the only thermoelectric coefficient that can be measured directly for individual materials. The heat absorption is a proof of an electromotive force moving in the same direction as the current, which means that electrical energy is supplied to the circuit. Therefore, the Thomson effect for iron would lead to a flow from heat to cold regions in iron. In addition to iron, many metals such as bismuth, cobalt, nickel, and platinum exhibit the same property called a negative Thomson effect. That is, in these metals, the high-temperature portion has a lower potential than a low-temperature portion, and therefore, heat energy is absorbed when the current flows from a high temperature point to a low temperature point. The opposite of this situation explains the positive Thomson effect. The potential at high temperature is considered to be higher than a section at low temperature. When the new current flows from a low-temperature point to a high-temperature point, the heat energy is absorbed. Metals showing positive Thomson effect are copper, silver, zinc, antimony, cadmium, etc. Lead is a metal with zero Thomson effect. In some metals, changes in temperature or crystal structure can reverse the effect sign [21]. The Thomson coefficient is positive for materials with positive Thomson effect. The term in Eq. (10.9) is simply the Joule heating that is always released when a current pass through an imperfect conductor. There is no relationship with the Thomson effect, but it is included in the equation for integrity. Studies have shown that the total Thomson emf along a conductor depends only on the temperatures of

10.4 Classification of Thin Film Thermoelectric (TE) Materials

the both ends and is not in any way dependent on a particular shape in which the temperature gradient changes.

10.4 Classification of Thin Film Thermoelectric (TE) Materials In this part of the book, after making a general classification for thermoelectric films, general characteristics of these films and some numerical values are mentioned. In this context, the experimental data obtained from the studies we have collected from the literature have been presented to the reader.

10.4.1 Inorganic Thermoelectric Thin Films 10.4.1.1 Bi–Te-Based Superlattices

The performance of Bi—Te materials has been tried to be improved by adding dopant material to crystal lattices in the last 20 years. Compared with pure Bi2 Te3 and Ag-doped BiTe3 , the results showed that the doped structure had lower thermal conductivity. It also has a low power factor. Thanks to the 2% silver dopant added to Bi2 Te3 , ZT value of 0.77 was obtained at 450 K temperature [30]. Similar studies have been carried out for materials produced in thin film form that showed great performance [30, 42]. In some similar studies that used low-temperature growth process in metal–organic chemical vapor deposition (MOCVD) technique to prepare Bi2 Te3 film with a thickness of 10 Å p-type Bi2 Te3 /Sb2 Te3 super lattices and n-type Bi2 Te3 /Bi2 Te2 .83Se0.17 super lattices, thin films showed ZT value of ∼2.4 and ∼ 1.4 at 300 K [30, 43]. In addition, there are also studies in the literature of BiTeSb thin film that have been formed by methods such as flash evaporation (FE) method [30]. However, it can be said that Bi—Te-based films are relatively more expensive than other conventional material. Recently, Zn- and Cu-based thermoelectric films, which can be produced at a lower cost than Bi—Te-based thermoelectric films, have started to attract attention [19, 44–46]. 10.4.1.2 Cobalt Oxide-Based Thin Films

Cobalt oxide-based thin films exhibit a high power factor in the in-plane direction [47, 48]. The NaCo2 O4 , an example of single-crystal thin film, showed the strength of 100 μV/K at 300 K [49]. In addition to these investigations, Ca3 Co4 O9 has been found to be used in terms of thermopower at room temperature. The single crystal of Ca3 Co4 O9 showed resistance of 10–40 mΩ cm, 125 μV/K thermopower, and 0.04–0.16 mW /(m K2 ) power factor, in order [50]. Bendable and flexible nanostructured Ca3 Co4 O9 thin films showed high power factor above 1 × 10−4 W/(m K2 ). They also did not lose significant performance when bended at high temperature. Porosity is one of the important properties for the thermoelectric futures. Nanoporous structures in the Ca3 Co4 O9 caused selective scattering phonons. In this way thermoelectric performance could be improved, yielding a power factor of 2.32 × 10−4 W/(m K2 ) near room temperature. The lowest electrical resistivity for CaCo4 O9 is around 7 mΩ cm [51].

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Furthermore, by using simple dry transfer, it is possible to transfer these materials from their initial substrates to arbitrary polymer platforms. This is one of the important signs that the potential for wearable applications is quite high. In addition, it has been reported in some studies that the layered calcium cobalt oxides and strontium cobalt oxides synthesized from sodium cobalt oxide starting material exhibit high power factor values. In these studies, the highest power factor values for layered Ca0.33 CoO2 and Sr0.29 CoO2 at 300 K were found to be 9 × 10 −4 W/(m K−2 ) and 3.5 × 10−4 W/(m K−2 ). This value for Ca0.33 CoO2 is 25% higher than that of Ca3 Co4 O9 [52–54]. In thermoelectric power generation applications, the power factor for a specified ZT value is considered to be a more important material property than thermal conductivity [55]. Therefore, in applications where low power is used, it is preferable to obtain high power factors before reducing thermal conductivity. 10.4.1.3 Zn-Based Thin Films

ZnO thin films can be used in a wide range of applications in energy-related area. One of them is thermoelectric film studies. Among thermoelectric materials, ZnO materials got great attention with its high Seebeck coefficient and high thermal stability. As in Bi–Te-based electrical materials, doping method is usually used in Zn-based thermoelectric materials to improve performance [56, 57]. ZnO thin films have a number of advantages to engineers compared with bulk materials as they can be produced at nanoscale. In the case of thermoelectric materials, major improvements have been achieved in Bi–Te-based materials with nano-sized defects to control phonon scattering [12, 30]. The main problems of Bi–Te-based materials are their decomposition at temperatures above 600 ∘ C and the limitation of possible practical applications [58]. Nano-defects in bulk oxide materials such as ZnO are randomly distributed, and it is difficult to control their density and size. ZnO thin films are good candidate to solve this kind of problems [58]. Studies have shown that ZnO thermoelectric properties can be improved by doping Al into its crystal structure. Besides aluminum, titanium, antimony, and nickel are used frequently as doping element [59–63]. 10.4.1.4 Cu-Based Thin Films

No matter whether it is bulk or thin film, Cu-based materials are of considerable interest for thermoelectric applications [64–68]. In example of study about Cu2 Se bulk materials, it has been reported reaching the highest ZT value of 2.4 at 1000 K [69]. This superior ZT value of Cu2 S material is associated with its low latency thermal conductivity [64, 65]. Already bulk Cu2 S materials exhibit better thermoelectric properties than Cu2 S materials produced in the form of thin films. A relatively new approach, the flexible Cu2 S, has recently begun to be tested. According to one of these studies, the Cu2 Se powder was dissolved in the organic solution, making an ink solution. This ink solution was then deposited on a flexible substrate using the wet coating method. The Cu2 S material produced in this way showed a power factor of 0.62 mW/(cm K) at 684 K [44]. Furthermore, thermoelectric materials that are capable of operating at low temperatures, such as room temperature, will provide

10.4 Classification of Thin Film Thermoelectric (TE) Materials

a significant improvement for different application such as wearable and portable devices. Another Cu-based thermoelectric materials is Cul. Recent research explored that γ-CuI has low thermal conductivity as well as high hole conductivity and Seebeck coefficient. The thickness of the films produced in this study was around 200–300 nm while the ZT values were obtained as 0.21 at 300 K [19]. The conductivity of the p-type Cul film obtained by the sputter method was 156 S/cm, while the Cul film with iodine doped was 283 S/cm [66].

10.4.2 Organic-based Thin Film TE Materials Organic polymers also have low thermal conductivity. We can also refer to these materials in the form of semimetallic polymers. Due to the low thermal conductivity of the polymers, they exhibit significant advantages over other conventional materials in the thermoelectric material fields [70–73]. The abundance of polymers and their light weight, flexibility, and low cost are also other advantageous aspects [74]. But one of the biggest disadvantages of the polymer materials is their low efficiency. Efficiency of such materials is tried to be increased by doping. Various polymers of p-type and n-type can be produced by this method. Several examples are polyacetylene, polyaniline, polypyrrole, polythiophene, and poly(3,4-ethylenedioxythiophene) [75–79]. 10.4.2.1 Polyacetylene and Polyaniline

In a study, iodine-doped polyacetylene film showed better conductivity than pure film. Its conductivity value was increased from 3 × 103 S/cm to 1 × 105 S/cm [80]. Metal chlorides FeCl3 , MoCl5 , NbCl5 , and ZrCl4 can also be added to the structure of this polymer to increase the efficiency of it. According to a study, conductivity measurements made at different temperatures, FeCl3 -doped polyacetylene film showed the best result with 3 × 105 S/cm conductivity at 220 K [81]. However, the polyacetylene material has a significant disadvantage, which is its instability under atmospheric conditions. Another remarkable polymer material is the composite of Bi0.5 Sb nanoplate-doped polyaniline. This material has a relatively high Seebeck coefficient, and therefore it possesses a power factor of 16.5 × 10−8 to 84.4 × 10−8 W/(m K2 ) at 400 K [82]. Polyaniline material, on the other hand, stands out as a material that can meet the stability requirement under atmospheric conditions and is produced on a large scale at low cost [83, 84]. The composite material consisting of polyaniline and camphorsulfonic acid (CSA) exhibits good electrical conductivity. This conductivity can go up to metallic levels with regard to the doping level. For example, a high conductivity value of 300 S/cm could be obtained in doping at 60% [85]. 10.4.2.2 Poly(3,4-ethylenedioxythiophene)

Poly(3,4-ethylenedioxythiophene) polymer is another remarkable material. This polymer can exhibit high electrical conductivity along the casting plane, especially when mixed with polystyrene sulfonate (PEDOT: PSS) [86]. Thermal and electrical

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in-plane conductivities of drop-cast films characterized by 20 μm are 1.0 W/(m K2 ) and 500 S/cm, respectively. Drop-cast dimethylsulfoxide (DMSO)-mixed PEDOT: PSS films showed the characterization of the correlation between electrical conductivity and anisotropic thermal conductivity. As increasing electrical conductivity, anisotropic thermal conductivity tends to rise. The power factor of the DMSO posttreatment films is 65% increased when compare with DMSO added PEDOT: PSS films. When using DMSO posttreatment instead of DMSO addition in the PEDOT: PSS films, the power factor of the film is 65% increased. The increase in electrical conductivity contributes to the development of the power factor. While these changes occur, changes in the Seebeck coefficient remain at lower levels [87]. 10.4.2.3 Polypyrrole and Polythiophene

Polypyrrole films, unlike the aforementioned films, can exhibit high conductivity values at very low temperatures of about 0 K. This conductivity is higher than 300 S/cm [88]. High tensile stress-resistant polythiophene and poly(3-methylthiophene) nanofilms showed a value of 0.03 ZT at 250 K, which is higher than that of many other conducting polymers [89]. According to Sun et al., high conducting polymers tend to exhibit conductivity from carriers that are close to the Fermi level. However, their experimental result did not prove of their claim. A thickness of 100 nm of poly(3-hexylthiophene) thin film on Si substrate did not show that much performance. Their studies illustrate theoretical route for reaching a high Seebeck coefficient [90]. 10.4.2.4 Other n-Type Polymers

Examples of n-type organic thermoelectric materials include poly(nickel-ethylene tetrathionate) (poly[Ni-ett]). This material can reach its maximum ZT value of 0.3 at room temperature. Different production techniques are available for this material, for example, potentiostatic deposition method. Different production parameters lead to obtaining different optical power factor values. Increases in optical power factor up to 131.6 μW/(m K2 ) were observed with synthesis potential down to 0.6 V. This value was lower than that of at 1 or 1.6 V [91, 92]. Another n-type organic thermoelectric material is poly(benzobisimidazobenzophenanthroline) (BBL). Electron mobility and electrical conductivity values of this polymer are 0.1 cm2 /(V s) and 2.4 S/cm, respectively, with the field effect [93, 94]. Lastly perylene diimide (PDI)-based materials need to be referred to n-type thermoelectric materials. Their power factor can reach to 1.4 μW/(m K2 ) [95].

10.4.3 Inorganic–Organic Composite Thermoelectric Thin Film Materials The polymeric materials exhibit advantages such as low density, low thermal conductivity, easy reproducibility, and low cost and a number of disadvantages. Polymeric materials unfortunately do not perform well in terms of longevity. Their low electrical conductivity and ZT values also prevent them from being

10.5 Applications of Thermoelectric Materials

used in thermoelectric power generation applications. One of the important methods of dealing with such problems is to produce composite materials as in many applications. Composite materials, which exhibit the superior properties of inorganic and organic materials together, are promising. In this way, it becomes possible to eliminate the obstacles that prevent these materials from being used in thermoelectric applications. Recently, there has been an increase in the number of studies added to the literature with increasing interest in this subject [96, 97]. 10.4.3.1 Metal–Organic Frameworks

Metal-organic frames are a new type of thermoelectric materials. After observing the electrical conductivity of such composite materials, they became the subject of research for thermoelectric applications. One of the remarkable and common studied metal–organic frameworks is tetracyanoquinodimethane (TCNQ) and Cu3 (BTC)2 (BTC = benzene tricarboxylate) [98, 99]. This material draws attention with its higher Seebeck coefficient than some of the aforementioned materials. According to the measurement under the controlled temperature gradient, the Seebeck coefficient of this material is 375 μV/K [100]. Although the electrical performance of this material is not satisfactory, it is predicted that it can be improved in the following years and studies on this subject are continuing increasingly. 10.4.3.2 Carbon Nanotube–Polymer Composites

Carbon nanotube materials are one of the most remarkable technological materials that have attracted attention recently. Thanks to their superior electrical properties, it is possible to see them in many areas related to energy storage and thermoelectrics [101, 102]. The power factor values of the single-walled carbon nanotubes, which may also have elasticity, were 1.5 × 103 and 1.84 × 103 μW/(m K) for n-type and p-type, respectively [18]. When single-walled carbon nanotube (SWCNT)–polyaniline composite films were tested for thermoelectric measurements, a high power factor value of 217 μW/(m K) was found [103]. Polyaniline/carbon nanotube (CNT)/graphene multilayer film is another promising thermoelectric material. The power factor value of this material is 1.825 × 103 μW/(m K2 ) that is comparable with the others [104].

10.5 Applications of Thermoelectric Materials Bulk thermoelectric materials have widespread application fields. Power generation and cooling process can be counted as one. Nevertheless, their convenience in microscale applications is a bit demanding. If so, it is necessary to resort to thin film approach by which the films can be applied on the substrate. Equal conditions are difficult to maintain during the creation of films. Therefore, the properties of the materials can be distinct from their bulk counterpart. Consequently, thin film application rather than the bulk material usage is said to cause considerable trend upward in the figure of merit [30].

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10.5.1 Thermoelectric Cooling One of the most important thermoelectric effects is the Peltier effect. It has many application fields such as refrigeration, cooling of electrical components, portable coolers, cameras, climate controlled jackets, spacecraft, satellites, etc. There are two main materials in thermoelectric devices; p-type and n-type, which are interconnected. The connections are made in electrical terms in series and in thermal terms in parallel. The main working principle of the process starts with applying a voltage to the lowest point of junction, and then free carriers start moving from the peak point of the junction downward. Therefore, the heat goes down to the bottom of the junction. Whether the device is effective or not is in relation with the figure of merit, as was described by Ioffe [105]. As can be seen from Eq. (10.10) [35], thermoelectric cooler performance of coefficiency can be calculated by 𝜑=

(sp − sn )ITC − kΔT − 12 I 2 R QC = P I[(sp − sn )ΔT + IR]

(10.10)

where Qc is the cooling rate, P is the power consumed, T C (T H ) is the cold side (hot side) temperature, I is the current, and R is total resistance. There come lots of conveniences with application of thermoelectric coolers such as being compact in volume, requiring minimal mend, having no mobile pats, and being trustworthy. Hence, they are convenient for micro-applications.

10.5.2 Thermoelectric Power Generation Seebeck effect is a way of generating useful power. Its main working principle is that it converts lost heat energy such as industrial plants, vehicles, boilers, stoves, or any kind of the heat-emitting item. Another usage of the effect is that it saves the lost heat from solar cells, powers, watches, and space vehicles [106]. The relationship of Seebeck coefficient, electrical conductivity, and thermal conductivity is shown in Figure 10.12. In this figure, 𝛼, 𝜎, 𝛼 2 𝜎, K e , and K l correspond to the Seebeck coefficient, electrical conductivity, power factor, and electronic and lattice parts of the thermal conductivity, respectively. The process of devices is to maintain a temperature gradient between junctions. The working principle is that while the heat source junction is keeping the temperature high, the heat sink junction draws the heat that keep the junction cold so that the source junction can be heated. Therefore, the outcome of this process is a maintained temperature gradient. As a result, charge carriers propagate from high temperatures to cold temperatures so that voltage or current can move through junctions [35]. Thermoelectric generators are a little less efficient than mechanical generators. Thus, it is essential to find the right materials to enhance TE efficiency.

10.5.3 Organic Inverter Circuit Presumably, an inverter can be appreciated as the simplest unit in digital circuitry. When compared with n-type materials, because of their higher field effect mobility

10.5 Applications of Thermoelectric Materials

Figure 10.12 The relationship of Seebeck coefficient and electrical and thermal conductivities for different materials.

Insulators

Semiconductors Metals

α

σ α2σ σ

α

opt

In(n) Ke

KI

Insulators

Semiconductors Metals

In(n)

and better intrinsic stability, p-type materials are highly preferred in designing organic thin film transistor (OTFT)-based inverters. Nevertheless, low static power consumption, high noise margin, and high gain and operational robustness are preferred properties of complementary organic inverters. Exhibiting mobility and threshold voltage is a must for p-type and n-type transistors. Nevertheless, we should remember that because n-type organic transistors have some drawbacks in high mobility, there is an arising need for an all-p-type organic inverter circuit. An all-p-type inverter also has some drawbacks. Low voltage swings, poor balance between pull-up and pull-down operations, higher power dissipation, and low noise margins are among the difficulties of those inverters. However, when we take those drawbacks into consideration, again there is an arising need for hybrid complementary circuits. Dodabalapur et al. [107] is the first person to maintain the technology of hybrid via adjusting n-type organic transistors with inorganic ones like a-Si: H TFT. All-p-type organic complementary inverters and hybrid inverters have different static and dynamic responses. The ideal value of Vdd/2 of noise margin can be maintained with a combination of pentacene and C60. It also has less propagation delay around 68% when compared with CuPc–F16CuPc combination. The main reason why it has less delay is that pentacene- and C60-based transistors have comparable mobility. On the other hand, when comparing CuPc-based transistors with F16CuPc-based ones, it can be seen that the former has 16 times higher mobility. Therefore, CuPc–F16CuPc combination brings about the highest delay ratio. Because F16CuPc has lower field effect mobility, the operational speed of transistors slows down and accordingly causes high delay ratio. The lowest propagation delay of 0.28 seconds

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can be maintained with the hybrid inverter circuit pentacene–ZnO combination. This ratio is nearly %50 less than pentacene–a-Si: H. Meanwhile, all-p-type organics exhibit only an average ratio. The static performances of all-p-type organics are VoH = 8.0, VıH = 6.8, VoL = 0.2, and VıL = 4.4. That is to say, the upper voltage is lesser than the Vdd(10v) in contrast to lower voltage of 0.2 V, which is desirable. Nevertheless, it must be admitted that the noise margins of all-p-type organic inverters are not matched enough with silicon-based transistors.

10.5.4 Organic Light-Emitting Diode (OLED) The organic light-emitting diodes (OLEDs), which have been marketed up till now, are providing some benefits such as low- cost, large-area displays. They have proven their benefits in small displays such as mobile phones, programmable digital arrays, MP3 players, and digital cameras. The main reason why they have been chosen is that they have low costs, are lightweight and flexible, have modification advantages, have larger color scale, and have better efficiency. In addition, further developments can be made in color quality, sharp image, intensive background, larger sight angle, swift switching, and low voltage. In the manufacturing process of good quality flexible displays of electronic papers, these properties can be used, which is very much suitable since it matches with properties of OLEDs. An example of an OLED structure and its OTFT-based driving circuit can be seen in Figure 10.13. The OTFT1 shown in Figure 10.13 demonstrates the charging and discharging capacities. On the other hand, OTFT2 is the main driver. OTFT1 must have a high on current and low off current for effective charging and discharging; otherwise, a charge leakage might occur, which of course would require a large-size capacitor that should be charged as well. In general, a large on/off current ratio is necessary to operate such a large display. A high-performance pentacene-based TFT with a mobility ratio of 1.5 cm2 /(V s) can enable the display to point minimum a thousand lines and maintain minimum charge leakage in longer frame times [108]. The first active matrix organic light emitting diode (AMOLED), the first fully organic active matrix organic light emitting diode, is said by Zhou et al. [109] in 2006

V

–

–

–

–

+

+

+

+

–

–

+

+

Electron transort layer Recombination region Hole transport layer Anode

C OTFT1

OTFT2

OLED

Transparent V Data substrate

Glass

Figure 10.13

–

+

ITO

Light

– Alq3 ETL

CuPc HTL

VGate

Cathode

Silver

Output

A schematic representation of OLED driven by an OTFTs.

VCathode

GND

10.5 Applications of Thermoelectric Materials

to contain 48 × 48 bottom emission OLED pixels with two pentacene transistor per pixel. Li et al. [110] is said to be the first person to report a fully printed multilayer OLED manufactured via polymer inking and stamping technique. The process has been conducted via transferring a layer of PEDOT material on polyestersulfone (PES) substrate with a stamp of polydimethylsiloxane (PDMS) material. A yellow light is emitted from the bottom of the device, from the transparent indium tin oxide (ITO) substrate with a voltage of around 7 V. OTFT devices with a low temperature generally have low mobility that in turn impedes the designing process of pixel circuitry [111]. As a result, what is needed to turn the table in this process is a compensatory unit that is necessary for low mobility of OTFT circuits. Only after that, a constant driving current can be maintained that in turn provides with enough electrical operation for AMOLED display.

10.5.5 Organic Radio Frequency Identification Tags Nowadays, in areas like defense sector, medical, toll bridge, and supply chain management, there is a new player. Over the years, organic radio frequency identification (RFID) tags have raised much interest around scientific circles since they provide with lots of advantages such as low cost and flexibility. They are three times more inexpensive than silicon ones. According to researchers, they can replace the former technology in inventory identification [112–114]. In Figure 10.14, it can be seen that the unit consists of a transmitter/receiver, rectifier/modulator, and RFID tag (12). The principle of this RFID system follows some simple steps. Primarily, the signal moves toward two directions. On the one side, the signal is sent from transmitter to read a code from the tag, and on the other side, the code read is sent back with the same signal to the receiver.

10.5.6 Organic DNA Sensors In medical electronics field, DNA hybridization sensors with organic transistors is a groundbreaking technique. The purpose of these organic sensors is to detect and quantify the nucleic acids. These kinds of system have much importance since they provide with lots of potential in fields such as pharmacogenomic research, drug discovery, forensic analysis, and molecular diagnosis [115, 116]. Among the highly advanced nucleic acid detection system comes pentacene-based label-free DNA sensors. Furthermore, they are applied in viral, forensic, and genetic symptom examination and detection. Formerly used techniques for sensing DNA molecules were mostly based on expensive, complex optical methods, which have long processing time. On the other hand, this new electronic DNA hybridization technique seems to be the convenient technique since it has better selectivity and sensitivity at on inexpensive way. This new organic material-based DNA sensor has proven its worth for transforming a chemical binding event into electric current. In addition, they can also be measured, analyzed, and amplified readily when compared with conventional techniques [117].

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Rectifier/Modulator

CCouple1

TAG A

GND

OTFT2

VRead VRec

Code Generator

OTFT1

Demodulator

CDecouple

CCouple2 Reader

–Vdd

Antenna

Clock generator (ring oseillator)

B Output shift register

Binary counterr (3-bit)

Output

Multiplexer (8*1)

Memory unit (ROM) D7D6D5D4D3D2D1D0Row 0 Line selector (8-bit) (3*8 decoder)

“ “ “ “ “ “

“ “ “ “ “ 7 Row

Figure 10.14 Schematic demonstrations of capacitive coupled organic rectifier/modulator (upper side) and transponder circuit of an organic RFID (lower side).

10.5.7 Limitations When they are compared with organic transistors, silicon-based transistors have higher operating speed, smaller size, environment stability, and durable performance. Therefore, over the last 10 years, they have strong position in the electronic sector. However, the mind started to blow to other direction, since, despite their drawbacks, organic semiconductors have extended their market share. Nevertheless, there is much to be done so that organic semiconductors could become feasible and applicable.

10.6 Techniques of Thin Film Deposition for Thermoelectric Device A lot of thin film deposition techniques are said to be used. However, when deciding whether a technique will be beneficial or not, lots of different parameters must be taken into account such as melting point of material, temperature of deposition, and the bond between the substrate and the material.

10.6.1 Sputtering The main principle of sputtering technique is that an inert gas like Argon is taken to deposit atoms on the substrate by using high energy. To ionize the Argon, RF power

10.6 Techniques of Thin Film Deposition for Thermoelectric Device

Substrate and film growth Sputterin gas

Ar+

Sputtering target

Figure 10.15

The main principle representation of sputtering technique.

sources or direct current is preferred so that energy can be supplied to spread from single element targets or a mix of them. Once the films got deposited at different temperatures ranging from high to average, then they are annealed at high temperatures [118]. In this case, mixed powders from different compositions were taken as sputtering agents to prepare Bi2 Te3 films. As a result, it can be seen that electrical conductivity is enhanced under high-temperature conditions, while Seebeck coefficient plummeted. Another thing that can easily be seen is that annealing temperature of 300 ∘ C is the best condition if the highest power factor is to be targeted [119]. This system is shown in Figure 10.15.

10.6.2 Molecular Beam Epitaxy (MBE) If single crystals are to be deposited, then among the most qualified methods comes the molecular beam epitaxy (MBE). The process begins with separating the single effusion cells from the substrate with shutters or valves, and then evaporation is carried out to extract pure materials while the substrate is heated. The films are grown in epitaxial manner as long as there is a slow deposition rate. Optimization to achieve a certain purity level is maintained by benefitting from high vacuum chambers [21]. In a trial, Seebeck coefficient of 180 μ V/K is maintained by means of this technique to deposit Bi2 Te3 [120]. In another trial, a similar kind of output was maintained by using this technique where Seebeck coefficient is 184 μV/K and power factor is 1.6 mW/(K2 m) while (BiSb)2 Te3 was being deposited [121]. This process is demonstrated in Figure 10.16.

10.6.3 Metal–Organic Chemical Vapor Deposition (MOCVD) MOCVD is another technique where you can produce highly qualified n-type and p-type materials. This technique is maintained with metal hydrides and organic compounds and with the surface reaction between them that have necessary chemical elements. At the surface of the substrate, final pyrolysis of the constituent

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Shutters switch beams on or off

Beams fire at substrate

Substrate

322

Effusion cells

Layers of atoms build up on substrate

Figure 10.16

Substrate heater

A schematic demonstration of molecular beam epitaxy.

chemicals takes place so that the required filming could be formed. The necessary pressure is the optimal rate needed for a successful reaction to occur. In addition, the necessary temperature must be between 300 and 500 ∘ C. However, the situation may change in relation to the reaction. Seebeck coefficient values under this technique’s circumstances are −210 and 100 μ V/K for Bi2 Te3 and Sb2 Te3 films in a row. Besides assessing values of Seebeck coefficient, resistivity values of Bi2 Te3 and Sb2 Te3 are 9 and 3.5 μΩ-m [122]. For maximum Seebeck coefficient, 115 μV/K,

CVD thin film growth

1

Main gas flow region

Gas phase reactions

Desorption of volatile surface reaction products

2

Transport to surface

3

Adsorbtion of film precursor

Figure 10.17 (MOCVD).

4

Surface diffusion

5

Redesorption of filim Nucleation and precursors island growth Step growth

6

Surface reactions

A schematic demonstration of metal–organic chemical vapor deposition

10.6 Techniques of Thin Film Deposition for Thermoelectric Device

Glass cover Anode

Zn foil

Cu foil

Electrolyte

Au contact

Cathode

Au contact Glass substrate

μA

Zn electrodeposited film

Au flash

Controlled current generator

I

+

–

V

Figure 10.18

A schematic representation of electrochemical deposition.

the deposition of Sb2 Te3 took place under 450 ∘ C. MOCVD system is represented in Figure 10.17.

10.6.4 Electrochemical Deposition (ECD) Electrochemical deposition (ECD) is a relatively cheap and suitable technique of producing thin films in comparison with vacuum-based techniques. Nevertheless, its applications are limited due to low-performance and low-segment materials. The components not only are smelted in nitric acid solution but also perform as chelating agent that helps in avoiding the deposit of irresolvable oxides. Deposition of n-type Bi2 Te3 was performed at a constant potential that is a regular three-electrode configuration [123]. While using ECD technique to deposit Sb2 Te3 films, the power factor was maintained at 0.57 mW/(K2 m) [124]. Electrodeposited Bi2 Te3 films at 50 mV showed a Seebeck coefficient of 51.6 μV/K and the power factor of 0.71 mW/(K2 m). However, electroplated Sb2 Te3 films at 20 mV showed a Seebeck coefficient of 52.1 μV/K and power factor of 0.17 mW/(K2 m) [125]. A schematic representation of ECD is shown in Figure 10.18.

10.6.5 Flash Evaporation (FE) When the components have various vapor pressures, it is flash evaporation that is preferred while depositing thin film alloys. The evaporation of the least volatile constituent of the alloy is maintained with a boat that has adequate pressure and temperatures. Crucial vapor pressure and temperature of components are not of particular importance, which is a very beneficial point of this technique. Rather than using sophisticated mechanical utensils or luxurious cleaning procedures or being contaminant, this technique is performed with simple evaporation materials and precursors [126]. Figure of merit ZT = 0.21 × 10−4 was yielded using this technique while evaporating (Bi2 Te3 )0.9 (BiSe3 )0.1 for n-type material powder and (Bi2 Te3 )0.25 (Sb2 Te3 )0.75 for p-type material powder. A schematic representation of flash evaporation is exhibited in Figure 10.19.

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Powder vessel Substrate Glass chamber Guide Tungsten boat

Vacuum

Figure 10.19

A schematic representation of flash evaporation.

10.6.6 Thermal Evaporation Thermal evaporation is the process of heating solid materials inside a highly powerful vacuum chamber to the point where they can produce vapor pressure. In thermal evaporation, thermal energy is used to vapor the material in contrast to MBE kind of techniques with electron beam. While using evaporation technique, Seebeck coefficient and electrical conductivity of p-type Sb2 Te3 thin film were equal to 160 μV/K, 3.12 × 10−3 Ω cm and of n-type Bi2 Te3 thin film were equal to 200 μV/K, 1.29 × 10−3 Ω cm [127]. With the application of this technique, 4.9 mW/(K2 m) for Bi2 Te3 and 2.8 mW/(K−2 m) for Sb2 Te3 high power factors have been maintained [128]. A schematic representation of thermal evaporation is depicted Figure 10.20.

10.6.7 Pulsed Laser Deposition (PLD) While using this technique, the deposition of thin films is maintained with ablation of a couple of targets. A focused pulsed laser beam is necessary for this ablation. Vaporization of the materials from the target is maintained, and then deposition process of thin films under highly powerful vacuum conditions in the presence of gas took place [129, 130]. A schematic diagram of a typical pulsed laser deposition (PLD) setup is shown in Figure 10.21. The process starts with the striking of targets with a 45∘ angle by pulsed and focused laser beam under highly powerful vacuum conditions. After that, the ablation of the atoms and ions took place that subsequently are deposited. In the face of

10.6 Techniques of Thin Film Deposition for Thermoelectric Device

Substrate holder Substrate Deposited thin film Vaporized material

Evaporator

Target material

Chamber

Heater

Figure 10.20

A schematic representation of thermal evaporation.

Substrate Pl a

m

as

Ta rg et

e

um

Laser pulse

pl

Figure 10.21 A schematic demonstration of a typical pulsed laser deposition (PLD) method.

UHV chamber

an inert gas such as Argon or a gas like oxygen, deposition is performed, as long as oxide is to be formed. In general, attachment of substrates is in line with the target surface with a distance of 2–10 cm. In 1996, Dauscher was the first man who attempted to deposit Bi2 Te3 film with PLD [131]. With a thickness of 60 nm, laser energy ranging from 300 to 680 mj, laser intensity ranging from 2 to 10 J cm2 , and substrate temperature ranging from 20 to 500 ∘ C, thin films were deposited [132]. With various temperatures changing from 30 to 400 ∘ C in a deposition chamber that permits inert gas pressure to be taken under control between 100 Pa to atmospheric pressure, on silicon and mica substrates, Bi2 Te3 film raising was made possible [133]. At a substrate temperature ranging from 300 to 500 ∘ C with the usage of pulsed laser, on the surface of mica and aluminum nitride, the depositions of thin films of p-type Bi0.5 Sb1.5 Te3 , n-type Bi2 Te2.7 Se0.3 , and n-type(Bi2 Te3 )90 (Sb2 Te3 )5 (Sb2 Se3 )5 were maintained [134]. Organic transistors have wide range of application fields such as organic inventors, ring oscillators, analog circuits, solar cells, and sensors [135–139]. Their main usage

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field is backplane driver in organic display circuits. In addition, they can also be used in organic radio frequency systems as rectifier/modulator unit.

10.7 Conclusion and Future Trends Thermoelectric devices have a wide range of personal electronic applications such as wearable devices (watches or those that can be attached to clothes) and biomedical materials (drug and equipment used to monitor vital signs). This up-to-date technology can provide us benefits. However, their development depends on cost-effective mass production techniques and performance optimization. A good thermoelectric material must have a high electrical conductivity and low thermal conductivity [140]. If electrical conductivity is to be increased, then what must be done is to enhance carrier concentration and/or improve carrier mobility. Enhancing material structure or fabrication processes improves mobility or concentration. Using the superlattice, quantum well structure enhances not only mobility but also Seebeck effect [141]. Quantum confinement is the key feature to enrich figure of merit; consequently nanowires and dots can be incorporated into the scope. Besides the technique mentioned above, using Si ions, various doses over a multilayer thermoelectric material are among the most beneficial ways of carrier concentration enrichment, thus, reducing thermal conductivity when considering quantum structures. In addition, ion usage is said to boost up electronic intensity of quantum structures, thus supporting electrical conductivity and Seebeck coefficient. With enhanced conductivity comes a certain default high thermal conductivity. As far as Seebeck effect is concerned, high thermal conductivity is something faulty. As for the solution, the one that makes sense is to separate them. That is, conductivities must be distinct from each other. Phonon glass electron crystal (PCEC) is a structure; among the processes that can be carried out, there are alloying and site substitution. However, it should not be forgotten that it can only be done with isoelectric elements such as Bi2 Te3 with Sb2 Te3 or Bi2 Se3 . As a result of this process, p-type Bix Sb1−x Te3 and n- Bi2 Sey Te1−y are formed. The interesting thing is that disorder can be introduced into a complex lattice structure as a different technique [7]. As was mentioned above, PCEG material is a perfect material, and it can be created by adding defected atoms in the empty space of the material structure. It does not have any impact on conductivity because the disorder is situated in the empty space and the transmittance occurs outside the empty space. Another beneficial side of it is that defected atoms suppress thermal conductivity. Decoupling process of both conductivities by spin Seebeck effect (SSE) is another way. The main working principle of this effect is that current can be obtained by converting thermally generated spin. You benefit from spin–orbit interaction [142]. Low thermal ferromagnetic materials with highly conductive properties are preferable when generating charges. In Figure 10.22, images respectively show the following: ●

Thermoelectric device with traditional Seebeck effect.

List of Abbreviations and Symbols Longitudinal spin Seebeck effect LSSE) Metalic film Conductor

Ese

∇T

● ●

Flexible device

Z

∇T

Magnetic M Y insulator X Enlarging area

Flexible substrate

Metalic film a1 – Ferromagnetic device ∇T

∇T

Figure 10.22

●

Coating method

Iy

Serial cnnection

Thermocuple

●

Developing LSSE devices

Ix

i3

Module structure

Fundamental element

Seebeck effect

Magnetic insulator

Multilayer stack

Ferromagnetic metal

An example of device structure for longitudinal spin Seebeck effect (LSSE).

Thermoelectric device with a module structure with Seebeck effect. Thermoelectric device with longitudinal spin Seebeck effect (LSSE). Thermoelectric device with a module structure LSSE. LSSE device structure.

The benefits of topological insulators have been proven useful when it comes to thermoelectric devices [143]. A mixture of Bi2 Te3 and Sb2 T3 is a perfect insulator. This material makes achieving a figure of merit ZT up to 1.86 feasible and possible, and that is much above traditional thermoelectric device [144]. In United States today, automobile manufacturers are considering using thermoelectric devices. Well-known firms such as Ford and GM are being funded by the Department of Energy to manufacture systems for heating, ventilation, and air-conditioning (HVAC) [145]. Their production leads to a wider range of industrial and electronic applications of thermoelectric devices. To sum up, thermoelectric devices for electricity generation have proven their worth not only in personal applications but also in industrial electronics. Nevertheless, there is much way to go further until they are more efficient and mass produced.

List of Abbreviations and Symbols BTC CSA DMSO ECD FE MBE MOCVD OLED OTFT

Benzene tricarboxylate Camphorsulfonic acid Dimethylsulfoxide Electrochemical deposition Flash evaporation Molecular beam epitaxy Metal–organic chemical vapor deposition Organic light-emitting diode Organic thin film transistor

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PCEC PDMS PEDOT: PSS PLD RFID TE ZT 𝜋 𝛼 𝜆 𝜌 𝜎 k Q

Phonon glass electron crystal Polydimethylsiloxane Polystyrene sulfonate Pulsed laser deposition Radio frequency identification Thermoelectric materials Figure of merit Peltier coefficient Seebeck coefficient Thermal conductivity Electrical resistivity Electrical conductivity Thermal conductivity Thomson heat

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141 Ismail, Y. and Alaskalany, A. (2014). Thermoelectric devices: cooling and power generation. arXiv preprint arXiv:1403.3836. 142 Uchida, K.-i., Adachi, H., Kikkawa, T. et al. (2016). Thermoelectric generation based on spin Seebeck effects. Proceedings of the IEEE 104 (10): 1946–1973. 143 Zhang, J., Feng, X., Xu, Y. et al. (2015). Disentangling the magnetoelectric and thermoelectric transport in topological insulator thin films. Physical Review B 91 (7): 075431. 144 Kim, S.I., Lee, K.H., Mun, H.A. et al. (2015). Dense dislocation arrays embedded in grain boundaries for high-performance bulk thermoelectrics. Science 348 (6230): 109–114. 145 Gonzalez, R. and Maronde, C. (2010). Automotive thermoelectric HVAC development and demonstration project. California Energy Commission 10, 10-10.

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11 Template-assisted Fabrication of Nanostructure Thin Films for Ultrasensitive Detection of Chemicals and Biomolecules: Part A – Template-assisted Nanoimprinting Technology for Functional Thin Films Xiaomin Zhu 1 , Xinhua Chen 2,5 , Andrey A. Voronov 3,4 , Vladimir I. Belotelov 3,4 , and Yujun Song 1,5 1 University of Science and Technology, Center for Modern Physics Technology, Applied Physics Department, School of Mathematics and Physics, 30 Xueyuan Road, Beijing 100083, China 2 Zhejiang University, Key Laboratory of Combined Multi-organ Transplantation, Ministry of Public Health, Department of Hepatobiliary and Pancreatic Surgery, the First Affiliated Hospital, Hangzhou, Zhejiang, 310003, China 3 Lomonosov Moscow State University, Faculty of Physics, Leninskie Gory, Moscow 119991, Russia 4 Russian Quantum Center, 45, Skolkovskoye shosse, Moscow, 121353, Russia 5 Zhejiang Key Laboratory for Pulsed Power Technology Translational Medicine, Hangzhou Ruidi Biotechnology Company, Hangzhou 310000, China

11.1 Development of Template-assisted Nanoimprinting Technology Template-assisted (TA) lithography (LIGA) has developed as a powerful physical technique that enables the production of surface morphology-confined nanoparticles (NPs) and NP arrays with controlled shapes, sizes, and interparticle spacing. Several templates have been developed for these purposes, such as porous polymers, porous Al2 O3 foils, and nanosphere (NS) arrays (polymers and ceramics), resulting in various TA LIGA, correspondingly as porous polymers lithography (PP-LIGA), porous anodic aluminum oxide (AAO) LIGA, or nanosphere lithography (NSL) [1]. Our team has been working on the development and research of TA LIGA for the preparation of nanostructured thin films for many years and has made outstanding progress in NSL and porous anodized alumina TA fabrication processes. We have invented the methodology of single nanomaterials identification via multi-hierarchy arrayed microwindows (MHMW) assisted NSL and the AAO template-assisted in situ sol–gel transfer nanoimprinting method, which can obtain nanostructured array thin films on hard or soft substrates. This chapter focuses on three TA LIGA methods developed by our team. Combined with AAO technology, different nanoarrays, nanowires (NWs), and nanorods (NRs) were fabricated. Additionally, their application for detecting ultrasensitive chemical and biological molecules is also summarized.

Inorganic and Organic Thin Films: Fundamentals, Fabrication, and Applications, First Edition. Edited by Yujun Song. © 2021 WILEY-VCH GmbH. Published 2021 by WILEY-VCH GmbH.

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11.2 Nanosphere Lithography (NSL) This section presents the recent progress in NSL for the controlled producing noble metal nanomaterials, including the size, shape, and surface morphology-controlled fabrication of noble metal NPs and nanoarrays. Then four distinct progresses in the development of NSL techniques are also showed: (i) fabrication of hierarchically ordered NW arrays on substrates by combination of NSL and porous AAO; (ii) identification of single NPs and nano-arrays by combining NSL and MHMW; (iii) fabrication of biosensing system based on the combination of the noble metal NPs and nanoarrays fabricated by NSL and microfluidic techniques; and (iv) synthesis of solution-phased NPs by transferring the NPs into solutions. In (ii) and (iii), the related 3D morphologies and arrangement dependent optical properties, and comparison between the numerical as well as experimental results are provided, revealing their intrinsic quantum mechanism. The research are fundamental pre-conditions for the discovery of novel properties and applications of noble metal NPs. Finally, issues and perspectives in the controlled fabrication of noble metal nanomaterials by NSL, and investigation of their 3D morphologies and arrangement dependent optical properties for future potential applications are also highlighted and discussed.

11.2.1 Size and Shape Controlled Fabrication of Nanomaterials via NSL NSs have been used to form uniformly arranged layers as templates to produce perfect triangle nanoprisms on substrates. The routine procedure for the production of triangular shaped nanoprisms, based on the NSL, is described in Figure 11.1 (a: cross-section view; b: top view). The hexagonal arranged NS monolayer is formed on the substrate by a coating process (e.g. dip-coating, rotating-coating, or spinning-coating) (Step 1a). The interstitial among any three adjacent NSs form triangular voids (Step 1b) as templates. The desired noble metal (e.g. Ag) is then deposited on the triangular interstitial among the NSs to form triangular Ag NPs (Step 2a, b). After NSs are released by sonication or other methods, surface-confined triangular Ag nanoprisms can be obtained (Step 3a). By this NSL process, uniform hexagonal-arrayed triangle nanoprisms can be fabricated on a variety of substrates (e.g. glass, mica, silica wafer, polymethyl methacrylate (PMMA), etc.). Step 3b is an atomic force microscope (AFM) image of Ag triangular nanoprisms fabricated by our group using a self-assembled monolayer of 300 nm polystyrene NSs as the template. The initial critical step in NSL is the formation of a uniform large-scale NS template. Both drop-coating and spin-coating can produce uniform templates on glass, silica wafer, or mica substrate. The uniformity of the NS template produced by drop-coating depends on the NS type and concentration, the hydrophilic properties of the substrate, the environmental humidity, temperature, and the drying speed. A monolayer colloidal polystyrene NS mask can be prepared by drop-coating of 3.0–4.0 μL, 2–15 times diluted NS solution (concentration 4.0 wt%) onto the glass

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Figure 11.1 The NSL process for triangular NPs fabrication. Step 1a: The hexagonal arranged nanosphere mono layer is formed on the substrate by coating process. Step 1b: The interstitial among any three adjacent nanospheres form triangle shaped voids as templates. Step 2a,b: The Ag metal is deposited on the triangular interstitials among the nanospheres to form triangle shaped Ag NPs; Step 3a: The nanospheres is released by sonication or other methods, leaving the triangular Ag nanoprisms on the substrates, by this nanosphere LIGA process, the hexagonal arrayed uniform triangular nanoprisms can be fabricated on a variety of substrates (e.g. glass, mica, silica wafer, PMMA, etc.). Step 3b: The AFM image for Ag triangle nanoprisms fabricated by monolayer template from 290 nm polystyrene nanospheres in our group; these nanoprisms have very uniform edge length of 67 ± 4 nm (STDEV% of 6%) and thickness of 20.0 ± 1.0 nm (STDEV% of 5%). (a) Crosssectional view; (b) top 3D view. Source: Adapted in part from Song, Y. China Patent, CN200910085973.9; Haynes, C. L.; van Duyne, R. P., J. Phys. Chem. B 2001 105, 5599, Copyright [1999] American Chemical Society; (b) Song [1], doi: 10.5772/25037.

support and leaving them to dry overnight. A detailed procedure to fabricate the NS mask using drop-coating is described. The glass is cleaned by sonication with a mixture of sulfuric acid and hydrogen peroxide (3 : 1 = concentration H2 SO4 : 30% H2 O2 , volume ratio) at 80 ∘ C for 30 minutes and washed using sufficient nanopure water. Then, the glass is sonicated in a mixture of ammonia and hydrogen peroxide (5 : 1 : 1 = H2 O:NH4 OH (37%):30% H2 O2 , volume ratio) to increase the hydrophilic property on the surface. Finally, the glass is washed using sufficient nanopure water again and then stored in the nanopure water for future use. When drop-coating is to be performed, the glass is picked up from the nanopure water from one of its edges. The remaining water droplets on the glass substrate are removed by drying

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Figure 11.2 Nanosphere templates based on 290 nm spherical polystyrene nanospheres for Ag nanoparticle fabrication. Source: Reprinted from Song and Elsayed-Ali [1], Copyright (2010) Elsevier.

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on the opposite edge with filter paper. The substrate is then left flat in a clean Petri dish with a tilt angle of 3∘ –5∘ , 15 μL of PS NS solution is added on the surface of the glass substrate using a droplet. The water spreads over the whole glass to form a semi-ellipsoidal shaped water spot. The Petri-dish is left long enough for full evaporation. During evaporation, the temperature is kept at 18 ± 3 ∘ C and the humidity is kept 50 ± 5%. In our group, a near-uniform monolayer NS template can be prepared on almost the whole glass (18 mm diameter). Figure 11.2 shows one typical area of a near-uniform monolayer template over scale of 20 μm. From the magnified image, a selected area shown in the inset, no lattice defects can be observed. Using this template, uniform Ag nanoprisms can be fabricated by vapor deposition process. One typical area fabricated by our group is shown in Step 3b in Figure 11.1, where these nanoprisms have very uniform edge length of 67 ± 4 nm (STDEV% of 6%) and thickness of 20.0 ± 1.0 nm (STDEV% of 5%). We recently developed a modified NSL process to fabricate Ag NPs with controlled shapes on substrates. The modification in NSL is performed by thermally annealing the triangular nanoprisms, and sonication to remove weak tips, followed by removing debris and small broken parts around the NPs on the substrates. The detailed process is shown as the following: (i) releasing the NSs by immersing the cover slip into a 5% HCl solution for 30 minutes, then immersing the glass substrates into CH2 Cl2 for 30 seconds, and then sonicating for 20–60 seconds; (ii) The fabricated Ag nanoprisms on the glass substrates are annealed at 100–300 ∘ C for 1–5 hours; (iii) Then Ag nanoprisms are cleaned by immersing the glass cover slip into 5% HNO3 for 10–20 seconds to remove any surface contamination and dissolve debris around the NPs, and then washed by large amount of nanopure water. Comparing the AFM images in Step 3b in Figure 11.1 showing the NPs without above posttreatment, tip-round triangle nanoprisms, square-shaped, and trapezoidal Ag NPs (Figure 11.3) can be obtained via one or two of the above treatment. Thermal annealing results in much more uniform NP surfaces without the thin, weak tips, and edges (Figure 11.3a). From the magnified AFM plane image in the inset of Figure 11.3a and

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Figure 11.3 Surface-confined Ag NPs with controlled shapes fabricated by the modified NSL process. (a) AFM image of triangular prism Ag NPs with round tips after thermal annealing at 200 ∘ C for 4 hours, cleaning by 5% nitric acid, and washing by nanopure water. (b) The 3D image of the triangular prism Ag NPs with round tips. (c) Flat trapezoidal Ag NPs after sonication to remove one tip, thermal annealing, cleaning by 5% nitric acid, and washing by nanopure water. (d) The 3D image of the trapezoidal Ag NPs with one snipped tip. (e) The quadrilateral or pentagon shaped Ag NPs after sonication intensively to remove two tips, thermal annealing, cleaning by 5% nitric acid and washing by nanopure water. Dashed circles: pentagonal Ag NPs with one sharp tip left; dashed squares: quadrilateral Ag NPs. (f) is the 3D image of the quadrilateral and pentagon shaped Ag NPs. Source: Song and Elsayed-Ali [7]. © 2010, Elsevier.

the 3D image in Figure 11.3b, the NPs still show triangular prism shape with round edges and little surface defects. Alternatively, if the NPs produced by NSL is sonicated for 30–45 seconds to remove a weak tip, anneal them at 200 ∘ C for 1–4 hours, then wash them with 5% nitric acid, the trapezoidal shaped NPs with round edges will be formed, as shown in Figure 11.3c,d. If the sonication time is increased to more than two minutes, the NPs will lose their two sharp tips and form quadrilateral or pentagon shaped NPs. After thermal annealing for 1–4 hours and washing with 5% nitric acid, their edges and corners become round, as shown in Figure 11.3e, which show quadrilateral NPs (in dashed squares) or pentagon (in dashed circles). The 3D AFM image in Figure 11.3f, shows NPs have round edges and corners. Clearly, even after thermal annealing, they are still showing prism shapes with increased thickness from their edges to centers according to their 3D AFM images. The work described above demonstrates that NSL, broadly defined to include angle-resolved nanosphere lithography (AR NSL) and some modified post treatment after deposition of the desired materials, is manifestly capable of creating far more than arrays of nanotriangles, nanodots as previously supposed. The progresses in NSL endow much potential in the size- and shape-controlled fabrication of NPs and nanoarrays, which gives NSL potential since the ability of NSL to synthesize

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monodisperse, size- and shape-tunable NPs can be exploited to precisely investigate the size- and shape-dependent physiochemical properties of nanooptics and nanoarrays.

11.2.2 Multi-hierarchy Micro Windows (MHMW) for Single Nanostructure and/or Array Identification The physicochemical properties of nanomaterials significantly depend on their 3D morphologies (sizes, shapes, and surface topography), their surrounding media and spatial arrangement. Systematically and precisely correlating these parameters with the related physicochemical properties of specific single NPs or nanoarrays is a fundamental requirement for the discovery of their novel properties and applications, as well as for advancing the fundamental and practical knowledge required for the design and fabrication of new materials. The lack of effective means of fabricating recognizable 3D morphology-controlled NPs and nanoarrays and correlating their structure parameters with their physicochemical properties has been observed in different characterization techniques. It is an obstacle for studying the 3D morphology-dependent properties of individual NPs and nanoarrays. Current studies investigate the physicochemical properties of the NP ensemble, but not of a single NP. Effective methods for 3D morphology-controlled fabrication of nanomaterials, in order to correlate their 3D morphology of a single NPs or nanoarrays with their physicochemical properties, are also essential to address fundamental and practical questions related to the single NPs. An important research area in nanoscale plasmonic optics is single NP identification and characterization of their 3D morphologies and space orientation-dependent physicochemical properties. Recently, much attention has been given to the localized surface plasmon resonance (LSPR) of metal NPs because of their promising applications in plasmonic circuits, optoelectronic transducers, optical bioprobes, and surface plasmon resonance (SPR) interference LIGA. Since the plasmonic properties of metal NPs intrinsically rely on their size, shape, surface topography, crystal structure, interparticle spacing, and the dielectric environment around them. Methods to correlate their plasmonic properties with the above structural and environmental parameters have become one of the most rapidly developing research directions. In the precise investigation of the relationship between the LSPR properties and their 3D morphologies of specific NPs and nanoarrays, two methods have been developed recently: the in situ method and the spatial localization method [2]. The in situ method combines at least two different instruments together to conduct the structure and property characterization simultaneously: one can be used to characterize the 3D morphology (e.g. AFM or STEM) [3] of NPs and the other to characterize the LSPR-related optical properties of the same NPs (e.g. dark-field microscope and spectroscopy). The spatial localization method requires using markers to recognize the same single NP in different instruments. We have also developed one spatial localization method to precisely investigate the 3D morphology dependent LSPR properties of specific NPs and nanoarrays by the combination of NSL and traditional

11.2 Nanosphere Lithography (NSL)

ultraviolet lithography (UV-LIGA), where Ag NPs and nanoarrays can be fabricated by NSL in the preformed multihierarchy arrayed transparent microwindows on the substrates (e.g. glass cover slip) by the UV-LIGA. This technique permits easy characterization of the 3D morphologies of single NPs by AFM or SEM and their LSPR spectra using dark field optical microscopy and spectroscopy (DFOMS). It is also possible to investigate the local morphology dependence of the LSPR spectra of the single NPs and nanoarrays. In this method, MHMW are first fabricated on a glass cover slip using the standard photolithography, whose details are shown in Ref. [4]. Figure 11.4A,B shows one example of the designed MHMW (three tiers) and the typical final microwindows (Figure 11.4C) pattern after printing. The MHMW on the glass cover slip are used to identify the location and orientation of single NPs, whose tiers can be determined by the observed field at desired resolution. For example, in the first tier of the MHMW (Figure 11.4A), each local area can be discerned by marking its X and Y number, such as the shaded area X1–Y2. Then, in the second tier of the MHMW (Figure 11.4B), the scale can be reduced by M or N times and each local area can also be marked by x and y number. If this area is the sub-tier in the shaded area of the first tier, it can be labeled as X1–Y2–x3–y3. In a similar way, step by step, we can reach the last tier with several transparent microwindows available (Figure 11.4C), in which the desired NP can be made by different fabrication methods (e.g. electron beam LIGA or NSL). NPs less than 10 nm of different shapes synthesized by a wet-chemical process can be

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Figure 11.4 The MHMW on the substrate (e.g. glass cover slip). (A) The first tier of the MHMW, each local area can be discerned by marking its X and Y number, such as the red-dashed square area of X1–Y2. (B) The second tier of the MHMW, whose scale can be reduced by M or N times, whose local area can also be marked by x and y numbers. If this net area is the sub-tier n the red area of the first tier, it can be labeled as X1–Y2–x3–y3. Step by step, the last tier with several uniquely shaped transparent windows can be reached. The open windows can be made with different shapes. (C) The nanoparticles can be fabricated on the micro-pattern by various methods (e.g. nanosphere lithography). In each window, the same nanoparticle can be identified by comparing the images taken by optical microscopy, AFM, or other microscopy methods. Finally, the structural parameters (size, shape, orientation, interparticle spacing, and thickness) can be correlated with their optical responses. Source: Reprinted from Song Y.; et al., Nanoscale 2011, 3, 31–44, Figure 7, copyright [2011] from the Royal Society of Chemistry.

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immobilized by a routinely diluted deposition. Consequently, the same NP in each window can be identified by comparing the images taken by the optical microscope with those characterized by the AFM. Finally, in each window, the same NP can be characterized by different techniques (e.g. DFOMS and AFM) allowing correlation of its 3D morphology with its optical response.

11.2.3 Identification of Localized Surface Plasmon Resonance (LSPR) of Single Noble Metal Nanoparticles and/or Nanoarrays by MHMW-assisted NSL A typical example to identify NPs and nanoarrays using both AFM and DFOMS is illustrated in Figure 11.5. Triangular Ag NPs and hexagon-arranged nanoarrays fabricated on the surface of glass cover slips within the nearly circle-shaped microwindow can be identified and characterized using AFM (Figure 11.5a,b is the 3D AFM image of the dash-squared area in Figure 11.5a) and DFOMS equipped with a color camera (Figure 11.5c) and charge-coupled device (CCD) camera (Figure 11.5d). The CCD camera offers higher spatial resolution than the color camera, while the color camera provides the real color of individual Ag NPs that are generated by LSPR. The center of each individual NP in the optical images recorded by the CCD is located with a single-pixel resolution (each pixel can be 125 or 67 nm depending on the CCD resolution and equipment setup) by determining the address of the pixel with the highest intensity of the NP. The positions of individual NPs of interest (e.g. the circled one) within the microwindow in the optical images (Figure 11.5c,d) are then determined with a spatial resolution limited by the optical diffraction limit (200 nm) and an orientation angle resolution of about 1.0∘ . This approach allows us to correlate AFM images of individual NPs (as the one circled in each image) with the same NP shown in its corresponding optical image and to investigate its 3D morphology-dependent LSPR properties. Clearly, these triangular NPs in this window almost show the same scattering color (Figure 11.5c) and intensity contrast (Figure 11.5d). By comparing their scattering color images (Figure 11.5c) with their AFM images (Figure 11.5a,b) of these NPs, it is once again showing that NSL is powerful method in the fabrication of uniform triangular NPs and nanoarrays. We have used it to investigate size- and shape-dependent LSPR spectra of single Ag NPs by the analysis of the experimental results with the theoretical calculation (i.e. discrete dipole approximation (DDA) simulation). Figure 11.6 gives the AFM images of one specific triangle-shaped Ag NPs characterized by MHMW. The AFM image of the triangular silver NP shows that it has the edge length of 375–420 nm (Figure 11.6A) and the out-of-plane height of about 16.1 nm (Figure 11.6B). This NP shows multi LSPR scattering colors (Figure 11.6C), as further evidenced by its multi-mode LSPR peaks at 562.3, 659.9, and 759.6 nm (Figure 11.6D-b). The peak wavelengths, peak ratios, and full wave at half maximum (FWHM) at 562.3 and 659.9 nm from experiment are in good agreement with DDA simulation for its LSPR scattering (Figure 11.6D-c), as have been summarized together with other shaped NPs fabricated by the modified NSL in Ref. [4]. In general, the DDA simulation

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Figure 11.5 One example for the identification of the specific nanoparticles and nanoarrays in different instruments via MHMW based on Ag triangle nanoparticles and nanoarrays fabricated by NSL in one nearly circle-shaped window. (a) The plan view of the hexagon-arrayed triangular Ag NPs in one circle microwindow scanned by AFM; (b) The 3D view of the hexagon-arrayed triangular Ag NPs marked in the large pink dash-square in (a); (c) The real scattering color of these hexagon arrayed triangular Ag NPs observed under a dark-field microscope; (d) The CCD images of the scattering light of these hexagon-arrayed triangle Ag NPs recorded by a CCD camera equipped in the dark-field microscope. The dashed circles in each image refer to the same specific particle and the dashed squares in each image refer to the same specific nanoparticle pair. Source: Y. Song, China Patent, Appl. No. CN200910085973.9.

shows best agreement with the experimental spectra for NPs; hence their shapes can be accurately modeled. However, it can also be seen that for wavelengths longer than 650 nm for the investigated NPs, the experimental result has a lower intensity than the simulation. By analysis the instrument errors and the wavelength dependent CCD quantum efficiency, these deviations are deduced by the precision in the shape construction during DDA simulations. From these results, it was also found that when the shapes and 3D morphology of the NPs became more complicated, the deviation between the DDA simulation and the experimental result increased. This is due to the geometrical deviation between the real NPs and

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Figure 11.6 (A) The AFM image for one single triangular shaped Ag nanoparticle with edge length of 375–420 nm; (B) the height mapping of the triangle shape Ag nanoparticle along the direction of the arrow in Figure 11.6A, showing the out-of-plane height of this nanoparticle of 16.1 nm; (C) the real scattering color image of this triangular Ag nanoparticle; (D-a) the LSPR absorption spectrum of this nanoparticle by DDA; (D-b) the LSPR scattering by experiment; (D-c) the LSPR scattering by DDA; (D-d) the LSPR extinction by DDA. In order to identify the location and the orientation of these positions around the NPs, the AFM image and color image were netted by dashed lines with each square unit of 125 nm × 125 nm after their distances and orientations are corrected. Source: (a, c) Adapted from references: Y. Song, China Patent, Appl. No. CN200910085973.9; (b, d) Song et al. [2]. Copyright [2011] from the Royal Society of Chemistry.

the regular species used in the calculations. If these two instrumental factors and the geometrical deviation of NPs are considered, the corrected experimental results will match with the DDA simulation very well. This result also confirms that our experimental method (DFOMS), based on the far field detection, preserves the ability to detect the near-field LSPR signal. This combined method based on the NSL and the MHMW also allows us to investigate the 3D morphology dependent tip–tip LSPR coupling of triangular NP pairs. The zoom-in AFM image for the detailed 3D morphology of one typical Ag nanoprism pair is shown in Figure 11.7A. The nanoprisms have almost the same edge size of 375 nm and maximum out-of-plane height 17.1 nm as shown in Figure 11.7B by the

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typical height map along the arrowed tip–tip direction in Figure 11.7A. The real scattered color for the nanoprism pair, taken from dark-field microscopy, is shown in Figure 11.7C. Both of the nanoprisms in the pair give red color with different brightness, which might be due to variation in their surface roughness, slightly difference in the underlying surrounding dielectrics, and the focusing distance during image recording. The middle area between the two nanoprisms clearly shows more reddish color than the optical centers of the two nanoprisms. Using their CCD image (not shown here) for the location identification, together with that obtained by the DDA calculation of the nanoprism pair, the LSPR spectrum for the middle area of the two optical centers (representing the tip–tip-coupling) is recorded in Figure 11.7D. Based on the 3D morphology of the NP pair, the two nanoprisms can be treated as regular triangular nanoprisms with the bottom edge length of 375 nm, the top edge length of 125 nm, and out-of-plane height of 17.1 nm for conducting the DDA calculation

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Figure 11.7 Tip–tip coupling of one typical pair of triangular Ag nanoprisms in the arrays with interspacing of 103 nm is characterized using (A) AFM, (B) the scan of out-of-plane height of the two nanoprisms along the arrow direction in (A), (C) the color image taken from darkfield microscopy, (D-a) the LSPR scattering spectrum at the central locations (three pixels) of the source nanoprisms, and (D-b) the LSPR scattering spectrum of the pair calculated by DDA. Source: (a, c) Y. Song.

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of the nanoprism pair. The recorded LSPR spectrum (Figure 11.7D-a) at the middle optical center of the two nanoprisms shows three distinct peaks: one strongest peak at 605 nm, one shoulder at 536 nm, and one secondary strong peak at 754 nm. By comparing the experimental result for the tip–tip coupling of the nanoprism pair with the DDA calculation (Figure 11.7D-b), it can be deduced that the peak at 605 nm represents the in-plane quadrupole resonances originated from the two source nanoprisms and the peak at 536 nm is from the out-of-plane quadrupole resonances of the two source nanoprisms. Although the DDA simulation does not show one distinct peak at 754 nm, our experiment result suggest one strong peak at this wavelength, which is probably from the strong tip–tip coupling. In order to reveal whether the peak at 754 nm is mainly from the tip–tip coupling or not, the LSPR spectra from the optical centers of the source nanoprisms, are recorded (not shown here), showing one strong peak at the same position. Generally, one can see that the peak positions and shape resonances for the two nanoprisms are almost the same, suggesting that the NSL process is very powerful in the fabrication of the nanoprisms with almost identical 3D morphologies and surroundings. Both of the two triangle nanoprisms do not give the peak at 754 nm as strong as the pair, confirming that the additional peak at 754 nm indeed is from the tip–tip coupling. However, previous investigations did not show additional strong peak due to tip–tip coupling. This significant coupling between the nanopair may be caused by the unique size of our particles that is just lying in the range of half wavelength of visible light, which can cause a strong long-range electrodynamic interaction among light and the collective electrons on the particle surfaces. In addition, our experimental observations show that nanoprism coupling does not affect the quadrupole mode in LSPR significantly, resulting in little shifts in the highest peak at 598–605 nm (the in-plane quadrupole mode). However, one additional peak (i.e. 754 nm) as compared with the in-plane quadrupole mode can be observed. This peak resulted from LSPR coupling is in good agreement with the prediction by the semi-analytical model by Schatz and coworkers [5]. In the present study, the edge lengths of the triangular nanoprisms are more than 𝜆/2𝜋 (64–128 nm), which is more than the critical scale in the semi-analytical model in the DDA. Therefore, the long-range electrodynamic interaction, not electrostatic effects, will be dominant in the LSPR of the two nanoprisms. The center-to-center interspacing of the two nanoprisms is 532 nm, more than the critical interspacing. As a consequence, the coupling will be mainly determined by the long-range radiative dipolar interactions (or radiative damping effects), and phase retard effects, resulting in one new peak with wavelength more than the highest peak for the two nanoprisms. Based on this combined method, we have investigated the distance dependent tip–tip coupling between triangular Ag nanoprism pairs with dimensions at the range of half wavelength of visible light and distance ranging from 100 to 400 nm. It has been found that the coupling peak wavelength increases and the coupling intensity decreases with the increased tip–tip distance, and finally the coupling disappears (no coupling peak) when the tip–tip distance is more than about 400 nm due to the coupling intensity becomes extremely low.

11.2 Nanosphere Lithography (NSL)

Generally, the combination of NSL and the MHMW fabricated by the routine UV-LIGA shows a powerful ability not only in the identification of NPs and nanoarrays but also in the precise investigation of the fundamental theory related to the 3D morphology dependent LSPR and LSPR coupling. In our study, the detector is far-field, while the DDA calculation is based on the near-field. Thereby, the results indicate that the near-field LSPR of single NPs and the coupling signals of nanoarrays can be detected by the far-field detector if the 3D morphologies of NPs or nanoarrays can be precisely accounted for in the DDA model.

11.2.4 Development of NSL for Nanomaterials Synthesis 11.2.4.1 Aqueous Phase Ag Nanoparticles with Controlled Shapes Fabricated by NSL

The chemical and physical properties (e.g. magnetic, catalytic, or optical) of NPs depend on their sizes, shapes, and spatial arrangements. Recent investigations have demonstrated that the optical properties of noble metal NPs significantly depend on their shapes and sizes. Therefore, the ability to productively control the shape of the fabricated NPs is of much interest. Chemical synthesis of noble metal NPs relies on the ability to reduce a metal salt in a controlled environment. An alternative to solution phase NP fabrication is to fabricate NPs on solid substrates and then release them into a solution. Recent progress in NSL has shown that this method provides a good template for shape-controlled fabrication of surface-confined NPs. This also allows for flexible institutionalization on the clean surface. van Duyne and coworkers have developed this process and used it to fabricate solution phase NPs in ethanol [6]. However, their results indicated that most of the NPs in the solution have nonuniform surface morphologies with truncated tips in addition to the presence of debris and some of the NPs attached together on the glass substrate surface causing the agglomeration of the released NPs. In addition, aqueous phase NPs are expected to be more biocompatible than those in ethanol. We present a modified NSL process to fabricate Ag NPs with controlled shapes on glass substrates. These modifications were made by thermal annealing of the triangular nanoprisms, sonication to remove weak tips, slight etching of the glass substrates under the NPs, removing debris and small broken parts around the NPs on the substrates. Thiol compounds were adsorbed on the NPs prior to dislodging them into water. These aqueous phase NPs show much more uniform shapes than those obtained by the traditional NSL process and give a narrower UV–Vis absorption peak [7]. Figure 11.8a shows AFM image of Ag NP array fabricated on a glass substrate using the established NSL process after removing the NS templates. The produced Ag NPs show hexagonally arranged triangular nanoprisms. The magnified image in the insert shows that for some NPs their tips and edges have many defects due to NP damage during liftoff by sonication. From the 3D images for some of these Ag NPs (Figure 11.8b), it can be clearly seen that the Ag NPs are triangular nanoprisms with increased thickness from edges to the center (30 ± 0.5 nm). However, as observed in the insert of Figure 11.8a, many of the NPs show rough surface morphology. NP tips

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11 Template-assisted Fabrication of Nanostructure Thin Films

or edges form weak parts in the NPs which can easily break during sonication. This significantly reduces the shape uniformity of the NPs and produce debris in the solution. Transmission electron microscopy (TEM) images for these aqueous phase NPs show many small NPs present with prism shape (Figure 11.8c). The calculated edge size of the triangular prism NPs is ∼78 nm when using 290 nm diameter NSs. This size is consistent with that observed from the AFM images of unreleased NPs shown in Figure 11.8a,b. For the released NPs, the TEM image in Figure 11.8c shows the expected triangular prism shape (dashed ellipses) and some small irregular spherical shaped NPs (dotted circles). Their size distributions are shown in Figure 11.8d,e. For the triangular prism NPs, a broad size distribution of 74.3 ± 31.0 nm (in addition to some smaller triangular NPs with an edge size of ∼20–30 nm) is observed. The circular NPs had a size distribution of 10.0 ± 4.6 nm. The Ag NPs observed with one or two truncated tips (some of the dashed ellipse) are broken tips from the original NPs produced by NSL. Some of these triangular prism NPs have rough surfaces and their

100 nm 200 nm

0.2 0.4 0.6

1 μm

0.8

200 nm 1.0

(b) Frequency (counts)

10 8 6 4 2

40 20

2 4 6 8 10 12 14 20 30

140

110

80

50

Size (nm)

Size (nm)

(d)

0.4

0 20

0

(c)

60 Absorption (a.u.)

(a) Frequency (counts)

352

(e)

(f)

100 nm

0.35 0.3

605

0.25

352 100 nm 0.2 300 400 500 600 700 800 900 Wavelength (nm)

Figure 11.8 Ag NPs fabricated by the routine NSL process. (a) AFM image of an area of surface-confined triangular prism Ag NPs. The inserted magnified image shows the weak tips and rough surface morphologies. (b) 3D AFM image of the triangular prism NPs. (c) TEM images of the released aqueous phase Ag NPs show different kinds of NPs: some are the triangular prism Ag NPs with or without truncated tips (labeled with dashed ellipses and shown in the top-right inset) and some are the small NPs from debris produced during sonication (some irregular spherical debris labeled with dotted circles and some small triangular debris from broken tips of the triangular nanoprisms labeled by dashed squares). (d) Histogram of triangular shaped Ag NPs gives mean sizes of 74.3 ± 31.0 nm. (e) Histogram of the irregular spherical shaped Ag NPs gives mean sizes of 10.0 ± 4.6 nm. (f) The UV–Vis absorption spectrum for the aqueous phase NPs show two distinct peaks; one sharp peak at 352 nm from the combined contribution of the LSPR of the small Ag NPs and the high-mode LSPR of the triangular Ag NPs, another broad peak at 605 nm mainly from the dipole LSPR of the triangular Ag NPs. Source: Song and Elsayed-Ali [7]. © 2010, Elsevier.

11.2 Nanosphere Lithography (NSL)

50 nm

100 nm

Frequency (Counts)

thickness varies considerably over the triangular surface of the NPs. This roughness appears to be caused by NP damage during sonication. These size and shape features of the aqueous phase NPs are also reflected by their UV–Vis absorption spectrum, which shows two distinct absorption peaks centered at ∼352 and 605 nm, as shown in Figure 11.8f. We next investigated the shape integrity of the heat-treated NPs after releasing them into water. Figure 11.9a shows TEM image of the Ag NPs after thermal annealing without pre-sonication. Most of those Ag NPs show triangular shapes with round tips (dotted circles in Figure 11.9a) and some with snipped tips (dashed circles in Figure 11.9a). The inset is a magnified image of these NPs, clearly showing a triangular shape with round tips. The histogram for these Ag NPs (Figure 11.9b) gives a mean size of 39.6 ± 4.9 nm with much narrower size distribution of STDEV% = 12.4% than those obtained from surface-confined Ag NPs without any post-annealing (Figure 11.8d, STDEV% = 41.7%). Figure 11.9c is a TEM image for Ag NPs that were thermally annealed after removing two tips by sonication, whose histogram gives a mean size of about 33.9 ± 6.8 nm (Figure 11.9d), less than that for those triangular shaped NPs with round tips after post-annealing. Most of these NPs show quadrilateral shapes (dashed circles) or pentagon shapes as shown more clearly in the inset of Figure 11.9c. These NPs have a similar shape as those

15 10 5 0

35

40 45 Size (nm)

50

55

50 nm

Frequency (Counts)

(b)

(a)

16 12 8 4 0

25

30

35

40

45

50

55

60

65

Size (nm)

100 nm

(c)

30

(d)

Figure 11.9 TEM images of the aqueous phase Ag NPs after surface modification by thiol compounds and dislodging from the glass substrate. (a) Triangle Ag NPs with round tips. Dashed circles: Ag NPs with round tips; dotted circle: Ag NPs with slightly round tips. (b) Histogram of triangular shaped Ag NPs with round tips based on 45 NPs giving a mean size of 39.6 ± 4.9 nm. (c) Quadrilateral and pentagon-shaped Ag NPs. Dashed circles: some typical Ag NPs with quadrilateral shapes. (d) Histogram of quadrilateral and pentagon-shaped Ag NPs based on 45 NPs giving a mean size of 33.9 ± 6.8 nm. Source: Song and Elsayed-Ali [7]. © 2010, Elsevier.

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11 Template-assisted Fabrication of Nanostructure Thin Films

476

672

(a)

Absorption (a.u.)

354

352

(b) 470

300

400

626

532

(c)

500

600

700

800

Wavelength (nm)

Figure 11.10 Optical absorption of Ag NPs. (a) Surface-confined Ag NPs before surface modification. (b) Surface-confined Ag NPs after surface modification with thiol. (c) Aqueous phase Ag NPs after releasing the surface-confined NPs into water. Source: Song and Elsayed-Ali [7]. © 2010, Elsevier.

observed by AFM images in Figure 11.3e,f. From the TEM images in Figure 11.9c, some of the NPs give less contrast in their central parts (NPs labeled by dashed circles). We believe that the lighter centers in these NPs are from a thinner center resulting from adhesion of the center of these NPs to the glass substrate during annealing. AFM observation of the glass substrate after removal of the NPs show debris forming hexagonal shaped arrangements. This observation is consistent with adhesion of the central part of the triangular prism NPs to the substrate. The UV–Vis absorption spectra of the surface-confined NPs change significantly before and after surface modification. Figure 11.10a shows the absorption spectrum of surface confined Ag NPs fabricated by NSL without any modification. It can be seen that the absorption spectrum for the Ag NPs yields two distinct peaks at 476 and 672 nm. Based on previous investigations on these NPs and their arrays [6, 8, 9], it is reasonable to expect that the absorption peak at 476 nm is primarily from the higher order mode SPR (e.g. quadrupole) of the NPs, and the peak at 672 nm is primarily from the dipole resonance of the NPs. We note that the higher order resonance peak has almost the same intensity as that for the dipole resonance, although the higher order modes are expected to be much weaker than the dipole resonance. Since the substrate is continuously covered by a hexagonally arranged array of Ag NPs with tip–tip distance less than 100 nm, we postulate that particle–particle coupling will contribute to the LSPR spectrum. This particle–particle interaction effect could be responsible for the observed spectrum. Before releasing the Ag NPs into water, their surfaces must be modified by a water-soluble stabilizer. We used the combination of 1-OT and 11-MUA. The surface modification of the Ag NPs was conducted using the routine thiol coupling chemistry. In order to confirm that the thiol compounds are linked on the Ag NPs, X-ray photoelectron spectroscopy (XPS) spectrum for the surface-confined Ag NPs after modification by 1-OT and 11-MUA was obtained. Before XPS was performed,

11.2 Nanosphere Lithography (NSL)

Ag3d5 Ag3d3 Ag3s Ag3p3

C2s

O1s

C1s

Ag4p S2p Si2p

Ag4s Ag3p1 1000

900

800

700

600

500

400

300

Si2s 200 100

0

Binding energy (eV)

Figure 11.11 XPS spectrum for the surface-confined Ag NPs after surface modification by 1-OT and 11-MUA. Source: Song and Elsayed-Ali [7]. © 2010, Elsevier.

the glass substrate with the modified Ag NPs was thoroughly washed by ethanol to remove the unbound thiol compounds (1-OT, 11-MUA, etc.). The obtained XPS spectrum in Figure 11.11 shows binding energy peaks for Ag3s (700 eV), Ag3p1 (608 eV), Ag3p3 (578 eV), Ag3d3 (376 eV), Ag3d5 (375 eV), Ag4s (334 eV), Ag4p (69 eV) in addition to peaks for C1s (292 eV), C2s (15 eV), O1s (537 eV), and S2p (170 eV) originating from the thiol compounds. The height of the Ag NPs before and after the thiol modification was also measured by AFM; it increased by ∼2.9 nm after the thiol modification. Since the calculated height of MUA is ∼1.7 nm, the adsorbate may be one layer formed by mono or double assembly of MUA and 1-OT molecules. The variation between the calculation and the AFM result may be resulted from the height increase of NPs during immersion in the thiol solution, similar to the solvent annealing effect [10–12]. These experimental results confirm that at least one layer of thiol compounds is bonded on the Ag NP surface. Since this adsorbed layer is only ∼2.9 nm thick, it is not expected to produce significant change to the surface dielectric properties of the Ag NPs. However, the silver–sulfur bonding between the NPs and the thiol compounds may affect the conductivity and the number of free surface electrons in the Ag NPs. The absorption spectrum for the modified Ag NPs (Figure 11.10b) shows a slight blue shift at the peak at 476 nm (to 470 nm) and a significant blue shift at 672 nm (to 626 nm) with reduced intensities. This spectrum was expected to give a red shift due to the increased dielectric constant from the adsorbed thiol compounds [13]. We attribute this blue shift to shape variation (e.g. increased height, round tips, smoother surface topography) during surface modification by immersion that was similar to solvent annealing which results in blue shift of LSPR since any solvent annealing has not been done on our NPs [10–12]. These variations are further observed by the slightly reduced NP size and round shapes observed in the TEM image in Figure 11.9 when compared with the AFM image in Figure 11.3. In addition, when the Ag NPs are covered by thiol groups, the surface free electron density may be reduced, leading

355

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11 Template-assisted Fabrication of Nanostructure Thin Films

to weaker SPR in single NPs and SPR coupling among NP arrays [14]. This will also result in a blue shift of the LSPR peak and a reduced LSPR intensity. The UV–Vis absorption spectrum of the Ag NPs after release in water, shown in Figure 11.10c, was compared to surface-confined NPs. The aqueous Ag NPs give a main peak at 532 nm and a very weak peak at 352 nm. The main peak at 532 nm appears to be from LSPR by the triangular prism NPs with round tips and is blue-shifted from that obtained for NPs fabricated by the routine NSL and released from the surface that showed LSPR peak 605 nm. This is attributed to the reduced size and round tips. The peak at 352 nm in Figure 11.10c becomes much weaker and narrower than that for the aqueous Ag NPs released from the surface-confined Ag NPs as fabricated by routine NSL (shown in Figure 11.8f), obviously due to the shape variation of NPs and almost no small spherical shaped debris observed in the aqueous Ag NPs released from the surface-confined Ag NPs fabricated by the modified NSL (Figure 11.10a). By comparing the TEM images for the two kinds of Ag NPs, it can be deduced that the peak at 352 nm in Figure 11.10c is mainly from the out-of-plane quadrupole resonance of Ag nanoprisms with round tips according to previous investigation. The peak intensity ratio between the main peak at 532 nm and the weak peak at 352 nm for these NPs is ∼11.5 : 1 (after subtracting the background), which is much higher than that for the NPs obtained by the routine NSL and releasing process (1 : 3.6). Clearly, the number of small debris caused by sonication is greatly reduced using the modified NSL and releasing process. The modified NSL process favors the formation of uniform Ag NPs with round tips with significant reduction in Ag debris, as shown in Figure 11.9. In addition, 1-OT and 11-MUA can be substituted by the combination of 1-BT and TP, or MCH and MUA for more water-soluble NPs. 11.2.4.2 Ultrathin Nanopore Arrays with Uniform Size

The magneto-optical properties and the related proximity effect rely on the detailed morphology of each component and the dielectric properties of the media. The correlation between these parameters and their magnetic properties, SPR and magneto-optical properties has not been well characterized, even though some progress has been achieved. Particularly, research on how to utilize proximity effects among different components to improve the magneto-optical effect at low energy loss, critical for many applications, is just in its infancy. Most current studies have investigated ensembles of NPs in which it is difficult to guarantee homogeneity of composition, structure, and morphology by conventional preparation methods. This makes it difficult to identify the intrinsic proximity effects from the nano-structures and associated interparticle interactions, which are necessary for precisely addressing the relationship between structures and their properties for high performance. Therefore, it is essential either to characterize single nanostructures or to develop controlled fabrication processes for size homogeneity [15]. In this chapter, we introduce an NS TA multistep sputtering technique to fabricate multi-layered uniform nanoporous films, using nanoporous Ag/CoFeB/Ag films as examples. Structural parameter dependent magnetic properties, LSPR and light reflection tunable by a magnetic field at different wavelengths (magneto-optical

100 Counts (numbers)

Counts (numbers)

11.2 Nanosphere Lithography (NSL)

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80 60 40 20 0 775 806 838 863 Diameter (nm)

934 952 967 988 1012 Diameter (nm)

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(a)

100

888

2 μm

(b)

Figure 11.12 SEM images of the monolayer nanosphere template (a) and the formed periodic nanoporous structure (b). The insets show the diameter histograms of nanospheres and nanopores. Source: From Song et al. [15]. Licensed under CC BY 3.0.

Kerr effect) in these hybrid nanostructures are experimentally investigated and theoretically analyzed. 11.2.4.3 Fabrication of Periodic Uniform Nanoporous Films with Controlled Layers

NSs with size dispersion below 4% can self-assemble into a uniform monolayer on a large scale (exceeding 5 mm × 5 mm) using a modified drop-coating process. Figure 11.12a shows a typical SEM image of the monolayer NS template with an NS diameter of 970 ± 19 nm. Using this template, uniform and periodic nanopore films with controlled layers can be fabricated by a multi-step magneto-sputtering process. As shown in Figure 11.12b, the resulting nanopores have a diameter of 840 ± 29 nm, and the thinnest pore wall is about 130 nm thick. The calculated area of the walls is around 37.5 V% of the total film. It is interesting to compare the uniform porous nanostructures shown in Figure 11.12 with our previous reports on the fabrication of separated triangular or square shaped NP arrays by thermal evaporation. The formation of a periodically arranged nanoporous film instead of separated NPs may be attributed to the effects from the high energy sputtered atoms/clusters having sufficient kinetic energy to penetrate the triangular holes between the NSs. This result provides us a new method to prepare surface confined uniform nanopore arrays using NS templates over a large scale, whose pore diameters and periods can be controlled by the diameter of the NSs. Results on the magnetic properties (Figure 11.13, Table 11.1) of these periodic nanoporous films suggest that enhanced magnetic response can be induced by the Ag layers and the nanopores even though they both hinder the rotation of magnetic domains away from the in-plane direction. The nanopores can result in a strong pinning of MDW, an increased local shape anisotropy and non-uniform spin distribution from the local dipolar field around the nanopores. The Ag layers mainly produce a strong interfacial magneto–electric coupling due to the increased conductivity, the pinning effect of Ag layers and the mixing of itinerant electrons with ferromagnetic d electrons at the Ag–CoFeB interface. Fabrication

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11 Template-assisted Fabrication of Nanostructure Thin Films

80

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5000

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Figure 11.13 Room temperature hysteresis loop of CoFeB film (A), nanoporous CoFeB film (B), Ag/CoFeB/Ag film (C), nanoporous Ag/CoFeB/Ag film (D), and nanoporous Ag/CoFeB/Ag without removing nanospheres (E). All nanoporous films have hexagonally periodic nanopores of circular shape. (a) In-plane magnetization (with the magnetic field parallel to the film surface); (b) out-of-plane magnetization (with the magnetic field perpendicular to the film surface). Source: From Song et al. [15]. Licensed under CC BY 3.0.

Table 11.1

| In-plane (IP) and out-of-plane (OP) magnetic properties of samples.

Magnetization direction parameters

Samples

IP Hc Oe

He Oe

Mr / Ms

OP HK Oe

Hc Oe

He Oe

Mr / Ms

HK Oe

CoFeB film

14

0

0.730

7

123

1

0.064

1958

CoFeB nanopore film

18

0

0.748

9

69

5

0.039

1487

0.213

965

Ag/CoFeB/Ag film

25

8

120

111

58

0.070

Ag/CoFeB/Ag nanopore film

35

20

0.182/ 221 0.101a)

122

91

0.091/ 1796 0.065a)

Ag/CoFeB/Ag nanopore film with nanospheres

32

13

0.173/ 290 0.088a)

95

59

0.092/ 1440 0.075a)

a) Right/Left: Mr./Ms. from the top/bottom half of the hysteresis loops. Source: From Song et al. [15]. Licensed under CC BY 3.0.

11.3 Anodic Aluminumoxide (AAO) Template-assisted Nanoimprinting

of nanopores in the CoFeB continuous thin film increases the field parallel to the film plane (IP) ferromagnetic character but reduces the field perpendicular to the film plane (OP) ferromagnetic character based on the changes in the squareness (remanence/saturation magnetization: M r /M s ) and incoercivity H c . However, from the M r /M s , H c and exchange bias H e of films with Ag layers, we infer that the formation of nanopores can increase the OP ferromagnetic character much more than the IP ferromagnetic character of the Ag/CoFeB/Ag film, due to the additional vertical magnetism caused by the much enhanced pore and Ag layer pinning effects perpendicular to the film plane. Therefore, Ag layers have the potential to improve the magnetic performance of the nanoporous magnetic film as a perpendicular recording media. According to the observed changes H c , H e and OP M r /M s , a nanoporous film with Ag layers has a strongly enhanced ferromagnetic character, exceeding that caused by the simple addition of the Ag layers or nanopores alone. Thus, there exists a synergistic effect between the nanopores and Ag layers in improving the ferromagnetic character of the magnetic film. In conclusion, a methodology based on NS templating and multistep magneto-sputtering process is successfully developed to fabricate surface confined uniform multilayered nanoporous arrays over a large scale. These hybrid nanostructures show unique magnetic and LSPR properties due to a synergistic interaction of periodic nanopores and Ag layers, compared with those films without the nanopores and/or Ag layers. This synergistic interaction can be used to improve the ferromagnetic character of the magnetic film or to tune their optical properties. By controlling the geometry of the formed nanostructures, it is possible to either enhance the light extinction by the synergistic effect of nanopores and surface magneto-plasmonic interference (such as in the nanoporous Ag/CoFeB/Ag film without removing NSs) or to foster the light transmission through the film (such as in the nanoporous Ag/CoFeB/Ag film) for different applications. The nanoporous Ag/CoFeB/Ag film shows strongly enhanced magneto-optic effects both in peak intensity and in peak wavelength changes as compared with the CoFeB film, the nanoporous CoFeB film and their precursors with NSs due to the magneto–-plasmonic interaction between the magnetic layer and the LSPR layer, endowing them with great potential for applications in high sensitivity magneto-optic sensing systems, such as optical transistor, circular isolator, active waveguides and gratings, or magnetic field sensors.

11.3 Anodic Aluminumoxide (AAO) Template-assisted Nanoimprinting 11.3.1 Introduce of AAO Template AAO templates have the advantages of low cost, high maneuverability, high temperature resistance, insulation, and highly ordered pore structure [16]. They have many applications in the field of nanomaterials, such as preparation of nanoarrays, nano-replication, preparation of quantum dots, etc. In 1953, Keller et al. first set up

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11 Template-assisted Fabrication of Nanostructure Thin Films

the theoretical model of anodic alumina, becoming a fundamental explanation for the growth mechanism of anodic alumina [17]. In the past few decades, there have been new theories, such as field dissolution model [18], volume expansion theory, isofield strength model, viscous flow theory, and so on, which have provided rich theoretical knowledge for the development of AAO template and brought new results to the preparation technology and practical application of AAO template. AAO is thin film material with unique nano-porous structure, which consists of substrate, aluminum barrier layer and porous layer (alumina). The porous layer is composed of hexagonal stacked cells. The center of the cell is a cylindrical hole growing vertically along the surface. The general pore depth refers to the thickness of the AAO alumina layer, the pore channels are parallel to each other, by the bottom barrier layer to connect the pore channel and the unoxidized metal, as shown in Figure 11.14. AAO template is prepared by anodic oxidation of aluminum sheet in acid (sulfuric acid, oxalic acid or phosphate acid, etc.) electrolyte under appropriate conditions. A highly ordered nanoporous array film with self-organization can directly regulate the structure of alumina film by changing the type of acidic solution, mediating the concentration of electrolyte, reaction time, adding voltage or other conditions in the oxidation process. Such as hole shape, hole depth, hole spacing, pore size, and so on. As a carrier of nanomaterials and templates for preparing ordered nanostructures, AAO has flexible and variable structural parameters, which can accurately regulate the size, morphology, distribution position, growth direction and nanostructure spacing of nanostructures, thus affecting the performance of nanostructures. AAO template is very inclusive to various materials, and it has important applications in chemical detection of photoelectric conversion devices such as plasma optical surface enhancement spectroscopy [19]. In terms of plasma optics, for example, using an AAO template as a carrier, aluminum NPs are supported on the AAO hole wall by vacuum deposition. Using

(a)

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Barrier layer Al base

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(d)

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(e)

150 nm 15

500 nm 50

Figure 11.14 (a) AAO template structure model; (b) AAO template side SEM; (c, d, e) are AAO templates SEM with each aperture and different structure, respectively. Source: (b–e) Zhu et al.

11.3 Anodic Aluminumoxide (AAO) Template-assisted Nanoimprinting

such a structure as a solar seawater desalination film can increase the absorption efficiency of visible light in the full band, improve the energy conversion efficiency, localize the generated heat, accelerate the evaporation desalination of seawater, and reduce the salinity of seawater by four orders of magnitude [20]. At the same time, this kind of seawater desalination device based on plasma optics is more portable and stable, and its conversion efficiency remains basically unchanged after 25 cycles. Through finite-difference time-domain (FDTD) simulation, it is found that the coupling between AAO pore wall and aluminum NPs with oxide layer is the key factor to improve the absorption efficiency of visible light in the full band. AAO as the waveguide layer can be coupled. Metal plasmon resonance enhances surface-enhanced Raman scattering (SERS) signal and is used for the detection of immune system [21]. The SPR mode of Au and the photonic mode of cadmium sulfide (CdS) complement each other, so that the device can exhibit high intensity broadband absorption in visible light region. Both of them are wavelength dependent, which can further improve the performance of the device by regulating the nanostructure of Au and the thickness of CdS [22]. In the aspect of surface enhancement spectroscopy, the pore structure based on AAO template, such as the construction of finite field with AAO and spheres cluster silver NRs. Under the action of capillary force, these silver NRs with high aspect ratio will come together to reduce the gap between NRs, enhance the local electric field intensity and improve the sensitivity of SERS.

11.3.2 Preparation Methods 11.3.2.1 Preparation of Anodic Alumina Template

In our experiment, anodized alumina template was prepared by two-step anodizing method. For the treatment of aluminum foil, an aluminum foil with purity of 99.999% and thickness of 0.3 mm was selected. A 3 cm × 3 cm square aluminum foil was cut and soaked and cleaned in acetone for 20 minutes at room temperature and then again in deionized water for 10 minutes at room temperature; this process was repeated three times to remove impurity particles after reaction with acetone. This was followed by ultrasonic cleaning with anhydrous ethanol for 10 minutes at room temperature for three times. Finally, the foil was soaked and cleaned with deionized water for 10 minutes at room temperature for three times. The cleaned aluminum foil was put into a tubular furnace with argon and treated at 400 ∘ C for 2 hours. To produce an anodizing reaction, a graphite was used as a cathode and treated aluminum foil as anode. In the first anodizing in oxalic acid solution (0.3 M) at 40 V for 2 hours, the whole reaction device should be placed in the cooling system to ensure the reaction temperature at 10 ∘ C and stirring repeatedly in the reaction process. The anodized aluminum foil was transferred to the mixed solution of chromic acid (0.2Mol/L) and phosphate (0.4Mol/L) and cleaned at 80 ∘ C for 1 hour, and the cleaned aluminum sheet was used as anode, graphite as cathode, and electrified in oxalic acid solution (0.3 M) for the second anodization. Final cleaning dry naturally for use.

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100 nm

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Figure 11.15 AAO template prepared by two-step anodic oxidation (a) The aperture is 30 nm. (b) The aperture is 50 nm. (c) The aperture is 200 nm. (d) Profile of anodized alumina. The hole depth was 300 nm and the hole spacing was about 110 nm. Source: Zhu et al.

The anodic alumina templates prepared by two-step anodizing at 30, 50, and 200 nm are shown in Figure 11.15, where we can see that the porous alumina template presents a vertical surface void, a more regular hexagonal honeycomb structure. Except for the change of pore size, all the anodic alumina templates prepared have the same parameters, the depth of holes is 300 nm, and the inter-hole spacing is about 110 nm. 11.3.2.2 Preparation of Thin Film Substrates for Surface-enhanced Raman Scattering

SERS has been widely used in interface, surface science, single molecule detection, cancer detection, sensing devices and microfluidic chip owing to its unique fingerprinting, high sensitivity, high accuracy, and simple operation. On the basis of the improvement of its enhancement mechanism, the SERS substrate with excellent performance is a key factor to promote the development of SERS technology. The results show that nanomaterials have great advantages in surface-enhanced Raman substrate, among which gold, silver, copper, aluminum, and other active precious metal nanomaterials have more outstanding properties, so the morphology

11.3 Anodic Aluminumoxide (AAO) Template-assisted Nanoimprinting

is diverse. Three-dimensional nanomaterials were designed. With the help of alumina template to assemble the metal porous structure of nanoarray, it can provide sufficient “hot spot” for SERS, which is based on SPR. In addition, in the past few years, researchers have designed and studied surface-enhanced Raman substrate based on a hard substrate (glass, quartz or silicon), but most of them have some limitations in practical detection because of their lack of flexibility, low transmittance, low contact area with detected molecules, and so on. Recently, researchers have begun to show great interest in substrate based on macromolecular transparency, flexibility, and other materials, which can improve the defects of traditional hard substrate, and are easier to prepare and cheaper to use. Therefore, we have designed a novel SERS substrates based on nano-porous gold film deposited on the transparent materials of polymethyl methacrylate and the transparent and flexible materials of polydimethyl siloxane, which are fabricated by magnetron sputter techniques and template transfer methods using AAO as templates [23]. The properties of substrate with different nanometer pore size under the same thickness of gold NPs were studied in detail. At the same time, gold NPs are introduced into the substrate system in order to produce the coupling resonance of local surface plasma and propagating surface plasma, so as to produce “hot spot” effects.

11.3.3 Preparation and Characterization of Nanoporous Gold Thin Films Based on Anodic Alumina Template (AAO) The anodic alumina templates with different pore sizes prepared by two-step anodic oxidation were selected for backup: (1) The AAO template is fixed on the cleaned slide with double-sided small film, and then placed on the sample table in the center of the magnetron sputter cavity. In order to ensure the uniform deposition of the thin film, the rotation speed of the sample table is fixed at 20 rpm. (2) The mechanical pump using magnetron sputter vacuum to 10−1 Pa, and then pump the vacuum to 10−4 Pa, Ar gas flow 80 cm by molecular pump, Au target, DC sputter, power at 50 W. (3) Nano-porous Au thin films with the same film thickness and different pore size were obtained by changing the pore size of AAO template. The pore size of AAO template was fixed, the sputtering time was changed, and the nanoporous. Thin films with the same pore size and different film thickness can be obtained. Figure 11.16 shows the SEM morphology of Au/AAO nanoporous thin films. The thickness of Au thin films is 100 nm, and the pore diameters of AAO are 30, 50, and 200 nm, respectively. It can be found from the SEM topography that all the nano-films have successfully inherited the structural characteristics of AAO and formed a large area array pore structure. The whole surface of the film is flat and uniform, but when the observation multiple is magnified, it can be seen that there is a slightly protruding hexagonal petal structure around the hole of the film. When the pore size is 50 or 30 nm or even smaller, each small petal of the hexagonal petal shows a more regular triangle. In shape, the gap between small petals is much

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Figure 11.16 Au/AAO thin films using AAO templates with pore size of (a) 30, (b) 50, and (c) 200 nm, prepared by magnetron sputtering vapor deposition process. Source: Zhu et al.

narrower than that between thin film petals when the pore size is larger 30 nm, and the more detailed gap is often present in a place where the plasmon resonance effect is stronger. With the increase of pore size, the hexagonal petal structure gradually degenerates into hexagonal structure, and the plump petal is gradually marginalized and the pore characteristics are highlighted, which means that the easier the target atom is to deposit and grow into the pore wall and hole, and the larger the gap between NPs, the SPR or coupling resonance effect may be weakened. 11.3.3.1 Substrate Transfer and Characterization of Nanoporous Gold Thin Films

The anodic alumina template attached Au thin film prepared by two-step anodizing method was selected as sample I. (1) A layer of PMMA solution with visible thickness was spin-coated on the surface of the gold nanofilms of sample I and then transferred to an electrothermal thermostatic blast dryer 50 ∘ C for drying and curing for 1.5 hours as sample II. (2) The II sample was immersed in 5–8% dilute HCl until the bottom AAO template was completely dissolved. In the whole process, it can be observed that the bright silver surface of the bottom layer gradually dissolves with the emergence of a large number of bubbles around the sample. (3) Ethanol and deionized water were used to clean and dry naturally, and the front of the gold film was inlaid in the cured PMMA, and the opposite side was exposed without the attachment of AAO. In this way, the nano-porous gold film based on

11.3 Anodic Aluminumoxide (AAO) Template-assisted Nanoimprinting

PMMA is obtained. Figure 11.17 shows the transfer process of nano-porous gold film substrate. Figure 11.18 is a PMMA/Au nanoporous film obtained by substrate transfer of Au/AAO nanoporous thin films. Because the substrate transfer is to immerse the front of Au film in PMMA and dissolve the AAO which binds directly to the opposite side of Au, what we can see directly on the front is the opposite side of Au film. The pores of the morphologically thin films change after the pore size is small (30, 50 nm). Most of the growth particles of 30 nm thin films are deposited on the front, so the pores on the opposite side are relatively larger. The growth particle deposition of 50 nm thin films is more uniform, so the pores on the opposite side are only a little smaller than when the substrate is not transferred, but the overall pores are still slightly larger than those on the reverse side of 30 nm thin films. For 200 nm

Pore

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Metal layer

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Figure 11.17

Substrate

Substrate transfer process of porous gold films.

100 nm

100 nm 50 nm

30 nm (a)

(b)

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Figure 11.18 Au/PMMA nanoporous thin films with PMMA as substrates and AAO as templates with pore sizes of (a) 30, (b) 50, and (c) 200 nm, respectively. Source: Zhu et al.

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11 Template-assisted Fabrication of Nanostructure Thin Films

thin films, because the pore size is too large, some of the NPs fall completely into the pores. PMMA is a hard material, so that the NPs in the pores and the area between the pores are similar to the reverse buckle and gently pressed on the PMMA adhesive. As a result, such a flat elliptical structure is formed, and small pores appear where there are fewer particles falling into the pores. The anodic aluminum template attached Au thin film prepared by two-step anodizing method was selected as one sample: (1) Polydimethylsiloxane (PDMS) and its own curing agent were equipped with PDMS, cold storage for 10 hours at the proportion of 10 : 1. On the surface of gold nanofilm of sample I, a layer of visible PDMS solution with thickness was rotated on the surface of gold nano-film, and then it was dried and solidified in 50 ∘ C in an electrothermal constant temperature blast drying box for 1.5 hours as I-1 sample. The I-1 sample was immersed in 5–8% dilute HCl until the bottom AAO template was completely dissolved. In the whole process, it can be observed that the bright silver surface of the bottom layer gradually dissolves with the emergence of a large number of bubbles around the sample. Ethanol and deionized water were used to clean repeatedly and dry naturally. At this time, the front of the gold film was inlaid in the film-forming PDMS, and the opposite side was exposed without the attachment of AAO. In this way, the nano-porous gold film based on PDMS was obtained. Figure 11.19 is a PDMS/Au nanoporous film obtained by substrate transfer of Au/AAO nanoporous thin films. Because the substrate transfer is to immerse the front of the Au film into the PDMS and dissolve the AAO directly bonded to the

100 nm

100 nm 30 nm

(a)

50 nm

(b)

100 nm 200 nm

(c)

Figure 11.19 PDMS/Au nanoporous thin films using PDMS as substrates and AAO as templates with pore sizes of (a) 30, (b) 50, and (c) 200 nm, respectively. Source: Zhu et al.

11.3 Anodic Aluminumoxide (AAO) Template-assisted Nanoimprinting

opposite side of the Au, what we can see directly on the front is the negative side of the Au film. PDMS is a flexible material, and the overall morphology and the PMMA as the substrate have obvious changes, the previous hexagonal petal has been completely immersed in the flexible PDMS film, and the reverse side is similar to that of half a ball with holes. The rule is most obvious at 30 nm, and the hole is smaller at 50 nm. It is worth noting that the morphology of 200 nm is almost the morphology of nano-lattice, but it is the shape of half a sphere, and the properties of nanomaterials are closely related to the morphology. 11.3.3.2 Preparation and Characterization of Aqueous Spherical Gold Nanoparticles

Here, 0.1 g sodium borohydrate (NaBH4 ) was dissolved into 50 ml deionized water as a strong inductance solution in the chemical reduction process. 0.25 g polyvinylpyrrolidone (PVP) was used as protective agent and 0.3 g HAuCl4 ⋅H2 O raw material was dissolved into 50 ml deionized water. The two solutions were simultaneously transported to two microfluidic channels for reaction at a temperature of 800 ∘ C and injection rate of 1 ml/min. A small amount of reaction solution was added with acetone (the proportion of 1 : 1–2) for centrifugation. Then mixed with deionized water and diluted for several times. After each dilution, ultrasonic examination was conducted for 120 seconds, and finally the non-agglomerated gold NPs with a concentration of 10 μm/ml were obtained. The morphology features were Figure 11.20a,b, and the particle diameter was about 50 nm. According to the UV–Vis spectroscopy absorption curve Figure 11.20c, it can be seen from the figure that the absorption characteristic peak of the spherical gold NPs is at 526 nm, which is very similar to the laser wavelength of 532 nm of the Raman spectrometer selected in the experiment. 11.3.3.3 Composite of Nanoporous Gold Thin Films and Aqueous Gold Nanoparticles

Ultrasound was performed for 120 seconds after 40 l × 10−5 mol/l R6G solution and 360 l × 10 m/ml water-based gold NPs were mixed. A uniform mixture of 10.6 mol/l R6G solution and 9 m/ml aqueous gold NPs was obtained. After sucking the mixture drop by drop to the pre-characterization of the gold film with a straw, the mixture was placed in an electric thermostatic blast drying oven, and the temperature was set at 40 ∘ C for 2 hours.

11.3.4 AAO Template-assisted Fabrication of Ultra-dense Nanoparticle Arrayed Thin Films Our group developed a new method to prepare ultra-dense NP arrayed thin films. It can be obtained by the following fabrication process: (i) deposition of a Au layer no more than 10 nm on the AAO templates with pore diameter of 10 nm and thickness below 150 nm and (ii) etching off the AAO templates without annealing the Au coated AAO template. Figure 11.21a is a wide angle SEM image of the Au NP array on a (100) Si substrate formed by thermal evaporation of 10 nm Au on

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11 Template-assisted Fabrication of Nanostructure Thin Films

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Figure 11.20 (a) SEM morphology of Au nanoparticles (NPs). (b) SEM morphology of Au(Ps). (c) The absorption spectrum of Au(Ps). Source: (a, b) Zhu et al. Copy permission from Nanoscale, Royal Society of Chemistry.

the AAO template. Figure 11.21b is a magnified SEM image of the close-packed Au NP array. Their size and interparticle edge distribution are summarized in supporting information Figure 11.21a,b, giving a mean diameter of 10.1 ± 1.9 nm and inter-edge spacing of 5.0 ± 1.7 nm. The area density of these Au NPs is as high as 1.2 × 1012 cm−2 , higher than the highest pore density of the AAO templates (∼1011 cm−2 ) and much higher than the reported NP arrays prepared using AAO templates (∼1010 cm−2 ) [24–27]. Energy dispersive X-ray (EDX) measurements on the NP arrays (Figure 11.21c) indicate that the AAO template is almost completely removed by the etching process since no Al is detected. Trace oxygen detected in the Au NP array is likely from the SiO2 formed during the anodizing process, consistent with previous studies [24–26]. Without annealing the physical vapor deposited layer, the crystallinity of these Au NPs is likely not so perfect since some defects and amorphous areas may be formed in the NPs. However, they may be

11.3 Anodic Aluminumoxide (AAO) Template-assisted Nanoimprinting 3500

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Figure 11.21 (a) A typical SEM image for the Au nanoparticle array on a Si substrate prepared by deposition of 10 nm Au on a AAO template with pore diameter of 13.8 ± 2.4 nm and etching off the AAO template, without annealing; (b) a magnified detailed view of the Au nanoparticle array in a; (c) an EDX spectrum of the Au nanoparticle array in a and b, showing Si, Au and O; (d) a typical SEM image of the Au nanoparticle array prepared by deposition of 6 nm Au layer on a AAO template with pore diameter of 13.8 ± 2.4 nm, and etching off the AAO template after annealing the 6 nm Au layer under flowing N2 at 400 ∘ C for 2 hours, and in air at 200 ∘ C for 2 hours; (e) a wide-angle view of the Au nanoparticle array in D; (f) a EDX spectrum of the Au nanoparticle array in a and b, showing Si, Au, Al, and O; (g) a typical SEM image of a 6 nm thick Au layer coated AAO template on a Si substrate after physical vapor deposition of the Au layer; (h) a typical SEM image of a 6 nm thick Au layer coated on a naked Si substrate, after annealing under flowing N2 at 400 ∘ C for 2 hours and then in air at 200 ∘ C for 2 hours; (i) a typical SEM image for the Au nanoparticles after annealing (under N2 at 400 ∘ C for 2 hours, and in air at 200 ∘ C for 2 hours) and etching and annealing at 800 ∘ C in air for 2 hours. Source: Song et al. [28]. © 2013, Royal Society of Chemistry.

desired as efficient catalysts due to these active amorphous sites existing in these NPs [28]. In order to improve the crystallinity of these Au NPs, annealing can performed prior to etching off the AAO templates, or annealing the Au coated AAO templates in flowing nitrogen for 2 hours at 400 ∘ C. Figure 11.21d is the wide angle view (∼7 μm × 10 μm) of the NP assemblies obtained by deposition of 6 nm Au on the AAO template and using the aforementioned annealing process, which can

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11 Template-assisted Fabrication of Nanostructure Thin Films

be uniformly dispersed over the whole substrate with an area of 2 cm × 3 cm. Figure 11.21e is a typical magnified SEM image for Au NPs arrays. Their size distribution and inter-edge spacing distributions are plotted in the supporting information, as Figure 11.21d,e; this gives a mean diameter of 12.5 ± 2.9 nm, a mean inter-edge spacing of 7.9 ± 3.4 nm and an areal density of 5.6 × 1011 cm−2 . As compared with the corresponding image taken without annealing (Figure 11.21b), the micrograph of these Au NPs prepared from the annealed Au layer on the template show superior contrast between the edge rims of these NPs and the substrate, suggesting an improved crystallinity. In order to further confirm the annealing effect, X-ray diffraction (XRD) measurements were conducted for these dense Au NP arrays prepared from the Au-coated AAO templates with and without annealing at 400 ∘ C in nitrogen flow. As shown in Figure 11.22b, two distinct peaks (open triangles) at 38.0∘ and 44.1∘ are clearly visible in the XRD pattern of the NP arrays prepared from the annealed 6 nm coated AAO template clearly, representing (111) and (200) planes of FCC Au [29]. The XRD pattern of the NP arrays prepared from the 10 nm coated AAO template without annealing (Figure 11.22a) only shows one broad peak at 38.0∘ . The FWHM of the peaks at 38.0∘ are about 0.60∘ for the samples without annealing and 0.94∘ for the annealed samples, giving the size of the samples without annealing of 8.9 nm and the size of the annealed samples of 14.0 nm by Scherrer equation [30]. The smaller size by XRD than by the SEM observation for the samples without annealing and the larger size by XRD than by the SEM observation for the annealing samples further confirm that the crystallinity of the Au NPs is indeed improved by post-annealing [30]. In addition, the weak peaks (solid triangles) at 32.2∘ and 42.0∘ indicate that some Au–Si phases are formed. The other trace peaks including 28.0∘ and 55.9∘ (void diamonds) may be from Al–Si–O or Au–Si–Al–O compounds or other Al compounds (e.g. α-Al2 O3 ) [31]. EDX measurements (Figure 11.21f) for the annealed samples indicate the presence of Al and O, besides Si and Au. This likely originates from a thin layer of Al–Si–O or Au–Al–Si–O on the Si substrate, probably formed during the second annealing process at 400 ∘ C and which cannot dissolve into the 5 wt% H3 PO4 etching solution completely. Prior to annealing, the Au-coated AAO templates were examined by SEM to discern at which fabrication stage the NPs began to form. As shown in Figure 11.21g, NPs with almost the same size and inter-edge spacing can be observed after just 6 nm thick of Au are coated onto the AAO templates, even though most of NPs are elliptical. This suggests that these small NPs form on the AAO templates immediately during the vapor deposition process. As a self-consistency experiment to check the effect of the AAO template, the same process was performed by depositing 6 nm of Au directly on to the naked Si substrates. We found that larger Au NPs with a variety of morphologies and a much broader size distribution were formed (Figure 11.21h). Clearly, the ultrathin AAO template plays an important role in the formation of the dense Au NPs arrays, possibly due to interface interactions between the Au layer and the underlying Al2 O3 , SiO2 and/or Si layer. By recalling the formation conditions for the Au NP arrays, it is apparent that thin AAO templates with pore diameters below 20 nm and thin Au films (below 10 nm) are two key parameters to form such

11.3 Anodic Aluminumoxide (AAO) Template-assisted Nanoimprinting

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Figure 11.22 XRD pattern for the Au nanoparticles arrays prepared from the 10 nm thick Au coated AAO template with pore diameter of 13.8 ± 2.6 nm without annealing before etching off the template in 5 wt% H3 PO4 aqueous solution (a), and XRD pattern for the Au nanoparticles arrays prepared from the 6 nm thick Au coated AAO template with pore diameter of 13.8 ± 2.6 nm and annealed at 400 ∘ C for 2 hours under N 2 before etching off the template in 5 wt% H3 PO4 aqueous solution (b). ∇: Au; ▾: Au(Si) alloys; ◊: Al–Si–O or Al–Si–O–Au compounds. Source: Song at al. [32]. © 2013, Elsevier. .

dense Au NPs. The Au atoms cannot form a continuous film but directly form NPs assisted by the underlying Al2 O3 porous membranes, due to the interfacial tension between the thin Au layer and the Al2 O3 porous surface during the deposition. During the slow etching of the AAO layer, some of the larger NPs may further break into small NPs assisted by the interfacial tension, forming more tiny particles. These Au NPs will settle down on the surface of the Si substrate rather than being released into the etching solution with the gradual dissolution of the AAO templates, since they are too thin (200%), heat resistance (up to 160 ∘ C), biocompatibility, and controllable degradability. Figure 13.5 schematically illustrates the fabrication process of protein-based electronic skin (PBES) based on SFCM. In the first step, SFCM with high transmittance and flexibility was prepared in Figure 13.5a. It is worth mentioning that degumming is committed to remove sericin of raw silkworm cocoon fibers, which is necessary for SF film formation. As for PU, they are incorporated into the neat SF solutions to reinforce the network structures within SFCMs. Primarily, as shown in Figure 13.5b, Ag nanofiber (NF) networks as the heater were fixed on SFCM with water and heating treatment (60 ∘ C) to form fused and solidified junctions.. These junctions can notably increase the conductivity and mechanical flexibility of the networks. Then, Pt NF networks, divided into 5 × 5 units, were utilized as the temperature sensor and transferred onto the opposite side of the membrane. Finally, the obtained 5 × 5 array-type PBES was integrated with electric circuits by inkjet printing Ag ink. This PBES with Ag NF/SFCM/Pt NF sandwich structure is capable of realizing temperature controlling and can be mounted to human neck or hand as shown in Figure 13.5c. Joule performances including heating efficiency and stability were investigated. Figure 13.6a shows similar temperature profiles of the Ag NF heater (Rs = 12 Ω/square) after applying voltage gradients, experiencing the climb within short time (12 seconds) and reaching the maximum temperature plateaus. The temperature of Ag NF heaters can reach up to 106.2 ∘ C by merely using a low voltage (4.5 V) and then recovered back to original temperature after approximately 20 seconds. The Ag NF heater (1.5 × 2.5 cm2 ) exhibited high thermal resistance, calculated as 152.2 ∘ C/W/cm2 in accordance with the fitting curve in Figure 13.6b. During the cyclic testing in Figure 13.6c, the Ag NF heater had the similar

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13 Polymer-based Films for Artificial Intelligence

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Ag NFs networks Pt Networks transferring Tearing tap

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Figure 13.5 Fabrication of PBES based on SFCM. (a) Schematic illustration of preparation of flexible transparent SFCM. (b) Schematic illustration of the fabrication of PBES realizing heating and temperature detection. (c) Photographic images of the PBES attaching to human neck and hand closely. Source: (a, b) Huang et al. [12]. © 2020, John Wiley & Sons.

climbing–descending temperature profiles versus time (98%). To more effectively desalinize brackish water with low salt content, an NF membrane with a pore size of less than or equal to 10 Å is called a “loose” or “low-pressure” RO membrane. They have also been developed, and they exhibit lower NaCl retention but higher water permeability compared with the conventional RO membranes. Generally, the sodium chloride retention rate of NF membranes is between 20% and 80%, and the retention capacity for polyvalent ions (such as Ca2+ , Mg2+ , and SO4 2+ ) is higher [61]. The ideal RO membrane should be resistant to chemical or microbial attack under operating conditions, good mechanical stability and excellent separation performance (such as salt rejection rate and flux). As mentioned above, the performance of current commercial polymer films needs to be improved. Currently, commercially available RO membranes are mainly derived from two basic types of polymers: CA and aromatic PA.

14.5 Application of the Selective Permeable Membranes

The first CA membrane was made of cellulose diacetate polymer in the late 1950s and was commercialized for seawater desalination in the 1960s. Current CA membranes are usually made of CA, CTA, cellulose diacetate, or mixtures thereof and have an asymmetric structure (Figure 14.5b) [62]. Due to their neutral surface and resistance to limited free chlorine, CA membranes often have stable performance in applications where the feedwater has high scaling potential (such as municipal sewage and surface water supplies) due to hydrophobicity. The increase in the number and size of sexual substituents hinders the movement of water and solute molecules through the CA membrane. Blending other polymers or monomers with cellulose-based materials may be an effective method for adjusting membrane separation properties. A slight increase in the degree of substitution leads to an increase in salt rejection and a decrease in flux. For example, adding a small amount of poly-(methyl-hydro siloxane)) to the casting solution of the CA film, due to the stronger hydrophobicity and compactness, can greatly increase the retention rate of NaCl and reduce the permeability. Although the porosity was increased and the pore size of the membrane was increased, one layer remained. Membrane made from a mixture of alkaline-hydrolyzed polymethyl methacrylate and CA exhibits higher RO performance in brackish water desalination and improves thermal stability. Besides, the cost of CA is relatively low because they are derived from abundant natural cellulose. However, the widespread development of CA membranes has encountered some disadvantages, such as a narrow pH range (4.5–7.5), sensitivity to biological attack, structural densification under high pressure, and lower upper temperature limit, and excessive addition in CA polymers (such as polypropylene) leads to a decrease in polymer compatibility, resulting in unfavorable phase separation and therefore poor desalination performance. In recent years, research on improving cellulose-derived membranes has only accounted for a small proportion, and because PA or related polymers are more attractive, research efforts have mostly turned to them. CA film was the industry standard from the 1960s to the mid-1970s, until Cadotte of North Star Research developed the interfacial polymerization method of producing PA TFC membranes (Figure 14.6) [62]. The PA TFC membrane has a thin layer of highly selective aromatic PA on top of the reinforced microporous sublayer. The top layer of this is different from the polymer (Figure 14.5b) [62]. Compared with CA film, the interfacially polymerized PA film exhibits excellent water flux, salt, and organic repellency, compaction resistance, a wider operating temperature range (0–45 ∘ C) and pH range (from 1 to 11), and higher stability to biological attack. In terms of membrane structure (Figure 14.5b), there have been only limited studies on the manufacture of flat monolithic asymmetric PA or polyamide–hydrazide membranes. The high PA concentration of the casting solution leads to an increase in salt rejection and a decrease in water permeability. Prolonged (40–60 minutes) high-temperature (for example, 100–120 ∘ C) processing of preformed film will increase the film’s desalination capacity [63]. However, compared with PA asymmetric membranes, more research has focused on the manufacture and characterization of PA and its derived TFC membranes. A typical

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COCI TMC in hexane

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COOH

Figure 14.6 The polyamide derived from m-phenylenediamine and trimesoyl chloride via interfacial polymerization. Two structures may occur, cross-linked portion (m) and linear moiety (n), which has an unreacted acid chloride groups that subsequently hydrolyze to a carboxylic acid group. Source: Li and Wang [62]. © 2010, Royal Society of Chemistry.

PA TFC membrane consists of two parts: (i) a porous substrate (mainly a polysulfone [PSf] UF membrane) and (ii) a thin barrier layer, which is usually formed on the substrate after polycondensation at the interface between the acid chloride and the diamine. Therefore, PA membranes are widely used in commercial one-way desalination plants worldwide, especially because they provide a combination of high throughput and high selectivity, which are their advantage over other types of RO membranes. But the PA membrane has some disadvantages in the desalination process, such as chlorine and scale sensitivity, which may affect the performance of the membrane, such as reducing the membrane life and reducing the flux of salt rejection. In the face of growing water scarcity, it is important to understand the potential of desalination as a long-term water supply option. Recent research by David Cohen-Tanugi et al. [64] highlights the prospect of a new type of membrane material that can desalinize water while showing greater permeability than conventional RO membranes – the potential of ultra-permeable membranes (UPMs), improving the performance and cost of RO. By modeling the mass transfer inside an RO pressure vessel, quantifying the membrane permeability of a given RO device three times reduces energy consumption or the required pressure vessel quantity. For a seawater reverse osmosis (SWRO) unit with a given capacity and recovery rate, a threefold increase in permeability will reduce pressure vessels by 44% or energy by 15%. Besides, three times the permeability will result in a 63% reduction in pressure vessels for saltwater RO or a 46% reduction in energy. They first considered the impact of UPM on energy consumption in SWRO. Consistent with previous work, we found that SWRO’s opportunities to reduce energy consumption were

14.5 Application of the Selective Permeable Membranes

limited by thermodynamics. By reducing the inlet pressure, energy consumption can be reduced, and the relationship between achievable pressure reduction and membrane permeability is shown in Figure 14.7a [64]. The inlet pressure (purple line) of the SWRO gradually decreased to the outlet brine osmotic pressure. This limit corresponds to the osmotic pressure of brine, and it is a fundamental limitation on the energy efficiency of desalination, which is completely independent of membrane performance. Figure 14.7a shows that increasing Am from 1 to 3 l/(m2 h bar) will reduce the inlet pressure from 70 to 63 bar. A 10% reduction in inlet pressure can reduce specific energy consumption by 15% [64]. Now let us look at the impact of permeability on SWRO equipment requirements. In this case, an increase in Am helps to increase the permeate flux, while a higher feed rate reduces the effect of concentration polarization because the mass transfer coefficient increases monotonically with the fluid velocity. We have plotted 100 000 m3 d in Figure 14.7, the relationship between the total number of pressure vessels required by the RO plant and the membrane permeability. Figure 14.7b shows that three times the Am enables the SWRO plant to produce the same total output (with the same inlet pressure and recovery) and the pressure vessel is reduced by 55% [64]. However, at high feed rates, the viscosity loss of more energy will be dissipated, so at this higher throughput rate, the specific energy consumption will also increase by 6%. Besides, by operating at lower pressures, the SWRO plant can maintain the same energy consumption while reducing the number of pressure vessels by 44%. They then continued to investigate the potential operational impact of UPM on brackish water reverse osmosis (BWRO). Figure 14.7 reveals a result that is largely unresolved in the literature, that is, compared with SWRO, there is more room for the inlet pressure to fall in the case of BWRO. Figure 14.7a shows that to increase Am , the inlet pressure required to achieve a 65% recovery in BWRO is rapidly decreased [64]. Since the permeation limit of BWRO is only the permeation limit of SWRO, three times the membrane permeability will achieve a greater reduction in inlet pressure, from 12 to 6.4 bar for TFC membranes. For more permeable membranes, when Am > 5 l/(m2 h bar), the pressure basically reaches the asymptotic limit. Therefore, for brackish water, UPM can almost reduce the energy consumption of the RO stage by half. For BWRO, the reduction in the number of pressure vessels is also greater than SWRO. Figure 14.7b shows that by increasing the feed flow per container from 139 to 378 m3 /d, for a given plant capacity, a triple-A m can reduce the pressure vessel by 63% [64]. In addition, the increase in feed flow involves little energy loss because the viscosity loss accounts for a negligible part of the total energy consumption of the BWRO system. They then considered the effect of concentration polarization. For SWRO, fouling has been observed when the concentration polarization factor (CPF) or the ratio of surface salinity to volume salinity exceeds 1.20. Fouling and the detailed effects of fouling are beyond the scope of this article but are related to the design constraints of RO equipment. In Figure 14.8, the change in CPF for each scene with z is calculated. Figure 14.8 shows that the maximum CPF of the UPM is higher than the TFC membrane, which is consistent with the fact that the former allows higher transmembrane flux [64]. CPF starts with a higher value from the front of the RO

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14 Selective Permeable Thin Films and Membranes

100 80

Seawater (pressure, bar)

6

60 4 Seawater (energy consumption, kWh/m3)

40

2

Energy (kWh/m3)

Inter pressure (bar)

8

(Constant RR, Qin, Lc)

20

Brackish water

(a) 0

0

2

6

4

8

10

0

Permeability (l/(m2 h bar)) 3000 Number of pressure vessels

466

2500

(constant RR, Pin, Lc) (for 100, 000 m3/d)

2000 Baseline

1500 1000

Brackish water

500 0 (b)

0

Seawater 2

4

6

8

10

3

Permeability (l/(m h bar))

Figure 14.7 Improvements in key performance criteria as a function of membrane permeability for seawater reverse osmosis (SWRO) at 42 000 ppm NaCl (purple) and brackish water reverse osmosis (BWRO) at 2000 ppm NaCl (orange). (a) Minimum required inlet pressure (solid lines) and energy consumption (dashed) as a function of membrane permeability at fixed recovery and feed flow rate. (b) Number of pressure vessels required as a function of membrane permeability for a total capacity of 100 000 m3 /d at fixed recovery ratio and pressure. Membrane width is held fixed in both subplots.

container, where the permeate flux is the highest, and then decays to 1 at the end of the container. In the reference case, corresponding to the traditional TFC membrane operation, the CPF curve is relatively flat in all three cases because the permeate flux is relatively modest. In contrast, the UPM system shows a larger CPF at the front of the container (consistent with higher membrane permeability), but the attenuation is steeper along the length of the container to unity. In the SWRO scheme, the CPF in the second half of the ship is lower than the reference case. Although the CPF value of UPM is slightly higher, the CPF remains below 1.20 in all three cases. It turns out that the development of super osmosis membranes can help reduce energy consumption and pressure vessel requirements for RO desalination. Such a membrane can also make the permeate water output of each ship the same as today, reducing the energy consumption of SWRO and BWRO by 15% and 46%, respectively.

14.5 Application of the Selective Permeable Membranes

Seawater

1.2

CPF = cw/cb

Brackish water TFC case UPM case (fixed pin, RR, le) 1.1

1.0 0.0

0.2

0.4

z/L

0.6

0.8

1.0

Figure 14.8 CPF as a function of distance down RO vessel for SWRO and BWRO in the low-permeability reference case and in the UPM scenarios at 3× higher permeability examined in this work. For a tripling in membrane permeability (dashed lines) compared with TFCs (solid lines), the maximum CPF remains below 1.20.

As the membrane permeability increases, the typical cross-flow velocity and mass transfer coefficient will also increase, and the permeate flux will increase monotonically with the membrane permeability, albeit linearly. Although novel system designs may be needed to take advantage of the greatly increased feed flow, the results suggest that advances in membrane science will continue to make seawater desalination a competitive advantage in freshwater supply for decades to come. In a recent study, Jay R. Werber et al. stated that desalination membranes need to improve selectivity rather than increase water permeability [65]. They first introduced the performance of the latest desalination membranes and then asked questions. RO is an advanced municipal wastewater treatment program [66], key steps in industrial and drinking water reuse. With the increasing use of desalination membranes, research on membrane materials has again attracted attention. Due to recent advances in nanomaterial synthesis and assembly, potential step-change performance improvements are possible. However, the existing literature lacks guidance on the practical impact of improving the performance of key active layers, namely, membrane permeability and water solute selectivity. In other words, what is the most desired active layer characteristic in the design of new desalination membranes? They then reviewed the recent analysis and modeling studies that found that increased membrane permeability has a limited impact on the performance of RO and RO processes. It then highlights several examples of important processes that are adversely affected by insufficient solute retention, demonstrating the need to improve water solute selectivity. Finally, how current and potential future membranes will adapt to this situation is discussed. RO desalination can consume up to 50% of SWRO’s operating and maintenance costs, depending on several factors, including membrane performance, process design and efficiency, and required preand posttreatment steps. Since the water permeability of the membrane affects only

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14 Selective Permeable Thin Films and Membranes

40

Minimum additional energy

Пbraine П1.2-stage

Theoretical minimum energy

0 25 50 75 100 (a)

0 25 50 75 100

Recovery ratio, r (%)

2.00 1.50

ПBW (r)

Energy for hydraulic overpress ure

Seawater

Current membrance performance

1.00

Пbrine

ПSW (r)

Brackish water

0.50

Seawater

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Single stage

0.00

60

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80

the desalination stage, the discussion here is limited to the exclusion of influent, pretreatment and posttreatment, and brine discharge, which together can consume more than 1 kWh of energy/m3 of produced water. Energy consumption can be further reduced through the development of TFC PA membranes, which have higher water permeability and selectivity than SWRO’s original CA membranes. Improved active layer performance reduces the hydraulic overpressure required to obtain enough water flux. Although the appearance of TFC membranes saves energy, the single-stage operation of RO limits its further development, in which the minimum water pressure and the minimum specific energy are equal to the osmotic pressure of the outflow brine. Figure 14.9a illustrates the impact of actual operations on energy demand. Single-stage operation increases the actual minimum specific energy (gray and yellow shaded areas together) to 1.56 kWh/m3 . Reinforced materials only affect the energy (cyan shaded area) associated with hydraulic overpressure, which is larger for low permeability membranes and smaller for high permeability membranes. As shown by BWRO, more energy can be saved by using multiple RO stages, which is a common practice in BWRO, allowing operation under multiple hydraulic pressures. Recent module-scale modeling studies have explicitly analyzed the impact of increasing a factor on specific energy requirements. The study was modified and expanded here, and the results are shown in Figure 14.9b. For SWRO and BWRO, as the A coefficient increases, the specific energy decreases sharply before finally reaching a plateau. The effect of increased water permeability in a single-stage BWRO using a 0.1 M NaCl feed is similarly limited. In addition, the effect of increased water permeability is slightly greater, but the absolute reduction in energy required is still small. Its size is similar to the change in SWRO. Using the latest TFC membrane simulation results proves that increasing the water permeability coefficient above the current value will only

0

468

0 2 4 6 9 10 Water permeability coefficient (b) A (l/(m2 h bar))

Figure 14.9 Energetics and the impact of increased water permeability for RO and forward osmosis (FO) desalination. (a) Impact of system design on the energy required in RO. (b) Specific energy requirements for RO desalination with an increasing water permeability coefficient.

14.5 Application of the Selective Permeable Membranes

contribute to a smaller reduction in RO energy consumption. This limited impact is due to two factors. First, when using TFC membranes, hydraulic overpressure is already small, limiting the potential for further efficiency improvements. Second, if kf is increased, concentration polarization can be reduced, but this usually requires greater frictional pressure loss due to increased turbulence. It is noted that higher water permeability may result in membrane area savings, as the model does not consider fouling, which can be exacerbated at higher water fluxes. By incorporating high-permeability elements only at the back end of the RO module, the membrane area can be reduced without increasing the level of dirt and without increasing the degree of pollution. The driving force of the RO module is relatively low. This is because of the osmotic pressure of the retained feed increases. According to their research discussions, desalination applications require membranes with increased water-solute selectivity rather than water permeability. RO membrane desalination has developed rapidly over the past 40 years and has gradually become the main choice for new plant installations. In recent years, significant progress has been made in improving the separation performance, chlorine resistance, and stain resistance of polymer RO membranes. The increasing performance of commercial RO membranes will reflect this, which will help reduce energy consumption and reduce the cost of desalination processes. However, there is still much room for improvement in the energy efficiency of RO desalination. In particular, the development of high-performance polymer-based RO membranes will continue to be a major focus in this area. In addition to new polymer film materials, the strategy of forming mixed matrix films by incorporating other functional materials into the polymer matrix will be very attractive for the development of thin film nanocomposite mixed matrix films with excellent performance and versatility. The new desalination membrane may provide a feasible way to achieve extremely high selectivity. Properly adjusting the size of the nanochannels and combining them into a sturdy, relatively impermeable membrane can produce a high-performance membrane. CNTs best illustrate the potential impact on the highly selective demand for new desalination membranes [67]. CNTs have long been promising due to the ultrafast water penetration of their smooth interiors. Finally, despite decades of RO seawater desalination, new processes such as FO seawater desalination are emerging. FO desalination uses osmotic pressure as the driving force. In principle, it consumes much less energy than the RO process. Similarly, the FO process is less prone to membrane fouling than the RO process. The desalination of high-performance FO membranes deserves more research. The development of a suitable FO membrane is critical to realizing the potential of FO desalination technology.

14.5.2 Application in Gas Separation For the past 30 years, membrane technology has been prominently used for various gas purifications to obtain a high purity gas. Membrane acts as semipermeable wall, in which the separation occurs by controlling the rate of movement of various molecules between two liquid phases or two gas phases or a liquid phase and a gas phase that passing through the membranes. Then, the advantages offered by

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membrane process such as simple operation with low energy consumption, low operating and capital cost, continuous process, and unnecessary regeneration process compared with absorption and adsorption processes have attracted considerable attentions of researchers [68]. Besides, traditional methods such as pressure swing adsorption, cryogenic distillation, and amine absorption that have high energy consumption, high cost, and adverse impact on the environment have even made membrane process as preferable method for gas separation. Polymeric materials were used to develop a membrane that can attain high selectivity and permeability with high chemical and thermal resistance. 14.5.2.1 Oxygen Separation

Oxygen is an important industrial feed gas that is utilized in many industries, such as chemical industries, ferrous metallurgy, glassmaking, and environmental protection. If the energy consumption for pure oxygen separation can be significantly reduced, then not only the production costs of the abovementioned processes will be decreased remarkably, but also less pollution and CO2 will be discharged into the environment [69]. Currently, the commercialized technology for the generation of pure oxygen is the cryogenic distillation of air. However, the cost of cryogenic distillation technology for oxygen separation remains high. Therefore, it is facing a great energy penalty, i.e. >10% loss in net output, for oxy-fuel and precombustion power plants using pure oxygen produced by cryogenic distillation factories [70]. Other than cryogenic distillation, pressure swing adsorption and polymer membranes are the commercialized technologies for oxygen separation from air, but they are inefficient for the production of oxygen with purity higher than 95% [71]. Recently, a production method based on mixed ionic–electronic conducting (MIEC) membranes, which can produce high-purity oxygen with higher energy efficiency and lower investment cost than the cryogenic distillation technology, has gained much attention from academic and industrial communities [72]. In contrast to the physical permeation behavior occurring on microporous membranes and polymer membranes, this mechanism is an electrochemical permeation of oxygen through MIEC membranes. This behavior demonstrates that the permeation selectivity of MIEC membranes toward oxygen is 100%. Although the MIEC membranes are dense ceramics, their oxygen permeability is comparable with that of microporous membranes and is much higher than that of polymer membranes. In the past 20 years, the US Department of Energy and Air Products & Chemicals, Inc. (APCI) have invested hundreds of millions of dollars to commercialize MIEC ceramic membranes to produce pure oxygen [73]. This revolutionary new technology can significantly reduce the production cost (>30%) compared with that of the current cryogenic distillation technology [74]. Macroscopically, the oxygen permeation process includes three steps, i.e. oxygen surface exchange on the feed side, ambipolar transport of oxygen ions and electrons across the membrane bulk (including the grains and grain boundaries (GBs), and oxygen surface exchange on the permeation side, as illustrated in Figure 14.10a. On the microscopic scale, the oxygen exchange kinetics on the gas–solid interfaces and the diffusion kinetics

14.5 Application of the Selective Permeable Membranes

(a) Macroprocess

(b) Micromechanism

(c) Membrane reactors

Oxygen exchange on interfaces Reactions CH4 + O2 O

2-

2O2-

2O2-

N2

r

Air

e-

O2+4e-

O2+4e-

O2

MIEC membrane

on

usi

Diff

in c

CO + H2

als yst

CnH2n+2 + O2 CnH2n + H2O H2O(CO2)

Dif fus

O2-

ion bo acro un da ss g rie r s ain

H2 (CO)/O2

O2-

(Air + H2o) / CH4 (N2 + H2)/(CO + H2)

Applications Syngas production Olefins production H2/syngas production Two types of syngas production

Figure 14.10 (a) Macroprocess of oxygen permeation. (b) Micromechanism of oxygen exchange on the gas–solid interfaces and oxygen transport in the crystals and across the grain boundaries of a MIEC membrane. (c) The reaction separation coupling in MIEC membrane reactors. Source: Zhu and Yang [71]. © 2019, John Wiley and Sons.

across the GBs, as shown in Figure 14.10b, cannot be neglected. In addition to their use in the separation of pure oxygen from air, MIEC membranes are applied as reactors in which the reaction and separation processes can be coupled to simplify the process and save energy, as shown in Figure 14.10c [69]. In MIEC membrane reactors, catalytic reactions are integrated with the oxygen separation process. For an oxidation reaction with a large negative free energy, the deep oxidation reactions limit the selectivity of the target product. MIEC membranes control the oxygen input into the reactor, which results in a low oxygen partial pressure in the membrane reactor, so a high selectivity for the target product can be obtained by avoiding the deep oxidation. For a reducing reaction with positive free energy, such as water decomposition for the production of hydrogen, its conversion is limited by the thermodynamic equilibrium. The MIEC membrane selectively removes oxygen from the reactor, thus shifting the reaction equilibrium to the product side. The requirements for MIEC membranes used for chemical reactions in membrane reactors depend on the reaction conditions. The permeation flux, permeation stability, and structural stability under catalytic reaction conditions are the most important performance indicators for the MIEC membranes. Therefore, many studies have focused on developing MIEC membranes that can satisfy the operational conditions of catalytic reactions. There are several excellent reviews and books that comprehensively discuss the progress in the study of MIEC membranes in recent years [75, 76]. In these studies, the basic principles, material development, membrane reactor properties, experimental methods, and theoretic models were well introduced. Composition change (or element doping) is the most commonly used strategy to develop new membrane materials. However, this strategy does not always effectively improve both the stability and permeability of MIEC membranes. Recently, the development of the microstructural and interfacial designs of MIEC membranes was suggested as an important strategy to enhance the membrane performance, but this approach

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has not been thoroughly discussed in a review article. With the help of theoretic model studies, the microstructural and interfacial effects can be well described [77]. With the advanced strategies, the stability and permeability of membranes can be improved simultaneously [78, 79]. In this work, we describe the recent efforts to build models to aid in the understanding of the important effects of the surface and GBs on the stability of the membranes, design novel MIEC membrane materials for oxygen transport and catalytic reactions, and utilize the developed membranes to construct membrane reactors for new separation and reaction processes with high efficiencies. The goal of this study is to understand the permeation mechanism; to design MIEC membrane materials with improved performance by controlling the bulk lattice, GBs, and surface structure; and to develop new reaction separation couplings for process intensification. Recent developments indicate that the technology based on MIEC membranes is promising for the production of pure oxygen and for process intensification via reaction separation coupling. 14.5.2.2 CO2 Separation

Global CO2 emissions have increased steadily in tandem with the use of fossil fuels. A paradigm shift is needed in developing new ways by which energy is supplied and utilized, together with the mitigation of climate change through CO2 reduction technologies. There is an almost universal acceptance of the link between rising anthropogenic CO2 levels due to fossil fuel combustion and global warming accompanied by unpredictable climate change. Therefore, renewable energy, non-fossil fuels and CO2 capture and storage (CCS) must be deployed on a massive scale. CCS technologies provide a means for reducing greenhouse gas emissions, in addition to the current strategies of improving energy efficiency. Coal-fired power plants are among the main large-scale CO2 emitters, and capture of the CO2 emissions can be achieved with conventional technologies such as amine absorption. However, this energy-consuming process, calculated at approximately 30% of the power plant capacity, would result in unacceptable increases in power generation costs. Membrane processes offer a potentially viable energy-saving alternative for CO2 capture because they do not involve any phase transformation. However, typical gas separation membranes that are currently available have insufficiently high permeability to be able to process the massive volumes of flue gas, which would result in a high CO2 capture. Polymer membranes highly permeable to CO2 and having good selectivity should be developed for the membrane process to be viable. This perspective review summarizes recent noteworthy advances in polymeric materials having very high CO2 permeability and good CO2 /N2 selectivity that largely surpass the separation performance of conventional polymer materials. Five important classes of polymer membrane materials are highlighted: polyimides, thermally rearranged polymers (TRs), substituted polyacetylenes, and polymers with intrinsic microporosity (PIM), which provide insights into polymer designs suitable for CO2 separation from, for example, the post-combustion flue gases in coal-fired power plants [80]. Of the multitude of polymeric membrane materials with high CO2 permeability and adequate CO2 /N2 selectivity, only a small number have been conveyed

14.5 Application of the Selective Permeable Membranes

O

F 3C

O

CF3

N

N O

O

Polyimide XH O

F3C

O

CF3

N

F3 C

CF3

F 3C

CF3

N

N

O

X

X H3C

O HX

CH3

TR polymer (x = O, S) CH3 C

O C

CN O

O

n

H3C Si CH3 CH3

O CN

PTMSP (Polyacetylenes)

nC16H33

O

O

n

PIM

O

O p

O n

N

O

O n q

nC16H33

nC18H37

Figure 14.11 [80].

Representative chemical structures of polymers with high CO2 permeability

into industrial practice as commercial membranes. We highlight five classes of highly permeable polymeric materials: polyimides, thermally rearranged polymers (TR polymers), substituted polyacetylenes, polymers of intrinsic microporosity (PIM) and poly (ethylene oxide) (PEO)-based polymers, because the majority of these polymers exhibit excellent characteristics for CO2 separation applications (Figure 14.11) [80]. High Permeability Polyimides

Polyimides are attractive materials for gas separation owing to their excellent gas separation and physical properties, such as high thermal stability, chemical tolerance, and mechanical strength [81]. They are commonly prepared by step polymerization involving a thermal or chemical imidization between a bis(carboxylic anhydride) and a diamine. The variation in structure property relationships of polyimide membranes has been studied, relative to molar volume, density, free volume, and gas permeabilities by means of group contribution theory [82, 83]. Generally, polymer chain rigidity determines the diffusivity selectivity while inter-chain spacing and chain mobility govern the diffusion rate. The main factors affecting the gas transport properties in polyimide membranes are (i) spatial linkage configurations, (ii) type of bridging groups, and (iii) bulky and polar

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groups incorporated into the structures. In the molecular design of polyimides for gas separation of commercially important gas pairs such as CO2 /CH4 , previous research suggests that the selectivity in polyimide membranes can be enhanced by incorporating (i) meta-linkages, (ii) swivel linkages comprising bulky groups, and (iii) polar and bulky pendant groups [84]. For instance, asymmetric polyimides with meta-linkages show higher chain packing efficiency and restricted rotational freedom compared with the corresponding symmetric para-linked isomers [85, 86]. As a result, the meta-isomers show moderately lower FFV and gas permeability but higher gas selectivity. Also, polyimides with bulky bridging groups reduce interchain rotation, resulting in less chain mobility and higher FFV. For example, polyimides derived from 4,4′ -(hexafluoroisopropylidene) diphthalic anhydride (6FDA)) with diamines such as 2,3,5,6-tetramethyl-1,4-phenyldiamine (4MPDA) [87, 88] and 3,30-dimethylnaphthidine (DMN) have increased chain stiffness due to the introduction of C(CF3 )2 linkages. This linkage is believed to serve as a molecular spacer and a chain stiffener; it reduces the intrasegmental mobility and limits the degree of chain packing, thereby increasing the FFV. As a result, aromatic polyimides incorporating C(CF3 )2 linkages tend to have both high CO2 permeability and high CO2 /CH4 selectivity. Thermally Rearranged (TR) Polymers

Membrane-based separation systems need to achieve both high gas throughput and high selectivity. For polymer membranes, separations depend on the size of the cavities that lead to porosity on the sub-nanoscale. These cavities, so-called free volume elements, generally exhibit a broad-scale distribution. Recently, rodlike polymers were derived from functionalized polyimides by a thermal post-membrane conversion process, i.e. TR polymers. These appear to have more uniform cavity sizes that create tailored free volume elements with well-connected morphology in amorphous polymers. The TR membranes have outstanding transport and separation properties for small gas molecules and ions. The TR membrane concept was proposed by Park et al. and Lee and coworkers [89, 90], which adopts a post-membrane fabrication polymer-modifying reaction to obtain dense polybenzoxazole (PBO) and polybenzothiazole (PBT) membranes by the thermal rearrangement of soluble aromatic polyimides containing ortho-linkage positioned functional groups (e.g. OH and SH). The TR polymers exhibit excellent separation performance, particularly for CO2 /CH4 mixtures, with high selectivity and permeability due to an unusual microstructure whose cavity size and distribution could be further controlled by the appropriate selection of template molecules and heat treatment protocols. The unexpected physical phenomena in TR polymers are of great importance in that the random chain conformations occurring in the condensed polymer phase lead to tuned microvoids, which contribute to performance enhancement in selective molecular transport. A benefit of the thermal rearrangement concept is the relatively easy degree of control over the average interchain spacing and free volume elements that directly lead to molecular sieving effects. Thermal rearrangement is a feasible method for producing polymeric membranes with high permeability and selectivity suitable for gas separations.

14.5 Application of the Selective Permeable Membranes

Highly Permeable Substituted Polyacetylenes

Highly permeable substituted polyacetylenes generally have many molecular scale voids, which are formed by the presence of bulky pendant groups. They are prepared by polymerization of acetylenic monomers using transition metal catalysts [91]. It is known that various metal catalysts yield polymers with different geometric structures and properties. For example, NbCl5 gives a more cis-rich poly(1-trimethylsilyl-1-propyne) (PTMSP) than TaCl5 [92]. The pendant groups inhibit rotation of the rigid backbone [93], which leads to inefficient chain packing. The large free volume distribution includes both small disconnected elements and larger continuous microvoids. However, the large free volume elements collapse with time owing to vapor sorption, contamination [94], and/or relaxation phenomena, resulting in significant decreases in gas permeability. PTMSP and related polymers in the class are among the most permeable polymers to any gases, having almost ten times higher permeability than the rubbery polymer poly(dimethylsiloxane). Hence, their gas transport properties have been extensively studied [95, 96]. Although glassy PTMSP exhibits some properties that are similar to rubbers, gas transport through polyacetylenes is described in terms of the dual-mode sorption mechanism [97]. The molecular design for highly permeable polyacetylenes has usually focused on incorporating different substituent groups. It has been deduced that the steric shape of the substituents attached to poly(diphenylacetylenes) plays a very important role in gas permeability [98, 99]. Polyacetylenes with tert-butyl substituents provide higher permeability. In 2008, Hu et al. reported an indan-containing poly(diphenylacetylene) derivative, which exceeded the oxygen permeability of even PTMSP, which previously had the highest permeability [100]. In some substituted polyacetylenes, especially those with very high P values, which were based on both high diffusivity and solubility contributions, long-chain n-alkyl substituents gave relatively high diffusivity, while those with phenyl substituents had relatively high solubility [101]. It is believed that large microvoids give rise to high diffusion coefficients compared with other glassy polymers and also lead to high apparent solubility coefficients. Although substituted polyacetylenes have characteristically high permeabilities, their selectivities are low, in accordance with performance trade-off behavior. Furthermore, significant aging problems in polyacetylenes impede their application in industrial membranes. Although polyacetylene-based membranes have high CO2 permeability, they have not been considered for CO2 separations because of low selectivity and strong physical aging. Nevertheless, there are opportunities for improving the properties of substituted polyacetylenes through macromolecular structural design. For example, approaches include grafting CO2 -philic groups onto the PTMSP backbone, in order to enhance the solubility selectivity [101], reversing aging by methanol treatment [102], or reducing aging effects by cross-linking the membrane [103]. Polymers with Intrinsic Microporosity

The term polymers of intrinsic microporosity (PIM-1) was designated for a fluorescent yellow high molecular weight polymer with one of the simplest structures in the PIM class of materials, prepared by polycondensation reaction of commercial

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monomers 5,5,6,6-tetrahydroxy-3,3,3,3-tetramethylspiro-bisindane with tetrafluorophthalonitrile. PIM-1 is soluble in a number of solvents such as chloroform, toluene, or tetrahydrofuran and can be cast from solution to form robust membranes. Low-temperature N2 adsorption/desorption analysis indicates that PIM-1 in powder or membrane form has a high apparent surface area (SBET = ∼800 m2 g−1 ) and exhibits microporous character. PIMs are considered as amorphous materials, since there is no evidence of crystallinity or a glass transition below the decomposition temperature. The latter observation may be expected, since there is no degree of chain rotational mobility, at least over short length scales. Initial data for PIM-1 membranes exhibited gas permeabilities exceeded only by very high free volume polymers such as PTMSP and Teflon AF2400. Combined with selectivities, the performance trade-off is typically located between the 1991 and the 2008 Robeson upper bounds for gas pairs, such as O2 /N2 and CO2 /CH4 , and it is an upper bound material for CO2 /N2 [100]. In the majority of glassy polymers, the typical order of permeability is He > CO2 . However, PIM-1 has an unusually high CO2 permeability, with the order of decreasing permeability being CO2 > H2 > He > O2 > CH4 > N2 . This is because CO2 selectivity for PIM-1 is dominated by solubility selectivity and less so by diffusivity selectivity. Subsequent studies showed that permeability could be substantially enhanced by methanol treatment, which helps to remove residual bound casting solvent [104].

14.5.3 Application in the Separation and Purification of Low Molecular Substances In the application of separation and purification of low molecular substances, peptides, oligosaccharides, antibiotics, and amino acids are often separated, extracted, and refined. Generally, factories use centrifugation, precipitation, adsorption, extraction, ion exchange, and chromatography to achieve their goals. The extraction process is time-consuming and labor-intensive and has the problems of not only large consumption of raw materials, high energy consumption, low product recovery rate, serious wastewater pollution, etc. but also deformation and inactivation of the product. In this context, polymer permeable membranes have huge development space in the pharmaceutical market due to their advantages of high-efficiency separation, simple equipment, energy saving, normal temperature operation, and no pollution. 14.5.3.1 Isolation and Purification of Oligosaccharides

Oligosaccharides are a class of small molecular weight sugars that are polymerized from 2 to 10 monosaccharides [105]. At present, most researchers pay attention to natural polysaccharides and oligosaccharides in the field of production technology, mainly including alcohol extraction, water extraction, acid–base precipitation, and other technologies, and relatively little research on separation and purification. Especially in terms of production scale and separation and purification (facing the problems of small production scale, low production efficiency, large environmental pollution, and high

14.5 Application of the Selective Permeable Membranes

cost), it is difficult to break the bottleneck of functional sugar production. Polymer permeable membranes have become more and more widely used in the separation and purification of functional oligosaccharides in recent years due to their high precision and low energy consumption. Membrane separation technology provides a technically reliable, economical, and environmentally friendly production route for the separation and purification of functional sugars. 14.5.3.2 Fructooligosaccharides

Fructooligosaccharides (FOS) is a typical prebiotic that can promote the proliferation of bifidobacteria, inhibit intestinal spoilage, promote the absorption of mineral elements such as calcium and magnesium, enhance immunity, reduce cancer risk, improve intestinal bacteria clusters and lipid metabolism, and more [106]. In the actual production process, semi-finished products of FOS may contain some impurities such as protein, bacteria, and particulate matter. Therefore, the UF or MF can be used to pretreat the feed liquid before NF purification, thereby improving the efficiency and product purity [107, 108]. Machado et al. [107] used a combination of MF and NF to separate FOS from concentrated artichoke extracts. The MF membrane can almost pass 100% of sugars and remove large particles of impurities. Ledur Alles et al. [108] combined UF and NF to extract and concentrate FOS from the rhizomes of snowdrop fruit. Although the clarification effect of UF is better, there is a certain loss of FOS (about 15%), which is not conducive to the improvement of product yield. 14.5.3.3 Galactooligosaccharides

Galactooligosaccharides (GOS) is a natural functional oligosaccharide. In nature, trace amounts of galactooligosaccharides are found in animal milk, while in human breast milk, they are more abundant. It plays an important role in the establishment of bifidobacteria groups in infants [109]. Sarney et al. used NF-CA-50 NF membrane to purify galactooligosaccharides in human breast milk. After four diafiltration operations, 6.7 g of galactooligosaccharide can be extracted per liter of breast milk, and the product yield is about 50% [110]. Goulas et al. used an NF membrane (DS-5-DL) to separate and purify a commercial galactooligosaccharide mixture (Vivinal® GOS). During continuous diafiltration, 98% of galactooligosaccharides were trapped on the raw material side by the NF membrane, and 82% of the monosaccharides passed through the NF membrane. 14.5.3.4 Isolation and Purification of Amino Acids

Amino acid is the basic unit of protein. It is widely used in food and medicine. The polymer osmosis membrane technology plays an important role in the clarification of amino acid fermentation broth, desalting and concentration of mother liquor, and purification of amino acid products. Different amino acids have different isoelectric points. Therefore, by adjusting the pH and using the charged properties of the polymer permeable membrane, amino acids with similar molecular weights and different isoelectric points can be

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separated, such as aspartic acid, isoleucine, and ornithine. Their relative molecular masses are 1 3 3, 1 3 1, 1 3 2, which are almost the same, but their isoelectric points are quite different, which are 2.8, 5.9, 9.7, respectively. When pH = 5.0, the membrane’s interception rate of aspartic acid is 40%, while the rejection rates of isoleucine and ornithine are less than 10%. When the pH is changed, the membrane is effective for the above three kinds. There are obvious differences in the retention rates of amino acids. Although not very large, they can still be separated. Garem et al. used an organic–inorganic composite NF membrane (the isoelectric point of the membrane is 10.8) of ZrO2 surface-grafted and cross-linked PEI to perform membrane separation experiments of nine amino acid mixtures (two of them are acidic, three are alkaline, and the rest are neutral). At pH = 2, the transmittance of positively charged basic amino acids is less than 25%, while the transmission of neutral and acidic amino acids is greater than 85%. At pH = 1, the transmission of negatively charged acidic amino acids is less than 30%, while the transmission rate of neutral and basic amino acids is higher than 80%. The wastewater of amino acid fermentation broth is a kind of harmful and nontoxic high-concentration organic acid wastewater. The content of organic matter is high, and the concentration of ammonium and sulfate is quite high. It is difficult to achieve the standard with traditional single biological treatment technology. Membrane separation technology is used to treat fermentation liquid wastewater mainly to recover valuable substances in wastewater. The components such as bacteria and macromolecular proteins removed by membrane separation technology can be reused after treatment, and the COD and BOD contents in the wastewater are significantly reduced, which reduces the load of biological treatment. 14.5.3.5 Application in the Isolation and Purification of Antibiotics

Antibiotics have a good sterilizing effect and have different therapeutic effects on viruses, parasites, tumors, etc. There are hundreds of antibiotics currently in the medical market. Antibiotics are mainly produced by microbial or higher animal and plant fermentation, and their content (w/v) accounts for 0.1–5% of the fermentation broth or even lower. The relative molecular mass of antibiotics is mostly concentrated in 300–1200, and most of them exist in the fermentation broth, so the isolation and extraction of antibiotics is a key link in the production process of antibiotics. Traditional separation methods are generally drum filtration, plate and frame filtration, and centrifugal separation. Compared with traditional processes, membrane separation technology for treating fermentation broth has the advantages of high yield, good quality, low cost, and low waste liquid. Penicillin fermentation broth has by-products, residual medium, soluble proteins, etc., and emulsification will occur during extraction, which affects the transfer of penicillin between two phases, prolongs the extraction time, reduces the yield of penicillin in the extraction section, and uses UF membranes to treat penicillin fermentation. The liquid can effectively remove protein and other macromolecular impurities and eliminate the emulsification in the extraction process.

14.6 Current Status and Recent Progress and Perspectives

In the production of kanamycin sulfate, a UF membrane was used to remove bacteria, oligosaccharides, and heat sources in the decolorizing solution [111], replacing the sterile plate and frame filtration in the original process. This method has a good sterilization and heat removal effect, which can overcome the disadvantages brought by the sterilization method of sterile plate and frame and can also remove small molecular impurities.

14.6 Current Status and Recent Progress and Perspectives As the demand for water separation grows, coupled with the increasing availability of nanomaterials and a deeper understanding of the structural characteristics of biofilms giving them excellent permeability and selectivity, substantial research aimed at overcoming the permeability/selectivity trade-offs has been stimulated. Molecular level design and insight, including advanced simulations and modeling, will be critical for future breakthroughs. For example, the methods used today to prepare membranes are not suitable for independent control of permeability, and water transport cannot be controlled independently of salt (or other solute) transport. Whereas traditional membranes for water and gas separation are essentially based on polymers and subject to permeability–selectivity trade-offs, many new materials and design methods (e.g. bioinspired, biomimetic) provide better ways to control pore size and size distribution, which can break the traditional upper limits. However, simplifying membrane fabrication and novel processing strategies will help accelerate the development of new membrane materials. The use of environmentally friendly, solvent-free membranes with cheap solvents, such as water, will benefit sustainable development. Solutions to such issues as process constraints, material machinability, and long-term stability in the process environment should be established in research efforts to identify materials that could be deployed for large-scale practical applications as early as possible. In recent years, membrane separation technology with its high separation efficiency and selectivity, as well as low energy consumption, has shown great application prospects in the fields of sewage treatment and desalination, fuel cell, petrochemical, and so on. Two-dimensional (2D) porous materials represented by covalent organic framework (COF) materials can achieve effective nanoscale separation because of their controllable porous and chemical functions and have become a new hope for the preparation of high-performance separation membranes. However, the COFs obtained by stable covalent bonding exhibit many inherent drawbacks in the formation of large and structured films, such as insoluble non-melting, disordered orientation, macroscopic structural defects, and so on. These factors lead to the inability of COFs to address the longstanding “trade-off” challenge between membrane permeability and selectivity. Aiming at the key problems in structural design and functionalization of membrane materials, Yonghua Ren team [35] designed and synthesized a novel honeycomb-like 2D supramolecular polymer by means of supramolecular

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self-assembly. Using a simple solution-casting membrane method (drop-cast), the authors successfully developed a structurally highly oriented supramolecular assembly composite membrane loaded on a polycarbonate filter membrane. Because of its thin thickness, the membrane showed a fast permeation rate. In addition, the function of the membrane is diverse and not only can the specific noble metal platinum divalent ions be efficiently and selectively adsorbed but also can realize the precise selection and screening of nanoparticles at sub-nanoscale, reflecting the effect of “one film multipurpose.” The development of membrane material market depends on the innovation drive of membrane material foundation research. Through cross-disciplinary with materials, chemistry, and so on, new high-performance membrane materials are constantly breaking through. At present, advanced multifunctional surface construction, membrane structure regulation, and the development of new conceptual membrane materials have become research hot spots in this field. Membrane fouling is one of the main obstacles to the application of membrane materials in wastewater treatment technology. The development of membrane materials with antipollution function is one of the most direct and core methods to control pollution. At present, hydrophilic modification of membrane surface is still the main way to improve the antifouling properties of membrane materials. In addition, the introduction of antimicrobial agents to make membrane materials have anti-biofouling function is one of the frontiers of the research (e.g. nanosilver, nanocopper, graphene, quaternary ammonium salt, etc.). The development of multifunctional membrane materials is another research hot spot and frontier, such as additional catalytic and adsorption functions on the basis of separation functions. The composite conductive material on the surface of the membrane can make the membrane material have catalytic ability and realize the enhanced removal of refractory organic matter at the same time. Grafting certain groups or segments with special response to external physicochemical stimuli (e.g. temperature, pH, etc.) on the membrane surface can achieve the identification and selective passage of specific ions (e.g. K+ , Ba+ , etc.) in the solution to be separated; the membrane material can have the function of specific identification and selective adsorption of pollutants and heavy metals by molecular imprinting. The precise regulation of membrane structure can greatly improve membrane performance. Using 3D printing technology based on reaction monomer electrospray instead of traditional interface polymerization technology, US scientists have successfully realized the nanoscale fine regulation of film thickness and surface roughness of PA RO membrane material, which has higher salt cross section and water flux compared with commercial RO membrane products. In addition, the ingenious design of nature also inspired researchers like Zhejiang University research team through the addition of polyvinyl alcohol in the water phase to improve the interfacial polymerization of active agent and inhibitor diffusion coefficient difference, resulting in tubular, bubble-like, and other different Turing structure, giving the membrane more permeable sites with the structure of NF membrane water permeability than the conventional NF membrane 3–4 times. The water channel proteins on the cell membrane regulating the entry and exit of water

Acknowledgments

molecules were introduced into the polymer-based membrane. The as-prepared biomimetic NF membrane water flux was increased by more than 50% with 95% rejection of MgCl2 (operating pressure 0.1 MPa). At present, the Aquaporin InsideTM series desalination membrane produced by Aquaporin A/S of Denmark has been successfully applied in municipal sewage treatment, and the industrialization of aquaporin biomimetic membrane has been successfully realized. Introducing nanomaterials into the polymer active separation layer can maintain a low active layer thickness while regulating the membrane surface performance and structure, effectively enhancing membrane flux and rejection. With the development of materials, chemistry, and other disciplines, new conceptual membrane materials have been proposed by researchers, such as graphene conceptual membrane materials, through the lamellar spacing of graphene membranes to achieve ion interception. The conceptual membrane materials of CNT water channel membrane have also been proposed, and water molecules can pass through CNT water channel membrane in a special form, thus greatly improving the filtration performance. The emergence of new conceptual membrane materials provides a continuous stream of impetus for the research and development of high-performance membrane materials in the future. At present, membrane industry in China is in an important period of strategic development opportunities; sewage treatment and resources and other areas have a broad development space. However, we also need to soberly realize that there is still a certain gap between the membrane materials in China and the other developed countries in terms of independent original innovation, core competitiveness, and so on. Firstly, the membrane industry supporting capacity is insufficient, and upstream main core raw materials are difficult to meet the requirements of membrane material production. Secondly, membrane material innovation to industry transformation efficiency is low, technology research and development and industry interaction still need to be strengthened; in the key high-end membrane material, preparation needs to further break through to improve the competitiveness of domestic membrane in the international market. In addition, in the membrane industry, technical standards and other aspects need to be further strengthened; the quality of membrane products in the market is uneven and needs to follow technical standards, so as to promote the healthy development of the membrane industry. The membrane fouling control, energy saving, and consumption reduction in the process of membrane process operation are still important concerns in the development and application of membrane technology.

Acknowledgments This chapter is supported by the NSFC-BRICS STI Framework Program (No. 51861145309), the National Natural Science Foundation of China (No. 51971029), the National S&T Major Project (No. 2018ZX10301201), the “All English teaching demonstration course construction project of University of Science and Technology Beijing” (No. KC2015QYW06, 2016), the “1125” Zhihui Zhengzhou Talent project

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of Henan province (Fund No. in USTB: 39080070), the “100 talent plan” fund of Fujian province (Fund No. in USTB: 39080067), and the development of a high sensitive magneto-optical biomolecular sensor experimental prototype (Fund No. in USTB: 2019-0649) by Hangzhou Ruidi Biotechnology Co. Ltd.

List of Abbreviations and Symbols APCI BWRO CA CCS CNTs COFs CPF CTA CVD ED FO FOS GBs GO GOS MED MF MIEC MPD MSF MST NF NF-TFC PA PA PBO PBT PCVD PEO PIM PPD PRO PRP PSf RO SWRO TFC

Air Products & Chemicals, Inc. brackish water reverse osmosis cellulose acetate CO2 capture and storage carbon nanotubes covalent organic framework concentration polarization factor cellulose triacetate chemical vapor deposition electrodialysis forward osmosis fructooligosaccharides grain boundaries graphite oxide galactooligosaccharides multi-effect distillation microfiltration mixed ionic–electronic conducting m-phenylenediamine multistage flash membrane separation technique nanofiltration thin film composite nanofiltration aromatic polyamide polyamide polybenzoxazole polybenzothiazole plasma chemical gas-sinking fit pressure-enhanced osmosis polymers with intrinsic microporosity p-phenylenediamine pressure-retarded osmosis piperazine polysulfone reverse osmosis seawater reverse osmosis thin film composite

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TMC TRs UF UPMs USDI VCD F Δx l ΔTMP Lp 𝜇 Rm J lim Cb Cp ΔΠb Rt W F M

trimesoyl chloride thermally rearranged polymers ultrafiltration ultra-permeable membranes US Department of the Interior vapor compression distillation driving force potential difference average thickness of the film transmembrane pressure difference permeability coefficient viscosity of the penetrating liquid membrane resistance limit fluxCg concentration of the gel layer on the membrane surface solute concentration in the raw material liquid solute concentration of the permeate osmotic pressure difference on both sides of the membrane total resistance when filtering the material liquid power flow rate molecular weight

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94 Robeson, L.M., Burgoyne, W.F., Langsam, M. et al. (1994). Polymer 35: 4970–4978. 95 Masuda, T., Isobe, E., Higashimura, T., and Takada, K. (1983). Journal of the American Chemical Society 105: 7473–7474. 96 Ulbricht, M. (2006). Polymer 47: 2217–2262. 97 Nagai, K., Masuda, T., Nakagawa, T. et al. (2001). Progress in Polymer Science 26: 721–797. 98 Kouzai, H., Masuda, T., and Higashimura, T. (1994). Journal of Polymer Science Part A: Polymer Chemistry 32: 2523–2530. 99 Tachimori, H. and Masuda, T. (1995). Journal of Polymer Science Part A: Polymer Chemistry 33: 2079–2085. 100 Hu, Y., Shiotsuki, M., Sanda, F. et al. (2008). Macromolecules 41: 8525–8532. 101 Ichiraku, Y., Stern, S.A., and Nakagawa, T. (1987). Journal of Membrane Science 34: 5–18. 102 Nagai, K., Higuchi, A., and Nakagawa, T. (1995). Journal of Polymer Science Part B: Polymer Physics 33: 289–298. 103 Kelman, S.D., Matteucci, S., Bielawski, C.W., and Freeman, B.D. (2007). Polymer 48: 6881–6892. 104 Budd, P.M., McKeown, N.B., Ghanem, B.S. et al. (2008). Journal of Membrane Science 325: 851–860. 105 Li, Y., Lu, F.P., and Wang, Z.X. (2013). Research progress on glycosyltransferases in functional oligosaccharide synthesis. Food Science 34 (09): 358–363. 106 Sangeetha, P.T., Ramesh, M.N., and Prapulla, S.G. (2005). Recent trends in the microbial production, analysis and application of fructooligosaccharides. Trends in Food Science & Technology 16 (10): 442–457. 107 Machado, M.T.C. et al. (2016). Clarification and concentration of oligosaccharides from artichoke extract by a sequential process with microfiltration and nanofiltration membranes. Journal of Food Engineering: 120–128. 108 Ledur Alles, M.J., Tessaro, I.C., and Zapata Norena, C.P. (2015). Concentration and purification of yacon (Smallanthus sonchifolius) root fructooligosaccharides using membrane technology. Food Technology and Biotechnology 53 (2): 190–200. 109 Cordova, A., Astudillo, C., Guerrero, C. et al. (2016). Assessment of the fouling mechanisms of an ultrafiltration membrane bioreactor during synthesis of galactooligosaccharides: effect of the operational variables. Desalination 393: 79–89. 110 Sarney, D.B., Hale, C., Frankel, G. et al. (2000). A novel approach to the recovery of biologically active oligosaccharides from milk using a combination of enzymatic treatment and nanofiltration. Biotechnology and Bioengineering 69 (4): 461–467. 111 Fang, X. (1998). Application of hollow fiber ultrafiltration membrane in the production of kanamycin sulfate. Medical Engineering Design (03): 3–5.

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15 Biomass-Derived Functional Films and Coatings Gao Xiao 1,2 1 Fuzhou University, Department of Environment Science and Engineering, School of Environment and Resources, 2 Xue Yuan St., Fuzhou 350108, Fujian Province, China 2 Harvard University, Department of Chemistry and Chemical Biology, John A. Paulson School of Engineering and Applied Sciences, 9 Oxford Street, Cambridge, MA, 02138, USA

15.1 Introduction Biomass-derived materials have attracted immense research interests in recent years because of their great potential for producing a variety of high-value products with low impact on the environment [1–3]. Biomass is a hodgepodge category of renewable sources that are popular or were popular recently [4]. Biomass waste is organic material disposed by humans that originated from agricultural processes, which includes waste from crops, chemicals, and rural households. Most biomass originating from agricultural waste, if improperly disposed, will cause emissions of SO2 , H2 S, N2 O, CH4 , and large amounts of smoke into the atmosphere, causing global pollution problems [5–7]. Apart from environmental pollution, economic losses caused by traditional disposal of biomass waste cannot be ignored. Burning biomass waste, including crop residues, in the open air or the kitchen is one of the main sources of serious air pollution [6]. On the other hand, biomass waste can be utilized as valuable materials, such as bioadsorbents, because it has loose and porous structural properties, and has functional groups like carboxyl and hydroxyl [8]. Meanwhile, biomass waste is available in large quantities, can be recycled, is biodegradable, and is environmentally friendly that it has good prospects for the comprehensive utilization of resources in environmental pollution control [6, 9]. In addition, biodegradable polymers broaden the range of waste management treatment option over traditional plastics and this is supported by the life cycle assessment [10]. The most favored end-of-life disposal options for these materials are domestic and municipal composting instead of landfill, which is the worst disposal option [11]. Therefore, biodegradable polymers can make significant contributions to material recovery, reduction of landfill, and utilization of renewable resources [10]. Because of the difficulty in recovering the conventional polyethylene mulching film after its use, where it can be buried in the soil along with the plant remains in order to be decomposed by microorganisms [12], the contributions Inorganic and Organic Thin Films: Fundamentals, Fabrication, and Applications, First Edition. Edited by Yujun Song. © 2021 WILEY-VCH GmbH. Published 2021 by WILEY-VCH GmbH.

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in this special issue of biomass derived functional films and coatings highlight the mastering of the fundamentals of film formation and characterization, which extends toward utilization. The applications covered in this special issue, supported by the state-of-the-art analysis, provide a dissection of the status of the biopolymer thin film and coating field [13]. We hope that the issue will inspire the readers to further contribute to this exciting field of fundamental and applied endeavors [14]. Biomass derived films are biodegradable and degrade easily when exposed to heat, moisture, and microbial actions, hence leave no traces of residue in the implantation [15]. This chapter covers the recent developments in the field of biomass derived functional films and coatings with special emphasis on the barrier properties. Many state-of-the-art biopolymers including pectin, starch, chitosan, xylan, galactoglucomannan, lignin, and cellulose nanofibrils (CNFs) are discussed [16]. In most cases, the barrier-related properties of single-layer biopolymer films are inadequate, and the fabrication process of thin film coatings, such as sol–gel and atomic layer deposition (ALD) and multilayer coatings, are also briefly touched [17]. For instance, bioplastics are well-known examples of renewable products obtained from biomass that has shown increasing potential to succeed the conventional plastics [18]. The “drop-in” plastics are the market leaders, which reduce the carbon footprint but continue to state the biodegradability concern attributed to most plastic products in the packaging sector. Designer polymers with new properties will broaden the application of bioplastics, not only in packaging (currently), but also in other applications (medicine, textile, electronics, etc). Improvements in biodegradation of bioplastics particularly combining with energy generation will maximize the benefits [19].

15.2 Biomass-derived Polymers Most recently, biodegradable polymers have become the most studied class in the field of polymer science, which have comparable properties with synthetic polymers. Depending on the source of origin, biodegradable polymers can be classified as either derived naturally through fermentation or by extracted biomass-based precursor through polymerization [20–22]. Various categories of biodegradable polymers derived from agricultural sources such as polysaccharides or protein-based polymers as shown in Figure 15.1. The first category of biopolymers are those which are derived directly from biosources and consist of polysaccharides, lipids, and proteins. Polysaccharides that include cellulose, chitosan, gum, starch, pectin, etc., are the most abundantly available and naturally derived biopolymers. These are generally extracted from renewable lignocellulosic biomass, sea creatures, and agricultural resources [23, 24]. Collagen, silk, keratin, and gelatin belong to the class of proteins which are generally isolated from animal extracts, insects, worms, animal hair, bird feathers, etc., through chemical treatment [25]. Recently, research on bio-derived polymers have been widely carried out to develop strategies to convert “wastes into wealth” due to their interesting physicochemical properties

15.2 Biomass-derived Polymers

Biodegradable polymers Directly extracted from biomass

Produced by microbes

Polymerization routes

Polysaccharides PHB Cellulose

Chitosan

Starch

PLA

Bacterial cellulose

Lipids Xanthan Proteins

Figure 15.1 Biodegradable polymers with various source of origin derived from different renewable resources.

such as higher structural properties, bio-origin, high functionality, nontoxicity, and improved biocompatibility.

15.2.1 Collagen As byproducts of the meat processing industry, nearly 100 million tons of bones, skin, and scales are generated from livestock, poultry, and fish every year that are generally discarded as waste [26–28]. However, these widespread and low-cost biomass materials are rich in collagen that is primarily composed of the elements C, N, O, and S. Collagen is a protein found in connective tissues of the body [29], and is significant in making certain parts of the body, such as nails and joints healthy. Collagen supplements come in various forms. One of the most common is called gelatin. For a long time, gelatin has been used in collagen replacement and has been thought to strengthen nails and hair [30]. Collagen molecules are produced by cells and are self-assembled into hierarchical structures to form collagenous tissues (shown in Figure 15.2) [31]. Figure 15.2 shows a schematic of the hierarchical structure of collagenous tissues [32]. The collagen molecule is a triple helical protein structure that consists of three chains with a characteristic repeating sequences (Gly–X–Y)n . A type I single collagen molecule has a diameter of about 1.6 nm with a length of about 300 nm. Collagen molecules form into collagen fibrils with a diameter of about 100 nm with a specific pattern known as D-period [33]. The collagen fibrils that form the fibers at the micron scale, which finally form the collagenous tissues, are the basic components of various collagenous tissues, while the alignments of collagen fibrils and the components in a collagen fiber varies in different collagenous tissues to provide various mechanical and biological functions. For example, bone contains minerals to provide higher strength. In contrast, there is no mineral in tendon which can exhibit more strain in our daily activities. Collagen fibrils exhibit a parallel alignment in tendon and bone but align in different orientations in the cornea to support varied loading directions [34].

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15 Biomass-Derived Functional Films and Coatings Collagen molecule

300 nm 1.5 nm α2 helix α1 helices

Collagen microfibrils Crosslinks

Collagen fibrils

D–spacing:~67 nm

80–100 nm

Multiple microns

Figure 15.2 Collagen structure and organization. Collagen molecules self-assemble in a quarter-staggered array into microfibrils to form collagen fibrils with characteristic periodic D-spacing. Source: Modified from Canelon and Wallace [31].

A statistical analysis of high-resolution X-ray crystal structures of triple-helical peptides has provided the molecular structure information of collagen molecules [35]. It has been revealed that the collagen molecule has a varied unit height of 0.853 nm for imino rich regions, 0.865 nm for amino rich regions, and inner radius of around 0.1–0.2 nm depending on the variation of the collagen sequences [36]. The collagen molecule is a heterogeneous structure along its twisting axis that the local conformation is controlled by the variation of sequences and each segment has varied mechanical and biological properties, and likely, biological functions.

15.2.2 Cellulose Nanofibrils CNFs can be isolated from plant cell walls. The abundant availability of plant biomass and the superior mechanical properties of cellulose made nanocellulose including CNCs and CNFs a desirable reinforcing material for polymer nanocomposites [37]. As shown in Figure 15.3, cellulose is a linear chain polysaccharide consisting of repeated β-(1→4)-D-glucopyranose units. Hydroxyl groups are abundant on CNF surfaces, allowing potential hydrogen bonding and surface modifications. CNFs can be produced by 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO)-mediated oxidation (2,2,6,6,-tetramethylpiperidine-1-oxyl radical), multi-pass high-pressure homogenization, enzymatic hydrolysis or direct mechanical fibrillation [38, 39]. The morphologies and dimensions of CNFs can vary substantially, depending on the degrees of fibrillation and any pretreatment involved [40]. Cellulose, one of the most versatile and widely found biopolymers in

15.2 Biomass-derived Polymers

OH OH O O

HO

HO

O O

OH OH

Tree

Plant cell

Macrofibrils

Microfibrils

100 μm

10 μm

1μm

Cellulose molecules 1nm

n

(a) Crystalline regions Nanocrystals Baterial cellulose nanofibers

Disordered region

Strong acid hydrolysis

Amorphous regions

(b)

(c)

Figure 15.3 Cellulose contained in plants or trees has a hierarchical structure from the meter to the nanometer scale, as shown in (a). A schematic diagram of the reaction between cellulose and strong acid to obtain nanocellulose is shown in (b). Bionanocellulose cultured from cellulose-synthesizing bacteria is shown in (c). Source: Miyashiro et al. [37]. Licensed under CC BY 4.0.

nature, has been used by humans for millennia as a building material, an energy source, a component of clothing, and for storing and sharing knowledge and culture [41]. Today, cellulose materials are used in a wide range of applications, and the paper and pulp industry constitutes a significant share of the economic output in many countries. One recent strong trend, on an international scale, is to focus on the isolation of fibrils and whiskers of cellulose with diameters in the nanometer range and to utilize their enhanced properties to develop novel cellulose-based materials with diverse advanced functionalities. CNF, also referred to as nanocellulose are typically generated by mechanical grinding or high pressure fluidization [42]. CNF consists of very thin (∼20 nm) and long (several μm) fibrils and in a low concentration of 98% montmorillonite), simply prepared by casting the solution onto Petri dishes. Pectin is a family of complex polysaccharides, which are found in high amounts in plant primary wall. The exact chemical structure of pectin is still under debate [48]. Pectins are a family of covalently linked galacturonic acid-rich polymers. Until now, three main pectic polysaccharides have been isolated from plant wall whose structure has been identified. They are homogalacturonan (HG), rhamnogalacturonan-I (RG-I), and substituted galacturonans (GS) [49]. HG which constitutes about 65% of pectin molecule is a linear chain of D-galactopyranosyluronic acid (GalpA) bound in α-1,4. The carboxyl group of some residues can be methyl-esterified [50]. According to the plant species, HGs may also be partly O-acetylated on C-3 or C-2 (Figure 15.4) [51]. The Galp residues forming the main chain can be O-acetylated in C-3 or C-2 but are usually not linked with monomers or lateral chains. Among these GS is rhamnogalacturonan-II (RG-II), HG

XG

RG II

RG I

AG

Lateral chain B

B OH COOH O

1

α

Lateral chain A O

4

O COOH

O

O

OH D-Galacturonic

O-Acetyl ester O-Methyl ester

acid

L-Rhamnsose D-Galactose L-Arabinose

D-Dha*

D-Xylose

D-Apiose

L-Galactose

L-Aceric

acid L-Fucose

Kdo** D-Glucuronic

acid

B Borate

Figure 15.4 Schematic representation of pectin structure. AG, arabinogalactan; HG, homogalacturonan; RG, rhamnogalacturonan; XG, xylogalacturonan. Source:Eliaz and Raz [51]. Licensed under CC BY 4.0.

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which has nothing to do with RG-I, its main chain is not composed of GalA-Rhap disaccharide but of a HG chain [52]. Four types of chains with structurally different oligosaccharides are linked to the main chain of RG-II, which are composed of 12 types of glycosyl residues bound together by at least 22 types of glycosidic bounds. One nonasaccharide (lateral chain B) and one octasaccharide (lateral chain A) are attached in C-2 of some GalA residues from the main chain and two different disaccharides are linked in C-3 of the main chain [53]. The localization of these lateral chains one in relation to the other is not yet determined (Figure 15.4). RG-II is often found in dimers thanks to a borate ion located on chain A. This dimerization seems essential for the integrity of the plant cell wall. Despite its complexity, the RG-11 structure is well conserved in vascularized plants [54]. RG-II have been identified until now, which indicates the importance to conserve its structure. Other GS have been described in a short number of plants. Xylogalacturonan contains β-D-xylosyl (Xylp) linked in C3 of the main chain and is present in reproductory tissues of plants like apple, carrot, and cotton [52]. Apiogalacturonan is found in some monocotyledons. The most accepted model for pectin structure is a main backbone of HG, which are intercalated regions of RG-I, RG-II, and GS [55].

15.2.4 Starch Starch is a carbohydrate which is composed of two polysaccharides called amylose and amylopectine [56], which are polymers of glucose. Starch is present as a resent food in plant bodies. Glucose, formed during photosynthesis, is mostly turned into starch. Starch is usually granular. The shape and structure of starch grains vary from plant to plant. Starch is composed of two polysaccharides mentioned below (shown in Figure 15.5) [57]: (1) Amylose. The glucose molecules of amylose mutually remain attached to carbon 1–4. Usually, an amylose is composed of 200–1000 molecules of glucose. Amylose has an unbranched chain of molecules. (2) Amylopectin. Besides the bonding of glucose molecules with carbon 1–4 of amylopectin, glucose molecules also remain attached to ∞-1-6. Usually, an amylopectin is composed of 2000–200 000 glucose molecules. Amylopectin has a branched chain of glucose molecules. Starch films can be produced either by extrusion or casting process [58]. Plasticizers, such as glycerol, sorbitol, or xylitol, are typically added for reducing

O

H C

H OH

H

C

C

H

OH

HO

Figure 15.5

CH2OH

CH2OH

CH2OH C

H C O

H

C

C

H OH

O H

C

C

H

OH

Structure of starch.

H C

H

C

O

C

H OH

H

C

C

C

OH

H

OH

O n

H

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15 Biomass-Derived Functional Films and Coatings

brittleness [59]. At low glycerol concentrations, both strain and strength decreased, but above 20% glycerol concentration, the elongation reached larger values [60]. Effects of glycerol, sorbitol, or xylitol on physical and mechanical properties of starch films are largest for glycerol and smallest for sorbitol. High contents of xylitol and sorbitol resulted in changes in physical and mechanical properties of films probably due to phase separation and crystallization [61]. Crystallization can be prevented by using binary polyol mixtures as plasticizers. Water sorption and water vapor permeability are lower for the films plasticized with xylitol–sorbitol mixture than for the films plasticized with glycerol–xylitol and glycerol–sorbitol mixtures at constant plasticizer contents [62]. Oxygen permeability of the films is strongly dependent on the water content. At low humidity conditions, the films are typically excellent barriers against oxygen transmission, but when the water content rises above 20%, the barrier is mostly lost [63]. The increased permeability is most likely due to increased polymer chain mobility, which facilitates the transport process. Oxygen permeability of starch films can be decreased by using below 21% of sorbitol [16]. This behavior is related to changes in secondary relaxations, which are hindered because of the connections established between starch and sorbitol, leading to decreased diffusion of oxygen molecules. For sorbitol contents above 21%, oxygen permeability increased slightly. Water has produced a classic plasticizing effect (an increase of oxygen permeability), but at low sorbitol content, this is only moderate due to the effect of sorbitol itself [64].

15.2.5 Chitosan Chitosan, the β-1-4-linked polymer of 2-amino-2-deoxy-β-D-glucose, is prepared by the N-deacetylation of chitin, the second most abundant natural biopolymer after cellulose, whose common molecular structure is schemed in Figure 15.6 [65]. Chitosan is soluble in dilute aqueous acid solutions (e.g. acetic acid) and has been widely studied due to its good film forming ability [66]. Chitosan dissolved in the 1% acetic acid solution is usually applied onto the copy paper by lab-scale bar coater and dried at 105 ∘ C [67]. Chitosan coatings clearly improve both the gloss and oxygen barrier properties of paper. The gloss value in the machine direction was increased as a function of added chitosan [68]. Chitosan can also be immobilized onto NH3− or CO2− plasma activated polypropylene films by exploiting either carbodiimide or glutaraldehyde chemistries to encourage crosslinking reactions [69]. Barrier properties Chitosan HO H

H O

HOH2C H HO H

NH2

H C

O H

H NH2

O C H

H

H

CH2OH

O

n

Figure 15.6

Chemical structure of chitosan. Source: Based on Kumar et al. [65].

15.2 Biomass-derived Polymers

can be further improved by using nanoclays incorporated into chitosan [70]. Biohybrid nanocomposite films and multilayer coatings have improved barrier properties against oxygen, water vapor, grease, and UV-light transmission. Oxygen transmission is significantly reduced under all humidity conditions. In the dry condition, more than 99% reduction and almost 75% reduction in oxygen transmission rates can be obtained at the 80% relative humidity. However, hydrophilic chitosan is lacking the capability to prevent water vapor transmission; thus total barrier effect of nanoclay containing films is no more than 15% as compared to the pure chitosan coating films [71].

15.2.6 Xylan Xylan is a group of hemicelluloses that represents the third most abundant biopolymer on Earth. It is found in plants, in the secondary cell walls of dicots and all cell walls of grasses [72]. Xylans are polysaccharides made up of β-1,4-linked xylose (a pentose sugar) residues with side branches of α-arabinofuranose and α-glucuronic acids and contribute to crosslinking of cellulose microfibrils and lignin through ferulic acid residues [73]. On the basis of substituted groups, xylan can be categorized into three classes: (i) glucuronoxylan (GX), (ii) neutral arabinoxylan (AX), and (iii) glucuronoarabinoxylan (GAX) [74]. Xylan is alkali extracted from kraft hardwood pulp, whose molecule structure is shown in Figure 15.7. It is almost as ubiquitous as cellulose in plant cell walls and contains predominantly β-D-xylose units linked as in cellulose. Xylan as such is not suitable for film formation due to its high internal cohesion, which results in fragile and fragmented films [75]. Thus, xylan needs to be plasticized externally or internally. It can be further hydroxypropylated, resulting in a water-soluble derivative with an improved film forming properties [76]. The inner plasticization with hydroxypropyl groups can be combined with external plasticizers, such as glycerol or sorbitol. The biodegradability, high degree of biomass-based raw materials, and nonfood origin of hydroxypropylated xylan (HPX) coatings –OOC α-Glucuronidase (GH67, 115)

O

H3CO HO

Xylanase (GH8, 10, 11, 30) O

HO

O

O O

O

OH

O OH

O H3CO

O

O

H3C

OH HO O

Acetyl esterase (CE1–7, 12)

O HO O

O O

n

α-L-Arabinofuranosidase (GH43, 51, 54, 62) OH

Ferulate esterase (CE1)

Figure 15.7 The schematic shows a basic xylan structure, the enzyme classes that hydrolyze the various linkages, and their respective major CAZy families. Simple glucuronoxylans predominate in the walls of dicots, whereas monocots usually contain more complex glucuronoarabinoxylans.

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and films make them an attracting alternative of films from petroleum for food packaging. Coatings of aqueous HPX solutions containing glycerol and sorbitol are applied with a lab-scale bar coater onto a pigment-coated board [77]. Furthermore, fatty acid esters of hemicelluloses and cellulose have been successfully coated onto paper/board. The coatings are hydrophobic and provide good grease and moisture barrier properties. The best water and water vapor proof can be achieved with the cellulose derivatives esterified with the longest fatty acid [78]. Benzyl ether derivatives of birch xylan with varying degree of substitution have recently been prepared. These derivatives are also hydrophobic and thermoplastic capable of forming free standing films from organic solvents [66]. The paper sheets prepared from the esterified cellulose fibers showed high hydrophobicity. The barrier properties of xylan may be further improved through nanoclay reinforcement in aqueous medium. The nanoclay dispersions were mixed with the plasticized (glycerol) xylan, resulting in homogeneous and viscous coating solutions. Xylan-nanoclay dispersion was further coated onto paper by lab-scale bar coater and the coatings significantly enhanced water vapor barrier properties [79].

15.2.7 Lignin Lignin is one of the most abundant natural polymers, which is a complex oxygen-containing organic substance that, with cellulose, forms the chief constituent of wood. It is second only to cellulose as the most abundant organic material on Earth, though it has found relatively few industrial uses other than as a fuel [80]. It is a mixture of complex, apparently polymeric compounds of poorly known structure. Lignin is concentrated in the cell walls of wood and makes up 24–35% of the oven-dry weight of softwoods and 17–25% of hardwoods. It is removed from wood pulp in the manufacture of paper, usually by treating with agents such as sulfur dioxide, sodium sulfide, or sodium hydroxide. Lignin has a number of industrial uses such as binder for particleboard and similar laminated or composite wood products; as a soil conditioner; as a filler or as an active ingredient of phenolic resins; and as an adhesive for linoleum [81]. Softwood lignin has been esterified with tall oil fatty acid (TOFA) and applied with a lab-scale bar coater onto the paperboard. According to Urbaniec et al. [9], the characteristics of lignin, due to phenolic hydroxyl groups, aliphatic hydroxyl groups, low molecular weight, and narrow polydispersity result in significant antioxidant activity. Lignin has an aromatic and highly crosslinked structure and is quite reactive due to its functional groups. Therefore, lignin is able to interact with many polymers and change their wettability, fire resistance, and mechanical properties. Oliviero et al. [82] revealed that the addition of lignin to zein protein matrix caused some changes in the secondary structure of the zein protein, α-helix, β-sheet, and β-turn. Therefore, lignin can modify physicochemical properties (other than antioxidant activity) in some films. The effect of different types of lignin on the functional and physicochemical properties of natural polymers at different film preparation conditions has been extensively studied [82]. However, the interaction between soy protein isolate (SPI) and lignin under transglutaminase enzyme treatment, which

15.2 Biomass-derived Polymers

may potentially crosslink them, has not been explored yet. There is also a decrease in oxygen transmission observed for the TOFA lignin ester as well as the TOFA coated paperboard samples. The coating materials do not affect the tensile strength of the paperboard. Esterification with palmitic and lauric acid chloride will result in coatings with greatly improved moisture barrier properties [83]. The localization of lignin and cellulose in plants along with the chemical structure of the biopolymers and the monolignols that form the highly heterogeneous lignin structure are schematically shown in Figure 15.8 [84]. The wood pulping techniques, such as the kraft, sulfide or organosolv processes, which are currently applied for the generation of cellulose by separation from lignin, are highly energy-intensive and demand a variety of additives [85]. The lignin produced this way as byproduct is mainly used as a low-grade fuel to compensate the energy demands of the pulping industry [86]. A more sustainable process for the pretreatment of wood to separate lignin and cellulose is therefore highly desirable from economic and ecological points of view. Furthermore, material usage of lignin in the form of biodegradable polymers could reduce the environmental problems compared to mineral oil-based polymers and minimize resource underutilization. Polyurethanes are an important material for coatings and use of lignin enables incorporation of the biomacromolecule into a higher value polymer material. The

Lignin (aromatic polymer) Lignin

Hemicellulose (polysaccharides) Hemicellulose

Lignin– carbohydrate limkage

O O B HO

γ

β

α O

A,B E,F

4' 6' HO O O 4'' HO C 5' O 2' D 3' 1' HO

Structural elucidation of α ether linkage between lignin and carbohydrates by 2D-, 3D- NMR Fundamental plant science plant biomass conversion Biomass refinery

Figure 15.8 A three-dimensional view of the lignin–carbohydrate complex (LCC) in the wood cell wall. An illustration of wood cell walls and their ultrastructure focusing on the ether linkage between lignin and carbohydrate, a structural feature that has remained unresolved for more than 150 years, and whose presence and location is elucidated in this study. Wood fiber networks consist of wood cell walls composed of cellulose, hemicellulose, and lignin, the most abundant organic resources on earth. These polymer combinations give the plant cell wall strength and structure; however, the cell wall’s robustness is a barrier for the separation and utilization of plant-based resources. Thus, the LCC is a key structure to elucidate physiologically, physically, and industrially. Source: Nishimura et al. [84]. Licensed under CC BY 4.0.

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rational choice of the isocyanate for prefunctionalization of lignin supports the preparation of the materials for better mechanical performance [87].

15.3 Coating Technologies of Biomass Thin Films 15.3.1 Sol–Gel Coating Method Sol–gel phase inversion coatings are ceramic or organically modified hybrid materials formed from a colloidal solution (sol) that acts as a precursor (shown in Figure 15.9). Typical process involves the hydrolysis of alkoxysilanes to produce hydroxyl groups, followed by polycondensation among these groups and residual alkoxy groups to form a three-dimensional polymeric network (or gel) [88]. Coatings with a longer chain are found to decrease the wettability and absorption of base paper more efficiently than coatings with a shorter alkyl chain. Sol–gel hybrid coatings applied with roll-to-roll pilot line onto poly lactic acid (PLA) laminated paperboard also improved other barrier properties [16]. In addition, the atmospheric plasma pretreatment has been used to activate PE-coated paper prior to applying the sol–gel coatings. Sol–gels functionalized with long-chain hydrocarbon Precursors solution

Dip-coating

Sol

Gel

Spin-coating

Electrodeposition +

–

Film gel Drying

Cure Coating

Figure 15.9 Preparation of coatings by sol–gel method. Source: From Nguyen-Tri et al. [88]. Licensed under CC BY 4.0.

15.3 Coating Technologies of Biomass Thin Films

groups or amino groups were sprayed and cured by drying at 110 ∘ C. Hydrophobic coatings containing long hydrocarbon chains were effective barriers against grease penetration whereas hydrophilic coatings with amino groups reduced oxygen transmission rates especially at low relative humidity. Plasma activation clearly improved adhesion and performance of both coatings [89].

15.3.2 Atomic Layer Deposition (ALD) ALD technique is a surface-controlled layer-by-layer deposition process based on self-limiting gas–solid reactions [90]. As shown in Figure 15.10, a typical ALD cycle usually consists of two half-cycles. It is well suited to produce inorganic barrier coatings, such as Al2 O3 , SiO2 and ZnO on various materials including synthetic and biomass-based plastics and biopolymers such as PLA, polyhydroxy butyrate (PHB), pectin, galactoglucomannan and CNFs. Atomic layer deposited Al2 O3 has proven to be effective in enhancing the moisture and gas barrier properties of various films and coatings. With ALD ZnO coatings both barrier and antimicrobial properties of biaxially oriented poly lactic acid (BOPLA) and biaxially oriented polypropylene (BOPP) films were improved. PLA films were coated with 20 nm polyelectrolyte multilayer film made from sodium alginate and chitosan, and additionally, with 25 nm ALD Al2 O3 layer. The double-coating is found to significantly enhance the water vapor barrier properties of the PLA film due to increased surface hydrophobicity. ALD, electron beam evaporation, magnetron sputtering, and sol–gel method have been used to deposit thin aluminum oxide coatings onto low density polyethylene (LDPE)- and PLA-coated board. Increased barrier performance against moisture and oxygen permeation rates were observed with each technique [91]. However, among the techniques tested ALD was found to be most suitable. Typically, ALD coatings have been produced with slow batch processes. Recently, 30–40 nm Al2 O3 barrier coatings were deposited onto cellophane, PLA, and polyimide films with continuous roll-to-roll technique [92].

Figure 15.10 Schematic illustration of a typical ALD cycle consisting of two half-cycles. Sequential precursor and co-reactant doses are separated by purge or pump steps, leading to self-limiting film growth. “M” indicates the metal atom, which can for instance be bound to oxygen or nitrogen atoms (in blue), to form a metal oxide or metal nitride, respectively. Precursor ligands are colored green and are eliminated by reaction with the co-reactant before being purged away [90].

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15.3.3 Multilayers Coating Method The biobased multilayer coating consisting of starch, nanoclay, and inorganic nanolayer has been found favorable in terms of CO2 emissions as compared to polyethylene terephthalate (PET)/aluminum coatings. In some cases, the individual biopolymer layers may even be attached to each other using biomass-based adhesives [93]. As single layer CNF films have relatively weak moisture resistance, flexibility, and zero sealing ability, the multilayer structures have been recently produced. CNF is first dispersion-coated onto PET film and further extrusion-coated with LDPE resulting in a 3-layer structure [94]. PET/CNF/LDPE film had excellent technical properties which fulfilled both oxygen and water vapor barrier as well as sealing ability requirements of modified atmosphere food packaging. Demonstrator pouches filled with nitrogen were produced with the packaging machine and the oxygen concentration inside the packages was monitored as a function of time. As a result, CNF containing multilayer films performed better as compared to commercial three-layer multilayer films containing ethylene vinyl alcohol copolymer (EVOH) barrier layer. The developed films could be used as packaging material for dry or low moisture content food products such as dry snacks, dried fruits, nuts, spices, etc. [95]

15.4 Degradable Biomass-derived Functionalized Films 15.4.1 Polysaccharide-based Films Generally, polysaccharide films are made from starch, alginate, cellulose ethers, chitosan, carrageenan or pectins and exhibit good gas barrier properties. Linear structure of some of these polysaccharides, for example, cellulose (1,4-β-D-glucan), amylose (a component of starch, 1,4-α-D-glucan), and chitosan (1,4-β-D-glucosamine polymer), renders their films tough, flexible, and transparent. Their films are resistant to fats and oils [96]. However, due to their hydrophilic nature, they are poor water vapor barriers. Among polysaccharide and biopolymers in general, starch is considered to be one of the most promising materials for use in biodegradable plastics [97]. Different approaches have been carried out to use this polysaccharide as natural biopolymers for the production of biodegradable thermoplastics, and different types of starch are processed in the blend systems with natural plasticizers and commercial fibers by conventional extrusion and injection molding techniques. Hydrophilic compounds, such as polyols (GLY and sorbitol) are commonly used in starch films, but some sugars, surfactants, amino acids, and fatty acids could also be employed to improve their mechanical and barrier properties. However, a limiting barrier to the development of starch materials is the brittle nature of blends containing high concentrations of starch. The recrystallization of starch restrains it from coming into real practical use, because it easily becomes rigid and brittle during long-term storage and loses its value. Overcoming the brittleness of starch while achieving full biodegradability in blends, can be accomplished with the addition of biodegradable plasticizers [98]. An ideal plasticizer for starch-based

15.4 Degradable Biomass-derived Functionalized Films

materials should impart flexibility and suppress retrogradation to thermoplastic starch (TPS) during aging. The crystallization of GLY-containing potato starch plastic sheets revealed a significant influence on their mechanical properties. The changes in crystallinity were clearly related to the initial amount of plasticizer and moisture migration during aging. The differences in material properties could be attributed to the formation of an entangled starch matrix and by starch chain-to-chain associations that are related to plasticizer content [98]. Zou et al. used chitosan to attach as an ultrathin biofilm on negatively charged colloidal lignin nanoparticles by electrostatic interaction. Such nanoparticles were especially suited for stabilizing oil-in-water Pickering emulsions for, e.g. food applications [99].

15.4.2 Protein-, Pig Skin Gelatin-, Lipid-based Films Proteins have a unique structure (based on 20 different monomers), which confers a wider range of functional properties, especially a high intermolecular binding potential. Protein-based edible films can form bonds at different positions and offer high potential for forming numerous linkages. Molecular weight, number, and positions of hydroxyl groups of a plasticizer are all variables that affect its ability to plasticize a protein-based polymer. Zein, the prolamine of corn, was investigated as a raw material for packaging materials [100]. Oleic and linoleic acids were added as plasticizers resulting in flexible sheets of high clarity, low modulus, high elongation and toughness, but low tensile strength. Fatty acid separation caused zein aggregation, resulting in loss of flexibility and increased water absorption. Linoleic acid was more effective than oleic acid at reducing water absorption of sheets. However, plasticization of zein with oleic acid resulted in relatively tough and water-resistant sheets that may be applied in thermoformed packaging trays. The plasticizing effect of traditional natural plasticizers such as polyols (GLY, polyethylene glycol [PEG], expansion reduced graphene [EG], diethylene glycol [DEG], triethylene glycol [TEG], and PNIPAm modified graphene [PG]), sorbitol and sucrose of varying type and concentration of plasticizer on protein- or lipid-based film properties is widely reported in the current literature [101].

15.4.3 Biomass Plastics Films Bioplastics are plastics derived from renewable biomass sources, such as vegetable fats and oils, corn starch, or microbiota. Bioplastic can be made from agricultural by-products and also from used plastic bottles and other containers using microorganisms [102]. Common plastics, such as fossil-fuel plastics (also called petrobased polymers) are derived from petroleum or natural gas. Production of such plastics tends to require more fossil fuels that will produce more greenhouse gases than the production of biobased polymers (bioplastics). Not all bioplastics are biodegrade nor more readily available than commodity fossil-fuel derived plastics. Bioplastics are usually derived from sugar derivatives, including starch, cellulose, and lactic acid. As of 2014, bioplastics represented approximately 0.2% of the global polymer market (300 million tons) [103].

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Biomass plastics utilize the biomass resources of regenerative biological origin. Currently, biomass plastics based on plant-derived materials are in practical use. The plastics are high-polymer materials made of raw materials consisting of starch, sugar, or cellulose contained in plants [104]. One example is polylactic acid, the chemically-synthesized lactic acid that is fermented from starch. Even if CO2 is ejected, when polylactic acid is incinerated, it can be utilized in photosynthesis by growing plants that are used as raw materials for the whole ecosystem recycling. Therefore, it will not significantly increase the CO2 in the atmosphere (carbon-neutral). It is an environmentally friendly material that does not contribute to global warming. Furthermore, this material is as strong and rigid as polystyrene. However, for polylactic acid to be used as a material in durable goods, issues related to heat resistance, impact resistance, and flame retardancy need to be addressed; the material must provide nonflammable parts for electric and electronic devices; and issues related to molding techniques for mass production of components have to be addressed [105]. Bioplastics is a key element in the transition to a low-carbon circular economy and can make a considerable contribution to increasing resource and recycling efficiency. Through the strategy, the European Commission is looking to assess how to decarbonize Europe’s plastics economy. By replacing a significant proportion of the conventional fossil-based feedstock with certified bio-based alternatives, bioplastics can help reduce greenhouse gas emissions and the EU’s dependency on imported fossil resources, as well as contribute to achieving the UN Sustainable Development Goals and EU Climate Protection Targets [106].

15.4.4 Cellulose-based Films Cellophane is a polymeric cellulose film made from the cellulose from wood, cotton, hemp, or other sources. The raw material of choice is called dissolving pulp, which is white like cotton and contains 92–98% cellulose. Its structure is composed of linear polymer chains of β-1,4-linked glucose residues from plant biomass, which is the richest resource for cellulose [107]. The use of cellulose in composites is mainly to exploit its high strength. Young’s modulus originated from the crystalline nature of the cellulose molecules, which in turn results from the hydrogen bonds extended along the molecular chains. Many researchers reported the generation of nanocellulose or cellulose microfibrils from various resources and their application in the composite Science and Engineering. The exploitation of cellulosic fibers as reinforcements in the composites has started more than a decade ago. The principal reasons for the utilization of cellulosic fibrils are (i) its high specific strength and modulus compared to other engineering materials and (ii) its reinforcing potential. The main challenge associated with the preparation of nanocellulose composite is their poor dispersion in the polymer matrix due to the agglomeration of the fibrils resulting from the hydrogen bonding within the cellulose fibrils. Most of the reported works used hydrophilic polymers to improve the dispersion, and plenty of research is going on in this direction to improve the dispersion and exploit the full potential of the nanocellulose. Cellulose acetate films could be prepared through

References S OH

OH

H2 C

H2C CH O

O

CH

CH

CH CH HO

H2C CH O

CH O

OH

O

OR

CS2

CH CH HO

OH

n

CH

O

S–Na+

H2C CH O

NaOH

CH O

C

CH O CH O

CH

CH CH RO

OR

Cellulose

CH O

CH CH RO

OR

n

Viscose S where

R=

C S–Na+

Figure 15.11

Conversion of raw cellulose to viscose.

acetylation of cellulose from sugarcane bagasse. The hemicellulose content (5%) present in bagasse was used as an internal plasticizer of the acetate cellulose films. Furthermore, residual xylan acetate acted as a plasticizer for cellulose acetate and the resulting film exhibited good mechanical properties without addition of an external plasticizer [108]. For example, the cellulose is dissolved in alkali in a process known as mercerization, where the resulting mixture is aged for several days. The mercerized pulp is treated with carbon disulfide to make an orange solution called viscose or cellulose xanthate. The viscose solution is then extruded through a slit into a bath of dilute sulfuric acid and sodium sulfate to reconvert the viscose into cellulose. The film is then passed through several more baths, one to remove sulfur, one to bleach the film, and one to add glycerin to prevent the film from becoming brittle [109]. The approximate chemical structures are shown in Figure 15.11.

Acknowledgments We gratefully acknowledge the financial support provided by the National Natural Science Foundation of China (Grant No.21506036), the Special Supported Project of China Postdoctoral Science Foundation (Grant No. 2020T130599) and the Natural Science Foundation of Fujian Province, China (Grant No. 2017J01412). Dr. G. X. further thanks the Chinese Scholarship Council (Grant No. 201606655030) and the Open Project Program of the State Key Laboratory of Photocatalysis on Energy and Environment (Grant No.SKLPEE-202013), Fuzhou University.

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16 Polymer Composite Coating for Anti-marine and Related Organism Corrosion Kaifeng Chen 1,2 , Zhipeng Xie 1,2 , Yu Liang 1,2 , Jingjing Wang 1,2 , and Haiyan Zhuang 1,2 1

Luoyang Ship Material research Institute, Xiamen Advanced Materials Academy, Xiamen 361101, China Ship Material research Institute, Science and Technology on Marine Corrosion and Protection Laboratory, Qingdao 266101, China 2 Luoyang

16.1 Introduction A wide variety of microorganisms exist in the ocean. They attach to the surface of engineering materials to form biofilm. Inside the biofilm, factors such as pH, dissolved oxygen, and organic and inorganic species are completely different from the marine environment in which the microorganism activity controls the rate and type of electrochemical reactions. The corrosion of metals and alloys affected by microorganisms is called microbiologically influenced corrosion (MIC). At present, the microorganisms related to metal corrosion in the marine environment include iron bacteria, sulfate-reducing bacteria (SRB), acid-producing bacteria, and slime-producing bacteria. Microbes directly or indirectly affect the metal corrosion process through activities in everyday life [1]: for example, Bacterial metabolism produces certain corrosive substances (such as sulfuric acid, organic acid, hydrogen sulphide and so on), thus affects the corrosion reaction process at the metal/solution interface; changes the surrounding environmental conditions, such as dissolved oxygen, salinity, and pH, etc.; and promotes or inhibits corrosion through metabolites [2]. The fouling-related microorganisms in the marine environment are called film-forming organisms, microfouling organisms, microbiological fouling organism, or microbiological fouling. They mainly formed a layer of viscoelastic film by bacteria and other microorganisms, such as diatoms and other seaweeds, protozoa, and so on, commonly known as marine biofouling film or marine biofilm. In fact, in addition, there are many fungi, yeasts, actinomycetes, seaweed spores, invertebrate larvae, etc. on the mucosa-forming organisms [3]. In short, the corrosion that occurs in the marine environment is complicated because the corrosion processes contain a variety of ways; the most special is that there are many marine organisms in the ocean, including microorganisms, animals, and plants. Most of the material corrosion that occurs in the marine environment is related to the interactions between marine life. Since Hilen published an article Inorganic and Organic Thin Films: Fundamentals, Fabrication, and Applications, First Edition. Edited by Yujun Song. © 2021 WILEY-VCH GmbH. Published 2021 by WILEY-VCH GmbH.

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on marine bacterial mucosa in the early 1920s, the world has summarized the application research and control of marine microfouling organisms in the past century. Among them, the ongoing researches that have made significant contributions include the following aspects: (1) The number, type, distribution, and community structure characteristics of microfouling organisms on the surface of submerged objects in each sea area. The attachment morphology, its formation, and succession change regular pattern. (2) The interaction between physicochemical properties of biological mucosa and the surface of its mucosal objects. (3) Correlation between mucosa and metal corrosion and its influence. (4) The relationship between biological mucosa and large fouling organisms. (5) Study on the attachment mechanism of bacteria and other microorganisms on the solid surface in the ocean. (6) Applied and control research on bacterial mucosa about ships, equipment, devices, instruments, and instruments directly related to the national economy as well as military department. (7) Study on the influence of various marine environmental factors on the formation and development of mucous membranes and their attachment mechanisms. (8) Research on various methods, feasibility measures, and assumptions of biological mucosal control. (9) Development, technology, and application of relevant biomucosal test methods. (10) New trends in the application of genetic engineering to microbial mucosa.

16.2 Microbial Corrosion and Protection 16.2.1 Primary Mucosa-forming Organisms and Their Attachment Marine biofouling comes from the undesirable settlement and accumulation of marine microorganisms, plants, and animals on submerged surfaces of materials, and it has huge adverse influence on the infrastructure and equipment served in marine industries [4–7]. Marine biofouling increases the weight and roughness of ship hulls, which increases the frictional resistance and then causes additional fuel consumption. It also initiates or accelerates the corrosion of metal and concrete structures, increasing the danger of failure of marine equipment and facilities [8]. Biofouling will attach on seawater pipelines used in near-sea industries and net cages used in aquaculture industry, which will decrease the efficiency of equipment and the production of aquatic products, respectively [9, 10]. In addition, biofouling organisms on ship vessels will migrate to different oceans where they do not naturally belong and disturb the ecological system [11]. Therefore, marine biofouling is a serious issue that needs to be prevented and solved for both marine economy and marine environment.

16.2 Microbial Corrosion and Protection

Conditioning film (protein, polysaccharide) Seconds to minutes

Figure 16.1 et al. [12].

Biofilm Minutes to hours

Diatoms, larva

Hours to days

Algae and invertebrates

Days to months

The typical growing process of marine biofouling. Source: Based on Yebra

Previous researches have come up with a general consideration about the formation and growth of marine biofouling that is related to biofilm as shown in Figure 16.1 including the following steps [12]: (i) an absorbed film quickly forms on the submerged surface due to the adsorption of proteins, glycoproteins, and polysaccharides; (ii) bacteria and other microorganisms adhere on the absorbed film and gradually develop into a biofilm by secreting extracellular polymeric substances (EPS) consisting of proteins and polysaccharides to envelope and fix themselves; (iii) marine organisms like diatoms, larva, and microalgae spores accumulate on the surfaces of materials because biofilm can provide nutrients for them; (iv) larvae of marine macroorganisms such as barnacles settle and grow on the surfaces of materials as macro-foulers. This common formation process for most biofouling organisms illustrates the relationship between microorganisms and macro-foulers like mussels and barnacles. Macro-foulers are major components and the last results of biofouling formation, while microorganisms are the origins of biofouling formation because of the proper settle sites and conditions they create and nutrients they provide to attract new organisms. The activities of microorganisms could regulate the formation of macro-foulers, while the accumulation of macro-foulers could bring some protection for microorganisms and biofilms from being eliminated. However, this general formation model is not applicable to all marine organisms because there are more than 4000 fouling organisms in the ocean and different marine environments may lead to different dominant fouling organisms and different biological habit. For example, D. Roberts et al. found that the cyprids of barnacle Amphibalanus amphitrite can settle on the surface of materials without the presence of a biofilm [13].

16.2.2 Overview of Fouling Biological Control MIC is the corrosion of metal materials that is accelerated directly by the life activities of microorganisms or indirectly by their metabolites [14, 15]. A large part of the economic losses in marine industry are caused by MIC. According to statistics, MIC accounts for about 20% of the total economic losses [16, 17]. MIC is often produced by a mixture of anaerobic SRB and aerobic iron-oxidizing bacteria (IOB) [18–21]. Under

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16 Polymer Composite Coating for Anti-marine and Related Organism Corrosion Microorganism

20–80 nmadsorbed film

Material

Material

Material

Metabolic products

Material

Figure 16.2 et al. [31].

Material

Material

Schematic process of the formation of biofilm and MIC. Source: Based on Liu

actual working conditions, these two microorganisms accelerate the corrosion of materials through synergistic actions. IOB consumes oxygen in the medium to create an appropriate growth environment for anaerobic SRB and then promote the corrosion of the matrix by SRB [22–24]. During this process, SRB and IOB are both used to form biofilms on metal surfaces that are usually composed of sessile cells, EPS, and corrosion products from these two bacteria [25–27]. Biofilm plays a very important role in MIC, and the development of biofilm theory and analytical techniques has led to a better understanding of the whole process of MIC [28–30]. Figure 16.2 shows the formation and development of biofilm affected by the metabolic activity of microorganisms and the corrosion caused by biofilms [31]. The whole process generally includes six steps: (i) an adsorbed film is created on the metal surface; (ii) planktonic microorganisms migrate to the material surface attracted by the adsorbed film; (iii) planktonic microorganisms attach to the active sites on the material surface and change into sessile microorganisms; (iv) the sessile microorganisms grow and produce metabolites to form biofilms; (v) with increase of metabolites and corrosion products, mature and stable biofilms are formed and start causing corrosion; and (vi) with the passage of time, the stability of biofilms decreases, and then part of them will fall off to create heterogeneous biofilms. Many studies have found that the composition of biofilms is complex, leading to complicated effects on the corrosion of materials, biofilms formed in different periods also have different effects on the corrosion, and finally, heterogeneous biofilms caused by the falling-off of unstable biofilms will create localized corrosion of materials to accelerate the corrosion rate [32–35]. It can be seen that microbial corrosion of metallic materials is the result of the combined effects of two factors, material and environment. Therefore, we should start from these two aspects to suppress and prevent microbial corrosion of materials. Common microbial corrosion protection measures in the marine environment include cathodic protection, electrolytic antifouling technology, antifouling coatings, and protective coatings. Different protection measures are often taken in different marine areas. Bactericidal and

16.2 Microbial Corrosion and Protection

antifouling coatings are chemical antifouling materials. Antibacterial ingredients are added to the protective paint or coating to achieve the antifouling and antibacterial effects. In recent years, many studies have put forward lots of new research ideas from the perspective of materials and environment, including antibacterial materials and fungicide synergists. Antibacterial materials are added with appropriate amount of antibacterial elements in the material design, so that the material itself has antibacterial properties, which is a new method to deal with microbial corrosion of metal materials. Taking stainless steel as an example, an appropriate amount of antibacterial Cu element is added to its composition. Once bacteria come into contact with the material or the material is eroded, its antibacterial element will be released into the environment, thereby playing a bactericidal effect. The use of germicidal synergist can greatly reduce the amount of germicidal and is environmentally friendly. At the same time, it can improve the germicidal effect of germicidal agents. Obviously, it is a new concept to prevent microbial corrosion. The surface of the antifouling coating on the metal is a special surface to let microorganisms adhere. Among microfouling organisms, only microorganisms that are resistant to toxicity, especially highly resistant to copper, adhered to the antifouling paint and formed mucous membranes. To date, further studies on the effect of antifouling paint on the biological mucous membranes of organisms are still required, due to the antifouling paint still being widely used. Bacteria and diatoms are two major microbial groups on antifouling coatings. Bacteria generally show a cream-colored, amorphous deposit on antifouling coating surface. Generally, coatings contain two main ingredients, nontoxic and toxic. Among the nontoxic components, some substances are beneficial to the formation of mucous membranes. In some cases, repeated formation and shedding of mucosa is often observed. A spongelike exudation layer of an insoluble lacquer-based coating was directly observed under the diatom surface layer. The diatom layer is firmly combined with the underlying oil and gas layer, and the diatoms are also connected to each other and covered with a layer of diatoms. These soluble and biodegradable lacquer bases in antifouling paints promote the growth of bacteria and diatoms. In antifouling coatings, the effects of toxic components on microorganisms with mucous membrane are manifested in toxicity resistance. On the effective cuprous oxide antifouling paint, many antitoxic bacteria and diatoms have become the main microorganisms of the formation of mucous membrane. In the cuprous oxide antifouling, the amount of copper exudation should be 1–2 orders of magnitude greater than that of large microorganisms anti-attaching, which would prevent diatom or bacterial contamination. This indicates that mucosal microorganisms are much more resistant to toxicity than large fouling organisms. On the other insoluble paint-based coatings containing poisonous materials, the exudation rate or concentration is based on preventing large-scale fouling organisms. For example, diatoms build up on some tributyltin (TBT)-containing compound coatings, or the mucous membranes are still formed at phenoxyphenol concentrations to prevent large fouling organisms. Some microorganisms have a strong adaptability and even quickly adhere to the cuprous oxide antifouling paint within a few hours of immersion in the sea.

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The effect of antifouling coatings on mucous membranes of organisms is manifold. It is both a promoter of mucosal growth and an effective suppressor of toxic material exudation rate. Bacteria on the mucous membranes degrade certain coating components to produce carbon dioxide, lowering the pH value and significantly increasing the exudation rate of cuprous oxide or organotin derivatives in the coating. The chemical influence of exudation rate of poisonous materials is not great, which is caused by diatom mucosa. However, the antifouling paint was found to have a large surface layer composed of diatoms with thickness between 2 and 30 μm. The layer will be an obstacle to diffuse the poisonous materials in the coating. This surface layer also grows rapidly and degrades insoluble lacquer-based coatings. Mucosa formation has undergone a development process from thin to thick. The thin mucous membrane prevents excessive copper release rate, which plays a role in preserving poisonous materials and extending the life of the coating. A thicker mucous membrane makes the released poisonous materials just right to prevent large fouling organisms from attaching. Thickening mucosa may prevent the release of poisons, which eventually make the antifouling performance of antifouling coatings ineffective. Mucosal microorganisms on antifouling coatings are affected by various environmental factors including light, temperature, nutrients, and water turbidity, all directly affecting the population attachment of bacteria and diatoms. The composition of photosynthetic paint is the main factor that affects the microorganisms on the antifouling paint. For example, a large number of diatoms gather in the waterline part of the hull where the sun is shining, but there are few keel parts on the bottom of the ship. Above all, antifouling coatings are still the main means to prevent marine fouling organisms. The coating could change the toxic components to continuously stimulate or inhibit and poison the mucous membrane of microorganisms. The microbial community on the coating is adjusted by adjusting the resin decomposition method to inhibit the attachment of large fouling organisms by water flow.

16.3 Anti-mold Coating Compared with corrosion in abiotic environments, marine microbial corrosion is a more complex process. It involves the attachment and reproduction process of microorganisms, the electrochemical corrosion process, and the interface chemical process, which enable the metal surface to adsorb more carbon sources and energy substances. The adsorption and attachment of these substances resulting in the metal surface form a layer suitable for microorganisms to sojourn, reproduce, metabolize, and develop. If the conditions for mold growth are provided on marine ship equipment, spores in the air will fall on them, and they will sprout into a large number of mycelia soon. These mycelia are composed of parenchyma cells, and the main component in these parenchyma cells is water. If a large number of hyphae are printed on the printed circuit board, the insulation resistance of the

16.3 Anti-mold Coating

electronic components can be reduced. Then, the impedance characteristics can be significantly changed, and some hyphae can cause serious leakage and even short-circuit accidents. In order to inhibit and prevent the corrosion caused by mold, the currently applied antifungal coatings should mainly contain silicone, acrylic, polyurethane, and epoxy coatings. Besides, modified coatings copolymerized with other organic resins, silicones, and organic fluorine have been widely used.

16.3.1 Silicone Anti-mold Coating Silicone “three-proof” coating is a block copolymer of linear polysiloxane and silicone resin. It uses linear polysiloxane under the action of a catalyst and different silicone monomers in the presence of a solvent through copolycondensation. The key technology is to synthesize a linear polysiloxane with sufficient molecular weight and a certain molecular weight to ensure the smooth progress of the copolycondensation reaction. At the same time, the ratio of linear polysiloxane to silicone resin and the choice of catalyst are all key factors to affect product performance.

16.3.2 Acrylic Mold-proof Coating Acrylic mold-proof coatings are usually one-component protective coatings with high volatile organic compounds (VOCs). They mainly protect printed circuit boards that work in harsh environments. They have good electrical and process properties and are suitable for batch assembly line operations. They are usually applied to the environmental electronic products with good temperature and humidity.

16.3.3 Polyurethane Anti-mold Coating Polyurethane anti-mildew coatings mainly include polyurethane varnish, polyurethane insulation paint, etc., which have relatively low viscosity and are suitable for brushing, spraying, and dipping. Its curing speed is relatively fast, and the adhesion to various circuit boards is relatively good. The products are suitable in environments that require resistance to humidity, heat, and salt spray. The coating has good toughness, high and low-temperature impact resistance, and excellent electrical and mechanical properties in environments such as salt spray, high- and low-voltage, and ultraviolet radiation environments.

16.3.4 Epoxy-based Mold-proof Coating Epoxy-based mold-proof coatings mainly include epoxy-polyamide coatings, insulating varnishes mainly composed of alkylene oxides, polybutadiene epoxy coatings and epoxy varnishes, etc., which are suitable for higher temperature parts and can withstand up to 150 ∘ C. These type coatings possess high hardness, wear resistance, and good dielectric properties.

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16.3.5 Development Trend of Anti-mold Coatings With the increasingly strict environmental protection requirements, in the future, anti-mold coatings will be developed into a series of new technologies such as low-VOC high-solid heat curing, water-based curing, and UV light curing. UV coatings refer to resin coatings that are cured by ultraviolet radiation and polymer resin coatings that use UV light as a curing energy source to rapidly cross-link at room temperature to form a film. It has a very low VOC content and is resistant to yellowing, moisture, antichemical corrosion, and other properties, which can protect the solder joints or pins. This type of product is suitable for spraying, dispensing, hand brushing, and other processes.

16.4 Antifouling Coatings 16.4.1 Main Types and Selection of Antifouling Agents The antifouling agent is the core component that exerts the antifouling effect. The role of the antifouling paint is mainly due to the antifouling agent gradually oozing out of the antifouling coating, forming a toxic thin layer on the coating surface, which will tend to avoid or prevent fouling organisms, thereby inhibiting the attachment of fouling organisms. The application of ship antifouling technology can be said to be a variety of antifouling agent applications, elimination, and the renew history. From the earliest mercury and other highly toxic antifouling agents, it is gradually replaced with less toxic or antifouling agent that could degrade gradually. The antifoulants currently used are mainly three types: inorganic antifoulants, organic antifoulants, and natural product antifoulants. Inorganic antifouling agents are mainly copper-containing inorganic compounds. The main varieties are cuprous oxide and cuprous thiocyanate. They are the most commonly used and most traditional antifouling agents that can meet the current environmental requirements [36]. Copper sulfate and copper naphthenate are also used to adjust the initial exudation rate of antifouling paint. In addition, zinc oxide is used as an auxiliary antifouling agent, which is often used in combination with copper antifouling agents to enhance the antifouling effect. Organic antifouling agents mainly include organic tin-based antifouling agents and non-tin organic antifouling agents. Organotin antifouling agents have great harm to the marine ecological environment and have been restricted and banned by various environmental protection regulations [36]. Non-tin organic antifouling agents have become the focus of current research and development. They are developing in the direction of low toxicity and even nontoxicity and easy degradation. The non-tin organic antifouling agent is generally added in a small amount and has been widely known for its antibacterial properties. The main organic antifouling agents are shown in Table 16.1. In recent years, researchers have carried out extensive research on natural antifouling products. Currently, they have studied a variety of terrestrial plants,

16.4 Antifouling Coatings

Table 16.1

Organic biocides and coating application area.

Trade name (chemical name)

Structure

Coating areas

N

ECONEA 2-(p-chlorophenyl)-3cyano-4-bromo-5-trifluoromethylpyrrole

Br

Antifouling

F

N H

F F

Cl O Cl N

TCPM N-(2,4,6-trichlorophenyl)maleimide

Antifouling O Cl

Cl Cl

O

Sea-Nine 211 4,5-dichloro2-n-octyl-4-isothiazolin-3-one

H3C

Tetramethylthiuram disulfide

Antifouling/anti-mold

N-C 8H

17

S

Cl S N

S S

H3C

Antifouling/anti-mold

S H C N 3 CH

3

S

Zinc dimethyldithiocarbamate

H3C

S Zn

N

S

S

CH3 O

Antifouling/anti-mold

N

S

O

O

S

N

Zn S

N

O

Cl

N ′ -(3,4-dichlorophenyl)-N,Ndimethylurea

Antifouling/anti-mold

S

Cu

1-Nitrogen oxidation-2mercaptopyridine zinc salt

CH3

CH3

N

1-Nitric oxide-2-mercaptopyridine copper salt

N

Antifouling/anti-mold

O

Cl

N H

N

Antifouling/anti-mold

CN

N ′ -(3,4-Dichlorophenyl)-N,Ndimethylurea

Cl

Cl

Cl

CN

Antifouling/anti-mold Cl S

Zinc ethylene di(dithiocarbamate)

CH2NHCS

Antifouling

Zn

CH2NHCS S S NH

C

S

CH2 NH

C

S

CH

2

Ethylene bisdithiocarbamate zinc

Mn x (Zn) y

Antifouling

H

Antifouling/anti-mold

S

N-Cyclopropyl-N ′ -(1,1-dimethylethyl)-6-(methylthio)-1,3,5-triazine2,4-diamine

H N

N

S N

N NH

N

Pyridyl triphenyl boron

C B C

C

Antifouling

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marine animals, and marine microorganisms. Then, they have obtained a series of natural products with antifouling activity, including spicy compounds and other natural compounds such as compounds, eucalyptus extracts, organic acids, inorganic acids, lactones, terpenes, phenols, sterols, and indoles [37]. Limited by current antifouling technology, cuprous oxide is still the most widely used antifouling agent for ships. But in the long run, the impact of cuprous oxide on the marine environment cannot be ignored. Therefore, the development of high-efficiency, low-toxicity or even nontoxic antifouling agents that can completely replace cuprous oxide will be the focus of future research and development.

16.4.2 Main Types and Selection of Antifouling Coatings The attachment of fouled organisms has always been a major problem that plagued the human navigation development. The history of antifouling methods can be traced back to ancient times. There are historical documents recorded that in the fifth century BC, coatings containing arsenic, sulfur, and oil were used to prevent ship bows. The ancient Greeks used asphalt and wax in the third century BC. From the thirteenth to the fifteenth centuries, asphalt, oil, resin (natural), and animal fat were used to protect ships. Antifouling coatings have been in a slow development stage until modern times, which is still important research topics. After entering the twentieth century, antifouling coating technology has made great progress, and its performance is getting higher and higher. According to the release mechanism of toxic materials in seawater and the development process of modern antifouling coating technology, it can be generally divided into several types such as abrasion type, self-polishing type, and fouling release type. 16.4.2.1 Abrasion-Resistant Antifouling Coating

The US Navy first applied antifouling coatings to warships in 1908, and they used several red mercury oxide-based coatings that suspended in rubber shellac, cereal alcohol, and turpentine. Zinc oxide, zinc powder, and Indian red are also added; the coating life is about nine months. Around 1926, the US Navy replaced shellac-type antifouling coatings with coal tar. Since then, it has been discovered that rosin is a cheap, rich, and successful substitute that can replace the increasingly expensive and scarce advanced rubber shellac. Based on that, the abrasive antifouling coatings have emerged as the times require. Base Material Is Completely Abrasion-resistant Antifouling Paint (Dissolved Antifouling Paint)

The traditional base material completely abrasive antifouling coating is often based on rosin. Rosin is a natural product obtained from the exudates of pine and fir [3], usually composed of about 85–90% acidic materials (resin acid), among which abietic acid (C30) and L-pelicic acid (C30) are the most important components [38]. In the antifouling mechanism, the rosin, which is slightly soluble in seawater (pH 8.1–8.3) in the paint film under the action of seawater, slowly dissolves and scours to continuously expose the fresh paint film surface, thus making the

16.4 Antifouling Coatings

Figure 16.3 The principle of antifouling agent bleed-out from the base completely antifouling coating.

antifouling agent (usually cuprous oxide) constantly in contact with the seawater. At the last, the antifouling agent dissolve, thereby preventing the attachment of marine life. Although natural rosin is mainly composed of resin acid components, the performance of natural rosin-based coatings is not stable due to the variability of various components in rosin. Simultaneously, each acid contains two double bonds and a carboxyl group. These conjugated double bonds affect the stability of the rosin, making it susceptible to oxidation in the air. Therefore, during new shipbuilding or dock repair, this characteristic must be taken into account, and antifouling paint can only be applied shortly before immersion. Once in contact with seawater, the carboxyl group reacts with sodium and potassium ions in the seawater, resulting in a highly soluble resin. And due to rosin being not able to prevent seawater from penetrating into the coating through hydrophobic interactions [39, 40], its dissolution rate is very fast. According to research, the dissolution rate of rosin in seawater can reach 230–260 μg/(cm2 d). The dissolution rate of cuprous oxide in the stained coating is 90–110 μg/(cm2 d), which is much higher than the minimum inhibitory concentration of cuprous oxide recognized in the later period. Therefore, the pollution effect is often short, and this product should be maintained every year. The principle of antifouling agent bleed-out from the base completely antifouling coating is shown in Figure 16.3. Antifouling Paint with Incomplete Abrasive

As people continue to increase the antifouling cycle, the rosin dissolves too quickly in the fully abrasive base antifouling paint that limits its long-term antifouling effect and further application. On the other hand, another antifouling coating has been prepared entirely from insoluble synthetic resin, as a resin base has appeared briefly, mainly including chlorinated rubber, vinyl chloride–vinyl acetate copolymers, and other resins with high content of cuprous oxide antifouling agent. This kind of base material has high mechanical strength and can get thicker coatings, but this kind of resin does not have the ability to dissolve. It is required to ensure the seepage through the ultrahigh cuprous oxide content. Because the antifouling agent seepage

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path is too long in the later period, the antifouling period effect is improved, which can be 18–30 months. However, the excessively high amount of antifouling agent would lead to high cost and easy to precipitate into agglomerates during storage. At the same time, the potential environmental hazard is greatly increased and quickly eliminated. In order to overcome this problem, people will replace a part of cuprous oxide with an appropriate amount of high-dissolution rate rosin, which is conducive to keeping the pores of the antifouling coating film open after immersion in the sea. The design will not block the poisonous material channel at the bottom of the paint film, thus forming long-lasting antifouling effect. At the same time, in the late 1960s, acrylic resins and vinyl resins were improved as the base material [41], while maintaining the mechanical strength of the antifouling coating. The working principle of the incomplete abrasive antifouling coating is that the antifouling agent continuously leaks and dissolves, forming a honeycomb structure on the surface and inside of the paint film so that the internal antifouling agent further leaks out and dissolves [42]. However, the shortcomings of this coating are also obvious. After the seawater has been applied for a period of time, a spongelike resin layer called a “skeleton” is left due to the seepage of the antifouling agent. In this type of antifouling paint, it is impossible to use it alone in long-term antifouling paint. In the meantime, the advent of organotin improved the performance of antifouling coatings and seemed to be the ultimate solution for fouling protection. Van de Kerk and Luitjen first reported the possibility of broad-spectrum, highly toxic organotin compounds as antifouling coatings in the mid-1950s [43]. These biocides did not initially react directly into the coating film-forming resin, and it exists in the so-called freely bound form [44], which has good affinity and compatibility with the carrier. Poisonous and base materials can form a fixing solution, which is dispersed in the entire coating-like molecules [45]. The utilization rate is higher, and the critical exudation rate required for the organic tin antifouling coating is small. Compared with general antifouling agents, the antifouling period is 2–3 years, and organic tin products can reach 4–5 years (Figure 16.4). However, with the application of organic tin antifouling coatings, it has also brought a series of environmental pollution problems. Due to biodegradation, chemical degradation, or photodegradation, the half-life of organotin compounds in

Minimum biocide release

Lifetime

Figure 16.4 The principle of antifouling agent exudation from incomplete abrasive antifouling coating. Source: Yebra et al. [12]. © 2004, Elsevier.

16.4 Antifouling Coatings

seawater is generally several days. But the organotin compounds become stable after being combined with suspended particles and transferred to the sediment. Even if the concentration of organotin in the water environment is low, most of the benthic organisms have been exposed to organotin pollution throughout their life. In addition, marine organisms have a strong ability to enrich organotin, approximately between 5000 and 10 000 times. This long-term accumulation of chronic toxicity can easily lead to the decline of the reproductive capacity of biological populations and even to the death of some sensitive biological populations, thereby destroying the normal structure and function of benthic biological communities. Therefore, it can cause cumulative poisoning of marine life or cause terrible reproductive changes at low concentrations. According to international conventions, from 1 January 2003, the use of paints containing TBT and other organotin in new and refurbished ships is prohibited worldwide; all ships must not contain such coatings since 2008. 16.4.2.2 Self-polishing Antifouling

The self-polishing antifouling coating mainly includes self-polishing resin, auxiliary resin, antifouling agent, thixotropic agent, solvent, etc. according to the composition of the coating. The antifouling mechanism mainly uses the hydrolysis or degradation of the self-polishing resin to achieve surface polishing and stability. Release antifouling agent inhibits marine organisms from attaching and achieving the purpose of antifouling. Self-polishing resin is the core and key component of antifouling coatings. It can be divided into ion exchange resins and degradable resins according to different release mechanisms of resins and antifoulants. At this stage, the main products that have been commercialized are ion exchange self-polishing antifouling coatings. Self-polishing antifouling coatings are rarely used, and most of them are still in the laboratory research stage. The auxiliary resin mainly controls the polishing rate of the coating and controls the release of the antifouling agent. It is mainly soluble resins such as rosin or polyethylene glycol (PEG). The antifouling agent is mainly cuprous oxide, and it is also compounded with other auxiliary antifouling agents. The auxiliary antifouling agents mainly include zinc oxide, cuprous thiocyanate, copper naphthenate, copper pyrithione, sulfur copper zinc, N-(2,4,6-trichlorophenyl)maleimide (TCPM), Irgarol 1051, diuron, zinc zeolite, isothiazolinone, etc. [46]. Antifouling performance can be optimized by adjusting the type and proportion of the compound antifouling agent. Thixotropic agents commonly used in antifouling coatings include polyamide wax, organic bentonite, fumed silica, etc., which are mainly used to improve the thixotropic performance of the coating and improve its workability. Common solvents for antifouling coatings include xylene, alcohols (xylene), esters (butyl acetate), ketones (cyclohexanone), etc., which are used to adjust the viscosity of the coating and improve the dispersion and construction performance of the coating. As a self-polishing antifouling coating, it must have the following characteristics: (i) the surface of the coating remains smooth during sailing; (ii) the saponification

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layer is thin and stable, and the antifouling agent has a stable release rate; (iii) there is a certain polishing rate during hull mooring in order to keep the coating active, and the polishing rate increases linearly as the sailing rate increases. Self-polishing antifouling coatings can be roughly divided into the following types according to the different resin base materials: (i) acrylic silicon, zinc, copper-based tin-free self-polishing antifouling coatings; (ii) self-polishing antifouling coatings with antifouling function side groups; (iii) copper-free self-polishing antifouling paint; and (iv) degradable self-polishing antifouling paint.

Acrylic Silicon, Zinc, Copper-based Tin-free Self-polishing Antifouling Paint

In the 1970s, organic tin self-polishing antifouling coatings brought a revolution because of their highly effective antifouling effect. This coating was made by grafting TBT onto the acrylic resin backbone through an ester bond. After brushing under the action of alkaline seawater environment, the ester bond is hydrolyzed, and TBT branched off the coating surface to play an antifouling effect. Due to the ionization of the resin main chain, it dissolves in the water, and a new surface layer is exposed to achieve self-polishing. However, TBT and its derivatives are a harmful substance to marine ecosystems. During the use of organic tin, it is found that while organic tin has a killing effect on marine foulants, it will also damage the marine ecological chain and cause biological genetic distortions. Tin is very stable and difficult to decompose. It enters the human body through the action of the food chain and directly endangers human health, which has attracted the attention of governments around the world. Various laws have been introduced to restrict or prohibit the use of organotin antifouling coatings. At the end of the 1980s, countries around the world began to accelerate the development of low-toxic or nontoxic antifouling coatings that do not contain organic tin. Acrylic silicon, zinc, and copper-based non-tin self-polishing antifouling coatings have been developed on this basis. Acrylic silicon, zinc, copper-based tin-free self-polishing antifouling coatings are mainly based on ion exchange resins, and most of them follow the technical route of organic tin self-polishing antifouling coatings. The resins used are mostly acrylic resins [47]. The difference is that the tin in the copolymer is replaced by other metals such as copper, zinc, and silicon. During the navigation of this type of antifouling coating, the resin side-chain silicon, zinc, copper, and other atoms in the coating will interact with sodium ions in seawater. The resin occurred self-polishing under the washing effect of seawater and the release of antifouling agent (Figure 16.5). Studies have shown that zinc acrylate self-polishing resins are mainly prepared by a two-step method [48]. First, acrylic monomers and acrylate monomers are prepared by free-radical copolymerization to form acrylic prepolymers with carboxyl groups on the side chains, and then small molecular acids are grafted to the side chain of acrylic prepolymer. The synthetic route is shown in Figure 16.6 The method for synthesizing copper acrylate self-polishing resin is similar to the method for synthesizing zinc acrylate self-polishing resin, except that the zinc compounds such as zinc oxide used in the grafting process are replaced with copper

16.4 Antifouling Coatings

O

Cr C

Zn

O

O

O

Na+

O

O

R

–

O Na+ + R

O– + Zn2+

C

O

Kill plankton Figure 16.5

Hydrolysis principle of self-polishing antifouling paint without tin.

O

H2C

R

O

C

C

OR1 +

H2C

R

O

C

C

C

OH

OH O

Acrylic monomer

Acrylic acid C

OR1

Acrylic prepolmer

O O C

OH

C +

O

ZnO

+

O.Zn.O–R

R O

C

OR1

Acrylic prepolymer

Figure 16.6

C

OR1

Zn acrylate resin

Synthesis route of zinc acrylate self-polishing resin.

compounds such as copper oxide. The following methods are commonly used to control the resin structure: the molecular weight of the resin is adjusted by controlling the content of the initiator, the glass transition temperature of the resin is adjusted

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by adjusting the ratio of the soft and hard monomers, and the hydrolysis rate and solubility of the resin was controlled by adjusting the grafting amount of zinc or copper and the type of small molecule acid. The abrasion rate of the self-polishing antifouling coating is an important index to measure its antifouling performance. Too low abrasion rate will cause a thicker release layer, reducing the antifouling agent bleed rate. When the antifouling agent leaching concentration is less than the minimum effective concentration, the antifouling coating will fail. If the abrasion rate is too high, the antifouling agent will be affected. The release concentration is too fast, which would reduce the service life of the coating under the condition of a certain coating thickness. The abrasion rate is affected by the resin structure and coating formulation but is mainly affected by the resin structure. The molecular weight, glass transition temperature, hydrophilicity and hydrophobicity, side-chain branch structure, and grafting rate of the self-polishing resin will affect the polishing rate of the coating. The above factors need to be considered during the design and synthesis of the resin. Many studies [49] have pointed out that if the molecular weight of zinc acrylate or copper self-polishing resin is relatively low, the disadvantages are poor resistance and easy peeling. These problems cannot be effectively solved by simply increasing the molecular weight of the base resin. Increasing the molecular weight will lead to the reduction of resin hydrolysis performance. When increasing the content of the metal compound on the resin side-linking branches or the length of the side-chain small molecule acid segment, the resin brittleness would increase similarly. By changing the content of pendant methyl groups in the resin, the hydrophilicity and glass transition temperature of the resin can be adjusted. The hydrolysis mechanism of zinc acrylate and copper acrylate self-polishing resins is similar to organotin antifouling coatings [49]. The resin base itself is not soluble in seawater, and metal ions in seawater undergo ion exchange with copper/zinc ions to generate water-soluble salts such as acrylate and copper/zinc ion salt. The antifouling agent in coatings plays an antifouling effect; the ionized resin on the surface layer is dissolved under the washing of seawater, thus exposing a new surface (Figures 16.7 and 16.8). Compared with zinc acrylate and copper acrylate resins, acrylic silicone resins have better flexibility and lower surface energy. The electronegativity of silicon is 1.74, which is very similar to the electronegativity of tin 1.72. Their chemical properties are also very similar. Silane is a functional group with hydrolytic activity. Since Polymer-COO-CuOOCR(s) + Na+

. . Polymer-COO Na+(s) + RCOO Na+(aq) + BCC

Cu acrylate polymer (insoluble)

Acid polymer (soluble)

Figure 16.7

Self-polishing mechanism of copper acrylic ion exchange resin.

Polymer-COO-ZN(s)-X + Na+ Zn acrylate (insoluble)

Figure 16.8

. . Polymer-COO Na+(s) + Zn2+(aq) + X Acid polymer (soluble)

Self-polishing mechanism of zinc acrylic ion exchange resin.

16.4 Antifouling Coatings

the polarity of the Si—O bond is much greater than the polarity of the CO bond, the silicon atom is susceptible to nucleophilic attacks such as hydroxyl ions, thus showing a hydrolytic type. However, the polarity of the silicon–oxygen bond is smaller than that of the tin–oxygen bond, so the tendency of silicon to be attacked by nucleophiles is slightly lower than that of tin. That is, the hydrolyzability of silane groups is weaker than that of organotin functional groups. Self-polishing antifouling coatings rely on the gradual hydrolysis of the base material, and the antifouling agent continuously oozes out to the coating surface for antifouling. If the hydrolysis is too slow, the antifouling effect will be affected. The surface energy of the silicone is low, and marine organisms are not easy to attach, which can make up for this deficiency. At the same time, the Si—O—Si bond has a long bond length and a large bond angle, making the silicon–oxygen bond easy to rotate, and the chain segment is very soft. Benefiting from Japan’s world-leading technical advantages in the field of silicone resins, the Japanese has done a lot of work in the synthesis and preparation of acrylic silane ester resins. Most acrylic silicone resins use (methyl) acrylic acid triorganosilane monomers and acrylate monomers through free radical copolymerization. Domestic units such as South China University of Technology and Marine Chemicals Research Institute have also studied the synthesis and preparation of acrylic silane ester resins, and their preparation processes are mostly similar to those in Japan. The structural formula of the (methyl) acrylic acid triorganosilane ester monomer is as follows (Figures 16.9 and 16.10). Gerigk and Schneider [50] and others proposed the action mechanism of self-polishing and antifouling resins of organic silicon in seawater. The saponification layer on the coating surface reacted slowly with seawater and released R3 SiCl. Compared with copper/zinc-based self-polishing antifouling coating, acrylic silicon self-polishing antifouling coating has more excellent linear self-polishing characteristics, and the polishing rate is more stable. When the coating thickness is not significantly different, the antifouling effect of zinc acrylate/copper self-polishing antifouling coating is generally between 5 and 7 years, and the structure-optimized antifouling effect of self-polishing silicon antifouling coating can be more than 10 years. In general, it is difficult for self-polishing coatings to achieve the same effect as organic tin self-polishing coatings. The side-chain groups of acrylic resins have a great impact on coating performance [51]. The first side-chain groups affect the Figure 16.9 The structure of (methyl) acrylic acid triorganosilane ester monomer. Source: Gerigk et al. [50].

R1 H2C

C

R2 C

O

O . Polymer-COO-SiR3 (s) +Na++Cl Silyl acrylate (insoluble)

Figure 16.10

Si

R3

R4

. Polymer-COO Na+(s)+ R3SiCl(aq) Acid polymer (soluble)

Self-polishing mechanism of acrylic alkyl silane type ion exchange resin.

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16 Polymer Composite Coating for Anti-marine and Related Organism Corrosion

hydrophilicity. At the same time, it also affects the glass transition temperature of the resin during hydrolysis, as well as water absorption and the expansion rate of the resin. In order to obtain a perfect controlled release system, in addition to having a good resin skeleton, complex chemical structures such as electronic interactions between binders, additives, filler materials, and spatial location also have a considerable impact on the coating system. Self-polishing Antifouling Coating with Antifouling Function Side Group

The efficient antifouling properties of organic tin self-polishing antifouling coatings benefit from the antifouling structure of TBT on the resin side chain, which increases the synergistic antifouling effect between the resin and the antifouling agent. Although the use of organic tin self-polishing antifouling coatings has been banned, the design concept of introducing structures containing antifouling active functions into the self-polishing resin side groups provides ideas for the development of new self-polishing antifouling coatings. Common side groups with antifouling activity include glyphosate derivatives, isothiazolinone derivatives, betaines, and so on. A glyphosate derivative containing a double bond can be obtained by combining acrylic acid chloride with a secondary amine of glyphosate and then copolymerizing with a double bond monomer such as acrylate to prepare an acrylic resin with branched glyphosate [52]. By reacting an acrylic derivative with an isothiazolinone derivative and copolymerizing it with a compound containing a polymerizable double bond, a functional resin with a side-chain branched isothiazolinone-derived structure can be obtained. Laboratory antibacterial tests show that isothiazolinone grafted zinc acrylate self-polishing antifouling coating has a significant antibacterial effect on Pseudomonas and Vibrio vulnificus [53, 54] (Figures 16.11 and 16.12). The betaine-type zwitterionic monomer is introduced into the self-polishing resin side group; the quaternary ammonium salt and sulfonate functional groups of the side group are used to break the marine microbial cell wall that is attached to the coating surface. It is easily to fall off under the shear action or other external forces O

O

O

O

O O

N S

N

O

H

S

O

COOH COOR

+ Zn (OH)2

OH

S

H +

O

Synthesis of 4-BIT-4-keto-2-enoic acid.

*

*

N

O

O

Figure 16.11

O

S

HO N

+ O

*

* O

O

COOR

O Zn

O

S N

O O

O

Figure 16.12

Synthesis of benzisothiazolinone grafted zinc acrylic resin.

16.4 Antifouling Coatings

Figure 16.13 Formula of betaine zwitterionic-modified acrylate resin structural.

R1 H2 C

C

m

H2 C

H C

p

H2 C

H C

x

H2 H C C

y

Ph

O

O

O

O

R2

O

O Zn

H3C

A N+

H3C O S O–

and effectively prevent green algae and diatom spores to form a microbial mucosa [54] (Figure 16.13). Copper-free Self-polishing Antifouling Paint

With the complete prohibition of organic tin antifouling coatings, antifouling coatings have gradually formed a compound antifouling system with Cu2 O as the main antifouling agent and organic antifouling agents as auxiliary antifouling agents. However, related research shows [55] that copper ions can also accumulate in marine organisms; their application and environmental protection issues have attracted much attention. Some countries and organizations are conducting in-depth and extensive research on them. The east coast of Sweden has banned the use of Cu2 O in yacht antifouling coatings. Some other countries also have restrictions on the copper exudation rate of antifouling coating products. Some states in the United States are also considering limiting the use of Cu2 O. At present, the development of copper-free antifouling coatings has become a new research hotspot. Copper-free antifouling coatings are mainly based on zinc acrylate, silicon, copper, and other self-polishing resins. The antifouling agents are mainly copper-free environmentally friendly antifouling agents, including Econea (2-(p-chlorophenyl))-3-cyano-4-bromo-5-trifluoromethyl-pyrrole), ZnPT (zinc pyrithione), Dcoit (4,5-dichloro-2-n-octyl-4-isothiazoline-3-ketones), etc. These antifouling agents are environment-friendly, certified by the EU BPD (EU Biocide Regulation) and the US Environmental Protection Agency (EPA) [56]. Through the combination of environmentally friendly antifouling agents and the screening and evaluation of antifouling coating formulations, it is possible to obtain a medium-term effect (three years) comparable with the copper-containing self-polishing antifouling coatings (three years). The most obvious advantage of copper-free antifouling coating is better environmental protection. But the disadvantages are also very obvious: first, the cost is higher, and second, the actual antifouling effect is not as good as the antifouling coating with Cu2 O main antifouling agent. The broad spectrum of

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environmental antifouling agents is not yet comparable with Cu2 O, and multiple organic antifoulants are often used in combination to ensure the antifouling effect of antifouling coatings [57]. Because cuprous oxide has a strong reducing ability, there is a risk of being oxidized in the atmospheric environment. It is not suitable to use cuprous oxide as an antifouling agent under some special occasions or working conditions, such as ship waterline parts and seawater tidal regions. At the same time, some marine equipment uses aluminum alloy substrate. If it is used with antifouling paint containing cuprous oxide, there is a risk of galvanic corrosion. Therefore, copper-free antifouling coatings can be applied to such special working conditions or the needs of specific substrates. Degradable Self-polishing Antifouling Paint

The base materials for degradable self-polishing antifouling coatings are generally biodegradable or hydrolyzable polymers in seawater. These polymers are generally divided into two categories: (1) Starch, cellulose, chitosan, and other natural polymers extracted from plants or animals. (2) Artificially synthesized polymers such as polycaprolactone, polylactide, and polyglycolide and its copolymers, polyester polyurethane, etc. Synthetic polymers have molecular designability, and resins with the required properties can be synthesized through structural design to meet different application requirements. Hydrolytic resin base for antifouling coatings is a new research direction. The introduction of degradable segments in the resin main chain would ensure that the surface resin is hydrolyzed without saponification layers. The stain can be released stably. In this way, it is only necessary to select appropriate monomers and control the synthesis process. The synthetic resin not only can ensure the service life of the antifouling layer but also can keep the antifouling agent from seeping out. Compared with ion-exchange self-polishing antifouling coatings, degradable self-polishing antifouling coatings have better mechanical elasticity and degradation performance (Figure 16.14). Studies abroad have shown [58, 59] that the resin that uses ε-caprolactone and L-lactide or ε-caprolactone and valerolactone as monomers and Ti(OBu)4 as copolymerization catalyst has good solubility in aromatic solvents. And the polymer shows a certain degree of hydrolysis in seawater. The coating based on these degradable polymers has a certain antifouling effect confirmed by hanging board experiments. Due to the rapid degradation rate of pure polyester copolymers, there is a problem that the polishing rate is too fast in antifouling coatings. Degradable polyurethane resin is prepared by preparing a hydroxyl-containing polyester copolymer and then curing it with an isocyanate, which can greatly improve the mechanical elasticity of the resin base. Since lactic acid has a certain autocatalytic degradation function, the static antifouling performance of the antifouling coating can be improved by introducing a lactic acid segment into the degradation resin.

16.4 Antifouling Coatings

H2O

Seawater

No leaching layer

O

CONH2 OH HO

The degradation of main chain Elastic substance

Figure 16.14

Principle of degradable antifouling coating.

16.4.2.3 Fouling Release Type Antifouling Paint

Fouling release coatings would make the coating surface difficult for marine organisms to attach or not adhere well. The attached fouling organisms can be easily washed away by seawater during the ship’s journey or can be cleaned by simple machinery. This type of antifouling paint mainly refers to an antifouling paint based on silicone resins and organic fluororesins [12, 60, 61] that basically contains no toxic agents and meets environmental protection requirements. From the perspective of environmental protection, fouling release antifouling coating has broad development prospects (Figure 16.15). At present, the applied fouling release coatings are mainly based on silicones and organic fluororesins, combined with cross-linking agents, stabilizers, low surface energy additives, and other pigments, fillers, and additives. In order to achieve a good antifouling effect, the fouling release type antifouling coating must meet the following conditions [4]: (i) low surface energy to prevent the initial attachment of marine life; (ii) low elastic modulus, which makes the fouling matter prone; (iii) suitable thickness to control interface fracture; (iv) smooth surface, easy for foul organisms to fall off; and (v) poor molecular fluidity, providing enough side-chain surface-active groups. If the surface energy of the coating is less than 3 × 10−4 N/cm, the surface tension is less than 22 dyn/cm2 , and the contact angle is greater than 98∘ , the marine microorganisms do not easily adhere to the coating, either the marine

Fouled FRC

After dynamic immersion at 10 knots for 1 min

After dynamic immersion at 20 knots for 1 min

Figure 16.15 Schematic illustration of the self-cleaning ability of fouling release coatings (FRCs). Source: Lejars et al. [4]. © 2012 American Chemical Society.

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16 Polymer Composite Coating for Anti-marine and Related Organism Corrosion

𝜃

𝜃

𝜃

Substrate Water contact angle Adhesion strength

Figure 16.16 Schematic view showing the influence of the water contact angle on the strength of adhesion. Source: Lejars et al. [4]. © 2012 American Chemical Society.

organisms and algae. Although biological attachment has occurred, it is easily to fall off under the impact of water flow (Figure 16.16). Silicone Series Fouling Release Type Antifouling Paint

Organosilicon fouling release type antifouling coating is mainly prepared by using polysiloxane as base material, mixed with cross-linking agent, other pigments, fillers, auxiliaries, etc. Since Si—O bonds have large bond angles and high bond energies and the main chain Si—O—Si is soft, silicone polymers have excellent properties such as low surface energy characteristics and excellent elasticity [62]. Silicone polymers usually use hydrosilylation and condensation reactions to obtain a cross-linked network structure for film formation [63]. Antifouling coatings made by cross-linking silicone resins can prevent the adhesion of sea creatures such as barnacles to a certain extent by virtue of their low surface energy characteristics [64]. Adding polymethylphenyl silicone oil or silicone oil with different molecular weight to the silicone resin can further improve the antifouling performance of the coating [65, 66] (Figure 16.17). Although the silicone resin has many advantages, it has poor mechanical properties and poor adhesion to the substrate. Common modified silicone resins including silicone-acrylate resins, which have better adhesion to the substrate [68]; silicone-polyurethane resins, which can be repeatedly deformed against the impact of ocean waves [69]; silicone-epoxy resin [70, 71]; silicone-polyether resin [72]; and so on. Modified silicone resin can overcome some shortcomings of pure silicone resin and has achieved certain application effects in stain release coatings. However, the disadvantages of these series fouling release type antifouling coatings are poor mechanical properties and the need for regular docking and cleaning. Organic Fluorine Series Fouling Release Type Antifouling Paint

Organic fluorine release type antifouling paint refers to an antifouling paint prepared by using an organic fluororesin as a base material, mixing with cross-linking agents, pigments, and fillers. Among them, the surface energy of the −CF3 group is only 15 mN/m. So, the organic fluoropolymer becomes the best candidate for antifouling release antifouling coating from its extremely low surface energy. The use of organic fluoropolymers, such as perfluoropolyether or fluorinated

Figure 16.17

Surface morphology of fouling release coatings with different silicone oil content. Source: Truby et al. [67]. © 2000, ELSEVIER.

16 Polymer Composite Coating for Anti-marine and Related Organism Corrosion

60 50 Relative adhesion

534

40 30 20 10 10

20

30 Surface energy

Figure 16.18 Berque [74].

40

50

(mJ/m2)

Relationship between adhesion and surface energy of coating. Source:

ethylene–propylene copolymers as antifouling coatings for ships can have a certain antifouling effect [73] (Figure 16.18). In order for the low surface energy characteristics of organic fluororesins to play a role in antifouling coatings, fluorocarbon resins are usually modified. For example, when introducing a fluorine-containing group into an acrylic polymer, changing the side-chain structure of the original polymer would produce a fluorine-containing acrylic resin. The low surface energy characteristics of the fluorine-containing group and the excellent acid and alkali resistance could be combined. The main modified resins are fluorine-containing acrylates, fluorine-containing epoxy resins, fluorinated polyols, and other fluoropolymers. Organofluorine fouling release type antifouling coatings should have the following characteristics [75, 76]: smooth surface and strong nonstickiness; sufficient fluorinated groups on the surface, and high enough fluorine content. The surface fluorinated group is sufficiently stable to resist molecular rearrangements induced by biomolecules. Because fluorocarbon resin is a rigid polymer, the release of surface-contaminated organisms requires higher energy. Therefore, some people think that organic fluororesins are not as good as silicone resins in terms of antifouling coatings [77]. Organic Fluorine Silicon Series Fouling Release Type Antifouling Paint

Organic silicon and organic fluororesin have their own advantages and disadvantages: organic fluororesin has low surface energy and good mechanical properties. However, fluorocarbon resin is a rigid polymer and takes a lot of energy to fall off after fouling organisms attach to silicone. The price of resin is lower than that of fluororesin, but its antifouling and mechanical properties are poor. Therefore, with the silicone chain as the main chain and the introduction of −CF3 or fluorinated groups into the side chain, it not only maintains the high elasticity and high fluidity of the linear polysiloxane but also retains the ultralow surface energy of the −CF3 group.

16.4 Antifouling Coatings

500 nm

1 μm (a)

(b)

Figure 16.19 (a) SEM and (b) TEM images of the synthesized PVP-Cu2 O microcapsules. Source: Mao et al. [79]. © 2020, ELSEVIER.

For example, the surface energy and antifouling performance of the fouling release type antifouling coating are improved when prepared by the copolymer obtained through organosiloxane, organofluoropolymer, and acrylic monomer [78].

16.4.3 Development Trend of Antifouling Technology With the increasingly strict marine environmental protection regulations, the current antifouling technology is gradually developing toward low-toxic or nontoxic environmental protection, which is mainly reflected in the following types of antifouling designs. 16.4.3.1 Antifouling Agent Design Low Copper Controlled Release Antifouling Design

Current popular antifouling coating is self-polishing antifouling coating containing cuprous oxide. The antifouling coating on the surface of the hull will release a large amount of copper ions in seawater, which will easily cause pollution of the marine environment. It will cause damage to the local marine ecological environment when copper would enrich in the port area. For the requirements of copper ion controlled release, a coating design method is generally used, such as coating with polyvinyl pyrrolidone [79], surfactant [80], or gel [81], which can delay the release rate of antifouling agents and improve the antifouling properties (Figure 16.19 and 16.20, Table 16.2). High-efficiency Broad-spectrum Degradable Antifouling Agent

Traditional antifouling agents are mainly poisonous to attached organisms, which are likely to cause pollution of the marine ecological environment. Ideal marine antifouling agents should meet the following requirements: active at low concentrations; harmless to humans and other organisms; have a broad spectrum; nonpolluting; and biodegradable. At present, natural antifouling active substances originate

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16 Polymer Composite Coating for Anti-marine and Related Organism Corrosion

20

0.5

S-1 S-2

16

y = 0.158x + 0.003 R2 = 0.995

0.4

18 A450 nm

Copper-ion release rate µg (cm2 d)

536

0.3 Standard curve

0.2 0.1

14

0.0 0.0

12

0.5

1.0 1.5 2.0 2.5 Cu2+ concenration (µg/ml)

3.0

10 0

20

40 60 Immersion time (d)

80

Figure 16.20 Standard curve and copper-ion release rate curve of samples. Source: Mao et al. [79]. © 2020, Elsevier. Table 16.2

The results of testing antifouling panels in shallow submergence.

Results

Panels P-0

P-1

P-2

P-3

Before

After

Source: Mao et al. [79]. © 2020, ELSEVIER.

from nature and are basically close to the above conditions. Therefore, research on natural antifouling active substances has received increasing attention, such as capsaicin extracted from terrestrial plants, pepper, marine bacteria, algae, sponges, corals, etc. [82], natural extracts such as furan derivatives [83], diterpene carboxamide compounds [84–86], indole derivatives [87], sterane derivatives [88], and pyridine salts [89]. But at present, the effect of synthetic artificial imitation [90] is still lower than the conventional antifouling agents currently used. So, the design and synthesis of high-efficiency spectral degradable antifouling agents still need to be further explored (Figures 16.21–16.24).

O

O

Figure 16.21 The compound synthesized according to the compounds extracted from North Sea Streptomyces strain GWS-BW-H5. Source: Xu et al. [83]. © 2010, Elsevier.

16.4 Antifouling Coatings

(a)

(b)

(c)

Figure 16.22 Filed tests of compound (a) painted pigment volume concentration (PVC) panel before submersion, (b) control PVC panel after submersion in seawater for three months, and (c) treated PVC panels after submersion in seawater three months. Source: Xu et al. [83]. © 2010, Elsevier.

(a)

(b)

Figure 16.23 Antibacterial activity of indole derivative against (a) E. coli and (b) S. aureus. Source: Terlizzi et al. [91]. © 2000, American Chemical Society. Control group

A-Zn

A-Zn-NPI-5

A-Zn-NPI-10

A-Zn-NPI-15

Figure 16.24 Images of coatings based on acrylate resins containing the synthesized indole derivative after immersion in the sea for three months. Source: Feng et al. [90].

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16 Polymer Composite Coating for Anti-marine and Related Organism Corrosion

16.4.3.2 Gel Mixed Fouling Release Type Antifouling Paint

Traditional organic fluorosilicone antifouling coatings mainly rely on the low surface energy and elastic characteristics of the material to reduce the adhesion of marine fouling organisms. However, based on the years’ analysis of actual seaboards, the organic fluorosilicone antifouling coatings have poor static antifouling. The antifouling effect of organic fluoropolymers on diatoms is slightly better but also needs to be improved [92], and the hull needs to maintain a high navigation rate, or high-speed water flow of more than 10 knots can avoid the attachment of fouling organisms. However, hydrophilic structures such as PEG [93] have anti-protein adsorption and cell attachment properties. The introduction of PEG in organofluorosilicone resin can improve the antifouling broad-spectrum antifouling coating, and PEG has hydrophilicity and high surface energy (>43 mJ/m2 ). The introduction of PEG to the fluorosilicone resin can provide hydrophilic groups, making the resin amphiphilic and more effectively to prevent marine fouling organisms from attaching (Figure 16.25). The antifouling coating prepared by introducing PEG into the fluorosilicone resin has the same coating adhesion resistance and ballast spore adhesion strength as polydimethylsiloxane (PDMS), but the biological adhesion strength is significantly lower [95, 96]. Antifouling coatings that contain PEG and hyperbranched fluorine chains show stronger antifouling properties against PDMS [97]. Therefore, by combining the anti-protein adsorption property of the PEG side chain and the hydrophobicity of the fluorine-containing side chain, the PEG-containing fluorine silicon fouling release type antifouling coating has unique antifouling properties.

(a)

(b)

Figure 16.25 Proposed mechanism for surface reconstruction of the ethoxylated fluoroalkyl side chains upon immersion of the surface in water. (a) the orientation of side chains in air; (b) the effect of water immersion. Source: Krishnan et al. [94]. © 2006, American Chemical Society.

16.4 Antifouling Coatings

Antifouling: cyprid - settlement

OTS

PEG

PG

Reference

Fouling release: zoospores - removal

PPG

OTS

PG

[

HO HO

OH O O HO HO

PG

4 nm

Glass

Reference

OH O OH OH

O HO

PG

OH O

HO

O O

O

O

HO

PEG

[

n

O OH

O

O

O

HO

O O

O

[

HO O

OH O OH OH

O

OH

O

O O

OH HO

HO

PPG

PG

[

n

O OH

OH

O

OH O OH OH

O

43 nm

PG

O

O

14 nm

HO O

OH OH

O

O O

PG

Figure 16.26 Dendritic poly(glycerols) against marine macrofouling organisms. Source: Krishnan et al. [93]. © 2006, American Chemical Society.

In response to this situation, current researches have used hydrophilic block materials such as PEG and silicone resins to modify the resin to ensure the resin amphiphilic properties. In addition, related modification work will also be performed on hydrophilic gel fragments, such as the combination of polyols and zwitterions [98, 99], can significantly improve the anti-protein adsorption properties, and improve their antifouling properties. HEMPEL’s X-series antifouling coating also adds a small amount of antifouling agent to the gel-mixed silicone antifouling coating system, leading to more excellent static antifouling properties (Figure 16.26). 16.4.3.3 Surface Microstructure Bionic Antifouling Paint

Although there have been reports in the literature, chemical etching or self-assembly methods can be used to prepare biomimetic antifouling coatings with certain microstructures, such as sharkskin structures. But the main work is still in the laboratory research stage. The size is small and cannot meet the needs of practical applications. In order to effectively solve the above problems, the current design of molecular brushes is commonly used on the antifouling material surface. For example, the IP company uses an elastic molecular brush structure design concept to make it difficult for fouling organisms to adhere. Functionally modified grafting on the surface of the filler [100] or amphiphilic functionalization on resin’s structure [101] can help improve the properties of inhibiting protein adsorption (Figure 16.27). Although the surface microstructure has a certain effect on the inhibition of living things, the existing technology has not yet been able to conduct microstructures in

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16 Polymer Composite Coating for Anti-marine and Related Organism Corrosion

Protens and cells

PEO domain Fluonnated domain

Protens and cells

Fence-like conformation and compositional heterogeneities Anti-fouling asymmetric polymer brush surface

Figure 16.27 Biomimetic asymmetric polymer brush coatings bearing fencelike. Source: Xu et al. [101]. © 2020 American Chemical Society.

large areas. It is hoped that in the future, advanced technologies such as 3D printing can be used for surface processing to achieve large-area designs. 16.4.3.4 UV Antifouling Technology

Ultraviolet light has a good killing effect on bacteria, especially ultraviolet light with a wavelength of 265–280 nm. Transplanting it as a marine antifouling concept will likely change the traditional antifouling design. At present, IP companies have already cooperated with Philips companies to carry out research on the antifouling of ultraviolet light emitting diodes (UV LEDs). Preliminary results show that it has certain antifouling effects, but the hydrolysis design of UV LEDs and the anti-UV aging design of adhesives still limit the development of this technology.

16.5 Summary and Prospects Microbiological corrosion and biofouling of materials are two main reasons of marine corrosion to cause the damage and failure of equipment and structures served in marine environment. The MIC is caused by SRB, IOB, and their biofilms. As a result, the strategies of preventing MIC and biofouling mainly focus on controlling the activity of microorganisms in biofilms, the adhesion of organisms, and the formation of biofilms. New materials or technologies are incorporated with traditional bactericide or antifoulant through synergistic action in order to reduce their dosage and achieve better bactericidal effect at the same time. The development tendency of preventing marine MIC and biofouling in the future is to find methods and materials that have highly efficiency, long service life, easy implementation process, low cost, and environment-friendly application under the complicated marine environment. The research approaches introduced in this article brought some new angles, but they may not match all of these requirements by themselves. As a result, the integration and synergistic action of different materials

References

and technologies to avoid the weaknesses of each other will be a very important research direction in marine anticorrosion and antifouling in the future.

List of Abbreviations EPS IOB MIC PEG PDMS SRB ZnPT

extracellular polymeric substances aerobic iron-oxidizing bacteria microbiologically influenced corrosion polyethylene glycol polydimethylsiloxane sulfate-reducing bacteria zinc pyrithione

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17 Anechoic Coating for Underwater Vehicles Weiwei Zhang 1 and Yujun Song 1,2 1 University of Science and Technology Beijing, Center for Modern Physics Technology, Applied Physics Department, School of Mathematics and Physics, 30 Xueyuan Road, Beijing 100083, China 2 Zhejiang Key Laboratory for Pulsed Power Technology Translational Medicine, Hangzhou Ruidi Biotechnology Company, Hangzhou 310000, China

17.1 Introduction More and more large ships and underwater vehicles (UVs) are used in the oceans with the development of the transportation of bulk cargo (e.g. crude oil, agriculture produces) across oceans, the marine source exploration (e.g. fish, sea grass, marine energy, seafloor minerals and energy, island exploration), and some military application (e.g. the reconnaissance/investigation for national defense or the invasion of foreign countries by destroying the corresponding vehicles, enhanced carrier strike capability, submarine-launched missile, frogman, unmanned undersea vehicle [UUV]). These ocean vehicles also produce great side pollution or effects on human and marine ecosystem due to the human-made activity (e.g. engine noise, engine blade cutting, direct waste releasing, strong electromagnetic [EM], and acoustic radiation to kill marine livings [e.g. coral] or to mislead marine animals [e.g. whales]). Marine life is an important ecosystem balance component of the underwater environment – as an indicator of ecosystem health, as a witness and marker for the potential impacts on human activity and global climate change, and traditionally, as a source of food (providing 40% of the world’s population with 20% of its protein intake). Since the 1980s, there have been regular catastrophic ecosystem collapses due to overfishing (e.g. in Grand Banks, Canada, during the 1990s) or marine pollution. They have also seen increasing efforts to monitor and manage marine ecosystems sustainably and ethically (e.g. European Union Good Environmental Status, 2008). This chapter aims at presenting through simple concepts and examples the two connected domains of fishery acoustics (how does one detect and monitor fish and other marine life?) and bioacoustics (how does marine life use sound? how sensitive are they to other sounds, including human-made sounds?). There are two types of key techniques are needed for these application developments in marine activities. One is the seeking, navigation, and position-localization Inorganic and Organic Thin Films: Fundamentals, Fabrication, and Applications, First Edition. Edited by Yujun Song. © 2021 WILEY-VCH GmbH. Published 2021 by WILEY-VCH GmbH.

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systems of desired targets and/or underwater sound landscapes. Another is the coating systems for reducing the hydraulic resistances and/or the ocean stealth of varieties of acoustic and EM noises to protect the marine ecology or avoid the detection by enemy’s vehicles for military and/or commercial activities. For those ships and vehicles moving over water surfaces, Global Positioning System (GPS) can be used for navigation and target seeking. However, it is very difficult for the UVs or submarines to use GPS to navigate because the EM (including visible light) signals decaying much more quickly in water can only transport in a much shorter distance compared with those in air due to the electric conductivity of water (particularly for ocean water). The aqueous system has a great shielding effect for EM wave by forming the Eddy current during the interaction between water and EM waves. Thanks to the sound and magnetic signals that can transport in water (the speed of sound in ocean water is about 1531 m/s at 25 ∘ C), these UVs or submarines can use sonar systems and/or magnetic induction systems (bionics from the magnetic compass and its use in long-distance navigation by animals [1] or the currently developing Metamaterial-Enhanced Magnetic Induction (M2I) method [2] to navigate underwater). Similar to radar, sonar is abbreviated as “sound navigation and ranging,” which is a powerful technique for detecting and determining the distance and direction of underwater objects by tracking acoustic echoes. Its emergence depends on two basic discovery: one is the precise measurement of the sound speed underwater in 1827 by two Swiss physicists, C. Daniel and S. Charles, and the other one is the carbon particle microphone invented in 1850s [3]. It is originally developed for the safely navigation by detecting and avoiding the submerged icebergs and reefs due to some maritime disaster (e.g. the Titanic disaster, 1812). The anti-submarine demand promotes the sonar research after the World War I (WW-I) occurred. However, the first sonar device for anti-submarine during the WW-I did not work well due to the theory and technique limitation, leading to almost no contribution in the elimination of threat from U-boats of Germany. With the technology progress, particularly in the theory of the echolocation, the magnetostrictive materials development for acoustic emission transducer, and the microelectronics for signal amplification and processing, the practical sonars were developed by Germany, the United Kingdom, and the United States in the 1930s, which were equipped in the warships. More than one thousand submarines destroyed in the World War II (WW-II) were mostly discovered by these sonars. After the WW-II, the low noise or silent submarines were developed very rapidly due to the intensive antagonism between the Warsaw Treaty Organization represented by the Soviet Union and the North Atlantic Treaty Organization represented by the United States due to their sudden and covert attacking feature, particularly the underwater launching intercontinental ballistic missiles with nuclear warheads as the second counterattack. As one of the efficient techniques for anti-submarines, the sonar systems obtain unprecedented rapid development from the beginning of the cold war up to now due to their military exploits in the WW-II and the increasing anti-submarine demands. Up to now, sonar is still the unique technique for UVs to obtain the surrounding information as they submerge into a certain depth under water for navigation

17.1 Introduction

and detection due to the excellent hydrophone features of acoustics with the development of the sonar system. In addition, sonar is the main efficient buster (bane) of submarines as considering anti-submarine war. Besides the indispensable military applications for the threat detection of hostile battleships, submarines, mines, and torpedo, sonar has been intensively used in the marine investigation and the fishery. After the WW-II, lots of sonar systems have been developed during the Cold War, and more and more advanced sonar systems are in progress with the blockade and anti-blockade by hostile forces in the world. There are also other efficient anti-submarine techniques further developed together with the sonar systems, such as the aircraft hunting technique using sono-radio buoy, smart mines and torpedoes, frogman, and autonomous underwater vehicle (AUV) [4, 5]. Thus, on the other hand, naval ships and submarines or commercial ships face increasingly serious threats from hostile integrated scouting sonars in modern oceanic battlefields. Acoustic waves of many sonar systems, particularly for those using the low-frequency sound waves also, do great harm to the marine life. Therefore, how to avoid the detection by those powerful advanced integrated sonar systems and how to reduce the acoustic noise from ships or sonar to protect marine ecology are becoming more and more urgent in the military application and the marine exploration. The techniques to reduce the radiation noise of naval ships or submarines, or the acoustic stealth, also become one urgent research field, together with the corresponding acoustic theory. Particularly, for avoiding the detection by the sonar system of hostile forces after the Cold War, techniques for acoustic stealth in submarines have to be developed in those countries of hostile forces, such as varieties of metamaterials and metasurfaces of unique acoustic absorbance or acoustic diffraction and disturbance, together with coatings of super sound-off or anechoic coatings. Up to now, many acoustic materials and structures have been developed with the anechoic theory, such as the external hull acoustic deflectors and anechoic coatings for the reduction of target strength. The optimized integration of a bow submarine array can also be used to the absorption of waves on the back plate and self-noise reduction with the sonar dome. Particularly the anisotropic materials, photonic crystal, acoustic cloaking, metamaterials, and metasurfaces have been recently developed for the efficient shielding of acoustic waves of broadband windows [6, 7]. This chapter firstly will briefly discuss the basic physical principle of sonar, including the echolocation and Doppler effects of acoustics and the applications of sonar systems. Then the infrastructure and the typical types of sonar systems will be summarized according to their function features and applications. And then the recent progress in the composition and microstructure for magnetoacoustic and acousto-optic transducer/detector will be discussed for the future ultrasensitive sonar system. After that, the basic physical principle for acoustic stealth will be summarized. Finally, the design theory of anechoic coatings and the development of the composition and structure of some typical coatings will be intensively discussed as the key points for the future highly perfect acoustic stealth.

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17.2 Fundamental Physics for Sonar System: Echolocation and Doppler Effect

(a)

up ∽4 seconds

∽4 second

s down

Sonar is a measuring instrument based on the bionic echolocation, such as dolphin or bat, which sends out an acoustic pulse in water and measures distances in terms of the time for the echo of the pulse to return from the target or their food (Figure 17.1a). The typical distance determination method for ships using the echolocation function of sonar is shown in Figure 17.1b. By measuring the time difference between the emission sound and the received echo sound, the distance can be detected by half of the time difference times the sound speed in water (here, the time difference is 8, and the distance from the water depth to the bottom of the ocean is about 6000 m based on the sound speed in the ocean water). Using multi-beam of sounds from different localized sensors in the sonar system, the location of the target can be precisely determined. Besides the distance and location determination of the target, the moving velocity vector of the target is another key parameter to trace the motion trail of the target. This function will rely on one interesting phenomenon in the acoustic wave – the Doppler effect. The Doppler effect is the change in frequency or wavelength of a wave in relation to observer who is moving relative to the wave source, which was discovered in 1842 by the Austria physicist and mathematician, Christian Johann Doppler, as he observed the sound of the whistle changes with the train approaching to and moving away from the passengers (Figure 17.2). The reason for the Doppler effect is that when the wave source is moving toward the observer, each successive wave crest is emitted from a position closer to the observer than the previous wave. Therefore, each wave takes slightly less time to reach the observer than the previous wave. Hence, the time between the arrival of successive wave crests and the sound sources is reduced at the observer, causing an increased frequency (f’ ) or high-pitched whistle in Figure 17.2. While they are traveling, the distance between successive wave fronts is reduced, so

∽6000 m depth

552

(b)

Figure 17.1 (a) Schemes of the bionic echolocation of the bat (top) and the dolphin (bottom). Source: Principles of echolocation, by Demon Mo. Baidu, 4th March, 2019. © 2019, Baidu, Inc.. (b) Scheme of the distance determination method using sonar by ship. Source: Sonar, Development history. Baidu. © 2020, Baidu, Inc.

17.2 Fundamental Physics for Sonar System: Echolocation and Doppler Effect

High-pitched whistle for observers (v0): f’

Sound sources: f, vs

Low-pitched whistle for observers (v0): f’

Figure 17.2 (a) Doppler effect. The frequency becomes high as the train approaching the passenger or the frequency becomes low as the train moving away from the passenger.

the waves “bunch together.” Conversely, if the source of waves is moving away from the observer, each wave is emitted from a position farther from the observer than the previous wave, so the arrival time between successive waves is increased, reducing the frequency. The distance between successive wave fronts is then increased, so the waves “spread out,” causing low-pitched whistle in Figure 17.2. The frequency between the observer and the source can be expressed as ) ( v ± v0 f′ = f v ∓ vs where f ′ is the observed frequency; f is the sound source frequency in the medium; v is the velocity of the acoustic wave in the medium; 𝜈 0 is the moving velocity of the observer, which is positive (+) if approaching the sound source but negative (−) if not; and 𝜈 s is the velocity of the sound source, which is negative (−) if approaching the observer but positive (+) if not. By this equation, we can understand the frequency changes as the sound source approaches to or moves away from you. If you know your velocity (𝜈 0 ), the velocity of the acoustic wave in the medium (e.g. 𝜈 = 1531 m/s in the ocean water), and the observed frequency f at two different velocities or as the distance is determined by one ranging sonar, the velocity of the target (or the sound source, 𝜈 s ) can be calculated by this equation. A Doppler radar is a specialized radar that is designed according to the Doppler effect and echolocation to produce velocity and position data about objects at a distance.

17.2.1 Types and Construction of Sonar Systems Based on the echolocation principle and the Doppler effect, lots of sonar types have been developed for the navigation of UVs or the detection of underwater targets either for industry or for military. Generally, there are two types of sonar systems according to their signal detection mechanism. One is the active sonar systems and another is the passive sonar systems. The active sonar process is the process for detecting and locating an object by receiving the echoes reflected from an object insonified in an echo-ranging process and then measuring its location and moving

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parameters. The distance can be calculated by the time difference between the originally transmitting sound wave to the target and the echo arrival from the target. The target direction can be measured by the location difference between the two sub-arrays in the receiving acoustic array. The passive sonar process is the process for detecting and locating an object by receiving the sound emitted by the object, whose detection mechanism is similar as the active sonar. However, only the target orientation can be measured by the passive sonar, which can be widely used not only for each arm of services in the navy but also for fishing, guidance, communication, oil exploration and submarine geological exploration, and so on. The active sonar system usually includes four components: transmitter (signal source), transducer (including acoustic matrix – sonar array), receiver (including signal amplification, identification, and processing), and display console (Figure 17.3). The passive sonar system is only composed of receiving acoustic array, receiver (including signal processing), and display console, which does not need transmitter. The key component is the sonar transducer for acoustic transmission and/or reception and signal transfer, which is formed by the transmitter, the acoustic matrix (sonar array), and the receiver in the active sonar system or by the receiving acoustic array and the receiver. The active sonars have broadbands, which can be divided as the low frequency (10 kHz). Sonars for military include all bands, and the civil sonars are limited in the middle and high frequency. The sonar intensity of the low frequency used in US Navy can be up to 215–220 dB, and the sonars in the middle frequency is about 230 dB. The intensity of the civil sonars is about 180–220 dB. Most underwater hydroacoustic transducers are based on electroacoustic materials (e.g. BaTiO3, piezoceramics, bismuth titanate-based ceramics, polyvinylidene fluoride [PVDF]) [8–10], magnetoacoustic materials (e.g. magnetostrictive alloys or Transmitter

Display console

Electric signal Electric

Transducer

signal

Receiver signal amplification and processing

sign

co

stic

ar e

Son

u Aco al

Target

Figure 17.3 Scheme of components forming the active sonar system and its working principle. Source: © Elsevier.

17.2 Fundamental Physics for Sonar System: Echolocation and Doppler Effect

single crystals: Tb0.27 Dy0.73 Fe2 ; galfenol [alloy based on gallium and iron]) [10], or optoacoustic materials (e.g. the Otto configuration based on semiconductor prism, air gap, and guiding layer and piezo transducer) [11], which are arranged by series of signal energy exchangers forming a certain arrayed structure. These transducers can be different shapes, such as sphere, cylinder, planar, or chain. They can include transmitter arrays (only in the active sonar system) and/or receiver arrays or transceiver in one array (usually in the active sonar system). The efficacy of each type of sonar system can be mainly evaluated by their transducer materials, projectors and hydrophones, single-element transducer geometries, and the transducer arrays. Now, single-beam echo sounders, multi-beam echo sounders, and side scan and the much recent synthetic aperture sonars and vector sonars have been developed for the high-efficient sonar systems [12, 13]. With the progress of transducers, particularly the functional materials and the logic operation software for transmitters and hydrophone receivers, the working frequency has also been extended to wide bands, from the high frequency (tens to hundreds of kHz) to the middle frequency (several kHz) and then to the low (100 Hz to kHz) and ultralow frequency (several Hz to 100 Hz), by the comprehensive consideration of the detection distance, target resolution, and anti-noise ability (the current status of the sonars for submarines).The detection distance of the TB-16 towed line array sonar owed by US Navy can reach 180 miles by the sound sea channel and the convergence effect if using the frequency as low as 10–20 Hz (review for the current status of the sonars for submarines: http:// www.360doc.com/content/19/0211/16/33540468-814264436.shtml).

17.2.2 Introduction of Some Typical Sonar Systems Based on the progress of the functional materials and the design theory of high-efficient transducers, lots of advanced sonar systems have been developed for modern submarines and/or ships according to their structures and functions for certain purposes. They are mainly as follows: (i) bow column integrated (CHA) sonar, (ii) active collision avoidance (MAA) sonar, (iii) shipboard passive direction finding (FA) sonar, (iv) shipboard passive ranging (PRA) sonar, (v) reconnaissance (BTA) sonar, (vi) self-noise detecting sonar (ONA + ACC), (7) towed line array (TA) sonar, and (8) environmental hydrologic detection (EHD) sonar (sonars necessary for modern submarines). With the diversity of tasks of modern ships and submarines and the strict requirement of the increasing concealment for underwater warfare, several sonar systems of different types and functions will be equipped into one ship or submarine. There are lots of sonar systems that have been integrated in one submarine. Figure 17.4 gives the Type 214 submarine (a) and the sonars equipped in it (b), including CHA, MAA, FA, PRA, TA, BTA, and ONA + ACC. The Type 214 submarine is a kind of conventionally powered submarines, being built for export by Kockums and HDW and using two air-independent propulsion (AIP) systems – fuel cell and Stirling. AIP can extend the underwater operation period of the diesel-powered submarines to five to six times longer than that of conventional submarines. For military applications, the advantages of an extremely quiet power source confer great tactical benefits,

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Isus 90

ESM

PERISCOPE

DM2A4

RADAR

IDRA DNA + ACC

BTA MAA

Winch with TA

MFCC CHA

FA PRA

(a)

(b)

Figure 17.4 (a) The Type 214 submarine and (b) its sonars equipped. Source: (a) GDK. Image taken from https://en.wikipedia.org/wiki/Type_214_submarine. Licensed under CC BY-SA 3.0.

(a)

(b)

Figure 17.5 (a) The MTK 300 ruby cylindrical sonar arrays equipped in the Class V-attack nuclear submarine and (b) the huge transversal CHA of the MTK 540 sonar system, usually equipped in the class S and class AK attack nuclear submarine. Source: 360doc.

and the current disadvantages of relatively low output compared with its size and weight mean that the first large units are in underwater systems. The Type 214 submarines have been sold and equipped in the navy of Greece, Indonesia, Portugal, and South Korea since its start of building on 16 January 2001 and the nomination of the first boat on 22 April 2004. Like the Type 214, the modern submarines (including the nuclear-powered type) are usually equipped with multiple sonar systems for improved operational performance during war. Bow multifunction sonar systems can be formed by cylindrical sonar arrays, spherical sonar arrays, and the common bow-shaped sonar arrays. Figure 17.5a is the MTK 300 ruby cylindrical sonar arrays of Class V-attack nuclear submarine of the former Soviet Union. There is one high-frequency collision avoidance mine-detecting sonar. Figure 17.5b is the huge transversal CHA of the MTK 540 sonar system, usually equipped in the class S and class AK attack nuclear submarine of the former Soviet Union. This kind of cylindrical sonar has huge volume of about 4 m diameter for the largest one, which can provide a large acoustic aperture, a low operation frequency, and a high output power, leading to long detecting distance. MTK 540 Raja3 CHA can have the detection distance for surface ships up to 60–80 km under the passive mode and precisely lock the target around 35 km. This kind of sonar also

17.2 Fundamental Physics for Sonar System: Echolocation and Doppler Effect

Vertical launch tubes

Battle Prevents sounds from the sub from interfering with the sonar sphere,

Sonar sphere with active a passive sonar. Bow dome bow plano tern planes subs depth.

(a)

(b)

‘Chin’ sonar array See box, far left,’SO ALL AROUND:

(c)

Figure 17.6 (a) The Ayax spherical sonar array with a diameter of 6 m equipped in Type 885 Yasen-class attack nuclear submarine, (b) the spherical array of the type BQQ10 sonar equipped in the Virginia attack nuclear submarine, and (c) the exposed spherical sonar in the Los Angeles-class Baton Rouge attack nuclear submarine as the asdic dome is removed. Source: 360doc.

has the narrow and broadband monitoring function and identifies the narrow and broadband signals from active sonars for the determination of azimuth parameters. Figure 17.6 gives some typical spherical sonar arrays in the nuclear submarines of Russia and the United States, which are (a) the image of the Ayax spherical sonar array with diameter of 6 m equipped in Type 885 Yasen-class attack nuclear submarine, (b) the spherical array of the type BQQ10 sonar equipped in the Virginia attack nuclear submarine, and (c) the exposed spherical sonar in the Los Angeles-class Baton Rouge attack nuclear submarine as the asdic dome is removed. In the cylindrical or spherical sonar arrays, the array elements will be arranged along the cylinder or the sphere and form the acoustic wave via the compensators to realize the beam scanning. Alternatively, similar to radar, they can realize the beam scanning with adjusting phase shift by adding one phase shifter in each element in the phased array mode. Thanks to the phased array digital multi-beam technology for the modern sonar system, the cylindrical or spherical sonar arrays have many advantages, such as the large detecting space, fast scanning speed, and multi-target tracing ability. They have greatly increased acoustic aperture and low operation frequency, which can receive the noise of low frequency that attenuates slowly in the seawater. Their detecting distance and identification ability can be further increased by utilization of reflection from the sea bottom reflection and the deep ocean channel. Compared with the conventional-powered submarine, the nuclear-powered submarine has more space and power, offering enough space and power for larger CHA or spherical array sonars that can have higher power transmitting array, resulting in longer detecting distance and enhanced seeking ability. The spherical array sonar for submarines in the United States in Figure 17.6c is usually designed by the famous high-tech equipment company: Raytheon Company Ltd. Based on the AN/BQS-11-12 spherical array sonars that was originally from the BQS-6 series of sonar systems, Raytheon has successfully designed and manufactured the much advanced AN/BQS-13DNA class spherical array sonar system equipped in the Los Angeles-class attack nuclear submarine. The AN/BQS-11-12 spherical array sonars have a diameter of about 4.6 m and power of 75 kW, which have a detection distance about 65 km under the active

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mode with a working frequency of 3.5 kHz. The AN/BQS-11-12 can have a detection of 30 km under passive mode with a working frequency of 1–3 kHz using surface reflection channel, 60 km using the first reflection of the seafloor, and 90 km using the convergence zone effect of submarine channels. Generally, the United States likes the spherical sonar much more than other types of sonars since it can have large volume that can support high power and increased acoustic aperture for low operation frequency. However, it also has some shortcoming. For example, the torpedo launching mode has to be shoulder launch mode, and the torpedo launchers have to be moved backward, which will result in the large ellipticity opening at the pressure hull. High technology has to be developed for the manufacture of the large spherical array sonar, which increases the manufacture cost of this kind of sonar. Therefore, the United States and Russia take the lead in developing the novel conformal array sonar to replace the large-volume spherical array sonar. The conformal array sonar uses a novel kind of transducer whose elements are installed according to the hull shape of the submarine. The array elements are appressed on the surface of the hull. Therefore, the elements can be mounted from the bow to the side to obtain the similar spatial gain as the spherical array. This kind of sonar can preform the scanning acoustic beam. However, it can also produce much more complex physical characteristics, making it difficult to form the scanning beam. Fortunately, both the United States and Russia have overcome the key issues in the manufacture of the large-volume conformal array and have realized the practical application. Figure 17.7a is the bow progressive plan of Virginia-class submarine of the United States, whose spherical array sonar (bottom) will be replaced by the novel conformal array sonar (top) that is more cheaper and flexible for armaments. Figure 17.7b is one LADA-class giant conformal array sonar under construction for Russian submarines. The Virginia-class submarine, the Amur-class submarine of Russia, and the Astute-class submarine of the United Kingdom all are committed to develop bow conformal array sonars of large acoustic aperture for long detection distance and precise localization and azimuth. This kind of sonar can have very low operation frequency. For example, the working frequency of the conformal array sonars can be 2.5–10 Hz in the Type 214 submarine of Korea and the Type 209 submarines (http://www.360doc.com/content/19/0211/ 16/33540468_814264436.shtml). In addition, the life can be also extended by designing a U-type large aperture array, such as that equipped in the third batch of Virginia-class attacking nuclear submarines of the United States, which can have a life of about 33 years (almost the same as the lifetime of the submarine), and the transducer can be used for 17 years (Zhao-ping Du, Gang Chen, Da Wang, 2019). It becomes more and more difficult for the bow sonars to further increase their acoustic apertures and reduce their operation frequency even using the conformal array sonar because of the limited space, leading to the restriction of the detection distance under passive mode. In addition, there are some blind areas for bow sonars, particularly the hull side and the areas behind the hull, which make it difficult for the comprehensive monitoring and in situ alert range for hostile objects. Therefore, side array sonars are equipped in the modern submarines, which is a kind of

17.2 Fundamental Physics for Sonar System: Echolocation and Doppler Effect

(a)

(b)

Figure 17.7 (a) Scheme for the bow progressive plan of Virginia-class submarine of the United States and (b) the LADA-class giant conformal array sonar under construction of Russia. Source: (b) 360doc.

(a)

(b)

(c)

Figure 17.8 (a) The side array sonar (red arrows) equipped in the Triumph-class strategic nuclear submarine of France. (b) The exposed large acoustic aperture side array sonar (red arrows) equipped in the Seawolf-class attacking nuclear submarine of the United States and (c) the exposed unique side array equipped in the Akula 971 type Russian submarine. Source: 360doc.

sonar systems, by longitudinally arranging lots of hydrophones on the right and left sides of the submarine hull. Figure 17.8a is the side hull image of the Triumph-class strategic nuclear submarine of France with the huge red arrowed side array sonar. Figure 17.8b is the exposed side array sonar (red arrowed) along the side hull of the Seawolf-class attacking nuclear submarine of the United States. Figure 17.8c is the opened part of the side array sonar in the Akula 971 type Russian submarine, which is also a kind of the conformal array sonar based on the side shape of the hull. The operation frequency can be further reduced since the acoustic aperture can be increased greatly by equipping the more arrays along the side of the hull (the length of the array can be up to 60 m long), leading to the extended detection distance. For example, the operation frequency of the modern side array sonars can be

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usually reduced to 500–2000 Hz, while some can reach 200 Hz, and the detection distance can reach 92.6 km (50 n mile) (http://www.360doc.com/content/19/0211/ 16/33540468_814264436.shtml). The side array sonar has good concealment under passive mode. Their detection area and distance are larger than the bow sonar since the wet parts are existing in both sides of the hull, which favors to detection the location of the target and greatly improves the maneuverability of submarines. Because of the complex seafloor morphology (e.g. underwater mountains, sea gulfs, underwater iceberg), some obstacles (e.g. reefs, shipwrecks, ocean surface ice layers), or other obstacles (e.g. mines laid by the hostile countries), the mine detection and obstacle avoidance array are usually equipped in the modern submarine for safe navigation. This kind of sonars are usually operated under active mode and have a high frequency about several tens or hundreds of kHz. The high frequency usually has high detection resolution although the detection distance is only about several hundred meters. In order to assure the detection range, one sonar is usually equipped in the top enclosure of submarines, and another is equipped under the bow passive sonar, such as the high-frequency obstacle avoidance array in the enclosure of the Seawolf-class submarine of the United States (red arrow position in Figure 17.9a) and the two red arrow pointed places in Figure 17.9 for the CHIN high-frequency obstacle avoidance sonar. These obstacle avoidance sonars can help the submarine passing through the complex underwater channel, obtaining the morphology of the seafloor for new channels and detecting the dangerous mines. Limited by the space of the hull, the array sizes cannot be expanded infinitely for both the bow sonars of various types and the side sonars, which restricts their acoustic aperture. Thereby, it is difficult to further reduce their operation frequency and improve their detection distance and precision, particularly difficult to discern those quiet-type submarines covered with advanced anechoic tiles. For overcoming the

(a)

(b)

(c)

Figure 17.9 The positions of the high-frequency obstacle avoidance array (red arrows) equipped in the Seawolf-class attacking nuclear submarine of the United States (a) and the AN/BQS-15 mine detection and obstacle avoidance sonar in the Los Angeles-class attacking nuclear submarine of the United States (b) and the two CHIN high-frequency sonars (red arrows) of the Virginia-class submarine. Source: 360doc.

17.2 Fundamental Physics for Sonar System: Echolocation and Doppler Effect

above limitation, the towed array sonar systems have been developed and equipped in the ships or submarines since the 1960s. The towed array sonar is formed by a succession of hydrophones arranged at regular intervals loaded into the acoustically transparent protection conduit, which is then dragged by ships or submarines at the rear of the hull to form a line array via the placement machine to detect the underwater targets. The typical construction parts of one towed line array sonar system includes the electronic cabinets for signal control and process, the winch machine and retractable device, the towed cable, and the aligned array, as shown in Figure 17.10a – the towed slender line array sonar of the Virginia-class attacking nuclear submarine [14]. The winch machine and retractable device in the modern submarine is usually placed in the sonar shelter along the side hull for secretiveness and safety. Figure 17.10b shows the side hull shelter (usually covered by the streamline shroud) for the TB-16 towed thick line array and the BQR-23/25 thin line array equipped in the Virginia-class attacking nuclear submarine, which was previously considered as the side array sonar. Figure 17.10c is the picture of the winched AN/SQR-18 towed array sonar [14]. The left translucent part is the acoustic section equipped with series of hydrophones, and the right part is its trail rope. There are two main kinds of towed array line sonars: the tactical-type towed linear array sonar and the monitoring-type towed linear array sonar. As showed in Figure 17.10c, the typical towed array sonar system is usually constructed by the tether cable, the first vibration isolation module (VIM), the non-acoustic module, the hydrophone section, the second VIM, and the drogue. The former is equipped in big medium anti-submarine sea surface ship and attacking submarines, whose total length of towed cable can be from 1000 to 2000 m. The detection distance of the TB-16 towed array sonar for lots of the submarines of the United States can be from 92.6 to 185.2 km by utilizing the sea acoustic channel and convergence effect under passive mode, and the maximum drag speed can reach to 30 km. TB-16 has the towrope up to 800 m, the hydrophone section up to 100–150 m and the diameter of 82.5–89 mm, the ultra-low operation frequency of 10–20 Hz, and a reliable detection distance up to 180 km. The monitoring-type towed linear array sonar is mainly installed on ocean surveillance ship, whose towed speed is relatively very low and towed cable with the total length of the line array can be above 5000 m. It can work in whispered band frequency and infrasound, whose passive detection distance can be up to 555.6 km. The existing active sonar cannot meet the detection requirements of submarine-laid anechoic tiles; for the quiet-type submarine, radiation noise is less than 100 dB at 1000 Hz frequency spectrum level. However, surface ships equipped with active and passive or united line array can efficiently find the quiet-type submarine that is covered with anechoic tiles at a very far distance. Submarines dragging the passive line array sonar can also detect the low-frequency signal, greatly improving the effectiveness of detection and alert. The significant progress of the submarine noise reduction and accurate position of underwater target. Anti-submarine warfare sonar is facing serious challenges, which promote the rapid development of the passive towed array sonar to the multi-beam array sonar. This multi-beam array sonar can further lower the work frequency and increase the number of hydrophone channel, which enhances the detection capability of

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Signal control and process system

Towing system

Towed cable Towed cable 1 Twin array Towed cable 2 Winch handling system

VIM 2×128 staves Transducer array (a)

(b)

(c)

DROGUE VIM

HYDROPHONE SECTION

TETHER CABLE NON-ACOUSTIC MODULE

VIBRATION ISOLATION MODULE, VIM (d)

Figure 17.10 (a) Scheme of the towed slender line array sonar system in the Virginia-class attacking nuclear submarine. (b) The winched AN/SQR-18 towed linear array sonar. (c) The shelter for TB-16 coarse linear array sonar and BQR-23/25 slender line array sonar in the side hull of Los Angeles-class attacking nuclear submarine. (d) The main parts of the towed linear array sonar. Source: (a, d) Charlie et al. [14], (b, c) 360doc.

the towed line array sonar system. In addition, the towed sonar system is usually working under the thermocline to overcome the adverse effect of the hydrological conditions on it efficiently. The Unite States has invested lots in the development of series of models of towed linear array sonars for the attacking nuclear submarines to improve the detection ability for dominating and destroying the enemy by striking first at a long distance. There are two kinds of towed linear array sonars according to their hydrophone diameters: the slender type (e.g. TB-29, TB-29A, BQR series) and the

17.2 Fundamental Physics for Sonar System: Echolocation and Doppler Effect

coarse type (e.g. TB-16, TB16D). The slender type has a thin diameter (e.g. 28.5 mm for BQR-23/25) and a longer hydrophone section, resulting a large acoustic aperture for longer-distance detection than the coarse type. However, the sea fluid noise enforces a great disturbance on the hydrophone section at the high-speed navigation. Therefore, the slender type has to be dragged at low speed. The coarse type can be dragged at high-speed navigation and be laid quickly, resulting in enhanced maneuverability. Usually, the submarines of the United States will be installed two kinds of towed linear array sonars, such as the 668 type Los Angeles-class submarine that is installed with TB-16 and BQR-23/25, the Seawolf-class and Virginia-class submarines that are equipped with TB-16D and TB-29 or TB-29A. Therefore, the submarines can have the good detection ability and the dragging maneuverability, resulting in the greatly enhanced operational superiority during the war. In addition, there are lots of function sonars developed, such as communication sonar, reconnaissance (BTA) sonar, self-noise detecting sonar (ONA + ACC), and EHD sonar besides the above main types of sonars to improve the detection ability for enhanced combating ability installed in the surface ships and submarines. Recently, due to the cheap price and the flexibility to lay out quickly, buoy sonars have caused great attention and been developed for surface ships, air forces, and near-shore defense to improve their anti-submarine ability. Buoy sonars are a kind of sonar devices used to detect submerged submarines that, when activated, relays information by radio. Based on their working mode and orientation mode, they can be active directional or nondirectional or passive directional or nondirectional, which are usually used together for the seeking of quiet submarines. According to the transmission mode of the signal, they can be cable sonar buoy or wireless sonar buoy. According to the equipment support objects, they can be divided into the aeronautical sonar buoy, the fixed surveillance sonar buoy, and the moored sonobuoy. Figure 17.11a shows the SSQ series of aeronautical sonar buoy (usually in passive mode) served for US Navy. Usually the aeronautical sonar buoy can weigh from 3 to 8 kg, and the maximum weigh can be 30 kg. Their diameter and height can be 10–20 cm and 50–90 cm, respectively, and the highest is 150 m. This kind of sonar buoy can be deployed by the helicopter one by one (Figure 17.11b,c) or by the patrol aircraft in batches to form sonar buoy array in a certain area. The array can be the linear form or the expanded form after they expand in the water, which can be divided by the surface part and the underwater part. As the buoy drops into the water, it will keep its straight status floating in the water, and its Yagi antenna can extend above the water surface. The sonar array will disengage from housing and sags down to the certain working depth under the gravity of the cable. After minutes, the power is on and the buoy start its work. As the buoy array receives the radiation noise (passive mode) or the echo signal (active mode) from the submarine, EM signal will be emitted to the sky after the audio frequency is amplified, processed, and tuned to ultrahigh EM frequency signal, which can be received by the buoy signal process device in the helicopter or the patrol aircraft. Since the array of the nondirectional sonar buoy works under the passive mode and has no orientation, the nondirectional sonar buoy can only detect the submarine around the buoy.

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(b)

(a)

(c)

Figure 17.11 (a) One single SSQ-101 aeronautical sonar buoy (usually passive mode). (b) Scheme of the sonar buoy opening process after it drops down into the water. (c) The anti-submarine helicopter is deploying one sonar buoy. Source: (a–c) 360doc.

17.2 Fundamental Physics for Sonar System: Echolocation and Doppler Effect

The sonar buoy is almost all wireless type, with about 30 or even hundred communication bands. The detection distance in water is about 2.8 km for active mode and can be 1.8–9.3 km, even 18.5 km if using the line spectrum detection mode. The wireless communication distance of the devices above water can be 10–100 km. The common life time is about from 15 minutes to 15 hours. For the advanced SSQ-101 type served for US Navy, the monitoring distance of the active mode can be 18 km as its submergence depth is about 150 m. It will be installed with a sonar pulse transmitter if the submergence depth is down to 600 m. The monitoring distance can be 37 km under the directional mode, which can reach 135 km at the submergence depth of 1500 m. The fixed surveillance sonar buoy has long lifetime and is deployed in some certain areas (i.e. off shore) for a very long time to monitor the area all day long, which usually has its power system (e.g. solar cells or long-term batteries). The moored sonobuoy is developed from the aeronautical sonar buoy, which is fixed under the seafloor deployed by aircrafts or surface ships for the monitoring of the blind area of the fixed surveillance sonar buoy. The obtained signal can be transported to the signal receiving and process devices of the sonar on the land by wireless or cable. The moored sonobuoy can be powered by the power station of the land or high-efficient long-term batteries. Thus it can work for a long time and has a long detection distance and depth. The sonar buoy of the MSS systems developed since the 1970s can preserve a submergence depth down to 5400 m. The collected information can be stored in a storage and then sent out according to the secret instruction, which can be further resent to the data process center for anti-submarine of the land. The anti-submarine aircraft or the ships around the moored buoy can obtain the information in the moored buoy by giving the instruction of the buoy. Usually, for the final goal of dominating and destroying the enemy by striking first at a long distance, the modern advanced submarines and surface ships are usually installed with series of different types of sonar system to enhance the detection ability for the hostile submarine. Figure 17.12a,b gives the typical sonar system equipped in the modern submarine and the surface ship, respectively. The types of sonar systems equipped in some worldwide known ships or submarines are also summarized in Table 17.1 for your references.

17.2.3 Threat from Advanced Hostile Sonar System Some applications may be on the behalf of human being, such as fishery, exploration of the ocean minerals, or mapping of the ocean floor for safe navigation. In many cases, sonars usually do harms to the local ecosystem since they emit lots of strong acoustic signals of wide bonds, particularly those operated at low frequency that can mislead lots of ocean animals by disturbing their echolocation system (e.g. dolphin) or even killing them directly (e.g. whales, tuna) [16–18]. The intensity of sonars used in the ship or submarines is usually more than 100 dB, which will do harm to any marine bios. Figure 17.13 gives the masking measures to reduce the effects of the sonars installed in the modern ships on the spawn of fishes and the predation of the killer whale.

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Aft fin underwater telephone array

UHF intercept array

Foerward fin underwater telephone array

Tow cable

Aft passive ranging patches mounted on both sides of hull Mid passive ranging patches UHF intercept mounted on both side of hull array Comprehensive set of large scale passive flank arrays mounted on both sides of the hull Forward passive ranging patches mounted on both sides of hull Passive towed array deployed from reelable cable system mounted under casing

Mine detection and obstacle avoidance array

Main conformal passive/ active bow array

(a) AN/SPS-67 surface search radar

Muld-function towed array (MFTA)

Active/passive sonobuoys

SQS-53C hull mounted array

(b)

AN/WQS-2 Hull mounted wideband phones Sparsely populated volumetric array

Acoustic intercept sonar (ACI)

Figure 17.12 Summary of the main sonar types usually installed in (a) the modern submarine and (b) the surface ship. Source: (a) 360doc, (b) © 2007, Defense Industry Daily, LLC.

17.2 Fundamental Physics for Sonar System: Echolocation and Doppler Effect

Table 17.1 The main battle sonar equipment of different countries and types of ships, submarines, and aircrafts (from (translation): [15]). Countries and ships or submarines equipped

Platform

Sonar class and type name

Ship borne

Active/passive bow sonar: SQS-53C(SQQ-89)

USA: Ticonderoga cruiser, Arleigh Burke destroyer Japan: Kong¯o destroyer, Atago destroyer

Passive towed array sonar: SQR-19(V)2(SQQ-89)

USA: Ticonderoga cruiser, Arleigh Burke destroyer

Integrated dual frequency active/passive bow sonar: SQS-60/61(SQQ-90)

USA: Zumwalt-class destroyer

Lightweight broadband and variable depth sonar: LBVDS(SQQ-90)

USA: Zumwalt destroyer, littoral combat ship

Multifunction towed array sonar: SQR-20(SQQ-90)

USA: Zumwalt destroyer, littoral combat ship

Mine avoidance sonar: ISMA

USA: Zumwalt-class destroyer

Variable depth sonar: VDS

USA: littoral combat ship

Active/passive towed array sonar: OQS-102

Japan: Kong¯o, Murasame destroyer

Towed array sonar: OQR-2

Japan: Kong¯o Destroyer

Active/passive middle frequency bow sonar: Thales TMS-WASS 4100CL

France: Horizon destroyer

Active/passive middle frequency bow sonar: Ultra/EDO MFS-7000

UK: 45 destroyer

SQS-56 hull sonar, SQR-19 passive towed array sonar: SQQ 89(V)2

USA: Oliver Hazard Perry frigate

Active/Passive towed array sonar: Marconi Spherion MRS 2000, Mk2CAPTA

Norway: Fridtjof Nansen frigate

Low-frequency active/passive towed array depth sonar: EDO980 ALOFTS

Singapore: Formidable frigate

Bow sonar: Atlas electronics: DSQS-24C

Netherland: De Zeven Provincien frigate

Active/passive bow sonar: Zarya-ME

Russia: Project 20 380 frigate

Active middle-frequency bow sonar: Atlas DSQS-21B

Germany: Sachsen-class frigate

Active/passive bow sonar: Thomson Sintra 2050

UK: 23 frigate

Passive ultralow-frequency towed array sonar: Ladder company 2031Z

UK: 23 frigate (continued overleaf)

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17 Anechoic Coating for Underwater Vehicles

Table 17.1

Platform

Submarine carrying

Helicopter lifting

(Continued)

Sonar class and type name

Countries and ships or submarines equipped

Active/passive low frequency towed array sonar: Thales 2087

UK: 23 frigate

Active/passive low frequency towed array sonar: SURTASS LFA

USA: Impeccable/Victory surveillance ship

Active/passive bow sonar, BQG-5A 3-element array shipboard light wide aperture passive ranging sonar, TB-16D coarse towed array sonar, TB-29A passive slender towed array sonar, WLY-1 acoustic detector: BQQ-10 kit

USA: Virginia attack nuclear submarine

Active/passive bow sonar, broad aperture passive side sonar, TB-16, TB-29(A) towed array sonar, WLY-1 acoustic detector: BSY-2 kit

USA: Seawolf attack nuclear submarine

Side sonar, towed array sonar, conformal bow sonar, mine avoidance sonar: integrated sonar kit

Russia: God of the north wind strategic missile nuclear submarine

Active/passive low-frequency bow sonar, passive ultralow-frequency side sonar, and towed array sonar: Irtysh-Amfora system

Russia: Yasen-class attack nuclear submarine

Bow sonar, side sonar, fin sonar, slender towed array sonar: Marconi 2076 integrated kit

UK: Astute-class nuclear-powered attack submarines

Bow sonar, wide aperture side sonar, slender towed array sonar: UMS3000

France: Huffron-class submarines

Low-frequency active sonar: AQS-13F

USA: SH-60F anti-submarine helicopter

Low-frequency active sonar: AQS-22

USA, France, Norway, Sweden: MH-60R/NH-90/ NFH/EH101 anti-submarine helicopter

Low-frequency active sonar: HELRAS DS-100

USA and Italy: MH-60R, EH-101 anti-submarine helicopter

Low-frequency active sonar: FLASH

France/Norway/Korea/Austria/ Switzerland: NFH90/MH-60/AW15 anti-submarine helicopter

Source: Based on Zhao [15].

17.2 Fundamental Physics for Sonar System: Echolocation and Doppler Effect

Sonar Masker

Spawn of fishes

Predation of the killer whale

Figure 17.13 Scenario used to evaluate the masking of spawning cod sounds from shipping noise and the killer whale (predator) calls from shipping noise.

In addition, more and more advanced geological exploration technology has been developed for more activities, precisely exploring marine geology and mineral resources and energy. The air gun is a novel device for low-pressure acoustic source, which has been used as the common equipment in the above exploration. It can produce high intensive noise with the frequency of 5–300 Hz and intensity of 220–250 dB. There will be a great acoustic environmental pollution as the frequency is more than 100 Hz and/or the intensity is above 100 dB, which will enforce a great threat on the normal life of the marine mammals and fishes. Furthermore, the gradually developed sonar systems equipped in the hostile ships and submarines are indeed the real threat to human being. Now the famous Raytheon Company Ltd. has designed and manufactured the integrated active/passive multiband sonar system, such as the integrated AN/BYG-1(V) submarine sonar system for Austria. Because it is much difficult to reduce the noise radiated from the submarine to a perfect level at the low frequency and it is also quite limited to use the anechoic tiles to reduce the radiation of the submarine, high-power and large-volume arrays with low operation frequency are continuously desired for the high-efficient sonar. Self-adaptive high-power and large-volume conformal array operated at much low frequency has been developed for the sonar system, such as that installed in the Amur submarine of Russia, which can fulfill the multi-base cooperative detection. The worst is that more large powerful sonars of the frequency harm to the marine animals have been developed rapidly based on the open architecture and COTS technology since the 1990s proposed by the United States under the rapid insertion of commercial acoustics (ARCI) program. These sonars, such as BQQ-5 and BSY-1 type, have been equipped in the Los Angeles attack nuclear submarine. The BSY-2 sonar and BQQ-6 sonar have also been installed in the Seawolf attack nuclear submarine and Ohio attack nuclear submarine, respectively. In addition, there are also some other new technologies already in the sonar system integration, such as vector sonar technology [13] and synthetic aperture sonar for more precise submarine pipeline inspection [12]. Currently, the operation frequency of the mainstream active

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17 Anechoic Coating for Underwater Vehicles

sonars is about 1.5–3.5 kHz, and that of the mainstream passive sonars is about 0.1–1.5 kHz. In order to increase the detection distance, particularly for detecting the hostile quiet submarines covered by anechoic tiles, the emitting power has to be increased as strong as possible. Simultaneously, the enough acoustic intensity is necessary to pass through the thermohaline and avoid the ocean current disturbance for clear acoustic image of submarines in the complex ocean environment. Up to now, the emitting power of the big active sonar can be up to 150–1000 kW, the intensity of the dynamite or laser explosion sound source can be 220 dB, and the intensity of the plasma sound source can be up to 240–260 dB. The marine animals have to be confounded at these sounds in the ocean area where lots of ships and submarines navigate around and work by using these powerful sonars. Some of them will go out of the water surface, and some of them will rush around blindly. Unfortunately, they die finally. Thereby, human being should try to shade this worse effect as much as possible, particularly for those fish during pawning and the fragile ecosystem for some endangered marine animals. In the future, there will be more and more powerful and efficient sonar systems developed due to the more and more intensive modern arms race among the hostile countries. Ironically, the progress of the novel science and technology of human being is usually used in this race, which inevitably harms our own marine environment firstly. Generally, the first one is the fully adaptive and intelligent cognition, which will overcome the shortcoming of the conventional sonar, or no consideration of the effect of the environment information, the target characteristics, and the prior knowledge on the transmitter. Thus, the reliability and identification ability detecting underwater targets can be increased significantly if combination of the above three influence factors. The second is the co-access and distributed MIMO sonar technology developed from the processing architecture of target detection and location in MIMO sonar proposed by I. Bekkerman et al. in 2006 MIMO. This technology can make it possible to identify the mines, the frogman, the quiet submarine, and other small target with low radiation by overcoming the complex acoustic background due to the large numbers of ships, strong background noises, and serious multipath and Doppler effects. The third technology is wide-area heterogeneous multi-sensor fuse technology. Since the detection efficiency of single sonar is too low to satisfy the requirement to obtain enough underwater information by monitoring the large area for a long-term detection, now the net monitoring technology deploying many sonars, radars, laser sensors, and IR sensors in multilocation has been in progress, which can realize the centralized or distributed data processing via the data exchange, distribution, and convergence rapidly. Thus, the distribution net detection system for underwater targets can be constructed, leading to the detecting, localization, tracing, and identification of targets. Similar to GPS, this kind of underwater net systems by varieties of sensors is flexible, convenient, and inexpensive, which can dramatically improve the information perception during the underwater war [15, 19]. In the practical application, multi-base exploration systems (MBESs) are usually used in the detection of the UVs, particularly the quiet submarine for enhanced

17.2 Fundamental Physics for Sonar System: Echolocation and Doppler Effect

Sonobuoy Ship on the water

Plane

Sonobuoy

Surface Submarine

Submarine Source (a)

(b)

Figure 17.14 (a) Scheme of multi-base exploration system consists of surface warships an airdropped buoy sonar system. Source: Du et al. [20]). © 2019, Ship Science and technology. (b) Scheme of aircraft anti-submarine using active explosion and sonobuoy method. Source: Wang [5]. © 2001, Tsinghua Tongfang Knowledge Network Technology Co., Ltd.

precision and reliability based on these advanced sonar technologies and combined anti-submarine strategy, as shown in Figure 17.14a. As for the anti-quiet-submarine by aircrafts and sonobuoys, the active explosion sound source method may be more powerful based on the dynamite, the laser explosion, and the plasma sound source, whose principle is schemed in Figure 17.14b. In addition, there are also other efficient anti-submarine techniques further developed together with the sonar systems, such as the aircraft hunting technique using sono-radio-buoy, smart mines and torpedoes, frogman, and smart AUV or drone [21]. Like an unmanned aerial vehicle (UAV) in the future sea war, the hostile AUV is a much higher threat than other UVs since it is an integrated system of multiple functions that include environment perception (including target identification), dynamic decision, planning, control and implementation (target locking and destroy), etc., which has become the very important method for underwater and/or oversea wars or the exploration of risky tasks. With the demands for the localization technique for different UVs, particularly for AUVs, a new technique called cooperative localization has been developed [22]. In this technique, a few of autonomous vessels (known as master or sensors) either are placed on water surface or frequently leave the operation area for getting GPS signals. Therefore, accurate positions of the sensors are known at all times. The remaining AUVs, operating underwater (known as slave or targets), maintain their accurate position by taking reference from the sensors at regular intervals. These AUVs can be replaced by the sonar buoys that are put on water surface by battleships or air antisubmarine aircraft for scouting the hostile UVs. With the AUV development, cooperative localization technique of UVs is recently developed for underwater localization, which is much more complex and challenging task compared with that of aerial or ground autonomous vehicles, much more than those man-controlled water surface ships, land vehicles, or airplane navigated by GPS [22, 23]. In this technique, few AUVs are deployed at water surface as reference sensors. Exact positions of sensors are known using GPS or other localization technique. The remaining AUVs, called targets, equipped with inertial navigation system are performing the actual underwater operation. The targets maintain their

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Table 17.2

Features of the four cooperative localization techniques for AUVs [22].

Localization techniques Advantages

Disadvantages

Applicable conditions

Time of arrival

High accuracy that is Time synchronization Suitable for long not much affected by between sensors and ranges, NLOS rang targets required operations

Angle of arrival

Required between targets and sensors; smaller number of sensors required

Accuracy decreased with range; high-accuracy directional antennas are to be installed

Suitable for short ranges when there is limitation on number of sensors

Time difference of arrival

High accuracy even at long ranges; no time synchronization between sensors and targets required

Larger number of sensors required as compared with other methods

Best option for long-range operations when larger number of sensors are available

Received signal strength

Does not need to install extra hardware in both targets and sensors

Severely affected by Suitable for short multipath, NLOS, and ranges and LOS communication only other underwater environmental conditions

Source: Xu Bo and Farid [22]. Licenced under CC BY 4.0.

accurate position by taking reference from sensors. The targets measure the received signal strength (RSS) of the sensors to estimate their position. The most important part of cooperative localization is to measure the relative position of the targets (RSS) with respect to the sensors using acoustic waves. However, these measurements are very noisy due to harsh underwater environment. While other limiting factors related to underwater environment cannot be controlled, the accuracy of cooperative localization can still be optimized by maximizing the observability of sensors by keeping them at maximum observable location. These positions are called optimal formation of the sensors. Now there are about four available kinds of cooperative localization methods: time difference of arrival (TDOA), RSS or received signal strength indicator (RSSI), time of arrival (TOA), and angle of arrival (AOA), whose features are summarized in Table 17.2 [22]. Each method has its advantages and disadvantages and application conditions, which can be selected according to the particular scenarios and application field using their unique algorithm. Based on the four kinds of the cooperative localization methods, a general form of cooperative localization of UVs is graphically shown in Figure 17.15 [22]. Recently, in order to remove threat from hostile submarines and mines in the shallow sea and coast, the integrated underwater monitoring system represented by so-called sea net has been developed by the United States. Based on this sea net, the Distributed Agile Submarine Hunting (DASH), Deployable Autonomous Distributed System, and helicopter anti-submarine system can be integrated together to realize underwater perception of the large area and pave to the Cross Domain Maritime Surveillance and Targeting (CDMaST), whose infrastructures

17.3 Basic Physics of Anechoic Coatings

Xn

Navigational coordinate system 0n Sensors

Yn

di Zn

Figure 17.15

Si = (xi,yi,zi)T

T = (x,y,z)T

Target

A typical scenario of cooperative localization.

Wide

Unmanned systems

D Depolymerization

Communications

C

B

PNT

Weapon systems

System of systems

Information management

Manned systems

Seafloor system

Cross-domain

Logistics

A (a)

(b)

Figure 17.16 (a) Scheme of the Cross Domain Maritime Surveillance and Targeting (CDMaST) of US Army. (b) The system infrastructure of CDMaST including the platform systems, the sensor systems, the weapon systems, the communication/information management systems, the Manned/Unmanned systems, the seafloor systems, and the Logistics systems. Source: (b) Zhao [15]).

and main systems are shown in Figure 17.16. The CDMaST is expected to utilize the underwater systems, the surface systems, the air systems, and the radar, optoelectronics, sonars of the manned or unmanned systems constructing cross-domain distributed detection, reorganization, localization, striking, and evaluation platform for the enhanced operational effectiveness.

17.3 Basic Physics of Anechoic Coatings As a major technical advance after the underwater vibration and noise reduction technology, the relay low-noise propeller technology, and the floating raft technology, anechoic coating that has been widely used on the underwater vehicles has also been used to avoid the detection of the sonar. Up to now, to guide meaning for engineering design righteousness, the theoretical research on anechoic coating is

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17 Anechoic Coating for Underwater Vehicles

helpful to better understand the acoustic characteristics of the sound absorption overburden. Three representative analytic mathematical models were generally taken into account when designing anechoic coatings to achieve the required sound characteristics. They are layered absorptive media model (used to obtain overall acoustic characteristics using the transfer matrix method), the effective media model (focused on particulate composites that were applied to convert sound to heat energy and induce waveform transformation), and the cavity resonances model. Compared with the above analytical models, a viscoelastic material layer, embedded with periodically distributed air cavities, offers more design feasibility and has been widely developed. At present, there are mainly analytical methods and numerical methods to study the sound absorption covering layer with the cavity. Early modeling studies for anechoic coatings with cavities were based on homogenization and effective medium properties [24]. More recently, multiple scattering among cavity scatters in a periodic diffraction grating has been taken into account in a rigorous way by, for example, finite element method (FEM) modeling and a faster semi-analytical method, borrowed from atomic physics and applied in recent years to studies of band gaps for photonic and phononic crystals [25–27].

17.4 Development of Structure and Materials Design for Anechoic Coatings The early anechoic coating illustrated in Figure 17.17 is called “Alberich” [27]. As shown in this system, the x- and z-axes of a Cartesian xyz coordinate system are indicated by dash line, and one of the cavities is centered at the origin. When sound reaches on the surface of the coating, part of the energy was scattered by the cavities. The scattered energy can subsequently be absorbed by the rubber material, and the reflected energy can be significantly reduced. The FEM method has been used to calculate the echo reduction. This method can be flexible to model different types of Alberich coatings. Additionally, utilizing periodicity, the computer-intensive FEM modeling can be restricted to an individual unit cell. Also, coating layer reflection and transmission coefficients can be theoretically calculated using the semi-analytical method. The calculated results show that simple one-dimensional plane-wave computations have been shown feasible coatings with vanishing reflectance at isolated frequencies. Coatings much thinner than quarter-wavelength ones are found, and the main loss of the absorption happened close to the cavities. Clearly, the scattering of compressional spherically symmetric waves is very important. Underwater sound reflections can also be reduced in magnitude by a rubber coating including three-dimensional (3D) cavities that form a doubly periodic diffraction grating [24]. As shown in Figure 17.18, both spherical and cylindrical scatters are in the lattice. In this model, Cartesian xyz coordinates are shown, with the y-axis going out of the paper plane. In the sphere case, the cavity lattice has period d in x

17.4 Development of Structure and Materials Design for Anechoic Coatings

Figure 17.17 The early rubber coating with spherical cavities in a quadratic lattice with the periodic distance between adjacent cavity centers. Plane-wave sound energy is normally incident from the water above. Source: Ivansson [27]. © 2006, AIP Publishing.

Normally incident plane-wave sound energy Water x

Coating Steel plate z

Figure 17.18 A thick steel plate in water is covered with a rubber coating with spherical or cylindrical scatterers of two sizes. Source: Ivansson [24]. © 2012, AIP Publishing.

Normally incident plane-wave sound energy Water Coating

Steel plate

x

d

z

as well as y, and the lattice for the small spheres is displaced d/2 not only in the x direction but also in the y direction. In the cylinder case, the cavity lattice has a single period d in x. Results show that absorption in the surrounding rubber solid can be distinctly enhanced by the monopole resonance for sphere-like cavities, and based on this system, a corresponding resonance for an infinite cylinder is studied. Results show that for the cylinder, the absorption peak appears at a considerably lower frequency than for a sphere with the same radius, and this suggests the possibility of much thinner anechoic coatings including cylindrical cavities, with axes in a lateral direction, forming a diffraction grating with a single period. Due to the invariance in the axial direction, this system is more effective than the 2D case. After the subsequent coating design computations, using the layer-multiple-scattering method and including cavities of different sizes, it shows improved reflection reduction with coatings only about one third as thick. Still accounting for multiple scattering among the cavities and capturing the essential physics, the monopole approximation is applied to advance the analytic study of the reflection reduction. An energy decomposition relation is derived and used to quantify the absorption of the incident sound energy by cavities of different sizes. Additionally, considering the coatings based on filled inclusions and other resonance effects, it is found that the 2D alternative with cylinders of mixed sizes can also show thinner coatings. Recently, by introducing locally resonant scatters with spherical shape proposed in crystals into the design of underwater sound absorption materials, the low-frequency underwater sound absorption phenomenon induced by the localized resonances is observed [28]. The model is composed of a viscoelastic polymer slab and backing. In this model, the viscoelastic polymer slab is composed of a plane of uniform spherical scatters arranged in a square lattice. In the present investigations, the backing can be half infinite water (air) or a steel slab followed by half infinite air. The calculated results from the FEM and LMSM method show that there are two peaks appearing at 870 and 2310 Hz in the absorbance spectrum. Furthermore, due to the presence of the respective displacement pattern, the mode conversion at the second absorption peak is more efficient than that at the first one. The vibration is primarily confined in the core, and the vibration of the whole

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17 Anechoic Coating for Underwater Vehicles

Y

Input Plane

Backing

Wave

Z

(a)

a2

X

a

576

a1

(b)

R

r

(c)

Figure 17.19 (a) The anechoic structure model. A plane wave is normally incident from the left along. (b) A plane of scatters with a square lattice, defined by the primitive vectors a1 , a2 . (c) The unit cell consists of a core with radius r and a coated rubber layer (hatched range) with outer radius R. Source: Zhao [29]. © 2007, Elsevier.

core is almost equal amplitude, which shows that the scatter exhibits a rigid body resonance. The polymer matrix and the core present have much less vibration than that of the coating, which means that the sound energy is mainly trapped in the coating. Transverse waves induced by mode conversions from longitudinal incidence channel the wave propagation along the longer dimension of the slab and damp rapidly in polymer, which enhances the energy dissipation. Another localized resonance in a photonic crystal, composed of 3D arrays of composite units, has been also discovered recently [29]. Similarly, as shown in Figure 17.19, the composite unit is a high-density sphere coated by soft silicon rubber. The absorptive properties induced by localized resonance are systemically investigated. The mode conversions during the Mie scattering of a single coated lead sphere in unbounded epoxy are analyzed by referring the elements of the scattering matrix. Then, the anechoic properties of a slab containing a plane of such composite scatters are investigated with the multiple-scattering method by accounting the effects of the multiple scattering and the viscous dissipation. The results show that the longitudinal to transverse mode conversion nearby the locally resonant region is an effective way to enhance the anechoic performance of the finite slab of photonic crystal, and most of the incident energy is absorbed in the desired frequency band [29]. Besides localized resonance in a photonic crystal, the underwater scattering and absorption on a coated plate with a finite signal conditioning plate using plane incident waves were also investigated to explore the changes ascribed as the induced structure in the scattered and absorbed sound fields and the mechanisms involved [30]. The results show that the signal conditioning plate may significantly have an effect on the local sound absorption and reflection, owing to the interaction between the induced pressure and the change of the vibration response of the coated plate. Such influence will become distinct at the resonance frequencies of the global modes and trapped modes of the coated plate with the signal conditioning plate. However, if the detection area is much larger than that of the signal conditioning plate, the overall absorption performance will approximately keep unchanged. In a word, compared with the structure without the signal conditioning plate, if the evaluation size of the area is small or comparable with the size of the signal conditioning plate, a reduced sound absorption coefficient will be found.

17.4 Development of Structure and Materials Design for Anechoic Coatings

The abovementioned work on sound scattering and absorption was further extended by a modified underwater coated plate with a single attached distributed inhomogeneity to that with periodically located distributed inhomogeneities. A comparison is made among cases of a plate without inhomogeneities, a plate with inhomogeneities, and one with inhomogeneities ignoring the mutual coupling. Results show that the coupling of the structural waves scattered by the inhomogeneities plays an important role in modifying the sound absorption and scattering of surface sound pressure, especially at low frequencies and/or the resonance frequencies of the trapped modes of the plate. The sound absorption of the plate is dependent on the distance between the adjacent inhomogeneities, the length of the inhomogeneity, and the angle of the incident sound. On the surface of the inhomogeneities, the scattered/total sound pressure is generally enhanced. On the surface in between the inhomogeneities, the pressure is also enhanced at low frequencies but is nearly unchanged at higher frequencies. Results also show that the coupling-induced variation of scattered/total pressure is significant only at the resonance frequencies of the global modes and trapped modes. A further modified underwater structure is designed for both detection and anechoic purposes. In a cross section of the periodic coated plate, the system was composed of periodic signal conditioning plates and two elastic coatings. Here, the signal conditioning plates are located at the interface of two elastic coatings that are attached to an elastic plate [31]. Results show that the embedded signal conditioning plates can enhance sound absorption at frequencies below the coincidence frequency of the plate (fc). An amplified acoustic signal of over 14 dB is observed at most frequencies within 0 – fc at the coatings’ interface close to the signal conditioning plates’ edges, owing to focused stress formed there. Peaks in the signal response at maximal 30 dB are also observed. The peak frequencies are coincident with or close to the peak frequencies of absorption, demonstrating that significantly enhanced acoustic signal and absorption can be achieved simultaneously through the use of embedded periodic signal conditioning plates. These findings are useful for the application of periodic signal conditioning plates for underwater sensing and stealth purposes. Besides the periodic signal conditioning plates devices, sub-wavelength and quasi-perfect underwater sound absorber composed of a viscoelastic coating layer embedded with periodically distributed plate scatter for multiple and broad frequency bands was also investigated. The system was shown in Figure 17.20. Results show that the embedded arrays cannot only slow sound waves in the coating, leading to a downshifted resonance frequency where the absorption is maximized, but also introduce multiple local bending modes and local longitudinal modes in the coating. Via a proper selection of the parameters of the plate scatters and the plate scatter array, multiple local resonance modes of different types in a coating unit can be excited, resulting in quasi-perfect absorption of incident sound at multiple frequencies whose wavelengths are much longer than the thickness of the coating layer. For example, absorption (89%) of underwater sound at 462.9 Hz is achieved by such a layer with a thickness of 6 cm, which is 1.9% of the wavelength of the incident sound. Broadband quasi-perfect absorption can also be realized by

577

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17 Anechoic Coating for Underwater Vehicles Z X

l l0

Water

y

i=I

pi(r, 𝜃i)

i=I

Untt cell 𝜃i Coating h1

Air

𝜃t pt(r, 𝜃t)

Figure 17.20 Schematic diagram of the absorber structure. Source: Zhang [32]. © 2018, AIP Publishing.

the coupling of those multiple local resonant modes. This quasi-perfect absorption property can also be observed for sound waves with different incident angles because a large number of local intrinsic modes could still be excited. The proposed design thus offers a new method for absorbing low-frequency underwater sound by devices with sub-wavelength thickness. Besides the abovementioned nanostructures, acoustic metasurface has recently grown into a burgeoning new field, and there has been a proliferation of work on the topic of the metasurface-based sound absorber, which has led to near-perfect absorption at deep sub-wavelength thickness [33]. It has been widely reported that the acoustic metasurface coupled with a decorated membrane resonator with a thin air chamber can be constructed to form hybrid resonances for the realization of the total absorption in the low-frequency range. For instance, by using a simple metasurface construction, an acoustically reflecting surface can acquire hybrid resonances and becomes impedance matched to airborne sound at tunable frequencies, leading to no reflection information [34]. It is a challenge to construct a perfect and stable acoustic absorber with deep sub-wavelength thickness due to the intrinsically physical dissipation mechanism. A metasurface-based perfect absorber capable of realizing the total absorption of the acoustic wave in an extremely low-frequency region was presented to solve this challenge. As shown in Figure 17.21, an incident acoustic wave along z direction normally impinges the perforated system with periodic holes (period a) and then penetrates the holes (diameter d and thickness t) to the back cavity (length l) with hard objects. It is demonstrated that the sound energy could be highly absorbed in the resonant frequency resulting from the energy dissipation. The contribution of the energy dissipation comes from the viscous friction on the wall of the perforated holes since its geometrical size is comparable with the thickness of the viscous boundary layer. The metasurface possessing a deep sub-wavelength thickness down to a feature size of 𝜆/223 is composed of a perforated plate and a coiled coplanar air chamber. Simulations based on fully coupled acoustic with thermodynamic

17.4 Development of Structure and Materials Design for Anechoic Coatings

d

a

b w d a l

t (a)

z a

y

w x

a t

(b)

Figure 17.21 (a) A perforated system composed of a perforated plate with centered holes placing in front of a hard object to form a straight back air cavity and (b) the metasurface composed of a perforated plate with a hole and a coiled air chamber. Source: Li and Assouar [35]. © 2018, AIP Publishing.

equations and theoretical impedance analysis are utilized to reveal the underlying physics and the acoustic performances, showing an excellent agreement. The total absorption is realized due to the optimized balance between the scattering wave and the dissipation. This structure can be easily fabricated with a 3D printing technique and takes advantage of the compact size, stable structure, and high efficiency. It has been widely reported that the porous films are capable of controlling the optical, magnetic, and magneto-optical effects [36–39]. Besides that, porous materials such as fibrous materials are traditionally used for sound absorption. Composite honeycomb sandwich meta-surface panel can be adopted in a wide range of applications, owing to their excellent mechanical properties. As shown in Figures 17.22a–c, the panel is composed of periodically and horizontally arranged honeycomb “super-cells,” which consist of unit cells of different geometric parameters (pore size). The orifice in the face sheet serves as the neck of Helmholtz resonator while the honeycomb core serves as the cavity. The results in Figures 17.22d show that the composite honeycomb meta-surface panel can achieve 90% sound absorption from 600 to 1000 Hz with a thickness of less than 30 mm. The relatively broadband sound absorption is found to be attributed to the coupling between unit cells. Such a device has advantages such as lightweight, broadband sound absorption, fine mechanical properties, and availability for mass production. Such a design is expected to find a wide range of applications for noise reduction. Investigation on a microlattice meta-material (Figure 17.23a–c) that consists of well-defined and organized material morphology in terms of pore size and porosity, for generating optimum sound dissipation, was presented. The sound absorption measuring device was shown in Figure 17.23d. It is found that the optimum pore size for maximum sound absorption is twice the viscous boundary. In addition, optimum combinations of pore size and porosity for maximal sound absorption over the entire

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17 Anechoic Coating for Underwater Vehicles

4 7

5 9 3 2 4 5

7

9

4 5

7

5

3 4 5

8 3 2

6 7

Supercell

9

1

2

3

6

9 8

1

8

(a)

3 2

6

4

1

8

1

8

4

Unit cell

5 9 8

3

(b) 1.0

2aj l

D

Absorption coefficient

580

0.8 0.6

HR modrl MPP modrl FEM

0.4 0.2

a 0.0 500

600

700

800

900

1000

1100

Frequency/Hz

(c)

(d)

Figure 17.22 (a) Schematics of the composite honeycomb sandwich panel. (b) A honeycomb unit cell. (c) A honeycomb supercell composed of nine unit cells of different orifice sizes. (d) Absorption coefficient of the composite honeycomb sandwich panels. Source: Xiuyuan et al. [33]. © 2018, AIP Publishing.

frequency band were also determined for practical applications. The findings in this work offer a useful guide for designing sound-absorbing materials with enhanced absorption performance. As shown in Figure 17.24, an ultra-broadband meta-material-based acoustic absorption material was designed. Compared with traditional acoustic absorbers, the designed device possesses a simple structure unrestricted by the material type and does not require any extra sound-absorbing material, suggesting the potential to have structural stiffness and environmentally friendly feature simultaneously. Such distinct characteristics stem from the localization and dissipation of waves with different frequencies at particular spatial positions. The proposed ultra-broadband absorption structure may provide design possibilities for various applications such as noise reduction and underwater anechoic. Considering the slow sound and critical coupling, an ultrathin acoustic metamaterial panel for perfect and quasi-omnidirectional absorption is conceived. As shown in Figure 17.25, the system consists of a rigid panel with a periodic distribution of thin closed slits. The upper wall of the system is loaded by Helmholtz resonators. Results show that the presence of resonators produces a slow sound propagation shifting the resonance frequency of the slit to the deep sub-wavelength regime. It is also demonstrated that by controlling the geometry of the slit and the Helmholtz resonator, the intrinsic isothermal losses can be tuned in order to

17.4 Development of Structure and Materials Design for Anechoic Coatings

(a)

(b)

Ridge

Rod

t

W

h

v

w

Ridge Rod

300 μm

h

s

(c)

x 100

Wire

a

t

a

w

(d)

Figure 17.23 (a) The microstructure of the microlattice meta-material. (b) SEM picture of longitudinal sectional view on solid frameworks. (c) Cross section of one layer with uniformly sized pores. (d) The sound absorption measuring device. Source: Xiaobing et al. [40]. © 2018, AIP Publishing.

2 4 Plane wave

l 3

(a)

z y x (b)

1

Plexiglass

Figure 17.24 (a) Photograph of the proposed structure sample. (b) Schematic of the experimental setup. Source: Jiang et al. [41]. © 2014, AIP Publishing.

exactly compensate the energy leakage of the system and fulfill the critical coupling condition to create the perfect absorption of sound in a large range of incidence angles due to the deep sub-wavelength behavior. These promising results open the possibilities to study different configurations based on these metamaterials and to extend the results to broadband and omnidirectional perfect absorption with deep sub-wavelength structures.

581

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17 Anechoic Coating for Underwater Vehicles

wc

Γx1=d

n=1

n=N

x1

lc ln d

wn

h

x1 (a)

x2

Γx =0 1 (b)

L

a

x2

Figure 17.25 (a) Conceptual view of the thin panel placed on a rigid wall with one layer of square cross-sectional Helmholtz resonators, N = 1. (b) Scheme of the unit cell of the panel composed of a set of N Helmholtz resonators. Source: Jiménez et al. [42]. © 2016, AIP Publishing.

17.4.1 Perspective for the Sonar System and Anechoic Coating Development Up to now, sonar is still the unique technique for underwater vehicles to obtain the surrounding information as they submerge into a certain depth underwater for navigation and detection due to the excellent hydrophone features of acoustics with the development of the Sonar system. In addition, sonar is the main efficient buster (bane) of submarines as considering the anti-submarine war. Besides the indispensable military applications for the threat detection of hostile battleships, submarines, mines, and torpedo, sonar has been intensively used in the marine investigation and the fishery. More and more advanced sonar systems are in progress with the blockade and anti-blockade by hostile forces in the world. Additionally, the design of sound absorption materials has been widely researched, which focused on different contents. Layered absorptive media model, the effective media model, and the cavity resonance model were three main types of models to be built to reduce the reflection of sound energy. Nowadays, the total absorption of sound using sub-wavelength structures or materials has always been a challenge, since the linear dynamics of dissipative systems dictates the fractional power to be linearly proportional to the elastic deformation energy. Furthermore, broadband underwater absorption together with signal enhancement/focusing is still a challenging but significant issue for hull-mounted hydrophones for sensing and stealth purposes.

Acknowledgments This chapter is supported by the NSFC–BRICS STI Framework Program (No. 51861145309), the National Natural Science Foundation of China (No. 51971029), the National S&T Major Project (No. 2018ZX10301201), the Postdoctor Research

References

Foundation of Shunde Graduate School of University of Science and Technology Beijing (No. 2020BH005), the Project funded by China Postdoctoral Science Foundation (No. 2020M680336), the “All English teaching demonstration course construction project of University of Science and Technology Beijing” (No. KC2015QYW06, 2016), the “1125” Zhihui Zhengzhou Talent project of Henan province (Fund No. in USTB: 39080070), the “100 talent plan” fund of Fujian province (Fund No. in USTB: 39080067), and the development of a high sensitive magneto-optical biomolecular sensor experimental prototype (Fund No. in USTB: 2019-0649) by Hangzhou Ruidi Biotechnology Co. Ltd.

List of Abbreviations 3-D AOA ARCI AUV CDMaST EM EHD FEM GPS MBESs RSS RSSI TOA TDOA UUV UVs VIM WW I

three dimensional angle of arrival rapid insertion of commercial acoustics program autonomous underwater vehicle cross domain maritime surveillance and targeting electromagnetic environmental hydrologic detection sonar finite element method global position system multi-based exploration systems received signal strength received signal strength indicator time of arrival time difference of arrival unmanned undersea vehicle underwater vehicles vibration isolation module world war I

References 1 Mouritsen, H. (2018). Long-distance navigation and magnetoreception in migratory animals. Nature 558 (7708): 50–59. 2 Guo, H.Z. et al. (2015). (MI)-I-2: channel modeling for metamaterial-enhanced magnetic induction communications. Ieee Transactions on Antennas And Propagation 63 (11): 5072–5087. 3 Urick, R. (2005). Principles of Underwater Sound, Rubber and plastic resources utilization, vol. 5, 44-44. 4 Tadahiro, H. Design of Autonomous Underwater Vehicle. International Journal of Advanced Robotic Systems 8 (1): 1. 5 Wang, B.H. (2001). Application and development of sonar technology. Physics 30 (08): 491–495.

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6 Assouar, B., Liang, B., Wu, Y. et al. Acoustic metasurfaces. Nature Reviews Materials. 7 He, H., Chunyin, Q., Liping, Y. et al. Topological negative refraction of surface acoustic waves in a Weyl phononic crystal. Nature 560 (7716): 61–64. 8 Yonghwan, H., Hongmin, A., Duc-Nam, N. et al. (2018). An underwater parametric array source transducer composed of PZT/thin-polymer composite. Sensors and Actuators A: Physical 279: 601–616. 9 Ahn, H., Hwang, Y., and Moon, W. (2016). Underwater parametric array source transducer composed of PZT rods and thin polymer plate with high power efficiency for wideband sound generation. The Journal of the Acoustical Society of America 140 (4): 3089–3089. 10 Bjørnø, L., Neighbors, T., and Bradley, D. (2017). Dedication. In: Applied Underwater Acoustics (eds. T.H. Neighbors and D. Bradley), 1–964. Elsevier. 11 Sopko, I.M., Ignatyeva, D.O., Knyazev, G.A., and Belotelov, V.I. Efficient acousto-optical light modulation at the mid-infrared spectral range by planar semiconductor structures supporting guided modes. Physical Review Applied 13 (3): 034076. 12 Hao, Y.U., Wang, P., Duan, K., and Zhang, X. (2015). Application progress of synthetic aperture sonar in submarine pipeline inspection. Hydrographic Surveying & Charting 3: 20–23. 13 Yang, D.S., Zhu, Z.R., and Tian, Y.Z. (2018). Theoretical bases and application development trend of vector sonar technology. Journal of Unmanned Undersea Systems 3: 185–192. 14 Charlie, W., Zhu, K.Q., Zhang, T.Y. et al. (2013). Evaluation current research status and development trend of towed multi-line array sonar of domestic. Ship Science & Technology 7: 1–7. 15 Zhao, P. (2016). Study status and development trend of foreign sonar technology. Modern Radar 8: 20–24. 16 Andre, M., Schaar, M.V.D., Sanchez, A.M. et al. (2016). Underwater acoustic observatories to reduce ship noise footprint: a risk assessment model to mitigate the impact of shipping noise on marine fauna. In: Techno-Ocean, 329–332. 17 Hurtós, N., Palomeras, N., Carrera, A., and Carreras, M. Autonomous detection, following and mapping of an underwater chain using sonar. Ocean Engineering 130: 336–350. 18 Audoly, C., Gaggero, T., Baudin, E. et al. Mitigation of underwater radiated noise related to shipping and its impact on marine life: a practical approach developed in the scope of AQUO project. IEEE Journal of Oceanic Engineering 42 (2): 373–387. 19 Liu, X.W. (2008). Harbin, and China, Network security situation awareness model based on heterogeneous multi-sensor fusion. Computer Science 35 (8): 69–73. 20 Zhaoping, D., Chen, G., and Wang, D. (2019). Foreign sonar technology development research summary. Ship Science and Technology. 41 (1): 145–151. 21 Shaohua, C., Chunxiao, Z., Guangzhou, W. et al. (2019). Development situation and military application of autonomous underwater vehicle. Ship Engineering 2: 79–84.

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22 Xu Bo, A.A.R. and Farid, G. (2019). A review on optimal placement of sensors for cooperative localization of AUVs. Journal of Sensors. 23 Zhang, L., Wang, T., Zhang, F., and Xu, D. Cooperative localization for multi-AUVs based on GM-PHD filters and information entropy theory. Sensors 17 (10): 2286. 24 Ivansson, S.M. Anechoic coatings obtained from two- and three-dimensional monopole resonance diffraction gratings. Journal of the Acoustical Society of America 131 (4): 2622–2637. 25 Liu, Z., Chan, C.T., Sheng, P. et al. Elastic wave scattering by periodic structures of spherical objects: theory and experiment. Physical Review B 62 (4): 2446–2457. 26 Psarobas, I., Stefanou, N., and Modinos, A. (2000). Scattering of elastic waves by periodic arrays of spherical bodies. Physical Review B 62: 278. 27 Ivansson, S.M. Sound absorption by viscoelastic coatings with periodically distributed cavities. Journal of the Acoustical Society of America 119 (6): 3558. 28 Wen, J., Zhao, H., Lv, L. et al. (2011). Effects of locally resonant modes on underwater sound absorption in viscoelastic materials. Journal of the Acoustical Society of America 130 (3): 1201. 29 Zhao, H., Liu, Y., Wen, J. et al. Tri-component phononic crystals for underwater anechoic coatings. Physics Letters A 367 (3): 224–232. 30 Zhang, Y. and Pan, J. (2013, 2082). Underwater sound scattering and absorption by a coated infinite plate with a distributed inhomogeneity. Journal of the Acoustical Society of America 133 (4). 31 Zhang, Y. and Pan, J. Enhancing acoustic signal response and absorption of an underwater coated plate by embedding periodical inhomogeneities. Journal of the Acoustical Society of America 142 (6): 3722–3735. 32 Zhang, Y., Pan, J., Chen, K., and Zhong, J. (2018). Subwavelength and quasi-perfect underwater sound absorber for multiple and broad frequency bands. The Journal of the Acoustical Society of America 144 (2): 648–659. 33 Xiuyuan, P., Jun, J., and Yun, J. (2018). Composite honeycomb metasurface panel for broadband sound absorption. Journal of the Acoustical Society of America 144: EL255–EL261. 34 Ping, S. Acoustic metasurface with hybrid resonances. Nature Materials 13 (9): 873–878. 35 Li, Y. and Assouar, B.M. Acoustic metasurface-based perfect absorber with deep subwavelength thickness. Applied Physics Letters 108 (6): 063502. 36 Zhang, W., Li, J., Ding, X. et al. (2017). Tunable magneto-optical Kerr effects of nanoporous thin films. Sci Rep 7 (1): 2888. 37 Zhang, W. et al. (2019). The optical cavity enhanced magneto-optical Kerr effect signals of AAO/Al-based CoFeB nanostructure arrays. Optics Communications 437: 44–49. 38 Zhang, W., Wang, Q., Zhao, C., and Song, Y. (2018). Surface and interface engineering multilayered nanopore films for enhanced Fabry–Pérot interferences. The Journal of Physical Chemistry C 122 (51): 29457–29463. 39 Song, Y., Yin, W., Wang, Y.-H. et al. (2014). Magneto-plasmons in periodic nanoporous structures. Sci. Rep. 4: 4991–4991.

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40 Xiaobing, C., Jun, Y., Gengkai, H. et al. (2018). Sound absorption by acoustic microlattice with optimized pore configuration. The Journal of the Acoustical Society of America 144: EL138–EL143. 41 Jiang, X., Liang, B., Li, R.-q. et al. (2014). Ultra-broadband absorption by acoustic metamaterials. Applied Physics Letters 105 (24): 243505. 42 Jiménez, N., Huang, W., Romero-García, V. et al. (2016). Ultra-thin metamaterial for perfect and quasi-omnidirectional sound absorption. Applied Physics Letters 109 (12): 121902.

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18 Thin Films and/or Coating for Electromagnetic Interference and Stealth Junmei Wang 1 and Yujun Song 1,2 1 University of Science & Technology Beijing, Center for Modern Physics Technology, Applied Physics Department, School of Mathematics and Physics, 30 Xueyuan Road, Beijing 100083, China 2 Zhejiang Key Laboratory for Pulsed Power Technology Translational Medicine, Hangzhou Ruidi Biotechnology Company, Hangzhou 310000, China

18.1 Introduction The rapid development in information and communication technology has led to severe electromagnetic (EM) radiation pollution, which is called electromagnetic interference (EMI). The radiations are emitted from both portable electrons, radar, and other highly integrated devices [1]. EMI affects the normal operation of various electronic devices, bringing huge economic losses. They can also interfere with surrounding electrical equipment, cause computer information leakage, and threaten national information and military security. In addition, powerful electromagnetic pulses can cause damage to the human body, causing neurological disorders. and uncontrolled behavior. To overcome these problems, varieties of electromagnetic shielding materials and absorbing materials are researched [2–4]. The shielding material can control the radiation of electromagnetic waves, magnetic fields, and electric fields by constructing a shielding body to prevent EMI. The absorbing materials can effectively absorb or attenuate the incident electromagnetic wave and convert the electromagnetic energy into heat energy or other energy consumption to eliminate the EMI. The electromagnetic spectrum is used to describe a continuous range of possible frequencies of electromagnetic radiation. Electromagnetic waves can be classified into radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, and γ rays. The frequency range is as shown in Figure 18.1. The EMI occurs mainly at the range of radio waves to microwaves. To describe radar operation range conveniently and guarantee military security, letter designations have been used as a short notation to describe the radar frequency band of operation. It is consistent with the recommended nomenclature

Inorganic and Organic Thin Films: Fundamentals, Fabrication, and Applications, First Edition. Edited by Yujun Song. © 2021 WILEY-VCH GmbH. Published 2021 by WILEY-VCH GmbH.

588

18 Thin Films and/or Coating for Electromagnetic Interference Visible light

Long radio waves

1

103

Figure 18.1 Table 18.1

Radio waves Microwaves

106

109

Infrared

1012

UV

1015

X-rays

1018

Gamma rays

1021

1024

Frequency (Hz)

The electromagnetic spectrum. Designation of radar bands.

Band designation

Nominal frequency ranges

HF

3–30 MHz

VHF

30–300 MHz

ITU assignment

138–144 MHz 216–225 MHz

UHF

300–1000 MHz

420–450 MHz 850–942 MHz

L

1–2 GHz

1215–1400 MHz

S

2–4 GHz

2300–2500 MHz

C

4–8 GHz

5250–5925 MHz

X

8–12 GHz

8500–10680 MHz

Ku

12–18 GHz

13.4–14.0 GHz

K

18–27 GHz

Ka

27–40 GHz

33.4–36 GHz

V

40–75 GHz

59–64 GHz

W

75–110 GHz

2700–3700 MHz

15.7–17.7 GHz 24.05–24.25 GHz

76–81 GHz 92–100 GHz

Mm

110–300 GHz

126–142 GHz 144–149 GHz 231–235 GHz 238–248 GHz

of the International Telecommunications Union (ITU) and been accepted by engineers. Letters such as L, S, C, etc., are used to stand for the radar-frequency bands as shown in Table 18.1 [5]. In the table, each radar frequency band has its own characteristics and can be used in different areas. This chapter is divided into two sections: EMI shielding materials and absorbing materials. The shielding and absorbing theories will be provided and different types of materials are presented to illustrate the research direction of materials used to prevent EMI.

18.2 EMI Shielding Materials

18.2 EMI Shielding Materials In order to reduce costs and facilitate large-scale industrial production, most of the shell materials of the electronic products are made of engineering plastics. However, the plastics themselves are not conductive and have no shielding ability against EMI. Other materials with good conductivity are required to replace plastics. In addition, the increasingly fierce competition in the shielding material market also promotes the research of new materials. Shielding composites, as a new type of shielding material, have received increasing attention. At present, the research on electromagnetic shielding composite materials is mainly focused on two categories, namely, surface conductive type and filled type. Commonly used methods for preparing surface conductive electromagnetic shielding composite materials include conductive coatings, metal spraying, surface metallization (vacuum gold plating, cathode sputtering, chemical plating, electroplating, etc.), surface metal foil coating, and the like. Filled electromagnetic shielding composite materials mainly refer to the materials obtained by adding metal-based (Cu, Ag, Fe, Ni, etc.), carbon-based (carbon black, graphite, fiber, etc.), conductive polymers, and other electromagnetic shielding agents to synthetic resins with good electrical insulation properties through blending, extrusion, and injection molding. Although a good shielding effect has been achieved, some issues such as the shielding of wideband, especially for low frequencies electromagnetic waves, the use temperature, and the preparation technologies remain to be addressed. In the following part, we will introduce the theory of EMI and then present several different kinds of EMI shielding materials, namely, surface coating film shielding material, ferromagnetic materials and good metallic conductor material, conductive paint shielding material, and composite shielding material.

18.2.1 Basic Theory of Electromagnetic Shielding Electromagnetic shielding is mainly used to prevent the influence of high-frequency electromagnetic fields, so as to effectively control the radiative propagation of electromagnetic waves from one area to another. The shielding effect is generated by the reflection of electromagnetic waves on the surface of the shielded conductor, the absorption inside the conductor, and the loss during transmission by using a low-resistance conductive material. It is usually expressed by the shielding effectiveness (SE), which is the ratio of the incident or emitted electromagnetic wave before shielded and the reflected or transmitted electromagnetic wave after being shielded at the same place. It is the attenuation value of the shielding material to the electromagnetic signal. We can express it using the following equation [6]: SE = 20 log(Eb ∕Ea ) SE = 20 log(Hb ∕Ha ) SE = 10 log(Pb ∕Pa )

589

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18 Thin Films and/or Coating for Electromagnetic Interference

where Eb and Ea are the electric field strengths before and after shielding, H b and H a are the magnetic field strength before and after shielding, and Pb and Pa are the intensity of the energy field before and after shielding. The unit of EMI SE is given in decibels (dB). The larger the attenuation value, the better the shielding effect. Electromagnetic waves are oscillating particle waves that are generated in space by electric and magnetic fields perpendicular to each other in the same phase. The electromagnetic field can be referred to as far field or near field, where 𝜆/2𝜋 is the transition line. In the far field, the electromagnetic wave is a plane wave and the characteristics of the wave are determined by the medium it propagates. While in the near field, the property of the electromagnetic wave mainly depends on the characteristics of the field source. To better understand the electromagnetic wave, a parameter of impedance should be known. It is the ratio of the transverse components of the electric and magnetic fields, which is represented using the symbol Z, where √ E0− (x) jw𝜇 = Z= − H0 (x) 𝜎 + jw𝜀 In the above equation, 𝜇 is the magnetic permeability, 𝜀 is the electric permittivity, 𝜎 is the electrical conductivity of the material that the wave is traveling through, j is the imaginary unit, and w is the angular frequency of the wave. In free space, √𝜇 the wave impedance can be illustrated as Z0 = 𝜀 0 = 377 Ω. The value of the wave 0

impedance determines the reflection intensity of the interface and it is closely related to the research of EMI. All the following theories are focused on the far filed electromagnetic wave. According to Schelkunoff’s electromagnetic shielding transmission line theory, the shielding effect of metallic materials can be expressed by the following formulas [7]: SE = R + A + B

(18.1)

R = 168 − log(f 𝜇r ∕𝜎r )

(18.2)

A = 1.314t(f 𝜇r 𝜎r )1∕2

(18.3)

( ) 2t B = 20 log 1 − e− 𝛿

(18.4)

where R, A, and B are the reflection loss (RL), absorption loss, and multiple reflection loss of the electromagnetic wave inside the shielding material, respectively. 𝜎 r , 𝛿, 𝜇 r , f , and t are the relative conductivity, skin depth, relative permeability, frequency of the electromagnetic wave (Hz), and the thickness of shielding film (cm), respectively. When A > 10 dB, B can be ignored [6]. The (18.1) can be expressed as SE = R + A

(18.5)

We can know from the above formulas that for good conductors such as silver, copper, and aluminum, the value of R is large due to the large 𝜎 r . That is, the SE mainly depends on the surface reflection loss. However, for high permeability materials,

18.2 EMI Shielding Materials

such as iron and iron–nickel alloys, a large 𝜇 r means a large A value, which means that absorption loss will play a major role. Furthermore, the reflection loss decreases with increasing frequency and the absorption loss increases with frequency. The shielding effect is measured according to the SE. Generally, if the SE value reaches 35 dB in the frequency range of 30–1000 MHz, it can play an effective shielding role in conventional electronic equipment. According to the principle of shielding, the contribution of shielding materials to SE is divided into three parts: (i) the reflection loss caused by impedance mismatch on the surface of the shielding body, (ii) the transmission loss or absorption loss caused by the absorption of electromagnetic energy when the electromagnetic waves are transmitted inside the shielding material, and (ii) multiple reflection losses caused by multiple reflections of the electromagnetic wave between the inner surfaces of the shielding material. It can be concluded that the electrical conductivity, magnetic permeability, and thickness are the three basic factors affecting the shielding effects of materials that lead to a breakthrough in the research of shielding materials. Additionally, the SE is also related to the structure, shape, and airtightness of the shielding material, and the frequency and field source characteristics of the electromagnetic waves to be shielded.

18.2.2 EMI Shielding Materials In recent years, great efforts have been made to develop materials with high electrical and thermal conductivity, excellent mechanical properties, and EMI shielding efficiency. In Sections 18.2.2.1–18.2.2.4, we will briefly review several researches regarding most widely used EMI shielding materials. 18.2.2.1 Surface Coating Film Shielding Material

This kind of material is made of plastic and other insulators with a conductive layer attached to the surface to achieve the purpose of shielding. Common preparation methods include chemical gold plating, vacuum sputtering, metal spraying, and metal foil attachment. These surface-layer conductive thin film shielding materials generally have advantages such as good conductivity and obvious shielding effects. However, the surface conductive film has low adhesion, is prone to peeling, and has poor secondary processing properties, which prevent its application. 18.2.2.2 Ferromagnetic Material and Good Metallic Conductor Material

Ferromagnetic material is a commonly used shielding material. It is suitable for shielding of low frequency (below 100 kHz) magnetic field. Its working principle is to use the high permeability to guide the magnetic lines of force through highly penetrating material and reduce the magnetic flux density in the nearby space to achieve magnetic shielding. Commonly used ferromagnetic materials are pure iron, silicon steel, permalloy (iron–nickel alloy), and so on. Soft magnetic alloy materials such as iron–cobalt (FeCo) alloy and iron–aluminum alloy can also be used for shielding of low-frequency magnetic fields. However, ferromagnetic materials are not suitable for shielding high frequency electromagnetic fields due to their low electrical conductivity.

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18 Thin Films and/or Coating for Electromagnetic Interference

Good metal conductors have high electrical conductivity and are suitable for shielding high and low frequency electromagnetic fields. Electrical conductivity is the main basis for selecting electromagnetic shielding materials. The most commonly used metal shielding materials are steel plate, galvanized steel plate, copper plate, aluminum plate, and other good conductivity materials, which have excellent mechanical properties. However, there are shortcomings such as high density, easy corrosion, and difficulty to process limit for their application. 18.2.2.3 Conductive Coating Shielding Materials

Conductive coating is a functional material. According to its composition and conductive mechanism, conductive coating can be divided into intrinsic conductive coating and blended conductive coating. The intrinsic conductive coating is made of an intrinsic conductive polymer as a film-forming substance, which are composed mainly of polyacetylene, polyphenylene sulfide, polypyrrole, polythiophene, polyaniline, and the like. However, these conductive polymers are hardly soluble, refractory, and difficult to process that they are limited to laboratory research. Current conductive coatings are mainly blended conductive coatings, which generally uses various synthetic resins as film-forming agents and fine metal powder or non-metal particles with good conductive properties as conductive fillers. After mixing and dispersing, it can be made into a paint that can be constructed, sprayed, or brushed on the plastic surfaces. It is then cured into a film under certain conditions. The main advantages of conductive coatings are that they are low cost, simple process, and practical. According to the different blended conductive fillers, conductive coatings mainly include silver-based, copper-based, nickel-based, and carbon-based conductive coatings. 18.2.2.4 Composite Shielding Materials

Traditional shielding materials have problems such as narrow application range, high density, poor processability, and susceptibility to environmental corrosion. With the development of electromagnetic shielding technology, the form of shielding materials is also developing. At present, there are two main categories of electromagnetic shielding materials: surface conductive materials and conductive composite materials. The former is to metalize the plastic surface to reflect electromagnetic waves. In the latter, conductive materials are filled in plastic to form a conductive network or blended with each other to achieve shielding effects. In Sections 18.2.2.4.1–18.2.2.4.2, we mainly focus on the researches of conductive composite materials. Polymer-Based Composite Materials

With the demand of low-cost, low density, and easy manufacturing of EMI shielding materials, polymer-based composites have become the most popular choice. The current conduction is realized through the conductive paths formed by mutual contact between conductive fillers. Therefore, the resistivity of the composite material depends on the amount of the conductive fillers and the existence of percolation threshold. With the increase of the conductive filler, the volume resistivity

18.2 EMI Shielding Materials

of the composite material decreases continuously. When the amount of conductive filler reaches the percolation threshold, the resistivity of the material drops sharply. After exceeding this threshold, the resistivity changes slowly with the amount of conductive filler. Different conductive fillers have different critical filling amounts, and the same conductive fillers have different percolation thresholds when they are filled with different polymer matrices. Therefore, different fillers and polymers are used to improve the EMI effectiveness. In the next section, we will introduce several widely studied composite fillers and their applications in electromagnetic shielding materials. Metal Fillers Generally, as the amount of conductive filler increases, the conductive

properties and EMI effectiveness of composite materials also increase accordingly. However, the amount of conductive filler affects not only the SE of the composite material but also the mechanical properties of the material. For conductive fillers with poor conductivity, in order to achieve a certain SE, the filling amount of the conductive filler must be increased. However, this will lead to a decrease in the mechanical properties of the composite material. The shape of the conductive filler, especially the aspect ratio, also has a significant effect on the SE of polymer-based electromagnetic shielding composites. According to the conductive mechanism of conductive composites, metal fibers with a large aspect ratio are more likely to form conductive pathways and have better conductive performance. For commonly used metal fibers, the dosage at 12–20% by volume is better, and 5–6% by weight can meet general shielding requirements. At present, the length-to-diameter ratio of the fibers in the conductive composites produced by the composite process can generally reach about 50–60, and the corresponding filling amount is about 10–15 (vol%). Lin et al. [8] demonstrate that Ag nanofibers with 89% transmittance in visible range and 1 μm thickness show about 20 dB shielding efficiency. When the thickness of Ag nanofiber increases to 10 μm, the total shielding can reach 50 dB and the transmittance in visible range can remain higher than 75%. Considering the advantages of one-dimensional Ag nanofibers or nanowires (NWs), they are successfully utilized in polymer-based composites [9, 10]. Hu et al. [11] fabricated an Ag NW-based transparent conductive film with sandwich structure as shown in Figure 18.2a–c. It is consisted of nanowire/polyethylene terephthalate (PET) film and a coated layer of poly(ethersulfone) (PES) (Figure 18.2d). The coating layer can protect the Ag NWs/PET films from dropping off during scratch and chemical corrosion. Their EMI SE at different thickness of silver layer were characterized (Figure 18.2e). It shows that the value reaches 38 dB at a frequency of 8 GHz for the film with the thickness of 60c. It can still be above 25 dB for the thickness of 25c, when the light transmission is 81%. This can also meet the commercial requirement. In addition, it is also critical to select reasonable mixing process parameters and mixing equipment technical parameters. For example, compared with ordinary composite processes, the use of special masterbatch method not only keeps the metal fibers as long as possible, but also distributes them evenly in the resin. Therefore, the filling amount of fibers can be reduced and the shielding effect of

593

18 Thin Films and/or Coating for Electromagnetic Interference

(a)

(d) PES film PES/PET film Silver NWs film PET film

(b)

(e) 70 Nothing PET 10c 15c 25c 40c 60c

60

(c)

EMI SE (dB)

594

50 40 30 20 10 0

8

9

10

11

12

Frequency (GHz)

Figure 18.2 (a) Photograph of a PES/PET double-layer transparent film; (b, c) photographs of a PES/silver nanowire/PET sandwich-structured film; (d) schematic diagram of a sandwich film; (e) EMI SE of the films with different thicknesses of the silver nanowire layer. Source: Hu et al. [11]. © 2012, American Chemical Society.

conductive composites can be improved. Gelves et al. [12] made nanowire/polymer nanocomposites with segregated nanowire networks resembling cell-like structures using copper nanowires as electrical fillers. To get this structure, they adopted a new method of miscible solvent mixing and precipitation (MSMP) method. The mechanism of the segregated network developed in Cu NWs/polystyrene (PS) nanocomposites by this method can be illustrated as Figure 18.3. The composites exhibit a high electrical conductivity of up to 104 S/m at a low concentration of NWs (∼13 wt%). The nanocomposite sheets of 0.21 mm in thickness exhibited EMI SE of more than 20 dB for copper nanowire concentrations of only 1.3 vol%. In addition to metal fibers or metal nanowires as fillers, metal-plated carbon fibers or graphite fibers also have good electromagnetic shielding properties [13]. The electromagnetic shielding materials prepared by this kind of composite fillers can not only reduce the cost but also overcome the disadvantage of excessive material density caused by using pure metal as fillers. The small density, thin diameter, large length-diameter ratio, high strength, high modulus, good chemical stability, and other advantages make carbon fibers easy to form a conductive network. It also has the function of reinforcing materials. Kumaran et al. prepared Au nanoparticle (NP) incorporated multiwalled carbon nanotubes (MWCNTs) reinforced polyvinylidene difluoride (PVDF) nanocomposites by a simple solvent casting method and found that the values of dielectric constants are 12.11 and 13.89 at 1 MHz upon the incorporation of 1 and 3 wt% MWCNT in PVDF. The

18.2 EMI Shielding Materials

Mixing nanowires + PS in miscible solvent mixture

Polymer precipitation and nanowire segregation

(a)

(b)

Separation of solvent mixture

(c)

Compression molding

(d)

Figure 18.3 Mechanism of segregated nanowire network development in CuNW/PS nanocomposites produced by MSMP method. Source: Gelves et al. [12]. © 2011, Royal Society of Chemistry.

polymer nanocomposites possess enhanced conductivity of 1.12 × 10−4 S/cm at 1 MHz, minimum return loss of 4.4 dB, and effective electromagnetic shielding of 26.7 dB at 12 GHz for 3 wt% Au NPs in 3 wt% MWCNT/PVDF thin film [14]. They also fabricated Ag–graphite/PVDF nanocomposites and found that the conductivity values of nanocomposites are increased up to 2.5 times when compared to neat PVDF at 1 MHz [15]. The existence of Ag increased the dielectric constant and reduced the dielectric loss of PVDF matrix. The EMI SE can be improved using optimal Ag and graphite contents. Ferromagnetic materials, due to their good microwave absorption and high energy dissipation properties, have attracted great attention in EMI shielding [16]. High permeability magnetic metals have high conductivities that support eddy currents. The eddy currents will reflect the incident wave before it can be absorbed, which makes the magnetic material difficult to be used as EMI absorber [17]. Combination of magnetic materials with polymers is a good way to reduce the eddy current and improve the EMI SE, which attracted wide attention. Gargama et al. [18] prepared PVDF/n-Fe composites using mechanical blending and hot molding process, and calculated their permittivity, permeability properties, and SE using scattering parameters measured in X-band (8.2–12.4 GHz). They demonstrated that a higher vol% of n-Fe in the PVDF matrix will lead to a stronger dielectric and/or magnetic losses and that the dominant mechanism for the SE was reflection loss. Carbon-Based Fillers Carbon-based materials are considered to be ideal reinforcements for polymer composites due to their excellent electrical and thermophysical properties, as well as high strength and stiffness, thereby significantly improving electrical conductivity, mechanical stability, and EMI efficiency. Among them, the most widely used are 1D and 2D carbon materials such as carbon nanotubes (CNTs) and graphene. Since the discovery of CNTs in 1991, they have attracted extensive attention from researchers in the field of chemistry, physics, biology, and medicine due to their excellent mechanical, thermal, and photoelectric properties [19, 20]. The spiral, tubular, and high aspect ratio structure of CNTs indicates their unusual electromagnetic properties. They may be conductors or semiconductors, which is

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related to the helical angle of CNTs. Because of their special structure and dielectric properties, CNTs exhibit strong broadband microwave absorption properties. Light weight, adjustable conductivity, high temperature oxidation resistance, and good stability make it an ideal microwave absorber, which can be used for stealth materials, electromagnetic shielding materials or dark chamber absorbing materials [21]. CNT-reinforced plastic has excellent mechanical properties, good electrical conductivity, corrosion resistance, and shielding of radio waves, which makes it popular as fillers in EMI materials [22]. Jia et al. [23] indicate that the incorporation of 50 wt% natural rubber (NR) leads to a tremendous mechanical improvement of the CNT-NR films compared to the pure CNT film, e.g. 3.1 and 486 times increase in tensile strength and toughness. Meanwhile, the CNT-NR film exhibits highly reliable EMI SE even after bending 5000 times at a radius of 2.0 mm. CNTs can be regarded as graphene sheet coils, so they can be divided into single-walled carbon nanotubes (SWCNTs) and MWCNTs according to the number of layers of graphene sheets. The SWCNT is a graphite-like planar hollow tube formed by a six-carbon ring, in which each carbon atom is completely bonded to the surrounding three carbon atoms through SP2 hybridization. It has a small diameter distribution range, fewer defects, and higher uniformity. The diameter of SWCNT is generally 0.8–2 nm. Compared with MWCNTs, SWCNTs have a lower percolation threshold, which means more loads are required to satisfy the electromagnetic shielding effect when they are used as fillers. However, more loads often lead to poor dispersion, which limit its application. To solve this problem, CNT foams or sponges are developed. Chen et al. [24] synthesized highly porous, 3D CNT sponge using chemical vapor deposition, then prepared epoxy nanocomposites by impregnating epoxy into this CNT sponge. They find that the composites exhibit a remarkable electrical conductivity of 148 S/m and a high EMI SE of around 33 dB in the X-band with only 0.66 wt% of CNT sponge. It is much higher than that of epoxy nanocomposites with 20 wt% separated CNTs. Additionally, the flexural and tensile strengths can be increased by 102% and 64%, respectively, as compared to those of neat epoxy. Furthermore, the tensile toughness of the nanocomposite material was increased by 250%, and the elongation at break was increased by 97%. Another solution to improve the EMI SE is to use MWCNT instead of SWCNT. MWCNT consist of multiple CNTs nested within each another. The number of nanotubes within a MWCNT can vary from 3 to over 20. MWCNTs have outer shells about 30 nm in diameter. They have a wider range of applications due to their ease of mass production, low cost per unit product, and higher thermal and chemical stability. Their high aspect ratio, large surface area, and low percolation threshold make them more favorable as fillers in polymer-based composites [25]. Zeng et al. [26] reported a facile freeze-drying method to fabricated lightweight, flexible, and anisotropic porous MWCNT/water-borne polyurethane (WPU) composites with exceeding 50 or 20 dB in the X-band at the density of 126 or 20 mg/cm3 . The relevant specific SE is up to 1148 dB/(cm3 g), which is attributed to the conductivity of the cell walls caused by MWCNT content, the anisotropic porous structures, and the polarization between MWCNT and WPU matrix.

18.2 EMI Shielding Materials

GO

RGO

PS

GO + PS in distilled water

(a)

In situ reduction of RGO+PS

(b)

s-rGO/PS composite

Solid phase formation under high-pressure

(c)

(d) rGO-embedded PS region

dense rGO region

Ps region 100 µm

(e)

(f)

200 nm

(g)

Figure 18.4 Schematic of the fabrication of the segregated rGO/PS (s-rGO/PS) composite under high pressure: (a) steps of the method shown are suspended GO and PS particles in distilled water; (b) in situ reduction of the GO/PS mixture in the presence of hydrazine hydrate; (c) obtained rGO/PS powder after the removal of water; (d) resulting s-rGO/PS composite; (e) optical photograph of the flexible s-rGO/PS thin film (20 μm in thickness) and the sample for EMI SE measurement; (f) cross-sectional optical microscopy image of the s-rGO/PS composite containing 0.14 vol% rGO; (g) transmission electron microscopy (TEM) image of rGO layers between PS regions. Source: Yan et al. [30]. © 2015, John Wiley and Sons.

Nowadays, graphene has attracted great attention due to its unique electrical, thermal, mechanical, and optical properties. It is the thinnest (about 0.3 nm) and strongest material with good elasticity. Its light transmittance is up to 97.7%. These excellent characteristics make graphene an excellent choice for EMI shielding materials used in portable electronic equipment, transparent electronics, and automobiles [1, 27, 28] Jan et al. [29] fabricated free-standing conducting composite films by dispersing the liquid-phase-exfoliated, pristine graphene nanosheets (GNS) in thermoplastic polyurethane. Their EMI shielding was tested in the X-band (8–12 GHz). The composites show a constant increase in electromagnetic attenuation as a function of GNS loading (0–0.12 V f [volume fraction]). The SE of the composite material can be improved by 14 times compared with the pure polymer at a load of 0.12 V f GNS. Fabrication methods often have a crucial impact on performance of composites. Yan et al. [30] obtained reduced graphene oxide (rGO)/PS composites via high-pressure solid-phase compression molding (Figure 18.4). The composites made by this method exhibits an average EMI SE of 45.1 dB with only 3.47 vol% rGO, which is higher than that made using in situ thermal reduction that they used and other methods in literatures. Just like CNT foams or sponges, graphenes with low-density, porous structure can also exhibit excellent EMI shielding effects [31, 32]. Wu et al. [33] prepared graphene

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90 80

35.2 S/cm

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20 10 0

90 80

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SET, (DB)

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5

Figure 18.5 EMI SEs of composites with different compositions: (a) SEA and SER , (b) SET as a function frequency, and (c) summary of SEs, spin seebeck effects (SSEs), and SSEs normalized by area density as a function of mass ratio of PEDOT:PSS to GF. Source: Wu et al. [33]. © 2017, American Chemical Society.

foam (GF)/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) composites with different PEDOT:PSS-to-GF mass ratios by drop coating of PEDOT:PSS on cellular-structured freestanding GFs. The GFs had an extremely low density of 9.3 × 10−3 g/cm3 and a porosity of 99.6%. They also functionalized the GF with 4-dodecylbenzenesulfonic acid to enhance the wettability and the interfacial bonds between the GFs and PEDOT:PSS. The absorption, reflection, and transmission SE as a function of frequency at different GFs/PEDOT:PSS ratios were measured as shown in Figure 18.5. The results show that with the increase of mass ratios, SEA increases and SER maintains a negligible change as shown in Figure 18.5a. Figure 18.5b indicates that the SET are 69.1 and 91.9 dB, respectively, at the mass ratio of 1 and 4.6. The composites show remarkable SE of 91.9 dB and specific SE of 3124 dB cm3 /g (Figure 18.5c). The EMI results analysis indicates that adsorption contributes more than 80% of the total EMI SE, which plays a dominant role in all ratio composites. In order to meet the rapid development of portable and wearable devices, it is necessary to continuously seek for fillers with better performance to prepare composites. In 2011, a large family of 2D transition metal carbides and/or nitrides, known as MXene (Mn+1 Xn Tx , where n = 1, 2, or 3, M is an early transition metal, X is C and/or N, and T is a terminating group), was discovered [34]. It opens a door to the research of 2D Mn+1 Xn structures. MXene is considered to be the most promising filler candidates to replace graphene due to its excellent mechanical properties, hydrophilicity,

18.3 Stealth Coatings

and metal conductivity [35]. Shahzad et al. [36] prepared MXene-SA composites with excellent EMI effects by combing MXene with sodium alginate (SA). They also compared the EMI SE of MXene and its polymer composites with other materials of previous literatures and find that ultrathin MXene films clearly outperform all of the known synthetic materials. They also analyzed the mechanism of EMI SE of the MXene. It indicates that the excellent EMI SE comes from the nacre-like (or laminated) structure that provides a multilevel shield and an alternating electromagnetic field to the local dipoles between Ti and terminating groups. Carbon-Based Materials

From Section 18.2.2.4.1, carbon materials like graphene and CNTs are good fillers that can improve the EMI SE of materials. In addition to the above functions, composites combined by carbon materials with metals or another carbon material also provide much higher EMI SE and have been widely researched. Wang et al. [37] designed a copper/large flake size graphene (Cu/LG) composite film by magnetron sputtering of Cu on an annealed large flake size graphene oxide (GO) film. This film exhibits ultrahigh thermal conductivity of over 1932.73 (±63.07) W/(m K) and excellent electrical conductivity of 5.88 (±0.29) × 106 S/m at a thickness of 9 ± 1.5 μm. Their EMI SE property was measured as shown in Figure 18.6a,b. It exhibits excellent EMI SE, especially at L-band (1–2 GHz) with a value range of 61.39–63.29 dB (Figure 18.6c). It is slightly larger than 52 dB in X-band (8–12 GHz), which can be used in space research (Figure 18.6d). From Figure 18.6e, we can see that the composite film can block and adsorb 99.999 34–99.999 59% of incident electromagnetic wave in Ku-band (12–18 GHz). The results in Figure 18.6f also indicate that EMI shielding mechanism of Cu/LG is dominated by adsorption. These properties are all much better than LG film and Cu foil. Two carbon materials can also be combined and used for EMI shielding due to their extraordinary electronic properties. Song et al. [38] fabricated CNT-multilayered graphene edge plane (MLGEP) core–shell hybrid foam using chemical vapor deposition. The MLGEP are seamlessly grown on the CNTs as shown in Figure 18.7. This special structure endows the hybrid foam excellent specific EMI SE of 47.5 dB in x-band at 1.6 mm thickness and 0.0089 g/cm3 density. It is confirmed that the hybrid foams have much higher absorption losses than CNTs due to the multiscaled reticulated conductive structure and the overlapped CNT-MLGEP nanowires on the microscale. All these properties also make it suitable to be used as a filler in polymer-based composites.

18.3 Stealth Coatings The development of modern attack weapons, especially the emergence of precision strike weapons, has promoted the study of stealth technology. It is an anti-detection technology, which is used to make objects less visible to radar wave, infrared, sonar, and other detection systems. Among them, the most popular researched is radar

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Figure 18.6 Remarkable EMI SE of Cu/LG film. (a) The optical image of network analyzer; (b) the diagrammatic sketch of coaxial line and three scenarios about EMI shielding mechanism of Cu/LG film; (c–e) the EMI SE of Cu foil, LG film, and Cu/LG film at L-band (1–2 GHz), S-band (2–4 GHz), C-band (4–8 GHz) for (c), X-band (8–12 GHz) for (d), Ku-band (12–18 GHz) for (e), respectively; (f) EMI SE-absorption (%) of Cu/LG film and Cu foil. Source: Wang et al. [37]. © 2018, John Wiley and Sons.

stealth. In Sections 18.3.1–18.3.2, we will mainly introduce the radar stealth and the development of stealth coatings.

18.3.1 Radar Stealth Coatings Radar stealth is designed to avoid the accurate detection of the target’s echo signal. The radar scattering area (RCS) of the target is used to represent the echo amplitude of the signal returning to radar. Therefore, the core problem of achieving radar

18.3 Stealth Coatings

(a)

(b)

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(c)

500 nm

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(d)

(e)

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(h)

(i)

(f)

(g)

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100 nm (k) 2L 3L 1L

1 µm (l)

200 nm m 4n

03

100 nm

5 nm

(m)

2 nm joint zone

Graphene plane 10 nm

2 nm

Figure 18.7 (a) Scanning electronic microscope (SEM) image of a CNT-MLGEP hybrid foam and (b) high-resolution SEM image and (c) a cross-sectional view of an individual CNTMLGEP hybrid; close-up SEM images of thin-layered graphene with different morphologies, such as (d) nanoflags, (e) nanokinks, (f) open-ended nanotubes and nanotrumpets, and (g) nanotriangles; (h) low-magnification, and (i) high-magnification TEM images of the CNTMLGEP hybrids; (j) TEM image of nanotrumpet-like MLGEP; (k) high resolution transmission electron microscopy (HRTEM) image of a nanotrumpet edge; (l) TEM image of a junction between CNT and MLGEP (inset shows the detailed structure of the junction); and (m) HRTEM image of a MLGEP. Source: Song et al. [38]. © 2017 John Wiley and Sons.

stealth is to reduce the RCS of the target. which is possible in two ways: through the shape design of aircrafts or objects and through research on the radar-absorbing materials (RAMs), which can interact with electromagnetic waves and lead to electromagnetic energy loss. However, the former method often results in a change in aircraft aerodynamic performance and a reduction in strength. Furthermore, shaping is achieved only in a limited angular region and is effective only at higher frequencies [39]. Therefore, research of high-performance RAMs has become an important task in military fields. 18.3.1.1 Radar-Absorbing Materials

The basic theory of the absorbing material is that the material absorbs incident electromagnetic wave, converts it to other forms of energy, and dissipates them. In this

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process, the electromagnetic wave is reflected, absorbed, and transmitted. Materials can be divided into impedance-matching absorbing materials and resonance absorbing material based on the above theory. Impedance matching means that surface impedance of the RAMs should be similar to that of free space to obtain minimal reflection. This can be derived from the following equation [40]: 𝛾=

ZM − ZO ZM + ZO

where 𝛾, Z M , and Z O are the reflection coefficient, intrinsic impedance of the reflecting substrate, and the intrinsic impedance of the incident medium, respectively. Zero reflection can be achieved when Z M equals Z O . In addition, materials must have sufficient dissipation attenuation including resistance losses, dielectric losses, and magnetic losses, to maximize the absorption of electromagnetic waves. Typical impedance-matching absorbing materials are pyramidal RAMs and matching layer RAMs. The pyramidal structure allows for a gradual transition of the impedance from the incident medium to the bottom of the RAMs. Some incident waves survive multiple reflections between the pyramids before reaching the solid absorber base, resulting in attenuation of the electromagnetic waves [41]. However, the thickness and easy fragmentation properties hinder its application. Matching layer RAMs are provided with an impedance-matching layer between incident and absorbing layer. The impedance value of the matching layer is between the impedance of the incident medium and the absorption layer. The best matching can be achieved at the thickness of 𝜆/4. Different from matching absorbing material, resonance absorbing materials uses destructive interference to shield electromagnetic waves. In this process, part of the electromagnetic wave is reflected on the surface of the absorbing layer, and another part of the electromagnetic wave passes through the absorbing layer and is reflected by the bottom layer before being emitted through the absorbing layer. The interference cancelation occurred when these two parts of the wave are in opposite phase. It means that 𝜙 = (2n + 1)𝜋, where 𝜙 is the phase difference of that two ( electromag) netic waves. Their optical path difference should be written as Δ = n + 12 𝜆 due to the relationship of phase difference and optical path difference. Therefore, the ideal thickness of the absorbing material for a vertically incident electromagnetic wave is 𝜆/4. They are much thinner than the other types of materials. Typical resonance absorbing materials are Dallenbach layer, Salisbury screen, and Iaumann layer materials. Good absorbing materials generally have strong absorption capacity of electromagnetic wave and wide coverage frequency range. They can be characterized by reflection loss (RL) of electromagnetic waves and the effective absorption width. The effective absorption width represents the frequency range capable of absorbing 90% energy of the electromagnetic wave, and generally refers to the frequency width corresponding to RL < −10 dB. Nowadays, many materials are used as absorbing materials such as ferrite absorbing coating, carbonyl iron absorbing coating, ceramic absorbing coating [42], radioisotope absorbing coating, polymer-based absorbing

18.3 Stealth Coatings

materials, and so on. In Sections 18.3.1.1.1–18.3.1.1.2, we will detail several commonly used absorbing materials used to absorb radar wave. Metallic Powder-Absorbing Materials

Metallic powder-absorbing materials are mainly magnetic metal powder, including metal ultrafine powder or metal oxide magnetic ultrafine powder and carbonyl powders like carbonyl iron powder, carbonyl nickel powder, cobalt nickel alloy powder, and so on. They are usually combined with a polymer to improve their absorbing performances. For metal ultrafine powder or metal oxide magnetic ultrafine powder, their physical properties such as magnetism, electricity, and light undergo qualitative changes due to the refining process. The electromagnetic parameters of the absorbing coating can be adjusted by changing the particle size, content, and mixing ratio of the powder so as to achieve a better absorbing effect. Among the carbonyl metal powders, carbonyl iron powder is the most widely used due to its high curie temperature, good temperature stabilization, high specific saturation magnetization intensity, and high permeability and dielectric constant [43, 44]. It is mainly produced by thermal decomposition of iron carbonyl iron compounds or by irradiating high-purity Fe(CO)5 directly with a laser beam. Metal powder absorbing material attenuates electromagnetic waves through magneto induced loss and eddy current loss, and mainly works at the ranges of MHz and GHz. It has been used on the surface and wings of F/A-18C/D aircraft in the United States to reduce its radar reflection cross section many years ago. At present, although the metal micropowder absorbing material has been widely used in stealth technology, the poor oxidation resistance, acid and alkali resistance, and the poor low-frequency band absorption performance limit its further utilization. Ferrites and Its Complexes

Ferrite is a kind of nonmetallic magnetic material made by sintering iron trioxide and one or more other metal oxides (i.e. nickel oxide, zinc oxide, manganese oxide, magnesium oxide, and barium oxide). Their resistivity is about 102 –108 Ω, so the eddy current loss is small. It is its good absorption performances and low price that make it commonly used in radar-absorbing coating [45]. The absorbing properties of ferrite are derived from its ferromagnetic and dielectric properties. Its relative permeability and relative conductivity are both complex forms, which can generate both dielectric and magnetic losses [46]. Zhu et al. [47] researched the permittivity and permeability of the nickel ferrite nanocrystal in the frequency of 2.0–18.0 GHz and found that the nanocrystals exhibit excellent microwave properties in the X-band (8.5–13.0 GHz) frequencies. The minimal RL is −24 dB at 9.5 and 12.5 GHz and the absorption band with RL under −10.0 dB is from 8.5 to 13.0 GHz at the thickness of 3.5 mm. Although ferrite has good wave absorption, its large weight and susceptibility to corrosion restrain its application. Therefore, it is necessary to combine it with other materials to solve this problem [48–50]. Panwar et al. [51] designed ferrite–graphene-based thin broadband radar wave absorber and measured their effective complex dielectric permittivity and effective complex magnetic permeability with different compositions. They found that increasing the

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graphene content in the composites will result in a reduction of film thickness and wide absorption bandwidth. In addition, the double layer absorber they designed shows a strong RL of −55.28 dB at 10.2 GHz with broad bandwidth of 3.1 GHz in the frequency range of 8.6–11.7 GHz. Gama et al. [52] researched the effects of MnZn ferrite content on the wave absorbing properties of MnZn ferrite/rubber composites and got the best absorbing performance at the MnZn ferrite volume fraction of 20%. The minimum reflection loss reaches −35 dB and the frequency bandwidth is 4.5 GHz at RL = −10 dB. 18.3.1.2 Radioisotope RAM

The plasma generated by radioactive isotopes is an effective electromagnetic wave absorption medium. Free electrons in the plasma region will generate a forced oscillation with a frequency equal to the carrier frequency of the electromagnetic wave under the action of the incident electromagnetic field. During the oscillation process, the moving electrons, molecules, atoms, and ions collide, increasing the kinetic energy of these particles, thereby transforming the energy of the electromagnetic field into the heat of the medium. The radioisotope absorbing coating is very thin and light and has the advantages of broadband absorption frequency, good durability, and high-speed aerodynamics. In addition, the radioactive isotope absorbing coating also has the functions of absorbing infrared radiation and sound waves, which is an ideal multifunctional absorbing coating. 18.3.1.3 Carbonaceous-Based RAM

Carbon material is a kind of light absorbing material with excellent dielectric properties and thermal conductivity. However, the impedance-matching characteristics of the absorbing layer are poor and the loss mechanism is single. To overcome this disadvantage, it is always combined with materials like magnetic metals and other metal oxides [53, 54]. Lin et al. [55] synthesized cobalt filled MWCNTs using a wet chemistry solution method and investigated their microwave-absorbing properties. An excellent absorbing property was obtained at an optimized thickness as shown in Figure 18.8. The maximum reflection loss is about −39.32 dB and the bandwidth corresponding to the reflection loss below −10 dB is 3.47 GHz. Fan et al. [56] use Ni to coat carbon fibers and get high frequency absorber with high performance. The RL of the Ni-coated carbon fiber less than −10 dB begins from 14.4 to 18 GHz and the RL is below −14 dB at 18 GHz. Liu et al. [57] fabricated CNTs/ZnO whisker composites using a simple mechanical mixing method. They investigated the wave absorbing properties at different CNTs and ZnO whisker mass fractions and found that 4 wt% CNTs mixed with 10 wt% ZnO whisker has the optimum microwave absorption ability with a thickness of 2.0 mm. The minimum reflection loss is −37.03 dB at 12.24 GHz and the bandwidth corresponding to the reflection loss below −10 dB is more than 4.04 GHz. Carbon materials, as the filler of polymer-based composites, can show excellent EMI shielding performances. As it is flexible to adjust the shapes of the carbon materials and the embedded matrix, the absorption property of materials that compose carbon materials and polymers can also be regulated. Therefore, it is also a good

18.3 Stealth Coatings

(a) 0 (d) Reflection loss (dB)

10 (c) (b)

(a) Unfilled CNTS (3.5 mm)

–20

(b) Co-filled CNTS (3.0 mm)

Band width = 3.47 GHz

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–30

RL = –39.32 fm = 15.7 1 GHz

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dm = 3.0 mm 8

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Figure 18.8 Absorption characteristics of raw MWCNTs and Co-filled MWCNTs composites. Source: Lin et al. [55]. © 2008, Elsevier.

candidate to be used as a RAM [58–60]. Carbon nanofibers (CNFs) [61], carbon black [62], CNT [63, 64], and grapheme [65] can all be used to combine with polymer materials [66]. The structure, contents, and configuration mode of carbon materials are all related to the absorbing performance of the composites [61, 63, 67]. Nanni et al. [68] prepared CNFs with two different dispersion methods and synthesized different CNF/epoxy composites. They evaluated the dielectric and absorbing properties and demonstrated that both the contents and the microstructure of the CNF are all key factors that can affect the absorbing properties. To further improve the absorbing property, carbon materials can be combined with both the magnetic metals and polymers [69]. Zhao et al. [70] made Fe-filled CNT/epoxy composites and found that the reflection loss of the Fe-filled CNT/epoxy composites is below −10 dB at 11.8–14.7 GHz, and the minimum value is −31.71 dB at 13.2 GHz. This good performance is attributed to both dielectric and magnetic losses. Their microwave absorbing peak moves to the higher frequency compared to the CNT/epoxy as shown in Figure 18.9. Besides the composites mentioned above, composites that only consist of carbon materials have also attracted much attention in the application of RAMs. In Chapter 2, we have mentioned that special structure (such as foam structure) of materials can improve the EMI effects of materials. Based on this structure, some other operations are taken to further improve the absorbing properties. Shu et al. [71] prepared nitrogen-doped rGO/MWCNT composite foams using GO as template by a two-step strategy of hydrothermal self-assembly and subsequent high-temperature calcination. They indicated that nitrogen doping, calcination temperature, and filler

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0 –5

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–25 –30 –35 2

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Figure 18.9 Microwave absorbing properties of the CNTs/epoxy composites and Fe-filled CNTs/epoxy composites. Source: Zhao et al. [70]. © 2009, Elsevier.

ratios can effectively control the microwave absorption properties of the composites. The microwave absorption of −69.6 dB at 12.5 GHz and absorption bandwidth of 4.3 GHz (13.2–17.5 GHz) were obtained under 600 ∘ C calcination with an 8 wt% mass filler ratio and extremely low matching thickness of 1.5 mm. Their absorbing mechanism is illustrated in Figure 18.10.

18.3.2 The Development of Stealth Coating 18.3.2.1 Negative Refractive Stealth

Negative refraction means that the light rays are reversely refracted from the normal direction. It was proposed theoretically by Victor Veselago and verified experimentally by David Smith and coworkers [72]. Materials with negative refractive index are also called metamaterials or left-handed materials, which have both negative dielectric constant and magnetic permeability. They are composed of periodic arrangement of unit cells. The permittivity and permeability of the materials can be changed by changing the shape, size, and configuration of unit cells, thereby controlling the transmission of electromagnetic waves. Besides, they also have high impedance, which is much useful for the application of absorbing materials [73]. D. Schurig et al. designed a metamaterial electromagnetic cloak at microwave frequency and demonstrated that it decreased scattering from the hidden object and reduce its shadow. This demonstrates the feasibility of using metamaterials to achieve stealth [74]. Pendry et al. plot the ray-tracing line figure from numerical integration of a set of Hamilton’s equation, which clearly illustrate the transmission mechanism of electromagnetic wave using metamaterials as stealth coating [75]. By

18.3 Stealth Coatings Re fl

ec ted wa ve s

nt

e cid

In

Transmitted waves

s ve wa Absorbed waves

MWCNTs

Defects polarization

RGO

Residual groups as polarized centers

Hopping electrons

N atom

Migrating electrons

Dipole polarization

Conduction loss

C atom

–COOH

–OH

Figure 18.10 Schematic diagram of the EM absorbing mechanisms of NRGO/MWCNT composite foams. NRGO; nitrogen-doped reduced graphene oxide. Source: Shu et al. [71]. © 2020, American Chemical Society.

backscattering light or radar waves, the negative refractive index material prevents the detection system from receiving reflected light or electromagnetic waves from the front, thereby technically realizing the stealth of a weapon system or combat platform. 18.3.2.2 Multiband Absorbing Material

Due to the continuous development of multi-mode composite guidance technology and the increasing diversity of detection means, weapons may face the threat of radar, infrared, laser, and visible light detection at the same time. The development of stealth materials compatible with each other in several wavebands becomes one of the main researches [76, 77]. Many multiband absorbing materials are designed based on the metamaterial due to its simple structure, light shape, and easily adjustable absorption band [78]. Different structures, such as compound structure [79] and multiporous square layer structures [80] were made to achieve excellent multiband absorbing properties. 18.3.2.3 Plasmon Active Stealth

Plasmon is an ionized gas that is generally electrically neutral and consists of positive ions, free electrons, and neutral atoms. It is generated by high-energy excitation of neutral gas using high particle bombardment, laser irradiation, gas discharge, and thermal ionization, etc. It is a special filter, which refracts and

607

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18 Thin Films and/or Coating for Electromagnetic Interference

absorbs electromagnetic wave and can be used in stealth technology. Plasma frequency plays an important role in plasma stealth technology. If the incident electromagnetic wave is smaller than the frequency of plasma frequency, 86% of its energy will be reflected. However, when the electromagnetic wave is larger than the frequency of plasma, it can enter the plasma and propagate in it. During the propagation process, part of the energy is transferred to the charged particles in the plasma, absorbed by the charged particles, and its own energy is gradually attenuated. Meanwhile, the higher electron density and oscillation frequency of the plasma, the greater the absorption of radar waves.

18.4 Summary and Prospects Generally, with the miniaturization and improvement of electronic devices, the advent of 5G era, and the development of military detection technology, researches on EMI shielding materials and absorbing materials will face more challenges and higher demands. Traditional materials that mainly focus on the strong attenuation of electromagnetic waves can no longer meet the needs. They should be replaced by composite materials with broadband, intelligent, and lightweight properties. In addition, they should be as thin as possible and able to adapt to the changes in temperature.

Acknowledgments This chapter is supported by the NSFC-BRICS STI Framework Program (No. 51861145309), the National Natural Science Foundation of China (No. 51971029), the National S&T Major Project (No. 2018ZX10301201), the “All English teaching demonstration course construction project of University of Science and Technology Beijing” (No. KC2015QYW06, 2016), the “1125” Zhihui Zhengzhou Talent project of the Henan province (Fund No. in USTB: 39080070), the “100 Talent Plan” fund of the Fujian province (Fund No. in USTB: 39080067), and the development of a highly sensitive magneto-optical biomolecular sensor experimental prototype (Fund No. in USTB: 2019-0649) by Hangzhou Ruidi Biotechnology Co. Ltd.

List of Abbreviations and Symbols dB EMI ITU MWCNTs PES PS PVDF

decibels electromagnetic interference International Telecommunications Union multiwalled carbon nanotubes poly(ethersulfone) polystyrene poly(vinylidene difluoride)

References

RAM RCS RL SE Eb Ea Hb Ha Pb Pa R A B 𝜎r 𝛿 𝜇r f t 𝛾 Zm Z0

radar-absorbing material radar scattering area reflection loss shielding effectiveness electric field strengths before shielding electric field strengths after shielding magnetic field strength before shielding magnetic field strength after shielding energy field intensity before shielding energy field intensity after shielding reflection loss of the electromagnetic wave absorption loss of the electromagnetic wave multiple reflection loss of the electromagnetic wave relative conductivity of electromagnetic wave skin depth of electromagnetic wave relative permeability of electromagnetic wave frequency of electromagnetic wave thickness of shielding film reflection coefficient of the reflecting substrate intrinsic impedance of the reflecting substrate intrinsic impedance of the incident medium

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19 Thermal Barrier Coating for Aerial and Aerospace Engine Zaidao Li 1 and Yujun Song 2,3 1 Brunel University London, Brunel Centre for Advanced Solidification Technology (BCAST), Uxbridge UB8 3PH, UK 2 University of Science and Technology, Center for Modern Physics Technology, Applied Physics Department, School of Mathematics and Physics, 30 Xueyuan Road, Beijing 100083, China 3 Zhejiang Key Laboratory for Pulsed Power Technology Translational Medicine, Hangzhou Ruidi Biotechnology Company, Hangzhou 310000, China

19.1 Introduction Thermal barrier coatings (TBCs) are advanced materials systems usually applied to metallic surfaces (e.g. superalloys) operating at much elevated temperatures (1400–1500 ∘ C), such as gas turbine or aeroengine parts, as a form of exhaust heat management [1, 2]. Figure 19.1 shows the different stages: intermediate-pressure compressor (IPC), high-pressure compressor (HPC), high-pressure turbine (HPT), intermediate-pressure turbine (IPT), low-pressure turbine (LPT), and the pressure and temperature profiles along the engine. Turbines should be operated at the temperature as high as possible to maximize the efficiency. The target of TBC application is to achieve an enhancement in fuel efficiency by increasing the operating temperature (∼100–300 ∘ C), thereby increasing the engine thrust and efficiency up to 5% and 1%, respectively. Until about 15 years ago, relentless increases in operating temperatures were achieved through improved superalloy (e.g. NbNi-based or Co-based superalloys) design [2], the development of blades composed of textured microstructures and subsequently single crystals, and the internal cooling by airflow through internal channels cast into the component [4, 5]. The use of TBCs along with internal cooling of the underlying superalloy component provides major reductions in the surface temperature (100–300 ∘ C) of the superalloy [2]. More recent increases in operating temperatures (up to 1500 ∘ C) have been enabled by deposition of TBCs on high-temperature gas turbine components [6, 7]. TBCs are complex, multifunctional thick films or coatings (typically 100 μm to 2 mm thick) of a refractory material that protect the metal part from the extreme temperatures in the gas (see Figure 19.2). These 100 μm to 2 mm thick coatings of thermally insulating materials serve to insulate components from large and Inorganic and Organic Thin Films: Fundamentals, Fabrication, and Applications, First Edition. Edited by Yujun Song. © 2021 WILEY-VCH GmbH. Published 2021 by WILEY-VCH GmbH.

19 Thermal Barrier Coating for Aerial and Aerospace Engine

Combustion chamber HPC

Fan

LPC

HPT

LPT

Temperature (°C) pressure (atm)

616

Figure 19.1 A jet engine (Rolls-Royce Trent 800). Source: Zare Shahneh [3]. Licenced under CC BY 4.0.

IPT

~40

~1400–1500

TBC

TGO

Figure 19.2 Cross-sectional image of a YSZ thermal barrier coating deposited by electron-beam evaporation on a superalloy. During use at high temperatures, a thermally grown oxide (TGO) of Al2 O3 forms on the metal beneath the TBC [8]. Source: Elsevier.

Superalloy 100 µm

prolonged heat loads and can sustain an appreciable temperature difference (∼170–200 ∘ C) between the load-bearing alloys and the coating surface [8, 9]. In the past few decades, TBCs have seen great increasing applications in gas turbine components such as vanes, blades, throats, and combustion walls [10]. The use of TBCs can ensure the engineer being operated at temperatures (1700–3500 ∘ C) much exceeding the normal melting point of the superalloy (e.g. 1150 ∘ C for Ni superalloy [9]), thus improving the efficiency and performance of engines. The gas turbine engine is considered to be the heart of many aircraft. As highly advanced material systems, TBCs are usually applied to metallic surfaces, such as gas turbine, aeroengine parts, rocket nose, or space return module, serving to provide oxidation

19.1 Introduction

1200

Mental advances

Benefit from cooling

Specific 900 core power [kW/(kg s)] 600

Ideal performance Inefficiency losses Development trend

300 Tm 800

Actual performance

1100 1400 1700 Turbine rotor inlet temperature (°C)

2000

Figure 19.3 The influence of turbine inlet temperature on specific core power for gas turbine engines. Source: Lemberg and Ritchie [17]. © 2012, John Wiley & Sons.

resistance and corrosion resistance and to insulate components from large and prolonged heat loads [9]. Up to now, new TBCs not only are needed for traditional advanced gas turbine engine applications but also have great potential applications for the armaments [11], aerospace [12], rocket nose, space shuttles [13], or vehicle shells for marine science [14] and polar region study [15]. In aerospace application, TBCs mainly enable the engines to operate at higher temperatures without raising the base metal temperatures using cooling systems inside the hot section components and, thus, enhance the operating efficiency of engines [16]. Therefore, continued development of TBCs is essential for further improving the performance of gas turbines by increasing the inlet gas temperature. As presented in Figure 19.3, the temperature of inlet gases is important, because it directly determines the efficiency of gas turbines [17]. Thereby, TBCs with low thermal conductivity, phase stability, and high resistance to sintering are enduringly in demand [18, 19]. Generally, TBCs consist of a ceramic (e.g. yttria partially stabilized zirconia [PSZ]) top coating and a NiCoCrAlY/PtAl-based metallic bond coating. A bond coating is deposited between the metallic substrate and the top coat to protect the metal substrate from oxidation and high-temperature corrosion, enhancing the coupling of the ceramic top coating and the metallic substrates [4]. There are two conventional fabrication methods used to deposit the ceramic top coating on the superalloys or other substrates [20]. These are the electron-beam physical vapor deposition (EB-PVD) and the atmospheric plasma spraying (APS) methods. TBCs with EB-PVD top coats generally provide longer thermal cycle lifetimes because of its more strain-tolerant columnar structures than those fabricated with APS TBCs. However, APS TBCs are widely applied due to lower thermal conductivity and lower processing costs [21]. Recently, various new processing techniques have been developed to deposit the ceramic coatings, such as solution precursor plasma spraying (SPPS) process and suspension plasma spraying (SPS) [22].

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19 Thermal Barrier Coating for Aerial and Aerospace Engine

19.2 Superalloy Substrates for TBCs “High-temperature materials” possess a remarkable ability to maintain their properties at elevated temperatures [23]. Their uses are many and varied, but good examples include the components for turbines, rockets, and heat exchangers. Superalloys are unique high-temperature materials used in gas turbine engines, which display excellent resistance to mechanical and chemical degradation [23]. When significant resistance to loading under static, fatigue, and creep conditions is required, the nickel-based superalloys [23] have emerged as the materials of choice for high-temperature applications. HPT blades and vanes of aeroengines are among the most highly loaded engineering components, which must endure extremely hot engine gases, an oxidative and hot corrosive environment, and large centrifugal loads and impacts by high-velocity foreign object [24]. For such extreme conditions only high-temperature Ni-based superalloys can be considered, which have the required combination of strength and toughness at high temperatures (>1100 ∘ C) [25]. The gas-turbine (GT) blades operate under the most arduous conditions of temperature and stress of any component in the engine. Nickel-based superalloys exhibit an excellent high-temperature creep resistance, thermal stability, good tensile strength, long fatigue life, microstructural stability at a high temperature, and good resistances to oxidation and hot corrosion [26]. For these reasons, they are used in the manufacturing of gas turbine hot section components. The use of HPT blades made of single-crystal (SC) nickel-based superalloys contributes efficiently to the continuous performance improvement of GT engines in terms of power and thermal efficiency [27, 28]. The employment of directional solidification (DS) metallurgy enables us to manufacture SC superalloy GT blades with an increased creep resistance (see Figure 19.4) [29]. Over the past five decades, the development of high-temperature alloys has been a primary driving force of increased operating temperatures for gas turbine engines, which has resulted in dramatic improvements in both their power and efficiency [25]. However, further improvements by alloy development approaches appear unlikely since in today’s engines, the flame temperature in the combustor reaches 2000 ∘ C, well above the melting point of superalloys (∼1300 ∘ C) [25]. The only way that the airfoils can survive in such an environment is by excessive internal and external cooling. Traditionally, this has been accomplished by designing more efficient cooling geometries within the component and by film cooling of the component surface using sophisticated arrays of holes, which will, however, minimize the overall thermal efficiency and increase the fabrication cost. In addition, these approaches have now matured, and alternate strategies to further improve the operating temperatures are required. Increasingly, engine designers are looking to other concepts to reduce the temperature of the superalloy during service. One of the most promising approaches is the use of TBCs whose primary function is to provide a low thermal conductivity barrier to heat transfer from the hot gas in the engine to the surface of the coated alloy component [30]. As shown schematically

19.3 TBC System Compositions

30 Single crystal

Creep strain (%)

Directionally solidifies

Equiaxed casting 0 0

100 Time (h)

Figure 19.4 Creep-to-failure behavior of equiaxed, directionally solidified, and single-crystal castings of Mar M200 Ni-based alloy at approximately 980 ∘ C and 207 MPa. Source: Versnyder and Shank [29]. © 1970, Elsevier.

in Figure 19.2, the structure of a TBC system consists of an yttria-stabilized zirconia (YSZ) coating deposited onto an oxidation-resistant bond coat alloy that is first applied to a nickel-based superalloy component. The use of TBCs, along with internal cooling of the underlying superalloy components, provides major reductions in the surface temperature (>500 ∘ C) of the superalloy. This has enabled modern gas turbine engines to operate at gas temperatures well above the melting temperature of the superalloy, thereby improving engine efficiency and performance.

19.3 TBC System Compositions The typical TBCs are primarily a two-layer system that consists of a porous (porosities generally in the range of 10–25%) ceramic top coat layer comprising zirconia (ZrO2 ) “partially stabilized” with about 6–8 wt% (∼3.4–4.5 mol%) yttria (Y2 O3 ) generally referred to as 7YSZ or YSZ and an alumina-forming bond coating layer, primarily of NiCoCrAlY- or NiAlPt-based compositions. As shown in Table 19.1, there are four main components with unique functions that influence the TBC life [22]: (i) Top coat layer that provides thermal insulation. (ii) Thermally grown oxide (TGO) layer that provides bonding of TBC to bond coat and slows subsequent oxidation. (iii) Bond coat layer that contains the source of elements to create TGO in oxidizing environment and provides oxidation protection. (iv) Superalloy substrate that carries mechanical load.

619

620

19 Thermal Barrier Coating for Aerial and Aerospace Engine

Table 19.1

Schematic of TBC system showing each layer, material used, and function.

TBC system

Material used

Function

Ceramic top coat

Oxide thermal barrier (6–8 wt% YSZ)

Thermal insulation

Thermally grown oxide (TGO)

Al2 O3

Oxidation barrier

Bond coat (BC)

MCrAlX (20%Cr–12%Al)

Bonding of TBC; oxidation protection

Substrate

Ni-based superalloy

Thermomechanical loading

Each of these components has markedly different physical, thermal, and mechanical properties that are strongly affected by processing conditions. During fabrication and most notably during use, these components interact chemically and mechanically. Dynamic relationships between these layers control the durability of TBC.

19.4 Applications of TBCs Applications of TBCs on cooled turbine engine components such as combustors, HPT blades, and HPT nozzles are increasing in commercial and military jet engines. This trend will certainly continue because the insulating capability of TBC enables high and high operating temperatures and/or permits a reduction in the required amount of cooling air, thereby leading to improved efficiency, reduced emission, and increased thrust/weight ratio. Performance of power generation turbine engines can also be enhanced using TBC [22]. A schematic of a jet engine with TBC applied to the turbine blades is shown in Figure 19.5 [31]. TBCs are currently used to provide metal temperature reductions of up to ∼100 ∘ C, while potential benefits are estimated to be greater than ∼200 ∘ C [22]. Factors that determine temperature reductions include part geometry and location, heat flux, heat transfer coefficients, backside cooling air, TBC thickness, and thermal conductivity [32]. The temperature benefits are highly significant and surpass other material technology advances including SC Ni-based superalloys achieved over a 30-year period [32]. The initial applications were driven by the need to suppress component degradation due to excessive surface temperatures in combustors and some selected turbine airfoils experiencing oxidation damage [22]. In spite of significant improvements, TBCs are yet considered “prime reliant” because of local spalls and reduction in thickness, and full utilization has eluded turbine designers. Research and technology developments have been carried out for the last two decades to address the performance and reliability while extensive research is being carried out for higher temperature applications beyond the baseline system and for TBC with lower thermal conductivity [22].

19.5 Processing Techniques for TBCs

Compressor

Combustor

Turbine

(A)

Top coat TGO Bond coat (C)

w d 40 μm

(B)

Figure 19.5 (a) A schematization of a jet engine. (b) A turbine blade. (c) A cross section of a specific TBC system. Source: Hille et al. [31]. © 2009, Elsevier.

TBCs are applied to components that are internally cooled by directing air though channels. Designs with TBC-coated parts need to consider part configuration and thickness, heat flux, heat transfer coefficients, combustion and turbine inlet temperatures, and total cooling air allowed by the system engineers. To minimize the increased excessive mass and cooling hole closure, thinner coatings are preferred.

19.5 Processing Techniques for TBCs There are many methods of preparing TBCs, such as EB-PVD, APS, high-velocity oxy-fuel (HVOF), sol–gel phase transition coating, laser-assisted chemical vapor deposition, SPS, SPPS, and so on [8, 22, 33]. There are two main types of commercial TBCs used in the aircraft engines. One is typically deposited on the rotating parts by EB-PVD with columnar grains normal to the coating/substrate interface and the intercolumnar porosity. The other is deposited typically on the stationary components by the low-cost process of APS) with the porosity generally parallel to the interface [34].

19.5.1 Electron Beam-Physical Vapor Deposition (EB-PVD) EB-PVD is one of the main methods of coatings manufactured to obtain the ceramic top coats of TBC systems in the industry [18]. Since the 1980s, EB-PVD has been intensively investigated for preparation of TBCs. A significant development in the 1990s was achieved by the invention of a new EB-PVD technology by Paton Electric Welding Institute in Ukraine, which greatly decreased the preparation cost of EB-PVD TBCs [35]. EB-PVD TBCs are more durable and are used primarily in

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EB gun for substrate heating Vaporizing EB guns

Vaccum chamber

Substrate

Horizontal manipulator

Vapor cloud

Crucible

Material coating ingots

Figure 19.6 EB-PVD coating chamber. Source: Sridhar et al. [37]. © 2013, International Journal of modern engineering research.

the most severe applications, such as turbine blades and vanes in aircraft engines. This technology has been widely used in the production of TBCs commercially [20]. EB-PVD is a form of physical vapor deposition in which a target anode is bombarded with an electron beam given off by a charged tungsten filament under high vacuum. The electron beam causes atoms from the target to transform into the gaseous phase. These atoms then precipitate into solid form, coating everything in the vacuum chamber with a thin layer of the anode material [36]. A characteristic columnar microstructure could be obtained in this technique, resulting in very high yielding strain tolerance during thermal cycle changes. It is realized by voids between columns, which can open and close during thermal cycles [18]. Figure 19.6 gives a diagram of EB-PVD equipment including in four electron guns, two sets of sample fixtures (one vertical and one horizontal), three ingots, and two chambers (one of which is a pre-vacuum chamber) [37]. Three electron guns are used to heat the materials in the ingots, respectively. The fourth electron gun is used to heat the substrate to achieve the right deposition condition that is able to ensure good adhesion between substrate and coatings. During depositing, the sample fixture keeps rotating to get a stable substrate temperature and uniform coating thickness. Three ingots can deposit from one to three types of materials and prepare coatings of different compositions and structures. There is a shaft below the materials in the ingot. During deposition, the shaft pushes the materials up to the ingot, thus increasing the deposition rate [20]. In the EB-PVD process, the electron beam plays a very important role as the thermal source of EB-PVD technology. EB-PVD is capable of depositing any type of materials. The deposition mechanism consists of an electron beam formed at 2000 ∘ C in an electron gun (either a Pierce gun or an electromagnetism deflexion gun), followed by acceleration of thermal electrons under high voltage. When the high-speed

19.5 Processing Techniques for TBCs Low modulus/strength Zone II High modulus/ strength Aliminum oxide layer Bond coat

(a)

Multiayered columnar TBC • Biasing parts (pulsed) • IBAD, EB-PVD (pulsed)

EB-PVD: Textured columnar structure (001 growth direction) • Columnar porosity Zone I

(b)

Figure 19.7 Schematic diagram illustrating (a) typical standard vapor-phase columnar microstructure and (b) modified columnar microstructure with multiple interfaces. Source: Wolfe et al. [38]. © 2005, Elsevier.

electrons hit materials in the ingot, energy is generated sufficient to melt and convert materials into the flying gas vapor and then deposit it onto the substrate [20]. The typical microstructure of a TBC produced by EB-PVD can be seen in Figure 19.7a [38]. The inner zone (zone I) is the early part of multiple nucleation and subsequent growth of the columnar microstructure having large number of interfaces, grain boundaries, microporosity, and randomly oriented grains. The thickness of inner zone ranges from 1 to 10 μm and exhibits lower thermal conductivity (∼1.0 W⋅(m⋅K)−1 ) [39]. The TBC microstructure is characterized by a high-aspect-ratio columnar grain with dominant crystallographic texture with increasing thickness. The thermal conductivity can be increased with the outer part of the coating becoming more crystallographically perfect (zone II) with fewer grain boundaries. In the outer zone (II), the thermal conductivity approaches that of bulk zirconia (2.2 W/m K). The best properties for commercial EB-PVD coatings (i.e. low thermal conductivity, high hemispherical reflectance, high strain tolerance, and good erosion resistance) can be achieved by modifying TBC microstructures. Thus, by modifying the macrostructure on the micron and submicron levels through periodically introducing strain fields (i.e. density changes by the incorporation of microporosity and surface restructuring), the thermal conductivity of TBC materials can be significantly reduced (Figure 19.7b) [38]. Due to its feasibility of varying coating structure, EB-PVD has great potential to produce functionally graded TBCs. This technology is based on the known phenomenon of fractionation of multicomponent systems during evaporation and subsequent condensation in vacuum. This approach, as applied to TBCs, allows replacing the flat interface between the metallic bond coat and the ceramic top coat by a graded transition zone and achieving a good adhesion of the coating to the substrate by introducing components with different melting temperatures and vapor pressures into the composition of the mixture being evaporated. A successful example is the Al2 O3 -YSZ graded thermal barrier coating (GTBC) [40–42], which basically has the structure MCrAlY-NiAl/NiAl + Al2 O3 /Al2 O3 -YSZ/YSZ. The GTBC significantly improved the thermal cycling lifetime compared with the MCrAlY/YSZ TBC because there are three microstructure features: (i) The

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formation of a predeposited α-Al2 O3 thin film and a high concentration of Al in the surface layer of the bond coat can improve oxidation resistance and hot-corrosion resistance. (ii) The formation of an “interlocking” interface contributes to the enhanced bonding between TGO and the bond coat. (iii) The formation of an Al2 O3 -YSZ graded transition zone helps to reduce the thermal stresses [43]. D.E. Wolfe et al. [38] proposed a new tailored microstructure of EB-PVD 8YSZ TBC by periodically interrupting the condensing vapor during the deposition process to create strain fields. Then, the phonon and photon scattering processes can be enhanced, leading to a lower thermal conductivity and a higher reflectance, respectively. The changes in the microstructure, mainly in the porosity and the grain boundaries, result in a reduction in the thermal conductivity from 1.8 to 1.2 W/(m K) (∼30% reduction) after two hours testing at 1316 ∘ C and an increase in the reflectance of up to 56%. Furthermore, an improvement of 100% in oxidation cyclic life as well as a better strain tolerance than the non-modified 8YSZ TBC structure has been obtained. The new tailored YSZ is expected to offer the potential of a longer turbine component life and a reduced fuel consumption within the turbine industry [20].

19.5.2 Plasma Spraying (PS) Process In the 1960s, plasma-sprayed thermal barrier coatings (PS-TBCs) were invented to reduce the surface temperatures of hot components by as much as 100–200 ∘ C. Since then, they have been widely used in the fields of diesel, aerospace, and land-based gas turbines and jet engines [44]. Typical PS-TBCs usually consist of three layers, namely, a top coat (PSZ), an overlay of bond coat (MCrAlY), and the substrate. PSZ containing 6–8 wt% of Y2 O3 coatings (YSZ) demonstrates excellent thermal shock resistance, low thermal conductivity, and relatively high coefficient of thermal expansion (CTE). The bond coat (MCrAlY) provides oxidation resistance [20]. Plasma spray processing utilizes electrically generated plasma to heat and melt the feedstock material, which is a very intensive process, requiring significant electrical power. It offers a method of depositing a feedstock material that is plasma-sprayed as a solid coating over an underlying target material. Deposits with a thickness from just a few micrometers up to several millimeters can be produced using a variety of feedstock materials, including metals and ceramics. In the plasma-spraying process, the material to be deposited, typically as a powder but sometimes as a liquid suspension, is introduced into the plasma jet emanating from a plasma torch. In the jet, where the temperature is about 10 000 K, the material is melted and propelled toward a substrate. Then, the molten droplets flatten and rapidly solidify to form a deposit. Usually, the deposits remain adherent to the substrate as coatings. There are a large number of technological parameters influencing the interaction among particles with the plasma jet and the substrate and therefore the deposit properties. These parameters include feedstock type, plasma gas composition and flow rate, energy input, torch offset distance, substrate cooling, etc. A typical plasma spray system consists of the following parts [20]: (i) the spray torch (or spray gun), the core device performing the melting and

19.5 Processing Techniques for TBCs Powder feeding

Fully-molten splat

Spraying direction

Inflight particle

Plasma gas

Pore Substrate

Air Cathode

Cooling water Anode

Plasma plume

Probable oxidation

Partially molten splat

Figure 19.8 Schematic of atmospheric plasma spraying process. Source: Kumar et al. [46]. © 2019, Elsevier.

acceleration of the particles to be deposited; (ii) the feeder, supplying the powder or liquid to the torch; (iii) the media supply, gases for generating the flame or plasma jet and carrying the powder. At present, there are several main processes for plasma spray preparation of TBC, such as APS, low-pressure plasma spraying (LPPS), laser cladding plus plasma spraying, SPPS, etc. [45]. APS is a coating process carried out at normal pressure, which is usually applied to the ceramic coating on metallic structural components. In plasma-sprayed coating deposition, particles melt in the process of being deposited, and when they impinge on the substrate, they flatten, undergo rapid solidification, and form deposits known as splats, as shown schematically in Figure 19.8 [46]. The high energy associated with the plasma-sprayed torches, and hence the impinging particles, gives rise to a rather rough and wavy interface that is fundamentally different from that produced by other methods such as EB-PVD. Furthermore, the presence of microstructural defects in the ceramics, i.e. porosities, generally introduces a long-term reliability problem since they tend to sinter after prolonged exposure at high temperatures, leading to an increase in the coating elastic stiffness. Currently, this technique is usually used to repair some used coatings after long-term operation [47]. In order to overcome the limitations of APS, LPPS and vacuum plasma spraying (VPS) technologies are being developed. In the LPPS coating process, the plasma spray gun, the work piece, and the running machine are put into an airtight room with a low-vacuum environment. A low-pressure environment decreases the oxidation rate of the feedstock powder as it is melting in the plasma plume, which also provides an increased acceleration of the melted powder due to the decreased air resistance. The lower pressure also extends the length of the plasma arc, so that spray distances can be increased from 3–5 in. for normal plasma spray to 11–13 in. for LPPS. Compared with the ordinary APS coating, the LPPS coating is characterized by uniform and compact microstructure, low porosity, high purity, no oxide inclusion, and smooth surface. The coating oxidation resistance is also distinctly enhanced. As an economic and versatile method, PS is widely used in the fabrication of metallic bond coats (e.g. MCrAlY) [48]. In the case of APS, the metallic or ceramic powder is injected into a plasma jet by the carrier gas, which makes it heated, accelerated,

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and deposited onto the surface of metallic substrate. Compared with EB-PVD coatings, APS coatings are relatively low cost and have a lower thermal conductivity [49]. The lamellar microstructures, oriented parallel to the surface of substrate, are obtained by the stacking of splats during APS process [50]. EB-PVD and APS are the two main methods used for TBC deposition with their own unique advantages. However, they have their own featured drawbacks [51]. Conventional plasma spraying allows rapid coating manufacture, and the produced coatings are thick (300–3000 μm) and porous (10–25%), which exhibits good thermal insulation but poor thermal shock resistance [52]. Conversely, columnar-structured TBCs by EB-PVD exhibit high strain tolerance and improved durability but high thermal conductivity as compared with TBCs by APS [9]. Moreover, due to their high investment costs and low deposition rates, EB-PVD processes are mainly used for mass production of thin film applications or production of thick TBC coatings on top of components subjected to high stress and temperature gradients, such as turbine vanes for aircraft engines. Besides, APS and EB-PVD are both line-of-sight processes [52]. A new processing technology, known as plasma spray physical vapor deposition (PS-PVD), has been developed in order to bridge the gap between conventional APS and PVD techniques to create unique microstructures [53]. PS-PVD combines the advantages of thermal spraying (high deposition rates and cost-efficiency compared with EB-PVD) with the possibility to produce columnar-structured coatings out of the vapor phase using plasma spray technology [53]. It was developed based on LPPS (also known as vacuum plasma spraying, VPS). However, the plasma jet of PS-PVD is greatly different from that of LPPS due to the low working pressure (ranging from 1 to 2 mbar) and high-power plasma gun [54]. The plasma plume of PS-PVD can expand to more than 2 m long and 200–400 mm in diameter [55]. Moreover, the temperature of the supersonic plume can exceed 6000 K due to the high power level (>100 kW) [53]. Therefore, when fine grain-sized powders are used, it is possible for the injected powders to be evaporated and obtain columnar microstructure. Besides, due to its high velocity and large dimension, the plasma gas stream is able to flow around complex geometries and “forced” through the shadowed areas, thereby making the PS-PVD a non-line-of-sight coating process [54]. The PS-PVD process is based on the ChamPro technology of Sulzer Metco that comprises all those thermal spray processes performing under a defined and controlled atmosphere, such as LPPS, VPS, and LVPS [56]. These processes operate under reduced pressure conditions, typically in an argon gas atmosphere. The common working pressure of vacuum thermal spray processes ranges between 3 and 20 kPa (30 and 200 mbar), which allows for the deposition of coatings, having a typical thickness range between 20 and 2 mm. This technology is widely adapted in the aerospace industry to deposit high-temperature oxidation-resistant layers and bond coats. The reduced pressure compared with atmospheric conditions enlarges the plasma plume from 50 to 500 mm in length and from 10 to 40 mm in diameter. Since a larger plume results in a bigger spray spot, particle velocities and temperatures are more homogenously distributed over the cross section of the plume [57]. This allows for production of coatings with a homogenous coating

19.5 Processing Techniques for TBCs

thickness distribution even on parts with complex geometries, particularly turbine components. Many investigations have been focusing on the process development of PS-PVD [58, 59]. Different microstructures of YSZ top coat, including dense coatings with splat-like microstructures, EB-PVD-like columnar coatings, quasi-columnar coatings, and mixed microstructure coating, could be formed by adjusting the feeding rate and the plasma gas composition [59]. In particular, significant differences in microstructures can be achieved by changing the spray distance slightly [60]. It is noticed that the microstructures of the coatings have a great influence on the operational parameters: the thermal conductivity, the thermal shock resistance, the high temperature oxidation resistance, the hot gas corrosion, etc. [52].

19.5.3 Solution Precursor Plasma Spraying (SPPS) Process The SPPS process was first reported and applied in the synthesis and deposition of nanopowders [61]. The process was further developed successfully in a joint effort by the researchers at the Institute of Materials Science and Mechanical Engineering Department at the University of Connecticut and the Inframat Corporation, making 7YSZ TBCs of high durability and low thermal conductivity, which were desired for marine, aircraft, and industrial engines [61–63]. In this process, an aqueous chemical precursor feedstock is injected into the plasma jet where the droplets undergo a series of physical and chemical reactions and then deposited on the substrates as coatings [10]. The SPPS process is schematically illustrated in Figure 19.9, which is fundamentally different from that of ceramic powders, while an aqueous solution of zirconium and yttrium salts is injected into the plasma gas environment [61–63, 65, 66]. Upon entrainment into the plasma, the subsequent vaporization of the water in the droplets and decomposition of the acetate and nitrite groups result in the formation of ceramic material, which contains the desired nanostructure in the final coating structure. The basic characteristics of the SPPS TBCs are uniformly dispersed porosity, absence of unwanted coarse splat boundaries, and straining tolerance, providing evenly spaced vertical cracks. As shown in Figure 19.10, the SPSS process enables the creation of some unique features to make it possible to manufacture high-strength TBCs [20, 63]. Thermal cyclic durability tests of TBCs deposited by APS, SPPS, and EB-PVD demonstrated that the spallation times of SPPS coatings were 2.5 times longer than those by EB-PVD coatings and 1.5 times longer than APS coatings under the same conditions. During these tests, vertical cracks and ultrafine splats in the SPPS coatings were found to remain stable. Microstructural observations of this type of TBC show fine splats and vertical cracks in a porous matrix. TBCs deposited by the optimized SPPS process exhibit superior durability relative to TBCs formed by the APS and EB-PVD processes. Thick and durable TBCs can be deposited by this process. Failure of these TBCs usually occurs by large-scale buckling of the ceramic top coat [63]. The efficiency of TBCs

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Solution precursor Atomizing nozzle

Cathode

Coating

Precursor mist

+ – +

Plasma flame Anode Substrate Gas

Figure 19.9 Elsevier.

Schematic illustration of the SPPS process. Source: Gell et al. [64]. © 2004,

Figure 19.10 SEM images of polished cross sections of SPPS-deposited TBC. Arrows indicate vertical cracks [63]. Source: Elsevier.

Metal Substrate

50 µm

used to protect and insulate metal components in engines increases with the thickness of the TBCs. However, the durability of thick TBCs deposited using conventional deposition methods has not been satisfied. Jadhav et al. [65] deposited highly durable 4-mm-thick ZrO2 –7 wt% Y2 O3 (7YSZ) TBCs on bond-coated superalloy substrates using the SPPS method. The average thermal cycling life of the SPPS TBCs was around 820 cycles. However, most of the conventional air plasma-sprayed coatings of the same composition and thickness deposited on similar bond-coated superalloy substrates were observed to be detached partially from the substrates in the as-sprayed condition. The APS TBC failed only after 40 thermal cycles. Due to the presence of the vertical cracks in the SPPS TBCs, significantly higher in-plane indentation fracture toughness and high degree of strain tolerance exist, leading to the dramatic improvement in the thermal cycling life of the SPPS TBCs over APS TBCs [65]. SPPS coatings are also different from traditional ones [67], i.e. the absence of large splat boundaries that are the main reason of delamination failure of conventional plasma-sprayed coatings, a better strain tolerance due to the existence of through-thickness vertical cracks [68], and the uniformly distributed porosity that reduces the thermal conductivity of coating. It is reported that the rich variety of SPPS coating features is determined by spray parameters [69]. Xie et al. [67]

19.5 Processing Techniques for TBCs

(a)

(b)

100 nm

50 nm

Figure 19.11 TEM images of SPPS YSZ coating: (a) dark-field image and (b) bright-field image [63]. Source: Elsevier.

studied spray pattern from SPPS process of 7 wt% YSZ precursors via a fixed scan spray method. The resulted patterns show that its central part is well adherent and relatively dense while its wings are porous and many powdery deposits are also found on both sides. A window-shield fixed scan spray test verifies that the dense area of deposited coatings derives from droplets that are injected into the plasma cores. Nanostructured YSZ coating using zirconium and yttrium salt aqueous solution was prepared via the SPPS process [68]. As shown in the dark-field image of the aggregate (Figure 19.11a), the average grain size is about 30 nm. Homogenously distributed polycrystalline grains can be observed in the bright-field image of the ion-beam-milled coating, which confirms that the grain size is about 20 μm (Figure 19.11b). Many vertical cracks and uniformly distributed pores were observed in the as-sprayed coatings. Due to the absence of “splat” boundaries/cracks and the superior strain tolerance, the SPPS coating showed a higher thermal cycling life than the other counterparts. Wang et al. deposited lanthanum zirconate (LZ) TBCs through the SPPS process, and the influence of addition of urea to the solution to the heat exchange on the microstructure of powder and coating was investigated [70]. Compared with the heat input by the plasma jet, the additional heat energy of urea can be neglected. The result suggests that the formation route and the microstructure of SPPS coatings are significantly related to the solid content of solution precursor [70].

19.5.4 Suspension Plasma Spraying (SPS) As a newly emerged method, SPS is used to deposit coatings with enhanced characteristics through submicron particles or nanoparticles. The suspensions contain fine particles that are dispersed in a liquid phase and are injected into the plasma through a liquid feedstock system as a liquid stream or a stream of droplets [50]. This method is intensively examined in very wide areas due to improved properties

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Atomizing gas

Atomizing gas

Suspension

Compressed gas

Twin-fluid atomizer Susprension

Plasma torch

Figure 19.12

Substrate

630

Schematic of the SPS process. Source: Zhao et al. [71]. © 2019, Elsevier.

of obtained coatings compared with their conventional equivalents (by APS) [33]. However, the SPS technique is more complex than APS method, and there are also more process parameters, which are collected in Figure 19.12 [71]. As discussed in some literatures, the typical features of SPS coating exhibit columnar structures or vertical cracks across the thickness of the coatings [72]. The thermal conductivity of SPS coating is significantly dependent on coating features, which can be much lower than that of EB-PVD or APS coating [73]. Some particles with sizes less than 1 μm are easily accelerated and then decelerated in the plasma jet, which results in a shorter standoff distance between the plasma torch exit and the substrate than the conventional processes [74]. Therefore, the substrate experiences a greater heat flux due to the shorter standoff distance in SPS than that in the conventional processes [75]. In addition, injector properties and spray parameters of the SPS technique must be accurately optimized to achieve a desirable coating microstructure. The structures of SPS coatings are influenced by a variety of parameters. Bernard et al. [76] stated that the substrate roughness can affect the coating microstructures greatly. Axial suspension plasma-sprayed TBCs under different process parameters were investigated by A. Ganvir et al. [77]. Columnar-structured coatings can be produced using axial SPS technique. However, the pore sizes, the density of columnar grains, and the intercolumn spacing are distinctly different under the different spray conditions. Zou et al. [78] studied the effect of bond coat surface on the microstructure and performance of YSZ-based TBCs. The microstructure of the YSZ coating was found to be strictly linked to the bond coat surface condition. The YSZ coating on a rough APS bond coat has a longer lifetime than that on a smooth HVOF bond coat. Li et al. [79] investigated the microstructure and oxidation behavior of SPS-deposited vertically cracked (VC) and columnar YSZ TBCs by axial injection. VC and columnar-structured coatings were fabricated via changing the standoff distance. It was observed that the columnar YSZ coating appeared a “cauliflower-like” morphology, while VC coating had a relatively smoother surface with more cracks. Meanwhile, the latter showed a longer lifetime than the former [79]. Sokołowski

19.5 Processing Techniques for TBCs

Figure 19.13 Cross section and surface microstructures of SPS coatings with low concentration deposited on different substrates prepared by (a) sand blasting, (b) laser treatment, and (c) grinding. Source: Sokołowski et al. [80]; Elsevier.

Figure 19.14 Cross section and surface microstructures of SPS coatings with relatively high concentration deposited on different substrates prepared by (a) sand blasting, (b) laser treatment, and (c) grinding. Source: Sokołowski et al. [80]; Elsevier.

(a)

50 µm

(b)

50 µm

(c)

50 µm

(a)

50 µm

50 µm (b)

50 µm (c)

et al. [80] systematically demonstrated the effect of parametric variation (solid loadings, substrate preparation methods, and particle size) on the formation of columnar microstructures in SPS YSZ coatings. As shown in Figures 19.13 and 19.14, the microstructure of coatings depends on the substrate surface morphology as well as the solid content. The increase of the solid content results in the formation of a relatively dense coating. It was found that low concentration of fine solids led to the formation of columnar structures. Due to its small size, the trajectory of fine solid particles in the vicinity of substrate can be influenced by the plasma jet. Therefore, the formation of columnar structures can be interpreted by the shadowing effect theory proposed by VanEvery et al. [81] in the next paragraph.

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Injection and fragmentation

Suspension Particles + agglomerates (i)

(ii)

Figure 19.15 Diagrams of deposition mechanism of SPS coatings. Source: Joulia et al. [82]. © 2014, Elsevier.

The formation mechanism of one SPS coating is shown in Figure 19.15 [82]. The suspension is firstly fragmented into liquid drops, followed by the solvent evaporation. After that, two different processes occur. (i) The single particles and agglomerates injected into the plasma jet are released and melted and then impact onto the substrate. Single particle yields ultrafine lamellae or non-flattened spherical particles. Due to the low inertia, most individual particles reach the substrate with a lower velocity and cannot be flattened. The agglomerates may form either small lamellae or unmelted sintered aggregates.(ii) The large-sized aggregates may form in the plasma jet. Some of them may break up, resulting in the formation of single particles or small agglomerates, which is similar to case (i), while others may produce large lamellae or sintered aggregates. The microstructures of SPS coatings are heterogeneous, such as highly dense, highly porous, and columnar-like or VC. Because of the improved performance of column-structured coating, it is desirable to produce this type of deposits. Therefore, it is necessary to understand the formation mechanism of column-structured coating deposited by SPS. The formation of columnar structures can be connected to the interaction of atomized droplets with the plasma jet [83]. If the sizes of droplets in the plasma are in the range from 1 to 5 μm or smaller in diameter, the impact trajectory of in-flight particles will be severely affected by the plasma gas stream [84]. VanEvery et al. [81] initially proposed that the column formation of SPS coating could be explained by the well-known shadowing effect. Ganvir et al. [77] further elucidated the dependence of droplet morphology on its size after it impacts onto a rough surface. As described in Figure 19.16 [85], the large droplet does not have the parallel

19.6 Thermal Transport in TBCs

Figure 19.16 Dependence of droplet morphology after it impacts onto a rough surface on the droplet size. The droplet morphology after it impacts onto the substrate depends on its trajectory and the resultant droplet velocity. Source: Ganvir et al. [85]. © 2015, Elsevier.

VNormal=0 VParallel=0 Asperity

Substrate

component of velocity and is hardly affected by the plasma gas. On the contrary, the impact trajectory of small droplet strongly depends on the plasma jet, which results in the formation of vertical cracks.

19.6 Thermal Transport in TBCs The TBC is a thermally insulating, “strain-tolerant” oxide, and its function is to provide a thermal barrier to heat transfer from the hot gas in the engine to the surface of the coated alloy components [25]. As seen from Figure 19.17, YSZ has emerged as the preferred material for the TBC due to its lowest thermal conductivity and minimum temperature sensitivity among all known ceramics [86, 87]. Engine temperatures are expected to continue to increase as higher-efficiency engines having higher power-to-weight ratios are being designed. In addition, weight is at a premium in an aircraft engine, and thin coatings with the lowest possible thermal conductivity are required for further application of TBCs. In part, the current YSZ TBC composed of the t phase becomes unstable at higher temperatures, decomposing to a mixture of tetragonal and cubic phases [87]. In crystalline solids heat is transferred by three mechanisms: (i) electrons, (ii) lattice vibrations, and (iii) radiation [88]. As zirconia and its alloys are electronic insulators (electrical conductivity occurring at high temperatures by oxygen ion diffusion), electrons play no part in the total thermal conductivity of the system, and thus conduction in zirconia is by lattice vibrations (phonons) or by radiation (photons). For an ideal crystalline material, the thermal conduction contributed by phonon transport in a temperature gradient can be described by the Debye theory: kp = CV vm Λ∕3 where CV is the specific heat, vm is the phonon velocity, and Λ is the mean free path for the scattering of phonons. In addition to phonon conductivity, zirconia is susceptible to radiative conduction (photon transport) at high temperatures due to its transparency in the infrared (0.3–5 μm). At the present gas temperatures, radiative heat transfer through the YSZ coating is only a minor contribution because of optical scattering within the coatings. For instance, at 1250 ∘ C, only 10% of the heat flux through the zirconia is induced by

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100.0 Thermal expansion coefficient (10–6/K)

634

Nikel alloys YSZ

ZrO2

10.0

BeO Mullites

Y2SiO5

Al2O3

MgO

Sialons Y2Si2O2

1.0 SiO2

0.1 1.0

10.0

100.0

1000.0

Thermal conductivity (W/mK)

Figure 19.17 The thermal expansion coefficients and thermal conductivities of some promising thermal barrier coating materials on nickel-based superalloys. Source: Sun et al. [86]. © 2009, Elsevier.

radiation [88]. However, the radiative heat transfer through the coating is expected to become a concern with increasing temperature. This contribution can be expressed as [89] kr = 16𝜎n2T3lr ∕3 where kr is the thermal conductivity due to radiation, 𝜎 is the Stefan–Boltzmann constant (5.67 × 10−8 W/[m2 k−4 ]), n is the refractive index, T is the absolute temperature, and lr is the mean free path for photon scattering. The total theoretical thermal conductivity of zirconia-based systems is the sum of the two terms due to phonons and photons: k = kp + k r To lower the intrinsic thermal conductivity of a TBC, reductions in both phonon transport and photon transport are needed. The first reduction can be achieved by lowering the density or elastic modulus of the coating, e.g. introducing pores, or increasing the mean atom weight of the coating materials, e.g. doping; the second reduction can be achieved by creating more reflective sites in the coating [25]. There are two efficient ways to decrease the thermal conductivity of the TBC: one is introducing microstructural defects such as pores, voids, and grain/interface boundaries that constitute obstacles against the through-thickness heat transfer propagation. Pores primarily decrease the net section area through which heat can be transported by phonons, and so the reduction in thermal conductivity depends not only on the volume fraction of the pores but also on their aspect ratio and their spatial distribution [90]. In addition, the interface disorder can scatter phonons at grain

19.7 Summary and Perspectives

boundaries or interfaces between similar materials [91]. Thus, it may be expected that materials with high interfacial densities should reduce the thermal conductivity of TBCs. A multilayered W/Al2 O3 film could give rise to a thermal conductivity of 0.6 W/(m K) [91]. For EB-PVD coatings, the introduction of interfaces into each column, which are parallel to the ceramic/metal interface, can reduce the thermal conductivity by 37–45% compared with the state-of-the art EB-PVD TBCs [87]. The other is that the thermal conduction by phonon transport can be reduced by increasing the mean atom weight (M/m) of the material. Following this concept, two important groups of low thermal conductivity materials have been examined: one based on co-doping of YSZ with one or more REO [92] and the other based on pyrochlore-type zirconates (M2 Zr2 O7 ) [93]. Although a great effort has been made in searching out low thermal conductivity materials for high-temperature applications, there are three difficulties in selection of candidate materials for TBCs. First, an effective model is needed to understand and predict the high-temperature conductivity, especially in chemically and microstructurally complex materials such as TBCs. Second, there is still a shortage of multielemental phase diagrams since most of the low thermal conductivity materials contain more than three components. Third, there is a difficulty in assessing the thermodynamic stability with alumina at high temperature [94].

19.7 Summary and Perspectives Application of TBC is one of the major developments in turbine industry that has revolutionized how turbine components are designed. The surface temperatures of the components can potentially be increased by more than 200 ∘ C [95]. The temperature benefit surpasses other material technology advances including single crystal Ni-based superalloys achieved over a 30-year period [22]. TBCs have become an integral part of turbine designs requiring higher efficiency, performance and reduced emissions, and noise. TBC applications continue to increase on components in the propulsion and power generation turbines. In the future, advanced applications of ceramic matrix composite (CMC) components will also depend on TBCs. TBCs have evolved from just simple insulating layers for temperature reduction to complex designs for more functions. However, the turbine designers have a love–hate relationship with TBCs because of unacceptable variation in the laboratory and the engine operation performance, even with the baseline YSZ TBCs due to a variety of reasons mostly related to processing and issues related to damage by the particulates in the turbine operating environment. The performance variation dictates that turbine components should be designed to the lowest spectrum of the scatter band. Enhanced reliability should lead to deriving greater TBC benefits [22]. The most commonly used TBC material is 6–8 wt% YSZ. However, the phase transformation of zirconia may occur when the temperature is above 1200 ∘ C, which greatly limits the application of YSZ under the higher temperature conditions. To

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meet the requirement of higher operation temperature of turbines, some new substitutes of YSZ are developed [96–99]. Among the candidates for TBCs, rare-earth zirconates (RE2 Zr2 O7 ) and rare-earth-doped YSZ have extensively attracted attention [100, 101]. In RE2 Zr2 O7 /YSZ double-layered systems, RE2 Zr2 O7 are mainly used as a top coat due to their lower thermal expansion coefficient, and YSZ as an interlayer [102]. For instance, La2 Zr2 O7 (LZ) is a promising ceramic material for TBCs. LZ has many good properties, including low thermal conductivity, high melting point, high sintering resistance, and high phase stability [102]. Duarte et al. [103] fabricated a LZ coating through SPPS technique, which revealed that the rheological properties, such as surface tension and viscosities, played a critical role in the formation of splats. Another potential material is gadolinium zirconate (GZ), which also shows high melting point, high-temperature phase stability, low thermal conductivity, and good calcia–magnesia–alumina–silicate (CMAS) resistance. Double-layered GZ/YSZ TBCs were deposited via SPPS process by Jiang et al. [104]. The microstructures of YSZ interlayer had inter-pass boundaries (IPBs), which reduced thermal conductivity of the coatings. It was confirmed that the thermal cycling life and CMAS resistance of SPPS TBCs was superior to APS TBCs. Rare-earth-doped YSZ coating has also attracted great interest of researchers [105, 106]. Gong et al. fabricated rare-earth-doped TSZ coatings through SPS method by dissolving different amounts of rare-earth nitrates into YSZ suspensions [105]. The thermal insulation of YSZ coating can be improved by doping CeO2 into it (YCSZ). YCSZ coatings can also be deposited via SPS process under different spray conditions [80]. It was found that the porosity of different samples was in the range of 15–21.5% and the corresponding thermal conductivity was in the range of 0.5–1.0 W/(m K). However, most of the present research is limited in the microstructures and thermophysical properties of these new-type TBCs prepared by SPS or SPPS process. Further investigations on the thermomechanical properties during thermal cycling are necessary. In addition, input process parameters such as power input of plasma jet, coating thickness, standoff distance, and type of coating play a major role in deciding TBC lifetime and performance. They are often very complicated, expensive, and time-consuming to sources of mechanical property values by experimental measurements, and it also can lead to erroneous results. Genetic programming (GP) is the most common approach from various evolutionary computation methods using multivariate regression fitting for the modeling of various systems [107]. M. Yunus and M.S. Alsoufi [107] reported that a new model was built for estimating the mechanical properties of TBC using GP. This model provides a more accurate prediction of mechanical properties compared to the empirical correlations, and the results can then be utilized to estimate a future set of parameters based on the historical data. In the future, the GP approach can be used as an advantageous tool that requires high proficiency to prepare high-performance TBCs.

References

Acknowledgments This chapter is supported by the NSFC-BRICS STI Framework Program (No. 51861145309), the National Natural Science Foundation of China (No. 51971029), the National S&T Major Project (No. 2018ZX10301201), the “All English teaching demonstration course construction project of University of Science and Technology Beijing” (No. KC2015QYW06, 2016), the “1125” Zhihui Zhengzhou Talent project of Henan province (Fund No. in USTB: 39080070), the “100 talent plan” fund of Fujian province (Fund No. in USTB: 39080067), and the development of a high sensitive magneto-optical biomolecular sensor experimental prototype (Fund No. in USTB: 2019-0649) by Hangzhou Ruidi Biotechnology Co. Ltd.

List of Abbreviations CMAS CMC DS EB-PVD GP GT GTBC GZ HPC HPT HVOF IPBs IPC IPT LPPS LPT PS SC SPPS SPS TBCs TGO YSZ

calcia–magnesia–alumina–silicate ceramic matrix composite directional solidification electron-beam physical vapor deposition genetic programming gas turbine graded thermal barrier coating gadolinium zirconate high-pressure compressor high-pressure turbine high-velocity oxy-fuel inter-pass boundaries intermediate-pressure compressor intermediate-pressure turbine low-pressure plasma spraying low-pressure turbine plasma spraying single crystal solution precursor plasma spraying suspension plasma spraying thermal barrier coatings Thermally grown oxide yttria-stabilized zirconia

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20 Perspectives for Thin Films and Coatings Yujun Song 1,2 1 University of Science and Technology Beijing, Center for Modern Physics Technology, Applied Physics Department, School of Mathematics and Physics, 30 Xueyuan Road, Beijing 100083, China 2 Zhejiang Key Laboratory for Pulsed Power Technology Translational Medicine, Hangzhou Ruidi Biotechnology Company, Hangzhou 310000, China

20.1 Introduction As forms of functional materials demonstrating key tactical performance, thin films and coatings, whether organic or inorganic, have been developed intensively to satisfy the requirement of academics and industrials, and some of them have innovated the traditional concepts of this field in the past decades. In this chapter, we have summarized their physicochemical fundamentals, their key fabrication methods, and the typical applications of some of the main and most important types of thin films and coatings. Their rapid development and the thrusts for great progress are analyzed and discussed according to their intrinsic application features. Here, we discuss some expectations for their future development in the theoretical breakthrough, novel fabrication techniques, the fundamental study, and their comprehensive application exploration for readers’ further consideration.

20.2 Development of the Subversive Novel Concepts for the Theoretical and Technological Breakthrough and New Findings of Thin Films and Coatings Devising strategies are always good approaches for high-throughput research and application by coupling with practical experiments and testing. These basic requirements and technology progress in thin films further boosts the development of algorithms and software for designing related materials and highly efficient devices based on thin films and coatings. Particularly, with the building up of big database and artificial intelligence incorporation, it will become possible to realize the combination of the first-principles theory (including the density functional theory (DFT) to calculate the related band energy and bandgap) for the Inorganic and Organic Thin Films: Fundamentals, Fabrication, and Applications, First Edition. Edited by Yujun Song. © 2021 WILEY-VCH GmbH. Published 2021 by WILEY-VCH GmbH.

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fundamental theoretical and mechanism study and the multiphysics coupling simulation (e.g. using COMSOL Multiphysics, finite-difference time-domain (FDTD) or other electromagnetic (EM) and/or thermodynamic-theory-based software) for the study of apparent phenomena. This coupling will favor to the molecule and microstructure design, and the study of the relationship between their multiscale or trans-dimensional structures and their physicochemical properties, as well as the prediction of the new performances of the desired thin films and coatings in a much reasonable and reliable approach. Based on these intensive studies, novel physical theories and functional materials emerge in an endless stream for the reasonable design of thin films and coatings to address the practical issues, such as the quantum entanglement theory for the information security, the string theory, and the multidimensional space concept to resolve the mass–energy exchange issues in the low dimension and trans-dimension. These novel concepts further promote the microstructure and component design of the basic surface and interface and their controlled fabrication, which possibly breaks the traditional classification of materials with the realization of the critical condition during research. The advanced topological concepts and the development of two-dimensional (2D) and layered materials are good examples even though topology has already progressed in the field of mathematics a long time ago by J. B. Listin in 1847 and since the situation analysis by Gottfried Wilhelm Leibniz in 1679. This topology has gradually become a subversive novel physics concept termed as the essential attribute (basic characteristic) of matter [1–34]. For theoretical discoveries of topological phase transitions and topological phases of matter, the Nobel Prize in Physics 2016 was awarded to David J. Thouless et al. [4]. Unlike geometry, topology is the study of physical space and is concerned not with exact dimensions and angles but with those properties of surfaces that remain unaffected by distortion such as contiguity, order, and relative position. Topology has journeyed from the purely mathematical arena to feature throughout physics, which can be used to explain several novel physic phenomena, such as quantum Hall effects and fermion-number fractionalization. Currently, topology has returned to prominence in condensed matter physics and materials, promoting great progress in the related layered thin films and 2D materials, such as the recent discovery of the topological insulators (a class of materials that are characterized by topological properties), and the semimetals (a class of materials that possess both properties of metals and nonmetals, such as Weyl semimetals (WSMs)) [5–8]. The appearance and development of the 2D and layered materials based on the new connotation of topology in physics and materials has been a good example since the successful synthesis of the free-standing graphene in 2004 by Kostya Novoselov and Andre Geim, who were also awarded the Nobel Prize in Physics in 2010 for “groundbreaking experiments regarding the two-dimensional material graphene.” They used a fabrication method, called as micromechanical cleavage, to synthesize graphene, a kind of 2D crystal that can be existing in room temperature and is very stable in a natural environment, thus debunking the old knowledge that the

20.2 Development of the Subversive Novel Concepts for the Theoretical and Technological

thermodynamic fluctuation does not allow any kind of 2D crystal existing stably at a finite temperature. Currently, graphene has been considered as the most promising material to substitute of silicon for the new generation of electronic devices (e.g. high-speed transistor) since it preserves many excellent properties, such as the high electron mobility due to its uniform atom structure and the special Dirac cone of the linear dispersion feature (or the conical dispersion relation between energy E and momentum kx and ky space). However, these intrinsic features also make pure graphene one zero-energy gap material, resulting in a constant “conducting” state, which is the huge shortcoming as idea electronic materials. It is impossible to obtain the stable “0” and “1” status using the pure graphene-based electronic devices, which limits its application. Just based on the novel concept of the bionic hybrid, this issue has been successfully addressed by layer stacking graphene with other 2D materials (e.g. hexagonal boron nitride, h-BN) forming non-covalent interfacial heterojunctions or the van der Waals heterostructures (vdWHs) [9–11]. Two-dimensional layered materials (2DLMs), particularly those vdWHs consisting of a covalently bonded, dangling-bond-free lattice and weakly bound to neighbor layers, have formed the current central focus of materials research [9–12]. Graphene/h-BN (Gp/h-BN) can be treated as a model vdWHs (van der Waals heterostructures) as the superlattice potential, which is induced by lattice mismatch and crystal orientation, gives rise to various novel quantum phenomena, such as the self-similar Hofstadter butterfly states [9]. H-BN preserves a similar honeycomb lattice structure as graphene, and the breaking of the inversion symmetry by distinct boron and nitrogen sublattices leads to an increased bandgap (5.97 eV) in the 𝜋 band, which is in sharp contrast to the gapless Dirac cones in graphene [9]. Gp/h-BN vdWHs not only exhibit greatly enhanced electronic properties for device applications, such as reduced ripples, suppressed charge inhomogeneities, and higher mobility, but also offer unique opportunities for band structure engineering of Dirac fermions by a periodic lattice potential [13]. The superlattice potential induced by the lattice mismatch and crystal orientation can distinctly tune the electronic property of graphene, which can result in varieties of novel quantum phenomena, such as the renormalization of the Fermi velocity [14], the topological currents [15], the gate-dependent pseudo-spin mixing [16], the gap opening at the Dirac point [14, 17, 18], and most importantly the emergence of second-generation Dirac cones, which are crucial for the realization of Hofstadter butterfly states under an applied magnetic field [17, 19]. Particularly, the topology concept, both ancient and innovative, has promoted people discovering many new thin films by creatively designing the 2D materials of unique space arrangement, such as the twisting two layers of graphene at a certain angle for the magic angle graphene and the rolling of 2D materials into 1D structural materials for unexpected electronic, electric, photonic, and magnetic properties, opening the new arena of functional thin-film design and fabrication [20–24]. It is no doubt for us expecting that with the development of the 2D materials, the metal materials have to be reconsidered now since the concept of semimetal appears and many layered 2D materials have been fabricated and exhibit unique electronic and magnetic properties [25, 26].

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Furthermore, by exploiting the novel properties in these vdWHs with diverse layering of metals, semiconductors, or insulators, new electronic devices have emerged, including tunneling transistors, barristors, and flexible electronics, as well as optoelectronic devices have been developed, including photodetectors, photovoltaics, and light-emitting devices with unprecedented characteristics or unique functionalities [9, 12, 27, 28]. Novel functional thin films based on the multilayered stacking strategy using 2D materials or nanostructured materials as building blocks have been expanded in the development of new energy and catalysis technologies, such as the highly efficient thin-film solar cell [29–31] and the electrochemical thin-film electrodes for fuel cells [32–34], and even in the ultrasensitive magneto-optical sensors [35–37]. Theoretically, these progress makes it both essential and urgent to develop a novel theoretical simulation strategy to correlate the intrinsic band energy mechanism with the apparent thermal-dynamic characteristics by simultaneously combining the first-principles theory (e.g. simulation by the current popular DFT) and the multi-physics coupling calculation (e.g. calculation by COMSOL). Finally, many constructive concepts based on quantum entanglement theory have been advanced for new technology development, particularly the quantum communication, quantum radar for ultralow-reflection objects, safe scanning, and high-resolution medical imaging of human tissues [38]. These courageous and subversive concepts and techniques undoubtedly depend on the progress of the quantum thin films with varieties of quantum microcavities for the transport, storage, and detangling of quantum, which also inversely promote the concept development of these functional thin films and the related large-scale highly precise fabrication methods.

20.3 Development of Highly Precise Fabrication Techniques for Thin Films and Coatings Development of novel fabrication and synthesis techniques with highly precise control of microstructure and component (e.g. in situ growth of single crystal thin films or layered 2D materials at atomic resolution) are the basis for the testing and realization of proposed novel concepts and theory. The traditional materials assemble strategy forming thin films are usually related to the formation of strong chemical bonds, for instance, the epitaxial or heteroepitaxial growth by molecular beam chemical vapor deposition (MBCVD), metal–organic chemical vapor deposition (MOCVD), or other kinds of vapor phase epitaxial processes [39]. This kind of process is intensively limited in those materials of highly microstructure matching and processing compatibility [11, 39, 40]. While the revolutionary concept of stacking atomic layers via van der Waals force interaction can overcome the above limitation of lattice and processing technology, realizing the non-chemical bond assembling to construct the desired 2D materials [39]. Most importantly, prefabricated building blocks can be physically assembled together flexibly [39], such as the fabrication of the magic angle graphene via the angle-oriented resolved stacking of two-layer graphene [21, 22].

20.3 Development of Highly Precise Fabrication Techniques for Thin Films and Coatings

Recent progress in the fabrication methods has already paved the road for the chiral synthesis or oriented growth along a controlled phase plane/surface at the atomic and nanoscale resolution for engineering microstructures and bands of thin films (including 2D and nanostructured materials) [35, 41–48]. Some methods have also shown the large-scale possibility for the synthesis of 2D or nanostructured thin films [11, 36, 39, 47, 49]. These successes continuously encourage scientists’ passion for the discovery of novel materials and the large-scale fabrication methods for novel functional thin films. For example, large-scale synthesis of high-quality solution-processable 2D semiconductors (e.g. MoS2 nanoplates) is much desired since they have great potential in large-area thin-film based electronic devices. However, preparing high-quality solution-processable 2D semiconductor nanosheets remains a challenge. For this goal, Duan and Huang in the University of California in Los Angeles invented a general approach to prepare highly uniform, solution-processable, phase-pure semiconducting nanosheets after several years’ study, which involves the electrochemical intercalation of quaternary ammonium molecules (such as tetraheptylammonium bromide) into 2D crystals, followed by a mild sonication and exfoliation process [49]. By precisely controlling the intercalation chemistry, phase-pure, semiconducting 2H-MoS2 nanosheets with a narrow thickness distribution can be obtained successfully. These nanosheets were then further processed into high-performance thin-film transistors, with room-temperature mobilities of about 10 square centimeters per volt per second and on/off ratios of 106 that greatly exceed those obtained for previous solution-processed MoS2 thin-film transistors. This approach can be used to prepare other kinds of solution-processible 2D materials and open a brand-new method for the scalable fabrication of large-area arrays of thin-film transistors that enabled the construction of functional logic gates and computational circuits using varieties of semiconductor nanosheets [49]. Another kind of atomically precise bottom-up methods for the fabrication of thin films (e.g. graphene nanoribbons) with large-scale ability or “on-surface synthesis technology” has been developed rapidly since it was advanced by Cai et al. since 2010 [41, 50]. The process involves the deposition of precursor monomers with structures that “encode” the topology and width of the desired ribbon end-product onto a metal surface and then surface-assisted coupling of the precursors into linear polyphenylenes followed by cyclodehydrogenation [41, 50]. The “on-surface synthesis” methods can currently provide a large-scale route to engineer the unusual nanostructured thin films with tuned chemical, electronic, and optical and catalytic properties for advanced electronics, catalysis, and transistors due to this method’s versatility and precision at the atomic resolution [44, 45, 50–54]. The typical examples are the precise synthesis of the graphene nanoribbons of different edge structures (e.g. zig-zag, armchair) [41, 50, 55] and the magic-angle graphene of unconventional superconductivity [22]. Recently, Steven G. Louie and coworkers advanced the armchair topological phase classification of grapheme nanoribbon, which classify them as topological nontrivial and topological trivial according to the difference in the width and the edge microstructure, showing unconventional topological properties [56]. As the two graphene nanoribbons of different types (e.g.

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nontrivial and trivial) are intersected, the interface state of topology protection (the zero-energy state) will be formed. The graphene nanoribbon with armchair edge preserves the width-dependent semiconducting properties and that with zig-zag edge preserves the magnetic side [51]. The simulated result suggests that the edge state at the same side has the same spin states and preserves the ferromagnetic property, while the spin state of the other edge state is contrary to this edge, leading to the antiferromagnetic property in the whole ribbon [51, 57]. These works are very important to promote people understanding electronic topological states, discovering novel physical phenomena, developing novel types of electronic and optical devices, and deeply unveiling the properties of basic particles. While the developed large-scale fabrication processes provide sufficient available materials for these studies and their commercialization, they also make it possible to tune their physicochemical properties and device performances. Now, many new concepts on topology and related 2D materials have been developed, such as the concept of fermion-number fractionalization (topologically nonequivalent) and 2D materials of tungston carbon (WC), molybdenum phosphide (MoP), PtBi2 , transition-metal dichalcogenides (TMDs) besides the graphene-based materials [25, 45]. Particularly, the combination of these materials of different types to construct novel 2D materials with varieties of disordered structures has emerged recently, breaking the property, structure, and concept limitation of the conventional 2D materials and/or functional thin films [28]. J. Hone and coworkers from Columbia University summarized the progress of varieties of the disordered structures and their effects on the properties of these 2D materials, such as those types formed by graphene and TMDs and particularly those materials that can form vdWHs by coupling them with h-BN [28]. Currently, high-quality graphene thin films can be obtained by mechanical stripping or epitaxial growth, whose disordered structures are induced externally. Progress in their synthesis, sealing, and device fabrication techniques can improve their performance gradually, and then, varieties of novel phenomena can be studied accordingly. As for TMDs, the intrinsic disorder and the external disorder are usually correlated. Even though the electronic and photonic property of TMDs can be enhanced via reducing the external disorder, the key problem is to address the crystal quality for the realization of progress similar as the progress of graphene, or the first issue is still to obtain TMDs of high crystalline quality [28]. In addition, the useful electronic, photonic, chemical, and magnetic properties can be obtained by the atom defects or disorders in the 2D heterostructures. The origin of the self-similar energy spectrum of Gp/BN heterojunctions has resulted from their energy band change [9]. An elegant theoretical model for Gp/h-BN heterojunctions has been built up targeting the calculation of their band structures, which depends on several parameters that are difficult to be determined theoretically. However, the measurement of these parameters is also experimentally since the distance between the Dirac cone and the secondary Dirac cone, which needs high-quality samples and the ultraclean surface. Therefore, whether the surface of the sample can be cleared completely determines whether the band structure can be measured correctly and precisely. Obviously, a study on the band

20.3 Development of Highly Precise Fabrication Techniques for Thin Films and Coatings

structures of the heterostructures of 2D materials is doomed to be a hard bone since it is difficult to obtain high-quality samples and ultraclean surfaces [9]. Generally, atomic thick 2D materials have caused great attention due to their unique intrinsic structures, whose properties and performance are affected by the disorder of materials to a large extent, including the intrinsic disorder caused by their crystalline defects and the external disorder from the environment. Particularly, the 2D material systems are very sensitive to external disorders. The least disorder is the key to realize their expected features and to improve the performance of the related devices during the practical application. Therefore, as for high-quality 2D materials or thin films with ultraclean surfaces, comparing with those chemical process, the physical fabrication methods (e.g. e-beam Lithographie, Galvanoformung, Abformung (LIGA), semiconductors grown using molecular beam epitaxy, probe-assisted chemical vapor deposition) in spite of expensive equipment and high cost are still the main trend in the highly precise fabrication of high-Q thin films or 2D materials and the related tiny quantum devices at the atom by atom resolution, which cannot be replaced up to now [48, 58]. Based on the development of the controlled synthesis of varieties of 2D materials and nanostructured thin films, the large-scale fabrication of nanoelectronic devices or even single-atom transistors of atomic resolution is just on the way [42, 50, 56, 58, 59]. For example, based on the novel quantum 2D graphene chains, precise nanotransistors (atomic resolution) could be manufactured in the future – a fundamental step on the way to nanoelectronics [41, 45, 56]. Whether the switching distance between the “1” state and the “0” state of the nanotransistor is actually large enough, depending on the bandgap of the semiconductor, and with the new method, this can be set almost at will [41, 45, 50]. Thus, these advanced fabrication methods developed from both physical processes and chemical solutions offer us more opportunities and challenges in the flexible integration of multiple material systems from 2D to a higher dimension. Besides the magic-angle graphene [21, 22], people also expect other magic thin films based on the superlattice assemble and the formation of artificial heterostructures including the combination of 2D materials and nanostructure template films for unexpected physical and chemical property [35, 36, 39], just as the theoretical and experimental breakthrough via the Gp/h-BN heterostructures [9]. Moreover, the discovery of the amazing physical and chemical phenomena needs the intimate combination between the theoretical calculation and the experiment testing ignited by the key scientific questions in the related field. The significance of the experiment results related to the intrinsic principle and physical mechanism needs to be revealed and expanded by coupling the theoretical analysis and calculation. The discovery of the existence of triply degenerate point (TP) fermions in trigonal layered PtBi2 is a good example [25]. It has been known that TP fermions in tungsten–carbide-type materials (e.g. MoP) represent new topological states of quantum matter, which have generated immense interest recently. However, the TPs in these materials are found to be far below the Fermi level, leading to the TP fermions having less contribution to low-energy quasi-particle excitations. To address this issue, Gao et al. theoretically predict the existence of TP fermions with

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TP points close to the Fermi level in trigonal layered PtBi2 by ab initio calculations and then experimentally verify the predicted band topology by magneto-transport measurements under high magnetic fields up to 40 T [25]. And then, analyses of both the pronounced Shubnikov–de Haas and de Haas–van Alphen oscillations by the combination of the experiment results and the ab initio calculations revealed the existence of six principal Fermi pockets and the interplay between transport behaviors and unique electronic structures, finally confirming the indeed existence of TP fermions in the trigonal layered PtBi2 [25]. It was further found that layered PtBi2 had intrinsic advantage in the fabrication of devices since they could be cleaved much easily, which shall pertain great potential in the fabrication of tiny micro electromechanical systems (MEMs) and nano electromechanical systems (NEMs) and their performance control. The topological insulators are a kind of novel thin-film materials; while maintaining the internal insulation of the material, they also have a highly stable energy gap or interfacial status. The current research on them is mainly focusing on the 2D or 3D topological insulators and the related topological crystal insulators. It has been predicted theoretically that one-dimensional symmetrical protection topological phase should exist in the graphene nanoribbons. If limiting some edge structures of graphene laterally, graphene nanoribon (GNRs) can be transferred into semiconductors realizing exchange between the “0” state and the “1” state, which made them scaffolds of a great perspective. However, confirmation of this theoretical study is experimentally difficult since the preparation limitation in the control of the edge structure of GNRs. For resolving this issue, Louie, Crommie, and Fisher developed the topological engineering strategy (or a kind of on-surface synthesis method) to modify GNR superlattices, realizing control of edge structures of graphene laterally and experimentally obtaining this kind of unique electronic structure [42]. The on-surface bottom-up synthesis method can realize the precise control of almost all the structure parameters of GNRs at atomic resolution via the self-assemble and rational design of the small-molecule precursor, which makes it possible to explore the relationship between energy gap and width of GNRs by the doping element mediating changes in electron affinity. This topological engineering strategy can further realize the design of the new final state directly to the end of one-dimensional GNR superlattice. The atomic precise topological GNR superlattices were formed on the Au(111) plane from the molecule precursor under high vacuum via the on-surface synthesis process. The experimental analysis by coupling the first-principles calculation further revealed that the boundary band energy structure (full band and empty band) of GNR superlattices was determined totally by the coupling between adjacent topological interface states. The newly discovered extraordinary one-dimensional topological phase provided an electron topological alternative in the precise band control of one-dimensional materials and a perspective physical platform for the study of one-dimensional quantum spin. Researches by the combination of theoretical calculation and experimental testing preserve great significance by promoting us to deeply understand electronic topological states and the nature of basic particles and to discover extraordinary physical phenomena for developing the new electronic, photonic, and quantum devices.

20.4 Perspective in the Development of High-Spatiotemporal-Resolution Characterization Methods

Another amazing success in the combination of theoretical calculation and experimental testing is the development of perfect single-photon sources. There are two key scientific issues for a perfect single-photon source: one is to realize quantum dots randomly emitting two polarized photons, and the other is to realize resonance excitation by eliminating the background laser [60, 61]. To address these two key scientific issues needs the simultaneous innovation of theory and experiment. Theoretically, fabrication of elliptical microcavities was proposed to break the symmetry by J.-W. Pan’s and coworkers [60–62], which can split the cavities mode into two nondegenerate vertical polarization modes and then selectively enhance the number of single polarized single photons. Experimentally, the vertical polarization lossless extinction technology has been developed, and the single-photon sources of deterministic polarization, high purity, high homogeneity, and high efficiency were fabricated in narrow-band microcolumns and wide-band target microcavities using the microcrystalline glass thin films as substrates [61, 63, 64]. Once again, this progress has set a new international record for the comprehensive performance of single-photon sources, which has made an important step toward the realization of “quantum hegemony” [60, 61, 65, 66]. However, there are still many great challenging issues for these techniques commercialization, such as how to trap photons for the desired periods, how to store single-photon quantum states in collective atomic excitations, and how to miniaturize the related facilities and devices [43, 60–62]. Let us expect that they will be resolved in the near future by the combination of the theoretical calculation and experimental testing and the rapid development of the related highly precise fabrication techniques of thin films and coatings [42, 43, 50, 55, 57, 58, 60–62, 67].

20.4 Perspective in the Development of High-Spatiotemporal-Resolution Characterization Methods for Microstructures and Properties Developing high-spatiotemporal-resolution characterization methods is the key to study the relationship between microstructures and properties of thin films precisely for the discovery of the intrinsic physicochemical mechanism. In the past decades, great signs of progress have been achieved in electron micrography (particularly in transmission electron microscopy [TEM]) and electromagnetic spectroscopy (EMS) based on the interaction between electromagnetic (EM) waves and matter (e.g. X-ray diffraction, imaging and absorbance, angle-resolved photoelectron spectroscopy) and the related characterization equipment development (e.g. X-ray electron-free laser, femtosecond laser, single-photon detector, atomic force acoustic microscopy [AFAM]), which provide us an ultra-highly precise characterization of microstructures and components of thin films and coatings [9, 68–71]. The essential condition to explore the intrinsic physicochemical mechanism of 2D materials and nanostructured thin films is to obtain their microstructure images at the atomic resolution. Up to now, high-resolution TEM is still the main instrument

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for the characterization of microstructures of thin films at atomic resolution that is determined by the numerical number and the energy of the electron beam. As for 2D materials formed by one or several layers of atomic superlattices, the imaging characterization has to be operated at low energy in case of displacement damage in 2D materials, leading to the resolution at about 0.1 nm [68]. To increase the spatial resolution, Muller and coworkers recently develop a new reconstruction technique of layered diffraction image (ptychography), which can realize 0.04 nm resolution imaging at low accelerate voltage, breaking the spatial resolution [71]. This kind of imaging technique uses an atom-sized electron beam collecting the diffraction pattern of each position in the 2D scanning areas and then reconstructs the microstructure of samples by imaging algorithm. Using MoS2 as the 2D material model, single sulfur atom defects in MoS2 can be clearly observed in the reconstruction image [71]. This method will definitely produce revolutionary impact in the study of more complicated materials defects and 3D microstructure reconstruction techniques. In addition, similar to the genes in biology, most of the chemical and physical properties of solid-state materials can be traced back to their energy band structures that are the relationship between the energy (E) and momentum (k) of an electron (or E(k)). The vdWHs can be formed by stacking two different kinds of 2D materials layer by layer based on weak van der Waals forces, whose properties are also determined by their energy band structures. For example, the origin of the self-similar energy spectrum of Gp/BN heterojunctions has resulted from their energy band change [9]. An elegant theoretical model for Gp/h-BN heterojunctions has been built up targeting the calculation of their band structures. However, several parameters have to be used to describe this kind of heterostructure systems. The challenge is that these parameters cannot be determined theoretically, and different results will be obtained due to the selectivity of these parameters. Therefore, it’s even more urgent to obtain the results experimentally. However, it was previously difficult to detect the energy band modulation in vdWHs (e.g. Gp/BN) using previous instruments. Developing the advanced testing method and the related facilities for this goal is much needed. Progress of the angle-resolved photoelectron spectroscopy (ARPES) shows the distinct advantage for this goal: electrons can be excited out of heterojunctions via photoelectric effects, and their band structures can be determined directly by measuring the energy and the momentum of the photoelectrons. Thus, it will be clear at a glance to determine the energy gap or the secondary Dirac cone. Zhou and coworkers from Qinghua University in 2016 used this advanced technique to address this issue by directly detecting this phenomena [9]. Their study indicated that the secondary Dirac cone was induced by the periodic potential modulation of superlattices and the resulting energy gap was caused by the symmetry breaking in the spatial inversion [9]. Furthermore, the X-ray-based methods are edge tools for materials’ microstructure and property characterization. Besides many characterization methods based on X-ray diffraction and X-ray absorbance, people also want to invent highly brilliant and/or superfast (picosecond or femtosecond) X-ray lasers of superhigh coherence for the characterization of the interaction between photon and matter to discover their physical nature. Particularly, instruments based on the dual-wave

20.4 Perspective in the Development of High-Spatiotemporal-Resolution Characterization Methods

sum-frequency oscillation using X-ray laser and optical laser are much desired since they have been theoretically advanced as atomic resolution probes for the light–matter interaction study prediction more than half of one century before [72–74]. However, this technique was significantly limited by the X-ray source of high brilliance and superhigh coherence [73]. With the progress in the X-ray laser materials and facility for highly coherent and highly brilliant X-ray source (e.g. light sources for synchrotron radiation), X-ray free-electron lasers (XFELs) have been invented in 2012 after the effort of half a century by scientists and engineers, whose brilliance of X-ray sources can be increased about 10 billion times [73, 75]. In addition, many process characterizations of XFELs related to the basic physicochemical mechanism among light–matter interaction require lasers of high time resolution up to only a few femtoseconds (10–15 s) or even angstroms (10–18 s) besides enough pulse duration and brilliance. To generate these pulses and to apply them in time-resolved experiments, synchronization techniques that can simultaneously lock all independent components, including all accelerator modules and all external optical lasers, to better than the delivered free-electron laser pulse duration, are needed. For this goal, Schulz et al. invented all-optical synchrotron radiation femtosecond technology of soft laser flash (or soft XFELs) to produce femtosecond pulses for high temporary resolution in 2015 [76]. As shown in Figure 20.1a, a macro-pulse of electron bunches is generated in a normal conducting photo-injector. Superconducting modules can accelerate bunches up to 1.25 GeV. Each bunch is compressed at intermediate energies of 150–450 MeV in magnetic chicanes. The arrival times of the electron bunches are measured with respect to the master laser oscillator after each compression stage and final acceleration (measurement stations indicated by orange dots in the schematic). The arrival times are incorporated in the feedback control loops for the amplitude and phase of the accelerating fields. The stabilized relativistic electron bunches are used to generate the self-amplified spontaneous emission XFEL pulses. Experiments can be carried out in conjunction with an external optical laser, which is synchronized to the master laser oscillator. As shown in Figure 20.1b, an external laser is optically locked to the accelerator reference clock signal and used to generate a single-cycle THz pulse by optical rectification in lithium niobate. The resulting picosecond THz pulse and the XFEL pulse are then overlapped in a neon gas jet, where the XFEL pulse profile and relative arrival time of the pulse with respect to the THz field are measured by streaking spectroscopy. As the THz pulse is phase-locked to the external laser, the arrival time with respect to the THz pulse is equivalent to the arrival time with respect to the external laser. Thus, the duration of the ultra-highly coherent and brilliant pulsed laser with wavelengths ranging from nanometer to sub-angstrom can be realized from a few femtoseconds to 100 fs, which provides a new method to develop external seed light sources for angstroms pulsed XFELs of arbitrary energy [76]. Currently, highly coherent and brilliant lasers have been realized from the extreme ultraviolet (EUV) to infrared (IR) wavelength, and the harmonic oscillation (sum-frequency generation, SFG) can be realized between these wavelengths and X-ray [76]. These XFEL-based facilities provide the atom resolution ability in

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20 Perspectives for Thin Films and Coatings ~300 m Photoinjector

Compressor

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SASE undulators experiment Photon beam

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Figure 20.1 (a) The setup of flash FEL facility, (b) the principle of relative timing between FEL and external laser. Source: Schulz et al. [76]. Licenced under CC BY 4.0.

the study of the light–matter interaction and shall preserve more perspective in fundamental studies and their application. For instance, the harmonic oscillation in diamonds caused by the coupling between X-ray and near IR (801.5 nm) confirmed that the nonlinear phenomena were originally from the light-induced charge and the corresponding reciprocal space of micro-field in the luminescent material, which also verified the correctness of the first-principles simulation [73]. In addition, the valence change can be directly measured using X-ray/EUV SFG at atom resolution and femtosecond for the study of the thermodynamic features of varieties of physicochemical processes [73]. Generally, XFELs realized the measurement of microstructure and kinetic features of atom nucleus and molecule systems at angstrom spatial resolution and femtosecond temporary resolution for the first time, providing the edge tools in the fields of physics, chemistry, biology, materials science, and high energy physics. With intensities 108 –1010 times greater than other laboratory sources and femtosecond resolution, X-ray free-electron lasers and the flash FEL facilities are currently opening up new frontiers across many areas of science [77, 78], such as X-ray quantum optics [79], X-ray nuclear optics [80], the precise characterization of atomic structures and properties of each layer of thin [78], and the in situ characterization of the transition state of catalytic reaction [81]. Finally, as one sophisticated technology for detecting ultra-weak optical signals, techniques for single-photon detection have been widely used in astronomical ranging, satellite/airborne remote detection, high-resolution spectral measurement, optical time domain reflection [82], bioluminescence analysis and other precision analysis [82], and quantum information communication [63, 65, 83, 84]. The single-photon field analysis founded the physical implementation for quantum information and quantum computation. Particularly for the quantum key system, utilization of single photons can ensure the absolute security of information

20.4 Perspective in the Development of High-Spatiotemporal-Resolution Characterization Methods

transmission [64, 85, 86]. Quantum cryptography is one of the most important components of quantum information technology. Based on the quantum no-cloning theorem and uncertainty principle, quantum cryptography provides an unconditional and secure way of communication. Single-photon detectors for the detection of single photons at the receiving end of the transmission channel are the most important key devices in the quantum communication system because single photons are the information carriers [63, 83, 86]. Clearly, single-photon sources of high-Q index are the most important core resources for the future photon quantum information technology [60, 61, 83]. However, it is an extreme challenge to obtain the ideal single-photon sources for application in the experimental system because perfect single-photon sources need to simultaneously satisfy four almost contradictory and harsh conditions: deterministic polarization, high purity, high homogeneity, and high efficiency [61, 63, 64]. Since 2000, universities in California, USA, have gradually made some progress in single-photon source research, but their quality cannot meet the practical needs [60, 61, 64]. It was widely used as a single-photon source that the average photon number is far less than 1 previously. The photon number still obeyed Poisson distribution, leading to multiphoton probability and thus leaving the eavesdropper a chance, which limited the range of safe transmission distance between two sides of communication [84, 85]. Therefore, the detection technology with high photon number resolution is of great significance, which is also based on the high-Q single-photon sources [60, 61, 63, 84]. Moreover, as transmitting quantum information based on single-photon phase coded key system, high-precision Fabry–Perot (FP) cavities are essential for high phase sensitivity for filtering information. But the FP cavities are too easily affected by external environment. Clearly, two extremely challenging issues have to be addressed. One is to develop an extremely precise single-photon detector to distinguish the number of photons, and the other is to fabricate high stable FP cavities and realize high-precision FP locking based on single-photon wavelength modulation [60, 61, 63, 84]. Generally, there are three important breakthroughs addressing these two extremely challenging issues: the subversive thinking, the related theory development, and the fabrication of the related devices. After the pulse resonance excitation of quantum dots were developed for the first time in the world in 2013 by J.-W. Pan and coworkers [62], they gradually overcome the other two key techniques of the above four contradictory and harsh conditions for the perfect single-photon sources. One is to realize quantum dots randomly emitting two polarized photons, and the other is to realize resonance excitation by eliminating the background laser [60, 61]. In 2019, collaborated with researchers from German and Demark, J.-W. Pan and coworkers from the University of Science and Technology of China, for the first time in the world, proposed a theoretical scheme to realize the deterministic polarization of single photons by coupling elliptical microcavities [60, 61]. The high-Q single sources with deterministic polarization, high purity, high homogeneity, and high efficiency were further realized successfully on two kinds of microcavities: narrowor wide-band. The successful realization of available high-Q single sources built up the solid theoretical and experimental foundation for the realization of “quantum

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hegemony” in the rapid development of photon quantum calculation, which is far beyond the classical computing power [60, 61, 65, 66]. Up to now, the three essential breakthroughs were finally obtained to address the two extremely challenging issues. Firstly, the subversive thinking has to be creatively obtained as follows: (i) using a single-photon detector to measure the single-photon counting information modulated by an acousto-optic modulator, (ii) obtaining the threshold value of amplitude (e.g. 0.4 V) of the acousto-optic modulator working near the linear bias point by the change of photon number counting, (iii) realizing the linear relationship between intensity modulation and modulation signal under the threshold, and (iv) assuring that single-photon modulation works in the nonlinear region while working voltage is higher than the threshold value. Secondly, there was a great breakthrough in the related theory to resolve the theoretical feasibility for subversive thinking. Transient response characteristics of a single-photon detector to the weak laser pulse were theoretically analyzed based on the response probability of the detector caused by the arrival time of the first photon and the number of different photons. The number of photons was precisely determined according to the different response time of the different photon number. Experimentally, the response time of a single photon, two photons, and triple photons can be measured precisely, which realized the accurate discrimination of photon numbers. And then, the effects of the detection time of the weak laser pulse on the measurement of the response time of the photon number were further analyzed. Third, there was a creative invention in the related fabrication method. The high-precision FP cavities (arrayed thin films) were constructed using the microcrystalline glass with a low thermal expansion coefficient as the cavity construction materials, which had free spectrum range of 750 MHz, fineness of 1173, and line width of 0.64 MHz. The frequency discrimination curve of the transmitted signal was obtained using single-photon wavelength modulation technology, which could lock the high-precision FP cavity on its peak value of the single-photon transmission. After locking, the jitter range of the transmission peak frequency of the FP cavity could be compressed to 0.25 MHz. Therefore, for the first time, the four demanding criteria (deterministic polarization, high purity, high homogeneity, and high efficiency) were simultaneously met experimentally for the perfect single-photon source based on their advanced theoretical design by the coupling of the coherent dichroic excitation [61] and the elliptical microcavity [60], which has been explored for 20 years in the world. These signs of progress showed groundbreaking research reporting a milestone achievement toward this ideality goal with the realization of a single-photon source. Moreover, based on the developed single-photon source with the highest efficiency and quality and multichannel optical interferometer with the largest scale and highest transmittance up to now, collaborating with Lixing You group from Shanghai Institute of microsystems and information technology, Chinese Academy of Sciences, J.-W. Pan and coworkers also made great progress in the quantum computing experiments by building up a Boson sampling platform via the coupling of the superconductive nanowires and highly efficient single-photon detector [65]. They developed solid-state sources of highly efficient, pure, and indistinguishable single photons and 3D integration of ultralow-loss optical circuits and realized

20.5 Perspectives in the Further Study of Relationship Between Microstructure and Property

experiments with 20 pure single photons fed into a 60-mode interferometer. In the output, up to 14 photons and samples over Hilbert spaces with a size up to 3.7 × 1014 can be detected, over 10 orders of magnitude larger than all previous experiments, which for the first time enters into a genuine sampling regime [65]. The results are validated against distinguishable samplers and uniform samplers with a confidence level of 99.9%. These signs of progress are indeed impressive technological achievements evaluated by Marric Stephens (a freelance science writer based in Bristol, United Kingdom), which is not only a powerful test of photonic strength but also a trampoline for the upcoming quantum computational supremacy. A summary of this work on the website of American Physical Society physics pinpointed that this work signifies a milestone in the field of quantum computation: approaching the point where a classical system cannot feasibly mimic a quantum system (https://physics.aps.org/articles/v12/s146). Recently, based on the single-photon sources and the related quantum technologies, they have further made marvelous achievements in the satellite-based quantum-secure time transfer [83] and entanglement of two quantum memories via fiber over dozes of kilometers [84]. However, it is clear that all these instruments and devices related to the quantum calculation and transmission are extremely related to the current optical and electronic devices and facilities. We are expecting more progress achieved in the quantum computing prototype and the commercialization of quantum communication in the near future with the great and rapid development of the functional thin-film-based quantum devices and facilities, just as the development of the large-scale integrated circuit.

20.5 Perspectives in the Further Study of Relationship Between Microstructure and Property The novel finding, invention and creation, and upgraded application exploration undoubtedly depend on the further deepening study of the relationship among their structures, properties, and application performance with the great progress in the subversive thinking and the related creative theory and fabrication methods in the field of thin films and coatings. There are several good examples for us to consider in the study of the relationship between microstructure and property of thin films and coatings. Firstly, new materials have to be discovered for the thin films and coatings during these innovations, whose structure–property relationship and potential application have to be further investigated deeply based on the engaged theory or the bold theoretical prediction [20, 21, 26, 49, 87]. Therefore, development of new materials for thin films and coatings itself can experimentally confirm the proposed theory (e.g. WSMs or Weyl fermion) and applications or favor to advancing the novel theory [8]. New physical properties of the related particles in these materials, such as electrons (requiring antisymmetric wave functions that are called fermions) and varieties of excitons (e.g. surface plasmons), should be further studied for novel electronic/magnetic/photonic properties, new physical phenomena, and expanded

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application [8, 20, 21, 26, 46, 49, 87–90]. They may promote the realization of the invention of the dreaming devices (e.g. spintronics,) and their assembly to highly efficient systems (e.g. quantum computer, quantum communication). Furthermore, the related surface phenomena and interface effects should be studied more fundamentally. The surface state is a very important concept in the topological insulators. As the band gas is open in the bulk band, the gapless well-defined surface states can be found in the bandgap, connecting the bulk states of the gap ends. As the fermion surface is in the bandgap, the physical characteristic of the system is determined by the surface state. However, as for WSMs, their bulk states are also gapless. Therefore, the bulk band and the surface state will be cut simultaneously whatever the fermion surface is regulated. As the Dirac point is cut, the state density of the bulk band is zero. However, in the real state, the thermal fluctuation and the quantum fluctuation will become distinct near the Dirac point, and the contribution of the bulk band cannot be neglected completely. Thus, we cannot define the surface states of WSMs. The discovery of nontrivial state in topographic insulators via the study of their surface and edge-dependent bandgaps is a good instance in information technology [9, 42, 91, 92]. WSMs have the topological nontrivial energy band structure, which is very important in the basic physical property research. At the same time, their linear dispersion relation entails them a kind of relativistic electronic systems, called “three-dimensional graphene,” which have great potential application in the electronic devices. The low energy excitation in the WSMs can be treated as Weyl fermion, which is one important member of the fermion family (Dirac fermions, Weyl fermions, and Majorana fermions). If two topographic insulators are topologically nonequivalent, their interface areas can produce local conductive states [93]. Therefore, their surfaces or edges can be conductive even though they have similar bulk bandgaps as other conventional insulators. These surfaces and edges sometime can have singular quantum states, which can be used in spintronics or quantum computing. Although WSMs are gapless, they still preserve topological nontrivial behavior. They are in a kind of novel quantum states, whose electron dispersion relation defines pairs of chiral opposite Weyl points [8]. These pairs of Weyl points can be treated as a pair of positive and negative magnetic charges (or magnetic monopole) in the reciprocal space. These magnetic charges can produce the energy splitting under the magnetic field, similar to the Zeeman effects, leading to the different density of electrons in different chiral states [8, 94, 95]. The topological semimetals preserve curious magnetic transport properties, such as chiral negative magnetoresistance, giant magnetoresistance, and the ultrahigh carrier mobility. They have great value in the future low-energy ultrafast electronic devices, which therefore have become one of the frontiers and the key hot research. As the external electric field is parallel to the magnetic field, the scattering of the different chiral states in the reciprocal space will reduce the corresponding intervalley scattering due to the increased distance between the Weyl points of the different chiral states, which will lead to the negative magnetoresistance, or the chiral anomaly [5, 8, 96]. These study also leads to the discovery of a new effect in the 2D materials, or the Alder, Bell, and Jackiw (ABJ) effect that was firstly proposed by Adler, Bell, and Jackiw in the particle physics

20.5 Perspectives in the Further Study of Relationship Between Microstructure and Property

theory [97], suggesting a pair of the pulled Dirac cones of different chiral states in the reciprocal space under the magnetic field [98]. Secondly, highly precise fabrication methods for thin films and coatings have to be developed preferentially since they are the materials fundamental for the deep study of the relationship between structure and property, particularly for the combined creative study by coupling the theoretical calculation and experiment testing for novel findings, phenomena, and applications by overcoming the key but much challenging scientific issues [20, 21, 26, 49]. We have discussed this point in the theory and device development for the quantum calculation and communication [60, 61] and the novel phenomena and physical property of 2D materials [25]. In addition, the current trend in the development of single magnetic atoms and molecules is also promising since they are the final solid candidates of miniaturized devices/sensors for the next generation of information technology [43]. We also hope that many new materials of amazing properties appear in the near future devices with the progress of highly precise fabrication methods for novel functional thin films and coatings, such as thin films or 2D materials of high-temperature superconductors, ultrasensitive magnetic/optical detectors, or ultrasensitive ferroelectric switchers desired by people for long times [22, 26, 46, 87, 99]. In addition, only the successful development of large-scale economical fabrication approaches (e.g. the solution-processable synthesis [49], the template transfer nanoimprinting method [46, 100], and the femtosecond laser micro/nanomanufacturing process [67]) can realize their commercialization possibility by overcoming the operation energy cost and spatial limits of memory storage, spintronics, quantum computing and qubits, and quantum-secure communication during the post-Moore era [43, 60, 61, 63, 65, 83, 84]. Thirdly, new precise characterization methods for microstructures and properties of thin films and coatings have to be developed for the deep study of the relationship between microstructure and property. They are the instrument and equipment foundation to explore new thin films and coatings and improve their performance by regulating and controlling their microstructures, components, and properties. In the following, we use the development of the facilities and instruments for light–matter interaction study as one example to elucidate its significance. The interaction between light and matter is universal, and its study has been promoting the development of a wide range of basic research fields (e.g. the discovery of magneto-optical effect, opto-acoustic effect, magnetoplasmon and quantum entangle, and their physical mechanism study) and application technologies (e.g. information technology, new energy and catalysis, sensing and imaging technology, LIGO) since the discovery of the photoelectric effect. However, many studies of the light–matter interaction and the related electronic and photon phenomena are usually based on the theoretical calculation and deduction due to the lack of a high-resolution direct experimental method. Lots of important phenomena and effects cannot be directly measured, making it difficult to verify the proposed theory and promote the upgrading of theory, particularly in the X-ray range [72–74]. The light-interaction study was often restricted by the availability of super-brilliant coherence and superfast lasers, particularly the X-ray lasers, infrared lasers, dark blue lasers, and extreme ultraviolet lasers and the related high-time-resolution laser

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(e.g. picosecond lasers or femtosecond lasers) [73, 77, 80, 81]. The highly brilliant coherent light sources are the key facilities for their instrumentation. For example, the two-wave SFG by X-ray and optical wave mixing can be used as an atomic-scale probe of optical interaction to detect the component and microstructure of materials and the energy band of the related excitons under light radiation simultaneously for light–matter interaction [73, 74, 77]. Just owing to a lack of sufficiently intense X-ray sources, the two wave SFG of X-ray and optical wave did not realize until 2012 as the XFELs were invented with the progress of laser materials and facilities of synchrotron radiation sources even though it has been theoretically advanced half-century before [73, 74, 77, 81]. The intensity of highly coherent X-ray sources can be increased about 1011 times higher than the conventional X-ray source, and the duration time can be maintained from several femtoseconds to 100 fs with the wavelength range from nanometer to sub-angstrom [73, 76]. The principle schemes for the XFELs and the flash FEL facilities can refer to Figure 20.1 [76]. XFELs provide a vast perspective to investigate the light–matter interaction at atom scale resolution for the development of a wide range of basic research fields and application technologies by assembling varieties of SFG facilities. Figure 20.2a shows one typical experimental layout for the generation of X-ray (8-keV) and optical (1.55-eV) SFG in diamond, and some results are shown in Figure 20.2b–e. Scattered X-rays are wavelength resolved using a Si(220) crystal. The combination of sample detuning and wavelength discrimination reduced the intensity of elastically scattered (background) light reaching our detector by 7–8 orders of magnitude relative to optimum elastic diffraction conditions. An SFG signal was detected at the expected sample and analyzer angles. The signal was observed depending on the simultaneous presence of X-ray and optical beams, and a signal rate of ∼4000 detected photons per second could be obtained on optimization. An energy analyzer scan (Figure 20.2b) confirms the detected photon energy equal to the sum of the input X-ray and optical photon energies and indicates the rocking-curve width (17 ± 2 μrad) in excellent agreement with the intrinsic analyzer rocking-curve width (∼17 μrad). The variation of SFG power with the sample angle can be measured (Figure 20.2c) as the analyzer is fixed in position. The signal is sharply peaked at the angle satisfying energy and momentum conservation, and the measured rocking-curve width (8.1 ± 0.7 μrad) is equal to half of the measured analyzer width (17 ± 2 μrad). A sample rotation of 𝛿𝜃 causes a 2𝛿𝜃 rotation of the output beam, which indicates that the sample rocking-curve width is set by the limited energy and angle acceptance of the analyzer. This observation supports the conclusion obtained from consideration of the analyzer rocking curve (Figure 20.2b): the SFG beam is narrow in energy and angle compared with the analyzer acceptance. These observations place a lower limit of ∼8 μrad on the SFG rocking-curve width because the SFG intensity is constant over an angular range (∼8 μrad) limited by the finite analyzer acceptance. Figure 20.2d gives one of the SFG signal (measured, blue; expected variation, black) versus angle of optical polarization vector (eo ) in diamonds, showing that how the SFG signal varies with rotation of the optical polarization vector. The SFG signal exhibits the expected

20.5 Perspectives in the Further Study of Relationship Between Microstructure and Property

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Figure 20.2 (a) Experimental layout for the generation of X-ray (8-keV) and optical (1.55-eV) SFG in diamond. a.u., arbitrary units. (b) SFG signal versus energy analyzer angle. Inset: Energy relative to 8 keV. The black line denotes a Gaussian fit (310 ± 35 meV, 17 ± 2-μrad full-width at half-maximum). The SFG energy exceeds the X-ray energy by one optical photon. (c) SFG signal versus diamond-sample angle: average of three data scans (width, ∼8 μrad; black line) and individual scans (markers). Energy and momentum conservation is satisfied at 𝜃 − 𝜃 Bragg ≈ 70 μrad. Inset: Sample rocking curve for elastic diffraction (width, ∼65 μrad). (d) SFG signal (measured, blue; expected variation, black) versus angle of optical polarization vector (𝜀o ). The signal is maximum when 𝜀o is in the diffraction plane (0∘ ) and is zero when 𝜀o is normal to the diffraction plane (±90∘ ). (e) SFG signal versus X-ray/optical time delay (raw data, blue markers; one-point-smoothed data, blue curve). Black cross-correlation curve (2.5-ps full-width at half-maximum): solution of the wave equation for an 80-fs X-ray pulse and a 1.7-ps optical pulse. Inset: SFG signal versus optical intensity. The red line is a fit to a linear dependence on optical intensity. Source: Glover et al. [73]. © 2012, Springer Nature.

dependence: it is maximized when 𝜀o is in the scattering plane, and it is zero when 𝜀o is normal to the scattering plane. Finally, an X-ray/optical cross-correlation is demonstrated. Within errors of ∼50%, the SFG signal is observed to vary linearly with optical intensity (Figure 20.2e, inset). The measured variation in the SFG signal with X-ray/optical relative time delay indicates a cross-correlation width of ∼2.5 ps (Figure 20.2e). By solving the wave equation, the SFG intensity is calculated as a function of relative time delay to determine that the measured cross-correlation is reproduced by a simulation with an 80-fs X-ray pulse and a 1.7-ps optical pulse, the second of which is consistent with the experimental optical pulse duration (2 ± 0.5 ps). The simulated cross-correlation is further shown in Figure 20.2e. The

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cross-correlation width is set by two factors: the duration of the (long) optical pulse and the fact that the X-ray and optical pulses move through the material at different speeds and in different directions. Results from this article suggest that the measured conversion efficiency (3 × 10–7 ) is consistent with first-principles calculations of atomic-scale optical polarization in diamond within experimental errors of 40% [73]. This measurement, in combination with ab initio simulations, supports the extension to atomic length scales of the long-held supposition that light selectively polarizes chemical bonds in covalent semiconductors [73]. Similarly, X-ray/optical SFG and the closely related X-ray/EUV SFG process create new opportunities to study dynamics by directly probing valence charge on atomic time and length scales, which has been used in the investigation of the thermodynamic features of physicochemical processes such as the transition state region in catalysis (e.g. the catalytic CO oxidation on Ru surface, the surface catalytic reaction mechanism on graphene thin films) by measuring the valence change and the transient state of chemicals at atomic resolution and femtosecond time resolution, which is called the “Holy Grail” in the catalysis [73, 78, 81]. Generally, X-ray wave mixing creates new opportunities for probing matter. X-ray/optical SFG provides access to two closely related and previously inaccessible quantities: the induced charges and excitons and the associated microscopic fields that arise when light illuminates a material. The ability to measure optically induced microscopic fields provides novel data for benchmarking condensed matter theories, such the direct measurement of the coupling factors of electrons and phonons in thin metal films and varieties of excitons in layered materials, which can be only obtained by the theoretical calculation previously [10, 73, 78, 81, 101]. Up to now, high-Q lasers have been available from X-ray to EUV, to optical wave, and to infrared wave. By coupling these lasers with the optical condenser and focusing technology, varieties of microscopes (e.g. dual photon microscope) have been developed for many basic research and application studies. Particularly, the imaging precise of X-ray computed tomography can be enhanced greatly as the high-Q X-ray lasers are coupled with focusing systems, which preserve much higher penetration depth than the optical wave and can obtain high-resolution images of surface morphology and 3D microstructures of thin films, especially the magnetization in the films [69]. With the progress in the facilities for light–matter interaction study, these new technologies shall be extended in the precise study of the intrinsic physical mechanism related to varieties of band energies of electrons and excitons in the frontier of 2D materials, such as topological insulators and topological semimetals, the related Dirac semimetals (DSMs) (e.g. graphene) [102], and the interesting phenomena of spin and momentum self-locking in the excited states of Dirac-like and fermion-like electrons in WSMs (TaAs, WTe2 ) [7]. Furthermore, some new amazing phenomena may be discovered and measured directly, such as the recently predicted Majorana bound state (a kind of unique electron status) that provides an experimental basis for the manipulation of quantum dots in future quantum communication [103]. Fourthly, new methods are expected to be developed for the significant enhancement of physicochemical properties and varieties of physical effects by the precise manipulation of electrons and their spin, the spin–orbital effect,

20.5 Perspectives in the Further Study of Relationship Between Microstructure and Property

photon, phonon, plasmon, varieties of excitons, and the related effects. The multi-physics-coupling-based methods are possibly much promising for future studies. For example, the Dirac-like and fermion-like electrons in WSMs will be much more complicated in the 3D structures of topological semimetal than in their 2D species. Particularly for those thin films formed by non-graphene WSMs (e.g. TaAs, WTe2 ) and DSMs (e.g. Na3 Bi, Cd3 As2 ), which have multicopies, the interface structures and the electron status of each layer have to be characterized together with the whole sample to develop the control method and to understand their intrinsic mechanism, such as the spin momentum locking and the self-locking of the spin–orbit torque control both in the surface and each layer and in the whole sample [8]. For this, the combination of the facilities for light–matter interaction with other fields (e.g. multiple laser fields, electric field, magnetic field, and thermal field) has to be performed to construct the testing equipment for limited quantitative characterization of multi-physics effects of each layer of thin films at atomic resolution [8, 104, 105]. It is one of the great challenges for the next generation of information technology using electric methods to control magnetic property, whose possibility has been explored in many thin films, such as ferromagnetic metal films, diluted magnetic semiconductors, multiferroic, and magnetoelectric (ME) materials. The recent discovery on electric-field switching of 2D van der Waals magnets opened another new platform to control the magnetic property via electric fields at the nanoscale [106]. For the first time, the magnetoelectric coupling effect completely controlled by spin order was discovered in the double-layer CrI3 with a crystal structure of center inversion symmetry, having a magnetoelectric coupling coefficient up to 110 ps/m, higher than most single-phase magnetoelectric coupling materials [107]. The magnetic properties of materials can be continuously and reversibly produced and reversed since a 30% saturation magnetic moment can be produced by a small voltage due to the large magnetoelectric coupling coefficient, which realized the possibility to control the magnetic property by the electric field in the 2D materials. Near the spin-flip transition of antiferromagnetism (AFM)–ferromagnetism (FM), reversible electric field switching of the magnetic sequence by field-effect devices was confirmed, which indicated the unique potential for 2D van der Walls magnetic thin films used in electronic nonvolatile memory, spin electron, and electronic devices [107]. Recently, Liao and coworkers built up equipment for controlling Fermi arc spin transport by electric field [104]. This equipment not only can detect the spin-polarized current in a single DSM semimetallic nanowire by separating surface electron and bulk electron but also can conveniently modulate the spin-polarized signal by the electric field. According to the mechanism, this phenomena was due to the special valence structure of electrons of DSMs, where energy can be used as a function of momentum to allow electrons to move in a solid or the DSM thin films [104]. Finally, the new application is expected with the new method (e.g. multi-physics coupling process) development and their achievements in the precise study of the structure–property relationship of thin films and coatings for the overcoming of the key issue. Room-temperature electrical control of exciton flux in a vdWH is a good example [108]. Devices that rely on the manipulation of exciton-bound

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pairs of electrons and holes hold great promise for realizing efficient interconnects between optical data transmission and electrical processing systems. Although exciton-based transistor actions have been demonstrated successfully in bulk semiconductor-based coupled quantum wells, the low temperature required for their operation limits their practical application. Recent discovery suggests that the spatial separation of electrons and holes in different layers in heterostructures may help to overcome this limitation and enable room-temperature operation of mesoscale devices because individual 2D materials have short exciton diffusion lengths. In July 2018, Andras Kis’s group from École Polytechnique Fédérale de Lausanne (EPFL) of Switzerland revealed that excitonic devices made of MoS2 –WSe2 vdWHs encapsulated in h-BN could electrically control transistor actions at room temperature [108]. The long-lived nature of the interlayer excitons in the device resulted in them diffusing over a distance of 5 mm. Within this device, the ability to manipulate exciton dynamics can be realized by creating electrically reconfigurable confining and repulsive potentials for the exciton flux. These results make a strong case for integrating 2D materials in future exciting devices to enable operation at room temperature, showing a great perspective for 2D materials used in devices controlling excitons via fields.

20.6 Perspectives of Thin Films and Coatings Promoting the Modern Technological Innovation and the Society Progress We have discussed lots of perspectives for the information technology as we analyzed the above perspectives in the subversive novel concepts, theory and experimental methods, the advanced fabrication methods, the development of the instruments and facilities, and the related precise study in the relationship between microstructures and properties. Besides information technology, applications of thin films and coatings almost span over all the basic academic research fields and industries closely related to the national economy, national defense, national military, and national living environment, as described in Chapter 1. The final goal of all the above achievements is for society to progress via promoting the modern technological innovation. These progress and innovation will no doubt bring new industries, new economic growth point, and high profit and then promote the accumulation and concentration of social wealth for great things. It is well known that energy, resource, environment, and public health are four issues facing people for sustainable and stable development. With the depletion of fossil-based energy and resource in Earth and the environment deterioration by the gradually discharge waste, people have to refer to find new sustainable and clean energy and source recycling and reduce the waste discharging into Earth. Technologies related to thin films and coatings are some of these promising technologies to address these problems. We still need some subversive thinking for their applications. Here are two interesting topics in new sources and new energy for us to consider.

20.6 Perspectives of Thin Films and Coatings Promoting the Modern Technological Innovation

Energy (kWh/m2-yr)

Figure 20.3 The semitransparent solar cells for the energy balance of the greenhouses via harvesting solar energy mainly from the wavelengths of light that are not used by plants for photosynthesis [29]. Source: Ravishankar et al. [29]. © 2020, Elsevier.

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Mixed-humid Climate

One is semitransparent organic solar cells (OSCs), which are related to the agricultural technologies that are the basis for us living on Earth. Semitransparent OSCs are particularly attractive given that their spectral absorption can be tuned to minimize the attenuation of sunlight over the plants’ photosynthetically active spectrum, which has great value to increase greenhouse-based agricultural land-use efficiency. The current greenhouses consume significantly more energy than conventional farming due in part to conditioning the greenhouse space. Integrating solar modules onto the greenhouse can mitigate the increase in energy consumption but requires semitransparent solar cells. Brendan T. O’Connor group advanced a detailed energy balance model by integrating semitransparent OSCs on the net energy demand of greenhouses, achieving net-zero energy [29]. As shown in Figure 20.3, these systems can have an annual surplus of energy in warm and moderate climates, and sunlight entering the greenhouse can be minimized with appropriate design [29]. Operation results of this kind of greenhouse demonstrate that semitransparent OSCs are excellent candidates for implementing in greenhouses and provide an opportunity to diversify sustainable energy generation technology. Many greenhouses could become energy neutral by using see-through solar panels to harvest energy – primarily from the wavelengths of light that plants don’t use for photosynthesis. In some places, this could make greenhouses energy neutral or even allow them to generate enough electricity to sell it back to the grid – creating a new revenue stream for growers. Another bold but interesting application study of thin films and coatings is based on direct electrolytic splitting of seawater by combination with the gradually matured renewable energy technology (e.g. technologies on solar cells, ocean energy, or wind energy). This shall be a much promising technology to obtain the potable dense hydrogen energy and fresh water directly from seawater simultaneously, whose process design and key technologies are shown in Figure 20.4a,b [109].

669

20 Perspectives for Thin Films and Coatings Energy consumption / (kWh Nm–3H O) 2

670

(a)

3370

3500 3000

Renewable energies

Photovoltaic

2500 2000

H2– storage

Electrolyzer

1500 1000

Fuel cell

Air / O2

Fresh water

Electricity

0

19.5

8.75

9.5

Local Reverse MSF fresh osmosis water

MED

MVC

3.25

Surplus energy Reverse Seawater H2 osmosis Electrolysis

Photovoltaic

Fuel cell

Reverse osmosis

Seawater

Seawater

500

0.2

City

Seawater

Freshwater Reverse seawater electrolysis

(b)

Renewable electricity H2

Seawater

Reverse osmosis

Other purification methods

Common electrolyzer PEM or alkaline O2

(c)

Figure 20.4 (a) Energy consumption of various desalination technologies and the local water treatment supplier per Nm3 . MSF, multistage flash distillation; MED, multieffect distillation; and MVC, mechanical vapor compression. The reversible seawater electrolysis is based on realistic conditions assuming 79% electrolyzer and 50% fuel cell efficiencies. Inset: Sketch for reversible seawater electrolysis based on renewable energy. (b) Process scheme of freshwater production based on seawater electrolysis. (c) In combination with a hydrogen fuel cell, a reversible seawater electrolysis scheme is possible that holds promise for the storage of surplus electricity in the form of molecular hydrogen, and more as a collateral process, purified water is formed during the fuel cell reaction. Source: Dresp et al. [109]. © 2019, American Chemical Society.

Hot, coastal, hyper-arid regions with intense solar irradiation and strong on- and offshore wind patterns are ideal locations for the production of renewable electricity using wind turbines or photovoltaics. Given ample access to seawater and scarce freshwater resources, such regions make the direct and selective electrolytic splitting of seawater into molecular hydrogen and oxygen a potentially attractive technology (Figure 20.4c). The key catalytic challenge consists of the competition between anodic chlorine chemistry and the oxygen evolution reaction (OER). Based on the energy efficiency of reversible electrolyzer–fuel cell schemes, the resulting freshwater production may remain limited with respect to more conventional freshwater technologies. Here, a new process concept for a sustained freshwater supply based on the combination of RO water purification and direct seawater electrolysis has been developed and suggested. Offshore wind parks and solar-energy-rich coastal desert regions would benefit the most from this combined process technology, where molecular hydrogen is transformed back into electricity and water. This perspective addresses some aspects related to direct seawater electrolyzers equipped with selective OER and hydrogen evolution reaction (HER) electrocatalysts. Starting from

List of Abbreviations

historical background to the most recent achievements, it will provide insights into the current state and future perspectives of the topic. This perspective also addresses the prospects of the combination of direct seawater electrolysis with hydrogen fuel cell technology (reversible seawater electrolysis) and discusses its suitability as combined energy conversion–freshwater production technology. The concepts and applications of thin films and coatings are extreme huge fields too broad to be summarized in one book. As these are used intimately in basic research and industrial applications, we included only some of the most important topics with great application prospects and/or worthy of further study. These two volumes offer commonplace remarks by way of introduction so that others can find their own interest and develop more about it. In fact, each topic itself can also be expended into one professional monograph. If readers are interested in any of them, they are welcomed to extend them. We hope this chapter can play a role as “casting a brick to attract jade.” Through investigation and research introduced in these topics, readers and/or researchers can initiate their interest in learning the fundamentals and starting their practical plans in this field with the help of this comprehensive study on subversive novel concepts, theoretic breakthroughs and new funding in surface/interface physics and chemistry, and large-scale economical fabrication techniques. We aim to update our book to satisfy the various requirements of readers from academics and industrials.

Acknowledgments This chapter is supported by the NSFC–BRICS STI Framework Program (No. 51861145309), the National Natural Science Foundation of China (No. 51971029), the National S&T Major Project (No. 2018ZX10301201), the “All English teaching demonstration course construction project of University of Science and Technology Beijing” (No. KC2015QYW06, 2016), the “1125” Zhihui Zhengzhou Talent project of Henan province (Fund No. in USTB: 39080070), the “100 talent plan” fund of Fujian province (Fund No. in USTB: 39080067), and the development of a high sensitive magneto-optical biomolecular sensor experimental prototype (Fund No. in USTB: 2019-0649) by Hangzhou Ruidi Biotechnology Co. Ltd.

List of Abbreviations 2D 2DLMs AFAM DFT DSMs EMS EPFL EUV

two-dimensional two-dimensional layered materials atomic force acoustic microscopy density functional theory Dirac semimetal electromagnetic spectroscopy École Polytechnique Fédérale de Lausanne extreme ultraviolet

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FDTD FP GNRs Gp/h-BN h-BN HER IR LIGO MBCVD ME MEMs MOCVD NEMs OER OSCs SFG TEM TMDs TP vdWHs WSMs XFELs

finite-difference time-domain Fabry–Perot graphene nanoribbons graphene/hexagonal boron nitride hexagonal boron nitride hydrogen evolution reaction infrared Laser Interferometer Gravitational-Wave Observatory molecular beam chemical vapor deposition magnetoelectric microelectronic mechanics metal–organic chemical vapor deposition nanoelectronic mechanics oxygen evolution reaction organic solar cells sum-frequency generation transmission electron microscope transition-metal dichalcogenides triply degenerate point van der Waals heterostructures Weyl semimetals X-ray free-electron lasers

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Index a AAO template-assisted nanoimprinting 359–376 abietic acid 520 abrasion-resistant antifouling coating 520–523 absorber-perpendicular TI structures 436 acid-producing bacteria 511 acoustic absorbance 551 acoustic diffraction and disturbance 551 acoustic intensity 79, 570 acoustic sensor 39, 268, 269 acoustic theory 77–79, 551 acoustic waves 39, 40, 79–81, 551–563, 572, 578 acoustooptic effect 82–83 acrylate monomers 524, 527 acrylic alkyl silane type ion exchange resin 527 acrylic mold-proof coatings 517 acrylic monomers 524 acrylic silicon, zinc, copper-based tin-free self-polishing antifouling coatings 524 active collision avoidance (MAA) sonar 555 active graphene–silicon hybrid diodes 438 active sonar system 553–555 Adaptive Cross Approximation (ACA) method 229, 232 adaptive integral method (AIM) 232

adhesive PDMS 423 advanced hostile sonar systems 565–573 aerobic iron-oxidizing bacteria (IOB) 513, 514, 540 aeronautical sonar buoy 563–565 Ag nanofibers 34, 419, 593 Ag nanoprisms 340–342, 348–350, 356 Ag NFs/SFCM/Pt NFs sandwich structure 419 Ag NW-based transparent conductive film 593 Ag NWs/PET films 593 Ag-graphite/PVDF nanocomposites 595 air dehydration 449 Air Independent Propulsion (AIP) systems 107, 286, 293–295, 387, 555, 575, 578–582 aircraft hunting technique 551, 571 AKULA 971 type Russian submarine 559 Al2 O3 -YSZ graded thermal barrier coating (GTBC) 623 Al2 O3 -YSZ graded transition zone 624 Alberich coatings 574 alkaline hydrolyzed polymethyl methacrylate 463 alternating field frequency effect 184–185 aluminum electrodes 203 aluminum plate 592 aluminum-ion batteries (AIBs) 21–23 amino acid 477, 478 2-amino-2-deoxy-β-D-glucose 496

Inorganic and Organic Thin Films: Fundamentals, Fabrication, and Applications, First Edition. Edited by Yujun Song. © 2021 WILEY-VCH GmbH. Published 2021 by WILEY-VCH GmbH.

682

Index

ammonia sensors 428 ammonium bifluoride 36 amorphous magnetic semiconductors 270––277 amorphous materials 264, 272, 279, 280, 285, 286, 476 AMR class submarine 558–560, 563 amylopectine 495 amylose 495, 502 anaerobic sulfate-reducing bacteria (SRB) 511, 513, 514, 540 AN/BQS-11-12 spherical array sonars 557 AN/BQS-13DNA class spherical array sonar system 557 anechoic coating 551 basic physics 573–574 sonar system and 582 structure and materials design for 574–582 angle of arrival (AOA) 572 angle-resolved photo-electronic spectroscopy (ARPES) 655–656 angular momentum (AM) 83, 222, 231, 233–239 anisotropic magnetoresistance (AMR) 95–96 anisotropic porous MWCNT/water-borne polyurethane (WPU) composites 596 annealing effect 355, 370 anodic aluminum oxide (AAO) Al substrate 382 cancer treatment 388–389 semiconductive nanowires thin films 371–376 structure model 360 surface enhanced Raman scattering 362–363 ultra-dense nanoparticle arrayed thin films 367–371 anodizing reaction 361 ANSYS Multiphysics 88–90 anti-biofouling function 480 anti-EpCAM antibody 392

anti-EpCAM binding cells 390 anti-mold coating acrylic mold-proof coatings 517 development trend 518 epoxy-based mold-proof coatings 517 polyurethane anti-mildew coatings 517 silicone 517 anti-submarine techniques 551, 571 antibiotics 476, 478–479 antiferromagnetic (AF) 97, 110, 135, 652 coupling 97 antifouling agents, main types and selection of 518–520 antifouling coatings 514 abrasion-resistant antifouling coating 520–523 development trend of 535–540 fouling-release coatings 531–535 main types and selection of antifouling agents 518–520 self-polishing antifouling coating 523–531 antiparallel alignment 59–60, 96–97, 106, 113 apiogalac-turonan 495 aquaporin biomimetic membrane 481 aqueous gold nanoparticles composite of nano-porous gold thin films and 367 aqueous phase Ag nanoparticles 351–356 aqueous spherical gold nanoparticles 398 preparation and characterization of 367 α-arabinofuranose 497 aromatic diamines 460 aromatic polyamide (PA) 452, 462–463 array computation 228 array-type PBES 35, 419 arrayed thin films 7, 367–371, 394, 660 artificial intelligence (AI) 411 high efficiency THz-wave modulators 438–439

Index

optical polymer based films 435–436 plastic liquid crystal display 436–438 thermal management for electronic equipment 439 artificially synthesized polymers 530 Arton film 436 as-prepared biomimetic nanofiltration membrane water flux 481 Astute class submarine 558 asymmetric polyimides 474 atmospheric plasma spraying (APS) method 617, 621, 625–628, 630, 636 atomic force acoustic microscopy (AFAM) 655 atomic force microscopy 129, 183 atomic layer deposited Al2 O3 501 atomic layer deposition (ALD) technique 112, 212, 490, 501 atomic thick 2D materials 653 atom-scale electronic mechanics (AEMs) 7, 8, 199–214 Auger recombination 19 auto antigen detection 100 autonomous underwater vehicle (AUV) 551, 571–572 auxiliary resin 523 axial suspension plasma sprayed TBCs 630 axial suspension plasma spraying technique 630 Ayax spherical sonar array 557

b bactericidal and antifouling coatings 515 bacteriorhodopsin (BR) 435 Ball Grid Array 207 Barkhausen’s jumps 162 batch wafer processing techniques 35 BaTiO3 433–434, 554 beer and wine demineralization 449 benzyl ether derivatives 498 betaine-type zwitterionic monomer 528 betaines 528–529

Bi-Te superlattices 311 bias magnetic field effect 110, 181, 185–186, 188, 190, 193 biaxially oriented polypropylene (BOPP) film 433, 501 bio-hybrid nanocomposite films 497 bio-integarted wearable sensors 422–425 bioacoustics 549 biodegradable polymers 489–491, 499 biomass derived polymers cellulose nanofibrils 492–494 chitosan 496–497 collagen 491–492 lignin 498–500 pectin 494–495 polysaccharides 490 protein based polymers 490 starch 495–496 xylan 497–498 biomass derived thin film coatings biobased multilayer coating 502 sol-gel phase inversion coatings 500–501 biomass plastics films 503–504 biomass waste 489 bionic echolocation 552 bioplastics 490, 503–504 biosensing configurations 386 biosensor film substrate 421–422 BiREIG 152 bis (carboxylic anhydride) 473 bismuth chalcogenides 434 bismuth titanate-based ceramics 554 bismuth-substituted iron garnet (BiIG) films 126 for biosensing 174–178 synthesis of 128 synthesis technology & conditions of 126–135, 137 Bloch theorem 228 blue shift 134, 143, 355–356 Bohr magnetic moment 83 Boltzmann constant 74, 83 Bombyx mori silkworms 419

683

684

Index

bond coat (MCrAlY) 624–625 boron nitride nanosheets (BNNs) 439 Bose–Einstein condensation 125 bottom-up methods 5, 651 bow column integrated (CHA) sonar 555 bow conformal array sonars 558 bow multi-function sonar systems 556 BQQ-6 sonar 569 BQQ10 sonar 557 BQR-23/25 slender line array sonar 562 BQR-23/25 thin line array 561 brackish water reverse osmosis (BWRO) 465–468 Bragg diffraction 82–83 Bragg gratings 81 Bragg mirrors (BM) 135–137, 139, 141–145 broadband quasi-perfect absorption 577 BSY-2 sonar 569 buffered oxide etch (BOE) 202 bulk dye film 435 bulk micromachining 209 buoy sonars 563, 571 1.4-butanediamine (BDA) 15

c calcia-magnesia–alumina-silicate (CMAS) 636 calcium-manganese-zirconiumgadolinium-gallium (CMZGG) 128 calcium-niobium-gallium (CNGG) 128 camouflage 420–421 camphor sulfonic acid (CSA) 313 capacitors 39–40, 210–211, 318, 432–434 ε-caprolactone 427, 530 carbodiimide 496 carbonaceous-based RAM 604–606 carbon-based fillers 595–599 carbon-based fossil fuels 439 carbon-based materials 595, 599 carbon black 26, 414, 416, 427, 589, 605 carbon nanofibers (CNFs) 490, 492–494, 501–502, 605 carbon nanotubes (CNTs) 457, 469, 595

polymer composites 315 reinforced plastic 596 water channel membrane 481 carboxylic acid 390, 464 catalyst nanoparticles 429 catastrophic failure temperature 286 cathode sputtering 589 cathodic protection 514 C(CF3 )2 linkages 474 cellophane 448, 501, 504 cellulose 448–449, 458, 496–499, 502–505, 530 cellulose acetate (CA) 449, 452, 456, 462–463, 468, 477, 504–505 cellulose based films 504–505 cellulose diacetate polymer 463 cellulose nanofibrils (CNF) films 490, 492–494, 501–502, 605 cellulose triacetate (CTA) 456, 463 cellulose xanthate 505 ceramic membrane 452, 470 ceramic–matrix composite (CMC) 635 cereal alcohol 520 CFTBOx thin films 274 chalcogenides 3, 11, 103, 211, 257, 258, 261, 265–268, 270, 277, 280, 434, 652 ChamPro technology 626 characteristic basis function (CBF) method 231 charge-coupled device (CCD) 10, 146, 346, 390 charging process 433 chemical etching 201, 202, 539 chemical modification method 456 chemical plating 589 chemical sensor 210, 269 chemical vapor deposition (CVD) 3, 24, 109, 202, 203, 211, 258, 260–261, 311, 321–323, 454, 596, 599, 621, 650, 653 chip design 206, 412 chip fabrication technology 412 Chip Scale Package 207 chiral negative magnetoresistance 662

Index

chiral opposite Weyl points 662 chitosan 490, 496–497, 501–503, 530 chlor-alkali process production 449 chlorinated rubber 521 circulating tumor cells (CTCs) 388–393, 404 cis-rich poly(1-trimethylsilyl-1-propyne) (PTMSP) 475–476 Class V-attack nuclear submarine 556 Clean & Clear® 422 closed-form relation 64, 225 CNF/epoxy composites 605 CNT-multilayered graphene edge plane (MLGEP) core–shell hybrid foam 599 CNTs/ZnO whisker composites 604 cobalt oxide thin films 311–312 CO2 capture and storage (CCS) 472 CO2 emissions 472, 502 CO2 reduction technologies 472 CO2 removal 449 CO2 separation high permeability polyimides 473–474 highly permeable substituted polyacetylenes 475 polymers of intrinsic microporosity (PIM-1) 475–476 thermally rearranged (TR) polymers 474 co-polarization 228 coefficient of thermal expansion (CTE) 286–287, 418, 624 coherent dichroic excitation 660 coil-type inducer 180 collagen 490–492 collagen fibrils 491–492 colloidal surfactants 33 colloids from crystalloids 448 colossal magnetoresistance (CMR) effect 59, 95, 102, 103 applications 105–106 multi-layered thin films fabrication 103–105 columnar structured TBCs 626

columnar-structured coatings 626, 630 common bow-shape sonar arrays 556 common modified silicone resins 532 communication sonar 563 complementary metal-oxide-semiconductor (CMOS) 2, 100, 201 fabrication process of 203 complementary split ring resonators (CSRRs) 235 composite honeycomb sandwich metasurface panel 579–580 composite membrane 28–30, 34, 37, 419, 440, 452, 456, 462, 480 composite shielding materials carbon-based materials 599 polymer-based composites 592–593 COMSOL Multiphysics 88–90, 191, 648 concentration polarization factor (CPF) 465–467 conducting polymers (CP) 314, 415–416, 418, 423, 428 conductive coatings 421, 589, 592 conductive coating shielding material 592 conductive filler 416, 423, 592–593 conductive polymers 12, 415, 589, 592 conformal array sonar 558–559 conjugated microporous polymer (CMP) 30 conjugated polymers 426–427, 431–432, 438 controlled breakdown (CBD) 36 cooperative localization 571–572 copper acrylate self-polishing resin 524, 526 copper naphthenate 518, 523 copper plate 592 copper pyrithione 523 copper-based tin-free self-polishing antifouling paint 524–528 copper-containing inorganic compounds 518 copper-free self-polishing antifouling paint 524, 529–530

685

686

Index

copper/large flake size graphene (Cu/LG) composite film 599 core-shell nanocomposites 440 covalent organic framework materials (COFs) 479 CPEN/F-BaTiO3 dielectric films 433 Cross Domain Maritime Surveillance and Targeting (CDMaST) 572–573 cross-linked polymer-based films 433–434 cross-polarization 228 crystal plates of gadolinium-gallium (GGG) 128–129, 131, 140, 146–147, 149, 151–154 Cu based thin films 312–313 Cu NWs/polystyrene (PS) nanocomposites 594 cuprous oxide antifouling agent 515, 521 cuprous thiocyanate 518, 523 Curie temperature 127, 132, 135, 270–271, 273–274, 277, 279, 603 cyclohexanone 523 cylindrical sonar arrays 556

d Dallenbach layer materials 602 dark blue lasers 663 dark field optical microscopy and spectroscopy (DFOMS) 345, 346, 348 DC magnetron sputtering technology 436 Dcoit (4,5-dichloro-2-n-octyl-4-isothiazoline-3-ketones) 529 Debye theory 633 decontamination 449 deep reactive ion etching (DRIE) 201 defective welds 180, 188–190 degradable biomass-derived functionalized films biomass plastics films 503–504 cellulose based films 504–505 polysaccharide-based films 502–503 protein-, pig skin gelatin-, lipid-based films 503

degradable self-polishing antifouling coatings 530 deionized water 110, 361, 364, 366, 367 demagnetization 71, 83, 162 dendritic poly(glycerols) 539 density functional theory (DFT) 31, 647, 650 Deployable Autonomous Distributed System 572 desalination 28–30, 37, 361, 430, 454, 457–458, 461–469, 479, 481, 670 D-galactopyranosyluronic acid (GalpA) 494 DI water dissociation system 416–417 diamine 287, 464, 473 dielectric storage capacitors 432 differential scanning calorimetry (DSC) method 286, 288, 289, 296 diluted magnetic semiconductors 259, 270, 271, 667 3,30-dimethylnaphthidine (DMN) 474 dimethylsulfoxide (DMSO)-mixed PEDOT 314 dinitrotoluene (DNT) 427 dipole resonance 354 Dirac fermions 75, 649, 662 Dirac point 649, 662 Dirac semimetals 2, 106, 666, 667 directional-solidification (DS) metallurgy 618 discrete dipole approximation (DDA) 346–351 dissolved antifouling paint 520–521 dissolving pulp 504 Distributed Agile Submarine Hunting (DASH) 272, 346, 347, 572, 574 diterpene carboxamide compounds 536 diuron 523 DNA detection 100 DNA sequencing 35–37, 392 Doctor blading and doping fabrication technique 12 doped photocatalysts 429 Doppler effect 551–573 Doppler radar 553

Index

Doppler, Christian Johann 552 double layered GZ/YSZ TBCs 636 double-layer graphene 29 doubly periodic diffraction grating 574 drop-coating 340–341, 357, 598 “drop-in” plastics 490 dry etching 202, 210 dry-wet method 456 dual band scatterer 227 dual photon microscope 666 dual-wave sum frequency oscillation 656–657 β-D-xylosyl (Xylp) 495 dynamic domain structure (DDS) 179, 186–187

e EA films 179, 182–184 physical properties of 184, 188 EA sensor 182–183, 185, 186, 188 E-beam lithography 2, 201 EC magnetic fields 180, 183 samples defects 190–193 echolocation 550–573 echolocation theory 550 Econea (2-(p-chlorophenyl))-3-cyano-4bromo-5-trifluoromethyl-pyrrole) 529 ecosystem recycling 504 eddy currents 80, 100, 113, 126, 178–193 efficient electron transport layer-free perovskite solar cells 18 electric and gas welding 180, 190 electric double layer (EDL) 40, 278 electric permittivity 71, 590 electrical conductivity 11–12, 24, 80, 210, 299, 301, 304, 306, 313–316, 321, 324, 326, 428, 432, 590–592, 594–596, 599, 633 electro-acoustic materials 554 electrochemical corrosion-resistance 203, 516 electrochemical deposition (ECD) 323 electrochemical reactions 24, 511

electrochemical reduction reaction 31 electrochemical/colorimetric sensors 415 electrode contact impedance 423 electrodialysis (ED) 447, 461 electroluminance OLEDs 286 electroluminescence effect 261–262 electrolytic antifouling technology 514 electromagnetic field 88, 90, 149, 204, 377, 386, 396, 402, 589–592, 599, 604 electromagnetic interference (EMI) 38, 587–608 electromagnetic spectroscopy (EMS) 655 electromagnetic spectrum 587–588 electromagnetic waves 38, 62, 65, 68, 95, 126, 175, 233–240, 586, 587, 589–592, 599, 601–604, 608 electron beam deposition method 97, 113 electron-beam evaporation 98, 136–137, 501, 616 electron-beam lithography 5, 146, 201 electron-beam physical vapor deposition (EB-PVD) 617, 621–627, 630, 635 electronic percolation 60–61 electrophysiological skin-integrated sensors 422 electroplating 200–201, 209, 589 electrostatic effects 350, 451 ellipsometer 286, 290, 295 elliptical microcavity 660 EMI shielding materials basic theory 589–591 composite shielding materials 592–599 conductive coating shielding material 592 ferromagnetic material 591–592 filled type 589 metallic good conductor material 591–592 stealth coatings 600

687

688

Index

EMI shielding materials (contd.) surface coating film shielding material 591 surface conductive type 589 endoscopic tooth-like tactile sensor 416 energy density 21–22, 24–25, 79, 81, 104, 433–434, 440 energy dispersive X-ray (EDX) 108, 152–153, 368–370, 374–375 energy storage capacitors 432 environmental hydrologic detection sonar (EHD) 555, 563 environmental pollution 401–402, 476, 489, 522, 569 environmentally friendly, solvent-free membranes 479 EP films sensory properties of 188 epitaxial ferrite garnet films (EPFG) 178–179, 190 epitaxial growth 3–5, 16, 650, 652 epithelial cell adhesion molecule (EpCAM) 390–392 epoxy-based mold-proof coatings 517 equivalent impedance model 65, 226 esters 498–499, 523–524, 527 ethylene vinyl alcohol copolymer (EVOH) 502 evaporative gel method 456 excitation coil 161 explosives detection 426–428 external hull acoustic deflectors 551 extracellular polymeric substances (EPS) 513–514

f Fabry–Perot (FP) fringes 135–136, 139–144, 381, 389–391, 393–394, 659 Fabry–Perot effect 404 Fabry–Perot interference (FP-IF) 8, 381, 390–391, 393–394, 405 Fabry–Perot one-dimensional magnetophotonic crystals (FP-1D-MPCs) 135, 136, 139–144

Faraday angle 72, 165 Faraday effect 69, 71–73, 125, 139, 145, 146, 149–151, 153, 164, 168, 179, 193 Faraday hysteresis loops (FHLs) 127, 129, 131, 132, 134, 139, 140 Faraday rotation (FR) 71, 73, 126–127, 129, 132, 134, 153–156, 164, 181 fast Fourier transform (FFT)-based method 172, 230, 232, 233, 390 fatty acid esters 498 F-BaTiO3 particles 433 Fe-filled CNT/epoxy composites 605 FeCo nanotubes 439 femtoliter 35, 36 femtosecond laser micro/nano manufacturing process 663 femtosecond lasers 655, 663, 664 Fermi energy 60, 106, 213, 308 Fermi’s rule 241 fermion-number fractionalization 648, 652 ferrite 125, 178, 190, 602–604 ferrite-garnet film 190 ferrite–graphene-based thin broadband radar wave absorber 603 ferromagnetic 97 material 61, 83, 326, 385, 388, 589, 591–592, 595 metals 61, 96, 112, 162, 175, 270, 273, 386, 667 ferromagnetism 268, 270–273, 277–280, 667 ferromagnets 60, 95, 96, 106, 113, 125, 270, 273, 276–278 ferulic acid residues 497 Fick equation 450 Fick’s 1st law of diffusion 87 Fick’s 2nd law of diffusion 87, 88 Fick’s law 87, 88 field emission transistors (FET) fabrication process of 203–205 field emission-scanning electron microscopy (FE-SEM) 374–375

Index

figure of merit (FM) 9, 126, 135, 139–140, 142, 299, 301–304, 306, 315–316, 323, 326–327 filled electromagnetic shielding composite materials 589 filled type EMI shielding materials 589 film capacitors 432–433 film form organisms 511 film preparation technology 412 finite element analysis (FEA) 15, 88–89 first principles theory 647, 650 fishery acoustics 549 fixed capacitors 433 fixed surveillance sonar buoy 563, 565 flash evaporation (FE) 311, 323, 324 flat membrane 452 flat monolithic asymmetric PA 463 flexible bioelectronics 418–419 flexible electronics 263–264, 419, 431, 650 flexible substrates or foldable OLEDs (FOLEDs) 8 Floquet theorem 228 Floquet theory 230 flow cytometry 389 fluorescence microscopy 389 fluorescent organic and inorganic conjugated polymers 427 fluorinated ethylene-propylene copolymers 534 flux-gate magnetometers 161–162 flux-gate magnetometry 164 flux-gate sensors 161 forward osmosis (FO) 452, 468 forward osmosis membrane 456–457, 469 fossil-fuel plastics 503 fouling biological control 513–516 fouling-related microorganisms 511 fouling-release coatings 531, 533 Fox equation 292–293, 295 FP-1D-MPCs 135–136, 139–144 free radical-triggered polymerization reaction 455 free-standing graphene 648

freshwater resources 31, 457, 670 frogman 549, 551, 570–571 fructooligosaccharides (FOS) 477 F4TCNQ 12, 13 fuel cells 10, 24–26, 89, 428–429, 479, 555, 650, 670–671 full-wave validation 228 full-width at half maximum (FWHM) 192, 346, 370, 665 fumed silica 523 functional membrane 453, 454, 456, 480 furan derivatives 536

g gadolinium–gallium garnet (GGG) 128–131, 136, 140, 146, 147, 149, 151–154 gadolinium zirconate (GZ) 636 galactooligosaccharides (GOS) 477 galfenol 555 galvanized steel plate 592 garnet film sensor 186, 188 gas diffusion electrodes (GDEs) 32 gas phase gel method 456 gate dielectric layer 264 gate-dependent pseudo-spin mixing 649 Gaussian beam 240 Gaussian ring basis function 231 Gaussian wave 237, 238, 240 GDE electrolyzers 32 gel mixed fouling release type antifouling paint 538–539 gel-polymer electrolyte 22 gelatin 448, 490, 491, 503 gene sequencing 35 generalized sheet transition conditions (GSTCs) 63, 224, 230 geometric-phase metasurfaces 234–237 germanium-on-nothing (GON) 16, 17 giant magneto resistance (GMR) effect 59, 95–97, 662 fabrication 204–206 magnetron-sputtering system 99–100 MBE method 99 multi-layered thin films 97–99

689

690

Index

giant magneto resistance (GMR) effect (contd.) sensors applications 100–102 glass transition temperature (Tg) 16, 285–295, 525, 526, 528 in organic thin films 287–291 of nano-organic composites 292–295 organic-organic composites 291–292 Global Position System (GPS) 550, 570, 571 glucose 415, 495, 496, 504 α-glucuronic acids 497 glucuronoarabinoxylan (GAX) 497 glucuronoxylan (GX) 497 glutaraldehyde 496 GLY-containing potato starch plastic sheets 503 glycerol 494–498, 539 glyphosate derivatives 528 GNR superlattices 654 GO membranes (GOMs) 29, 30, 37 gold nanoparticles 367, 397–399, 402 Gordon-Taylor equation 292 grain boundaries (GBs) 470, 471, 623, 624 graphene 18, 19, 29, 430, 480, 595 conceptual membrane materials 481 -doped mesoporous electron 20 one zero energy gap material 649 graphene foam (GF)/poly(3,4-ethylenedioxythiophene):poly (styrenesulfonate) (PEDOT:PSS) composites 598 graphene nanoribbons (GNRs) 6, 7, 651, 654 graphene nanosheets (GNS) 597 graphene oxide (GO) composites 24, 37 graphene/BN heterojunctions 652, 656 graphene/h-BN heterojunctions 652, 656 graphene/h-BN vdWHs 649 graphene/hexagonal boron nitride (Gp/h-BN) 3, 439, 649, 652, 653, 656 graphite fibers 594

graphite oxide (GO) 24, 29, 30, 37, 456, 597, 599, 605 Green’s function 231, 232, 242 greenhouse gas emissions 31, 472, 504 greenhouses 669 gyrotropy parameter 149, 152, 153, 155

h heat absorption 310, 435 helicopter anti-submarine system 572 Helmholtz coils 166, 172 Helmholtz resonators 580, 582 heterodyne frequency downconversion method 210 heterojunction theory 73–74 4,40-(hexafluoroisopropylidene) diphthalic anhydride (6FDA) 474 hexagonal boron nitride 3, 439, 649 hierarchical dipole approximation (HDA) 231 high efficiency THz-wave modulators 438–439 high permeability polyimides 473–474 high precision Fabry-Perot (FP) cavities 659, 660 high pressure compressor (HPC) 615 high pressure membrane filtration 457 high pressure turbine (HPT) 615, 618, 620 high quality solution-processable 2D semiconductors 651 high resolution transmission electron microscopy (HR-TEM) 23, 274, 279, 601 high spatiotemporal resolution characterization methods 655–661 high time resolution laser 663–664 high-performance polymer-based reverse osmosis membranes 469 high-performance RO 449 high-pressure membranes 457 high-Q lasers 666 high-Q single sources 659 high-Q thin films 653

Index

high-quality solution-processable 2D semiconductor nanosheets 651 high-temperature materials 618 highly permeable substituted polyacetylenes 475 highly uniform, solution-processable, phase-pure semiconducting nanosheets 5, 651 Hilbert dissipation 162 Hilbert space 75, 661 Hippocrates 417 H-matrix algorithm 232 2H-MoS2 nanosheets 651 H-NH4 /N wastewater 461 hole transport material 286 hole-transpoting materials (HTMs) 15, 16 hollow fiber membrane 452 Holzer effect 59, 60, 95–114 homogalacturonan (HG) 494, 495 homogeneous membrane 452 honeycomb “supercells” 33, 60, 111, 113, 362, 436, 479, 522, 579, 580, 649 honeycomb-like 2D supramolecular polymer 479 hot electron injection 61 human thermoregulation 418–421 hybrid organic-inorganic perovskite-based cells 19 hydrogel microphones 38–40 hydrogen evolution reaction (HER) 26, 27, 32, 670 electrocatalysts 670 hydrogen fuel cell 10, 25, 670, 671 hydrogen purification 449 hydrolytic resin base 530 hydrophilic and hydrophobic effects 451 hydroxypropylated xylan (HPX) coatings 497, 498 hyperbolic metamaterial (HMM) 242, 243 hyperbranched polymers (HBPs) 33

i ideal reverse osmosis membrane 462 III-V semiconductor solar cells 16 immersion gel method 456 immobilized photocatalytic multiphase reactor 429 impedance spectroscopy (IS) 86 impregnation 454 imprinted (MIP) materials 430 indium tin oxide (ITO) 12, 18, 393, 431 indole derivatives 536 infrared lasers 663 infrared spectroscopy 395 inorganic antifouling agents 518 inorganic membrane 452 inorganic semi-permeable membrane 448 inorganic thermoelectric thin films 311–313 insoluble synthetic resin 521 integral equation FFT (IE-FFT) 232 integrated active/passive multi-band sonar system 569 integrated AN/BYG-1(V) submarine sonar system 569 interfacially polymerized PA film 463 intermediate pressure compressor (IPC) 615 intermediate pressure turbine (IPT) 615 intrinsic conductive coating 592 inverse Faraday effect 125 ion exchange membrane 454 Irgarol 1051 523 iron-aluminum alloy 591 iron bacteria 511 iron-cobalt (FeCo) alloy and 591 iron garnets 125, 126 iron-nickel alloy 162, 591 irreversible Joule heating 309 isothiazolinone 523, 528 isothiazolinone derivatives 528 isothiazolinone grafted zinc acrylate self-polishing antifouling coating 528

691

692

Index

j Jones matrix 62, 223–224 Jones vector 62, 223–224

k kanamycin sulfate 479 Kelvin relationship 310 keratin 490 Kerr coefficient modulus 384 ketones 523 kinetic energy 79, 85, 357, 432, 604 Kittel formulas 184 Knudsen cell 99, 287 Kretschmann configuration 66, 67, 387

l label-free reflectometric interference microchip 389–393 LADA class giant conformal array sonar 558 Laguerre-Gaussian (LG) beams 233 Landau factor 83 Langmuir-Blodgett method 454 lanthanum zirconate (LZ) TBCs 629 large spherical array sonar 558 laser cladding plus plasma spraying 625 lattice and processing technology 650 layer-by-layer (LBL) deposition method 428 layered bimetallic hydroxide (LDH) 26 layered 2D materials 22, 649, 650 layered PtBi2 653, 654 layer materials 602 layer-multiple-scattering method 575 La2 Zr2 O7 (LZ) 636 lead telluride 434 left circularly polarized (LCP) 236 left-handed materials 606 left/right-handed circular polarization (L/RHCP) 233 Li batteries 22, 23 light emitting devices 261–263 lignin 498 limit flux 450 limits of detection (LOD) 386

linear regression analysis 293 linear regression fit 290 β-1-4-linked polymer 433, 496 β-1,4-linked xylose 497 linoleic acid 503 liquid crystal membrane 454 liquid membrane 454 liquid phase epitaxy (LPE) method 179 Lithographie, Galvanoformung, Abformung (LIGA) 200, 201, 653 L-lactide 530 local density of state (LDOS) 241 localized surface plasmon resonance (LSPR) 346, 350, 351, 382 longitudinal magneto-optical Kerr effect (L-MOKE) 70, 71, 385 longitudinal magneto-optical Kerr rotation 383, 385 longitudinal magnetophotonic intensity effect (LMPIE) 168, 169 “loose” or “low pressure RO membrane 461, 462 Lorentz force 69, 80 Los Angeles class attack nuclear submarine 557 low density polyethylene (LDPE) 502 low dimensional polymer-nanocomposite materials 434 low-frequency underwater sound absorption phenomenon 575 low molecular substances 476 low pressure chemical vapor deposition (LPCVD) 202 low-pressure membrane filtration 457 low-pressure plasma spraying (LPPS) 625 low pressure turbine (LPT) 615 low-resistance conductive material 589 L-pelicic acid (C30) 520 luminous solar concentrator (LSC) 18

m macro basis functions (MBF) 231 macro-foulers 513 magic angle graphene 650

Index

magnet-acoustic materials 554 magnetic circular dichroism (MCD) 133, 134 magnetic current 224 magnetic nanomaterials 38 magnetic nanoparticle detection 101 magnetic nonopole 662 magnetic permeability 590 magnetic pressure sensing 102 magnetic semiconductors (MSs) 271 magnetic semiconductor thin films 269–270 amorphous 271–275 diluted 270–271 phase change amorphous 275–277 spintronic devices 277–279 magnetic sensor 100, 101, 110, 113, 162, 164, 169, 179, 269 magnetic thin films 59–60 magnetoacoustic and acousto-optic transducer/detector 551 magnetoacoustic coupling 79–81 magnetocardiography (MCG) signals 163 magnetocrystalline anisotropy 164, 277 magnetoelastic effect coupled 79 magneto-electric coupling 59, 60, 357 magnetoelectric (ME) materials 667 magneto-optical (MO) 126 effects 68, 71, 126, 134, 356, 663 magnetometry 168 response 145–155 magneto-optical Kerr effect (MOKE) 8, 69, 70 nanostructural thin films with 381–385 surface plasmon polaritons 386–388 magneto-optic eddy current (MOEC) 178 defective welds 190 experiment 179–180 introscope 180 non-defective welds 188, 190 physical properties of 181 magneto-optic surface Plasmon resonance (MOSPR) 386

magnetophotonic crystals (MPC) 126 magnetophotonic plasmonic heterostructure 175 magnetoplasmon and quantum entangle 663 magneto-plasmonic microinterferometry 393 magneto-plasmonics 382 magnetoplasmonic sensor 176 magnetoplasmonic thin films 66 magnetoresistance (MR) 59, 96, 276 magnetoresistive random access memory (MRAM) 102 magneto-sputtering process 359 magnetostrictive alloys 554 magnetothermal effect 83, 84 magnetron effect 260 magnetron sputtering (MS) 98, 259 method 103, 412 process 432 system 98, 99, 104, 107 Majorana fermions 76, 662 manganese-based perovskite oxides 60, 95, 103, 113 marine biofilm 511 marine bio-fouling 512 marine bio-fouling film 511 marine microbial corrosion 516 Maxwell’s equations 72, 139, 204, 221, 229 Maxwell theory 62–65 MCrAlY-NiAl/NiAl+Al2 O3 /Al2 O3 YSZ/YSZ 623 mechanical sensor 269 membrane fouling 480 membrane pore screening effect 451 membrane resistance 450 membrane separation technique (MST) 31 challenges 447 high-performance RO 449 polymer-based synthetic membranes 449 pressure driven membrane processes 448

693

694

Index

membrane separation technique (MST) (contd.) principle of 449 schematic representation 447 thermosensitive substances 449 ultrafiltration 448 uses 447 mercaptoundecanoic acid (MUA) 390 mercerization 505 mercerized pulp 505 metaloendash nitrogenoendash carbon (MNC) compounds 26 metal fillers 593 metallic good conductor material 591 metallic powder absorbing materials 603 metal membrane 452 metal nanoparticles (MNPs) 39 metal organic chemical vapor deposition (MOCVD) 258, 260, 311, 321–323, 650 metal-organic frameworks 315 metal-oxide-semiconductor field-effect transistor (MOSFET) 4 metal-plated carbon fibers 594 metal spraying 589 metamaterial electromagnetic cloak 606 Metamaterial-Enhanced Magnetic Induction (M2I) method 550 metamaterials 62, 65, 221, 551, 606 enhance spontaneous emission 242–243 metasurfaces 222 based perfect absorber 578 based sound absorber 578 enhance spontaneous emission 243–244 method of moments (MoM) 229 (methyl) acrylic acid triorganosilane ester monomer 527 (methyl) acrylic acid triorganosilane monomers 527 methylammonium lead iodide (MAPbI3) 15 microbial corrosion and protection

overview of fouling biological control 513–516 primary mucosa-forming organisms and their attachment 512–513 microbiological fouling organism 511 microbiologically influenced corrosion (MIC) 511, 513 microcrystalline glass thin films 655 micro-electromechanical systems (MEMS) 199 microelectronic mechanics (MEMs) 7, 411, 415 microfiltration (MF) membranes 452 microfouling organisms 511, 512, 515 micro lattice metamaterial 579 micromechanical cleavage 648 microporous filter membrane 454 microstructures, fabrication of 200–203 microsystem packaging technology 207 microsystems technology 207, 209 micro system technology 205, 206 MIEC ceramic membranes 470 miscible solvent mixing and precipitation (MSMP) method 594 mixed ionic electronic conducting (MIEC) membranes 470 modified positively charged FeCo nanotubes 439 molar extinction coefficient 426 molecular beam chemical vapor deposition (MBCVD) 650 molecular beam epitaxy (MBE) 97–99, 103, 104, 108, 321, 322 molecularly imprinted polymers (MIPs) 429, 430 monolithic integrated circuit (MMIC) 207 moored sonobuoy 565 Moore’s law 7, 199, 207, 280 MoS2 nanoplates 651 MoS2 -WSe2 van der Waals heterostructures 668 m-phenylenediamine (MPD) 458 MTK 300 ruby cylindrical sonar arrays 556

Index

mucosal microorganisms 516 multiband absorbing materials 607 multi-based exploration systems (MBESs) 570 multi-beam echo sounders 555 multi-beam reflect array 227 multichannel optical interferometer 660 multi-dimension space concept 648 multi-effect distillation (MED) 461 multi-hierarchy arrayed micro windows (MHMW) 345 assisted NSL 346–351 nanosphere lithography 348 for single nanostructure and/or array identification 344–346 multilayer coating 502 multilayered W/Al2O3 film 635 multi-physical field coupling theory 88–90 multi-physics coupling calculation 650 multi-physics coupling simulation 648 multiple chip package 207 multiplexed allergen detection assay 100 multi-stage flash (MSF) 461 multivalent/monovalent salt ions 457 multi-walled carbon nanotubes (MWCNT) 594, 596 municipal sewage 463 municipal wastewater treatment program 467 MXene 598, 599 mycotoxin 429

n nanoarrays 344 nanocellulose 493 nanoclay 493 nanoconfined ferromagnetic material 385 nanocopper 480 nanoelectronic mechanics (NEMs) 7, 210, 211 nanofilms 213 nanofiltration (NF) 461 membranes 452

nanomaterial synthesis and assembly 467 nanooptics 344 nanoparticle patterning 33 nanoparticles 345 nanopores fabrication 359 nanoporous Ag/CoFeB/Ag films 356, 359 nanoporous carbon composite 30 nano-porous gold thin films aqueous gold nanoparticles, composite of 367 substrate transfer and characterization of 364–367 nanoporous silicon nitride (NPN) 35 nano-powders 627 nanopure water 341 nanorods (NRs) 371, 372 nanoscale plasmonic optics 344 nanoscale preconfinement 35 nanoscale ultrathin films 428 nanosilver 480 nanosphere lithography (NSL) 340 Ag NPs fabricated 352–354 aqueous phase Ag nanoparticles 351–356 NPs 352 size and shape controlled fabrication of nanomateials 340–344 ultrathin nanopore arrays 356–357 uniform nanoporous films 357–359 nanosphere mono layer 340 nanosphere templates 340, 342 nanostructure thin films 381 with enhanced magneto-optic Kerr effect 381–385 for SERS 394–396 nano-titanium dioxide (TiO2 ) 456 nanowire/polymer nanocomposites 594 nanowires (NWs) 371, 372 nano-zinc oxide 456 natural polysaccharides 476 natural rosin 521 natural rubber (NR) 596 NbNi-based/Co-based super-alloys 615

695

696

Index

negatively charged acidic amino acids 478 negative refractive stealth 606 negative Thomson effect 310 neutral and acidic amino acids 478 neutral arabinoxylan (AX) 497 NF-CA-50 nanofiltration membrane 477 nickel-base superalloys 618 nickeloendash aluminium (Nioendash Al) metal film 418 NiCoCrAlY/PtAl-based metallic bond coating 617 nitroaromatic explosives 426, 427 nitroaromatic explosives testing 426 nitrobenzene (NB) 427 nitrogen-doped rGO/MWCNT composite foams 605 nonacoustic module 561 non-anti-EpCAM binding cells 390 non-covalent interfacial heterojunctions 649 non-defective welds 188, 190 nondirectional sonar buoy 563 non-fossil fuels 472 non-graphene Weyl semimetals 667 non-imprinted (NIP) materials 430 non-magnetic semiconductor thin films 261 light emitting devices 261–263 phase change semiconductor thin films 265–268 thin film transistors (TFTs) 263–265 non-polarized capacitors 433 non-porous membranes 451 non-tin organic antifouling agents 518 NPs fabrication 341 NTEGRA scanning probe microscope 183 n-type polymers 314 nylon 449

o octamethyl trisubstituted silane 455 ohmic injection 61 oligosaccharides 476

one-dimensional GNR superlattice 654 one-dimensional inorganic nanofibers 433 one-dimensional magneto-photonic crystals (1D-MPC) 126 “one-point contact, multi-point response” 426 O2 /N2 separation 449 O2 /N2 silicone polymers 455 on-surface synthesis technology 651 optically pumped magnetometers (OPMs) 161 optical polarization vector 664 optical polymer based films 435 opto-acoustic effect 663 opto-acoustic materials 555 orbital angular momentum (OAM) detection 238–240 generation 222–234 multiplexing mode 227 organic antifouling agents 518 organic-based thin film TE materials 313 organic bentonite 523 organic DNA sensors 319 organic fluorine release-type antifouling paint 532 organic fluorine silicon series fouling release type antifouling paint 534 organic fluoropolymers 532 organic fluororesins 531 organic-inorganic composite nanofiltration membrane 478 organic inverter circuit 316–318 organic light emitting diodes (OLED) 8, 285, 318, 319 organic membrane 452 organic monomer 455 organic-organic composites 291, 292 organic radio frequency identification (RFID) tags 319, 320 organic solar cells (OSCs) 669 organic sol-gel inversion method 413 organic thin film transistor (OTFT) 317 organic tin antifouling coatings 522

Index

organic tin self-polishing coatings 527 organofluorine fouling release type antifouling coatings 534 organosilicon fouling release type antifouling coating 532 organotin antifouling agents 518 original single-layer graphene 430 osmosis 452 oxygen evolution reaction (OER) 26, 28, 670 oxygen reduction reaction (ORR) 25, 26 oxygen separation 470

p partially stabilized zirconia (PSZ) 624 passive sonar process 554 patulin (PAT) 429 PDMS substrate 399–401 pectin 494 Peltier coefficient 300, 308 Peltier effect 10, 84, 302, 307, 308 penicillin fermentation 478 penicillium expanse 429 pentafluorophenylethyl ammonium (FEA) 15 perfect electric conductor (PEC) layer 236 perfect magnetic conductor (PMC) layer 236 perfluoropolyether 532 periodic boundary condition (PBC) 228 permalloy 591 permeability coefficient 450 permeability-selectivity trade-off 453 permeability theory 87, 88 perovskite 77 single-junction solar cells 14 solar cells 11, 12, 15, 16 perovskite nanoflakes (PNPL) 17 perylene diimide (PDI) 314 petrobased polymers 503 phase change amorphous magnetic semiconductors 275–277 phase change semiconductor thin films 265–268

phase-inversion film-forming methods 456 phase inversion method 456 phase shift 234 [6,6]-phenyl C61 butyric acid methyl ester (PCBM) 431, 432 phonon-mediated colossal magnetoresistance 106 phonon thermal theory 83, 84 photoacoustic coupling 81 photocatalyst immobilized polymer film 428 photocrosslinking step 33 photodetector noise 174 photodetector signal 172 photolithography 200, 201 photoluminescence quantum yield (PLQY) 18 photonic band gap (PBG) 241 photonic crystals 237, 239 photovoltaic (PV) technology 11 phthalocyanine dye waste water 428 physical vapor deposition (PVD) 258, 285, 287, 454 picosecond lasers 664 picosecond THz pulse 657 piezoceramics 554 piezoelectric (PZT) based sonar systems 39 effect 208 polymer-based, skin-like tactile sensor 416 piezoresistance coefficient 208 piperazine (PRP) 458 planar polymer films 389 Planck’s constant 111, 170, 233, 241 plane heterojunctions (PHJ) 15 plasma chemical gas-sinking fit (PCVD) 454 plasma enhanced chemical vapor deposition (PECVD) 203 plasma-enhanced chemical vapor deposition (PECVD) 258 plasma induced polymerization 455 plasma polymer films 454–456

697

698

Index

plasma polymerization reaction 455 plasma-sprayed thermal barrier coatings (PSoendash TBC) 624 plasma spray oendash physical vapor deposition (PS-PVD) 626 plasma state polymerization 455 plasma-state polymerization reaction 455 plasmic gratings 170 plasmon active stealth 607 plasmonic polymers 455 plasticizers 495 plastic liquid crystal display 436–438 platinum–ionomer interface 25 PMMA substrate 398–399, 402, 403 PNIPAm modified graphene (PG) 31, 503 polar ferroelectic (PFE) 15 polarizability model 62, 64, 224–225 polarization density 63, 64, 224, 225 polarization electric current 62, 224 polarized capacitors 433 polar magneto-optical Kerr effect (P-MOKE) 69, 70 poly(3,4-ethylenedioxythiophene) (PEDOT) 313, 432 poly(3-hexylthiophene) (P3HT) 314, 431, 432 poly(diphenylacetylenes) 475 poly(ethersulfone) (PES) 593 poly(vinyl alcohol) (PVA) 423 polyacetylene 313, 592 polyacetylenes 475 polyamide (PA) 458, 464 hydrazide membranes 463 wax 523 polyamide thin film composite (PA TFC) membranes 463 polyaniline (PANi) 313, 428, 592 polyanions 459 polybenzothiazole (PBT) 474 polybenzoxazole (PBO) 474 polycaprolactone 427, 530 polycarbonate filter membrane 480 polycations 459

polydimethylsiloxane (PDMS) 397, 416 polyether alum 452 polyethylene 427 polyethylene glycol (PEG) 538 polyethylene terephthalate (PET) 431, 502 polyglycolide 530 poly 3-hexylthiophene (P3HT/PCBM) polymer solar cells 432 polyhydroxy butirate (PHB) 416 polylactic acid 504 polylactide 530 polymer-based composites 432, 592–593, 596, 599, 604 polymer-based organic solar cells 431 polymer-based pyroelectric infrared sensor 418 polymer-based strain sensor 421 polymer-based synthetic membranes 449 polymer chain rigidity 473 polymer films 454, 457 polymer functional membrane 453, 454 polymer materials 452 polymer membrane-based seawater desalination RO 461 polymer/mineral oil 422 polymer osmosis membrane technology 477 polymer permeable membranes 477 polymer poly(carbonate) (PC) 436 polymer separation membrane 453 polymer thin films 411 poly[2-methoxy-5-(2’-ethylhexyloxy)-1,4phenylene vinylene] (MEH-PPV) 438 poly-(methyl-hydro siloxane) 463 polymeric materials 314, 436, 470, 472, 473 polymer-inorganic nanocomposites 434 polymerization induced phase separation (PIPS) method 438 polymers of intrinsic microporosity (PIM-1) 472, 473, 475 polymesoy chloride 458

Index

polymethyl methacrylate (PMMA) 110, 288, 340, 363, 397, 427, 463 polymethylphenyl silicone oil 532 polyols 502, 503, 534, 539 polyphenylene sulfide 592 polypropylene (PP)/mineral oil film 422, 463, 651 polypyrrole 313, 314, 592 polysaccharide-based films 502–503 polysaccharides 32, 411, 476, 490–492, 494, 495, 497, 502 polystyrene 313, 340, 341, 342, 427–429, 504, 594 polystyrene sulfonate (PEDOT: PSS) 313 polysulfone (PSF) 458 ultrafiltration membrane 454 polythiophene 313, 314, 432, 592 polyurethane anti-mildew coatings 517 polyurethanes 34, 419, 499, 517, 530, 532 polyvinylidene fluoride (PVDF) 416, 452, 554 polyvinylpyrrolidone (PVP) 203, 367 POMA/V2 O5 hybrid thin films 428 pore size screening effect 451 porous base film 456 positively charged basic amino acids 478 post-annealing crystallization 128, 129–130 potential energy 79 power conversion efficiency (PCE) 15, 16, 18, 302, 311, 445 p-phenylenediamine (PPD) 458 precise nano-transistors 653 pre-deposited α-Al2 O3 thin film 624 pressure-driven membranes 448, 450, 457 pressure-enhanced osmosis (PEO) 452 pressure-retarded osmosis (PRO) 452 primary mucosa-forming organisms and their attachment 512–513 printed circuit board (PCB) technique 222, 516, 517 probing magnetometry 164 programmable microcontroller 180

prostate specific antigen (PSA) detection 404 protective coatings 514, 517 protein-based electronic skin (PBES) 34, 419 protein based polymers 490, 503 protein concentration and buffer exchange 449 protein-, pig skin gelatin-, lipid-based films 503 proton exchange membrane (PEM) 10, 25 ptychography 656 pulse deposition system 413 pulsed laser deposition (PLD) 324, 326, 413 pure iron 591 P(VDF-TrFE) powder 416 P(VDF-TrFE) thin film 417 PVP-Cu2 O microcapsules 535 pyridine salts 536 pyrochlore-type zirconates (M2 Zr2 O7) 635 pyroelectric material (PVDF film) 418

q quantum anomalous Holzer effect (QAHE) 59, 60, 95–114, 111, 112 quantum cryptography 659 quantum 2D graphene chains 653 quantum entanglement theory 648, 650 quantum films 199–214 quantum hall effects 648 quantum hegemony 655, 660 quantum mechanical magnetoresistance effect 59, 96 quantum no-cloning theorem 659 quantum optics 245, 658 quantum-well (QW) 15 quasi-perfect absorption property 578 quasi-periodic EM surfaces 227–233 design 228–229 full-wave simulation 230 simulation algorithms 229–230

699

700

Index

quaternary ammonium molecules 5, 651 quaternary ammonium salt 480, 548

r radar absorbing material 38, 601–604 radar-frequency bands 588 radar stealth coatings 38 absorbing material 601–604 carbonaceous-based RAM 604–606 radioisotope RAM 604 radiation sensor 269 radio frequency identification (RFID) tag 319 radioisotope RAM 604 radioisotope thermoelectric power generator 10 Raman effect 394 Raman-Nath diffraction 82 Raman scattering 68, 361–363, 394, 395, 396, 401 rapid insertion of commercial acoustics (ARCI) program 569 rare-earth doped YSZ 636 rare earth zirconates (RE2 Zr2 O7 ) 636 Rayleigh scattering 395 reaction monomer electrospray 480 reactive ion-beam sputtering (RIBS) 127 received signal strength (RSS) 572 received signal strength indicator (RSSI) 572 receiver arrays 555 reconnaissance (BTA) sonar 555, 563 red mercury oxide-based coatings 520 reduced graphene oxide (rGO) 17, 597, 607 reference sensors 571 reflect-array antennas 228 reflectometric interference spectroscopy (RIfs) 389–391 refractive index 67, 69, 70, 81, 82, 139, 153, 154, 175, 176, 241, 244, 286, 386, 387, 389, 437, 634 reinforced polyvinylidene difluoride (PVDF) nanocomposites 594 related layered thin films 648

renewable energy 22, 299, 472, 669 residual xylan acetate 505 resin acid 520, 521 reverse osmosis (RO) 28, 37, 430, 447, 448, 452, 461, 464–466 desalination process 462 membrane desalination 469 reversible electrolyzer-fuel cell scheme 670 rhamnogalacturonan-I (RG-I) 494 rhamnogalacturonan-II (RG-II) 494 right circularly polarized (RCP) 236 rigorous coupled-waves analysis (RCWA) 147, 154 roll membrane 452 rosin 520, 521–523 rubber shellac 520

s Salisbury screen materials 602 satellite based quantum-secure time transfer 661 saturation magnetization 126, 127, 146, 165, 166, 169, 275, 361, 603 scanning electronic microscope (SEM) 24, 459, 601 scatterer simulation 228–230 Schelkunoff’s electromagnetic shielding transmission line theory 590 Schrodinger equation 81, 213 seawater desalination 361, 430–431, 454, 458, 461, 463, 467, 469 seawater reverse osmosis (SWRO) 464, 466 secondary-ion-mass-spectroscopy (SIMS) 152 Seebeck coefficient 11, 84, 86, 299, 300, 301, 302, 304, 305, 306, 308, 312–314, 316, 321, 322 Seebeck effect 10, 84, 85, 300, 301–306, 307, 310, 316, 326, 327 Seebeck experiment 301 selected area electron diffraction (SAED) 274 selective diffusion process 448

Index

selective permeable membranes chemical modification method 456–457 gas separation 469–470 low molecular substances 476–479 phase inversion method 456 plasma polymer film 454–456 polymer functional membrane 454 polymer separation membrane 453–454 purification of water 457–469 types and characteristics of 452–453 selective xenon difluoride 35 self-absorbing 28μm PVDF pyroelectric sensor 418 self-assembled membranes 419 self-assembled monolayers (SAMs) 340, 390, 391, 427 self-noise detecting sonar (ONA+ACC) 555, 563 self-polishing antifouling coatings 523–531, 528 self-polishing resin 523, 524–526, 528 semiconductive nanowires thin films AAO template-assisted fabrication of 371–376 semiconductor thin films magnetron sputtering 259–260 metal-organic chemical vapor deposition (MOCVD) 260–261 molecular beam epitaxy (MBE) 259 non-magnetic semiconductor thin films 261–269 sensors 268–269 semi-metals 74–77, 648 semi-permeable membrane 453, 454 semipermeable synthetic membrane 448 semitransparent OSCs 669 sensors, semiconductor thin films 268–269 separation membrane 37, 454, 456, 472 SERS 394, 395, 404 aqueous gold nanoparticles 399–401 in cancer cell detection 402–404

composite aqueous gold nanoparticles in 398–399 development nanostructure thin films for 396–397 SERS-based immunoassays (SIA) 404 SF composite membranes (SFCMs) 419 shielding composites 589, 593 shielding effectiveness (SE) 589 shipboard passive direction finding (FA) sonar 555 shipboard passive ranging (PRA) sonar 555 Shockley-Queisser limit 14, 19 Shubnikov-de Haas and de Haas-van Alphen oscillations 654 signal-to-noise ratio (SNR) 147, 386 silane-based components 434 silicone-acrylate resins 532 silicone anti-mold coating 517 silicone-epoxy resin 532 silicone oil 532 silicone-polyether resin 532 silicone-polyurethane resins 532 silicone resins 517, 526, 527, 531, 532, 538, 539 silicon isotropic dry etchants 209 silicon steel 591 silk 34, 419, 490 silk fibroin composite membranes (SFCMs) 34 silk fibroins (SF) 34, 419 silk protein 34 simple solution-casting membrane method 480 Si nanomembrane diode sensors 414 single atom catalysts (SACs) 25 single-beam echo sounders 555 single-crystal diamond (SCD) 210 single crystal thin films 650 single-molecule detection 35, 397 single NPs 344, 377 single photon detector 655, 659, 660 single photon phase coded key system 659

701

702

Index

single-walled carbon nanotubes (SWCNTs) 596 slime-producing bacteria 511 small-celled commercially available TI structures 436 smart display 436–438 smart mines 551, 571 smart polymer films 411 smart polymer thin films and coatings for AI ammonia sensors 428 bio-integarted wearable sensors 422–425 biosensor film substrate 421–422 capacitors 432–434 degradation of complex phthalocyanine dye waste water 428–429 explosives detection 426–428 human thermoregulation 418–421 patulin 429–430 preparation methods 412–413 seawater desalination 430–431 smart device integration 413–415 solar energy 431–432 tactile sensor 415–417 thermal sensor 417–418 thermoelectric devices 434–435 ultraviolet radiation monitoring 425–426 sodium alginate (SA) 501, 599 soft magnetic alloy materials 591 softwood lignin 498 solar energy 9, 11, 12, 14, 405, 431, 435, 436, 670 sol-gel method 412, 413, 500, 501 sol-gel phase inversion coatings 500 solid electrolyte interfaces (SEIs) 21 soluble aromatic polyimides 474 solution precursor plasma spraying (SPPS) 617, 625, 627–629 solution-processable synthesis 663 solvent casting 448, 454, 594 sonar buoy 563, 565, 571 sonar detection 39

sonar systems advanced hostile 565–573 anechoic coating 573–574 Doppler effect 552–573 echolocation 552–573 types and construction of 553–555 typical 555–565 sono-radio-buoy 551, 571 sorbitol 495, 496, 498, 502, 503 sound navigation and ranging 38, 550 soy protein isolate (SPI) 498 spatial-localization method 344, 572 specific Faraday rotation (SFR) 73, 145, 153, 154, 181 spectrally selective film 435 spherical sonar arrays 556, 557 spin angular momentum (SAM) 233 spin-coating 340, 416, 427 spin detection technology 61 spin polarization 60, 61, 75, 106, 107 spintronic device magnetic semiconductors 277, 279 spintronic devices 61, 112, 113, 257, 270, 271, 279 spintronics 3, 6, 60, 61, 76, 95, 112, 113, 270, 271, 278, 279, 662 spintronic theory 60–62 spontaneous emission rate (SER) calculation 242 inhomogeneous electromagnetic environment 241–242 metamaterials 242–243 metasurfaces 243–244 SPP dispersion 147, 148 sputtering technique 320, 321, 356 starch 490, 495–496, 502, 503, 504 stealth coatings multiband absorbing materials 607 negative refractive stealth 606–607 plasmon active stealth 607–608 radar 600–606 steel plate 592 steered cathodic arc plasma evaporation (steered CAPE) 432

Index

Stephen-Boltzmann’s constant 634 sterane derivatives 536 steric hindrance effect 451 sterile filtration 449 strain tolerant oxide 633 stretchable, elastic conductive material 416 stretchable SF/CaCl2 electrodes 419 string theory 648 strong electron acceptability 426 structurally highly oriented supramolecular assembly composite membrane 480 substituted galacturonans (GS) 494 substituted polyacetylenes 472, 475 sub-wavelength and quasi-perfect underwater sound absorber 577 sulfate-reducing bacteria 511 sulfonated-polysulfone/PSf alloy ultrafiltration membranes 458 sulfonated-PSf/PSf alloy 458 sulfur copper zinc 523 sum frequency generation (SFG) 657, 664 superalloy substrates 38, 86, 618–619, 628 supercapacitors 23, 24, 30, 428, 432 surface-assisted cyclodehydrogenation 6 surface coating film shielding material 591 surface conductive type EMI shielding materials 589 surface enhanced Raman scattering (SERS) 362–363 surface metal foil coating 589 surface metallization 589 surface micromachining 209, 269 surface microstructure bionic antifouling paint 539–540 surface plasmon polaritons (SPP) 146, 174, 175, 386, 388 surface plasmon resonance (SPR) 8, 9, 66–73, 344, 346–351, 363

surface water eutrophication 449 surface water supplies 463 susceptibility model 64, 65, 225–226 suspension plasma spraying (SPS) 617, 629–633 symmetric para-linked isomers 474 synthetic aperture sonar 555, 569 synthetic basis function (SBF) method 231

t tactile sensor 415–417 tall oil fatty acid (TOFA) 498 Tamm 1D-MPC with Bi, IG bi-layer 136–138 TB-16 coarse linear array sonar 562 TB-16 towed array sonar 561 TCPM 523 tear and stack technique 5 TE materials 10, 11, 311–315 temperature ellipsometry 287, 288 template transfer nanoimprinting method 663 terahertz (THz) technology 438 ternary organic solar cells (OSCs) 16 tetracyanoquinodimethane (TCNQ) 315 tetraheptylammonium bromide 5, 651 Themis aberration-corrected 152 thermal annealing 108, 262, 342, 343, 353 thermal barrier coatings (TBCs) 38, 86, 615–617 aerospace application 617 applications 620–621 electron-beam physical vapor deposition 621–624 gas turbine/aero-engine parts 615 gas turbine components 616 plasma-sprayed 624–627 solution precursor plasma spraying process 627–629 superalloy substrates 618–619 suspension plasma spraying 629–633 system compositions 619–620

703

704

Index

thermal barrier insulation theory 86 thermal conductivity 11, 299, 301, 304, 311, 312, 314, 326, 591, 634 thermal evaporation 137, 203, 260, 324, 325 thermal expansion reduced graphene 24 thermal gel method 456 thermally rearranged (TR) polymers 472–474 thermal management for electronic equipment 439 thermal sensor 269, 417–418 thermal stability 34, 35, 108, 268, 286, 295, 419, 473 thermoacoustic effect 80 thermocouple 287, 306, 307, 308 thermoelectric devices 11, 300, 304, 305, 316, 320–326, 434 thermoelectric effect 84–86, 300, 303–305, 307, 309 thermoelectric generators 10, 299, 301, 316 thermoelectric heating and cooling devices 439 thermoelectricity 11, 300–301 thermoelectric materials (TE) 299 cooling 316 limitations 320 organic DNA sensors 319–320 organic inverter circuit 316–318 organic light emitting diode (OLED) 318–319 organic radio frequency identification (RFID) tags 319, 320 power generation 316 thermoelectric (TE) process 10 thermoplastic starch (TPS) 503 thermopower 305, 311 thermosensitive substances 449 thin film composite nanofiltration (NF-TFC) membranes 458 thin film composite polyamide based reverse osmosis membranes 458 thin-film materials 286, 412

thin-film nanocomposite mixed matrix films 469 thin film nanomaterials 21 thin films biomedical engineeringand life science 32–37 and coatings findings of 647–650 high precise fabrication technique 650–655 high spatiotemporal resolution characterization methods 655–661 microstructure vs. property 661–668 modern technology innovation and the society progress 668–671 ecological environment of Earth 28–32 fabrication methods 3 of information technology 2–7 national defense and homeland security 38–41 for sustainable energy application 9–28 for ultrasensitive sensing devices 7–9 thin film transistors (TFTs) 5, 257, 263–265 thin-layer composite films 456 thin polyimide film 416 thiol compounds 351–354, 355 third thermoelectric effect 309 thixotropic agents 523 Thomson coefficient 309, 310 Thomson effect 84, 85, 309–311 Thomson heat 309, 310 three dimensional graphene 662 THz pulse 657 time difference of arrival (TDOA) 572 time of arrival (TOA) 572 time-reversal symmetry (TRS) 60, 75, 111, 113 TMOKE effect 387 topological insulators (TIs) 3, 5, 60, 74–77, 111, 112, 648, 654, 666

Index

torpedoes 551, 571 total energy of sound 79 towed array sonar systems 561 towed line array (TA) sonar 555, 561, 562 towed slender line array sonar 561, 562 transimpedance amplifier 170, 171 transition-metal dichalcogenides (TMDs) 211, 652 transmembrane pressure difference 450, 451 transmission electron microscopy (TEM) 23, 146, 212, 274, 352, 597, 655 transmitter arrays 555 transparent conductive oxide (TCO) films 431 transparent heat-insulating film 435 transparent insulation (TI) structures 436 transparent plastics 436 transverse Kerr effects 168, 169, 176, 177 transverse magneto-optical Kerr effect (T-MOKE) 71, 175, 382, 387 tributyltin (TBT) 524 trigonal layered PtBi2 653, 654 trimesoyl chloride (TMC) 458, 464 trinitrotoluene (TNT) 427 triply degenerate point (TP) fermions 653 trivinylcyclopentadienyl manganese dye-doped ethylene/styrene copolymer 427 tubular membrane 452 tungsten-carbide-type materials 653 tunneling magnetoresistance (TMR) 59, 95, 106, 107 applications 110–111 multi-layered thin films fabrication 107–110 tunnel junction injection 61 turpentine 520 two-dimensional layered materials (2DLMs) 649

two-dimensional van der Waals magnets 667 two-layer graphene 650 T-W wastewater 461 type I single collagen molecule 491 Type 214 Submarine 555, 556, 558 type 885 Yassen class attack nuclear submarine 557

u ultra-broadband metamaterial-based acoustic absorption material 580 ultra-dense nanoparticle arrayed thin films AAO template-assisted fabrication of 367–371 ultrafiltration (UF) 447, 448, 454 membranes 452 ultra-high vacuum (UHV) 259 ultra-permeable membranes (UPMs) 464 ultrapure biological treatment 449 ultrasensitive ferroelectric switcher 663 ultrasensitive magnetic/optical detector 394, 663 ultrasensitive magnetometry 161–174 ultrasensitive sonar system 551 ultra-small transistors 6 ultra-stable glass 289 ultra-thin acoustic metamaterial panel 580 ultrathin BiIG films 126, 127 ultrathin films, magneto-optical (MO) response in 145–155 ultrathin nanopore arrays 356–357 ultrathin polymer films 455 ultraviolet lasers 663 ultraviolet lithography (UV-LIGA) 345 ultraviolet radiation (UVR) monitoring 425 uncertainty principle 659 underwater hydro-acoustic transducers 554 underwater sound reflections 574

705

706

Index

uniaxial anisotropy constant 126, 183 uniform Ag NPs formation 356 uniform nanoporous films, with controlled layers 357–359 unmanned aerial vehicle 571 U type large aperture array 558 UV antifouling technology 540 UV-curable acrylic resin 436

v vacuum evaporation coating method 412 vacuum gold plating 589 vacuum plasma spraying (VPS) 625, 626 vacuum thermal spray processes 626 valerolactone 530 vanadium dioxide 413, 438 van der Waals force 3, 4, 650, 656 van der Waals heterostructures (vdWHs) 649 Van der Ziel criterion 164 vapor compression distillation (VCD) 461 vdWHs heterojunctions 656 vector sonar technology 569 Vegard’s law 127 vibration isolation module (VIM) 561, 562 vinyl chloride-vinyl acetate copolymers 521 Virginia attack nuclear submarine 557, 568 Virginia class attacking nuclear submarine 558, 561, 562 Virginia class submarine 558–560, 563 viscoelastic polymer slab 575 viscose 505 viscosity of the penetrating liquid 450 Vivinal® GOS 477 Voigt effect 169 voltage difference 305, 306 VO2 thin film smart windows 413

w waste polystyrene 429 wastes into wealth 490 wastewater treatment and resource utilization 457 water flux/magnetic permeability 461 water permeability 37, 461–463, 467, 468, 469, 480 water purification 31, 32, 430, 449, 453, 457, 670 water resources 457 water/salt permeability selectivity 461 water vapor delamination 35 wave impedance 590 well-crystallized ITO films 431 wet etching process 201, 203 wet method 456 Weyl/Dirac node 77 Weyl equation 76 Weyl fermion 661, 662 Weyl points 662 Weyl semimetals 2, 112, 648, 661, 662, 666 winched AN/SQR-18 towed array sonar 561 towed linear array sonar 562

x X-ray diffraction (XRD) 370, 371, 384, 433, 655 X-ray electron-free laser 655 X-ray free electron lasers (XFELs) 657, 658, 664 X-ray lasers 656, 657, 663, 666 X-ray lithography 200, 209 xylan 490, 497–498 xylene 397, 523 xylitol 495, 496 xylogalacturonan 495

y yttria-stabilised zirconia (YSZ) 619, 633

Index

z Zeeman effects 662 zero-index-metamaterials (ZIM) 222 zinc acrylate self-polishing resins 524, 525

zinc oxide 518, 520, 523, 524, 603 zinc selenide 262

707