Nanotubes and Nanowires [3 ed.] 9781788017824

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Nanotubes and Nanowires 3rd Edition

Nanoscience & Nanotechnology Series Editor-­in-­chief:

Nguyễn T. K. Thanh, University College London, UK

Series editors:

Gabriel Caruntu, Central Michigan University, USA Shinya Maenosono, Japan Advanced Institute of Science and Technology, Japan Neerish Revaprasadu, University of Zululand, South Africa

Titles in the series:

1: Nanotubes and Nanowires 2: Fullerenes: Principles and Applications 3: Nanocharacterisation 4: Atom Resolved Surface Reactions: Nanocatalysis 5: Biomimetic Nanoceramics in Clinical Use: From Materials to Applications 6: Nanofluidics: Nanoscience and Nanotechnology 7: Bionanodesign: Following Nature's Touch 8: Nano-­Society: Pushing the Boundaries of Technology 9: Polymer-­based Nanostructures: Medical Applications 10: Metallic and Molecular Interactions in Nanometer Layers, Pores and Particles: New Findings at the Yoctolitre Level 11: Nanocasting: A Versatile Strategy for Creating Nanostructured Porous Materials 12: Titanate and Titania Nanotubes: Synthesis, Properties and Applications 13: Raman Spectroscopy, Fullerenes and Nanotechnology 14: Nanotechnologies in Food 15: Unravelling Single Cell Genomics: Micro and Nanotools 16: Polymer Nanocomposites by Emulsion and Suspension 17: Phage Nanobiotechnology 18: Nanotubes and Nanowires, 2nd Edition 19: Nanostructured Catalysts: Transition Metal Oxides 20: Fullerenes: Principles and Applications, 2nd Edition 21: Biological Interactions with Surface Charge Biomaterials 22: Nanoporous Gold: From an Ancient Technology to a High-­Tech Material 23: Nanoparticles in Anti-­Microbial Materials: Use and Characterisation 24: Manipulation of Nanoscale Materials: An Introduction to Nanoarchitectonics 25: Towards Efficient Designing of Safe Nanomaterials: Innovative Merge of Computational Approaches and Experimental Techniques 26: Polymer–Graphene Nanocomposites 27: Carbon Nanotube-­Polymer Composites 28: Nanoscience for the Conservation of Works of Art 29: Polymer Nanofibers: Building Blocks for Nanotechnology 30: Artificial Cilia

31: Nanodiamond 32: Nanofabrication and its Application in Renewable Energy 33: Semiconductor Quantum Dots: Organometallic and Inorganic Synthesis 34: Soft Nanoparticles for Biomedical Applications 35: Hierarchical Nanostructures for Energy Devices 36: Microfluidics for Medical Applications 37: Nanocharacterisation, 2nd Edition 38: Thermometry at the Nanoscale: Techniques and Selected Applications 39: Nanoceramics in Clinical Use: From Materials to Applications, 2nd Edition 40: Near-­infrared Nanomaterials: Preparation, Bioimaging and Therapy Applications 41: Nanofluidics, 2nd Edition 42: Nanotechnologies in Food, 2nd Edition 43: ZnO Nanostructures: Fabrication and Applications 44: Diatom Nanotechnology: Progress and Emerging Applications 45: Nanostructured Materials for Type III Photovoltaics 46: Chemically Derived Graphene: Functionalization, Properties and Applications 47: Graphene-­based Membranes for Mass Transport Applications 48: Carbon Nanostructures for Biomedical Applications 49: Surface Chemistry of Colloidal Nanocrystals 50: Reducing Agents in Colloidal Nanoparticle Synthesis 51: Carbon Nitride Nanostructures for Sustainable Energy Production and Environmental Remediation 52: Nanotubes and Nanowires, 3rd Edition

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Nanotubes and Nanowires 3rd Edition By

C. N. R. Rao

Jawaharlal Nehru Centre for Advanced Science Research, India Email: [email protected]

A. Govindaraj

Jawaharlal Nehru Centre for Advanced Scientific Research, India Email: [email protected] and

Leela Srinivas Panchakarla

Indian Institute of Technology Bombay, India Email: [email protected]

Nanoscience & Nanotechnology Series No. 52 Print ISBN: 978-­1-­78801-­782-­4 PDF ISBN: 978-­1-­78801-­963-­7 EPUB ISBN: 978-­1-­78801-­964-­4 Print ISSN: 1757-­7136 Electronic ISSN: 1757-­7144 A catalogue record for this book is available from the British Library © C. N. R. Rao, A. Govindaraj and Leela Srinivas Panchakarla 2022 All rights reserved Apart from fair dealing for the purposes of research for non-­commercial purposes or for private study, criticism or review, as permitted under the Copyright, Designs and Patents Act 1988 and the Copyright and Related Rights Regulations 2003, this publication may not be reproduced, stored or transmitted, in any form or by any means, without the prior permission in writing of The Royal Society of Chemistry or the copyright owner, or in the case of reproduction in accordance with the terms of licences issued by the Copyright Licensing Agency in the UK, or in accordance with the terms of the licences issued by the appropriate Reproduction Rights Organization outside the UK. Enquiries concerning reproduction outside the terms stated here should be sent to The Royal Society of ­ Chemistry at the address printed on this page. Whilst this material has been produced with all due care, The Royal Society of ­ Chemistry cannot be held responsible or liable for its accuracy and completeness, nor for any consequences arising from any errors or the use of the information contained in this publication. The publication of advertisements does not constitute any endorsement by The Royal Society of Chemistry or Authors of any products advertised. The views and opinions advanced by contributors do not necessarily reflect those of The Royal Society of Chemistry which shall not be liable for any resulting loss or damage arising as a result of reliance upon this material. The Royal Society of Chemistry is a charity, registered in England and Wales, Number 207890, and a company incorporated in England by Royal Charter (Registered No. RC000524), registered office: Burlington House, Piccadilly, London W1J 0BA, UK, Telephone: +44 (0) 20 7437 8656. For further information see our web site at www.rsc.org Printed in the United Kingdom by CPI Group (UK) Ltd, Croydon, CR0 4YY, UK

Preface to the Third Edition We are delighted that the previous editions of this book were well received. The subject has expanded greatly in the past few years. We now have new types of nanotubes, for example misfit nanotubes. There have been many advances in the area of nanowires. These developments are noteworthy and add a new direction to nanoscience. Many new properties and applications of nanotubes of carbon and other materials as well as of inorganic nanowires have been discovered. We have, therefore, decided to provide a new edition of the book which covers the various developments that have occurred since the second edition was published. Some of the important topics included in this edition are: chirality-­ dependent synthesis of SWNTs, metal chalcogenide nanotubes, misfit layered nanotubes, novel properties of inorganic nanotubes, new inorganic nanowires including perovskite nanowires, and photonic, thermoelectric and other properties of nanowires. We believe that with the addition of these topics, this book provides a satisfactory account of the status of nanotubes and nanowires. We have considered it important to divide the book into smaller chapters devoted to specific areas. The new edition has ten chapters instead of three. We trust that this edition will be found useful by all concerned. C. N. R. Rao A. Govindaraj L. S. Panchakarla

  Nanoscience & Nanotechnology Series No. 52 Nanotubes and Nanowires, 3rd Edition By C. N. R. Rao, A. Govindaraj and Leela Srinivas Panchakarla © C. N. R. Rao, A. Govindaraj and Leela Srinivas Panchakarla 2022 Published by the Royal Society of Chemistry, www.rsc.org

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Preface to the Second Edition Since we first wrote this monograph in 2005, the science of nanotubes and nanowires has had an explosive growth. Carbon nanotubes have attracted the attention of chemists, physicists, materials scientists and biologists because of the various fascinating properties that they exhibit. Raman spectroscopy and other techniques have been effectively employed to understand the structure and properties of nanotubes. Functionalization and solubilization of carbon nanotubes have been explored widely. Aspects related to potential applications of nanotubes have been widely investigated and they include not only electronics and sensor technology, but also nanobiotechnology and catalysis. In this revised edition, we have covered the varied developments in carbon nanotubes up to 2010 citing over 2000 references. Inorganic nanotubes were discovered post-­1991 starting with molybdenum and tungsten sulfides. Nanotubes of a variety of inorganic materials have since been synthesized by employing novel strategies. Properties as well as potential applications of these materials are yet to be fully investigated. Inorganic nanowires have constituted a popular topic of study. There has been a surge of publications in this area reporting synthesis, properties and applications. In this revised edition, we have expanded these chapters extensively and covered much of the recent advances. The numbers of references cited are 500 and 900, respectively, in the case of inorganic nanotubes and nanowires. With the wide coverage of the topics and the large number of references cited, the revised monograph has become bigger and at the same time more purposeful. We do hope that the monograph will be found useful by all those who work in nanomaterials and also by students and teachers. C. N. R. Rao A. Govindaraj   Nanoscience & Nanotechnology Series No. 52 Nanotubes and Nanowires, 3rd Edition By C. N. R. Rao, A. Govindaraj and Leela Srinivas Panchakarla © C. N. R. Rao, A. Govindaraj and Leela Srinivas Panchakarla 2022 Published by the Royal Society of Chemistry, www.rsc.org

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Preface to the First Edition The science of nanomaterials has become the flavour of the day, the research being driven both by academic curiosity and the promise of useful applications. Amongst the nanomaterials, nanocrystals, nanowires and nanotubes constitute three major categories, the last two being one-­dimensional. Since the discovery of the carbon nanotubes in 1991, interest in one-­dimensional nanomaterials has enhanced remarkably and a phenomenal number of research articles has been published on nanotubes as well as nanowires. The nanotubes are not only those of carbon but also inorganic materials. Several strategies have been developed for the synthesis of these materials and a range of interesting properties reported. Thus, the electronic and mechanical properties of carbon nanotubes have been studied extensively and several of them directly relate to potential applications. Typical of the important properties of carbon nanotubes are high mechanical strength, good electrical and thermal conductivity and excellent electron emission characteristics. The electronic and Raman spectra of carbon nanotubes have helped immensely in the characterization as well as in understanding some of the intrinsic structural characteristics. While nanotubes of several inorganic materials, many of which possess layered structures, have been synthesized and characterized, the literature on inorganic nanowires is much more extensive. Every conceivable inorganic material seems to have been prepared in nanowire form. Properties and possible applications of these inorganic one-­dimensional materials have been investigated to some extent, but there seems to be ample scope for study. This monograph provides an up-­to-­date survey of various aspects of carbon nanotubes, inorganic nanotubes and nanowires. Nanotubes of lipids, peptides, polymers and DNA are known, but they have not been discussed in this monograph due to its limited scope. We have found it difficult to cover   Nanoscience & Nanotechnology Series No. 52 Nanotubes and Nanowires, 3rd Edition By C. N. R. Rao, A. Govindaraj and Leela Srinivas Panchakarla © C. N. R. Rao, A. Govindaraj and Leela Srinivas Panchakarla 2022 Published by the Royal Society of Chemistry, www.rsc.org

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Preface to the First Edition

the entire gamut of properties and applications of the nanotubes and nanowires in detail in view of the immense literature that has accumulated in the last three to four years. We have been selective, emphasizing more the chemical aspects of nanotubes and nanowires, specially those related to synthesis and characterization to a greater extent. We have provided an extensive list of references to enable those who would like more complete information on the properties and other aspects of these materials. It is possible that we have failed to cite some important references by oversight or error in judgement, and we would like to be excused for such omissions. We have done our best to make the monograph contemporary and we do hope that students, teachers and practitioners of nanoscience will find it useful. C. N. R. Rao A. Govindaraj

Contents Chapter 1 Carbon Nanotubes  1.1 Introduction  1.2 Synthesis  1.2.1 Multi-­walled Carbon Nanotubes  1.2.2 Aligned Nanotube Bundles and Micropatterning  1.2.3 Single-­walled Carbon Nanotubes  1.2.4 Direct Spinning of Nanotube Yarns  1.2.5 Selective Preparation of Semiconducting and Metallic SWNTs  1.2.6 Chirality-­defined Synthesis of SWNTs  1.2.7 Junction Nanotubes  1.2.8 Peapods and Double-­walled Nanotubes  1.2.9 Mechanism of Formation of Nanotubes  1.2.10 Purification of SWNTs  1.2.11 Separation of Metallic and Semiconducting SWNTs  1.3 Structure, Spectra and Characterization  1.3.1 General Structural Features  1.3.2 Raman and Other Spectroscopies  1.3.3 Pressure-­induced Transformations  1.3.4 Electronic Structure  References  Chapter 2 Chemically Modified Nanotubes  2.1 Introduction  2.2 Doping with Boron and Nitrogen   Nanoscience & Nanotechnology Series No. 52 Nanotubes and Nanowires, 3rd Edition By C. N. R. Rao, A. Govindaraj and Leela Srinivas Panchakarla © C. N. R. Rao, A. Govindaraj and Leela Srinivas Panchakarla 2022 Published by the Royal Society of Chemistry, www.rsc.org

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1 1 3 3 14 23 35 35 39 43 45 47 52 56 61 61 66 77 81 86 111 111 111

Contents

xii



Chapter 3

2.3 Intercalation by Alkali Metals  2.4 Metal ↔ Semiconductor Transitions Induced by Molecular Interaction  2.5 Opening and Filling of Nanotubes  2.6 Decoration and Coating  2.7 Reactivity, Solubilization and Functionalization  2.8 Covalent Functionalization  2.8.1 Halogenation  2.8.2 End-­group Functionalization  2.8.3 Cycloaddition  2.8.4 Radical Addition  2.8.5 Nucleophilic Addition  2.8.6 Covalent Polymer Composites  2.8.7 Other Covalent Functionalization Methods  2.9 Noncovalent Functionalization  2.9.1 Noncovalent Polymer Composites  2.9.2 Functionalization Using Surfactants and Polyaromatics  2.9.3 Interaction with Biomolecules  2.9.4 Endrohedral Filling  2.10 Functionalization Using Fluorous Chemistry and Click Chemistry  References   roperties and Applications of Carbon Nanotubes  P 3.1 Electronic Properties  3.2 Phase Transitions and Fluid Mechanics  3.3 Carbon Nanotube Composites  3.4 Applications, Potential and Otherwise  3.4.1 Electronic Applications  3.4.2 Field-­effect Transistors (FETs) and Related Devices  3.4.3 Electromechanical Properties  3.4.4 Field Emission  3.4.5 Energy Storage and Conversion: Supercapacitors, Solar Cells and Actuators  3.4.6 Sensors and Probes  3.4.7 Biological Aspects  3.4.8 Mechanical Properties and Related Devices  3.4.9 Lithium Batteries  3.4.10 Gas Adsorption and Hydrogen Storage  3.4.11 Other Useful Properties and Devices  References 

115 117 118 120 121 127 127 128 129 130 131 134 135 140 141 141 144 149 150 151 164 164 171 173 182 182 187 193 194 200 204 208 214 216 217 220 221

Contents

Chapter 4

xiii

I norganic Nanotubes  4.1 Introduction  4.2 Synthetic Methods  4.3 Nanotubes of Different Materials  4.3.1 Nanotubes of Elemental Materials  4.3.2 Metal Chalcogenide Nanotubes  4.3.3 Pnictide Nanotubes  4.3.4 Nanotubes of Carbides and Other Materials  4.3.5 Metal Oxide Nanotubes  4.3.6 Misfit Layered Nanotubes  4.4 Properties  4.4.1 Mechanical Properties  4.4.2 Electronic, Magnetic, Optical and Related Properties  4.4.3 Tribological Properties  4.4.4 Thermal Properties  4.5 Solubilization and Functionalization  4.6 Applications  References 

240 240 244 246 246 256 268

Chapter 5

 ynthetic Strategies for Inorganic Nanowires  S 5.1 Introduction  5.2 Synthetic Strategies  5.2.1 Vapour-­phase Growth  5.2.2 Growth Mechanisms  5.2.3 Solution-­based Growth  5.3 Growth Control and Integration  References 

357 357 358 358 361 367 370 371

Chapter 6

Elemental Nanowires  6.1 Introduction  6.2 Silicon  6.3 Germanium  6.4 Boron  6.5 In, Sn, Pb, Sb and Bi  6.6 Se and Te  6.7 Gold  6.8 Silver  6.9 Iron and Cobalt  6.10 Nickel and Copper  6.11 Other Metals and Alloys  6.12 Trimetallic Nanowires  6.13 Segmented Heterojunction Nanowires  References 

374 374 374 382 384 384 386 390 394 398 401 404 408 409 410



272 274 302 312 312 315 323 323 324 326 335

Contents

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Chapter 7 M  etal Oxide Nanowires  7.1 MgO  7.2 A  l2O3, Ga2O3 and In2O3  7.3 SnO2  7.4 CeO2, SiO2 and GeO2  7.5 TiO2  7.6 CrO2, MnO2 and Mn3O4  7.7 CuxO  7.8 ZnO  7.9 Vanadium and Tungsten Oxides  7.10 Other Binary Oxides  7.11 Ternary and Quarternary Oxides  References 

421 421 425 433 438 443 448 449 451 456 458 461 462

Chapter 8

469 469 472 474 483 487 488 488 490 494 494 496

 etal Nitride, Carbide and Boride Nanowires  M 8.1 BN  8.2 AlN  8.3 GaN  8.4 InN  8.5 Si3N4 and Si2N2O  8.6 Metal Carbide and Boride Nanowires  8.6.1 BC  8.6.2 SiC  8.6.3 Other Carbide Nanowires  8.6.4 Borides  References 

Chapter 9 N  anowires of Metal Chalcogenides, Phosphides and Other Semiconductor Materials  9.1 Metal Chalcogenide  9.1.1 CdS  9.1.2 CdSe and CdTe  9.1.3 PbS, PbSe and PbTe  9.1.4 CuS and CuSe  9.1.5 ZnS and ZnSe  9.1.6 NbS2, NbSe2 and NbSe3  9.1.7 Bismuth Chalcogenides  9.1.8 Other Chalcogenides  9.2 GaAs, InP and Other Semiconductor Nanowires  9.2.1 GaAs  9.2.2 InP and GaP  9.3 Miscellaneous Nanowires  9.4 Coaxial Nanowires and Coating Nanowires 

503 503 503 506 508 510 513 515 515 517 518 518 520 522 524

Contents



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9.5 Perovskite Nanowires  9.5.1 Vapor-­phase Synthesis  9.5.2 Solution-­phase Synthesis  9.5.3 Template-­assisted Methods  References 

527 527 529 531 531

Chapter 10 F  unctionalization and Useful Properties and Potential Applications of Nanowires  10.1 Self Assembly and Functionalization  10.2 Useful Properties and Potential Applications  10.2.1 Optical Properties  10.2.2 Photonic Applications of Perovskite NWs  10.2.3 Electrical and Magnetic Properties  10.2.4 Transistors and Devices  10.2.5 Field Emission  10.2.6 Energy Storage and Conversion  10.2.7 Electromechanical Devices  10.2.8 Sensor Applications and Other Aspects  10.2.9 Mechanical Properties  10.2.10 Thermoelectric Properties  10.2.11 Biological Aspects  References 

541 541 549 549 554 557 559 563 563 566 567 570 570 573 574

Subject Index 

585

Chapter 1

Carbon Nanotubes 1.1  Introduction Graphite and diamond are the two well-­known crystalline allotropes of carbon. Three-­coordinate sp2 carbons in graphite form planar sheets with the flat six-­membered benzene ring as a motif. Carbon atoms have a four-­ coordinate sp3 nature in diamond whose motif is the chair conformation of cyclohexane that forms a three-­dimensional network. Fullerenes are the new crystalline allotropes of carbon, consisting of three-­coordinate carbon atoms having closed-­cage structures with spherical or nearly-­spherical surfaces, C60 being the best-­known example. C60 consists of a truncated icosahedral structure formed by twenty hexagonal rings and twelve pentagonal rings (Figure 1.1(a)). In 1985, Kroto et al.1 discovered fullerenes while studying the origin of carbon in outer space. In fullerenes, the coordination at each carbon atom is somewhat pyramidalized due to the inclusion of certain sp3 character in basically sp2 carbons. The coordination at every carbon atom in fullerenes is slightly pyramidalized with the presence of some sp3 character in essentially sp2 carbons. The presence of five-­membered rings is the key which provides the curvature necessary for developing a closed-­cage structure. In 1990, Krätschmer et al.2 found that arcing graphite electrodes produces C60 and other fullerenes. The ability to generate fullerenes in gram amounts in the laboratory utilizing a reasonably simple apparatus spurred intensive research work on these molecules and prompted a revival in carbon research. Iijima3 detected graphite nanotubules at the negative electrode during direct current arcing of graphite for the preparation of fullerenes in 1991. Because of the existence of five-­membered rings, these nanotubes are concentric graphitic cylinders that are sealed at both ends. Nanotubes may be multi-­walled, with a core tubule (with a diameter in the nanometer range) surrounded by   Nanoscience & Nanotechnology Series No. 52 Nanotubes and Nanowires, 3rd Edition By C. N. R. Rao, A. Govindaraj and Leela Srinivas Panchakarla © C. N. R. Rao, A. Govindaraj and Leela Srinivas Panchakarla 2022 Published by the Royal Society of Chemistry, www.rsc.org

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Chapter 1

graphitic layers separated by ∼3.4 Å. Single-­walled nanotubes (SWNTs), on the other hand, consist of only the tubule and no graphitic layers. Figure 1.1b shows a typical transmission electron microscope (TEM) image of a multi-­walled nanotube (MWNT). In this nanotube, graphite layers surround the central tubule. The structure of a nanotube formed by two concentric graphitic cylinders, which is obtained by force-­field calculations, is shown in Figure 1.1c. Imagine a single-­walled nanotube by cutting C60 down the middle and spacing the hemispherical corannulene end-­caps apart by a single-­layer graphitic cylinder of the same diameter. Carbon nanotubes (CNTs) are the only carbon allotropes with expanded bonding but no dangling bonds. CNTs are tubular fullerenes or bucky tubes since they are derived from fullerenes. Since the discovery of carbon nanotubes,3 several methods of preparing them have been explored.4,5 Arc-­evaporation of graphite electrodes, laser ablation, chemical vapor deposition (CVD) and vapor phase decomposition or disproportionation of carbon-­containing molecules have all been used to produce single-­walled (SWNTs), double-­walled (DWNTs) and multi-­walled (MWNTs) carbon nanotubes.6,7 Kumar and Ando8 have reviewed CNT synthesis, mechanism of formation and mass production by CVD. MWNTs and SWNTs have also been synthesized by other techniques including electrochemical synthesis9 and pyrolysis of organic molecular precursors.10 Among the various types of CNTs, SWNTs are of special interest due to their unique

Figure 1.1  (a)  Schematic diagram of a C60 molecule; (b) A TEM image of a multi-­ walled carbon nanotube; (c) Minimum energy structure of a double-­ walled carbon nanotube; (d) Electron diffraction pattern of a multi-­walled carbon nanotube. (a–c) Reproduced from ref. 31a with permission from John Wiley and Sons, © 2001 WILEY‐VCH Verlag GmbH, Weinheim, Federal Republic of Germany. (d) Reproduced from ref. 31c with permission from Elsevier, Copyright 1994.

Carbon Nanotubes

3

properties and potential applications. Specific methods have been applied to get long SWNTs11 as well as diameter-­control of the nanotubes.12 Various forms of SWNTs such as rings,13 brushes14a and films14b have been synthesized. Vertically aligned15 as well as horizontally aligned16 CNTs have been obtained on different substrates by several workers. Strategies have been developed for the enrichment or selective generation of SWNTs with metallic, semiconducting and other unique electronic properties.17 Diameter selective dispersions of SWNTs have been reported.18 High-­resolution electron microscopy has been used extensively to study the structure of carbon nanotubes.19–21 Nanotubes are closed at both ends but can be opened by different oxidants after being formed by arc discharge between graphite electrodes.22,23 The filling of nanotubes with various materials has made significant progress.24 Aside from opening and loading, CNTs have also been doped with boron and nitrogen, yielding p-­t ype and n-­t ype products, respectively. CNTs were used as removable templates to create carbidic, oxidic and other nanostructures. For particular applications, aligned nanotube bundles have been developed. Carbon nanotube properties and phenomena, as well as many potential and probable uses, have been widely published. As a result, it's not surprising that these nanomaterials have sparked a lot of curiosity. Carbon nanotubes have been the topic of many review articles, special issues of journals and conference proceedings.25–31 A few of the articles discuss potential technological applications, including an emphasis on the electronic properties,30,31 with the book by Reich et al.32 dedicated to a systematic presentation of the fundamental mechanics of carbon nanotubes. There are also other reviews and novels, several of which are referred to as guides.33–38 After the discovery of carbon nanotubes, researchers have focused on inorganic layered materials such as MoS2, WS2, and BN to investigate the creation of nanotubes from these materials. Many nanotubes have been produced and characterized.39–42 In Chapter 4, inorganic nanotubes are thoroughly discussed. Here, we shall present different aspects of carbon nanotubes, which include their preparation, structure, formation mechanism, chemical modification, functionalization, properties and applications. We will go into their electronic structure and associated effects, as well as their vibrational, thermal, and mechanical characteristics. These factors are linked because both thermal and mechanical properties represent the bonding in the carbon network, which also governs their electronic structure.

1.2  Synthesis 1.2.1  Multi-­walled Carbon Nanotubes Multi-­walled carbon nanotubes can be readily prepared by striking an arc between two closely separated graphite electrodes in helium. The pressure of arc apparatus is ∼0.7 atm (∼500 Torr), which is considerably higher than the helium pressure used to prepare fullerene soot. Figure 1.2 shows a schematic

Chapter 1

4

Figure 1.2  Schematic  diagram of an arc discharge apparatus. Reproduced from ref. 43b with permission from Elsevier, Copyright 2004.

illustration of an arc discharge apparatus. A high yield of carbon nanotubes can be achieved by a current of 60–100 A across a potential drop of about 25 V. The optimized arcing process converts a major portion of the carbon anode into carbon nanotubes and graphitic nanoparticles that get deposited on the cathode.43a Arc evaporation of graphite has also been conducted in several ambient gases (He, Ar and CH4).43b Hydrogen is found to be effective in making MWNTs of high crystallinity. Arc-­produced MWNTs in a hydrogen atmosphere contain fewer carbon nanoparticles. Large quantities of carbon nanotubes have been made using plasma arc-­jets by adjusting the quenching route in an arc between a graphite anode and a cooled copper electrode.44,45 If both the electrodes used in arc-­discharge are of graphite, MWNTs are the main products, along with side products of fullerenes, amorphous carbon and graphite sheets.

Carbon Nanotubes

5 46

Arc-­discharge in liquid nitrogen produces highly crystalline MWNTs. In this process, the arc discharge chamber is filled with liquid nitrogen. Typically, direct current is delivered to the apparatus using a power supply. The cathode and anode are pure graphite rods of 10 mm diameter and 8 mm diameter, respectively. A Dewar flask is filled with liquid nitrogen and the electrode assembly immersed in the nitrogen. Arc discharge happens when the separation between the electrodes is progressively made smaller (∼1 mm), and a current of ∼80 A flows between them. When the arc discharge is completed, carbon deposits near the cathode are collected for analysis. Liquid nitrogen inhibits the electrodes from contamination with undesirable gases and also lowers the overall temperature of the electrodes. Further, nanotubes do not stick to the walls of the arc discharge chamber. Through this method, the yield of MWNTs can reach up to 70% of the reaction product. Auger-­spectroscopy reveals that nitrogen is not incorporated into the MWNTs. Synthesis under a magnetic field produces high purity (>95%) and defect-­free MWNTs, which can be used for device fabrication as nanosized electric wires.47 Here, the magnetic field around the arc plasma controls the arc discharge between electrodes of pure graphite (purity 99.999%) (Figure 1.3a and b). Mass production of MWNTs can be achieved economically by a rotating arc discharge technique.48 The centrifugal force produced by the rotation creates turbulence and accelerates the carbon vapor perpendicular to the anode (Figure 1.3c). Rotation generates a stable plasma and increases the plasma volume and the plasma temperature. At 5000 rpm rotation and 1025 °C, a yield of 60% is obtained (without the use of a catalyst). The yield increases up to 90% at 1150 °C, if the rotation speed is increased. The MWNTs obtained by this procedure generally have an inner diameter of 1–3 nm and an outer diameter of ∼10 nm. The deposition of carbon vapor on cooled, highly oriented pyrolytic graphite gives tube-­like structures.49 Carbon nanotubes have been prepared by electrolysis in molten halide salts under argon with carbon electrodes.50 They have also been obtained under hydrothermal conditions at 800 °C in

Figure 1.3  (a)  and (b) Schematic diagrams of the system for the synthesis of MWNTs in a magnetic field, (c) Schematic diagram of a plasma rotating electrode system. (a–b) Reproduced from ref. 47 with permission from AIP Publishing, Copyright 2002. (c) Reproduced from ref. 48 with permission from Elsevier, Copyright 2002.

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Chapter 1

the presence of a nickel catalyst under 60–100 MPa pressure, using a polyethylene–water mixture.50 Besides the conventional arc-­discharge technique, carbon nanotubes have been produced by chemical vapor deposition (CVD), where hydrocarbons such as CH4 and C2H2 are decomposed under inert conditions at ∼700 °C over Fe/graphite,51 Co/graphite52 or Fe/silica53 catalysts. Transition metal particles are generally used for forming nanotubes by the CVD or pyrolysis process, and the nanotube diameter is mostly determined by the metal particle size.54

1.2.1.1 Chemical Vapor Deposition (CVD) One of the most popular thin film deposition methods is chemical vapor deposition (CVD). CVD varies from other popular methods for growing carbon nanotubes, such as electric arc discharge and laser vaporization.55,56 Catalytic CVD is a medium temperature (500–1100 °C) and long-­period reaction (typically minutes to hours) method, while arc discharge and laser vaporization are high temperature (>3000 °C) and short-­time reaction (µs–ms) techniques. The CVD mechanism converts energy to the molecule by utilizing a carbon supply (e.g., CO, CH4, C2H2) in the gas phase and plasma or a resistively heated coil. The molecules decompose into atomic carbon. The produced carbon then diffuses to the substrate, which is preheated and coated with the desired catalyst (typically a first row transition metal such as Ni, Fe or Co) and binds to it. The main technological drawbacks with arc discharge and laser vaporization is that the carbon nanotubes are produced as stand alone.57,58 The carbon nanotubes do not grow on a conventional or patterned substrate. Good alignment59 along with positional control on a nanometric scale60 is attained by using the CVD process. Control over nanotube diameter as well as the growth rate, is also achieved. Use of a suitable metal catalyst permits superior growth of single-­walled over multi-­walled nanotubes.61 CVD synthesis of CNTs is basically a two-­step process, involving preparation of a catalyst step followed by synthesis of the nanotube. The catalyst is usually prepared by sputtering an appropriate transition metal onto a substrate, followed by etching by chemicals such as ammonia, or thermal annealing, to allow the nucleation of the catalyst nanoparticles. Thermal annealing helps in the formation of metal clusters on the substrate from which the CNTs grow. The synthesis temperature of nanotubes by CVD is generally in the 650–900 °C range.59–62 The typical yield of nanotubes from CVD is around 30%. Several CVD processes developed for carbon nanotube synthesis include plasma-­enhanced CVD, alcohol catalytic CVD, thermal chemical CVD, laser-­assisted CVD and aerogel-­supported CVD. A major advantage of CVD is that the CNTs can be used without postpurification unless the catalyst nanoparticle is required to be removed. Combustion of polypropylene in the presence of Ni catalyst and an organically modified clay gives high yields of MWNTs.63 In this procedure silica–alumina is used as a support and combustion is conducted at 600 °C. Thin wall (two to four graphitic walls) MWNTs are prepared by methane

Carbon Nanotubes

7

decomposition over Mo/Ni/MgO catalysts. The formation of thin walls are attributed to in situ formation of Ni–Mo catalyst nanoparticles with sizes in the range of 2–16 nm.64

1.2.1.2 Plasma-­enhanced Chemical Vapor Deposition Plasma-­enhanced chemical vapor deposition (PECVD) is one of the suitable techniques to grow high yield vertically aligned nanotubes with desirable positions. In flat panel displays and field emitters, vertically aligned CNTs play an important role. Compared to thermal CVD, the temperates of the substrates are low in the PECVD technique as the high energy electrons dissociate the precursors. Several plasma techniques are used to deposit CNTs, including microwave PECVD, hot filament PECVD, radio frequency (RF) PECVD, inductively coupled PECVD and dc (glow discharge) PECVD. The glow discharge plasma-­enhanced CVD process involves a glow discharge in a reaction furnace or a chamber through which a high-­frequency voltage is applied to the electrodes. A representative diagram of a plasma CVD device is shown in Figure 1.4. A substrate is positioned on the grounded electrode. Fe, Co and Ni sputtered on Si/SiO2/glass substates are employed as catalysts. The formation of nanoparticles of the catalyst help the growth of CNTs. A gaseous carbon source, such as CH4, C2H2, C2H4, C2H6 or CO is supplied into the chamber during discharge.65 The catalyst has a strong influence on the nanotube diameter, growth rate, morphology, number of walls and microstructure. Nickel is found to be the most appropriate catalyst in this method for the growth of aligned carbon nanotubes.66 The diameter of the carbon nanotubes is about 15 nm. Chen et al. succeeded in getting a high yield of MWNTs (∼50%) at a fairly low temperature (Co>Pt>Cu or Nb and Co/Ni, Co/Pt>Ni/Pt>Co/Cu, respectively. SWNTs generally form in bundles due to van der Waals forces. SWNTs with more than 70% yield have been produced by the condensation of a laser-­vaporized carbon–nickel– cobalt mixture at 1200 °C.56,136 There are emission spectral similarities in the excited species in laser ablation of a composite graphite target and in laser-­irradiated C60 vapor. This suggests that fullerenes may be formed by laser ablation of catalyst-­filled graphite. However, subsequent laser pulses may excite the fullerenes to emit C2 that adsorbs on the catalyst particles forming SWNT growth. However, there is not sufficient evidence to support this conclusion. Laser ablation and arc discharge are similar since the catalyst mix and the optimum background gas are the same in the two processes. Condensates generated in laser ablation are often contaminated with carbon nanoparticles and MWNTs. Pure graphite electrodes yield MWNTs, but uniform SWNTs are formed when a mixture of graphite with Co, Ni, Fe or Y is used instead of pure graphite. SWNTs so synthesized exist as ropes. Optimization of catalyst composition (Ni : Y is 4.2 : 1) appears to improve the yield. With a Ni/Co catalyst and a pulsed laser at 1470 °C, SWNTs with a diameter of 1.3–1.4 nm are produced. Use of a continuous laser with the Ni/Y catalyst (Ni : Y is 2 : 0.5 at%) at 1200 °C produces single-­walled nanotubes (average diameter of 1.4 nm) with an yield of 20–30%. Because laser ablation yields good-­quality nanotubes, efforts have been made to scale up the process. However, the results are not as good as from the arc-­discharge method. Laser vaporization results in high-­quality SWNTs with better properties and narrower size distribution than those produced by arc discharge. Nanotubes prepared by laser ablation have greater purity (up to ca. 90%) than those produced by the arc-­discharge process. SWNTs are efficiently produced by using ultrafast pulses from a free-­ electron laser (FEL; pulse width is ∼400 fs) with a pulse repetition rate of 75 MHz.133 Here, the laser bundle intensity behind the lens approaches ∼5 × 1011 W cm−2, which is ∼1000 times higher than in the Nd : YAG system. A preheated (1000 °C) argon passing as a jet through a nozzle tip is placed close to the rotating graphite target, containing the catalyst. The jet of argon deflects the ablation plume approximately 90° away from the incident FEL beam direction, clearing the carbon vapor from the region in front of the target. The SWNT soot is collected on a cold finger. This process is shown in Figure 1.15b. The yield is about 1.5 g h−1 at 20% of the maximum power. With this method, the maximum yield with the current lasers is up to 45 g h−1 (with a Ni/Co or Ni/Y catalyst) in an argon atmosphere at 1000 °C at a wavelength of ∼3000 nm.

28

Chapter 1

The SWNTs are in bundles each contaning 8–200 SWNTs, with a diameter of 1–1.4 nm and a length of 5–20 microns. A continuous-­wave laser has also been used for SWNT synthesis, based on the laser ablation of graphite mixed with a metallic catalyst by a 2 kW continuous wave CO2 laser in nitrogen or an argon stream.134 The absorbed laser power is more effectively utilized for material evaporation and the technique yields 5 g h−1 with a Ni/Co catalyst (Ni : Co is 1 : 1) at 1100 °C. The yield of SWNTs is 20–40%, with an average diameter of 1.2–1.3 nm. The progress of SWNT production by the laser ablation technique has been reviewed by Arepalli.137a Lasers of various forms are now widely used to prepare single-­walled carbon nanotubes. The Smalley group's original system employed a double-­pulse laser oven technique. Several researchers have used laser combinations such as one laser pulse (green or infrared), varying pulse widths (ns to ms, as well as continuous wave), and different laser wavelengths (e.g., CO2, or free electron lasers in the near to far infrared). Any of these variations have been used for various metal catalyst combinations and quantities, buffer gases (e.g., helium), oven temperatures, flow pressures and graphite target porosities. Nikolaev et al.137b investigated the effect of laser vaporization temperature (on Co/Ni/C and Rh/Pd/C catalysts) on the diameter and chiral angle distributions of SWNTs and found that lower temperatures lead to smaller nanotube diameters and narrow (n,m) type distributions, with preference towards large chiral angles. Kingston et al.137c have also reviewed advances in laser synthesis of SWNTs and discussed the variants in terms of optimizing the growth.

1.2.3.4 Pyrolysis or Vapor Phase Deposition Under controlled pyrolysis, dilute hydrocarbon–organometallic mixtures produce SWNTs.93,138a Pyrolysis of metallocene–acetylene mixtures at high temperatures (∼1100 °C) yields SWNTs138a (see Figure 1.6c). The diameter of the SWNTs here is 1.4 nm. Figure 1.6d shows the SWNTs achieved similarly by the pyrolysis of ferrocene–CH4 at 1100 °C. It may be recalled that the pyrolysis of nickelocene–benzene under similar conditions primarily produces MWNTs. Figure 1.6c shows an amorphous carbon coating around the SWNT, common in such preparations. The amorphous carbon can be avoided by decreasing the amount of the hydrocarbon and mixing H2 into the Ar stream. Pyrolysis of acetylene and binary mixtures of metallocenes also produces SWNTs, due to the beneficial effect of such binary alloys.138b Good yields of SWNTs can be achieved by pyrolysis of acetylene mixed with Fe(CO)5 at 1100 °C. Pyrolysis of ferrocene–thiophene mixtures also gives SWNTs, but the yield is found to be low. Pyrolysis of ferrocene along with benzene and thiophene produces SWNTs with a high yield.139

1.2.3.5 Chemical Vapor Deposition (CVD) Unlike laser vaporation and arc-­discharge methods, CVD is used to scale up production to industrial levels. Colomer et al.140 produced high yields of SWNTs with CVD by using transition metal-­supported MgO substrates with

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29 141

methane as a carbon source. Flahaut et al. used CVD to prepare SWNTs where decomposition of methane was carried out over oxide spinels incorporated into a transition metal (produced by the combustion route). Here, hydrogen was used as a carrier gas. The quality of SWNTs was characterized through adsorption measurements. An enhanced CVD growth approach was used by Franklin and Dai142 to get highly oriented SWNTs in high yields (150 µm length) and extensive networks of suspended SWNTs. They introduced a conditioning step where, in a CVD growth environment, large amounts of catalyst (Fe–Mo/Al2O3) is positioned upstream of the patterned catalytic substrate. Extensive quasi-­ordered networks of nanotubes formed at 900 °C under methane flow (1000 sccm, 20 min). Here, the orientation of the nanotubes was directed by the pillar or tower structures on the patterned substrate. These authors propose that small concentrations of benzene that are catalytically produced flow to the downstream substrate, providing an effective carbon-­feedstock for SWNT production. Vertical growth of SWNTs with small diameters (0.8–1.6 nm) has been accomplished by heating small iron nanoparticles in the presence of an activated gas.143 The experimental set up is schematically shown in Figure 1.16a. The SEM images in Figure 1.16b–d confirm the growth of aligned SWNTs produced by this method. A thin layer of iron on a substrate was heated quickly in the presence of an activated gas. The activated gas was produced by rapidly passing a gas mixture of CH4 (40 sccm) and H2 (400 sccm) at pressures of 15–25 Torr, over a hot filament having temperatures exceeding 2000 °C to create activated gas mixtures of carbon-­containing species and hydrogen. The effect of precursor hydrocarbons on carbon nanotube formation was studied by Li et al.144 Of the six hydrocarbons (methane, cyclohexane, benzene, hexane, naphthalene and anthracene) used as carbon precursors, methane was found to be best for the formation of high-­purity SWNTs. Isolated SWNTs (1.45 nm diameter) have been fabricated using discrete nickel catalyst nanoparticles with the CVD technique.145 Ultralong SWNTs (4 cm) can be obtained by CVD at a high growth rate of 11 µm s−1.146 These studies indicate the possibility of uninterrupted SWNT growth without any length constraint. By a direct-­floating CVD process, Song et al. produced tough non-­woven material from SWNTs up to several tens of square centimetres.147 The non-­woven product shows a very high Young's modulus of ca. 0.4–0.7 TPa. SWNTs have been synthesized using a fluidized-­bed process requiring hydrocarbon flow fluidization of a catalyst/supply at high temperatures.148 By using silica-­gel particles coated with nickel nitrate and methane, SWNTs are found to grow densely at 760 °C over the whole surface of the support particles. This technique appears to offer advantages over the fixed-­bed method. These workers have also produced SWNTs in a hot fluidized-­bed reactor by rapid insertion of nickel formate (catalyst) and silica gel (support).149 Bimetallic catalysts such as Fe/Pt and Fe/Ru in the 0.5–3 nm size range are effective for the efficient growth of SWNTs on flat surfaces.150 The compatibility of various catalysts in CVD synthesis on Si substrates has been studied, and Fe catalysts found to be better for ethylene CVD and Co catalysts

30

Chapter 1

Figure 1.16  (a)  Schematic drawing of a 1″ hot filament CVD furnace system used for aligned SWNT growth. SEM images of carpet SWNTs grown over different times: (b) 1.3 min, scale bar 1 µm; (c) 20 min, scale bar 10 µm; (d) 40 min, scale bar 10 µm; (e) inside view of the sample grown for 40 min, scale bar 10 µm, inset image scale bar 1 µm. Reproduced from ref. 143 with permission from American Chemical Society, Copyright 2006.

for ethanol CVD.151 Pender et al.152 have used spin-­on catalysts to prepare SWNTs on various substrates using CVD at 900 °C with methane and hydrogen. The catalyst precursor was composed of iron nitrate and a solution of siloxane polymer. The matrix of siloxane helps in the controlled creation of catalytic nanoparticles by phase separation. A smooth continuous SiOx film is formed after heating, which prevents the coarsening and agglomeration of the catalysts. A simplified production process developed by Okazaki and Shinohara153 for SWNTs employs hot-­filament assisted chemical vapor deposition (HFCVD) using alcohol vapor as the carbon source. Plasma-­enhanced chemical vapor

Carbon Nanotubes

31 154

deposition has been used by Kato et al. to prepare thin bundles of SWNTs as well as individually grown SWNTs. Plasma-­enhanced CVD (PECVD) at 600 °C yields mainly semiconducting SWNTs (>85%).155 Zeolites containing 1D channels have been employed to synthesize monosized SWNTs.156 The synthesis route suggested by Endo et al.157 for SWNT production involves the use of a template and a floating catalyst. Nanosize zeolite has been used as the template, where metal nanoparticles are anchored in the exposed pores to prevent aggregation of the particles. Such seeding leads to high-­purity SWNTs. The three-­dimensionally floating template in the reaction chamber enables the growth of nanotubes in a semicontinuous process with a wide range of diameters (0.4–4 nm). In an efficient method of synthesis of SWNTS by CVD, the lifetime of the catalyst is enhanced by water.158 Enhanced catalytic activity by the presence of water results in super growth of vertically aligned and superdense nanotube forests up to 2.5 mm thick, readily separable from catalysts, with a purity of over 99.98%. A controlled supply of steam into the CVD reactor acts as a weak oxidizer and selectively removes amorphous carbon without damaging the growing CNTs. Controlling the relative ratios of ethylene and water is crucial to maximize the catalyst lifetime. Zhong et al.159 have shown that a reactive etchant such as water or hydroxyl radicals is not required in cold-­wall CVD reactors if the hydrocarbon activity is low. These studies show that the proper choice of the precursor is crucial for obtaing good growth as well as quality and yield of nanotubes. A low-­temperature method for the growth of SWNTs by water plasma chemical vapor deposition (CVD) has been reported.160 The water plasma lowers the growth temperature to 450 °C. A study on the kinetics of SWNT growth by water-­assisted CVD has revealed the catalytic role of water.161 Water-­assisted CVD solves some of the issues faced in the synthesis of carbon nanotubes. SWNTs can be grown continuously from an ordered array of open-­ended SWNTs.162 A root-­growth mechanism is seen to operate in the synthesis of vertically aligned SWNTs by microwave plasma CVD.163 SWNTs grow preferentially when CVD is carried out over SiO2 spheres.164 Aligned assembly of SWNTs on solid substrates is favored by the application of an electric field.165 Many metals serve as a catalyst for the growth of SWNTs. Besides widely applied transition metals (Ni, Co, Fe), a variety of other metals (Cu, Mn, Mo, Cr, Mg, Al, Pt, Pd, Sn, Au) have also been identified as catalysts for growth of horizontally-­aligned SWNTs on quartz substrates.166 Unlike Fe, Co and Ni, the solubilities of carbon in Au, Ag, Pt, Pd etc. are low, but improve with decreasing particle size, especially below 5 nm.167,168 Au nanoparticles deposited on Si have been used as a substrate to grow SWNTs.169 In this case, Si/Au alloy (about 80 at% Au) has been proposed to be the active catalyst. Cu has also been employed to grow CNTs efficiently. High densities of highly crystalline SWNTs have been grown on Cu nanoparticles supported on silicon substrates using methane and ethanol as carbon sources at 825–925 °C. The lengths of the nanotubes reached 1 cm.170 The synthesis of Cu nanoparticles involved the reduction of CuCl2 in the presence of Cu2O nanoparticles.171

32

Chapter 1

CNTs have also been grown in a CVD process without using metallic particles. In this process, ethanol is decomposed at 850 °C over graphite substrate coated with nanodiamond (5 nm).172 A mixture of layered CNTs, isolated CNTs and high-­density CNT mats were present in the product. Fusion of the nanodiamond particles has not been observed even at high temperatures during the CVD process, suggesting that they maintain a solid state during the CVD process. Oxygen has been shown to activate the growth of CNTs in some studies. Many metals, which are inactive as a catalyst to grow CNTs, have been shown to be active in the respective oxide forms.173 Directional growth of nanotubes has been observed on sapphires without any templates.174 Semiconductor nanoparticles such as Si and Ge have also been employed to grow CNTs (though C has little solubility in bulk Si or Ge). In these experiments the nanoparticles are preheated in the air before performing CVD.175 High-­efficiency growth of single-­walled carbon nanotubes has been achieved over Si/SiO2 substrate (sputtered 30-­nm-­thick SiO2) in CVD without using any further metal particles. In this process CH4 was used as the carbon source.176 Using scratched Si/SiO2 wafer as the substrate, patterned growth of CNTs has been achieved. The scratch produces SiO2 nanoparticles, which are active centres for the growth of SWNTs.177 Several other oxides (such as TiO2, Al2O3 and rare earth oxides) have been used to grow SWNTs. Both SWNTs and MWNTs are grown by CVD using ZrO2 nanoparticles.178

1.2.3.6 Alcohol Catalytic CVD Alcohol catalytic CVD has enabled the production of high-­quality SWNTs on a large-­scale at low cost. In this method, at a relatively low temperature of 550 °C, vaporized alcohol (ethanol, methanol) is passed over catalyst particles (cobalt and iron) supported on zeolites. The carbon atoms with dangling bonds are removed by alcohol hydroxyl radicals that are barriers to the preparation of high-­purity SWNTs. This method yields SWNTs with ∼1 nm diameter.179 The high purity of the product and low reaction temperature suggest the possibility of upscaling. Low reaction temperature often means that the procedure is appropriate for direct growth of SWNTs on aluminium-­patterned semiconductor chips. Vertically-­aligned SWNTs have been obtained on Mo/Co-­coated silicon and quartz substrates.180,181 The intermittent supply of acetylene in ethanol in CVD appears to preserve catalyst activity and enhance the growth rate.182

1.2.3.7 Aerogel-­supported Chemical Vapor Deposition SWNTs are synthesized using this process by decomposing carbon monoxide on an aerogel-­supported Fe/Mo catalyst at 860 °C. The surface area of the supporting material, reaction temperature and feed gas are all variables that impact the yield and efficiency of SWNTs. The efficiency of the catalyst is much higher than in other processes owing to the large surface area,

Carbon Nanotubes

33 183

ultralight density and porosity of the aerogels. High-­purity (99%) SWNTs are produced after a basic acid treatment and oxidation. The yield of the nanotubes is poor when CO is used as the feed gas, but the total purity of the substance is high. The nanotubes have a diameter distribution of 1.0 to 1.5 nm.184

1.2.3.8 Laser-­assisted Thermal Chemical Vapor Deposition A continuous-­wave CO2 laser with medium power is perpendicularly focused onto a substrate in laser-­assisted thermal CVD where acetylene and Fe(CO)5 vapor is pyrolyzed in a flow reactor. Silica substrate is generally used. Small iron particles act as the catalyst for the growth of nanotubes.185 Both SWNTs and MWNTs can be prepared in this process by using a reactant gas mixture of ethylene, acetylene and iron pentacarbonyl vapor. Diameters of the SWNTs and MWNTs range from 0.7 to 2.5 nm and 30 to 80 nm, respectively.

1.2.3.9 CoMoCat Process Here, SWNTs are grown by the disproportionation of CO at 700–950 °C.186 This technique was developed at the University of Oklahoma and it is being commercialized by South West Nanotechnologies (SweNT) Inc. The method is dependent on a Co/Mo catalyst formulation that prevents the sintering of Co particles, preventing the creation of undesirable sources of carbon. Cobalt is steadily lowered from the oxidic to the metallic state during the reaction. Molybdenum is simultaneously transformed into the carbide form (Mo2C). Cobalt is the active species in the activation of CO, while MO has a dual function. It can stabilize Co as well-­dispersed Co2+, thereby avoiding its reduction, and function as a carbon sink to moderate carbon development, thereby preventing the formation of undesirable types of carbon. Critically, for successful reactor activity, the space velocity must be large enough to hold CO conversion to a minimum. Fluidized bed reactors have the benefit of allowing constant inclusion and removal of solid particles from the reactor without stopping the process. The method can be scaled without sacrificing SWNT efficiency. SWNTs of various diameters may be created by changing the operational conditions. The CoMoCat catalyst has a strong selectivity for SWNTs (80–90%).

1.2.3.10 High-­pressure CO Disproportionation A gas-­phase catalytic method was used by Nikolaev et al.187 to produce SWNTs involving pyrolysis of CO along with Fe(CO)5. SWNTs were prepared by the decomposition of CO on a Co/Mo catalyst supported on silica.188 The “high-­pressure CO” disproportionation process (HiPco), is an improvized technique using continuous gas flow for catalytic production of SWNTs using Fe(CO)5 as the precursor and CO as the carbon feedstock. By passing a small

Chapter 1

34

amount of Fe(CO)5 mixed with CO through a heated reactor, SWNTs are produced. The layout of the CO flow-­tube reactor is shown in Figure 1.17. By controlling the CO pressure, the nanotube size and diameter distribution can be controlled. For bulk production of carbon nanotubes, this process is promising.187 The method yields SWNTs with a diameter as small as 0.7 nm (which is considered to be the chemically smallest achievable SWNTs).189 The HiPco method produces SWNTs with an average diameter of ∼1.1 nm and with around 70% yield. This technique can produce SWNTs with 97% purity up to 450 mg h−1.190 A parametric analysis has been performed on the gas-­ phase growth of SWNTs through the HiPco method.191 This process for producing large amounts of SWNTs fulfils its promise.190 In the HiPco process, nanotubes grow on catalytic iron clusters at high-­temperatures. Thermal decomposition of Fe(CO)5 produces the catalyst in situ, which interacts with flowing cold CO combining in the reaction zone with hot CO. These Fe particles surface catalyze SWNTs nucleation and growth in the gas phase through disproportionation of CO (Boudouard reaction). The Boudouard reaction rate scales as a square of CO pressure, and hence the significance of high pressure for efficient production of SWNTs. Fe 2CO   CO2  C  SWNT 

Current production rates exceed 450 mg h−1 (or 10 g day−1), and nanotubes do not have more than 7 mol% of iron impurity. A second-­generation HiPco reactor can run continuously for 7–10 days. HiPco is an appealing gas-­phase method that, unlike other CVD methods, does not use presynthesized catalyst particles. A closed-­loop reactor has been designed to increase the yield and to prevent the emission of CO into the environment.

Figure 1.17  Layout  of a CO flow-­tube reactor, showing water-­cooled injector and “showerhead” mixer. Reproduced from ref. 187 with permission from Elsevier, Copyright 1999.

Carbon Nanotubes

35

1.2.3.11 Flame Synthesis Flame synthesis is based on the use of a regulated flame atmosphere in which carbon atoms are produced together with metal catalyst aerosols from hydrocarbon fuels.192 On metal islands, SWNTs grow in the same way as in arc discharge and laser ablation. A metal (cobalt) submonolayer film (510 nm) is deposited by direct vapor deposition (PVD) on a stainless steel mesh (SS-­304). Droplet-­like metal islands formed on the mesh support act as catalyst particles. Once subjected to a flame, these small islands form an aerosol. Another way is to create aerosol-­t ype metal particles by burning a filter paper rinsed with a metal ion solution (e.g., iron nitrate).193 Laplaze et al. have developed a method that utilizes concentrated solar radiation to vaporize graphite in order to synthesize SWNTs.194

1.2.3.12 Sonochemical Route A sonochemical route has been developed to produce high-­purity single-­walled carbon nanotubes under ambient conditions.195 In this process, ferrocene is mixed with p-­xylene followed by addition of silica powder. High-­purity SWNTs with diameters of 1.3–1.8 nm are grown by performing ultrasonication under ambient conditions. The SWNTs are collected by filteration. Ferrocene is a source of the Fe catalyst for the growth of nanotubes and both ferrocene and p-­xylene act as the carbon source, while silica powder offers nucleation sites for nanotube growth. This technique also offers a simple means to high-­purity SWNTs without need of any complex purification processes.

1.2.4  Direct Spinning of Nanotube Yarns Postsynthesis methods are available for production of continuous nanotube fibres. However, directly producing such fibers would be advantageous. Windle and coworkers196 developed a process, which consists of spinning of the CNT fibres from the CVD synthesis zone of a furnace onto a rod or spindle. In this process, ethanol containing 0.23–2.3 wt% ferrocene as well as 1.0 and 4.0 wt% thiophene is used as starting material. This solution is introduced into the furnace in a hydrogen carrier gas. MWNTs are formed in an aerogel in this process at a temperature of 1100–1180 °C and are wound onto a rotating rod. SWNTs can also be produced by tuning the temperature, hydrogen flow rate and the ratio of thiophene ethanol concentration. Further improving the process yielded fibres with high toughness and stiffness.197

1.2.5  S  elective Preparation of Semiconducting and Metallic SWNTs Methods for the selective preparation of semiconducting and metallic SWNTs have been reported.17 Significantly, some of them require only a single step. Selective growth of semiconducting SWNTs have been produced by

Chapter 1

36 15,155,198,199

plasma-­enhanced chemical vapor deposition (PECVD). Dai and coworkers155,192 have demonstrated the preferential growth of semiconducting SWNTs up to 90 percent purity by PECVD at 600 °C (for a schematic of the experimental set-­up see Figure 1.18a). The TEM and atomic force microscopy (AFM) micrographs in Figure 1.18b reveal the growth of SWNTs from the iron island. The semiconducting nature of nanotubes can be characterized by Raman spectroscopy (Figure 1.18d and e). They attribute the preferential formation of semiconducting SWNTs to the low temperature conditions which predominantly produce smaller diameter nanotubes. The proportions of metallic and semiconducting nanotubes and the diameter of the SWNTs obtained by plasma-­assisted CVD seem to be connected.155,192 At lower temperatures, a high proportion of semiconducting SWNTs are produced, which is a result of the formation of smaller diameter nanotubes at lower temperatures. Pyrolysis of acetylene at 750 °C over an iron-­coated (0.5 nm) Al layer (10 nm) over Si/SiO2 substrate yields vertically aligned semiconducting SWNTs.15,193 By combining PECVD and fast heating, a proportion of the semiconducting SWNTs (∼96%) are obtained. High-­density horizontally aligned semiconducting nanotubes SWNTs (over 95%) have been produced on single-­crystal quartz substrates.199b These researchers suggested that introducing methanol during the growth process leads to the predominant growth of aligned semiconducting nanotubes. Selectively etching out metallic SWNTs from a mixture of metallic and semiconducting SWNTs has been achieved by gas-­phase reactions.200 Thus, gas-­phase plasma hydrocarbonation has been found to etch metallic SWNTs selectively by gasifying and keeping the semiconducting SWNTs.200a Hydrogen plasma also etches preferentially metallic nanotubes over semiconducting nanotubes.200b Irradiation of SWNT thin films in air by laser predominantly wipes out metallic SWNTs.201 Xenon-­lamp irradiation of densely packed aligned SWNTs also preferentially etches metallic SWNTs over semiconducting SWNTs.202 The authors have characterized SWNT arrays before and after irradiation by Raman, AFM and electrical transport measurements. The field effect on/off ratios increased from 10 to above 2000 after irradiation. Chemical extractions have also been used to selectively separate metallic tubes from semiconducting tubes by using long alkyl-­chain containing benzenediazonium compounds.203 Reacting SWNTs with perfluorinated polyolefines selectively removes metallic nanotubes via cycloaddition reactions.204 Fluorine gas preferentially reacts to the metallic nanotubes over semiconducting nanotubes.205 Hydrosilylation of SWNT-­FET device networks selectively eliminates metallic SWNTs.206 Zhang et al.207 studied the reaction of SWNTs with SO3 gas. Exposure of SO3 gas to SWNTs at 400 °C etch selectively semiconducting SWNTs. Applying dielectrophoretic force fields in a microfluidic channel to a mixture of metallic and semiconducting nanotubes, selectively and continuously extracts metallic SWNTs (Figure 1.19).208 This method is highly selective as well as nondestructive. Enriched metallic SWNTs (up to 65 percent) have been produced by pyrolysis of long-­chain monohydroxy aliphatic alcohols over a

Carbon Nanotubes

37

Figure 1.18  (a)  Schematic of the PECVD reactor. (b) SWNTs grown at 600 °C.

(A) AFM image of nanotubes grown from low-­density ferritin deposition on a SiO2 substrate. (B) AFM image of a tube grown from an iron-­ film island (nominal thickness 1 Å). (C) TEM image of an as-­grown SWNT (diameter 1.2 nm). (d) Left and right panels give the RBM band and the G-­band respectively. Reproduced from ref. 155 with permission from American Chemical Society, Copyright 2004.

38

Chapter 1

Figure 1.19  Metallic  nanotube extraction by microfluidic channel using dielec-

trophoretic force. (a) An H-­shaped channel with two inlets and two outlets. M and S designate samples of pure metallic and enriched semiconducting SWNTs collected at each outlet. (b) Separation mechanism based on dielectrophoresis. Metallic SWNTs (light color rods) were subjected to a significantly larger dielectrophoretic force, perpendicular to the direction of the flow, than the semiconducting SWNTs (dark color rods). (c) UV-­vis–near IR spectra of nanotube suspensions before and after separation. Reproduced from ref. 208 with permission from American Chemical Society, Copyright 2008.

Fe–Co/MgO catalyst.209 Selectivity is found to be dependent on the ratio of carbon to oxygen atoms in the precursor. The formation of hydroxyl radicals during the reaction are assumed to responsible for the selective etching. Selective synthesis of metallic SWNTs has been achieved by Voggu et al.17,210a by the arc-­discharge method (Figure 1.20). In this process, Ni + Y2O3 catalyst-­filled graphite electrodes are DC arc evaporated in the presence of Fe(CO)5. Fe(CO)5 is introduced continuously into the arc system by flowing He bubbled through an Fe(CO)5 solution. Arc evaporation deposites cobweb-­ like structures in the chamber comprising predominently up to 94% pure metallic SWNTs. Metallic nanotubes have been identified by both optical absorption and Raman spectroscopy studies. Optical absorption spectra show a band related to only metallic SWNTs (M11) and the bands corresponding to the semiconducting SWNTs are nearly absent (Figure 1.21a). The RBM band in the Raman spectrum reveals that the diameter of SWNTs produced

Carbon Nanotubes

39

Figure 1.20  Schematic  showing formation of SWNTs in the different regions of the arc-­dischargechamber.(A)TEMand(B)FESEMimagesofpurifiedSWNTs.Reproduced from ref. 210 with permission.

in the presence of Fe(CO)5 vapor changes and leading to the formation of metallic SWNTs (Figure 1.21b). The Breit–Wigner–Fano peak of the metallic nanotubes also broadens. The electrical properties are also indicative of formation of enriched metallic nanotubes. The SWNTs produced by the arc method using Fe(CO)5 show much lower sheet resistance. This technique can be scaled up to get bulk enriched metallic SWNTs. Liu and Hersam210b have reviewed the advances in SWNTs sorting and selective growth.

1.2.6  Chirality-­defined Synthesis of SWNTs Even after the separation of metallic SWNTs from semiconducting SWNTs, each semiconducting nanotube shows a different band gap, thus properties depend on their chirality and diameters. Selectively synthesizing SWNTs of particular chirality or separating them from one another after synthesis is extremely important for electronic applications.

1.2.6.1 Direct Controlled Synthesis Many factors influence the chirality and growth of SWNTs, including the catalyst, reaction temperature, carbon source and carrier gas. However, the chirality of SWNTs gets fixed at the nucleation step and will not alter during

40

Chapter 1

Figure 1.21  (a)  Electronic absorption spectra A and B are respectively spec-

tra of SWNTs obtained in the absence and presence of Fe(CO)5 and (b) Raman spectra of SWNTs collected from walls of the arc chamber: (A) RBM bands (B) G-­bands. (i) With the Ni + Y2O3 catalyst alone and (ii) with the Ni + Y2O3 catalyst in the presence of Fe(CO)5 vapor. Reproduced from ref. 210 with permission.

the growth process. It is therefore essential to control the nucleation step to achieve chirality-­controlled SWNTs. In this direction, Smalley et al. reported the growth of much longer SWNTs from predeposited open-­ended short SWNTs of the same type docked with iron nanoparticles.211 These authors showed from the AFM images that the short SWNTs served as templates to grow longer SWNTs via the vapor–liquid–solid (VLS) mechanism, with the same diameter. Raman measurements indicate similar radial breathing mode (RBM) frequencies before and after the growth confirming the growth of the same (n,m) SWNT as the template.212 Many other workers have used templates for the growth of chirality-­ defined SWNTs. Yao et al.213 used open-­ended nanotubes as templates and showed that they could be further elongated by VLS amplification growth without using a metal catalyst. The above reports prove the point that SWNTs can be cloned by predepositing SWNT templates, but the template itself is not chirality enriched. Liu et al.214 used single-­chirality enriched SWNT seeds (specific-­chirality SWNT seeds were achieved by postsynthesis DNA

Carbon Nanotubes

41

separation) as templates and achieved 100 times longer SWNTs in the CVD process. Growth of the same chirality nanotubes was confirmed by Raman and field-­effect transistor measurements. Care should be taken in assigning chirality with Raman measurements if the SWNT diameter is large (>1.2 nm) as more than one chirality is possible for a given RBM frequency. By this cloning method, the authors achieved different chirality SWNTs including (6,6), (7,7), (6,5), (7,6), (8,3), (9,1) and (10,2).215 Using metal nanoparticles to grow chirality-­specific SWNTs is extremely challenging due to the difficulty in achieving identical diameters and morphology as well as maintain the diameter and morphology at high SWNT growth temperatures. Bimetallic nanoparticles such as Co–Mo show good anchoring over silica support and are used successfully as catalysts to achieve selective growth of (6,5) and (7,5) SWNTs.216 It has been possible to make 57% of (6,5) and (7,5) together in semiconducting nanotubes, as estimated by photoluminescence measurements. The (n,m) distribution of SWNTs can also be controlled by gas feed composition, reactant temperature and type of catalyst.217 By optimizing growth conditions, ∼55% enriched (6,5) SWNTs have been achieved. When CO is used as a feed over Co–Mo/SiO2 catalysts, an increase in nanotube diameter is observed as the temperature rises, with no difference in chiral angles. Nanotubes of equal diameters but different chiral angles are obtained by flipping the support from SiO2 to MgO. Under the same reaction conditions, varying the nature of the gaseous feed results in various (n,m) distributions. Under high-­pressure CO or vacuumed C2H5OH and CH3OH on a Co–Mo catalyst, a narrow (n,m) distribution tends to be obtained.218 Wang et al.219 have reported selective growth of bulk SWNTs enriched with three different dominant chiralities including (6,5), (7,5) and (7,6) through the adjustment of the pressure of CO (2–18 bar) on Co–Mo catalysts. Bimetallic Fe–Ru supported on SiO2 has been found to be a good catalyst for (6,5) chirality enriched SWNTs under CVD conditions, with methane as the carbon source at 600 °C.220 When the growth temperature is increased to 800 °C, the selectivity shifts to (7,5), (7,6) and (8,4). MgO supported Fe–Cu catalyst,221 and SiO2 supported Co–Pt catalyst222 yield (6,5) chirality enriched SWNTs under CVD conditions at 600 °C and 800 °C, respectively. Fouquet et al.223 achieved (6,5)-­enriched SWNTs by using thermally evaporated cobalt on a Si/SiO2 wafer as the catalyst followed by annealing and exposing to C2H4. It has been proposed from XPS analysis that interaction between Co and Si helps to keep the narrow distribution of the catalyst. MgO is also found to be a good support for Co nanoparticles.224 This catalyst enabled 55% enriched (6,5) SWNTs to be produced under CVD conditions. Cobalt-­incorporated mesoporous silica (TUD-­1)87 and CoSO4 on SiO2 catalysts have been used to synthesize (9,8)-­enriched SWNTs. In the latter case, sulfur is found to have a crucial role in the selectivity of (9,8) SWNTs via the formation of Co–S bonds, thus preventing agglomeration of cobalt atoms.225 One of the important challenges in chirality-­specific synthesis is to prevent the catalyst from changing its shape and melting under high-­temperature

Chapter 1

42 226

CVD conditions. Li et al. have proposed a bimetallic catalyst based on tungsten. These authors have synthesized a W6Co7 catalyst on silicon wafers by molecular cluster decomposition, which was found to have high selectivity (∼92%) towards (12,6) nanotubes under ethanol CVD conditions. From XRD and TEM analysis, the authors found that the (0 0 12) plane of the catalyst is a perfect match for the open end of the (12,6) SWNTs responsible for chiral selectivity. By changing the reaction conditions, the authors later achieved a majority of (1 1 6) planes on the W6Co7 catalyst, suitable for zigzag (16,0) SWNTs with nearly 80% selectivity.227 These authors have also noticed the importance of a high flow of hydrogen gas for stabilizing (16,0) SWNTs. Raman spectroscopy, electron diffraction as well as photoluminescence, were used to confirm chirality. By tuning the same catalyst these authors have also achieved (14,4) SWNTs with high purity.228

1.2.6.2 Postsynthesis Separation Approaches Postsynthesis separation techniques have been effectively applied to separate different chirality nanotubes from a mixture. Density gradient ultracentrifugation (DGU) is found to be an effective way to separate nanotubes by their chirality and diameter as well as electronic structure.229 In this technique, SWNTs are treated with suitable surfactants such that the surfactants wrap around the SWNTs. Organization and packing density of a particular surfactant around the SWNTs depends on the structures and electronic properties of the SWNTs. These soluble surfactant-­SWNT hybrids are subjected to DGU to sort SWNTs by their diameter, electronic structure and band gap. Hersam's group used bile salts and mixtures of bile salts with anionic-­ alkyl surfactants to separate SWNTs by their diameter and electronic type.229 By the orthogonal iterative DGU separation strategy, Hersam's group have demonstrated separation of >98% single‐chirality (6,5) SWNT samples from mixtures.230 Weisman's group has demonstrated the separation of 10 different semiconducting single chirality SWNTs by nonlinear DGU that has allowed them to separate enantiomerically enriched fractions of the (n,m) species.231 Kataura's group applied gel chromatography to separate surfactant wrapped SWNTs to individual chiral SWNTs.232 By careful choice of the mobile phase and surfactant along with the temperature more than ten different chiral SWNTs have been obtained in milligram quantities. Aqueous two-­phase (ATP) separation233 is a simple and scalable technique that has become popular in the last few years to separate DNA or polymer-­dispersed SWNTs. In this technique, the phase separation phenomena of two polymers are used. Dispersions of polymer-­wrapped SWNTs are introduced into one of the phases and allowed to move to the other phase via differential preference of the polymer. Single-­stranded DNA (ssDNA) can wrap around helical SWNTs, and interaction between them depends on the ssDNA sequence as well as the diameter and electronic properties of the SWNTs.234 Ion exchange chromatography (IEC) can be used to separate SWNTs from DNA–SWNT hybrids based on

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43

both their diameter and conductivity (metallic vs. semiconducting). Zheng et al. have used this technique to get enriched (6,5) SWNTs from a mixture of CoMoCAT nanotubes.235 Zheng et al. also used a combination of size-­ exclusion chromatography (SEC) and IEC to obtain enriched (6,4), (9,1), and (6,5) SWNTs.236 Even though (9,1) and (6,5) SWNTs have the same diameter, their interaction with ssDNA differs due to the differences in chirality, thus enabling them to separate from one another. Zheng et al. have identified about 20 different ss-­DNA sequences and purified specific (n,m) SWNTs. The selection in the IEC method depends on electrodynamic and electrostatic interactions between the ion exchange resin and the ssDNA–SWNT hybrid.237 Chemical reactivity of SWNTs depends on their diameter, metallicity and chirality.238,239 Smaller diameter SWNTs are more prone to air oxidation, owing to the presence of the high strain induced by the misalignment of pi orbitals due to the larger curvature. Miyata et al.238 have revealed that the oxidative combustion of SWNTs also depends on the chiral angle of SWNTs. This strategy can be used for chirality selection. These authors have found that the initial oxidation by cycloaddition and breaking of the C–C bond is the rate-­determining step. These experiments have been done on a bundle of nanotubes. Liu et al., on the other hand, have conducted combustion experiments on individually suspended SWNTs and have revealed that for a similar diameter and metallicity, higher chiral angle of SWNTs are more stable than lower chiral angle ones.240

1.2.7  Junction Nanotubes Theoretical predictions suggest that junction carbon nanotubes such as Y-­junctions would hold unique electronic properties that could be applicable in different electronic circuits.241–243 Electronic characteristics of SWNT/SWNT junctions could be semiconductor–semiconductor (S–S) heterojunctions, metal–semiconductor (M–S) Schottky junctions or metal–metal (M–M) junctions. Both M–S and S–S junctions made from two different band gaps would show a rectifying diode with nonlinear transport characteristics.244–247 Junctioned single-­walled nanotubes are frequently observed in SWNT samples prepared by different techniques, including laser ablation,248 CVD249 and arc discharge.250 Highly branched Y-­junction CNTs have also been produced by an aerosol-­based method.251 Sequential temperature variations while the formation of the SWNTs leads to sequential intramolecular junctions along nanotubes.245,252 Preparation of CNTs with junctions has been an important challenge. Template methods have been used previously to generate junction multi-­walled carbon nanotubes.253 Introducing additives such as thiophene along with precursors during the growth yields large quantities of branched nanotubes. This was first studied by Rao et al. in 2000.254 The pyrolysis of nickelocene with thiophene yields up to 70% junction MWNTs. Here, nickelocene serves as both the carbon source and the catalyst (Figure 1.22a and b). The electrical characterization of the juctioned nanotubes show linear I–V characteristics along the arms, but show an interesting asymmetric I–V curve at the Y-­junction (Figure 1.22c).

44

Chapter 1

Figure 1.22  (a)  A typical TEM image of a Y-­junction nanotube, (b) HREM image of a Y junction showing the fishbone-­t ype stacking of graphene sheets and (c) I–V curves collected from different points in a Y-­junction carbon nanotube: (A) from one of the arms away from the junction and (B) from the junction of the three arms. Insets show plots of dI/dV vs. bias voltage. The observed gaps are indicated by arrows. Reproduced from ref. 254a with permission from AIP Publishing, Copyright 2000.

Different additives have been introduced into the CVD process to produce branched MWNTs.255 Sulfur (thiophene) plays a key role in producing junctions in the nanotubes with stacked-­cone morphologies.256 The presence of sulfur, even at minute amounts, promotes the creation of heptagons (negative curvature) and pentagons (positive curvature). Multi-­terminal SWNTs possess more advantages over MWNTs. Thus, there have been some efforts to produce junctioned SWNTs. The thermal decomposition of C60 has also been found to yield some amount of Y-­junction SWNTs in the presence of Fe, Co, Ti, Cr and Ni, which were identified by STM.257,258 Choi and coworkers259,260 used thermal CVD to produce Y-­junction SWNTs by using methane as the carbon scouce and Mo-­or Zr-­doped Fe nanoparticles supported on Al2O3 as a catalyst. It is believed that nucleation and growth of CNTs are promoted by the presence of Mo or Zr in the Fe catalyst, and formation of branches when they are connected to the sidewalls of the existing tubes. Voggu et al.261 have used the arc-­discharge method in the presence of He and thiophene to synthesize Y-­junction SWNTs using graphite filled with Ni/Y2O3 as an electrode (Figure 1.23).

Carbon Nanotubes

45

Figure 1.23  TEM  image of Y-­junction carbon nanotubes prepared by arc discharge

of graphite over a Ni/Y2O3 catalyst in an atmosphere of thiophene and helium. Reproduced from ref. 17a with permission from the Royal Society of Chemistry.

1.2.8  Peapods and Double-­walled Nanotubes Single-­walled carbon nanotubes (SWNTs) filled with fullerenes have been discovered by Smith et al.262a These authors used acid-­purified SWNTs grown by a laser oven technique56 (inset of Figure 1.14a). Fullerene filled nanotubes are peapods because the arrangement looks like miniature peas in a pod. In the peapods, fullerenes are organized one-­dimensionally with a constant intermolecular distance that is slightly lower (by 3%) than the nearest C60 molecular distance in the solid. The vapor-­phase reaction of C60 with an acid-­purified SWNT bundle gives C60 peapods in high yields.263a The reaction is conducted in a vacuum-­sealed glass ampoule at 400 °C, the reaction time being one day or more to fill the tube with C60 (attaining a yield close to 100%). The acid-­purified SWNTs have to be dried in dry air at 420–480 °C for 1 h in order to burn off contaminated carbon. Such burning also ensures nanotubes with open ends. The temperature of burning depends on the cleanliness of the surface of the nanotubes. C60 molecules, organized in the nanotube, fuse in vacuum above 800 °C and form an inner nanotube. A TEM study of the merging mechanism of C60 in the 800–1000 °C range shows that, the internal tube forms due to structural relaxation of C60 dimers, trimers, tetramers and so on in the “bottleneck” region. Fullerenes inside a nanotube are difficult to remove. The electronic structure calculations on C60@(10,10) peapod show that the formation is exothermic with an energy gain of 0.51 eV.264

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Chapter 1

Metallofullerenes (endohedral fullerenes) have also been incorporated in the hollow region of SWNTs and examined by TEM and other techniques.263b A vapor-­phase doping technique has been used to encapsulate C60 and metallofullerenes (Gd@C82) into SWNTs on Si substrates.265 Before the vapor-­phase incorporation of fullerenes, SWNTs produced on a SiO2/Si substrate are annealed in dry air to open the ends. From Raman spectroscopy, the smallest diameter SWNT capable of encapsulation of fullerenes is estimated to be 1.28 nm for C60 and 1.43 nm for Gd@C82. By refluxing SWNTs with n-­hexane solutions of C60 and C70, Simon et al.266a could obtain peapods at temperatures as low as 69 °C. Using a similar procedure, they synthesized (N@C60 : C60)@SWNT. Pantoş and coworkers266b have put C60 molecules into helical CNTs (studied through circular dichroism). The arc-­discharge method has been used to synthesize double-­walled carbon nanotubes (DWNTs) in the presence of a catalyst and a mixture of Ar and H2 (1 : 1/v : v) at 350 torr.267 DWNTs synthesized from MWNTs + carbon nanofibres have a higher purity than those from graphite powders.268 The yield of DWNTs is more than 80%, the rest being SWNTs. The isolated DWNT ends were uncapped. The high-­temperature pulsed arc-­discharge technique at 1250 °C with Y/Ni alloy catalyst yields high-­quality DWNTs.269 Optimal parameters for the synthesis of DWNTs are similar to those of SWNTs, suggesting that the growth pathways of the two are interrelated. Outer and inner diameters of the DWNTs are 1.6–2.0 and 0.8–1.2 nm, respectively, as revealed by high-­resolution TEM and Raman spectroscopy studies. Chains of C60 molecules inside SWNTs can be converted to DWNTs in quantitative amounts.263,270 Coalescence between C60 units inside SWNTs occurs on heating at ∼1200 °C. The structural transformation of C60-­encapsulated SWNTs (C60 peapods) into DWNTs has been monitored via X-­ray diffraction by Abe et al.271 The intertube spacing between the inner and outer tubes was found to the 0.36 ± 0.01 nm. Bandow et al.263a have reviewed the mechanism of formatation of DWNTs from peapods. Production of DWNTs has been achieved by catalytic CVD in the 900–1000 °C range using carbon-­containing molecules such as THF, hexane, propanol, benzene, and alcohol over Fe–Mo on MgO or Al2O3 support.272a–c Figure 1.14b and c display TEM images of an isolated as well as a bundle of DWNTs (77% yield), prepared by methane CVD over a solid solution of Mg1–xCoxO containing Mo oxide.272d The diameter of DWNTs so obtained depends on the conditions of the process and the source of carbon. SWNTs as well as DWNTs are formed by the decomposition of methane over Mg0.9FexCoyO (x + y = 0.1) solid solutions.273 Aligned DWNT ropes are obtained directly by sulfur-­assisted floating catalytic decomposition of methane.274 Synthesis of DWNTs by CVD of alcohol over Fe/Co embedded in mesoporous silica was reported by Shinohara and coworkers.275a Various silica materials with the required pore diameter and morphology have been investigated for DWNT production. The reaction temperature, thermal stability and pore size of the support material affect the selectivity and diameter

Carbon Nanotubes

47 275b

distribution of the DWNTs. Endo et al. have produced DWNTs by a conditioning catalyst of molybdenum (Mo/Al2O3), which is placed at one end of the reactor and a nanotube catalyst of iron (Fe/MgO), which is kept in the central part of the reactor. By feeding a methane + argon mixture (1 : 1) into the reactor for 10 min (kept at 875 °C), two sets of DWNT pairs with an inner-­to-­outer diameter ratio of 0.77 : 1.43 and 0.90 : 1.60, respectively, were obtained. Use of the conditioning catalyst allows the growth of more DWNTs than SWNTs. DWNTs have been prepared by the decomposition of a CH4 + Ar mixture over a Mo0.1Fe0.9Mg13O catalyst, prepared by the combustion route.276 It is significant that Fe/Mo/MgO catalyst prepared by the combustion route yields a high percentage of DWNTs (>90%) compared to other catalyst preparation techniques. Catalysts prepared by other techniques such as sol–gel and coprecipitation yield 10–15% DWNTs, the rest being SWNTs and MWNTs.

1.2.9  Mechanism of Formation of Nanotubes For carbon nanotubes prepared by pyrolysis of hydrocarbons on metal surfaces, multiple growth models have been suggested. A four-­step mechanism was suggested by Baker and Harris.277 In the first step, the hydrocarbon decomposes to unleash hydrogen and carbon on the metal surface, which dissolves in the particle. The second step includes carbon diffusing through the metal particle and depositing on the rear side to form the filament's core. The flow of carbon to the front face is greater than the diffusion through the material, allowing the front face to absorb the gas. Surface diffusion of the carbon creates a layer around the central filament structure, in the third step. In the fourth step, the catalyst is overcoated and deactivated, and the tube growth terminated. Utilizing controlled atmosphere electron microscopy, Baker et al.278 have demonstrated the mechanism of carbon filament growth by the decomposition of acetylene at 600 °C over a silica/graphite supported by Ni. During the process, the shapes of the metal nanoparticles change and they move up with a trail of carbon deposit. This is called the tip-­growth mechanism (Figure 1.24A). They observed a similar tip-­growth process with Fe, Co and Cr catalysts.279 The change of metal shape suggests a liquid state during the growth of the nanotube. The calculated activation energy of the growth is close to the activation energy needed for carbon diffusion in liquid nickel suggesting that carbon diffuses through the bulk metal during the process and the rate of growth is diffusion-­controlled. In the case of acetylene decomposition over a bimetallic Pt–Fe catalyst, the catalyst remained static on the substrate, while the carbon filament kept growing. This led them to propose a base-­growth model (Figure 1.24B).280 The interaction between the substrate and the Pt–Fe is strong so that catalyst particles are strongly anchored on the SiO2 surface during growth and carbon precipitates via the free upper face of the nanoparticle. Growth dynamics are controlled by concentration gradient and temperatures.

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Figure 1.24  An  illustration of the tip growth mechanism (A) and the base/root growth mechanism (B) of nanotube growth. Reproduced from ref. 278 with permission from Elsevier, Copyright 1972. (B) Reproduced from ref. 280 with permission from Elsevier, Copyright 1975.

Oberlin et al.281 suggested a model where there is negligible bulk diffusion, and carbon is transferred through surface diffusion around the particle. Dai et al.122 suggested a mechanism wherein carbon forms a hemispheric graphene cap (yarmulke) on the catalyst particle, and the nanotube emerges from the cap. The size of the catalytic particle controls the diameter of the nanotube, nanometre-­size particles producing SWNTs. A central feature of this model is that at all stages of growth, it avoids dangling bonds. SWNTs could form in arc vaporization by the yarmulke mechanism. Based on the ejection of C2 from C60 formed by mass spectrometry, Endo and Kroto indicated that the production of MWNTs required the creation of fullerenes.282 Nevertheless, Smalley283 noted that such a system would only enable the growth of outer layers of multi-­walled tubes. Iijima et al.20 provided evidence for open-­ended growth of carbon nanotubes by electron microscopy and proposed that the closure of incomplete carbon layers seen on the tube surface could result from the expansion and thickening of the nanotubes by the development of graphite islands on the surfaces of existing tubes. The nucleation of heptagons and pentagons on the open tube ends contributes to a change in the direction of the expanding tube, and novel morphologies occur, including one where the tube turns about 180° during growth. The growth is self-­similar and fractal-­like with the smaller tubules telescoping out of the bigger ones. Isotope scrambling experiments by Ebbesen et al.284 showed that the plasma had vaporized carbon atoms under the conditions of fullerene production. A mechanism related to that of Saito et al.285 was proposed based on

Carbon Nanotubes

49

tube morphologies, in which the carbonaceous material reaching the cathode anneals into polyhedral particles. The tip could open and continue to grow, under the right conditions. Ebbesen et al. proposed that tubes could develop directly from the closure of a large sheet of graphene. Such a hypothesis gains credence from the simulations of Robertson et al.286 who looked at curling and closing graphitic ribbons. Both entropy and enthalpy seem to support the formation of tubes. The concept of a spatial velocity hodograph is used by Amelinckx et al.287 to define the extrusion of a carbon tubule from a nanoparticle catalyst. The model is compatible with the tubule shapes seen and describes how a tubule can undergo spontaneous plastic deformation. A model in which both concentric cylinders and scroll-­t ype structures are formed as been proposed. Nanotubes nucleate from a wide fullerene cap. Two kinds of catalytic growth can occur in the case of SWNTs formed by the arc method: (a) one where multiple nanotubes develop outwards from a single catalyst particle that is much greater than the diameter of the nanotube and (b) another where the catalyst particles are of the same size or smaller than the diameter of the nanotube and a single nanotube grows from the nanoparticle. The yarmulke mechanism122 described explains SWNT growth by mode (b). Based on calculations of molecular dynamics and total energy, Maiti et al.288 suggest a model for (a), in which nanometer-­sized protrusions on the surface of the metal particles contribute to SWNT nucleation. A graphene layer caps the metal nanoparticle and protrusions of nanometer scale on the metal surface leave holes in the graphene sheet. Carbon atoms are introduced to the graphene layer, in the shape of handles between a pair of closely adjacent carbon atoms. These handles travel along the sheet of graphene until they reach the edge of the initial hole to form seven-­membered rings producing a curving geometry. The seven-­membered rings are contained in the high curvature area and hexagons continue to grow to form the body of the tube. TEM analysis of the carbonaceous materials produced in hydrocarbon pyrolysis and organometallic precursors indicates that the size of the catalyst particle plays a role in relation to the nature of the product created. Figure 1.25a schematically illustrates the effect of the size of the catalyst particle on the nature of the carbon nanostructure. While nanotubes with diameters >50 nm do exist, this should not be taken as a law. When the catalytic metal nanoparticle is about 1 nm in diameter, SWNT are primarily formed.97,138 Dai et al.122 have observed that SWNTs are formed in the size range of 1–4 nm on Mo nanoparticles. Metal nanoclusters of about 1 nm diameter are generated under controlled conditions from organometallic precursors. When the organometallic content is high, MWNTs are produced around catalytic particles with sizes between 5 and 20 nm. This is the case with nanotubes produced by the metallocene method.71,72 Graphite-­covered metal particles are primarily produced in the higher size range of ∼50 nm.97 Moisala et al.289 have reviewed the advancement of SWNTs synthesis by CVD and aerosol methods with emphasis on the role of catalytic nanoparticles. The active catalyst is in the reduced metalic form based on measurements

50

Chapter 1

Figure 1.25  (a)  Schematic showing the dependence of the carbon nanostructure obtained by hydrocarbon pyrolysis on the size of the metal nanoparticles. Reproduced from ref. 31a with permission from John Wiley and Sons, Copyright © 2001 WILEY-­VCH Verlag GmbH, Weinheim, Federal Republic of Germany; (b) Sequence of in situ TEM images showing the growth of a bamboo-­like carbon nanotube catalyzed by a Ni particle at 650 °C. Reproduced from ref. 295 with permission from American Chemical Society, Copyright 2007.

of the carbon and oxygen concentrations. It was found that the carbon that is precipitated depends on the diameter of the catalyst particle, the conditions of the reactor and the carbon feed rate. In aerosol synthesis, the precipitation rate affects the form of precipitated carbon. In CVD, surface groups on the support play a role in particle size distribution. In aerosol synthesis, the gradients of velocity and temperature in the flow reactor are related to particle size distribution. For efficient generation of SWNTs, it is necessary to prevent aggregation of amorphous material, and regulate the feed rate of carbon to the metal particles. Helveg et al.290 reported in situ growth of MWNTs using HRTEM. They decomposed methane over a Ni catalyst at 500 °C. The in situ TEM observation revealed that the Ni cluster maintains crystallinity with well-­faceted shape throughout the growth process. The growth of graphitic layers is a result of a dynamic interaction between Ni and carbon atoms. The surface structure of Ni nanoparticles continuously changed during the process, where the Ni atoms moved in and out, up and down. The shape of the nanocluster changed

Carbon Nanotubes

51

periodically from spherical to cylindrical to align the graphitic layers around them. The atomic steps on the catalyst boundary were suggested to play a crucial role in attaching carbon atoms and constructing the graphene network. The above observation shows that solid phase of the catalyst is maintained during growth, while carbon diffuses over the surface of the catalyst. Molecular dynamics simulation studies of the early stages of SWNT growth on metal nanoparticles also suggest surface diffusion of the carbon atoms.291 At first a graphene cap forms on the metal and floats over the catalyst, while the edge atoms of the cap remain attached to the catalyst. New carbon atoms then attach to the edge atoms, driving the cap to move up and creating a cylindrical wall. Hofmann et al.292 also reported similar findings by in situ TEM observations. Much of the experimental evidence shows that formation of SWNTs is from the small catalytic nanoparticles. Steep sharp edges over the small metal clusters (1–2 nm) have high catalytic activity. The atomic step sharpness decreases with increasing cluster size, thus decreasing catalytic activity. Bigger catalytic particles (5–20 nm) produce less-­strained MWNTs. Very big metal nanoparticles (∼100 nm) possess a nearly spherical periphery and do not show any catalytic activity towards carbon nanotube growth. Terrones, Kroto and coworkers studied in situ formation of nanotubes from a metal-­encapsulated MWNT by exposure to 300 kV electrons inside a TEM and keeping at 600 °C for 90 min.293 Carbon atoms from the side walls diffuse into the metal and emerge in the form of small-­diameter nanotubes, coaxial to the original MWNT. Such bulk diffusion cannot, however, be a general CNT growth mechanism. Real-­time growth dynamics of SWNTs have been observed using a ultra-­high vacuum TEM at 650 °C.294 Growth of bamboo nanotubes on a Ni/MgO catalyst by the catalytic decomposition of C2H2 at 650 °C has been examined in this manner.295 A series of images obtained is presented in Figure 1.25b. Nucleation of an internal cap begins in image (c), and the new graphitic sheet develops around the bottom border of the catalyst. Nickel nanoparticle elongates during nanotube growth until it is expelled from the nanotube, giving a completely shaped internal cap. Hydrocarbons decompose at high temperatures over nanometer-­size metal particles to produce nanotubes. The solubility and diffusion of carbon is high in iron, cobalt and nickel at elevated temperatures. Thus, these metals are most commonly used for growth of carbon nanotubes. Further, these metals possess high melting points and low equilibrium-­ vapor pressures, thus making them useful in CVD with the wide variety of temperature ranges and carbon sources. Ding et al.296 found that Fe, Co and Ni have stronger adhesion with the growing carbon nanotubes compared to the other transition metals and hence they are more efficient in forming high-­curvature (i.e. low-­diameter) nanotubes. The substrate material, its surface morphology and textural properties greatly affect the yield and quality of the resulting CNTs. Metals have high affinity for Al2O3-­based materials. Thus, Al2O3 is found to be a better catalytic support over silica to prevent agglomeration and lead to a high density of metal nanoparticles over

Chapter 1

52 297

298

substrates. Ago et al. examined the importance of metal-­support interaction in the growth of SWNTs and DWNTs with MgO-­supported diameter-­ controlled iron particles and found that iron particles were smaller during CVD and that the small particles gave SWNTs and DWNTs when methane was used. In situ X-­ray photoelectron spectroscopy studies of CNT growth from different precursors on iron catalysts supported on alumina and silica substrates have confirmed the role of metal–support interactions.299 Iron-­ loaded alumina flakes (0.04–4 µm thick) were found to give high aspect ratio aligned carbon nanotubes with high yields.12,300 The oxide substrate, basically used as physical support for the metal catalyst, could participate in the chemistry of CNT growth.301

1.2.10  Purification of SWNTs As synthesized SWNTs possess various impurities including amorphous carbon, catalytic metal particles and graphitic nanoparticles, they cannot be directly used for further applications.302 Hence, purification is an important step to separate nanotubes from the other unwanted products. A simple and effective strategy for purifying arc-­grown MWNTs is oxidation at high temperatures in air. Attempts to apply such a method to purify SWNTs failed, due to the presence of metal catalyst particles in the material. The metal particles indiscriminately catalyze the low-­temperature oxidation of carbons in the presence of oxygen and other oxidizing substances, damaging the SWNTs. Therefore, the first step preceding gas-­phase oxidative purification is the elimination of the metal particles. A general method for dissolving metal nanoparticles from the nanotubes is by treating with dilute mineral acids. Use of concentrated acids functionalize nanotubes and in some cases even damage them. As indicated before, entire removal of amorphous carbon is not possible by air oxidation. Most methods of nanotube purification include one or more of the following steps: oxidation in the gas or vapor form, wet chemical treatment/ oxidation, filtration or centrifugation (including chromatographic processes). To achieve complete purification, a combination of these steps is used. Refluxing of acids and strong oxidation have an impact on tube structure. Purification by wet chemical methods (e.g., acid digestion of impurities) gives rise to reaction products on the surface of the nanotube and make it a natural surfactant. Although separating the impurities from the catalyst is essential in purification, it is not enough. Through heating the nanotubes in the air near 300 °C, amorphous carbon is readily burnt away. Hydrothermal treatment may be used to eliminate amorphous carbon from the surface of nanotubes.303,304 Microfiltration can be used to eliminate SWNTs from other impurities.305 A mixture of acid digestion and air oxidation was employed by Martinez et al.306 to purify SWNTs produced by the arc-­discharge method. High-­temperature annealing of the purified samples was done to remove surface functional groups.307,308

Carbon Nanotubes 309,310

53

Chiang et al. first pointed out the need to remove metals from nanotubes. Their process starts with long (18 h) oxidative cracking of the carbonaceous shells encapsulating the metal particles at low temperature (225 °C). Wet oxygen (20% O2 in argon pushed through a water-­filled bubbler) is passed over the nanotubes in the hot zone of a flow tube furnace. The product is then agitated in HCl to remove iron particles. The oxidation and acid extraction processes are repeated at 325 °C after washing the acid and drying, accompanied by an oxidative bake at 425 °C. The step at 325 °C may, however, be unnecessary. The effect of oxidation treatment on the purity of SWNT films grown by the arc-­discharge method has been examined by Sen et al.311 Vivekchand et al.312 reported a purification technique for SWNTs using high-­temperature hydrogen treatment of the acid-­treated nanotubes at 700–1000 °C. H2 treatment efficiently removes amorphous carbon from acid-­ treated SWNTs. A typical method for purifying SWNTs prepared by the arc technique is air oxidation of nanotubes at 300 °C, then acid washing followed by heating in H2 at 700–1000 °C. The heat-­treatment temperature in H2 may vary for SWNTs prepared by different procedures. The procedure was successful for the purification of both SWNTs and MWNTs. Acid washing dissolves the metal particles and treatment with hydrogen eliminates carbon coating on the metal particles and amorphous carbon. This technique allows for the removal of other carbonaceous material along with amorphous carbon in the SWNTs without the need for microfiltration etc. This work should be compared with other literature procedures. All the techniques use acid cleaning to eliminate the metal particles. In air oxidation procedures, SWNTs undergo heat treatment in the range 350–500 °C, depending on the synthesis method. Foror most SWNTs and MWNTs, the method of Vivekchand et al.,312 employs hydrogen treatment around 1000 °C, except for HiPco SWNTs that require a lower temperature. Amorphous carbon is converted into CH4 at high temperatures with hydrogen treatment. Figure 1.26 shows the electron microscope images corresponding to the hydrogen purification process. Figure 1.26a shows TEM micrographs of SWNT bundles prepared by the arc-­discharge method containing nanoparticles of metal and amorphous carbon. Although acid washing dissolves most of the metal nanoparticles, the nanotubes are still covered with amorphous carbon (Figure 1.26b). High-­ temperature hydrogen treatment removes the amorphous carbon and the small metal catalyst particles agglomerate to large particles. The TEM image in Figure 1.26c shows the absence of amorphous material. TEM micrographs also show that after the treatment of hydrogen, bundles expand and have diameters between 20 and 50 nm. The nanoparticles get agglomerated in the region of 750–850 °C (Figure 1.26c) and are dissolved during the second acid treatment. After this treatment with hydrogen takes place at 1000 °C for pure SWNTs. Figure 1.26d displays a TEM micrograph of the purified SWNTs. Hollow onion-­like structures are not often seen, but are found in other approaches to SWNTs purification.

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Figure 1.26  TEM  images of (a) as-­synthesized SWNTs obtained by the arc-­discharge method, (b) after the first acid treatment, (c) after the first hydrogen treatment and before the second acid treatment and (d) after the second hydrogen treatment. Reproduced from ref. 312 with permission from American Chemical Society, Copyright 2004.

Figure 1.27 shows the Raman spectra of single-­walled nanotubes at different steps of purification. After purification, the intensities of the radial breathing modes and G-­band increase and the intensity of the D-­band falls as the amorphous carbon is removed. Carbonaceous impurities are also removed by high-­temperature CO2 treatment. A regulated, scalable multi-­ stage purification process for eliminating carbon-­based and metal impurities in raw HiPco SWNTs has been reported.313 Multi-­stage oxidation at increasing temperatures uncovers and oxidizes carbon-­coated iron nanoparticles, with the use of either C2H2F4 or SF6 to deactivate the oxidized iron nanoparticles. Metal nanoparticle impurities have been removed from SWNTs by magnetic filtration.314 Further purification of nitric acid-­treated SWNTs by centrifugation has been investigated at constant pH.315 Determination of the purity of the SWNTs after the purification procedure is an important task. In order to determine the purity of SWNTs,

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55

Figure 1.27  Raman  spectra of SWNTs (obtained by the arc-­discharge method) at

different stages of purification: (a) as-­synthesized SWNTs, (b) acid-­ refluxed SWNTs, (c) H2 tre ated SWNTs at 800 °C followed by acid washing and (d) final product after H2 treatment at 1200 °C followed by acid washing. Reproduced from ref. 312 with permission from American Chemical Society, Copyright 2004.

thermogravimetric analysis (TGA), Raman spectroscopy, electron microscopy and visible NIR spectroscopy are used. Electron microscopy cannot determine the purity of bulk SWNTs,316 due to the small volume of the analyzed sample and the absence of algorithms for converting the micrographs to numerical data. It is necessary to use Raman and NIR spectroscopy for the quantitative assessment of purity. A schematic of the electronic spectra of SWNTs is shown in Figure 1.28. The main characteristics in the spectra arise from the electronic transitions between the inter-­van Hove singularities in both the semiconducting and the metallic SWNTs. Relative purity of SWNTs can be evaluated based on the electronic absorption bands.317a,b Sorting lengths of SWNTs is an important aspect for various applications. Ultrasound treatment breaks SWNT bundles into open-­ended small fragments of 100–300 nm long.318 Different techniques have been used to obtain SWNTs sorted by length.319–323 Chromatographic strategies are effective for the fractionation of shorter SWNTs with sizes below 300 nm in length.319a,320,321,323 On the other hand, techniques like capillary electrophoresis284 and field-­flow fractionation322 are more applicable for longer SWNTs.

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Figure 1.28  Schematic  illustration of the electronic spectrum of a typical SWNT

sample produced by the electric arc method. The inset shows the region of the S22 interband transition utilized for NIR purity evaluation. In the diagram, AA(S) = area of the S22 spectral band after linear baseline correction and AA(T) = total area of the S22 band including SWNT and carbonaceous impurity contributions. The NIR relative purity is given by RP = (AA(S)/AA(T))/0.141. Reproduced from ref. 316 with permission from American Chemical Society, Copyright 2005.

1.2.11  Separation of Metallic and Semiconducting SWNTs The stochastic nature of SWNT growth produces metallic and semiconducting SWNT mixtures in a 1 : 2 ratio, bundled together, and their separation is challenging.187 In the last few years, several methods have been devised to separate semiconducting and metallic nanotubes.17,324 Methods to isolate semiconducting and metallic SWNTs include dielectrophoresis, selective destruction of one type of nanotube by irradiation or by chemical means, ultracentrifugation, noncovalent or covalent functionalization and selective interaction with molecules. Metallic SWNTs with small diameters are reported to be removed selectively by treatment with nitric and sulfuric acid mixtures (Figure 1.29).325

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Figure 1.29  Raman  spectra for HiPco SWNTs. (a), (b) RBM and G-­band Raman spec-

tra from the leftover on the filter for batch 1 and 2 HiPco SWNTs at 514 nm excitation wavelength. The bottom, middle and top lines are from the pristine, HNO3/H2SO4 (1 : 9) treatment for 12 h, and HNO3/H2SO4 (1 : 9) treatment for 48 h, respectively, followed by heat treatment at 900 °C in Ar atmosphere. (c) Raman spectrum of the filtrated sample after HNO3/H2SO4 (1 : 9) treatment for 12 h at 514 nm excitation wavelength. (d) Corresponding RBM and G-­band Raman spectra for batch 1 and 2 HiPco SWNTs at 633 nm excitation wavelength similar to (a). The RBMs were normalized with respect to the G-­band. Mii and Sii correspond to metallic and semiconducting interband transitions. Reproduced from ref. 325 with permission from American Chemical Society, Copyright 2005.

Semiconducting single-­walled nanotubes have been separated from metallic ones by applying selective interaction of aliphatic amines with functionalized nanotubes.326,327 Based on the improved chemical affinity of semiconducting SWNTs to octadecylamine [ODA, CH3(CH2)17NH2] separation has been achieved.326,328 The amine interacts more strongly with semiconducting SWNTs as revealed by spectroscopic and thermogravimetric studies. This decreases the ability of semiconducting SWNTs to accumulate as the concentration increases. Samsonidze et al.329 have developed an octadecylamine-­ assisted procedure and used changes in the integrated intensities of the radial-­breathing mode region in the Raman spectrum for the quantitative assessment of the separation efficiency. This separation technique enhances the proportion of semiconductor nanotubes by a factor of five in the case of SWNTs produced by high-­pressure CO decomposition.

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Large-­scale separation of semi-­conducting and metallic SWNTs were accomplished by a dispersion-­centrifugation technique using long-­chain alkyl amines, where the amine functional group strongly adsorbs on the metallic nanotubes.330 In this technique, SWNTs (prepared by the HiPco method) have been dispersed in a solution containing THF and 1-­octylamine. Centrifugation selectively yields enriched metallic SWNTs. Preferential charge-­transfer interaction between metallic nanotubes and bromine has been used to separate metallic nanotubes from semiconducting ones by centrifugating surfactant-­stabilized SWNTs.331 Typically, the separation is done by exposure of an aqueous SWNT suspension to a bromine solution in Triton X-­100 surfactant, followed by centrifugation at 24 000 g over twelve hours, resulting in an enrichment of the supernatant in semiconducting nanotubes and metallic nanotube as the sediment.331,332 This separation is significant, in that SWNTs in the water–surfactant suspensión stay suspended throughout the high-­speed centrifugal processes.333 By this technique, narrow size distributed SWNTs can be obtained with specific electronic properties in the presence of surfactants (Figure 1.30).334 Sucrose has

Figure 1.30  Sorting  SWNTs using density gradient ultracentrifugation (DGU).

(a) Small-­diameter (0.7–1.1 nm) SWNTs encapsulated with sodium cholate are sorted by diameter following DGU. Under these conditions, smaller-­diameter SWNTs, being more buoyant, settle at higher points in the centrifuge tube. Larger diameter and bundled SWNTs, being less buoyant, settle at lower points in the centrifuge tube. The observation of visible colours in the topmost bands verifies sorting by the diameter of SWNTs (b) Optical absorption spectra of different fractions confirm sorting by diameter. Reproduced from ref. 334 with permission from Springer Nature, Copyright 2006.

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been used as the gradient medium along with surfactants to obtain semiconducting SWNTs with near 95% purity.335 Aromatic molecules such as naphthalene, anthracene and TCNQ interact strongly with metallic SWNTs.336 A simple method of separating metallic nanotubes from semiconducting ones has been developed in an aqueous medium by interacting a mixture of SWNTs with the potassium salt of coronene tetracarboxylic acid.337 After the treatment, the semiconducting nanotubes remain in the dispersion, while the metallic ones precipitate out (Figure 1.31). In the optical absorption spectra, pristine SWNTs show bands at about 750 nm (M11) related to the metallic nanotubes and about 1040 nm (S22) and 1880 nm (S11) related to the semiconducting nanotubes (Figure 1.31a and b). These results are supported by Raman spectra, (Figure 1.31c and d). After separation, the G-­band of the semiconducting nanotubes shows two features around 1570 cm−1 (radial) and 1590 cm−1 (longitudinal), whereas metallic ones show features around 1585 cm−1 (radial) and 1540 cm−1 (longitudinal). The method avoids centrifugation and is amenable for large-­scale separation. Based on alternating current dielectrophoresis, Krupke et al.338 have developed a process to isolate metallic SWNTs from semiconducting ones. The relative dielectric constants of the two species being different from the solvent leads to opposite movement of metallic and semiconducting tubes due to the electric field gradients. From the suspension of SWNTs, metallic nanotubes are drawn to the microelectrode, leaving the solvent with semiconductive nanotubes.338,339

Figure 1.31  Optical  absorption spectra of pristine SWNTs (top), precipitate (middle), and SWNTs from solution (bottom) obtained with (a) 5 mM and (b) 10 mM of potassium salt of coronene tetracarboxylic acid, (spectra obtained after background subtraction). Raman (c) G-­band and (d) radial breathing mode (RBM) of pristine SWNTs (top), precipitate (middle), and SWNTs from solution (bottom) obtained with 10 mM solution. Reproduced from ref. 337 with permission from American Chemical Society, Copyright 2010.

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Nanotubes are highly aligned along the electric field lines, which is evident from Rayleigh scattering in the green wavelength region. Agarose selectively adsorps semiconducting tubes and separates metallic nanotubes.340 This process is straightforward, where a gel comprising sodium dodecyl sulfate and SWNTs is frozen, thawed and squeezed. The subsequent solution holds 70% metallic nanotubes, while the gel possesses almost pure semiconducting nanotubes. The selective noncovalent intractions between semiconducting SWNTs and derivatized porphyrins have been used to separate semiconducting nanotubes from metallic ones.341 Upon light irradiation, osmium tetroxide reacts selectively with metallic SWNTs via osmylation. Successive self-­aggregation separates the metallic SWNTs from semiconducting ones.342 Strano et al.343 investigated the reaction pathway by which covalent chemical functionalization is regulated by variations in the nanotube electronic structure. They have demonstrated that diazonium reagents are highly selective for the functionalization of SWNTs suspended in aqueous medium. Under controlled conditions, only metallic nanotubes undergo the reaction. Hydroxybenzenediazonium salt selectively functionalizes metallic SWNTs. Further deprotonation in basic solutions trailed by electrophoretic separation yields enriched semiconducting as well as metallic fractions separately.344 Density gradient separation of the hydroxybenzenediazoium salt-­functionalized SWNTs produce two different density sections consisting of functionalized metallic and pure semiconducting single-­walled carbon nanotubes. Covalent functionalization with azomethine ylides has been employed to separate metallic from semiconducting nanotubes.345,346 Laser irradiation has been found to destroy selectively metallic SWNTs.201,347 Bundling shows less effect on optical absorption of metallic SWNTs over the semiconducting nanotubes causing an improved absorption ratio of metallic SWNTs to semiconducting SWNTs.348 Single-­stranded DNA (ssDNA) interacts with CNTs forming a stable CNT– DNA hybrid which is easily dispersed in aqueous solution.349 Ion-­exchange chromatography can separate DNA-­coated nanotubes into fractions with electronic properties. Wrapping SWNTs in single-­stranded DNA depends on the electronic properties as well as the diameter of the nanotubes. This characteristic has been employed to get enriched metallic nanotubes. Zheng et al.350 have shown that an oligonucleotide sequence is self-­assembled on CNTs into a highly ordered structure, causing improved separation of metallic nanotubes from semiconducting ones and helps in diameter-­dependent separation. A testing method of the electrostatic characteristics of nanoscaled DNA–CNT hybrids is provided by anion exchange chromatography. As measured by spectral variations in the different fractions, anion exchange-­based separation of DNA–CNT depends on the sequence of the DNA. Raman spectroscopy and optical absorption spectroscopy reveal that smaller diameter and metallic nanotubes are enriched in early fractions, whereas larger diameter and semiconducting nanotubes are enriched in later fractions. Ghosh and Rao351 applied fluorous chemistry to separate semiconducting from metallic nanotubes existing in a mixture of nanotubes.

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The diazonium salt of 4-­heptadecafluorooctylaniline selectively interacts with metallic SWNTs. The subsequent fluoro derivative was extracted in perfluorohexane, while the semiconducting SWNTs remained in the aqueous phase. The products were characterized by electronic absorption and Raman spectroscopy. In this process there is no need for an extra centrifugation step that is usually needed in other methods but is somewhat cumbersome compared to the process of Voggu et al.337 Haddon et al.316,352 have discussed the approaches available for assessing the purity of SWNTs, as well as the existing state of purification processes. They stress the importance of developing a hierarchy of purification steps in order to produce high-­quality content appropriate for advanced applications. The electronic (near IR) absorption spectra seem to be the best means of establishing purity.

1.3  Structure, Spectra and Characterization 1.3.1  General Structural Features Findings from transmission electron microscopy clearly show that carbon nanotubes formed by arcing typically comprise multi-­layered, concentric cylinders of single graphitic sheets. The inner tube diameter is normally of the order of a few nanometers. The outer diameter can be 10–30 nm (Figure 1.1b). Helicity is introduced while curling a graphic sheet into a cylinder. Electron diffraction patterns demonstrate the existence of helicity, suggesting that nanotubes grow in the same manner that crystals grow spirally. MWNTs contain concentric cylinders ∼3.45 Å apart which is close to the separation between the (002) planes of graphite. These are the lowest energy surfaces of graphite with no dangling bonds. In electron microscopic images, nanotubes are typically viewed along their length with the electron beam perpendicular to the nanotube axis. Due to the lattice planes that run along the length of the nanotubes, spots can be seen in high-­resolution images. Such an image was published by Iijima353 for the (110) planes, 2.1 Å apart. Ring-­like patterns attributable to individual tubes that comprise individually oriented cylindrical graphic sheets (with no registry between sheets) of helical symmetry are observed. Zhang et al.31c have interpreted the electron diffraction patterns of MWNTs in terms of the reciprocal space. The peaks appear as streaks due to the curvature of the graphene sheet. They found that the streaks are perpendicular to the axis of the tubule. The streaks exhibit a fine structure. The geometery and reciprocal space of electron diffraction patterns can be examined by tilting the experiments about an axis and corelating tilting angle and reciprocal vectors. A typical selected area diffraction pattern of a MWNT consisting of 18 layers is shown in Figure 1.1d. The diffraction spots collected by aligning the tube perpendicular to the electron beam direction corresponding to both nonchiral and chiral nanotubes. The {000l} spots due to the parallel graphene layers

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(normal to the beam direction) are observed horizontally on both sides of the central spot. The other reflections indicated by arrows relate to achiral (i.e. armchair or zigzag) nanotubes and are of {1120} or {101Ī0} types. The other low-­intensity spots are due to chiral nanotubes. The chiral angle can be determined by analyzing these spots. In such analysis, the diffraction has to be collected on the nanotube by aligning exactly perpendicular to the electron beam. Figure 1.32A illustrates models of the three kinds of nanotubes created by bisecting the C60 molecule and then attaching a graphite cylinder. A chiral angle and a chiral vector Ch can be used to define nanotubes.   



Ch = na1 + ma2 …

(1.1)

  

Figure 1.32  (A)  Models of (a) armchair, (b) zigzag, and (c) chiral nanotubes. Repro-

duced from ref. 28 with permission from Elsevier, Copyright 1996. (B) A 2D graphene sheet showing chiral vector Ch and chiral angle.

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63

The vector Ch connects two crystallographic equivalent sites of a 2D graphene sheet, and the chiral angle is regarded as the angle it makes with the direction of zigzag direction (Figure 1.32B). A nanotube is created by rolling up the graphene sheet so that the two points connecting the chiral vector align. Eqn (1.1) defines various possible chiral vectors in terms of pairs of integers (n,m). Figure 1.32B shows several such pairs and each pair (n,m) describes a distinct way to roll up the graphene sheet to create a certain chirality. The limiting cases are n = m ≠ 0, (armchair tube) and n ≠ 0, m = 0 (zigzag tube). Eqn (1.2) and (1.3) are used for a carbon nanotube described by the index (n,m). Here θ and d are the chiral angle and the diameter of the tube, respectively, and a = 1.42 (3)1/2, 0 ≤ θ ≤ 30°.   



d = a(m2 + mn + n2)1/2/π

(1.2)

θ = arctan (–(3)1/2m/2n + m)

(1.3)

  

  

The electronic structure of a nanotube is determined by the (n,m) indices. SWNTs are metallic if n = m (armchair), metals or small band-­gap semiconductors if n – m = 3q, where q is a nonzero integer, and all others are semiconductors. The nanotube diameter is inversely proportional to the band gap.28 Due to this relation, where SWNTs with |n – m| = 3q are metallic and |n – m| = 3q ± 1 are semiconducting, we observe that 1/3 correspond to metallic nanotubes and 2/3 are semiconducting nanotubes. This remarkable characteristic of carbon nanotubes is a result of the special symmetry of 2D graphene. It is not, however, straightforward experimentally to identify (n,m) SWNTs and then to perform property measurements on the same nanotube, as synthesized SWNTs comprise a mixture of semiconducting (66%) and metallic species. Qin354,355 has studied electron diffraction patterns of nanotubes to determine their chiral indices (n,m). Meyer et al.356 have shown the presence of two separate sets of peaks in the electron diffraction pattern of SWNTs, corresponding to the upper and lower graphene layers of the nanotube. For zigzag and armchair nanotubes, the two sets of spots overlap. The equatorial line passes along the center of the pattern. The periodicity of the intensities on this line is related only to the nanotube diameter, and is independent of the chiral angle θ. The chiral angle may be measured by taking the angle between the two sets of peaks or “rolling angle”, however, this is not always correct. The cylindrical curvature of the nanotube results in a discrepancy between the measured angle and the true angle. A difference can also occur if the diffracting nanotube is not exactly normal to the electron beam. Qin et al.357 have calculated the correction essential to define the measured rolling angle and the true helicity. Meyer et al.356 have suggested a different method, where the relative distances between the peaks to the equatorial line determine the chiral angle. In this approach, the measured distances are not dependent on the incidence of the electron beam. Further, the authors carried out the electron diffraction analysis of individual single-­walled carbon nanotubes.

Chapter 1

64 358

Liu and coworkers have reported studies of the determination and mapping of diameter and helicity of SWNTs using nanobeam electron diffraction. Lucas and Lumbin359 studied extensively the diffraction of carbon nanotubes and other helical nanostructures. Colomer et al.360 have analyzed SAED patterns from SWNT bundles obtained by CVD and laser ablation techniques and the results are indicative of a narrow range of chiralities. Graphitic cylinders have dangling bonds at the ends, but dome-­shaped fullerene-­t ype hemispheric structures cover the carbon nanotubes. The capping units are made up of pentagons to facilitate the requisite curvature for closure. Ajayan et al.19a have studied pentagon distribution on carbon nanotube caps, and found that the caps do not have to be conical or hemisphere-­ like and can form skewed structures. By bisecting the C60 structure, and adding a graphite cylinder of the same diameter, the simplest possible single-­walled carbon nanotube is formed. A zigzag nanotube is created if C60 is bisected normal to a three-­fold axis, and an armchair tube is made if it is bisected normal to a five-­fold axis. Both armchair and zigzag nanotubes are nonchiral. Likewise, several chiral nanotubes can be formed with the screw axis along the axis of the nanotube. The structure and formation energy of caps of carbon nanotubes have been studied by Reich et al.361 Their studies show that that the structure of a cap exclusively governs the chirality of the tube that can be joined to it. The pentagon assembly in the cap specifies the chirality of the nanotube that matches to it. Generating new caps for different chirality nanotubes can be rationally achieved by relocating one or more pentagons relative to the hexagonal lattice. The diameter distribution in CVD-­produced nanotubes is dependent on the formation energy of adjacent pentagon pairs. Wang et al.,362 on the basis of abounding density function calculations, proposed a mechanism to explain SWNT growth and chirality selection caused by addition of a carbon atom and C2 dimer in the absence of a catalyst. The study shows two competing reaction paths, where addition of a single carbon atom changes the chirality, while addition of a C2 dimer affects the nanotube growth. Inclusion of a carbon atom or C2 dimer progress via a low-­energy barrier suggests that chirality and growth of SWNTs are kinetically and thermodynamically feasible under catalyst-­free growth conditions. The study also indicates that chiral selection is also affected by the availability or concentration of C atoms and C2 dimers during the growth. MWNTs consist of capped concentric cylinders separated by 3.45 Å (which is slightly bigger than the graphite interlayer distance). The number of carbon atoms increases as we pass from the inner to the outer cylinder, and perfect ABAB… stacking is not possible. The interlayer distance in MWNTs is, however, similar to that in turbostratic graphite. Carbon nanotubes may also have heptagons in addition to pentagons and hexagons. The heptagons cause a negative curvature. Therefore, nanotubes with heptagons and pentagons will have unique shapes and curvatures. Bent nanotubes arise from the presence of pentagons and heptagons on opposite sides of the tube.353 Suenaga et al.363 imaged active topological defects (pentagon–heptagon pair defects)

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in an SWNT. In this work, topological defects were deliberately introduced into SWNTs by heating at 2000 °C and cooling rapidly. The topological defects accumulate on a deformed nanotube especially near the kink, implying that active topological defects or motion of dislocations induce plastic deformation of SWNTs. Using atomistic simulations, Kotakoski et al.364 examined the energetics, structure and long-­range interaction of vacancy-­t ype defects in carbon nanotubes. SWNTs generally have low defect densities.365 Based on force field calculations, Tersoff and Ruoff366 propose that the nanotubes become cylindrical bundles in a crystal and that large tubes are hexagonal in order to maximize the van der Waals interaction among the nanotubes. Simulation experiments at the sites of local deformation suggest p-­orbital nature and broad angles of pyramidalization.367 The rings in the electron microscope images are an interesting observation with SWNTs.368 The rings are formed in acidic media during ultrasound processing when nanotube dispersions are deposited on the substrate. In catalytically formed, multi-­wall nanotubes, similar ring morphologies are observed in SEM and atomic force microscopy (AFM) images.369 On pyrolyzing Fe(ii)phthalocyanine, Huang et al.370 found “crop circles” from aligned nanotubes in a perpendicular direction to the substrate. The presence of encapsulated fullerenes in the SWNTs, as shown by high-­resolution TEM, is another important nanostructure.262 After annealing SWNTs produced by the laser method, fullerenes were observed within the nanotubes. The inset of Figure 1.14a displays a TEM picture showing the presence of C60 in SWNTs. An analysis of the size distribution in SWNTs prepared by the arc method shows significant amounts of fullerenes in the size range C36–C120 in the capillaries of the nanotubes.371 The double-­walled nanotubes discussed earlier are another remarkable finding (Figure 1.14b and c). Carbon nanotubes are routinely characterized by X-­ray diffraction (XRD).372,373 CNT XRD patterns show only the (001) and (hk0) reflections but no general (hkl) reflections. This is the case for turbostratically modified graphite.374 Warren375 has proposed different methods to study the (hk0) reflection, but not in any combination, where structural similarities occur along the directions perpendicular to, and within, the carbon nanotube axis. The length of the correlation obtained in the XRD pattern analysis is similar to that in the microscopic one. Scanning electron microscopy (SEM) is widely used for the investigation of carbon nanotubes and to identify mass yields as well as nanotube alignment. Nanotubes appear to be formed as bundles at the cathode and nanotube alignment in the bundles appears to be dependent on the stability of the arc.376–378 In cross-­sectional SEM micrographs of the cathode deposit, Wang et al.378 observed tightly packed bucky bundles. When the arc electrodes are kept at low separations, fractal-­like structures are formed. The electronic structure of nanotubes has been investigated by STM on various substrates.379–381 Closure of the CNT tips, sp3 defect structures and pentagon-­ induced changes in the electronic structure in CNTs can be probed using STM.382 Venema et al.383 have obtained STM images of SWNTs, in which the

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chirality is unambiguously defined, which in turn affects the electronic property of nanotubes. Lambin and coworkers384 have compared electron diffraction and scanning tunnelling microscopy (STM) as methods for determining the helicity of SWNTs. Atomic-­resolution imaging and site-­specific force measurements on a single-­walled nanotube have been achieved by dynamic force microscopy and 3D force field spectroscopy. The imaged topography reveals the trigonal arrangement of the hollow sites.385 Electron diffraction intensities have been employed to obtain an atomic-­resolution image of a double-­walled nanotube.386 The presence of image-­potential states in carbon nanotubes has been shown experimentally by Zamkov et al.387 Because of their quantized centrifugal motion, the observed features reflect a new type of surface picture state. Using femtosecond time-­resolved photoemission, the binding energies and temporal evolution of image state electrons can be determined. The related lifetimes are far longer than those of the n = 1 picture state on graphite, suggesting a significant variation in electron decay dynamics between tubular and planar graphene sheets. These states provide a novel means for the investigation of nanotube surface phenomena, and structural and optical properties at interfaces, as well as electron transport at nanotube heterojunctions.

1.3.2  Raman and Other Spectroscopies Raman spectroscopy has historically played an important role in the study and characterization of graphitic materials388–391 such as pyrolytic graphite, carbon fibers, glassy carbon, pitch-­based graphitic foams, nanographite ribbons and fullerenes. For sp2 nanocarbons, Raman spectroscopy provides information on the diameter of nanotubes, crystallite size, presence of sp3 hybridization, optical energy gap, doping, chemical impurities, edge structure, strain, defects and other types of disorder. It offers vital insights into the structure of carbon nanotubes. Jishi et al.392 have used zone folding for 2D graphene sheets to measure Raman-­active phonon modes and have shown 15 possible Raman modes per each tube diameter. The allowed mode frequencies depend on the tube diameter and the chiral angle, and the number of modes depends on the diameter. Hiura et al.393 reported a small line width in the range of 20 cm−1 for the Raman peaks of CNTs. Due to the nanotube curvature, the frequencies of the Raman phonons in nanotubes are softer compared to highly oriented pyrolytic graphite. Phonon softening can be related to the bigger nanotube c-­axis lattice parameter compared to graphite. Holden et al.394 studied Raman spectra SWNTs and related them to the calculations of Jishi et al.392 Rao et al.395 demonstrated diameter-­selective resonance behavior of the normal modes of the armchair (n, n) carbon nanotubes. The resonance is induced by electrons in the nanotubes which are quantum confined in one-­dimensional space. Kasuya et al.396 presented the first proof of a diameter-­dependent dispersion resulting from the cylindrical structure of nanotubes. The Raman features of SWNTs are listed in

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67

Table 1.1. Raman scattering of SWNTs with diameters of 1.1, 1.3 and 2 nm show that the optical phonon peak belonging to the E2g graphite mode is split into multiple peaks based on tube diameter. Polarized Raman experiments with aligned multi-­walled carbon nanotubes demonstrate a strong polarization dependence of the graphite-­like G-­band and disorder-­induced D-­band.397 Resonance Raman spectra of isolated nanotubes enable the determination of (n,m). A remarkable property of carbon nanotubes is their potential to use resonance Raman spectroscopy to establish geometrical structure.399 Raman spectroscopy, in general, determines phonon frequencies. The technique offers knowledge about the electronic structure under resonance conditions. Since the electronic structure of a nanotube is uniquely determined by its (n,m) indices, it becomes possible to determine the geometrical structure of an SWNT from the resonance Raman spectrum. The observation of the Raman spectrum from an isolated SWNT is an important development that has become possible because of the large density of electron states close to the van Hove singularities. The Raman cross-­section rises due to the tight coupling of electrons and phonons as incident or dispersed photons are in resonance with an electrical transition between the singularities in the valance and conduction bands. This has enabled researchers to investigate the dependency of various Raman spectrum features on nanotube diameter and chiral angle, as well as obtain spectroscopic information for each Raman feature. In Figure 1.33, we show the radial mode Raman bands of three isolated single-­walled nanotubes of different diameters. Thus, the electronic transition energies and the radial mode frequencies of different types of metallic and semiconducting SWNTs can be discussed on the basis of geometrical parameters.400 Dresselhaus and colleagues demonstrated that the G-­band and the radial breathing feature can be used to differentiate semiconducting from metallic nanotubes. The G-­band in SWNTs is composed of two elements, one peaking at 1590 cm−1 (G+) and the other at about 1570 cm−1 (G−); additionally, the lineshape of the G feature is extremely sensitive to whether the SWNT is semiconducting or metallic. Eii, or electrical transition energies of individual tubes can be measured using the radial breathing mode. This was accomplished through careful examination of the laser energy dependence of nanotube Stokes and anti-­Stokes Raman spectra.400c Table 1.1  Vibrational  modes observed for Raman Scattering in SWNTs (Reproduced from Dresselhaus et al.)403

Notation a

RMB D-­band G-­band G′-­band a

Frequency (cm−1)

Symmetry

Types of modes

248/dl ∼1350 1550–1605 ∼2700

A — A, E1, E2 —

In-­phase radial displacements Defect-­induced dispersive Graphite-­related optical modeb Overtone of D-­band, highly dispersive

RBM denotes radial breathing mode. The related 2D graphite398 mode has E2g symmetry, In 3D graphite, the corresponding mode is denoted by E2g2.

b

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68

Figure 1.33  Superposition  of the three Raman spectra (solid, dashed, and dash–

dotted curves) from three different spots on the Si substrate, showing the presence of only one resonant nanotube and one RBM frequency for each of the three laser spots. The RBM frequencies (line widths) and the (n,m) assignments for each resonant SWNT are displayed. The 303 cm−1 feature comes from the Si substrate and is used for calibration purposes. Reproduced from ref. 398 with permission from American Physical Society, Copyright 2001.

1.3.2.1 The G-­band C–C bond stretching in sp2 carbon systems produces the G-­band (see Figure 1.34 and 1.35). This band is similar for nanotubes and graphenes and yet can be distinguished between them. The hexagonal symmetry can be modified either by strain, external petrubations or the substrate, which affect the G-­band.389,401 Curvature effects give rise to multiple peaks in the G-­band of SWNTs,402 while a single peak (ωG ≈ 1582 cm−1) is observed for a 2-­dimensional graphene sheet (see Figure 1.35).398,403 Two (the totally symmetric A1 modes, see Figure 1.35) out of six Raman allowed modes in chiral SWNTs dominate the spectra. This curvature dependence also creates diameter dependence.404 Using density functional theory, Dubay et al.405 studied the radial dependency of the G band (1500–1600 cm−1) frequency. The frequency of a mode with A1 symmetry is considerably lower in metallic nanotubes than the corresponding mode in semiconducting nanotubes or graphite. The down-­shift of phonon band in metallic tubes is explained by a Peierls-­like mechanism. In addition, at room temperature, the energy increase from opening the gap is too little for static lattice distortion, so the frequency of the corresponding vibration is lowered. Piscanec et al.406 have demonstrated that graphite phonon dispersions have two Kohn anomalies (two sharp kinks) at the Γ-­E2g and K-­A1′ modes, where the slope of these kinks is proportional to the square of the electron–phonon coupling, which can be measured from experimental dispersions. Coupling between electrons and phonons in nanocarbons405,406 is of great interest because of the breakdown of the adiabatic approximation, thus changing both the electron energies (Peierls instability) and the phonon

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Figure 1.34  Raman  spectra from different types of sp2 nanocarbons. Reproduced from ref. 390 with permission from American Chemical Society, Copyright 2010.

energies (Kohn anomaly effect) and thereby offering means of analyzing differences in the properties of different carbon nanostructures. Since temperature and the Fermi level strongly affect these characteristics, the study of nanocarbon electronic dispersion relations with respect to doping can be probed through changes in the G-­band. The frequency and spectral width of the G-­band in metallic and semimetallic systems, including graphene and metallic SWNTs, are influenced by the Kohn anomaly. Due to quantum confinement effects, the spectral changes are more predominant in metallic SWNTs over graphene. The changes are also dependent on the chirality and diameter of the nanotube.407a Phonon energy renormalization is also observed in semiconducting SWNTs due to electron–phonon coupling, however, these effects are weaker than those in metallic SWNTs and graphene. In semiconducting nanotubes, when Eg > pωG, no real anomaly takes place. In metallic SWNTs and graphene, the G-­band line width is extremely sensitive to whether or not the Fermi level equals the energy of the Kohn anomaly.407a Kohn anomalies and nonadiabaticity in doped carbon nanotubes have been examined using density-­functional theory.407b Transport measurements were carried out with a field-­effect transistor (FET) device and the ambipolar

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Figure 1.35  (a)  The G-­bands for highly oriented pyrolytic graphite (HOPG), one

semiconducting SWNT and one metallic SWNT. (b) The RBM and G-­band Raman spectra for three semiconducting isolated SWNTs with the indicated (n,m) values. (c) Frequency vs. 1/dt for the two most intense G-­band features (ωG− and ωG+ ) from isolated SWNTs. Reproduced from ref. 399 with permission from Elsevier, Copyright 2002.

behavior of nanotube studied. A sharp increase in current is observed in semiconducting SWNTs for large negative (positive) Vg because of the population of holes (electrons) in the first van Hove singularity (VHS) on the valence (conduction) band side. Das et al.407c have carried out in situ Raman experiments together with transport measurements on SWNTs (as a function of gate voltage) which yielded an accurate measure of the doping of both metallic and semiconducting nanotubes. They used electrochemical gating of the nanotubes with a solid polymer electrolyte to shift the Fermi level. The G-­band frequency is observed to increase in both hole doping and electron doping in the metallic SWNTs. The spectral linewidth also decreases in either case. In addition, hardening of both the G– and the G+ band is observed in semiconducting SWNTs. The G bands in semiconducting SWNTs are G– (∼1567 cm−1) due to the TO (circumferential) mode and G+ (∼1590 cm−1) due to the LO (axial) mode. In the case of metallic nanotubes, the modes are the opposite of the semiconducting case, thus G– (∼1540 cm−1) is due to the LO (axial) mode and G+ (∼1580 cm−1) to the TO (circumferential) mode. The broader linewidth of the G– peak in metallic tubes (∼60 cm−1) than the semiconducting ones (∼10 cm−1) is a result of the electron–phonon coupling (EPC) interaction. Figure 1.36a shows how the Raman spectra recorded at different Vg give a

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Figure 1.36  (a)  Tangential Raman modes of SWNTs recorded using an excitation

energy of 1.96 eV at several Vg. The open circles show the raw Raman spectra, and the black lines are fitted. The shaded Raman spectra show the LO phonon component of the metallic nanotubes. Vg dependence of (b) L1, (c) L2 and (d) L3 modes: Frequency (top panel), FWHM (middle panel) and total integrated area (bottom panel). Reproduced from ref. 407c with permission from American Physical Society, Copyright 2007.

measure of the shift in the G bands and therefore the doping level in metallic and semiconducting SWNTs. The three major bands in the Raman spectra are fitted with three Lorentzians: L1 (∼1540 cm−1), L2 (∼1567 cm−1) and L3 (∼1590 cm−1). The linewidth, area and the Raman frequency of these bands are plotted in Figure 1.36b–d, as a function of Vg. The L1 band is assigned to the 1.4 nm diameter metallic nanotubes (the LO mode). The L1 band/line can also be fitted with a Fano-­resonance line shape. The spectral linewidth of the L2 band is large (∼40 cm−1), and shows a similar doping dependence to L1. Hence, the L2 band is assigned to a combination of the LO mode of metallic nanotubes and of the TO mode of semiconducting tubes with a diameter of 1.6 nm. The L3 band is attributed to a combination of the TO of metallic and the LO of the semiconducting nanotubes. The L1 band linewidth decreases and frequency increases for both electron (15 cm−1) and hole (10 cm−1) doping by Vg (Figure 1.36b) because of the dependence of the Kohn anomaly (KA) in metallic tubes. Similar effects are also observed in the L2 band due to the association of this band to metallic tubes. At high doping (negative or positive) levels, the L1 band intensity drops to zero, suggesting that the Raman signal is negligible for metallic nanotubes at high doping levels. The doping dependency of the L3 band is largely because of semiconducting tubes.

1.3.2.2 The Radial Breathing Mode (RBM) The RBM frequency (ωRBM) can be used to determine the diameter of the nanotube (dt) and the intensity of the RBM modes are dependent on the resonant optical transition energies Eii. The relation ωRBM = A/dt + B has been used

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to fit the experimental data of RBM frequency to tube diameter. The values of A and B for nanotubes grown by water-­assisted super-­growth are respectively 227.0 ± 0.3 nm cm−1 and 0.3 ± 0.2 cm−1. The results are explained by elasticity theory.408 Taking account of graphite elastic properties, the theory connects 1D nanotubes with 2D graphene from which the nanotubes are formed. Due to van der Waals interactions, the RBM frequencies in the literature are usually blue-­shifted from these values of A and B, expressed by:  RBM

227 1  Ce dt2 dt

RBM bands are important for understanding the effect of tube–tube interactions within multi-­walled carbon nanotubes. The prototype material for studying such interactions is provided by DWNTs. Through the RBM-­based analysis, we can also study the (n,m) dependence of the optical transition energies (Eii). Using a noninteracting electron model,409,410 the role of electron–electron interactions in finding the optical transition energies has become clear.

1.3.2.3 Dispersive G′-­band (2D band) A strong Raman band is exhibited by several sp2 carbon materials in the range 2500–2800 cm−1 (Figure 1.34). This Raman band is also characteristic of graphitic sp2 materials besides the G-­band (1582 cm−1). This band is called the G′ band or also the 2D band (since it is the overtone of the defect-­related D-­band). Unlike other Raman modes, the frequency of 2D band shows strong unusual excitation laser energy (Elaser) dependency, attributed to a second-­ order two-­phonon process. The 2D band is associated with a phonon near the K point in graphene, activated by double resonance (DR) processes.411 This feature enables sensitively specific sp2 nanocarbons to be examined. For instance, the 2D band can be used to probe the electronic structures of SWNTs as well as indentifying differences between monolayer graphene and double-­layer graphene with AB stacking.389 The 2D band also varies with the layer number in a graphene sample.389 The frequency and number of 2D peaks change with quantum confinement, curvature-­induced strain and chirality in SWNTs.402 The 2D band can also be utilized to designate n-­t ype and p-­doping in SWNTs.412

1.3.2.4 Disorder-­induced D Band The presence of disorder in sp2-­hybridized carbon systems gives rise to an important signature in the Raman spectra. This defect-­related band (D-­band) generally appears around 1350 cm −1 and acts as a key tool to establish the structures of various carbon materials such as diamond-­like carbon, amorphous carbon, carbon nanofibers, nanotubes and nanohorns.391,413 Raman spectra of individual SWNTs produced by different techniques have been evaluated and the properties of G-­bands of semiconducting and metallic

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nanotubes determined (Figure 1.35). The tangential mode frequencies with respect to the diameter of the individual semiconducting single-­walled nanotube have been established.415 Resonant Raman spectroscopy on a semiconducting SWNT of mod(n – m, 3) = 1 shows disproportionately smaller Raman cross-­sections.416 The effect of temperature on Raman spectral features have been studied for individual SWNTs on a SiO2 substrate.417 Ultra-­long metallic and semiconducting nanotubes have been compared via both Raman and microscopy.418 Step-­like dispersive behavior is observed for metallic and semiconducting SWNTs in the 600–1100 cm−1 range using resonance Raman spectra.419 The effect of uniaxial strain on resonant Raman spectral features has been studied on individual semiconducting and metallic SWNTs.420 The changes in spectral features of various Raman bands (D, G and 2D) are different for individual nanotubes suggesting dependency of relative shifts on the chirality of the nanotubes. The frequency of intermediate modes of single-­as well as double-­walled carbon nanotubes have been studied.421 Raman spectra of double-­walled nanotubes have been investigated and the features associated with the interior nanotube identified.263,422 In situ Raman spectroscopy has been employed to study the changes in the radial mode with electrochemical biasing, to understand the effect of changes in the π overlap integrals with the energy gaps between the van Hove singularities.423

1.3.2.5 Optical Spectroscopy Optical absorption as well as fluorescence spectroscopies have been used to characterize SWNTs. The transitions between van Hove singularities of metallic and semiconducting SWNTs are observed in optical spectroscopic processes. Because of the curvature of the graphene when rolling into a nanotube, even metallic nanotubes show a small energy gap at the Fermi level.424 A typical optical absorption spectrum of SWNTs (containing a mixture of both metallic and semiconducting) in the visible and near IR regions containing bands characteristic of each electronic type is shown in Figure 1.37b.425 The transition designated as M11 in the visible region arises from the inter-­van Hove energy levels in metallic SWNTs, while semiconducting tubes show transitions in the near-­infrared region (S11 and S22). Due to the small gap at the Fermi level, metallic nanotubes exhibit a band (M00) in the infrared spectrum. With decreasing SWNTs diameter, the absorption spectra blue shifts. A combination of Raman and electronic spectra is an excellent means for specifying the purity and chirality aspects of nanotubes.398,402,403,425,426 As described earlier, the Raman G-­band of SWNTs gives features centered at 1580 cm−1 and at ∼1540 cm−1 (due to metallic SWNTs) (Figure 1.37a). By deconvoluting the G-­band, relative proportions of semiconducting and metallic species can be estimated. Due to both types of nanotube, absorption spectra of SWNTs show features in the near IR and visible regions (Figure 1.37b). The diameter and (n,m) values of the nanotube can be estimated by the RBM frequency. Kataura plots can be used to predict electronic characteristics of the SWNTs using the RBM bands (Figure 1.38).425,426

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Figure 1.37  (a)  Typical Raman spectra showing radial breathing mode (RBM) and

G-­band of SWNTs grown by the arc-­discharge method. Reproduced from ref. 399 with permission from Elsevier, Copyright 2002. (b) Optical absorption spectra of films of purified SWNTs. Here S and M stand for electronic transitions of semiconducting and metallic SWNTs, respectively. Reproduced from ref. 425 with permission from American Chemical Society, Copyright 2001.

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Figure 1.38  The  Kataura plot showing calculated gap energies between mirror-­image

spikes in the density of states for single-­walled nanotubes with different diameters. Solid circles indicate metallic SWNTs and open circles semiconducting ones. Double circles indicate armchair SWNTs. Arrows show the diameter distributions for tubes produced by arc evaporation or laser vaporization using RhPd and Ni/Y “catalysts”. Two horizontal lines in each catalyst area show a “metallic window” in which only the optical transitions of metallic tubes would be observed. Reproduced from ref. 426a with permission from Elsevier, Copyright 2002.

The individual SWNTs are prepared by dispersing the bundles using micelles. Fluorescence from these isolated nanotubes is studied.333 A series of emission peaks are observed in the near infrared (800–1600 nm) region. The emission is assigned to semiconducting SWNTs across the band gaps. The origin of fluorescence can be understood from the density of states for a semiconducting nanotube. Spectrofluorometric measurements show how the absorption and emission transitions are distinct for different SWNTs depending on (n,m).333,427 Thus, fluorescence spectroscopy can be used to identify the electronic properties of the individual nanotubes.428,429 Photoluminescence (PL) of semiconducting nanotubes is quenched in the presence of metallic nanotubes (electron–hole pair nonradiative decay), thus bundled nanotubes do not show photoluminescence. Photoluminescence in isolated nanotubes occurs in three steps: (a) an absorption of light at photon energy E22 (S22) followed by (b) a relaxation from E11 (S11) using

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a phonon–electron interaction and (c) a spontaneous emission at E11. The values of E11 and E22 depend on the tube structure. PL emission is feasible with the S11 (E11) transition, whereas a broad range of energies are suitable for excitation. The chirality of the nanotubes determines the maxima of the energy of the van Hove singularities, thus, two-­dimensional photoluminescence mapping can be used to map the (n,m) values of the various SWNTs in a sample. The Kataura plot depicts the similarity of E11 values among SWNTs with different structures. It is therefore essential to consider spectral features of both the emission and excitation spectra to characterize a particular nanotube. Using 2D plots, the emission and excitation wavelengths can be designated. These spectral characteristics cannot yet be designated to (n,m) structures. To assign these features, resonance Raman spectroscopy has been performed. By relating the PL data and the Raman results, each optical transition can be assigned to a particular (n,m) nanotube structure. As PL emission comes from semiconducting tubes only, metallic tubes cannot be identified by this process. A large propensity to form SWNTs in near-­ armchair structures with high chiral angle is observed in CVD-­synthesized nanotubes using CO feedstock and Co/Mo supported on silica.430 Measurement of optical spectra in magnetic fields has revealed the dependence of the band structure on magnetic flux threading, consistent with the Aharonov– Bohm effect.431 Exciton–phonon bound states in isolated SWNTs can be found experimentally in photoluminescence excitation studies.432 The excitonic origin of the transient absorption spectra of SWNTs can be studied by femtosecond spectroscopy on optical excitation.433 Simultaneous Raman and near-­ field photoluminescence spectroscopies are employed to image nanoscale optical excitons in SWNTs.434 The optical transition energies and the physical structure of individual SWNTs have been determined by combining Rayleigh scattering spectroscopy and electron diffraction.435 By comparing simulated electron diffraction with experimentally collected electron diffraction patterns, the structure of the SWNTs can be assigned. The Rayleigh spectra of the S33 and S44 transitions for (16,11) and (15,10) SWNTs show two nanotubes with different diameters (1.83 and 1.71 nm) showing similar chiral angles (23.9° and 23.4°). The smaller diameter nanotube shows an upward shift in energy for both the transitions, but shows little change in the ratio of the S44/S33 transition energies. Temperature effects on the PL spectrum of semiconducting SWNTs have been studied.436 The emission shows a small shift with respect to temperature suggesting moderate chirality dependence. Polymer-­w rapped nanotubes show chiral dependency due to strain, when the temperature effects are removed. Functionalization effects on the visible luminescence of nanotubes have been studied.437 The emission was found to be higher in the case of functionalized nanotubes with better dispersion. Noncovalent functionalization has been adopted to tune the optical properties of SWNTs. In these studies, SWNTs were wrapped with poly(vinylpyrrolidone) (PVP-­1300), which has been

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fluorescently labeled. Time-­resolved PL spectroscopic studies on water-­ suspended individual SWNTs and DWNTs were conducted.439 The emission pathways in smaller diameter tubes are influenced by the diameter, chirality and environment. The band-­gap energies and exciton binding energies of individual semiconducting SWNTs have been probed by two-­photon excitation spectroscopy.440 The band-­gap energies are higher than the values that are suggested by tight-­binding calculations. The exciton binding energies are high in these nanotubes and alter with the diameter of the nanotube. SWNTs have been optically manipulated using linearly polarized infrared tweezers.441 Using cw polarized photoluminescence and polarized pump-­probe photomodulations, ultrafast relaxations of photoexcitations in semiconducting SWNTs have been studied with 10 fs time resolution.442 Excited-­state carrier lifetime studies have also been conducted on different chirality SWNTs using near-­infrared photoluminescence decay time measurements.443 Unlike nanotube bundles, in semiconducting nanotubes, the carrier lifetime in the first excited state is significantly higher (exceeds 30 ps). The degree of order and alignment in SWNTs have been investigated by near-­edge X-­ray absorption fine structure spectroscopy.444

1.3.3  Pressure-­induced Transformations Phase transformations of many allotropes of carbon such as graphite, diamond, C60 and C70 and their polymeric as well as amorphous forms under static and dynamic pressure are of considerable importance. Raman spectroscopy can be used to probe pressure-­induced effects on SWNT bundles in a diamond anvil cell up to a maximum pressure of 25.9 GPa (1 GPa = 109 Nm−2).445,446 The radial and tangential modes at 0.1 GPa are identical to those reported at atmospheric pressure.395 The radial bands are at 172 and 182 cm−1. The variation of the radial mode frequencies, ωR[cm−1], of isolated SWNTs with nanotube diameter d [nm] is given by ωR = 223.75/I.447 For the (10,10) nanotube, this gives ωR = 164 cm−1 and for the (9,9) nanotube, ωR = 183 cm−1. The radial mode frequency of the (9,9) nanotube shifts from 171.8 cm−1 (for an isolated tube) to 186.2 cm−1 in bundles due to van der Waals interactions between the nanotubes. This blue shift of 14.4 cm−1 is due to interactions between the tubes and does not depend on the tube diameter.448 Thus, the empirical formula of radial mode frequency in a SWNT bundle (which is diameter dependent) is given by, ωR = 14.4 + 209.9/d, which preserves the 1/d dependence of ωR and reproduces ωR = 186.2 cm−1 for the (9,9) nanotube.449 In terms of the irreducible representations of Dnh (Dnd) for even n (odd n), the tangential modes are defined, with 1531 cm−1 as E1g, 1553 and 1568 cm−1 as E2g, 1594 cm−1 with unresolved doublet A1g + E1g and 1606 cm−1 with E2g symmetry.450,451 With increasing pressure, the intensities of the radial modes decrease rapidly, and are not visible beyond 2.6 Gpa. These characteristics are reversible. With pressure the intensities of the tangential modes also decrease.

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Figure 1.39 displays the pressure dependence of the 172 cm radial mode along with the measured curves for three models in increasing and decreasing pressure runs.448 Figure 1.40(a–c) gives plots of tangential mode frequencies as a function of pressure. The modes at ωT = 1568, 1568 and 1594 cm−1 show that the band frequency increases with pressures beyond 16 GPa in Figure 1.40a–c, softening at ∼10–16 GPa. Remarkably, the peak positions (open symbols) follow the same pattern when the pressure is lowered from the highest pressure of 25.9 GPa. The slope at values (dω/dP) obtained by fitting ω(P) = ω(0) + aP to the tangential modes and a second-­order polynomial to the radial mode were used to measure a mode Gruneissen parameter, γ = Bd(lnω)/dP, where B (136 Pa) is the bulk modulus. There is a large difference between the values of g for radial and tangential modes just as in layered as well as molecular crystals. Under pressure, the energy positions of the valence and conduction band singularities of nanotubes change, resulting the enhancement of resonance Raman cross-­sections being minimized, leading to lower intensities of the Raman bands. Increased intertube interactions at high pressure, leading to increased electronic density of states at the Fermi level can contribute to increased Raman linewidths and reduced intensities. To understand the pressure dependence of radial and tangential modes of nanotubes, Venkateswaran et al.448a used generalized tight binding

Figure 1.39  Pressure  variation of the radial mode Raman bands ωR. Squares ■ (□)

indicate a run with increasing (decreasing) pressure. Theoretical curves for models I, II, and III are also shown. Reproduced from ref. 445 with permission from American Physical Society, Copyright 2000.

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Figure 1.40  Pressure  variation of (a) 1531, 1553 and 1606, (b) 1568 and (c) 1594 cm−1

Raman frequencies. Data points for increasing (decreasing) pressure are filled (open) symbols. Inset: Faceting in SWNT bundles leads to the conversion of (a) a circular cross-­sectional bundle to that of (b) hexagonal and (c) elliptical cross-­sections. Reproduced from ref. 445 with permission from American Physical Society, Copyright 2000.

molecular dynamics simulations, taking three pressure transmittance conditions for the SWNT bundle. In debundled SWNTs, Schlecht et al.448b recorded pressure dependence in the pressure range from 0 to 4.8 GPa for the ωR and ωT frequencies and compared them with the corresponding dependencies of bundled SWNTs. The results indicated that the magnitude of van der Waals interactions on ωR, as observed in Raman-­active radial breathing mode measured under high pressure, is almost the same whether an SWNT resides within a small bundle (three to seven tubes) or inside in a large bundle (with 100 neighboring tubes). At high pressures it is necessary to take into account electron hopping between the tubes and hence the results of Venkateswaran et al.448a cannot be extrapolated to higher pressures. High-­pressure SWNT studies

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confirm them to be comparable to the strongest carbon nanofibers, and also their impressive resilience.449,452 Valence-­force model calculations448 assume that the tubes flatten against each other, creating a honeycomb structure. Under pressure, smaller diameter nanotubes may also facet451 and the cross-­section of the tube may change from circular to elliptical or hexagonal as shown in Figure 1.40c inset. This will significantly affect the intensities of the Raman modes, especially in the radial mode. Faceting will result in substantial overlap of the surfaces of the adjacent tubes, similar to the packing of two AB planes in graphite. The frequencies of tangential modes in the SWNT bundles will then be close to the in-­plane graphite vibration at 1579 cm−1. The thick solid line in Figure 1.40c shows the pressure dependence of the E2g 1579 cm−1 graphite mode measured up to 14 GPa.453 The 1594 cm−1 mode of the SWNT bundles approaches the graphite frequency at ∼10 GPa, becoming equal at ∼16 GPa. The pressure derivative dω/dP is close to that of graphite above 16 GPa.453 At 23 GPa, graphite becomes amorphous, and recrystallizes when the pressure is released.454 Single-­walled nanotube bundles under hydrostatic pressure do not convert into bulk graphite, as is apparent from the reversibility of the pressure-­dependent Raman spectra. X-­ray diffraction experiments under pressure show that SWNT bundles lose translational order at about 10.4 GPa.455 This may be the origin of the anomaly observed around that pressure in the tangential Raman modes. The bending of SWNTs and MWNTs during mechanical stress has been studied using high-­resolution electron microscopy as well as atomistic simulations. Results suggest that bending of nanotubes is entirely reversible up to very high bending angles, signifying the flexibility of the hexagonal network, which resists bond stretching and breaking up to large strain values.456 Pressure-­and temperature-­induced transformation studies of SWNTs at 1.5, 8.0 and 9.5 GPa and temperatures ranging from 200 to 1500 °C show irreversible changes (i.e., polymerization) in the SWNT structure, in contrast to the reversible effect of high pressure alone.457 Covalent interlinking of SWNTs by sp3 C–C bonds increases with increasing temperature and pressure. Under higher pressures (8.0 and 9.5 GPa), new carbon structures such as nano-­ and microcrystalline diamond (cubic and hexagonal) and nanographite phases are observed. Tangney et al.458 used simulations with a classical force field to study the transformation of isolated SWNTs under hydrostatic pressure from a circular to a collapsed cross-­section. SWNTs with larger diameters show hysteresis and exhibit a first-­order-­like transformation, while small-­diameter tubes deform continuously under pressure. Abouelsayed et al.459 studied the properties of unoriented SWNTs by infrared spectroscopy as a function of pressure. With increasing pressure, the hybridization and symmetry of nanotubes change resulting in a decrease in the optical transition energies corresponding to the Van Hove singularities. The changes in the optical transition due to the pressure show an anomaly around 2 GPa because of a structural phase transition. With increasing pressure, the low-­energy absorbance monotonically decreases, indicating an increase in carrier localization.

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In situ synchrotron X-­ray diffraction measurements have been carried out on SWNTs and C60-­peapod samples under high pressures (up to 13 GPa) and at different temperatures.460 Anisotropical shrinkage of 2D lattices by compression at room temperature were observed, and upon releasing the pressure, the lattices returned to their initial structure reversibly. By increasing the temperature at the highest pressure, an irreversible phase transformation occured. At high temperatures and high pressures, C60-­peapods form hexagonal diamond.460 In the C60-­peapods (C60@SWNTs), the C60–C60 distance decreases with pressure from 0.956 nm at 0.1 MPa down to 0.845 nm at 25 GPa.461 The C60–C60 distance on complete release of pressure remained smaller than the initial value due to the polymerization of C60 molecules. The structural changes of carbon peapods (C60 trapped in SWNTs) have been examined under high-­pressure/ high-­temperature (HPHT) conditions by in situ X-­ray diffraction.462a The formation of one-­dimensional polymer chains of C60 inside single-­walled carbon nanotubes has been demonstrated. When returned to ambient conditions, the C60 chain remained polymerized. Although C60 molecules polymerize within nanotubes at polymerization temperatures and pressures, no polymerization is observed in C70 peapods, even at higher pressures and temperatures.462b

1.3.4  Electronic Structure As with fullerenes, the curvature of the graphitic sheets in nanotubes is predicted to influence the electronic structure. Since the coupling between the cylinders is poor in MWNTs, their electronic properties are identical to those of perfect SWNTs. Since a variation in the helicity degree and the amount of six-­membered rings per turn around the tube will change the electronic properties within the spectrum of metal-­semiconductors, theoretical simulations show that nanotubes can be as strong electrical conductors as copper.463–465 As a result, the electronic properties of CNTs are affected by their geometric structure. While graphene is a zero-­gap semiconductor, theory suggests that carbon nanotubes (CNTs) may be metals or semiconductors with varying energy gaps, depending delicately on the diameter and helicity of the tubes, i.e. the indices (n,m). The electronic properties of CNTs are adaptive to their shape, which can be described using a band-­folding model. This is attributed to the unusual band configuration of the graphene sheets, with states crossing the Fermi stage at just two inequivalent points in k-­space, as well as the quantization of the electron wave vector along the circumferential direction. An isolated sheet of graphene is a zero-­gap semiconductor with an occupied π band and an empty π* band near the Fermi energy. These two bands have linear dispersion and converge at the Fermi stage in the Brillouin region at the K point (Figure 1.41). The Fermi surface of a perfect graphene layer is made up of six corner K points. Due to the periodic boundary conditions applied in the circumferential direction, only a limited number of k states of the planar graphene sheet are permitted when forming a tube. The permitted collection of k (indicated by the lines in Figure 1.41) is defined by the tube's diameter and helicity. Where the permitted k includes the point K, the structure is a metal with a nonzero

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Figure 1.41  (a)  Tight-­binding band structure of graphene. Two-­dimensional Brill-

ouin zones of graphene showing the quantized ky values (dashed lines) for (b) metallic and (c) semiconducting carbon nanotubes. (a) Reproduced from ref. 485 [https://doi.org/10.1080/14686996.2018.1494493] under the terms of a CC BY 4.0 license [https://creativecommons.org/ licenses/by/4.0/]. (b) and (c) Reproduced from ref. 486 with permission from American Physical Society, Copyright 2000.

density of states at the Fermi point, yielding a 1D metal with two linear dispersing bands. Since point K is omitted, the system is a semiconductor with varying energy gaps. The states near the Fermi energy in both metallic and semiconducting tubes are all from states near the K point, and hence their transport and other electronic properties are linked to the properties of the permitted lines. The conduction and valence bands of a semiconducting nanotube, for example, are derived from states along the line nearest to the K position. The following are the basic rules governing the metallicity of SWNTs. The (n,n) tubes (armchair) are metals; the (n,m) tubes with n – m = 3q, where q is a nonzero integer, are minimal-­gap semiconductors; and the remaining tubes are broad-­gap semiconductors. The n – m = 3q tubes will be metals under the band-­folding system, but a slight distance opens due to tube curvature if q is nonzero (Figure 1.42). As a result, CNTs may be classified into three types: big gap, minimal gap and no gap. Because of their symmetry, the (n,n) tubes are often metallic inside the single-­electron frame, regardless of curvature (Figure 1.42). The band difference decreases with a 1/R or 1/R2 dependency as the tube radius, R, rises. As a result, for the majority of experimentally observed carbon nanotube sizes, the difference in the narrow-­gap variety caused by curvature effects is small, and the n – m = 3q tubes may be called metallic at room temperature for all purposes. A (12,0) tube, for example, is a narrow-­gap semiconductor (with an energy gap of 8 meV), while a (11,0) tube is semiconducting (with an energy gap of 0.8 eV); a (6,6) armchair tube is often metallic. Based on the tight-­binding technique, such a band-­folding image tends to be true for larger-­diameter tubes. Because of the radial confinement of the wave function, the continuous electronic density of states (DOS) in graphite splits into a sequence of spikes in SWNTs, which are referred to as van Hove singularities (Figure 1.42). Electronic transitions between these singularities give rise to prominent features of scanning tunnelling and electronic spectroscopy.

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Figure 1.42  Band  structures of a (12,0) zigzag tubule (a), a (11,0) zigzag tubule (b),

and a (6,6) armchair tubule (c). (d) One-­dimensional electronic densities of states of metallic (14,5) and semiconducting (16,2) chiral nanotubes, compared to the DOS of a metallic (10,10) achiral nanotube. The Fermi level is positioned at zero energy. Reproduced from ref. 487 with permission from Springer Nature, Copyright 1998.

The transparency of diamond is attributed to the fact that all of its electrons are connected by sp3 bonds, which absorb only infrared radiation. Visible light, therefore, passes right through it. Their sparkle is the consequence of an extremely high refractive index, and the dispersion ability of visible light into its spectral components. Graphite does not have the matt black colour of more finely divided sp2 carbons, due to the metallic character. Bulk nanotube powder usually appears black. However, thin layers of nanotubes can be transparent, due to the large penetration depth for nanotubes. The penetration depth for visible light changes inversely with the density of free carriers. The density of carriers is low in nanotubes, resulting in a high penetration depth. As seen in Figure 1.43, the majority of SWNTs undergo the first and second electronic transitions in semiconducting nanotubes (S11 and S22), as well as the first transition in metallic nanotubes (M11).425 The Fermi level transitions of metallic SWNTs (M00) are noticeable in the far-­IR portion of the electromagnetic spectrum.466 This is an informative region of the spectrum, and the low-­energy feature in acid-­purified SWNTs arises from the doping of the SWNTs, presumably by nitric acid. A peak appears about 0.01 eV, which is most likely due to the curvature-­induced pseudogap in chiral SWNTs. The absorption spectrum in Figure 1.43 shows three high intensity peaks around 1.7, 1.2 and 0.68 eV, superimposed on the broad π plasmon absorption. SWNTs with varying diameter distributions show expected shifts in their absorption peaks. The locations of the singularities in the densities of states are defined by the structure and diameter of the nanotube. In other words, Kataura and colleagues concisely introduced the inverse association between band-­gap energies and tube diameter as a systematic plot. The resulting graph, shown in Figure 1.38, has come to be recognized as the Kataura plot.426a Kataura and coworkers determined the theoretical gap energies between mirror-­image spikes in the densities of states for a large number of single-­walled nanotubes. The plot displays where absorption bands should be observed in a spectrum

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Figure 1.43  Fermi  level electronic structure of common forms of SWNTs. The low-­ energy (∼0.01 eV) features arise from a combination of transitions that are intrinsic to the metallic SWNTs, transitions due to the curvature-­ induced gap (M00) in the chiral metallic SWNTs and transitions due to purification-­induced acid doping of the semiconducting SWNTs. Reproduced from ref. 425 with permission from American Chemical Society, Copyright 2001.

for tubes with a given span of diameters. For instance, 1 nm diameter nanotubes would show bands around 2.3, 1.6 and 0.8 eV (higher-­energy bands would be drowned in the π plasmon). The Kataura plot explains why optical absorption spectroscopy alone cannot identify the structure of a nanotube: nanotubes with different structures can have similar absorption features. The plot was subsequently modified on the basis of the available energy gap data for semiconducting SWNTs.426b,427,467 Electrostatic force microscopy in atomic force microscopy (AFM) has been used study the charge density in carbon nanotubes under local perturbation.468 Such measurements provide the local band gap information for an intratube quantum-­well structure that could be used to assign the chiral angle of the nanotube. A high photoresponse has been observed from suspended SWNT films in vacuum and this enhancement is due to heating of the nanotube networks.469 A field-­effect transistor was fabricated on semiconducting nanotubes and in this geometry, their photoconductivity excitation spectra were measured.470 The spectrum exhibited a main band (excitation of an exciton) and a weak sideband at higher energy (∼200 meV) due to the second van Hove singularity. The electronic structure and chemical reactivity of carbon nanotubes can also be understood based on traditional chemical concepts.471 Thus, the frontier orbital picture of Hoffmann can be used to understand the electronic structure. Interactions between occupied orbitals as well as those

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between empty orbitals (zero electrons) would be different in metallic and semiconducting nanotubes. In Figure 1.44, we show the frontier orbital representation of zero electron interactions between a molecule and (a) a metallic and (b) semiconducting nanotube. The molecular orbitals are denoted by the letters HOMO and LUMO. The density of state plots provide the nanotube orbitals. Because of charge transfer, interaction with the metallic nanotube becomes attractive. The frontier orbitals of a semiconducting nanotube are related to the first van Hove singularities in the valance and conduction bands. Resonant inelastic soft X-­ray scattering has been used to study the electronic structure of bulk SWNTs. The electronic states are derived from the K point and are positioned near the Fermi level, indicating the presence of metallic nanotubes in the bundles, beside a significant fraction of semiconducting nanotubes.472 13C nuclear magnetic resonance (NMR) spectroscopy of SWNTs reveals two types of nuclear spin with spin–lattice relaxation at different rates. The rapidly relaxing component follows the relaxing behavior of metals.473 Studies of femtosecond time-­resolved photoemission on SWNTs show a reduction in the lifetime of the photoexcited electrons with a rise in energy relative to the Fermi level. This in turn causes lifetime-­induced broadening of van Hove singularities in the density of states of the nanotubes.474 Ago et al.475 have extracted the work function and DOS of multi-­walled nanotubes by ultraviolet photoelectron spectroscopy. The work function of pure MWNTs is lower compared to the acid-­oxidized nanotubes. The higher work function in functionalized nanotubes is due to disruptions of π-­conjugation and the creation of surface dipole moments. Herrmann et al.476 fabricated nanotube based double quantum dots by joining two nanotubes with a central superconducting finger and measured their conductance. By operating

Figure 1.44  Frontier-­  orbital picture representation of zero-­electron interactions

between a molecule and a metallic (a) or a semiconducting (b) single-­ wall carbon nanotube. The molecule frontier orbitals are designated as HOMO and LUMO, and the nanotube orbitals are represented by the density of states plots. The interaction with the metallic nanotube becomes attractive due to charge transfer, but is not effective with the semiconducting nanotube. Reproduced from ref. 471 with permission from John Wiley and Sons, Copyright © 2004 WILEY-­VCH Verlag GmbH & Co. KGaA, Weinheim.

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the device as a beam splitter, they detected proof of in situ tunable crossed Andreev reflections. Exitionic phase transitions are observed in pairs of semiconducting carbon nanotubes using density-­functional theory calculations. Authors observed the closing of the Kohn–Sham gap of the system by applying 0.05 eV Å−1 electric-­field strength.477 Application of 0.06 eV Å−1 closes the excitonic gap but fails to close the quasiparticle gap. This can induce an excitonic phase transition in the system, where the ground state is populated with repulsive gas of quasi-­one-­dimensional excitons. Lüer et al.478 have studied mobility and exciton size in carbon nanotubes. Studies on individual SWNTs show direct proof of a breakdown of the Born–Oppenheimer approximation.479 Tunnelling measurements conducted by Leturcq et al.480 on quantum dots formed in suspended SWNTs have shown decreases in the current flow at low biases suggesting an extremely large electron–vibron coupling (Franck–Condon blockade). In pure CNTs, the Mott insulating state has been detected.481 Impact excitation in SWNT photodiodes generates a high multiple electron–hole pair.482a A strong isotope effects in spin-­ blockaded transport has been observed by Churchill et al.482b via studying the electron–nuclear interactions in 13C nanotube double quantum dots. Shtogun et al.483 studied the electronic structure of radially deformed SWNTs under transverse external electric fields by self-­consistent density functional theory methods. Structural and electronic properties of carbon nanotubes have been probed by using transmission electron energy-­loss spectroscopies, which are complementary to optical and X-­ray absorption techniques.484

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462. (a) M. Chorro, S. Rols, J. Cambedouzou, L. Alvarez, R. Almairac, J. L. Sauvajol, J.-­L. Hodeau, L. Marques, M. Mezouar and H. Kataura, Phys. Rev. B, 2006, 74, 205425; (b) M. Chorro, J. Cambedouzou, A. Iwasiewicz-­Wabnig, L. Noé, S. Rols, M. Monthioux, B. Sundqvist and P. Launois, Europhys. Lett., 2007, 79, 56003. 463. J. W. Mintmire, B. I. Dunlap and C. T. White, Phys. Rev. Lett., 1992, 68, 631. 464. N. Hamada, S. Sawada and A. Yoshiyama, Phys. Rev. Lett., 1992, 68, 1579. 465. R. Saito, M. Fujita, G. Dresselhaus and M. S. Dresselhaus, Appl. Phys. Lett., 1992, 60, 2204. 466. M. E. Itkis, S. Niyogi, M. Meng, M. Hamon, H. Hu and R. C. Haddon, Nano Lett., 2002, 2, 155. 467. (a) R. B. Weisman and S. M. Bachilo, Nano Lett., 2003, 3, 1235; (b) M. S. Strano, S. K. Doorn, E. H. Haroz, C. Kittrell, R. H. Hauge and R. E. Smalley, Nano Lett., 2003, 3, 1091. 468. J. Heo and M. Bockrath, Nano Lett., 2005, 5, 853. 469. M. E. Itkis, F. Borondics, A. Yu and R. C. Haddon, Science, 2006, 312, 413. 470. X. Qiu, M. Freitag, V. Perebeinos and P. Avouris, Nano Lett., 2005, 5, 749. 471. E. Joselevich, ChemPhysChem, 2004, 5, 619. 472. S. Aisebit, A. Karl, W. Eberhardt, J. E. Fischer, C. Sathe, A. Agui and J. Nordgren, Appl. Phys. A, 1998, 67, 89. 473. Z. P. Tang, A. Kleinhammes, H. Shimoda, L. Fleming, K. Y. Bennoune, S. Sinha, C. Bower, O. Zhou and Y. Wu, Science, 2000, 288, 492. 474. T. Hertel and G. Moos, Chem. Phys. Lett., 2000, 320, 359. 475. H. Ago, T. Kugler, F. Cacialli, W. K. Salaneck, M. S. P. Shaffer, A. H. Windle and R. H. Friend, J. Phys. Chem. B, 1999, 103, 8116. 476. L. G. Herrmann, F. Portier, P. Roche, A. L. Yeyati, T. Kontos and C. Strunk, Phys. Rev. Lett., 2010, 104, 026801. 477. J. D. Sau and M. L. Cohen, Phys. Rev. B: Condens. Matter Mater. Phys., 2008, 78, 115436. 478. L. Lüer, S. Hoseinkhani, D. Polli, J. Crochet, T. Hertel and G. Lanzani, Nat. Phys., 2009, 5, 54. 479. A. W. Bushmaker, V. V. Deshpande, S. Hsieh, M. W. Boekrath and S. B. Cronin, Nano Lett., 2009, 9, 607. 480. R. Leturcq, C. Stampfer, K. Inderbitzin, L. Durrer, C. Hierold, E. Mariani, M. G. Schultz, F. Von Oppen and K. Ensslin, Nat. Phys., 2009, 5, 327. 481. V. V. Deshpande, B. Chandra, R. Caldwell, D. S. Novikov, J. Hone and M. Bockrath, Science, 2009, 323, 106. 482. (a) N. M. Gabor, Z. Zhong, K. Bosnick, J. Park and P. L. McEuen, Science, 2009, 325, 1367; (b) H. O. H. Churchill, A. J. Bestwick, J. W. Harlow, F. Kuemmeth, D. Marcos, C. H. Stwertka, S. K. Watson and C. M. Marcus, Nat. Phys., 2009, 5, 321. 483. Y. V. Shtogun and L. M. Woods, J. Phys. Chem. C, 2009, 113, 4792.

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484. P. Castrucci, M. Scarselli, M. De Crescenzi, M. A. El Khakani and F. Rosei, Nanoscale, 2010, 2, 1611. 485. G. Yang, L. Li, W. B. Lee and M. C. Ng, Sci. Technol. Adv. Mater., 2018, 19, 613. 486. M. S. Ferreira, T. G. Dargam, R. B. Muniz and A. Latge, Phys. Rev. B, 2000, 62, 16040. 487. J.-­C. Charlier and J.-­P. Issi, Appl. Phys. A, 1998, 67, 79.

Chapter 2

Chemically Modified Nanotubes 2.1  Introduction Many of the applications of carbon nanotubes require their chemical modification in order to use them suitably. This can be done by chemical doping or by attachment of molecules covalently or noncovalently to the nanotube surfaces.

2.2  Doping with Boron and Nitrogen There has been a great deal of interest in substituting carbon with other elements since the discovery of carbon nanotubes. Exohedral, endohedral and in-­plane doping (substitution) are the three major types of doping. Atoms such as nitrogen and boron will dope the carbon nanotube wall substitutionally (in-­plane doping). As a consequence, nanotubes of boron–carbon (B–C), boron–carbon–nitrogen (B–C–N), and carbon–nitrogen (C–N) have been prepared and characterized. Nitrogen substitution of carbon nanotubes results in n-­t ype doping, while boron substitution results in p-­t ype doping. These doped nanotubes are predicted to have novel electron transport properties.1 Boron-­doped carbon nanotubes have been generated by pyrolyzing acetylene and diborane mixtures, and characterized using microscopic and spectroscopic techniques.2 C35B is the average composition of these nanotubes. B–C–N nanotubes have been prepared by striking an arc between a graphite anode filled with B–N and a pure graphite cathode in helium.3 Boron and nitrogen codoped BN-­SWNTs have been prepared on a large scale using an electric arc-­discharge method and it has been shown that the band gaps of SWNTs can be tuned with such doping.4   Nanoscience & Nanotechnology Series No. 52 Nanotubes and Nanowires, 3rd Edition By C. N. R. Rao, A. Govindaraj and Leela Srinivas Panchakarla © C. N. R. Rao, A. Govindaraj and Leela Srinivas Panchakarla 2022 Published by the Royal Society of Chemistry, www.rsc.org

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B–C–N nanotubes were also generated by laser ablation of a composite target comprising B–N, carbon, Ni and Co at 1000 °C in the presence of flowing nitrogen.5 The laser ablation of graphite electrodes filled with boron as well as NiB yields B-­doped SWNTs.6 Terrones et al.7 produced B–C–N nanotubes by pyrolyzing the addition compound CH3CN : BCl3 over Co powder at 1000 °C. Sen et al.8 produced B–C–N nanotubes as well as C–N nanotubes by the pyrolysis of appropriate precursors. C–N nanotubes are formed by pyrolysis of aza-­aromatics such as pyridine over Co catalysts (C33N on average). B–C–N nanotubes are formed by pyrolysis of the 1 : 1 addition compound of BH3 with (CH3)3N. Figure 2.1 depicts typical TEM photographs of a few nanotubes with bamboo shape, nested

Figure 2.1  TEM  images of some of the unusual carbon nanotube structures obtained

by the pyrolysis of pyridine (flow rate = 30 cm3 min−1) over Fe/SiO2 substrates at 900 °C for 1.5 h under Ar (120 cm3 min−1) flow. The nanotubes show (a) bamboo shape, (b) nested cone, (c) and (d) other unusual morphologies. (d) Shows the TEM image of a coiled nanotube. Reproduced from ref. 10a with permission from Elsevier, Copyright 2000.

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cone-­shaped cross-­sections and peculiar morphology, like coiled nanotubes. The structure of the B–C–N nanotubes differs based on the process of preparation. Furthermore, the composition of any given batch of B–C, B–C–N or C–N nanotubes formed by pyrolysis of precursors differs greatly. Zhi et al.9 discuss the design, synthesis and properties of boron carbonitride in some detail. Nath et al.10a generated aligned carbon–nitrogen nanotube bundles by pyrolyzing pyridine over cobalt/silica or iron/silica substrates derived from sol–gel. C–N nanotubes are generated utilizing electron cyclotron resonance CVD and anodic alumina, as well as C2H2 and N2.10b CVD of Ni-­phthalocyanine yields aligned C–N nanotubes.11 Open-­ended nitrogen-­doped MWNTs are obtained by the direct pyrolysis of the metal organic framework, [Co(HTTG) (H2O)2]n (TTG = N,N′,N″-­1,3,5-­triazine-­2,4,6-­triyltris-­glycine).12 Thus large surface area and high CO2/CH4 adsorption selectivity, both increasing with increasing nitrogen content, are observed. SWNTs have been thermally processed with boron trioxide in a nitrogen environment by Goldberg et al.13 to create boron-­ or boron and nitrogen-­ doped SWNTs. According to the EELS study, the boron content in B–C nanotubes is 10%. The phase isolation of the BC3 islands in the graphene sheets is an intriguing feature of the B–C–N nanostructures. Tunneling conductance measurements of doped nanotubes show acceptor-­like states near the Fermi level, originating from the BC3 islands.14 According to Efsarjani et al.15a a nanotube with donor atoms on one side and acceptor atoms on the other will act as a nano-­diode. The detection of rectification in a SWNT is an experimental condition that is similar to this effect.15b The existence of an impurity in a SWNT segment influences its nonlinear transport nature. Xu et al.16 used a plasma-­assisted HFCVD process to achieve codoping of boron/ nitrogen (3 to 8%) in metallic SWNTs, where selective etching of a gas-­phase plasma hydro-­carbonation reaction with metallic nanotubes resulted in conversion to semiconducting B/N codoped SWNTs. Synthesis and characterization of boron-­ and nitrogen-­doped DWNTs have been carried out in view of their potential applications.17 In this work, a low-­doping regime (∼1 atom %) was explored as it does not change the fundamental band structure. Mo0.1Fe0.9Mg13O was used as a catalyst, which was obtained by combustion route. The mixture of CH4–pyridine–Ar or CH4– NH3–Ar was decomposed over a catalyst at 950 °C to yield DWNTs preferentially. The nitrogen concentration in the DWNTs was around 1 atom % in both cases. The outer and inner diameters of the DWNTs were analyzed by TEM or by radial breathing modes in the Raman spectra. The thermal decomposition of a CH4–B2H6–Ar mixture at 950 °C over the same catalyst yielded boron-­doped DWNTs. The diameters of the nanotubes obtained from the Raman RBM modes and transmission electron microscopy are comparable (Figure 2.2). The N-­doped nanotubes show the G-­band in the Raman spectrum at a lower frequency than the undoped ones, while the B-­doped nanotubes show an increase in the frequency. The proportion of metallic nanotubes appears to decrease on N-­ or B-­doping, but the average diameter is substantially larger in the B-­doped DWNTs.

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Figure 2.2  (a)  G-­bands in the Raman spectra of undoped and doped DWNTs. Inset shows the D-­bands of the same. (b) RBM bands of undoped and doped DWNTs. Reproduced from ref. 17 with permission from American Chemical Society, Copyright 2007.

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Vertical N-­doped CNT arrays with tunability of the carbon wall number have been grown using PECVD from nanopatterned iron catalysts in an NH3 environment.18 Optimized growth conditions produced 52 µm long N-­doped CNTs within one minute. Terrones et al.19a published a literature review on the construction and characterization of B-­ and N-­doped carbon nanotubes (CNTs) and nanofibres. They also show how CNTs doped with B or N will exhibit novel chemical, electronic and mechanical properties not found in pure carbon counterparts. To take advantage of the novel properties, low amounts of dopants (e.g., 0.5%) can be inserted into these tubes, so that the electrical conductance is significantly increased while the mechanical properties remain unchanged. Their surface will become more receptive due to the existence of holes (B-­doped) or donors (N-­doped). This kind of reactivity may be useful in the construction of field-­emission sources, nanoelectronics, sensors and strong composite materials. Ayela et al.20a present a comprehensive overview of the experimental and theoretical topics related to the doping of nitrogen into both SWNTs and MWNTs. Ewels and Glerup20b have reviewed nitrogen doping of single-­ and multi-­walled carbon nanotubes to explore the influence of dopants on quasi-­1D electrical conductors and for applications such as lithium storage, composites, field emission tips and nanoelectronic devices. Terrones and coworkers21a have carried out the substitutional doping of phosphorus in SWNTs and investigated it by density functional theory and resonance Raman spectroscopy. These workers have also reported the synthesis of N-­, P-­ and Si-­doped SWNT bundles by CVD of ethanol/ferrocene solutions in the presence of different precursors (benzylamine, pyrazine, triphenylphosphine and methoxytrimethylsilane) containing the doping elements (N, P and Si) at various concentrations.21b Sulfur-­substitutional defects in SWNTs have been examined by first-­principles calculations.22 By estimating the formation energy, which is comparable with that for a nitrogen-­doped SWNT, they suggest the possibility of introducing sulfur into the SWNT framework by employing sulfur-­containing heterocycles during the synthesis process.

2.3  Intercalation by Alkali Metals MWNTs can be intercalated either by electron acceptors or electron donors in intershell spaces, and in between the individual tubes or inside the tubes in the case of SWNTs. Intercalation can be carried out in the liquid or vapor phase, and electrochemically. The first doping reactions by K and Rb were performed on MWNTs produced by the electrical arc-­discharge method.23 Using the two-­bulb process, the samples were intercalated in the vapor phase, producing the saturation MC8 composition (M = K or Rb). Interlayer distances were comparable to the first stages of graphite intercalation compounds (Ic = 0.568 nm in RbC8), meaning that the host was completely intercalated with the alkali cations occupying all intershell spaces. HREM

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experiments after successive intercalation with K and deintercalation revealed the existence of individual nanoparticles, showing that intercalation in the graphitic shell of nanotubes caused the development of defects and partial disintegration of the tubes, which is consistent with a Russian doll morphology. Mordkovitch et al.24 found a distinctive beadline pattern of swollen (intercalated) areas alternating with nonintercalated necks on intercalation with K, rather than the MC8 saturated composition, with no proof of residual pristine buckybundles in the XRD pattern. K intercalation is followed by a decline in conductivity and a transition in inelastic scattering processes. The metallic character of SWNTs is induced by alkali metal intercalation. Liang et al.25 studied substantial debundling of SWNT ropes due to alkali metal intercalation (Li, Na and K) in dodecylated SWNTs produced from HiPco nanotubes and 1-­iodododecane. Using the intercalation technique as in exfoliation of graphite, Tanaike et al.,26 short-­circuited SWNT paper and lithium metal foil in an electrolyte containing lithium ions. The diffusion of the dopant inside the host nanotube along the defects (the edge planes accessible at the surface of the cylindrical layers of the tube) is needed to conduct intercalation of a perfect MWNT (Russian doll tube closed at the extremities). This implies the dopant diffusion around the defects limits intercalation. Dopant intercalation is therefore defined by the structural order and morphology of the tubes (basal planes parallel with the tube axis or at an angle to the tube axis). Thus, the reactivity of MWNTs is determined by whether they are Russian dolls, scroll nanotubes or catalytic nanotubes (straight or conical). For instance, the fast diffusion of dopant in scroll nanotubes (buckybundle helical nanotubes) enables intercalation with tube preservation even though only certain disordered domains are intercalated, resulting in intercalated beads.24,27 Furthermore, large compounds, such as metal chlorides, intercalate less in the scroll tubes than in graphite. Alkali metals intercalate in disordered graphite-­like regions of Russian doll tubes, which are not reactive with Br2 or metal chlorides.28 Catalytic tubes react with alkali metals to give compositions and metallic properties similar to graphite intercalation compounds.23,29 Intercalation in MWNTs creates a decrease in the coherent energy of the tubes, resulting in their partial or total collapse. Russian doll tubes are distorted by mechanical distortion of the cylindrical planes used for the accommodation of the dopant species in the intershell spacing, as observed by TEM.23 MWNTs can be opened longitudinally to form graphene sheets and nanoribbons by intercalation of lithium and ammonia followed by exfoliation.30 Acid treatment followed by microwave heating also opens up nanotubes to yield nanoribbons. By Li doping of SWNTs in THF solution31 or by electrochemical means,32 the compositions of LiC6 and Li1.2C6 reversible capacity are obtained, respectively. Ball-­milling of SWNTs causes tube fracturing, and the electrochemical doping (Li2.7C6 reversible capacity) is improved due to the absorption of Li

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+

atoms through the inner cores of the broken tubes. Low-­energy Na ion implantation and Na atom intercalation in SWNTs and MWNTs result in significantly different dopant spatial distributions.34a Iwai et al.34b have studied alkali-­metal (Li, K) intercalated SWNTs by XRD and Raman measurements. As the alkali metal is doped into open-­end SWNTs, the Raman G-­band moves to the higher-­energy side initially due to the tension created by the incorporation of the alkali metal between the tubes, and then exhibits a down-­ shift primarily due to the impact of charge transfer from the alkali -­metals inserted within the tubes. Developments in the storage of lithium in various carbon forms have been reviewed by Kaskhedikar and Maier.35a The novel fabrication of SWNT Schottky diodes has been recognized by mass transport of lithium ions intercalated in functionalized SWNTs by pyrene.35b

2.4  M  etal ↔ Semiconductor Transitions Induced by Molecular Interaction The interaction between molecules or metal nanoparticles and SWNTs has been identified as a route to transform metallic tubes into semiconducting ones and vice versa. The alteration of the electron structure of SWNTs is due to the interaction with acceptor and donor molecules, which can be probed by studying electronic properties and analyzing Raman spectra. Electron acceptor molecules such as tetracyanoethylene (TCNE), nitrobenzene and electron donor molecules such as tetrathiafulvalene (TTF) and aniline modify the electronic structure of SWNTs.36,37 The G-­band at 1540 cm−1 due to metallic species increases upon interaction of SWNTs with electron donors such as TTF and aniline, while the band nearly disappears on interaction with electron acceptor molecules such as tetracyanoethylene (TCNE) and nitrobenzene (Figure 2.3a).36a It seems that metallic SWNTs interact selectively with electron acceptor molecules and semiconducting SWNTs with electron donor molecules giving rise to semiconductor ↔ metal transitions. The variations in the Raman spectra are complemented by variations in the electrical conductivity (Figure 2.3b). Semiconducting SWNTs are also converted to metallic species by depositing gold and platinum nanoparticles.36b Metallic SWNTs are transformed to semiconducting species by helical wrapping of DNA.38a Water has been found to be vital to this reversible transition, which is accompanied by formation of hybrids with DNA. Opening up of a band gap is predicted in metallic SWNTs upon wrapping with ssDNA in the presence of water molecules, owing to charge transfer. Sgobba et al.38b have reviewed the optical, optoelectronic, electrochemical, redox and photoelectrochemical features of CNTs with specific stress on the important applications as hole conductor/electron conductor materials or as electron acceptors/electron donors in solution and in the solid state.

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Figure 2.3  (a)  G-­bands in the Raman spectra of SWNTs on interaction with elec-

tron donor and acceptor molecules. The inset shows RBM bands in the Raman spectra of SWNTs on interaction with (a) TTF and (b) TCNE. (b) I–V characteristics of the SWNTs in air and in the presence of different aromatic molecules with electron-­withdrawing and electron-­donating groups. Reproduced from ref. 36a with permission from American Chemical Society, Copyright 2008.

2.5  Opening and Filling of Nanotubes Multi-­walled nanotubes are usually closed at both ends, which is rendered possible by the inclusion of five-­membered rings. At high temperatures, MWNTs may be uncapped by oxidation using oxygen or carbon dioxide.39–41 However, high yields of uncapped MWNTs can be produced by boiling them in concentrated HNO3. HNO3 treatment causes carbonyl and other functionalities to form in the nanotubes. Metals have been injected into opened

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nanotubes. The conventional approach involves treating the nanotubes with boiling HNO3 in the presence of metal salts such as Ni(NO3)2.42 The nanotubes are opened by HNO3 and filled by the metal salt. The metal salt converts into the metal oxide during drying and calcination, and reduction of the encapsulated oxide in hydrogen at about 400 °C yields the metal within the nanotubes. MWNTs have been uncapped using different oxidants,43–45 and the opened MWNTs have been loaded with Pt, Au, Ag or Pd by different chemical methods, as well as high-­temperature hydrogen reduction.45 Bower et al.46 detected alkali-­metal intercalation into SWNTs using in situ techniques in a TEM. Metal-­intercalated SWNTs are also formed by sealed tube reactions of SWNTs and metal salts.47 Hsu et al.48 handled potassium-­ intercalated MWNTs with CCl4 hydrothermally in an attempt to convert the nanotube sp2 carbon to sp3. They observed crystallization of KCl within the nanotubes and inside the tube walls. Possible methods of closing the nanotubes exposed by oxidants have been investigated.45 Aside from opening, loading and closing nanotubes, acids have been used to prepare highly functionalized MWNTs.45,49 SWNTs are easily opened and filled with metals after mild acid treatment.50,51 Metal nanoparticles such as Au, Ag or Pt may be used to decorate acid-­treated nanotube surfaces.52 The enhanced reactivity of SWNTs with acceptor dopants such as HNO3,53 H2SO4,54 bromine55 and iodine56 can be explained on the basis of intercalation on both the outside and inside of the nanotubes. Prior to intercalation, the tubes are opened through iodine oxidation. HREM images demonstrate that iodine is intercalated not only between the tubes, but also inside the tubes, creating helical chains. Green and coworkers57a used HREM to study molecules and 1D crystals within SWNTs and collected atomic scale images. Typically, traditional HREM may be used to view molecular species and aligned 1D crystals produced inside SWNTs using direct lattice imaging. Indeed, after the microstructure and helical existence of multi-­layer nanotubes were correctly illustrated by a combination of HREM and electron diffraction in early 1993, the technique has proven invaluable. Significant progress has been made in imaging SWNT-­incorporated molecules and 1D crystals using direct, super-­ resolved and spectroscopic approaches. Figure 2.4a displays a conventional HREM micrograph, produced at close to optimum Scherzer defocus settings, of a 2 × 2 potassium iodide (KI) crystal formed within an SWNT ∼1.4 nm in diameter. In this image, the I–K or K–I columns are clearly resolved (see the structure model in Figure 2.4b), whereas the graphene walls appear as continuous and featureless parallel black lines. In the example in Figure 2.4a, only the intensely scattering I atoms (Z = 53) add greatly to the picture comparison, whereas the K atoms (Z = 19) make a marginal contribution. Figure 2.4c shows a super-­resolved picture of 3 × 3 KI shaped inside a ∼1.6 nm diameter SWNT, and Figure 2.4d shows the corresponding derived structure model. Since the encapsulated crystal is noticeable in a [110] projection compared to bulk KI, all of the atom columns are visible as pure atom columns. In a conventional image, the strong I atoms are visible, while the K atoms

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Figure 2.4  (a)  High-­resolution transmission electron microscope image of a 2 × 2 KI crystal formed within a ∼1.4 nm-­diameter single-­walled carbon nanotube (SWNT). (b) Structural model derived from (a). (c) Super-­resolved HRTEM image of a 3 × 3 KI crystal formed in a ∼1.6 nm-­diameter SWNT. (d) Corresponding structural model derived from (c). Reproduced from ref. 57a with permission from John Wiley and Sons, Copyright © 2012 John Wiley & Sons, Ltd.

are invisible. The I columns are even more evident in the reverse-­contrast restored picture (Figure 2.4c), but they are now interspersed with the weaker K columns. The intensities of the light spots referring to the I and K columns vary with atom column thickness. Along the SWNT, we therefore identify two alternating layers, corresponding to I–2K–3I–2K–I and K–2I–3K–2I–K, which is consistent with the 3 × 3×∞ 1D KI crystal. Notably, both 1D crystals display considerable lattice distortions compared with their bulk structures. In the 2 × 2 case, a lattice expansion of ∼17% occurs across the SWNT capillary, whereas in the 3 × 3 case a differential expansion is observed, with the I columns being more compressed than the K columns (Figure 2.4d). Recently, Eliseev et al.,57b reviewed various methods for filling SWNTs both in situ and ex situ approaches based on both the growth of nanoparticles inside SWNTs as a result of chemical reactions and intercalation of inorganic substances inside the channels from solution or melts and gas phase.

2.6  Decoration and Coating Carbon nanotube surfaces have been decorated with several metal nanoparticles. Satishkumar et al.52 decorated acid-­treated carbon nanotubes with Au, Pt and Ag nanoparticles by employing several procedures. The carbon

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nanotubes were refluxed with concentrated nitric acid and were washed with distilled water and dried. These acid-­treated nanotubes were treated with metal compounds of gold, platinum or silver followed by reduction with an appropriate reducing agent yielding decorated nanotubes. By tuning the metal compound concentrations or by mild sonication, the extent of metal coverage and nature could be varied. The surface acidic sites on the nanotubes act as potential nucleation sites for metal deposition. This is different from the decoration of oriented graphite crystals where the metal clusters preferentially get deposited along dislocations and grain boundaries. The electrodeposition method was applied to deposit the noble metal nanoparticles on to the surface of SWNTs.58 Using platinum(ii) acetylacetonate as the precursor, composites of Pt-­CNTs were produced in supercritical CO2.59 Decoration of Ru nanoparticles on CNTs was achieved in supercritical water using RuCl3.60 Self-­assembly of Au nanoparticles on SWNTs were achieved by using DNA.61 These hybrid structures are achieved by hybridizing Au nanoparticles attached to single-­stranded DNA with complimentary DNA-­ attached carbon nanotubes (Figure 2.5). Fluorinated silica-­coated SWNTs show a red shift in the UV-­visible-­NIR spectra (at the first van Hove singularity transition) suggesting that the SiO2 coating makes a more polarizable and inhomogeneous environment around the SWNTs than that of surfactant solutions.62 Cadmium chalcogenides63 and Fe3O4 nanoparticles64 have also been attached to SWNTs. Nanohybrids of SWNTs and MWNTs with a photoactive donor–acceptor were obtained by linking thioglycolic acid-­capped CdTe nanoparticles.65 SWNTs coated with nanodiamond were achieved by employing a CVD reactor.66 The inner tubular structures comprise bundles of single-­walled tube with lengths up to 15 µm, and the outer deposits contain well-­shaped diamond crystallites having diameters in the 20–100 nm range (Figure 2.6). Carbon nanotubes coated with chemically bonded ceramic oxide are prepared by reacting acid-­treated CNTs with vapors of metal halides such as SiCl4 or TiCl4, followed by water treatment and calcination (Figure 2.7).67 The thickness of the metal oxide is tuned by the number of cycles of reaction with the metal halide and H2O.

2.7  Reactivity, Solubilization and Functionalization The enormous pressure caused by the spherical geometry of fullerenes, as expressed in the pyramidalization angles of the carbon atoms, drives their reactivity. Planarity is highly favored for an sp2-­hybridized (trigonal) carbon atom, suggesting a pyramidalization angle of θP = 0°, while θP = 19.5° is needed for an sp3-­hybridized (tetrahedral) carbon atom (Figure 2.8). Many of the carbon atoms in C60 have θP = 11.6°, and their geometry lends itself to tetrahedral rather than trigonal hybridization. The chemical transformation of a trivalent carbon in C60 to a tetravalent carbon relieves pressure at the connection stage and mitigates strain at the remaining 59 carbon atoms. Strain relaxation facilitates reactions that saturate the carbon atoms, which greatly supports fullerene addition chemistry. A perfect SWNT, like a fullerene, lacks

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Figure 2.5  (a)  Schematic representation of the procedure for DNA-­directed self-­ assembly of multiple carbon nanotubes and nanoparticles. Typical AFM image of (b) the self-­assembly of ssDNA-­MWNTs and cDNA-­Au nanoparticle (scanning area: 0.55 µm by 0.55 µm; vertical scale bar: 50 nm) and (c) the 3-­D surface plot of (b) with different color codes (scanning area: 0.85 µm by 0.85 µm). Reproduced from ref. 61 with permission from American Chemical Society, Copyright 2005.

functional units, making these cylindrical aromatic macromolecules chemically inert. The curvature-­induced pyramidalization and misalignment of the carbon π-­orbitals results in local strain (Figure 2.8), and the nanotubes are predicted to be more reactive than a flat graphene layer.68 The end caps and sidewalls of carbon nanotubes can be conceptually divided into two regions. Carbon nanotube end caps mimic hemispherical fullerenes. Since it is impossible to reduce the maximum pyramidalization angle of a fullerene below about θPmax = 9.7°, the end caps are always fairly reactive, regardless of the nanotube diameter. In Figure 2.8a, a (5,5) SWNT covered with a hemisphere of C60 is shown, the angles of pyramidalization are as follows: θP ≈ 11.6° (end cap) and θP ≈ 6.0° (sidewall). The pyramidalization is the main cause for strain in the fullerenes, and the large strain caused by the spheroidal

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Figure 2.6  SEM  images showing: (a) a general view of nanodiamond coated SWNTs;

(b–d) nanotube bundles covered in diamond nanocrystallites. Reproduced from ref. 66 with permission from American Chemical Society, Copyright 2005.

structure is credited with their well-­advanced (addition) chemistry. Similar to the fullerenes, the chemical reactivity of carbon nanotubes results from their topology but for different reasons. Moreover, since the π-­orbital misalignment angles and the pyramidalization angles of SWNTs scale inversely with the nanotube diameter, variation in reactivity with respect to diameter of nanotubes is expected. Although Chen et al.69 and others have attempted chemistry with as-­ prepared SWNTs, the difficulties in characterizing the products made it necessary to develop a dissolution process for the SWNTs by chemical attachment of organic functional groups by other means. SWNTs are highly resilient to wetting.70 They usually present as highly dense bundles or ropes of 10–25 nm in thickness, which are entangled to form complex network structures. Additionally, these tubes do not possess any surface functional groups, making them challenging to disperse in any medium. Carbon nanotubes are not soluble in most common solvents. It is important to have carbon nanotubes in solution in various solvents for use in different applications. The extremely low solubility of as-­prepared carbon

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Figure 2.7  The  ceramic coating process for (a) carbon nanotubes and (b) metal

oxide nanowires. (c), (d) TEM images of the same sample after calcination at 350 °C, (e) TEM image of the nanotubes and nanowires after the removal of carbon nanotubes and (f) TEM image of TiO2-­coated SWNTs after calcination at 350 °C. Reproduced from ref. 67 with permission from John Wiley and Sons, Copyright © 2005 WILEY-­VCH Verlag GmbH & Co. KGaA, Weinheim.

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Figure 2.8  Diagrams  of (a) metallic (5,5) SWNT, (b) pyramidalization angle (θP), and (c) the θP–orbital misalignment angles (ϕ) along C1–C4 in the (5,5) SWNT and its capping fullerene, C60. Reproduced from ref. 68 with permission from American Chemical Society, Copyright 2002.

nanotubes limits their manipulation and use. Therefore, it is essential to modify the nanotube surface to facilitate their solubility in different solvents. Functionalization not only allows the dispersion of CNTs in different solvents but also protects them against agglomeration. Dispersions of functionalized CNTs can be readily used to make composites with different polymers. Carbon nanotubes can be functionalized via covalent modification or through noncovalent interactions, both leading to means of solubilizing them in aqueous, polar and nonpolar media. Chemical manipulations are essential aspects of the chemistry of carbon nanotubes and functionalization and solubilization are also necessary for many of the applications. Derivatization of nanotubes offers products with sidewall substituents, wrapped with polymers, or with the insertion of guest molecules. Various methods have been developed for the functionalization of CNTs and several reviews cover this work.68,71–74 These include covalent functionalization of defects and the sidewalls, noncovalent exohedral functionalization and endohedral functionalization (Figure 2.9). Refluxing SWNT raw soot in the presence of nitric acid results in oxidization of the end caps of the tubes to carboxylic acid and additional weakly acidic functional groups.75,76 An ultrasonic force treatment disperses the acid-­ treated SWNTs in different amide-­t ype organic solvents. The nitric acid treatment of CNTs purifies them, but leads to the formation of defects on the CNT

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Figure 2.9  Different  functionalization possibilities for SWNTs: (a) defect-­group functionalization, (b) covalent sidewall functionalization, (c) noncovalent exohedral functionalization with surfactants, (d) noncovalent exohedral functionalization with polymers and (e) endohedral functionalization with C60. Reproduced from ref. 74 with permission from John Wiley and Sons, Copyright © 2002 WILEY-­VCH Verlag GmbH, Weinheim, Fed. Rep. of Germany.

surface, and produces impurity states (hole dopes) (Figure 1.43 in Chapter 1) at the Fermi level of the nanotubes.77 This latter effect can be described as oxidizing agents such as nitric acid intercalating the nanotube lattice with partial exfoliation, with consequent impact on the electronic properties of the nanotubes, as shown by Raman scattering.53,78 Under oxidizing conditions, the defect sites inserted into carbon nanotubes may be used to shorten and even degrade the nanotubes.75,79,80,81a Shorter nanotubes (s-­SWNTs) dissolve faster in amide solvents than full-­length SWNTs. Microwave heating is commonly used to functionalize carbon nanotubes.81b Acid–base titration methods have been applied to define the basic and acidic characters of the surfaces of diverse carbon-­based structures, including MWNTs.45,49,82,83 The intensity of surface acidic groups in nitric acid-­treated MWNTs is in the range of 0.2–0.5 at.%. Titration with NaOH will estimate the

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overall acidic sites (including carboxylic acids, lactones and phenols), while titration with NaHCO3 can determine the carboxylic acid groups separately. Boiling MWNTs in an HNO3/H2SO4 mixture dissolves them, and the solid obtained from the solution is strongly functionalized by acid groups, with the density of acidic groups present reaching 1%.45,49 Hu et al.76 assessed the acidic sites in full-­length nitric acid purified SWNTs. Titration with NaOH and NaHCO3 yields the following cumulative percentage of acidic sites in full-­length, filtered SWNTs : SWNT–COOH functionality is 1–2%, overall SWNT acid functionality is 1–3%.

2.8  Covalent Functionalization 2.8.1  Halogenation Carbon nanotubes produced by different techniques have been fluorinated by elemental fluorine at various temperatures. Fluorination occurs well at temperatures between 150 and 400 °C.84–89 Both experimental and theoretical investigations have been carried out to understand the probable addition pattern in fluorinated CNTs. Both 1,2-­addition and 1,4-­addition are found to be equally probable and sidewall carbon atoms functionalized with F atoms are tetrahedrally coordinated and adopt sp3 hybridization. This functionalization destroys the electronic band structure of semiconducting or metallic SWNTs. Mickelson et al.90a studied the solvation of fluorinated SWNTs in alcohol solvents having functionalization up to F/C = 1/2. Fluorinated single-­walled carbon nanotubes (F-­SWNTs) are obtained by reaction with elemental fluorine at elevated temperatures. By ultrasonication, the fluorinated carbon nanotubes can be solubilized in alcoholic solvents. Among the interesting properties of F-­SWNTs are their solubility in alcohols and their further derivatization with nucleophiles. Defunctionalization can be achieved by hydrazine treatment and annealing. Bettinger90b covers the experimental and computational investigations of F-­SWNTs with a focus on the nature and the strength of the C–F linkage. STM studies of fluorinated SWNTs reveal an interesting banded structure followed by atomically resolved regions, indicating sidewall functionalization.91a Starting from fluorinated SWNTs, Boul et al.91b have carried out alkylation by reaction with alkyl-­magnesium bromides or alkyllithium. Fluoronanotubes can be reacted with several diols or diamines, similar to Grignard or organolithium reagents, via nucleophilic substitution, to form alkylated carbon nanotubes.73 Aminoalkylated CNTs are soluble in dilute acids and water due to the presence of terminal amino groups. Figure 2.10 shows some of these reactions. CNTs have been functionalized with chlorine or bromine electrochemically, and the functionalized CNTs are soluble in polar solvents.92a Treatment of SWNTs with SOCl2 affects the electrical and mechanical properties thereby indicating a Fermi level shift into the valence band.92b DWNTs have been fluorinated to get CF0.3.93

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Figure 2.10  Reaction  scheme for fluorination of nanotubes, defunctionalization and further derivatization. Reproduced from ref. 73 with permission from American Chemical Society, Copyright 2006.

2.8.2  End-­group Functionalization The attachment of a long-­chain hydrocarbon at the ends of shortened (100– 300 nm) carbon nanotubes, according to Liu et al.94 could make the functionalized SWNTs soluble in organic solvents. They transformed the acid functionality of small SWNTs to the amide of octadecylamine, resulting in shorter soluble SWNTs. Mid-­IR spectroscopy can quickly track the formation of the amide bond. To demonstrate that the substance is in solution, the SWNTs possess a range of distinct spectroscopic signatures. In order to remove SWNT fragments in organic solvents, Chen et al.95 derivatized them with halogen and amine moieties. The development of zwitterions resulted from the direct reaction of acid-­purified short SWNTs with long-­chain amines.96 Functionalization with 4-­tetradecylaniline and aniline provided some insight into the process of dissolution. Although tetradecylaniline surface modification generated short SWNTs that were soluble in THF, CS2 and aromatic solvents, aniline derivatives were only soluble in aniline. The long hydrocarbon long appears to be critical in disrupting and compensating for the loss of van der Waals attraction between carbon nanotubes.

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Full-­length SWNTs are suitable for use in composites and nanoscale conductors. However, the direct reaction of the SWNT–COOH with octadecylamine culminates in the breakdown of unshortened, full-­length SWNTs.97 There are some advantages of ionic functionalization. To begin with, the acid–base reaction is the shortest possible path to soluble SWNTs and can be quickly scaled up at a low cost. Secondly, unlike the covalent amide bond, the cation (+NH3(CH2)17CH3) in the ionic bond of SWNT– COO− +NH3(CH2)17CH3 can be easily exchanged with organic and inorganic cations. This property enables electrostatic interactions between SWNTs and biological molecules, which can be used to produce biocompatible SWNTs. An ester bond forming protocol has been used to connect a bulky side chain to SWNT ends through amide or other groups. Similarly, water solubilization of SWNTs by functionalization with glucosamine was achieved by Pompeo and Resasco,98 while sonication-­assisted functionalization and solubilization of SWNTs was carried out by Huang et al.99 Interaction of oxidized SWNTs with vaporous aliphatic amines was studied by Basiuk et al.100

2.8.3  Cycloaddition Haddon et al. reported carbene [2 + 1] cycloaddition to CNTs followed by nucleophilic addition of carbenes by Hirsch et al.69,95,101,102,103 [2 + 1] thermal cycloaddition by nitrenes has also been investigated.104–106 By this method, carbon nanotubes have been attached to different dendrimers, alkyl chains and crown ethers, which can form complexes with metal ions. Brunetti et al.107a have reported a new synthetic strategy based on a simple and fast microwave-­induced method to make multi-­functionalized CNTs using a combination of two addition reactions, the addition of diazonium salts and the 1,3-­dipolar cycloaddition of azomethine ylides. Using the microwave method, Vázquez et al.107 reported functionalization of CNTs using a variety of procedures, especially 1,3-­dipolar cycloaddition, esterification and amidation reactions. Azide photochemistry has been employed to functionalize CNTs via the formation of the nitrene group, which connects to the CNTs to form aziridine adducts.108,109a Reaction of amino acids and aldehyde yields azomethine in situ that can be added to CNTs to form pyrrolidine-­fused rings via 1,3-­dipolar cycloaddition, which are useful in DNA–peptide interactions. Thermal 1,3-­dipolar cycloaddition of nitrile imines with CNTs has been carried out using microwaves.109b Figure 2.11a–d shows some of these reactions. CNTs bearing pendant amino groups (NH2–CNT) can be attached to nucleic acids, amino acids, peptides or electron donors like ferrocene.110 Silylation of CNTs with different silane reagents has been achieved by photochemical means.111 Sidewall functionalization and the subsequent solubilization of MWNTs has been accomplished using microwave-­assisted cycloaddition reaction of azomethine ylides.112

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Figure 2.11  Schemes  showing (a) cycloaddition reaction with in situ-­generated

dichlorocarbene, (b) photoinduced generation of reactive nitrenes in the presence of nanotubes, (c) 1,3-­dipolar cycloaddition of azomethine ylides, (d) 1,3-­dipolar cycloaddition of nitrile imines to nanotubes. Reproduced from ref. 73 with permission from American Chemical Society, Copyright 2006.

2.8.4  Radical Addition A general method of functionalization is through diazonium salts.113–115 Typically, SWNTs have been functionalized by diazonium salts formed in situ by the reaction of sodium nitrite with substituted aniline in a mixture of ammonium persulfate and 96% sulfuric acid.116 This method has been used extensively in the separation of semiconducting SWNTs from metallic ones. By using fluorous chemistry, Ghosh and Rao have reported separation wherein

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the diazonium salt of 4-­heptadecafluorooctylaniline is reacted selectively with metallic SWNTs and the fluoro derivative is extracted in perfluorohexane.117 SWNTs can be exfoliated and functionalized by grinding them for a few minutes at room temperature with aryldiazonium salts in the presence of ionic liquids and K2CO3 (Figure 2.12).118 Functionalized product shows a higher D-­band in the Raman spectrum (Figure 2.12c) due to the increase in the number of sp3 carbons that form on the SWNTs after functionalization. A rapid and efficient procedure to functionalize SWNTs was reported by Martínez-­Rubí et al., wherein free radicals generated at room temperature by a redox reaction between reduced SWNTs and diacyl peroxide derivatives are covalently attached to the side walls.119 Guo et al.120 have reviewed methods for the functionalization of CNTs through free radical reactions. The covalent attachment of phenyl groups has been accomplished by a coupling reaction to provide electrochemical modification.121–123 Two kinds of coupling reactions have been investigated, namely oxidative coupling of aromatic amines and reductive coupling of aryl diazonium salts (Figure 2.13a and b). Electrochemical functionalization of SWNTs has been carried out at room temperature.124 An ionic liquid supported three-­dimensional network SWNT electrode is employed in this method. Covalent functionalization of CNTs with radicals has been achieved by employing thermal and photochemical routes. Thermal decomposition of alkyl or aryl peroxides results in radicals and these radicals are heated with CNTs in the presence of peroxides and alkyl iodides or Fenton's reagent.125–127 Photo-­induced addition of perfluoroalkyl radicals to CNTs has been achieved.104 A solvent-­free method has been implemented where in aniline and isoamyl nitrite are added to SWNTs to obtain arenediazonium species in situ that react with SWNTs. Some of these radical reactions are given in Figure 2.13c. Functionalization of SWNTs have been carried out using microwave nitrogen plasma.128 Functionalization of MWNTs at preselected locations can be achieved by initial irradiation with an ion beam followed by functionalization. This methodology may be helpful to create hybrid devices.129

2.8.5  Nucleophilic Addition Basiuk and coworkers have carried out amination of the closed caps of MWNTs with octadecylamine in solvent-­free conditions.130 Amination occurs only on the five-­membered rings of the CNTs and the six-­membered rings are inert to direct amination. Treatment of pristine CNTs with sec-­BuLi followed by reaction with carbon dioxide gives rise to –COOH groups, which render CNTs water dispersible.131 Covalent attachments of amino groups to the sidewalls of SWNTs can be achieved by the nucleophilic addition of in situ-­ generated lithium n-­propylamide and these products lead to soluble derivatives.132 Syrgiannis et al.133 have carried out a mechanistic investigation of the nucleophilic addition of organolithium reagents onto SWNTs and shown that functionalization of the sidewalls of SWNTs is reversible and that the introduced substituents can be detached by reduction.

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Figure 2.12  (a)  Schematic showing the functionalization of SWNTs in ionic

liquid using a mortar and pestle and Raman spectra (633 nm, solid) of (b) purified SWNTs and (c) SWNTs functionalized with 4-­chlorobenzenediazonium tetrafluoroborate in BMIPF6. Reproduced from ref. 118 with permission from American Chemical Society, Copyright 2005.

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Figure 2.13  Schemes  showing (a) electrochemical functionalization resulting in

C–C bond formation, (b) electrochemical functionalization by oxidative coupling resulting in C–N bond formation and (c) derivatization reactions of carbon nanotubes by addition of carbine, nitrene and photoinduced addition of fluoroalkyl radicals.

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2.8.6  Covalent Polymer Composites Reactions of CNTs with polymers help them to dissolve in different solvents. There are two methodologies known as “grafting to” and “grafting from” for the covalent attachment of polymers. Figure 2.14a and b show examples of these two methods. The “grafting to” method involves polymer synthesis, conversion of the end group followed by connection of the polymer chain to the nanotube surface. Nucleophilic reaction of polymeric carbanions, use of halogenated polymers, reaction of organometallic reagents, radical coupling reactions and cycloaddition reactions are used to create polymer–nanotube composites.134–139 On the other hand, polymer precursors are covalently immobilized on the surface of CNTs and subsequent in situ polymerization is involved in the “grafting from” method. Chemical as well as radical polymerization methods are employed in this procedure besides utilizing electrostatic interactions.140,141 MWNT surfaces have been grafted with thermosensitive poly(N-­isoproplyacrylamide) through chain-­transfer polymerization to produce water-­soluble MWNTs.142 Several block copolymers with different functional side groups have been used for the preparation of aqueous dispersions

Figure 2.14  Simple  schemes showing (a) “grafting to” approach for nanotube-­ polystyrene composites and (b) “grafting from” a polyelectrolyte by an in situ process for obtaining water-­soluble nanotubes.

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of MWNTs as well as SWNTs. Cross-­linking of an amphiphilic α-­peptide to SWNTs increases the solubility of the CNTs in water.144 Polyaniline–carbon nanotube composites have been fabricated via the in situ chemical polymerization of aniline.145–147 These polymer composites produce stable solutions in both organic and aqueous solvents. Polyethylene has been periodically crystallized over carbon nanotubes by adjusting crystallization conditions.148 Utilizing both the end group and the sidewall, Riggs et al.149 covalently attached highly soluble linear polymers (poly(propionylethylenimine-­ co-­ethylenimine)) to SWNTs, solubilized them in water and studied their luminescence behavior. Sun et al.150 have investigated the preparation, characterization and properties of soluble dendron-­functionalized carbon nanotubes. Huang et al.151 attached proteins to carbon nanotubes via diimide-­activated amidation. Self-­organization of PEO-­graft-­single-­walled carbon nanotubes in solutions and Langmuir–Blodgett films was generated by Shinkai and coworkers.151,152a Synthesis of water-­soluble MWNTs with grafted temperature-­ responsive shells has been accomplished by surface reversible addition and fragmentation chain transfer (RAFT) polymerization.152b

2.8.7  Other Covalent Functionalization Methods Hydrogenation of carbon nanotubes has been carried out by Birch reduction with Li metal and methanol dissolved in liquid ammonia.153 Nanotubes have been functionalized with atomic hydrogen by proton bombardment or glow discharge.154–157 Increased solubility of CNTs can be achieved by the electrophilic addition of chloroform in the presence of a Lewis acid and subsequently hydrolyzed to make hydroxy groups.158 Covalent modification of carbon nanotubes with imidazolium salt-­based ionic liquids has been reported.159 Banerjee and Wong160 have reported the synthesis and characterization of carbon nanotube-­CdSe (capped with mercaptothiol derivatives) heterostructures. Ozonolysis of single-­walled CNTs affords primary CNT-­ozonides, which after treatment with hydrogen peroxide renders the surface of the CNT with high proportions of alcohol, carboxylic acid/ester and ketone/aldehyde groups.161–164 Oxidation of SWNTs by ozone causes an irreversible increase in their electrical resistance.165 Treatment of nanotubes with osmium tetroxide vapor under UV irradiation yields osmylation of CNTs.166 Reaction between CNTs and trans-­ IrCl(CO)(PPh3)2 forms nanotube–metal complexes. Chemical modification of CNTs has also been achieved by radio frequency glow-­discharge plasma activation.167 Production of short nanotubes containing different chemical functional groups such as thiols, amines and amides has been achieved by ball-­milling of nanotubes in appropriate reactive atmospheres.168 A simple solid-­phase milling of SWNTs with potassium hydroxide,169 produces nanotube surfaces covered with hydroxyl groups, resulting in increased solubility. Using a similar approach, nanotubes can be attached to C60.170 SWNTs can be readily dispersed in alcohol using sodium hydroxide.171

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Defect functionalization: Smalley et al. demonstrated a purification process for carbon nanotubes in which the as-­prepared CNTs were treated with a mixture of HNO3 : H2SO4 (1 : 3), where long nanotubes with closed tips are converted into shorter, open-­ended nanotubes with carboxylic functionalized sidewalls. The defect sites are functionalized with carboxylic groups and this has enabled simple strategies to functionalize CNTs.75,172 Cutting SWNTs into short segments is achieved by controlled oxidation by piranha (HNO3/H2SO4) solutions.173 Piranha attacks the existing damage sites at high temperatures creating vacancies in the nanotube sidewalls, and consuming the oxidized vacancies produces short, cut nanotubes. Dilute HNO3 treatment under supercritical conditions yields direct sidewall functionalization of MWNTs.174 Asymmetric end-­functionalization (each end is functionalized with a different chemical group) of individual MWNTs has been reported.110 Figure 2.15 shows the procedure used for the asymmetric end-­functionalization of an aligned carbon nanotube film. At first, functionalization of aligned nanotubes is carried out at one end with 3′-­azido-­3′-­deoxythymidine using UV radiation. After washing, perfluorooctyl chains are attached to the unmodified side of the nanotubes by treating the above functionalized tubes with perfluorooctyl iodide in tetrachloroethane. Water-­dispersible SWNTs have been achieved by microwave-­assisted treatment of SWNTs in a mixture of sulfuric and nitric acids.175 SWNTs containing COOH groups can be transformed to amino groups.176 Chen and coworkers studied the acylation reaction of long-­chain alkylamines with oxidized CNTs to produce soluble CNTs in various organic solvents.95 Direct thermal mixing of SWNTs containing COOH groups with alkylamines yields functionalized material via the formation of zwitterions (Figure 2.16A).

Figure 2.15  A  free-­standing film of aligned MWNTs floating on the top surface

of (a) an AZT solution in ethanol (2%) for UV irradiation at one side of the nanotube film for 1 h, and (b) a perfluorooctyl iodide solution in TCE (2%) for UV irradiation at the opposite side of the nanotube film for 1.5 h. (c) Asymmetrically modified aligned CNT film. Reproduced from ref. 109a with permission from American Chemical Society, Copyright 2005.

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Figure 2.16  (A)  Zwitterionic functionalization of short-­length-­SWNT. (B) Schemes

showing different ways of defect-­end covalent functionalization on carbon nanotubes by amide bond formation, making the product soluble to organic solvents. (A) Reproduced from ref. 97 with permission from American Chemical Society, Copyright 2001. (B) Reproduced from ref. 96 with permission from John Wiley and Sons, Copyright © 1999 WILEY-­VCH Verlag GmbH, Weinheim, Fed. Rep. of Germany.

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Semiconducting SWNTs exhibit greater affinity towards amine groups, over their metallic counterparts, which offers a way for separation.96,97,177,178,179,180 Using defect site chemistry, fluorescent pyrene molecules have been attached to CNTs, which induce interesting photophysical properties.181 CNTs have also been attached to modified porphyrins and their photovoltaic properties studied.182,183 Soluble and processible nanotubes are commonly produced by esterification or amidation. Treating oxidized nanotubes with taurine (2-­aminoethanesulfonic acid), Gu and coworkers made water-­soluble nanotubes.184 Pompeo and coworkers185 reported soluble CNTs with short lengths made by connecting glucosamine moieties, while other researchers have produced mannose-­ and galactose-­modified CNTs.186,187 Amine-­modified oxidized nanotubes have been reported by Kahn and coworkers.188 Gold nanoparticles were linked to the soluble amine-­modified CNTs, by cross-­linking with a thiol-­pyrene derivative. Alkyl-­modified nanotubes were produced by treating oxidized nanotubes with a base followed by alkyl halides. CNTs were terminated with thiol functional groups and deposited on gold surfaces. Acid-­functionalized CNTs have been deposited on a silver surface through adsorption of the –COOH groups onto the surface. Treatment of the acid-­functionalized CNTs with octadodecylamine (ODA) produced the corresponding SWCNT–ODA amide, soluble in organic solvents. Figure 2.16B shows representative reactions of amidation through defect-­end group functionalization. Langa and coworkers189 treated functionalized CNTs with N-­anilinopyrazolino-­fullerene by an amidation reaction to synthesize hybrid conjugated SWCNT–fullerene materials (Figure 2.17a). Prato and coworkers applied a similar strategy to functionalize SWNTs with strong electron donor molecules such as tetrathiafulvalene (TTF) or its π-­extended analogues (exTTF) (Figure 2.17b).190 Different photoactive functionalities including ferrocenyl or porphyrinyl have been attached to nanotubes. The photo and electroactive species (ferrocenyl and porphyrinyl fragments) serve as potential chromophores for photovoltaic and optoelectronic applications.191 Functionalization and solubilization of DWNTs are of value because the outer nanotube is functionalized whereas the inner tube remains intact. Covalent functionalization of DWNTs has been achieved by amidation, resulting in organically soluble DWNTs.192 Rao and coworkers covalently functionalized multi-­walled carbon nanotubes with organotin reagents such as trioctyltinchloride and dibutyldimethoxytin, as well as organosilane reagents like HDTMS to produce stable dispersions in toluene and CCl4.193 Using defect site chemistry, CNTs have been attached to different biomolecules. CNTs have been studied as bio-­active component carriers that transport and deliver into cells giving rise to new biosensors.194 By using proteins, SWNTs have been solubilized. Water-­soluble CNTs-­protein conjugates have been prepared through the diimide-­activated amidation reaction. Covalent attachment of biotin to the carboxylic sites of oxidized nanotubes has been achieved by Dai and coworkers, and incubated with streptavidin.195

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Figure 2.17  (a)  HRTEM image of covalently linked nanohybrid material SWCNT–[60]

fullerene along with the computer generated image of the hybrid. (b) Schematics of TTF–SWCNT and ex-­TTF–SWCNT derivatives. (A) Reproduced from ref. 189 with permission from Elsevier, Copyright 2007. (B) Reproduced from ref. 190 with permission from John Wiley and Sons, Copyright © 2006 WILEY-­VCH Verlag GmbH & Co. KGaA, Weinheim.

CNTs have been covalently attached to glucose oxidase (GOD)196 composites involving covalent attachment of DNA strands to CNTs to moderate solubility in aqueous solution. Defect site reactivity has also been exploited by grafting polymers to oxidized nanotubes. Sun and coworkers converted carboxylic acid groups on CNTs to acyl chlorides then attached poly(ethyleneimine) chains.149,197

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CNT–polymer composites are soluble in common organic solvents. Solubilization of oxidized CNTs by poly(ethylene glycol) (PEG) has been investigated by several groups. Direct thermal reaction of the reactants, carbodiimide-­activated coupling and acylationamidation have all been implemented successfully. These polymer–CNT composites show fairly good dispersibility in polar solvents. CNT–nylon composites have been prepared by Haddon and coworkers through covalent attachment of acidic groups and the polymer chain.198 Poly(vinyl alcohol)-­oxidized CNT composites have been obtained by a carbodiimide-­ activated esterification reaction. The resultant adducts are soluble in polar solvents.198 Incorporation of chemically oxidized MWNTs into a polymer matrix has been achieved by in situ polymerization of methyl methacrylate monomer.199 Song et al.200 have reported evidence for the formation of a lyotropic liquid crystalline phase of surface-­treated MWNTs (with a HNO3/H2SO4 mixture) in aqueous dispersions. They found a phase transformation from isotropic to a Schlieren texture characteristic of lyotropic nematic liquid crystals above a crucial concentration of 4.3% by volume by analyzing a sequence of aqueous dispersions of surface-­treated/highly functionalized MWNTs of various concentrations (vol%). Covalent chemistry on the walls of SWNTs is a viable route to soluble materials. The ability to carry out controlled (covalent) chemistry on the sidewalls of the SWNTs may become useful for several applications. Covalent sidewall chemistry of SWNTs was first achieved by carbene reactions with short soluble SWNTs using phenyl(bromodichloromethyl)mercury in toluene to obtain modifications of the band electronic structure of the SWNTs.95 Characterization of functionalized material is a major challenge in carbon nanotube chemistry, but solution spectroscopy is helpful in the case of wall chemistry, where the band electronic structure is destroyed. The results of sidewall functionalization can also be analyzed using Raman spectroscopy. Dichlorocarbene is an electrophilic reagent that reacts with double bonds and fullerenes. Electronic spectroscopy has shown that at a functionalization frequency of 2% of the usable SWNT carbon atoms, the band-­gap transitions in semiconducting tubes are totally disturbed. Other chemical processes that can be carried out with nanotubes include nitrene attachment, hydrogenation through the Birch reduction, fluorination, alkylation, arylation and 1,3-­dipolar cycloaddition. The chemistry of SWNTs has been reviewed comprehensively by Niyogi et al.68 Kharisov et al.201 have reviewed recent advances in the functionalization methods leading to soluble carbon nanotubes. Singh et al.202 have reviewed the covalent modification of SWNTs with organic moieties, and illustrate the major analytical techniques routinely used to characterize these functionalized SWNTs.

2.9  Noncovalent Functionalization Non-­covalent functionalization of carbon nanotubes is important as it does not alter the original electronic structure of the CNTs and permits them to be soluble in polar and nonpolar solvents. Noncovalent functionalization has been carried out with polymers, surfactants, polynuclear aromatic

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compounds and biomolecules. Zhao and Stoddart have reviewed developments in the preparation, characterization and applications of noncovalently functionalized SWNT hybrids.

2.9.1  Noncovalent Polymer Composites Due to their excellent mechanical strength, CNTs are used in reinforcing polymers. Surfactant-­assisted processing or physical mixing in solution or in situ polymerization of monomers in the presence of nanotubes prepares CNT–polymer composites. Nanotube–epoxy composites have been widely investigated. Ajayan et al. prepared aligned arrays of MWNTs within an epoxy resin matrix.204 Surfactant-­assisted processing of CNTs with an epoxy matrix has been employed for improved dispersion and interfacial bonding of the nanotubes.205 CNT–PMMA (poly(methyl methacrylate)) composites dispersible in organic solvents have been prepared by mixing the components in a solution under ultrasonication. In situ radical polymerization of the monomer has been applied by Putz and coworkers to prepare nanotube-­PMMA composites.206 CNT–polystyrene composites can be prepared by shear mixing in solution. CNT–conjugated-­polymer composites are an interesting class of composites with appreciable conductivity. The polymer chain essentially wraps the nanotube and the large nonpolar chain induces solubility of the composite in common organic solvents. Nanotube–polymer composites with different properties and applications have been produced by mixing carbon nanotubes with polycarbonates, polyacrylonitrile, fluoropolymers, aminopolymers, poly(vinylalcohol), poly(ethyleneglycol), polyesters, polyelectrolytes, polyamides, poly(p-­phenylene benzobisoxazole), poly(vinylcarbazole), phenoxy resin, natural rubber and petroleum pitch. Water-­soluble CNT–polymer complexes are obtained by wrapping nanotubes with polymers that bear polar side-­chains, such as polystyrenesulfonate (PSS) or polyvinylpyrrolidone (PVP).73

2.9.2  Functionalization Using Surfactants and Polyaromatics By using surface-­active molecules such as sodium dodecylsulfate, carbon nanotubes can be transferred to the aqueous phase. The stable dispersions in the aqueous phase could be a result of micelle formation, where the CNTs are in the hydrophobic core. The strong π–π-­stacking interactions between nanotube sidewalls and a surfactant containing an aromatic group such as N-­succinimidyl-­1-­pyrenebutanoate ester, can be used to functionalize CNTs. Patterned assembly of SWNTs has been achieved over pyrene-­modified oxide surfaces.207–209 The approach relies on distinctive molecular recognition properties of pyrene towards the graphitic carbon structure (Figure 2.18a). An attractive example of noncovalent modification of nanotubes based on π–π pyrene–CNT interactions have been reported by Guldi et al.210 using pyrene containing fullerene-­bisadduct. These functionalized nanotubes are

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Figure 2.18  (a)  PYBS irreversibly adsorbs onto the sidewall of an SWNT via π-­stack-

ing and that can be used in immobilization of proteins bearing amino groups. (b) Computer-­generated image of the first supramolecular hybrid [60]fullerene–pyrene–SWCNT. (c) Schematic view of Sn(iv)-­ porphyrin functionalized SWNTs. (a) Reproduced from ref. 232 with permission from American Chemical Society, Copyright 2001. (b) Reproduced from ref. 72 with permission from the Royal Society of Chemistry. (c) Reproduced from ref. 213 [https://doi.org/10.3390/ ijms9010045] under the terms of a CC BY 4.0 license [https://creativecommons.org/licenses/by/4.0/].

soluble in nonpolar solvents (Figure 2.18b). By this approach, CNTs have been attached supramolecularly with several organic addends including electro-­and photo-­active groups such as porphyrins211 and TTF.212 Soluble nanohybrids of SWNT–dihydroxotin(iv) porphyrin have been achieved via simple sonication (Figure 2.18c). Spectroscopic investigations show the occurrence of electron transfer suggesting possible uses of these materials in photoelectronic devices.213 Meso-­(tetrakis-­4-­sulfonatophenyl) porphine dihydrochloride disperses SWNTs in aqueous solutions by interacting via noncovalent means.214 These dispersions are stable for a few weeks. Noncovalent functionalization and water solubilization of HiPco-­SWNTs are achieved with ionic pyrene and

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naphthalene derivatives. A liquid–liquid extraction method has been developed using tetraoctylammonium bromide to extract nanotubes from aqueous phase to organic solvents.216 The electrostatic interactions between tetraoctylammonium bromide and nanotube surface functional groups play a role during the extraction. Noncovalent functionalization and dispersion of SWNTs using self-­assembling peptide amphiphiles have been reported.217 Reversible cyclic peptides are useful in the noncovalent functionalization and solubilization of SWNTs in aqueous solutions.218 Polycyclic aromatic ammonium amphiphiles such as trimethyl-­(2-­oxo-­2-­phenylethyl)-­ ammonium bromide are used to make aqueous solubilization of SWNTs.219 A series of peptides that can noncovalently solubilize, as well as individually disperse, SWNTs in water without varying their electronic structure has been reported.220 Noncovalent functionalization of double-­walled carbon nanotubes has also been achieved by using substances such as 1-­pyrenebutanoicacid succinimidylester (PYBS), polyethylene glycol (PEG) and polyoxyehylene(40) nonylphenyl ether (IGPAL). PYBS-­functionalized DWNTs are soluble in nonpolar solvents. Noncovalent functionalization of DWNTs with IGPAL and PEG results in solubilization in water (Figure 2.19a–g).192 Unlike the previously mentioned perpendicular alignment of carbon nanotubes, the post-­synthesis ordering of chemically and/or physically manipulated carbon nanotubes has not been extensively studied. However, perpendicularly aligned carbon nanotubes have been created utilizing end-­ functionalized carbon nanotubes on unique substrates. For example, Liu et al.221 prepared aligned SWNTs on a gold substrate by self-­assembling nanotubes end-­functionalized with thiol groups. The carboxy-­terminated short SWNTs obtained by acid oxidation were used as the starting material for further functionalization with thiol-­containing alkyl amines through the amide linkage. The self-­assembly of aligned SWNTs was accomplished by immersing a gold (111) ball in the thiol-­functionalized SWNT suspension in ethanol, accompanied by ultrasonication and nitrogen drying. The resulting self-­assembled aligned nanotube film was stable, and ultrasonication was unable to extract it from the gold substrate. The packaging density of self-­ assembled aligned carbon nanotubes was highly based on incubation period. Shimoda et al.222 demonstrated the formation of ordered/micropatterned nanotubes through self-­assembly of preformed nanotubes on glass and other substrates by immersing the substrate vertically in an aqueous solution of acid-­oxidized short SWNTs. Under the circumstances, carbon nanotube deposition did not occur on a hydrophobic substrate (e.g., a polystyrene spin-­coated glass slide). Using glass substrates with prepatterned hydrophobic and hydrophilic areas, such selectivity allowed the fabrication of carbon nanotube patterns. We must recall here that Chen and Dai223 generated nanotube micropatterns by patterned growth on surfaces prepatterned with plasma polymer or region-­specific adsorption of some chemically modified nanotubes. In addition to solution-­phase functionalization,51,91a MWNT probes in atomic force microscopy (AFM) have been functionalized by a

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Figure 2.19  Photographs  of stable dispersions of (a) amide functionalized dou-

ble walled nanotubes (DWNT) dispersed in CH2Cl2, (b) in THF, (c) PYBS-­functionalized DWNT dispersed in DMF, (d) PEG-­functionalized DWNT dispersed in water and (e) DWNT solubilized in water by adding IGPAL. (f) Comparative IR spectra of pristine, acid functionalized and amide-­functionalized DWNT. (g) TEM image of DWNTs in solution showing de-­bundling. Reproduced from ref. 192 with permission from Springer Nature, Copyright 2009.

discharge process, in the presence of various gases.224 These functionalized carbon nanotube probes have the potential to be used for chemically sensitive imaging of materials, particularly biomolecules.

2.9.3  Interaction with Biomolecules Carbon nanotubes are thought to be a suitable transporter for drug delivery. However, appropriate chemical functionalization of CNTs is necessary to lower their toxicity, and improve function and water solubility, which will make them useful in biomedical fields. Applications of CNTs in biological

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and biomedical research are being actively investigated by many research groups.225,226 However,the use of nanotubes is inhibited by numerous difficulties, such as: (i) the existence of nanotubes in entangled bundles, (ii) the chemically passive nature of carbon nanotubes, (iii) the very low solubility of carbon nanotubes in aqueous and organic medium, etc. Chemically modifying nanotubes via covalent means has partially resolved these problems and is also one of the most powerful ways of manipulating and handling CNTs, by introducing more sophisticated functionalities to bind biomolecules. Many bio-­applications of CNTs rely on successful outer/inner surface functionalization of the CNTs for controlling biomedical functions.227–229 Many biomolecules can also interact with CNTs via noncovalent means. The specific recognition properties of biosystems with the electronic properties of nanotubes with could yield miniaturized biosensors.230,231 Proteins interact with external walls of nanotubes and adsorb strongly. Metallothione proteins adsorb onto the carbon nanotube surface. Nanotubes immobilize streptavidin on surfaces probably via interaction between the CNT graphitic surface and the hydrophobic parts of the biomolecule or through charge-­transfer interactions. A bifunctional linker based on pyrene has been used for non-­covalent modification of CNTs with biomolecules.232,233 Due to van der Waals interactions, the anchor molecule is irreversibly adsorbed onto the CNT graphitic surfaces. In a subsequent step, the activated pyrene is attached covalently to an enzyme. The process occurs with basic amino acid residues via nucleophilic attack. These biocomposites have shown excellent biosensor properties, which could be applicable in nanobiotechnology and medicinal chemistry. DNA noncovalently interacts with carbon nanotubes and makes stable dispersions in aqueous media.73 The separation of semiconducting nanotubes from metallic nanotubes is achieved by wrapping single-­walled nanotubes with a DNA sequence of alternating thymine and guanine bases. Diameter-­dependent sorting is also possible via ion-­exchange chromatography. By decorating the CNT surface with carbohydrate macromolecules, many groups have achieved aqueous soluble CNT composites. CNTs can be dispersed in an aqueous solution of arabic gum. Light-­harvesting molecules such as porphyrins, phthalocyanines and dyes of phenazine and thionine types have also been immobilized onto CNTs. Noncovalent adsorption of porphyrins enables dissolution of CNTs in organic solvents or aqueous media. DNA molecules can be used to prepare stabilized nanotube suspensions with liquid crystalline properties.234 Zhao et al.235 have studied the structure and characteristics of carbon nanotube-­ionic liquid gel biosensors and have concluded that noncovalent (π–π) interactions between the imidazole loop of the ionic liquid and the nanotube sidewall is responsible for the excellent electrochemical detection of glucose oxidase (GOD) and NADH. Su et al.236 have reported the sequence and conformational requirements of peptides for high-­affinity binding to SWNTs. Constrained conformations of motif are essential for high-­affinity binding to SWNTs. Constrained conformations are attained either by adding

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constrained disulfide bonds (e.g., CGHPWTKC) or by extending the amino acid sequence (e.g., LLADTTHHRPWT). Zhang et al.237a have reported a method for the immobilization of biomolecules on CNTs based on ionic interaction, wherein GOD and SWNTs are integrated into a unitary bionanocomposite by means of an ionic liquid-­like unit. Functionalization of carbon nanotubes with biodegradable supramolecular polypseudorotaxanes from grafted-­poly(ε-­caprolactone) and α-­cyclodextrins has been achieved. This process yields poly(ε-­caprolactone)-­grafted MWNTs (MWNT-­g-­PCL) by ring-­opening polymerization of ε-­caprolactone in the first step, followed by creation of α-­CD-­NTPCL hybrids via forming inclusion complexes between α-­cyclodextrins (α-­CDs) and grafted-­PCL chains.237b Hu and Hu238 studied the surface design of carbon nanotubes obtained from different sources for optimizing the adsorption and electrochemical response of small biomolecules and glucose oxidase. By dissolving CNTs in water using Congo red (CR) via strong noncovalent π-­stacking interactions, uniform, compact and stable films on various substrates were obtained. Further, the electrochemical behavior of several small biomolecules and GOD on various CR-­functionalized CNT films was examined. They concluded that the adsorption of hydrophobic analytes and the electrochemical response are favored by the presence of hydrophobic defect sites on CNTs, which are formed during the growth or workup of CNTs. On the other hand, the adsorption and direct electrochemistry of redox proteins are facilitated by the presence of oxygen-­containing hydrophilic groups created by acid treatment. Suri et al.239 have investigated the reciprocal interactions of SWNT with bovine serum albumin with pristine and carboxylated nanotubes and point out that detailed understanding of the interactions between biomolecules and SWNTs is important for the design and applications of biosensors that employ SWNTs for transduction of the response of the analyte. Kang et al.240 observed a spontaneous encapsulation of a globular protein into the CNT through molecular dynamics simulations. A stepwise conformational change in the protein has been observed during the insertion, to increase its affinity to the nanotube walls. The interactions between CNT and protein cause deformation of β-­sheets in the protein. Functionalization of carbon nanotubes with cleavable disulfide bonds has been applied for intracellular drug delivery of short-­interfacing RNA (siRNA) and gene silencing.241 The short SWNTs are noncovalently functionalized to improve the stability of aqueous suspension. At first, poly(ethyleneglycol) chains are functionalized with phospholipid molecules (PL-­PEG, MW of PEG = 2000) via terminal amine or maleimide groups. van der Waals interactions makes PL-­PEG bind strongly to SWNTs, with the PEG chain spreading into the aqueous phase to impart solubility in water. Functionalized SWNTs are coupled with siRNA and DNA molecules through the incorporation of a disulfide bond. Through heparination, blood-­compatible carbon nanotubes have been prepared for in vivo applications.242 Glucose sensing with SWNTs has been possible by functionalizing the sidewalls or ends of oxidized nanotubes with colloidal particles or polyamine

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dendrimers through carboxylate chemistry. Proteins adsorb separately around the surface of the nanotube. AFM is used to classify the nanotube– protein conjugates at the molecular stage. Several metalloproteins and enzymes are attached to SWNT sidewalls and termini. Though coupling may be modulated to some extent, AFM studies indicate that immobilization is solid and does not involve covalent bonding. Protein binding tends to be correlated with the maintenance of native biological structure. Nanotube electrodes have valuable voltammetric properties, allowing for direct electrical contact between a redox-­active biomolecule and the delocalized divice of its carbon nanotube support. Figure 2.20 shows how a biomodified SWNT works well as a glucose biosensor. Assembly of surfactants and synthetic lipids on surfaces of carbon nanotubes has been accomplished by Richard et al.243b Above a critical micellar concentration, sodium dodecyl sulfate (SDS) develops supramolecular structures on the nanotube surface composed of rolled-­up half-­cylinders. The carbon nanotube may form loops, helices or double helices based on its

Figure 2.20  (a)  Tapping mode atomic force microscopy (TMAFM) amplitude micro-

graph of a GOD modified SWNT in which a high degree of enzyme loading is apparent. Scale bar 200 nm. (b) Voltammetric response of such nanotubes in the absence (lower curves) and presence (upper curves) of substrate, β-­d-­glucose. (c) Schematic representation of the SWNT glucose biosensor. Solution-­phase d-­glucopyranose is turned over by oxidase enzymes immobilized on the nanotubes. This redox process at the enzyme flavin moieties is communicated to the nanotube system through the diffusive mediator ferrocene monocarboxylic acid. The redox action of the ferrocene at the nanotube surface ultimately generates a quantifiable catalytic current characteristic of substrate detection and turnover. Reproduced from ref. 243a with permission from John Wiley and Sons, Copyright © 2003 WILEY-­VCH Verlag GmbH & Co. KGaA, Weinheim.

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symmetry and diameter. Several synthetic single-­chain lipids engineered for the immobilization of histidine-­tagged proteins may also be used to achieve such self-­assemblies. After dialysis of the surfactant, permanent assemblies were formed at the nanotube–water interface from mixed micelles of SDS and various water-­insoluble double-­chain lipids. Such configurations may be used to create biosensors and bioelectronic nanomaterials. Kartz and Willner244 have reviewed biomolecule-­functionalized CNTs and their applications in nanobioelectronics. The authors discuss different synthetic techniques for chemically modifying sidewalls or tube ends with molecular or biomolecular materials. The tailoring of hybrid systems made up of CNTs and biomolecules (proteins and DNA) has evolved steadily and drawn a lot of interest. The combination of biomaterials and carbon nanotubes allows hybrid structures to be used as active field-­effect transistors or biosensor units (enzyme electrodes, immunosensors or DNA sensors). Their combination also aids in the development of complex nanostructures and nanocircuitry with controlled properties and functions. Takeda et al.245 have produced a sensitive carbon nanotube sensor for detecting anti-­hemagglutinins based on antigen–antibody interactions. Gruner has investigated biosensing and biodetection characteristics of biomolecules by making field-­effect transistor (FET) devices with carbon nanotube conducting channels.246 Star and coworkers247 reviewed the use of nanotube FET (NTFET) instruments in protein identification, antibody–antigen assays, DNA hybridization and enzymatic reactions containing glucose. Integrating distinct multi-­element nanostructures into a single molecular electronic device has been reported by Guo et al.248 to detect biological assembly at individual event level. This was attained first by bridging a probe molecule with two ends of two different SWNTs via functionalization. The property of a probe that is placed between the SWNTs is complementary in nature to the biomolecule that would be detected. The probe was chemically adjusted to make oximes, and their formation was seen in the ON-­state resistance modulation and device threshold voltage. Holzinger report electrogenerated functional SWNT composites for biosensor applications.249 In this work, during electropolymerization to make copolymer SWNTs, glucose oxidase (GOD) enzyme has been entrapped. The signal sensitivity for glucose biosensors was drastically enhanced in the presence of nanotubes compared to a similar device using electrogenerated polymer films alone. Functional nanotube composites via electropolymerizing adamantane-­ and biotin-­ pyrrole derivatives along with the SWNT have shown particularly interesting results. The high-­affinity interaction of avidin and β-­cyclodextrin with the highly porous nanotube-­polypyrrole layer provided a platform to attach gold nanoparticles modified with β-­cyclodextrin and adamantane-­cyclodextrin to the functionalized SWCNT as an intermediary layer. Furthermore, adamantane tagged GOD has been immobilized to collect a biosensor response towards glucose via potentiostating the altered electrodes so as to oxidize the enzymatically produced H2O2 in the presence of glucose and oxygen. The structure constructed using gold particles modified with β-­cyclodextrin

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as an intermediate layer showed the maximum current density and highest sensitivity. Wu et al.250 synthesized biocompatible carbon nanotubes by functionalization with glycodendrimers. These protective coatings of glycodendrimers can function as homogeneous bioactive coatings that mitigate the cytotoxicity of SWNTs. Haddad et al.251 have reported noncovalent biofunctionalization of SWNTs via biotin attachment by π-­stacking interactions and pyrrole polymerization. Noncovalently functionalized biotin-­nanotubes show better permeability for enzymatically generated hydrogen peroxide.

2.9.4  Endrohedral Filling A unique class of composites is produced by introducing guest molecules into the cavities of SWNTs, including fullerenes, metallofullerenes, various biomolecules, metal salts and metals. Peapods, (i.e., fullerene-­filled SWNTs) exhibit transformations, such as C60 diffusion, dimerization and coalescence on irradiation with an electron beam. Complete coalescence of C60 leads to the formation of double-­walled CNTs. Nanotubes have been filled with several other fullerenes including C70, C78, C80, C82 and C84,73 and metallofullerenes including Gd@C82, Dy@C82, Sm@C82, Ti2@C80, La@C82, Gd2@C92, Ca@C82, Sc2@C84, and Ce@C82. Other than fullerenes, the capillaries of nanotubes have been filled with several metals, including Pt, Ru, Ag, Au, Bi, Pd, Co and Ni. These metals are converted to metal nanothreads inside the nanotube. Iodine atoms have been incorporated inside single-­walled nanotubes in a helical chain fashion.252 Bando and coworkers253,254 demonstrated the smallest nanothermometer by filling CNTs with liquid gallium. The height of a column varies linearly and reproducibly with temperature. Liquid-­Ga-­filled carbon nanotubes act as a miniaturized temperature sensor and electrical switch. The electrical resistance of the nanotube decreases linearly with increasing temperature as the metallic Ga column expands inside the tube channel. Metallocenes, such as ferrocene, vanadocene, cobaltocene, chromocene and ruthenocene have been inserted in nanotubes. Carborane as well as hydroxides of K and Cs have been put inside the cavities of carbon nanotubes. Metal oxides and metal halides were placed within the nanotube channel, as well as small proteins, such as lactamase, in addition to RNA and DNA.255 The interaction of the interior of CNTs with water molecules has been studied by molecular dynamics simulations. CNTs have been found to be useful in nanofluidic applications.256–258 Lattice defects and distortions in alkali metal iodides (MI, M = K, Cs) encapsulated in DWNTs have been examined.259 Bendall et al.260 studied thermal stability and reactivity to oxidation of several nanocomposite systems obtained by encapsulation of metal halides in SWNTs. Crystallization of PbI2 in discrete and bundled SWNTs, DWNTs and thicker-­walled nanotubes has been studied.261 Costa and coworkers262 have demonstrated the stepwise current-­driven release of attogram quantities of copper iodide encapsulated in carbon nanotubes. A simple two-­step approach for the filling of SWNT

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channels with chalcogenide 1D nanocrystals was reported. The method is based on the formation of intermediate metal iodide in SWNTs using a molten media technique with a subsequent chalcogenation reaction performed in molten chalcogen. Friedrichs et al.264 have demonstrated an approach to fill ex-­situ synthesized silicon nanocrystals embedded in SiO2-­based spin on glass within MWNTs at room temperature and atmospheric pressure. A simple method has been developed for the containment and purification of filled open-­ended SWNTs using C60 molecules.265 Kim et al.266 have reported filling CNTs with fluorescent particles. Fluorescent signals emitted by the particles were visible through the walls of the nanotubes. CNTs loaded with magnetic nanoparticles are produced via chemical vapor deposition using commercial ferrofluids in alumina membranes.267 Filling of nanotubes with aqueous ferrofluid based on superparamagnetic iron oxide nanoparticles (SPIONs) prepared by a controlled coprecipitation technique has been reported.268 Wang et al.269 have reviewed progress in the filling of CNTs using various preparation methods and the mechanisms involved in the filling processes.

2.10  F  unctionalization Using Fluorous Chemistry and Click Chemistry Purified SWNTs can be solubilized in a fluorous medium (perfluorocarbon) by reacting them with heptadecafluoroundecylamine.270 To solubilize the nanotubes, SWNTs, heptadecafluoroundecylamine (SWNTs/amine, 2 : 1) and perfluorohexane were sealed in a Teflon-­lined stainless steel autoclave and heated at 130 °C for 48 h. This process produced a clear solution of SWNTs in perfluorohexane, as can be seen from Figure 2.21a. The solubilization of SWNTs appears to occur by a zwitterion-­type interaction of the –NH2 groups of the amine with the –COOH groups present on the surface of the SWNTs. The Raman spectrum of the solution gives the characteristic bands of SWNTs, especially the radial breathing modes (155 and 177 cm−1). TEM images in Figure 2.21b and c show the presence of SWNTs in the fluorous medium. Using microwave irradiation, cycloaddition of fluorous-­tagged pyrrolidine moieties onto the SWNT surface (1/108 functional coverage) in ionic liquids has been achieved.271 The click reaction provides another strategy to functionalize nanostructures by 1,3-­dipolar cycloaddition of azide with alkynes. Voggu et al.272 have assembled nanorods of gold and CdSe using click chemistry. By using this strategy, they have decorated gold nanoparticles over the surface of single-­ walled carbon nanotubes. Complex assemblies of nanorods were obtained by carrying out click reactions between gold nanorods capped with azidoalkane-­ and alkyne-­thiol chains. In order to decorate gold nanoparticles on SWNTs, 4-­azidobutylamine-­treated SWNTs were reacted with gold nanoparticles capped with hexanethiol. Campidelli et al.273 functionalized SWNTs with phthalocyanines via click chemistry, by initially reacting SWNTs with 4-­(2-­trimethylsilyl)ethynylaniline, with subsequent attachment of a zinc-­ phthalocyanine (ZnPc) derivative using 1,3-­dipolar cycloaddition. Palacin

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Figure 2.21  (a)  Raman spectrum of SWNTs solubilized in perfluorohexane. The inset shows a photograph of the solution of SWNTs in perfluorohexane. TEM images of the solubilized SWNTs are shown in parts (b) and (c). Reproduced from ref. 270 with permission from American Chemical Society, Copyright 2006.

et al.274 have reported functionalization of SWNTs with zinc porphyrins (ZnP) via click chemistry under very mild conditions, resulting in a series of SWNTs-­ZnP electron donor–acceptor conjugates.

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Chapter 3

Properties and Applications of Carbon Nanotubes 3.1  Electronic Properties Carbon nanotubes exhibit various interesting properties, and they are useful for many applications. The use of SWNTs in electronics, sensor technology and other fields is dependent on whether they are metallic or semiconducting.1,2 Nanotubes may be used as AFM or STM tips. Interesting electron transport features of as-­prepared, bent and shortened single-­walled carbon nanotubes have been revealed by a combination of STM–STS studies and tight–binding calculations,3a,4 and show how a range of electronic transport characteristics can be attained with SWNTs. Fundamentals of electronic properties of carbon nanotubes have been reviewed by Lieber and coworkers5 and Dresselhaus et al.6 Electron transport properties of SWNTs have been reviewed by McEuen and Park.7 Scanning tunneling microscopy (STM) in ultrahigh vacuum has allowed atomic resolution imaging of the surface of SWNTs, and I–V spectroscopy has enabled direct measurement of the electronic band structure. Depending on their diameter and chirality, SWNTs may act like metals, semiconductors, or narrow band-­gap semiconductors. Traditional spectroscopic techniques will detect electronic transitions between the energy bands of SWNTs (Figure 1.37b in Chapter 1). Aside from sample variability in terms of nanotube diameters and helicities, impurity doping adds to the width of the absorption characteristics. Since the band gaps are inversely proportional to the nanotube diameters, the band transition energies may be used to extract structural details.8,9 Figures 3.1 and 3.2 show typical results obtained by employing STM. The STM images in the figures show atomically resolved   Nanoscience & Nanotechnology Series No. 52 Nanotubes and Nanowires, 3rd Edition By C. N. R. Rao, A. Govindaraj and Leela Srinivas Panchakarla © C. N. R. Rao, A. Govindaraj and Leela Srinivas Panchakarla 2022 Published by the Royal Society of Chemistry, www.rsc.org

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Figure 3.1  (a),  (b) STM images of atomically resolved SWNTs. (c), (d) The current–

potential (I–V) characteristics of metallic and semiconducting SWNTs, respectively. (e) The inverse scaling of tunneling gap with the nanotube diameter. Reproduced from ref. 4 with permission from American Chemical Society, Copyright 2000.

images of SWNTs.3a,4 The I–V characteristics show both metallic and semiconducting behavior with the tunnelling gap varying inversely with nanotube diameter. Calculated and observed density of states showing van Hove singularities can be seen from the figure. Optical measurements have shown the existence of excitons in semiconducting nanotubes. Electronic structure calculations and scanning tunneling spectroscopy (STS) experiments report the same gap values as optical experiments. The band gaps of SWNTs have been measured via STS by Lin et al.7b The results indicate screening of the gap due to the metal substrate, resulting in reduction of the gap, which demonstrates many-­body interactions in these systems. Note that the optical transitions happen at lower energies (due to the exciton binding energies) than the intrinsic nanotube band gap obtained in STS.

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Figure 3.2  (a),  (b) STM images of SWNTs. (c), (d) The corresponding I–V character-

istics of the tubes shown in (a, b). The calculated and observed density of states curves are also included. Reproduced from ref. 4 with permission from American Chemical Society, Copyright 2000.

Bockrath et al. have measured electrical transport properties of bundles of SWNTs.10 Electron transport measurements on individual SWNTs by Tans et al.11 have shown that conduction occurs through well-­separated, discrete electron states, which are quantum-­mechanically coherent over long distances (∼140 nm). By employing STM and STS studies, the electronic wave functions of short metallic nanotubes were imaged by Venema et al.12 These wave functions resemble the quantized energy levels in SWNTs. Chico et al. studied the implications of topological defects in CNTs.13 The presence of pentagon–heptagon defects in carbon nanotubes gives rise to confined electronic states, similar to a quantum dot. Fully suspended micrometer-­long single-­walled carbon nanotubes have been grown between metal contacts and their device characteristics examined. These devices show well-­defined characteristics over much wider energy ranges than SWNTs deposited on substrates (Figure 3.3).14

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Figure 3.3  (a)  Schematic of a device with a local gate at the bottom of the trench.

(b) An SEM image of the actual device, the scale bar is 0.5 µm. (c), G–Vg characteristics of a SWNT (Dev1) recorded at T = 300 mK under V = 1 mV and B = 0 T. The heights of the bars along the top axis correspond to peak spacings ΔVg (right vertical axis) between conductance peaks along the Vg axis. A four-­peak shell is highlighted (by dashed lines) for the p-­channel (negative Vg side) and n-­channel (positive Vg side). (d) Energy dispersion E(k) for the valence band. Quantization of wavevectors along the length of the carbon nanotube (kn = nπ/L) is indicated by evenly spaced vertical lines. Each kn gives rise to a shell (represented by the horizontal red levels), each consisting of four states corresponding to the K and K′ sub-­bands and spin-­up and spin-­down. (e) Details of two of the shells in (c). Four electrons fill each shell with a charging energy of Ueff. To reach the next shell, in addition to Ueff, an energy difference between the quantized shells Δn needs to be paid. Reproduced from ref. 14 with permission from Springer Nature, Copyright 2005.

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Several low-­temperature transport regimes, and large and small band-­gap semiconducting as well as metallic nanotubes have been detected. A magnetic field-­induced Aharonov–Bohm effect has been observed. Transport data show a correlation between the transport regimes and the contact junction resistance in SWNT devices. Single-­walled carbon nanotubes exhibit a linear dependence of resistivity on temperature.15 The conduction in SWNTs is affected by the twistons (long-­wavelength torsional fluctuations). The contact resistance influences true transport properties and their use in devices. SWNTs exhibit high contact resistance in measurements at low temperatures, possibly due to the weak electronic coupling of CNTs with the Fermi level of the metal contacts.16 At high pressures, the resistance of SWNTs decreases, due to van der Waals compaction of the intertube spacing.17 Kong et al. employed CVD18 to synthesize SWNTs on patterned catalyst islands and measured the low-­temperature electric properties. A two-­terminal metallic SWNT device has shown around 16.5 kΩ resistance at 4.2 K. Transport characteristics have been measured using AFM tips to contact the nanotubes with Ti electrodes.19 Avouris et al.20 have used the AFM tip to manipulate multi-­walled nanotubes, and measured radial and axial structural deformations due to nanotube–substrate interaction. Strong interactions between the substrate and the nanotube have been exploited to place individual nanotubes on electrical contacts and measure their transport characteristics, as well as for their use as AFM tips for lithography. Tans et al. observed the field-­effect transistor action of semiconducting SWNTs21 at room temperature. A field-­effect transistor based on semiconducting MWNTs has also been reported.22 The transport characteristics of metallic SWNTs are usually not dependent on gate voltage, while those of semiconducting SWNTs are more complex, exhibiting a more diffusive rather than ballistic nature. However, semiconducting SWNTs do show high mobilities.23 SWNTs with highly reproducible Pd ohmic contacts and lengths varying from a few microns to tens of nanometers have been studied.24 The mean free path (MFP) for acoustic phonon scattering is assessed to be lap ∼300 nm, and that for optical phonon scattering lop ∼15 nm. Transport across very small (tens of nanometers) nanotubes is free of significant acoustic and optical phonon scattering, and hence is ballistic and quasi-­ballistic at low-­ and high-­bias voltage limits, respectively. A short nanotube can hold high currents of up to 70 µA. The processes that could contribute to the subsequent electrical breakdown of short nanotubes at high fields have been examined. These observations have implications for high-­performance nanotube transistors and interconnects. Coskun et al.25 presented results from studies on quantum dot single-­electron-­tunnelling (SET) transistors consisting of short multi-­wall nanotubes threaded by magnetic flux. The method allowed the authors to investigate the electronic energy spectrum of the nanotube and its relationship to the magnetic field. Evidence for interconversion between gapped (semiconducting) and ungapped (metallic) states was obtained. The tubes exhibited h/e-­period magnetic flux dependence, in agreement with simple tight-­binding calculations.

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Likodimos et al. have studied the electronic properties of SWNTs using electron spin resonance (ESR) and dc magnetization. A weak diamagnetic susceptibility (≈10−7 emu g−1) has been observed in dc magnetization, and ESR results exhibit a low-­intensity narrow resonance line with a metallic line shape. The spin susceptibility results show a substantial Curie component at low temperature and a major Pauli contribution at above >150 K. At low temperatures (T < 14 K), the spin susceptibility clearly drops indicating opening up of a spin gap. The underlying mechanism is attributed to the electronic instability of the SWNTs as well as to the occurrence of defect-­mediated spin magnetism. Albrecht et al.27 have applied both ab initio density-­functional calculations and experimental techniques to study the electronic properties of SWNTs. Isolated SWNTs have been deposited over the surface of p-­ and n-­doped hydrogen-­passivated Si(100). Experiments were conducted using STS at ultrahigh vacuum. The dI/dV plots suggest that relative band alignments of semiconducting SWNTs over a substrate surface are sensitive to the polarity of the Si(100) doping. Moreover, The experimental data agree with theoretical calculations on a (12,4) s-­SWNT, whereas the surface bands of SWNTs shift to lower (higher) when they are measured over substrates doped with n-­t ype (p-­t ype). The (12,4) s-­SWNT physisorbed onto unpassivated Si(100) showed higher adsorption energy and charge transfer compared to over H–Si(100). Low-­energy electron irradiation on SWNTs induces a metal–semiconductor transition, due to the inhomogeneous electric field developed by charging during irradiation.28 The electronic characteristics of p-­doped SWNTs have been investigated by various measurements,29 and the results indicate a downshift of the Fermi level due to doping. Electronic properties of fluorinated nanotubes made by CF4 plasma treatment and amino functionalization have been studied.30 Field-­effect transistor measurements reveal the p-­t ype semiconducting nature of CF4-­functionalized SWNTs, while amino-­ functionalized SWNTs are n-­t ype. Electrical characteristics of metallic nanotubes at high bias voltages between 300 and 800 K indicate the thermal conductivity to be 3500 Wm−1 K−1.31 Thermal properties of individual MWNTs have also been studied through transport measurements.32 Conventional metal nanowire interconnects with a small diameter fail when passing high current through them due to current-­induced electromigration. The covalently bonded structure of CNTs helps to prevent such a breakdown with nanotubes, and the inherent resistance of the nanotube nearly vanishes due to ballistic transport. Experimental results show that normal metals carry current densities of 105 A cm−2 while metallic SWNTs can carry up to 109 A cm−2.33,34 Due to the large contact resistances, the ballistic current-­carrying capability of nanotubes is less useful for applications. An electronic circuit connecting electrical leads to and from a single-­walled carbon nanotube has a resistance of at least h/4 × 102 or 6.5 kΩ, where e is the charge of an electron and h is Planck's constant.35 Contact resistance can be reduced by contacting all layers in an MWNT, but it cannot be totally eliminated.

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Mann et al. have also observed quantum conductance in metallic SWNTs. These workers fabricated SWNT devices on Si/SiO2 wafers prepared by the CVD method. Depositing Pd metal as source/drain contacts over the nanotubes shows reliable ohmic contacts. High current-­carrying capability (∼25 µA per tube) and conductance near the ballistic transport limit have been observed at room temperature. The acoustic phonon scattering mean free paths have been measured to be ≫4 µm at low temperatures and ∼500 nm at room temperature. Herrmann et al.37 have reported conductance measurements in carbon nanotube-­based double quantum dots connected to two normal electrodes and a central superconducting finger. SWNTs show large thermoelectric power (TEP) with hole-­like behavior at elevated temperatures.38 The predicted TEP values of a single metallic SWNT are much lower than measured ones, because of breaking of electron–hole symmetry owing to the self-­assembly of nanotubes into crystalline ropes. The electrical resistance and TEP of SWNTs are strongly affected by transition metals. This transport performance has been assigned to the Kondo effect39 (i.e., the interaction between the spin of the conduction electrons of nanotubes and the magnetic moment of the metal). A pure orbital Kondo effect is found in CNTs.40 The orbital quantum number conservation during the tunneling was found by modification of the spin-­polarized states into orbital degeneracy under applied magnetic field. At finite field, when both orbital degeneracy and spin degeneracy exist simultaneously, an enhanced Kondo effect has been detected with multiple splitting of the Kondo resonance, due to SU(4) symmetry. The magnetoresistance behavior of SWNTs in the temperate range 4–300 K is consistent with 2D variable-­range hopping and weak localization.41 Farajian et al. have investigated transport through doped nanotube junctions42a and found sections of negative differential resistance (NDR) present in the I–V characteristics of metallic nanotubes, whereas semiconducting nanotubes show an asymmetric transport characteristic with respect to the applied bias. Mann et al.42b have shown electrically driven thermal light emission from individual quasi-­metallic SWNTs. Under low bias voltages, suspended quasi-­metallic SWNTs emit light owing to Joule heating, displaying strong peaks in the visible and infrared, corresponding to interband transitions. Due to excitons and interband transitions, photoluminescence and electroluminescence are detected in semiconducting SWNTs, whereas nonradiative relaxation means that metallic SWNTs are not expected to show luminescence. At low temperatures under an applied magnetic field, an interesting manifestation of charge transport in carbon nanotubes occurs in that the magnetoresistance shows oscillations as a function of field (Aharanov–Bohm effect). This effect has been observed in MWNTs.43 Some nanotubes display short-­period oscillations attributed to defect-­induced anisotropic electron currents. The magnetic field required for one Aharonov–Bohm period was about 6 T. Even at high magnetic fields (55 T), Lassagne et al.44 have detected the Aharonov–Bohm effect in MWNTs. Due to the requirement for very high fields, it was thought that detecting the Aharonov–Bohm effect was difficult in SWNTs. However, Zaric et al.44b reported measurements that provide

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evidence for Aharonov–Bohm behaviour in SWNTs at 45 T (matching 1% of a full Aharonov–Bohm period). The nanotube electronic properties are determined by optical absorption and photoluminescence spectroscopies. The band gap of semiconducting nanotubes shrinks in the presence of a magnetic field. Cao and coworkers45 observed large modulations to the valence band conductance of the nanotubes on application of a relatively low magnetic field parallel to the SWNT axis. Doping changes the electrical properties of pristine carbon nanotubes. Bockrath et al. measured the electrical properties of semiconducting SWNT ropes doped with potassium, through reversible intercalation.46 Doping can tune carriers in the nanotubes from holes to electrons. The majority of carriers are holes in pristine MWNTs.22 Optical as well as transport conductivity measurements on films of potassium-­doped SWNT show that doping influences on-­tube and intertube transport differently.47 Four-­probe transport measurements on ropes of SWNTs show that doping involves charge transfer and that SWNTs are inherently p-­t ype in character, due to the presence of defects or unintentional doping by exposure to air.48 By employing redox reactions, the transport properties of SWNTs can be modified.49a On intercalation, loss of long-­range ordering is observed, leading to disappearance of Van Hove singularities characteristic of 1D systems. Ballistic electron transport occurs in metallic SWNTs and MWNTs over long nanotube lengths, because of the nearly 1D electronic structure, enabling them to carry high currents with essentially no heating.33,50 Propagation of phonons also occurs easily along the nanotube. The thermal conductivity of an individual MWNT measured at room temperature (>43 000 W m−1 K−1) is superior to that of natural diamond and graphite along the basal plane (both 2000 W m−1 K−1).51 The thermal conductance and thermopower of an individual suspended SWNT have been measured.52 The torque generated by an electric field gives transient-­induced birefringence in SWNTs.53 Atomic-­scale imaging of individual MWNTs with concurrent transport measurements has been carried out.54 The study offers evidence for different breakdown successions of nanotube walls. The current-­carrying capacity of a vertical MWNT (7.27 mA) is high, with low resistance of 34.3 Ω, which is considerably higher than that of metallic SWNTs (∼25 µA).55 This is due to the participation of multiple walls in electrical transport, with each wall behaving as a quasi-­ ballistic conducting channel. Spin and orbital contributions to the magnetic moment have been resolved by electronic transport spectroscopy measurements on nanotubes in a magnetic field.56 SWNTs exhibit superconductivity at low temperatures. The transition temperature of SWNTs with a 1.4 nm diameter is ∼0.55 K57 and that of 0.5 nm diameter nanotubes grown in zeolites is ∼5 K.58

3.2  Phase Transitions and Fluid Mechanics A lyotropic phase in carbon nanotubes was identified some time ago,59 and a isotropic–nematic phase transition has been observed in dispersions of MWNTs.60 SWNTs dispersed in acid media also show this type of phase

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transition. With increasing acid strength, the critical concentration of the isotropic–nematic transition increases. By using constant-­pressure molecular dynamics simulations, structural transitions in DWNTs have been examined under hydrostatic pressure.62 During the transition, the nanotube cross-­section transforms from circular to elliptical along with a large drop in the radial bulk modulus. By using liquid crystal–CNT dispersions, electrically controlled CNT switches have been fabricated.63 At high temperatures, SWNTs experience superplastic deformation of up to 280% of their original length, as seen in Figure 3.4.64 Aligned carbon nanotube membranes allow fluids to flow through their channels with speeds approaching those of biological channels.65 The extremely fast flow creates CNT membranes, a potential mimic of protein channels, for selective chemical sensing and transdermal drug delivery. To recognize the nanotube sensor mechanism, integrated SWNT microfluidic devices have been created, wherein analytes can be introduced for sensing.66 This simple method can be applied to make thin-­film transistors from nanotubes within the microfluidic channels. Aligned CNT arrays have been coated with fluorocarbons to make superhydrophobicity on two-­tier (nanometer and micrometer scales) rough surfaces.67 Stable superhydrophobicity is achieved by coating a thin film of ZnO on aligned carbon nanotubes.68a The

Figure 3.4  In  situ tensile elongation of individual single-­walled carbon nanotubes

viewed in a high-­resolution transmission electron microscope. (a)–(d) Tensile elongation of a single-­walled carbon nanotube (SWNT) under a constant bias of 2.3 V (images are all scaled to the same magnification). Arrowheads mark kinks; arrows indicate features at the ends of the nanotube that are almost unchanged during elongation. (e)–(g) Tensile elongation of a SWNT at room temperature without bias (images (e) and (f) are scaled to the same magnification). Initial length is 75 nm (e), length after elongation (f) and at the breaking point (g) is 84 nm. (g) Low-­magnification image of the SWNT breaking in the middle. Reproduced from ref. 64 with permission from Springer Nature, Copyright 2006.

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surface wettability can be changed reversibly between superhydrophobicity and hydrophilicity by repetition of ultraviolet irradiation and dark storage. Jung et al.68b have reported CNT composite structures possessing properties such as superhydrophobicity, self-­cleaning and low drag using a spray method. The fabricated CNT composite surfaces are compared with structured surfaces such as lotus wax by various tests for durability as well as loss of superhydrophobicity. CNT composite structures showed good mechanical durability, superior to the structured surfaces. Electrowetting studies of single-­walled carbon nanotubes with mercury shows reversible wetting in the open nanotubes.69 Dierking et al.70 have dispersed SWNTs in a nematic liquid crystal and studied the temperature-­ dependent reorientation dynamics. They identified two characteristic timescales, the longer timescale associated with the reorientation of the carbon nanotubes and the short timescale correspond to the reorientation of the liquid crystal texture. Zhang and Kumar71 have reviewed developments in liquid-­crystalline phase fabrication and the production of macroscopic nanotube fibers and films via liquid-­crystal routes. Weiss et al.71,72a have carried out the preparation and characterization of carbon nanotube lyotropic liquid crystal (LC) composites by using different surfactants for the dispersion of SWNTs and LC formation. Carbon nanotubes have been dispersed in hexagonal lyotropic liquid crystals formed at room temperature using ionic liquids (e.g. ethylammonium nitrate) and the tribological properties of the dispersed composites studied.72b

3.3  Carbon Nanotube Composites Nanotube–polymer composites have been investigated by several workers.73 Of interest are the electrical and mechanical properties of the composites, specially the latter. Determining the mechanical properties of SWNTs is difficult, but some measurements have been conducted using a nanostressing stage.74 These measurements have yielded Young's modulus values in the range 320–1470 GPa (mean 1002 GPa) with an average breaking strength of 30 GPa, which is notworthy. For example, the Young's modulus of the stiffest conventional carbon fibers is ∼800 GPa, while glass fibres typically show around 70 GPa. Atomic force microscopy provides direct measurement on individual nanotubes and reveals that they can withstand extreme deformations without fracturing.75 After deformation, CNTs show the extraordinary ability of returning to the original structure. The excellent current-­carrying capacities (current densities of up to 1011 Am−2) as well as high thermal conducting properties of carbon nanotubes have been exploited by incorporating them into different matrixes.36,51 Preparation and characterization of nanotube composites are therefore of value.76 Polymer matrices are generally used in the composites, while ceramics and metals are also of some interest as matrix materials. Nanotube–polymer composites are commonly prepared by mixing solutions of the polymers with nanotube dispersions followed by evaporation of

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the solvents in a controlled manner. Surface functionalization of CNTs with carboxyl and other groups is an effective way, which enables the dispersion of nanotubes in solvents and improves bonding with the matrix. As mentioned in the previous sections, prechemical treatment of carbon nanotubes enables solubilization. Shaffer et al.77 have achieved stable aqueous dispersions of catalytically produced MWNTs by acid treatment. A nanotube–PVA [poly(vinyl alcohol)] composite can be prepared simply by mixing an aqueous solution of the polymer with an aqueous nanotube dispersion, followed by casting the mixture as a film and controlled evaporation of the water.78 Nanotube–polystyrene composites have been prepared by solution-­based methods. Other preparation techniques such as in situ bulk polymerization of polystyrene in the presence of MWNTs,79 and melt processing of mixtures such as SWNT/polyethylene80a or CNT/polycarbonate,80,81 yield homogeneous and dense composites, making extrusion and injection molding possible. The effects of sidewall functionalization on the dispersion as well as interfacial properties of fluorinated SWNT-­polyethylene composites have been studied.80b Hill et al.82 have studied polystyrene copolymer-­functionalized SWNTs and MWNTs and their solubility in organic solvents. The functionalization was done by creating carboxyl groups on the nanotube surface through acid treatment followed by esterification. These polymer–carbon nanotube composites are soluble in common organic solvents. To produce the composites, polystyrene and nanotubes were dissolved in a solution, and thin films prepared using wet casting. To produce a polymer composite, usage of prefunctionalized nanotubes is not always necessary. Qian et al.83 have dispersed MWNTs in toluene by applying high-­energy ultrasonication and mixing with a dilute solution of polystyrene in toluene under ultrasonic agitation. Due to the low viscosity of the polymer solution, nanotubes move freely through the matrix. Composite films were obtained by casting the mixture on glass followed by evaporation of the solvent. Nanotube–polystyrene composites have been prepared by in situ polymerization. Using thermoplastic polymers, shear mixing of the melt with the nanotubes can be employed to produce homogeneous dispersions. Extrusion produces aligned nanotubes while artefacts can be produced by injection molding. Catalytically produced nanotubes are dispersed in various polymers such as high-­impact polystyrene, polypropylene and acrylonitrile by shear mixing.84 A combination of solvent casting and melt mixing methods has been used to disperse SWNTs in poly(methyl methacrylate) (PMMA).85 Melt spinning has been employed to obtained composite fibres having a high degree of nanotube alignment. Composites of CNTs with a thermoplastic polymer such as polycarbonate have been proposed.86 Melt processing techniques were used by Sennett et al.87 to disperse and orient CNTs in polycarbonate. This was done by mixing polycarbonate resin with SWNTs or MWNTs in a conical twin-­screw extruder, with alignment achieved by fiber spinning. Mixing time and fiber draw rates were optimized to get excellent dispersion and alignment. Epoxy–nanotube composites have been

Properties and Applications of Carbon Nanotubes 88,89

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90

prepared. Ajayan et al. embedded nanotubes in an epoxy resin, which was then cut into fine slices with a diamond knife. The knife movement was found to align the nanotubes (Figure 3.5). The alignment is considered to be primarily a consequence of extensional or shear flow of the matrix created by the cutting. Orientation is more readily attained when the epoxy is in a liquid state. Using spin coating methods, alignment of nanotubes has been achieved in nanotube–epoxy composites.89c Nanotube–epoxy thin films show excellent mechanical properties. When 0.1 wt% MWNTs are added into the resin, there is about 20% increase in elastic modulus as compared to neat resin thin films (elastic modulus was 4.2 GPa). The enhancement was attributed to spin coating-­induced partial alignment of the MWNTs. Fracture characteristics of the films examined by SEM indicated that the pulled-­out tubes were often covered with the polymer, implying strong interfacial adhesion. Several other polymer–nanotube composites have been prepared by in situ polymerization, including polystyrene–MWNT91 and polyimide–SWNTs.92a Fabrication of polyetherimide (PEI) nanocomposite films has been achieved using commercially functionalized MWNTs and novel properties have been formed with only 0.5 wt% of multi-­walled carbon nanotubes. They observed twelve orders of magnitude increase in electrical conductivity and 86 °C increase in thermal decomposition temperature. Porous MWNT–polypyrrole composite films have been grown by employing electrochemical polymerization and used as supercapacitors.93 In most composite preparations, the aim is to produce samples with evenly distributed nanotubes throughout the polymer (either aligned or randomly oriented). For some purposes, however, a layered arrangement is beneficial. Layers of nanotubes or polymer–nanotube composites have been used for

Figure 3.5  Alignment  of nanotubes in a polymer matrix following cutting with microtome; arrows indicate buckled nanotubes. Reproduced from ref. 90 with permission from John Wiley and Sons, Copyright © 1995 Verlag GmbH & Co. KGaA, Weinheim.

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photovoltaic devices. Multi-­layer polymer–SWNT composites have been prepared with excellent mechanical properties.94 The method was to deposit the polymer and the SWNTs onto a substrate in a layer-­by-­layer manner, followed by cross-­linking. Composites with as high as 50 wt% SWNT loadings could thus be obtained. Coating was performed by dipping a silicon wafer or a glass slide alternately in the polymer solution and dispersions of SWNTs (subjected to acid treatment). van der Waals forces along with electrostatic attraction between the SWNTs (with negative surface charge) and a positively charged polyelectrolyte such as branched polyethyleneimine (PEI) held the layers together. After completing this process, cross-­linking was accomplished by heating the multi-­layer films at 120 °C. Uniform free-­standing membranes were obtained by lifting off the films from the substrate. Multi-­ layer SWNT–polyelectrolyte polydiallyldimethylammonium chloride composite films could also be obtained by using the above procedure.95a Lin et al.95b have used only functional polymers that are maximally similar to the matrix polymers or structurally identical for the functionalization and solubilization of CNTs. Mechanical performance of carbon nanotube composites depends on whether the nanotubes are from the arc or catalytically produced. Qian et al.83 have obtained dispersion of catalytically produced nanotubes throughout a polymer matrix with no clustering. During composite fracture, the nanotubes often broke, suggesting that they had relatively poor strength. Watts and Hsu,96 however, employed more perfect nanotubes, prepared by the arc-­discharge method, and observed better mechanical properties, with no breaking. The CNTs tended to cluster together so that during crack growth the nanotubes could easily slide past one another. The best mechanical performance was achieved with arc-­grown nanotubes. Interfacial bonding between the nanotubes and a polymer matrix can be enhanced by functionalizing the nanotubes before incorporating them into the matrix. Gojny et al.97 have shown that functionalization of arc-­produced MWNTs with amine groups improves bonding with the epoxy matrix. The nanotube outer shells often remain embedded in the matrix. The perfect cylindrical structure of the nanotubes is disturbed and becomes weak upon functionalization, which is disadvantageous. Functionalization is not always required for bonding between nanotubes and the polymer matrix. Liao and Li98 have examined the nanotube–polystyrene composite interface, assuming no covalent bonding between the matrix and the nanotubes. They found significant contributions from van der Waals and electrostatic interactions. Simulations of nanotube pull-­out experiments suggest that the interfacial shear stress of the nanotube–polystyrene composites is around 160 MPa, which is significantly greater than for most carbon fiber-­reinforced polymer composites. Wrapping SWNTs with poly{(5-­alkoxy-­m-­phenylenevinylene)-­ co-­[(2,5-­dioctyloxy-­p-­phenylene)vinylene]} (PAmPV) derivatives forms strong noncovalent bonds.99 Single-­walled nanotube–epoxy composites with high Vickers hardness have been prepared.100 Thermal conductivities were also increased. Thus, 1 wt% loading of samples with unpurified SWNTs revealed

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a 70% improvement in thermal conductivity at 40 K, increasing to 125% at room temperature. Pristine SWNTs create a physical gel when combined and ground with an imidazolium ion-­based room-­temperature ionic liquid.101 Inside the gel, the heavily intertwined nanotube bundles untangle to create even finer bundles. Physical cross-­linking of the nanotube bundles, induced by local molecular ordering of the ionic liquids, tends to shape the gels rather than nanotube entanglement. The gels were thermally stable and did not shrivel, except at low pressure due to the nonvolatility of the ionic liquids, but they easily transitioned from gel to solid on absorbent substances. The use of a polymerizable ionic liquid as the gelling medium enabled the development of a highly electroconductive polymer–nanotube composite material with a significant increase in dynamic hardness. Mamedov et al.94 have prepared SWNT–polyelectrolyte composites by the layer-­by-­layer process. Films prepared by this process have shown exceptional mechanical properties. The ultimate tensile strength of the films is as high as 325 MPa with an average of 220 MPa. The tensile strength is several orders of magnitude higher than strong industrial plastics, and suggests that the layer-­by-­layer process has great promise for the fabrication of strong nanotube-­containing composites. Fibers and ribbons of carbon nanotubes have been spun directly from the CVD synthesis zone of a furnace using a liquid source of carbon and an iron nanocatalyst.102 MWNT yarns with high strengths could be spun from aerogels during nanotube synthesis by CVD. This process was realized through an appropriate choice of reactants, control of the reaction conditions and continuous withdrawal of the product with a rotating spindle used in various geometries. Direct spinning from a CVD reaction zone is extendable to other types of fiber and to spin coating of rotating objects in general. The resulting fibers can be post-­impregnated with epoxy to make composites. These fibers have failure strengths up to 1 GPa, compared with an expected ultimate strength of 30–50 GPa for a single nanotube. In fact, fibers made by each method creep before failure, suggesting that a great deal of straightening and aligning occurs before failure. It also shows that optimization is needed to share the load better across the individual CNTs. By introducing a twist during the spinning of multi-­walled carbon nanotubes, yarn strengths greater than 460 MPa have been achieved.103 These yarns deform hysteretically over large strain ranges, providing up to 48% energy damping, and are as tough as the fibres used for bulletproof vests. Unlike ordinary fibers and yarns, te nanotube yarns are not degraded in strength by overhand knotting. They also retain their strength and flexibility after heating in air at 450 °C for an hour or when immersed in liquid nitrogen. High creep resistance and high electrical conductivity are found after polymer infiltration, which substantially increases yarn strength. PVA (poly(vinyl alcohol)) infiltration decreases yarn electrical conductivity by only ∼30%, leading to MWNT–PVA composite yarns whose electrical conductivity is more than 150× that of coagulation spun nanotube composite

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fibers containing the insulating polymer. A hot-­drawing method for treating wet-­spun composite fibers made of MWNTs and SWNTs with PVA has been reported.104 Nanotube–PVA composite fibers produced by this process show a large strain-­to-­failure, and their toughness surpasses commercially available polyacramide fibers including Kevlar or Twaron. Fibers produced from hot-­drawn nanotube/PVA composites are useful in making bulletproof vests, helmets, protective textiles and so forth. Continuously spun fibers of carbon nanotubes have been formed from an aerogel generated during the chemical vapor deposition process.105 Conditions including catalyst concentration were selected to lie within the range suitable for continuous spinning. As the iron concentration decreased in the process, the proportion of SWNTs increased and yielded higher-­quality fibers with high stiffness and strength. The highest tensile strength attained was 1.46 GPa (corresponding to 0.70 N/tex, considering a density of 2.1 g cm−2). The starting materials can be monomers instead of polymers to produce nanotube–polymer composites. Cochet et al.106 applied in situ polymerization to prepare a composite of MWNT with polyaniline (PANI). In this procedure, CNTs were produced by the arc-­discharge method, and sonication carried out in an HCl solution to achieve dispersion. An HCl solution containing aniline was transferred to the above suspension followed by slow addition of a solution of an oxidant under constant sonication for 2 h in an ice bath. The composite was attained after filtering, rinsing and drying. Using this process, composites with high nanotube loadings (up to 50 wt%) were achieved. Strong interaction between the polymer and nanotubes induces major changes in the electronic behavior, as confirmed by transport measurements. The resistivity values of the composite were an order of magnitude lower than that of pure polyaniline at room temperature or that of MWNTs. The temperature dependence of the resistivity was also lower than that of pure polyaniline. The lower resistivity of composites was explained in terms of charge transfer between polyaniline and MWNTs facilitated in the in situ polymerization process. Vivekchand et al.107 prepared composites of PANI via in situ polymerization of aniline by ammonium persulfate with pristine multi-­walled and single-­ walled nanotubes, as well as with nanotubes subjected to acid treatment and subsequent reaction with thionyl chloride. The PANI–nanotube composites exhibited electrical resistivities different from those of the parent nanotubes and of PANI. The electrical resistivity of the PANI–nanotube composites could be manipulated by variation of the composition as well as by prior treatment of the nanotubes. The resistivity of 2 : 1 PANI–MWNT composites falls between that of PANI and MWNTs. The resistivity of 1 : 2 composites shows similar behavior. PANI–SWNT (1 : 1) composite shows a slightly higher resistivity than PANI, while PANI-­acid-­treated-­SWNT composite shows a resistivity close to that of PANI. The resistivity of the PANI-­chlorinated SWNT composite is close to that of the SWNTs.107 Bauhofer and Kovacs108 have reviewed the experimental and theoretical studies of electrical percolation of CNTs in polymer composites.

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Electrical properties of nanotube–epoxy composites have been studied by Sandler et al.89a Resistivities of about 100 Ω m were achieved even at 0.1 vol% filler fractions. These values are higher than the best conductivities reported previously on the same epoxy matrix containing carbon black. These and other investigations suggest that CNTs have great potential in decreasing electrostatic charging of bulk polymers. The advantage of using CNTs over conventional fillers, such as carbon fibers and carbon black, is that nanotubes are more readily dispersable throughout the matrix and amenable to processing. In addition to the reports available in the open literature, there is considerable work being carried out within industry on such problems and commercial products based on nanotube-­containing plastics are indeed available,109 including fuel lines in automobiles where the CNTs help to dissipate any dangerous charge that may build up. Composites of thermoplastic polymers with nanotubes are used in automobile parts. Several groups have reported composites of nanotubes with the poly(p-­ phenylene vinylene) (PPV) and its derivatives. The electroluminescent properties of these polymers are widely used in light-­emitting diodes. These polymers are also used in photovoltaics. In an early report, researchers presented the synthesis of a composite material containing MWNTs produced by arc evaporation and a PPV derivative, PmPV.110 A solution mixing method was used to prepare the composite and good wetting of the nanotubes was achieved by the polymer. One of the objectives of adding nanotubes is to increase the electrical conductivity of the polymer. However, previous attempts to improve electrical conductivity by doping had shown unwanted degradation of the optical properties. CNTs, on the other hand, showed no degradation of optical properties and increased the electrical conductivity of the polymer by up to eight orders of magnitude. This was, apparently, because the nanotubes act as nanometric heat sinks, inhibiting the build up of large thermal effects that damage the polymers. This work also showed the potential of this composite as the emissive layer in organic light-­emitting diodes (LED). Nanotube–polymer composites have been tried in photovoltaic devices. Another method to make a nanotube-­conjugated polymer photovoltaic device is to drop cast or spin coat SWNT-­poly(3-­octylthiophene) composite films in solution onto a quartz substrate coated with indium–tin oxide (ITO).111a In the dark, a clear diode characteristic is obtained, while a photocurrent is observed upon illumination through an aluminium electrode. About two-­fold improvement in the quantum efficiency is observed compared to the standard indium–tin oxide device. Pure P3OT polymer has also been used to make devices. The P3OT–SWNT blend device showed considerable enhancement in the photovoltaic effect over the pure polymer device. A bulk heterojunction photovoltaic cell was made with immobilized C60–SWNT as the photoactive layer.111b By taking advantage of the high electron transport properties of SWNTs and the electron accepting properties of C60, enhancement in performance of polymer photovoltaic cells can be achieved.

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Nonlinear optical properties have been observed in both SWNT and MWNT suspensions. Thus, polymer composites with nanotubes may find applications in optical devices. However, only limited work has been reported on the optical limiting properties of nanotube-­containing composites. Chen et al.112a showed third-­order optical nonlinearity in a composite of SWNT– polyimide. A composite of MWNTs with poly(9,9-­di-­n-­octylfluorenyl-­2,7′-­diyl) (PFO), reported by O'Flaherty et al.113 exhibited excellent optical limiting at CNT loadings in excess of 3.8 wt% relative to the polymer. Chen et al.112b have reviewed optical limiting properties of small molecule-­doped carbon nanotubes, solubilized carbon nanotubes, carbon nanotube suspensions and carbon nanotube–polymer composites. Thermally stimulated solvent-­ bubble creation and sublimation of CNTs shows nonlinear scattering in the optical limiting responses of CNT suspensions, while solubilized CNTs show a nonlinear absorption mechanism in optical limiting processes and major concentration-­dependent optical limiting responses. Optical limiting, in the former case, does not dependent on the concentration of nanotubes. Different amounts of MWNTs have been incorporated into polymers by melt-­shear mixing and solution dispersion.114 Better wrapping of polymers around nanotubes occurs generally when MWNTs are dissolved in organic solvents. Thus, CNTs are functionalized with alkylbenzoic acids to achieve better dispersions in ethylene glycol.115 MWNT-­poly(ethylene terephthalate) nanocomposites have been prepared by in situ polycondensation of terephthalic acid and ethylene glycol in the presence of functionalized MWNTs. Functionalized nanotube–polyurea has been prepared and characterized.116 Functionalized SWNT–poly(ethyleneimine) has been used as a substrate for neuronal growth.117 Water-­soluble graft copolymers of SWNTs have been produced by covalent attachment.118 Anisotropic films with amine polymers of SWNTs were produced and their electrical characteristics studied.119 An electrochemical method has been reported to produce MWNT-­polyaniline composite films in aqueous acidic solutions and their electrochemical capacitance measured.120 Aligned SWNT composite films have been produced by connecting Au nanoparticles to single-­walled nanotubes followed by compression in a Langmuir–Blodgett trough.121 Controlled nanotube dispersion and alignment in polymer composites have been achieved by a two-­step process, involving aligned MWNT growth via CVD followed by in situ polymerization.122 Electrostatic interaction can be used to coat nanotubes, wherein layers of oppositely charged polyelectrolytes are mixed with CNTs.123 Composites consisting of a nanotube core coated with four functionalized layers comprising iron oxide nanoparticles encapsulated with protein followed by streptavidin (tetravalent biotin-­binding protein), 24-­base three-­stranded biotin-­terminated oligonucleotide duplexes and oligonucleotide-­coupled Au nanoparticles have been prepared. A hybrid nanocomposite based on polyaniline–CNT–nickel hexacyanoferrate has been made, which exhibits selectivity towards Cs ions, good ion exchange capacity and stability.124 Carbon nanotube composites with metal nitrides show better electrical properties.125

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Polymer-­free carbon nanotube fibers were obtained by wet spinning using solutions of nanotubes, water and a surfactant.126 A continuous spinning process was employed to prepare fibers of SWNT–nylon composites with improved mechanical properties.127 Nanotube brushes can be produced by covalent attachment between polystyrene and N-­doped MWNTs using grafting techniques and subsequent polymerization.128 Ajayan and Tour described the importance of multi-­functional components of the next generation of composite materials and studied both the reinforcing effect of CNTs in composites and interfacial contacts between nanotubes and the surrounding polymer matrix.129 Hernández et al.130a reviewed processing, grafting, and mechanical and thermal properties of CNTs in polymer-­based composites. The mechanical and thermal properties of CNT–polymer composites synthesized with various forms of carbon nanotubes were discussed in detail, as well as processing methods for developing CNT–polymer composites, the solubility behavior of CNTs, and interactions developed between CNTs and the polymer grafted. In addition, the impact of various chemical modifications on nanotubes, with a particular emphasis on those produced to enhance the compatibility of these nanostructures with engineering polymers, as well as their impact on the final composite properties, is discussed. Green et al.130b have reviewed the structure and behavior of SWNTs and show that they are essentially polymeric. This perspective of “SWNTs as polymers” allows methods, applications and theories of polymer science to be used in the case of nanotubes. CNT–inorganic hybrids form a class of composites which combines the multi-­phase characteristics of nanocomposites with the synergistic functions of hybrid frameworks. Recently, Eder has reviewed a new and promising class of functional materials.131 In these CNT hybrids, synergistic effects occur via size domain effects as well as charge transfer process via the inorganic–CNT interface. The advantage of nanotubes in these hybrids is manifold, including electron traps for magnetic shields, intrinsic capacitors, photosensitizers and electrodes. The high surface area of nanotubes support small nanostructures and their exceptional thermal conductivity prevents nanoparticle growth during annealing treatments. Furthermore, new phases can be stabilized resulting in functional hybrids with novel characteristics. Chu et al.132 summarized the handling of carbon nanotubes, the preparation of inorganic nanomaterial–carbon nanotube composites and the application of such composites in a wide variety of fields. A practical method to fabricate SWNT-­conducting polymer nanocomposites through a poly(ionic liquid)-­mediated process has been developed.133a The poly(ionic liquid) layer helps nanotube dispersion as well as interlinking conducting polymers with nanotubes. Hasan et al.133b have discussed different aspects of nanotube–polymer composite production, characterization, device implementation and operation. Byrne and Guin'Ko134 have discussed advancements in research into the production of CNT–polymer composites, with a focus on electrical and mechanical properties.

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A novel nanotube composite with silica has been described with possible beneficial optical properties.135a Silica spheres with diameters ranging 200– 650 nm were used to prepare films. Catalyst particles (molybdenum/cobalt) were then deposited on the silica spheres for SWNT growth. As SWNTs show high nonlinear behavior and fast switching, they can be incorporated into an optically confining environment to attain these characteristics at fairly low levels of laser intensity. Curtin and Sheldon135b have provided a brief overview of the integration of carbon nanotubes (CNTs) into metal matrices and ceramic to construct composite structures, with a focus on processing techniques, mechanical performance and applications. Lau et al.135c have given a summary of nanotube and nanotube/nanoclay-­based polymer composite materials. A novel fabrication process (molecular-­level mixing) has been reported to fabricate nanocomposites of carbon nanotube-­reinforced metal (Cu), exhibiting high strength.136a CNTs/Cu nanocomposite powders sintered by spark plasma show an unusual strengthening effect, superior to that of any other kind of reinforcement. Carbon nanotube-­reinforced aluminium matrix composites have been fabricated by isostatic pressing followed by hot extrusion techniques.136b A nanotube content of 1.0 wt% in the composite gives rise to maximal tensile strength and Young's modulus, (35.7% and 41.3%, respectively). Cho et al.136c have presented an overview of CNT-­loaded ceramic matrix composites. Bakshi et al.136d have reviewed aspects of CNT-­reinforced metal matrix composites that include processing techniques, nanotube dispersion, strengthening mechanisms and mechanical properties.

3.4  Applications, Potential and Otherwise Carbon nanotubes' diverse set of interesting properties opens up exciting possibilities for scientific applications.137–141 Some are realistic and likely to become commercial in the near future, while others are in the development stage. Notably, however, numerous patents and prototype devices have been reported in the last few years.

3.4.1  Electronic Applications Applications based on electronic properties have been reviewed142–145 and some of them, such as transistor action was discussed in earlier sections. We shall survey some of the device applications and the properties related to them in this section. In semiconductor technology, device miniaturization is expected to reach its limits because of quantum effects as Si channels move towards smaller sizes. In such a scenario, nanoelectronics based on molecules would be an alternative. The use of carbon nanotubes in nanoelectronics has produced some interest, with the possibility of connecting CNTs of various diameters and chiralities.146 The crystalline interfaces of nanostructures are promising for use in electronic devices. Tans et al. have reported I–V curves

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21

of SWNTs and demonstrated field-­effect transistor action (Figure 3.6). SWNTs can be interfaced with other materials to produce novel heteronanostructures. Thus, Zhang et al. produced heterostructures of SWNTs with carbide nanorods.147 Interfaces of SWNTs with carbides such as TiC, SiC or NbC have attractive electronic properties. The simplest device one can envision with CNTs is that comprising a bend or a kink. An intramolecular device can be realized by connecting two nanotubes of different chiralities. Such a device behaves like a diode rectifier.148,149 Another way to produce nanotube diodes is to use doping to make a p–n junction as in a usual silicon diode. Zhou et al.150 first demonstrated this approach by modulated chemical doping of individual carbon nanotubes, wherein one half of a semiconducting single-­walled nanotube was doped with potassium, leaving the other half undoped. The undoped part of the nanotube behaved

Figure 3.6  (a)  Tapping-­mode AFM image of an individual carbon nanotube on

top of three Pt electrodes. (b) Schematic side view of the nanotube FET device. A nanotube is contacted by two electrodes. The Si substrate, which is covered by a layer of SiO2 acts as a back-­gate. (c) Current– potential characteristics at various gate voltages (Vgate) over a −3 to +6 V range for an SWNT-­based field-­effect transistor (300 K). Inset: The conductance S versus gate voltage at bias Vbias = 0 V. Reproduced from ref. 21a with permission from Springer Nature, Copyright 1998.

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as a p-­t ype semiconductor, while the doped part was n-­t ype. Under certain conditions, the junctions displayed behavior consistent with that of a tunnel diode. For example, negative differential conductance was observed over certain voltage ranges. An SWNT p–n junction diode with superior characteristics has been fabricated, wherein “doping” was carried out by applying different charges to different parts of the tube rather than using a dopant.151 Three-­point nanotube junctions such as Y-­and T-­junctions have been proposed as prototypes for complex building blocks in nanoelectronics.152,153 The T-­and Y-­junctions go against established models, which require the presence of an equal number of five-­and seven-­membered rings to generate nanotube junctions. Instead, an equal number of five-­and eight-­membered rings can create such junctions.153 Crossed-­junction nanotubes have been fabricated and examined for transport properties.154 By using a Y-­shaped alumina nanochannel as a template, Y-­junction nanotubes have been prepared.155 On the other hand, Y-­junction nanotubes have been prepared in good quantities without using any templates by simple pyrolysis of nickelocene in the presence of thiophene.156,157 A typical TEM image of such a Y-­junction nanotube is shown in Figure 3.7a. Figure 3.7b and c show TEM images revealing the presence of several Y-­junction carbon nanotubes. Interesting electrical characteristics have been observed at the Y-­junction in STM and STS studies. Using tight-­binding molecular dynamics, Menon et al.158 simulated the formation of single-­walled carbon nanotube T-­junctions via the fusing of two nanotubes. They presented an efficient all-­sp2 pathway for the formation of SWNT junctions. Silicon nanowire–carbon nanotube heterojunctions also exhibit rectification behavior.159 To realize multi-­functional devices from carbon nanotubes, it is necessary to develop microprocesses capable of identification as well as manipulation of carbon nanotubes. Nishijima et al.160 have developed a unique microprocess that can help to join individual nanotubes to the cantilevers of a scanning probe microscope (in an SEM), which are used as probes to image biological and industrial samples. Lafebvre et al.161 have developed a method by employing tapping-­mode AFM, which allows controllable manipulation of individual SWNTs. The assembled SWNT circuits are connected to metal electrodes using electron beam lithography to obtaining their transport characteristics. Frank et al.50 attached MWNTs with a nanotube fiber and investigated their quantum transport properties after establishing electrical contacts with liquid metal. A conductance of 77.5 µS (= 2 × 102 h−1) that is equal to one unit of quantum conductance, was observed in the MWNTs. Even at room temperature, the charge transport was ballistic. By studying atomically flat titanium surfaces on an α-­Al2O3 substrate using a AFM cantilever attached to SWNTs as the probe, Cooper et al.162 demonstrated that terabits per square inch could be stored. This is a high areal data storage density. As this method uses SWNT-­based lithography, it offers nanofabrication capabilities under 10 nm. Instead of using electronics, where devices operate based on charge transport, spintronics has been explored with nanotubes. Injection

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Figure 3.7  (a),  (b) and (c) TEM images of Y-­junction carbon nanotubes obtained

by the pyrolysis of nickelocene and thiophene at 1000 °C. Reproduced from ref. 156a with permission from AIP Publishing, Copyright 2000.

of spin-­polarized electrons from a ferromagnetic metal into carbon nanotubes (in contact with metal) has been investigated, and coherent electron spin transport observed.163a The phase coherence length is 250 nm. It would be of interest to inspect the behavior of spin transport through SWNTs. Kuemmeth et al.163b have reviewed electrically gated carbon nanotubes, individual nanotube-­based spin-­electronic devices and have made attempts to recognize their nuclear and electronic spin degrees of freedom for quantum applications.

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Carbon nanotube quantum dots with multi-­electrostatic gates have been developed, and the enhanced control that resulted was used to investigate a nanotube double quantum dot.164a Transport measurements revealed honeycomb charge stability diagrams as a result of two nearly independent gate voltages. The device's interdot tunnel-­coupling regimes can be adjusted from weak to strong, and the transparency of the leads can be regulated independently. For this method, they calculated energy-­level spacings, capacitances and interaction energies. This capacity to regulate electron interactions in a molecular conductor's quantum regime is critical for applications such as quantum computation. A large magnetoresistance (61% at 5 K), has been observed in nanotube devices placed between the extremely spin-­polarized La0.7Sr0.3MnO3 epitaxial electrodes.164b Dierking and coworkers165 exploited the ability of nematic liquid crystals to self-­organize to induce alignment of dispersed nanotubes. In addition, director reorientation on the application of electric or magnetic fields to liquid crystals, commonly known as the Freedericksz transition, was employed to manipulate the direction of the nanotubes. Thus the reorientation of the nanotube follows the reorientation of the liquid crystal through elastic interactions via the liquid crystal director field. Such dynamically changing nanotube orientation is due to elastic interactions with the anisotropic host, which leads to the realization of nanosized on–off (i.e., conducting to nonconducting) electrical switches. The range of findings discussed above opens up the possibility of assembling CNTs into multi-­functional circuits with novel device-­like properties21,22,149,157,166 and the ultimate realization of a computer chip based on a carbon nanotube. Rueckes et al.166a have explained the concept of nonvolatile random access memory based on carbon nanotubes for molecular computing. The feasibility of the concept has also been verified. Gaufrès et al.166b experimentally demonstrated room-­temperature high intrinsic optical gain of 190 cm−1 at a wavelength of 1.3 µm in a thin film doped with s-­SWNTs. These results signify a development towards fabricating laser sources based on CNTs for future high-­performance integrated circuits. Rutherglen et al.166c have reviewed the progress in nanotube electronics for radiofrequency applications in terms of device physics, circuit design and the manufacturing challenges. Microwave devices based on carbon nanotubes have been reviewed by Dragoman et al.166d A broad range of applications utilizing unique electronic properties of carbon nanotubes for terahertz (THz) optoelectronics have been proposed. Hot electrons in quasi-­metallic nanotubes generate THz frequencies. Chiral-­ nanotube-­based superlattices have been proposed to show frequency multiplication that can be controlled via a transverse electric field. Armchair nanotubes are expected to show emission and THz radiation detection under strong magnetic fields.167a Hüttel et al.167b have fabricated devices with suspended carbon nanotubes and measured the single electron tunneling current at millikelvin temperatures to examine the transversal vibration mode. By applying a contact-­free radio frequency electric field, suspended

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nanotubes can be actuated. The time-­averaged current via CNT is measured to detect the mechanical resonance. The bending mode resonances of the nanotube can be tuned with gate voltage. The measured Q factor magnitude and its dependency on temperature were in agreement with the values that have been predicted for suspended nanotube. The responsiveness of neurons with the cell membranes can be improved by carbon nanotubes that help in making tight contacts between them. These measurements can be probed by electron microscopy and by using single-­cell electrophysiology techniques, which are also understood with theoretical modeling. These tight contacts would help in making electrical shortcuts between the proximal and distal sections of a neuron.167c

3.4.2  Field-­effect Transistors (FETs) and Related Devices An SWNT-­based FET comprises a substrate (gate), two microelectrodes (source and drain) and an SWNT (or SWNT network) bridging the electrodes.21b We showed the field-­effect transistor action of carbon nanotubes in Figure 3.6. The functions of transistors are associated with the diffusive electron transport properties. In nanotube field-­effect transistors (NT-­FETs), application of gating is through the electrode, which is placed beneath an SWNT that is contacted at opposite nanotube ends by a metal source and drain leads.21 A standard nanoelectronic NT-­FET device comprises a semiconducting nanotube, which is placed on top of an insulating layer (either aluminium oxide or SiO2), where both ends of the nanotube are connected to a metal electrode. The nanotube is operated by applying a potential to the aluminium gate, which is placed under the nanotube and aluminium oxide. NT-­FETs have been fabricated by using a lithographic technique where the electrodes are placed precisely onto the nanotubes that are either distributed randomly on a substrate or positioned on the substrate using an atomic force microscope.22,168a A NT-­FET fabricated in this manner may or may not work, as it depends upon whether the selected nanotube is semiconducting or metallic. Peeling outer layers selectively from a multi-­walled nanotube can be achieved until a nanotube cylinder with the required electronic properties is attained,168b but this method is not reliable and is not suitable for mass production. The aim of using nanoscopic NT-­FETs is to substitute the source–drain channel structure with a nanotube. A more profound approach is to use interconnected nanotubes to construct entire electronic circuits. As helicity defines the electronic properties, it should be possible to create a diode, by grafting a semiconducting nanotube to a metallic nanotube. Such a device has been validated. However, the bihelical nanotube was not rationally created; it was accidentally recognized in a normal nanotube sample by its kinked structure.141 Several groups have fabricated single-­walled carbon nanotube-­based integrated logic circuits that exhibit a range of digital logic operations, which include inverters, random-­ access memory cells and ring oscillator circuits.168a,169,170 Articles and reviews are available on the electronic and transport properties of nanotubes, as well

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as aspects such as chirality-­dependence, the one-­dimensional nature of electrostatic screening and the presence of several direct band gaps.171–173 Yaish et al.174 probed the electronic properties of semiconducting carbon nanotube transistors locally using an atomic force microscope (AFM) tip. In the three-­probe measurement configuration, a gold-­coated AFM tip acts as a voltage or current probe. They explored scaling of the device properties with channel duration by utilizing the tip as a movable current probe and investigated the characteristics of the contacts with the same tip as a voltage probe. Au was found to have good interactions in the p zone with no Schottky barrier. Wide contact resistances were discovered in the n zone, which dominated the transport characteristics. To investigate characteristics as a function of gate length, a carbon-­nanotube field-­effect transistor with several individually addressable gate segments was designed.175 When the systems are switched using gate segments regulating the device interior and those near the source and drain, the transistor characteristics are noticeably different. This distinction is attributed to a change from Schottky-­barrier modulation at the contacts to bulk switching. They also realized that for every gate voltage, the current through the bulk component is independent of gate length, presenting clear proof of ballistic transport in semiconducting carbon nanotubes over at least a few hundred nanometres, and also at very low carrier velocities. Furthermore, it was easy to differentiate between Schottky-­ barrier switching and bulk switching in CNFETs, indicating possible potential nanoelectronic applications. Appenzeller et al.176 provided a thorough investigation into the effects of multi-­mode transport in carbon nanotube field-­effect transistors. Electrical properties of tube systems are a function of the influences of more than one 1D sub-­band under certain field conditions. They address the significance of scattering for a stepwise shift in current as a consequence of gate voltage and describe how their findings affect the efficiency of nanotube transistors. While no stepwise shift in current as a function of Vgs may be observed due to the limited or disappearing scattering probability within the nanotube, current transport for high Vgs may require more than one sub-­band. Sub-­bands have been rendered clear in experiments by systematic potassium doping. A model was developed to clarify the findings, and the consequences of these findings for the electrical properties of nanotube-­based FETs were addressed. Carbon nanotubes are suitable building blocks for molecular electronics due to their mixture of electrical properties and dimensions. The evolution of carbon nanotube-­based electronics, on the other hand, necessitates assembly techniques that make for specific localization and interconnection. Keren et al.177 have reported the realization of a self-­assembled carbon nanotube field-­effect transistor running at room temperature using a scheme dependent on identification of molecular building blocks. A DNA scaffold molecule acts as both an address for the exact localization of a semiconducting SWNT and a template for the extended metallic wires that contact it. The realization of a FET in a test tube promotes self-­assembly as a viable method for building carbon nanotube-­based electronics. This method may be used

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to create a working circuit on a scaffold DNA network. Numerous molecular devices could be positioned at various network addresses and linked together using DNA-­templated wires. The scheme tends to be stable and general enough to allow for the versatile incorporation of other active electronic components into circuits. However, understanding a working circuit will necessitate improving the electronic properties of the transistor as well as individual gating to each unit. The above is possible by using a three-­armed DNA junction as an example, with the SWNT located at the junction, and creating a procedure for converting one of the arms into a gate. CNT-­based FETs have been coupled with biomaterials to create nanodevices capable of detecting biorecognition events and biocatalytic processes. A streptavidin-­sensitive FET was created by bridging two microelectrodes (source and drain) with a biotin-­functionalized carbon nanotube; Figure 3.8a.178 The SWNT used as a gate in the CNTFET system was coated with a polymer blend of polyethyleneimine (PEI) and polyethylene glycol (PEG). The former provided amino groups for further binding of

Figure 3.8  (A)  A carbon nanotube field-­effect transistor with a biotin-­functionalized

SWNT operating as a gate sensitive to streptavidin. (B) Biotinylation reaction of the polymer layer (PEI and PEG) on a sidewall of the SWNT. (C) AFM image of the polymer-­coated and biotinylated CNTFET after exposure to streptavidin labeled with gold nanoparticles (10 nm diameter). (D) The source–drain current Isd dependence on the gate voltage of an FET device based on carbon nanotubes functionalized with biotin in the absence and presence of streptavidin. Reproduced from ref. 178 with permission from American Chemical Society, Copyright 2003.

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biotin-­N-­hydroxysuccinimidyl ester, whereas the latter blocked nonspecific protein adsorption on functionalized carbon nanotubes (Figure 3.8b). Figure 3.8c portrays an AFM image of the unit after it had been subjected to streptavidin labeled with gold nanoparticles (10 nm). The light dots reflect gold nanoparticles, signaling the existence of streptavidin attached to the biotinylated carbon nanotube. The dependency of the FET source–drain current, Isd, on the gate voltage modified drastically after streptavidin binding to the biotin-­functionalized carbon nanotube (Figure 3.8d). Control tests showed that streptavidin binding occurs only at the biotinylated interface. The mechanism of the effect was explored in terms of charge transfer at the nanogate caused by the attachment of charged streptavidin molecules.179 Thin-­film anodic aluminium oxide (AAO) templates on silicon wafers can be used to make vertically aligned carbon-­nanotube arrays and fabricate devices with Schottky behavior at room temperature.180 This geometry can be applied to make nanotransistors operating at room temperature. Using high-­ density SWNT thin films, field-­effect transistors have been fabricated and the effects of adsorption of ionic surfactant on their surfaces examined.181 By modifying the surface properties of the SiO2 substrates, SWNT device characteristics can be altered to be sensitive to cationic or anionic surfactants. The performance of such devices is associated with the surface charge densities around the SWNTs in aqueous solutions. Device characteristics such as the conductance can be tuned by adsorption of ionic surfactants. Thus, this effect can be used to design chemical and biological sensors. Controlled alignment of individual metallic or semiconducting SWNTs between two electrical contacts with high repeatability has been achieved by a floating-­ potential dielectrophoresis method.182 Fabrication of field-­effect transistors using chemically functionalized carbon nanotubes at preselected locations has been achieved.183 An integrated logic circuit has been constructed using single carbon nanotubes.170 FETs have been fabricated with superconducting leads to investigate quantum supercurrent through carbon nanotubes.184 Guided growth of large-­scale, horizontally aligned SWNT arrays on single crystal quartz substrates have been reported for thin-­film transistors.185 Chemically doped contacts have been used to fabricate high performance n-­t ype carbon nanotube FETs.186a With potassium-­doped source and drain regions and using short channel (∼80 nm) SWNT-­FETs and high-­k gate dielectrics, n-­MOSFET-­like devices have been demonstrated to show high on-­currents because of suppression of Schottky barriers at the contacts by chemical modification. These devices have shown small ambipolar conduction, a sub-­threshold swing of 70 mV decade−1 and high on/off ratios up to 106 at 0.5 V bias voltage. High-­speed memory has been reported from carbon nanotube field-­effect transistors coated with an atomic layer of high-­k gate dielectric hafnium oxide.186b High gate efficiencies along with low operating voltages and the absence of hysteresis is achieved by a polymer electrolyte in carbon nanotube network transistors.187 A on/off current ratio of >106 has been demonstrated in short channel sub-­20 nm carbon nanotube FETs.188 These nanotube transistors show

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on-­currents exceeding 15 µA at 0.4 V source–drain bias. Self-­aligned carbon nanotube transistors with p-­t ype charge-­transfer doping, employing oxidizing molecules have been investigated.189 Flexible and transparent carbon nanotube transistors have been fabricated where both the conducting channel and the bottom gate are CNT networks. Parylene N has been used as the gate dielectric in these devices.190 The transistors show on/off ratios of 100 and mobilities of 1 cm2 V–1 S−1, with the former affected by the characteristics of the insulating layer. Transparent and flexible FETs by SWNT films showing mobilities of 0.5 cm2 V−1 s−1 and an on/off ratio of ∼104 have been fabricated.191 A transparent flexible organic thin-­film transistor using printed SWNT electrodes and pentacene has been described. Spatially selective guided grown carbon nanotubes have been used to fabricate FETs.192 Performance enhancement in the nanotube-­FETs was realized by chemical tuning of the interfaces between substrate/nanotube and electrode/nanotube.193 Self-­assembled CNT transistors have been constructed placing individual SWNTs selectively on a patterned aminosilane monolayer. Doping level and carrier injection of SWNTs can be increased via the reactivity of an aminosilane monolayer. The Schottky barrier height between the nanotube and the metal interface can be reduced by these chemical treatments reaching close to the ohmic contact. Highly sensitive gas sensors, reaching a sensitivity of 20 ppb of triethylamine, have been realized using such self-­assembled transistors. Nanoscopic 3D σ–π self-­ assembled superlattices can be used as good organic nanodielectrics for p-­ and n-­channels for low-­voltage SWNT thin-­film transistors and complementary logic gates.194 Single-­electron transistor properties have been realized in C60 peapods.195 The current behavior on application of source–drain biases and gate voltages revealed that the conductivity is improved by applying negative gate biases, whereas on application of positive gate voltages, the conductivity decreased and oscillated. These current characteristics are attributed to variation in the density of states of SWNTs in the presence of C60. SWNT-­FETs can sense molecular conformation when the molecules are photoswitched.196 Fabrication of CNT–molecule–silicon junctions has been achieved by covalent attachment of individual SWNTs via orthogonally functionalized oligo(phenylene ethynylene) (OPE) aryldiazonium salts to the Si surfaces.197 Unique electrical switching performance and logic in Y-­junction carbon nanotubes has been reported.198 The new logic device works without the need for an external gate, which is a result of the common contact among the currents in the three branches of the Y-­junction nanotube. These characteristics would be applicable in unique transistor technologies. Y-­junction CNT devices show differential current amplification.199 FETs based on Y-­junction SWNTs have been made and the devices show a small off-­state leakage current of ∼10−13 A with an on/off ratio of 105.200 Two electrostatic gates have been applied to a single suspended SWNT device. The device shows slight hysteresis and works as a single-­electron transistor or Fabry–Perot interferometer.201 FETs based on double-­walled carbon

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nanotubes exhibit an ambipolar to unipolar transition by the adsorption of molecular oxygen.202 Band engineering can be achieved in CNT-­based FETs by exposing the channel or the contact of the CNT devices to either oxidizing or reducing gases.203 By using CNT thin-­film transistors, a printing process has been developed for the high-­resolution transfer of all the constituents for organic electronic devices onto plastic substrates.204 Ambipolar random telegraph signals have been detected in SWNT-­FETs.205 The ambipolar RTS can be utilized to obtain a small band gap in the SWNT. CNTs can potentially be used in devices to improve neural signal transfer through supporting dendrite elongation and cell adhesion.206 These studies suggest that the development of neuronal circuits on a nanotube grid is supplemented by a high increase in the network function. Improvement in the efficacy of the neural signal transmission can be connected to certain characteristics of nanotube. Burghard et al.207a have reviewed advances in carbon-­based FETs for nanoelectronics devices. It was observed that CNTs are possibly the nearest to real applications in high-­performance FET device technologies among the different carbon nanostructures. This development was supported by the fact that semiconducting carbon nanotubes are the only carbon nanostructure that encompasses many critical characteristics, including the existence of a band gap, a large enough size to allow electrical contacting with considerable diligence, high stability and low defect density. Theoretical calculations have suggested that it is beneficial to make electrical contacts with carbon-­based electrically conductive materials over metal contacts.207b This is attributed to the likelihood of specific bonds forming between the nanostructure and the carbon material, thereby guaranteeing a well-­matched bonding channel and a strong continuation of the electronic structure. Another benefit would be that the comparatively small work-­function gap between carbon materials prevents shifts in the electrically treated nanostructure because of contact doping. In this scenario, the nanotubes have a small diameter of 1–2 nm, which makes them highly favorable for contacting molecules.208 Additionally, metallic SWNTs were used to make contact with pentacene nanocrystals.209 Due to the promising gate electrostatics of the sharp one-­dimensional structure, thin field-­effect transistors are capable of showing conducting modulation with orders of magnitude on just a few molecules. Another notable breakthrough is the use of carbon nanotubes as patterned growth scaffolds for the direct growth of organic single crystals into organic electronic devices.210a Aside from integration methods, innovative interface models are expected to grow in importance in the future. Byon et al.210b fabricated network SWNT-­FETs with a larger Schottky contact area for highly sensitive biosensor applications. Tunable nanotube-­based quantum-­dot devices have been fabricated using an ultraclean suspended nanotube.211 In this case, the local gate voltage of the nanotube confines a single electron in both a single quantum dot and a tunable double quantum dot. Field-­effect transistors from isolated semiconducting carbon nanotubes have been fabricated and their 1f noise studied

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both in ultrahigh vacuum and exposure to atmosphere. The amplitude of the spectral noise density is inversely related to gate voltage, to length of the channel and therefore to the carrier density and does not dependent on source–drain current. Electroluminescence (EL) properties of SWNT network field-­effect transistors (NNFETs) and small bundle carbon nanotube field-­effect transistors (CNFETs) have been studied.213 CNFETs emit sharper (∼80 meV) NIR spectra at room temperature, whereas NNFET emissions are broad and structured (∼180 meV). The wavelength of EL emission by NNFETs is increased with respect to the associated absorption wavelength, and it is situated near the minority carrier injecting interface (drain). The spectral characteristics detected are reproduced by a phenomenological model dependent on a Fermi–Dirac distribution of carriers in the CNT network. A general method for fabricating stable high-­performance photosensitive FETs from self-­assembled columns of polycyclic aromatic hydrocarbons using SWNTs as point contacts has been identified.214a The molecules are tetra(dodecyloxy)hexabenzocoronenes (HBCs), liquid crystalline materials that can spontaneous order into columnar nanostructures with a diameter close to that of single-­walled nanotubes and then create nanoscale columnar transistors. This research shows that nanoscale stimuli-­responsive transistors could be useful for solar energy harvesting and making sensors with very high sensitivity. Li and Zhang214b have observed room-­temperature negative differential conductance phenomenon in a bundle of SWNTs in a CNT-­based field-­effect transistor. Balasubramanian et al.215a present an overview of the advances in the field of carbon nanotube-­based FETs. Various methods reported for fabricating CNT-­FETs are presented along with the importance of chemical modification methods to improve the device performance. They have also listed several gate insulator materials including solids, liquids or solid polymer electrolytes. Using FETs based on random networks (RNs) of SWNTs, Peng et al.215b detected nonpolar molecular patterns and discrete nonpolar molecules in the presence of polar molecules in the same environment. They used specially built RN-­CNT sensor arrays to identify lung cancer and kidney disease as indicators of the technical impact.

3.4.3  Electromechanical Properties Strain greatly influences band structure and electrical properties of carbon nanotubes.216 These properties of CNTs can be used to make nanoelectromechanical devices. It is predicted that the electrical properties of metallic armchair nanotubes are less sensitive to tensile strain, whereas semiconducting or quasi-­metallic nanotubes are sensitive and show band gap changes under tensile strain. Change of electrical resistivity occurs by applying axial strain via atomic force microscopy (AFM) tips. Experiments by Tombler et al. have revealed a reversible change in electrical resistivity of up to two orders of magnitude increase in metallic SWNTs when deflected by an AFM tip.217

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Theoretical simulations on (5,5) nanotubes by the authors suggested that a large local deformation converts sp2 carbons to sp3, causing a drastic decrease in conductivity of the nanotubes. Cao et al.218 extended electromechanical studies on several suspended SWNTs and found that the resistance change of SWNTs under uniform tensile stretching is greater in small band-­ gap SWNTs and lower in metallic SWNTs and high band-­gap SWNTs. Other than uniaxial strain (bond stretching), torsional strain (bond twisting) is also found to influence the conductivity of carbon nanotubes. Washburn et al.219a measured the torsional strain-­induced electrical resistivity on several suspended SWNTs. The AFM tip was used to push a pedal placed on suspended SWNTs to create a torsion on the SWNTs (see Figure 3.9)219b and simultaneously measure the resistivity. The torsional strain decreased the electrical resistance of some nanotubes but increased the resistance of others. This variation in resistance was attributed to the increase or decrease in the band gap of the SWNT, defined by its unique chiral structure.219 Electromechanical torsional measurements in MWNTs show oscillations in the conductivity of each tube. Joselevich et al. conducted measurements on different diameter MWNTs and found that these conductance quantum oscillations were an intrawall effect due to Fermi momentum-­shifting across quantization lines (leading to periodic closing and opening of the band gap).220 This observation implies that MWNTs are a set of independent coaxial SWNTs from an electromechanical viewpoint. Twisting not only affects an individual carbon nanotube but also influences the conducting properties of CNT yarns (bundles of CNTs). Twisting of CNT yarns increases the specific conductivity due to the increased mechanical densification of CNTs.221

3.4.4  Field Emission Field emission is the emission of electrons from a solid in the presence of a strong electric field. Field enhancement at the edge of a sharp point is the easiest way to construct such a field. Initially, anisotropic etching or deposition was used to build Si or W tips. CNTs have benefits over Si or W tips in that they are mechanically inert to sputtering, chemically inert to contamination and can hold a massive current density of 109 A cm−2 until electromigration thanks to their tight covalent bonding.222–224 Furthermore, when driven at high currents, their resistivity decreases, preventing electric-­field-­ induced sharpening, which creates instabilities in metal-­tip field emitters.225 CNTs outperform other sources of carbon, such as diamond and diamond-­ like carbon, in terms of FE performance. It is possible to grow aligned arrays that are large enough to act as flat-­panel displays utilizing CVD and PECVD processes. Bonard and coworkers,222 Fan and coworkers224 and Nilsson and coworkers223 were the first few to study field emission from CNTs and self-­ oriented regular arrays of CNTs produced in this way. Closely packed arrays are not suitable for field-­emission systems, according to these reports, since the applied field is screened by the close packing of the tubes. Milne et al.225

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Figure 3.9  (a)  SEM image of a typical electromechanical SWNT torsional device

under backgate voltages −9, 0 and 9 V, respectively (scale bar = 500 nm). (b, c) Size of the measured paddle deflection (open circles) as well as differential resistance both before (light gray) and after (dark gray) etching of the backgate vs. backgate voltage for a device representing a positive (b) and a negative (c) relation between deflection and resistance. Reproduced from ref. 219b with permission from Springer Nature, Copyright 2007.

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discovered that an array of vertically arranged tubes positioned twice their height apart would provide the highest emitted current density. As a result, strategies for increasing patterned arrays of well-­separated tubes have been established. Rapid thermal annealing of nanotube arrays in high vacuum has increased both contact efficiency and crystallinity, according to Minoux and coworkers226 Several researchers have studied closed and open nanotube emission, showing conflicting results.226–228 Open tips seem to need less voltage to turn on since they are sharp, but they are more fragile in terms of current emission, whereas nanotubes with closed caps appear to be more robust, with a smooth emission pattern, but they need higher voltages to switch on. Field-­emission displays based on nanotubes have been reviewed by deJonge and Bonard and others.229,230 There has been much research on the possible uses of SWNTs and MWNTs as field emission electron sources227,231 for flat panel displays,232 lamps,233 gas discharge tubes providing surge protection234 and X-­rays.235 Due to the sharp tips and high aspect ratio of carbon nanotubes, they produce high local fields under applied potential between a carbon nanotube-­coated surface and an anode. The local field causes electrons to tunnel from the CNT tip into the vacuum. The emitted electrons are directed by an electric field towards the anode where a phosphor creates light for the flat panel display (Figure 3.10). The picture is not, however, trivial. Unlike in bulk metals where electron emission arises from continuous electronic bands, nanotube tips emit electrons from discrete energy states.236 The structure of the nanotube tip affects the emission behavior. For instance, emission is enhanced in the case of

Figure 3.10  (a)  Schematic drawing of a longitudinal cross-­section of a field-­ emission fluorescent display with a field-­emission cathode made of carbon nanotubes. The front glass forms a convex lens to condense emitted light in the forward direction. (b) A field-­emission fluorescent display based on carbon nanotubes emitting visible light. The anode current and voltage are 200 mA and 10 kV, respectively. Reproduced from ref. 239 with permission from Springer Nature, Copyright 1998.

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open-­ended SWNT or MWNT tips. Manufacturing nanotube-­based field-­ emitting surfaces is carried out by screen-­printing nanotube pastes that do not fade in moderate vacuum (10−8 torr), unlike tungsten-­ and molybdenu-­ based tip arrays which require a high vacuum of 10−10 torr.237 Nanotubes offer stable emission, low emission threshold potentials and long lifetimes.231,233 High current densities up to 4 A cm−2 have been attained, compared to required current densities of 10 mA cm−2 for field emission flat panel displays and the >0.5 A cm−2 needed for microwave power amplifier tubes.238 Nanotube displays possess advantages of over liquid crystal displays such as low power consumption, fast response rate, wide viewing angle, high brightness and wide operating temperature range. Feld-­emission lighting devices based on MWNTs have been fabricated.239 Figure 3.10 shows a field-­emission lighting device. A sealed 4.5 inch2 field-­ emission display device has been assembled by Choi et al.240a using vertically aligned SWNTs along with organic binders. The display in three primary colours shows 1.5 mA emission current at 3 V µm−1 along with 1800 Cd m−2 brightness. Lee et al.240b have shown high emission current density of around 2.9 mA cm−2 at 3.7 V µm−1 from aligned nanotube bundles. By employing field ion microscopy, Lovall et al.241 have studied the emission properties of SWNT ropes. The field-­emitted electron energy distribution (FEED) of single-­walled nanotube field emitters displays a large density of states near the Fermi energy. Groning et al.,242 have examined emission characteristics of MWNTs as well as SWNTs produced by the CVD process, with an emission site density of 10 000 emitters cm−2 at fields of around 4 V µm−1. MWNTs show a work function of 5 eV and SWNTs show a smaller value. CNTs produced by pyrolysis of ferrocene yields dense, quasi-­aligned carbon nanotubes on a pointed tungsten tip that display high emission current densities with good functional characteristics.243 A typical I–V plot for a carbon nanotube-­covered tungsten tip is shown in Figure 3.11a, with currents ranging from 0.1 nA to 1 mA. The applied voltages were 4.3 kV and 16.5 kV for total currents of 1 µA and 1000 µA, respectively. Figure 3.11b shows the Fowler–Nordheim (F–N) plot of a device having two distinct regions. In the low-­field region, it shows metal-­like behavior, while it saturates at higher fields as the voltage is raised. A field-­emission current density of 1.5 A cm−2 has been obtained at a field of 290 V mm−1, which is significantly higher than that observed with planar cathodes. Accordingly, the field enhancement factor estimated from the slope of the Fowler–Nordheim plot in the low-­field region is also large. Carbon nanotube bundles produce lobe structure symmetries in field-­emission micrographs. Remarkably stable emission currents are achieved over an operating period of more than 3 h for several currents in the 10–500 mA range. With increasing current levels, relative fluctuations decrease, and the emitter can be continuously operated for at least 3 h at high current levels with no degradation in the current. Several prototypes have been produced by Samsung (Figure 3.12), including a red–blue–green colour 9 inch display that can reproduce moving images.232

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Figure 3.11  I–V  characteristics showing field-­emission currents in the range 0.1 nA to 1 mA. (b) Fowler–Nordheim plot corresponding to the data in (a). Reproduced from ref. 243 with permission from Elsevier, Copyright 2001.

Chemically anchored SWNT patterns have been studied for field-­emission properties.244 According to Charlier et al.245 B-­doped MWNTs show improved field emission (turn-­on voltages of 1.4 V µm−1) as opposed to carbon MWNTs (turn-­on voltages of 3 V µm−1). This is due to the inclusion of B atoms at the nanotube tips, which causes a rise in DOS at the Fermi level. Tight binding and ab initio calculations show that the work function of B-­doped carbon nanotubes (1.7 eV) is much smaller than that of pure carbon MWNTs. Similarly, Golberg et al.246 found that N-­doped MWNTs would emit electrons at low turn-­on voltages (2 V µm−1) and large current densities (0.2–0.4 A cm−2). As a consequence, both B-­and N-­doped CNTs could be ideal for use as stable and strong field-­emission sources. Milne and coworkers have used CNTs as cathodes and studied their electron emission properties.247 A microwave diode has been fabricated using

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Figure 3.12  (a)  Schematic illustration of a flat panel display based on carbon nano-

tubes. ITO, indium tin oxide. (b) SEM image of an electron emitter for a display, showing well-­separated SWNT bundles protruding from the supporting metal base. (c) Photograph of a 5 inch (13 cm) nanotube field-­emission display made by Samsung. Reproduced from ref. 232 with permission from AAAS, Copyright 2002.

CNTs as a cold cathode electron source operating at high frequency as well as high current densities. By using direct microwave irradiation, flexible 3D arrays of CNT field emitters have been produced on organic polymer substrates.248a Electron field-­emission characteristics of functionalized SWNT– polymer composites with an electron emission using as little as 0.7% volume fraction of nanotubes in the composite have been reported.248b Furthermore, by tailoring the nanotube concentration and the type of polymer, improvement in the charge transfer through the composite is attained. A simple approach to improve the spatial emission uniformity, as well as enhanced field emission of carbon nanosheets has been demonstrated by coating them with a chromium oxide thin film.248c Well-­aligned open-­ended carbon nanotube architectures have been prepared using CNT transfer technology.249a This technique has potential applications in the manufacturing of field emitters, thermal control systems in microelectronics packaging and electrical interconnects, as well as placing nanotubes on temperature-­sensitive substrates. Wang et al.249b have prepared uniform CNT films by electrophoretic deposition and obtained better emission properties of the cathode. Hybrid CNTs developed in situ with a number of carbon allotropes (e.g., thin nanotubes, nanocones, carbon nanoonions, etc.) bound to the carbon nanotube surfaces have shown enhanced electron field-­emission properties.249c

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200 250a

Colón et al. produced a composite nanotube film with regulated nanotube orientation and improved adhesion as well as surface density, using an electrophoretic technique. Under strong operating voltages, the cathodes display long-­term stability and improved macroscopic field-­emission current density. Lv et al.250b have observed enhanced field emission of open-­ended, thin-­walled carbon nanotubes filled with ferromagnetic nanowires. An in situ synthesis technique has been suggested for thin-­walled, open-­ended nanotubes, packed with long ferromagnetic (FeNi, FeCo, FeCoNi, etc.) nanowires. Thin-­walled FeNi-­filled nanotubes (FeNi-­CNTs) have a significantly lower threshold field of 0.65 V m−1 (at 1.0 mA cm−2) and turn-­on field of 0.30 V m−1 (at 10 A cm−2) than their thick-­walled equivalents. At low vacuum (10−6 torr), FeNi-­filled thin-­walled carbon nanotubes exhibit strong field-­emission stability. Field emission properties of screen-­printed carbon nanotube films have been improved by depositing ZnO nanostructures on multi-­walled carbon nanotubes, via a vapor phase transport technique.251a A low threshold field of 2.3 V m−1 (at 1 mA cm−2), a low turn-­on field of 0.7 V m−1 (at 0.1 A cm−2), a high field enhancement factor of 8.2 × 103, and a homogeneous emission picture with emission spot density of ∼105 cm−2 were obtained based on the combined impact of geometrical structure of ZnO/MWNT. Ghosh et al.251b found enhanced field emission for carbon nitride (CNx) nanotubes/ZnO heterojunctions by the nitrogen-­mediated growth of ZnO nanocrystals on carbon nitride (CNx) nanotubes.

3.4.5  E  nergy Storage and Conversion: Supercapacitors,   Solar Cells and Actuators Porous nanotube arrays consist of a high electrochemically accessible surface area along with high electronic conductivity and useful mechanical properties. They are promising candidates as electrodes for devices that utilize electrochemical double-­layer charge injection. They can be used for making supercapacitors, where the capacitances are much higher than those of ordinary dielectric-­based capacitors and can be used to make electromechanical actuators for use in robots. CNT-­based supercapacitors252–254 and electromechanical actuators255 typically have two electrodes separated by an electrolyte, which is an electrically insulating material and ionically conducting. The separation between the electrode and electrolyte is very small (about a nanometer) for CNTs, as compared to ordinary dielectric capacitors (micrometer or larger) and leads to a high surface area of the nanotubes accessible to the electrolyte, resulting in high capacitances. Depending on the nanotube array surface area, the capacitance can be ∼15 to ∼200 F g−1, resulting in huge amounts of charge injection with only a few applied volts. The charge injection leads to the storage of energy in CNT-­based supercapacitors and provides electrode expansions and contractions, which does mechanical work in electromechanical actuators. The capacitances (102 and 180 F g−1 for MWNT and SWNT electrodes, respectively) and power densities

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252,253

(20 kW kg at energy densities of ∼7 W h kg for SWNT electrodes) are appealing, and the numbers can be improved by using unbundled SWNTs instead of SWNT bundles. An extremely short discharge time (7 ms) was reported254 for ten MWNT capacitors connected in series that operated at up to 10 V. Electromechanical actuators based on nanotubes function at a few volts, as compared to ∼100 V used in piezoelectric stacks and more than 1000 V used in electrostrictive actuators. Zakhidov et al.256a have demonstrated the tunable electrochemical charge injection and charge retention properties of carbon nanotube sheets in the absence of an applied field, in which neither volumetric intercalation of ions nor maintained electrolyte contact is needed. Use of this material in supercapacitors may extend their charge-­ storage times. Pumera256b has reviewed the electrochemistry of carbon nanotubes and discussed the fundamental reasons behind the electrochemical and electrocatalytic activity of CNTs and applications of CNTs for sensing, biosensing and energy storage systems. Nanotube-­based actuators have been found to function up to 350 °C. Operating them above 1000 °C should be possible, and would be useful as industrial carbon electrode electrochemical applications above this temperature.3b The observed nanotube actuator strains can exceed 1% improvement with commercial actuators, if the mechanical properties of the CNT sheets can be improved to match those of the actual individual CNTs.3b SWNT actuators currently have a maximum isometric actuator stress of 26 MPa.3b This stress is more than ten times that originally recorded for these actuators and approximately one hundred times that of natural muscle, reaching the stress generation potential of high-­modulus industrial ferroelectrics (∼40 MPa). The predicted modulus of the individual SWNTs should generate higher stress, but it is still >100× lower than that predicted. Improvement in CNT-­based actuator technology will depend on enhancement of the mechanical properties and high surface area, which can be achieved by improving the CNT alignment and the binding between the nanotubes. Fennimore et al.257 have constructed the first rotational nanoscale electrochemical actuator based on CNTs by incorporating a rotatable metal plate, with an MWNT serving as the key motion-­enabling element. SWNT sheets have been used for electrochemical actuators for some time and these devices produce stresses higher than natural muscle with excellent strains, even at low applied voltages. Actuation behavior has been observed with MWNT mats in which the CNTs are oriented randomly within the plane of the film.258 MWNT-­based nanoscale torsional actuators have been fabricated.259 Polymer–nanotube composites exhibit photo-­induced mechanical actuation upon IR irradiation.260 CNT-­reinforced polyaniline fibers have been used to construct high-­strength actuators for possible use as artificial muscles.261a Aliev et al.261b developed giant-­stroke, superelastic, pure carbon nanotube aerogel sheets as muscles that have 220% and 3.7 × 104% elongations per second, respectively, at operational temperatures ranging between 80 and 1900 K.

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CNT films with ordered structures made from concentrated H2SO4 could be applied to make electrodes for supercapacitors with high power density.262 In supercapacitors, pyrrole-­treated functionalized CNTs have been used as electrode materials.263 In 6 M KOH with a double layer capacity of 154 F cm−2, these electrodes had a capacitance of 350 F g−1, with energy densities of 3.3 kJ kg−1 and power density of 4.8 kW kg−1. SWNTs that have had their ends acid-­ treated to add oxygenated carbon tend to have good electrochemical properties.264 The use of aligned RuO2–MWNT nanocomposites as supercapacitors has been studied.265 Pyrolysis of polyacrylonitrile/CNT blends created a composite with higher electrical conductivity and pseudo-­capacitance properties.266 High power-­density supercapacitors have been made using thin films of MWNTs with high packing density and alignment.267 The conductance and capacitance of SWNTs has been investigated in the presence of various chemical vapors.268a Yu and Dai268b have produced graphene sheets modified with PEI to make them water soluble and for sequential self-­assembly with acid-­oxidized MWNTs, resulting in hybrid carbon films showing high supercapacitor performance, with a nearly rectangular cyclic voltammogram, even at an extremely high scan rate of 1 V s−1 and an average specific capacitance of 120 F g−1. Frackowiak269a has reviewed carbon materials for supercapacitor applications and reported CNTs as ideal conducting additives and/or supports for materials with pseudo-­capacitance properties, such as conducting polymers and MnO2. Heteroatom substitution (oxygen, nitrogen) in the carbon network also can improve capacitance. The effects of chemical and physical properties of nanotubes, such as size, shape, defect, functionalization, purity and annealing, on supercapacitance were reviewed by Pan et al.269b Further, to enhance the supercapacitance and to keep the stability of the supercapacitor intact, various composites, including CNT/oxide and CNT/polymer were discussed, especially by optimally engineering the composition, particle size and coverage. Thin-­film supercapacitors have been fabricated using printable materials to make flexible devices on plastic.270a The efficiency of devices utilizing a printable organic liquid electrolyte as well as an aqueous gel electrolyte is similar to that of other SWNT-­based supercapacitor devices produced using various methodologies (6 W h kg−1 for both electrolytes and 70 and 23 kW kg−1 for organic electrolyte and aqueous gel electrolyte, respectively). Electrodeposited manganese oxides on three-­dimensional carbon nanotube substrates show supercapacitive behavior in aqueous and organic electrolytes.270b The pseudo-­capacitive characteristics of MnO2 film and CNT/MnO2 electrodes have been investigated in both organic (1.0 M LiClO4/in propylene carbonate) and aqueous (1.0 M KCl) electrolytes. Compared with an MnO2 film electrode, the MnO2/CNT electrode shows a higher specific capacitance and better high-­rate capability. Li et al.271 have reviewed some of the current developments in CNT and nanotube-­based composite actuators and sensors, with a focus on electromechanical performance. Hierold et al.272a discuss nano electromechanical

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sensors based on CNTs. The processing methods for incorporating SWNTs into nano-­ and microsystems are discussed. A membrane-­based CNT pressure sensor and suspended single-­walled carbon nanotube-­based cantilever structures have been compared and evaluated. The use of 1D nanostructures for enhancing solar cell efficiencies has been reported.272b Photochemical solar cells have been constructed from stacked-­cup carbon nanotubes.273 Single-­walled carbon nanotubes have been used as interconnected building blocks in solar energy conversion.274 SWNTs were mixed with porphyrin in this analysis to create these devices. Nanocomposites of CdSe–SWNT have also been investigated for light harvesting and photo-­ induced charge transfer.275 SWNT–polymer composites have been studied for near infrared photovoltaic devices as an active layer.276 SWNTs were incorporated into polythiophene films for possible use in photoconversion.277 Porphyrin polymer-­wrapped SWNTs show long-­lived intracomplex charge separation, and can be used as photoactive materials.278 Solution-­based synthesis of polyaniline–MWNT composite films have been investigated for optical properties.279 A transfer printing technique has been applied to deposit transparent conducting SWNT films on various substrates.280 SWNT films can be used as hole-­conducting transparent electrodes in polymer–fullerene photovoltaic devices.281 The properties of polypyrrole–SWNT nanocables have been investigated for electrical transport and chemical sensing.282 High-­density multi-­layer structures can be achieved using patterned SWNT layers. These structures show excellent surface adhesion properties due to direct bonding to the substrates, yielding high electrical conductivity. Ideal p–n junction diodes can be formed by individual SWNTs.283 Under illumination, they display substantial power conversion efficiencies. Such photovoltaic effects could be useful in the development of electronic materials. Kymakis and Amaratunga284a reported photovoltaic properties of functionalized SWNT-­conjugated polymer, poly(3-­octylthiophene) (P3OT), blend composites. Photovoltaic characteristics of 1% nanotube-­incorporated diodes (ITO/PEDOT : PSS/P3OT-­SWNTs/Al) demonstrated 0.75 V open-­circuit voltage, 0.25 mA cm−2 short-­circuit current and 0.48 fill factor, resulting in an AM 1.5 power conversion efficiency of 0.1%. It was hypothesized that the device's photovoltaic action is dependent on the incorporation of internal nanotube– polymer junctions within the polymer matrix, which lead to improved charge separation and collection due to photo-­induced transfer of electrons from the polymer to the nanotube. SWNTs and functionalized MWNTs can be used together for photovoltaic application in a poly(3-­octylthiophene)/n-­Si heterojunction solar cell yielding better performance of the device.284b Ammonium ion–crown ether interactions have been exploited to form porphyrin–SWNT donor–acceptor hybrids.284c On irradiation with a flash light, splitting of water into oxygen and hydrogen occurs in the channels of SWNTs.285 High electrocatalytic activity can be achieved by depositing Pt nanoparticles on nitrogen-­doped carbon nanotubes, which can be useful in fuel cells.286 Pt nanoparticles decorated on

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SWNTs show selective electrocatalytic activity for O2 reduction, implying possible use in H2 and methanol fuel cells.287 Superhydrophobic films of catalytic Pt nanoparticle-­decorated carbon nanotubes have been made by a simple filtration method.288 The films exhibit improved electrocatalytic activity as well as improved mass transport within the film. Nanocomposites of CNTs with Pt/Ru can be used in methanol fuel cells.289 If the costly platinum-­based electrocatalysts for oxygen reduction reactions cannot be substituted by other effective, low-­cost and reliable electrodes, large-­scale functional implementations of fuel cells would be difficult to achieve. Dai and coworkers290 have reported good electrocatalytic activity of vertically aligned nitrogen-­containing CNTs for oxygen reduction in alkaline fuel cells, which can act as a metal-­free electrode with better long-­term durability and resistance to crossover effects than platinum. They found a current density of 4.1 mA cm−2 and a steady-­state peak potential of −80 mV at −0.22 V in air-­saturated 0.1 molar KOH, relative to −85 mV and 1.1 mA cm−2 for a platinum–carbon electrode at −0.20 V.

3.4.6  Sensors and Probes Chemical sensor applications require semiconducting carbon nanotubes, because the electronic transport and thermopower of nanotubes are sensitive to substances that affect the amount of charge injection.291–293 Advantages are the small amount of substance sufficient to observe the response and the tiny size of the nanotube sensing element. However, device selectivity (differentiating absorbed species in complex mixtures) and quick response remain major challenges. Development of CNT-­based gas sensors and sensor arrays has attracted interest for selective and rapid detection of various gaseous species. The most promising forms of gas nanosensors are chemical field effect transistors and chemiresistors. When exposed to target gas analytes, the electrical characteristics of the nanostructures in these sensors alter significantly. Several studies291,292,294a have shown that pure carbon SWNTs and MWNTs can be used to track poisonous gases and other molecules because low doses can induce significant shifts in nanotube conductance, moving the Fermi level to the valence band and producing hole-­enhanced conductance. In this sense, N-­doped MWNTs have proven to be efficient. When exposed to poisonous gases and organic solvents, CNx MWNTs react rapidly, on the order of milliseconds (Figure 3.13), and achieve saturation in 2–3 seconds.295 The presence of molecules attached to the pyridine-­like sites found inside CNx nanotubes induces an improvement in electrical resistance (Figure 3.13). A permanent transition to higher resistance has been noted for ethanol, acetone and NH3. A strong decrease in DOS at the Fermi level is observed, indicating lower conduction and chemisorption. As a consequence, CNx nanotubes tend to be more efficient at detecting toxic gaseous species. Obviously, B-­doped CNTs should be screened for similar applications as well. CO and HCN2O do not interact with the surface of pure carbon SWNTs, according

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Figure 3.13  Plots  of resistance vs. time for NH3 on N-­doped MWNT sensors. (a)

It is clear that the sensor is sensitive to 1% NH3. Chemisorption is clearly observed, which can be attributed to the strong interactions between the pyridinic sites of the tube surface with the NH3 molecule; (b) graph indicating the response time for NH3 gas (4.7%). The results demonstrate that N-­doped MWNTs could be used in the fabrication of novel and fast responsive gas sensors. Reproduced from ref. 295 with permission from Elsevier, Copyright 2004.

to theoretical ab initio calculations.296 If the surface of the tube is doped with a donor or acceptor, modifications in the electronic properties should arise as a function of the attachment of molecules to the doped positions,295,296 implying that either CNx nanotubes or B-­doped MWNTs could be useful for detecting low ethanol concentrations. SWNT-­based conductometric NO2 gas sensors have been developed.293b SWNT properties are systematically controlled by adjusting the annealing temperature under vacuum from 350 to 550 °C in order to maximize gas-­sensing abilities. Sensors that have been annealed at 400 °C have the highest NO2 sensitivity and the least amount of humidity interference. A miniaturized CNT-­based gas sensor array for monitoring landfill gas at a temperature of 150 °C has been developed.294b Unaltered CNTs and CNTs equipped with 5 nm nominally thick sputtered nanoclusters of Pt, Ru and Ag have been used in a vertically aligned CNT sensor array. Chemical study of H2, NO2, NH3, CO, CO2 and CH4 multi-­component gas mixtures has been carried out. SWNT hydrogen gas sensors based on exfoliated/debundled SWNT have been achieved by using gum arabic (GA) followed by Pd-­functionalization, and showed a better response to H2 over purified SWNTs.297 At room temperature, the assembled thin-­film sensors showed a strong reversible and repeatable response to hydrogen. Zhang et al.298 has reviewed progress in CNT-­based gas sensors, wherein improvement of the sensing performance (sensitivity, selectivity and response time) has been carried out via the appropriate functionalization of carbon nanotubes using various methods (noncovalent and covalent) and materials (metals and polymers). In these gas sensors, such as chemiresistors and chemical field-­effect transistors, the

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electrical properties of the nanostructures are dramatically changed when exposed to the target gas analytes. In FET-­based sensors functionalized CNTs have been used for a variety of specific functions.299–301 The current flow in an SWNT-­FET is sensitive to the adsorption of the analyte and related events on which sensing is based. SWNTs are functionalized with different attachments for specific sensing. As biological macromolecules attach to a nanotube, a shift in charge state perturbs the current flow of the nanotube, resulting in noticeable signals for sensing. SWNT-­ FETs have been used in a variety of applications, including protein recognition, single nucleotide polymorphism, DNA–DNA hybridization and antibody–antigen interactions. Sensing of proteins or protein–protein interactions has detection limits ranging from 100 pM to 100 nM.302 In FET for highly selective sensing, antibodies are often bound to SWNTs as basic targeting agents.303,304 Amino acids, small molecule drugs and hormones have also been detected using synthetic oligonucleotides, like aptamers.305 The binding efficiency of the attached sensing agents and the target molecules determines the detection limit. Carbon nanotubes are used in enzyme-­based electrochemical sensors as electron conductors. Surface functionalization of nanotubes improves their biocompatibility. Amperometric biosensors are dependent on the ability of an enzyme adlayer to convert substrate turnover into an observable, repeatably quantifiable current. This transduction is accomplished by controlling the enzyme's direct voltammetric reaction or the catalytic enhancement in diffusive voltammetry of a suitable mediator (redox iron complexes, such as ferrocene). Glucose oxidase is a flavin enzyme that is used commercially on a large scale to control blood glucose levels in diabetics. Incubating SWNTs (oxidized or not) with glucose oxidase for a prolonged period of time results in successful coating of the nanotube with enzyme306a,307 (Figure 2.20a in Chapter 2). Immobilization may occur without a significant loss of enzyme function. When such bio-­SWNT electrodes are treated with both a diffusive mediator and an equilibrated glucose substrate, a catalytic anodic wave is generated (Figure 2.20b and c). The frequency of the catalytic reaction is more than one order of magnitude greater than that observed with an active macrocarbon electrode (for the same enzyme, substrate and mediator concentrations). Since metalloproteins in solution can interact electrochemically with oxidized SWNTs at an electrode surface, and bioimmobilization at high loading occurs with preservation of action, metalloproteins immobilized on a nanotube surface can communicate directly with the nanotube π system. Since the tunneling gap between the (redox) active site and the underlying support/electrode is far too high, direct electrochemical contact between the flavin active site of glucose oxidase and the nanotube itself is not feasible. Since CNTs have a high conductivity and surface area-­to-­weight ratio, they can be used as supports for the immobilization of biomolecules, which can then be used for electrochemical and quartz crystal microbalance sensing of biorecognition processes such as immunosensing and DNA sensing. As a result, immunosensing systems with an electrochemiluminescence read-­out signal based on CNTs have been created. Liposomes have been used as biocatalyst

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carriers and as identification sites in biosensor systems. Functionalized carbon nanotubes have also been used to transport various enzyme labels for electrochemical DNA sensing and immunosensing.308 By covalently binding the protein to the carboxylic groups of the oxidized CNTs, CNTs were filled with approximately 9600 alkaline phosphatase (AlkPh) molecules per CNT. The modification of enzyme-­functionalized CNTs with a biorecognition system (such as an oligonucleotide or an antibody for DNA sensing and immunosensing, respectively) allows for amplified electrochemical identification of biorecognition events. For example, magnetic particles functionalized with the DNA primer have been reacted with complementary analyte DNA, resulting in a double-­stranded DNA complex (as seen in Figure 3.14a and b). The magnetic bead–DNA–CNT assembly shown in Figure 3.14c was formed during a 20 minute hybridization with a 10 pg mL−1 DNA target sample. Figure 3.14 illustrates

Figure 3.14  (A)  Electrochemical DNA sensing and (B) immunosensing using alka-

line phosphatase-­functionalized CNTs as the biocatalytic amplifying tags. (C) TEM image of the magnetic bead–DNA–CNT assembly produced following a 20 min hybridization with a 10 pg mL−1 DNA target sample. (D) Chronopotentiometric signals for various concentrations of the DNA target: (a) 0.01; (b) 0.1; (c) 1; (d) 50; (e) 100 pg mL−1. (PSA = potentiometric stripping analysis). Bottom: The derived calibration plot. Reproduced from ref. 308 with permission from American Chemical Society, Copyright 2004.

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the chronopotentiometric responses obtained after examining various concentrations of the DNA analyte using a CNT-­modified electrode and a nucleic acid-­functionalized CNT–enzyme combination as the amplifying sticker (d). The calibration plot (bottom) reveals that the detection maximum for the analyte DNA was about 1 fg mL−1. Similarly, enzyme-­loaded CNTs functionalized with an antibody have been used in amplified electrochemical immunosensing. Katz and Willner308b have reviewed nanobioelectronic applications of carbon nanotubes. An electrochemical sensing stage has been fabricated based on carbon nanotubes in a polymer matrix integrated with redox mediators for the detection of β-­nicotinamide adenine dinucleotide (NADH).309 Transistors based on DNA-­decorated CNTs act as chemical sensors.310 Due to favorable π–π stacking interactions, single-­stranded DNA (ss-­DNA) shows high affinity for SWNTs. An FET sensor consisting of ss-­DNA attached to an SWNT involves chemical recognition by the ss-­DNA and an electronic read-­out component provided by the SWNT FET. By a meticulous choice of the base sequence of ss-­DNA, the response of ss-­DNA/SWNT FETs can be tuned in sign and magnitude for distinctive gases as well as odors. Low concentrations of dopamine have been detected by ss-­DNA-­functionalized SWNTs, even in the presence of excess ascorbic acid.311 Reaction with H2O2 by water-­soluble SWNTs suggests possible use as an H2O2 optical sensor.312 An SWNT-­based microelectromechanical system detecting CO2 has been demonstrated.313a SWNT films can be used for hydrogen sensing at room temperature.313b Wang and Lin314a have reviewed electrochemical biosensors based on functionalized CNTs and carbon nanofibers with an emphasis on applications such as measurement of small biological molecules and environmental pollutants, detection of DNA and immunosensing of disease biomarkers. Hu and Hu314b have reviewed key aspects of electrochemical sensors in biomedical systems based on CNTs, including methods for dispersing CNTs in solution, methods to immobilize functional sensing CNT films on electrodes and the electrochemical nature of CNTs. Barone et al.315 developed and tested near-­infrared optical sensors, utilizing d-­glucose sensing as a model device and SWNTs as the material of choice. New forms of noncovalent functionalization utilizing electron-­withdrawing molecules have been demonstrated to include locations for electron transmission in and out of the nanotube. The signal transduction processes of fluorescence quenching and charge transfer were also demonstrated. Electroactive species irreversibly adsorb on the surface and shift the Fermi levels into the valence bands or act to quench the emission after photo-­excitation.

3.4.7  Biological Aspects Some aspects of biosensors were mentioned earlier. Ion transport through aligned CNT membranes and reversible biochemical switching are found to mimic protein ion channels.316 Protein immunosensors have been developed from SWNT forests,317 where the carboxylate ends of the nanotube

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forest are attached to antibodies. Plastic implants may be made from SWNT– polyelectrolyte composite films, which have a biochemical framework.318 The thin films are biocompatible with neuronal cell cultures and mechanically compliant with tissues under constant flexural and shear stresses. Chemically functionalized SWNT scaffolds have been examined for osteoblast proliferation and bone growth.319,320 DNA oligonucleotide-­encapsulated SWNTs can be used as markers in live cells, and are resistant to photobleaching for up to 3 months.321 Functionalized carbon nanotubes are used for targeted delivery of amphotericin B, an antibiotic for chronic fungal infections.322 A single-­walled carbon nanotube-­based optical sensor has been developed for in vivo fluorescence detection of glucose.323 Redox polymer–CNT–enzyme composites can be used as amperometric biosensors.324 DNA can be detected by band-­gap fluorescence modulation of SWNTs.325 Conformational polymorphism in DNA can be detected optically using SWNTs.326 Conformational changes in DNA affect the dielectric environment of DNA-­coated SWNTs, resulting in a red shift in the SWNT fluorescence. An aligned MWNT-­based electrochemical sensor has been developed for the detection of cholesterol in blood.327 An electrochemical sensor based on enzyme–carbon nanotubes has been developed for potential use in the detection of V-­t ype nerve gases.328 Cytotoxicity of MWNTs and SWNTs has been studied in comparison with C60 and quartz.329 SWNTs show significant cytotoxicity in alveolar macrophages (AMs) after exposure for 6 h in vitro. A ∼35% increase in the cytotoxicity is observed when the SWNT dosage is increased by 11.30 µg cm−2. In contrast, C60 up to a dose of 226.00 µg cm−1 shows no significant toxicity.2 Apparently, the cytotoxicity seems to adopt a mass-­based sequence: C60 < quartz < MWNT < SWNTs. At a low dose of 0.38 µg cm−2, SWNTs greatly decrease AM phagocytosis, while MWNT and C60 cause damage at a large dose of 3.06 µg cm−2. Macrophages subjected to MWNTs or SWNTs at a concentration of 3.06 µg cm−2 displayed necrosis and degeneration. Carbon nanotubes can be modified to be biocompatible using suitable biomimetic polymers, while the uncoated CNTs lead to cell death.330 Acid-­treated SWNTs adsorb several proteins on the sidewalls leading to the development of noncovalent protein–nanotube conjugates331 which can be used to transport proteins into various mammalian cells, where the nanotube acts as the transporter. FETs based on SWNTs can be applied for biosensing with the use of artificial oligonucleotides (aptamers).332 The aptamers being small (1–2 nm) make it feasible for protein–aptamer binding inside the electrical double layer at millimolar salt concentrations. A drug delivery device is typically created to enhance a drug's pharmacological and clinical profile. It aims to address issues such as reduced solubility, lack of selectivity, unfavorable pharmacokinetics, low bioavailability and healthy tissue damage associated with the administration of free medicines. CNTs have been pursued as one of the recently discovered methods because of their possible high loading capability and ability to enter cells without the use of an external transporter device. CNTs have made substantial advances in the delivery of multiple peptides, antigens, nucleic acids and small molecular drugs into living cells in recent years.331,333–338

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The nonselective destruction of healthy cells that divide quickly under normal conditions is a big issue in cancer chemotherapy. The most prominent side effects of chemotherapy are myelosuppression (lower blood cell production), alopecia (hair loss) and mucositis (inflammation of the digestive tract lining). The aim of several forms of study is to transmit drug molecules precisely to the tumor target with limited side effects on normal cells. Functionalized carbon nanotubes have been investigated as drug carriers and have been engineered to be tumor-­t argetable.339 In vivo positron emission tomography (PET), ex vivo biodistribution and Raman spectroscopy were used by Dai and colleagues to examine the biodistribution of radio-­labeled SWNTs in mice (Figure 3.15).339a RGD-­functionalization (RGD: a cyclic arginine–glycine–aspartic acid peptide) of SWNT-­PEG5400 and basic RGD-­integrin v3 identification were used to effectively target integrin v3-­ positive U87MG tumors. SWNT-­PEG5400-­RGD had a fast tumor absorption of 10–15% injected dose (ID) g−1, which was significantly higher than the 3–4% ID g−1 for SWNT-­PEG5400 without RGD. The same group compounded SWNTs with a large number of doxorubicin (DOX, a common cancer medication), which were stacked with a DOX aromatic hydroxyl anthraquinone on the surface of the nanotube.340 In vitro toxicity tests revealed that DOX-­loaded phospholipid (PL)-­SWNTs caused significantly more U87 cancer cell death and apoptosis than PL-­SWNTs, despite the fact that the IC50 (half-­maximum inhibitory concentration) for nanotube-­bound DOX (8 mM) was greater than that of free DOX (2 mM). RGD was complexed to the terminal classes in PL-­ SWNT to directly address U87 cancer cells. The RGD positive U87 cancer cells had a lower IC50 value (3 mM), indicating that the targeted delivery of nanotube-­bound DOX was successful. Bhirde et al.339b report the targeted, in vivo killing of cancer cells using a drug–SWNT bioconjugate, and demonstrate efficacy superior to nontargeted bioconjugates. Regression of tumor growth was rapid in mice treated with targeted SWNT– cisplatin–EGF relative to nontargeted SWNT–cisplatin. Villa et al.341 used regioselective chemistry to create various SWNT structures that can be used for selective targeting with RGD ligands, radiotracking with radiometal chelates and self-­assembly with oligonucleotides. The melting temperature of the SWNT-­oligonucleotide adduct annealed with a complementary oligonucleotide series was 54 °C. Radiolabeled SWNT–oligonucleotide conjugates were used to measure biodistribution in mice. A v3-­positive human coronary artery endothelial cell line has been applied as a model in a flow cytometric assay for binding specificity to validate precise antigen targeting of CNT–oligonucleotide conjugates (SWNT-­RGD-­ODNFAM). The SWNT-­RGD-­ODNFAM treatment resulted in a substantial improvement in median fluorescence strength as compared to the isotype monitor SWNT-­RAD-­ODNFAM. The insertion of the RGD targeting moiety to the nanotube–oligonucleotide build enabled precise tumor targeting, and this result suggests that the SWNT-­RGD-­ ODNFAM could be used as an anchor structure in a neovasculature targeted self-­assembly strategy.

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Figure 3.15  Water-­  soluble carbon nanotubes functionalized with PEG, radiolabels

and RGD peptide targeting integrin avb3-­positive U87MG tumor in mice by RGD-­functionalized SWNTs. (a) Schematic drawings of noncovalently functionalized SWNT-­PEG2000-­RGD, SWNT-­PEG5400-­RGD with DOTA-­64Cu. (b) MicroPET images of mice. The arrows point to the tumors. High tumor uptake (>15% ID g21) of SWNT-­PEG5400-­RGD is observed in the U87MG tumor (second column), in contrast to the low tumor uptake (first column) of SWNT-­PEG2000-­RGD. The third column is a control experiment showing blocking of SWNT-­PEG5400-­ RGD tumor uptake by coinjection of free c(RGDyK). The fourth column is a control experiment showing low uptake of SWNT-­PEG5400-­RGD in an integrin avb3-­negative HT-­29 tumor. (c) U87MG tumor uptake curves for mice injected with SWNT-­PEG5400, with and without RGD. All data shown represent three mice per group. Reproduced from ref. 339a with permission from Springer Nature, Copyright 2007.

Large functional biomolecules may also be introduced into cells using functionalized CNTs, in addition to small molecule medications. Pantarotto et al.342 used mono-­and bis-­derivatized CNTs to covalently connect a neutralizing B cell epitope from the foot-­and-­mouth disease virus (FMDV). While experimental observations have shown the effectiveness of CNT-­ enabled transmission, the process of CNT cellular uptake remains a mystery. The entry process that controlls the cellular incorporation of CNTs and their cargos is the main concern. Centered on the temperature dependency

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of CNT cellular uptake, Dai and colleagues suggested an endocytosis uptake process.336 They studied the absorption of shortened SWNTs (50–200 nm in length) by HeLa (adherent) and HL60 (nonadherent) cells by noncovalently functionalizing them with DNA and proteins. According to their findings, the bioactive cargos were transported into living cells in an energy-­dependent manner by these short SWNTs. As a result, they proposed that clathrin-­coated holes, rather than caveolae or lipid rafts, were the endocytosis mechanism for these widely spread, short SWNTs with bioconjugation. The same team tested larger aggregates of DNA-­functionalized SWNTs (up to 15 nm in diameter and 200 nm–2 mm long) for absorption by HeLa cells and found that the results were close to those for shortened SWNTs, indicating endocytosis. Both SWNTs and MWNTs were functionalized with a variety of molecules and bioactive species, including ammonium, anticancer medications, small molecule fluorescent probes and antibiotics, according to Kostarelos et al.343 Also, under endocytosis-­inhibiting conditions, many of these functionalized CNTs were observed to be taken up by a large range of cells and intracellularly trafficked via various cellular barriers to the perinuclear zone. They proposed that CNTs function as nanoneedles, passively piercing or penetrating the membranes of a variety of cell types. The findings of the molecular simulations seemed to support the nanoneedle function hypothesis.344 When SWNTs and MWNTs with surface defects are chemically functionalized at the defect sites, the photoluminescence is comparatively solid, and the emission is brighter with improved functionalization.345 In the visible and near-­IR ranges, defect-­derived photoluminescence is wavelength based. As a result, optical bioimaging technologies will benefit from well-­functionalized carbon nanotubes. Lacerda et al.346 used confocal laser scanning microscopy to visualize the association of SWNT-­NH3+ with human caucasian lung carcinoma A549 cells using SWNTs that were covalently associated with NH3+-­ terminated aliphatic appendages (SWNT-­NH3+, emission peak at 485 nm with 395 nm excitation) (Figure 3.16). At doses up to 500 g mL−1 and 24 hours after incubation, SWNT-­NH3+ was found intracellularly and perinuclearly, but no cell plasma membrane harm was observed. This was the first paper on imaging SWNTs in cells using visible fluorescence without the use of broad fluorescent labels applied to biological macromolecules. The imaging technique may be used to further understand the intracellular transport process of carbon nanotubes. CNTs are effective carriers for fluorescent labels in indirect bioimaging for CNTs. CNTs have been shown to reduce the fluorescence of the fluorophores they are bound to. CNTs, on the other hand, function as intracellular transporters, carrying fluorescent labels into cells and allowing fluorescence to be detected within the cells.347 Similarly, in investigations of the translocation and delivery of CNT conjugates, radioactive labels in conjunction with CNTs are equally effective. Porter et al.348a used a modern method named low-­loss light-­filtered transmission electron microscopy in tandem with electron energy loss continuum imaging to image individual SWNTs in cells. CNT applications in biology and medicine have

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Figure 3.16  Multiple  stain confocal images of A549 cells (a) incubated for 2 h (37 °C,

5% CO2) in the absence of SWNT-­NH3+ (control), (b) in the presence of 20 µg of SWNT-­NH3+ (scale bar: 20 µm), (c) z-­stack imaging data obtained from the preparation where A549 cells were incubated with 20 µg of SWNT-­NH3+. Cellular membranes are stained with WGA-­ TRITC and nuclei are counterstained with TO-­PRO 3. Reproduced from ref. 346 with permission from John Wiley and Sons, Copyright © 2007 WILEY-­VCH Verlag GmbH & Co. KGaA, Weinheim.

been reviewed by Liu et al.348b They noted that surface functionalization is essential for carbon nanotube activity in biological systems, and that ultrasensitive identification of biological species with carbon nanotubes can be achieved after surface passivation, which prevents nonspecific biomolecule binding on the hydrophobic CNT surface. Water-­soluble and serum-­ stable nanotubes are biocompatible, nontoxic and suitable for biomedical applications, according to in vitro and in vivo toxicity experiments utilizing electrical nanosensors based on nanotubes. They also report that carbon

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nanotube-­based drug delivery shows promise. Johnston et al. have reviewed biological mechanisms underlying the in vitro and in vivo toxicity of CNTs.

3.4.8  Mechanical Properties and Related Devices Nanoindentation has been used to study the nanomechanical properties of SiO2-­coated MWNT-­poly(methyl methacrylate) (PMMA) composites.349 A substantial improvement in the hardness (a factor of two), as well as Young's modulus (a factor of three) was detected in PMMA upon the incorporation of 4 wt% of SiO2-­coated MWNTs. A large enhancement in damping was achieved by incorporation of SWNTs in the polymer without losing the mechanical strength and stiffness.350 Incorporation of SWNTs into electrostrictive poly(vinylidene fluoride-­trifluoroethylene-­chlorofluoroethylene) produced higher strain response at reduced electrical fields.351 Thermal conductivity, as well as interface resistance of epoxy-­SWNT composites has been investigated.352 Aligned carbon nanotube-­polymer composites exhibited improved thermal conductivity.353 Composites based on nanotubes can be used in microelectromechanical systems. Mechanical tests reveal improvement in the composite modulus by a factor of 20 compared with pristine nanotubes.354 Both SWNTs and MWNTs transcend nanoclays as efficient flame-­retardant additives in polymer nanocomposites if they produce a jammed network structure in the polymer matrix.355a The fire-­retarding power of polymer nanocomposites containing different nanofillers has been investigated.355b Microcatheters have been fabricated with carbon nanotube–nylon composites and their thrombogenicity and blood coagulation have been inspected.356 A systematic investigation of the electromechanical properties of SWNTs under tensile strain has been carried out.357 Wide band-­gap semiconducting (or quasi-­metallic) nanotubes show the greatest resistance improvements or susceptibility to tensile stretching in the small strain range (βGF up to 600–1000), whereas metallic nanotubes are the least susceptible, which is qualitatively consistent with current theoretical predictions. The findings indicate that, at room temperature, quasi-­metallic SWNTs may be useful for extremely sensitive electromechanical sensors and may represent a different form of strain gauge material (conventional doped-­Si strain gauges have βGF ∼200). Mylvaganam and Zhang358a reviewed several articles and licensed patents on carbon polymer–nanotube composites manufactured in order to improve material properties such as mechanical strength, electrical conductivity and detection of radiation, which are widely used for the production of nanoelectronic, space and medical devices. Nishijima et al.160 have attached individual nanotubes to scanning probe microscope tips, which were then used as probes to image biological and industrial specimens. Carbon nanotube-­based scanning probe tips for

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atomic probe microscopes are commercially available. The small tube diameter and cylindrical shape permit imaging in narrow, deep crevices, showing improvement in resolution in comparison with conventional nanoprobes, particularly for high sample feature heights.358b,358c Nanotube tips modified covalently with biologically responsive ligands allow mapping of chemical and biological functions.186 Kim and Lieber359 have developed nanoscopic tweezers that are guided by the electrostatic interaction between two carbon nanotubes on a specimen tip. They coated glass micropipettes with carbon nanotubes. Closing and opening of the free ends of the nanotubes was achieved by applying voltages to the electrodes to enable the grabbing and manipulation of sub-­micron clusters and nanowires. They can be used as nanoprobes for assembly. These uses may not have practical applications, but are likely to improve the measurement system for characterization and manipulation on the nanometric scale. Prasad et al.360a have observed significant synergy when binary combinations of nanocarbons such as nanodiamond, few-­layer graphene and SWNTs are incorporated into a polyvinyl alcohol polymer matrix. As opposed to single nanocarbon reinforcements, the mechanical properties of the resultant composites demonstrate outstanding synergy in stiffness and hardness by up to 400%. The mechanical properties of the different PVA composites developed with two nanocarbons are summarized in Table 3.1. Carbon nanotubes may also be integrated into scaffolds to offer structural reinforcement whilst also imparting novel properties including electrical conductivity. The construction of extremely porous scaffolds consisting of diverse materials has been accomplished.360b Harrison and Atala360c studied carbon nanotube applications in tissue engineering, where possible cytotoxic effects of carbon nanotubes were mitigated by chemical surface functionalization. Seidlits et al.360d have reviewed the methods adopted to fabricate nanostructured scaffolds. Edwards et al.360e have presented the suitability of carbon nanotubes as a biomaterial for scaffold fabrication, as well as the fabrication and efficiency of carbon nanotube-­based scaffolds. Table 3.1  Mechanical  properties of the reinforced composites incorporating two

nanocarbon. Reproduced from ref. 360a with permission from National Academy of Sciences, Copyright 2009.

Composite wt%

Hardness, MPa

Synergy, % Elastic Elastic modulus, Hardness modulus GPa

Degree of crystallinity, %

PVA-­0.4SWNT + 0.2FG PVA-­0.4SWNT + 0.2ND PVA-­0.4FG + 0.2ND PVA-­0.4FG + 0.2SWNT PVA-­0.4ND + 0.2FG PVA-­0.4ND + 0.2SWNT

366.5 ± 23.99 314.2 ± 10.15 66.4 ± 5.43 336.9 ± 22.21 61.18 ± 0.002 352.9 ± 34.78

9.3 ± 0.43 36.4 7.5 ± 0.05 12.5 1.6 ± 0.1 141.4 8.6 ± 0.34 20.4 1.3 ± 0.07 71.1 9.3 ± 0.36 23.6

56.5 55.1 55.1 57.5 54.8 57.2

30.1 2.5 397.7 25.6 90.7 33.6

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3.4.9  Lithium Batteries As graphite-­like materials are used in Li+ batteries, ions are intercalated between the graphite layers, enabling Li+ to pass from a graphitic anode to a cathode (usually LiCoO2, LiNiO2 or LiMn2O4). The potential Li storage ability of graphite is 372 mA h g−1 (LiC6), and the charge and discharge phenomena in these batteries are based on Li+ intercalation and de-­intercalation.361a These batteries are commonly used in portable computers, cell phones, compact cameras and other electronic devices. Endo and colleagues showed that B-­doped vapor-­grown carbon fibers and nanofibers outperform all other carbon sources present in the graphitic anode. This may be because Li+ has a greater preference for B-­doped locations, resulting in improved energy capacity for the battery.361,362 N-­doped CNTs and nanofibers often provide high reversible Li storage (480 mA h g−1) compared to commercial carbon products used in Li+ batteries (330 mA h g−1).362c Carbon nanotubes have potential as lithium ion battery material owing to their distinct electrochemical and mechanical characteristics.363 Carbon nanotubes may be used as a conductive filler at a lower weight load than traditional carbons like graphite and carbon black, resulting in a more efficient electrical percolation network. Furthermore, CNTs may be assembled into free-­standing electrodes (without any current collector or binder) to serve as an active lithium ion storage material or as a physical support for ultrahigh-­capacity anode materials such as silicon or germanium. Depending on experimental conditions, estimated reversible lithium-­ion capacities for carbon nanotube-­based anodes will reach 1000 mA h g−1, which is a threefold increase over traditional graphite anodes. The lack of copper current collectors, which can result in a 50% improvement in real energy capacity for the total battery design, is the main benefit of using free-­standing CNT anodes. To solve some of the issues, such as first-­c ycle charge failure, further research is required. Future CNT batteries are projected to have higher energy capacity thanks to efforts to use prelithiation methods and modify SWNT bundling. Lee et al.364 have dispersed MWNT bundles in natural graphite and prepared a composite anode material for lithium-­ion batteries. Anode material with 2 wt% of MWNTs exhibited a capacity of 300 mA h g−1 at 3C rate with excellent cyclability. Composites of CNTs with graphene nanosheets have been examined for use as high-­ capacity anode materials in rechargeable lithium secondary batteries.365 They obtained large reversible Li storage capacity, which was an increase from 540 mA h g−1 (for pure graphene), to 730 mA h g−1 (for composite). The possible use of CNTs as electrodes in lithium batteries has been investigated due to the high reversible component of storage capacity at high discharge rates. SWNTs have shown a maximum reversible capacity of 1000 mA h g−1 when mechanically milled so as to enable the filling of nanotube cores, as compared to graphite (372 mA h g−1)256 and ball-­milled graphite (708 mA h g−1).291

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3.4.10  Gas Adsorption and Hydrogen Storage Liquids or gases can adsorb in nanosized channels of SWNTs. Eswaramoorthy et al.366 have studied the adsorption of nitrogen, methane and benzene in SWNTs. The studies suggest a good microporous nature of SWNTs with a total surface area >400 m2 g−1. Typical adsorption isotherms for SWNTs are shown in Figure 3.17. SWNTs packs in a hexagonal fashion in the bundles, which are ideal channels for 1D adsorbates. Studies of helium adsorption in SWNTs reveal a high binding energy, which is considered to be due to 1D adsorption in the SWNT bundle interstitial sites.367 Thermal activation can be applied to opening the SWNTs, which raises both saturation adsorption and the kinetic rate of nanotubes for Xe atoms at 95 K.368 CNTs have not been used as catalysts themselves, but they have been used as metal supports. Carbon nanotubes were believed to be good hosts materials for hydrogen storage (e.g. for fuel cells that power electric vehicles), but there is considerable doubt about the magnitude of the hydrogen uptake and, hence the use of CNTs for storage.3a,369,370,371,372,373 Carbon nanotubes were first used as hosts for reversible adsorption of molecular hydrogen by Dillon et al.374 These workers studied the capacity of hydrogen adsorption of as-­prepared

Figure 3.17  N  2 adsorption isotherms of SWNTs at 77 K: as-­prepared (▲), HCl treated (■) and HNO3 treated (ο). Inset: The hysteresis in the adsorption–desorption isotherms for SWNTs. Reproduced from ref. 366 with permission from Elsevier, Copyright 1999.

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SWNT bundles (0.1–0.2 wt%) comprising unidentified carbonaceous materials and large fractions of cobalt catalyst particles (20 wt%). Composition (H/C) vs. pressure isotherms of as-­synthesized SWNTs, sonicated SWNTs and a high surface area carbon (Saran) at 80 K (193 °C) were reported by Ye et al.373 These workers found that arc-­derived SWNTs have a hydrogen storage capacity of 8.25 wt% at 80 K and ∼4 MPa. Liu et al. reported 4.2 wt% of hydrogen storage capacity in SWNTs.375 at 27 °C and 10.1 MPa. SWNTs with a large mean diameter of 1.85 nm were used in this study. Under ambient pressure at room temperature, 78.3% of the adsorbed hydrogen (3.3 wt%) could be released. Heating of the sample was required for the release of the residual hydrogen (0.9%). Gundiah et al.376 performed a comparative study of high-­pressure hydrogen uptake and electrochemical hydrogen storage with different types of carbon nanotube samples. For hydrogen storage studies they used the following samples: I, arc-­produced SWNTs (as-­synthesized), II, treated with conc. HNO3; III, MWNTs prepared by the pyrolysis of acetylene (as-­synthesized); IV, conc. HNO3-­treated MWNTs synthesized by the pyrolysis of acetylene; V, arc-­ produced MWNTs; VI, aligned MWNT bundles prepared by the pyrolysis of ferrocene (as-­synthesized); VII, conc. HNO3-­treated aligned MWNT bundles prepared by the pyrolysis of ferrocene; VIII, aligned MWNT bundles prepared by the pyrolysis of a mixture of ferrocene and acetylene (as-­synthesized); IX, conc. HNO3-­treated aligned MWNT bundles prepared by the pyrolysis of a mixture of ferrocene and acetylene. Hydrogen adsorption vs. time plots of various carbon nanostructured samples are shown in Figure 3.18a. These workers obtained a maximum hydrogen storage capacity of 3.75 wt% (143 bar, 27 °C) with densely aligned nanotubes synthesized by the pyrolysis of ferrocene–hydrocarbon mixtures. Arc-­generated MWNTs and SWNTs exhibited a high-­pressure hydrogen storage capacity, (less than 3 wt%). Electrochemical charging capacity plots of several types of carbon nanotubes are shown in Figure 3.18b. Aligned MWNT electrodes exhibit greater electrochemical charging capacities, up to 1100 mA h g−1, equivalent to a hydrogen storage capacity of 3.75 wt%. Arc-­generated MWNTs and SWNTs, however, demonstration capacities in the range 2–3 wt%. Hydrogen adsorption capacities of SWNTs are generally less than 1 wt% at 25 °C for pressures up to 110 bar.377a SWNT nanotube bundles can be engineered as scaffolds for hydrogen storage.377b CNT fibers were swelled in oleum (fuming sulfuric acid) and organic spacer groups were covalently bound between the nanotubes to create 3-­dimensional (3D) frameworks for hydrogen adsorption utilizing diazonium functionalization chemistry. In comparison to macroporous carbon fabrics, these 3-­D nanoengineered fibers physisorb double that much H2 per unit surface region. Yürüm et al.378a have provided an outline of hydrogen storage in nanostructured carbon products, focusing on microporous powders and carbon nanotubes. The number and position of dopants have been optimized using local density approximation calculations, to find that a configuration of eight Li dispersed at the hollow sites above the hexagonal carbon rings of CNTs gives rise to high H2 storage capacity (13.45 wt%).378b At room temperature, CNTs impregnated with TiO2 nanorods and nanotubes were studied for

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Figure 3.18  (a)  Amount of hydrogen adsorbed in wt% as a function of time for the var-

ious carbon nanostructures (I–IX). The broken curve represents the blank data obtained in the absence of a carbon sample. (b) Plot of the charging capacity against the number of cycles for different carbon nanostructures. Also shown are the corresponding weight percentages of H2 stored. Reproduced from ref. 376 with permission from the Royal Society of Chemistry.

Chapter 3

220 379a

hydrogen uptake. The hydrogen storage capacities of hybrid composites of acid-­treated MWNTs and metal-­organic frameworks (MOF-­5) have been studied.379b

3.4.11  Other Useful Properties and Devices Ge et al.380a presented a synthetic gecko tape made by moving micropatterned CNT arrays onto a versatile polymer tape that mimics the organizational arrangement seen on a gecko's foot. The gecko tape can withstand shear stress four times greater than the gecko foot and adheres to a variety of surfaces. CNT bristles grafted on fiber handles have been used to produce multi-­functional conductive brushes.380b Cleaning of nanoparticles from sensors and small gaps, selective chemical adsorption, coating of the internal openings, movable electromechanical brush contacts and switches, and elimination of heavy metal ions are all possible applications for these brushes with nanotube bristles. Chemically modified CNT–hydroxyapatite nanocomposites have been used as electrodes for determination of Cd2+ by anodic stripping voltammetry with a linear range of 20 nM–3 µM.380c In nonlinear optics, optical limiting is a valuable property for protecting human eyes, optical components and optical sensors from extreme laser pulses. An optical limiter is a device that strongly attenuates high-­intensity light or possibly harmful light, such as directed laser beams, thus allowing for high ambient light transmission. Carbon nanotubes have been studied for optical limiting properties, which are important for high-­power laser applications. Nonlinear scattering is responsible for the optical limiting behavior of visible nanosecond laser pulses in SWNT suspensions.381a Yoshino et al.381b observed increased conductivity in polymer composites at relatively low nanotube concentrations and photoconductivity enhancement, implying potential use in optoelectronic devices. ZnO nanoparticle–MWNT nanohybrids exhibit ultrafast nonlinear optical switching.382a Photothermal current microscopy wherein scanning the focal spot of a laser across the surface of a device provides spatially resolved conductance images of both single and arrayed nanotube transistors.382b Kis and Zettl383 examined the nanomechanics of carbon nanotubes in relation to promising prototypes of nanoelectromechanical devices built on carbon nanotubes, such as high-­performance nanomotors, switches and oscillators. SWNT-­sheet-­based electromechanical actuators appear to produce stresses greater than natural muscles and higher strains than the ferroelectric counterparts.255 This behavior of nanotubes could be applied in robotics as it converts electrical energy into mechanical energy. Wood and Wagner384 detected a significant shift in the Raman frequencies of SWNTs upon soaking in liquids, which allows nanotubes to be used as molecular sensors. The flow of liquids and gases over nanotubes generate a voltage in the tubes.293,385 The voltage so generated fits a logarithmic velocity dependence over nearly six decades of velocity. The amount of voltage/current depends on the ionic conductivity of liquid as well as its polar nature. The dominant

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mechanism responsible for this strongly nonlinear reaction appears to entail the fluctuating Coulombic field of the liquid flowing past the nanotubes pressing the free charge carriers in the nanotubes. This research shows that nanotubes have the ability to be used as flow sensors. Although this is a fascinating property, it is doubtful that it will be used to produce electricity. For atom transfer, Karl and Tomanek386a have proposed a molecular pump based on CNTs. The principle is based on the coherent excitation of a CNT by two laser beams, which generates an electron current, driving the intercalated atoms. Controllable, reversible atomic-­scale mass transfer (electromigration) along carbon nanotubes has been reported by Regan et al.386b Seidel et al.387 created SWNT-­based systems that can power macroscopic devices like light emitting diodes and electric motors. The use of SWNT networks as transparent, conducting electrodes in versatile organic light-­emitting diodes (FOLEDs) has been reported by Hu et al.387c These networks are comparable to widely utilized electrodes such as PEDOT : PSS and ITO suggesting that SWNT networks have excellent optical and superior mechanical properties. From a single carbon nanotube, Jensen et al.388 created a completely functioning, integrated radio receiver. The nanotube can act as an antenna, an amplifier, a tunable band-­pass filter and a de-­modulator all at the same time. The radio is driven by a direct current voltage source, such as a battery. They presented music and speech reception using carrier waves in the commercially applicable 40–400 MHz range and both frequency and amplitude modulation techniques. Later, Rutherglen and Burke published experimental findings for a CNT-­based amplitude-­modulated (AM) demodulator with modulation frequencies of up to 100 kHz.389 Furthermore, the CNT-­based demodulator was realized in an actual AM radio receiver capable of demodulating high-­fidelity audio and running at a carrier frequency of 1 GHz. The nonlinear current–voltage (fcs vs. l/bs) feature of the CNT causes rectification of a portion of the applied RF signal, resulting in demodulation. The demodulation effect can be maximized by correctly biasing the CNT such that the operational point is located on the highest nonlinear portion of the I–V curve. This is a basic example of CNTs and nanotechnology being used in the wireless realm. CNT thin-­film loudspeakers that are magnet-­free, stretchable, flexible and translucent have been made.390 Carbon nanotube thin films have a broad frequency response spectrum and a high sound pressure level due to their ultralow heat power per unit field.

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Chapter 4

Inorganic Nanotubes 4.1  Introduction One-­dimensional (1D) nanotubes and nanowires, as well as zero-­dimensional nanocrystals, are essential types of nanomaterials.1,2 Iijima is accredited with discovering carbon nanotubes, the first family of nanotubes.3 The discovery of fullerenes,4 led researchers to look at other layered materials that could generate tubular structures. Many inorganic materials have structures identical to graphite, including metal dichalcogenides (sulfides and selenides), halides, oxides and hydroxides. Metal dichalcogenides, such as MoS2, WS2, MoSe2, NbS2 and HfS2, consist of a metal sheet sandwiched between two chalcogen layers, with the metal in octahedral or trigonal pyramidal coordination. The MX2 (M = Metal, X = S or Se) layers are packed in an ABAB pattern in the c-­direction, analogous to the graphite structure's single graphene sheets (Figure 4.1). The absence of an M or X atom at the edges of the layers leads to dangling bonds (when viewed parallel to the c-­axis). While in the bulk, the metal atom is six-­fold coordinated to the chalcogen atoms in the MX2 layer, at the edge of the MS2 nanocluster, it is four-­fold coordinated. The chalcogen atom is three-­fold coordinated to the metal atoms in the bulk, but at the rim, it is two-­fold coordinated (Figure 4.1). Graphite also possesses such unsaturated bonds at the edges of the layers. The relative number of rim atoms with dangling bonds increases with decreasing size of molecular sheet. The layers of transition metal dichalcogenide therefore become unstable to bending and have a high tendency to roll into curved structures, producing hollow fullerene-­like structures. Chianelli et al.5 and Sanders6 identified rolling of layered transition-­metal chalcogenides before the discovery of carbon fullerenes and nanotubes. Rag-­like as well as tubular structures of MoS2 were reported by Chianelli­ et al., along with their catalytic properties. Low-­magnification TEM images­   Nanoscience & Nanotechnology Series No. 52 Nanotubes and Nanowires, 3rd Edition By C. N. R. Rao, A. Govindaraj and Leela Srinivas Panchakarla © C. N. R. Rao, A. Govindaraj and Leela Srinivas Panchakarla 2022 Published by the Royal Society of Chemistry, www.rsc.org

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Figure 4.1  Comparison  of the structures of (a) graphite and inorganic layered

compounds such as (b) NbS2/TaS2, (c) MoS2 and (d) BN. In the layered dichalcogenides, the metal is in trigonal prismatic (TaS2) or octahedral coordination (MoS2). Note that the rim atoms have lower coordination to the neighbouring atoms as compared with bulk atoms. From ref. 11 with permission from the Royal Society of Chemistry.

show folded sheets that look like crystalline needles due to layers folding onto themselves. Indeed these structures exemplify nanoscrolls. Tenne and coworkers7–10 demonstrated that Mo and W dichalcogenide nanosheets are unstable to folding and closure and can generate fullerene-­ like nanoparticles and nanotubes. These novel nanoparticles were called inorganic fullerenes (IF). Dichalcogenide systems are made up of cylinders of concentrically nested fullerene and have lower structural order than carbon nanotubes. Typical TEM images of MoS2 fullerene and WS2 nanotube are shown in Figure 4.2a and b, respectively. Defect-­free MX2 nanotubes are rigid and do not permit plastic deformation. Figure 4.3 shows a schematic representation of the folding process of an MS2 layer into a nanotube. There has been some debate about the sources of curvature and folding in layered metal dichalcogenides. Stoichiometric chains and layers, such as those found in TiS2, have an inherent propensity to bend and fold, as shown by intercalation reactions. Variable stoichiometry and coordination in the chalcogenide layers may also cause folding. The stoichiometric change within the material leads to the formation of closed rings. There has been significant development in the synthesis and characterization of nanotubes of molybdenum and tungsten dichalcogenides. Thus nanotubes of a variety of inorganic materials11,12 and peptides13,14 have been realized. Detailed reviews on inorganic nanotubes have appeared.11,15–19

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Figure 4.2  TEM  images of (a) a multi-­walled nanotube of WS2 and (b) hollow­

particles (inorganic fullerenes, IF) of WS2. Reproduced from ref. 7 and 8 ­ with permission from Springer Nature, Copyright 1992 and 1993.

Transition-­metal chalcogenides exhibit a diverse collection of appealing physical properties. They are widely used as lubricants as well as in catalysis. They are metallic, semiconducting and superconducting in nature. Following the synthesis and characterization of MoS2 and WS2 nanotubes and fullerene-­like structures, a new avenue of investigation into other metal

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Figure 4.3  Schematic  illustration of the bending of a MoS2 layer; n1 and n2 are­ the lattice parameters of the nanotube. Reproduced from ref. 15 with permission from American Chemical Society, Copyright 1998.

chalcogenides and 2D layered structures has opened up. Indeed, many group IV and V metal dichalcogenides possess layered structures suitable for making nanotubes. Apart from carbon and dichalcogenides of Mo and W, other compounds are also known to form curved structures. Well-­known examples are asbestos minerals (kaolinite, chrysotile) which form nanotubular structures that contain fused tetrahedral and octahedral layers along the c-­direction.20 Numerous inorganic materials have been synthesized in a nanotubular form. This includes elemental materials, binary oxides, halides and nitrides.1,2,12 Different synthetic methods are employed to synthesize nanotubes of oxides of transition metals and other metals.17,21–27 Polycrystalline silica nanotubes are produced during the hydrolysis of tetraethylorthosilicate in a mixture of water, ethanol, ammonia and d, l-­tartaric acid to produce silica spherical particles.21 Metal oxide nanotubes have been synthesized by employing carbon nanotubes as templates.23–25 Chemical methods have been employed to synthesize V2O5 nanotubes,26 to study their catalytic activity. Due to the structural similarity of boron nitride and graphite, synthesis of BN nanotubes has received considerable attention. BN crystallizes in a graphite-­like structure and can be viewed as exchanging the C–C pair with the iso-­electronic B–N pair in the graphene sheet. It is therefore a suitable material for the formation of BN nanotubes. Replacement, partly or entirely, of the C–C pairs by B–N pairs in the hexagonal network of graphite (for example in C60) offers a array of two-­dimensional phases that can produce hollow cage structures and nanotubes.27–30 BN nanotubes of different wall thickness and morphology have been created using a variety of techniques.31–34 Metal halides like NiCl2, oxides like Tl2O, nitrides like GaN and chalcogenides like GaSe crystallize in layered systems, and their nanotubes have been examined.35,36

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As well as elemental or binary compounds, complex compounds such as spinels and perovskites have also been synthesized in nanotubular form. It would seem that one is able to synthesize nanotubes from almost any inorganic material. At present, there are only a few reports of single-­walled inorganic nanotubes. With the accessibility of sufficient quantities of inorganic nanotubes, there is scope to measure their physical and chemical properties and investigate phenomena exhibited by them.

4.2  Synthetic Methods Carbon nanotubes have been created using arc-­evaporation and pyrolysis techniques, laser ablation of graphite, and electrochemical and templating techniques.37 Boron nitride and metal chalcogenide nanotubes may also be synthesized utilizing methods close to those used to produce carbon nanotubes, with the fact that the nanotubes of MoS2 and BN may involve complex reactions involving the constituent elements or compounds comprising the elements. One alternative path is the decomposition of substances containing precursor elements. Fullerene-­like nanoparticles and nanotubes of metal dichalcogenides such as MoS2, WS2 and MoSe2 have been synthesized by arc discharge38 and laser ablation39,40 which employ processes that are far from equilibrium. By far, chemical reactions are the most successful routes to make them.15,41 Thus, stable oxides MoO3 and WO3 can be used as the starting materials for making MoS2 and WS2 nanotubes. The oxide precursors are subjected to high temperatures in a reducing atmosphere followed by reaction with H2S. Selenides are obtained by reacting with H2Se.9,42 As trisulfides are likely to be intermediates in the formation of disulfide nanotubes, nanotubes can be prepared by the direct decomposition of trisulfides such as MoS3 and WS3.43 The decomposition of metal triselenides yields diselenide nanotubes.44 The trisulfide process can be used to create nanotubes from a number of metal disulfides, including HfS2 and NbS2.45,46 Decomposition of ammonium salt precursors, (NH4)2MX4 (X = S, Se; M = Mo, W), has been employed to synthesize nanotubes of Mo and W dichalcogenides.43 Other methods include a hydrothermal method for the preparation of dichalcogenide nanotubes where an organic amine taken as one of the components in the reaction mixture acts as a structure-­directing agent for one-­dimensional crystal growth. Hydrothermal and solvothermal routes are important methods used to synthesize nanotubes and related structures of inorganic materials. These processes are often carried out in the presence of surfactants or other additives. Thus, the hydrothermal method can be employed to produce nanotubes of several metal oxides (e.g., SiO2, V2O5, ZnO).26,47,48 Nanotubes of oxides such as V2O5 have also been prepared from an appropriate oxide precursor in the presence of a surfactant or an organic amine.49 CdSe and CdS nanotubes have been synthesized using a surfactant. Here, the reaction between the metal oxide and the sulfidizing or selenidizing agent was carried out in the presence of a surfactant such as Triton X.50

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A template-­assisted method is popular for the synthesis of inorganic nanotubes. Porous membranes of alumina (AMs) or polycarbonate are commonly used templates. The general procedure is to deposit the relevant materials or appropriate precursors of the materials in the pores, followed by high-­ temperature annealing and removal of the template. An electrochemical or sol–gel method is used for the deposition of the material in the porous channels. Sol–gel chemistry with the use of a template has been broadly used for metal oxide nanotube synthesis such as silica and TiO2.21,51 Other templates including carbon nanotubes, polymer gels, surfactants and liquid crystals have been used. The templates are filled with the precursor material and then annealed before removal (dissolution or burning). Thus, oxide gels in the presence of suitable templates or surfactants produce nanotubes. For instance, burning off carbon from oxide gel-­coated carbon nanotubes yields nanotubes and nanowires of a variety of metal oxides such as SiO2, ZrO2 and MoO3.24 Oxide nanotubes have been prepared employing the sol–gel method and pores of alumina membranes. Notably, MoS2 nanotubes were synthesized in alumina membrane pores by the decomposition of a precursor.52 A layer-­by-­layer film-­forming method in the pores of alumina membranes, involving alternate immersion of the template in a diorganodiphosphonate and then in a solution of ZrO2+ has been developed to obtain ZrO2 nanotubes.53 Another commonly used method for the synthesis of nanotubes is electrochemical anodization. Nanotubes of TiO2, ZnO and other oxides have been synthesized by employing electrochemical anodization. High-­temperature laser ablation of NiCl2 in the presence of CCl4 at 700– 940 °C gives NiCl2 IF structures and nanotubes.54 To form germanium-­filled silica nanotubes, Hu et al.55 used a two-­stage process by combining simultaneous thermal evaporation of SiO powder and laser ablation of the Ge target. Nanotubes of boron nitride and other materials have been synthesized by electric-­arcing and laser-­ablation techniques. Striking an electric arc between HfB2 electrodes yields BN nanotubes in a N2 atmosphere.56 Arcing between B/C electrodes yields BCN and BC nanotubes. Considerable efforts have been made to synthesize BN nanotubes starting with various precursor molecules comprising B and N. Some of the nanotubes are obtained by employing chemical vapor deposition. Thus, decomposition of borazine in the presence of transition-­metal nanoparticles or of the 1 : 2 melamine-­boric acid addition compound yields BN nanotubes.31,32 Reaction of B2O3 or boric acid with NH3 or N2 at high temperature in the presence of catalytic metal particles or carbon produces BN nanotubes.34 Goldberger and coworkers.57 have employed an epitaxial casting approach in a chemical vapor deposition (CVD) system to synthesize single-­crystal GaN nanotubes with wall thicknesses of 5–50 nm and inner diameters of 30–200 nm. They have used nanowires of hexagonal ZnO as templates for the epitaxial overgrowth of thin GaN layers. In a typical reaction, a trimethylgallium precursor was heated at 600–700 °C in the presence of NH3 with argon or nitrogen as the carrier gas. A carbon-­free CVD process using B2O3–Ga2O3 mixtures and NH3 has also been employed for BN nanotubes.58 Template-­free synthesis of InP nanotubes starting with In2O3, In metal and red P has been reported.59

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The use of organic templates (organogels) to form morphologically interesting inorganic materials has been reviewed by Shinkai and coworkers.60 Organogels form fibrous, tubular, ribbon-­like, lamellar and hollow spherical morphologies. The organogels comprise an organic liquid and low concentrations ( 1) 2SiO → Si + SiO2 The above decomposition reactions result in the creation of silicon nanoparticles, which serve as nuclei for the formation of silicon nanowires covered in silicon oxide. As precipitation, nucleation and growth of the nanowires occur near the cold finger, the temperature gradient has been discovered to be the external driving force for their formation and growth. The TEM images of nuclei at the early stages of Si nanowire formation are shown in Figure 5.6a–c. Si nanoparticles enclosed by a thin amorphous Si oxide layer are shown in Figure 5.6a. A high concentration of defects is found at the tip of the Si crystalline core. The growth mechanism of the nanowire is schematically shown in Figure 5.7. The silicon nanowire growth is determined by three factors: (1) the catalytic effect of the SixO (x > 1) layer on the nanowire tips, (2) hindrance of nanowire growth in the lateral direction by the SiO2 component in the shells formed by the decomposition of SiO, (3) stacking faults along the〈112〉direction and micro-­twins at the tip areas. Only the nuclei that have the〈112〉direction parallel to the growth direction grow fast (Figure 5.7b). Oxide assisted one-­dimensional growth of Si is also explained by an electric-­field mechanism.28 This model explains the observation that SiO vapors condense only on the tip of the nanowire. It proposes the presence of a strong electric field at the tip of the nanowires, which attracts the SiO molecules strongly from the vapor and facilitates one-­dimensional growth.

5.2.2.3 Vapour–Solid Growth Whisker growth from a vapor–solid (VS) process also holds for nanowire growth.10 In this process, no additional metals or templates are required, the vapor is carried and condenses on a substrate. The vapor is produced

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Figure 5.6  TEM  micrographs of (a) Si nanowire nuclei formed on the Mo grid and

(b), (c) initial growth stages of the nanowires. Reproduced from ref. 25 with permission from Cambridge University Press, Copyright Materials Research Society 1999.

by evaporation, chemical reduction or gaseous reaction. At the early stage of crystallization, there is competition in growth rate among different crystallographic planes. When conditions favor anisotropic 1D growth, the feed elements begin to attack on a particular favorable expanding plane to increase and continue the 1D growth, leading to nanowire formation. Crystallographic planes with comparatively higher surface free energies are more unstable thermodynamically, and susceptible to attack by external atoms. VS growth has been used to make micron-­diameter oxide whiskers as well as metal whiskers. It is possible to produce one-­dimensional nanostructures using this process by controlling the nucleation and growth process. Using the VS method, several metal oxide and metal nanowires have been obtained. Anisotropic reactivity of the different planes in the crystal does not lead to high aspect ratio one-­dimensional materials. Crystal defects and self-­ catalyzed growth helps in producing high aspect ratio nanowires. Screw

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Figure 5.7  Schematic  describing the nucleation and growth mechanism of Si

nanowires. The parallel lines indicate the [112] orientation. (a) Si oxide vapor is deposited first and forms the matrix within which the Si nanoparticles are precipitated. (b) Nanoparticles in a preferred orientation grow quickly and form nanowires. Nanoparticles with nonpreferred orientations may form chains of nanoparticles. Reproduced from ref. 25 with permission from Cambridge University Press, Copyright Materials Research Society 1999.

dislocation-­induced crystal growth is well known in the literature.29 In this mechanism, crystal growth happens via addition of atoms/ions/molecules at the kinks of a surface step and the step advances. This process is self-­ perpetuating, as screw dislocation cannot terminate inside a perfect crystal.

5.2.2.4 Carbothermal Reactions Carbothermal reactions have been used to synthesize nanowires of carbides, oxides and nitrides. For instance, an oxide mixed with carbon (activated carbon or carbon nanotubes) produces sub-­oxidic or metal vapor species that react with C, O2, N2 or NH3 to yield the desired nanowires. Thus, reacting a mixture of Ga2O3 and carbon at high temperature in N2 or NH3 yields GaN nanowires. The following steps are involved in the carbothermal reaction: metal oxide + C → metal sub-­oxide + CO metal sub-­oxide + O2 → metal oxide nanowires

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metal sub-­oxide + NH3 → metal nitride nanowires + CO + H2 metal sub-­oxide + N2 → metal nitride nanowires + CO metal sub-­oxide + C → metal carbide nanowires + CO Depending on the desired product, the sub-­oxide is heated in the presence of O2, NH3, N2 or C to produce nanowires of oxides, nitrides or carbides.

5.2.3  Solution-­based Growth The solution-­based method to grow anisotropic one-­dimensional structures, depends on the crystallographic structure of the material, use of templates to confine and direct, kinetic control or the use of a suitable capping agent.

5.2.3.1 Anisotropic Structures Solid materials consisting of anisotropic bonding in the structure such as poly(sulfur nitride), (SN)x, grow as 1D nanostructures.30,31 Other materials including selenium,32,33 tellurium34 and molybdenum chalcogenides35 are readily achieved as nanowires owing to anisotropic bonding, which dictates that the crystallization occurs along the c-­axis, favoring the stronger covalent bonds over the weak van der Waals forces between the chains.

5.2.3.2 Template-­based Synthesis One-­dimensional nanostructures can be generated conveniently by template-­ directed synthesis. The template acts as a support against which other materials are formed in situ with complementary morphologies to that of the template. Templates such as porous alumina, mesoporous materials or polycarbonate membranes consist of nanoscale channels. The nanoscale pores are filled with the precursor solution using electrochemical or sol–gel methods. Free-­standing nanowires are obtained by removing the templates.36 Unlike polymer membranes prepared by track etching, anodic alumina membranes (AAM) containing 2D arrays of hexagonally packed cylindrical channels of uniform size are prepared by anodization of aluminium foil in an acidic medium (see Figure 5.8). Nanowires of several materials including Au, Pt, Ag, TiO2, ZnO, SnO2, In2O3, MnO2, CdS, CdSe, CdTe, electronically conducting polymers including polyaniline, polypyrrole and poly(3-­methylthiophene) as well as CNTs have been prepared using AAMs. However, it is often challenging to achieve single-­crystalline materials by this method. Surfactants can be self-­assembled into mesophase structures (Figure 5.9) that offer another class of useful, versatile templates for producing large quantities of one-­dimensional nanostructures. Surfactant molecules can self-­assemble into rod-­shaped micelles over the critical micellar concentration (c.m.c).3,37 The organization of surfactants into different shapes can be

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Figure 5.8  TEM  micrograph of an anodic alumina membrane (AAM). Reproduced from ref. 36c with permission from Elsevier, Copyright 2001.

Figure 5.9  Schematic  illustration showing the formation of nanowires by templat-

ing against mesostructures which are self-­assembled from surfactant molecules; (a) formation of cylindrical micelles, (b) formation of the desired material in the aqueous phase encapsulated by the cylindrical micelle, (c) removal of the surfactant molecule with an appropriate solvent (or by calcination) to obtain an individual nanowire. Reproduced from ref. 37 with permission from John Wiley and Sons, Copyright © 2003 WILEY-­VCH Verlag GmbH & Co. KGaA, Weinheim.

controlled by tuning the hydrophilic head group, hydrocarbon chain length and volume. Templates from these soft anisotropic structures have been used to produce nanorods when coupled with a suitable electrochemical or chemical reaction. Nanorods or nanowires are formed after removal of the surfactants. By using surfactants such as Na-­AOT or Triton X-­100, various nanowires have been synthesized including CdS, CdSe, CuS, CuSe, ZnS and ZnSe.38,39 Nanowires themselves are used as templates to produce nanowires of other materials. The nanowire may be coated by another material, growing coaxial nanocables,40 or it might react with the nanowires producing a new material.41 Nanotubes of the coated materials may also be produced by dissolution of the original nanowires. The sol–gel coating process is a common route to produce coaxial nanocables that comprise electrically conductive metal cores and insulating sheaths.

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Several metal nanowires of 1–1.4 nm diameter have been prepared by filling SWNTs. SWNTs are opened by acid treatment.42 Sealed-­tube reactions as well as solution-­based methods are employed to synthesize nanowires of Au, Ag, Pt and Pd. Thin layers of different metals have also been incorporated into the intertubular space of the SWNT bundles.

5.2.3.3 Solution–Liquid–Solid Process A low-­temperature solution–liquid–solid (SLS) method has been developed by Buhro and coworkers43 for the synthesis of crystalline nanowires of group III–V semiconductors.44 In a standard procedure, a metal with a low melting point (e.g. In, Sn, or Bi) acts as the catalyst, and the preferred material is synthesized through the decomposition of an organometallic precursor. Nanowires of InP, InAs and GaAs have been synthesized by solution-­phase reactions at a low temperature (≤203 °C). The schematic diagram in Figure 5.10 clearly depicts the SLS system for growing nanowires. Generally, the process produces single-­crystalline materials. Holmes et al.45 synthesized defect-­free silicon and germanium nanowires in large quantities using the supercritical-­ fluid–liquid–solid (SFLS) method. By using selective capping with mixed surfactants, it is possible to extend the growth of group II–IV semiconductor nanoparticles to nanorods.46

5.2.3.4 Solvothermal Synthesis The solvothermal methodology is extensively used as a solution route to prepare semiconductor nanorods and nanowires. In this process, certain metal precursors are mixed with a solvent mixture and crystal growth is regulated by agents such as amines. Crystal growth from the solution mixture is carried out inside an autoclave. The choice of temperature, solvent and filling fraction play a role in the crystal growth. The methodology is versatile and has been widely used to produce crystalline nanowires of semiconductors and other materials. The term “hydrothermal” is used when the solvent is water in this process.

Figure 5.10  Schematic  illustration showing the growth of a nanowire through the

solution–liquid–solid (SLS) mechanism which is similar to the vapor– liquid–solid (VLS) process. Reproduced from ref. 43 with permission from the AAAS, Copyright 1995.

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5.3  Growth Control and Integration One of the challenges in chemical synthesis of nanowires is how to rationally control assemblies of the nanostructure so that their dimensionality, size and their 2D and 3D superstructures can be tailor-­made. Several physical and thermodynamic properties are diameter-­dependent. Using clusters with narrow size distributions, numerous workers have produced uniform-­sized nanowires by the vapor–liquid–solid method. Wang et al.12 have reviewed methods and mechanisms of growth of one-­dimensional nanostructures of different materials through self-­organized growth including VLS growth, oxide-­assisted chemical vapor deposition (without a metal catalyst), thermal evaporation, metal-­catalyzed molecular beam epitaxy, chemical beam epitaxy, laser ablation and solvothermal reactions. For various applications, control over the growth orientation of nanowires is essential. By employing the conventional epitaxial crystal growth technique in the vapor–liquid–solid process, a VLS epitaxy technique has been developed for the controlled synthesis of nanowire arrays. Nanowires usually have preferred growth directions. For instance, zinc oxide nanowires prefer to grow along the direction.47,48 The preferred growth direction of silicon nanowires is the direction when produced by the VLS growth method, but growth can also occur along the or directions by the oxide-­assisted growth process. Clearly, in the VLS nanowire growth process, the positions of the nanowires are determined by the initial positions of Au thin films or Au clusters. By using the lithographic technique, it is feasible to produce the desired Au nanoparticle patterns on substrates, from where ZnO nanowires can grow in the same designed pattern since the nanowires grow vertically from the Au catalyst.47,48 By the surface-­patterning strategy, nanowires networks with precise placement of individual nanowires on substrates with the chosen configuration are attained.47,48 One of the challenges is to integrate nanowire building blocks, in a controlled manner, into complex functional networks. In the direct one-­ step growth process, catalyst particles are deposited selectively on the substrate and nanowires are grown selectively from the catalyst particles by the vapor–liquid–solid technique. Another approach to acquire a hierarchical assembly is to place the nanowire building blocks together into the functional structure. Nanowire surface patterning and alignment have been accomplished using a microfluidic-­assisted nanowire integration method in which a solution/suspension containing nanowires is put into the microchannels generated between a flat Si substrate and a poly(dimethylsiloxane) (PDMS) micro mold.49a,b Figure 5.11 illustrates the microfluidic-­ assisted nanowire incorporation mechanism schematically. Additionally, the Langmuir–Blodgett method has been used to build aligned, high-­ density nanowire assemblies.50

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Figure 5.11  Schematic  illustration of the microfluidic-­assisted nanowire integra-

tion process for nanowire surface patterning. Reproduced from ref. 49b with permission from John Wiley and Sons, Copyright © 2002 WILEY-­VCH Verlag GmbH, Weinheim, Fed. Rep. of Germany.

References 1. S. Iijima, Nature, 1991, 354, 56. 2. P. Yang, Y. Wu and R. Fan, Int. J. Nanosci., 2002, 1, 1. 3. (a) Y. Xia, P. Yang, Y. Sun, Y. Wu, B. Mayers, B. Gates, Y. Yin, F. Kim and H. Yan, Adv. Mater., 2003, 15, 353; (b) H. Ringsdorf, B. Schlarb and J. Verzmer, Angew. Chem., Int. Ed., 1988, 27, 113. 4. C. N. R. Rao, F. L. Deepak, G. Gundiah and A. Govindaraj, Prog. Solid State Chem., 2003, 31, 5. 5. Nanowires Nanobelts, ed. Z. L. Wang, Kluwer Academic Publishers, 2003, vol. 1-­2. 6. C. N. R. Rao, A. Govindaraj and S. R. C. Vivekchand, Annu. Rep. Prog. Chem., Sect. A: Inorg. Chem., 2006, 102, 20. 7. C. N. R. Rao, S. R. C. Vivekchand, K. Biswas and A. Govindaraj, Dalton Trans., 2007, 34, 3728. 8. S. R. C. Vivekchand, A. Govindaraj and C. N. R. Rao, Nanomaterials Chemistry, ed. C. N. R. Rao, A. Mueller and A. K. Cheetham, Publ. Wiley-­VCH Verlag GmbH Co. KGaA, Weinheim, Germany, 2007, vol. 45–118. 9. L. Cademartiri and G. A. Ozin, Adv. Mater., 2009, 21, 1021. 10. (a) R. S. Wagner, Whisker Technol, ed. Levitt A. P., Wiley-­Interscience, New York, 1970; (b) R. S. Wagner and W. C. Ellis, Appl. Phys. Lett., 1964, 4, 89. 11. P. S. Shah, T. Hanrath, K. P. Johnst and S. A. Korgel, J. Phys. Chem. B, 2004, 108, 9574. 12. N. Wang, Y. Cai and R. Q. Zhang, Mater. Sci. Eng., R, 2008, 60, 1. 13. W. Q. Han and A. Zettl, Appl. Phys. Lett., 2002, 80, 303. 14. F. Qian, Y. Li, S. Gradečak, D. Wang, C. J. Barrelet and C. M. Lieber, Nano Lett., 2004, 4, 1975. 15. S. Hofmann, C. Ducati, R. J. Neill, S. Piscanec and A. C. Ferrari, J. Appl. Phys., 2003, 94, 6005.

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16. W. Shi, Y. Zheng, H. Peng, N. Wang, C. S. Lee and S. T. Lee, J. Am. Ceram. Soc., 2000, 83, 3228. 17. L. Schubert, P. Werner, N. D. Zakharov, G. Gerth, F. M. Kolb, L. Long and U. Gösele, Appl. Phys. Lett., 2004, 84, 4968. 18. Z. H. Wu, X. Y. Mei, D. Kim, M. Blumin and H. E. Ruda, Appl. Phys. Lett., 2002, 81, 5177. 19. E. I. Givargizov, J. Cryst. Growth, 1975, 31, 20. 20. H. Wang and G. S. Fischman, J. Appl. Phys., 1994, 76, 1557. 21. G. Neumann and G. M. Neumann, Surface Self-­diffusion of Metals, Diffusion and Defect Monograph Series, Diffusion Information Center, 1972, vol. 1, p. 5. 22. Y. Wu and P. Yang, J. Am. Chem. Soc., 2001, 123, 3165. 23. S. Kodambaka, J. Tersoff, M. C. Reuter and F. M. Ross, Science, 2007, 316, 729. 24. J. L. Lensch-­Falk, E. R. Hemesath, D. E. Perea and L. J. Lauhon, J. Mater. Chem., 2009, 19, 849. 25. S. T. Lee, N. Wang, Y. F. Zhang and Y. H. Tang, MRS Bull., 1999, 36. 26. N. Wang, Y. H. Tang, Y. F. Zhang, C. S. Lee and S. T. Lee, Phys. Rev. B, 1998, 58, R16024. 27. R. Q. Zhang, T. S. Chu, H. F. Cheung, N. Wang and S. T. Lee, Phys. Rev. B, 2001, 64, 113304. 28. S. W. Cheng and H. F. Cheung, J. Appl. Phys., 2003, 94, 1190. 29. W. K. Burton, N. Cabrera and F. C. Frank, Nature, 1949, 163, 398. 30. J. J. Stejny, R. W. Trinder and J. Dlugosz, J. Mater. Sci., 1981, 16, 3161. 31. J. J. Stejny, R. W. Dlugosz and A. Keller, J. Mater. Sci., 1979, 14, 1291. 32. H. R. Kruyt and A. E. V. Arkel, Kolloid-­Z., 1928, 32, 29. 33. B. Gates, B. Mayers, B. Cattle and Y. Xia, Adv. Funct. Mater., 2002, 12, 219. 34. B. Mayers and Y. Xia, J. Mater. Chem., 2002, 12, 1875. 35. (a) B. Messer, J. H. Song, M. Huang, Y. Wu, F. Kim and P. Yang, Adv. Mater., 2000, 12, 1526; (b) J. Song, B. Messer, Y. Wu, H. Kind and P. Yang, J. Am. Chem. Soc., 2001, 123, 9714. 36. (a) C. R. Martin, Science, 1994, 266, 1961; (b) D. Almawlawi, C. Z. Liu and M. Moskovits, J. Mater. Res., 1994, 9, 1014; (c) M. Zheng, L. Zhang, X. Zhang, J. Zhang and G. Li, Chem. Phys. Lett., 2001, 334, 298. 37. Y. Xia, P. Yang, Y. Sun, Y. Wu, B. Mayers, B. Gates, Y. Yin, F. Kim and H. Yan, Adv. Mater., 2003, 15, 353. 38. C. N. R. Rao, A. Govindaraj, F. L. Deepak, N. A. Gunari and M. Nath, Appl. Phys. Lett., 2001, 78, 1853. 39. A. Govindaraj, F. L. Deepak, N. A. Gunari and C. N. R. Rao, Isr. J. Chem., 2001, 41, 23. 40. Y. Yin, Y. Lu, Y. Sun and Y. Xia, Nano Lett., 2002, 2, 427. 41. B. Gates, Y. Wu, Y. Yin, P. Yang and Y. Xia, J. Am. Chem. Soc., 2001, 123, 11500. 42. A. Govindaraj, B. C. Satishkumar, M. Nath and C. N. R. Rao, Chem. Mater., 2000, 12, 202.

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43. T. J. Trentler, K. M. Hickman, S. C. Geol, A. M. Viano, P. C. Gibbons and W. E. Buhro, Science, 1995, 270, 1791. 44. (a) T. J. Trentler, S. C. Goel, K. M. Hickman, A. M. Viano, M. Y. Chiang, A. M. Beatty, P. C. Gibbons and W. E. Buhro, J. Am. Chem. Soc., 1997, 119, 2172; (b) P. D. Markowitz, M. P. Zach, P. C. Gibbons, R. M. Penner and W. E. Buhro, J. Am. Chem. Soc., 2001, 123, 4502; (c) O. R. Lourie, C. R. Jones, B. M. Bartlett, P. C. Gibbons, R. S. Ruoff and W. E. Buhro, Chem. Mater., 2000, 12, 1808. 45. J. D. Holmes, K. P. Johnston, R. C. Doty and B. A. Korgel, Science, 2000, 287, 1471. 46. (a) L. Manna, E. C. Scher and A. P. Alivisatos, J. Am. Chem. Soc., 2000, 122, 12700; (b) C. Chao and C. Chen Lang, Chem. Mater., 2000, 12, 1516. 47. M. H. Huang, Y. Wu, H. Feick, N. Tran, E. Weber and P. Yang, Adv. Mater., 2001, 13, 113. 48. M. H. Huang, S. Mao, H. Feick, H. Yun, Y. Wu, H. Kind, E. Weber, R. Russo and P. Yang, Science, 2001, 292, 1897. 49. (a) Y. Huang, X. Duan, Q. Q. Wei and C. M. Lieber, Science, 2001, 291, 630; (b) Y. Wu, H. Yan, M. Huang, B. Messer, J. H. Song and P. Yang, Chem. -­Eur. J., 2002, 8, 1260. 50. F. Kim, S. Kwan, J. Arkana and P. Yang, J. Am. Chem. Soc., 2001, 123, 4360.

Chapter 6

Elemental Nanowires 6.1  Introduction A variety of elemental and alloy nanowires have been synthesized. In this chapter, we will deal with the synthesis strategies employed to make elemental and alloy nanowires.

6.2  Silicon Various methods have been employed to synthesize silicon nanowires (SiNWs), which include chemical vapor deposition (CVD), physical evaporation of the metal, SiOx and the use of precursors. Thermal evaporation was employed for the first time by Yu et al. to synthesize SiNWs,1 where a hot-­ pressed Si powder target mixed with Fe was sublimed at 1200 °C in flowing Ar gas at ∼100 Torr. By employing this simple method, they could obtain ∼15 nm diameter SiNWs, with length ranging from a few tens to hundreds of microns (Figure 6.1a). The selected-­area electron diffraction (SAED) pattern (see inset in the figure) resembles that of bulk silicon. About 2 nm of amorphous oxide layer covers the nanowires, and is etched out by dilute HF solution treatment. Figure 6.1b shows a typical TEM image of Si nanowires after the HF treatment. The diameter of the Si nanowires can be tuned by adjusting the ambient pressure from 150 to 600 Torr.2 With increasing gas pressure, the average diameter of the nanowires increases. By using a Si substrate with prepatterned iron, nanowires can be produced at the patterned positions by a thermal evaporation method.3 Using electron beam evaporation and lithography, 5 nm thick Fe film was patterned onto silicon substrates, and SiNWs selectively grown on them. Nanostructures of Si and SiOx (x = 1 to 2) can be obtained by thermal treatment of pure Si powder at 1373   Nanoscience & Nanotechnology Series No. 52 Nanotubes and Nanowires, 3rd Edition By C. N. R. Rao, A. Govindaraj and Leela Srinivas Panchakarla © C. N. R. Rao, A. Govindaraj and Leela Srinivas Panchakarla 2022 Published by the Royal Society of Chemistry, www.rsc.org

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Figure 6.1  (a)  TEM image of the SiNWs with an average diameter of around 15 nm.

The inset shows the SAED. (b) TEM of the SiNWs after etching the outer oxide layer in dilute HF. Reproduced from ref. 1 with permission from AIP Publishing, Copyright 1998.

K on quartz substrate coated with Fe(NO3)3 under Ar flow.4 This procedure yields tree-­like as well as tadpole-­like SiOx nanofibres with a Si core. Carbon-­assisted synthesis of single-­crystal SiNWs has been accomplished with Si powder as well as solid Si substrates.5 Here carbon is coated lightly on Si and the material heated to ∼1300 °C. The carbon reacts with the surface oxide to produce the sub-­oxide species. The VLS method employs relatively low temperatures to produce SiNWs and has been successful in producing nanowires in large quantities.6 Using Ga as the molten solvent, uniform diameter (∼6 nm) SiNWs were synthesised at direction.

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Figure 8.4  (A)  Typical cross-­sectional SEM image of GaN nanowires grown on a

silicon substrate. (B) Typical HREM image of a nearly defect-­free GaN nanowire. Inset in (B) shows its corresponding electron diffraction pattern. Reproduced from ref. 37 with permission from American Chemical Society, Copyright 2001.

GaN nanorods are obtained by a carbon nanotube-­confined reaction in which ammonia is reacted with Ga2O vapor.41 The nanowires are 4–50 nm in diameter and up to 25 microns in length. Heating Ga(acac)3 with activated carbon or carbon nanotubes at 910 °C in NH3 vapor over catalytic Fe/ Ni particles also yields GaN nanowires.6,42,43 Ga(acac)3 generates in situ fine Ga2O particles which react with the NH3 vapor. Use of SWNTs or a lower catalyst ratio helps to produce lower-­diameter nanowires. The diameter and length of the nanowires are 30–50 nm and 1–2 microns, respectively. This technique yields high quantities of single-­crystalline nanowires with a wurtzite structure.

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The VLS mechanism has been shown to synthesize high-­quality, ultra-­fine GaN nanowires using HFCVD, where liquid Ga placed over Ni-­coated p-­t ype Si substrate is heated in NH3 at 700 °C.44 CVD has also been carried out with a 30 Å Au-­coated Si/SiO2 substrate. On this substrate, GaN nanowires of 20–100 nm in diameter were grown using a source of Ga heated to 900° C in NH3 flow.45 Direct and precise regulation of the nanowire diameter is beneficial for electronic and optoelectronic properties. The process has been modified to create catalyst islands in specific patterned regions by using electron-­beam lithography to facilitate the growth of nanowires between two catalyst islands. In order to make nanowire-­integrated systems, position-­controlled growth is necessary. Devices can be made on a chip in parallel by utilizing the advantages of batch processing. Raman spectra reveal significant broadening of the bands due to phonon-­ confinement effects.37,46 Novel GaN nanostructures such as nanobelts are produced through the reaction of Ga/Ga2O3 and ammonia over large area Si substrates, using Ni/ Fe and B2O3 as catalysts. The nanobelt diameter varies from 100 nm to 1 µm with a thickness of approximately one-­tenth of the width.47,48 Reacting GaCl3 with NaN3 under solvothermal conditions gives an azide precursor that decomposes to nanocrystalline or amorphous GaN in superheated THF and toluene solvents near their critical points at or below 260 °C. The product include nanorods, nanoparticles and faceted crystallites.49 Starting from ball-­milled Ga2O3 and NH3, GaN nanowires and nanobelts were obtained over Si and LaAlO3 substrates.50 A SEM micrograph of GaN nanobelts obtained by this method is shown in Figure 8.5a. The thickness varies between the 10–30 nm and the lengths in the 10–50 µm range. A high-­ magnification image of a belt is shown in the inset in Figure 8.5a, revealing the morphology clearly. A nan-­ring and the other nanostructures shown in Figure 8.5b are produced by twisting the nanobelts. An HREM lattice image in Figure 8.5c displays 0.32 nm spacing corresponding to the distance between two [001] lattice planes. There are many defects (indicated by arrows) that result from the growth process. Pyrolysis of gallium dimethylamide and ferrocene in NH3 yields GaN nanorods.51 MOCVD using (N3)2Ga[(CH2)3NMe2] in the presence of pure N2 (100 sccm) at 950 °C yields oriented GaN nanopillars and randomly distributed nanowires. The suspended GaN nanopillars show a strong, broad emission peak ∼430 nm in the PL spectrum at room temperature.52 Using urea complexes formed with the trichlorides of Al, Ga and In, single-­crystalline GaN (as well as InN and AlN) nanowires can be grown on Si substrates filled with Au islands.53 The thermal evaporation of Ga2O3 powders in the presence of ammonia onto a Au coated Si wafer at 1423 K yields GaN nanowires.54 Direction-­dependent homoepitaxial growth of GaN nanowires (Figure 8.6) has been achieved by controlling the Ga flux during direct nitridation with NH3.55 When Ga droplets are nitridated at a high flux, GaN nanowires grow in the c-­direction (), while when the Ga flux is smaller, GaN nanowires grow in the a-­direction () (Figure 8.6d).

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Figure 8.5  (a)  SEM images of GaN nanobelts. Inset is the image of a belt with a

higher magnification. The scale bar corresponds to 200 nm. (b) A nanoring. (c) HREM image of a selected GaN nanobelt. Inset shows TEM image of the nanobelt. Reproduced from ref. 50 with permission from Elsevier, Copyright 2003.

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Figure 8.6  (a)  SEM images showing GaN nanowires with diameters less than 30

nm obtained from the direct reaction of Ga droplets and NH3. The inset shows the spontaneous nucleation and growth of multiple nanowires directly from a larger Ga droplet. (b) High-­resolution transmission electron microscopy (HREM) image of a GaN nanowire from the sample shown in (a) indicating that the growth direction is . The inset is a fast Fourier transform of the HREM image, and the zone axis is . (c) SEM image showing GaN nanowires with diameters less than 30 nm resulting from the reactive-­vapor transport of a controlled Ga flux in an NH3 atmosphere. (d) TEM image of a GaN nanowire from the sample shown in (c) indicating that the growth direction is . Reproduced from ref. 55 with permission from John Wiley and Sons, Copyright © 2006 WILEY-­VCH Verlag GmbH & Co. KGaA, Weinheim.

Conventional thermal CVD is used to synthesize composites of GaN nanorods coated with layers of graphitic carbon.56 GaN nanorods are first produced by treating Ga2O3 vapor with NH3 gas at 950 °C. In the second step, the nanorods are treated with methane at 900 °C in CVD to produce the graphitic layers. Insulating BN layers are also coated over GaN nanorods by heating amorphous B powder, Ga, Ga2O3 and an iron oxide catalyst in nitrogen followed by ammonia.57 Large-­scale GaN nanowires have been grown within AAM nanochannels starting with Ga and Ga2O3 using catalytic In nanoparticles.58

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The anisotropy in the growth rates of the {1–100} and the (0001) facets of GaN is used to grow 50 nm diameters GaN nanowires on patterned GaN/sapphire (0001) substrates by MOCVD.59 GaN NWs have been grown on silicon substrates by plasma-­assisted hot-­filament chemical vapor deposition using a thin gold film as catalyst, GaN powder as the Ga source, and hydrogen and nitrogen as the reactive gases.60 The nanowires have diameters in the range of 40–150 nm. Plasma-­enhanced chemical vapor deposition (PECVD) has been applied to grow GaN NWs using gallium oxide and nitrogen.61 The nanowires have a single crystalline wurtzite structure and have diameters and the lengths of 140 nm and 20 µm, respectively. The direct use of Ga and N2 in PECVD results in GaN nanowires on graphite substrates without the use of a catalyst.62 The polycrystalline nanowires have diameters ranging from 90 to 200 nm and lengths ranging from 4 to 20 µm. GaN NWs can be grown epitaxially on graphene without any catalyst using molecular beam epitaxy.63 The GaN nanowires grow vertically along the c-­axis and show an epitaxial relationship with graphene where the ⟨ 2110 ⟩GaN direction is parallel to the directions of the carbon zigzag chains. Plasma-­ assisted molecular-­beam epitaxy is used to grow all-­epitaxial and scalable growth of single-­crystalline GaN nanowires on graphene, grown on SiC.64 This techniques have also been applied to grow other nitride nanowires such as AlN and AlGaN.65 GaN NWs have also been grown epitaxially on (111) single-­crystalline diamond without any catalyst or buffer layer. In this case the in-­plane epitaxial relationship of ( 1010 )GaN ∥ (011̄ )Diamond was observed.66 Self-­assembled vertically aligned GaN nanowires on a flexible Ti foil are achieved by plasma-­assisted molecular-­beam epitaxy.67 This technique also helps to grow GN NWs on metallic TiN films without compramizing the quality of nanowires compared to those grown on Si substrates.68 The pulsed-­mode MOCVD growth technique has been used to grow position-­controlled GaN NW arrays.69 The orientation, location, diameter and length of each GaN nanowire are controlled in this process via pulsed-­ mode growth parameters such as precursor injection, growth temperature and interruption durations. The nanowires have diameters of more than 240 nm and lengths between 250 and 1250 nm. Very long GaN NWs have been prepared on c-­plane sapphire substrates by VLS coupled with a near-­equilibrium hydride vapor-­phase epitaxy technique.70 The nanowires have uniform diameters and show very low defects. This process gives an exceptional growth rate of 130 µm h−1. Ordered arrays of GaN NWs are achieved by selective area sublimation of prepatterned GaN(0001) layers.71 The nanowires have diameters from 50 to 90 nm and spacings between nanowires in the arrays are 0.1 to 0.7 µm. Guided growth of coherent, horizontal GaN NWs has been achieved on atomically flat singular SiC (0001) as well as on periodically stepped vicinal (cutting substrate with a slight tilt with respect to a low-­index crystallographic plane) SiC (0001) substrates.72 All nanowires show the same epitaxial relationship with both singular and vicinal (0001) substrates.

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Metal-­assisted chemical etching has been applied to prepare GaN NWs.73 In this process n-­t ype GaN wafers are electrochemically etched in a AgNO3/ HF solution where simultaneous deposition of Ag nanoparticles happens, which act as a catalyst and initiate the etching process. Optical properties of wurtzite GaN nanowires have been studied. A blue shift in the energy gap of nanowires is observed compared to bulk GaN. The effect of temperature on the energy gap is observed in bulk GaN, however this effect is weaker in the nanowires. The density of free carriers can be evaluated in the nanowires by the infrared response.74 Photoconductivity studies on GaN nanowires has been reported.75 In single-­crystalline GaN nanowires, ultraviolet-­blue laser activity was recorded using near-­field and far-­field optical microscopy to characterize radiation spectral properties and waveguide mode structure at room temperature.76 The far-­field image in Figure 8.7a shows laser emissions pumped optically (310 nm, 4.0 eV) from an individual, isolated GaN nanowire. Localizing bright emission at the nanowire ends indicates good waveguiding activity and the cavity modes are Fabry–Perot (axial) rather than whispering gallery modes. The image has low contrast below the lasing threshold and the PL spectrum is wide and without features (Figure 8.7b, curve A). Some sharp (3380 nm) is observed at high pump fluences, suggesting a change in the gain curve as a result of band-­gap renormalization. This is probably due to the creation of an electron–hole plasma, which is the dominant laser mechanism for GaN at high temperatures due to the weakly bound excitons (almost 25 meV) and high excitation-­intensity Coulombic screening. Figure 8.7c shows the dependence of the laser emission on pump fluence. Below the threshold, the PL dependency is linear, but at about 700 nJ cm−2, a superlinear rise in pump fluence emission strength occurs. This is characteristic of stimulated emission, and a log–log plot above the power-­dependence threshold shows a quadratic dependence on pump fluence (inset). Even at high excitation fluence (Figure 8.7c, filled circles), the power dependence of the PL from nonlasing GaN content is linear. Doping GaN nanowires with different elements is of great interest due to their tunable optical and magnetic properties. In the presence of single-­ walled carbon nanotubes, multi-­walled GaN nanowires doped with Mn have been produced by reaction of acetylacetonates with NH3 at 950 °C. The nanowires are considerably small in diameter (25 nm) and show ferrowmagnetic behavior.77 The coercivity in nanowires of GaMnN is higher than those observed in thin films. The Mn-­doped samples (diameters up to 50 nm) reveal a PL band around 420 nm, and a red-­shift due to hole-­doping (Figure 8.8a). However, the characteristic emission of Mn2+ is at ∼610 nm due to the 4T1 → 6A1 transition of Mn2+ (3d5). The FWHM of the Mn2+ emission band decreases

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Figure 8.7  Individual  isolated GaN nanowire laser. (a) Far-­field image of a single

GaN nanolaser. The nanowires was excited at about 3 µJ cm−2. Laser emission is seen at the ends of the nanowire, (b) photoluminescence spectrum (curve A) using 1 mW continuous-­wave excitation and lasing (curve B) using about 1 µJ cm−2 pulsed excitation, (c) power dependence of lasing near threshold (solid squares) and of photoluminescent emission from a nonlasing region (solid circles). Inset shows the logarithmic plot of lasing power dependence. Reproduced from ref. 76 with permission from Springer Nature, Copyright 2002.

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Figure 8.8  (a)  Photoluminescence spectra of GaMnN nanowires prepared from MWNTs. (b) Photoluminescence spectra of GaMnN nanowires prepared from SWNTs. The spectrum of the undoped nanowires is also shown in each of the cases. Mn2+ emission in the case of the 25 nm nanowires exhibits a blue shift compared to the bands of the larger-­diameter nanowires, as seen in (b). Reproduced from ref. 77a with permission from Elsevier, Copyright 2003.

as the concentration of Mn increases. A feature at 410 nm in the PL spectrum of 25 nm diameter GaN nanowires is shown in Figure 8.8b. In the spectra of the small-­diameter nanowires, the characteristic Mn2+ emission is blue-­ shifted. Mn emission intensity also increases with the Mn content. Si-­doped (n-­t ype) as well as Mg-­doped (p-­t ype) GaN nanowires have been produced.77 These nanowires show PL bands at 390 and 442 nm. Optical emission properties of a phosphor such as YAG : Ce-­coated Mg-­doped GaN nanowires have been studied (Figure 8.9). The Mg-­doped nanowire PL spectrum displays a band at 442 nm (curve (a) shown by filled circles in Figure 8.9), due to the recombination of the donor–acceptor (D–A) pair. The blue emission excites the coating of yellow phosphor, emitting yellow fluorescence (curve (b), solid line in Figure 8.9). The yellow and blue emissions from the phosphor and the Mg-­doped nanowire, respectively, result in white emission (curve (c), broken line in Figure 8.9). Field-­effect transistors (FETs) have been fabricated on individual GaN nanowires.78 Gate-­dependent measurements of electrical transport indicate that the GaNNWs are n-­t ype and that the FET conductance can be amplified by more than three orders of magnitude. At equal carrier concentrations, electron mobilities in the nanowires are slightly better than or equivalent to thin-­ film materials. These properties make nanowires suitable for components in

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Figure 8.9  White-­  light emitting Mg-­doped nanowire coated with YAG : Ce phos-

phor: curve. (a) In closed circles is the blue emission from the Mg-­doped GaN nanowire, curve (b) shown as a solid curve is the yellow emission from the YAG : Ce phosphor and curve (c) shown as a broken curve is the white-­light emission. Reproduced from ref. 77a with permission from Elsevier, Copyright 2003.

future nanoscale electronics. GaN NW FETs (Figure 8.10a) have been fabricated by dispersing a GaN NW suspension in ethanol on the surface of a Si/ SiO2 substrate. Here Si serves as a global back gate. Electron-­beam lithography is used to prepare source and drain electrodes. A set of typical current vs. source–drain voltage (I–Vsd) data acquired at different gate voltages (Vg) from a single GaN nanowire FET is shown in Figure 8.10b. The two-­terminal I–Vsd data are all linear, showing the ohmic contacts of the metal electrodes to the GaN NWs. The n-­t ype behavior of nominally undoped GaN originates from vacancies of nitrogen and/or oxygen. GaN NW FET transfer characteristics have been studied. The I vs. Vg plot for the FET measured at various source– drain voltages (Figure 8.10c) is indicative of the n-­t ype character of the device. Using metal–organic chemical vapor deposition (MOCVD), single-­crystalline GaN nanowire heterostructures, which show blue light emission, have been fabricated and they could be potential candidates in nanophotonics.79

8.4  InN By using azido-­indium precursors, InN fibers have been produced in solutions at temperatures as low as 203 °C. The InN fibers are polycrystalline and have diameters of ∼20 nm and lengths between 100 and 1000 nm. A solution–liquid–solid (SLS) mechanism is proposed for the growth of the nanowires, where metal nanodroplets generated in situ act as a catalyst for one-­dimensional growth.80 Mixtures of In2O3 powder and In metal heated at 700 °C under flowing NH3 produce nanowires with a single-­crystalline wurtzite structure (10–100 nm diameter) via the VS process (Figure 8.11a–c).81 InN

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Figure 8.10  (a)  Schematic of an NW FET, and (inset) FE-­SEM image of a GaN NW

FET, scale bar 2 µm. (b) Gate-­dependent I–Vsd data recorded on a 17.6 nm diameter GaN NW. The gate voltages for each I–Vsd curve are indicated. (c) I–Vg data recorded for values of Vsd = 0.1–1 V. (inset) Conductance, G, vs. gate voltage. Reproduced from ref. 78 with permission from American Chemical Society, Copyright 2002.

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Figure 8.11  (a)  A typical SEM image of InN nanowires, (b) TEM image and (c)

HREM image of an InN nanowire. Reproduced from ref. 81 with permission from the Royal Society of Chemistry.

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nanowires have been produced from the thermal evaporation of pure In in a NH3/N2 mixture on gold-­patterned silicon substrates.82 The diameters of the nanowires are in the range of 40–80 nm (depending on the size of the Au clusters). Typical lengths of the nanowires are about 5 µm. Nanowires grow through the VLS mechanism as evidenced by the observation of catalytic particles at the tips of the nanowires. Indium acetylacetonate vapor decomposition over Au islands coated on Si(100) substrates was used to create InN nanowires by reacting indium acetate with hexamethyldisilazane.83 The reaction of In2O3 powders in ammonia will produce large amounts of InN nanowires with standardized diameters.84 Nebulized spray pyrolysis has been used to create single-­crystalline GaN, AlN and InN nanowires on gold islands deposited on Si(100) substrates (NSP).83 A dilute (∼1 mM) solution of metal acetylacetonate in methanol is atomized and reacts with NH3. InN nanowires develop at a small temperature range of 585–600 °C, while GaN nanowires grow at temperatures ranging from 750 to 900 °C. The diameter of the nanowires is solely defined by the particle size of the catalyst (Au), while the length of the nanowires can be regulated by adjusting the deposition period. Figure 8.12 depicts SEM (Figure 8.12a and b) and TEM (Figure 8.12c and d) images of nanowires grown on a patterned Au-­ coated Si substrate. The low-­magnification SEM picture shows that nanowires expand only where Au is present (white circles with average diameter

Figure 8.12  (a,  b) SEM images of InN nanowires obtained by NSP, (c, d) TEM and

HREM images, respectively, of the nanowires. Top left inset in (c) shows the SAED pattern, bottom right inset in (c) shows a low magnification TEM image, and inset in (d) shows the HREM image at the tip of the nanowire. Reproduced from ref. 83 with permission from John Wiley and Sons, Copyright © 2005 WILEY-­VCH Verlag GmbH & Co. KGaA, Weinheim.

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0.6 nm). High-­magnification SEM photographs of such positions reveal the existence of a significant number of hexagonal InN nanowires (9% morphological yield), as reported by XRD. The nanowires are 10–50 nm in diameter and a few microns in length. They are single-­crystalline, as shown by the HREM picture (Figure 8.12d), which reveals fringes with a spacing of 0.27 nm, referring to the (101) planes of the wurtzite form, as well as the spots in the SAED pattern corresponding to the (100), (002) and (101) planes (inset in Figure 8.12c). The nanowires expand in the direction (the white arrow in Figure 8.12d shows the growth direction of the nanowire). GaN nanowires have a morphology identical to InN nanowires.85 On the basis of metal nanocluster-­catalyzed CVD, a general method for the synthesis of Mn-­doped nanowires of CdS, GaN and ZnS has been established.86 Localized laser-­assisted metal organic vapor-­phase epitaxy has been used to control the length of InN nanowires during growth. Gold nanoparticles serve as a catalyst and the laser irradiation time helps to control the length of the nanowires. Well-­ordered, vertically aligned and high aspect ratio InN nanorods are grown using selective area growth by vapor phase epitaxy approach at 640 °C using fom NH3 and InCl3 as nitrogen and In sources, respectively.87 The growth happens via openings in a SiNx masked Ga-­polar GaN/c-­Al2O3 template. Molecular-­beam epitaxy and plasma-­assisted molecular beam epitaxy (PA-­ MBE) have been used extensively to grow InN nanowires on substrates.88–94 Substrate temperature and choice of buffer layer play a major role in controlling the quality of the nanowires. Si is frequently used as substrate in this process. Chang et al. have grown InN nanowires on a Si (111) substrate using In as a seed. Position-­controlled InN nanowires have been grown on Si (111) substrates by PA-­MBE without any buffer layer. This process also allows the growth of nanowires on different substrates.95 In the Raman spectrum, InN nanowires exhibit bands at 445, 489 and 579 cm−1, which are attributed to the A1 (transverse optical), E2 and A1 (longitudinal optical) phonon modes of the wurtzite crystal structure.82 The room-­ temperature PL range of the nanowires reveals a wide band at 1.85 eV. The broad half-­width is caused primarily by thermal excitations and a wide size distribution.82 The characteristic absorption band of InN nanostructures has been determined to be about 0.7 eV, rather than the 1.85 eV previously mentioned.83,85

8.5  Si3N4 and Si2N2O Carbothermal reduction of silica in flowing N2+3% H2 around 1300 °C produces silicon nitride and β-­sialon whiskers.96 CNT templates have been used to prepare silicon nitride nanorods.97 A mixture of SiO2 and Si powders reacts with CNTs in a nitrogen atmosphere to yield nanorods of β-­Si3N4, α-­Si3N4 and Si2N2O, several microns long and 4–40 nm in diameter. Reaction of silicon oxide nanoparticles with active carbon in nitrogen flow at 1450 °C produces coaxial nanowires of length of 15 µm and diameter of 45 nm.98 The coaxial

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nanowires consist of a core of silicon nitride, with an amorphous outer shell of silicon and silicon dioxide. Si powder or Si/SiO2 mixtures heated at 1200 °C in N2 or NH3 produce Si3N4 nanowires of 10–70 nm diameter.99 The nanowires are single-­crystalline and covered with a layer of amorphous SiO2. Nanowires of α-­Si3N4 and of Si2N2O have been produced by reacting SiO2 gel with NH3 in the presence of activated carbon or carbon nanotubes and a Fe catalyst.6,100 The reaction yields Si3N4 nanowires as a major product when MWNTs are reacted with silica gel at 1360 °C for long periods. The composition of the final product depends on the duration of the reaction. The silica gel reacts with the nanotubes within 4 h in the presence of catalytic Fe particles, giving a good yield of pure Si3N4 nanowires without Si2N2O. When the catalyst has 0.1% Fe, a mixture of the α-­phase and hexagonalβ-­phase is obtained, while a catalyst with 0.5% Fe yields nearly monophasic nanowires of Si3N4. Figure 8.13a and b show SEM micrographs of Si3N4 nanowires produced using Fe catalysts. The reaction between silica and NH3 with MWNTs prepared by ferrocene decomposition at 1360 °C produces a mixture of α-­and β-­Si3N4. A SEM micrograph of product prepared by this method is shown in Figure 8.13c. The nanowires are hundreds of microns long and have a large diameter (5–7 µm).

8.6  Metal Carbide and Boride Nanowires 8.6.1  BC Plasma-­enhanced CVD of close-­1,2-­dicarbadodecanborane yields boron carbide nanowires along with nanoparticles.101 The carrier gas is argon and typical deposition temperatures are between 1100 and 1200 °C. Using a single molecular precursor 6,6′-­(CH2)6(B10H13)2 and a porous alumina template, aligned, monodisperse boron carbide nanofibers are generated.102 The SEM micrograph of the nanofibers in Figure 8.14 reveals the diameter to be ∼250 nm with a length of ∼45 µm. The fibers are amorphous up to 1000 °C and become crystalline on heating at 1025 °C. High-­purity BCNWs are produced by the thermal evaporation of C/B/B2O3 powders with or without a catalyst in an Ar atmosphere.103 Nanowires synthesized using an iron catalyst (via a VLS mechanism) have diameters in the range 10–30 nm, whereas nanowires prepared without a catalyst have diameters of 50–200 nm. Synthesis of carbide nanorods via a carbon nanotube-­confined reaction was demonstrated as early as 1995, by Lieber and coworkers.104 Nanorods of TiC, Fe3C, NbC, BCx and SiC had diameters in the range 2–30 nm with lengths up to 20 µm. A partial substitution reaction was used to produce B-­doped CNTs.105 At 1150 °C, reaction of CNTs with B powder produces boron carbide nanorods with the formula B4C.106 In the presence of Fe, BCNWs transform from linear to helical structures.107 Copyrolyzing methane and diborane at as low as 879 °C in a low-­pressure CVD system yields boron carbide nanowires using Fe or Ni as catalyst via a VLS growth mechanism. The nanowires have diameters in the range 15 and 90 nm with lengths up to 10 µm. The single-­crystalline boron carbide nanowires are covered with thin amorphous oxide sheaths.108

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Figure 8.13  SEM  images of Si3N4 nanowires prepared by the reaction of multi-­ walled carbon nanotubes with silica at 1360 °C for 4 h in the presence of 0.1% Fe and 0.5% Fe catalysts, respectively, are shown in (a) and (b). (c) SEM image of the Si3N4 nanowires prepared by the reaction of aligned multi-­walled carbon nanotubes with silica at 1360 °C for 4 h. Reproduced from ref. 100 with permission from the Royal Society of Chemistry.

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Figure 8.14  SEM  image of aligned boron carbide nanofibers obtained upon

pyrolysis of a filled template at 1025 °C, followed by dissolution of the alumina matrix. Reproduced from ref. 102 with permission from American Chemical Society, Copyright 2000.

Figure 8.15  SEM  image of SiC nanowires obtained by the laser ablation of a SiC target. Reproduced from ref. 109 with permission from John Wiley and Sons, Copyright 2004 The American Ceramic Society.

8.6.2  SiC Laser ablation of a SiC target yields SiC nanowires at 900 °C via a VLS growth mechanism.109 Figure 8.15 shows an SEM micrograph of SiCNWs produced by this method. Carbothermal reduction of silica xerogels containing carbon nanoparticles in an Ar atmosphere yields β-­SiC nanorods with and without

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491 110

amorphous silica layers on heating at 1650 and 1800 °C, respectively. Heating a mixture of Fe nanoparticles embedded in silica with activated carbon produces β-­SiC nanowires.111 The nanowires have a core–shell structure comprising a crystalline SiC core covered with an amorphous silicon oxide layer. The nanowires have diameters between 10 and 30 nm and lengths of several tens of microns. Several approaches have been used by Gundiah et al.100 to produce SiC nanowires. Heating silica gel at 1360 °C with activated carbon in H2 or NH3 yields β-­SiC nanowires. An SEM image of SiCNWs produced by heating silica gel at 1360 °C for 7 h with activated carbon in NH3 is shown in Figure 8.16a. Figure 8.16b shows an HREM image of a single nanowire, which reveals (111) planes. The Bragg spots in the SAED pattern (shown in inset) correspond to the (111) planes, with some streaking due to stacking faults.

Figure 8.16  β-­  SiC nanowires obtained by heating a gel containing activated carbon and silica at 1360 °C for 7 h: (a) SEM image and (b) HREM of a single nanowire. The inset shows the SAED pattern. The arrow denotes the normal to the (111) plane and the direction of growth of the nanowire. Reproduced from ref. 100 with permission from the Royal Society of Chemistry.

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Silicon–carbon nanotubes and nanowires of various structures and shapes were produced by reacting Si (prepared by the disproportionation of SiO) at different temperatures with MWNTs.112 β-­SiC nanowires are produced by reacting Si with CCl4 in the presence of Na at 700 °C.113 Heating SiC powder in Ar at 1700 °C in the presence of an Al catalyst, yields SiC nanowires via a VLS mechanism.114 These nanowires have diameters between 10 and 50 nm and are 1–2 µm long. Using a similar procedure, nanojunctions of SiC were produced.115 Helical SiC nanowires coated with a silicon oxide sheath have been synthesized by the CVD technique.116 The diameter of the SiC core is between 10 and 40 nm (with a helical periodicity of 40 to 80 nm) and the amorphous silica layer is 30–60 nm thick. BN-­coated β-­SiC nanowires have been synthesized by a VLS process.117 A carbon nanotube-­confined reaction of aligned CNTs with SiO produces oriented SiCNWs.118 Figure 8.17 shows a SEM image of the nanowires (2 mm length, 10–40 nm diameter). The field-­ emission properties of these nanowires have been reported. At applied fields as low as 2.5–3.5 V µm−1, emission current densities are ∼10 µA cm−2. Aligned SiCNWs are also grown from Si substrates via a catalytic reaction at 1100 °C with a methane–H2 mixture.119 Needle-­shaped SiCNWs have been produced by the thermal evaporation of SiC powders over an Fe catalyst.120 SiCNWs have also been prepared on a SiC thin film coated with a thin Ni catalyst layer via a CVD route at 950 °C.121 β-­SiC nanowires are produced on graphite flake surfaces by partly reacting silicon powders in NaCl–NaF salts at 1150–1400 °C in argon. The nanowires have diameters in the range of 10–50 nm with various lengths.122 SiC NWs have been synthesized via carbothermal reduction using electronic waste (e-­waste) components at 1550 °C in an inert atmosphere. A computer monitor served as a silica source and the computer's plastic shell as a carbon source. The nanowires had diameters in the range 30–200 nm with lengths up to 10 µm.123 A hybrid anodic and metal‐assisted chemical etching method has been developed to grow SiC NWs under atmospheric pressure at room temperature.124 In this process SiC wafers are coated with a 3 nm layer of Pt and these substrates are etched electrochemically using aqueous HF and H2O2. A one-­ step electrosynthesis process was developed to produce SiCNWs from silica/ carbon precursors in molten CaCl2 at 900 °C under Ar atmosphere.125 In this process ∼3.1 V dc current was applied between a cathode pellet and a graphite anode that are assembled in an alumina crucible. The diameters of the nanowires are between 30 and 50 nm. A large-­scale flexible SiC sponge consisting of nanowires has been produced in a carbon thermal reduction of gangue at 1500 °C for 2 h in an Ar atmosphere.126 The nanowires are cubic crystalline and having diameters between 100 and 500 nm. Vertically aligned SiCNWs have been synthesized using Si powder and SiO2-­infiltrated vertically aligned carbon nanotubes at 1150–1350 °C.127

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Figure 8.17  SEM  images of oriented SiC nanowires. (a) Low-­magnification image

of an oriented SiC array. (b) High-­magnification image of the oriented SiC array. The particles present on the array surface are silicon-­ containing particles formed and deposited during the reaction. (c) Bottom surface of the SiC nanowire array showing a high density of well-­separated, oriented nanowire tips. Reproduced from ref. 118 with permission from John Wiley and Sons, Copyright © 2000 WILEY-­VCH Verlag GmbH, Weinheim, Fed. Rep. of Germany.

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CVD was used to create self-­assembled multi-­coaxial SiC nanowires inside wires.128 Thermal grooving along grain boundaries is caused by recrystallization of the copper substrate prior to vapor deposition, providing a natural template for self-­assembled development of nanowires. The heart of the wires generated is made up of various faceted SiC nanowires with diameters ranging from 20 to 50 nm, which are encapsulated by wider hollow wires with diameters ranging from 0.2 to 1.0 nm. The optical and electrical transport properties of SiC nanowires, which are n-­t ype semiconductors, have been investigated.129 The nanowires may be used as sensors, detectors and actuators. The field-­emission properties of SiC nanowires have been identified.120

8.6.3  Other Carbide Nanowires Mo2C nanowires can be synthesized by calcination of a Mo3O10(C6H5NH3)2·2H2O precursor at 750 °C for 5 h.130 The precursor is synthesized by reacting (NH4)6Mo7O24·4H2O with aniline under acidic conditions at 50 °C for 4 h. These nanowires are further doped with Co to make efficient electrocatalysts in hydrogen evolution reactions in both acidic and basic conditions. Nanoporous Mo2C nanowires have been synthesized by calcining MoOx/amine hybrid nanowires with sub-­nanometer periodic structures at 725 °C.131,132 WC nanowires integrated with nanosized Mo2C have been synthesized by a combination of a hydrothermal method and a carburization route.133 These composite nanowires are realized in an interwoven nanostructure. TiC nanowires were produced using a chloride-­assisted carbothermal reaction with TiO2, sucrose and NaCl as precursors and Ni(NO3)26H2O as a catalyst. The nanowires had diameters in the range of 200–400 nm with lengths extending up to tens of micrometers.134 Single-­crystalline HfC nanowires have been synthesized by a CVD method at 1280 °C using Ni as a catalyst.135 HfCl4 and methane gas were used as Hf and C sources, respectively. The nanowires have diameters of several tens of nanometers and lengths of several tens of microns and grow via a VLS mechanism. Bimetallic Ni–Mo carbide nanowires on carbon cloth have been synthesized by an electropolymerization-­assisted method starting from NiMoO4 nanowires.136 In this process NiMoO4 nanowires are prepared on carbon cloth by a hydrothermal method, and further coated with polypyrrole by electropolymerization. These are transformed into Ni3Mo3C nanowires by a carbothermal reduction reaction. These nanowires have shown to be potential electrocatalysts for hydrogen evolution reactions.

8.6.4  Borides The reaction of rare-­earth chlorides with BCl3 in the presence of hydrogen results in the formation of single-­crystalline nanowires of metal borides (MxBy, x = 1,3 and y = 1,6) such as LaB6, CeB6 and GdB6 over a Si substrate.137 Heating B and B2O3 powders with LaCl3 powders at 1000–1100 °C in a mixed gas of Ar and H2 under CVD conditions yields LaB6 nanowires.138

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SmB6 nanobelts and nanowires are prepared by a chemical vapor deposition technique at 1100 °C under a mixed gas of Ar and H2.139 In this process B, B2O3 and anhydrous SmCl3 powders are used as precursors and a Ni-­coated Si substrate as catalyst. The nanowires have diameters ranging from 20 to 110 nm with lengths reaching up to several hundreds of micrometers. A VLS mechanism has been proposed for the growth of the nanowires, while the synergistic effect of VLS and VS mechanisms are thought to be responsible for nanobelt formation. SmB6 nanowires have also been produced without using a catalyst, where heating acid-­washed SmB6 particles deposited on a Si plate at 1100 °C was found to grow nanowires via aVS mechanism.140 Heating powders of Sm, Mg, H3BO3 and I2 in an autoclave in inert conditions at 220–240 °C produces SmB6 nanowires. The nanowires have diameters ranging from 50 to 120 nm, with lengths varying from 1 to 8 µm.141 Figure 8.18a shows a SEM image of SmB6 nanowires synthesized at 240 °C. The TEM image of nanowires in Figure 8.18b shows uniform diffraction contrast indicating the single-­crystalline nature

Figure 8.18  SmB  6 nanowires prepared at 240 °C, 6 h: (a) SEM image, (b) TEM

image, (c) HREM image, (d) SAED pattern. Reproduced from ref. 141 with permission from the Royal Society of Chemistry.

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of the nanowires. The corresponding nanowire HREM images in Figure 8.18c and SAED patterns in Figure 8.18d also confirm the single-­crystalline nature. Nanowires grow along the [001] direction. Using a similar high-­pressure solid–state reaction, CeB6 nanowires are produced starting from Ce, Mg, H3BO3 and I2 powders. Here Mg serves as a reducing agent to produce elemental B from H3BO3. The reaction is conducted in an autoclave at 260 °C and kept for 12 h using I2 as a catalyst.142 ErB6 nanowires have been grown on TiN-­coated Si substrates decorated with Pd nanoparticles by chemical vapor deposition at 900–925 °C using ErCl3·6H2O and gaseous decaborane as precursors.143 The nanowires show a cubic crystal structure and have high aspect ratios with lengths up to several micrometers and diameters between 30 and 150 nm. Fe3B nanowires were produced on sapphire substrates coated with Pt and Pd (Pt/Pd) by CVD at 800 °C, starting with BiI3 and FeI2.144 The morphology of the Fe3B nanowires could be controlled by manipulating the Pt/Pd film thickness and growth time, the typical diameters were 5–50 nm with lengths ranging from 2–30 µm. Fe2B nanowires are prepared by a simple chemical reduction route under an applied magnetic field. These nanowires deposited onto three-­dimensional nickel foam show promising electrocatalytic activity towards oxygen evolution reactions in basic conditions.145 PtB nanowires have been produced by the dc arc-­discharge method using a mixture of boron nitride (BN) and platinum (Pt) powders. The nanowires have diameters of 30–50 nm with lengths of 20–30 µm.146 Superhard W0.5Ta0.5B nanowires are synthesized in an aluminum flux at ambient pressures starting from W, Ta and B metal powders. Controlling the flux-­to-­metal-­boride ratio controls the aspect ratio of the nanowires.147 Oriented assemblies of TaB nanowires have been prepared by a controlled thermal process starting from Ta and BN. The nanowires have diameters in the range 60–100 nm.148 MgB2 nanowires have been prepared by the sol–gel technique using MgBr2·6H2O, CTAB and NaBH4 followed by heating the gel at around 810 °C. The nanorods have diameters in the range of 50–100 nm.149

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131. Q. Gao, C. Zhang, S. Xie, W. Hua, Y. Zhang, N. Ren, H. Xu and Y. Tang, Chem. Mater., 2009, 21, 5560. 132. L. Liao, S. Wang, J. Xiao, X. Bian, Y. Zhang, M. D. Scanlon, X. Hu, Y. Tang, B. Liu and H. H. Girault, Energy Environ. Sci., 2014, 7, 387. 133. P. Xiao, X. Ge, H. Wang, Z. Liu, A. Fish and X. Wang, Adv. Funct. Mater., 2015, 25, 1520. 134. X. Yuan, L. Cheng, L. Kong, X. Yin and L. Zhang, J. Alloys Compd., 2014, 596, 132. 135. J. Yuan, H. Zhang, J. Tang, N. Shinya, K. Nakajima and L. C. Qin, J. Am. Ceram. Soc., 2012, 95, 2352. 136. L. Guo, J. Wang, X. Teng, Y. Liu, X. He and Z. Chen, ChemSusChem, 2018, 11, 2717. 137. H. Zhang, Q. Zhang, G. Zhao, J. Tang, O. Zhou and L.-­C. Qin, J. Am. Chem. Soc., 2005, 127, 13120. 138. Q. Xue, Y. Tian, S. Z. Deng, Y. Huang, M. S. Zhu, Z. X. Pei, H. F. Li, F. Liu and C. Y. Zhi, Mater. Today Energy, 2018, 10, 28. 139. H. Gan, B. Ye, T. Zhang, N. Xu, H. He, S. Deng and F. Liu, Cryst. Growth Des., 2019, 19, 845. 140. Y. Chu, Y. Cui, S. Huang, Y. Xing and H. Xu, Nanomater, 2019, 9, 1062. 141. W. Han, Y. Qiu, Y. Zhao, H. Zhang, J. Chen, S. Sun, L. Lan, Q. Fan and Q. Li, CrystEngComm, 2016, 18, 7934. 142. Z. Wang, W. Han, Q. Kuang and Y. Zhao, Adv. Powder Technol., 2020, 31, 595. 143. Z. C. Gernhart, R. M. Jacobberger, L. Wang, J. R. Brew, M. A. Dar, D. R. Diercks, W. N. Mei and C. L. Cheung, J. Am. Ceram. Soc., 2012, 95, 3992. 144. Y. Li, E. Tevaarwerk and R. P. H. Chang, Chem. Mater., 2006, 18, 2552. 145. Q. Liu, H. Zhao, M. Jiang, Q. Kang, W. Zhou, P. Wang and F. Zhou, J. Mater. Chem. A, 2020, 8, 13638. 146. Z.-­H. Ding, L.-­X. Qiu, J. Zhang, B. Yao, T. Cui, W.-­M. Guan, W.-­T. Zheng, W.-­Q. Wang, X.-­D. Zhao and X.-­Y. Liu, J. Alloys Compd., 2012, 521, 66. 147. M. T. Yeung, G. Akopov, C.-­W. W. Lin, D. J. King, R. L. Li, Z. C. Sobell, R. Mohammadi and R. B. Kaner, Appl. Phys. Lett., 2016, 109, 203107. 148. Y.-­S. Zhou, Y.-­C. Zhu, W. Dong and J.-­J. Chen, Micro Nano Lett., 2012, 7, 936. 149. Shumaila, M. Alam, A. M. Siddiqui and M. Husain, Polym. Int., 2014, 63, 1465.

Chapter 9

Nanowires of Metal Chalcogenides, Phosphides and Other Semiconductor Materials 9.1  Metal Chalcogenide 9.1.1  CdS The nucleation and growth of CdS nanowires can be achieved by the thermal evaporation of CdS nanoparticles via a VS route.1 Thermal evaporation of CdS powders produces CdS nanowires in the presence of an Au catalyst,2 with diameters of 60–80 nm and several tens of micrometres in length. The nanowires exhibit strong red emission centered around 750 nm, attributed to the surface states.2 Simple thermal evaporation produces a variety of CdS 1D nanostructures on Si substrates.3 By tuning the experimental parameters such as the position of the substrate and temperature, one can control the shapes of the 1D nanoforms. By electrochemical deposition in AAMs, uniform arrays are obtained.4 The diameter of the nanowires is as low as 9 nm, while the length is in the 1 µm range. CdS nanowires exhibit strong features in resonant Raman spectra depending on the particle size. Electrochemical deposition of CdCl2 and thioacetamide or analogous precursors also yields single-­crystalline CdS nanowires within the pores of AAM templates.5,6 The nanowires exhibit Raman bands resultant from the first-­, second-­, and third-­order transverse optical (TO) phonon modes at 304, 606 and 908 cm−1, respectively. CdS nanowires have been synthesized by using other methods, which include a template-­based sol–gel synthesis,7 using an Au/Si substrate8 and sulfurization of Cd electrodeposited within nanopores.9   Nanoscience & Nanotechnology Series No. 52 Nanotubes and Nanowires, 3rd Edition By C. N. R. Rao, A. Govindaraj and Leela Srinivas Panchakarla © C. N. R. Rao, A. Govindaraj and Leela Srinivas Panchakarla 2022 Published by the Royal Society of Chemistry, www.rsc.org

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An ion-­exchange process is employed to synthesize mesoporous CdS nanorods starting from nanocrystallites of CdS as precursors.10 The mesoporous nanorods exhibit bands at 296 and 594 cm−1 in the resonant Raman spectrum due to the fundamental and the overtone of the longitudinal optical phonon mode. Mesoporous CdS nanorods show fairly high specific surface areas (e.g. 57.5 m2 g−1). CdS, CdSe and CdTe nanorods have been synthesized by employing a nonaqueous synthetic route by reacting elemental precursors in ethylenediamine at 120–190 °C.11 Oriented nanocrystal bonding may be used to build one-­dimensional as well as complicated nanostructures. Surfactants were used to build nanotubes and nanowires of II–VI semiconductors.12 CdS nanorods and nanotubes, as well as other materials developed in this manner, are generally nanocrystals. Using triton X-­100 [t-­octyl–C6H4–(OCH2CH2)xOH] (x = 9, 10) as the surfactant, high aspect ratio CdS nanowires were obtained.12,13 The SEM micrographs of the hexagonal CdS nanowires in Figure 9.1a were prepared by using a low concentration of Triton X-­100. Figures 9.1b–d show TEM images of the CdS nanowires. These nanowires have lengths up to 4 µm and diameter in the 80–160 nm range. The inset of Figure 9.1c shows the ED pattern, revealing the nanowire to be polycrystalline, the spacing of 3.66 Å corresponding to the (100) planes. The absorption maximum of surfactant-­assisted CdS nanowires shows a blue shift as compared with the bulk.13 In CuS, ZnS

Figure 9.1  (a)  SEM image of CdS nanowires (80–160 nm diameter) obtained by using Triton X-­100 as the surfactant, (b), (c) and (d) TEM images of the CdS nanowires. Inset in (c) shows the ED pattern of the nanowires. Reproduced from ref. 13 with permission from John Wiley and Sons, Copyright © 2001 WILEY-­VCH Verlag GmbH & Co. KGaA, Weinheim.

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and CdS nanorods have been made with hydrogels as templates, where oriented attachment-­like growth has been observed.14 The nanotubes or nanorods of CdS and other materials obtained in this process mostly consist of nanoparticles. CdS, CdSe, PbSe and ZnSe nanorods were synthesized using n-­butylamine.15 An in situ micelle–template interface reaction route was employed to prepare CdS nanowires.16 The authors could estimate the diameter of the nanowires by the absorption peak. The peak at 452 nm in the UV-­vis absorption spectrum was indicative of CdS nanowires with an average diameter of 5 nm (which are quantum-­confined). Size-­tunable CdS nanorods were achieved at low temperatures (25–65 °C) through the reaction of air-­insensitive precursors such as cadmium acetate and sodium sulfide with (EO)x(PO)y(EO)x triblock copolymers as surfactant templates in the aqueous phase.17 CdS nanorods have been synthesized by a single-­source cadmium thiosemicarbazide complex.18 The nanorods exhibit a slightly blue-­shifted absorption band edge at 490 nm and in the PL spectrum at 515 nm.18 Carboxylic acid-­functionalized monolayers can be used to grow long CdS nanowires in situ on a substrate with lengths ranging from several micrometers to tens of micrometres and widths of 70–80 nm.19 Reaction of cadmium nitrate with thiourea in ethylenediamine under solvothermal conditions yields CdS nanorods.20 A solvothermal treatment of CdCl2 and thiourea in the presence of ethylenediamine and dodecylthiol at 160 °C yields multi-­ armed CdS nanorods.21 The PL spectrum of multi-­armed CdS nanorods exhibits a strong self-­activated emission peak at 718 nm. This band blue-­ shifts with decreasing arm length and shifts to the red with an increase in the length.22 Injecting octylamine solutions of anhydrous cadmium acetate and sulfur into hexadecylamine yields nanorods of luminescent cubic CdS.23 CdS nanohelices were generated by mineralizing supramolecular organic ribbons.24 By integrating semiconductor semimagnetic nanowires into SiO2 MCM-­41 hosts, ordered arrays of magnetic Cd1−xMnxS nanowires with high crystallinity are obtained.25 The PL spectrum of Cd0.985Mn0.015S in MCM-­41 exhibits an IR band at 1.38 eV as well as a red band at 2.02 eV, which could arise from the internal Mn2+ (3d5) transition (4T1 → 6A1).25 Raman spectrum of CdS nanowires in AAMs shows a small blue shift with decreasing particle size due to quantum containment. The lack of polarization dependence is attributed to the low aspect ratios of microcrystallites in the nanowires.26 Using a modified template-­directed technique, Zhang and Wong27 have presented the preparation and characterization of (a) isolated, individual motifs and (b) crystalline and semiconducting transition metal sulfide (CuS, PbS, and CdS) nanowire arrays. According to their findings, photocatalytic degradation of hexagonal-­phase CdS cactus-­like nanotubes is higher than that of cubic CdS nanowires and industrial bulk equivalents. Jung et al.28 described the fabrication of three-­dimensional branched single-­ crystalline nanowire heterostructures with CdS and ZnS branches and backbones, respectively.

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9.1.2  CdSe and CdTe A hydrothermal method has been used for the synthesis of CdSe nanorods and dendritic fractals, starting from cadmium nitrate and sodium selenite.29 CdSe fractals and nanorods exhibit absorption edges at 1.78 and 1.68 eV in comparison to the bulk bandgap of 1.74 eV.29 The Cd–Se–Cd vibration gives rise a strong band at 209 cm−1 in the Raman spectrum.29 A poly(vinyl alcohol)-­ assisted solvothermal method using ethylenediamine has been employed at ∼170 °C to produce nanowires of CdSe and CdTe with a zinc blende structure.30 Monodisperse CdSe nanorods are prepared at 160 °C, using cadmium naphthenate.31 These nanorods show an emission maximum in the PL spectrum between 500 and 600 nm. The macroscopic alignment of CdSe nanorod superlattice structures in a nematic liquid-­crystalline phase has been determined. These phases were found after depositing the nanorods on the substrate and observing the phases up to complete solvent evaporation.32 Because external fields or pretreated surfaces influence the liquid-­crystalline phases, a high degree of control can be achieved over the deposited films of nanorods. Direct current electrodeposition within AAM pores can be used to produce nanowire arrays of group II–VI semiconductors such as CdSe and CdTe.33–35 By using Triton X-­100 as the surfactant, high aspect ratio polycrystalline CdSe nanowires are obtained.12,13 Compared to bulk CdSe, these nanowires show a blue shift of the absorption maximum. By employing e-­beam lithography along with electrochemical deposition techniques, CdSe nanopillar 2D arrays with a large height-­to-­width ratio are produced.36 By injecting molecular precursors into a hot surfactant, colloidal quantum rods of CdSe have been synthesized.37 With time, the aspect ratios of CdSe nanorods change from 1.9 to 3.8 and, consequently, the polarization, radiative lifetime and the global Stokes shift changes reveal the transformation of the electronic structure from a 0D quantum dot into a 1D quantum wire.37 Treating CdS nanowires with Se in tributylphosphine for 24 h at 100 °C produces core–shell CdS/CdSe nanowires with diameters of 7.7/0.75 nm.38 The profile of the absorption spectrum of CdS/CdSe core–shell nanowires is slightly different from that of CdS nanowires.38 The major component of the core–shell structure generally dominates the absorption spectrum. The PL spectra of CdS nanowires and CdS/CdSe core–shell nanowires are different. In the latter case, the shell quenches the CdS nanowire fluorescence completely. The fluorescence is restored nearly to previous levels when excess Cd2+ cations are introduced into a dispersion of CdS/CdSe nanowires.38 It has been reasoned that excess Cd2+ destroys nonradiative sites on the surface leading to the restoration of fluorescence. Photoconductive metal–CdSe–metal (metal = Ni, Au) nanowires have been prepared by template-­based electrochemical process. Either porous Al2O3 (pore size ∼350 nm) or polycarbonate track etch membranes (pore size ∼700 nm) are used as templates.39 In this process, metal, CdSe and metal are sequencially electrodeposited into the pores. Free-­standing metal–CdSe–metal nanowires are obtained by etching the template. Spontaneous reorganization of CdTe

Nanowires of Metal Chalcogenides, Phosphides and Other Semiconductor Materials 507

nanoparticles into luminescent CdTe nanowires are observed by controlled removal of an organic stabilizer protective shell.40 The driving force for self-­ organization of nanoparticles is believed to be strong dipole–dipole interactions. During the self-­organization of CdTe nanoparticles into a nanowire, a red shift of the emission band occurs.40 By controlling the nature of the precursor and the monomer concentration, shape-­evolution and shape-­control of colloidal semiconductor CdSe nanocrystals can be achieved.41 Figure 9.2 shows CdSe nanorods generated by redox-­assisted asymmetric Ostwald ripening of CdSe dots to rods.42 The cation-­exchange method was used to create CdSe nanowires.43 Ag2Se nanowires are converted to single-­crystal CdSe nanowires using the cation-­exchange reaction between Ag2+ and Cd2+. At low

Figure 9.2  Representative  low-­ and high-­magnification TEM images of CdSe NCs

before (a,b) and after (c–f) annealing at 135 °C in 3-­amino-­1-­propanol/ H2O (v/v = 9/1) with 0.1 M CdCl2 for 48 h. Representative HREM images of rods (e–h) depict their zinc blende (ZB) tip(s) and stacking faults along the 002 axis. Scale bar = 2 nm. Reproduced from ref. 42 with permission from American Chemical Society, Copyright 2006.

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temperatures, a single-­source molecular precursor has been used to make blue-­emitting cubic CdSe nanorods (2.5 nm diameter and 12 nm length).44 Alivisatos and coworkers have synthesized arrow-­, teardrop-­and tetrapod-­ shaped CdSe nanocrystals by thermal decomposition of the organometallic precursors.45 This has been followed by the synthesis of tetrapod structures of chalcogenides by other workers. The charge-­separating properties of branched tetrapods with CdSe core tetrapods and terminal CdTe branches are unique.46 Under identical circumstances, CdSe nanorods and CdTe tetrapods all emit well-­defined bandgap luminescence. Electrons seem to be localized in CdSe tetrapods. A high aspect ratio organometallic preparation of CdTe nanowires has been identified.47 Guided growth of horizontal CdSe nanowires, which are orientationally controlled, are achieved on several planes of sapphire.48 The crystallographic orientations as well as the growth direction have been controlled by a graphoepitaxial effect of nanosteps and grooves on the surface and the epitaxial relationship with the substrate. In this process a two-­zone horizontal tube furnace is used where CdSe is evaporated in the first stage at 800 °C and the growth of the nanowires happens in the second furnace on a gold-­coated sapphire wafer, which is held at 600 °C. Nanowires grow via a VLS process. High-­density epitaxial growth of CdSe and CdS nanorod arrays on selective facets of 2D hexagonal-­shaped nanoplates has been achieved by a seeded growth approach.49 By tuning compositions and crystal structures of 2D seeds, selective growth of nanorods can be achieved either on one basal facet or both basal facets as well as on both basal facets and six side facets.

9.1.3  PbS, PbSe and PbTe A biphasic solvothermal interface reaction is employed to synthesize rod-­ like PbS nanocrystals at 140–160 °C.50 The nanorods have lengths up to several micrometres and diameters of 30–160 nm. PbS nanowires are prepared inside mesoporous silica SBA-­15 channels. The nanowires are 6 nm in diameter and several micrometres in length.51 PbS-­wire-­SBA-­15 shows a band-­edge emission maximum around 665 nm in the PL spectrum, corresponding to 1.87 eV, which is blue-­shifted compared to the bulk PbS (0.41 eV), due to a strong quantum size effect. Reacting Pb(NO3)2 with Se powder at 60 °C yields PbS nanowires under ambient pressure with an average size of 8 × 350 nm.52 Atmospheric-­pressure CVD was used to obtain uniform PbS nanowire arrays and networks.53 The reaction of PbCl2 with Se in the presence of KBH4 in ethylenediamine produces high aspect ratio PbSe nanowires at room temperature.54 Oriented attachment of nanoparticles can produce PbSe nanowires as well as complex 1D nanostructures (Figure 9.3).55 Starting with a lead salt and TeO2 along with nitrilotriacetic acid, sonoelectrochemical methods are used to create PbTe nanorods with diameters of less than 10 nm.56 PbTe nanowires are synthesized on a large scale with high yield by a two-­step solution-­based synthesis method.57,58 In this method Te nanowires are produced in the first step by reducing TeO2 with N2H4 in the presence of polyvinylpyrrolidone KOH, and ethylene glycol at 120 °C. In the

Nanowires of Metal Chalcogenides, Phosphides and Other Semiconductor Materials 509

Figure 9.3  (a)  Star-­shape PbSe nanocrystals and (b–e) radially branched nanowires.

(d) TEM image of the (100) view of the branched nanowire and (inset) the corresponding selected-­area electron diffraction pattern. (e) TEM image of the (110) view of the branched nanowire and (inset) the corresponding selected-­area electron diffraction pattern. Reproduced from ref. 55 with permission from American Chemical Society, Copyright 2005.

second step, Te nanowires are converted to PbTe nanowires by reacting with Pb(CH3CO2)2·3H2O/polyvinylpyrrolidone solution in ethylene glycol at 120 °C. The diameter of the PbTe nanowires is approximately 12 nm. The CVD technique has been used to grow PbTe nanowires.59 In this method, Te is evaporated at 400 °C and the vapors are carried by a mixture of Ar(90%)/H2(10%) gas onto Pb film, which is heated at 300 °C and the tube furnace maintained at 50 Torr. In the case of chemical vapor phase formation of PbS60 and PbSe61 nanowires, dislocation-­driven nanowire growth has been observed. Chirality is introduced into the structure by the axial screw dislocation elastic tension, which results in an Eshelby twist. Zhu et al.62 reported the synthesis and electrical properties of hyper-­branched PbSe nanowire networks. Hyper-­branched PbSe nanowire networks are obtained via a VLS mechanism. Constantly supplying the PbSe reactant with the vapor of a low-­melting-­point metal catalyst such as

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510

In, Ga or Bi causes branching. Standard orientation relationships can be seen in the branches. Hyperbranched networks can be grown epitaxially on NaCl. Electrical measurements through the branched nanowires show development of charge carrier transport with distance and degree of branching.

9.1.4  CuS and CuSe By using Na-­AOT (sodium bis(2-­ethylhexyl)sulphosuccinate) or Triton X-­100 as the surfactant, high aspect ratio nanowires of CuS and CuSe have been prepared.13,63 SEM and TEM micrographs of CuS nanowires produced by employing AOT as the surfactant are shown in Figures 9.4a and b. The lengths of the nanowires extend up to 0.2 µm or longer and the average diameter varies between 5 and 20 nm. SEM and TEM micrographs of CuS nanowires produced by employing Triton X-­100 as the surfactant are shown in Figures 9.4c and d. The lengths of the nanowires vary between 150 and 900 nm and average diameter is in the 5–15 nm range. The inset in Figure 9.4d displays an ED pattern, with a lattice spacing of 2.8 Å between (103) planes. The CuS nanowires are found to be single-­crystalline. Using Triton X-­100 seems to give better CuS nanowires. Figure 9.5a shows a SEM image of hexagonal CuSe nanowires attained with Triton X-­100. Figure 9.5b shows corresponding TEM images. The inset in Figure 9.6b shows the ED pattern with a lattice spacing of 1.90 Å equal

Figure 9.4  (a)  and (b) SEM and TEM images of CuS nanowires obtained by using

AOT as the surfactant, (c) and (d) SEM and TEM images of CuS nanowires obtained by using Triton X-­100 as the surfactant. Inset in (d) shows the SAED pattern. Reproduced from ref. 13 with permission from John Wiley and Sons, Copyright © 2001 WILEY-­VCH Verlag GmbH & Co. KGaA, Weinheim.

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Figure 9.5  (a)  and (b) SEM and TEM images of CuSe nanowires obtained using Triton X-­100 as the surfactant, (c) HREM image of a CuSe nanorod showing lattice resolution. Inset in (b) is the SAED pattern. Reproduced from ref. 13 with permission from John Wiley and Sons, Copyright © 2001 WILEY-­VCH Verlag GmbH & Co. KGaA, Weinheim.

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corresponding to the (110) planes. The average diameter of the nanowire is about 70 nm, with the length ranging from 1 to 2 µm. Figure 9.5c shows the HREM image of a CuSe nanowire with an interlayer spacing of 0.33 nm corresponding to (101) planes. CuS nanowires of 5–20 nm show two wide bands in the absorption spectrum. One ranges from 300 to 600 nm, with a distinct band around 400 nm and another peaking around 1000 nm. The near-­infrared band is characteristic of the electron-­acceptor states lying within the band gap. CuSe nanowires with an average diameter of ∼70 nm show a broad feature, centered around 540 nm, along with an intense band above 1000 nm.63 Metal particles with rough surfaces generate arcs under microwaves. Cu metal particles are subjected to microwaves in the presence of sulfur and carbon/C3N4 in Teflon substrate to yield CuS nanorods coated with amorphous fluorinated carbon.64

Figure 9.6  (a–c)  TEM images of ZnS nanowires obtained using Triton X-­100 as the surfactant. Inset in (c) is the SAED pattern showing the single-­crystalline nature of the nanowires. Reproduced from ref. 13 with permission from John Wiley and Sons, Copyright © 2001 WILEY-­VCH Verlag GmbH & Co. KGaA, Weinheim.

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A surfactant-­assisted route for the synthesis of core–shell CuxS nanowires has been developed.65 Nanowire growth was achieved by exposing the Na-­ AOT-­covered copper surface to H2S for 12 h. The growth of highly oriented and uniform Cu2S nanowire arrays occurs by an oxide-­assisted mechanism on a copper substrate in a H2S/O2 atmosphere.66 The Fowler–Nordheim plot from field-­emission measurements on Cu2S nanowire arrays indicates nonlinearity, suggesting that the nanowires can potentially be cold cathodes.66 At 433 K, thermal decomposition of copper-­diethyldithiocarbamate (CuS2CNEt2) in a mixture of oleic acid and dodecanethiol yields single-­crystalline Cu2S ultra-­thin nanowires with a high aspect ratio.67

9.1.5  ZnS and ZnSe Thermal evaporation of ZnS powder onto silicon substrates, sputter-­coated with a thin (∼25 Å) film of Au, can be used to build wurtzite ZnS nanowires and nanoribbons.68 High aspect ratio ZnS nanowires have been developed using a surfactant such as Triton X-­100 or AOT, in a process close to that used in the synthesis of CuS and CuSe nanowires.26,63 TEM micrographs of the hexagonal ZnS nanowires obtained by using Triton X-­100 are shown in Figure 9.6. The length of the nanowires is in the range 8–10 µm and the diameter in the range 10–50 nm. The inset in Figure 9.6c shows an ED pattern of 2.854 Å spacing attributable to the (101) planes. ZnS nanowires with typical diameters varying from 10 to 50 nm show an absorption band with a peak about 340 nm. The PL spectrum has a wide band about 430 nm that ranges up to 550 nm.63 Decomposition of zinc xanthate provides a convenient means of producing uniform, thin ZnS nanorods and nanowires.69 Large-­scale ZnS nanowire bundles have been synthesized by reacting thiourea with Zn foil at 140 °C in the presence of hydrazine.70 Reaction between Zn2+ and S2− in a reverse micellar template yields winding ZnS nanowires.71 Nanowire formation takes place through a cycle of directional aggregation of nanoparticles and oriented growth. ZnS nanorods are produced by annealing ZnS nanoparticles, prepared in an NaCl flux by grinding ZnCl2 and Na2S at ambient temperature.72 The nanorods thus prepared are 40–80 nm in diameter and up to several micrometers in length. The Raman spectrum displays a band at 349 cm−1 due to the LO phonon. In addition, there is a weak 276 cm−1 band due to the TO phonon mode.72 Single-­crystalline ZnS nanowires have been produced on a silicon substrate by thermal evaporation of ZnS particles using the Au catalyst.73 PL spectra of the nanowires obtained by the above procedure show emission in the blue and green regions due to surface states.73 A CVD procedure for the synthesis of 1D metal sulfides including ZnS and CdS, has been described.74 A virus-­based scaffold for the synthesis of nanowires of ZnS, CdS and other materials has also been reported.75 Metal– organic chemical vapor deposition has been applied to grow vertically aligned ZnS NWAs on GaAs (111)B substrates. Here Ga serves as a catalyst.76 High-­density, vertically aligned ZnS nanowire arrays of a centimeter long have been produced at 1100 °C by evaporating ZnS powders on a silicon

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substrate coated with gold-­ion containing ionic liquid. The ability to self-­ organize ionic liquids on a solid surface, generating a uniform pattern of gold nanoparticles on the substrate, plays a crucial role in the nanowire alignment. A template method was used to synthesize single-­crystal ZnS nanosheets with lateral micrometer sizes and flake-­like ZnO dendrites by using a lamellar molecular precursor, ZnS.(NH2CH2CH2NH2)0.5.77 Nanostructured rectangular porous ZnS nanocables were synthesized by the chemical reaction of ZnO with H2S by using ZnO nanobelts as templates.78 The PL spectrum of nanocables shows a small blue-­shift due to quantum confinement. Lasing by ZnS nanowires,79 as well as optical properties and Raman spectra of cross-­ sectional ZnS nanowires80,81 have been reported. Cubic ZnS nanorods are obtained by an oriented attachment mechanism starting with diethylzinc, sulfur and an amine.82 A surfactant-­assisted process was employed to prepare ZnSe nanowires using Triton X-­100 as the surfactant, starting with ZnO and Se powder.13,63,83 This method yields ZnSe nanowires with a hexagonal structure with diameters in the 10–50 nm range and the lengths of up to several µm. Figure 9.7a and b shows TEM micrographs of the nanowires prepared by this procedure. The inset in Figure 9.7a displays the SAED pattern with a spacing of 1.97 Å, corresponding to the (110) planes. Size-­dependent, periodically twinned

Figure 9.7  (a)  and (b) TEM images of ZnSe nanowires (50–150 nm diameter)

obtained using Triton X-­100 as the surfactant. Inset in (a) shows the ED pattern of the nanowires. Reproduced from ref. 13 with permission from John Wiley and Sons, Copyright © 2001 WILEY-­VCH Verlag GmbH & Co. KGaA, Weinheim.

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ZnSe nanowires have been fabricated by a VLS process.84 ZnSe tetrapods have been obtained by Sn-­catalyzed thermal evaporation.85 A laser-­ablation technique has also been used to produce ZnSe nanowires.86 ZnSe nanowires formed by the surfactant route displays a broad band around 480 nm in the absorption spectrum. The emission peak in the PL spectrum is blue-­shifted compared to the bulk, with a peak at 425 nm and a shoulder at 435 nm.63 The longitudinal and transverse optic phonons of ZnSe nanowires show Raman bands at 257 and 213 cm−1, blue-­shifted with respect to the bulk due to compressive strain. A surface phonon mode is seen at 237 cm−1.83 Starting with the right precursors, microwave-­assisted methods can generate thin aligned nanowires and nanorods of CdS CdSe, ZnS and ZnSe.87 Thermal evaporation of ZnSe mixed with activated carbon powders yield ZnSe tetrapods in the presence of a tin-­oxide catalyst.88

9.1.6  NbS2, NbSe2 and NbSe3 Thermal decomposition of NbS3 at 1000 °C in H2 yields NbS2 nanorods.89 The nanorods extend up to several microns (Figure 9.8a). NbS2 nanorods have also been prepared by treating NbCl4-­coated carbon nanotubes with H2S.90 NbS2 nanowires have been generated by heating Nb and S powders in sealed tubes in the presence of iodine.91 Decomposing NbSe3 at 970 K under Ar flow yields NbSe2 nanorods.92 The HREM micrograph in Figure 9.8b shows an NbSe2 nanorod approximately 20 nm in diameter and resolving planes with an interlayer spacing of ∼6.2 Å. The ED pattern (see inset in Figure 9.8b) reveals Bragg spots corresponding to the (002) planes. The NbSe2 nanorods show no stacking faults or dislocations. These nanostructures exhibit a PL band at 820 nm, blue-­shifted from the bulk value by about 5 nm. They are metallic from room temperature down to low temperatures and superconducting below 8.3 K.92 The measured Tc is close to the bulk value as the nanostructures have large diameters. The bulk sample, however, exhibits steeper resistance change in the metallic state. Nanowires and nanoribbons of NbSe3 are obtained by the reaction of Nb and Se powders.93

9.1.7  Bismuth Chalcogenides Microwave irradiation of bismuth nitrate and thiourea in formaldehyde solution produces Bi2S3 nanorods with lengths up to 300 nm and 10 nm in diameter.94 The reaction of BiCl3 with thiourea at 140 °C in solvothermal conditions using polar solvents yields Bi2S3 nanorods.95 Bi2S3 nanorods can be produced by the hydrothermal reaction of Bi(NO3)3·5H2O and Na2S·9H2O in the 100– 170 °C range.96 In the presence of complexing agents such as ethylenediaminetetraacetic acid, sodium tartrate and triethanolamine, sonochemical treatment of an aqueous solution of bismuth nitrate and sodium thiosulfate yields Bi2S3 nanorods.97 Bi2S3 nanorods with uniform urchin-­like patterns were produced in ethylene glycol at 197 °C after 30 min.98 The complex

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Figure 9.8  (a)  SEM image of the nanorods of NbS2, (b) HREM image of a NbSe2

nanorod. Inset in (b) shows the ED pattern of the nanorod. (a) Reproduced from ref. 89 with permission from American Chemical Society, Copyright 2001. (b) Reproduced from ref. 92 with permission from Elsevier, Copyright 2003.

formed between Bi(NO3)3·5H2O and thiourea is decomposed in this reaction. The nanorods have diameters in the 50 nm range. By refluxing a DFM solution of bismuth citrate and thiourea, well-­segregated, crystalline Bi2S3 nanorods have been obtained.99 Biomolecule-­assisted synthesis of ordered structures of Bi2S3 nanorods has been reported.100 Thus, lysozyme is used to create single-­crystalline Bi2S3 nanowires by controlling the morphology and directing the formation of 1D structures.101 Bi(NO3)3·5H2O, thiourea and lysozyme are reacted at 433 K under hydrothermal conditions in this method. The thermal decomposition of bismuth alkylthiolate precursors in air at 500 K resulted in a solvent-­free synthesis of orthorhombic Bi2S3 nanorods and nanowires with high aspect ratios (>100).102 Direct electrodeposition of dissolved Bi and Te into AAO template pores produces dense, continuous Bi2Te3 nanowires.103–105 SEM micrographs of

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Figure 9.9  (a)  and (b) SEM images of a Bi2Te3 nanowire array composite obtained

by electrodeposition. The bright regions in the image are the filled pores, (c) Bright-­field TEM images of a single Bi2Te3 nanowire. Reproduced from ref. 103 with permission from American Chemical Society, Copyright 2001.

the bottom surface of the nanowire arrays produced by the above method are shown in Figure 9.9a and b. The images show pore filling to be approximately 20%. Figure 9.9c shows a TEM micrograph of a single Bi2Te3 nanowire. The addition of l-­c ysteine or thioglycolic acid to a bismuth chloride solution, accompanied by mixing with orthotelluric acid, was used to make single-­crystalline Bi2Te3 nanorods using a template-­free process at 100 °C.106 Reacting presynthesized Te nanowires with Bi(NO3)3·5H2O in the presence of PVP and ethylene glycol as well as N2H4 yields Bi2Te3 nanowires.58 Metal telluride nanowires are also synthesized starting from Te nanowires and the corresponding metal precursor and using ascorbic acid as a reducing agent.107 This process yields nanowires of Bi2Te3, PbTe, CdTe, Ag2Te and Cu1.75Te.

9.1.8  Other Chalcogenides MoS2 fibers can be prepared by a low-­temperature solution route, in which the reaction between (NH4)2Mo3S13 and ethylene diamine is carried out under stirring, followed by annealing the product in N2 at 400 °C.108 MoS2 fibers

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show a broad band centered at 386 nm in the electronic absorption spectrum and two weaker bands at 610 and 652 nm, similar to the bulk sample.108 A solvothermal method is employed to synthesize β-­La2S3 nanorods with 30–50 nm diameter and a micrometer long.109 The process involves the reaction of LaCl3 and thiourea in an autoclave for 10 h at 260–280 °C. β-­La2S3 nanorods emit a strong band around 420 nm.109 Reaction of FeCl2·4H2O with thioacetamide under solvothermal conditions in the presence of ethylenediamine (en) yields Fe7S8 nanowires.110a Nanowires of the Fe7S8.en adduct are first formed and then decompose at a low temperature to give the sulfide nanowires. GeS and GeSe nanowires have been prepared by the decomposition of organoammonium derivative precursors such as organoammonium selenide.110b GeTe nanowires are obtained by a VLS process starting with GeTe powder and an Au nanoparticle catalyst.111 CuInS2 nanorods are prepared by a solvothermal route at 280 °C by the reaction of Cu, In and S powders in ethylenediamine.112 Under hydrothermal conditions reaction of CuCl, SbCl3 and NH2CSNH2, yields famatinite (Cu3SbS4) nanofibers and tetradrite (Cu12Sb4S13) nanoflakes.113 The reaction of anhydrous CuCl2 and In2Se3 with Se powder under solvothermal conditions formed copper indium diselenide nanowires.114 Micrometre-­long, nanometre-­wide LiMo3Se3 fibers were prepared by the reaction of the corresponding elements at 1000 °C in sealed, evacuated quartz ampoules, followed by intercalation of Li in the Mo3Se3 chains.115 A soft chemical route has been developed for the development of MMo3Se3 nanowires with different M cations ((CnH2n+1)N+(CH3)3,12 ≤ n ≥ 18, and (C10H2n+1)2N+(CH3)2, 10 ≤ n ≥ 18).116a This involves a low-­temperature cation-­exchange process, where Li cations are extracted from the LiMo3Se3 by adding 12-­crown-­4 and the other cations incorporated. A redox templating process has been developed to prepare nanowires of Ag, Pd, Au and Pt of 10–100 nm diameter using LiMo3Se3 nanowires as sacrificial templates.117

9.2  GaAs, InP and Other Semiconductor Nanowires 9.2.1  GaAs Semiconducting nanowires have been synthesized widely by using laser ablation. By ablation of an acceptable target with a laser source, catalytic nanometer-­diameter clusters are produced, in which a VLS mechanism directs the size and growth of the nanowires. To predict catalysts and growth conditions, equilibrium phase diagrams have been used. Thus, Duan and Lieber118 prepared nanowires of group III–V semiconductors such as GaAsP, InAs, GaAs, InP and InAsP, II–VI semiconductors such as CdS, CdSe, ZnS and ZnSe, and IV–IV alloys of SiGe. GaAs nanowires (GaAsNWs) were produced without the use of any metal catalyst by laser ablation of GaAs powders mixed with Ga2O3 by an oxide-­assisted process.119 Figure 9.10a shows a SEM micrograph of GaAs nanowires with a diameter

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Figure 9.10  (a)  A SEM image of GaAs nanowires synthesized by an oxide-­assisted method. (b) An HREM of a GaAs nanowire. The growth axis, denoted by the white arrow, is close to the [1̄11̄] direction. The inset is the corresponding ED pattern recorded along the [110] zone axis perpendicular to the nanowire growth direction. Reproduced from ref. 119 with permission from John Wiley and Sons, Copyright © 2001 WILEY-­VCH Verlag GmbH, Weinheim, Fed. Rep. of Germany.

of approximately 5 nm and length up to 10 µm. The electron diffraction pattern (Figure 9.10b) as well as the HREM image of a single nanowire shows that it is single-­crystalline and grows along 〈111〉. The nanowire is covered by a thin amorphous oxide layer. By employing a similar method, GaP and GaN nanowires have also been synthesized. The nanowires of GaAs show a shift and broadening of the Raman band as well as the PL band due to stress, impurities and defects.120

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Using In as the catalyst, low-­temperature solution–liquid–solid growth was employed to produce GaAs nanowires with lengths of several microns and diameters in the range 6–16 nm.121 The nanowires had a small range of diameters depending on the size of the metal particles. MBE has been employed to grow ordered GaAs nanowires by VLS growth in porous alumina templates.122 The wires show a narrow distribution of diameter. Due to quantum confinement, PL measurements on individual GaAsNWs exhibit large blue shifts relative to the bulk.123 Joyce et al.124 used an Au nanoparticle-­catalyzed metal organic chemical vapor deposition to produce vertically arranged epitaxial GaAs nanowires of outstanding crystallographic accuracy and ideal form. A two-­temperature growth protocol was used to do this, with an initial high-­temperature growth stage followed by a longer period of growth at lower temperatures. On the (111)B GaAs substrate, the initial high-­temperature stage was critical for obtaining smooth, vertically oriented epitaxial nanowires. By limiting radial growth and removing twinning defects, the lower temperature used for subsequent growth produced superior nanowire morphology and crystallographic accuracy. Uniform nanowire arrays of GaAs (as well as CdS, CdSe, CdSxSe1-­x, CdxZn1-­xS and metallic (Ni, Fe)) nanowires have been prepared using porous membranes.125 Conductance oscillations and Coulomb staircase behavior are observed in the I–V properties of two terminal devices made from the nanoarrays. In comparison to the development of a cubic zinc blende phase in bulk form, Glas et al.126 created a nucleation-­based model to describe the formation of a preferential wurtzite phase during the catalyzed growth of freestanding nanowires of zinc blende III–V semiconductors. They demonstrated that nucleation occurs preferentially at the triple phase line in VLS nanowire formation. At high liquid supersaturation, wurtzite nucleation is preferred based on the appropriate interface energies. This explains why zinc blende is observed during the early stages of the growth of gold-­catalyzed GaAs nanowires.

9.2.2  InP and GaP Laser-­assisted catalytic growth has been employed to obtain single-­crystalline InP nanowires. The technique also allows n-­ and p-­t ype doped nanowires to be obtained. They are 10 nm in diameter and up to tens of microns in length.127 Gate-­voltage-­dependent transport experiments show that it is possible to synthesize both n-­and p-­t ype nanowires. The doped nanowires act as nanoscale FETs. They can also be arranged into p–n junctions that can show rectifying behavior. Integrated device arrays from nanowires can be achieved by an electric-­field-­driven assembly process. InP, GaP and other oxide, sulfide, and tungstate nanowires are formed by an aqueous-­solution route utilizing inorganic-­surfactant intercalated mesostructures.128 These mesostructures are formed by cocondensation of cetyltrimethylammonium cations with anionic inorganic species. The mesostructures act as both reactants and microreactors.

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Lieber and coworkers129a have studied hierarchical assemblies of a 1D nanostructure into well-­defined functional networks. Assembling InP nanowires into parallel arrays has been achieved. The average separation was controlled by combining fluidic alignment with surface-­patterning. With layer-­by-­layer assembly and specific flow directions for sequential phases, complex crossed nanowire arrays can be prepared and their transport properties examined. With monodisperse nanocluster catalysts, the diameters and lengths of InPNWs developed by laser-­assisted catalytic growth can be regulated.130 InPNWs of 10–0 nm diameters have been developed from the same diameter nanocluster catalysts. A growth pattern is shown in Figure 9.11a. One can monitor the length of the nanowires by changing the time of ablation, as shown in Figure 9.11b. Figure 9.11c and d show TEM micrographs of a nanocluster catalyst at the nanowire end and a crystalline wire core prepared by Au nanoclusters with a diameter of 20 and 10 nm. A bottom-­up method

Figure 9.11  (a)  Schematic depicting the use of monodisperse colloid catalysts for

the synthesis of diameter-­selective InP nanowires. (b) Schematic illustrating the effect of the variation of growth time on nanowire length. (c) TEM image showing the nanocluster catalyst at the end of an InP nanowire grown from a 20 nm Au cluster (scale bar is 50 nm). (d) HREM showing the crystalline core of the InP nanowire grown from a 10 nm colloid (scale bar is 5 nm). The (111) lattice planes resolved perpendicular to the growth axis have an average spacing of 0.59 ± 0.05 nm, which is in good agreement with the bulk value for zinc-­blende InP of 0.5869. Reproduced from ref. 130 with permission from American Chemical Society, Copyright 2001.

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to produce ordered and uniform arrays of InP nanowires, employing nanoimprint nanolithography, has been demonstrated.131 Vertically aligned, catalyst-­free InP nanowires have been grown by CVD of trimethylindium and phosphine at 623–723 K.132 The VLS approach was used to create homogeneous InAs1−xPx nanowires and InAs1−xPx heterostructure segments in InAs nanowires with P concentrations ranging from 22% to 100%.133 PL spectroscopy and imaging have been used to examine individual InPNWs.134 Due to radial quantum containment, the emission maximum shifts to higher energies with decreasing nanowire diameter for diameters below 20 nm. Polarization-­sensitive studies show a strong anisotropy (order of magnitude) in PL intensity of individual InP nanowires when recorded perpendicular or parallel to the long axis of the wire.135 The polarization anisotropy is attributed to large dielectric contrast between free-­standing nanowires and the surrounding environment. Algra et al.136 have shown that using impurity dopants, they can regulate the crystal structure of indium phosphide (InP) nanowires. They discovered that zinc lowers the activation barrier for two-­dimensional nucleation growth of zinc-­ blende InP, allowing InP nanowires to crystallize in the zinc-­blende rather than the wurtzite crystal structure. Once the zinc-­blende crystal structure is formed, it induces twinning in superlattices with long-­range order in InP nanowires. Semiconductor nanowire superlattices of group IV and group III–V materials were developed into nanowires by repeated modulation of vapor-­phase semiconductor reactants during wire production.137 Compositionally controlled superlattices consisting of 2 to 21 layers of GaP and GaAs have been prepared. Modulation-­doped n-­InP/p-­InP and n-­Si/p-­Si nanowires have also been achieved. These were characterized by single-­nanowire photoluminescence, electroluminescence and electrical transport measurements. Single-­crystalline gallium phosphide nanowires were synthesized by laser-­ assisted catalytic growth.138 A laser was used to ablate Ga and P and the nanowires were formed on Au nanoclusters supported on a SiO2 substrate. The nanowires grow in the [111] direction. Laser ablation of a mixture of GaP and Ga2O3 utilizing oxide-­assisted growth yields GaPNWs of 22 nm in diameter and hundreds of microns in length.139 GaPNWs are also produced on Ni-­coated alumina substrates by subliming ball-­milled GaP powder.140 An SEM image of the nanowires is shown in Figure 9.12a and TEM images in Figure 9.12b–f. It has been documented that single-­crystalline CoP nanowires with uniform diameters can be synthesized in one pot using metal–organic chemistry.141 The process entails the thermal decomposition of cobalt(ii)acetylacetonate and tetradecylphosphonic acid in a TOPO/hexadecylamine mixture.

9.3  Miscellaneous Nanowires AgCl, AgBr and AgI nanowires and nanorods have been prepared in water/ oil microemulsions.142 The reactant ratio plays a role in determining the morphology. In nonionic surfactant reverse micelles (Triton X-­100–cyclohexane–water), nanowires of CaCO3, BaCO3 and CaSO4 have been produced.143

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Figure 9.12  (a)  SEM image of GaP nanowires grown in Ni-­catalyzed alumina sub-

strates via the sublimation of GaP powders. (b) TEM image showing the general morphology of the nanowires. (c) Higher-­magnification image showing all the nanowires to be straight and cylindrical. (d) HREM of a single nanowire with the SAED (inset) pattern showing a single crystalline zinc blende structure. (e) TEM image showing the curled GaP nanowires obtained. (f) HREM image showing a polycrystalline nanowire with the SAED pattern (inset). Reproduced from ref. 140 with permission from the Royal Society of Chemistry.

The nanowires of CaCO3 are 5–30 nm in diameter and are more than 10 µm long. Figure 9.13 displays TEM images of CaCO3 nanowires. Reaction of trioctylphosphine oxide and trioctylphosphine with Fe-containing precursors gives FeP nanowires with large aspect ratios.144 Using conventional optical lithography and silicon processing procedures, epitaxial CoSi2 nanowires are produced on Si(100) and Si–O substrates.145 NiSi2 nanowires have been produced by the decomposition of silane on Ni surfaces.146 Highly ordered nanowire arrays of Prussian blue (Fe43+[Fe2+(CN)6]3·xH2O), ∼50 nm in diameter and up to 4 µm long, have been produced by electrodeposition on AAO films.147 Optically active, uniform, near-­monodisperse worm-­like nanowires of potassium and lithium rare-­earth fluoride nanocrystals (KCeF4) were synthesized via cothermolysis of metal trifluoroacetate precursors [K(CF3COO) and Ce(CF3COO)3] in a hot oleic acid/oleylamine/1-­ octadecene solution.148

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Figure 9.13  TEM  images of CaCO3 samples prepared in Triton X-­100 reverse micelles after aging for different times: (a) 10 min, (b) 1 h, (c) 4 h and (d) 48 h. Reproduced from ref. 143 with permission from Elsevier, Copyright 2002.

Nanourchins can be made from iron, aluminum and indium oxyhydroxides. Indium oxyhydroxide urchin-­like nanostructures, composed of nanorods with a diameter of many nanometers and a length of 100 nm, have been developed via polymer-­assisted hydrolysis of In3+ in a water/ethanol mixed solvent.149 A simple one-­step wet-­chemical route was used to selectively prepare γ-­AlOOH architectures of hollow and self-­encapsulated structures.150 A template-­free hydrothermal process at low temperature was used to synthesize α-­FeOOH nanocrystals (rod-­like, bundle-­like and urchin-­like) in high yield.151 Eu(OH)3 nanorods are produced in the first stage of the hydrolysis of Eu(NO3)3 in the presence of hexamethylenetetramine.152 Heating the Eu(OH)3 nanorods in air converts them to Eu2O3, which is then transformed to EuO by a decrease in Eu vapor. ZnF2 nanorods are synthesized by treating Zn metal powder with sharp edges in a Teflon beaker under microwaves. In the similar conditions Ni powder yields NiF2 nanorods. These rods are coated with amorphous carbon.64

9.4  Coaxial Nanowires and Coating Nanowires Core–shell or core–multi-­shell nanowire heterostructures entail several advantages.153 Importantly, heterojunctions are not restricted to lattice-­ matched materials in the nanowire geometry as one-­dimensional geometry

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tolerates for a partial release of the interface strain. These core–shell materials are designed for surface passivation of the channel to improve optical properties of nanowires or for improving thermoelectric properties, as charge carriers are less scattered by surface defects (thus improving electrical conductivity) and, simultaneously, more phonons are scattered in the shell (thus reducing thermal conductivity). These core–shell architectures are also used for confinement in the shell, which is relatively easy compared to stand-­ alone nanowires. Surrounding-­gate architectures of field effect transistors can also be made by this geometry. Lauhon et al. reported Si–Ge or Ge–Si core–shell nanowire heterostructures as early as 2002 by using a CVD method.154 Silane and germane are used as Si and Ge sources, respectively. A shell coated at low-­temperature is found to be amorphous, and is crystallized by subsequent thermal annealing at high-­temperatures. Si/Ge/Si, p-­Si/i-­Si/n-­Si and i-­Ge/p-­Ge/i-­Si core–double shell nanowires have also been realized.155–157 III–V core–shell or core–multi-­shell nanowires are well explored, because of their potential applications in optoelectronic and emitting devices. A metal-­organic chemical vapor deposition technique is employed to obtain core–multi-­shell nanowires of n-­GaN/InGaN/p-­GaN and a dislocation-­free single crystal n-­GaN core with InxGa1−xN/GaN/p-­AlGaN/p-­GaN shells.158,159 Here emission wavelength is tuned by the variation of the indium mole fraction. Selective-­area metal-­organic vapor-­phase epitaxy was used to build InP/ InAs/InP core–multi-­shell heterostructure nanowire arrays.160 These nanowires were created to house a stretched InAs quantum well layer inside a higher band-­gap InP nanowire (Figure 9.14). Precise control over the nanowire growth path and heterojunction creation provided for the efficient fabrication of a nanostructure in which all three layers were epitaxially developed without the use of a catalyst. The molecular-­beam epitaxy (MBE) technique has been used to prepare GaP nanowires via Au-­catalyzed VLS method followed by adjusting the As and P precursor flux ratio to obtain GaP/GaAsP core–shell nanowires. This technique is extended further to get GaP/GaAsP/GaP nanowires.161 MBE is also used to grow AlGaAs/GaAs/AlGaAs multi-­shell nanowires without using a catalyst seed layer, thus solving the major concern of metal contamination.162 Interestingly, the corner between two quantum wells in a GaAs/ AlGaAs core–multi-­shell nanowire system behaves as a quantum wire.163 Spontaneous alloy composition ordering in the AlGaAs shell of GaAs/ AlGaAs has been reported,163–165 where Al is accumulated at the corners of the six equivalent {110} sidewall facets of the AlGaAs hexagonal shell. The AlGaAs shell thickness is found to be closely related to the minority carrier lifetimes.166 High-­mobility field effect transistors based on InGaAs/InP/InAlAs/InGaAs core–multi-­shell nanowires have been reported.167 A radial p-­i-­n photodiode based on n-­GaAs/InGaP/p-­GaAs multi-­shell structure was reported by Gutsche et al.168

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Figure 9.14  (a)  Schematic cross-­sectional image of an InP/InAs/InP core–multi-­ shell nanowire. (b) SEM image ofa periodically aligned InP/InAs/InP core–multi-­shell nanowire array. (c) Low-­angle inclined SEM image showing highly dense ordered arrays of core–multi-­shell nanowires. (d) Schematic illustration and (e) high-­resolution SEM cross-­sectional image of a typical core–multi-­shell nanowire observed after anisotropic dry etching and stain etching. Reproduced from ref. 160 with permission from AIP Publishing, Copyright 2006.

Metal–organic vapor-­phase epitaxy (MOVPE) has been employed to grow multiple InGaN/GaN quantum wells wrapped around a ZnO nanotube on n+-­ GaN/Al2O3(0001) substrates, which allow tuning of the light emission from green to violet by tuning the In content in the InxGa1−xN layers.169 MOVPE on SiO2 has been used to develop nanowires with several GaP-­GaAs junctions.170 The vapor–liquid–solid (VLS) growth kinetics of GaP and GaAs in heterostructured GaP-­GaS nanowires have been investigated as a result of temperature, and arsine and trimethylgallium partial pressures. PbCl2 and S were used in CVD to deposit silica-­coated PbS nanowires on silicon substrates at temperatures ranging from 650 to 700 °C.171 A novel silica-­coating technique for CTAB-­stabilized gold nanorods and the hydrophobization of a silica shell with octadecyltrimethoxysilane (OTMS) has been developed.172 The polyelectrolyte layer-­by-­layer (LBL) technique combined with the hydrolysis and condensation of tetraethoxyl orthosilicate (TEOS) in a 2-­propanol-­water mixture results in homogeneous coatings with tight control over shell thickness. The heavy attachment of CTAB molecules to the gold surface, on the other hand, renders surface hydrophobization difficult; however, functionalization with OTMS, which includes a long hydrophobic hydrocarbon chain, enables the particles to be moved into nonpolar organic solvents such as chloroform.

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Chemically fused ceramic oxide coatings on inorganic nanowires and carbon nanotubes have been suggested as a general treatment.173 Acid-­treated carbon nanotubes or metal oxide nanowires combine with reactive metal chlorides to create ceramic oxide-­coated surfaces, which are then hydrolyzed with water. Ceramic coatings with the appropriate thickness may be produced by repeating the previous process multiple times with calcination. Using a porous anodic alumina membrane as a template, an electrochemical method was used to fabricate core–shell CdS and polyaniline (PANI) coaxial nanocables with enhanced photoluminescence.174 SiC nanowires may be coated with Ni and Pt nanoparticles (3 nm) using plasma-­enhanced CVD.175 Thermal CVD was used to produce single and double-­shelled coaxial core–shell nanocables of GaP with SiOx and carbon (GaP/SiOx, GaP/C, GaP/SiOx/C) with unique morphology and form (GaP/SiOx, GaP/C, GaP/SiOx/C).176 At 873–1073 K, silica-­sheathed 3C– Fe7S8 was prepared on silicon substrates using FeCl2 and sulfur precursors.177

9.5  Perovskite Nanowires Organic–inorganic metal halide perovskites with an ABX3 structure (A = monovalent cations, B = divalent cations and X = halide) are new-­generation semiconductors with exceptional optoelectronic properties. These materials have a direct band-­gap with high optical absorption coefficients as well as long carrier diffusion lengths, important for different applications, including photovoltaics, photodetectors, light-­emitting diodes and lasers. Even though perovskites of CsPbX3 (X = Cl, Br or I) and CH3NH3PbX3 (X = Cl, Br, I) have been known for a long time,178 real attention to these materials has been due to the discovery of the photovoltaic properties of MAPbBr3 and MAPbI3.179 These materials have shown remarkable improvements in power conversion efficiencies from 4% to more than 22%. The properties of these compounds are sensitive to their composition and morphologies.180 Nanorods and nanowires of halide perovskites to have the longer carrier lifetimes and diffusion lengths required for optoelectronic devices, such as photodetectors, solar cells, waveguides and lasers.181–183

9.5.1  Vapor-­phase Synthesis Xiong et al. synthesized CH3NH3PbCl3 (MAPbCl3) perovskite nanowires by a vapor-­phase method. In this procedure, PbCl2 nanowires were prepared on a mica substrate and then reacted with CH3NH3Cl at 100 °C, as shown in Figure 9.15a.184 This procedure yields fewer nanowires and more nanosheet structures. The yield of nanowires was improved by taking silicon oxide as the substrate. Similar reaction conditions yield CH3NH3I and CH3NH3Br at 100 °C. The structure and SEM images of PbI2 and MAPbI3 synthesized on a SiO2 substrate are shown in Figures 9.15b and c, respectively. The vapor-­transport method has been used to synthesize CsPbX3 (X = Cl, Br, I) nanowires. Park et al. synthesized them by placing a mixture of CsX and

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Figure 9.15  (a)  Schematic synthesis setup of vapor-­transport system, (b) simulated structure and (c) TEM images of a MAPbI3 nanowire. Inset in (c) shows the corresponding SAED pattern. (a) Reproduced from ref. 184 with permission from John Wiley and Sons, Copyright © 2014 WILEY-­VCH Verlag GmbH & Co. KGaA, Weinheim. (b–c) Reproduced from ref. 219 with permission from American Chemical Society, Copyright 2015.

Figure 9.16  SEM  images of vertically aligned growth of nanowires of CsPbBr3 with

a rectangular cross-­section grown on a substrate. Reproduced from ref. 185 with permission from American Chemical Society, Copyright 2016.

PbX2 powder inside a quartz tube, which was subsequently heated at 570–600 °C.185 A Si substrate was placed 10 cm away from the powder mixture that was heated to 350–380 °C. Under argon flow, the powders were carried to the Si substrate and reacted to form CsPbX3 nanowires. SEM images of the vertically aligned CsPbBr3 nanowires on a Si substrate are shown in Figure 9.16. A similar strategy using a sapphire substrate (instead of a Si substrate) yields horizontally aligned CsPbBr3 nanowires.186 A nanofaceted sapphire

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Figure 9.17  Guided  graphoepitaxial growth (a) the planes and directional growth

are schematically shown (b) optical microscope image of a typical growth. SEM images of an assembly of nanowires (c) and higher magnification images of (d and e). (f) A 3D AFM image indicating the triangular cross-­section of the NWs. Reproduced from ref. 186 with permission from American Chemical Society, Copyright 2018.

substrate contains nano steps and nanogrooves (the steps are created by heating thermodynamically unstable flat M ( 1010 ) sapphire to 1600 °C), which allows the nanowires to grow along these steps causing graphoepitaxy. Graphoepitaxial growth is shown to be dominant over epitaxial growth for nanowires. The planes and growth direction, and various microscope images of horizontally grown CsPbBr3 nanowires on a sapphire substrate are shown in Figure 9.17. Parallel arrays of guided growth of nanowires allow production of multiple photodetector devices in a single metal evaporation step using a custom-­made mask. Epitaxial growth of nanowires is achieved on flat C (0001) sapphire. Similar strategies have been used to grow horizontally aligned CsPbCl3, CsPbBr3 and CsPbI3 nanowires on annealed M ( 1010 ) sapphire by Pan and coworkers.186,187. By these means, several-­millimeter-­long nanowires of CsPbBr3 have been observed. The individual nanowires show excellent photo-­detecting properties with a responsivity of 4400 A W−1 and a response speed of 252 µs.

9.5.2  Solution-­phase Synthesis Solution-­based synthesis is cost-­effective and allows nanowires to form deposits on desired substrates.188–193 Horváth et al. have reported a slip-­ coating method to prepare MAPbI3 nanowires.194 In this process, the saturated MAPbI3 solution in DMF was placed between two microscope glass slides. The top glass slide progressively slides while the bottom glass slide is left fixed. Instantaneous crystallization of the nanowires happens when the solvent evaporates as evidenced by a color change from yellow to brown-­red.

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The kinetics of crystallization are sensitive to temperature, concentration, fluid publicity and sliding speed. The width and length of the nanowires are 50–200 nm and 16 µm, respectively. Zhu et al. have reported the synthesis of highly crystalline MAPbX3 nanowires of 20 µm length by a two-­step solution process.195 In the first step, a lead acetate (PbAc2) solution is spin-­coated onto a glass substrate; in the second step, the glass substrate is aged in a solution of CH3NH3X (X = Cl, Br, I or mixed halide precursor) for 20 h at room temperature. A dissolution-­ recrystallization growth mechanism is proposed to occur to yield the nanowires. Other halide perovskite nanowires synthesized by this method include formamidinium lead halide perovskite nanowires and inorganic CsPbX3 (X = Cl, Br, I) perovskite nanowires.196–201 Yang et al. used a similar procedure to obtain single crystalline, well-­faceted nanowires of MAPbBr3 arrays at room temperature.202 In this procedure, a saturated PbAc2 methanolic solution was spin-­coated on a PEDOT : PSS/ITO/ glass substrate followed by ageing in CH3NH3Br solution. The nanorods are converted to MAPbI3 nanorods by annealing at ∼150 °C in CH3NH3I vapor. A similar strategy and anion exchange have been used to get inorganic perovskite nanowires.203,204 A hot injection method was used by Zhang et al. to get high-­quality CsPbX3 nanowires with well-­defined morphologies.205 In this procedure, Cs-­oleate solution was injected at 150 °C into a solution of PbX2 (Cl and Br) in octadecene (ODE), oleic acid (OA) and oleylamine (OLAM). Nanocubes of CsPbCl3 and CsPbBr3 with a small uniform diameter were formed initially, which then converted to nanowires. CsPbl3 nanowires were synthesized similarly at 180 °C. TEM images of CsPbBr3 and CsPbl3 nanowires synthesized by this procedure are shown in Figure 9.18. The hot injection method has been used further to obtain ultra-­thin (2–3 nm diameter) nanowires of CsPbBr3.206 This technique has been well explored by others to prepare perovskite nanowires.207–212 Liu et al. used microwave-­induced heating of a solution of Cs-­oleate and PbX2 in organic solvent to get CsPbX2 nanowires.213 The diameter and length were found to depend on the microwave power and irradiation time.

Figure 9.18  (a)  Schematic crystal structure representations of three CsPbX3 (X = Cl,

Br, I) orthorhombic crystal structures. Cs, shown inside octahydra; Pb, dark and X, light gray. TEM images of nanowires of (b) CsPbBr3 and (c) CsPbI3. (a) Reproduced from ref. 220 with permission from American Chemical Society, Copyright 2016. (b) Reproduced from ref. 205 with permission from American Chemical Society, Copyright 2015.

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Solvothermal synthesis has been applied to prepare lead-­free perovskite quantum rods of CsSnX3 (X = Cl, Br and I) with tunable photoluminescence emissions.214 In a typical synthesis, SnX2 is dissolved in ODE, OA and OLAM. This solution, along with Cs-­oleate, TOPO and diethylenetriamine, is transferred into a Teflon-­lined autoclave for solvothermal treatment at 180 °C for 6 h to yield CsSnX3.

9.5.3  Template-­assisted Methods Preparation of perovskite nanowires with ordered nanostructures without using additional ligands can be achieved by template-­assisted methods. In this process, well-­defined nanoporous cylindrical templates such as anodized aluminium oxide (AAO) or porous membranes are used. Mirkin and coworkers synthesized MAPbI3 nanowires with diameters from 50 to 200 nm using AAO as the template.215 In a typical process, aluminium was evaporated onto TiO2-­deposited FTO glass. A DMF solution containing CH3NH3I and PbCl2 was allowed to penetrate the AAO template followed by thermal annealing in air at 100 °C for 45 min to obtain MAPbI3 nanowires. This process does not require additional surfactants to control the size. A vapor–solid–solid reaction process has been used to grow ordered nanowire arrays of MAPbI3 in an AAO template.216 Pb clusters were used inside the AAO to start the growth of the nanowires. Lead-­free MASnI3 and other inorganic halide perovskite nanowires have also been synthesized using templates.217,218

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170. M. A. Verheijen, G. Immink, T. de. Smet, M. T. Borgström and E. P. A. M. Bakkers, J. Am. Chem. Soc., 2006, 128, 1353. 171. M. Afzaal and P. O'Brien, J. Mater. Chem., 2006, 16, 1113. 172. I. Pastor, J. Pérez-­Juste and L. M. Liz-­Marźan, Chem. Mater., 2006, 18, 2465. 173. A. Gomathi, S. R. C. Vivekchand, A. Govindaraj and C. N. R. Rao, Adv. Mater., 2005, 17, 2757. 174. Y. Xi, J. Zhou, H. Guo, C. Cai and Z. Lin, Chem. Phys. Lett., 2005, 412, 60. 175. A. D. LaLonde, M. G. Norton, D. N. McIlroy, D. Zhang, R. Padmanabhan, A. Alkhateeb, H. Man, N. Lane and Z. Holman, J. Mater. Res., 2005, 20, 549. 176. S. Y. Bae, H. W. Seo, H. C. Choi, D. S. Han and J. Park, J. Phys. Chem. B, 2005, 109, 8496. 177. H.-­X. Zhang, J.-­P. Ge, J. Wang, Z. Wang, D.-­P. Yu and Y.-­D. Li, J. Phys. Chem. B, 2005, 109, 11585. 178. C. K. Møller, Nature, 1958, 182, 1436. 179. A. Kojima, K. Teshima, Y. Shirai and T. Miyasaka, J. Am. Chem. Soc., 2009, 131, 6050. 180. C. Zhou, H. Lin, Q. He, L. Xu, M. Work, M. Chaaban, S. Lee, X. Shi, M.-­H. Du and B. Ma, Mater. Sci. Eng., R, 2019, 137, 38. 181. W. Zhang, G. E. Eperon and H. J. Snaith, Nat. Energy, 2016, 1, 16048. 182. J. S. Manser, J. A. Christ and P. V. Kamat, Chem. Rev., 2016, 116, 12956. 183. H. C. Kwon, W. Yang, D. Lee, J. Ahn, E. Lee, S. Ma, K. Kim, S. C. Yun and J. Moon, ACS Nano, 2018, 12, 4233. 184. S. T. Ha, X. F. Liu, Q. Zhang, D. Giovanni, T. C. Sum and Q. H. Xiong, Adv. Opt. Mater., 2014, 2, 838. 185. K. Park, J. W. Lee, J. D. Kim, N. S. Han, D. M. Jang, S. Jeong, J. Park and J. K. Song, J. Phys. Chem. Lett., 2016, 7, 3703. 186. E. Oksenberg, E. Sanders, R. Popovitz-­Biro, L. Houben and E. Joselevich, Nano Lett., 2018, 18, 424. 187. M. Shoaib, X. H. Zhang, X. X. Wang, H. Zhou, T. Xu, X. Wang, X. L. Hu, H. W. Liu, X. P. Fan, W. H. Zheng, T. F. Yang, S. Z. Yang, Q. L. Zhang, X. L. Zhu, L. T. Sun and A. L. Pan, J. Am. Chem. Soc., 2017, 139, 15592. 188. K. Hong, Q. V. Le, S. Y. Kim and H. W. Jang, J. Mater. Chem. C, 2018, 6, 2189. 189. H. Deng, D. D. Dong, K. K. Qiao, L. L. Bu, B. Li, D. Yang, H. E. Wang, Y. B. Cheng, Z. X. Zhao, J. Tanga and H. S. Song, Nanoscale, 2015, 7, 4163. 190. H. Deng, X. K. Yang, D. D. Dong, B. Li, D. Yang, S. J. Yuan, K. K. Qiao, Y. B. Cheng, J. Tang and H. S. Song, Nano Lett., 2015, 15, 7963. 191. L. Gao, K. Zeng, J. S. Guo, C. Ge, J. Du, Y. Zhao, C. Chen, H. Deng, Y. S. He, H. S. Song, G. D. Niu and J. Tang, Nano Lett., 2016, 16, 7446. 192. J. Zhang, X. K. Yang, H. Deng, K. K. Qiao, U. Farooq, M. Ishaq, F. Yi, H. Liu, J. Tang and H. S. Song, Nano-­Micro Lett., 2017, 9, 36. 193. S. Wang, K. Y. Wang, Z. Y. Gu, Y. J. Wang, C. Huang, N. B. Yi, S. M. Xiao and Q. H. Song, Adv. Opt. Mater., 2017, 5, 1700023.

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194. E. Horvath, M. Spina, Z. Szekrenyes, K. Kamaras, R. Gaal, D. Gachet and L. Forro, Nano Lett., 2014, 14, 6761. 195. H. M. Zhu, Y. P. Fu, F. Meng, X. X. Wu, Z. Z. Gong, Q. Ding, M. V. Gustafsson, M. T. Trinh, S. Jin and X. Y. Zhu, Nat. Mater., 2015, 14, 636. 196. Y. P. Fu, F. Meng, M. B. Rowley, B. J. Thompson, M. J. Shear, D. W. Ma, R. J. Hamers, J. C. Wright and S. Jin, J. Am. Chem. Soc., 2015, 137, 5810. 197. W. Tian, C. Zhao, J. Leng, R. Cui and S. Jin, J. Am. Chem. Soc., 2015, 137, 12458. 198. R. Xiao, Y. Hou, Y. P. Fu, X. Y. Peng, Q. Wang, E. Gonzalez, S. Jin and D. Yu, Nano Lett., 2016, 16, 7710. 199. X. X. He, P. Liu, S. N. Wu, Q. Liao, J. N. Yao and H. B. Fu, J. Mater. Chem. C, 2017, 5, 12707. 200. W. M. Tian, J. Leng, C. Y. Zhao and S. Y. Jin, J. Am. Chem. Soc., 2017, 139, 579. 201. Y. X. Wang, R. X. Lin, P. C. Zhu, Q. H. Zheng, Q. J. Wang, D. Y. Li and J. Zhu, Nano Lett., 2018, 18, 2772. 202. A. B. Wong, M. L. Lai, S. W. Eaton, Y. Yu, E. Lin, L. Dou, A. Fu and P. D. Yang, Nano Lett., 2015, 15, 5519. 203. S. W. Eaton, M. L. Lai, N. A. Gibson, A. B. Wong, L. T. Dou, J. Ma, L. W. Wang, S. R. Leone and P. D. Yang, Proc. Natl. Acad. Sci. U. S. A., 2016, 113, 1993. 204. L. Dou, M. L. Lai, C. S. Kley, Y. M. Yang, C. G. Bischak, D. D. Zhang, S. W. Eaton, N. S. Ginsberg and P. D. Yang, Proc. Natl. Acad. Sci. U. S. A., 2017, 114, 7216. 205. D. D. Zhang, S. W. Eaton, Y. Yu, L. T. Dou and P. D. Yang, J. Am. Chem. Soc., 2015, 137, 9230. 206. D. D. Zhang, Y. Yu, Y. Bekenstein, A. B. Wong, A. P. Alivisatos and P. D. Yang, J. Am. Chem. Soc., 2016, 138, 13155. 207. M. Imran, F. Di Stasio, Z. Y. Dang, C. Canale, A. H. Khan, J. Shamsi, R. Brescia, M. Prato and L. Manna, Chem. Mater., 2016, 28, 6450. 208. F. Di Stasio, M. Imran, Q. A. Akkerman, M. Prato, L. Manna and R. Krahne, J. Phys. Chem. Lett., 2017, 8, 2725. 209. D. Amgar, A. Stern, D. Rotem, D. Porath and L. Etgar, Nano Lett., 2017, 17, 1007. 210. C. Y. Wang, Y. K. Zhang, A. F. Wang, Q. Wang, H. Y. Tang, W. Shen, Z. Li and Z. T. Deng, Chem. Mater., 2017, 29, 2157. 211. P. Vashishtha, D. Z. Metin, M. E. Cryer, K. Chen, J. M. Hodgkiss, N. Gaston and J. E. Halpert, Chem. Mater., 2018, 30, 2973. 212. T. Guner, G. Topcu, U. Savaci, A. Genc, S. Turan, E. Sari and M. M. Demir, Nanotechnology, 2018, 29, 927. 213. W. N. Liu, J. J. Zheng, S. Cao, L. Wang, F. M. Gao, K. C. Chou, X. M. Hou and W. Y. Yang, Inorg. Chem., 2018, 57, 1598. 214. L. J. Chen, C. R. Lee, Y. J. Chuang, Z. H. Wu and C. Y. Chen, J. Phys. Chem. Lett., 2016, 7, 5028. 215. M. J. Ashley, M. N. O'Brien, K. R. Hedderick, J. A. Mason, M. B. Ross and C. A. Mirkin, J. Am. Chem. Soc., 2016, 138, 10096.

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216. L. L. Gu, M. M. Tavakoli, D. Q. Zhang, Q. P. Zhang, A. Waleed, Y. Q. Xiao, K. H. Tsui, Y. J. Lin, L. Liao, J. N. Wang and Z. Y. Fan, Adv. Mater., 2016, 28, 9713. 217. A. Waleed, M. M. Tavakoli, L. L. Gu, Z. Y. Wang, D. Q. Zhang, A. Manikandan, Q. P. Zhang, R. J. Zhang, Y. L. Chueh and Z. Y. Fan, Nano Lett., 2017, 17, 523. 218. A. Waleed, M. M. Tavakoli, L. L. Gu, S. Hussain, D. Q. Zhang, S. Pod, Z. Y. Wang, R. J. Zhang and Z. Y. Fan, Nano Lett., 2017, 17, 4951. 219. J. Xing, X. F. Liu, Q. Zhang, S. T. Ha, Y. W. Yuan, C. Shen, T. C. Sum and Q. H. Xiong, Nano Lett., 2015, 15, 4571. 220. D. Zhang, Y. Yang, Y. Bekenstein, Y. Yu, N. A. Gibson, A. B. Wong, S. W. Eaton, N. Kornienko, Q. Kong, M. Lai, A. Paul Alivisatos, S. R. Leone and P. Yang, J. Am. Chem. Soc., 2016, 138, 7236.

Chapter 10

Functionalization and Useful Properties and Potential Applications of Nanowires 10.1  Self Assembly and Functionalization In the presence of adipic acid, gold nanorods that are protected with surfactant self-­assemble into ordered structures.1 By using molecular bridges such as cysteine, gold nanorods are connected to each other in an end-­to-­ end fashion.2 End-­to-­end assembly of gold nanospheres and nanorods is also obtained by hybridization of a oligonucleotide.3 The reason behind the choice of mercaptoalkyloligonucleotide is due to the fact that the thiol group attaches to the ends of the nanorods, connecting through with the target oligonucleotide in an end-­to-­end manner. Au nanorods can be aligned by using MWNTs as templates, as shown in Figure 10.1a.4 The longitudinal absorption band of Au nanorods shifts to higher wavelengths as in Figure 10.1b, suggesting a preferable string-­like orientation on the MWNT surface. Using isothermal titration calorimetry, the assembly of Au nanorods into nanonecklaces was examined by Varghese et al.5 A solution of gold nanorods was titrated against the ligand solution and the heat of reaction measured as a function of time. The assembly process occurs in two steps in the case of mercaptopropionic acid, whereas in the case of cysteine, the whole process takes place in one step. The alignment of gold nanorods was investigated in polymer composites and on polymer surfaces.6 It was observed that the long axis of the nanorods was aligned along the polymer stretch path.   Nanoscience & Nanotechnology Series No. 52 Nanotubes and Nanowires, 3rd Edition By C. N. R. Rao, A. Govindaraj and Leela Srinivas Panchakarla © C. N. R. Rao, A. Govindaraj and Leela Srinivas Panchakarla 2022 Published by the Royal Society of Chemistry, www.rsc.org

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Figure 10.1  (a)  TEM image of Au nanorods assembled on MWNTs surface and (b) UV-­visible spectra of Au nanorods (dashed lines) and nanorods attached on MWNTs (solid lines). Reproduced from ref. 4 with permission from John Wiley and Sons, Copyright © 2005 WILEY-­VCH Verlag GmbH & Co. KGaA, Weinheim.

Ordered gold nanostructure assemblies have been obtained by droplet evaporation from single-­ and two-­component systems by Ming et al.7 The Au nanorods display nematic/smectic-­A kind liquid-­crystal packing arrangements, as shown in Figure 10.2. The ordering of the assemblies is dependent on the shape and size of the gold nanostructures. The two-­photon-­excited

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Figure 10.2  (a)  Schematic showing a Au nanorod stabilized by a cetyltrimeth-

ylammonium bromide (CTAB) bilayer, and the droplet-­evaporation-­ induced formation of ordered Au nanorod superstructures, (b) low-­magnification SEM image of a nematic Au nanorod (AR¼4.4) superstructure, (c) SEM image of a Au nanorod (AR¼4.4) superstructure at higher magnification, (d) low-­magnification SEM image of a smectic-­A Au nanorod superstructure, (e) SEM image of a smectic-­A Au nanorod superstructure at higher magnification. Reproduced from ref. 7 with permission from John Wiley and Sons, Copyright © 2008 WILEY-­VCH Verlag GmbH & Co. KGaA, Weinheim.

luminescence of ordered gold nanorod assemblies is larger than that of disordered nanorod assemblies. Rings of nanorods were found by drying solutions of Au nanorods attached to polystyrene.8 The production of well-­dispersed metal nanowire suspensions has been documented. For example, Xu et al.9 used template-­assisted synthesis to

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create gold nanowires, which were then functionalized with thiols terminated with a sulfonate group. The degree of aggregation of the functionalized nanowires was determined by the solvents used, with isopropyl alcohol providing stronger dispersions. Wu et al.10 studied the dispersing properties of gold, silver and copper high aspect ratio metal nanowires synthesized from anodic alumina membranes in different solvents. Gold nanorods synthesized with the cationic surfactant cetyltrimethylammonium bromide were exchanged with thiolated ligands, allowing the nanowires to be suspended in buffer solutions.11 Nandana et al.12 used polymers extracted from chitosan oligosaccharides, which contain various thiols as well as primary amine groups, to modify the surface of gold nanorods and nanospheres. Gold nanorods can be conjugated to antibodies for biological applications.13 The use of antigens explicitly bound to antibodies seems to be a viable method for creating a linear, directed assembly of gold nanorods.14 Antimouse IgG was immobilized through thioctic acid with a terminal carboxyl group, on the {111} end faces of gold nanorods. The assembly of biofunctionalized nanorods was achieved by using mouse IgG for biorecognition and binding. By using an engineered M13 bacteriophage as a template, nanowires and other nanostructures have been assembled.15 Phage clones with gold-­binding motifs at one end of the capsid and streptavidin-­ binding motifs can be used to assemble Au and CdSe nanoparticles into one-­ dimensional ordered arrays on more complex geometries. Palladium nanowires were stabilized with thiol-­functionalized ionic liquids after being produced by reducing aqueous solution H2PdCl4 with NaBH4 in the presence of gold nanoparticles.16 To diffuse copper nanowires in nonpolar solvents, alkanethiols were added to them.17 Surface modification of magnetic copper–nickel nanowires with trioctyl phosphine and oleic acid favors dispersion in toluene.18 DNA oligonucleotides can be covalently attached to silicon nanowires, which show biomolecular recognition properties.19 CVD-­synthesized Ge nanowires were functionalized with alkanethiols rendering them dispersible in organic solvents and assembled readily into close-­ packed Langmuir–Blodgett films, as shown in Figure 10.3.20 WO3 nanowires with diameters in the 20–100 nm range and lengths of 300–1000 nm have been prepared by tungsten isopropoxide decomposition in a solution of benzyl alcohol and self-­assembled into bundles.21 The bundles are made up of crystalline nanowires with diameters of around 1 nm and aspect ratios of more than 500. NO2 has a high sensitivity in gas-­sensing experiments. The WO3 nanostructures grow and assemble under different reaction conditions in the presence of deferoxamine mesylate.22 By employing a facile solution method, assembly of SnO2 nanorods into arrays was achieved on an α-­Fe2O3 nanotube surface.23 By using a scanning tunnelling microscope, SiO2 nanowires have been assembled on silica aerogel substrates.24 Metal oxide nanostructures could be surface modified by grafting a desired functional group onto a preformed nanostructure (post-­synthesis

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Figure 10.3  Langmuir–Blodgett  film of GeNWs. (a) SEM image of a GeNW film

with dodecanethiol (C12) functionalization. Inset: photo of a GeNW suspension in chloroform. (b) SEM image of a GeNW film with octyl (C8) functionalization. Inset: TEM image of GeNWs functionalized with thiol. Reproduced from ref. 20 with permission from American Chemical Society Copyright 2005.

modification) or by adding the functionality during synthesis (in situ modification). While organic compounds including thiols, carboxylic acids and amines can be used as surface modifiers for metal oxide nanostructures, there have been few records of amines or thiols being used. Silanes and phosphonates are two compounds that have been used to alter metal oxide nanostructures. Surface hydroxyl groups on the surfaces of as-­synthesized metal oxide nanostructures may help them diffuse in polar solvents. The sedimentation behavior of dispersions of Al2O3, ZnO and MgO nanowires has been examined in DMF, DMSO and acetonitrile.25 DMF was found to be the best solvent for stable oxide nanowire dispersions, except in the case of Al2O3 nanowires.

Chapter 10

546 26

Ghosh et al. investigated the dispersion of metal oxide nanostructures (ZnO, Fe3O4 and TiO2) in polar solvents (water, dimethylformamide (DMF) and toluene) in the presence of surfactants (polyethylene oxide (PEO), polyethylene glycol (PEG), sodium bis(2-­ethylhexyl) sulphosuccinate (AOT), polyvinyl alcohol (PVA) and Triton X-­100 (TX-­100)). Surfactant solutions of 1% (in the case of nanoparticles) or 2% (in the case of nanowires) (weight/volume) strength were prepared in various solvents for this reason. A known quantity of the nanostructures (5 mg in each case) was distributed in 20 ml of the solutions and sonicated for 1 hour in a water bath, resulting in nanostructure dispersions with differing degrees of stability depending on the solvent and surfactant. The following are the best solvent–surfactant/polymer formulations for different oxide nanowires, according to these authors. PEO, PVA, and TX-­100 were able to stabilize TiO2 nanowire dispersions in both water and DMF, while AOT developed strong dispersions in both DMF and toluene. When PEO was used as the binding agent, the authors were able to prepare stable dispersions in all three solvents using ZnO nanowires. In both water and DMF, AOT kept the nanowires in solution for long periods of time. SDS also developed stable ZnO nanowire dispersions in water, while PEG, PVA and TX-­100 stabilized the dispersions in DMF. AOT stabilized Fe3O4 nanoparticles in toluene, whereas PEO, AOT, PVA and PEG held the nanoparticles for longer in water. Similarly, when AOT and PEO were used in water, stable dispersions of TiO2 nanoparticles were produced. AOT formed stable dispersions in the case of toluene. Organosilanes with various functionalities can be used effectively to modify metal oxide surfaces. Fadeev et al.27 have reported surface modification of silica with oligo(dimethylsiloxane). Zinc oxide nanostructures have been coated with 3-­aminopropyltriethoxysilane (APTES) and used for plasmid DNA purification, polymerase chain reactions and delivery under different reaction conditions.28,29 Alkoxysilanes were used by Byrne et al.30 to functionalize titania nanorods and make nanocomposites of titania-­reinforced polystyrene. Rao and coworkers26,31,32 used hexamethyltrimethoxysilane (HDTMS) to covalently functionalize nanowires of ZnO, TiO2 and Al2O3, and ZnO, TiO2, Fe3O4 and CeO2 nanoparticles to diffuse the nanostructures in nonpolar solvents. In a standard reaction, the metal oxide nanostructures and organosilane were mixed in a 1 : 1 ratio and stirred or refluxed in dry toluene before being functionalized and washed in dry hexane to remove excess silanes. Infrared spectroscopy was used to determine the covalent form of the functionalization. The dispersions of HDTMS-­coated CeO2 and Fe3O4 nanoparticles and TiO2 and ZnO nanowires are seen in Figure 10.4a–d. The above-­mentioned technique can be used to functionalize metal oxide nanostructures with organotin reagents, according to these authors. Organotin reagents including dibutyldimethoxytin and trioctyltinchloride were used for this. The organotin reagents form covalent bonds with the metal oxide surfaces, allowing nanostructures to be dispersed in nonpolar solvents. Photographs of dispersions obtained by covalent functionalization of TiO2 and ZnO nanowires with dibutyldimethoxytin are seen in Figure 10.4h and i. The

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Figure 10.4  Photographs  showing dispersions of HDTMS-­coated (a) CeO2 nanopar-

ticles, (b) Fe3O4 nanoparticles, (c) TiO2 nanowires, (d) ZnO nanowires, (e) MWNTs, (f) BN nanoparticles, (g) BNNTs and DBDMT-­coated (h) TiO2 nanowires, (i) ZnO nanowires and ( j) MWNTs in toluene. Reproduced from ref. 31 and 32 with permission from Springer Nature, Copyright 2007, and the Royal Society of Chemistry.

organic component of the organosilane/organotin-­coated nanostructures decomposes when heated in air/oxygen, resulting in silica/tin oxide-­coated nanostructures. As seen by the X-­ray diffraction (XRD) pattern, the silica coating was amorphous, while the tin oxide coating obtained by heating the organotin-­coated nanostructures was crystalline. Nd2O3 nanoparticles functionalized with dimethyldichlorosilane and dispersed in dimethylsulfoxide serve as a liquid laser medium with excellent fluorescence.33

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In2O3 nanowires have been preferentially functionalized by a mild self-assembly method for biosensing applications, using 4-­(1,4-­dihydroxybenzene) butyl phosphonic acid (HQ-­PA) (Figure 10.5).34 A high-­throughput technique for lithographic processing of one-­dimensional nanowires has been reported.35

Figure 10.5  (a)  Schematic representation of an In2O3 nanowire mat device. The

NW is functionalized with a SAM of HQ-­PA and is placed between two gold electrodes protected by a SAM of dodecane-­1-­thiol. (b) (i) The monolayer of HQ-­PA, deposited on the In2O3 nanowire or ITO, can be reversibly oxidized to Q-­PA in an electrochemical cell. (ii) Addition of the probe, thiol-­terminated DNA (HS-­DNA) to Q-­PA. (iii) Attachment of complementary DNA strand (dye-­DNAj) to the probe DNA. (c) Fifteen consecutive CVs showing the reversible oxidation/reduction of a SAM of HQ-­PA on an ITO glass sheet. (Inset) Chronoamperometry shows the amount of charge necessary to oxidize a predefined area of HQ-­PA: an average of 56.8 mC. (d) A fluorescence image of ITO surface that was oxidized to Q-­PA, reacted with SH-­DNA, and the DNA paired to its complementary DNA strand labeled with a fluorescent dye. (e) A fluorescence image of ITO with a HQ–PA monolayer that went through the same DNA attachment procedure as that of the ITO sheet in (d), showing little or no DNA binding. Reproduced from ref. 34 with permission from American Chemical Society, Copyright 2005.

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10.2  Useful Properties and Potential Applications In the previous section, we addressed synthesis as well as some of the useful properties of inorganic nanowires with possible technical applications. Several workers have shown an interest in the numerous applications related to electronics. As a result, working unit components and module arrays have been assembled using semiconducting nanowire building blocks.36 Nanowire junction arrays with large gain have been designed as OR, AND and NOR logic-­gate structures and used to implement simple computation.37 Bottom-­up assembly of engineered segments of various III–V semiconductor nanowires resulted in functional resonant tunneling diodes.38 Wang39 presented some exciting applications in areas such as sensors, photon detectors and nanogenerators. Electrical, optoelectronic, field emission and mechanical properties of individual nanowires, as well as some novel devices and applications such as nanogenerators, nanopiezotronic devices, solar cells, light-­emitting diodes, and ultra-­sensitive chemical and biological nanosensors based on these properties have been reported.40 Lieber and coworkers41 described the nanowire transistor performance limits and applications. Basic methods for organizing nanowires used for fabricating devices and circuit arrays, as well as a novel application of thin-­film nanowire transistor arrays on low-­cost substrates, have been demonstrated with results for comparatively high-­frequency ring oscillators and fully transparent device arrays. Applications of metal oxide nanowires in electronic and optoelectronic devices, and chemical sensing have been reviewed.42 Hochbaum and Yang43 have described how semiconductor nanowires containing heterojunctions promote charge separation and directional transport and hence can be used for energy conversion as well as for selectively positioning catalysts. These studies were carried out in 2010 or earlier.

10.2.1  Optical Properties Using Mie theory as well as discrete dipole approximation, the scattering and absorption properties of gold nanoparticles of different shapes and sizes were calculated.44 For the three widely used classes of nanoparticles (gold nanospheres, silica–gold nanoshells and gold nanorods) absorption, scattering efficiencies and optical resonance wavelengths were calculated. The study reveals well-­established dependencs of the optical properties on the dimensions of the nanoparticles. Gold nanorods exhibit optical cross-­ sections analogous to nanoshells and nanospheres, (however, at far smaller effect sizes). To relate the effectiveness of different sizes of nanoparticles for biomedical applications, size-­normalized optical cross-­sections have been calculated. Gold nanorods exhibit coefficients of scattering and absorption per micron which are higher by an order of magnitude than those for nanoshells and nanospheres. Higher-­order plasmon resonances were studied using cylindrical colloidal gold nanorods that were electrochemically deposited in AAO.45 Homogeneous suspensions of nanorods have been used

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with 85 nm average diameter and varying lengths. The experimental optical spectrum is in agreement with discrete dipole approximation (DDA) calculations that were modeled from the basis of the gold nanorod dimensions. The patterns produced via lithographical means, (the even as well as the odd modes) were identified up to seventh order, in strong agreement with DDA calculations. Aspect-­ratio-­dependent fluorescence of Au nanorods has been examined in detail.46 The relaxation mechanism of the excited state tends to be dominated by nonradiative mechanisms. O'Dwyer et al.47 explained the formation of layers of indium tin oxide nanowires with optimal electronic and photonic properties, illustrating their use as clear top contacts for light-­emitting devices in the visible to near-­infrared range. For the same layer resistance, Lee et al.48 showed that solution-­processed transparent electrodes made up of random meshes of metal nanowires have optical clarities similar to or greater than metal-­oxide thin films. The efficiency of organic solar cells deposited on these electrodes is comparable to that of devices operating on a typical metal-­oxide transparent electrode. Schubert et al. have mapped the polarization pattern of anisotropic nanostructure plasma modes using single-­particle spectroscopy.49 Optical transmission characteristics of silver nanorods prepared through electrodeposition in AAO membranes have been studied.50 Wurtz et al.51 observed strong coupling between a plasmon assisted by an aligned Au nanorod assembly and a molecular exciton.51 Yao et al.52 reported negative optical refraction in bulk metamaterials of nanowires. Claudon et al.53 created a high-­efficiency single-­photon source using an InAs quantum dot embedded in a GaAs photonic nanowire with precisely tailored ends. A record source efficiency of 0.72 was observed under optical pumping, with pure single-­photon emission. This nonresonant method allows for broadband spontaneous emission regulation, allowing for the creation of single-­photon sources based on spectrally large emitters, wavelength-­ tunable sources or effective entangled photon-­pair sources. Gautam et al.54 discovered two distinct concentric assemblies of zinc oxide rods, each with an inherently positive and negative polar end caused by the noncentrosymmetric Zn and O atom structure. Rods from a single assembly all radiate from a central area, retaining a single polar orientation. These assemblies are distinct in their intrinsic properties due to growth along the two polar surfaces of different atomic configurations, and show heavy UV luminescence in the exterior of Zn-­polar assemblies, unlike O-­polar assemblies. These findings indicate that hierarchical structure with respect to internal asymmetry could be common in natural crystal assemblies, including the possibility of novel applications. Control over the interaction between single photons and individual optical emitters is a problem of importance in quantum science and engineering. Via sub-­wavelength trapping of optical fields near metallic nanostructures, Akimov et al.55 demonstrated a cavity-­free, broadband solution for engineering photon–emitter interactions. When a single CdSe quantum

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dot is optically stimulated near a silver nanowire, the quantum dot emission couples directly with the nanowire surface plasmons, allowing the ends of the wire to light up. Effective coupling is followed by a 2.5-­fold increase in quantum dot spontaneous emission, according to results from a large number of instruments. The lasing characteristics of nanowires of ZnO was mentioned earlier. ZnS nanoribbons also exhibit good lasing characteristics.56 Room-­temperature ultraviolet lasers can be produced by using ZnO nanowires with low defect density, synthesized by a low-­temperature aqueous pathway.57 ZnO nanowires prepared under optimal synthesis conditions exhibit a low threshold of pump fluence. Optimal microstructure and density of defects of ZnO nanowires are critical for realizing room-­temperature ultraviolet lasing. The quantum efficiency of ZnO nanowires and other characteristics of nanolasers have been studied.58 ZnO nanowires on sapphire and Si produced by pulsed laser ablation exhibit a deferential external quantum efficiency of 60%. Continuous-­wave laser excitation of aligned CdS nanowires show optical waveguide action.59 Using time-­resolved and temperature-­dependent photoluminescence measurements, the lasing action mechanism of single CdS nanowire cavities has been explained (Figure 10.6).60 Temperature-­ dependent photoluminescence measurements exhibit rich spectral features and disclose that, down to 75 K, exciton–exciton interactions are critical for lasing, whereas at higher temperatures the exciton–phonon process dominates. Using integrated, microfabricated electrodes based on CdS and GaN nanowires, electric-­field modulation of ultraviolet and visible nanoscale lasers has been achieved.61 GaN nanowires show optically pumped lasing with a low lasing threshold at room temperature.62 Multi-­quantum-­well (MQW) core–shell nanowire heterostructures built on well-­defined III-­nitride materials lase over a wide range of wavelengths at room temperature, according to Qian et al.63 Ring resonator lasers based on GaN nanowires have been reported.64 Recent developments in nanomanipulation have permitted the shape of GaN structures to be modified from a linear to a pseudo-­ring conformation. The ring-­mode red shift for a given cavity increases with decreasing diameter of the ring. Significant differences are found during the optical pumping of a hole resonator nanolaser compared to the linear counterpart. The change arises from cavity conformation changes rather than band-­gap renormalization. The photoluminesce of a CdSxSe1−x nanobelt can be controlled with emissions varying from 500 nm to 700 nm.65 The band gap is engineered in these nanowires by controlling the composition.66 Using a microreactor technique, development of a self-­organized photosensitive chain of gold nanoparticles encapsulated in a dielectric nanowire has been accomplished.67 Such a nanowire exhibits marked surface plasmon resonance (SPR) absorption. More interestingly, in a two-­terminal system, a clear wavelength-­dependent, reversible photoresponse was demonstrated using an ensemble of gold nanopeapod silica nanowires under illumination. Plain silica nanowires do not exhibit any photoresponse. These

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Figure 10.6  (A)  Schematic of single NW optical experiments. (B) PL image show-

ing luminescence from the excitation area (lower left) and one end (upper right) of a CdS NW. The NW was excited with a focused beam (5 mm in diameter) with a power of 10 nJ cm−2; scale bar, 5 mm. (C) PL spectra of CdS NW end emission recorded at 4.2 K with excitation powers of 0.6, 1.5, 30 and 240 nJ cm−2 as indicated on each curve. The inset shows the peak intensity of I1 (squares) and P (circles) bands vs. incident laser power. The solid lines are fits to experimental data with power exponents of 0.95 for I1 and 1.8 for P. Reproduced from ref. 60 with permission from American Chemical Society, Copyright 2005.

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findings suggest that gold nanopeapodded silica nanowires may be used as wavelength-­controlled optical nanoswitches. A number of one-­dimensional metal–dielectric hybrid nanostructures can be prepared using the microreactor process for use as practical building blocks for nanoscale waveguiding devices, optoelectronics and sensors. Because of their improved nonlinear third-­order susceptibility near the SPR frequency, these noble-­metal nanoparticles incorporated in dielectric matrices are expected to have uses in ultrafast all-­optical switching instruments. It has been shown how light can be used to operate a basic nanorotor, regardless of the polarization of the light68 (Figure 10.7). When a single-­beam optical trap gradient force is used to hold an asymmetric nanorod, a torque on the nanorod is produced by the scattering force, making it rotate around the optical axis. Morphological asymmetry or intrinsic textural irregularity gives rise to the torque under pressure from the radiation. Even a small nonzero chirality surface irregularity is enough to produce sufficient torque to regulate the rotational speed. Semiconductor nanowire optical trapping and integration assemblies in water have been achieved.69 A single-­beam optical, infrared trap is used to capture, move and assemble high-­aspect-­ratio semiconductor nanowires in a fluid atmosphere. Nanowires with small diameters

Figure 10.7  Time  sequences of different sized and shaped rotors are shown

here. In each time frame the orientation of the rotor is indicated by an arrow. Panels (A), (B) and (C) represent rotations of three Al–O– rotors. The rotor in panel (A) is a typical nanorod, whereas the rotor in panel (B) is a bigger asymmetric nanorod and in panel (C), the rotor is a nanorod bundle. The predicted structures of the rotors in panels (B) and (C) have been depicted in the rightmost column. A size bar has been shown at the bottom right-­hand corner. Magnification factors of all the images are the same. The images shown in panel (A) are diffraction-­limited images and hence they do not convey the real size of the rotor. Reproduced from ref. 68 with permission from IOP Publishing.

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of 20 nm and aspect ratios of more than 100 can be trapped and transported in three dimensions, allowing the creation of nanowire architectures, which can work as active photonic devices. Under physiological conditions, nanowire systems afford new ways of chemical, optical and mechanical stimulation of living cells. The properties and functions of oxide nanoribbons that serve as sub-­ wavelength optical waveguides have been identified, and their potential use as nanoscale photonic elements has been evaluated.70 Oulton et al.71 reported a hybrid plasmonic waveguide for sub-­wavelength confinement and long-­ range propagation. These authors propose a method that combines plasmonics and dielectric waveguiding. A dielectric nanowire is isolated from a metal surface by a nanoscale dielectric difference in the hybrid optical waveguide. When the plasmonic and waveguide modes are coupled, capacitor-­like energy storage is allowed, allowing for efficient sub-­wavelength propagation in nonmetallic regions. Thus, surface plasmon polaritons can move over long distances (40–150 µm) with confinement of the strong mode varying from λ2/400 to λ2/40. This approach may lead to nanoscale semiconductor-­ based plasmonics and photonics. Oulton et al.72 also reported an experimental demonstration of nanometer-­scale plasmonic lasers, generating optical modes a hundred times smaller than the diffraction limit. Manjavacas and Abajo73 have prepared robust plasmon waveguides in strongly interacting nanowire arrays by designing plasmon-­based interconnects and achieving a high degree of integration. Efficient exciton–plasmon–photon conversion and guiding are demonstrated with plasmonic nanostructures such as silver nanowire functionalized with CdSe nanocrystals.74

10.2.2  Photonic Applications of Perovskite NWs Due to their favorable optical absorption coefficients and low defect density, perovskites have gained importance in optoelectronics for use in solar cells and optical devices such as lasers and photodiodes.75 Due to the one-­ dimensional nature of nanowires, they can be good candidates over other-­ dimensional counterparts for photon waveguides that are important in lasing applications.

10.2.2.1 Perovskite NW Lasers Owing to long carrier lifetimes and small nonradiative recombination rates, organic–inorganic hybrid perovskites are used in high-­performance solar cells. These properties are equally important for semiconducting lasing. Zhu et al. have reported lasing action for the first time in high-­quality NWs of CH3NH3PbBr3, CH3NH3PbI3 and CH3NH3Pb(Cl, Br)3.76 These perovskite nanowires form ideal Fabry–Pérot cavities for lasing. The NWs exhibit low a lasing threshold of 220 nJ cm−2 and a high quality factor of 3600 at room temperature. The wavelength of lasing can be tuned by changing the halide or

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77

changing the organic cation in the nanowires. Emission spectra and optical emission images of CH(NH2)2PbI3 nanowires are shown in Figure 10.8. Optical images of (FAxMA1−x)Pb(Br3−yIy) nanowires are shown in Figure 10.8c. Large-­scale arrays of CH3NH3PbBr3 NWs fabricated by Liu and coworkers show low-­threshold lasing with uniform emission.78 Performance of halide perovskite NW lasers is tabulated in Table 10.1. Inorganic halide perovskites have gained attention due to their greater stability compared to their organic counterparts.79–81 Eaton et al. have reported the solution-­based synthesis of CsPbX3 (X = Cl, Br, I) and good-­quality laser cavities based on them.79 CsPbBr3 NWs show a threshold of 5 µJ cm−2 with a quality factor of ∼1009 under femtosecond pulsed laser excitation. CsPbCl3 NWs have a lasing threshold of 86.5 µJ cm−2. Wavelength-­tunable lasing (420–710 nm) from single-­crystalline CsPbX3 (X = Cl, Br, I), nanowires and CsPb(Cl,Br)3 and CsPb(Br,I)3 nanowires has been reported by Fu et al.82 Stable lasing emission up to 8 h under femtosecond pulsed laser illumination is realized in these nanowires at room temperature. Huang et al. have realized dual-­colour lasing from a single nanowire of compositionally graded CsPbBrxI3−x NWs.83 Plasmonic enhanced exciton–photon coupling is observed in the presence of a Ag layer in perovskite nanowires,

Figure 10.8  (a)  Emission spectra of a CH(NH2)2PbI3 nanowire around the thresh-

old. Integrated PL intensity as well as emission peak FWHM as a function of power is shown in the inset. (b) Single-­nanowire optical image (top) and emission images below (middle) and above (bottom) the threshold. Scale bar: 10 µm. (c) Series of optical images of (FAxMA1−x) Pb(Br3−yIy) nanowires. Reproduced from ref. 77 with permission from American Chemical Society, Copyright 2016.

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Table 10.1  Performances  of nanowire lasers of halide perovskites. Reproduced from ref. 75 with permission from John Wiley and Sons, Copyright © 2018 WILEY-­VCH Verlag GmbH & Co. KGaA, Weinheim.

Materials

Wavelength [nm]

Threshold [µJ cm−2]

Quality factor

CH3NH3PbX3 NW CH3NH3PbBr3 NW CH3NH3PbX3 NW CH(NH2)2PbX3 NW CH3NH3PbBr3 NW CsPbX3 NW CsPbX3 NW CsPbX3 NW CsPbBr3 NW CsPbX3 NW CsPbBrxI3−x NW

∼490–∼770 543 ∼550–∼780 ∼500–∼820 ∼550 ∼430, ∼535 425, 525, 725 ∼425–∼710 540 ∼430–∼730 521, 556

0.22 12.3 11 ≈6 5 5 6 6.2 8 4 16

3600 500 405 2000 1009 1300 1000 2256

which in turn are used for low-­threshold plasmonic and polariton lasing in CH3NH3PbBr3 and CH3NH3PbI3 perovskite NWs.84,85 In these measurements, nanowires are placed on Ag layers separated by a thin SiO2 layer.

10.2.2.2 Photodetectors High optical absorption, quantum yield and well-­confined carrier transport channels of perovskite nanowires make them prominent materials for photodetectors.86,87 Zhuo et al.88 have reported highly sensitive and fast response photodetectors of porous CH3NH3PbBr3 perovskite NWs. The poor environmental stability of hybrid perovskites is improved by capping with oleic acid. Gao et al. have reported good sensitivity (4.95 A W−1) and small response time (0.1 ms) along with improved stability on the oleic acid-­passivated surface of CH3NH3PbI3 NWs.89 Oleic acid passivation also improves carrier lifetime up to 51 ns by decreasing the nonradiative recombination process. Xu et al. have reported high responsivity (460 A W−1) and detectivity (2.6 × 1013 Jones) along with an ultrafast response time of 180 µs from high-­quality CH3NH3PbI3 NW.90 Other than individual nanowires, networks of perovskite nanowires have also been realized. These are useful for wafer-­scale applications.91,92 Photodetectors from a network of CH3NH3PbI3 nanowires have been fabricated by Horváth et al.93 The detector showed low dark current (tens of pA). The current of the nanowires reached up to a factor of 300 under laser illumination. The short response time (