Nanomaterials: Basics to Applications 9811938806, 9789811938801

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Yarub Al-Douri

Nanomaterials Basics to Applications

Nanomaterials

Yarub Al-Douri

Nanomaterials Basics to Applications

Yarub Al-Douri American University of Iraq Sulaimani, Iraq Bahcesehir University Istanbul, Turkey University of Malaya Kuala Lumpur, Malaysia

ISBN 978-981-19-3880-1 ISBN 978-981-19-3881-8 (eBook) https://doi.org/10.1007/978-981-19-3881-8 © The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2022 This work is subject to copyright. All rights are solely and exclusively licensed by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, expressed or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. This Springer imprint is published by the registered company Springer Nature Singapore Pte Ltd. The registered company address is: 152 Beach Road, #21-01/04 Gateway East, Singapore 189721, Singapore

Preface

Nanomaterials describe, in principle, materials in which a single unit is sized (in at least one dimension) between 1 and 1000 nm (10–9 m) but is usually 1–100 nm. Nanomaterials research takes a materials science -based approach to nanotechnology, leveraging advances in materials metrology and synthesis which have been developed in support of microfabrication research. Materials with structure at the nanoscale often have unique optical, electronic or mechanical properties. Biological systems often feature natural, functional nanomaterials. The structure of foraminifera (mainly chalk) and viruses (protein, capsid), the wax crystals covering a lotus or nasturtium leaf, spider and spider-mite silk, the blue hue of tarantulas, the “spatulae” on the bottom of gecko feet, some butterfly wing scales, natural colloids (milk, blood), horny materials (skin, claws, beaks, feathers, horns, hair), paper, cotton, nacre, corals and even our bone matrix are all natural organic nanomaterials. Natural inorganic nanomaterials occur through crystal growth in the diverse chemical conditions of the Earth’s crust. For example, clays display complex nanostructures due to the anisotropy of their underlying crystal structure and volcanic activity can give rise to opals, which are an instance of naturally occurring photonic crystals due to their nanoscale structure. Fires represent particularly complex reactions and can produce pigments, cement, fumed silica, etc. Inorganic nanomaterials, (e.g. quantum dots, nanowires and nanorods) because of their interesting optical and electrical properties, could be used in optoelectronics. Furthermore, the optical and electronic properties of nanomaterials which depend on their size and shape can be tuned via synthetic techniques. There are possibilities to use those materials in organic material-based optoelectronic devices such as organic solar cells, OLEDs, etc. The operating principles of such devices are governed by photoinduced processes like electron transfer and energy transfer. The performance of the devices depends on the efficiency of the photoinduced process responsible for their functioning. Therefore, a better understanding of those photoinduced processes in organic/inorganic nanomaterial composite systems is necessary in order to use them in organic optoelectronic devices. Nanoparticles or nanocrystals made of metals, semiconductors or oxides are of particular interest for their mechanical, electrical, magnetic, optical, chemical and v

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other properties. Nanoparticles have been used as quantum dots and as chemical catalysts such as nanomaterial-based catalysts. Recently, a range of nanoparticles are extensively investigated for biomedical applications including tissue engineering, drug delivery and biosensor. Nanoparticles are of great scientific interest as they are effectively a bridge between bulk materials and atomic or molecular structures. A bulk material should have constant physical properties regardless of its size, but at the nanoscale, this is often not the case. Size-dependent properties are observed such as quantum confinement in semiconductor particles, surface plasmon resonance in some metal particles and superparamagnetism in magnetic materials. Nanoparticles exhibit a number of special properties relative to bulk material. For example, the bending of bulk copper (wire, ribbon, etc.) occurs with the movement of copper atoms/clusters at about the 50 nm scale. Copper nanoparticles smaller than 50 nm are considered super hard materials that do not exhibit the same malleability and ductility as bulk copper. The change in properties is not always desirable. Ferroelectric materials smaller than 10 nm can switch their magnetisation direction using room temperature thermal energy, thus making them useless for memory storage. Suspensions of nanoparticles are possible because the interaction of the particle surface with the solvent is strong enough to overcome differences in density, which usually result in a material either sinking or floating in a liquid. Nanoparticles often have unexpected visual properties because they are small enough to confine their electrons and produce quantum effects. For example, gold nanoparticles appear deep red to black in solution. The often very high surface area to volume ratio of nanoparticles provides a tremendous driving force for diffusion, especially at elevated temperatures. Sintering is possible at lower temperatures and over shorter durations than for larger particles. This theoretically does not affect the density of the final product, though flow difficulties and the tendency of nanoparticles to agglomerate do complicate matters. The surface effects of nanoparticles also reduce the incipient melting temperature. The chemical processing and synthesis of high-performance technological components for the private, industrial and military sectors require the use of highpurity ceramics, polymers, glass ceramics and material composites. In condensed bodies formed from fine powders, the irregular sizes and shapes of nanoparticles in a typical powder often led to non-uniform packing morphologies that result in packing density variations in the powder compact. Uncontrolled agglomeration of powders due to attractive van der Waals forces can also give rise to microstructural inhomogeneities. Differential stresses that develop as a result of non-uniform drying shrinkage are directly related to the rate at which the solvent can be removed, and thus highly dependent upon the distribution of porosity. Such stresses have been associated with a plastic-to-brittle transition in consolidated bodies and can yield crack propagation in the unfired body if not relieved.

Preface

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In addition, any fluctuations in packing density in the compact as it is prepared for the kiln are often amplified during the sintering process, yielding inhomogeneous densification. Some pores and other structural defects associated with density variations have been shown to play a detrimental role in the sintering process by growing and thus limiting end-point densities. Differential stresses arising from inhomogeneous densification have also been shown to result in the propagation of internal cracks, thus becoming strength-controlling flaws. It would therefore appear desirable to process a material in such a way that it is physically uniform with regard to the distribution of components and porosity, rather than using particle size distributions which will maximise the green density. The containment of a uniformly dispersed assembly of strongly interacting particles in suspension requires total control over particle-particle interactions. It should be noted here that a number of dispersants such as ammonium citrate (aqueous) and imidazoline or oleyl alcohol (nonaqueous) are promising solutions as possible additives for enhanced dispersion and deagglomeration. Monodisperse nanoparticles and colloids provide this potential. Monodisperse powders of colloidal silica, for example, may therefore be stabilised sufficiently to ensure a high degree of order in the colloidal crystal or polycrystalline colloidal solid which results from aggregation. The degree of order appears to be limited by the time and space allowed for longer range correlations to be established. Such defective polycrystalline colloidal structures would appear to be the basic elements of submicrometre colloidal materials science, and, therefore, provide the first step in developing a more rigorous understanding of the mechanisms involved in microstructural evolution in high-performance materials and components. The academics, researchers, universities and research institutes will be advantaged and benefited from this textbook of nanomaterials for their own under- and postgradute programmes of nanotechnological curriculums as the following: the general properties of physical and chemical ones are given in Chap. 1. Chapters 2 and 3 are focused on the synthesis of nanomaterials using different methods followed by characterisation and analysis. The main studies of mechanical, magnetic, electrical and optical properties are elaborated in Chaps. 4 and 5. The modern concepts that are applicable in different sectors of industry and medicine are explained in Chaps. 6 and 7. Whilst Chaps. 8 and 9 are focused on nanomaterials-based technology of quantum dots and superconductivity. Finally, the last chapter displays different applications of nanomaterials with predicting their future aspects. Sulaimani, Iraq/Istanbul, Turkey/Kuala Lumpur, Malaysia

Yarub Al-Douri

Contents

1

Nanomaterials Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1.1 What Are Nanomaterials? . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1.2 Where Are Nanomaterials Found? . . . . . . . . . . . . . . . . . . . 1.2 Advances in Nanomaterials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3 Nanomaterials Classification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.4 Why Are Nanomaterials Important? . . . . . . . . . . . . . . . . . . . . . . . . . 1.5 Nanomaterials Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.6 The Nanoscience and Nanotechnology . . . . . . . . . . . . . . . . . . . . . . 1.6.1 Nanomaterials Characteristics . . . . . . . . . . . . . . . . . . . . . . 1.7 Nano-effect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.7.1 Exceptional Optical Properties . . . . . . . . . . . . . . . . . . . . . . 1.7.2 Exceptional Thermal Properties . . . . . . . . . . . . . . . . . . . . . 1.7.3 Exceptional Magnetic Properties . . . . . . . . . . . . . . . . . . . . 1.7.4 Exceptional Mechanical Properties . . . . . . . . . . . . . . . . . . 1.7.5 Exceptional Electrical Properties . . . . . . . . . . . . . . . . . . . . 1.7.6 Natural Nano-effect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.8 Physical Principles of Nano-effect . . . . . . . . . . . . . . . . . . . . . . . . . . 1.8.1 Discontinuity of Electron Levels . . . . . . . . . . . . . . . . . . . . 1.8.2 Kubo Theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.8.3 Small Size Effect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.8.4 Surface Effect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.8.5 Dielectric Confinement Effect . . . . . . . . . . . . . . . . . . . . . .

1 1 1 1 3 4 4 5 6 8 9 10 11 12 13 13 14 16 16 17 18 20 21

2

Nanomaterials Synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.1 Nanoparticles Preparation . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2 Icroemulsion-Based Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3 Carbon Fullerenes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4 Synthesis of Nanowires, Nanorods and Nanotubes . . . . . . . . . . . . 2.4.1 Rods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

23 23 23 26 29 31 32

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Nanomaterials Characterisation and Analysis . . . . . . . . . . . . . . . . . . . . 3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2 Particle Size Detection and Analysis . . . . . . . . . . . . . . . . . . . . . . . . 3.3 Detection and Analysis of the Electrical Properties . . . . . . . . . . . . 3.4 Detection and Analysis of Magnetic Properties . . . . . . . . . . . . . . . 3.5 Detection and Analysis of the Mechanical Properties . . . . . . . . . . 3.6 Detection and Analysis of Thermal Properties . . . . . . . . . . . . . . . . 3.7 Detection and Analysis of Optical Properties . . . . . . . . . . . . . . . . . 3.8 Scanning Probe Microscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.9 Principles of Scanning Tunneling Microscopy . . . . . . . . . . . . . . . . 3.9.1 Operating Mode of STM . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.9.2 STM Application: Atomic Manipulation . . . . . . . . . . . . . 3.9.3 STM Advantages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.10 Atomic Force Microscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.10.1 AFM Principle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.10.2 Comparison of the AFM Scanning Modes . . . . . . . . . . . . 3.10.3 Application Examples of AFM . . . . . . . . . . . . . . . . . . . . .

39 39 40 42 43 45 45 48 50 50 50 51 54 54 54 55 56

4

Mechanical and Magnetic Properties of Nanomaterials . . . . . . . . . . . 4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2 Mechanical Behaviour . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.1 Elastic Properties of Nanocrystalline Metals . . . . . . . . . . 4.2.2 Hardness, Yield and Ultimate Strengths . . . . . . . . . . . . . . 4.2.3 Mechanical Properties at Room and Elevated Temperatures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.4 Strength of Amorphous Alloys Containing Nanoscale Particles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.5 Deformation Behaviour of Nanostructured Alloys . . . . . 4.3 Structure and Soft Magnetic Properties . . . . . . . . . . . . . . . . . . . . . . 4.3.1 Effect of Grain-Size Distribution and Curie Temperature of Intergranular Amorphous Phase on Soft Magnetic Properties . . . . . . . . . . . . . . . . . . . . . . . . 4.4 Magnetic Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

59 59 60 61 62

Electrical and Optical Properties of Nanomaterials . . . . . . . . . . . . . . . 5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2 Metals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.1 Quantum Transport of Electrons . . . . . . . . . . . . . . . . . . . . 5.2.2 Electrical Conductivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.3 Surface Plasmons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3 Semiconductor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3.1 Band Gap Modification . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3.2 Quantum Size Effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3.3 Quantization and Energy Level Spacing . . . . . . . . . . . . . .

75 75 76 79 82 86 94 95 95 98

5

63 64 64 65

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5.3.4 5.3.5

Electrical Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99 Optical Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102

6

Nanodevices and Nanostructures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2 Nanodevices General Scheme . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.3 Nanocomponents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.3.1 DNA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.3.2 Carbon Nanotubes and Fullerenes . . . . . . . . . . . . . . . . . . . 6.4 Nanoelectronics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.5 Nanostructured Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.5.1 Nanoparticle Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.5.2 Nanoalloys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.6 Future Modelling Prospects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

105 105 106 106 107 107 108 108 108 111 114

7

Carbon Nanotubes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.1.1 Carbon Allotrope’s Structure . . . . . . . . . . . . . . . . . . . . . . . 7.1.2 Single-Layer Graphite Material (Graphene) . . . . . . . . . . . 7.2 CNTs Types and Nature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.2.1 CNTs Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.2.2 CNTs Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.3 CNTs Electronic Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.3.1 π-Electron Orbital and the Energy of the Conjugated Molecule in Planar Structure . . . . . . . 7.3.2 Graphite Electronic Structure . . . . . . . . . . . . . . . . . . . . . . . 7.4 CNTs Preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.5 CNTs Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.5.1 CNTs Electronics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.6 Single-Electron Transistor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.7 CNTs Electronics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.7.1 Quantum Wire . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.7.2 CNT-Based Junction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.7.3 SET with CNTs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.7.4 CNT-Based FET . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.7.5 Complementary Nongate (Inverter) Circuit with CNTs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.8 Other Applications of CNTs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.8.1 Nano Test Tubes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.8.2 Nanobalance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.8.3 Nanomolds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.8.4 CNTs: Field Emission Cathode Materials . . . . . . . . . . . . 7.8.5 CNTs Application in Hydrogen Storage . . . . . . . . . . . . . .

117 117 118 120 120 120 121 124 124 127 127 129 129 132 136 137 138 140 141 142 144 144 144 144 144 145

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7.8.6 7.8.7 7.8.8 7.8.9 7.8.10

High-Energy Microbattery . . . . . . . . . . . . . . . . . . . . . . . . . High-Energy Capacitor . . . . . . . . . . . . . . . . . . . . . . . . . . . . Chip Thermal/Heat Protection . . . . . . . . . . . . . . . . . . . . . . Nanoreactor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Nanocomposite Materials . . . . . . . . . . . . . . . . . . . . . . . . . .

146 146 146 147 147

8

Semiconductor Quantum Dots . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.2 The Physical Basis of Semiconductor QDs . . . . . . . . . . . . . . . . . . . 8.2.1 Quantum Confinement Effect . . . . . . . . . . . . . . . . . . . . . . . 8.2.2 Excitons and Luminescence . . . . . . . . . . . . . . . . . . . . . . . . 8.3 Semiconductor QDs Preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.4 Laser Devices Based on QDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.5 Single-Photon Source . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

149 149 150 150 153 158 161 165

9

Superconductivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.2 The Physical Principles of Superconductivity . . . . . . . . . . . . . . . . . 9.3 The Superconductors Classification . . . . . . . . . . . . . . . . . . . . . . . . . 9.3.1 Low-Temperature Superconductors . . . . . . . . . . . . . . . . . . 9.3.2 High-Temperature Superconductors . . . . . . . . . . . . . . . . . 9.3.3 Other Novel Superconductors . . . . . . . . . . . . . . . . . . . . . . . 9.4 Nanosuperconductors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.4.1 Incredible Magnetic Nanoclusters . . . . . . . . . . . . . . . . . . . 9.4.2 Quantum Fluctuations and Strong Correlation in Nanowires . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.4.3 Ultrathin Film . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.4.4 Nanosuperconductors and Hybrid Structures . . . . . . . . . . 9.4.5 Links Between Superconductors and Nanostructure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.5 Nanosuperconductor Applications . . . . . . . . . . . . . . . . . . . . . . . . . . 9.5.1 Quantum Computers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.5.2 Nanosuperconductor Quantum Bits . . . . . . . . . . . . . . . . . .

169 169 171 172 172 172 173 174 177

10 Nanomaterial Multi-application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.2 Amorphous Silicon/Oxide Superlattice . . . . . . . . . . . . . . . . . . . . . . 10.3 Single-Electron Transistor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.4 Quantum Dot Laser . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.5 Epilogue . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.6 Chemical and Biological Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.7 Optical Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.8 Catalysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.9 Future Issues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

187 187 188 189 193 195 196 197 198 198

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Blurb . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 201 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 203 Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 209

Chapter 1

Nanomaterials Properties

1.1 Introduction Nanomaterials are cornerstones of nanoscience and nanotechnology. Nanostructure science and technology is a broad and interdisciplinary area of research and development activity that has been growing explosively worldwide in the past few years. It has the potential for revolutionising the ways in which materials and products are created and the range and nature of functionalities that can be accessed. It is already having a significant commercial impact, which will assuredly increase in the future [1] as depicted in Fig. 1.1.

1.1.1 What Are Nanomaterials? Nanoscale materials are defined as a set of substances where at least one dimension is less than approximately 100 nm. A nanometre is one-millionth of a millimeter— approximately 100,000 times smaller than the diameter of a human hair. Nanomaterials are of interest because at this scale unique optical, magnetic, electrical and other properties emerge. These emergent properties have the potential for great impacts in electronics, medicine and other fields (Fig. 1.2) [2].

1.1.2 Where Are Nanomaterials Found? Some nanomaterials occur naturally, but of particular interest are engineered nanomaterials (EN), which are designed for, and already being used in many commercial products and processes. They can be found in such things as sunscreens, cosmetics, sporting goods, stain-resistant clothing, tires, electronics, as well as many other

© The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2022 Y. Al-Douri, Nanomaterials, https://doi.org/10.1007/978-981-19-3881-8_1

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Fig. 1.1 Evolution of science and technology and the future [1]

Fig. 1.2 The nanomaterials types

everyday items, and are used in medicine for purposes of diagnosis, imaging and drug delivery. Engineered nanomaterials are resources designed at the molecular (nanometre) level to take advantage of their small size and novel properties which are generally not seen in their conventional, bulk counterparts. The two main reasons why materials at the nanoscale can have different properties are increased relative surface area and new quantum effects. Nanomaterials have a much greater surface area-to-volume ratio than their conventional forms, which can lead to greater chemical reactivity and affect their strength. Also at the nanoscale, quantum effects can become much more important in determining materials’ properties and characteristics, leading to novel optical, electrical and magnetic behaviours [3]. Nanomaterials are already in commercial use, with some having been available for several years or decades. The range of commercial products available today is very

1.2 Advances in Nanomaterials

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broad, including stain-resistant and wrinkle-free textiles, cosmetics, sunscreens, electronics, paints and varnishes. Nanocoatings and nanocomposites are finding uses in diverse consumer products, such as windows, sports equipment, bicycles and automobiles. There are novel UV-blocking coatings on glass bottles which protect beverages from damage by sunlight, and longer lasting tennis balls using butyl-rubber/nanoclay composites. Nanoscale titanium dioxide, for instance, is finding applications in cosmetics, sun-block creams and self-cleaning windows, and nanoscale silica is being used as filler in a range of products, including cosmetics and dental fillings.

1.2 Advances in Nanomaterials The nanostructures are formed to represent the start of nanomaterials history in the early big bang. Subsequently, numerous alternative nanostructures, such as seashells and skeletons, amongst others, arose naturally. Primitive humans generated smoke particles on a nanoscale when they utilised fire. However, the scientific narrative relating to nanomaterials did not commence for some considerable time. Michael Faraday published one of the initial scientific descriptions, producing colloidal gold particles in 1857. Silica nanoparticles, which were precipitated and fumed, were synthesised in the early 1940s and marketed in the United States (US) and Germany as alternatives to ultrafine carbon black for the fortification of rubber. Nanoconfigured catalytic agents have been the subject of study since the 1950s. The nanoparticles of amorphous silica have multi-application in different fields of daily life like automobile tires, non-dairy coffee creamer, catalyst supports and optical fibres. Metallic nanopowders for use with magnetic recording tapes evolved in the 1960s and the 1970s. Granqvsit and Buhrman reported the initial synthesis of nanocrystals in 1976, utilising the now frequently used inert-gas evaporation method. Maya blue paint has been discovered to comprise a nanoconfigured hybrid substance. The colour derivation and its ability to withstand acids and biocorrosive agents require further delineation, but examination of genuine specimens from Jaina Island has determined that the paint is formed from spine-shaped palygorskite clay crystals that generate a 1.4 nm period superlattice. This is interposed with an amorphous silicate substrate which encompasses nanoparticles of magnesium. The attractive blue hue is achieved when the nanoparticles and superlattice exist concurrently, as has been demonstrated by the production of manmade specimens [4]. To manipulate catalytic, mechanical, magnetic, electric, electronic and optical functions, nanoengineering grows rapidly for both inorganic and organic structural and functional materials. Typically, synthesis of nanophase or cluster-assembled substances is founded on the generation of clusters that are segregated and diminutive in size, and which are then merged into a bulk form or entrenched within condensed liquid or solid matrices, such as nanophase silicon. The latter exhibits physical and electronic characteristics at variance with normal silicon which can then be utilised for macroscopic semiconductor applications to produce de novo devices. If routine

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Fig. 1.3 Nanomaterials classification a 0D spheres and clusters, b 1D nanofibres, wires, and rods, c 2D films, plates, and networks, d 3D nanomaterials

glasses were doped using quantised semiconductor colloids, this would transform them into a high-performance vehicle that has the potential to be deployed for optical computing purposes.

1.3 Nanomaterials Classification The specific dimension of nanomaterials is 100 nm or less. The adjective nanoscale can apply to nanomaterials in one, two or three dimensions, such as in surface films, strands or fibres and particles, respectively. Nanomaterials are present in single, fused, aggregated or agglomerated versions, with spherical, tubular or irregular morphology. Nanotubes, dendrimers, quantum dots and fullerenes are formats that are frequently encountered. Their functionality is related to the sphere of nanotechnology; they exhibit disparate physicochemical traits from their normal counterparts, e.g. silver nano, carbon nanotube, fullerene, photocatalyst, carbon nano and silica [5]. Based on Siegel, the nanostructures are classified into 0D, 1D, 2D and 3D nanostructures. The nanomaterials are characterised by grain size as mentioned earlier. Nanomaterial engineering can be achieved using a range of modulation dimensionalities, as described by Siegel [5], i.e. zero, one, two or three, as evidenced by atomic clusters, filaments and cluster assemblies, multilayers, ultrafine-grained overlayers or buried layers, and nanophase substances comprising equiaxed grains of nanometre parameters (Fig. 1.3).

1.4 Why Are Nanomaterials Important? These materials have created a high interest in recent years by virtue of their unusual mechanical, electrical, optical and magnetic properties. Some examples are given below [6]:

1.5 Nanomaterials Types

(i) (ii)

(iii)

(iv)

(v)

(vi)

(vii)

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Nanophase ceramics are of particular interest because they are more ductile at elevated temperatures as compared to coarse-grained ceramics. Nanostructured semiconductors are known to show various non-linear optical properties. Semiconductor Q-particles also show quantum confinement effects which may lead to special properties, like the luminescence in silicon powders and silicon–germanium quantum dots as infrared optoelectronic devices. Nanostructured semiconductors are used as window layers in solar cells. Nanosized metallic powders have been used for the production of gas-tight materials, dense parts and porous coatings. Cold welding properties combined with the ductility make them suitable for metal–metal bonding, especially in the electronic industry. Single nanosized magnetic particles are mono-domains and one expects that also in magnetic nanophase materials the grains correspond with domains, whilst boundaries on the contrary to disordered walls. Very small particles have special atomic structures with discrete electronic states, which give rise to special properties in addition to the super-paramagnetism behaviour. Magnetic nanocomposites have been used for mechanical force transfer (ferrofluids), high-density information storage and magnetic refrigeration. Nanostructured metal clusters and colloids of mono- or plurimetallic composition have a special impact on catalytic applications. They may serve as precursors for new types of heterogeneous catalysts (Cortex-catalysts) and have been shown to offer substantial advantages concerning activity, selectivity and lifetime in chemical transformations and electrocatalysis (fuel cells). Enantioselective catalysis is also achieved using chiral modifiers on the surface of nanoscale metal particles. Nanostructured metal oxide thin films are receiving a growing attention for the realisation of gas sensors (NOx , CO, CO2 , CH4 and aromatic hydrocarbons) with enhanced sensitivity and selectivity. Nanostructured metal oxide (MnO2 ) finds applications for rechargeable batteries for cars or consumer goods. Nanocrystalline silicon films for highly transparent contacts in thinfilm solar cells and nanostructured titanium oxide porous films for their high transmission and significant surface area enhancement leading to strong absorption in dye sensitised solar cells. Polymer-based composites with a high content of inorganic particles leading to a high dielectric constant are interesting materials for photonic band gap structure.

1.5 Nanomaterials Types Nanomaterials (gold, carbon, metals, metal oxides and alloys) with a variety of morphologies (shapes) are depicted in Fig. 1.4.

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Au nanoparticle

Titanium nanoflower

Buckminsterfullerene

Silver nanocubes

FePt nanosphere

SnO2 nanoflower

Fig. 1.4 Nanomaterials with a varity of morphologies

1.6 The Nanoscience and Nanotechnology Transliteration is a way to define the nanometre (nm). In keeping with millimetre and micron parameters, the definition of a nanometre is a gradation of length, which has no particular physical reference. It represents a billionth of a metre and can be written as 10–9 m. It reflects the distance taken up by a combined configuration of two or three metal atoms, or by the breadth taken up by 10 lone hydrogen atoms. The average virus measures between 60 and 250 nm in diameter, the diameter of an erythrocyte is in the region of 2000 nm, and a hair’s diameter is within the range of 30,000–50,000 nm (Fig. 1.5). The “nano” has been used in the 1980s to define the particle within a range of 1– 100 nm. The inaugural conference of the International Symposium on Nanoscience and Nanotechnology took place in Baltimore, MD in July 1990; an official promulgation was made declaring the science of nanomaterials to be a de novo domain of materials science. Consequently, numerous individuals skilled in scientific and technological aspects entered the nanotechnology research sphere, which gave rise to an accelerated global interest in this novel scientific area. In 1962, Kubo has developed the theory of quantum confinement on ultrafine particles to promote nanoparticle exploration in experimental physics. In Germany, in 1984, Professor H. Gleiter and co-workers manufactured nanocrystals comprising palladium and iron, amongst additional metals. In the US, at the Argonne National Laboratory, nano-TiO2 polycrystalline ceramics were engineered by Dr. Siegel.

1.6 The Nanoscience and Nanotechnology

1 mm

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10 µm 1000 nm

1 µm

100 nm 10 nm 1 nm 0.1 nm

1Å

Fig. 1.5 Comparison of physical scales

These are extremely robust, with no loss of tensile strength or cracking in temperatures equal to or greater than 100 °C. This ground-breaking work inspired the initial global escalation in nanotechnology, thereby formally establishing it as a materials science sphere. For the most known elements, the unique orbital of carbon bonding consists an abundant family of carbon. Originally, it was thought that just three natural carbon allotropes existed, i.e. diamond, graphite and amorphous carbon. However, in 1985, cage-like C60 molecules were recognised by Kroto and co-workers; 60 carbon atoms were sited superior to the football-shaped polyhedrons comprised of 20 and 15 hexagons and pentagons, respectively. Kratschmer initially acquired a macroquantity of manufactured C60 by employing a graphite electrode arc discharge. This discovery stimulated a further spate of nanotechnology studies, which uncovered a sizeable cohort of carbon allotropes of spherical and spheroidal morphology. In 1991, Iljima has discovered a hollow tube in the cathode rod within carbon black deposition resulted in discharge DC arc in the Ar atmosphere. Transmission electron microscopy demonstrated that the diameter of the hollow tube was between 1 and tens of nanometres, whereas the length was from tens of nanometres to a millimetre. Multiple tubes were configured in a coaxial plane, giving rise to a radial distance in the region of 0.34 nm between the neighbouring hollow tubes, e.g. graphite’s plane spacing (002). This structure has now been termed a carbon nanotube. Its distinct configuration has generated a de novo sphere within the science of single-dimensional nanomaterials. The identification of carbon nanotubes gave rise to a further surge in nanotechnological research [7].

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The nanoscale researches are involving three areas: nanomaterials, nanodevices and characterisation and detection of nanotechnology. Studies pertaining to nanostructure and nanomaterials are relevant in that they have enabled individuals to gain a new comprehension of nature; the subject per se has emerged as an enriched resource of de novo knowledge. The dimensions of nanoscale configuration match numerous typical distances within substances, e.g. the de Broglie electron wavelength, length of superconducting coherence, tunnelling barriers depth and the critical dimension of magnetic iron. Nanomaterials and their respective structures therefore have varied physicochemical traits to normal atoms, molecules and macro-substances. The possibilities for investigating nature and generating novel information have been expanded to a middle ground between macro- and micro-items. Within the nanotechnology sphere, the recognition of de novo concepts, comprehending unfamiliar laws and generating new constructs and hypotheses form the bedrock for the construction of a scientific basis for studies relating to nanophysics, nanochemistry and additional new domains. The nanotechnology presents a production mode on nanoscale with novel skills and tools different than in the traditional sense. For instance, if the goal were to construct robots to be introduced into the human vasculature, they would have to be diminutive and thus, any instruments employed by these robots would have to be engineered using nanomaterials. Nanoshovels and nanospoons have been created by scientists which can be utilised by robots for intravascular interventions. This is a typical case of nanotool application. The devices are built in a nanoscope for double-cutting and operation of atoms and molecules, manufacturing technology of nanomaterials applied to various fields and understanding of new laws of the material transfer and energy transfer within the nanoscope are under the umbrella of nanotechnology. Thus, the nanotechnology nomenclature does not simply reflect nanomaterials per se; the term nanomaterials does not only indicate nanopowder. In addition to the latter, nanomaterials encompass a range of substances, e.g. nanofilaments, nanotubes, nanowires, nanocables, nanothin films, three-dimensional nanoblocks and composite materials, amongst others. They can exist in either solid or liquid phases, e.g. nanowater comprises tiny water molecule clusters generated following treatment with high-frequency ultrasound [8].

1.6.1 Nanomaterials Characteristics “Imagine that if one day, atoms and molecules could be arranged as what people want them to be, how different the world might be! There is no doubt that if we could control things on the very tiny scale, the scope of physical properties we can get can be greatly expanded” Feynman said in 1959. Currently, it is recognised that individuals are unable to configure atoms and molecules as they desire in order to create nanomaterials as their generation necessitates certain criteria to be fulfilled, e.g. the “perfect law of nanomaterials”.

1.7 Nano-effect

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The materials structure is consisting and describing the electronic structure. The principal measurements for atomic and electronic configurations include the lattice constant, bond length and angle, and the energy band, quantum state and distribution function, respectively. These factors are constants which have been established for the well-recognised macrosystem. However, within the nanosystem, alterations in atomic number give rise to parameter variations. This reflects a distinctive trait noted in substances and devices within the sphere of nanotechnology that underlies nanomaterial heterogeneity. The nanosystem is governed by a key rule, which is referred to as the “perfect law of nanomaterials”. This can be stated in a straightforward fashion, i.e. “existence is perfect and only the perfect can exist”. This tenet encompasses a “magic number” regulation that applies to nanocrystals, which states that clusters of atoms are deemed to be stable if they have the atomic numbers, 13, 55 and 147. For instance, within a fullerene configuration, C60 and C70 have the highest likelihood of being present; configurational components, e.g. C59 or C71 , are not in existence. This is why the Nobel Prize was conferred to Smalley et al. [9] who determined that C60 and C70 are present in several fullerene structures. Comparable regulations relate to one-dimensional nanomaterials, e.g. nanotubes and nanowires. The one-dimensional nanostructure is made up of shells, each of which comprises a more complex configuration referred to as a unit; the unit is formed by an atomic chain. A single unit comprises the core; 7 units give rise to a parcel layer. The defects melting rule applies to two-dimensional membranes, i.e. multiple defects are not permitted. When the number of defects becomes critical, additional flaws arise naturally, thus obliterating the two-dimensional crystalline configuration. These properties of low-dimensional configurations reflect the precise meaning of the Perfect Law.

1.7 Nano-effect For the nanoscale materials of 1–100 nm, the properties of the materials have some exceptional properties. Substances with particular characteristics that are at variance with the atomic and molecular constituents from which they are derived and the macroscopic material, are referred to as nanomaterials. However, if the material’s dimensions lie within the nanometre spectrum but the substance itself does not exhibit these unique traits, the substance cannot be termed a nanomaterial. Interest has typically been drawn to microscopic and macroscopic phenomena, e.g. atoms and molecules, or the universe, respectively; the middle ground has frequently remained unheeded. However, numerous substances naturally lie within this sphere, but the characteristics of items of this size have previously been unrecognised. Japanese scientists first attempted to generate a restructuring of the properties of items of these dimensions, thus proposing the notion of nanotechnology. They

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formed progressive micro-ions in the 1970s deploying the evaporation technique and evaluating their properties. They noted that the typical electrical and thermal characteristics of metals, e.g. copper and silver, were attrited when they were diminished to nanoscale materials; the nanomaterials exhibited non-conductive and non-thermal traits. The iron-cobalt alloy as magnetic material is true. If this alloy were engineered with a dimension in the region of 20–30 nm, the magnetic domain would be altered to a single magnet domain; this would display a coercivity greater than the original alloy by a factor of a 1000. Compared with a normal metal, a nanomagnetic metal has a 20-fold efficacy in relation to magnetic susceptibility, and a saturation magnetic moment that is only 50% of the original. When a stratum in a multilayer film achieves nano-dimensional width, a magneto-resistive force of considerable magnitude may be generated. The paraelectric properties for nanoscale are for PbTiO3 , BaTiO3 and SrTiO3 ferroelectrics. The absence of a routine covalent band and the presence of only partial interface bond polarisation with a modest AC resistance are typical properties of nanosilicon nitride ceramics. Nanoparticle catalytic agents, with outstanding performance, can be synthesised from inert platinum metals, e.g. platinum black. The nanomaterials sensitivity is higher than materials volume due to body surface area changes. Nano-optical substances have atypical abilities for absorption; notable diminished light-reflective traits characterise nanometals. These observations can be ascribed to the diminutive dimensions and surface properties of nanoparticles that render them highly efficacious for light absorption [10]. The Cu nanocrystals have self-diffusion 1016 –1019 times than traditional crystals and 103 times than crystal boundary spread. Nano-Cu has a specific heat that is double that of conventional copper. The thermal expansion rate of Pd as a nanosolid is twice that of the normal material. Ag nanocrystals demonstrate nearly a third greatest efficacy than typically utilised substances when employed as heat exchanger for dilution refrigeration liquids. The nanoscale crystals have limited dislocation slip to the border and display bigger hardness than materials volume. In its nanocrystalline form, copper has a more robust hardness by a factor of five compared to the micron scale. The fracture threshold for nano-Fe crystals, which have a dimension of 6 nm, can be augmented to 12-fold greater than that evident in polycrystalline Fe. The next section offers an in-depth discourse relating to the unique characteristics of nanomaterials, i.e. their optical, thermal, magnetic, mechanical and electrical properties.

1.7.1 Exceptional Optical Properties It has well known that nanomaterials have colours changes. For instance, a crimson or yellow hue can be generated by CaSe depending on whether the particles are greater or more diminutive in size (Fig. 1.6). If gold were reduced to a dimension lower than

1.7 Nano-effect

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Fig. 1.6 Changes of colour for CaSe particles of different sizes (upper: the powder in larger particles presents a red colour; lower: the smaller particles are a yellow powder) [11]

the wavelengths of light, the typical colour would be replaced by a black appearance. Indeed, any metal, as an ultrafine particle, presents as black. The colour becomes darker as the dimensions decrease. Silver-white platinum and chrome transform into platinum black and chrome black, respectively. The metals of ultrafine particles have a low rate of light reflection, estimated by less than l%. Light can be eradicated entirely at only a few microns in width. This characteristic can be applied to generate extremely efficacious solar energy transmission to produce heat and electricity; it may additionally be employed in infrared-sensitive devices or in infrared stealth techniques. A range of nano-ultrafine particles are used in the US F-117A stealth fighter; these have a potent capacity for electromagnetic absorption at a variety of wavelength spectra. This technology can confuse radar and conceal planes [11].

1.7.2 Exceptional Thermal Properties The substance of solid has a fixed melting point at big size, whereas this point is reduced in ultrafine forms. These reductions are especially of import where the size of the particles is under 10 nano-orders of magnitude. So, the gold melting point is 1064 °C. If the particle dimensions were diminished to 10 nm or 2 nm, the respective melting points would be 27 and 327 °C.. Ultrafine silver particles have a melting temperature of under 100 °C.; traditional silver melts at 670 °C. (Fig. 1.7). Thus, it is possible to sinter the conductive paste obtained from ultrafine silver powder at notably reduced temperatures. This temperature facilitates the use of more frequently utilised materials, e.g. plastic, for the device substrate, rather than high-temperature-resistant ceramic substances.

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Fig. 1.7 Changes of the Ag melting point at different sizes [11]

The surface atoms of metal nanoparticles are active. Catalysts or solid rocket fuel can be engineered with the use of nanoparticle powder, e.g. the addition of 1% (by weight) of ultrafine particles of aluminium or nickel to rocket fuel increases the combustion heat by a factor of 2.

1.7.3 Exceptional Magnetic Properties There is a significant change in nanomaterials that guides to important changes in magnetic properties. Rhodium forms an excellent illustration of this principle, which can be validated by upscaling the rhodium atom frequency within Rh clusters (Fig. 1.8). Coercivity demonstrates a rise by a factor of 1000 when the particle dimensions are decreased to 2 × 10–2 μm, but if the size were diminished to under 6 × 10–3 μm, paradoxically, the coercive force would fall to zero, thus producing excessive paramagnetism. Fig. 1.8 Magnetic properties changes of rhodium clusters with different numbers of rhodium atoms [11]

1.7 Nano-effect

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The magnetic properties of ultrafine particles can be utilised for recording magnetic powder production with high-density storage that is used in manufacturing of disks, tapes and magnetic cards. According to the superparamagnetic characteristics, magnetic fluids suitable for a broad spectrum of applications can be engineered from ultrafine magnetic particles.

1.7.4 Exceptional Mechanical Properties The nanomaterial particles have a good surface effect. A surface area measuring up to several hundred metres square can be achieved with 1 g of nanomaterial. Products engineered with the use of nanotechnology have enhanced robustness, plasticity and scalability. The analogy of a caterpillar with thousands of legs is relevant; the contact area of the caterpillar on an even glass veneer would enable it to withstand a level 12 typhoon. The ceramic materials are very brittle. A similar tensile strength to that found in spring can be achieved using nanoceramic substances generated from nanoultrafine particles. Research has demonstrated that human dental intensity is related to the presence of nanomaterials, e.g. calcium phosphate, amongst others. Metal nanocrystals exhibit hardness in excess of their conventional coarse-grain counterparts by a factor of between 3 and 5. There is a wide spectrum of utility for composite nanomaterials, i.e. those combining metals and ceramics [12].

1.7.5 Exceptional Electrical Properties The electronic movement of nanomaterials is restricted inside nanoparticles as an electronic energy quantisation. It is therefore possible to engineer specific metal particles which, under various voltages, may either be conductive or non-conductive. When the size of conducting metals, e.g. copper, is diminished to several nanometres, their conductivity becomes attrited. Conversely, the well-established properties of insulating substances, e.g. silicon dioxide, are ameliorated and they acquire conductive abilities. The negative electricity is shown when a metal nanoparticle acquires extra electron from the external circuit. The electron’s Coulomb force has sufficient strength to prevent the subsequent electron from gaining access to the metal particle from the outside circuit; this therefore halts current continuity and is referred to as the Coulomb blocking effect. This principle gave rise to the concept that a master electronic device could be created that is governed by a lone electron, i.e. a single-electron device. These have a diminutive dimension that enables them to become an integral component of computer chips with a capacity and computing rate that is much greater than the currently used chips.

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1.7.6 Natural Nano-effect The nanomaterials are synthesised materials, also there are natural nanomaterials that demonstrate specific properties. For instance, foliage from certain vegetation stays clean regardless of contamination within the surroundings. It has been determined that this property reflects hydrophobic substances that are present on an even surface and the most external inferior waxy veneer ranked according to the nano-order regulations (100–200 nm). The nanorough veneer is the main element that safeguards the foliage from filth. Any dirt settling on the foliage can simply be rinsed away with water, a phenomenon referred to as a self-cleaning or lotus effect. The nanoscale cilia exist on the lotus leaves surface. Electron microscopy has revealed outpouchings formed by cilia on the foliage surface which safeguard the leaves against water or dirt particle adherence; hence, the self-cleaning observation made with respect to lotus foliage. The lotus leaf veneer contains villi that fortify its hydrophobic properties. The hydrophobic effect indicates that an object’s surface has only a weak attraction to water molecules, and that water molecule absorption is challenging. It is similar to the self-cleaning effect of insects. During flight, insects have to sustain corporeal equilibrium. If any dirt particles were present on their wings, their weight distribution would be impacted and cause issues with flying. Thus, insects are obliged to clean their wings. Those with larger wings cannot perform this process with their limbs; however, since the wing veneer in the majority of insects is a nanostructure, it has self-cleaning properties (Figs. 1.9, 1.10 and 1.11). It is proved that magnetic ultrafine particles are attributed to features of dolphins, butterflies, bees, pigeons and water magnetotactic bacteria particles that enable them to navigate under a geomagnetic field. Bees have ultrafine particles within their bodies (Fig. 1.12g) which form a biological magnetic compass that can detect the geomagnetic field, magnetic declination and inclination with precision. This system allows the bee to navigate during flight. Electron microscopy research has demonstrated that aquatic magnetotactic bacteria Fig. 1.9 Lotus effect: plant leaves are usually able to remain clean without being polluted [11]

1.7 Nano-effect Fig. 1.10 Self-cleaning effect of insect wings [11]

Fig. 1.11 Bee wings [11]

Fig. 1.12 Magnetic nanoparticles in the body of bees [11]

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encompass magnetic oxide particles of dimensions in the region of 2 × 10–2 μm. These enable these microbes to descend underwater to obtain rich nutrients.

1.8 Physical Principles of Nano-effect The nanomaterials have different chemical and physical properties for molecules and atoms. Nanoscale and volume materials demonstrate notably disparate characteristics. The former have distinctive traits that arise when the dimensions of the substance constituents are downsized into the nanoscale spectrum. At this level, the engagements between atoms and molecules exert a potent modulatory effect on the substance’s macroscopic characteristics, i.e. mechanical, electrical and optical features. Fundamentally, these nanomaterial properties include small size, surface interface, quantum size and quantum tunnelling effects, respectively. Kubo et al. have presented a theory which offers a qualitative description of these empirical properties [13].

1.8.1 Discontinuity of Electron Levels It is suggested according to band theory that a single atom has discrete energy levels. When the atom number within a solid, of conduction electron number N, rises, the atomic levels may become a single band (Fig. 1.13). There is an unrestricted atom number within macroscopic items, i.e. for N electrons, conductivity demonstrates a trend towards infinity. Sizeable particles or macroscopic items may exhibit a void between the macrolevels which approaches zero. Thus, at elevated temperatures or as a consequence of general dimensions, electrons in the proximity of the metal Fermi level are typically maintained at a continuous level.

Fig. 1.13 From the discrete atomic level to the level in solid band

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According to nanoparticles, the N-value is quite small. This gives rise to a finite value for the energy void, and so the amounts of energy are distinct at cool temperatures. Since the energy level spacing is higher than thermal, magnetic, static, electrostatic, photon or concentration energies, respectively, within the superconducting condition, the considerable disparity will be observed in the magnetism, optics, sound, heat, electricity, superconductivity and macroproperties of the nanoparticles [14].

1.8.2 Kubo Theory The nanoscale particles have small size and minimum mass. The characteristics of bulk matter, which typically comprise an unrestricted number of atoms, cannot explicate numerous events. This concept is often called the volume effect; a wellestablished hypothesis, the Kubo theory, describes a routine instance. The Kubo theory indicates the distribution of the state of electron energy levels close to Fermi surface of metallic ultrafine particles. As particles are introduced to nanoscale dimensions, the quantum size effect stimulates the quasi-individual events on the parent bulk metal’s continuous energy levels. Initially, electronic energy amounts in the region of a lone diminutive particle’s Fermi surface at cool temperatures are considered to be energy levels at equivalent spacing. The term Fermi surface in this context reflects the equal energy surface, where energy is indicated by εF in the space, k, at absolute zero temperature. It operates as a divisor between filled and unfilled electronic orbits. The expression below describes the specific heat of a lone ultrafine particle: C(T ) = kB exp(−δ/ kB T )

(1.1)

where δ, k B and T represent level spacing, the Boltzmann constant and absolute temperature, respectively [15]. In raised temperatures, kB T ≫ δ, C(T ) → kB . This implies that the specific heat is autonomous of temperature, which is in keeping with this feature in bulk metals. However, at cool temperatures, (T − 0), kB T ≪ δ. This is entirely at variance with bulk metals, as the specific heat and temperature are associated by T 3 . The approximate model of equal energy level is utilised to derive the specific heat formula of the single ultrafine particle at low temperatures; therefore, it cannot be confirmed experimentally due to can perform experiments on ultrafine particle collection only. Kubo contributed significantly to this domain, utilising a de novo paradigm with respect to ultrafine particles in order to seek an explanation for the disparities between observed and theoretical findings. This author formulated two major assumptions regarding the electronic conditions of sizeable aggregates of diminutive particles.

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1.8.3 Small Size Effect The magnetic exchange length, magnetic domain wall width and de Broglie wavelength of conduction electrons will be destroyed at ultrafine particle size smaller than the light wavelength. Simultaneously, in proximity to the particle’s surface stratum of amorphous nanoparticles, there is a fall in atomic density together with a de novo impact on physical traits, e.g. alterations from magnetic order to disorder, adjustments in magnetic coercivity and a fall in the melting temperature of the metal. This gives to heightened optical absorption, alongside the plasmon resonance absorption zeniths, adaptations from a condition of ordered to disordered magnetism, from a superconducting to a normal conducting stage and variations in the phonon range, amongst others. The nanoscale particles have opened new horizons for technological application. For instance, particles exhibiting strong magnetic properties within the nanoscale, e.g. Fe–Co alloy or iron oxide, could be utilised in the production of magnetic cards, keys, tickets and fluids. Of these merchandises, magnetic liquid has significant utility in electroacoustic or damping devices, in rotary seals and for lubrication. Since the melting temperatures of nanoparticles are markedly lower than for conventional bulk metals, these materials offer a de novo technological domain for the powder metallurgy sector. The alteration in plasmon frequency with dimension can be utilised together with regulated translocation of the absorption limit in order to synthesise nanomaterials encompassing microwave absorption properties within a particular bandwidth [16]. The optical properties of the materials are depending on their properties of reflectance or absorbance. For instance, green foliage appears this colour as it reflects light wavelengths from the green spectrum, but absorbs light from all the other wavelength spectra. Similarly, a red hue implies reflection of light from the red bandwidth, but absorption of all other wavelengths. Nanoparticle dimensions may be as diminutive as a few to multiple nanometres, demonstrating distinct small size and surface interface effects. The optical characteristics are therefore at variance from those of traditional block and coarse substances. Nanometal powders have particular electromagnetic wave absorption properties; these are deployed for optimal performance military products, e.g. in millimetre-wave infrared and structural stealth materials, in addition to their usage as radiation guards for mobile phones. An insulating material, glass is unable to liberate any electromagnetic wave absorption. In contrast, vaporisation of heavy metals can produce nanomaterials with extreme conductivity. Thus, static electricity absorption could be obtained using a ground wire attached to the shield in order to eradicate any static electricity that could potentially be injurious to human well-being. Electromagnetic waves are also emanated by computer screens at non-standard wavelengths, indicating that the glass veneer is not uniformly layered with nanomaterials. This means that the coating can mitigate electromagnetic wave frequency fluctuations. Screens treated with this material can safeguard the ophthalmic system from damage by blinking lights and optimise the clarity of the display.

1.8 Physical Principles of Nano-effect

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The nanoparticles have coupled properties of quantum size effect and surface effect, whilst nanoparticles diameter is equivalent to the wavelength of superconducting coherence, Bohr radius and de Broglie wavelength of electrons. Simultaneously, particle surface atoms and electrons are at greater variance than those in the particle’s interior. This trait notably affects nanoparticles’ optical properties. Bulk metals exhibit a spectrum of colours, demonstrating their heterogeneous ability to reflect and absorb a range of visible light wavelengths. If they were condensed to nanometre dimensions, the majority of metals would appear black as a consequence of their poor visible light-reflecting properties. The nanoparticles have a specific role against electromagnetic and infrared waves due to the followings; Firstly, nanoparticle dimensions are infinitely more diminutive than wavelengths in the infrared and radar ranges; their wave transmission is far superior to that of traditional substances. This can markedly diminish wave reflection properties and so signals from the infrared and radar spectra are ameliorated and challenging to detect, thus attaining the feature of stealth. Secondly, nanoparticle materials have a much higher surface area than their traditional counterparts which again, attenuates the strength of their infrared and radar emanations. Engineered stealth coatings have found their role in contemporary military combat. The body of the fourth version of supersonic fighter planes is assembled from composite substances, wing-body assimilation and radio-absorbing coatings [16]. Overlays with electromagnetic wave absorption properties and shielding paints are now being added to stealth planes. Studies in the US, Russia, France, Germany, Japan and additional nations have encompassed nanomaterials in their most recent research into the next phase of stealth material engineering. This is because of the superlative properties of the nanomaterials, i.e. absorption, broadband, compatibility and dimensions. In the case of metals, metal oxides and several non-metallic substances, a rising atom number may be placed on the nanoscale ultrafine powder surface during refinement, thus enhancing their functionality. When exposed to microwave radiation, atomic and electronic motion is exaggerated which increases magnetic strength. Electronic energy is converted into heat energy, thus augmenting wave absorption, as seen in innovative nanometal automotive paint. The latter comprises a de novo version of a high-performance coating substance which encompasses a range of nanometal powders together with progressive technology relating to nanometal automotive paint manufacture. The coating has remarkable adherence and can withstand the impacts of chemicals including acids, alkalis and antioxidants. The automotive paint has specific features of flip-flop effect to provide marvelous protection against physical issues like collisions, scratches and wear. Furthermore, it has the ability to absorb injurious radiation, thus safeguarding human welfare, and also prolongs the paint’s longevity.

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1 Nanomaterials Properties

1.8.4 Surface Effect The nanomaterials surface effect refers to the ratio of surface atoms and total atomic number of nanoparticles, which exhibits an abrupt rise, generating property changes as the particle dimensions are reduced. Diminutive dimensions and marked surface energy characterise nanoparticles; surface atoms are responsible for a notable percentage (Table 1.1). As shown in Fig. 1.14, the surface atoms percentage rapidly increases due to the particle size is less than 10 nm. A 90% surface atomic proportion is achieved when the particle dimensions are less than a single nanometre; there is therefore a high atom content on nanoparticle surfaces. Thus, with decreasing nanoparticle dimensions, the proportion of surface atoms becomes greater in comparison to the atom population within the particle’s interior. The rise in atom number on the surface leads to poorer atom synchrony; the raised energy on the particle’s surface generates increased surface functionality. However, such atoms lose their individual integrity, and have a greater propensity for bonding with adjacent atoms, e.g. metal nanoparticles may combust in air or adsorb and react with vapours. Copper particles of 100 nm, 10 nm and 1 nm in diameter exhibit surface area ratios of 6.6 m2 /g, 66 m2 /g and 660 m2 /g, respectively [17]. Surface atoms activity induces changes in surface nanostructure and atomic transport, and may additionally cause spin and electron spectroscopy configurational variations. The carbon nanotube is the most exaggerated example as it lacks internal atoms, being comprised of surface atoms alone. Table 1.1 Relationship between the size of nanoparticles and number of surface atoms

Fig. 1.14 Percentage of surface atoms and the total number of atoms of nanoparticles of different sizes [11]

Size of nanoparticle, d (nm)

Number of surface atoms

Proportion of surface atoms (%)

10

3 × 104

20

4

4 × 103

40

2

2.5 × 102

80

1

30

9

1.8 Physical Principles of Nano-effect

21

1.8.5 Dielectric Confinement Effect The materials have dielectric properties and dielectric loss that constitute the physical characteristics of dielectric materials. Traditional substance polarisation is dependent on an orderly configuration; however, there is notable architectural variation between nanomaterials and regular coarse-grained substances. Distinctive dielectric activity, i.e. dielectric constant and loss, is present in nanomaterials, which reflects particle dimensions; this is markedly impacted by electric field frequency. When the nanoparticles are distributed in heterogeneous dielectric materials, the interface produces the dielectric enhancement of the system. This is referred to as the dielectric confinement effect [17]. It predominantly stems from the particles’ surface and interior regions. If notable variations are present in the refractive indices of differently sized particles, a refractive index boundary will apply. This generates a rise in field strength internally and on the surface of the particle. This regional increase is termed dielectric confinement. The effect of dielectric confinement of nanoparticles is important for absorbing and non-linear optical properties. The impact on reflected light within the absorption spectrum is demonstrated by a transparent red shift, an observation that is straightforward to comprehend. Since the carrier’s free range is larger than the nanoparticle dimensions, loading of the photovoltaic composite can be achieved. With diminishing particle dimensions, the particles’ characteristics will be notably affected by the condition of the surface. When substances with a lower dielectric constant are altered on the surface of a semiconductor comprising ultrafine particles, additional modifications occur in contrast to those impacting the exposed ultrafine particles. This arises owing to the fact that the charge carrier’s power line, which is embedded in ultrafine particles, can traverse this film stratum without difficulty in comparison to the matrix in which the bare particles lie. Thus, there is amelioration of the shielding influence and concurrent strengthening of the Coulomb force amongst the charged particles. There is consequently augmentation of the binding energies and oscillation forces of the excitons. These processes are observed as a transparent red shift within the absorption spectrum. In the Brus formula, we can give a qualitative or quantitative analysis [18] of the effect of dielectric confinement on optical absorption band edge shift (red shift, blue shift). The Brus equation can be expressed as E(r ) = E g (r = ∞) +

e2 h2π 2 − 1.786 − 0.248R Ry 2 μr εr

(1.2)

where E(r ) indicates the absorption nanoparticle band gap, E g (r = ∞) describes the band gap, r refers to the particle radius and μ = [1/m e + 1/m h ]−1 represents the equivalent particle quality, where me and mh are the effective electron and hole masses, respectively. The second and fourth items reflect the quantum confinement item or blue shift, and the effective Rydberg energy, respectively. The dielectric

22

1 Nanomaterials Properties

confinement is described by the third item and has a negative value, thus indicating a red shift within the absorption spectrum. The transition metal oxides like Fe2 O3 , Co2 O3 , Cr2 O3 and Mn2 O3 nanoparticles distributed in sodium dodecyl benzenesulfonate, will display enhanced nonlinear optical effects of third order. In Fe2 O3 nanoparticles, parameters have inferred that the third-order non-linear coefficient, χ (3) , can reach 90 m2 = V 2 ; this is elevated by a factor of two in water. The augmentation of this coefficient can, to some extent, be ascribed to the dielectric confinement effect. Problems 1. 2. 3.

4. 5.

For 2.1 nm cube of MgO (100) calculate the number of five-, four- and threecoordinate sites. List five types of scanning probe microscopies and give a brief description of each. Investigate the following analytical technique and provide a description of each, including the information provided and limitations: TEM, SEM, powder XRD, XPS, EXAFS, XANES and nitrogen physisorption. Explain what happens to the melting point and specific heat of metals as the size changes from the bulk to the nanoscale. “Top-down” approaches to nano refer to those in which larger systems are broken down until they reach nanoscale, whilst “bottom-up” involves building nanoscale materials by putting together atoms or molecules.

Chapter 2

Nanomaterials Synthesis

2.1 Introduction Different types of sensors, ferrofluids, adsorbents and catalysts have used metal nanoparticles. These offer significant utility with respect to optical, electronic and magnetic apparatus. The majority of these indications are reliant on nanoparticle morphology and dimensions. Thus, the manufacture of nanoparticles merits a high degree of regulation with respect to these attributes so that they are apposite for their designated purpose. A frequently utilised method is metal complex reduction so as to generate metallic colloid dispersions. A number of antecedents, reducing compounds and polymeric stabilisers are employed in order to create the latter (Table 2.1). The metallic colloid size is correlated with the reducing agent type. A rapid reaction can be instigated by the use of a potent reducing compound; typically, a swift reaction generates diminutive nanoparticles. Conversely, a less potent reducing agent will engender a more prolonged reaction, which yields nanoparticles of a greater size.

2.1.1 Nanoparticles Preparation To generate an abrupt surge of the growth species concentration, a strong reducing agent is required to result in a very high supersaturation. This gives rise to the immediate formation of abundant nuclei, which for a standard metal antecedent titre, generate a diminutive nanoparticle dimension. The polymeric stabiliser is used in order to establish a monolayer on the nanoparticle surface and thus circumvent aggregation; this substance is referred to as a capping material. However, growth may be retarded by the polymer monolayer if it engages with the growth loci. If it cloaks the evolving particle’s surface in its entirety, it may obstruct the diffusion of growth species from the solution medium to the particle surface.

© The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2022 Y. Al-Douri, Nanomaterials, https://doi.org/10.1007/978-981-19-3881-8_2

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Table 2.1 Precursors, reducing agents and polymeric stabilisers used in the preparation of metallic nanoparticles Category

Name

Precursor

Metal anode Palladium chloride Potassium tetrachloroplatinate II Silver nitrate Chloroauric acid Rhodium chloride

Reducing agent

Hydrogen Sodium citrate Citric acid Carbon monoxide Methanol Formaldehyde Hydrogen peroxide Sodium tetrahydroborate

Polymeric stabiliser

Poly(vinylpyrrolidone) Polyvinyl alcohol Sodium polyphosphate Sodium polyacrylate

The nanoparticles’ shape can be varied by using varying amounts of polymeric stabiliser. The dimensions and configuration of platinum nanoparticles have been regulated through alteration of the polymer (sodium polyacrylate): platinum cation titre ratio [19]. Tetrahedral, cubic, irregular-prismatic, icosahedral and cubo-octahedral morphology was reported (Fig. 2.1). They have been utilised in enamels and glasses as colouring agents as shown in Fig. 2.2. A number of techniques have been deployed in order to engineer nanoparticles from gold. A frequently utilised method is sodium citrate-induced chloroauric acid reduction at 373 K. To synthesise a rhodium colloidal dispersion, the reducing and stabilising agents, methanol and polyvinyl alcohol, respectively, are utilised. The reaction can be expressed as follows: 2RhCl3 + 3CH3 OH → 2Rh + 3HCHO + 6HCl

(2.1)

The nanoparticle size is depending on the conditions of reaction. An important process in dictating nanoparticle dimension is Ostwald ripening. Hydrogenmediated reduction can be employed to engineer platinum and palladium nanoparticles; hydrolysis of K2 PtCl4 and PdCl2 yields hydroxides, which then undergo reduction. PdCl2 + Na2 CO3 + 2H2 O → Pd(OH)2 + H2 CO3 + 2NaCl(OH)2 + H2 → Pd + 2H2 O

(2.2)

2.1 Introduction

25

Fig. 2.1 TEM images of platinum nanoparticles: a cubic nanoparticles formed when the initial ratio of the concentration of polymer (sodium polyacrylate) to that of the metal cation in the solution is 1:1, and b tetrahedral nanoparticles are formed when the initial ratio is 5:1. The insets show high-resolution images of the particles [19]

Fig. 2.2 Glass carafe in which gold nanoparticles are dispersed (this glass is known as ruby glass)

Non-oxide semiconductor nanoparticles can be prepared by organometallic precursor pyrolysis dissolved in a water-free solvent in a heated vacuum, together with a polymeric stabilising compound. This technique has been used in order to engineer nanocrystals from CdS, CdSe and CdTe. GaN nanocrystals have been generated from a pressurised thermal reaction attained by combining Li3 N and GaCl3 at 553 K; the solvent used was benzene, and the reaction was conducted in argon gas [20]. GaCl3 + Li3 N → GaN + 3LiCl

(2.3)

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2 Nanomaterials Synthesis

The GaN yield is 80% and the particle size is 30 nm. The majority of the yield contained hexagonal GAN, together with a modest quantity of rocksalt GaN. Sol–gel processing is a well-recognised technique for the production of oxide nanoparticles, and encompasses antecedent hydrolysis and condensation, on occasion promoted by a catalyst. Metal alkoxides or inorganic and organic salts are common precursive compounds, which are then made up into solutions in either aqueous or organic solvents. GaCl3 + Li3 N → GaN + 3LiCl

(2.4)

M(Oet)4 + xH2 O ↔ M(Oet)4−x (OH)x + xEtOH

(2.5)

Hydrolysis:

Condensation: 2M(Oet)4−x (OH)x ↔ (Oet)4−x (OH)x−1 MOM(Oet)4−x (OH)x−1 + H2 O

(2.6)

where the metal is indicated by M. Frequently, organic moieties are bonded to the nanoclusters resulting from the condensation reaction, which may be associated with only partial hydrolysis. The nanoclusters size of the ultimate product’s configuration can be customised by dictating the reaction conditions in an apposite manner. Metal hydrous oxide colloidal dispersions containing particles, largely homogeneous with respect to dimensions and configuration, have been produced by maintaining the various metal salt solutions at a high temperature for a range of durations. Particle morphology and constituents are most robustly associated with the acid–base balance and the type of anions present within the maturing systems. Ageing ferric salt solutions can be used in conjunction with the matching acid at a temperature of 373 K for a single day in order to yield ferric oxide nanoparticles. Figure 2.3 presents electron micrographs of ferric oxide nanoparticles generated from Fe(NO3 )3 and HNO3 , and Fe(ClO4 )3 and HClO4 solutions, respectively. The influence of the anions on the surface properties and nanoparticle interface energies contribute to nanoparticle evolution. These factors can additionally impact interparticle electrostatic double-layer repulsion, which consequently affects their integrity.

2.2 Icroemulsion-Based Methods The nanoparticles of ultrafine metal of diameter between 5 and 50 nm can be synthesised using microemulsions, comprising water and oil. Water nanodroplets undergo

2.2 Icroemulsion-Based Methods

27

Fig. 2.3 Electron micrographs of iron oxide nanoparticles obtained by ageing the solutions at 373 K for one day: a Fe(NO3 )3 (18 mol/m3 ) + HNO3 (104 mol/m3 ) and b Fe(ClO4 )3 (18 mol/m3 ) + HClO4 (104 mol/m3 ) [21]

dispersion within the oil phase; their dimensions can be adjusted within a 550 nm range by altering water:surfactant proportions. The latter offers particle nucleation loci which add stability to the evolving particles. The salts of reactant metal and agents of reduction can generally be dissolved in an aqueous solution, and so particle nucleation typically occurs within the microemulsion’s water pockets. Metal salts and reducing agents are in separate microemulsions; nanoparticles are harvested following a microemulsion combination (Fig. 2.4). During the water droplets collision, there is an extremely rapid reactant interchange during the admixing of the metal salt and the reducing compound. Nanoparticle nucleation and evolution occur within the droplets. Interdroplet exchange of nuclei or particles is impeded, as it would necessitate the generation of a sizeable defect in the process of droplet collision which would involve a notable curvature alteration in the layer of surfactant surrounding the droplets, a process which, from an energy perspective, is suboptimal. Given that the solubility of inorganic salts is extremely poor within the oil phase, the dynamic substitution of reaction components amongst various droplets via the continuous phase is unlikely. Once particles reach their ultimate dimensions, the surfactant molecules bind to the particle veneer, thus adding a stabilising layer and inhibiting additional growth. Percolation is a very important step in the nucleation of particles. Figure 2.4b demonstrates this process for a reaction involving two molecules, i.e. A + B → C, where A, B and C indicate the metallic salts, e.g. FeCl3 , the reducing agent, e.g. NaBH4 , and the metal particle, respectively. In order for the reaction to proceed, A and B have to collide; however, they are both sequestered within separate water droplets. There are two processes through which they can connect, i.e. the exit of A from its water pool, its translocation through the oil phase and access to the water reservoir where B is located, or by an immediate interchange of the reactants from one water pocket to the other when the two water pools connect following droplet collision.

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2 Nanomaterials Synthesis

Fig. 2.4 a Mechanism for the synthesis of metal nanoparticles by the microemulsion approach and b the percolation mechanism in detail [22]

If the collisions are strongly and energetic interactive, the second mechanism would be favoured. Overall, the rate of reaction between the metal salt and the reducing compound is rapid when judged against the speed of droplet engagement. Thus, the second-order communication stage is the rate-limiting step, which has a rate constant between 106 and 107 dm3 mol−1 s−1 for an aerosol OT-water-heptane microemulsion system [22]. The reactant concentrations affect the reduction rate. The likelihood of collisions between two atoms, a single atom and nucleus, and a pair of nuclei dictate the speed of nucleation and evolution. The initial form of collision pertains to nucleation; growth is governed by the latter two events. In order to engineer particles with varying diameters, the proportions of water and surfactant can be adjusted. Barnickel et al. [23] have prepared varying dimensions of silver nanoparticles in this manner. The reverse micelles were stabilised using dodecyl-pentaethyleneglycolether, which is a non-ionic surfactant. AgNO3 and N NaBH4 , in solution, were present in the two water/oil microemulsions, respectively. Droplet dimensions and mean particle diameter were enhanced within the microemulsion once the water:surfactant ratio was augmented from 0.05 to 0.2 (Fig. 2.5).

2.3 Carbon Fullerenes

29

Fig. 2.5 Transmission electron micrographs of silver nanoparticles synthesised in w/o microemulsions. The micrographs depict the effect of water/surfactant ratio on the size of silver nanoparticles. The most probable particle diameters are indicated on the micrographs [23]

However, with increasing the ratio of water/surfactant further, the opposite process becomes apparent with a reduction in particle diameter; nevertheless, some extremely big nanoparticles are generated. The latter reflects secondary growth as a consequence of minor temperature deviations which potentially lead to emulsion destabilisation. The harvest with the lowest range of dimensions, and consequently the most homogeneous, was achieved with a water:surfactant ratio of 0.2. There have been numerous publications pertaining to the manufacture of nanoparticles from iron, platinum, palladium, cadmium, silver, copper, nickel and gold which have followed the water-in-oil microemulsion technique.

2.3 Carbon Fullerenes Fullerenes are carbon allotropes family: molecules composed entirely of carbon in the form of a hollow sphere, ellipsoid, tube or plane. Fullerenes are similar in structure to graphite, which is composed of stacked sheets of linked hexagonal rings, but may also contain pentagonal or sometimes heptagonal rings. The most familiar carbon fullerene is a molecule with 60 carbon atoms, represented as C60 . It is discovered in 1985 by Kroto et al. [24] and named Buckminsterfullerene. The name is coined after the American architect Richard Buckminster Fuller who is famous for the geodesic domes built by him, i.e. carbon molecules, which are shaped like a hollow sphere, or are ellipsoidal, tubular or planar in appearance. They have a similar configuration to graphite, which comprises stacked arrays of connected hexagonal rings. The fullerene that is most well known is C60 which, as the name suggests, includes 60 carbon atoms within the molecule. It was first reported by Kroto et al. [24] in 1985, and labelled buckminsterfullerene; Richard Buckminster Fuller was an American architect who was renowned for his geodesic domes.

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2 Nanomaterials Synthesis

Fig. 2.6 The truncated-icosahedral structure of C60 Buckminsterfullerene [24]

The C60 molecule has a structure of truncated icosahedral It is engineered by substituting each football seam apex with a carbon atom (Fig. 2.6). The molecule has 20 hexagonal and 12 pentagonal faces, respectively, 0.144 nm is the closest C–C distance, which approximates that of graphite, i.e. 0.142 nm. In a similar manner to graphite, the carbon atoms all demonstrate trigonal bonds to the others. Of the three bonds regenerating from the individual carbon atoms, two are single and one is double. Rotating single and double bonds are present on the hexagonal interface; single bonds are associated with the pentagonal veneer. The latter is 0.146 nm in length, a distance greater than the mean bond length of 0.144 nm; the double bonds are 0.14 nm. The C60 molecule diameter measures 0.71 nm; a range of atoms can be encompassed within its interior. This unusual molecule is prepared by the carbon species vaporisation from a solid graphite disk surface into a flow of high-density helium; a focused pulse laser is utilised. A supersonic molecular beam expands the carbon clusters. An excimer laser and time-of-flight mass spectrometry are subsequently employed for photoionisation and detection. The vaporisation chamber is illustrated in Fig. 2.7. The laser beam vaporisation is concentrated through the spout to hit the gently rotating graphite disk. High-density carrier helium vapour is directed across the vaporisation areas in pulsed jets. It facilitates the thermalising collisions required to reduce the temperature of the species, enabling engagement and clustering within the gaseous graphite plasma. The cluster-rich vapour at the outlet’s terminal undergoes

Fig. 2.7 Schematic of the pulsed supersonic nozzle used to generate carbon cluster beams [24]

2.4 Synthesis of Nanowires, Nanorods and Nanotubes

31

free expansion, generating a supersonic beam which can be investigated at a point 1.3 m away from the origin using time-of-flight spectroscopy. Other fullerenes of smaller as well as larger numbers of atoms of carbon are available. They can be expressed as Cn . A collection of straightforward, basic chemical geodesic rules have been published by Kroto et al. [24], which associate the carbon cage integrity predominantly to the state of the pentagonal rings, or to a range of immediately integrated pentagonal ring structures. This research group has demonstrated improved stability with respect to adjacent structures in fullerenes where n is 24, 28, 32, 36, 50, 60 and 70. Fullerenes can be prepared in large scale by Krätschmer et al. [25]. The precursor for this technique comprises pure graphitic carbon soot, which includes a small number of C60 molecules. It is engineered through the evaporation of graphite electrodes; the conditions include a helium atmosphere at approximately 0.14 times atmospheric pressure. The arising black soot is extracted from the collecting interfaces within the evaporation chamber and then dissolved in a solvent, e.g. benzene, carbon tetrachloride or carbon disulphide. The C60 molecules are dissolved to give colours that vary from maroon to brown according to the solution strength. The liquid and soot are segregated and then desiccated over low heat; a dark brown- or black-coloured crystalline residue is obtained. A second technique is to place the soot in either a vacuum or inert atmosphere and then heat it to 673 K, following which there is sublimation of the C60 molecules out of the soot. These coatings appear brownish-grey according to their thickness. The purification techniques have been described in depth by Krätschmer et al. [25]. The low solubility of fullerenes in aqueous solution limits their applications in biology. By appropriate substitution, the fullerenes can be transformed into stabilised anions that are water-soluble and can form large aggregated structures. A laser light scattering study of the association behaviour of the potassium salt of pentaphenyl fullerene (Ph5 C60 K) in water reveals that the hydrocarbon anion, Ph5 C− , associate with bilayers, forming stable spherical vesicles with an average hydrodynamic radius and a radius of gyration of ~17 nm (Fig. 2.8).

2.4 Synthesis of Nanowires, Nanorods and Nanotubes One-dimensional nanomaterials synthesis can be achieved by different techniques including: (i) spontaneous growth, such as evaporation–condensation, vapourliquid–solid (VLS) growth and stress-induced recrystallisation; (ii) template-based synthesis, e.g. electroplating, electrophoretic deposition, colloid dispersion, melt or solution filling and chemical reaction; (iii) electrospinning; and (iv) lithography. Spontaneous growth typically leads to single-crystal nanowire or nanorod generation in a favoured trajectory of crystal evolution, which is governed by the configurations of the crystals and nanowire surface characteristics. Anisotropic growth is essential in order to engineer nanowires or nanorods since more rapid growth is required along a particular vector. Flaws and contaminants on growth surfaces

32

2 Nanomaterials Synthesis

Fig. 2.8 A bilayer vesicle model, consisting of 6693 molecules in an outer shell (of radius 17.6 nm) plus 5973 molecules in an inner shell of radius 16.7 nm. A sector has been cut out for enhanced visibility. The hydrophobic fullerene bodies are shown in green, the hydrophilic charged cyclopentadienide regions are in blue, and the five substituents are schematically represented as yellow sticks [26]

may engender heterogeneous materials. Manufacture founded on templates generally yields polycrystalline amorphous substances.

2.4.1 Rods The terms, nanorod and nanowire, are fairly random; however, their use within the field’s publications implies that they are differentiated according to diameter, with the former having a greater diameter than the latter, and a diameter in the region of 20 nm forming the cut-off point. Nanobelts, i.e. elongated ribbon-type nanostructures, have been produced by Pan et al. [27], using semiconducting metal oxides from zinc, tin, indium, cadmium and gallium; a method encompassing high-temperature evaporation of commercially obtained metal oxide powders was employed (Fig. 2.9). The oxide powder is placed at the alumina tube centre. The tube is subsequently placed in a horizontal tube furnace; parameters for temperature, pressure and evaporation time can be regulated. In the course of evaporation, the resultant materials are dropped onto an alumina plate placed at the alumina tube’s downstream side. The properties of the obtained substance can then be analysed using X-ray diffraction, scanning and transmission electron microscopy, and energy-dispersive X-ray spectroscopy. Oxide nanobelts have no impurities, have a consistent configuration and comprise a single crystalline. The majority have no flaws or dislocations. The cross section, which measures within the range 30–300 nm, is typically rectangular; the length is several millimetres. A width:thickness ratio lies between 5 and 10. The appearance of nanobelts is unique; however, they contain a number of configurational traits typical of the semiconducting oxide substance class. ZnO nanobelts,

2.4 Synthesis of Nanowires, Nanorods and Nanotubes

33

Fig. 2.9 TEM images of several straight and twisted ZnO nanobelts, displaying the shape characteristics of the belts [27]

formed from a single crystal, can be manipulated into left-handed helical nanostructures and nanorings, an occurrence ascribed to the outcome of making the energy sum relating to spontaneous polarisation and elasticity negligible. A simple and well-characterised technique to produce nanowires is by creating a template with multiple cylindrical perforations. For instance, metal nanowires can be engineered by metal salt reduction within a slim membrane’s tubular pores. If a template encompassing uniform perforations could be accessed, this method would yield nanorods and nanowires of consistent dimensions. Numerous porous templates meet these requisites, e.g. microporous zeolites, which can be packed with alternative substances. Further examples encompass chemically or electrochemically produced templates, e.g. liquid–crystal-templated mesoporous aluminosilicates, or anodically etched aluminium or silicon, respectively, self-organised systems, e.g. microphasedivided block copolymers and track-etched polymer membranes; these can be utilised to produce nanowires (Fig. 2.10) [28, 29]. Silicon nanochannel membranes with impeccably laid out pores have been synthesised by a range of methods, and can be utilised to engineer gold nanorods of desired diameter. The techniques have been modified in order to generate nanorods comprising portions of different metals, using electrodeposition. Such composite nanorods are referred to as nanobarcodes, the term inspired by the stripes created. Their applications include molecular labelling in the disciplines of analytical chemistry and biology. Orthogonal self-assembly refers to the anchorage of particular moieties of chosen molecules to a nanobarcode metal rod. For example, if a nanorod were comprised of two portions, one gold and one platinum, an organic molecule, with identical metal affinity, would bind randomly to either of the two metals. However, if a subsequent organic molecule had an increased bonding affinity with gold, it would dislodge the previous molecule from the gold areas. Thus, the two metals within an identical nanorod could have separate chemical species as monolayers.

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Fig. 2.10 A schematic representation of high-density nanowire fabrication in a polymer matrix. a An asymmetric diblock copolymer annealed above the glass transition temperature of the copolymer between two electrodes under an applied electric field, forming a hexagonal array of cylinders oriented normal to the film surface. b After the removal of minor component, a nanoporous film is formed. c By electrodeposition, nanowires can be grown in the porous template, forming an array of nanowires in a polymer matrix [29]

The ability to anchor functional molecules for selective locations of nanorods offers intriguing openings with respect to nanorod self-assembly into a preestablished operational configuration. This process has utility for DNA endfunctionalisation and recognition of gold nanorods. The bare terminals of gold nanorods, enclosed within an alumina membrane, are bound with thiolated DNA. These are then suspended in NaOH; the nanorods then undergo end-coupling with complementary DNA, labelled with rhodamine. DNA-side functionalised gold nanorods have also been engineered in this way and guided in order to undertake self-assembly on soft, lithographically patterned gold surface loci, which were operationalised using complementary DNA. This technique of gold surface coverage with nanowires made from gold through DNA linkage is promising for wire configurational assembly with specific connectivity (Fig. 2.11). Organisation of nanowires via electrospinning techniques can be used to create operational devices. A polymer solution, e.g. polyvinyl pyridine or a polymer sol–gel admixture, which traverses a metal capillary of high voltage yields a slim charged stream from the outlet. As the process continues, a cohort of charged nanofibres is forced onto a substrate-embedded ground electrode. Parameters, such as solvent viscosity, conductivity, surface tension and strength of the precursor solution, dictate the size of the electrospun nanowires. The electrodes gathering the nanowires can

2.4 Synthesis of Nanowires, Nanorods and Nanotubes

35

Fig. 2.11 A strategy for the orthogonal assembly of DNA-functionalised nanoparticle building blocks by dip-pen nanolithography [29]

be set out in different formats in order to persuade the emerging nanowire to form desired configurations. Single- or multilayered nanowires can be engineered using this technique, which is straightforward and easily reproduced. Hollow nanofibers of inorganic-polymer hybrid and inorganic substances have been created using electrospinning techniques. These encompass the release of an enduring coaxial jet, made up of a dense mineral oil centre enclosed in a coating of ethanol-acetic acid-PVP-Ti(OiPr)4 , from a double-capillary spinneret. An aluminium or silicon substrate is employed to gather the composite core–shell nanofibres; these are left to hydrolyse in ambient conditions. Octane is utilised to extract the oil centre; an array of nanotubes, with mural components comprising amorphous TiO2 and PVP, remains. The organic polymer and amorphous titania are subsequently transformed into the anatase polymorph via a calcination reaction conducted in air at a temperature of 773 K [30]. Zeolites, membranes or nanotubes can be employed for techniques using templates; these regulate the growth of the crystals but typically generate heterogeneous and polycrystalline structures. Generally, the VLS growth method offers a positive outcome with respect to manufacturing vectored single-crystal semiconductor nanowires, offering governance over their length and diameter, as well as constituents. This concept originates from the 1960s when vapour growth methods were evolved in order to use heated vapourised reactants to generate crystalline semiconductors. It was established that semiconductor hairs would automatically evolve from gold particles inserted into the reaction chamber. During VLS nanowire preparation, a droplet is initially formed by melting a catalyst. This is supersaturated with the reaction antecedents, which give rise to a singlecrystal nanowire which pokes out from the catalyst droplet following elemental precipitation. Silicon nanowires have been engineered by laser ablation of a Fe-Si substrate in buffer vapour; this generates a thick gas that forms nanoclusters during the condensation process; the temperature of the Se and Fi species falls as they collide with the buffer vapour (Fig. 2.12).

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2 Nanomaterials Synthesis

Fig. 2.12 a A TEM image of the nanowires produced after ablation of a Si0.9 Fe0.1 target. Scale bar, 100 nm. b Diffraction contrast TEM image of an Si nanowire; crystalline material (the Si core) appears darker than amorphous material (SiOx sheath) in this imaging mode. Scale bar, 10 nm. (Inset) Convergent beam electron diffraction pattern was recorded along the [211] zone axis perpendicular to the nanowire growth axis [30]

The schematic for the equilibrium pseudo-binary phase is well established. Thus, the reaction conditions that engender preferential liquid Fe-Si in equilibrium with solid Si can be determined, including temperature and constituents. The furnace heat is regulated in order to sustain the liquid phase of the Fe-Si. The Si nanowire starts to evolve once the Fe-Si liquid nanocluster undergoes supersaturation with Si, and continues until the Fe-Si nanoclusters are no longer in the liquid phase or the Si nutrient becomes exhausted. The nanowire diameter and length are dictated by the nanocluster diameter and the rapidity of growth, respectively. Nanowire development halts when it leaves the reactor’s temperature zone. A silica sheath covers the wire surface and the catalytic Fe-Si nanocluster is bound to the wire terminal. Doping can be achieved by presenting regulated quantities of PH3 or BH3 for n-Si or p-Si nanowires, respectively, in the growth process vapour phase. The vectored development of the single crystal is a consequence of epitaxy at the point of contact between the nanocluster and nanowire. A single-crystal silicon nanowire, with limited dimensional distribution and a diameter of down to 3 nm can be engineered by VLS using a SiH4 /H2 gaseous mixture on gold nanocluster catalysts. The nanowire growth arises following condensation of the silicon species on the droplet veneer which facilitates droplet supersaturation with silicon. The latter undergoes diffusion from the liquid–gas interface; silicon growth is initiated following its precipitation at the solid–liquid interface and occurs along a 90 degree trajectory. The uneven liquid surface is comprised of ledge, ledge-kink or kink loci, all of which can ensnare the encroaching growth species. The function of the catalyst is to form a sink for the growth species during the gaseous phase and to facilitate deposition [30].

2.4 Synthesis of Nanowires, Nanorods and Nanotubes

37

Consequently, the nanowire growth rate using the VLS technique is extremely rapid when a catalyst is added. Surface energy and the curvature radius dictate solubility; this can be expressed by the Kelvin equation. When facets evolve in the course of nanowire development, the speed of lateral and longitudinal evolution is related to the growth of each facet. Owing to their sizeable curvature, nanowires have a tiny radius, so to attain consistent nanowires and to circumvent notable lateral surface growth, a low supersaturation is desirable. High supersaturation could generate additional facets, together with secondary nucleation on the growth surface which could halt development in an epitaxial direction. In situ microscopy of a germanium-gold system has demonstrated the existence of nanowire development under the eutectic temperature in the presence of catalysis mediated by liquids or solids at identical heat [31]. It was noted that growth pressure and thermal history influenced the catalyst phase, and it was proposed that the kinetic enhancement of the eutectic alloy constituents may explain these observations. Chemical dopants, i.e. contaminants that either deposit or extract electrons, may be encompassed during nanowire evolution. There is the potential to n-dope or p-dope the nanowires, i.e. donating additional conduction electrons or subtracting electrons to form voids with a positive charge, respectively. The long configuration of a nanowire and its susceptibility to electrical polarisation mean that it is drawn towards a strong electric field with which it is aligned. Thus, if a voltage were applied across a pair of electrodes, a nanowire in proximity to liquid suspension is attracted to breach the intervening void. Parallel single-nanowire bridges can be arranged in organised rows in this manner. Positioning a p- and an n-doped nanowire in opposition can generate a junction [32]. Problems 1. 2. 3.

4.

5.

Derive an expression for the average binding energy per atom of metallic NP and the dependence on its diameter. Using the equation derived above estimate the melting temperature of 1 nm and 10 nm diameter Au NP. Note the cohesive energy of Au is 3.9 eV. The specific surface area of NP is defined as the total surface area per unit mass, usually measured in m2 g−1 . Calculate the specific surface area of cubic MgO NP with 3 nm diameter. Note the density of MgO is 3.58 cm−3 . Calculate the relative proportion of 3C (corner), 4C (edge), 5C (terrace) and 6C (bulk) ions in MgO NPs assuming perfect cubes of length L and lattice constant a. Low coordinated atoms on metallic NPshave been proposed to be catalytically active sites. How many atoms with a coordination of six or lower are there on icosahedral, octahedral and truncated Au NPs.

Chapter 3

Nanomaterials Characterisation and Analysis

3.1 Introduction Complete characterisation and analysis of nanomaterials include particle composition, particle size distributions, morphology/shape, structural analysis, surface characterisation, surface area analysis, optical properties, magnetic properties and others [33]. Conventional characterisation methods for nanomaterials can vary from system to system but commonly include transmission electron microscopy (TEM), Xray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), energy-dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), X-ray absorption fine structure (XAFS), inductively coupled plasma mass spectroscopy (ICP-MS), vibrating sample magnetometer (VSM), Auger electron spectroscopy (AES), Mossbauer spectroscopy and differential scanning calorimetry (DSC), to name a few. For nanoparticles with a size less than 10 nm, different techniques are required, such as high-resolution electron microscopy (HRTEM), Raman spectroscopy, nuclear magnetic resonance (NMR), ultraviolet photoemission spectroscopy (UPS), scanning tunneling electron microscopy (STEM), secondary ion mass spectroscopy, second neutral-atom mass spectroscopy (SNMS) and field emission scanning transmission electron microscopy (FE-STEM). Table 3.1 highlights characterisation techniques used for nanomaterials with the type of information obtained and the resolution of the instruments (TFXRD indicates thin-film XRD and TC indicates texture cradle) [11]. For nanoparticles with a size less than 10 nm, different techniques are required, such as high-resolution electron microscopy (HRTEM), Raman spectroscopy, nuclear magnetic resonance (NMR), ultraviolet photoemission spectroscopy (UPS), scanning tunneling electron microscopy (STEM), secondary ion mass spectroscopy,

© The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2022 Y. Al-Douri, Nanomaterials, https://doi.org/10.1007/978-981-19-3881-8_3

39

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3 Nanomaterials Characterisation and Analysis

Table 3.1 Comparison of performance of different testing and analytical instruments for nanomaterials Categories Surface analysis

Instrument

Functions

AES

Surface composition, chemical bonding

AFM

Resolution 1 μm 5 nm

XPS

Surface structure

5 nm

SIMS

Surface composition, chemical bonding

50 μm

SEM (X2)

Surface composition, in-depth analysis

0.1 μm

Electron microscopy SEM (X2)

Surface microstructure, composition analysis, 0.4/20 nm materials analysis

TEM/STEM Internal microstructure, crystal structure, composition analysis

XRD

HREM

Crystal (atomic/molecular) structure, interface structure

XRD

Crystal structure, phase identification

TFXRD

Phase identification of films, film thickness

TC

Hole image

0.18 nm

second neutral-atom mass spectroscopy (SNMS) and field emission scanning transmission electron microscopy (FE-STEM). Table 3.1 highlights characterisation techniques used for nanomaterials with the type of information obtained and the resolution of the instruments (TFXRD indicates thin-film XRD and TC indicates texture cradle).

3.2 Particle Size Detection and Analysis Firstly, we need to find a way to define the size of nanomaterials. For spherical nanoparticles, the diameter is defined as the size of the nanoparticle. As for nanomaterials of asymmetric shapes, the following four definitions are usually used: geometric diameter, equivalent diameter, SSA (specific surface area) diameter and refraction diameter. Geometric diameter: For particles of any geometric shape, the largest projected area can be converted into a circle of the same size. The diameter of this circle is the geometric diameter of particles. Equivalent diameter: The size of powder particles can be measured by using the sedimentation method, centrifugal method, mechanical method or hydraulic method. Homogeneous spherical particles, for example, have the same terminal settling velocity as nanoparticles; their diameter shall have an equivalent diameter of the nanoparticles.

3.2 Particle Size Detection and Analysis

41

Table 3.2 Determination of particle size Size definition

Determination

Range (μm)

Distribution benchmark

Geometric diameter

Optical microscopy

500–0.2

Number distribution

Electron microscopy

10–0.01

Number distribution

Equivalent diameter

Gravity sedimentation

50–1

Mass distribution

Centrifugal sedimentation

10–0.01

Mass distribution

Gas precipitation

50–1

Mass distribution

Proliferation

0.5–0.001

Mass distribution

Adsorption (gas)

20–0.001

Average SSA size

Infiltration (gas)

50–0.2

Average SSA size

Wetting heat

12–0.001

Average SSA size

Refraction

12–0.001

Volume distribution

X-ray line width

0.05–0.0001

Volume distribution

X-ray scattering at small angle

0.1–0.001

SSA size

Refraction diameter

SSA diameter: Using a variety of possible techniques, the SSA of the nanoparticle can be determined. From the surface area, a diameter can be calculated by one of the even spherical particles with the same formula: ⎡ ds =

6

ρ ρm

S

⎤ V (3.1)

Here, ds is the SSA particle size, ρ is the sample density, V is the volume of material tested, ρ m is the density of the bulk materials and S is the calculated surface area of the particles. Refraction diameter: The diameter of the nanoparticle is determined using XRD techniques. Typically, the calculated diameter will vary depending on the method used, as shown in Table 3.2. The diameter of SSA can be determined utilising both chemical and physical adsorption techniques; their respective attributes are presented in Table 3.3. The method of BET which was published by Brunauer, Emmett and Teller, employs multilayer gas adsorption; volumetric and gravimetric forms are frequently utilised in order to quantify solid phase substance SSA. Disparities between the sample volumes of a predetermined amount of vapour prior to and following adsorption are exploited by the volumetric method so as to measure surface vapour adsorption. The equivalent variations in weight are utilised by the gravimetric method for the same purpose. Stringent pregassing treatment and a strong vacuum are required by both BET modes. Technique precision is dictated by particle morphology and the presence of flaws, e.g. pores and fissures, amongst others, which may lead to a negative output error.

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3 Nanomaterials Characterisation and Analysis

Table 3.3 Comparison of physical adsorption and chemical adsorption Properties

Physical absorption

Chemical absorption

Adsorbability

van der Waals force

Atomic bonding force

Heat of adsorption

10 kcal/more

10–100 kcal/more

Optionality

None (applicable to any system Selectivity at low temperatures)

Absorption rate

Quickly (unable to be determined)

Usually not too fast (able to be determined)

Adsorption layer

Adsorption on multi-molecular layer

Adsorption on single-molecule layer

Fixed temperature adsorption

Decreased at high temperatures (decreasing with temperature increase)

Increased at high temperature (increasing with temperature increase)

Reversibility

Easy to fall off

Not easy to fall off

3.3 Detection and Analysis of the Electrical Properties Analysis of the nanomaterials’ electrical properties exploits the materials’ electrical resistance and conductivity, dielectric and piezoelectric traits, together with surface and volume impedance, respectively. Typically, in comparison to bulk substances, resistance is higher in nanomaterials as their size restricts the motion of multiple electrons through the diminutive crystal. The width of the crystal border increases with rising crystal atom disarray and restricts the movement of electrons; this interfacial energy boundary gives rise to heightened resistance. The association between resistance and temperature is notably disparate from that of bulk substances; unusual properties may be observed, e.g. in Pd nanomaterials. The configuration of nanomaterials is also at considerable variance from their bulk equivalents. Their dielectric characteristics are individual and predominantly reflect the fact that particle dimensions govern the dielectric constant and dielectric loss. These parameters are also significantly influenced by the frequency of the electric field. The formula, D = εE can be applied, where E and ε represent the applied field strength and the dielectric constant, respectively, and which indicates that the electric displacement is correlated with E by a factor of ε. Due to available extrinsic electric field, polarisation may lead to the conversion of electrical energy to heat, i.e. dielectric loss. This is generally notated as loss tangent or loss factor tan δ, i.e. tan δ =

εr′ εr′′

(3.2)

where εr′ and εr′′ indicate the real and imaginary components, respectively, of εr, the dielectric constant.

3.4 Detection and Analysis of Magnetic Properties

43

Charge can be segregated by a number of materials when they are subject to mechanical forces, such as stress or strain; this is termed the piezoelectric effect, where the first word is derived from the Greek for pressure, ‘piezo’. Voigt noted that the piezoelectric effect had been reported in the late nineteenth century when it was described as being limited to crystals generated by a non-centrosymmetric lattice. Contemporary studies on the micro-theory underlying this phenomenon remain plagued by difficulties, particularly with respect to disparities in experimental data. However, it appears transparent that this effect is mainly owing to crystal medium polarisation. A substance can be termed piezoelectric when it produces electrical charge in response to mechanical deformation. If a voltage were applied to the material, the opposite would occur; it would give rise to physical stress or strain processes. Substances exhibiting this property are referred to as piezoelectric materials. Nanomaterials’ electrical properties can be recognised and appraised by employing the tools listed below: (i) (ii) (iii) (iv)

(v)

Digital multimeter, with three modes, which enables the investigation of AC and DC voltage, current and resistance, and includes a diode; High impedance meter, which appraises the electrical insulation type; Four-point probe, to measure surface resistance of the substance; Metal insulator semiconductor component; this provides information on the dielectric constant, threshold voltage, carrier concentration and insulating layer width and capacitance, amongst other parameters; and Scanning probe microscopy, which has evolved into a key method for the quantification of electrical characteristics associated with nanomaterials.

3.4 Detection and Analysis of Magnetic Properties The magnetic properties are dictated by each material’s constituents, configuration and condition. Crystal dimensions, morphology, distribution and flaws may influence magnetisation and susceptibility, whereas crystal phase and material number may impact saturation magnetisation and Curie temperature. The nanomaterials differ greatly from conventional bulk materials properties, particularly with respect to their magnetic characteristics, such as magnetic transformation, superparamagnetism, coercive force, Curie temperature and susceptibility. An abridged description of these traits is presented below. The Néel temperature, T n , is a property of a number of metals, alloys and salts under which the substance exhibits spontaneous magnetic ordering of a non-parallel nature, together with anti-ferromagnetism, and above which, acquires paramagnetic properties. It is influenced by the closest atom coordination number, atomic spacing and the forms of adjacent atoms. Magnetic susceptibility can be defined as the material magnetisation:magnetic field strength ratio. The alignment of these two parameters along a different or similar vector determines whether the ratio is a tensor or a straightforward scalar, respectively.

44

3 Nanomaterials Characterisation and Analysis

The materials have a tendency to acquire magnetisation within an extrinsic magnetic field. In order to diminish the magnetisation intensity to zero, the opposite magnetic field strength has to be imposed; this is referred to as coercive force. In the superparamagnetic condition, if the nanoparticle parameters were to exceed the critical dimension, the coercive force would decrease as particle size diminished. This observation can be read in two ways. Firstly, the diminutive dimension means that an individual particle is essentially a lone magnetic domain, in which nanoparticles exhibit permanent magnetic behaviour. A strong reverse magnetic field would be necessary to reverse the magnetic moment created by the contributions of several particles in concert. Secondly, chains may be generated from spherical nanoparticles as a consequence of the magnetostatic influences, leading to elevated coercivity. The Curie temperature, T C , mentions the specific temperature where strong magnetism or ferromagnetism is lost; at higher temperatures, paramagnetism is present. It comprises a key material property, which is influenced by a substance’s atomic configuration and spacing; these form the principal parameters used to appraise the magnetic characteristics of nanomaterials. They can be recognised by techniques including magnetic force microscopy (MFM) and NMR. A surface detection technique, MFM deploys the magnetic interplay between silicon probes which are covered in a slim magnetic layer; the specimen is utilised in order to acquire the surface magnetic configuration. Martin and Wickramasinghe were responsible for its design, in 1987 [34]. At the outset, just a magnetic picture was acquired; material configuration delineation was not possible. Contemporary MFM requires a two-stage scanning process. Initially, intermittent contact scanning is utilised in order to depict the specimen’s surface morphology; subsequently, the probe is lifted to a specific distance above the sample so as to acquire the magnetic data. The technique uses a similar paradigm to that underlying noncontact atomic force microscopy (AFM) although it offers improved resolution, in the region of approximately 50 nm, and is straightforward to apply in a range of circumstances. The method has become a key assessment tool in studies in the magnetic material sector; it can be utilised in relation to magnetic film, memory devices and recording instruments, amongst others. A microscope for MFM was created by Rugar et al. that places a low bias voltage of between 1 and 10 V between the probe and specimen; this enhances the attraction between the tip and consequently, stability. Strong magnetic fields cause some nuclei to exhibit split spin energy levels that have the capacity to absorb electromagnetic waves or radiation in order to generate nuclear magnetic resonance (NMR). This mandates certain assumptions: (i) the nuclear magnetic moment cannot be zero; (ii) the external magnetic field is strong and homogeneous; and (iii) the required electromagnetic wave frequency is applied in order to generate nuclear resonance. The NMR technique utilises a mode of detection that causes no damage; it can be used to assess the physicochemical characteristics of substances. When judged against comparable non-invasive tools, e.g. X-ray and infrared, NMR exhibits a number of benefits: (i) rapid rate of detection; (ii) a dedicated functional moiety which can be fixed in order to determine whether a partial reaction has caused residual antecedents or byproducts, and which can be utilised to govern yield purity, and to

3.6 Detection and Analysis of Thermal Properties

45

guide purification and separation technique development; and (iii) use of an electromagnetic wave detection process, which is unaffected by the dimension, morphology or hue of the specimen.

3.5 Detection and Analysis of the Mechanical Properties The point at which stress causes permanent deformation is referred to as yield strength. Once an applied stress surpasses the elastic limit, the degree of deformation rises quickly and encompasses both elastic and plastic distortion. At a certain stress, the plastic strain becomes markedly elevated, and the stress–strain curve displays a modest area of fluctuance, which is termed yielding. The upper and lower yield points indicate the highest and lowest stress values at this juncture. The lower yield point tends to exhibit greater stability so it is also referenced as the lower yield point or strength. The upper yield point is the highest strength restriction of the substance where it can be utilised without hazard. The yield strength of substances can be represented by the Hall Petch equation: C1 σ y = σ0 + √ d

(3.3)

where σ0 , d and C 1 reflect the friction stress, the mean crystal dimension and an empirical constant, respectively. Studies employing TEM and AFM have demonstrated the superlative mechanical characteristics of carbon nanotubes, i.e. elevated elastic modulus, and elastic and rupture strains, respectively, which are in keeping with those calculated experimentally. When contrasted against bulk substances, nanomaterials commonly exhibit strength and hardness properties increased by a factor of 2. Nanocomposites evidence marked enhancement of a number of mechanical traits, such as strength and toughness, together with abrasion, ageing, pressure and water resistance, respectively, and compactness. Nanoceramics obtained from nanoparticles have excellent toughness and ductility.

3.6 Detection and Analysis of Thermal Properties According to the nanomaterials’ thermal properties, physical characteristics that merit appraisal encompass the thermal conductivity coefficient, specific heat, thermal expansion, thermal stability and melting temperature. Once the substance’s fine layer of film achieves a particular depth, the grain boundary effect gradually exerts a notable influence on thermal conductivity. As the film layer depth diminishes, the thermal conduction coefficient at 90 degrees to the film demonstrates a tendency to fall.

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3 Nanomaterials Characterisation and Analysis

Hypothetical forecasts and laboratory findings confirm that the specific heat data for nanomaterials are markedly elevated in comparison to those of traditional bulk compounds. In comparison to the latter, nanosubstances demonstrate a relatively random spread of atoms within their configuration, which occupy a greater volume. Entropic effects impacting the non-crystalline veneer, therefore, give rise to greater specific heat than in typical coarse crystalline substances, thus causing the specific heat to be higher. The nanocrystals are twice larger than the mean crystals in the thermal expansion coefficient; the raised t is attributed to the crystalline border constituents. The principal tool used to quantify the thermal expansion coefficient is called a thermal expansion analyser; alternative nomenclatures include thermal dilatometer or thermomechanical analyser. Measuring this parameter can offer comprehension of molecular movement, configurational alterations and thermal expansion activity. A thermal expansion analyser is generally the optimal tool for the resolution of queries relating to the heat bonding properties of various substances during semiconductor device production. The melting point is the temperature at which a substance is converted from a solid to a liquid phase. The definition of this may be transparent, as in crystalline materials, or less well delineated, as in non-crystalline substances. The temperature may rise to a point where a modest proportion of atoms within the macrostructure exhibit synchronous motion in the manner of a liquid, i.e. the glass transition temperature, T g . Glass material exists in a solid phase at temperatures lower than the T g and as a supercooled liquid in those above the T g . From a mechanical perspective, elastic and viscous deformations begin at temperatures lower and higher than the T g, respectively (Table 3.4). Table 3.4 Melting point of several materials at different scales Material type

Particle size: diameter (nm) or total number of atoms

Melting point (K)

Au

Conventional bulk materials

1340

300 nm

1336

100 nm

1205

20 nm

800

2 nm

600

10–30

555

500

480

Conventional bulk materials

600

30–45

583

Conventional bulk materials

1678

2 nm

≈910

1.5 nm

≈600

Conventional bulk materials

1358

20 nm

≈312

Sn Pb CdS

Cu

3.6 Detection and Analysis of Thermal Properties

47

The nanomaterials’ thermal properties are identified and analysed utilising thermogravimetric analysis (TGA) and derivative thermogravimetry (DTG). TGA offers information derived from the ongoing measurement of the mass modification of materials whilst they are heated. In particular, these mass alterations are observed in relation to the temperature, which is adjusted at a pre-established speed and then associated with the mass reductions and thermal changes in the substance under investigation. Various treatments can be performed simultaneously. Documentation of the alterations in parameters that occur from the DTG method. Using TGA or DTG, several thermal characteristics can be recognised, e.g. the ageing temperature, dynamics and behaviours, respectively, during pyrolysis in various temperatures and vapour conditions, in addition to the IC packaging substances utilised in the production of executable semiconductor apparatus, plastic printed circuit boards and glass and ceramic substrates, together with alternative constituents. In colloidal systems, Brownian motion, diffusion and sedimentation equilibrium are included amongst the associated thermal traits. Mean particle displacement in Brownian motion, X , can be represented by the following equation: / X=

RT Z N0 3π ηr

(3.4)

where R represents the ideal gas constant, T indicates absolute temperature, N 0 is the Avogadro constant, Z reflects the time observation interval, η is the dispersion medium viscosity and r defines the particle radius. Brownian motion confers notable properties onto colloidal particles, significantly impacting the stability of the dispersion system. Sedimentation is the result of colloidal aggregation arising from interparticle collisions rather than gravimetric forces. The diffusion phenomenon is associated with mass transfer, which stems from Brownian motion in conditions where there is a stepwise change in solution strength. Diffusion becomes less obvious in systems in which particles are large and thermal velocity is low. Typically, diffusion velocity is expressed by the diffusion coefficient, D, which can be defined as the physical amount of a substance that reflects its ability to undergo diffusion. In a colloidal system, D can be described by the following equation, where the parameter definitions are as for Eq. 3.4: D=

RT 1 . N0 6π ηr

(3.5)

Since D is related to the mean displacement, Eq. 3.5 can be rewritten as follows: 2

D=

X 2Z

(3.6)

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3 Nanomaterials Characterisation and Analysis

Table 3.5 Diffusion coefficient of sol resulting from nano-Au particles at 291 K

Nano-Au particles size (nm)

Diffusion coefficient (109 m2 /s)

1

0.213

10

0.0213

100

0.00213

where Z and X indicate a certain observation time interval and mean particle displacement caused by Brownian motion, respectively. Diffusion coefficients relating to sol arising from different-sized gold nanoparticles at a temperature of 291 K are presented in Table 3.5. Once the suspended particles within the solution demonstrate a settling speed that is equivalent to the rate of diffusion, an equilibrium is achieved, which is referred to as the sedimentation equilibrium. At this point, the Gaussian distribution law governs the colloidal particle concentration. This can be formulated as: N0

n 2 = n 1 e− RT . 3 r 4

3

(ρ p −ρ0 )(x2 −x1 )g

(3.7)

where n1 and n2 are the particle concentrations at the cross section at the heights x 1 and x 2, respectively, ρ 0 and ρ p indicate the density of the colloidal particles and dispersion medium, respectively, g refers to gravity acceleration, and R, I, A and r are as previously defined.

3.7 Detection and Analysis of Optical Properties The principal variations in optical characteristics between nanomaterials and their traditional bulk equivalents are the followings: (i) robust nanoparticle broadband optical absorption; (ii) optical absorption band frequency shift; (iii) the quantum confinement effect; (iv) nanoparticle luminescence; and (v) optical features of the dispersion system. 1.

Robust nanoparticle broadband absorption characteristics

2.

Nanoparticles exhibit enriched light absorption at a number of visible frequency wavelengths; however, their light-reflective properties are diminished. Nanoparticles which display potent broadband absorption traits include the powders, silicon nitride (Si3 N4 ), silicon carbide (SiC) and alumina (Al2 O3 ). Robust absorption within the ultraviolet (UV) spectrum is exemplified by zinc oxide (ZnO), iron oxide (Fe2 O3 ) and titanium dioxide (TiO2 ). Optical absorption band frequency shift Nanoparticle optical absorption band frequency shift can be observed as blue or red translocations of absorption ranges, a feature which is attributed to both intraand interparticle surroundings. A red shift can be achieved in the absorption

3.7 Detection and Analysis of Optical Properties

49

3.

spectra of semiconductor nanoparticles following chemical adjustments at their surface. A blue shift can be induced by quantum size or surface effects; a red shift in absorption frequency can arise from heightened electron wave functional imbrication. Quantum confinement effect

4.

An exciton absorption band can be produced when semiconductor nanoparticle dimensions are lower than the exciton’s Bohr radius, aB , as there is imbrication of electron wave functions which generates a void. The latter is augmented by a reduction in nanoparticle dimension, a phenomenon that can be expressed as (aB = r)3 . A blue shift occurs as a result of heightened exciton absorption. Nanoparticle luminescence

5.

In ambient conditions, visible light may be emanated by silicon particles under 6 nm in size; reducing particle dimension heightens the emission band saturation and gives rise to a blue shift. Tabagi has postulated that silicon nanoparticle luminescence occurs owing to the carrier’s quantum confinement effect. Brus reasoned that the translational mirroring present in traditional bulk configurations is lost once the size of the silicon nanoparticles is lower than 6 nm; hence, the described characteristics of luminescence are observed. Optical features of dispersion system For nanoparticles in dispersed systems, the Tyndal effect becomes relevant [35]. This relates to the dispersed particles’ dimensions and the light projection wavelength.

For the characterisation of nanomaterials’ optical properties, the most frequently applied techniques encompass infrared (IR) and UV–visible spectroscopy, respectively, and scanning near-field optical microscopy (SNOM). Dispersive and nondispersive forms of IR are represented by conventional IR and Fourier transformed IR spectroscopy, respectively. The latter is characterised by a greater output of energy, has excellent performance parameters in terms of frequency or wave number recognition, and additionally includes multiplicity. The basis of UV–vis spectroscopy is the electronic transitions principle into molecules. Electronic, vibrational and rotational forms of energy all contribute to molecular energy. Nanomaterials have individual traits, e.g. various optical absorption characteristics, which distinguish them from the traditional bulk forms. An admixture of SPM and a fibreoptic probe, SNOM offers a high spatial resolution above the diffraction limit for the quantitative and qualitative assessment of optical parameters. It encompasses the benefits of both optical and electron microscopy. High-resolution depiction is provided by SNOM of the nanomaterial surface with respect to spectral analysis, with the capacity to discern molecular aggregation. In combination with time-resolved fluorescence microscopy, SNOM can additionally be employed in order to investigate nanoscale optical features. Contemporary applications of NFOM, which offers a resolution of approximately 50 nm, include studies within biological and clinical spheres, together with research exploring semiconductor optoelectronic high-density optical media.

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3 Nanomaterials Characterisation and Analysis

3.8 Scanning Probe Microscopy SPM is highly benefitted in the nanomaterials study due to the fact that nanomaterial veneers can be appreciated with a resolution at the level of the atom. A range of surface characteristics can be investigated, such as their magnetic, electrical, mechanical, thermal and optical traits. A number of scientists have used SPM extensively in their research in this sphere. SPM requires no specimen preparation and can be used in a range of settings, factors which add to the benefits of this technique. Transparent drawbacks to SPM include its slow rate of image acquisition, substandard data reproducibility and its inability to offer elemental analysis. STM involves the application of a low voltage between a metal probe and the conductive specimen placed at a distance of between several and tens of angstroms. A fixed tunnelling current level can be sustained in relation to the probe terminal atoms and the specimen veneer; surface morphology can be resolved at atomic level. This method offers the most optimal resolution of all recognised microscopy techniques; however, it cannot be applied to substances with insulating properties [36].

3.9 Principles of Scanning Tunneling Microscopy Metal tips are employed to survey the specimen surface, a process which, if standard functional settings were in place, would generate a quantum tunnelling current. The atom tip experiences a more powerful current which therefore gives rise to lateral resolution at the level of the atom. The representation of the specimen surface is attained by utilising the tunnelling currents as signals of quantification. Thus, both sample and probe require properties of conduction, and the veneer of the specimen must be planar from the atomic perspective in order to achieve an accurate representation. These requisites restrict STM applicability (Fig. 3.1).

3.9.1 Operating Mode of STM During STM, the motion vector of the tip is documented as it surveys the specimen surface. The atomic representation of the veneer is obtained from the computed surface morphological density distribution, although this evidences a number of fluctuations. The height parameters of the latter are computed using voltage application in the z-drive. When a surface exhibits only a modest fluctuation level, the height of the tip can be governed in order to conduct a conservation scan; alterations in tunnelling current are again noted in order to depict surface morphology distribution. This method of scanning is characterised by its rapidity, high signal:noise ratio and thermal drift associated with the measurement. In principle, it is unsuitable for

3.9 Principles of Scanning Tunneling Microscopy

51

Fig. 3.1 Basic structure of STM

the analysis of specimens that exhibit fluctuations in excess of 1 nm on their surface (Fig. 3.2). A.

B.

Fixed current image acquisition: a tunnelling current of approximately 1 nA is established as a feedback cue. This necessitates fixing the distance between the probe and specimen surface, as this is highly responsive to any change in the tunnelling current’s predetermined magnitude. During surface scanning, the probe height, or z-value, is modified in keeping with any surface fluctuations. Thus, the change in probe height is representative of the specimen’s surface morphology [37]. Fixed height image acquisition: in this mode, the image is obtained immediately from the tunnelling current parameters. The probe is kept at a fixed height during imaging, and so any surface fluctuations impinge on the distance between the probe and specimen surface, consequently, modifying the tunnelling current parameters (Table 3.6).

3.9.2 STM Application: Atomic Manipulation Thirty years ago of continuous dream to see individual atoms and to spatially manipulate individual atoms. Three decades later, this dream has become a reality; globally, numerous laboratories possess the information and apparatus to conduct techniques that were originally deemed beyond reach. Precise atomic engineering was initially achieved by Eigler et al., at IBM in 1990. They employed an STM scanning needle to relocate Xe atoms on nickel surfaces in a controlled fashion. In order to showcase

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3 Nanomaterials Characterisation and Analysis

Fig. 3.2 Schematic diagram of scanning modes: a fixed current mode and b fixed height mode. S is the spacing between the tip and sample, I and V b are the tunnelling current and bias voltage and Vz is the feedback voltage to control the tip height in the z-direction Table 3.6 Advantages and disadvantages of the STM scanning modes Mode

Principle

Advantages and disadvantages

Fixed current mode

Probe is maintained at a fixed current value height and adjusted with the morphology of the sample surface. The probe height variation is used to present the image of the sample surface

Advantages: large depth of field Disadvantages: scan speed is slower and vulnerable to low-frequency noise

Fixed height mode

With the probe being set at a fixed height, tunneling current values are directly used to present changes in surface morphology

Advantages: quick scanning rate, which can potentially observe surface dynamics Disadvantages: probe damage is possible if large fluctuations in the sample height are present

Current density mode Combination of the two above-mentioned materials, wherein bias modulation is used to gather images

Disadvantages: larger data sets, time-consuming

3.9 Principles of Scanning Tunneling Microscopy

53

Fig. 3.3 Examples of atomic manipulation

their abilities, they wrote the firm’s title in Xe atoms (Fig. 3.3). At Hitachi Ltd., in Japan, Hosoki et al. inscribed PEACE’91HCRL on the surface of molybdenum disulphide (MoS2 ) using diminutive single-atom voids, a year later. Subsequently, a further group created an array of almost 100 atoms of iron to create a pair of Chinese symbols, 原子 (Fig. 3.3). These images inspired many scientists and were frequently publicised in scientific communications and at seminars during this era [38]. There are three techniques that can be employed for the surface manipulation of single atoms. Firstly, the force of the engagement between the atoms on the STM tip with those on the specimen surface can be exploited, facilitating the transfer of the latter to a predetermined de novo locus, as illustrated by Eigler. Secondly, evaporation of the surface atom domain to the scanning tip can be attained utilising an electric field; the atoms on the tip can be translocated and returned to the surface, a technique deployed by Hosoki. Finally, an electric field gradient can be used to relocate the atoms. The latter technique is presently subject to extensive study and is considered promising by numerous scientists in the sector. The potential applications of STM for reforming surface atomic configurations and manoeuvring lone atoms are broad. At present, these include molecular and quantum devices, data storage as well as biological and material science spheres. Control of a lone atom necessitates three empirical phases, i.e. movement, extraction and placement. Such technological processes are additionally key for future utilities of these techniques and the preparation of structural veneers for devices at an atomic scale. The atomic manipulation method is principally applied in order to engineer structures at either the nanoscale or atomic level, e.g. high-density data storage. The previously described translocation of xenon atoms that was presented by researchers at ABM can be utilised immediately in the synthesis of high-density memory at atomic level. If the individual surface loci with the presence or absence of an atom were designated 1 and 0, respectively, the surface would have the capacity to be utilised as binary memory. This type of storage density was previously unheard of, being far superior to that attained by contemporary semiconductors and magnetic

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3 Nanomaterials Characterisation and Analysis

disks. Images of the participating atoms are straightforward to acquire using STM; this process is analogous to reading data bits but on an atomic scale [39].

3.9.3 STM Advantages The apparatus of scanning tunnelling microscope has numerous benefits which engender this method as an efficacious experimental instrument for the examination of the surface configurations of substances and biological microelectronicsassociated specimens. Biologists can deploy STM to investigate single molecules of protein or DNA, crystal flaws, as well as the configuration and appearance of microelectronic devices comprising nanoscale constituents. Prior to the advent of STM, only injurious techniques were available to study structures at this level. STM is non-invasive, maintaining the integrity of the specimen veneer. Furthermore, it is not subject to optical microscopy’s diffraction restriction, which prohibits imaging of materials at 50% of the optical wavelength’s dimension [40].

3.10 Atomic Force Microscopy AFM is mainly utilised to quantify the atomic van der Waals forces in order to perform surface atom imaging. Since AFM has the ability to image any substance irrespective of its electrical characteristics, it has some superiority over STM. The technology underlying AFM evolved in 1985 through collaboration between Binnig and Quate, working at IBM and Stanford University, respectively, [40] on scanning non-conducting substances. If the AFM probe were combined with a metal layer, it would be able to recognise electronic traits, a process referred to as static electricity microscopy. Analogous strategies can be utilised in order to upgrade the probe to measure a broad spectrum of parameters. The evolution of this series is termed SPM, of which the AFM is most commonly employed.

3.10.1 AFM Principle Principally, AFM operates on the force principles between the probe and the atoms on the material surface; an optical offset is used to document these, obtain representative images and compute a range of measurements. The microcantilever is a crucial part of AFM; its head entails a slim, pointed probe which is utilised for specimen surface scanning. Typically, the cantilever is constructed from silicon or silicon nitride; its curvature is of nanoscale dimensions. Since the probe is sited in proximity to the specimen surface, any distortion will follow Hook’s law and produce an offset as a consequence of the probe-surface engagement. The latter can be documented by using

3.10 Atomic Force Microscopy

55

Fig. 3.4 Diagram of the AFM working principle

Fig. 3.5 Basic structure of AFM

a light-sensitive diode array to quantify the laser beam reflection that is projected onto the microcantilever. During surface scanning, the angle of laser reflection alters together with the light-sensitive diode current. Measurement of the latter facilitates computation of any buckling or displacement of the cantilever. The data can then be processed in order to generate a three-dimensional representation of the specimen veneer (Figs. 3.4 and 3.5).

3.10.2 Comparison of the AFM Scanning Modes There are three AFM scanning modalities, i.e. contact, non-contact and tapping. A comparison is presented in Table 3.7.

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Table 3.7 Comparison of the AFM scanning modes Scanning mode

Basic principle

Advantages and disadvantages

Contact (2–3 nm)

In image scanning, the tip has a gentle contact with the sample surface. A very weak repulsion arises between atoms to produce the probe offset. Feedback is used to control a fixed value. The probe position change is obtained corresponding to all the scanning points, which renders the topographic images of the sample surface

The contact area is small, but access force may damage the sample, especially for soft materials. However, the larger force can usually get better resolution. Therefore, an appropriate level of force is needed. As the repulsion is very sensitive to the distance, the atomic resolution is easier to obtain

Non-contact ( D(m , then )} Dm can be regarded as infinity and we obtain L ex = L 40 / (1 − Va ) ⟨D⟩3 exp 3 σD2 , ⎛

ln2 Dr f ( Dr ) = √ exp − 2 σD2 2π σD Dr 1

⎞ (4.4)

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4 Mechanical and Magnetic Properties of Nanomaterials

( ) where ⟨D⟩ = D0 exp DD2 /2 is the mean grain size. Then Eq. (4.4) indicates that ⟨ K l ⟩ increases with increasing σD , i.e. soft magnetic properties of nanocrystalline alloys deteriorate with increasing σD , even if ⟨D⟩ is constant. It should be noted that this result is essentially established in other distribution functions. Naturally, the effective anisotropy in nanocrystalline alloys may have contributions from induced anisotropies such as magnetoelastic anisotropy other than the random magnetocrystalline anisotropy and / hence the effective anisotropy constant in actual materials is

K u2 + ⟨ K l ⟩2 [85] where K u is the induced uniaxial anisotropy / √ ∫ constant and ⟨ K 1 ⟩ = (1 − Va ) K 1 / N = K 1 (1 − Va ) D 3 f (D)dD/ L 3ex and N is the number of grains in a magnetically coupled volume). Most of the nanocrystalline Fe–M–B (M = Zr, Hf, Nb) alloys with good soft magnetic properties exhibit a remanence ratio (J r /J s ) of around 0.5. This means that the magnetization process of the alloys is mostly governed by the induced anisotropies, i.e. √ K u > ⟨ K 1 ⟩.. A limiting condition of K u2 ≫ ⟨ K 1 ⟩2 enables us to arrive at L ex = L 0 K 1 / K u and more correctly ⟨K ⟩ =

√ ⟨K ⟩ ≈ K u +(1 − Va )

Ku K1 2

⎛

⟨D⟩ L0

⎞3

) ( exp 3 σ D2 .

(4.5)

The D3 behaviour of H c is observed for nanocrystalline Fe–Zr–B(–Cu) alloys with sufficiently small D [85]. The grain-size distribution evaluated by counting (α-Fe grains in TEM images is shown in Fig. 4.10) [83]. The log-normal distribution function reproduces well the observed grain-size distribution. In order to reproduce grain-size distribution of the Fe85 Nb6 B9 alloy more accurately, we consider the bimodal distribution function (f b (D)) expressed by superimposing the two log-normal distribution functions with different medians (D0 and d b Do , d b > 1), the geometric standard deviations (σ D and σ b ) and the ratio of the distribution function for the large grains to that of the small grains (r b ) [83]. When the induced anisotropies are dominant, the coercivity is given as Hc = 0.64(⟨K ⟩ − K u )/ Js [86] where J s is the saturation magnetization. The relative observed H c values, adjusted for the difference of D and J s , for the alloys, Fe84 Nb7 B9 and Fe84.9 Nb6 B8 P1 Cu0.1 , annealed at 823 K are indicated by closed circles. Rising va and diminishing Tcam lead to elevated coercivity; specifically, the influence of the latter on H c is notably larger where the alloy va is of sizeable magnitude. Moreover, the computed and experimentally obtained measurements exhibit a good agreement. Thus, it can be surmised that for the Fe84.9 Nb6 B9 P1 Cu0.1 alloy, the principal factors that engender optimal soft magnetic characteristics are a low va and elevated Tcam ; this includes application within the low T a area at approximately 800 K.

4.4 Magnetic Properties

71

Fig. 4.10 Grain-size distribution of Fe-Nb-B(-P-Cu) alloys. The histograms and mean grain size (D) are obtained from transmission electron microscopy (TEM) images. The solid lines indicate fitting a, b and d unimodal or c bimodal log-normal distribution functions. The inset in (c) is an enlarged view of 30–50 nm grain size. The fitting parameters and calculated results are also shown [86]

4.4 Magnetic Properties Figure 4.11 illustrates the reliance of the composition of as-deposited Fe–Hf–O films on Bs and H c which have undergone sputtering in the absence of magnetic field application. Open circles indicate the lone bcc phase; solid circles represent the single amorphous stage. Oxide, and mixed bcc and amorphous phases are represented by the double open and half-solid circles, respectively. There is a tendency for the B values to diminish with rising Hf and O constituency; a ridge in relation to the Hf proportion is seen at 10–15 at%. The H c parameter falls with elevated Hf and O inclusion and a dip is displayed at a similar Hf content to the Bs ridge. The Bs ridge and the H c valley

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4 Mechanical and Magnetic Properties of Nanomaterials

Fig. 4.11 Compositional dependence of saturation magnetic flux density (Bs ) and coercivity (H c ) for as-deposited Fe–Hf–O films [86]

Fig. 4.12 Temperature dependence of saturation magnetic flux density (Bs ) for as-deposited Fe76 Hf24 , Fe55 Hf11 O34 and Fe49 Hf16 O35 films. The arrows indicate the first crystallisation temperature (T X ) of the films [87]

are at a comparable point. There is an area where Bs and H c parameters above 1.0 T and under 160 A/m, respectively, are observed concurrently and in which the true portion of the initial permeability is 4 × 102 despite the as-deposited condition; the configuration of the film encompasses fine bcc and amorphous phases represented by the filled-in areas.

4.4 Magnetic Properties

73

The temperature dependence of Bs for as-deposited Fe–Hf–O films in comparison with a Fe-Hf amorphous alloy film is shown in Fig. 4.12. Two-phase crystallisation activity is seen in Fe–Hf–O films containing an admixture of amorphous and bcc configurations. The initial temperature of crystallisation, relating to grain development within the bcc phase, is shown by the arrows, which is measured by DSC. The value of BS for both films decreases with increasing temperature with bending points in the course of changes, at 500 and 750 K for the Fe49 Hf16 O35 and Fe55 Hf11 O34 films, respectively. An amorphous Fe76 Hf24 film has a low T c , (below room temperature) owing to the Invar effect [87]. For the Fe49 Hf16 O35 film, the bending point is thought to result from the T c of the amorphous phase not from crystallisation, because the temperature is lower than that of the first crystallisation. It is difficult to conclude certainly that the bending point is attributed to the T c of the amorphous phase for the Fe55 Hf11 O34 film, because the temperature of the bending point is close to the first crystallisation temperature. However, there is no bending point below that temperature, and the T c of the amorphous phase for the Fe55 Hf11 O34 film can be regarded as higher than 700 K. As a consequence, the T c of the amorphous phase including Fe and Hf increases with the dissolution of O in the Fe–Hf–O films. Furthermore, we confirmed [88] that the T c of the amorphous phase for the Fe–Hf–O films increased after annealing. The film compositions are the same as those of the films shown in Fig. 11.28. In all systems, Bs above 0.9 T and high ρ RT above 4 μ Ωm are simultaneously obtained. The k, values are 0.1–2.9 × 10–6 . Relatively low H c values below 400 A/m are obtained in M = Hf, Zr and Re systems, which have a mixed structure of nanogranular bcc and amorphous phases. The properties of soft magnetism with respect to as-deposited Fe–(Hf, Z: Re)–O films potentially reflect the nanodimensions of the grains and the intergrain ferromagnetic coupling, which leads to an averaging of the magnetocrystalline anisotropy of the bcc-Fe phase during the high-T c amorphous stage (Fig. 4.9) [88]. Notably, acceptable magnetic characteristics are evident despite the incorporation of rare earth (Re) elements within the films; these exhibit notable magnetocrystalline anisotropy and suppress soft magnetism owing to a reduction in the magnetic engagement of Fe and Re arising from the favoured Can = 0. The frequency dependence of p and the quality factor (Q = μ′ /μ′′ ) of the Fe– Hf–O and Co–Fe-Hf–O films prepared by various methods, together with the data on other metallic soft magnetic alloy films developed to date is shown in Fig. 4.13. From a utility perspective, the Q value is an essential parameter. In fact, at 1 MHz, traditional soft magnetic films have a high permeability; in the region of 104 has been achieved for nanocrystalline soft magnetic films, e.g. Fe-Si–Al–Hf–C [89]. Nevertheless, with rising instances of lower values of ρ RT , μ′ diminishes. Conversely, in the spectrum under 30 MHz, the Fe–Hf–O films have a μ′ less than that of traditional films, although above 30 MHz, flat μ′ traits are observed owing to the elevated ρ RT parameters and medium H K . In an as-deposited condition, Bs and μ′ values of 1.3 T

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4 Mechanical and Magnetic Properties of Nanomaterials

Fig. 4.13 Frequency dependence of the real part of initial permeability (μ′ ) and the quality factor (0 = μ′ /μ′′ ) for Fe62 Hf11 O27 , film (as-deposited), Fe61 Hf13 026 film (after uniaxial field annealing (UFA) at 673 K for 10.8 ks) and Co44.3 Fe19.1 Hf14.3 O22.1 film (as-deposited) compared with the other soft magnetic [89]

and 1.4 × 103 at 100 MHz, respectively, are associated with Fe62 Hf11 O27 film. The values of Q are additionally elevated compared to those seen in traditional films. Problems 1. 2. 3. 4. 5.

How mechanical could behave in nanocrystalline metals? Explain hardness and ultimate strength of nanomaterials. What is the deformation behaviour of nanostructured alloys? Elaborate the soft magnetic properties. How the grain size and Curie temperature affect the magnetic properties?

Chapter 5

Electrical and Optical Properties of Nanomaterials

5.1 Introduction The nanocrystalline particles represent a matter state which lies within the transition area between a bulk solid and a lone molecule. Their physicochemical characteristics therefore evolve progressively from those of a solid to those representative of a molecule as the particle dimensions are reduced. The underlying rationale of this process can be attributed to two key observations. Firstly, as indicated previously, the diminutive size of the particles augments the surface:volume ratio, and so the surface atom population may have an equivalent or greater size that sited within the centre of the crystalline lattice; thus, the surface traits become significant. If no additional molecules underwent adsorption onto the nanocrystalline particles, the atoms on the surface would be extremely unsaturated and their electronic impact on the particles’ traits would be entirely disparate from that arising from the core atoms. If the surface atoms were ligated, these influences would become more evident. Thus, nanocrystalline particles exhibit distinctive electronic transfer and catalytic features. Secondly, a sole electronic effect is observed in metal and semiconductor nanoparticles. As particle dimensions grow, the band configuration progressively develops, a process in which the molecular orbital transforms into delocalised band conditions. The size quantisation effect that underlies the conversion to cluster species from a bulk metal or semiconductor is illustrated in Fig. 5.1. Within a metal, distinct electronic levels are generated following the division of the quasi-continuous density of states in the valence and conduction bands; the distances between these levels and the band gap rises as particle dimensions diminish. A mild variation of this behaviour is seen in semiconductors, as in their bulk condition, a band gap is already present. An increment in this band gap is also demonstrated as the particle dimensions decrease; progressive transformation of the energy bands into distinct molecular electronic levels occurs [90]. Where the particle dimension is lower than the De Broglie electron wavelength, the carriers of the charge may be viewed from a quantum mechanics perspective as ‘particles in a box’, where the box dimensions are determined by the size of the crystallites [91]. © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2022 Y. Al-Douri, Nanomaterials, https://doi.org/10.1007/978-981-19-3881-8_5

75

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5 Electrical and Optical Properties of Nanomaterials

Fig. 5.1 Size quantization effect. Electronic state transition from bulk metal/semiconductor to small cluster

In semiconductors, the quantization effect of optical gap is seen where the dimensions of clusters fall into the spectrum of 1 nm to nearly 10 nm. The metal behaviour of metal particles, comprising between 50 and 100 atoms and of 1–2 nm in diameter, begins to attrite and they acquire semiconducting properties [92]. Particles that evidence this size quantisation effect can be referred to as Q-particles or quantum dots; these will be described subsequently. At clusters of extremely diminutive particles are dictated by the’magic numbers’, these are more often seen than alternatives. A cubic or hexagonal dense configuration comprising a single core atom, encompassed by an inner and outer shell comprising 12 and 42 atoms, respectively, characterises metals. This configuration can be expressed empirically as 10n2 + 2 atoms in the nth shell. A gold particle, containing 5 atoms, i.e. Au55 , and first described by Schmid in 1981, is an extremely well-known stabilised metal cluster [93].

5.2 Metals The nanocrystals and nanowires have a mean radius, R, within the Fermi wavelength spectrum, λF , which can, in fact, be synthesised; they demonstrate atypical mechanical and electronic characteristics. The impact of their diminished measurements and dimensionality, together with their quantum traits, has been the subject of much

5.2 Metals

77

study over the last ten years. Extremely thin nanowires, together with monatomic chains poised between a pair of metal electrodes have been manufactured. Electrical conduction and force have been measured concomitantly in the course of stretching of nanowires. Initially, much of the work is carried out on GaAs-AlGaAs heterostructures which are grown to contain thin conducting layer at the interface. The conducting layer is treated as a two-dimensional (2D) electron gas in which a narrow constriction of desired width w, and length l can be created by applying a negative gate voltage. The conductivity σ, which relates the electric current density to the electric field by j = σ E, is expressible in terms of the areal charge density ρ S for 2D electron gas of effective mass m∗ through the equation σ =

ρs e2 τ m∗

(5.1)

The experimental quantity of interest, however, is the conductance G = I/V, which is the ratio of the total current I to the voltage drop V across the sample of length l in the direction of current flow. For 2D, since I = wj, one can also write G=σ

w l

(5.2)

For a 3D conductor, this relationship is valid provided that w is replaced by the cross-sectional area A orthogonal to the current flow direction. Similar expressions are also valid for the thermal transport of energy. The thermal conductance related to the energy (or heat) current J x through a sample between two reservoirs is given by k x = J x ΔT, where ΔT is the temperature gradient. Depending on whether the energy is carried by electrons (x = e) or by phonons (x = p) the thermal conductance is identified as electronic K e or phononic K p . Here our focus is on quantum transport through materials of very small dimensions. Novel size-dependent effects emerge as w and l are reduced towards nanometric dimensions in the nanometre range. The relationship expressed by G=σ

w l

(5.3)

stands true in the diffusive transport system in which w and l are larger than the average free path. When the constriction width diminishes, there is a juncture at which quantum mechanics comes into play. The discrete conversion of energy levels induced by the carrier’s quantum confinement in a band width, w, can be expressed as pn = n2 h2 /(8 m w2 ). The conductance is dictated by the quantity of occupied transverse modes reliant on w. Instead of a straightforward linear relationship between G and w, a secondary dependence is opposed on w by quantum mechanics. Changes in w lead to alterations in the energy spectrum which in turn, modify the frequency of occupied modes under the Fermi energy level and thus, conductance.

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5 Electrical and Optical Properties of Nanomaterials

Straightforward physical principles demonstrate that the quantity of transverse occupied conditions, i.e. N ∼ 2w/λF , rises with constriction width. Since conductance is generated by influences from all these modes, it could be anticipated to exhibit a linear increase with w despite being within the nanosphere. This is almost correct apart from the fact that in theory, it is possible for an ongoing alteration in w to occur; however, as the mode number / is an integer, this can only change in discrete increments. Reorganising N ≈ n EεnF makes this notion clearer from a physical perspective. N can be obtained from the highest occupied level by straightforward quantification of the discrete eigenvalue frequency. Since direct comparability between the eigenvalue and Fermi energy would be an unusual occurrence, N is assumed to represent the integer reflecting the greatest occupied level immediately inferior to E F . Consequently, the quantum mechanics influence on the diminished parameter, w, leads to an alteration in conduction in a step-wise manner, although the height of each step is yet to be determined. This straightforward outlook requires amendment when subsequently discussed additional elements become major actors in this process; a more in-depth interpretation is then necessary. λF is the typical length involved as in low temperatures, these are the sole electrons which have energy in proximity to the Fermi energy carry current. If w ≫ λF , there would be an abundance of conducting modes and the conductance would be high. This is characteristic of metals in which λF ∼ 0.2 nm, i.e. extremely diminutive; thus, w ∼ λF is necessary in order to note discrete variations in conductance. This is the rationale underlying the fact that discrete conductance activity is initially seen in semiconductor heterojunctions which have an extremely low density of electrons and where λF is 100-fold greater than the value observed in metals. The effect of a decreased length, l, on conductance is more apparent. If the ohmic system were true for the expression: G=σ

w l

(5.4)

It is seen that a finite residual resistance would always be present. The average free path, l e , describes a natural typical length in accordance with Heisenberg’s uncertainty principle. If there were no momentum attrition during the propagation of carriers, defined by l < le , this sphere would be named the ballistic transport regime. This is usually defined by the principle that the conductance is equal to the current passing through the specimen with the voltage difference as a divisor: G=

I ΔV

(5.5)

The electrical conductance quantum arises spontaneously in keeping with Heisenberg’s uncertainty principle. The formula, I = ΔQ/t transit , describes the current, where I indicates the charge flow rate. Charge is measured in units of elementary charge, e. In the outer quantum limit, ΔQ = e can therefore be established; the time taken for transit should then fall within the time spectrum inferred by Heisenberg’s uncertainty

5.2 Metals

79

principle. I =

e Δt

(5.6)

When Eqs. 5.5 and 5.6 are amalgamated and the definition of potential difference, ΔV, i.e. equal to the electrochemical potential difference, ΔE, divided by e, electronic charge, is exploited, the following expression becomes valid: G=

e2 ΔEΔt

(5.7)

Following Heisenberg’s uncertainty principle once more, where ΔE Δt ≫ h, the formula for ballistic conductance encompassing the spine degeneracy becomes G = 2e2 /h in the perfect situation, for which 8 × 10−5 Ω−1 is the highest possible value and reflects the height of the step or the conductance for each transverse mode. The equivalent resistance value, computed by h/2e2 , is 12.9 kΩ; the value 12,345 Ω can be utilised as an aide memoire. This is ascribed to the resistance at the points of contact where the conductor is bonded to the electrodes or electron reservoirs, and represented traditionally by G ∝ w. The effect of quantum mechanics rises in discrete stages, increasing by 2e2 /h when there is a sufficient augmentation of w to enable an additionally transverse mode to become occupied and thus able to offer conduction. Currently, a range of quantum effects have been exhibited by the electronic and transference characteristics of metallic point contacts and wires manufactured by STM with a mean size within the metallic λF spectrum, as well as by mechanical break junctions. Stepwise differences in the wire’s two-terminal electrical conductance, G, were observed with the stretch.

5.2.1 Quantum Transport of Electrons Landauer [94], in 1957, has presented an innovative outlook with respect to conduction. He proposed the perception of conduction as transmission and published his well-recognised equation, which has made ground-breaking contributions to the identification of properties of conductance and to mesoscopic domain physics. Landauer described conduction as a scattering phenomenon, and that the incident current flux dictates transport. Founded on self-consistency rationales for reflections (R) and transmissions (T ), his equation for a single-dimensional conductor’s conductance states: ) ( G = 2 e2 / h T /R

(5.8)

where the transmission coefficient is represented by T, and the reflection coefficient, R = 1 − T. Subsequently, Sharvin [95] has investigated the electron transport through

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5 Electrical and Optical Properties of Nanomaterials

a small contact between two free-electron metals. Since the length of the contact is negligible, i.e. l → 0, the scattering in the contact is absent and hence T ∼ 1. By using a semiclassical approach he found an expression for the conductance, ( )( ) G s = 2 e2 / h A A2F /4π

(5.9)

this is now recognised as Sharvin’s conductance, where A indicates area. The initial Landauer formula (Eq. 5.8) and Sharvin’s conductance have appeared to be inconsistent, since the Landauer equation gives rise to G → ∞ as R → 0 when scattering is absent. The resultant perplexity arising in the literature has been resolved by acknowledging that the latter only represents the barrier conductance within a conductor that has one dimension. A notable point was raised by Engquist and Anderson, i.e. that there is a tight correlation with the conductance and the method of quantification. The formula generated by Landauer was extrapolated to include conductance appraised between two external reservoirs in which the finite-length conductor was situated. The analogous two-terminal equation for conductance, i.e. G = (2e2 /h)T, includes the resistance that arises from the contacts with the reservoirs. Thus, for flawless transmission, where T ∼ 1, the conductance remains finite and has a value of 2e2 /h. The step-wise activity observed in G has been evaluated in a number of publications. A straightforward and official description is offered below, utilising an optimised constant constriction of width, w. The electrons are restricted to the transverse trajectory; the quantised energy of their conditions is represented by pi . The electrons exhibit unrestricted propagation along the constriction length. For an electron with pi < E F , the propagation constant, γ i , can be defined as ⎡ γi =

2m ∗ ( E F − εi ) ℏ2

⎤ 1/2 (5.10)

If the constriction width rises by λF /2, a de novo subband, whose energy can be expressed as pi + è2 γ 2 i/2 m, would fall under the Fermi level and be added to the current via the modest bias of the voltage, ΔV. The current can be expressed as I =

J Σ

⎡ ( ) ⎤ 2n ie ϑγ i Di E f + eΔV − Di ( E F )

(5.11)

i=1

Here j is the index of the highest subband that lies below the Fermi level, i.e. pi ≤ E F + eΔV and pj + 1 > E F + eΔV, and ni is the degeneracy of the state i. By assuming perfect transmission in the absence of any contact resistance and barrier inside the constriction, i.e. T = 1, and by expressing the group velocity ϑγ i and the density of states Di (p) in terms of the subband energy p = pi + è2 γ 2 i/2m* and dividing I by ΔV we obtain

5.2 Metals

81

G=

J Σ 2 e2 i=1

h

ni

(5.12)

Accordingly, each current-transporting state with energy in the range E F < p < E F + eΔV contributes to G an amount 2e2 ni /h. For a consistent, infinite wall constriction, non-degenerate conditions are required where ni = 1; a rise in w by a factor of λF /2 causes a step-wise increase in G, by a factor of 2e2 /h. Consequently, the G(w) curve has a step-wise configuration. Given that the transverse quantisation level spacing is relatively diminutive for the low electron density found in the two-dimensional electron vapour domain, and Δp ∼ λF −2 , the incremental configuration has the potential to be smothered out if T ∼ 10 K or the bias voltage were finite. In the presence of a constriction of finite length, the scattering at the reservoir contacts, or the contact resistance, and the heterogeneities or possible boundaries within the constriction, influence transmission. Lone channel conductance can be described by ) ( G = 2 e2 / h T

(5.13)

may diverge from ideal quantised parameters. Consequently, the steep step-wise configuration of G(2 = w) is smoothed out by factors, such as the arising contact with the possible boundary, roughness of the surface and scattering of impurities. Regional broadening of the constriction or likelihood of contaminants can lead to quasi-bound zero-dimensional conditions in the constriction and to resonance tunnelling influences. The constriction length, l, is another key variable. In order to obtain a steep step configuration, l has to be elevated in comparison to λF , yet more diminutive than the electron average free path, le. There is then an evening out of G(w) within a short constriction, where l < λF . Thus, when there is a constant constriction, G(w) has a steep step-wise configuration; the relevant parameters have the relationships, w ∼ λF and λF ≪ l < l e . Conversely, hypothetical research suggests that resonance configurations arise on the even plains owing to wave interference reflected from the sudden links with the reservoirs. The discrete incremental change in G as a function of w or E F is referred to as the quantisation of conductance and comprises the reflection of the constriction conditions quantised in the electrical conductance. The above discourse can subsequently be expanded in order to explore the ballistic electron conductance that is present across a point contact or nanowire; in this case, the electronic movement is restricted within two dimensions but is able to propagate in an unrestricted manner within the third. Objects that have drawn attention encompass the point or quantum contact, generated by STM indentation, a nanowire or connective neck manufactured by tip retraction from such a dent, and additionally, a metallic SWNT. Where STM is used to synthesise nanowires, the products are anticipated to be rounded, although their cylindrical shape contains flaws, and to have a neck radius of R ∼ λF . Horn-like terminals link the neck and the electrodes, and

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5 Electrical and Optical Properties of Nanomaterials

so there is a rise in the radius with the distance remote from the neck. An ultimate example, where l → 0, encompasses Sharvin’s conductance, GS = (2e2 /h)(π R/λF )2 , where R indicates the contact radius. In this instance, the discrete increments are practically smoothed out in their entirety and the plateaus are lost. In the quantum domain, where the cross section is defined as A ∼ λF 2 , GS would be expected to disappear when A becomes lower than the key cross-sectional parameter established by the uncertainty principle. Thus, when l rises, Gs acquires a step-wise activity.

5.2.2 Electrical Conductivity Gimzwewski and Möller initially designed a contact of atomic proportions with a connective neck utilising the STM tip which evidenced sudden differences in the conductance range on tip movement. To start with, the conductance behaviour noted was ascribed to conductance quantisation. However, a higher standard of nanowires has been synthesised utilising STM and additionally, by deploying the mechanical break junction. Following the stretching, sudden variations in the two-terminal electrical conductance of such wires were observed. The narrowest cross-sectional radius of wire before the break measures just several Ångströms; it is described by the length scale of λF , for which the metal’s discrete character takes precedence over its continuum definition. Given that at this scale of length, the level spacing, Δp, is approximately ∼1 eV, the density zeniths of the connective neck’s conditions, D(p), are distinctly segregated. Thus, resolution of the conditions’ transverse quantisation occurs in a straightforward fashion in an ambient temperature. Additionally, an alteration in atomic configuration of the radius leads to notable modifications of the level spacing and in state occupancy, which is subsequently reflected by identifiable adjustments in the associated characteristics. Thus, the ballistic electron transport along a nanowire should have a tight correlation with the atomic configuration and most slender portion of the radius. Conversely, incongruities of atomic configuration and electron potential promote scattering, which disrupts the typical step-wise architecture. In the next section, data recently published from contemporary experimental and hypothetical research on the topic of stretched nanowires are presented.

5.2.2.1

Atomic Structure and Mechanical Properties

Computer simulations of the atomic structure of connective necks created by STM are first performed in the seminal works by [96]. Generally, a two-phase process occurs when there is deformation or elongation of the wire; these are two dissimilar but serial steps that are reiterated during the stretching process. In the initial phase, i.e. the quasi-elastic stage, there is a rise in the stored strain energy and mean tensile force as the stretch is applied without disturbing the integrity of the atomic strata. At this juncture, the change in added tensile force, Fz(s),

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is roughly linear, but it diverges from this linear relationship as the neck atom count is lowered. Changes in Fz(s) may arise owing to dislodgement and resiting of atoms within the identical layer, or by atoms swapping between neighbouring strata. Furthermore, repositioning of atoms both within and between strata may induce oscillations in conductance. The second phase is referred to as the yielding stage. A wire undergoes yielding as a result of a range of processes according to its diameter. The dislocation movement and or the slides on the glide planes typically dictate any yielding in conditions in which the integrity of the ordered crystalline configuration is retained by the wire and it has a fairly sizeable cross section. The format of the ordered architecture and its alignment with respect to the z-axis, or stretching trajectory are anticipated to influence yielding. Conversely, yielding may arise through order–disorder conversion and swapping of a single atom if the wire cross section were reasonably diminutive. At the point where the elongation becomes equivalent to the spacing between the layers at the conclusion of the quasi-elastic phase, disorder becomes evident within the structure. However, following additional stretching some reparation occurs and a de novo stratum is created. During the yielding phase, there is a sudden drop in |Fz| and the layer cross section generated at the conclusion of the yielding phase is suddenly diminished by several atoms. A standard instance of this is depicted in Fig. 5.2, in which the force change and atomic configuration computed using the MD technique are shown.

Fig. 5.2 The variation of the tensile force Fz (in nanonewtons) with the strain or elongation s along the z-axis of the nanowire having Cu(001) structure. The stretch s is realised in m discrete steps. The snapshots show the atomic positions at relevant stretch steps m. The MD simulations are performed at T = 300 K [96]

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Yielding is confirmed at the point where the neck is only 3–4 atoms in width by a lone atom leaping from a neighbouring stratum to the interlayer territory. In specific conditions, a pentagon is created by atoms in the region of the neck which is alternating in configuration within various strata. The atoms between the layers form a chain which goes through the heart of the pentagon rings. In this de novo stage, the elastic and yielding phases are admixed and elongation, i.e. to a greater degree than the distance between two strata, is facilitated. The pentagons are converted into a triangle when the wire cross section becomes even more diminished. At the beginning of the pulling off stage, the single-atom mechanisms, during which there is migration of lone atoms from the central to terminal strata, can induce an alteration in cross section which is modest and more continuous. There is an inclination to reduce the surface area and thus to decrease the system’s strain energy, and this forms the impetus for this form of neck generation. Although the quasi-elastic and yielding phases can be differentiated at the outset, the changes in force acquire complexity and become increasing reliant on atomic drift for atomic necks. These atomic migrations have significant connotations, e.g. falls in conductance changes alongside stretch. Simulation experiments on wires, which have neck diameters of ∼λF but which rise gradually with distance remote from the neck area, have shown that these wires lack configurational integrity, evidencing unprovoked neck thinning despite the lack of added tensile strain. When stretching is enduring, immediately prior to the break, the neck cross section is diminished so as to encompass around 2 or 3 atoms. In this instance, the hollowsite registry may alter to the top-site registry, which gives rise to a collection of atomic chains or rope, or of a lone atomic chain (Fig. 5.3). This represents a marked alteration in the wire’s atomic configuration that has significant connotations, e.g. it can be computed on first principles that Young’s modulus for a chain of lone Al atoms is more robust than that for the bulk material.

5.2.2.2

Electrical Conductance of Nanowires

The overall feature of electrical conductance has been acquired from statistical data analysis of numerous serial parameters measured in order to determine the frequency of a measured value of conductance. In metal nanowires, a number of distribution curves demonstrated a zenith in the proximity of GO = 2e2 /h and a wide apex close to 3GO , but practically an absence of notable structure with conductance values of greater magnitude. It seems that the connective neck cross section can be lowered to just a lone atom immediately prior to wire fracture. In specific circumstances, a monatomic chain, comprised of a line of a small number of atoms, may form the connective neck. In this instance, a transverse state, which is quantised in the neck at the Fermi level, is linked to electrode conditions and generates a conducting channel which gives rise to a flat portion within the G(s) curve, and thus a zenith in the curve of statical distribution. The step-like configuration for wider necks made up of several atoms cannot arise at integer multiples of Go as a result of the scattering from incongruities and thus, channel integration.

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Fig. 5.3 The top view of three layers at the neck showing atomic positions and their relative registry at different levels of stretch. m = 15 occurs before the first yielding stage. The atomic positions in the layers 2, 3 and 4 are indicated by plus, black circle and diamond, respectively. m = 38 and m = 41 occur after the second yielding stage. m = 46, 47 and m = 49 show the formation of bundle structure (or strands). In the panels for m = 38–49 the positions of atoms in the third, fourth and fifth (central) layers are indicated by plus , black circle and diamond, respectively. Atomic chains in a bundle are highlighted by square boxes [96]

It should be appreciated that during stretching, the elastic and yielding phases are reiterated, the nanowire surface becomes rougher and diverges significantly from a symmetrical circular configuration. The slimmest portion of the neck typically only comprises a maximum of two interlayer distances and is linked to the horn-like terminals. In this scenario, quantisation is incomplete and there is a degree of tunnelling influence. Data from atomic simulations indicated that the neck cannot encompass either adiabatic growth of distinct electronic conditions or flawless circular symmetry. Thus, the anticipated quantised pointed configuration will be smoothed by channel integration and tunnelling. Nevertheless, contrary to the above observations, alterations in conductance are sudden. This paradox has been explicated by the concurrent evaluations of changes in conductance and force with stretch. The alterations in the latter during stretching occur with the abrupt liberation or cessation of the applied tensile force. This occurs immediately after the yielding stage; the wire cross section is diminished in a discontinuous fashion, and thus both the electronic configuration and level spacing are subject to rapid modification in the region of the neck. An example of conductance change, G(s), computed for a nanowire, is illustrated in Fig. 5.4.

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Fig. 5.4 Variation of the conductance calculated for the stretching nanowire described in Fig. 5.2 [96]

5.2.3 Surface Plasmons The bright crimsons and yellows observed in stained-glass windows are generated by gold and silver colloid nanoparticles, respectively. For several hundred years, these characteristics have drawn attention; studies evaluating metal nanoparticles can be traced back as early as Michael Faraday. Thus, it was a major breakthrough in traditional physics when a solution to Maxwell’s equations that define the extinction spectra, i.e. extinction = scattering + absorption, of randomly sized spherical particles was published by Mie in 1908, relating to derivation of surface plasmon resonance in noble metal nanoparticles. Metal free electrons, e.g. d electrons in silver and gold metals, are at liberty to move throughout the substance. For silver and gold the average free path is approximately 50 nm, and so in particles of a lower dimension, the bulk would not exhibit scattering and any engagements would be anticipated to occur at the surface. Standing resonance states can be established where the light wavelength is of a notable order of magnitude greater than nanoparticle dimensions (Fig. 5.5). Oscillation of the liberated electrons within the metal occurs when light resonates with the surface plasmon oscillation. The passage of the light wavefront leads to the polarisation of particle electron density to a single surface; this oscillates in keeping with the frequency of the light generating a standing oscillation. This state of resonance can be appraised using absorption and scattering spectroscopy and is dictated Fig. 5.5 Schematic of plasmon oscillation for a sphere, showing the displacement of the conduction electron charge cloud relative to the nuclei [96]

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by the morphology, dimension and dielectric constants of the metal and the encompassing substance. This phenomenon is called the surface plasmon resonance, since it arises at the particle’s veneer. Alterations in morphology or dimension precipitate geometrical modifications which in turn, create a shift in surface electric field density. This influences the electrons’ oscillation frequency, creating a range of optical trait cross sections, such as absorption and scattering. An alteration in the encompassing material’s dielectric constant will impact the oscillation frequency owing to the surface’s changing capacity to take up electron charge density from the nanoparticles. The dielectric constant can be modified by substituting a different solvent; however, the capping substance is key in dictating the plasmon resonance shift owing to the fact that it has a local impact on the nanoparticle surface. The recognition of molecules that are chemically bound is achieved through identification of the alterations they evoke on the surface electron density and the resultant change in peak surface plasmon absorption. This forms the foundation for the application of noble metal nanoparticles as responsive sensors. Surface plasmon resonance was initially computed by Mie, who offered a solution to Maxwell’s formulae for diminutive spheres engaging with an electromagnetic field. Gan expanded this hypothesis to encompass ellipsoidal configurations. Contemporary techniques, which utilised discrete dipole approximation, facilitate computation of surface plasmon resonance absorption in random geometrical shapes. For gold nanorods, a rise in intensity and peak wavelength has been computed for the longitudinal plasmon resonance as the aspect ratio, i.e. length/width, is elevated. Adjusting the latter parameter can enable the plasmon resonance to be refined within the visible spectrum. Numerous applications have made use of the rise in surface plasmon resonance absorption intensity and its resultant electric field augmentation.

5.2.3.1

Dipole Plasmon Resonances

When small spherical metallic nanoparticles undergo light irradiation, the electric field oscillation induces coherent oscillation of the conduction electrons (Fig. 5.5). When the electron cloud is displaced relative to the nuclei, a restoring force arises from Coulomb attraction between electrons and nuclei that results in oscillation of the electron cloud relative to the nuclear framework. The oscillation frequency is determined by four factors: the density of electrons, the effective electron mass, and the shape and size of the charge distribution. The collective oscillation of the electrons is called the dipole plasmon resonance of the particle (sometimes denoted ‘dipole particle plasmon resonance’ to distinguish from plasmon excitation that can occur in bulk metal or metal surfaces). Higher modes of plasmon excitation can occur, such as the quadrupole mode where half of the electron cloud moves parallel to the applied field and half moves antiparallel. For a metal like silver, the plasmon frequency is also influenced by other electrons such as those in d-orbitals, and this prevents the plasmon frequency from being easily calculated using electronic structure calculations. However, it is not hard to relate the plasmon frequency to the metal dielectric

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constant, which is a property that can be measured as a function of wavelength for bulk metal. To relate the dipole plasmon frequency of a metal nanoparticle to the dielectric constant, we consider the interaction of light with a spherical particle that is much smaller than the wavelength of light. Under these circumstances, the electric field of the light can be taken to be constant, and the interaction is governed by electrostatics rather than electrodynamics. This is often called the quasistatic approximation, as we use the wavelength-dependent dielectric constant of the metal particle, pi , and of the surrounding medium, p0 , in what is otherwise an electrostatic theory. Let’s denote the electric field of the incident electromagnetic wave by the vector Eo . We take this constant vector to be in the x-direction so that E o = E o x, where x is a unit vector. To determine the electromagnetic field surrounding the particle, we solve LaPlace’s equation (the fundamental equation of electrostatics), ∇ 2 ϕ = 0, where ϕ is the electric potential and the field E is related to ϕ by E = −ϕ∇. In developing this solution, we apply two boundary conditions: (i) that ϕ is continuous at the sphere surface and (ii) that the normal component of the electric displacement D is also continuous, where D = pE. It is not difficult to show that the general solution to the LaPlace equation has angular solutions which are just the spherical harmonics. In addition, the radial solutions are of the form r l and r −(l+1) , where l is the familiar angular momentum label (l = 0, 1, 2, …) of atomic orbitals. If we restrict our considerations for now to just the l = 1 solution and if E o is in the x-direction, the potential is simply ϕ = Ar sin θ cos Φ within the sphere (r < a) and ϕ = (−E o r + B/r 2 ) sin θ cos Φ external to the sphere (r > a), where A and B are constants requiring evaluation. If the boundary conditions were applied to these solutions and the calculated ϕ is utilised to establish the sphere’s extrinsic field, E out , the following equation could be stated: ⎡ E out

= E0 x − α E0

x 3x − 5 (x x + y y + zz) r3 r

⎤ (5.14)

where α is the sphere polarizability and x, y, z are the usual unit vectors. We note that the first term in Eq. (5.14) is the applied field and the second is the induced dipole field (induced dipole moment = αEo) that results from polarisation of the conduction electron density. For a sphere with the dielectric constants indicated above, the LaPlace equation solution shows that the polarizability is α = gd a 3

(5.15)

with gd =

εi − ε0 εi + 2ε0

(5.16)

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Although the dipole field in Eq. 5.14 that for a static dipole, the more complete Maxwell equation solution shows that this is actually a radiating dipole, and thus, it contributes to extinction and Rayleigh scattering by the sphere. This leads to extinction and scattering efficiencies given by Q ext = 4xIm(gd )

(5.17)

8 4 x |gd |2 3

(5.18)

Q ext =

where x = 2πa(p0 )1/2 /λ. The efficiency is the ratio of the cross section to the geometrical cross-sectional πa2 . Note that the factor gd from Eq. (5.16) plays the key role in determining the wavelength dependence of these cross sections, as the metal dielectric constant pi is strongly dependent on wavelength.

5.2.3.2

Quadrupole Plasmon Resonances

For larger particles, higher multipoles, especially the quadrupole term (l = 2) become important to the extinction and scattering spectra. Using the same notation as above and including the l = 2 term in the LaPlace equation solution, the resulting field outside the sphere, Eout, now can be expressed as E out = E 0 x + ik E 0 (x x + zz) ⎡ x 3x − α E 0 3 − 5 (x x + y y + zz) r r ⎤ ⎤ ⎡ 5z x x + zz −β E 0 x + y y + zz) − (x r5 r7

(5.19)

and the quadrupole polarizability is α = gd a 5 gd =

εi − ε0 εi +3/2 ε0

(5.20) (5.21)

Note that the denominator of Eq. (5.21) contains the factor 3/2 whilst in Eq. (5.16) the corresponding number is 2. These factors arise from the exponents in the radial solutions to Laplace’s equation, e.g. the factors r l and r −(l+1) that are discussed above. For dipole excitation, we have l = 1, and the magnitude of the ratio of the exponents is (l + 1)/l = 2, whilst for quadrupole excitation (l + 1)/l = 3/2. Higher partial waves work analogously.

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Following the same derivation, we get the following quasistatic (dipole + quadrupole) expressions for the extinction and Rayleigh scattering efficiencies: ⎡ Q ext Q ext =

5.2.3.3

= 4xIm gd +

⎤

x2 x2 gq + (εi −1) 12 30

⎡ ⎤ x 2 || ||2 x 4 8 4 gq + x |gd |2 + |εi −1|2 3 240 900

(5.22)

(5.23)

Extinction for Silver Spheres

We now evaluate the extinction cross section using the quasistatic expressions, Eqs. (1.24), (1.25), (1.29) and (1.30) as well as the exact (Mie) theory. We take dielectric constants for silver that are plotted in Fig. 5.6a and the external dielectric constant is assumed to be 1 (i.e. a particle in a vacuum). The resulting efficiencies for 30 and 60 nm spheres are plotted in Fig. 5.6b, c, respectively. The cross section in Fig. 5.6b shows a sharp peak at 367 nm, with a good match between quasistatic and Mie theory. This peak is the dipole surface plasmon resonance, and it occurs when the real part of the denominator in Eq. (5.16) vanishes, corresponding to a metal dielectric constant whose real part is -2. For particles that are not in a vacuum, the plasmon resonance condition becomes Re pi /p0 = −2. Since the true portion of the silver dielectric constant diminishes as the wavelength is elongated (Fig. 5.6a), the plasmon resonance wavelength for p0 > 1 is greater than within a vacuum. It would also be elevated if the particle dimension were to exceed 30 nm, owing to further electromagnetic influences that will form the subject of subsequent discourse. The l = 2 quasistatic (dipole + quadrupole) cross section, together with the entire Mie paradigm output for a sphere with a 60 nm radius, is illustrated in Fig. 5.6c. The quasistatic data incorporate a finite wavelength amendment. It can be observed that there is a red shift of the dipole plasmon wavelength, and a discrete quadrupole resonance zenith at 357 nm. The latter arises when the true portion of the denominator in Eq. 5.20 disappears, reflecting a metal dielectric constant with a real component of −3/2. In a sphere of this dimension, transparent disparities are evident between the outcomes from the quasistatic and the Mie hypothesis data, although the key traits are preserved. Despite the fact that the Mie hypothesis is not a costly computation, the quasistatic formulae are expedient when solely qualitative data are required.

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Fig. 5.6 a Real and imaginary part of silver dielectric constants as function of wavelength. b Extinction efficiency, i.e. the ratio of the extinction cross section to the area of the sphere, as obtained from quasistatic theory for a silver sphere whose radius is 30 nm. c The corresponding efficiency for 60 nm particle, including for quadrupole effects, and correcting for finite wavelength effects. In (b) and (c), the exact Mie theory result is also plotted [96]

5.2.3.4

Electromagnetic Fields for Spherical Particles

Up to this point, the computation of extinction and Rayleigh cross-sectional computations have been highlighted. Nevertheless, for particular characteristics, e.g. surfaceenhanced Raman spectroscopy (SERS) and hyper-Raman scattering (HRS) intensities, it is the electromagnetic field at or near the particle surfaces that determines the measured intensity. Thus, if ⟨ E(ω) is the local ⟩ field for frequency ω, then the |2 | SERS intensity is determined by |E(ω)|2 | E(ω′ )| where ω’ is the Stokes-shifted frequency and the brackets are used to denote an average over the particle surface. The

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⟩ ⟨ HRS intensity is similarly (but approximately) determined by |E(ω)|4 |E(2ω)|2 . Also, when one makes an aggregate or array of metal nanoparticles, the interaction between the particles is determined by the polarisation induced in each particle due to the fields E arising from all of the other particles. At the dipole (dipole + quadrupole) level, the field outside a particle is given by ⎡ E out = E 0 x − α E 0

x 3x − 5 (x x + y y + zz) r3 r

⎤

E out = E 0 x + ik E 0 (x x + zz) ⎡ x 3x − α E 0 3 − 5 (x x + y y + zz) r r ⎤ ⎤ ⎡ 5z x x + zz −β E 0 − x + y y + zz) (x r5 r7

(5.24)

(5.25)

These expressions determine the near-fields at the particle surfaces quite accurately for small enough particles; however, the field beyond 100 nm from the centre of the particle exhibits radiative contributions that are not contained in these equations. To describe these, we need to replace the dipole or quadrupole field with its radiative counterpart. In the case of the dipole field, this is given by E dipole = k 2 eikr

r 2 P − 3r (r · p) r × (r × p) + eikr (1 − ikr ) 3 r r5

(5.26)

where P is the dipole moment. Note that this reduces to the static field in Eq. (5.24) in the limit k → 0 where only the term in square brackets remains. However, at long range, the first term becomes dominant as it falls off more slowly with r than the second. Figure 5.7 shows the outlines of the augmented electric field, |E|2 , surrounding silver spheres with a radius of between 30 and 60 nm, founded on computations from the Mie hypothesis which encompassed all multipoles. The plots incorporate two selected planes, i.e. the zx-plane, generated by polarisation and k vectors, and the yzplane which lies perpendicular to the polarisation vector. The wavelength chosen for the 30 nm particle is the dipole plasmon peak, so since the dipole field dominates, we see the characteristic p-orbital shape around the sphere in Fig. 5.7a,c. Note that a small quadrupole component to the field makes the p-orbital lobes slightly asymmetrical. At long range, the radiative terms in Eq. (5.26) become more important, and then, the field has a characteristic spherical wave appearance. The wavelength for the 60 nm particle has been chosen to be that for the peak in the quadrupole resonance, and as a result, the field contours close to the particle in Fig. 5.7b look like a dxz-orbital (slightly distorted by a small dipole component that is also present). In addition, Fig. 5.7d, which is a nodal plane for the dxz-orbital, only shows the weak dipolar component. Note that the peak magnitude of the field for the 30 nm particle occurs at the particle surface, along the polarisation direction. This

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Fig. 5.7 E-field contours for radius 30 and 60 nm Ag spheres in a vacuum. Two cross sections are depicted for each sphere. a, b The plane containing the propagation and polarisation axes and c, d the plane perpendicular to the propagation axis. The 30 nm sphere refers to 369 nm light, the main extinction peak for this size, whereas the larger sphere is for 358 nm light, the quadrupole peak for this size. Labelled points 1 and 2 illustrate locations for Fig. 5.8 [96]

peak is over 50 times the size of the applied field, whilst that for the 60 nm particle is over 25 times larger. This is responsible for the electromagnetic enhancements that are seen in SERS, and they also lead to greatly enhanced HRS. Figure 5.8 illustrates the surface-averaged E-field augmentation for the 30 and 60 nm diameter spheres plotted against wavelength, together with extinction efficiency. The illustration also encompasses the E-field augmentation relating to two loci on the sphere, i.e. point 1, which lies in alignment with the polarisation vector, and point 2, which exhibits a 45° rotation away from the trajectory of polarisation. The E-field enhancement related to the particular surface loci would be apt for comprehending a lone molecule SERS study if this were achievable for a spheroidal particle. For the two-sphere dimensions, the field augmentation owing to the dipole resonance reaches a maximum towards the extinction peak’s red aspect. Nevertheless, the finite wavelength amendments to the quasistatic data give rise to plasmon excitation depolarisation towards the extinction zenith’s blue spectrum, leading to a more diminutive mean field and an apex that exhibits a red shift. The E-field augmentation in the sphere with the lower diameter (Fig. 5.8, top panel) is associated with point 1, which is of a threefold higher magnitude than the mean surface value, although the line morphology is similar. A lesser degree of enhancement is seen for point 2, for which the zenith inclines towards the blue spectrum, reflecting the impact of modest quadrupole resonance. In the sphere with the greater diameter (Fig. 5.8, lower panel), the peak for point 1 is 3.5-fold higher than the dipole zenith’s surface average. Point 2 exhibits a peak at the quadrupole resonance wavelength, giving an augmentation that is thrice that of the mean surface

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Fig. 5.8 Comparison of extinction efficiency, surface-averaged E-field enhancement, and E-field enhancement for specific points for radius 30 nm (top) and 60 nm (bottom) Ag spheres in a vacuum. The two points chosen are point 1, along the polarisation direction, and point 2, at a 45° angle relative to the polarisation direction and in the xz-plane [96]

data. In the latter sphere, it is feasible for the greatest SERS enhancement to be at a surface site that contravenes the polarisation vector.

5.3 Semiconductor Over years ago, the capability to control the semiconductors surfaces with an accuracy almost down to the level of the atom has given rise to additional innovation in relation to semiconductor configurations, i.e. quantum wells, wires and dots. If the in-depth atomic architecture were fleetingly overlooked, there would be the potential to envisage straightforward geometric items of varying dimensionality, i.e. zero, 1 and 2, which were constructed from uniform semiconductor substances and which would have a flawless surface termination. These configurations would display the imagined changes in electronic state densities as anticipated by the uncomplicated ‘particle in a box’ paradigms described in basic quantum mechanics; continuous degrees of

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Fig. 5.9 Idealised density of states for one band of a semiconductor structure of 3, 2, 1 and ‘0’ dimensions. In the 3D case the energy levels are continuous, whilst in the ‘0D’ or molecular limit the levels are discrete [97]

the three-dimensional example would develop into the individual conditions of the zero-dimensional instance (Fig. 5.9). ‘ From the materials science and solid-state physics perspective, it is possibly unexpected that inorganic solid nanocrystals, overlaid with organic ligands, may give rise to one of the most varied and potent archetypes of a quantum dot. This occurs as in this context, the nanocrystal molecule can be viewed as a constituent entrenched within a solid-phase device surface and as a chemical reagent. In this format, the nanocrystal can undergo a spectrum of interventions, e.g. dissolution in liquid, spinning into a polymer, incorporation into an electrical circuit, attachment to additional nanocrystals in the form of dimers or trimers amongst others, or ultimately, affixing to biological compounds.

5.3.1 Band Gap Modification Independent large number of semiconductor nanocrystals and surface atoms with the same interior bonding geometry as a known bulk phase exhibit strong variations in their electrical and optical properties with size. Such alterations are generated via logical conversions of the electron energy level densities based on internal properties, i.e. quantum size effects. Nanocrystals sit amid the atomic and molecular limits of the electronic conditions’ specific density and the expanded crystalline boundary of continuous bands (Fig. 5.10).

5.3.2 Quantum Size Effects The semiconductor nanocrystals properties are notable alterations in optical characteristics as a function of their dimension. As the latter is diminished, there is an excitation change towards higher energy, and the oscillator strength becomes focused on only a small number of transitions. These empirical physical behaviours occur

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Fig. 5.10 Density of states in metal (a) and semiconductor (b) nanocrystals. In each case, the density of states is discrete at the band edges [97]

via modifications in the electronic conditions’ densities and can be appreciated by studying the association between position and momentum in particles that are both liberated and confined: ΔpΔx ≥ ℏ/2

(5.27)

With respect to a particle that is free or within a periodic potential, it is possible to give an exact description of the energy and the momentum of the crystal, hk; however, the position cannot be clearly defined. Since the particle is regional, it is possible to describe the energy, but the locational ambiguity falls leading to a lack of clarity in the description of momentum. The particle’s energy eigenfunctions can then be considered as superpositions of bulk k conditions. In the expanded instance, the energy change, as a function of dimension, can be gauged in a straightforward manner by appreciating that the confined particle energy occurs by bulk k condition superpositions of varying energy states. The energy dependence of the wave vector in a liberated particle is quadratic: E = ℏ k 2 /2m

(5.28)

In the approximation of effective mass, there is an assumption that this association is true for an electron or hole in the semiconductor’s periodic potential, with a

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97

diminished mass which has an inverse relationship with the band width. In view of the correlation between space confinement and momentum superposition, there is an immediately estimated energy reliance on dimension, given by 1/r 2 , as would be anticipated for a straightforward particle in a box. For dimensions of greater magnitude, this estimation is almost true but falls down for particles of medium size as there is no quadratic dependence of energy on k in genuine crystallites. In order to acquire a physical comprehension of the way in which E changes with K, at this juncture, it is helpful to change to a molecular image of bonding inside the solid (Fig. 5.11), together with quantum confinement mechanisms. The single-particle wave functions for electrons and holes in the extended solid can be viewed as linear combination of unit cell atomic orbitals, multiplied by phase factors between the unit cells. When all the cells are in phase, the wavevector, k = (2π/λ), is equal to 0; when adjacent cells are out of phase, k takes on its maximum value, π/a. In a simple one-dimensional single band tight binding model, the dependence of E on k is

Fig. 5.11 a A simplified MO diagram for the electronic structures of zinc blende CdSe and diamond structure Si. b A comparison of the HOMO–LUMO transitions for CdSe and Si. In CdSe the transition is dipole allowed, whilst in Si it is not [97]

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E = α + 2β cos(ka)

(5.29)

where α, the energy of the linear combination of atomic orbitals inside the unit cell, determines the centre position of the band in energy. 2β gives the width of the band and is directly related to the strength of nearest-neighbour coupling and inversely proportional to the effective mass. An expansion of the cos(ka) term for small k yields a quadratic term as its first term, so that one can see why the effective mass approximation only describes the band well near either its minimum or its maximum. Considering now a real binary semiconductor, such as CdSe, the single-particle states can be viewed as products of unit cell atomic orbital combinations and phase factors between unit cells (Fig. 5.11). For example, the highest occupied molecular orbital or top of the valence band may be viewed as arising from Se 4p orbitals, arranged to be in phase between unit cells. This will be the maximum of the valence band, since adjacent p orbitals in phase are σ-anti-bonding (and π-bonding). Similarly, the lowest unoccupied molecular orbital will be comprised of Cd 5s atomic orbitals, also in phase between unit cells. This is the minimum of the conduction band, since s orbitals in phase constructively interfere to yield a bonding level. In this case, the minimum of the conduction band and the maximum of the valence band have the same phase factor between unit cells.

5.3.3 Quantization and Energy Level Spacing As the size is reduced, the electronic states may be viewed as superpositions of bulk states. Hence, there is a shift to higher energy, the development of discrete features in the spectra, and concentration of the oscillator strength in just a few transitions.

Fig. 5.12 Optical absorption versus size for CdSe nanocrystals shows the shift to higher energy in smaller sizes, as well as the development of discrete structure in the spectra and the concentration of oscillator strength in just a few transitions [97]

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99

Qualitatively, all of these effects can be readily observed in the spectra of Fig. 5.12, which show data for CdSe. The quantitative analysis of these spectra remains a difficult subject, for several reasons: • The foregoing picture is a single-particle one and does not include the substantial effects of correlation. In molecules this is analogous to trying to use the highly approximate molecular orbital theory, instead of more advanced quantum chemistry methods. Regrettably, the nanocrystals are too large to describe using even moderately advanced methods that are routinely applied to small molecules. • Further, in CdSe at least, the large atomic number of the Se ensures that the coupling between spin and orbital momenta is very strong in the valence bands (p bands). This coupling is in the j-j, and not the Russell–Saunders, L-S, coupling regime. When translational symmetry is removed, the mixing of k vectors can also result in different bands mixing together. • The shape of the crystallites, which is regular (tetrahedral, hexagonal prisms), or spherical, or ellipsoidal, will determine the symmetry of the nanocrystals and will influence the relative spacing of the levels. • Finally, surface energy levels are completely excluded from this simple quantum confinement picture. Yet it seems apparent that surface states near the gap can mix with interior levels to a substantial degree, and these effects may also influence the spacing of the energy levels. Theoretical approaches that directly include the influence of the surface, as well as electronic correlation, are being developed rapidly.

5.3.4 Electrical Properties Single-electronics are discussed where there is the potential to regulate the position and movement of a single or several electrons. In order to appreciate the way in which a lone electron could be controlled, a comprehension of electric charge motion within a conductor is required. Flow of an electric current along a conductor occurs as a number of free electrons have the ability to migrate through the atomic nuclei lattice. The charge transference across the conductor dictates the current. Unexpectedly, the former could have nearly any value and specifically, it could equal only a proportion of a lone electron’s charge. Thus, there is no quantisation of this parameter. Initially, this concept appears paradoxical, but it represents the outcome of the electron cloud shift with respect to the atom lattice. This displacement can be altered in an ongoing manner, such that the transferred charge becomes a continuous variable (Fig. 5.13, left panel). If a regular conductor were disrupted by a tunnel function, electric charge would propagate through the system using discrete and continuous processes. Individual electrons alone have the ability to tunnel across junctions, and so charge will accrue at the electrode surface abutting the isolating stratum until a sufficient quantity of bias has amassed across the tunnel junction (Fig. 5.13, right). This leads to the transfer of

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Fig. 5.13 The left side shows, that the electron cloud shift against the lattice of atoms is not quantized. The right side shows an accumulation of electrons at a tunnel junction

a single electron. This process has been likened to a dripping tap by Likharev. Thus, if a lone tunnel junction were biased with a constant current, I, Coulomb oscillations would be generated which have a frequency, f , defined as I/e, where e reflects an electron’s charge (Fig. 5.14). Accretion of charge on the tunnel junction is ongoing until the energetic conditions favour tunnelling of an electron, leading to tunnel junction discharge via a fundamental charge, e. Superconductors exhibit comparable properties; Cooper pairs form the charge carriers, and the typical frequency is defined as ω = I/2e, which is associated with fluctuations referred to as Bloch oscillations. The current-biased tunnel junction is an extremely straightforward circuit to show regulated electron transfer. The electron-box forms a further circuit, which is illustrated in Fig. 5.15. The tunnel junction links with the particle on just one aspect, where electrons have the capacity to tunnel back and forth. An example would be an oxide-coated metal particle (Fig. 5.16). The superficial oxide coating is sufficiently slender to enable the tunnelling of electrons. The necessary energy to move a single electron onto the particle can be defined as the Coulomb energy, which can be expressed as E C = e2 /2C, where C reflects the capacitance of the particle. If thermal and additional energy modes were ignored, the bias voltage, V b , would become the sole accessible source of energy. If

Fig. 5.14 Current biased tunnel junction showing Coulomb oscillations

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Fig. 5.15 The electron-box can be filled with a precise number of electrons

Fig. 5.16 Metal particle embedded in oxide. Tunnelling is only possible through the thin top layer of oxide

this were sufficiently diminutive, i.e. less than the threshold, V th = e/C, this would prevent the tunnelling of any electrons as an inadequate quantity of energy would be present to charge the island, a property referred to as the Coulomb blockade. Increasing V b can facilitate the serial transfer of individual electrons which generates a step-wise property. It is understanded that such single-electron behaviours, e.g. Coulomb oscillations and the Coulomb blockade, are only relevant when the thermal energy is lower than the Coulomb energy. Where this does not apply, thermal flux will interrupt the electron movement and ameliorate the impact of quantisation. The required condition can be expressed: Ec =

e2 > kB T 2C

(5.30)

where k B and T indicate Boltzmann’s constant and absolute temperature, respectively. Thus, at liquid nitrogen or ambient temperature, it is necessary for the capacitance, C, to be lower than 12 nF or 3 nF, respectively, in order to achieve charging effects. A second criterion required to appreciate the charging effects is that there

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should be minimal quantum fluctuations in the island population of electrons; the latter needs to be discretely regionalised. Failure of island electron localisation would prevent the charge effects as the islets would not comprise individual particles, but instead, a single large homogeneous domain. It would be unattainable to establish island charging with an elementary charge defined by an integer number since this would suggest that islets had single electrons in common. The Coulomb blockade would disappear as the lower limit of charge available to an islet would no longer be in existence. Thus, all tunnel junctions must be sufficiently obscured to restrict the electrons to the islets. A tunnel junction’s transparency can be expressed in terms of the resistance of the tunnel, RT , which has to meet the criteria for discrete charging effect discernment, where h indicates Planck’s constant. This should be appreciated as a measure represented by an order of magnitude, rather than a specific threshold. RT >

h = 25813 Ω e2

(5.31)

Therefore, these effects are experimentally verifiable only for very small highresistance tunnel junctions, meaning small particles with small capacitances and/or very low temperatures. Advanced fabrication techniques, such as the production of granular films with particle sizes down to 1 nm, and deeper physical understanding allow today the study of many charging effects at room temperature. Based on the Coulomb blockade many interesting devices are possible, such as precise current standard [98], very sensitive electrometers [97], logic gates [99] and memories [100] with ultra-low power consumption, down-scalability to atomic dimensions, and high speed of operation. Altogether, single-electronics will bring new and novel devices and is a very promising candidate to partly replace MOS technology in the near future.

5.3.5 Optical Properties As a consequence of their being a new type of material, the properties of semiconductor nanocrystals can be expected to evolve with improvements in sample preparation. Spectra which at first seemed featureless and diffuse have gradually acquired definition, with multiple discrete states apparent in the latest generation of samples. Essentially all of this progress derives from narrowing the distribution of sizes in the sample. Since the energies of the transitions depend so strongly on the size, size variation is a special form of inhomogeneous broadening at work here, which over time has been largely reduced. Today it is really the intrinsic or single particle, line widths that are a matter of greater concern. Narrow band (15–20 nm), size-tunable luminescence, with efficiencies at least of order 10%, is observed at room temperature from semiconductor nanocrystals. The origin of this luminescence remains the topic of some controversy. For some

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time researchers thought that this luminescence arose from partially surface trapped carriers. Other experiments strongly suggest that the luminescence in fact arises from low-lying ‘dark’ states of the nanocrystal interior, and surface modifications only influence the quantum yield by modulating the non-radiative rates. Just as in organic molecules a singlet state may be optically prepared, followed by rapid relaxation to triplet states with long decay times, so in semiconductor nanocrystals, where j-j coupling dominates, an angular momentum allowed state is initially prepared, and on the time scale of picoseconds or longer, there is a decay to a lower lying angular-momentum-forbidden state, which decays relatively slowly (nanoseconds to microseconds). One success of this dark state model is the accurate prediction that the magnitude of the exchange splitting increases as r −3 , thus explaining one long puzzling feature of the luminescence. As the size is reduced, the shift between the absorbing and emitting state is observed to increase. The rigorous separation of interior and surface states is somewhat artificial in nanocrystals in any case, since substantial mixing may be expected. Indeed, other features of the spectra suggest that there may well be substantial surface character associated with the emitting state. For example, electric field modulation of the luminescence yields signals a factor of 100 or larger than modulation of absorption, indicating that the emitting state is not as well confined spatially. Further evidence for surface localization of the emitting state comes from low-temperature studies of the vibronic coupling of the emission, which show that there is a well-defined localization temperature. Independent of the exact origin of the luminescence, it does appear to be one property which can be manipulated in useful ways. For example, two reports of light-emitting diodes made with polymers and CdSe nanocrystals have appeared within the past year. In the first instance, nanocrystals are assembled in layers a few nanocrystals thick on the surface of PPV, an electroluminescent polymer. The PPV itself is grown on a layer of indium tin oxide, a transparent hole-injecting contact. Finally, the nanocrystal layer is coated with a film of Mg/Ag, the electron-injecting contact. This complete assembly electroluminescence when a voltage is applied. The recombination of electrons and holes may take place either in the polymer layer (which emits green light) or in the nanocrystal layer. The nanocrystal emission shifts with size. Thus, these LEDs provide a variety of means for tuning the output colour. This advance is particularly important, since it constitutes the first example of electrical, rather than purely optical, investigation of semiconductor nanocrystals. Problems 1. 2.

What capacitance is needed to permit the exchange of exactly one electron at 273 K? The absorption coefficients of human (arota) tissue and of water as a function of excitation wavelength are known. The minimum of the tissue absorption can be found at approximately 800 nm. Some studies have suggested that multiplephoton absorption of Au nanospheres at this wavelength might be advantageous for plasmonic heating. What shape of Au nanostructures do you suggest instead of spheres? What would be the most notable advantage of the shape you suggest?

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Why are quantum dots ideal for simultaneous in vitro and in vivo fluorescence detection? What are the most important differences between AU NPs and CdSe quantum dots? Why are the luminescence lifetimes of Au NPs/rods in the fs range?

Chapter 6

Nanodevices and Nanostructures

6.1 Introduction Devices and machines at nanometre scale, once a scientific fantasy, are now becoming a reality. Fundamental types of nanometre-scale devices (nanodevices) are rapidly being developed, the principles of their operation are being established, the performance of the prototypes of some nanoscale machines (nanomachines) is being demonstrated, and more sophisticated nanoscale machines with more complex structures and/or networks are now envisioned [101]. The working principles of nanodevices and nanomachines are, in most cases, fundamentally different from their macroscale counterparts. Nanodevices and nanomachines are correlated with a significant degree of hierarchy. And some nanodevices (especially biological ones) actually show the capability of work to be done. The uniqueness of nanodevices and nanomachines can be better addressed this way, and their relationship with nanofabricated and nano-integrated systems, which in some cases is not easy to distinguish, can be clarified, too. Most importantly, this definition can provide a clear scheme for understanding the role of self-assembly in their fabrication and operation. A general view of nanodevices and nanomachines can be provided with their typical operating ranges. They operate at up to yocto (10–24 ) Joule of energy with usually nano (10–9 ) to pico (10–12 ), but sometimes up to atto (10–18 ), Newton of force. Detection up to a single small molecule is not uncommon. Movement of their parts is usually in nm ranges with frequency of a few nm to nanoseconds, which makes their moving speed ~nm to ~µm per second. Voltage is also applied in the range of nV. For example, the biological motor kinesin generates ~5 × 10–18 W per molecule. Also, a variety of nanoscale spaces that can be used for nanoscale reactions or as nanochambers can have capacities down to femtolitre (10–15 ) and sometimes to yoctolitre.

© The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2022 Y. Al-Douri, Nanomaterials, https://doi.org/10.1007/978-981-19-3881-8_6

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Fig. 6.1 Conceptual schematic representation of nanodevices

6.2 Nanodevices General Scheme When fabricated systems of nanocomponents show the capability of unit operations, they can be defined as nanodevices. Their operation principles are mainly established at the nanometre scale and their operating components are in the range of nanometres. But sometimes their operation is at the nanoscale even though the whole device is above the nanometre scale. Figure 6.1 shows the schematic representation of nanodevices. This is a conceptual one to provide a general view; their actual structural features can vary significantly. The functional component is to generate the push and pull element motions whose balance and correlation induce the desired unit operations. This is mainly done by the input of external energy, but it is not uncommon for the energy source to be incorporated as a part of the nanodevices. The results of the unit operations are reflected in the work done or signal output. Usually, it is required for the nanodevices to have the component that can generate the guide element motion, whose role is to make the unit operations controllable, such as directional rotation or directional movement. The auxiliary component is for this purpose, and the incorporated power supply can be a part of this component, too. Networking of the individual nanodevices is critical to create nanomachines that can eventually communicate with the macroworld. The communication component is used to make effective connections with adjacent nanodevices. In many cases, nanodevices have to be anchored on the surfaces. When this is necessary, the nanodevices can be immobilised on the surfaces (platform) through the proper connector, such as a shaft component for molecular motor.

6.3 Nanocomponents It would be ideal if all available atoms, molecules and nanostructured objects could be used as nanocomponents for the construction of nanodevices. However, the major

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working principles and the size range of their operations are mainly at the nanometre scale; this leads to the critical requirement that the nanocomponents should ensure the proper interaction of intermolecular and colloidal forces amongst them while maintaining the integrity of their structural features. The interaction of intermolecular and colloidal forces should be controllable but strong enough to hold entire structures during their operation. These are requirements of rigidity and flexibility. As surprisingly, many self-assembled aggregates are strong enough that they endure unit operations and even perform mechanical work under the right conditions. For example, self-assembled lipid bilayers have mechanical strength parable to stainless steel of the same thickness, yet they are extremely flexible [102]. External force-induced self-assembly systems are especially important for proper operation of functional components (Fig. 6.1) under the control of external energy. For electrochemical energy, ferrocene-based and sexithiophene-based derivatives are typical examples. Limited representative groups of examples for nanocomponents. These are DNA, and carbon nanotubes and fullerenes.

6.3.1 DNA DNA has perfect site-specific, programmable self-assembly capability through basepairing. It also shows incredible molecular recognition and can self-assemble with chiral and helical structures. This is important to generate the force for element motions by releasing the energy stored within these conformational features. They are rigid but conductive, come with a variety of structural diversities, and can be conformationally responsive to a variety of external forces. They are easy to synthesise, easy to functionalise and easy to manipulate. They have enough stiffness but also flexibility to be a nanodevice. Forces from DNA-based nanodevices are mainly generated through push or pull element motions that are strongly involved with hydrogen bonding during hybridisation and dehybridisation. This may be the reason that DNA-based nanodevices are strongly anticipated to generate a force comparable to the protein-based molecular motors with high controllability of directionality. A variety of DNA-based nanodevices have been demonstrated [103].

6.3.2 Carbon Nanotubes and Fullerenes Carbon nanotubes have remarkable mechanical toughness and electrical conductivity along with many other useful properties up to a diameter as small as 1 nm. So do fullerenes. They also come with structural diversity and easiness of diverse surface functionalisation. Diverse ranges of nanodevices have been demonstrated with these. A nanoscale rotational actuator whose metal plate rotor is operated by an external voltage is a good example. This takes advantage of the mechanical toughness and

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electrical conductivity of the multiwalled carbon nanotube, which works here as the support shaft and as the source of rotational freedom [104].

6.4 Nanoelectronics The concept of nanoelectronics (or molecular electronics) is being developed to fabricate a variety of nanoscale components into working electronics, so we can ultimately overcome the inevitable physical limitation that we are going to face with the current silicon-based electronics. Excellent achievements have been made over the past decades, which show a powerful possibility for nanoelectronics. The fabrication and performance of singlemolecular transistors have been demonstrated [105], and it is shown that the carbon nanotube-based field effect transistor (FET) outperforms silicon-based FETs [106]. Also, logic-gate performance has been demonstrated with nanowire- and carbon nanotube-based systems [107]. Unit fabrications of jointing, crossing, deposition, and hybridisation have been used for the nanofabrication of these systems. The biggest challenge at this stage is the massive parallel fabrication of those promising individual nanocomponents into actual working nanomachines, which means nanoelectronic devices that can communicate with the macroworld. Continuous development of new and effective unit fabrications and unit operations that most likely should be based on self-assembly will be critical [108].

6.5 Nanostructured Materials The nanostructures have unique properties that are useful for multiapplication. Nevertheless, numerous intricacies relating to the mechanisms and configurations that generate these characteristics require further elucidation. The advent of improved computer power, together with more dependable and enhanced analytical potentials and instruments, has facilitated the development of computer paradigms that can discern essential minutiae that are frequently unavailable from alternative methods. These in-depth observations are therefore giving rise to de novo and vital perceptions regarding the associations between the configurations and features of nanosystems.

6.5.1 Nanoparticle Properties Understanding the unique properties of nanoclusters will have a long-standing goal for the scientific community. Experimentally, it is well-recognised that a magic number of configurations exists that demonstrate an atypical stability within a cluster

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class of varying dimensions [109]. This observation has been ascribed to the occupancy of mesoelectronic shells and the establishment of configurations containing a high degree of symmetry. An associated computer modelling sphere has offered formative insights into the comprehension of nanocluster phase stability compared to the bulk substance. Additional instances encompass simulations of vibrational conditions and their reliance on dimensions, mode widening and thermal transport, and the dynamic properties of phase alterations generated by compression and surface modifications. Despite the fact that nanosolids are not addressed immediately by such research, it does offer an outlook and context with respect to the way in which configuration and properties engendered at the nanometre scale may add impetus to processes, e.g. the sintering of nanoparticles and creation of amorphous grain borders. The intricacy of these domains can be appreciated through these studies; simulation minutiae, e.g. potential energy function, may impact the outcome when a simulation is run. The relative stability of different phases can change in dramatic and often nonintuitive ways as particles are reduced to nanometre-scale dimensions. Landman et al. [110] have utilised molecular dynamics simulations to characterise the dynamics and structure of gold clusters containing 75, 146 or 459 atoms as a function of temperature. The embedded-atom technique leads to the attainment of interatomic forces and energies. Thus, simulations fail to encompass any influences from electronic conditions. In comparison to transitions in the solid phase where the configuration of the whole cluster is transformed, following premelting, the liquid stayed at the surface whilst the internal portion of the clusters retained their crystalline properties. Complete cluster melting occurred after additional heat was applied. Analogous data have been published for copper nanoparticles [111]. It is mentioned to remain some uncertainty in relation to gold cluster temperature reliance, with contemporary simulations appearing to verify the overall results published by Landman et al. [110], whereas additional simulations have presented contradictory data. The equilibrium morphology of gold clusters of dimensions between 3 and 100 nm was computed by Barnard et al. [112], employing firstprinciples data and a thermodynamic paradigm. Their outcome indicated that an abridged octahedron is a configuration that is energetically preferable for clusters within this dimensional spectrum; these data are contrary to two simulations conducted utilising analytical potentials. Koga et al. [113] reported the highresolution electron microscopy findings for gold clusters sized between 3 and 14 nm and confirmed that the configuration altered from an icosahedral to a decahedral shape at a temperature immediately under the melting point. Meyer et al. have analysed the vibrational states of silver, nickel and copper clusters, contrasting these conditions against those of their bulk equivalents and with nanosolids containing a range of porosity levels [114]. A consequence of the dimension of these systems is that instead of the vibrational frequencies being acquired through the diagonalisation of a force matrix, they are computed via a Fourier transformation of the velocity autocorrelation function for atoms recognised as pertaining to either the cluster centre or surface. In a 791-atom copper cluster comprised of an underpinning face-centred cubic lattice, investigation of the conditions of vibration

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exposed marked disparities for atoms within the two locations. For those in the centre, two distinct but wide zeniths were computed, i.e. one at a low frequency correlating with the bulk crystal’s transverse modes, and one at a higher frequency, which reflected the bulk vibrational modes in the longitudinal plane. It is wised to probe different sets of properties induced by nanoscale. Simulation, as a function of cluster dimension and morphology, of the conversion under the pressure of gallium arsenide nanocrystals from a zinc blend to a rock salt configuration was performed by Kodiyalam et al. [115]. The pressure required was determined to be highly dependent on the cluster dimension; more diminutive clusters necessitated a reduced conversion pressure. Cluster dimension also dictated the ultimate cluster architecture. For those under 4.4 nm in size, the rock salt lattice was laid down in a single orientation. Increasing cluster dimensions gave rise to grain boundaries between areas comprising varied orientations of the crystal. It appeared that the conversion pressures and related strains lacked homogeneity within clusters of higher dimensions in the course of configurational change. Campbell et al. [116, 117] have tested the detailed dynamics of the oxidised aluminium nanoparticles. These researchers exploited a variable-charge interatomic interaction paradigm that encompassed the influence of both ionic and covalent types of bonds in order to model the quick oxidation process of aluminium clusters with a diameter of 20 nm. Given that there is no process for dissipating kinetic energy, oxidation reactions have an incendiary potential owing to the huge energy liberation caused by the bonding process between the aluminium and oxide atoms, together with their swift diffusion as a result of regional oxide stresses (Fig. 6.2, upper panel). Conversely, the modelling of a canonical ensemble mediated by a thermostat engendered a simulation revealing that the depth of the oxide commences development in a temporally linear fashion; it concludes with a 40-A saturation depth generated at 466 ps (Fig. 6.2, lower panel). In-depth analysis of the oxygen atom vectors in the course of the reaction led to the postulation of a three-stage oxidative percolation mechanism that describes the retardation of oxide evolution in the canonical ensemble. The carbon nanostructures could provide an excellent system since the system dimension is robustly coupled to phase integrity [118]. Proximate energy levels exist in bonds linked with the atomic hybridisations, sp2 and sp3 . Macroscopically, the practically equivalent bonding energies lead to the approximation of the respective stabilities of the diamond and graphite phases when translation symmetry is present in the system. At the nanometre level, numerous structures exist with rivalling energy levels, such as fullerenes, nanotubes, nanodiamond clusters and bucky onions, amongst others. Since the binding energies of the individual configurations are in proximity, and as the surface:bulk atomic ratio is extremely high, fairly modest influences, e.g. disparate surface stress, can cause unexpected phase stability modifications in configurational groups with respect to bonding and integrity; phase schematics for nanoscale systems have been put forward [118].

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Fig. 6.2 Snapshots (at 50 ps) of variable-charge molecular dynamics simulations of the oxidation ´ where small and large spheres are aluminium panels. of an aluminium nanoparticle of radius 100 Å, The bottom panel is at 400 K [117]

6.5.2 Nanoalloys The results of simulation on nanostructures have implied opulent key behavioural properties in nanostructured alloys. There are two contemporary pressing matters. Firstly, atomic contaminants affect the processes and characteristics of dislocation emanation from nanostructured metal grain boundaries. This is the subject of a subsequent more in-depth discourse. Early data have indicated that lead contamination within an aluminium nanostructure comprising grain dimensions greater than the conversion to inverse-Hall–Petch activity may augment the stress application required for partial dislocation emission into the nanograins, thus leading to substance fortification with respect to the uncontaminated nanometal.

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Another issue is the impurity’s influence over grain development at the grain boundaries. It is especially pertinent to establish if such contamination could inhibit grain evolution and thus promote nanostructure stability. Nevertheless, if the energy of the relevant grain boundary could be diminished to a maximum of zero, grain development would be retarded and thermodynamic integrity of the nanostructure would be achieved [119]. Theoretically, this situation could be attained if the contaminating atoms contained a significant positive heat of segregation, with the caveat that the generation of a second stage incorporating the contaminants could be circumvented. A study by Millett et al. [119] utilised simulations of molecular behaviour in order to analyse the integrity of a conceived nanocrystalline metal designed to imitate copper versus grain growth, incorporating contaminants, in terms of the impurity titre and the extent of the metal and contaminant atom radius disparity. The segregation energy alters as a function of the impurity atom radius in these simulations. At the outset, in a material undergoing annealing at 800 K without contaminants and a maximum mean grain dimension of 10 nm, grain evolution was recognised via rotation coalescence, as aforementioned, and grain boundary diffusion. Once the grain dimensions exceeded 10 nm, the rotation dissipated and grain growth occurred owing to grain boundary movement. In order to ascertain the impact of contaminants, equivalent simulations were performed in which contaminant titres within the range 1–2 at% were positioned at regular intervals amongst the grains. At the lowest impurity level, the grain dimensions at simulation conclusion were equivalent to those for the pure system. At the highest impurity concentrations, the grain dimensions remained unaltered and it could be surmised that the grain development was suppressed by their presence. Varying levels of coarsening for the identical simulation period were noted for middle contaminant titres. The preliminary simulations are achieved for aluminium-containing lead contaminants in order to acquire a greater comprehension of their possible impact on the mechanical and grain development characteristics within this system. The machinability of bulk steels and brass is frequently improved by the creation of alloys containing lead. However, in bulk aluminium, lead is mostly immiscible; it is wellrecognised that lead contaminants separate out and generate clusters. Current experimental work has suggested that the separation of lead contaminants leads to the formation of grain boundaries within nanocrystalline aluminium; only diminutive quantities of lead, i.e. under 1 at%, have the capacity to exert a significant effect on the traits of nanostructured aluminium [113]. It is not yet clear whether lead can maintain the integrity of this system with respect to grain evolution; the exact mechanisms through which lead impurities influence this system’s mechanical characteristics also merit future investigation. The simulations have utilised an adapted embedded-atom technique potential in order to discern the engagements that occur between atoms in alloys comprised of lead and aluminium [120]. Since lead and aluminium are immiscible, there is a dearth of experimental data relating to the configurations and characteristics of the arising alloy. Thus, the fitting of the potential function onto lattice constituents, cohesive energies and elastic constants can be carried out for a number of theoretical alloys; first-principles techniques can be employed for computation. Monte Carlo

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Fig. 6.3 Illustrations from a Monte Carlo simulation of lead impurities in aluminium. The lead atoms are shaded lighter. Top left: 1 at% lead in bulk aluminium. A tendency for clustering of lead impurities is apparent in the figure. Top right: bulk system containing a tilt grain boundary. Bottom left: the lead impurities in a fully dense three-dimensional aluminium nanocrystal. Bottom right: the lead impurities in an aluminium nanocrystal with a columnar nanostructure. In all cases with grain boundaries, the lead impurities segregate to and wet the interfaces [117]

simulations can facilitate the exploration of lead’s segregation behaviours within aluminium; these comprise motion, including modest single atom translocations and the interchange of lead and aluminium atom individualities. The paradigm is able to forecast the evolution of lead contaminant clusters within a bulk specimen which agrees with experimental observations (Fig. 6.3). Nevertheless, where a grain boundary exists, clustering is absent and the lead disseminates in relation to the boundary (Fig. 6.3). Evaluation of parameters which reflect grain boundary stress infers that the stress is the impetus responsible for the lead distribution. Since atoms of lead are of greater size than aluminium atoms, they are inclined to separate towards loci subject to tensile hydrostatic stress. Equivalent data have been obtained with respect to columnar and completely dense three-dimensional nanostructures (Fig. 6.3, lower panel), which demonstrate the dissemination of lead contaminants in proximity to the grain boundaries. This reduces the energy associated with the latter, implying that grain evolution may be suppressed by lead in this setting.

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6.6 Future Modelling Prospects It is obvious that atomic simulations provide exciting and new insights into the distinct characteristics of nanosystems, both overall and specifically, in relation to nanoconfigured solids. Nevertheless, a number of domains remain where enhanced modelling techniques are required in order to attain a more detailed comprehension and to improve the data standards generated by such simulations. An initial clear problem is the timeframe of the simulations of molecular behaviours. Modelling research has transparently demonstrated that plastic deformation as a result of dislocation emissions may necessitate the occurrence of nucleation; furthermore, diffusion may be a requisite for mechanisms underlying grain boundary accommodation. Thus, strain rate is likely to exert a profound effect on these two processes and on occasion, they may need a prolonged duration that exceeds the pragmatic timeframe of a straightforward convergence study which offers simulated data in terms of strain rate. A second issue that seems more straightforward is utilising more transferable and accurate engagements between atoms as simulation input. The discourse above clearly implies that although qualitative data are mostly autonomous of the intricacies of interatomic interplay, alternative data forms necessitate precise energies for a spectrum of substance characteristics. A patent association lies between the stacking fault energies and the key grain dimension required for partial dislocation emissions; however, less well-defined relationships may be present that are yet to be recognised. Over the last 20 years, the standard of analytical potential energy functions for the definition of energies within a broad spectrum of structures has evolved notably; however, generalisation of these to architectures beyond those incorporated within the fitting data set still causes consternation. The Car-Parrinello technique [121] is a potential clear answer; this employs forces originating from computation according to first principles. From a computational perspective, such methods are costly, and the output is still subject to estimates utilised in the foundation set, the pseudopotential and density functional type; these factors may yield data erraticism. However, in contrast to the temporal issue, which has the simple answer of needing high-speed processors, the pragmatism of computation using forces ascertained from first principles can be enhanced by using a larger number of processors. When the practical capacity to synthesise nanoscale crystallines of sufficient dimensions for bulk utilities evolves, improved multiscale paradigms for these substances will be required. Problems 1.

2.

Why are aerogels more commonly prepared from base catalysis rather than acid catalysis? Describe how the acid catalysis method can be amended so that a highly porous polymeric silica aerogel can be achieved. Monolithic aerogels are often quite fragile. Describe two methods that can be employed to prepare stronger colloidal silica aerogel and indicate how these methods might otherwise impact the native properties of aerogel.

6.6 Future Modelling Prospects

3.

4.

5.

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Recent efforts have focused on developing electronically conducting aerogels. Provide two methods by which this can be achieved. What is the motivation for preparing these kinds of network of applications are envisioned? Magnetic aerogels may prove useful for preparing O2 from air. Considering the magnetic of O2 relative to those of other principal components of air (N2 , Ar), describe how this may be achieved. Why should aerogels be more suitable for this application than bulk (non-nanostructured) materials? Metal chalcogenide aerogels prepared by consideration of metal chalcogenide nanoparticles (quantum dots) exhibit unique optical properties. What are these properties and how does the aerogel dimensionality determine the extent of quantum confinement?

Chapter 7

Carbon Nanotubes

7.1 Introduction The carbon regards a nonmetallic element, it represents an element from group 14 which is assigned to the periodic table’s second period. Represented by the chemical symbol, C, carbon’s atomic number is 6, and the formula representing its electron structure is [He]2s2 2p2 . Naturally, this element is found abundantly, and in all living beings; it is present within the air and in the planet’s outer layer. It has been wellestablished that carbon can be found as both crystalline carbon, i.e. diamond and graphite, and amorphous carbon, e.g. anthracite, bituminous coal, peat and lignite; there are numerous allotropes. This state of knowledge was challenged in 1985, when C60 and C70 were recognised; carbon nanotubes (CNTs) were first described in 1991. The 2010 Nobel Prize was earnt by a Manchester astrophysics professor, Andre Geim who, in 2010, described the presence of a lone atomic stratum of graphite, or graphene. Graphene has been determined to exhibit a range of fascinating characteristics which have utility in numerous sectors, e.g. electronic engineering, nanomechanics and quantum computing. The carbon allotrope family includes diamond, graphite, fullerene (C60 ), CNTs and single-layer graphite or graphene [11]. As mentioned earlier, the nanotubes were first described by Iijima; multiple coaxial carbon fullerene shells or multi-wall nanotubes (MWNTs) were recognised. Subsequently, lone fullerene shells, also referred to as single-wall nanotubes (SWNTs), were manufactured in 1993 with the aid of transition metal catalysts. A fundamental definition of a nanotube is a panel of graphite or graphene that is rolled along its coaxial plane in order to configure a cylindrical surface (Fig. 7.1a). In this way the 2D hexagonal lattice of the grapheme is mapped onto a cylinder of radius R. The mapping can be realised with different helicities resulting in different nanotubes. Each nanotube is characterised by a set of two integers (n, m) indicating the components of the chiral vector C = na1 + ma2 in terms of the 2D hexagonal Bravais lattice vectors of graphene, a1 and a2 , as illustrated in Fig. 7.1b. The chiral vector is a circumferential vector and the tube is obtained by folding the graphene © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2022 Y. Al-Douri, Nanomaterials, https://doi.org/10.1007/978-981-19-3881-8_7

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Fig. 7.1 a Asingle-wall nanotube is graphene wrapped on a cylinder surface. b Nanotubes are described by a set of two integers (n, m) which indicate the graphite lattice vector components. A chiral vector can be defined as C = na1 + ma2 . Tubes are called ‘zigzag’ if either one of the integers is zero (n, 0) or called ‘armchair’ if both integers are equal (n, n) [91]

such that the two ends of care are coincident. The radius of the tube is given in terms √ of (n, m) through the relation R = a0 n 2 + m 2 +nm /2π , where |a1 | = |a2 | = a0 . When C Involves only a1 (corresponding to (n, 0)), the tube is called ‘zigzag’, and if C involves both a1 and a2 with n = m (corresponding to (n, n)), the tube is called ‘armchair’. The chiral (n, n) vector is rotated by 30° relative to that of the zigzag (n, 0) tube. SWNTs are found in the form of nanoropes, each rope consisting of up to a few hundred nanotubes arranges in a hexagonal lattice structures.

7.1.1 Carbon Allotrope’s Structure The fullerene (C60 ), graphite, diamond, and CNTs have differences in their bonding orbitals and structure. The lone atomic stratum of C in graphite is referred to as graphene. In this section, emphasis is placed on the configurational properties of graphite, fullerenes and CNTs (Fig. 7.2). The graphite is consisting of a planar configuration encompassing sp2 carbon bonding; carbon is incorporated amongst the layers by covalent bonds and van Der Walls attraction. This configuration renders graphite an excellent electrical conductor. The carbon present in diamond has a three-dimensional configuration; |sp3 covalent bonding is present. Diamond lacks effective electrical conducting properties. It is known that the diamond is a non-equilibrium structure which gradually becomes converted to graphite, albeit over the extremely prolonged timeframe of 5 million years. This transformation can be hastened by heat or bombardment with high-energy particles. The C60 was co-reported by Kroto from the UK’s University of Sussex, aside, and by Smalley and Carl from America’s Rice University, another side; these researchers were awarded the Nobel Prize for Chemistry in 1996. C60 has an interesting configuration; the bonding conditions of the carbon atoms are represented by sp2 ; there are 12 and 20 hexagons, respectively, and 30 and 90 π and σ bonds, respectively.

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Fig. 7.2 Bonding orbitals of carbon atoms: a graphite sp2 and b diamond sp3

In 1991, Iijima has found that the arc discharge obtained from high-resolution electron microscopy during carbon fibre analysis could be exploited for the synthesis of MWNTs. A MWNT is a type of tubular C molecule; the individual carbon atoms have a hybridised sp2 configuration in relation to the tube. Amongst them, σ C– C attachments generate a hexagonal honeycomb architecture which forms the tube framework. Each C atom has an unbound π electron pair that creates a cloud of conjugated π electrons which traverse the whole CNT. The number of layers dictates whether CNTs are classified as either SWNTs or MWNTs. The radial dimension of nanotubes is extremely narrow and within the nanoscale. The width of a strand of hair is comparable to that of tens of thousands of nanotubes. SWNTs were recognised in 1993 by NEC and IBM [122]. The CNT’s diameter is within the range 0.4–50 nm; the length is generally greater than 1 μm, and the density has a value between 1.3 and 1.4 g/cm3 , characteristics that are similar to wool or cotton. The thermal conductivity of CNTs is analogous to that of diamond; the electrical conductivity is dictated by the CNTs’ diameter and lies between 1023 and 1024 Ωcm and 5.1 × 106 Ωcm, which are equivalent to germanium, a semiconductor, and copper, respectively. The previous works have maintained that diameter limits exist for CNTs. Their diameter typically lies between 1 and 6 nm; more diminutive measurements of 0.4 nm can occur, although this is the lowest attainable diameter, hypothetically. If the parameter were decreased below this, then the angle of the carbon atoms would be insufficiently large to retain structural integrity. SWNTs with a large diameter also lack stability; if above 6 nm, the wall tends to subside. Regarding the CNTs length, contemporary laboratory synthetic techniques can engineer a SWNT length that is markedly less than that possible for MWNTs, i.e. within hundreds of nanometers to several microns, compared to the length of the latter which ranges from tens of microns to up to several millimetres. The spacing layer

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within MWNTs is in the region of 0.34 nm; the diameter falls within the spectrum of 0.4 to several thousand nm; the exact measurement is dictated by the layer number.

7.1.2 Single-Layer Graphite Material (Graphene) The graphene has a C atom monolayer which is tightly packaged within a honeycomb crystal lattice. It can be synthesised by quickly segregating an atomic stratum from graphite. Its properties include stability, flexibility and strength, together with good conductivity. It is well-established that graphite is comprised of C. Its configuration can be represented diagrammatically by vertically ripped graphite atom strata, in a manner analogous to paper. Utilised as a model system in condensed matter physics, graphene’s charged particles have the capacity to move at relativistic rates as they lack mass. This observation means that graphene possesses unusual electronic transport characteristics, e.g. if, in low temperatures, a layer of graphene were interposed between two superconducting electrodes, a superconducting current could be generated, fed by either the electrons or the voids, and dependent on gate voltage and graphene layer charge densities [122]. Interestingly, there is a specific limitation of current that could pass along graphene despite a zero charge density. Such properties offer insight into relativistic constructs, e.g. time-reversal symmetry and graphene’s mode of electron transport. There is no scattering seen within the sub-microdistance in the graphene stratum, which offers the requisites for the production of speed distribution transistors. Currently, laboratory construction of a graphene single-electron transistor is possible.

7.2 CNTs Types and Nature 7.2.1 CNTs Types The CNTs are continuous nanocrystal tubules which are engineered within the curved layer of graphite. The latter encompasses sp2 C bonds within a planar configuration. A honeycomb format and the planar carbon are covalently attached in CNTs; the hexagonal C twists to form the CNTs. The planar carbon atoms hexagonal crimping could commence from a range of angles, giving rise to diverse CNTs. Three categories of SWNT chiralities have been recognised, i.e. the armchair, zigzag and chiral (Figs. 7.3 and 7.4).

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121

Fig. 7.3 Three kinds of CNTs. a Armchair type. b Zigzag type. c Chiral type

Fig. 7.4 Curling of CNTs

7.2.2 CNTs Characteristics 7.2.2.1

Mechanical Properties

The CNTs sides are comprised of six-sided C rings which form a graphite sheet. At the portion resembling a cap, where there is buckling of the tube body and the tube has a cap, both five- and seven-sided C atom rings are present. Since these rings encompass covalent C–C bonds which are naturally extremely robust, it could be

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anticipated that CNTs exhibit excellent mechanical features, which have a strength similar to the C–C bond per se. Research has demonstrated that CNTs have a strength greater than steel by a factor of 100, although their density is only a sixth of that of steel. The CNTs have outstanding features of flexibility displaying a maximum extension rate of 30%. They can be contorted into diminutive angles or ring configurations. When this stress is ceased, the structural distortion recovers entirely back to the initial condition. Thus, under large extrinsic stresses, there is no evidence of brittle fracturing. Fibrous substances, which are made of nanotubes with favourable mechanical characteristics, are likely to have numerous future utilities within the industrial domain, e.g. cables in a space lift. This is hypothetical apparatus that could ferry merchandise and humans from the planet to space stations. CNT cables are the only potential substances that would not disintegrate owing to their mass.

7.2.2.2

Electrical Characteristics

There are different types of CNTs have variations in their conduction characteristics, i.e. in SWNTs exhibiting metallic conductivity, the one-third saw-toothed and two-thirds palm morphologies are associated with electrical traits typical of metals and semiconductors, respectively. The latter will exhibit a smaller band gap as the diameter is augmented. A band gap of zero is seen with a big diameter; metallic conducting traits are noted. Due to the CNTs electronic flow has restrictions as a result of quantum confinement. Typically, CNT electrons display axial motion along the graphite sheet layer vector; radial motion is notably restricted. Such electrical features could facilitate CNT utility in a broad spectrum of nanoelectronic applications, e.g. metallic and semiconductor types of CNTs could form connection lines within nanometreintegrated circuits (ICs), and nanoelectronic switches and additional quantum apparatus, respectively. MWNT field emission effects were demonstrated to comply with Ohm’s law with respect to determining parameters of resistance. The 1996 Nobel laureate, R. E. Smalley has conducted a study in 1995 in which he applied bias voltages within the range −100 to −110 V to patent nanotubes, which gave rise to field emission currents between 0.5 and 1.5 mA. Using CCD photography, the nanotube heads were noted to emanate low-strength incandescent light. Also in 1995, De Heer, Chatelain and Urgate suggested the concept of utlising CNTs to create field emission devices.

7.2.2.3

Thermal Properties

The studies of CNTs have revealed enhanced thermal conduction properties. At the university in Michegan State, David Tomanek reported that CNT thermal conductivity parameters reached 3000 W/m/K, comparable to those of diamond. He forecasted that ultimately, a value of 6600 W/m/K could be attained. This level of

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123

thermal conductivity could be eventually used in circuits as a method of directing heat remotely from dense circuits; this is a contemporary problem in IC engineering. Also, the researchers at University of Pennsylvania have claimed that CNTs comprise, at present, the optimal thermal conductivity substances available globally, as a result of their maximum transmission rate of 10,000 m/s via ultrasonic heat transfer. This research also demonstrated that if CNTs were connected, no heat transfer would occur between individual CNTs, indicating that the heat transfer vector only lies in a single dimension. Additionally, the studies have revealed that CNTs form wires and conductance which are ballistic in nature. In ambient conditions, electrons can travel along wires lacking resistance and backward scattering and generate heat. Investigations at the Georgia Institute of Technology determined that the application of 10 million amps/cm to CNTs led to a uniform resistance in MNWTs which was autonomous of their lengths and breadths.

7.2.2.4

CNTs Superconducting Phenomenon

Mathieu Kociak has asserted that nanotube ropes exhibit the property, superconductivity. This concept was initially reported in a system comprising negligible conduction conduits, as a requisite for supercurrent is to travel through substrate material CNTs within a two-dimensional network configuration without any constituents. These workers anticipated that appropriate ion instigation would cause a rise in transition temperature; presently, the latter is between 300 and 400 mK.

7.2.2.5

Chemical Properties

The CNTs utilising could enhance AFM potency, e.g. John Hafner, at Harvard, sited a CNT measuring between 0.9 and 2.8 nm in diameter at the AFM probe’s terminal. Deploying nanotubes within microprobes improves resolution and additionally, the utility of AFM with respect to the examination of the type of adherence between the specimen and the tip of the scanner. Hafner referred to this type of process as chemical force microscopy. Additionally, Masako Yudasaka has positioned molecules of C60 into CNTs, giving rise to extremely high-pressure parameters. This type of carbon can only give rise to a micronewton force. Nevertheless, if the CNT were used as a divisor, a pressure of 0.1 GPa would be attained, a finding that could stimulate innovative alterations in fullerene utility in the chemical domain. Yudasak additionally postulated that those CNTs which had a cone-like tip, i.e. diameter, length and open angle of 2 nm, 50 nm and 20°, respectively, could become a substitute for activated carbon in filters employed for gas adsorption.

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7.3 CNTs Electronic Structure 7.3.1 π-Electron Orbital and the Energy of the Conjugated Molecule in Planar Structure During 1931, E. Hückel has published the Hückel equation, which was derived from the Schrödinger formula. Its eigenvalue and eigenvector solutions give rise to the planar configuration’s conjugated molecule π electron orbital and energy parameters. It was demonstrated that the nature of benzene’s ground state carbon electron cloud distribution meant that it is enclosed on both aspects. This logic additionally provided a rationale underlying some of the current’s features. The Hückel formula is of utility in computing the electron configuration of carbon materials, e.g. graphite, graphene, C60 and CNTs. An abridged version of this hypothesis is presented below [123]. Within a molecule, a molecular orbital, MO, is occupied by an electron. Following the assumption that the MO wave function is given by Ψ , a linear collection of atomic orbitals can be used to define Ψ : Ψ =

n Σ

Ci ψi

(7.1)

i=1

where ψi reflects the function of the orbital wave. The left portion of the Schro¨dinger formula can be multiplied to acquire the MO’s energy, W: Ψ Hˆ Ψ = Ψ W Ψ

(7.2)

A volume integral can be utilised to give: ∫

Ψ Hˆ Ψ d V W = ∫ 2 Ψ d V

(7.3)

Equation 7.1 can be inserted into Eq. 7.3 in order to generate: ∫ W =

⎛

n Σ

⎞ ⎛n ⎞ Σ ˆ Ci ψi H Ci ψi d V

i=1

∫

⎛

n Σ

i=1 ⎞2

Ci ψi

(7.4) dV

i=1

Following recombination of the sum, the following formula is obtained: ΣΣ

C j ψk Hˆ ψk d V ∫ W =ΣΣ C j Ck ψ j ψk d V j

j

k

k

(7.5)

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125

Employing Eqs. 7.6 and 7.7, Eq. 7.5 can be rewritten as Eq. 7.8. ∫ H jk

=

ψ j Hˆ ψk d V

(7.6)

ψ j ψk d V

(7.7)

∫ S jk W

ΣΣ j

=

C j C j S jk =

ΣΣ

k

j

(7.8)

C j C j H jk

k

Further sum term recombination produces the expression: W

Σ

Ck Sik + W

Σ

j

C j S ji =

Σ

k

Ck Hik +

k

Σ

C j H ji

(7.9)

j

With respect to H jk = Hk j , S jk = Sk j . Equation 7.9 can be reformulated to give: W

Σ k

Ck Sk j =

Σ

Ck H jk

(7.10)

k

or alternatively: Σ

Ck ( Hk j −W Sk j ) = 0

(7.11)

k

From Eq. 7.11, together with the non-mediocre solution criteria, the derived secular formula can be written: | | | H11 −W S11 H12 −S S12 . . . H1n −W S1n | | | | H −W S H −S S . . . H −W S | 21 22 22 2n 2n | | 21 | | |. | | |=0 (7.12) |. | | | | | |. | | | | H −W S H −S S . . . H −W S | n1 n1 12 n2 nn nn The molecules energy has the secular formula’s solution, which can be made more straightforward using an approximate technique proposed by Hückel, referred to as the Hu¨ckel MO (HMO) method. This requires a triad of assumptions: (i) (ii) (iii)

Hii = α For adjacent and non-adjacent atoms, H jk = β, and Hjk = 0, respectively. Sii = 1, Sjk = (j /= k)

If these assumptions were applied to ethylene, Eq. 7.12 could be condensed:

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| | | α − W . . . . . . β| | | | |=0 | β . . . . . . α − W|

(7.13)

Since x = (α − W )/β, Eq. 7.13 can be transformed to give: | | |x ...1| | | |1...x | = 0

(7.14)

x 2 −1 = 0

(7.15)

which becomes:

therefore x = ± 1. It can then be assumed that W =α±β

(7.16)

where W = α + β and W = α − β are equivalent to the bonding and anti-bonding states, π-MO and π *-MO, respectively. In order to compute the wavefunction coefficient, the energy parameter can be introduced to the secular formula: C1 (α − W ) + C2 β = 0, C1 β + C2 (α − W ) = 0

(7.17)

Since W = α + β, C1 = C2 and W = α − β, C1 = −C 2 are both provided, normalisation condition usage offers ∫ ψ 2d V = 1

(7.18)

Equation 7.1 is comparable to the Hu¨ckel formal, and so Eq. 7.18 can be stated as ΣΣ j

C j Ck S jk =

Σ

Ci2 = 1

k

which yields the following: √ |C1 | = |C2 | = 1/ 2

(7.19)

7.4 CNTs Preparation

127

7.3.2 Graphite Electronic Structure Via the approach described above, the graphite electronic energy dispersion relationship could be computed [124]: ⎡

⎛√ ⎞ ⎛ ⎞ ⎛ ⎞⎤1/2 3 kx a ky 2 ky a + 4 cos cos W2D (k x , k y ) = ± 1 + 4 cos 2 2 2

(7.20)

This relationship is important for CNT electronic structure calculation. The associations of energy dispersion with respect to graphite can be utilised in the computation of the electronic structure pertaining to CNT electronics. The electronic configuration of CNTs with an armchair morphology and periodic boundary criteria is illustrated in Fig. 7.5. CNTs with this shape form the optimal substance for the manufacture of quantum wires with a single dimension (Fig. 7.5).

7.4 CNTs Preparation The known techniques utilised for the CNTs preparation predominantly encompass arc discharge, laser ablation, chemical vapour deposition, solid-phase pyrolysis, glow discharge, gas combustion and polymerisation synthesis. The former is the longest established principle technique for engineering CNTs (Fig. 7.6). It was initially utilised by the Japanese scientist, Sumio Iijima, in order to synthesise carbon fibre; this led to the first recognition of CNTs. The principle underlying the

Fig. 7.5 Electronic structure of armchair-shaped CNTs [11]

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Fig. 7.6 Diagram of arc discharge device

arc discharge approach is to position a graphite electrode within a reaction vessel containing helium or argon gas in order to trigger an arc across the two poles; at this juncture, a temperature of up to 4000 °C may be attained. The graphite evaporates within these parameters to yield products that encompass fullerenes (C60 ), amorphous carbon, SWNTs or MWNTs. Within the reaction vessel modification of the catalyst and quantity of hydrogen can adjust the proportions of the reaction’s products. Practically, this is a fairly straightforward approach for the manufacture of CNTs, although the synthesised CNTs may be combined with C60 amongst additional products; acquiring CNTs with a high degree of allotrope purity may prove challenging. MWNTs as opposed to SWNTs tend to be manufactured, this restricting the experimental applications of this technique. A high quantity of energy is utilised via the reaction. More recently, lithium chloride has been chosen for the anode; this has diminished the energy requirement and facilitated manufacture of a purer product. In the last decade, attempts to surmount the limitations of the arc discharge process have led to the evolution of the chemical vapour deposition technique. This allows atoms of vapourised hydrocarbons to traverse a template to which catalyst particles are bound. The hydrocarbons are then broken down within a range of temperatures, i.e. 800–1200 °C, to yield CNTs. A major benefit of this technique is that the remaining vapour reactants can exit the system thus facilitating the production of CNTs containing minimal contaminants. Limitations include the fact that the CNT harvest demonstrates considerable dimensional and morphological heterogeneity; a catalyst is a requisite. Further studies are being conducted in an endeavour to regulate CNT configuration, e.g. by adjustment of the catalyst template’s array. Other method includes solid-phase pyrolysis, which is a de novo technique in which traditional sub-solid body inclusive of carbon is subjected to high-temperature pyrolysis in order to induce the development of CNTs. This strategy needs no catalyst and is a reasonably stable method. Nevertheless, accessibility of the empirical substances may be a restriction, and so upscaling and ongoing manufacture are challenging. Ion or laser sputtering techniques have additionally been employed, although whilst enduring manufacture is possible, apparatus restrictions have stalled production upscaling.

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7.5 CNTs Applications 7.5.1 CNTs Electronics 7.5.1.1

The Limits of Microelectronics Technology and the Emergence of Nanoelectronics

The most frequent necessary constituent within electronic circuits is the transistor. From the range of transistors available, the metal oxide semiconductor field-effect transistor (MOSFET) is of especial import owing to the straightforward manner in which it can be assimilated within an IC. A MOSFET comprises a triad of electrodes, i.e. source, drain and gate; the latter controls the apparatus through its influence on the current volume transferred between source and drain. Many of the transistors alongside with other electronics like capacitors, resistors, wiring and diodes are incorporated into just one chip and comprise an IC. IC combinations with diverse operational capacities will form the foundations of electronic devices within complicated apparatus, e.g. computers, televisions and communications hardware. This is comparable to physiological viscera, which are composed of multiple cells that make up the whole being. Currently, transistor dimensions continue to diminish; in contrast, the rise in electronic device density with respect to unit chips is sustained (Fig. 7.7). Such components are required by the profound miniaturisation of devices together with computer capacity augmentation. The subsequent pattern of computer chip evolution was proposed by Gordon Moore, the co-founder of Intel, in 1965. He opined that the quantity of transistors positioned at low cost within an IC has undergone an exponential rise, increasing two-fold biennially. This sentiment constitutes the well-known Moore’s law. There are facts that have proved the way to present a precise prediction of the evolutionary pattern of chip technology during the last 3 decades. The greater frequency Fig. 7.7 Basic structure of the traditional transistor

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of IC integration has precipitated a drop in transistor dimensions; IC line widths are progressively decreasing, thus challenging Moore’s law. In 2010, the most diminutive microelectronic device and IC width were each under 100 nm, which is in the region of the physical restrictions of lithographic processing capabilities in contemporary microelectronics. This therefore threatens the validity of Moore’s law in relation to subsequent semiconductor developments. Utilising advanced photolithographic methods, transistors with a slender dimension and line width can be scored onto silicon, thus decreasing the dimension of the transistor channel to 100 nm. Nevertheless, continuing to downscale will be challenging owing to photolithographic optical and chemical restrictions. During lithography, the lowest processing line width is dictated by the chosen light wavelength. Contemporary lithography employs wavelengths from the deep ultraviolet spectrum, i.e. 240 nm, and so it is referred to as deep ultraviolet (UV) lithography. 100 nm is the hypothetical lithography limit; a lower minimal line thickness is not processable by contemporary lithographic techniques. In order to enhance lithographic precision, studies have evaluated the possibilities of light sources which exhibit greater stability and have lower wavelengths. American and Japanese chip production firms and some Chinese research establishments are investigating extreme UV lithography which applies a laser source from the ultra-UV spectrum. A lower ultra-UV wavelength would facilitate a minimum lithographic line breadth as low as 70 nm or less. As lithography techniques become more exact, surface heterogeneity between the substrate and photolithography mask becomes necessary in order to enhance the process as it is downscaled. A further rate-limiting step may be necessary in order to promote lithography scale and precision further. The beam spot diameter of electron-beam lithography can be engineered to be diminutive, with the etching accuracy attaining the lowest line width limit of 10 nm. Nevertheless, these techniques are overly time-consuming and too costly to be upscaled. The STM and AFM nanoprocessing methods, unconstrained by the restrictions of photo- and e-beam lithography, have the potential to be favoured future instruments for the manufacture of ultra-large scale ICs. The principal components for IC synthesis have additionally been evolved in order to heighten output. For instance, IBM has substituted copper for aluminium in order to link circuit transistors. The former has improved conductivity, which enhances the interdevice transmission speed and additionally, facilitates chips of lower dimensions and cost. The current technology relating to microelectronics is additionally affected by device quantum effect and power dissipation limitations, as well as those referred to above relating to lithographic procedures. Contemporary electronic devices that use electrical current encompass a plethora of electronic properties that comply with the traditional electron motion hypothesis. In the following section, a microprocessor chip is used to form the basis of an exemplar discourse on electronic behaviour restrictions in this sphere. The chip processor operates via logic gate on or off, which relies on the accessibility of flowing current. A lower-sized chip width causes a notable decrease in the electron population traversing the logic gates within a defined time interval. Once a maintainable decrease in electron number is achieved,

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131

the logic gate assumes a condition of uncertainty with respect to being on or off and is unable to operate effectively. A device being downsized to nanoscale may also exhibit alternative quantum effects, thus giving rise to several non-traditional electron dynamics. There is another important restriction in microelectronic technologic growth that is the high power usage required by these devices. IC integration can be optimised to some degree by developments in chip blueprints and production, and by enhancing lithographic techniques. However, the significant quantities of current power required for the function of contemporary microelectronic devices necessitates considerable power usage. Given the notable rise in chip integration and communication rates, electronic flow within the circuit will be augmented which, in turn, will ultimately cause an exponential upsurge in the power required to operate the chip effectively, which could cause chip malfunction. To defeat the microelectronic technology limits, an abundance of research is being conducted within the spheres of electronics at molecular and nanoscale levels. The latter is principally contributed to by electronic, atomic and molecular movement within nanoscale configurations, and emphasises nanoscale films and wires, together with additional nanostructures. It is founded on quantum characteristics in relation to the electronic aspects, as well as the manufacturing and construction requirements. Output encompasses some of the empirical problems, e.g. amplification, oscillation, pulse technology and computing processing. The novel paradigms are predominantly grounded by the quantum minutiae of electronics, the electronic quantum tunnelling effect, the electronic energy level discontinuity, quantum size effect and statistical fluctuation traits. The quantum effect in nanoelectronic devices presents working currents of between 1 and 10 electrons which therefore have negligible energy usage. In contrast to the current microelectronic devices in extremely large scale ICs, their power usage is markedly diminished. A further notable benefit of nanoelectronic devices is that their operational frequency can be markedly upgraded. The diminutive solid-state electronics channels can be forced to alter electron motion through voltage regulation. Conductance demonstrates a sharp incremental change as a function of G0 . This occurs owing to the entry of only a single electron into the conduit, referred to as the single electron effect, a concept which has the most potential for future utility with respect to the electronic characteristics of distinctive conductance substances, e.g. CNTs, C60 and quantum dots (QDs). Such materials can be employed in order to engineer single-electron devices within the domain of molecular electronics which can be applied to single-electron transistors, switches, circuits and logic devices, respectively. Thus, a de novo sphere in molecular electronics has evolved. The emergence of SET offers promise for the ability to diminish electronic device dimensions further.

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7.6 Single-Electron Transistor The SETs exhibit notable differences from conventional transistors with respect to the rules underlying their operation. Innovative electronic devices, SETS are founded on the essential physical tenets associated with the Coulomb blockade and single-electron tunnel effects, respectively, which were recognised over 70 years ago. However, these principles were not exploited until the 1980s when they were applied successfully to the manufacture of electronic circuitry. The principal cause of the time lag was production limitations. Figure 7.8 illustrates the SET diagram. The most notable variation from a conventional transistor is that the SET only permits a single electron to travel from source to drain. This necessitates conduit access and the capacity to switch off the source via voltage modification. The electron can then remain for the period required on a QD, which is effectively an islet; when a voltage is placed across the drain, electron motion is resumed. This is not unlike the Panama Canal, where the water depth is governed by opening and shutting the gate to enable a boat to pass through [125]. A slender layer comprising the tunnel junction interposes between the QD island, and the drain and source. An insulating substance segregates the conductive devices. As the depth of this insulation diminishes, electrons will transfer charge across the insulating stratum, and the device will fail, a scenario referred to in quantum mechanics as the quantum tunnelling effect. If a tunnel junction links two nanoparticles, i.e. an extremely diminutive gap or ultrathin insulating deposit, then a lone electron would traverse the tunnel junction and pass from the first nanoparticle to the second, an occurrence labelled quantum tunnelling. In order to facilitate this process, the electron requires sufficient energy to surmount the electron Coulomb blockade, a phenomenon recognised as the single-electron tunnelling effect. Typically, the middle island, or Coulomb island, comprises a QD of between 10 and 100 nm in dimension. As the substance dimension is in the region of the electron wavelength, the electrons that are confined within the QD hold definitive energy levels, which is the reason that the QD can be referred to as an artificial atom. Nevertheless, Heisenberg’s uncertainty principle indicates that there is no precision associated with this energy. The electrons can only undergo binding at a particular level when the energy accuracy, WE, is notably reduced compared to the QD’s energy gap, Eg. This principle can be written ΔE ≪ E g = e2 /C Fig. 7.8 Diagram of SET

(7.21)

7.6 Single-Electron Transistor

133

where e and C represent the basic charge quantity and QD capacitance, respectively. The uncertainty principle permits ΔE to be gauged: ΔEΔt ≥ h

(7.22)

where Planck’s constant is indicated by h and time uncertainty, by Δt. The latter can be roughly defined as the typical time of the comparable RC circuit created by the junction and the QD: Δt = RC

(7.23)

where R reflects junction resistance. If Eqs. 7.22 and 7.23 were interchanged into Eq. 7.21, this could be expressed: R >> h/ e2 ≈ 26kΩ

(7.24)

Thus, junction resistance must be above 26 kΩ in order to confirm electron stability on the equivalent level. From an energy perspective, the raised energy on both aspects of the QD and tunnelling junction can create a possible well (Fig. 7.9a). Of note, is that this energy boundary exhibits restricted height laterally to the QD. Furthermore, the voltage can be modified so as to elevate or to diminish the height parameter specifically (Fig. 7.9b). A negative bias is interposed on the source so that its potential energy can be augmented to a level greater than the QD’s lowest vacant energy level. The electrons then have the chance to traverse the barrier and transfer from source to drain (Fig. 7.9c). Figure 7.9 shows a quantum energy well created from a pair of tunnelling junctions and a QD. The well depth alters in keeping with any applied voltage modification so as to govern the flow of electrons. A SET has three separate components, i.e. a source, drain and gate. In a manner comparable to a traditional SET, current travels from the source to the drain through an electric field control, operated by a gate.

Fig. 7.9 The quantum well formed by two tunnel junctions and a quantum dot

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However, in contrast to the FET, only a single electron can be introduced to the SET at once; this is markedly at variance with the FET, through which tens of thousands of electrons can cross simultaneously. This disparity is generated by the Coulomb blockade effect. The effect of the Coulomb blockade was one of the most significant physical observations reported in the 1980s within the domain of solid-state physics. In a nanoscale dimension, the physical system is characterised by a discontinuous charge and discharge process, i.e. quantisation. In order to add charge to an electron, the requisite amount of energy, E c , is equal to the charging energy, which can be defined as e2 /2C, where e and C represent the electron charge and physical system capacitance, respectively. Reducing the system dimension yields a lower value of C and a higher E c parameter. The energy is labelled the Coulomb blockade. In the course of the nanoscale system’s charging and discharging activities, the electron is subject to single-electron transfer as opposed to continuous mass transfer. Within nanomaterials, the former is referred to as the Coulomb blockade effect [126]. The introduction of an electron into a QD from a source leads to a corresponding rise in QD energy levels (Fig. 7.9b, c), which is represented by the charging energy, Ec: E c = e2 /2C

(7.25)

The increasing of entry of first electron equals the construction of a barrier to the second electron. When E c is higher than thermal perturbation-induced electron energy, then: Ec > k B T

(7.26)

where the Boltzman constant is indicated by kB. In this instance, the second electron is unable to acquire enough energy to traverse the boundary in order to enter the QD until after the exit of the first electron and restoration of energy levels back to baseline. This process characterises the Coulomb blockade effect. Reducing the QD dimension yields a lower value of C and a higher E c parameter. For instance, where the diameter, capacitance and relative E c of a QD are 100 nm, 10– 16 F and 1023 eV, respectively, Eq. 7.26 can only be met if the functional temperature were less than 4 K in order to be certain that the Coulomb blockade effect would be operational. This low temperature is evidently not apposite for pragmatic use. In order to be able to increase the functional temperature, QD dimensions have to be decreased in order to enhance E c . The estimation suggests that in order to attain the Coulomb blockade effect in ambient conditions, i.e. 300 K, the QD requires a low C value within the range 10−18 and 10−19 F; the requisite dimension is therefore under 10 nm. Contemporary technological limitations mean that it is currently challenging to engineer this type of SET function at ambient temperature; this is an ongoing topic for investigation in the research sphere.

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Now, SETs can be classified into four groups according to their baseline material, i.e. semiconductor, e.g. silicon and GaAs, metal, superconductor and SET [127]. A CNT, in which semiconductors are used for construction (Fig. 7.10), is straightforward to install and can be upscaled for extremely large ICs. Composite semiconductors, e.g. AlGaAs/GaAs, with disparate energy gap widths are initially layered in order to generate a quantum well; lithographic electrode blueprints are then added. Different to the previously described top-down method of synthesis, Fig. 7.11 illustrates 10 nm gold particles positioned on a silicon dioxide substrate; an elongated chain comprising three particles of gold connects the metal source and drain over a 30 nm length, operating as the SET’s requisite Coulomb island. This approach is not as good as that of precise positioning of QD, although diminutive particles are utilised by this method in order to exemplify ambient temperature SET manufacture. In SETs, the middle island comprises a lone molecule, e.g. C60 , in the schematic depicted in Fig. 7.12. An atom is the most diminutive item able to be positioned at the island centre, although this would have a negligible impact on the capacitance of the tunnel junction. In reality, it is challenging to interpose an atom with a lone electron between two electrodes remote to the system’s exterior. When a sole atom of the STM tip is in close proximity to a specimen, this set-up can be considered to reflect the fundamental configuration of a SET at the level of a single atom (Fig. 7.13). In contrast to traditional transistors, the benefits of SETs encompass diminutive dimensions, rapid transmission, notable responsiveness and crucially, a low level of power usage. Microelectronics have officially gained access to the nanoscale sphere from the sub-micron level; the logistic limitations imposed by fundamental physics principles are already in jeopardy, e.g. Moore’s law is on the brink of compromise. The SET represents a de novo electronic device innovation representative of twenty-first century technological progress. Fig. 7.10 SET made from compound semiconductor heterostructure junction

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Fig. 7.11 SET made from GCP chain on silica substrate

Fig. 7.12 Basic pattern of SET at the molecular scale made from C60

7.7 CNTs Electronics FETs, diodes and other components of electronics are fundamental requisites of microelectronic systems grounded on silicon. Comparably, the evolution of the nanoelectronic domain necessitates a novel materials construct; electronics using CNTs demonstrate much future potential. The latter will arise essentially via a number of steps: (i) complete delineation of the electrical characteristics of CNTs; (ii) recognition and innovation relating to the fundamental components of CNT-based electronic devices, e.g. those equivalent to silicon-based microelectrons, including diodes and

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Fig. 7.13 Basic structure of SET with the centre island of a single atom

FETs; (iii) construction of a logic circuit with specific operations using CNT-based electronic devices; and (iv) reliable upscaled circuit integration. Contemporary CNT electronics research has demonstrated significant advances in relation to phases one and two, and work on the third stage is ongoing. Prototype CNT-based logic circuits have already been constructed in laboratory settings.

7.7.1 Quantum Wire The quantum wires study focuses on a range of CNTs. The latter are the optimal material for the synthesis of quantum wires; the latter are then known as nanotube quantum wires. The conductivity of these molecules is currently the subject of wideranging investigations. Different CNTs regard candidates for a selection of electronic devices that operate in a non-linear manner. CNT chirality dictates whether CNTs exhibit conductor or semiconductor properties. Additionally, various regional flaws, e.g. the presence of 5- or 6- membered rings, may impact their electronic characteristics. CNTs can be classified as SWNTs or MWNTs in keeping with their configuration. The former are constructed by curling a graphite layer; the latter are made up of several rolled strata which form concentric graphite tubes [128]. At low temperatures, SWNT conductivity indicates that there is in the region of 1 MΩ resistance across the two terminals. When various voltages are applied, the characteristic current–voltage curves demonstrate a transparent diode rectification effect, i.e. a one-way pass and a curve which demonstrates significant non-linearity

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at a higher bias. When a voltage is placed across alternative electrons, the identical CNT can be recognised but with various electronic activities in dissimilar sites. The conductivity of single MWNT indicates that SWNTs exhibit a conductance of 2G0 , which is autonomous of diameter or length. The conductance of MWNTs would be expected to rise with the layer count but studies have reported a conductance value of only 1Go in MWNTs, as just a single-CNT layer generates the conductance parameter. For MWNTs to act as conductors, the initial stratum has to demonstrate conductivity. If the second layer were insulating, this would generate a high axial resistance, and this layer and subsequent strata would not add to conductivity. It is important to realise that there are alternative choices to CNTs for the manufacture of molecular wire, e.g. the electrochemical growth technique can be employed to link two metal nanoparticles to metallic copper wire.

7.7.2 CNT-Based Junction The CNTs can form a p–n junction, cross-junction and/or molecular junction. A molecular junction can be created by adding a couple of pentagon-heptagon defects, abridged as a 5–7 pair, onto a SWNT; at least 2 SWNT portions are then linked to establish the junction. Two crossing SNTs comprise a cross-junction. A p–n junction results from deliberate modulation-doped SWNT activity; the latter display traits in common with silicon-based microelectronic diodes. There are different studies of SWNT hexagonal networks that can be incorporated, together with a 5–7 pair defect, in order to attain a smooth link between at least 2 CNT portions comprising varied atomic and electrical configurations. This process leads to the formation of a molecular CNT junction [129]. CNTs include two material formats, i.e. metal (M) and semiconductor (S). The addition of the 5–7 pair and the resulting molecular junction can be classified into three subgroups, i.e. MM, MS and SS. The electrical characteristics of MS and MM were described back in 1999. The MS molecular junction was noted to display non-linear current-voltage (I-V ) traits typical of a rectifier diode; in contrast, the MM molecular junction demonstrated temperature-dependent conductance. The AFM representation of 5–7 pair defects in a molecular junction of CNTs is illustrated in Fig. 7.14. An estimated angle of 40° lies between the CNT pair. A schematic of a molecular junction comprising a heptagonal recess and pentagonal convex is presented in Fig. 7.13c. Figure 7.14a indicates the electrode triad positioned on a molecular junction, together with the two linear CNTs. This configuration has promise for use in electronic switch production [130]. Doping practice to modify the energy band plays a major part in microelectronics. The process of control doping is essentially comprised of the semiconductor mechanism per se. In the contemporary microelectronic sector, the most fundamental constituents, e.g. diodes, bipolar transistors and FETS, are attained in their entirety via

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Fig. 7.14 Molecular junction with CNTs. a An AFM image of 5–7 pair defects in a molecular junction with a carbon nanotube. b Another example of 5–7 pair defects in a molecular junction. c Diagram of a molecular junction, which has a heptagonal recess and a pentagonal convex [131]

doping of the core semiconductors. Founded on these conventional devices, similar characteristics and operationality are observed in doped CNTs. The Dai group described the synthesis and profile of a p–n junction that was derived from modulation-doped CNTs. Axial modulation doping was carried out on a SWNT founded on a semiconductor; the two halves were retained as p- and n-types, respectively, with the latter being doped with potassium. A p–n junction was therefore attained, for which the typical I-V g curve is illustrated in Fig. 7.15. Fig. 7.15 I-V g and I-V characteristic curves of CNT p–n junction. a I-V g characteristic curve. b I-V characteristic curve for gate voltage at the ‘star’ point. c I-V characteristic curve for gate voltage at the ‘triangle’ point [132]

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Changes in gate voltage produced four conditions, as seen by the divisions in the curve: state I, where V g is 212–220 V, and the conductance is near zero; state II, where V g is 27–12 V, and the conductance exhibits a zenith; state III, where V g is 21–27 V, and the conductance returns to zero; and state IV, where V g is elevated to 20–21 V, and the conductance is observed to rise in a monotonic manner with the augmentation in V g .

7.7.3 SET with CNTs The CNTs nature is tightly linked with their configuration. For the purposes of electrical conductivity, CNTs can be comprised of either semiconductor or metal materials. Conductivity disparities can also be observed in various regions of an individual CNT, which is caused by structural homogeneities. A characteristic quantum confinement effect affects electron motion along a radial direction, but they can move freely along an axial trajectory. Thus, CNTs can be perceived as singledimensional quantum wires, which demonstrate a classic Coulomb blockade effect in low-temperature conditions. When the outer electron is introduced to the diminutive CNT capacitor, for which the voltage change can be expressed as ΔV = Q/C, where Q and C represent CNT injected charge and capacitance, respectively, those capacitors of sufficiently low dimension to receive an electron injection will yield a reverse voltage of a magnitude capable of producing the circuit block. Once the injected electron traverses the CNT, the opposite blocking voltage disappears in order to facilitate ongoing receipt of injected electrons. As alluded to previously, future progress of conventional microelectronics, based on silicon, appears promising with the use of SETs. Nevertheless, most SETS require extremely low temperatures for their operation, which causes pragmatic limitations. Contemporary research has demonstrated the marked bending capacity of metallic CNTs, which is observed as a nanoscale tunnelling barrier for electron transport. Depending on SET configuration, there is the potential for two robust metallic CNT bends to merge and to create an SET. Postma et al. have, in 2001, indicated that functionality in ambient conditions could be achieved in SETs derived from a metallic single-CNT molecule. Figure 7.16a demonstrates a metallic form of CNT evolving on a gold electrode on a substrate comprising Si/SiO2 . A portion of the CNT is towed along the trajectory shown by the arrow in order to generate robust bending. A further CNT segment is then moved by the AFM probe along the arrow vector to create a second bend (Fig. 7.16b). The SET is illustrated in Fig. 7.16c, d. The interposed Coulomb island amid the couple of bending parts forms a segment of CNT of approximately 25 nm.

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141

Fig. 7.16 SETs with CNTs. a The AFM probe is used to drag a part of the carbon nanotube along the direction of the arrow to produce strong bending. b The AFM probe drags another part of the carbon nanotube to produce a second strong bend. c Image of the single-electron transistor. d A larger version of the ‘Coulomb island’ formed in (C) [127]

7.7.4 CNT-Based FET Early in 1998, Dekker et al. proposed an FET that was composed of an SWNT which worked at ambient temperatures. It encompassed a semiconductor SWNT, linked to a triad of metal electrodes. Modification of the voltage placed across the gate facilitated CNT on- and insulating state transformations. Previous descriptions of equivalent properties seen in metallic SWNTs operating in very low temperatures have been published. Engineering a FET founded on CNTs offers the opportunity to make significant progress in the field of CNT-derived electronics. Fig. 7.17 CNT-based FET. a AFM image. b Side sectional view of the FET [133]

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Figure 7.17a illustrates the image obtained by AFM of the link between a SWNT and a triad of Pt electrodes. A thermal method is employed in order to facilitate the growth of a 300 nm deep SiO2 layer on the semiconductor’s Si substrate. The latter forms the back gate (Fig. 7.17b). The illustrated CNT-derived FET is constructed from a semiconductor form of CNT, whereas metallic CNT types are utilised in the CNT-based SET. An important consideration is to be able to segregate these two forms efficaciously.

7.7.5 Complementary Nongate (Inverter) Circuit with CNTs Avouris et al., in August 2001, have published a positive outcome with respect to the design of two corresponding CNT-based nongate circuits; one comprised a double CNT configuration, the other just a single CNT. As noted earlier, a routine FET made up of intrinsic CNTs is defined as a p-type; however, doping with potassium renders n-type traits. Contemporary research by this group has additionally determined that the p-type CNT FET can be annealed in a vacuum so as to be converted to an n-type; the presence of oxygen transforms the CNTs into p-type. Thus, exploitation of these characteristics enables the identical substrate to be located with n-type and p-type CNT-based FETs, with the use of the apposite protocols, in order to attain a complementary inverter configuration. Figure 7.18c depicts

Fig. 7.18 Complementary nongate circuit based on two CNTs. a Ids-Vg curve for the case annealed in a vacuum. b I ds -V g curve for the case exposed to an oxygen environment. c The complementary nongate circuit. d The transmission characteristic curve [134]

7.7 CNTs Electronics

143

Fig. 7.19 Complementary nongate circuit based on a single CNT [134]

the initial position of a SWNT on gold electrons coated with a SiO2 layer; the gate electrode is formed by the Si substrate. The reaction is conducted within a vacuum environment of under 1024 Pa at ambient temperature. The output is a 2-p-type CNT FET. The left CNT FET is then coated with a polymethyl methacrylate (PMMA) photoresist; the right is left bare in the vacuum ready for the annealing process, which converts the two CNT FETs into n-type. Figure 7.18a shows the I ds -V g curves for the two annealed CNT FETs. The circuit is placed in oxygen, at 1021 Pa, for a predetermined period. The CNT FET on the right that is not safeguarded by PMMA revers to p-type, whilst the other, protected by the PMMA, remains as a n-type. Figure 7.18b shows the resultant I ds -V g curves for the respective CNT FETs. The final product is a CNT inverter, which operates identically to a standard complementary metal oxide semiconductor (CMOS) inverter; its transmission profile is presented in Fig. 7.19d. Figure 7.19 A illustrates the image acquired by AFM of an inverter designed using a lone nanotube on three gold electrodes. A portion of the CNT is coated with PPMA in order to preserve it as p-type, whereas the remaining area is left bare and doped with potassium so as to achieve n-type. This process yields a CNT inverter. The comparable inverter circuit and transmission properties are demonstrated in Fig. 7.18b. The former can also be described as a CMOS circuit. When the input voltage, V in , which is positive, is high, the n-FET is activated and the p-FET is off, thus giving rise to a low negative voltage output, V out . When the positive input voltage is low, the reverse occurs, and the output is an elevated negative voltage To outline, the underlying principles and configurations of several CNT-derived logic circuits have been described. However, numerous issues are yet to be resolved. The logical components of CNTs require implementation and validation, e.g. a key logical storage unit to construct dynamic RAM has not come into fruition. The majority of reported CNTs function on resistor-transistor logic, which is suboptimal for current ICs. In theory, corresponding logic factors would be anticipated, e.g. CMOS. Two major potential limitations relating to the evolution of CNT electronics need to be considered. Despite the fact that CNTs derived from both semiconductors and metals can undergo screening when engineering devices and single-logic circuits, it is

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challenging to ensure homogeneous electrical characteristics when their manufacture is upscaled. Additional problems relate to the siting and manoeuvring of CNTs in large-scale circuits. Once these concerns have been addressed, the CNT electronic sphere will advance swiftly [131].

7.8 Other Applications of CNTs The CNTs unique nature offers the promise of a practically infinite number of utilities, as aforementioned.

7.8.1 Nano Test Tubes CNT provides the most diminutive capillary in order to investigate the processes underlying capillarity, and test tubes of minimal dimensions in which chemists can conduct chemical reactions.

7.8.2 Nanobalance The CNTs microparticles can induce beat frequency alterations in related current. In 1999, exploitation of this property led to the nanobalance innovation, which boasts a precision of 10–17 kg, and can quantify the mass of a single virus or a lone atom.

7.8.3 Nanomolds The materials like oxides and metals can be used to fill the interior of CNTs, essentially treating the CNT as a mould. A CNT can initially be filled with metal, together with additional compounds; etching can then remove the carbon layer. This allows the most diminutive wires or de novo single-dimensional substances to be engineered and potentially utilised in future electronic devices at either nanoscale or molecular levels.

7.8.4 CNTs: Field Emission Cathode Materials The use of CNTs as field emission cathode components is anticipated to resolve issues restricting the utility of cathode-ray tubes. A monitor constructed from silicon or

7.8 Other Applications of CNTs

145

metal, a cathode field emitter is generally synthesised by techniques including chemical etching and electron-beam deposition. Owing to microprocessing constraints, the smallest dimension of the produced silicon or metal field emission device is in the region of 20–50 nm, and lacks homogeneity. The dimensions of field emission devices are typically quite large as in order to emit electrons, a threshold voltage of 100 V has to be attained. Cathode field emission devices with elevated working voltages and suboptimal consistency will be inapposite for subsequent flat-panel monitor generations. Use of CNTs for field emission cathodes is a favoured option as the single-wall CNTs offer a mean diameter of 1–2 nm, i.e. ten- to 20-fold more diminutive than the peak diameter of the silicon-based devices in widespread utility. A field emission cathode device constructed from WNTs will have high resolution and an optimal source of electron emanation; the voltage threshold will be diminished to in the region of 10 V. The de novo generation of wall-mounted flat-panel monitors will exhibit lower energy requirements. Japanese and South Koran researchers, in 1999, have designed CNT-derived monitors of just several centimetres in depth. Compared with conventional products, these items were of small dimension and reduced weight, and exhibited high quality, reduced power usage and a dynamic reaction time of only several microseconds. Furthermore, they were able to function in a broad temperature spectrum, i.e. between 185 and 245 ˚C. It is therefore transparent that for the purposes of flat-panel monitors, CNTs show much promise [132].

7.8.5 CNTs Application in Hydrogen Storage The hydrogen is often considered to be the optimal source from which future clean energy can be derived. Nevertheless, the substance per se is only of low density; compression into liquid phase is neither expedient nor without hazard. Low weight CNTs, that have a hollow configuration, could make ideal storage modules for hydrogen, being able to retain hydrogen at a density that is greater than either the liquid or solid phases. Under thermal regulation, the stored hydrogen could be liberated gradually and harnessed as energy. Scientists are endeavouring to deploy CNTs for this purpose. For the absorption substance which displays the greatest storage capacity for hydrogen, CNTs could additionally influence the design of vehicles that are fuelled by hydrogen. The four principal global automobile firms, i.e. General Motors, Ford, Toyota and Daimler-Benz AG, are fast-tracking their research in this area. In March 2000, a second-generation hydrogen fuel cell was released by Toyota; this comprises an innovative solid polymer fuel cell which runs on hydrogen. The vehicle’s peak output power and speed are 90 kW and 150 km/h, respectively; a single hydrogen tank gives a distance range of 300 km. In order to exploit hydrogen as an automobile energy source, a number of criteria need to be fulfilled. A driving range of 500 km would be anticipated from 3.2 kg

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hydrogen, which would necessitate containers capable of an extremely sizeable hydrogen storage density. These would have to attain 6.5 wt%, i.e. the hydrogen weight percentage reflecting the total of the container and the weight of the hydrogen, and offer a stored hydrogen weight per unit volume capacity of 62 kg H2 per cubed metre. What is the CNT future of hydrogen storage? In 1997, a trial of the hydrogen storage capacity of CNTs was conducted by the National Renewable Energy Laboratory and IBM; hydrogen adsorption was enhanced with rising diameter. Thus, SNWTs with high purity exhibit a hydrogen storage capacity within the spectrum 5–10 wt%; with a diameter of between 1.63 and 2.0 nm, high purity SWNTs meet the required 6.5 wt%.

7.8.6 High-Energy Microbattery Displaying obvious characteristics of electrical conductivity, CNTs can also operate as a cathode or replace conductive polymer substances, thus forming a medium of conductivity in order to generate batteries of high-energy potential, which are of small dimension and heightened longevity. These would be optimal for use in computers and vehicles.

7.8.7 High-Energy Capacitor Following pressurisation into sheets, CNTs could be deployed as capacitors exhibiting high energy. The addition of low quantities of CNTs to various substances could enhance the latter’s conductivity. A defined aliquot of CNTs can reduce the resistivity of polymers by over three orders of magnitude.

7.8.8 Chip Thermal/Heat Protection Exhibiting distinguished. properties of thermal conductivity, it is anticipated that CNTs could be used as the heat plate for computer chips in subsequent high-speed computer generations. They could additionally form protective substances for a range of high-temperature devices, including engines and rockets.

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7.8.9 Nanoreactor The researchers have set that in view of the distinctive porosity and enhanced selective surface area of several hundred square metres per gramme CNTs, these nanostructures could be engineered into nanoreactors, which would operate with respect to chemical reactions limited to a low range.

7.8.10 Nanocomposite Materials Based on the CNTs nature, it is possible to engineer high-performance composite substances. CNT-fortified plastics demonstrate superlative mechanical characteristics, excellent conductivity and resistance to corrosion, and the ability for radio wave shielding. CNT composites incorporating a cement matrix offer notable resistance to impact, anti-static and anti-wear traits, together with improved stability. They are also environmentally sound. Robust strength and impact resistance are seen in CNT-reinforced ceramic composite substances. CNT reactivity is improved by the presence of the fivemembered ring defect. In heat, and when alongside additional materials, terminal CNTs have a tendency to configure a tube, which is susceptible to penetration by metals, thus generating composite substances with a metal matrix. In addition to exhibiting notable strength as well as an elevated modulus and temperature, such materials have minute thermal expansion coefficients and high resistance to thermal alterations [133]. Problems 1.

2.

3. 4. 5.

The figure shows the unit cell of (10,10) SWNT. How long would (10,10) SWNT that weight 1 g be? How does this distance compare with the distance from the earth to the sun? In many works of covalent functionalization of nature of pure SWNT, the Dband in Raman spectrum is observed to increase significantly upon functionalisation. When the functionalised sample is heated in He at 400 °C, the band decreases again to low D/G ratio, similar to that of pristine sample. Explain these observations. Why is it necessary to use a surfactant in order to see the fluorescence of SWNT? Do you expect that a solid sample of pure SWNT would fluoresce? Why? Why the number of C atoms dissolved in a cluster initially increase during growth, reach a maximum, and then decrease to a stable concentration? Nanohorns are being considered as vehicles for therapeutic drug delivery. What physical and chemical modifications should be done on them to optimise their efficiency for this important application?

Chapter 8

Semiconductor Quantum Dots

8.1 Introduction Last century, the transistors and silicon-based semiconductors have combined for the successful development of integrated circuits derived from silicon, which stimulated major technological advances within the electronics industry sector. The early 1970s were characterised by the innovation of quartz fibre-optic materials and GaAs lasers, which encouraged the swift progress seen in technology relating to fibreoptic communications. The notion of superlattices [135] was introduced by Esaki and Tsu in 1970, who were both at IBM; these facilitated the rapid evolution of semiconductor substances of reduced dimensions. This revolutionised the blueprints of optoelectronic devices; the concepts changed from addressing any impurities to the sphere of band engineering. The growth and utility of nanoscience and technology will offer the opportunity to regulate, to engineer and to synthesise potent de novo devices and circuits on atomic, molecular and nanoscale levels; these developments could notably alter contemporary standards of living. The research on small-size semiconductor quantum structures has been intense for over three decades. Electronic or optoelectronic devices in a III-V quantum well configuration have reached the contemporary marketplace. These substances have facilitated the development of ultrafast electronic devices and luminescence detectors with a high output. A group of zero-dimensional materials, quantum dots (QDs) are additionally labelled as artificial atoms. QD studies have revealed their utility for light-emitting apparatus and detectors [11]. Addition to electronics or optoelectronics applications, QDs can be deployed in order to generate qubits. For instance, a single QD’s charge can be utilised in the quantum bit definition, where one extra electron or none refers to 1 and 0, respectively. It has been proposed that the electron spin conditions, i.e. up or down, could also be deemed 0 and 1. Current work has taken more interest in the utilisation of the QD exciton state. Energy absorption by a QD that is equal to the photon excitation causes a self-excited electron to be introduced to the conduction band, thus leaving a void in the valence band. Coulomb attraction enables the pairing of electrons and © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2022 Y. Al-Douri, Nanomaterials, https://doi.org/10.1007/978-981-19-3881-8_8

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holes in order to diminish energy; these have fairly sharp lines of radiation, and are straightforward to excite and to identify with techniques of optical analysis. There is a great optimistic surrounding the QDs potential for quantum computing. However, although this possibility is highly anticipated and supported by academic theories, quantum-bit logic operations have yet to be tested. Experimental methods are still insufficient with respect to the regulation and quantification of the QD process.

8.2 The Physical Basis of Semiconductor QDs 8.2.1 Quantum Confinement Effect Due to the electrons have limited movement along a number of vectors, quantisation of electron energy in that trajectory would occur since the bound electrons would generate standing waves. Materials can be classified as dictated by their binding dimension (D) count, i.e. 0D, 1D, 2D and 3D, exemplified by bulk material, quantum well, quantum wire and QD, respectively. Energy quantisation is likely to have a significant influence in relation to the electronic density of states (DoS), which can be described as [136]: DoS =

dN d N dk = dE dk d E

(8.1)

In a 3D example, where N ðkÞ 5 k space volume/volume of each state, N (k) =

4/3π k 3 (2π )3 / V

(8.2)

The variations in carrier density difference with configurations of various sizes are presented in Table 8.1 and Fig. 8.1. Quantum confinement gives rise to discrete conditions, which are obtained by solving the energy level from the Schro¨dinger formula: Table 8.1 Expressions of carrier DoS in the structure with different dimensions Structure

Limited number of dimensions

Bulk material

0D

dN/dE √ E

Quantum well

1D

Quantum wire

2D

1 √ 1/ E

Quantum dots

3D

δ(E)

8.2 The Physical Basis of Semiconductor QDs

151

Fig. 8.1 Carrier DoS in the structure with different dimensions compared with bulk materials and nanocrystals in electronic DoS

−

ℏ2 2 ∇ Ψ + V (r )Ψ = EΨ 2m

(8.3)

The solution for the example of a 1D infinite square potential well can be written as follows: ⎛ nπ x ⎞ Ψ (x) ≈ sin (8.4) L where an integer is represented by n. The ground-state wave function is depicted in Fig. 8.2. If limitations were present along only a single vector, e.g. x, then the alternative two energy trajectories would remain continuous. The total energy can be expressed: Fig. 8.2 Electronic ground-state wave function of one-dimensional infinitely deep square potential well

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p 2y n2 h2 PY2 + + 8m L 2 2m 2m

(8.5)

With respect to the 3D infinitely deep square potential well or quantum box, the following applies: ⎛ Ψ (x, y, z) ≈ sin

nπ x Lx

⎞

⎛ sin

nπ y Ly

⎞

⎛ sin

nπ z Lz

⎞ (8.6)

where integers are represented by n, m and q. The energy level is given by: E n,m,q =

q 2 h2 m2 h2 n2 h2 + + 8m L 2x 8m L 2y 8m L 2z

(8.7)

Instead of the box having an infinite depth, the real potential barrier may additionally be restricted, spherical or display limitations in harmonic oscillator potential. This example only addresses the situation with a single election. In reality, multiple particles and electron-hole pairings are frequently involved. The mass of the particle should additionally be taken into account, together with potential lack of correspondence at particle borders. The QD dimensions lie within the spectrum 10 to 1000 nm, which is analogous to the electron de Broglie wavelength in semiconductors. Three-dimensional quantum confinement effects, therefore, impact the electron or hole which would then acquire quantised energy levels, thus leading to the generation of an electronic system which is described as zero-dimensional. As alluded to previously, QDs are sometimes called artificial atoms, as they have an extremely comparable electronic structure. Thus, QD energy levels are frequently preceded by the letters s, p or d, which indicate QD ground conditions and the excited-state levels [136]. One example is the InAs QDs that have a geometric appearance like a flat convex lens. Thus, along the r vector, a two-dimensional paraboloid can be used to gauge the potential energy. This estimation indicates that the ground and excited conditions each have a degeneracy number, e.g. 2, 4 and 6, etc., which encompasses spin degeneracy. Filling of QDs is only with an electron pair, and of p energy levels with four electrons, a pattern that is continued. Transparently, the QDs vary in electronic structure compared to atoms. Orbital energy levels, s, p and d, can receive the atomic energy levels numbered 2, 6 and 10, etc., a variation predominantly arising in QDs with various atomic potential energy types. The latter is principally generated by Coulomb interactions, and displays symmetry in three dimensions. Since the geometric structure is comparable to a convex lens, the height is more diminutive than the diameter. Thus, QD electrons exhibit only planar two-dimensional symmetry, and so the degeneracy energy level form is dissimilar to that found in atoms.

8.2 The Physical Basis of Semiconductor QDs

153

8.2.2 Excitons and Luminescence 8.2.2.1

The Excitons Concept

Because of Coulomb forces, attraction of excited electrons and holes can occur to form attached electron-hole pairs, e.g. the excitons. In the case of a semiconductor, the electron and hole both draw potential; in comparison, the effective mass of the hole is larger than that of the electron, and so an atomic system similar to hydrogen is created. The Bohr theory dictates exciton binding energy [137]: En = −

e2 ε h2 a ; = 0 2ε a0 n 2 4 π 2 e2

(8.8)

where the reduced mass is represented by μ. In QDs, excitons may be generated internally, and become influenced by these; their dimensions underlie the degree of limitation. Discrete energy levels are also present in an exciton; an absorption zenith comparable to the δ function is observed in the exciton absorption spectrum.

8.2.2.2

Exciton’s Energy Band Structure

In a temperature of zero, the semiconductor band is configured with an empty conduction band and filled valence band. A void without any electrons, termed the band gap, is interposed between the superior and inferior aspects of the valence and conduction bands, respectively. The former has a condition comparable to angular momentum, where l = 1. A quadruple state degeneracy is generated by the engagement of the spin with the orbital: ( j = l ⊕ s = 3/2, m j = ±1/2, ±3/2)

(8.9)

This is frequently referred to as heavy hole (Eq. 8.10) and light hole (Eq. 8.11). | \ |3 | , ±3 |2 2 | \ |3 | , ±1 |2 2

(8.10) (8.11)

The heavy and light holes can be categorised with respect to the vertical interface’s effective mass dimension. Considering the part played by restrictions along the vertical vector, typically, the lateral breadth of QDs is significantly greater than in the vertical trajectory, displaying robust restrictions in the latter. The light hole band is tethered owing to its diminutive effective mass. Thus, light absorption in proximity to the energy gap arises predominantly as a consequence of the heavy holes [138].

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When the electron excites from valence to conduction bands, a residual comparable positively charged void is created. Compelled by Coulomb forces, the electrons are merged into an exciton condition, a process that liberates binding energy; this is just 6 meV in bulk substances. In the case of two-dimensional quantum wells, the restriction effects can augment this parameter by 15 meV. A straightforward explanation of the process is that there is compression of the positive and negative electron distance which consequently, strengthens the Coulomb attraction. In exciton binding within a QD configuration, there is an additional liberation of binding energy in the region of 20 meV. The exciton band energy can be computed by adding the semiconductor band gap, Eg, to the electron and hole binding energies from limited effects, i.e. Ee and Eh, respectively, and then subtracting Eb, i.e. the binding energy required to create excitons. This is written as: E ex = E g + E e + E h − E b

(8.12)

In particular, Fig. 8.3 demonstrates exciton e 1 s and h 1 s: 1s1s = E g + E e1s + E h1s + E b1s1s E ex

(8.13)

Exciton e 1p and h 1p are illustrated in Fig. 8.4: 1p

1 p1 p

1 p1 p = E g + E e1 p + E h + E b E ex

(8.14)

At the juncture where the exciton hole and electron reassociate, there is emission of a photon equivalent to the difference in energy between the valence and conduction conditions. The lifetime of an exciton may be up to a nanosecond, i.e. 10−9 s, in duration, and is therefore easily appreciated with a range of experimental methods, thus facilitating examination of exciton generation through quantum evolution. When the electron-hole generation increases due to excitation light, multi-exciton conditions have to be taken into account. Since several electrons and holes exist, two Fig. 8.3 Diagram of single-particle energy spectrum in bulk materials (left) and small QDs (right)

8.2 The Physical Basis of Semiconductor QDs

155

Fig. 8.4 Diagram of transition of a Single-electron-hole pair in semiconductor QDs

distinct adjacent excitons may congregate to generate coupled exciton state pairs. These are characterised by an energy level of 1 meV, which is less than that observed in autonomous exciton state pairs. Stable laser frequencies can be set at 4 neV, i.e. 4 × 10−9 eV, and so it is easy to differentiate experimentally between the two conditions. An energy band schematic for multiple excitons in normal GaAs QDs is illustrated in Fig. 8.5; g and ε1 indicate the electronic valence bands in the vacuum condition, and a D-light-excited exciton state, respectively. | | \ \ |3 |1 3 1 | | hole| , − &electron| , − 2 2 2 2

(8.15)

An exciton condition excited by L-light is represented by E: | | \ \ |3 |1 3 1 hole|| , + &electron|| , + 2 2 2 2

(8.16)

where ε + 1 + ε − − 1:0 for the additive condition of D- and L-excitons, respectively; the energy is 1.0 meV lower than that for each exciton independently.

8.2.2.3

Exciton Binding Energy Calculations

In QDs that are spherical, and ignoring the Coulomb interaction case, the Schro¨dinger formula in terms of the hole or electron can be expressed as [139]:

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Fig. 8.5 Energy spectra of GaAs QDs in multiple exciton states of regular hexahedron 40 × 35 × 5 nm ε+ = ε− = 1764 meV

−

ℏ2 ∇ 2 ςi (r ) = εi ςi (r ) 2 mi

(8.17)

where the electron or hole is represented by i. In conditions of the optimal quantum constraints, the boundary condition can be described as: ςi (r ) = 0 for r = R

(8.18)

The boundary conditions in Eq. 8.18 can be applied to the Schro¨dinger formula to give the following solution: / ςi (r ) =

1 jl (αnl r/R) m Y (θ, φ) 4π R 3 jl+1 (αnl ) l

(8.19)

where jl and α nl indicate the l-order and nth root of l-order, respectively, spherical Bessel function. Using the latter, the boundary conditions for Eq. 8.18 can be

8.2 The Physical Basis of Semiconductor QDs

157

determined: r || =O )| R r=R

jl (αnl

(8.20)

This formula can be rewritten as: jl (αnl ) = 0

(8.21)

In order to obtain a discrete eigenvalue, Eq. 8.19 can be swapped into Eq. 8.17 as follows: εi =

ℏ2 ⎛ αnl ⎞2 2 mi R

(8.22)

The electron and hole energy levels can be described by Eqs. 8.23 and 8.24, respectively, utilising the band zero point at the peak valence band and the format of Eq. 8.22: εe = E g + εh = −

ℏ2 ⎛ αn e le ⎞2 2 me R

ℏ2 ⎛ αn h lh ⎞2 2 mh R

(8.23)

(8.24)

It is, therefore, indicated that the single-particle and optical absorption spectra are not aligned, since this fails to consider the Coulomb forces impacting the electrons and holes. The electron-hole pairings can be represented using the Schro¨dinger formula as follows: ⎞ ⎛ ℏ2 ℏ2 (8.25) − ∇e2 − ∇h2 + Vc φ(r ) = εφ(r ) 2 me 2 mh Here, the typical spherical coordinates together with the conditions of the boundary, ϕ(r = R) = 0, are included, where the Coulomb potential is reflected by V c . If the latter parameters were omitted, the electron-hole pair energy could be computed by solving Eq. 8.25: ε = εe + εh = E g +

ℏ2 ⎛ αn e le ⎞2 ℏ2 ⎛ αn h lh ⎞2 + 2 me R 2 mh R

(8.26)

The wave function is defined as: ϕ(re , rh ) = ς (re )ς (rh ) where the following definition applies:

(8.27)

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Fig. 8.6 Linear absorption spectra of CdS crystals and QD in glass at room temperature [11]

Fig. 8.7 Linear absorption spectra of CdS crystals and QD in glass at 10 K [11]

/ ς (r ) =

1 jl (αnl r/R) m Y (θ, ϕ) 4π R 3 jl+1 (αnl ) l

(8.28)

If the Coulomb principle was taken into account, a precise numerical solution could not be obtained from Eq. 8.26. The variation technique is frequently applied [140]. The linear absorption spectra for CdS crystals and QD in glass are presented in Figs. 8.6 (ambient temperature) and 8.7 (10 K). The spectrum lines for the QD are sharper and more straightforward to discern (Fig. 8.7).

8.3 Semiconductor QDs Preparation The QD nanostructure lacks uniformity. It is layered with semiconductor nanostructures and an additional substance; the two coatings exhibit lower and higher energy gaps, respectively. Contemporary techniques for the manufacture of QDs can be classified as described below [141].

8.3 Semiconductor QDs Preparation

(i)

(ii)

(iii)

(iv)

159

Chemical colloidal technique: Chemical sol production is utilised to generate multi-layered QDs. This method is straightforward and can be upscaled for industrial use. Self-assembly technique: This method employs either molecular beam epitaxy (MBE) or a chemical vapour deposition. It is founded on the concept of lattice mismatch that facilitates the evolution of QDs via a process of selfpolymerisation in a chosen substrate. It is an opposite technique for the upscaled manufacture of QDs with an organised configuration. Lithography and etching: The substrate is etched in order to generate the blueprint using a beam or electron beam directed immediately onto the substrate. This method is onerous and therefore unsuitable for upscaled QD synthesis. Split-gate strategy: Two-dimensional confinement with respect to a quantum well’s two-dimensional axis is created by the placement of an extrinsic voltage. The gate can be regulated in order to modify QD morphology and dimension. This method is more apt for experimental studies as opposed to upscaled QD manufacture.

Synthesis techniques for QDs are based on the generation of a thin film. Three conventional paradigms to achieve this are frequently referred to by contemporary publications: 1.

Layered growth (Frank-van der Merwe (F-M)) mode

Founded on a single layer at process commencement, the F-M model then incorporates the evolution of a second stratum, thus involving growth of a layer at a time. Crystalline thin film development is essentially determined by the crystalline alignment in the initial stratum. Where hetero-epitaxy is present, a degree of lattice mismatch may be interposed between the growth film and the substrate, which causes thin film stress. 2.

Island growth (Volmer-Weber (V-W)) mode

V-W mode requires the initial placement of atoms on a bare substrate veneer; these ultimately become configured into an islet which may simultaneously expand or disintegrate into its component atoms. Before the thin film is generated, realignment can be performed across a broad spectrum; this leads to coarsening. It is not possible to establish the crystalline vectors within the growth film during their evolution. 3.

Mixed growth (Stranski–Krastanov (S-K)) mode

This technique encompasses growth that occurs in both a layered manner and via islet formation. Typically, layered growth is used at initiation, with subsequent islet development on one of the multiple growth strata. The QDs have been highlighted due to the fact that self-initiated development can facilitate the engineering of QDs with improved optical properties, and which can be utilised in devices based on traditional technology. The self-patterned growth can be achieved using MBE or by employing a metal organic chemical vapour deposition

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(MOCVD) technique. Three-dimensional islet materials can be generated automatically between two substance types, which have misaligned lattices; these can be produced with a particular growth array. An example of a growth strategy to produce indium arsenide QDs is described below; this illustrates the production methods for semiconductor QDs. Through using MBE methodology and the S-K mode of growth, indium arsenide QDs can be synthesised using the following process. Typically, a substrate is formed by GaAs with an n-doped 100 surface. The oxide on the veneer is initially eliminated with the use of hydrogen-ion plasma. Buffer layer growth then arises to a depth in the region of several hundred nanometres. Growth of the epitaxial stratum of indium arsenide is facilitated by a temperature within the range 450–550 ˚C; the depth of the layer rises in proportion to its evolution rate and over time. Patent QDs are not evident until 1.8 monolayers (ML) are present. This represents the threshold thickness and when this is achieved, a blueprint of self-assembled QDs (SAQDs) is created. Their parameters, i.e. dot dimensions, density, homogeneity and shape, amongst other characteristics, are dictated by the technique used for their evolution and process conditions. Processes that produce QDs of high quality are considered to be cutting-edge technology. If the QDs were coated with a gallium arsenide layer, this could produce a QD device comprising a sandwich configuration, which forms the active stratum in laser diodes. Additionally, in order to achieve carrier restrictions and optical confinement, in which resonant cavities can be engineered so as to obtain luminescence, both aspects of an active stratum can undergo barrier layer growth of the component with varying n or p titres of arsenic aluminium gallium alloy constituents. This enables the development of efficacious carriers that can enter the active layer in order to facilitate the presence of compound-emanating radiation. The extreme outer pl or nl layer, which is highly doped, is predominantly for the configuration of ohmic contacts with electrodes. According to QDs growth, the QD geometrical shape and surface density (QD/cm2 ) can be appreciated via field-emission scanning electron microscopy (FESEM). For QDs used in lasers, the developmental quality of QDs is crucial and highly responsive to the growth techniques and parameters utilised. In the case of vapour-phase MBE growth, the gallium arsenide substrate (100 or 111) crystalline size, doped configuration, growth temperature, arsenic gas pressure, III:IV element ratio, interruption type and duration, configuration and depth of cap stratum and the sub-monolayer growth array of indium and gallium may impact the standard of InAs/GaAs QDs produced. A two-dimensional FE-SEM image of these QDs is shown in Fig. 8.8. In addition to the utilisation of FE-SEM to appraise QDs density, high-resolution TEM can quantify QD dimensions with precision (height3 base area). AFM, and scanning tunnelling or probe microscopy, can additionally be employed in order to evaluate the size and shape of QDs. A two-dimensional AFM image of InAs/GaAs QDs is depicted in Fig. 8.9. The density of QDs normally lies within the spectrum 1 × 109 –1 × 1011 cm−2 . These figures represent tens of nanometres to only a few, respectively. The relevance of QD dimensions, density and growth parameters are

8.4 Laser Devices Based on QDs

161

Fig. 8.8 FE-SEM images of InAs/GaAs QDs [141]

Fig. 8.9 Two-dimensional AFM images of InAs/GaAs QDs [141]

discussed in depth below. Further information can be acquired from contemporary nanotechnology literature [142].

8.4 Laser Devices Based on QDs The QDs can be utilised for numerous key utilities. This is exemplified below in terms of the contemporary position of QDs with respect to the laser device sphere. Globally, since quantum well configurations were first designed [135], scientists have

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made numerous hypothetical endeavours to facilitate the incorporation of quantum mechanisms into the technology underlying semiconductor lasers. The focus then switched to the use of smaller sized quantum wires and QDs. Hypothetical computations were published in 1982 by Arakawa and Sakaki at Tokyo University that proposed that three-dimensional limitations apply to QDs, together with the energy DoS as a function of δ. They additionally forecast the output characteristics of a QD laser, which encompassed a low current threshold, high-temperature features and enhanced glow efficacy. In contrast to traditional lasers constructed from semiconductors, those using QD technology would exhibit improved thermal stability. It was postulated that the threshold current density within the QD configuration would be notably less than for a single-dimension quantum well configuration. This offered a novel avenue through which to resolve the issue of an excessively elevated threshold current density, which is often problematic in semiconductor lasers. At this juncture, QD synthesis was reliant on lithographic processes which impeded production of high-standard QDs of nanodimensions. Although experimental studies have gradually verified the hypothetical calculations, the manufacture of high-performance lasers using QDs has yet to succeed. For the last four decades, following the advent of semiconductor epitaxy methods, i.e. MOCVD and MBE, the growth of semiconductor thin film substances can now be regulated accurately in order to generate 2-dimensional restricted quantum well and superlattice substances. Constructed from two-dimensional materials including a confinement heteroconfiguration, optoelectronic device output, e.g. of lasers and detectors, has been notably enhanced. These merchandises became available for purchase and have been used in many applications. The positive results from this approach have inspired scientists to endeavour to limit electron motion in different planes and initiated numerous experimental approaches to the examination of quantum wires and QDs. Marzin et al., in 1984, reported that geometric stress has resulted in the formation of SAQDs as a result of the lattice mismatch amongst the arsenide and indium gallium arsenide heteroconfigurational strata. Concurrently, Kirstaedter and Ledentsov described the first ever self-patterned QD lasers on edge emission [144]. A gradient refractive rate was incorporated into the self-organised QDs together with single-layer index InGaAS/GaAs; this facilitated segregation of the heterostructure and the quantum well laser structure. The former offered an active medium, giving rise to a low current density threshold of 120 A cm2 and a temperature of only 77 K. Following these endeavours, much interest was drawn to SAQDs. A plethora of research has encompassed a spectrum of related areas, including the empirical characteristics of quantum devices and fabrication methodologies that have translated the research into positive outcomes. After this time, QD laser technology advanced swiftly. Increasing numbers of scientists are researching their low threshold current density and enhanced thermal stability together with a range of additional novel characteristics. 1996 and 1997 are 2 years that saw accelerated progress in QD laser technology. Numerous international research groups combined forces in order to stimulate concentrated research within this sector. In order to attain the baseline QD laser

8.4 Laser Devices Based on QDs

163

state, ideal parameters for QD growth have to be established in order to enhance QD dimension and morphological homogeneity. Contemporary detailed investigations have led to the manufacture of QD lasers with the potential to display a markedly lower current density threshold than that recognised in traditional and quantum well lasers. An active region for QD layers comprising ten In0.5 Ga0.5 As/A10.15 Ga0.85 As strata was employed by Ledentsov in 1996 in a QD superlattice configuration that in ambient conditions, led to a fall in threshold current density to 90 A/cm2 . Three years later, an InAS/In0.15 Ga0.85 As QD laser was produced by Liu et al. which exhibited a threshold current density of 26 A/cm2 . To date, HR-coated QD lasers have achieved threshold current density values of between 10 and 20 A/cm2 , i.e. two- to four-fold less than those derived from optimised quantum well lasers. In lasers incorporating an active region made up of multilayer QDs, the QDs from the individual layers may display a threshold current density as low as 7–10 A/cm2 . QD lasers exhibit increased temperature stability. The initial electric-pumped QD lasers were described in 1994 by Kirstaedter; these displayed good temperature stability between 150 and 180 ˚C although in ambient temperatures, the current density threshold’s thermal stability was less than that of industrial GaAs quantum well devices. QDs were positioned into GaAs/AlGaAs quantum wells by Maximov et al. in 1997. This strategy caused a rise in the carrier escape barrier height within the QDs, thus reducing carrier escape likelihood. Additionally, current leakage was diminished in order to generate a laser characteristic temperature, T 0 , of up to 385 K at a functional temperature within the spectrum 80–330 K, which is notably elevated compared to that of the quantum well laser. However, a rise in T 0 caused marked augmentation of the current density threshold. The first-ever GaAs-based QD laser was presented by Shernyakov in 1999; this was associated with an operating wavelength of 1.3 μm and was the first device to be operational at ambient temperatures whilst simultaneously exhibiting a T 0 of 160 K, together with a low current density, J th , of 65 A/cm2 , with triple layer QD arrays. For InP-based quantum well lasers that function within the same band, the greatest T 0 is between 60 and 70 K range, and the smallest J th between 300 and 400 A/cm2 . In order to engineer the most optimal QD lasers, the QDs should be of identical shape and size, i.e. comprising a sole single-electron and hole energy level, and be straightforward to attain in a single-mode process. The latter was achieved by Kirstaedter et al. in 1996, within a 77 K temperature and with the density value just above that of the current density threshold, i.e. < 1.1 × J th . Conversely, single-mode operationality of quantum well lasers necessitates them to be markedly above the threshold current density. Researchers, in 2004, at Tokyo and Fjuitsu University have had a positive outcome regarding the pilot evaluation of QD lasers that operated at a wavelength of 1.3 μm; the optical power vacillation as a result of the temperature was able to be modified to about one-sixth less than the original. These lasers have the ability to emit optical signals at 10 Gbit/s; no optical power alterations occurred as a consequence of the temperature adjustment. The lack of requisite for extrinsic circuitry facilitated an optimal transmitter of a smaller dimension and diminished the overall expense of the initiative. The scientists are now attempting to expand the laser’s modifiable working

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temperature spectrum to between 0 and 85 °C. These advances have added to the innovation of inexpensive diminutive optical signal transmitters that require lower power usage. This is anticipated to be advantageous for both the metropolitan (MAN) and local (LAN) area networks. The substances for the construction of QD lasers have now been recognised; however, to date, arsenide indium or gallium indium arsenide alloy remain the most optimal QDs, with III-V class growth on GaAs substrates. In order to engineer QD structures within the nanoscale, the narrow energy gap of indium arsenide can be modified via the x variable following the addition of a tiny quantity of the gallium constituent, i.e. Inx Ga1-x As, 0 ≤ x ≤ 1. It can additionally be utilised to generate a laser light source or devices that detect light using wavelengths within the 1.33– 1.55 μm spectrum, i.e. apposite for optical communication applications. It should be highlighted that these wavelengths are essential in laser lights as the optical fibres exhibit extremely low energy attrition in this band, thus rendering them ideal for optical communications over large distances. III-V QDs are principally appreciated because of their optical electronic utility, e.g. as high-frequency ultra-rapid electronic devices, high-frequency light-emanating devices and efficacious light detectors. Analytical instruments to evaluate the optical characteristics of QDs encompass photoluminescence, time-resolved photoluminescence, and the studies on temperature alterations that accompany power variations in the optical excitation or pumping, or in relation to cryogenic systems. The latter may acquire the radiation photon ranges arising from the recombination of the e-hole, the relaxation process and carrier time, life data and additional properties pertaining to a device. The QD laser development has made considerable recent advances; although the sector has staunchly challenged conventional semiconductor lasers, a significant void between output and postulated forecasts remains. A number of issues require resolution in order to promote QD laser performance further. The first problem is the growth of dimensionally heterogeneous QD patterns. Despite the fact that QD substances offer numerous possible benefits, their irregular dimensions and arrangements mean that the QD light-emanating zenith has an inconsistent widening. The luminescence apex is broad and higher than that observed for quantum well substances (meV). Indeed, just a tiny fraction of the QDs are involved in light emanation. This, therefore, restricts the optical advantage and so additional decreases in the lasing threshold are hard to attain. The second issue is the requirement to enhance QD surface and volume densities, respectively, in order to optimise material QD advantage. Thirdly, the configurations of QD lasers require upgrading so that they are favourable to QDs in terms of carrier capture and attachment. Finally, it is important to be able to regulate the dimensions of the QD or to choose a de novo material system in order to expand the functionality of the QD laser wavelength into the spectrum, 1.4–1.6 μm, for wavelength division multiplexing (WDM) networks [145]. For the InAS QD, it has contributed to the innovations relating to diode devices. Using a 1.3 μm wavelength and a low critical current density of approximately 19 A/cm2 , it has the capacity to drive an operative continuous single-mode wave at ambient temperature with 210 mW power. Currently, QD lasers outperform the

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165

InP quantum well laser, although the former are not entirely successful. In order to enhance the efficacy of the indium arsenide QD lasers, further experiments are merited which evaluate the effects of diminishing the critical current, upgrading the working device temperature, safeguarding against the ground-state and excited-state transformation, and promoting the performance of the laser luminescence. QDs derived from indium arsenide require additional investigation to examine a range of properties, e.g. high density, dimension and homogeneous QD epitaxial layer development, together with accurate quantification of their morphology and dimensions, as well as their internal and external constituents. Additional characteristics requiring appraisal are stress distributions of the various parameters, hypothetical data relating to the effects of various morphologies on the quantum band and energy levels, electrons present in the optical transition across levels of energy, relaxation processes underlying carrier energy, and carrier lifetime definition, amongst additional physical traits. Following future research endeavours, high-power and singlemode high-quality laser devices should ultimately exhibit greater efficacy and be able to function in higher temperatures within a wavelength spectrum of between 1.3 and 1.55 μm.

8.5 Single-Photon Source Specific to light nature, quantum optics describes empirical light-matter interactions. Semiconductors relate to physical laws and the applications of electronic materials and devices. Over the last 40 years, these two domains have evolved independently. In particular, studies within the sphere of quantum optics have emphasized optics at atomic and molecular levels. More recently, swift advances in relation to semiconductors have been achieved within the science and technological aspects of nanomaterials. Attention has been drawn to a number of the semiconductor mesoscopic and quantum systems, with the realisation that they encompass novel quantum optical characteristics. Thus, a de novo research domain was born, referred to as semiconductor quantum optics. Within the latter sphere, a subspecialty exists encompassing mesoscopic systems, which relates to the complexities of mesoscopic quantum optics. The most remarkable system within this field is the QD system. As stated previously, QDs are often referred to as artificial atoms, and possess almost comparable energy level configurations. Thus, it is conceivable that the quantum optical characteristics present in several atomic or molecular systems additionally exist in QD semiconductor systems. Semiconductor substances or devices that emanate light, e.g. lasers and lightemitting diodes, are serving as an intermediary to transform the carriers, i.e. the electron and the hole, into photons. If this process were rapid and efficacious, then the statistical carrier activity would undergo conversion to that of the photons. It is well established that electrons and holes follow Fermi-Dirac statistics, whereas photons are a type of boson. If devices were constructed appropriately, or present in certain mesoscopic semiconductors and quantum systems, the photon radiation

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statistics would be at variance to traditional electromagnetic waves; this type of light is frequently called non-classical light. The utility of quantum optics encompasses numerous spheres, of which the most predominant is the field of quantum data, such as quantum computing and communication. Deploying just a single-photon source is anticipated to promote the advent of quantum cryptography, which has much potential. Semiconductor quantum optics is particularly relevant for the evolution of the pragmatic utilities of quantum data, e.g. as a consequence of contemporary technology, the concept has been effectively utilised for the generation of extremely efficacious single-photon radiation. Furthermore, the operational wavelength can additionally be expanded to include the communication band of optical fibres, i.e. 1.3 μm. Advances have already been accomplished in the use of quantum cryptography for optical communications. A theoretical quantum optics basis was initially conceived in 1963 by Roy Glauber, who was awarded the Nobel Prize in Physics in 2005 for his superlative work in quantum optics. The notion of optical quantisation potentially originates from the blackbody radiation hypothesis postulated in 1900 by Max Planck, and from Einstein’s 1905 perspective of photoelectric effects. Nevertheless, the true advent of quantum optics theory is represented by an optical interferometer, frequently called the HB-T interferometer, which was innovated by two physicists, Hanbury-Brown and Twiss, over the period 1952–1956. This gadget was first utilised to observe Sirius and subsequently, to evaluate mercury’s coherent properties, which gave rise to the unexpected finding of a degree of positive coherence amongst the recognised photons. The schematic in Fig. 8.10a illustrates a HB-T interferometer. The incident light is separated into two parts by a beam splitter. Two-photon detectors are utilised to assess the incident light’s intensity coherence characteristics, typically described by the second-order coherence function, g(2) (τ ), where τ represents the time period between photon recognition by the two detectors (Fig. 8.10b). The parameter, g(2) (τ = 0), can be employed to differentiate the coherent traits. When contemplating particles’ incident light, g(2) (τ = 0) can be viewed as the likelihood of the two detectors picking up photons at the same time. This likelihood is

Fig. 8.10 a Schematic diagram of an HB-T interferometer device. A beam splitter divides the incident light into two beams. b Diagram of second-order coherence function on the photon bunching and anti-bunching phenomenon [145]

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heightened when g(2) (τ = 0) > 1. This indicates that the incident light waves exhibit positive coherence in the time between them. At this juncture, Purcell postulated that quantum statistics could explicate this observation. Since photons are a type of boson, when they are in the identical quantum condition they have a tendency to cluster and thus arrive at the two-photon detector simultaneously. Thus, the light phenomenon is frequently called a coherent cluster or bunching photon effect. Indeed, light’s coherent traits can also be viewed through the lens of the traditional electromagnetic theory, which created some debate at this time. A plausible rationale for the photon cluster effect seen in the HB-T studies was offered by Glauber in 1963, who presented the quantum hypothesis of optical coherence. Contemporary scientists are aware that the bunching effect is in fact linked with photon attributes from thermal radiation. It has no positive association with coherent light, such as a laser, i.e. g(2) (0) = 1. The quantum optics hypothesis offers an explanation of the photon clustering effect, and it is anticipated that the opposite would occur in some contexts, i.e. an antibunching effect. The latter was initially described in 1977 by Kimble, Dagenais and Mandel with respect to a lone sodium atom’s fluorescence. Since the anti-bunching light observation could not be explicated by the electromagnetic wave theory, immediate evidence relating to electromagnetic waves’ quantum nature was assumed to be relevant. Light with these traits has additionally been termed non-classical light. If a physical system were to display an anti-bunching effect, then two or more photons would not be radiated simultaneously, thus defining it as a single-photon source. Fermions typically display the anti-bunching phenomenon. If photons with antibunching characteristics were radiated by a single-photon light source, the light source would have fermion properties. Indeed, any lone autonomous quantum system may release single-photon radiation. A two-level system is illustrated in Fig. 8.11 to illustrate this point. It encompasses both ground and excited conditions. A ground state electron, when excited by light or electricity, can leap into the excited state and then, following instant emission, return to its original condition. Although this is a straightforward concept, it infers the phenomenon of non-classical light. Since the electron lies within the fermion range when it is excited but has not so far released any spontaneous radiation, a further electron cannot be promoted to the excited state. The period the initial electron is within the excited state is associated with the spontaneous radiation’s lifetime. Thus, within this duration, even if an enduring excitatory stimulus were applied to this system, the system could still not generate Fig. 8.11 Schematic diagram of a separate two-level quantum system [145]

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photons. Thus, an autonomous two-tier system would not be capable of yielding two or more photons and simply comprises a single-photon source. It is well recognised that a single-photon source can be manufactured with the use of numerous systems, such as single-atom or single-molecule systems. Nevertheless, the regulation of the stability of the latter necessitates involved technological processes that may be challenging to apply pragmatically. In addition to systems at the atomic and molecular levels, multiple solid phase systems can generate singlephoton radiation, e.g. the nitrogen vacancy core in diamond chemical combinations and semiconductor QDs. They have a trait in common with identical atomic and molecular energy levels. Problems 1. 2. 3. 4. 5.

Define the quantum confinement effect. Detail the calculation of exciton binding energy. Describe the methods of quantum dots preparation. Elaborate the laser devices-based quantum dots. What are the quantum optics applications?

Chapter 9

Superconductivity

9.1 Introduction The definition of superconductivity is the electrical flow along with a substance with an electrical resistance of zero. This is a property seen in metals at extremely low temperatures. However, superconductivity can be observed in alternative materials, e.g. ceramics, at higher temperatures. Substances that exhibit this characteristic are referred to as superconducting materials. Superconductivity was initially recognised in 1911 by Heike Kamerlingh Onnes who was a scientist at the University of Leiden. He noted that at the transition or critical temperature, T c ), i.e. 4.2 K, or 2268.8 °C, mercury abruptly lost its electrical resistance. Almost twenty years later, in 1933, Meissner reported that following the application of a magnetic field to a material in the superconducting condition, there was no penetration of the specimen by the magnetic field lines whilst sustaining the magnetic flux in the superconductor’s interior as zero. This was referred to as the Meissner effect; this forms a further property of superconducting substances. Their final feature is known as the Josephson effect. Superconducting electrons may be configured as Cooper pairs; these can traverse the barrier with a specific likelihood, referred to as the Cooper pair tunnelling effect. Thus, the Josephson effect, postulated in 1962, is the situation in which current penetrates two types of superconducting materials segregated by an interposing slender insulating layer. In 1973, Josephson won the Nobel Prize for this ground-breaking research. The superconductors are suitable for key utilities, e.g. functioning as low-loss current conductors, generating extremely potent magnetic fields and operating in microwave systems. Attention has been drawn to these materials as a consequence of their notable economic advantages. Pivotal data have been presented and Nobel Prizes have been won on several occasions during the last century for work in this field, e.g. in 1913, 1972, 1973, 1987 and 2003 [11].

© The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2022 Y. Al-Douri, Nanomaterials, https://doi.org/10.1007/978-981-19-3881-8_9

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A considerable number of advances have been made as a result of the numerous studies performed to examine superconducting substances. Copper oxide hightemperature superconducting materials generate the most elevated transition temperature, i.e. a T c of approximately 130 K. This can be applied in numerous systems as this is higher than liquid nitrogen’s boiling point. Globally, superconducting devices are unable to function in the absence of liquid nitrogen or liquid helium cooling systems; this is costly and impractical. If it was possible to attain superconductivity in ambient conditions, contemporary science and technology would be revolutionised (Fig. 9.1). For the last two decades ago, the investigation and recognition of de novo forms of superconductors have altered current perspectives on superconductivity. Previously, this feature was deemed to be an infrequent occurrence that was only observed at extremely low temperatures. It is now appreciated that superconductivity is the empirical condition of numerous substances, including those that have few metallic characteristics. Fig. 9.1 Development of superconductor materials [11]

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9.2 The Physical Principles of Superconductivity Since the superconductivity phenomenon was recognised, there has been much interest in delineating its underlying physical characteristics. After a number of years of research, Bardeen, Cooper and Schrieffer postulated that the property was derived from interactions between the electrons and the lattice, a hypothesis referred to as the BCS theory. Figure 9.2 demonstrates that as an electron traverses the crystal lattice as a consequence of the mutual attraction of opposite charges, positive charges within the crystal lattice’s positive ion dot matrix will be in proximity to electron A, leading to a rise in the positive charges’ partial density and a surplus of positive charges within that locality. This partial positive charge disruption may be disseminated as a crystalline lattice wave, which would then impact a further electron. Thus, electron A could, through a secondary mechanism, attract electron B. Surely, Coulomb repulsion exists amongst the electrons. Nevertheless, whilst the electron-lattice engagement remains robust, the secondary influence of attraction between the electrons is likely to be greater than that of Coulomb repulsion; thus, there is a net quantity of interelectron attraction such that a bound condition can occur. The latter gives rise to an electronic pair, which comprises two electron pairs, referred to as Cooper pairs. In relation to the momentum space, the two electrons are assigned a total momentum of the value, K. Cooper proposed that if K = 0, the binding energy would attain its peak, and the electron pairing would be characterised by an energy nadir. A quantum condition could be present within the two Cooper pair

Fig. 9.2 Diagram of the BCS theory: a as electrons get close to the crystal lattice, they will attract the positive charge on the lattice because of their negative charge; b the attracted cation may cause lattice deformation, while part of the electronic energy will be transmitted onto the lattice; c when the electrons pass through the lattice, the lattice deformation still remains. This creates a positively charged area to attract another electron, which may receive energy from the crystal lattice [11]

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electrons, which has an equivalent dimension but an opposing trajectory, additionally encompassing those with a reverse spin. In this scenario, the Cooper pair energy is lower than that of the two electrons in their natural state and so they exhibit greater stability [146]. Within Cooper pairs, each single-electron may exhibit a dissimilar momentum or velocity, although, for each Cooper pair, the sum of the momentum is uniform. The typical electron condition within the crystal lattice wave dispersion dictates metallic resistance. In the superconducting condition, the crystal lattice wave dispersion by the generated Cooper pairs is uninterrupted. The total momentum is preserved and so the current is unaltered, hence the reason why superconducting current is unchecked. A thermal vibration is intrinsic in the crystal lattice’s positive ions; when challenged, these act as lone warriors, a process which engenders energy usage in the course of electronic motion. This forms the mechanism underlying the resistance present in traditional metals.

9.3 The Superconductors Classification 9.3.1 Low-Temperature Superconductors The superconductors of a critical temperature below that of liquid helium are referred to as low-temperature superconductors. Following the recognition of the characteristics of mercury, scientists identified superconducting features in tin, lead and numerous additional elemental metals and alloys, as well as in composite substances, although their critical temperatures are still below that of liquid helium. After a long period of endeavour, it has now been appreciated that superconductivity is present in the majority of elemental metals. The advent of dedicated methodology, e.g. highpressure techniques, low-temperature thin film precipitation and ultrafast cooling, have enabled the condition of superconductivity to be exhibited by many metals, originally not thought to be able to display this property, when specific criteria are fulfilled.

9.3.2 High-Temperature Superconductors The low-temperature superconducting materials with a critical temperature less than that of liquid helium have restricted industrial utility. Thus, progress relating to their use was static for a considerable period prior to the recognition of high-temperature superconductors (HTSs). This pivotal discovery was made in 1987, in relation to iridium, barium and copper oxide, by the American academics, Mow-Kuen Wu and Paul Chu. They synthesised a HTS comprising YIBa2 Cu3 O7 , which had a T c between 90 and 100 °C, i.e. higher than nitrogen’s boiling temperature of 77 K. Substances

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with this property are referred to as HTSs. The recognition of HTSs incorporating copper was highly influential in this scientific and technological domain. This discovery demonstrates the evolution of superconducting substances within the last decade. Numerous utilities for these materials were already appreciated; however, they also transformed the fundamentals of condensed matter physics. The formulation of a de novo hypothetical framework is ongoing and the subject of some debate. A number of scientists support the concept that comprehension of the mechanism underlying HTSs is an unresolved issue and one that is essential to condensed matter physics. This is exemplified by the recognition of HTSs incorporating copper, which has demonstrated that superconductivity is not just present at low temperatures; it is now feasible in ambient conditions. Although the majority of HTSs that have been identified in the last three decades are ceramics inclusive of copper with an enhanced critical temperature, HTSs still lack the requisite stability. Their carrying capacity was additionally modestly diminished. The substances with the highest critical temperature are compounds of smallsized configuration; they all encompass light elements from the chemical formula. Their properties of high phonon density conditions and elevated energy are extremely appealing, but a general theory pertaining to HTSs cannot be postulated until the true major factor is delineated. Thus, considerable further investigation is required within the superconductor sphere [147].

9.3.3 Other Novel Superconductors Newly recognised superconductors are distributed across the periodic table, ranging from the lighter molecules, e.g. boron and lithium, to the transition metal uranium cohort and the numerous transfer salt complexes. In this setting, C60 exhibits future promise as it has a higher degree of elasticity than oxide ceramics, which are hard and brittle. This property means that the carbon compound is more straightforward to mould. Additionally, C60 is characterised by a more sizeable critical current, magnetic field and coherence length, characteristics which engender C60 superconductors more apposite from a pragmatic perspective. C60 is being recognised as a shining example of a novel twenty-first century substance, as it exhibits a broad spectrum of de novo characteristics together with the potential for utility in machinery, light electricity, magnetism and chemistry. A number of scientists are anticipating that when C240 and C540 are ultimately engineered they will display superconducting properties in ambient conditions [147]. There are specific superconductors with a more elevated T c , such as the alkali metal, ‘bucky ball’ or Ax C60 , and RNi2 B2 C and YPd2 B2 C, which have critical temperatures of 33 K and 23 K, respectively. There are some atypical superconductors, e.g. the heavy fermion materials, CeMIn5 (M = Co, Rh, Ir), CePt3 Si and PuCoGa5 , which exhibit a T c of 18 K. The measurements pertaining to Sr2 RuO4 relate to p-wave symmetry. In its antiferromagnetic order, ruthenium copper oxide is not restricted to HTSs despite them only being a few angstroms in distance.

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Magnesium diboride, MgB2 , is characterised by dual-band superconductivity and a superconducting T c of 39 K. The identification of the latter offered a novel approach for the exploration of a previously unrecognised cohort of HTSs which have uncomplicated constituents and configurations. MgB2 can be produced without difficulty for processing; it is a simple matter to engineer MgB2 films or wires. This material can be utilised for power transmission, supercomputer devices and computerised tomography imagers. Its identification created much interest within the condensed matter physics domain. Within the electronic sphere, superconducting boron-doped diamond has drawn the attention of many scientists. H2 O is linked with a further notable system of de novo substances. Scientists have observed that the T c of numerous elements becomes elevated in high pressure. When subjected to a pressure of 1 million Pascals, several elements exhibit a T c above 10 K, e.g. Li and S, Ca, and B, which have a T c of 17 K, 15 K and 11 K, respectively. The newly identified superconductor, CaC6 , exhibits a T c of 11.5 K, which is higher by a factor of 2 than the T c associated with electrode compounds derived from graphite.

9.4 Nanosuperconductors The attention of researchers has been drawn to the superconducting properties of nanomaterials for a range of reasons. Firstly, they can be utilised to engineer extremely diminutive superconducting devices, e.g. quantum bits, which are described subsequently. Secondly, when a system’s material dimensions, either all or several, are equivalent to or less than the characteristic dimensions of length, e.g. depth of penetration, coherence length, and grain and cell dimensions, de novo quantum effects will be observed. Superconductivity has the potential to be generated in conditions of ultrathin film, wires and QDs, i.e. in 2D, 1D and 0D systems, respectively. This holds true for artificially constructed configurations, e.g. knots, mesh or multi-strata heterostructures. An ultrathin film nanoconfiguration is observed in 2D superconductors. The material’s characteristics when it is in the superconductor condition are mostly dictated by its spatial parameters. 3D substances exhibit the most powerful superconducting potential. In wires and thin films that display superconductivity, thermal and quantum vacillations are key actors, with the ability to alter electron transport characteristics via a weak connection. Attrition of superconducting features is frequently observed when the material becomes diminutive or thin. Although the presence of superconduction has not been established in ultrathin films, the data from single-dimension superconducting coils show promise. As previously described, superconductors comprise current generated by electrons in the format of Cooper pairs. The individual electrons within each pairing generally have reverse spin and momentum; at a particular juncture, one electron has spin and shift vectors of up and to the left and the second, down and to the right,

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respectively. Application of the magnetic field will endeavour to disrupt the equilibrium of the electron motion whilst maintaining the spin trajectories of each electron in a direction towards the magnetic field. Owing to the requisite for sufficient energy to disrupt the Cooper pairing, superconductors have the capacity to tolerate a specific magnetic field strength prior to transformation into a normal conducting condition. This point is referred to as the critical magnetic field, which may restrict the utility of superconducting coils and negate the ability of such a structure to conduct high current; the magnetic fields engendered by such currents could therefore ameliorate or eradicate the superconducting properties of the coil [148]. Adams et al. have engineered thin beryllium with superconducting properties with a depth of between 2 and 30 nm in glass substrates, covered with a 0.4 nm precipitated gold layer. They determined that when the application of a magnetic field was in a vector parallel to the film, this product was able to tolerate a field much stronger than the field tolerated by a film synthesised from beryllium alone. In the films with the lowest thickness, i.e. 2 nm, the critical magnetic field was enhanced by a factor of 10. These researchers were of the opinion that, in this instance, the major factor was the engagement between the electron spin and the gold’s positive nuclear charge. Owing to the relative motion of the electrons and the nuclei, the impact of two charges on each other is analogous to the generation of current which, in turn would create a magnetic field. The gold nuclei induce a much greater magnetic field than the beryllium nuclei, and so the former would lead to a modest rearrangement of each Cooper electron pair towards the opposing vector. These disrupted electrons are more easily modified by an extrinsic magnetic field as their spin along the new vector is more in proximity to the magnetic field. Nevertheless, a magnetic field that was not in parallel with the film would eliminate the superconductivity through its impact on the electron motion equilibrium as opposed to the spin. Despite the fact that common utility is not likely in the immediate future, Adams et al. recognised the promise of the multi-layered sandwich configurations in the ultra-wire blueprint. The ultra-wire coils have a higher probability of initiating a more robust magnetic field than the currently utilised magnet, which forms a component of nuclear magnetic resonance tools and physics research domains. Singular effects, shown by these researchers, have offered the opportunity to assist scientists in their investigation of Cooper pairs within THSs, for which the property of superconductivity requires further elucidation. It is known that superconductivity is derived from the Cooper pairs created by spin-pairing electrons. These are eradicated when a strong magnetic field is applied to a superconductor. The electron spin is also impacted, and so the superconducting properties will be ameliorated or dissipate entirely. However, as the dimensions of the superconductor are diminished, the detrimental influence of the magnetic field will become less potent such that at the nanoscale, the magnetic field no longer affects the Cooper pairs. A physics professor at Illinois University, Bezlia Kim, affirmed this theory. His group positioned a carbon nanotube monolayer on a wafer and etched a channel with a width of 100 nm. The carbon nanotube surface was covered with a layer of superconducting material, i.e. MoGe. The temperature was diminished to below the critical temperature, and during the nanoscale superconducting material’s

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response, a forceful magnetic field was generated. It was determined that the influence of the latter was notably attenuated when superconductors were of nanodimensions. These workers postulated that given the ultrawire was only 10 nm in diameter, the interplay between the electronic Cooper pairs countered the impact of the magnetic field on the superconducting characteristics. This property of nanomaterials offers a wider spectrum of potential utilities for superconductivity. Previously, for instance, high current could not be carried by superconducting coils as the current-induced magnetic field could diminish or eradicate the coil’s superconducting properties. If the nanoscale superconducting coil were combined with superconducting filaments, then the transportation of strong current could be achieved in a straightforward manner. Furthermore, nanoscale superconducting materials can additionally be utilised in magnetic resonance imaging, as well as for additional purposes. Indeed, the nanoscale superconductors cannot be diminished indefinitely; if that were attainable, then the reciprocated interference amongst the Cooper pairs would mitigate against the superconductivity. Furthermore, it is not possible for the nanoscale superconducting substances to achieve zero resistance reliably, a property that is comparable to bulk superconductors. Over the last two decades, advances have been made relating to the engineering and definition of superconducting nanostructures, e.g. nanoparticles, QDs, nanowires, ultrathin film and monolayer superlattice substances. The length of numerous HTSs is only within the nanometre range, e.g. single-crystal domain dimensions and coherence length, amongst others, and so the experimental investigation of these nanoconfigurations is vital in order to facilitate the comprehension of the mechanisms underlying the observed properties of materials within this size spectrum. Multiexperiments have been conducted with respect to the zero-dimensional tunnel junction, e.g. studies in the system referred to as completely confined. The capacity in readiness for low capacitance and diminutive tunnel junctions inspired research on the conversion between the comparable Josephson and charge properties in a range of Josephson junctions and junction arrays. The second feature represents the phase paradigm of a physical example of the change from superconductor to insulator; the first demonstrates the dissipation that diminishes the quantum vacillations, thus alternating the characteristics of the superconductor. Via the latter process, the superconducting to insulation changeover can be regulated. The technology associated with the extremely diminutive tunnel junction configurations underpins the understanding of the quantum bit or qubit, which is a fundamental component required to engineer a quantum computer. Investigations into the double tunnelling junction have unveiled Coulomb blockade properties and odd–even effect that may be present in superconductors. The e-beam lithography development has facilitated the ability to engineer configurational design at a scale in the region of the superconducting coherence length. This has inspired studies within the physics sphere that have evaluated superconducting nanowires. These are synthesised utilising evolutionary strategies including lithography and electrochemical templates, the latter formed by carbon nanotubes.

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The inverse proximity effect has recently been described in superconducting wire, i.e. when the electrodes touching the wire additionally demonstrate superconducting features; the superconductivity of the wire per se is subsequently diminished. This may represent another instance of the dissipation regulation of superconductivity. Further research has demonstrated that once the superconducting wire’s random nature achieves a specific point, the superconductivity vanishes entirely and the wire undergoes transformation from superconductor to insulator, a quantum shift that is recognised in the generated nanowire template. The critical point can arise as either the critical resistance or diameter, where the former is in proximity to the quantum resistance (h/4e2 ), although the experiment demonstrated a value of 6 nm to be the critical diameter. The innovation of ultrathin superconductors that are two-dimensional has offered a means to study the topological phase transitions, e.g. the Kosterlitz-ThoulessBerezinskii transition [150], which is a classic instance of modern condensed matter physics. In films that exhibit disorder, an additional reduction in thickness induces transition to a superconducting insulator as opposed to metal transition. This can be moderated by altering the level of disorder, magnetic field strength or the electrostatic charge. This property, representative of a quantum phase transition, has attracted a lot of recent attention, despite the fact that the underlying theory behind it remains to be fully elucidated. The superlattices and heterostructures comprise configurations of a number of strata, with an interface interposed amongst the varied substances. Their characteristics are notably at variance with those of the raw constituents. Superconductivity has been evaluated deploying sandwich junctions, together with rival order measurements and their interplay. Specific focus has been placed on the border interposed between the superconductor (S) and ferromagnetic (F) or antiferromagnetic (AF) substances. In a configuration with three levels, e.g. FSF, the transport process arising via the magnetosphere caused the generation of a π-junction. Numerous substances comprised of a superlattice containing thin nanoscale strata have been engineered and assessed. Contemporary studies have been drawn towards the study of SFSFStype superlattices. Several non-traditional quantum properties have been identified in relation to superconducting thin films, e.g. critical temperature and current density vacillations in relation to the depth of the magnetic iron, which were anticipated and have now been verified experimentally.

9.4.1 Incredible Magnetic Nanoclusters A contemporary hypothesis has implied that globular clusters, containing a certain proportion of superconducting electrons, will exhibit properties of superconductivity at elevated temperatures. This theory may be proven for some straightforward metallic clusters, e.g. aluminium, gallium, zinc and cadmium; Ga56 is anticipated to have a critical temperature in the region of 150 K or −123 °C. Surface mesh metal

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formed from these clusters may be generated on the surface and carry the superconductor current at raised temperatures. A configuration, subsequently referred to as the shell structure and recognised in 1984 by Knight, underlies this observation, although at the time, this discovery failed to draw the attention of the superconductivity research sphere, being overlooked in favour of the exciting discovery, 2 years later of copper-incorporating HTSs. Current research evaluating nanoclusters is progressing, and there is potential for the identification of superconducting characteristics within these structures. Studies in this domain necessitate the artificial quantification of the excitation spectrum of the chosen cluster at temperatures that are low, but higher than the critical temperature. Potentially, at the latter, changes in the excitation spectrum will be observed as a result of the superconducting electron pairing processes. Pertinent instruments for the evaluation of these structures have evolved, e.g. mass spectrometry, creation of an energy beam within various temperatures, and photoelectron spectroscopy. A further advance is the development of isolated clusters within the matrix, together with molecular crystals that can lead to the evolution of an organised lattice which is three-dimensional.

9.4.2 Quantum Fluctuations and Strong Correlation in Nanowires The researchers did fail to come to a conclusion with respect to the superconducting nanowires regarding results related to the precise part enacted by quantum fluctuations. Much attention has been drawn to this question within this research sector, as it may restrict the length of the nanowire. The conventional Fermi liquid theory does not apply with a single-dimensional limit. In this case, the Tomonaga-Luttinger liquid theory [151] is relevant, although this does not encompass a quasi-particle; entirely decoupled and individually excitable spin and charge degrees of freedom define the excitation system. This has been reported in filaments founded on GaAs heterostructures. Any association between superconductivity and the Luttinger liquid, or the potential of the latter being a superconductor per se, are questions yet to be resolved.

9.4.3 Ultrathin Film Within this domain, ongoing dilemmas are including: (i) the presence of an intermediary metallic scheme in systems that are two-dimensional, comprising Cooper pairs in a manner similar to that of ultrathin films as opposed to the non-electronic quasi-particle format, e.g. the charge carrier; (ii) the properties of the insulating state,

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i.e. a de novo condition of Bose insulator matter; and (iii) the existence of any comparable features between the superconducting characteristics of ultrathin films and the pseudo-gap regime within a copper-containing superconductor that is underdoped.

9.4.4 Nanosuperconductors and Hybrid Structures There are multiconcept within the nanoconductor sphere, such as the Josephson array, shot noise and dynamics, Andreev state and associated activities, diminutive Josephson circuit and the vortex dynamics relating to the Josephson array, nonstationary effect in mesoscale superconductors and ferromagnetic superconducting heterogeneous configurations. MBE, used at the atomic level, has led to the marked growth of formats comprising multiple strata. This technique can additionally be employed to create innovative functional composite configurations, encompassing spin effects. This type of structure would offer an empirical system for studying the interplay between the magnetic order and the areas displaying superconductivity.

9.4.5 Links Between Superconductors and Nanostructure In combination, nanotechnology and the proximity effect have the ability to enhance superconductors’ conventional electron carriage and magnetic properties. This influence was anticipated and then validated experimentally many years previously, but only contemporary work has led to the widening of the potential spectrum of applicability owing to the technological advances made with respect to nanostructure manufacture. One instance would be the interplay between nanoconfigured magnetic particle arrays and thin film superconductors, during which concurrent engagement with the vortex lattice may cause a pinning effect. Electron-beam lithography or selfassembly, together with sputtering or MBE and additional methods of thin film deposition, can be used to facilitate the engineering of magnetic particle arrays. Cyclical alterations in the degree of magnetic particle array resistivity and magnetisation are exhibited as a result of the pinning array geometrical configuration. A ratchet effect is induced by asymmetric magnetic nanostructures. The interplay between the vortex and nanostructured configurations is assumed to arise within a specific frequency spectrum in order to promote a Josephson effect such that the extrinsic magnetic field can regulate the frequency band. Regarding to the proximity effect relating to the two substances, the boundary states and nanointerface geometry in contact with the superconductors will engender the creation of a de novo electronic configuration. For instance, the frequently arising electronic reflection from the superconductor’s interface may transform to include Andrew reflections, which would give rise to de novo standing-wave conditions in specimens of diminutive dimensions.

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9.5 Nanosuperconductor Applications Nanomaterials can be utilised for the purposes of ultra-small superconducting devices, e.g. quantum computers. The nanosuperconducting Josephson junction, acting as a qubit, forms a perfect contender for use in this context, together with several quantum bits [152].

9.5.1 Quantum Computers Quantum computers are a category of physical devices capable of ultrafast logical and mathematical functionality that can hold and process quantum data in accordance with quantum mechanics laws. If a device’s processing methods and computation were underpinned by quantum data, and quantum algorithms were operated, it could be deemed to be a quantum computer. This notion originates from research relating to reverse computers, with the underlying incentive of dealing with the problems associated with computer energy usage. During the years encompassing the 1960s and 1970s, the researchers have noted that computer chips could overheat as a result of energy consumption. This impacted the integration scale and restricted their processing speed. Research revealed that energy usage arises from irreversible computational functions, an observation which led to the question of whether every computation requires an irreversible operation that must be concluded. In relation to traditional computers, there is the potential for a matching reversible computer with unaltered computing power. Since the individual operational phases can be transformed into a reversible procedure, this, in quantum mechanics, can be described as a unitary transformation. Primitive quantum computers were traditional computers defined by quantum mechanics linguistics. Key characteristics of quantum mechanisms, e.g. quantum superposition and coherence, did not contribute to their functions. The empirical data unit in traditional computers is the bit; the object of computing is a range of bit sequences. Comparably, in a quantum computer, the data unit is the quantum bit; qubit sequences are targeted by the various functions. They diverge owing to the siting of quantum bit sequences at orthogonal superposition conditions and additionally, at the entangled condition. Such quantum states not only offer the potential for parallel quantum computing but also give rise to a number of advantageous properties. Compared to traditional devices, quantum computers have the capacity to carry out arbitrary unitary conversion. Acquiring the output condition, quantification can generate the computing data, thus extending the spectrum of conventional computations significantly. Within the mathematics sphere, traditional computing can be viewed as a selective category of quantum computation. Quantum computers have the ability to convert a stack of each constituent; these actions are concluded simultaneously and would be stacked at a particular probability rate in order to generate the output, a function that is beyond traditional computer capabilities (Fig. 9.3).

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Fig. 9.3 Comparison of traditional bits and qubits [11]

The combined use of quantum simulation computing or quantum parallel computing is necessary in order to exploit quantum coherence, although at present it is challenging to sustain this property within a practical setting. Quantum bits within quantum computers do not form a segregated system, but engage with the extrinsic surroundings. This behaviour causes quantum coherence to be attrited, a characteristic referred to as decoherence. Thus, a key objective is to engineer a quantum computer that surmounts this issue. The most efficacious method to achieve this was recognised with the identification of quantum coding. The principal quantum coding categories include quantum error-correcting and error-avoiding codes. The former reflects the traditional analogue equivalent that is a group of codes that has received much scientific interest. Its advantage is its broad spectrum of utility, but it has the disadvantage of a lower degree of efficiency. To date, globally, quantum computers are not available in a practical context. Nevertheless, numerous institutions are fervently endeavouring to meet this objective, with a plethora of techniques being put forward relating to ways in which to make quantum computers a reality. The main issue is that experimental engineering within

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the micro-quantum condition is extremely arduous. Contemporary programmes are predominantly founded on the interplay between the atoms and cavity, together with effects from cold trap bound ions, electron or nuclear spin resonance, QD manipulation and superconducting quantum interference. Which option holds the most potential is hard to appreciate. An admixture of the quantum point and superconducting Josephson junction schema may be most apposite for amalgamation and downscaling. It is likely that a de novo blueprint will be created, which will supersede the present ideas, potentially based on a completely new substance. The notion of quantum computers is not conceived as a substitute for the present traditional variety but instead, represents a new approach to computation offering diverse phenomena, including optical computers and biocomputers. Thus, their contribution is likely to be greatly in excess of simply addressing the unresolvable issues that plague their traditional counterparts.

9.5.2 Nanosuperconductor Quantum Bits The control and production of a qubit system that is stable is a major problem in the pragmatic utilisation of contemporary quantum data and the physical instigation of quantum computing techniques. In non-solid qubit systems, positive results relating to the function of quantum logic gates and straightforward quantum algorithms have been reported. However, pragmatic quantum computing has to surmount numerous challenges, both defined and undefined, and to surpass principles and non-principles. In order to perform computations on quantum computers for pragmatic physics purposes, there are a number of requisites, i.e. the organic amalgamation of universal quantum logic gates, maintenance of quantum entanglement amongst qubits, and the capacity to manoeuvre them. If the number, N, of qubit integration rises, the quantum decoherence would be augmented exponentially by the power, e. Contemporary research has demonstrated that the attrition of quantum coherence of a lone particle may diminish at the index of e. However, if simultaneous quantum entanglement were in place, it would nearly be eradicated in a restricted time interval, an outcome that implies that a multi-qubit integrated in a straightforward manner would exhibit more susceptible quantum coherence. Essentially, this is a difficulty faced by the present hypothetical and experimental material [152]. Quantum systems at the microscopic level, e.g. photons, spin and atoms, are simple to segregate from the environment, which thus diminishes decoherence and represents a benefit of this particular quantum system. Nevertheless, the microscopic quantum system fails to enable the integration required to produce true quantum computing devices. From this perspective, the macrosystem exhibits greater pliability in the utilisation of the technological properties of routine integrated circuits. The presently postulated macroscopic quantum bits are founded on nanoconfigured electronic circuits, QD constituents or the superconducting Josephson junction. Quantum bits relating to the latter can be categorised into three cohorts, i.e. charge, flux and matching hybrid qubits, respectively.

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Fig. 9.4 Diagram of a two-level system. It is constituted by the spin 1/2 particles in a magnetic field. Two basic quantum states correspond to spin orientations upward and downward [11]

Actually, any physical system can be utilised as qubits if it can be described without ambiguity within the two-tier condition, e.g. the spin ½ particle within a magnetic field with the spin only +1/2 or −1/2 aligned along the field’s axis comprises a two-level system (Fig. 9.4). Previously, scientists reported the property of macroscopic quantum transport in superconductors. Owing to the superconductor coherence seen in Cooper electron pairs, the entire population of microelectron pairs can be viewed as a lone microwave function. The Josephson junction system demonstrates the tunnelling characteristic exhibited by the macroscopic quantum wave function. Currently, the latter can be exploited to manufacture quantum devices that offer several benefits, e.g. simple functionality, more prolonged coherence time and capacity to integrate for synthesis as a possible quantum bit system. Previous superconducting quantum devices that have been engineered encompass the quantum interference (SQUID) and single-electron devices which include flux and charge qubits, respectively. Early verification has indicated that they form outstanding quantum bits. Their underlying principles and properties are described subsequently [153]. A flux qubit is illustrated in Fig. 9.5a. The SQUID quantum interference device comprises a superconducting ring associated with a single or a number of Josephson

Fig. 9.5 a A typical circuit diagram of magnetic flux quantum bits. b SQUID energy changes with the internal magnetic flux. External magnetic flux Φext = Φ0 /2, and the horizontal level is representative of a mixed energy state [11]

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junctions. The SQUID level is a two-tier system (Fig. 9.5). The Josephson junction makes two contributions, i.e. to segregate the superconducting phases on the left and right aspects, and to enable the passage of superconducting Cooper pairs via electron tunnelling. The superconducting phase difference between the left and right aspects, γ , dictates the superconducting current traversing the Josephson junction. Where γ = 0, or an integer multiple of 2π, there is zero current and the system is in the lowest energy condition. Typically, the energy of the Josephson junction can be described as −E J cos γ . Of note, is that when placed in an extrinsic perpendicular magnetic field, the latter will generate a rise in a single phase of superconducting quantum wave functions in the trajectory of the ring. From a mathematical perspective, this heightened phase is described as the line integral of vector spaces, comparable to the magnetic flux passing across the superconducting rings. Owing to the singlevalued property of the superconducting wave function, the phase combined with the extrinsic magnetic field must be countered by the phase difference amongst the two Josephson junction terminals. If the phase difference was not equal to zero and if an enclosing positive current, referred to as a persistent current, were generated by the superconducting ring in a manner equivalent to a shielding current, then the persistent current would initiate a magnetic flux which can be expressed as Φeff = L I , where L represents the superconducting ring self-inductance. The superconducting phase difference of the Josephson junction fulfils the criteria of the following formula: Σ k

γk =

2π φ + 2nπ φ0

(9.1)

where the magnetic flux within the ring is represented by Φ, and encompasses the applied extrinsic magnetic field, Φext , and the summed effect of continuous current. Φ0 indicates a constant and Φ0 = h/2e is the formula for the magnetic field’s magnetic flux that causes a 2π phase increase. The charge qubits are another kind of quantum bit that has superconductivity. The phase conjugate physical quantities, i.e. electric charge, can be utilised as two fundamental quantum bit conditions. The theory of charge qubits is comparable to that of single-electron devices. A standard superconducting single-electron device is shown in Fig. 9.6a. Additionally, the more diminutive Josephson junction region of linked devices, together with the lesser volume of the core superconducting area, e.g. the islet or box, may subsequently diminish the comparable capacitance of the latter. This generates Cooper pairs that exhibit a charging energy, E, for introduction to the central islet that is higher than the heat energy disruption. Thus, at low temperatures, the access of the Cooper pairs is impeded; the entire line becomes involved in a breaking circuit activity or charge effect. However, if the gate voltage was used in order to modify the core islet’s potential energy, the electron energy for the Cooper pair access would be altered to zero. The line then transforms into an access but the charge transfer has to be transported an electron at a time [154]. As the charge effect is enhanced, the degree of islet charge rises in import as a physical amount. Overall, the system’s

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Fig. 9.6 a Typical circuit diagram of charge qubits. b The dotted line denotes charging energy at n = −1, 0, 1. Solid line represents the energy curve of the charge qubit in quantum state at different ng. Josephson coupling energy makes it possible to produce anti-crossing of energy levels at ng = ½ [11]

Hamiltonian can be expressed as: ( )2 H = 4 E c n − n g − E J cos θ

(9.2)

where the islet’s charge quantity defined in the Cooper electron population is indicated by n, θ reflects the core islet’s superconducting phase and ng indicates the comparable charge quantity generated by the gate voltage. Since θ and n are conjugates, and since Josephson energy gives rise to a contribution of +1 or −1 of the Cooper electron pair number on the islet, the following can be written [154]: H=

Σ n

Σ ( )2 4 E c n − n g |n⟩ ⟨n| − E J (|n + 1⟩ ⟨n| + |n − 1⟩ ⟨n|)

(9.3)

n

Since between 0 and 1, ng is altered, n = 0 and n = 1 are the two lowest energy conditions; if only these were taken into account, then a two-tiered condition would be present. The magnetic fields along the z and x vectors are defined in a spin ½ model, i.e. 4E C (1 − 2ng) and E J , respectively. The perfect superconducting charge qubit would be the link between the left and right aspects to engineer a Josephson junction comprising two superconducting rings, equivalent to a quantum interference device, and referred to as the controlled SQUID. The advantage is that when the magnetic flux passing across the superconducting rings is modified, the Josephson coupling can be adapted using the previous theory, which gives rise to E J = E 0J cos|π φ/ φ0 |. Thus, the dimension and vector of the analogous magnetic field can be governed through adjustment of the gate voltage and magnetic flux. Such devices do not exist in a straightforward two-tier condition. In order to abridge the infinite charge conditions to just two, the energy gap for the individual energy states has to be augmented, which necessitates E C ≫ E J . Really, many forms of superconducting devices can be utilised as quantum bits. An opposite two-tier condition that can be determined is not automatically needed to represent the magnetic flux or charge as, for instance, a lone Josephson junction can be

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employed as qubits. In the latter system, there is a natural √ generation of energy levels which have equal intervals described by: ℏw p = 2 E c E J . The current passing through the Josephson junction slants the potential energy wells, which have a wavelike form, in a manner analogous to the Josephson effect. In specific circumstances, an additional two phase states that exhibit stability can be attained, whereas the alternative higher energy states would attenuate rapidly. The two residual stable conditions form the requisite quantum bit systems. These can be engineered further by adjusting the current dimension and the Josephson coupling energy. However, the consequences are more complex, as for the magnetic flux and charge qubits alluded to earlier. A further potential enhancement is to exploit a Josephson π junction to take over from the current 0 junction. If the phase difference was zero, then the general Josephson junction and π junction would exhibit currents of zero, respectively. The latter is comparable to translation of the 0 junction’s potential energy curves over the distance, π. Without a magnetic field, this enables the superconducting rings utilising a π junction to attain the two magnetic flux conditions naturally from energy dissipation. Currently, three forms of π junctions exist able to be utilised in experimental work: (i) the d-wave characteristics of HTSs where the bonding is concluded following a 90° rotation of the superconductor crystal plane; (ii) the addition of a magnetic metallic stratum to the superconductor junction centre so that the flipping of the magnetic material’s electron spin alters the phase of the tunnelling Cooper electron pairs; and (iii) which also involves a metal layer being interposed within the superconductor junction centre, but which has undergone high-energy electron injection. Within the metal strata, the non-thermal equilibrium condition’s electronic distribution can instigate alterations in the association between the superconducting current and phase. Since the latter is sited within an extremely high-energy excited condition, such a system is likely to exhibit a brief coherence time. Thus, studies evaluating quantum bits are more susceptible to impediments. For an individual device, they can be refined from the 0 to π junction. Hypothetically, a further specific Josephson junction has been postulated, comprising an admixed s-wave-d-wave-s-wave; −E J cos(2γ ) defines the junction’s recipient energy. When connected in parallel state with the general Josephson junction, this could additionally form a quantum bit device [155]. Problems 1. 2. 3. 4. 5.

Define the superconductivity. Classify types of superconductivity. Elaborate the nanosuperconductivity with unconventional quantum phenomena. What are the nanosuperconductor applications? What is the Josphon junction’s role in nanosuperconductor?

Chapter 10

Nanomaterial Multi-application

10.1 Introduction According to Canham [156], there have been ongoing accounts of porous diodes based on silicon, i.e. LEDS, together with photodetectors and opticallogic gates, amongst other devices. PSi LEDs were first described by Richter et al. [157] and Koshida and Koyama [158]. The configuration of the device is illustrated in Fig. 10.1; it reflects the characteristic design of a preliminary form of LED, demonstrating an EL spectrum zenith at approximately 680 nm. The depicted LED has a semi-transparent metal-porous silicon layer p-type silicon-Al electrode; the device has extremely restricted longevity with an extrinsic quantum efficiency of just 10−5 %. Subsequently, efficacy has been enhanced in the region of 0.2%, which is trending towards the pragmatic industrial requisite of approximately 1% [159–161]. Such devices are configured from indium tin oxide (ITO)-p + Psi layer-n− and Al/poly-as-contact-n+ PSi layer-p− substrates, respectively, and a structure of p+ nn+ PSi. Typically, the EL and PL PSi spectra are wide, equivalent to alternative forms of nanostructures and nanoclusters. A notable decrease in the breadth of the EL spectrum and the radiative decay time is attained through the admixture of micro-cavities in order to improve the output of the LED [162]. This gives rise to the potential for PSi LED incorporation into regular bipolar circuitry. The longevity of the device has been extended to some extent [164, 165] by encasing the porous silicon within aluminium and aluminium oxide. The porous silicon veener is oxidised in order to circumvent additional oxidative processes and generates stable passive layers with radiative cores. Improved stability is generally seen with the oxide passivated configurations, but these tend to be less efficacious. The marked fragility and reactivity of porous silicon is a major drawback; this is challenging to surmount, although enhanced morphology has been achieved using PSi that has undergone cautious etching [166]. In comparison to an UV-enhanced Si photodiode which, at 740 nm, was observed to have an external quantum efficiency of 75%, it was reported that a photodetector of a Al/RTO structure comprising rapidly thermally oxidised PSi/p-Si/Al had a greater © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2022 Y. Al-Douri, Nanomaterials, https://doi.org/10.1007/978-981-19-3881-8_10

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Fig. 10.1 Schematic of one of the first Psi LED [157]

sensitivity at 350 nm [167]. Various etching parameters can give rise to specific PSi porosities, enabling refinement of the substance’s refractive index, which, in turn, facilitates the synthesis of the disseminated Bragg reflectors through the interspersion of high-porosity strata, 0.25 wavelength in thickness, between low-porosity layers.

10.2 Amorphous Silicon/Oxide Superlattice Researchers have documented robust visible PL arising from a superlattice comprised of amorphous-Si/Si02 . If the depth of the superlattice amorphous silicon layer was altered from 3 to 1 nm, the PL zenith could be relocated from 1.7 to 2.3 eV. The rationale of the visible light emanation encompasses the quantum confinement properties of the electrons within the two-dimensional strata. Whilst the depth of the amorphous silicon is less than the coherence length, from a quantum trait perspective, it becomes less critical as to whether interference by the restricted electrons occurs within either an amorphous or crystalline layer. Nevertheless, in contrast to crystalline bonding, amorphous bonding enhances plasticity and gives rise to a greater range of bonding flaws, which eventually generate heightened recombination and scattering cores. Over time, gradual progress has been made with respect to the synthesis of hydrogenated amorphous silicon photovoltaic substances.

10.3 Single-Electron Transistor

189

10.3 Single-Electron Transistor A broad spectrum of quantum devices are now in existence [170]. The single-electron transistor (SET) has been chosen as an example as its principles encompass a number of the previously described topics. The context of this device includes the current transport of discrete electronic conditions within a QD. A SET has the potential to offer notably high density with extremely diminutive power attenuation as well as functionality at ambient temperatures. It could be utilised in nanoscaled memory, capacitance and logic gate devices, amongst others. The single-electron phenomenon has been recognised for some time [172]. The first description of single-electron charging in association with a field effect transistor (FET) was published over a decade previously. Chou and Wang presented the initial experimental report of a novel FET of nanoscale dimensions with a lone barrier in a one-dimensional channel. It was demonstrated to be paramount to restrict the electrons using a couple of barriers although, under apposite circumstances, a sole barrier could give rise to the single-electron charging effects. Tiwari et al. described a memory device in 1996 that had the capacity to use nanocrystallites composed of silicon [175]. Stability of the device was demonstrated with no loss of output following in excess of 109 operational cycles, which although notable, is still under the requisite 1013 cycles. Diagrammatic cross-section and band profiles obtained during the write and erasure cycles representing device electron injection and extraction, respectively, are illustrated in Fig. 10.2. The inversion surface of an n-channel Si FET is segregated by a slender tunnelling oxide of between 1.1 and 1.8 nm thickness from a Si nanocrystal film comprising 5 nm crystallites of 1 × 1012 cm−2 density disseminated across the whole surface channel area. A deeper tunnelling oxide of a minimum depth of 4.5 nm is interposed between the nanocrystals and the FET control gate. The electron injection arises through immediate tunnelling from the inversion layer; this occurs when there is forward bias of the control gate in relation to the source and drain. The consequent stored charge obscures the gate charge and thus diminishes conduction within the inversion stratum. Nanostructures fabricated using a of electron beam lithography and reactive ion etching offer a further strategy to attain single-electron MOS memory [176], which can function at ambient temperatures [177]. The configuration comprises a narrow channel MOS FET; its breadth is approximately 10 nm, a dimension that is lower than the single-electron Debye screening length, together with a polysilicon dot of nanodimensions, i.e. 7 × 7 nm. The latter forms a floating gate entrenched between the channel and control gate (Fig. 10.3). There is a slim natural oxide stratum at the grain boundary that forms a potential barrier even in the absence of the addition of tunnel oxide between the channel and floating gate. This strategy restricted the variation in the dimensions of the channel, floating gate and tunnel barrier [101], and of the overall dimension of the silicon nanocrystal spread [175]. Storage of a single electron causes a stepwise incremental change in threshold voltage; this staircase association between the charging voltage and threshold alteration is typical of a charging process that is self-restricting.

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Fig. 10.2 a A Schematic cross-section and b band diagram during injection, c storage, and d removal of an electron from a nanocrystal [175]

Fig. 10.3 Schematic of a single-electron MOS memory that has a narrow silicon channel and a nanoscale polysilicon dot as the floating gate. The cross-section view illustrates the floating gate and the channel region [176]

Nanocapacitance devices based on the single electron are utilising nanoscale semiconductor and metal particles as the functional device components [178]. Where the particles are of apposite low dimension, i.e. within a diameter of between 2 and 4 nm, the particle’s typical single-electron charging energies are notably higher than k B T in ambient conditions, indicating the potential functionality of single-electron devices derived from metal nanocrystals at room temperature.

10.3 Single-Electron Transistor

191

A method for the synthesis of single-electron capacitance devices has been described by Markovich et al. [179], which employed a two-dimensional tightly packaged organically operationalised silver nanocrystal stratum. They described properties suggestive of a Coulomb blockade, a stepwise configuration analogous to the Coulomb staircase, and memory charging properties. The empirical configuration of the device, together with its comparable circuit and operation, is presented in Fig. 10.4; the dielectric space layer is formed by polymethylmethacrylate (PMMA). A novel strategy with respect to digital circuitry was postulated by Ohshima and Kiehi; this was founded on bi-stability in locked single-electron tunnelling oscillations [180–182]. Binary logic states are fundamental to this blueprint; these are linked with the single-electron tunnel junctions’ tunnelling phases and injected at a two-fold rate compared with the tunnelling frequency and triggered by recognising the do bias. The diagram in Fig. 10.5 illustrates the circuit suggested; a shared ac source, V P , is used to pump tunnelling junctions, defined by a capacitance, C. Oscillation locking and the circuit timing reference are driven by the pump at ωp . The empirical mechanism underlying this process is the creation of single-electron tunnelling vacillations

Fig. 10.4 a Cross-sectional schematic of a device (not to scale); b equivalent RC circuit; c equivalent energy-level diagram. In (b), C represents the Al/Al2 03 -nanocrystal junction capacitance, R is the same junction’s tunnelling resistance and C 2 represents the nanocrystal-PMMA-AI junction capacitance. In (c), regions 1 and 5 are AI electrode layers, region 2 is an Al2 03 layer (1 nm), and region 3 is the metal nanocrystal monolayer (5 nm) and region 4 is the PMMA insulating spacer layer (30–40 nm) [44]

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Fig. 10.5 TPL of three coupled stages. Ultrasmall tunnel junctions (double-box symbols) with capacitance C are pumped by a common AC source and do bias by clocked sources through a load resistance [44]

at ωset . These are phase-synchronised with the pump, generating a multi-stability effect owing to the indeterminate phase association described as ωset and ωset = ωp = n ωset , where n is equal to 2. Clocking the do bias for the individual gates facilitates the clicking of phase data along the gates. If there were a rise in the do bias, the gate would be triggered, causing subharmonic locking. The capacitive coupling of the gates means that the ultimate condition is impacted by the former gate’s condition. Thus, the characteristics required for the possible utilities in digital logic circuitry are offered by the single-electron tunnelling phenomena and pump coordination, which engender phase bi-stability. Self-generated Si QDs have been investigated for use in resonant tunnelling devices which function in ambient conditions [182]. Developed using low-pressure CVD, the hemispherical Si QD dimensions, i.e. diameter and height ranges of 5– 20 nm and 1–10 nm, respectively, give rise to robust carrier confinement within the Si nanodot, thus facilitating detection of a negative differential conductance in ambient conditions. The tunnelling device is made up of a two-fold barrier comprising n+ Si(100), SiO2 (3 nm in thickness), Si nanodots and native SiO2 . This represents a key advance and requires reproduction at reduced temperatures in order to optimise the NDC. Nanopillars of silicon can be used in order to generate a SET that functions with the use of a nanoscale vibrating arm [138]. This form of device is referred to as a nanoelectromechanical system (NEM). With a length of approximately 200 nm, the features of transistor silicon arm are solely achieved by the application of an AC voltage with an analogous frequency to the arm’s resonant frequency in order to achieve arm vibration across the electrodes. In this instance, this was within the range 350–400 MHz. An electron flow from

10.4 Quantum Dot Laser

193

source to islet was attained; from the islet, tunnelling of the electrons occurred in the direction of the drain.

10.4 Quantum Dot Laser Stranski-Krastanow (SK) mode and island growth within hetero-epitaxial systems that are notably strained, e.g. InGaAs on GaAs, represent a utility of self-assembled QDs within the optoelectronic domain. These flawless QDs can be manufactured in a straightforward manner utilising techniques, such as traditional molecular beam epitaxy (MBE), metal-organic vapour phase epitaxy (MOVPE), or metal-organic chemical vapour deposition (MOCVD). There is a homogeneous dimensional spread of these dots of approximately 90%; densities of up to 1011 cm−2 are observed. Their PL efficiencies have been measured as equivalent or superior to those of the reference quantum well [184]. Edge-emitting lasers with a low threshold have been constructed using these QDs as the active stratum. Within a short space of time, the QD verticalcavity surface-emitting laser (VCSEL) was presented. This, which again exploits QDs in the active portion, has greater appeal as it has the benefit of regulating the electron and photon conditions within a microcavity configuration. The coordination of the cavity mode with the QD slim bandwidth light emanation gives rise to a superior output light source with an extremely small-sized threshold current. The bandwidth governs the VCSEL’s temperature properties and can be modified by regulating the distribution of the dot dimensions. Thus, the QD VCSEL may reflect an ideal optical device, which uses QDs. The properties of InGaAs QD VCSEL have been described when functioning within ambient conditions using a continuous-wave current of 32 pA [185]. The configuration of VCSEL evolved using MBE is illustrated in Fig. 10.6. AlAs/GaAs distributed Bragg reflectors (DBRs, n-doped, spacer layer of AlGaAs and bottom 18 period are initially developed using a GaAs substrate. Within the DBR,

Fig. 10.6 Quantum dot VCSEL with 10-period InGaAs dots in the active region. On average, the dots are 28 nm in diameter and 3 nm in height with a density of 2 × 1010 cm−2 , covering 12% of the surface [180]

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Fig. 10.7 Schematic illustration of the oxide-confined VCSEL containing the In0.50 Ga0.35 Al0.15 As active region [86]

the AlAs stratum has a depth of 64.9 nm, and the GaAs, a depth of 51.5 nm. The interposed layer is a graded constituent stratum of AlGaAs, which is 18 nm thick. Subsequently, growth, at a temperature of 520 °C, of 10 periods of In0.5 Ga0.5 As dot/Al0.25 Ga0.75 is performed. There are consecutive depositions of alternating 0.2 monolayer coatings of GaAs and InAs, respectively, which encompasses a pause of 2 s. The AlGaAs outer stratum on the dots is predisposed to smoothing out, thus giving rise to an equivalent growth quality as that induced by the first layer. Once this is achieved, a p-type AlGaAs spacer layer and top 14.5-period DBRs are developed within a temperature of 600 °C. A wafer of the material as evolved exhibited a reflectivity spectrum, which indicated that a 960 nm wavelength characterised the core of the DBR mirror and the resonant cavity. Given the QD height dimension of 3 nm, in ambient conditions, the quantum confinement effects are predominant compared with those induced by thermal energy. Huffaker et al. [186] have resulted a low threshold, oxide-confined (7 µm aperture) VCSEL based on a self-assembled QD active region. The operating condition is at room temperature under the pulsed threshold of 560 µA. The schematic cross-section is shown in Fig. 10.7. The structure consists of a lower DBR of 26 n-type AlAs/GaAs quarter-wave pairs, a half-wave cavity spacer consisting of lower n-type and upper p-type Al0.99 Ga0.01 As layers cladding the QD active region, and a single upper ptype GaAs quarter-wave layer followed by heavily p-type Al0.75 Ga0.25 As and GaAs contact layers. The superior DBR is concluded by half a dozen further MgF/ZnSe quarter-wave pairs placed by deploying electron beam evaporation. The final densely p-doped strata are fabricated in order to be specifically etched from the cavity of the VCSEL before deposition of the metals. The active portion of the QD is generated by the deposition of 5 In0.50 Ga0.35 A10.15 As monolayers interposed between GaAs and AlGaAs barriers, which are without doping and graded, respectively. During the evolution of the DBRs and cavity spacer, the temperature of the substrate is maintained at approximately 620 °C and then diminished to in the region of 500 °C for QD deposition and development. The QDs are allowed to assemble during a 20-s pause following the quintuple monolayer placement; RHEED is utilised for close observation. AFM

10.5 Epilogue

195

images revealed the QDs, comprising In0.50 Ga0.35 AI0.15 As and engineered with the six deposited monolayers, to have a lateral dimension of approximately 20 nm, and a sizeable density of around 1011 cm−2 . Narukawa et al. [187] examined a laser diode, which was in the purple spectrum of 420 nm, and which contained self-assembled QDs in its active portion. Seven periods of undoped In0.20 Ga0.80 N (3 nm)/In0.05 Ga0.95 N (6 nm) MQW comprised the active layer, which was interposed between individual 0.1 µm and 0.41 µm waveguiding and cladding strata of GaN and A10.15 Ga0.85 N, respectively. These QDs were sited within the In-dense well sections; their dimensions ranged from 3 to 5 ran, with a diameter zenith of 3 nm. In a manner comparable to that of a QD laser, the self-constructed islet forms a straightforward approach [188, 189], which includes a triad of InAs islet monolayers interspersed amongst 5.5 nm GaAs strata. These LEDs emanate light over a wide range of wavelengths, with a 120 nm characteristic line width and a zenith within the range 1000–1100 nm. In self-organised ln(Ga)AS QD lasers in ambient conditions, there is a restriction on the 1.0–1.3 µm modulation bandwidth of approximately 6–8 GHz owing to hot carrier effects produced by the principal wetting strata/barrier condition occupation by the electrons injected into the active area. Adjuncts, such as tunnel injection and p-doping, have enhanced the output of such lasers which emit at 1.1 and 1.3 pm. A small-signal modulation bandwidth and compression factor of 22 GHz and 8.2 × 10−16 cm3 , respectively, were described in association with undoped 1.1 µm tunnel injection lasers [190]. Using the electronic component molecules as a de novo trajectory in nanoscale research and technological spheres, the eventual goal would be to engineer a device in which electrons had the ability to jump on and off a lone atom between a pair of contacts. This notion has been investigated with two associated molecules comprising a Co ion attached to polypyridyl ligands, linked to insulating chains of various lengths [191]. Altering the dimensions of the latter changes the ion-electrode coupling, facilitating the synthesis of devices that display single-electron characteristics, e.g. Coulomb blockade.

10.5 Epilogue As nanoscale device, there are a number of restrictions to downscaling. For instance, doping barriers cannot be down-sized owing to the solid solubility restrictions applicable to the doping agents utilised. The immediate nature of heterojunctions permits diminution to the nanoscale, but they then become subject to quantum effects, of which some are detrimental to the output of a device. Diminishing carrier mobility causes a reduction in the coherence length of the carrier which can eradicate quantum effects, but at the cost of the dissipation of power and velocity. Essentially, the wave characteristics of electronics dictate quantum effects; these govern both positive and negative interactions, generating de novo plasticities and opportunities. Discrete relationships between energy and momentum are typical of

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quantised systems and facilitate novel utilities. The attrition of dielectric screening and capacitance, and the rise in the binding energies of doping energies can lead to challenges in the doping of nanoparticles, and induce unwanted properties. The empirical principle of charging a capacitor needs to be adapted to encompass the construct that it is not charge but an electron that is stored, and that in a quantum context, the electron possesses kinetic energy. Within a nanoparticle, the broadly segregated electron energy conditions create stepwise current changes or zeniths. However, with improved comprehension and blueprints, numerous apparently detrimental traits could be utilised productively. This is a vast topic. This review is, therefore, somewhat restricted, and predominantly concentrates on contemporary studied systems [188–191].

10.6 Chemical and Biological Sensors Figure 10.8 shows a schematic illustration of these types of applications. Ligand is attached covalently or sometimes physically onto the nanoparticle surface and the acceptor is attached to the target substrate in a similar way. Targets can include chemical entities, biological subjects and colloidal particles. The recognition between the ligand and the acceptor occurs when there is a chemical process such as complexation, biological complementarity, or geometrical matching. Detection is confirmed through the identification of the properties of the nanoparticles that are selectively attached to the target substrates. For example, when the anthrax virus is targeted by using DNA-modified gold nanoparticles, the colour of the sample is changed as a result of their aggregation [192].

Fig. 10.8 Schematic illustration of nanoparticles in chemical or biological sensory applications

10.7 Optical Sensors

197

In addition, nanoparticles with different sizes/shapes that are modified with different ligands can be applied as a noninvasive optical (fluorescence) marker both in vitro and in vivo. Those with one size/shape with a certain ligand will bind on the specific target that has the right acceptor; then the properties of the nanoparticles such as their given colour can indicate the detection of that target.

10.7 Optical Sensors Figure 10.9 shows a schematic illustration of these types of applications. The band gap (Eg ) of the semiconductor nanoparticles increases as their size decreases. This induces the increase of the emission energy, meaning the blue shift as the size decreases. Since the degeneracy of the electronic structure is also delicately dependent on the shape and composition of the nanoparticles, the band gap can be even more precisely tuned by manipulating these factors, mainly because of the different surface polarisation. Applications can range from simple optical sensors to fabricated devices such as photovoltaic cells and light-emitting diodes (LEDs) [193]. The surface scattering from nanoparticles is dependent on the above structural factors, too. This leads to applications ranging from consumer products such as sunscreen to surface-enhanced Raman scattering (SERS). The surface plasmon band, which is a broad absorption band of noble metal nanoparticles in aqueous solution, is located in the visible region [194].

Fig. 10.9 Schematic illustration of nanoparticles in optical sensor applications

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10.8 Catalysis Based on the surface atom effect, nanoparticles have (1) increased total surface area, (2) increased number of atoms accessible on the surface, (3) increased catalytic activity of those atoms and (4) different (tunable) surface catalytic properties by the change in shape, size and composition. These are the keys to their use as catalysts [195].

10.9 Future Issues We have discussed the applications of semiconductor, metal and oxide nanoparticles. Self-assembly plays a crucial role in the synthesis of nanoparticles by providing geometrical confinement during decomposition, nucleation and growth of precursor molecules. This confinement is spatial or surface, depending on the types of self-assembled systems. By tuning the intermolecular forces of the self-assembly processes or self-assembled aggregates, the structural properties of the nanoparticles synthesised, including size, shape and composition, can be easily controlled. The noble properties of nanoparticles can be precisely tuned by controlling these structural factors [196]. In the sense that there are plenty of elements that can be explored as a possible form of nanoparticles, and that there are plenty of new unexplored self-assembly systems available, further advances in nanoparticles look promising. Here are some of the future issues: 1.

2.

Structure-Property Relationship. Once the structure of the self-assembly building unit and the environmental conditions are acquired, the self-assembly process and the structure of the subsequent self-assembled aggregates can be obtained with a reasonable accuracy. A well-established relationship of the structures of the self-assembly building units (ligands, capping agents, and amphiphiles) to the physicochemical properties of the nanoparticles synthesised will provide better control of their structural/chemical features. This will also help expand the scope of the nanoparticles that can be synthesised through self-assembly. For nanoparticle properties, the structure-property relationship between spherical properties, and also the relationship between nonspherical properties, will be among important future challenges. Even a slight alteration in the structure of the nanoparticles often ends up causing a dramatic change in their physical and chemical properties. Systematic studies and the construction of the generalised relationship for the properties of the nanoparticles across the wide range of their structures will provide a much better pathway for expediting their applications. Force Balance within Confined Space or Surface. There are constant changes in the intermolecular forces during the synthesis of nanoparticles. This is due to the

10.9 Future Issues

3.

199

constant change in the concentration of the precursors and to the constant evolution of the intermediate species. This strongly affects the force balance for the self-assembled aggregates, and therefore the packing geometry. For nanoparticle synthesis, this new balance will be in the confined space or surface, which means that the effect will be more dramatic than in the case of the synthesis of nanostructured materials. Along with the above structure-property relationship issue, a proper understanding of this fact will provide better insight into the general synthetic mechanism for the synthesis of nanoparticles. Surface Functionalisation. The ultimate applications of nanoparticles will come in the form of nanodevices. This will require the controllable assembly of nanoparticles across desired and, in many cases, varied species of them into unified but functionalised contents. This is controllable nanofabrication. In regard to advancements in nanoparticles, this issue challenges us to develop the concept of control of the intermolecular/colloidal forces between nanoparticles. Controlled functionalisation of the surface of nanoparticles coupled with well-developed organics (ligands, capping agents and amphiphiles) and proper control of their surfaces even without the organics will provide diverse routes to fulfilling this purpose. Also, the often-revealed ‘coupled properties’ (noble properties of nanoparticles that are coupled with organics or that are induced by changes in the interparticle distances) will provide a wider spectrum for their application.

Problems 1. 2.

3. 4. 5.

Describe three methods for treating Cl-containing volatile organic compounds (VOC) and identify their characteristic features. Transition metals such as Fe and V (added to MgO) are said to promote the mineralisation of Cl-containing VOC into MgO. What is the role of the transition metal? Calculate the volume % of spherical balls that occupy a space by the closepacking structure. Identify advantages and disadvantages by comparing fixed bed system for the reaction between gas and nanoscale materials for industrial use. Compare reversible adsorption, irreversible adsorption and mineralisation reactions with respect to the process.

Blurb

Nanotechnologies use very small objects or artefacts. The nanomaterials have increasingly interest as a product of nanotechnologies. They have nanoparticles with a size less than 100 nm in at least one dimension. The nanomaterials have multiuse in healthcare, electronics, cosmetics and others. Their physical and chemical characteristics have different properties than bulk structure. This is needing to cover health risks for the workers, consumers and potential risks to the environment. It is done on a case-by-case basis, but risk assessment method needs to be kept updated as utilising of nanomaterials expands, especially for the consumer products. What do we know about the health risks exposing to nanomaterials, and how can improv them? The assessment includes synthesis, analysis and characterization to explore many of advantages for the readers and interests.

© The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2022 Y. Al-Douri, Nanomaterials, https://doi.org/10.1007/978-981-19-3881-8

201

References

1. H. Hosono, Y. Hishima, H. Takezoe, K.J.D. Mackenzie, Nanomaterials: From Research to Application (Elsevier, the Netherlands, 2006) 2. D. Vollath, Nanomaterials: An Introduction to Synthesis, Properties and Application (Wiley, Germany, 2008) 3. H.-G. Rubahn, Basics of Nanotechnology (Wiley, Germany, 2008) 4. C.N.R. Rao, A. Muller, A.K. Cheetham, Nanomaterials Chemistry (Wiley, Germany, 2007) 5. R.W. Siegel, Nanostrucutred materials, in Advanced Topics in Materials Science and Engineering, ed. by J.L. Moran-Lopes, J.M. Sanches (Plenum, New York, 1993) 6. K.J. Klabunde, R.M. Richards, Nanoscale Materials in Chemistry (Wiley, New Jersey, 2009) 7. J.K. Klabunde, Nanoscale Materials in Chemistry (Wiley, Canada, 2001) 8. C. Kumar, Semiconductor Nanomaterials (Wiley, USA, 2010) 9. H.W. Kroto, J.R. Heath, S.C. O’Brien, R.F. Curland, R.E. Smalley, Nature 318, 162 (1985) 10. D.S. Yan, D. Feng, Nova of Materials: ¾ Nano Materials Science (Hunan Science and Technology Press, Changsha, 1998) 11. D. Shi, Z. Guo, N. Bedfor, Nanomaterials and Devices (Elsevier, Amsterdam, 2015) 12. A.J. Cox, J.G. Louderback, S.E. Apsel, L.A. Bloomfield, Phys. Rev. B 49, 12295 (1994) 13. M.S. Cao, Introduction to Nano-materials, 1st edn. (Harbin Institute of Technology Press, Harbin, 2007) 14. Z.K. Zhang, Z.L. Cui, Nano-technology and Nano-materials (National Defence Industry Press, Beijing, 2000) 15. L.D. Zhang, J.M. Mou, Nano-materials and Nano-structures (Science Press, Beijing, 2001), p. 144 16. R. Denton, B. Muhlschlegel, D.J. Scalapino, Phys. Rev. Lett. 26, 707 (1971) 17. T. Takagahara, Phys. Rev. B 47, 4569 (1993) 18. L. Brus, J. Chem. Phys. 79, 5566 (1983) 19. T.S. Ahmadi, Z.L. Wang, T.C. Green, A. Henglein, M.A. El-Sayed, Science 272, 1924 (1996) 20. Y. Xie, Y. Qian, W. Wang, S. Zhang, Y. Zhang, Science 272, 1926 (1996) 21. E. Matijevi´c, J. Colloid Interface Sci. 58, 374 (1977) 22. I. Capek, Adv. Colloid Interface Sci. 110, 49 (2004) 23. P. Barnickel, A. Wokaun, W. Sager, H.F. Eicke, J. Colloid Interface Sci. 148, 80 (1992) 24. H.W. Kroto, J.R. Heath, S.C. O’Brien, R.F. Curl, R.E. Smalley, Nature 318, 162 (1985) 25. W. Krätschmer, L.D. Lamb, K. Fostiropoulos, D.R. Huffman, Nature 347, 354 (1990) 26. S. Zhou, C. Burger, B. Chu, M. Sawamura, N. Nagahama, M. Toganoh, U.E. Hackler, H. Isobe, E. Nakamura, Science 291, 1944 (2001) 27. Z.W. Pan, Z.R. Dai, Z.L. Wang, Science 291, 1947 (2001) 28. C.R. Martin, Science 266, 1961 (1994) 29. T. Thurn-Albrecht, J. Schotter, G.A. Kästle, N. Emley, T. Shibauchi, L. Krusin-Elbaum, K. Guarini, C.T. Black, M.T. Tuominen, T.P. Russell, Science 290, 2126 (2000) © The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2022 Y. Al-Douri, Nanomaterials, https://doi.org/10.1007/978-981-19-3881-8

203

204

References

30. 31. 32. 33.

A.M. Morales, C.M. Lieber, Science 279, 208 (1998) S. Kodambaka, J. Tersoff, M.C. Reuter, F.M. Ross, Science 316, 729 (2007) X.F. Duan, Y. Huang, Y. Cui, J.F. Wang, C.M. Lieber, Nature 409, 66 (2001) Y.F. Zhu, Characterization and Testing of Nano-materials (Chemical Industry Press, Beijing, 2006) Y. Martin, H.K. Wickramasinghe, Appl. Phys. Lett. 50, 1455 (1987) M. Kerker, The Scattering of Light and Other Electromagnetic Radiation (Academic, New York, NY, 1969) J. Colchero, H. Bielefeldt, A. Ruf, M. Hipp, O. Marti, J. Mlynek, Phys. Stat. Sol. (a) 131, 73 (1992) S.N. Magonov, M.-H. Whangbo, Surface Analysis with STM and AFM (VCH, Weinheim, 1996) Y.B. Cai, Y.C. Liang, Charact. Struct. Chem. 20, 425 (2001) R.M. Overney, E. Meyer, J. Frommer, D. Brodbeck, R. Luthi, L. Howald, Nature 359, 133 (1992) G. Binnig, C.F. Quate, C. Gerber, Phys. Rev. Lett. 56, 930 (1986) T.Q. Zhao, W.M. Zhou, Y.J. Liang, S. Dong, Lett. Biotechnol. 13, 68 (2002) E.O. Hall, Proc. Roy. Soc. London B 64, 474 (1951) N.J. Petch, J. Iron Steel Inst. 174, 25 (1953) C.C. Koch, Nanostructured Materials: Processing, Properties and Applications (William Andrew, USA, 2007) Y.S. Lee, Self-assembly and Nanotechnology (Wiley, New Jersey, 2008) Y. Yoshizawa, S. Oguma, K. Yamauchi, J. Appl. Phys. 64, 6044 (1988) N. Hasegawa, M. Saito, J. Mag. Soc. Jpn. 14, 313 (1990); IEEE Trans. J. Magn. Jpn. 6, 91 (1991) A. Makino, T. Bitoh, A. Kojima, A. Inoue, T. Masumoto, J. Magn. Magn. Mater. 215–216, 288 (2000) A. Makino, T. Bitch, A. Kojima, A. Inoue, T. Masumoto, J. Appl. Phys. 87, 7100 (2000) A. Makino, T. Bitoh, A. Kojima, A. Inoue, T. Masumoto, Mater. Sci. Eng. A304–306, 1083 (1997) A. Makino, in Novel Nanocrystalline Alloys and Magnetic Materials, ed. by B. Cantor. (Institute of Physics Publishing, Bristol and Philadelphia, 2004) H. Gleiter, Prog. Mater. Sci. 33, 223 (1989) G.W. Nieman, J.R. Weertman, R.W. Siegel, J. Mater. Res. 6, 1012 (1991) V. Kristic, U. Erb, G. Palumbo, Scripta Met. Mater. 29, 1501 (1993) P.G. Sanders, J.A. Eastman, J.R. Weertman, Acta Mater. 45, 4019 (1997) J.S. Cao, J.J. Hunsinger, O. Elkedim, Scripta Mater. 46, 55 (2002) T.D. Shen, C.C. Koch, T.Y. Tsui, G.M. Pharr, J. Mater. Res. 10, 2892 (1995) K. Zhang, J.R. Weertman, J.A. Eastman, Appl. Phys. Lett. 87, 061921 (2005) K. Zhang, J.R. Weertman, J.A. Eastman, Appl. Phys. Lett. 85, 5197 (2004); W.A. Soer, J.Th.M. De Hosson, A.M. Minor, J.W. Morris, E.A. Stach, Acta Mater. 52, 5783 (2004) E. Bonetti, E.G. Campari, L. Del Bianco, L. Pasquini, E. Sampaolesi, Nanostruct. Mater. 11, 709 (1999) Z. Huang, L.Y. Gu, J.R. Weertman, Scripta Mater. 37, 1071 (1997) S. Sakai, H. Tanimoto, H. Mizubayashi, Acta Mater. 47, 211 (1999) K.M. Youssef, R.O. Scattergood, K.L. Murty, J.A. Horton, C.C. Koch, Appl. Phys. Lett. 87, 09104 (2005) A. Budrovic, H. Van Swygenhoven, P.M. Derlet, S. Van Petegem, B. Schmitt, Science 304, 273 (2004) R.A. Masamura, P.M. Hazzledine, C.S. Pande, Acta Mater. 46, 4527 (1998) P.G. Sanders, C.J. Youngdahl, J.R. Weertman, Mater. Sci. Eng. A 234–236, 77 (1997) S. Cheng, E. Ma, Y.M. Wang, L.J. Kesckes, K.M. Youssef, C.C. Koch, U.P. Trociewitz, K. Han, Acta Mater. 53, 1521 (2005) Y.M. Wang, K. Wang, D. Pan, K. Lu, K.J. Hemker, E. Ma, Scripta Mater. 48, 1581 (2003)

34. 35. 36. 37. 38. 39. 40. 41. 42. 43. 44. 45. 46. 47. 48. 49. 50. 51. 52. 53. 54. 55. 56. 57. 58. 59. 60. 61. 62. 63. 64. 65. 66. 67. 68.

References

205

69. E.O. Hall, Proc. Phys. Soc. London B 64, 747 (1951) 70. N.J. Petch, J. Iron Steel Inst. 174, 25 (1953) 71. J.R. Weertman, D. Farkas, K. Hemker, H. Kung, M. Mayo, R. Mitra, H. Van Swygenhoven, MRS Bull. 24, 44 (1999) 72. C.C. Koch, D.G. Morris, K. Lu, A. Inoue, MRS Bull. 24, 55 (1999) 73. S.G. Kim, A. Inoue, T. Masumoto, Mater. Trans. JIM 31, 929 (1990) 74. A. Inoue, T. Nakamura, N. Nishiyama, T. Masumoto, Mater. Try. JIM 33, 937 (1992) 75. A. Inoue, N. Nishiyama, S.G. Kim, T. Masumoto, Mater. Trans. JLV 33, 360 (1992) 76. A. Kato, H. Horikiri, A. Inoue, T. Masumoto, Mater. Sci. Eng. A 179/180, 707 (1994) 77. A. Kato, T. Suganuma, H. Horikiri, Y. Kawamura, A. Inoue, T. Masumoto, Mater. Sci. Eng. A 179(180), 112 (1994) 78. A. Inoue, A. Takeuchi, Mater. Trans. 43, 1892 (2002) 79. W. Unsworth, Mater. Product. Technol. 4, 359 (1989) 80. A. Uoya, T. Shibata, K. Higashi, A. Inoue, T. Masumoto, J. Res. 11, 2731 (1996) 81. A. Makino, T. Bitoh, A. Inoue, T. Masumoto, Scr. Mater. 48, 869 (2003) 82. A. Niakino, T. Bitoh, J. Appl. Phys. 93, 6522 (2003) 83. T. Bitoh, A. Makino, A. Inoue, T. Masumoto, Mater. Trans. 44, 2011 (2003) 84. G. Herzer, Scr. Metall. Mater. 33, 1741 (1995) 85. K. Suzuki, J.M. Cadogan, Phys. Rev. B 58, 2730 (1998) 86. C.S. Kim, S.B. Kim, J.S. Lee, T.H. Noh, J. Appl. Phys. 79, 5459 (1996) 87. N. Saito, H. Hiroyoshi, K. Fukamichi, Y. Nakagawa, J. Phys. F16, 911 (1986) 88. Y. Hayakawa, A. Makino, Nanostruct. Mater. 6, 985 (1995) 89. N. Hasegawa, M. Saito, A. Makino, J. Mgn. Soc. Jpn. 18, 750 (1994) 90. H. Weller, Adv. Mater. 5, 2 (1993) 91. G. Schön, U. Simon, Colloid Polym. Sci. 273, 202 (1995) 92. U. Simon, G. Schön, G. Schmid, Angew. Chem. Int. Ed. Engl. 232, 250 (1993) 93. G. Schmid, R. Pfeil, R. Boese, F. Bandermann, S. Mayer, G.H.M. Calis, J.W.A. Van der Velden, Chem. Ber. 114, 3634 (1981) 94. R. Landauer, IBM J. Res. Dev. 1, 223 (1957) 95. Yu. V. Sharvin, Sov. Phys. JETP 21, 655 (1965) 96. G. Schön, U. Simon, Colloid Polym. Sci. 273, 101 (1995) 97. A.N. Korotkov, D.V. Averin, K.K. Likharev, S.A. Vasenko, in Single-Electron Tunneling and Mesoscopic Devices, ed. by H. Koch, H. Lübbig (Springer, Berlin, Heidelberg, New York, 1992) 98. J.M. Martinis, M. Nahum, H.D. Jensen, Phys. Rev. Lett. 72, 904 (1994) 99. P.D. Tougaw, C.S. Lent, J. Appl. Phys. 75, 1818 (1994); J.R. Tucker, J. Appl. Phys. 72, 4399 (1992) 100. K. Nakazato, R.J. Blaikie, H. Ahmed, J. Appl. Phys. 75, 5123 (1994); K. Yano, T. Ishii, T. Sano, T. Mine, F. Murai, K. Seki, in IEEE International Solid-State Circuits Conference, 1996, pp. 266–267 101. W.R. Browne, B.L. Feringa, Nat. Nanotechnol. 1, 25 (2006) 102. U. Seifert, Adv. Phys. 46, 13 (1997) 103. F.C. Simmel, W.U. Dittmer, Small l, 284 (2005) 104. A.M. Fennimore, T.D. Yuzvinsky, W.-Q. Han, M.S. Fuhrer, J. Cumings, A. Zettl, Nature 424, 408 (2003) 105. S. De Franceschi, L. Kouwenhoven, Nature 417, 701 (2002) 106. S.J. Wind, J. Appenzeller, R. Martel, V. Derycke, P. Avouris, Appl. Phys. Lett. 80, 3817 (2002) 107. Molecular Electronics: Nanocomputing Marches Forward, C&EN 7 (2001) 108. J.R. Heath, M.A. Ratner, Phys. Today 43 (2003) 109. A.W. Castleman, S.N. Khanna, Cluster and nanoscale science: overview and perspective, in Quantum Phenomena in Clusters and Nanostructures, ed. by S.N. Khanna, A.W. Castleman (Springer, New York, 2003) 110. C.L. Cleveland, W.D. Luedtke, U. Landman, Phys. Rev. B 60, 5065 (1999) 111. F. Delogu, Phys. Rev. B 72, 205418 (2005)

206

References

112. 113. 114. 115. 116.

A.S. Barnard, X.M. Lin, L.A. Curtiss, J. Phys. Chem. B 109, 24465 (2005) K. Koga, T. Ikeshoji, K.-I. Sugawara, Phys. Rev. Lett. 92, 115507 (2004) R. Meyer, L.J. Lewis, S. Prakash, P. Entel, Phys. Rev. B 68, 104303 (2003) S. Kodiyalam, R.K. Kalia, A. Nakano, P. Vashishta, Phys. Rev. Lett. 93, 203401 (2004) T.J. Campbell, R.K. Kalia, A. Nakano, P. Vashishta, S. Ogata, S. Rodgers, Phys. Rev. Lett. 82, 4866 (1999) T.J. Campbell, G. Aral, S. Ogata, R.K. Kalia, A. Nakano, P. Vashishta, Phys. Rev. B 71, 205413 (2005) O.A. Shenderova, V. Zhirnov, D.W. Brenner, Crit. Rev. Solid State Mater. Sci. 32, 347 (2002) P.C. Millett, R.P. Selvam, A. Saxene, Acta. Mater. 54, 297 (2006) M.I. Baskes, Phys. Rev. B 46, 2727 (1992) R. Car, M. Parrinello, Phys. Rev. Lett. 55, 2471 (1985) A.K. Geim, K.S. Novoselov, Nat. Mater. 6, 183 (2007) R. Iser, Nanoelectronics and Information Technology—Advanced Electronic Materials and Novel Devices, second, corrected edn. (Wiley, Weinheim, Germany, 2005) T. Chakraborty, F. Peeters, U. Sivan, Nano-physics & Bio-electronics: A New Odyssey (Elsevier, Amsterdam, 2002) Q.D. Chen, Physics 26, 483 (2004) J.-Y. Zhang, Y.-Z. Cai, An introduction to single-electron transistor (SET). Nanosci. Nanotechnol. 19 (2006) H.W.C. Postma, T. Teepen, Z. Yao, Science 293, 76 (2001) J.F. Jiang, Q.Y. Cai, Nature 23, 10 (2001) Y. Zheng, S. Yi, G.U. Shu-Lin, S.N Bo, Z. Rong, Z. You-Ke, Physics 31, 624 (2002) Z. Yang, Y. Shi, S.L. Gu, B. Shen, R. Zhang, Y.D. Zheng, Res. Prog. Solid State Electron. 22, 131 (2002) Z. Yao, H.W.C. Postma, B. Leon, C. Dekker, Nature 402, 273 (1999) C. Zhou, J. Kong, E. Yenilmez, H. Dai, Science 290, 1552 (1999) S.J. Tans, A.R.M. Verschueren, Dekker. Nature 393, 49 (1997) V. Derycke, R. Martel, J. Appenzeller, P. Avouris, Nano Lett. 9, 453 (2001) L. Esaki, R. Tsu, IBM J. Res. Dev. 14, 61 (1970) A.P. Alivisatos, Science 271, 933 (1996) Y. Fu, W. Lu, Semiconductor Quantum Devices, 1st edn., vol. 1 (China Science Press, Beijing, 2005) Y.T. Lu, Physics 25, 497 (2003) A.S. Edelesten, R.C. Cammarata (eds.), Nanomaterials—Synthesis, Properties and Applications (Institute of Physics Publishing, Bristol, 1996) Z.Z. Guo, X.X. Liang, S.L. Ban, Phys. Stat. Sol. (b) 238, 173 (2003) G.Z. Zhan, Physics 25, 1 (2003) M. Sugawara, Self-assembled InGaAs/GaAs Quantum Dots (Academic, 1999) J.-Y. Marzin, J.-M. Ge´rard, A. Izrae¨l, D. Barrier, G. Bastard, Phys. Rev. Lett. 73, 716 (1994) N. Kirstaedter, N.N. Ledentsov, M. Grundmann, D. Bimberg, V.M. Ustinov, S.S. Ruvimov, Electron. Lett. 30, 1416 (1994) W.H. Zhang, Z.M. Xu, Physics 28, 851 (2006) K. Huang, R.Q. Han, Solid State Physics (Higher Education Press, Beijing, 2003) G.C. Che, A.F. Dong, Z.X. Zhao, J. Low Temp. Phys. 27, 97 (2005) National Science Library of Chinese Academy of Sciences (Wuhan branch), Dyn. Monit. Bull. Sci. Res. (New Mater. Adv. Manuf.) 15, 1 (2006) X.S. Wu, P.W. Adams, Y. Yang, R.L. McCarley, Phys. Rev. Lett. 96, 127002 (2006) S.S. Yan, Basis of Solid State Physics, 2nd edn. (Tsinghua University Press, Beijing, 2000) J.M. Luttinger, J. Math. Phys. 4, 1154 (1963) H.C. Guo, Q.D. Chen, Physics 25, 510 (2003) R. Iser (ed.), Nanoelectronics and Information Technology—Advanced Electronic Materials and Novel Devices, 2nd edn. (Wiley, Weinheim, 2005) Y. Makhlin, G. Schon, A. Shnirman, Rev. Mod. Phys. 73, 357 (2001)

117. 118. 119. 120. 121. 122. 123. 124. 125. 126. 127. 128. 129. 130. 131. 132. 133. 134. 135. 136. 137. 138. 139. 140. 141. 142. 143. 144. 145. 146. 147. 148. 149. 150. 151. 152. 153. 154.

References 155. 156. 157. 158. 159. 160. 161. 162. 163. 164. 165. 166. 167. 168. 169. 170. 171. 172. 173. 174. 175. 176. 177. 178. 179. 180. 181. 182. 183. 184. 185. 186. 187. 188. 189. 190. 191. 192. 193. 194. 195. 196.

207

G.F. Mao, Y. Yu, Prog. Phys. 26 (2007) L.T. Canham, Appl. Phys. Lett. 57, 1046 (1990) A. Pichter, P. Steiner, F. Kozlowski, W. Lang, IEEE Electron Device Lett. 12, 691 (1991) N. Koshida, H. Koyama, Appl. Phys. Lett. 60, 347 (1992) A. Loni, A.J. Simons, T.I. Cox, P.J.D. Calcott, L.T. Canham, Electron. Lett. 31, 1288 (1995) L. Tsybesko, K.L. Moore, D.G. Hall, P.M. Faucjet, Phys. Rev. B 54, R8361 (1996) J. Linro, N. Lilic, Appl. Phys. Lett. 66, 3048 (1995) L. Pavesi, R. Gardini, C. Mazzoleni, Solid State Commun. 97, 1 (1996) K.D. Hirschman, L. Tsybeskov, S.P. Duttsgupta, P.M. Fauchet, Nature 384, 338 (1996) S. Lazarouk, P. Jaguiro, S. Katsouba, G. Masisni, S. La Monica, G. Maiello, A. Ferrari, Appl. Phys. Lett. 68, 2108 (1996) L. Tsybeskm, S.P. Duttagupta, K.D. Hirschman, P.M. Pauchet, Appl. Phys. Lett. 68, 2058 (1996) A. Filios, S. Hefner, R. Tsu, JVST B 14, 3431 (1996) C. Tsai, K.H. Li, J.C. Campbell, A. Tasch, Appl. Phys. Lett. 62, 3431 (1993) Z.H. Lu, D.J. Lockwood, J.M. Barlbeau, Nature 378, 258 (1995) J.M. Baribeau, D.J. Lockwood, Z.H. Lu, Mater. Res. Soc. Symp. Proc. 382, 79 (1995) M. Cahay, Adv. Electron. Electron. Phys. 89, 93 (1994) M.A. Reed, J.N. Randall, R.J. Aggarwal, R.J. Matyi, T.M. Moore, A.E. Wetsel, Phys. Rev. Lett. 60, 535 (1988) T.R. Zeller, I. Giaever, Phys. Rev. 20, 1504 (1968) U. Meirav, M.A. Kastner, S.J. Wind, Phys. Rev. Lett. 65, 771 (1991) S.Y. Chou, Y. Wang, Appl. Phys. Lett. 61, 1591 (1992) S. Tiwari, F. Rana, H. Hanafi, A. Hartstein, E. Crabbe, K. Chan, Appl. Phys. Lett. 68, 1377 (1996) K. Yano, T. Ishii, T. Hashimoto, T. Kobayashi, E. Murai, K. Seki, IEEE Trans. Electron Devices 41, 1628 (1994) L. Guo, E. Leobandung, S.Y. Chou, Appl. Phys. Lett. 70, 850 (1997); V.L. Covin, M.C. Schlamp, A.P. Alivisatos, Nature 370, 354 (1994) H. Schmidt, G. Medeiros-Ribiero, M. Oestreich, P.M. Petroff, G.H. Dohler, Phys. Rev. B 54, 11346 (1996) G. Markovich, D.V. Leff, S.W. Chung, H.M. Soyez, B. Bunn, J.R. Heath, Appl. Phys. Lett. 70, 3107 (1997) R.A. Kiehi, T. Ohshima, Appl. Phys. Lett. 67, 2494 (1995) T. Ohshima, R.A. Kiehl, J. Appl. Phys. 80, 912 (1996) M. Fukuda, K. Nakagawa, S. Miyazaki, M. Hirose, Appl. Phys. Lett. 70, 2291 (1997) D.V. Scheible, R.H. Blick, App. Phys. Lett. 84, 4632 (2004) D. Leonard, K. Pond, P.M. Petroff, Phys. Rev. B 50, 11687 (1994) H. Saito, K. Nishi, I. Ogura, S. Sugou, Y. Sugimoto, Appl. Phys. Lett. 69, 3140 (1996) D.L. Huffaker, O. Baklenov, L.A. Graham, B.G. Streetman, D.G. Deppe, Appl. Phys. Lett. 70, 2356 (1997) Y. Narukawa, Y. Kawakami, M. Funato, S.Z. Fujita, S.G. Fujita, S. Nakamura, Appl. Phys. Lett. 70, 981 (1997) M. Tabuchi, S. Noda, A. Sasaki, in Science and Technology of Mesoscopic Structures, ed. by S. Namba, C. Hamaguchi, T. Ando (Springer, Tokyo, 1992) G.S. Solomon, M.C. Larson, J.S. Harris, Appl. Phys. Lett. 69, 1897 (1996) S. Fathpour, Z. Mi, P. Bhattacharya, J. Phys. D 38, 2103 (2005) J. Park, A.X. Pasupathy, J.I. Goldsmith, C. Chang, Y. Yaish, J.R. Petta, M. Rinkoski, J.P. Sethna, H.D. Abruna, P.L. McEuen, D.C. Ralph, Nature 417, 722 (2002) M. Antonietti, D. Kuang, B. Smarsly, Y. Zhou, Angew. Chem. Int. Ed. 43, 4988 (2004) M.G. Bawendi, M.L. Steigerwald, L.E. Brus, Annu. Rev. Phys. Chem. 41, 477 (1990); C. Burda, X. Chen, R. Narayanan, M.A. El-Sayed, Chem. Rev. 105, 1025 (2005) B.L. Cushing, V.L. Kolesnichenko, C.J. O’Connor, Chem. Rev. 104, 3893 (2004) M.-C. Daniel, D. Astrue, Chem. Rev. 104, 293 (2004) J.H. Fendler, F.C. Meldrum, Adv. Mater. 7, 607 (1995)

Index

A Absorption coefficient, 103 Analysis, 21, 84, 99 Annealing, 69, 73, 74, 143 B Band gap, 5, 21, 75, 122 C Characterization, 8, 39 Compounds, 64, 174 Crystalline, 9, 31, 32, 46 D Deposition, 7, 31, 128, 159 Diameter, 6, 7, 40, 130, 176 Dielectric constant, 5, 21, 87 E Efficiency, 89 Electrical, 42, 81, 122 F Fabrication, 105 Ferroelectrics, 10 G Gap, 21, 164, 197 H Heating, 46, 64, 109

I Impurity, 113, 149 Integration, 84, 130, 182 Interface, 10, 40, 179

L Luminescence, 5, 164, 165

M Morphology, 26, 51, 165

N Nanostructure, 4, 9, 63

O Oxide, 3, 5, 32

P Phase, 19, 36, 64, 73, 110, 137, 168, 184, 186, 192 Production, 8, 18, 130, 162

Q Quantum, 4, 17, 48, 53, 77, 78, 94, 135, 165

R Reflection, 11, 19, 179

© The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2022 Y. Al-Douri, Nanomaterials, https://doi.org/10.1007/978-981-19-3881-8

209

210 S Semiconductor, 21, 43, 135, 158, 168, 190, 197 Sol-gel, 26, 34 Synthesis, 3, 28, 81, 183, 199

T Temperature, 26, 36, 65, 73, 109, 128, 153 Thickness, 31, 62, 107, 130, 177

Index U Ultraviolet (UV), 48, 130

V Valence, 75, 98, 149, 153, 154

X X-ray Diffraction (XRD), 39–41, 65, 66