INORGANIC NANOWIRES Applications, Properties, and

Characterization

M. Meyyappan NASA Research Center, Moffett Field

California, USA

Mahendra Sunkara University of Louisville

Kentucky, USA

LßP) CRC Press Nv* / Taylor & Francis Group

Boca Raton London NewYork

CRC Press is an imprint of the Taylor & Francis Group, an informa business

Contents

Preface xi Authors xv

1. Introduction 1 References 5

2. Historical Perspective 7 References 15

3. Growth Techniques 17 3.1 Introduction 17 3.2 Liquid-Phase Techniques 17

3.2.1 Template-Based Methods 18 3.2.1.1 Template Preparation 18 3.2.1.2 Deposition Methods 23

3.2.2 Template-Free Methods 32 3.2.2.1 Hydrothermal Method 32 3.2.2.2 Sonochemical Method 34 3.2.2.3 Surfactant-Assisted Growth:

Soft Directing Agents 35 3.2.2.4 Catalyst-Assisted Solution-Based Approaches 36

3.3 Vapor-Phase Techniques 37 3.3.1 One-Dimensional Growth Concepts 38

3.3.1.1 Vapor-Liquid-Solid Schemes Using Foreign Metal Clusters 38

3.3.1.2 Vapor-Liquid-Solid Schemes Using Low-Melting Metal Clusters 39

3.3.1.3 Vapor-Liquid-Solid Schemes Using Large Size, Molten Metal Clusters 40

3.3.1.4 Vapor-Solid-Solid Scheme 40 3.3.1.5 Oxygen-Assisted Growth (OAG) Scheme 40

3.3.2 Source Generation and Reactors for Vapor-Phase Synthesis of Nanowires 40 3.3.2.1 Thermal Evaporation 41 3.3.2.2 Laser Ablation 42 3.3.2.3 Metal Organic Chemical Vapor Deposition 44 3.3.2.4 Chemical and Molecular Beam Epitaxy 47 3.3.2.5 Plasma Arc Discharge-Based Techniques 49

3.4 Bulk Production Methods 50

v

VI Contents

3.4.1 Hot Filament CVD Method 50 3.4.2 Supercritical Fluid Approach 52 3.4.3 Direct Oxidation Schemes Using Plasma 52 3.4.4 Direct Gas-Phase Reactions Using Plasma Discharges 53

3.5 Future Developments 55 References 57

4. Thermodynamic and Kinetic Aspects of Nanowire Growth 61 4.1 Introduction 61 4.2 Thermodynamic Considerations for Vapor-Liquid-Solid

Growth 63 4.2.1 Thermodynamic Considerations of Nucleation from

Molten Metal Droplets 63 4.2.1.1 Gibbs-Thompson Relationship 63 4.2.1.2 Nucleation from Molten Metal Alloy Droplet 65 4.2.1.3 Nucleation from Various Molten

Metal Droplets 66 4.2.1.4 Thermodynamic Estimation of

Supersaturation for Spontaneity of Nucleation 70 4.2.1.5 Rational Choice of Metal for Tip-Led Growth

of Nanowires (Avoiding Nucleation) 73 4.2.1.6 Experimental Conditions for Promoting

Tip-Led Growth Using Any Molten Metal 76 4.2.2 Interfacial Energy and Tip-Led Growth 77

4.2.2.1 Role of Interfacial Energy in the Nanowire Growth Stability 77

4.2.2.2 Role of Interfacial Energy in Nanowire Faceting 81

4.2.2.3 Role of Interfacial Energy on the Nanowire Growth Direction 85

4.3 Kinetic Considerations of Nanowire Growth under VLS Growth 87 4.3.1 Kinetics of Vapor-Liquid-Solid Equilibrium 87 4.3.2 Role of Direct Impingement in Growth Kinetics 89 4.3.3 Role of Surface Diffusion in Growth Kinetics 91 4.3.4 Direct Impingement and Diffusion 94 4.3.5 Role of Surface Diffusion on the Metal Droplet 95 4.3.6 Role of Interwire Spacing 97

References 98

5. Modeling of Nanowire Growth 101 5.1 Introduction 101 5.2 Energetics of Stable Surface Faceting: Silicon

Nanowire Example 102 5.3 Simulation of Individual Nanowire Growth 104

vii

5.3.1 Simulation Methodology 105 5.3.2 Kinetic Monte Carlo Simulation Results 108 5.3.3 Experimental Results on Growth Direction

and Surface Faceting 112 5.4 Modeling of Multiple Nucleation and Growth of

One-Dimensional Structures 115 5.5 Modeling Nanowire Array Growth 117 References 121

6. Semiconducting Nanowires 123 6.1 Introduction 123 6.2 Silicon Nanowires 123

6.2.1 SiCl4/H2 System 124 6.2.2 Silane Feedstock in VLS Growth 134 6.2.3 Other Sources 135 6.2.4 Oxide-Assisted Growth 136 6.2.5 Template-Assisted Synthesis 137 6.2.6 Plasma Enhancement 138 6.2.7 Doping of SiNWs 139 6.2.8 Properties of SiNWs 140

6.3 Germanium Nanowires 142 6.3.1 Synthesis Using Germanium Powder 143 6.3.2 Germane and Related Sources 146

6.4 Catalyst Choice 147 6.5 III-V Nanowires 148

6.5.1 GaAs Nanowires 149 6.5.2 InAs Nanowires 152 6.5.3 InP Nanowires 152 6.5.4 GaP Nanowires 154

References 155

7. Phase Change Materials 161 7.1 Introduction 161 7.2 Phase Change Nanowire Growth 162 7.3 Properties Relevant to PRAM 167 References 169

8. Metallic Nanowires 171 8.1 Bismuth Nanowires 171 8.2 Silver Nanowires 173 8.3 Copper Nanowires 174 8.4 Nickel Nanowires 176 8.5 Zinc Nanowires 178 References 180

V l l l Contents

9. Oxide Nanowires 183 9.1 Introduction 183 9.2 Synthesis Methodologies 184

9.2.1 Catalyst-Assisted Synthesis 184 9.2.2 Direct Oxidation Schemes Using

Low-Melting Metals 190 9.2.2.1 Direct Oxidation of Molten Metal Clusters 190 9.2.2.2 Direct Chemical/Reactive Vapor Deposition

of Low-Melting Metal Oxides 193 9.2.3 Chemical Vapor Transport or Deposition of

High-Melting Metal Oxides 196 9.2.4 Plasma and Thermal Oxidation of Foils 202

9.3 Directed Growth and Morphological Control 206 9.3.1 Branched Nanowire Structures 206 9.3.2 Networking of Nanowires 208 9.3.3 Nanobelts 209 9.3.4 Tubular Nanostructures 212

9.3.4.1 High-Melting Metal Oxides 212 9.3.4.2 Low-Melting Metal Oxides 213

9.4 Oxygen Vacancies, Doping, and Phase Transformation 214 9.4.1 Oxygen Vacancies 214 9.4.2 Doping and Alloying 216 9.4.3 Phase Transformation of Metal Oxide Nanowires 217

References 220

10. Nitride Nanowires 225 10.1 Introduction 225 10.2 Synthesis of Group Ill-Nitride Nanowires 225

10.2.1 Catalyst-Assisted Synthesis 226 10.2.1.1 Choice of Precursors 229 10.2.1.2 Substrates for Epitaxial Array Growth 230 10.2.1.3 Choice of Catalysts and Process Variables 231 10.2.1.4 Control of Nanowire Growth Direction 232

10.2.2 Direct Reaction and Self-Catalysis Schemes 233 10.2.2.1 Control of Growth Direction 239

10.2.3 Synthesis of Nanotubes 240 10.2.4 Micro/Nanomorphologies 243

10.2.4.1 Ill-Nitride Nanobelts 243 10.2.4.2 Tapered Morphologies 245

10.3 Branching of Nanowires 247 10.3.1 Homobranching or "Tree-Like" Structures 247 10.3.2 Heterobranching 248

10.4 Diameter Reduction of Ill-Nitride Nanowires 249 10.5 Direction-Dependent Properties 252 References 254

ix

11. Other Nanowires 257 11.1 Antimonides 257 11.2 Selenides 260

11.2.1 Zinc Selenide 260 11.2.2 Other Selenides 263

11.3 Tellurides 264 11.3.1 Bismuth Telluride 264 11.3.2 Cadmium Telluride 265 11.3.3 Other Tellurides 265

11.4 Sulfides 266 11.4.1 Zinc Sulfide 266 11.4.2 Other Sulfides 267

11.5 Suicides 269 References 269

12. Applications in Electronics 275 12.1 Introduction 275 12.2 Silicon Nanowire Transistors 278 12.3 Vertical Transistors 280 12.4 Germanium Nanowire Transistors 284 12.5 Zinc Oxide and Other Nanowires in Electronics 286 12.6 III-V Transistors 289 12.7 Memory Devices 290

12.7.1 Phase-Change Random Access Memory 292 References 296

13. Applications in Optoelectronics 299 13.1 Introduction 299 13.2 Photodetectors 299 13.3 Light-Emitting Diodes 303 13.4 Nanoscale Lasers 306 References 310

14. Applications in Sensors 313 14.1 Introduction 313 14.2 Chemical Sensors 314

14.2.1 Sensor Requirements and the Role of Nanomaterials 314

14.2.2 Nanowires in Sensor Fabrication 317 14.2.3 Sensing Mechanisms 327 14.2.4 Selectivity and Electronic Nose 331

14.3 Biosensors 337 14.3.1 Nanoelectrode Arrays 340

References 344

Contents

15. Applications in the Energy Sector 349 15.1 Introduction 349 15.2 Solar Cells 350

15.2.1 Dye-Sensitized Solar Cells 350 15.2.1.1 Titania Nanowire-Based DSSCs 353 15.2.1.2 ZnO Nanowire-Based DSSCs 357 15.2.1.3 Sn02 NW-Based DSSCs 358 15.2.1.4 Inorganic Nanotubes, Polymers, and

Nb2Os Nanowires for DSSCs 358 15.2.1.5 Quantum Dot Sensitizers for

Nanowire-Based Solar Cells 360 15.2.1.6 Hybrid/Composite Structures 360 15.2.1.7 Transport and Recombination 363

15.2.2 Direct Absorption PEC Cells 366 15.2.3 p - n Junction Solar Cells 366 15.2.4 PEC Cells for Chemical Conversion 368

15.3 Electrochromic Devices 373 15.4 Li-Ion Batteries 377

15.4.1 Challenges with Anode Materials 378 15.4.2 Challenges Facing Cathode Materials 379 15.4.3 One-Dimensional Materials for Anodes 380

15.4.3.1 Carbon Nanotubes (CNTs) 380 15.4.3.2 Metal/Metal Oxide Nanowires 381 15.4.3.3 Nanowires of Silicon and Related Materials ...386

15.4.4 Rational Concepts for Nanowire-Based Architectures 387 15.4.4.1 A Concept of Nanometal Cluster-Decorated

Metal Oxide Nanowires 387 15.4.4.2 Nanowire Arrays on Conducting Substrates... 389 15.4.4.3 Miscellaneous Concepts of

3-D Geometries 390 15.4.5 Nanowire-Based Materials for Cathodes 391

References 393

16. Other Applications 399 16.1 Field Emission Devices 399

16.1.1 Background 399 16.1.2 Work Function (Ф) 400 16.1.3 Field Emission Testing 401 16.1.4 Field Emission Characteristics of

Nanowire-Based Materials 404 16.2 Thermoelectric Devices 414 References 416

421