Molecular Electronics From Principles to Practice

Michael C. Petty School of Engineering and Centre for Molecular and Nanoscale Electronics, Durham University, UK

DICENTENNIAL 1 8 0 7 n

eWILEY z 2007 r DICENTENNIAL John Wiley & Sons, Ltd Contents

Series Preface xv

Preface xvii

Acknowledgements xix

Symbols and Abbreviation xxi

1 Scope of Molecular Electronics 1 1.1 Introduction 1 1.2 Molecular Materials for Electronics 2 1.3 Molecular-Scale Electronics 5 1.3.1 Evolution of Microelectronics 5 1.3.2 Moore's Laws 7 1.3.3 Beyond Moore 8 1.4 The Biological World 12 1.5 Future Opportunities 13 1.6 Conclusions 15 Bibliography 15 References 16

2 Materials' Foundations 19 2.1 Introduction 19 2.2 Electronic Structure 19 2.2.1 Atomic Structure 19 2.2.2 Electrons in Atoms 20 2.2.3 Filling of Orbitals 24 2.2.4 The Periodic Table 24 2.3 Chemical Bonding 26 2.3.1 Bonding Principles 26 2.3.2 Ionic Bond 27 2.3.3 Covalent Bond 29 2.3.4 Metallic Bonding 32 2.3.5 Van der Waals Bonding 33 2.3.6 Hydrogen Bonding 34 viii CONTENTS

2.4 Bonding in Organic Compounds 35 2.4.1 Hybridized Orbitals 35 2.4.2 Isomers 37 2.4.3 Double and Triple Bonds 42 2.5 Crystalline and Noncrystalline Materials 45 2.5.1 States of Matter 45 2.5.2 Phase Changes and Thermodynamic Equilibrium 47 2.5.3 The Crystal Lattice 48 2.5.4 Crystal Systems 49 2.5.5 Miller Indices 50 2.5.6 Distance Between Crystal Planes 52 2.5.7 Defects 52 2.5.8 Amorphous Solids 56 2.6 Polymers 57 2.6.1 Molecular Weight 57 2.6.2 Polymer Structure 59 2.6.3 Polymer Crystallinity 60 2.7 Soft Matter: Emulsions, Foams and Gels 63 2.8 Diffusion 63 Bibliography 64 Reference 64

3 Electrical Conductivity 65 3.1 Introduction 65 3.2 Classical Theory 65 3.2.1 Electrical Conductivity 66 3.2.2 Ohm's Law 68 3.2.3 Charge Carrier Mobility 69 3.2.4 Fermi Energy 70 3.3 Energy Bands in Solids 72 3.3.1 Quantum Mechanical Foundations 73 3.3.2 Kronig—Penney Model 80 3.3.3 Conductors, Semiconductors and Insulators 85 3.3.4 Electrons and Holes 86 3.3.5 Intrinsic and Extrinsic Conduction 88 3.3.6 Quantum Wells 92 3.3.7 Disordered Semiconductors 93 3.3.8 Conductivity in Low-dimensional Solids 94 3.4 Organic Compounds 96 3.4.1 Band Structure 96 3.4.2 Doping 107 3.4.3 Solitons, Polarons and Bipolarons 109 3.4.4 Superconductivity 112 3.5 Low-Frequency Conductivity 112 3.5.1 Electronic Versus Ionic Conductivity 113 3.5.2 Quantum Mechanical Tunnelling 114 CONTENTS ix

3.5.3 Variable Range Hopping 116 3.5.4 Space-Charge Injection 118 3.5.5 Schottky and Poole–Frenkel Effects 120 3.6 Conductivity at High Frequencies 121 3.6.1 Complex Permittivity 121 3.6.2 Impedance Spectroscopy 125 Bibliography 127 References 127

4 Optical Phenomena 129 4.1 Introduction 129 4.2 Electromagnetic Radiation 129 4.3 Refractive Index 130 4.3.1 Permittivity Tensor 132 4.3.2 Linear and Nonlinear Optics 133 4.4 Interaction of EM Radiation with Organic Molecules 135 4.4.1 Absorption Processes 135 4.4.2 Aggregate Formation 140 4.4.3 Excitons 141 4.4.4 Effect of Electric Fields an Absorption 143 4.4.5 Emission Processes 144 4.4.6 Energy Transfer 147 4.5 Transmission and Reflection from Interfaces 149 4.5.1 Laws of Reflection and Refraction 149 4.5.2 Fresnel Equations 150 4.5.3 Ellipsometry 152 4.5.4 Thin Films 152 4.6 Waveguiding 154 4.7 Surface Plasmons 156 4.7.1 The Evanescent Field 156 4.7.2 Surface Plasmon Resonance 157 4.8 Photonic Crystals 162 4.8.1 Subwavelength Optics 165 Bibliography 166 References 166

5 Electroactive Organic Compounds 169 5.1 Introduction 169 5.2 Selected Topics in Chemistry 169 5.2.1 Moles and Molecules 169 5.2.2 Acids and Bases 170 5.2.3 Ions 171 5.2.4 Solvents 173 5.2.5 Functional Groups 173 5.2.6 Aromatic Compounds 178 5.3 Conductive Polymers 180 x CONTENTS

5.4 Charge-Transfer Complexes 185 5.5 Buckyballs and Nanotubes 188 5.5.1 Fullerenes 188 5.5.2 Carbon Nanotubes 191 5.6 Piezoelectricity, Pyroelectricity and Ferroelectricity 194 5.6.1 Basic Principles 194 5.6.2 Organic Piezoelectric, Pyroelectric and Ferroelectric Compounds 197 5.7 Magnetic Materials 201 5.7.1 Basic Principles 201 5.7.2 Organic Magnets 208 Bibliography 210 References 211

6 Tools for Molecular Electronics 213 6.1 Introduction 213 6.2 Direct Imaging 214 6.2.1 Optical Microscopy 214 6.2.2 Electron Microscopy 216 6.3 X-ray Reflection 218 6.3.1 Electron Density Profile 221 6.3.2 Keissig Fringes 222 6.3.3 In-Plane Measurements 223 6.4 Neutron Reflection 223 6.5 Electron Diffraction 223 6.6 Infrared Spectroscopy 224 6.6.1 Raman Scattering 231 6.7 Surface Analytical Techniques 232 6.8 Scanning Probe Microscopies 233 6.9 Film Thickness Measurements 236 Bibliography 238 References 238

7 Thin Film Processing and Device Fabrication 241 7.1 Introduction 241 7.2 Established Deposition Methods 242 7.2.1 Spin-Coating 242 7.2.2 Physical Vapour Deposition 243 7.2.3 Chemical Vapour Deposition 251 7.2.4 Electrochemical Methods 252 7.2.5 Inkjet Printing 253 7.2.6 Sol–Gel Processing 255 7.2.7 Other Techniques 258 7.3 Molecular Architectures 258 7.3.1 Langmuir–Blodgett Technique 258 7.3.2 Chemical Self-Assembly 268 7.3.3 Electrostatic Layer-by-Layer Deposition 270 CONTENTS xi

7.4 Nanofabrication 275 7.4.1 Photolithography 275 7.4.2 Nanometre Pattern Definition 276 7.4.3 Soft Lithography Techniques 278 7.4.4 Scanning Probe Manipulation 278 7.4.5 Dip-Pen Nanolithography 280 7.4.6 Other Methods 282 Bibliography 283 References 283

8 Liquid Crystals and Devices 287 8.1 Introduction 287 8.2 Liquid Crystal Phases 287 8.2.1 Thermotropic Liquid Crystals 287 8.2.2 Lyotropic Liquid Crystals 293 8.3 Liquid Crystal Polymers 295 8.4 Display Devices 297 8.4.1 Bifrefingence 297 8.4.2 Freedericksz Transition 299 8.4.3 Twisted Nematic Display 300 8.4.4 Passive and Active Addressing 302 8.4.5 Full-Colour Displays 303 8.4.6 Super-Twisted Nematic Display 304 8.5 Ferroelectric Liquid Crystals 305 8.6 Polymer-Dispersed Liquid Crystals 306 8.7 Liquid Crystal Lenses 308 8.8 Other Application Areas 309 Bibliography 311 References 311

9 Plastic Electronics 313 9.1 Introduction 313 9.2 Organic Diodes 313 9.2.1 Schottky Diode 313 9.2.2 Ohmic Contacts 318 9.3 Metal—Insulator—Semiconductor Structures 318 9.3.1 Idealized MIS Devices 319 9.3.2 Organic MIS Structures 320 9.4 Field Effect Transistors 321 9.5 Integrated Organic Circuits 327 9.5.1 Radiofrequency Identification Tags 328 9.6 Organic Light-Emitting Displays 330 9.6.1 Device Efficiency 334 9.6.2 Methods of Efficiency Improvement 336 9.6.3 Full-Colour Displays 343 9.6.4 Electronic Paper 345 xii CONTENTS

345 9.7 Photovoltaic Cells 346 9.7.1 Organic Semiconductor Solar Cell Dye-Sensitized Solar Cell 348 9.7.2 Luminescent Concentrator 350 9.7.3 9.8 Other Application Areas 351

Conductive Coatings 351 9.8.1 9.8.2 Batteries and Fuel Cells 351

Xerography 354 9.8.3 Bibliography 356

References 356

10 Chemical Sensors and Actuators 359 10.1 Introduction 359 10.2 Sensing Systems 360 10.3 Definitions 361 10.4 Chemical Sensors 363 10.4.1 Calorimetric Gas Sensors 364 10.4.2 Electrochemical Cells 365 10.4.3 Resistive Gas Sensors 369 10.4.4 Dielectric Sensors 376 10.4.5 Acoustic Devices 380 10.4.6 Optical Sensors 382 10.5 Biological Olfaction 388 10.6 Electronic Noses 390 10.7 Physical Sensors and Actuators 391 10.7.1 Touch Sensors 391 10.7.2 Polymer Actuators 392 10.7.3 Lab-on-a-Chip 395 10.8 Smart Textiles and Clothing 399 Bibliography 399 References 400 11 Molecular-Scale Electronics 403 11.1 Introduction 403 11.2 Nanosystems 403 11.2.1 Scaling Laws 403 11.2.2 Interatomic Forces 404 11.3 Engineering Materials at the Molecular Level 405 11.3.1 Polar Materials 406 11.3.2 Nonlinear Optical Materials 408 11.3.3 Photonic Crystals 410 11.4 Molecular Device Architectures 411 11.5 Molecular Rectification 415 11.6 Electronic Switching and Memory Devices 417 11.6.1 Resistive Bistable Devices 418 11.6.2 Flash Memories 421 CONTENTS xiii

11.6.3 Spintronics 424 11.6.4 Three-Dimensional Architectures 426 11.7 Single-Electron Devices 427 11.8 Optical and Chemical Switches 429 11.8.1 Fluorescence Switching 430 11.8.2 Photochromic Systems 431 11.8.3 Chemical Control 435 11.9 Nanomagnetic Systems 436 11.10 Nanotube Electronics 437 11.11 Molecular Actuation 440 11.11.1 Dynamically Controllable Surfaces 440 11.11.2 Rotaxanes 442 11.11.3 Optical Tweezers 443 11.12 Logic Circuits 443 11.13 Computing Architectures 447 11.14 Quantum Computing 449 Bibliography 450 References 451

12 Bioelectronics 455 12.1 Introduction 455 12.2 Biological Building Blocks 455 12.2.1 Amino Acids and Peptides 455 12.2.2 Proteins 458 12.2.3 Enzymes 459 12.2.4 Carbohydrates 461 12.2.5 Lipids 462 12.3 Nucleotides 465 12.3.1 Bases 465 12.3.2 DNA 466 12.3.3 RNA 467 12.3.4 ATP, ADP 467 12.4 Cells 468 12.5 Genetic Coding 469 12.5.1 Replication, Transcription and Translation 470 12.6 The Biological Membrane 474 12.6.1 Transport Across the Membrane 475 12.7 Neurons 480 12.8 Biosensors 482 12.8.1 Biocatalytic Sensors 483 12.8.2 Bioaffinity Sensors 484 12.9 DNA Electronics 487 12.10 Photobiology 487 12.10.1 Bacteriorhodopsin 487 12.10.2 Photosynthesis 493 xiv CONTENTS

12.11 Molecular Motors 498 12.11.1 Nature' s Motors 498 12.11.2 Artificial Motors 501 Bibliography 502 References 503

Index 505