Nanoscience the science of the small in physics engineering chemistry biology and medicine

212 5.1.3 Electronic Properties of Carbon Nanotubes.. 219 5.1.6 Thermal Properties of Carbon Nanotubes.. © 2000 Materials Research Society nanoscience and nanotechnology one must underst

The Science of the Small in Physics, Engineering, Chemistry, Biology and Medicine

123

Prof Dr Hans-Eckhardt Schaefer

Universität Stuttgart

Fak Mathematik und Physik

Institut für Theoretische und

Springer Heidelberg Dordrecht London New York

Library of Congress Control Number: 2010928839

© Springer-Verlag Berlin Heidelberg 2010

This work is subject to copyright All rights are reserved, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilm or in any other way, and storage in data banks Duplication of this publication

or parts thereof is permitted only under the provisions of the German Copyright Law of September 9,

1965, in its current version, and permission for use must always be obtained from Springer Violations are liable to prosecution under the German Copyright Law.

The use of general descriptive names, registered names, trademarks, 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.

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Springer is part of Springer Science+Business Media (www.springer.com)

Nanoscience is an interdisciplinary field of science which has its early beginnings

in the 1980s At small dimensions of a few nanometers (billionths of a meter) newphysical properties emerge, often due to quantum mechanical effects During thelast decades, additionally novel microscopical techniques have been developed inorder to observe, measure, and manipulate objects at the nanoscale It rapidly turnedout that nanosized features not only play a role in physics and materials sciencesbut also are most relevant in chemistry, biology, and medicine, giving rise to newfenestrations between these disciplines and wide application prospects

The early precursors to this book on Nanoscience date back to the 1990s when

the author initiated a course on Nanoscience and Nanotechnology at StuttgartUniversity, Germany, based on his early studies of nanostructured solids whichwere performed due to most stimulating discussions in the early 1980s with HerbertGleiter and the late Arno Holz, at that time at Saarbrücken University

Together with the growing interdisciplinarity of the field, the author’s researchand teaching activities in nanoscience were extended at Stuttgart University and

at research laboratories in South America, Japan, China, and Russia During theseresearch and teaching activities it became clear that a comprehensive yet concisetext which comprises the current literature on nanoscience from physics to mate-rials science, chemistry, biology, and medicine would be highly desirable Such atextbook or monograph should be a valuable source of information for students andteachers in academia and for scientists and engineers in industry who are involved

in the many different fields of nanoscience

In the present book, the state of the art of nanoscience is presented, ing in addition to the width and interdisciplinarity of the field the rapid progress inexperimental techniques and theoretical studies The text which focuses to the fun-damental aspects of the field in 12 chapters is supported by more than 600 figuresand a bibliography of nearly 2000 references which may be useful for more detailedstudies and for looking at historical developments and which cover with their ownreferences the wealth of the literature A number of textbooks and review articlesare quoted as introductory literature to the various fields

emphasiz-The book starts inChap 1 with some general comments, physical principles,and a number of nanoscale measuring methods with the subsequent Chap 2onmicroscopy techniques for investigating nanostructures Chapter 3 is devoted to

vii

viii Preface

the synthesis of nanosystems whereasChap 4surveys dimensionality effects with

Chap 5 focusing to carbon nanostructures and Chap 6 to bulk nanocrystallinematerials In the Chaps 7 and 8 the topics of nanomechanics, nanophotonics,nanofluidics, and nanomagnetism are raised before inChap 9nanotechnology forcomputers and data storage devices are overviewed The text is concluded with

Chap 10on nanochemistry andChap 11on nanobiology with finally an extendedsection on nanomedicine inChap 12

These 12 chapters are closely linked and intertwined as demonstrated by manycross-references between the chapters Although a particular chapter is dedicated,e.g., to synthesis (Chap 3), some synthesis aspects reappear in other chapters Thesame is true for nanomagnetism In addition to the particular chapter on this topic(Chap 8), nanomagnetic features appear in the introductory chapter, in the chapters

on nanocrystalline materials (Chap 6), on nanotechnology for computers and datastorage (Chap 9), on nanobiology (Chap 11), or nanomedicine (Chap 12) TheSubject Index may additionally help the reader to find the appropriate information

in his field of interest quickly

The wide application prospects of nanoscience are discussed in the variouschapters The importance of risk assessment strategies and toxicity studies innanotechnology is emphasized inSect 12.11

Stuttgart, Germany Hans-Eckhardt SchaeferDecember 7, 2009

The author is indebted to highly competent colleagues for critically reading gle chapters of the present book: W Sprengel, Graz Technical University, Austria;

sin-B Fultz, California Institute of Technology, Pasadena, USA; H Strunk, StuttgartUniversity, Germany; L Ley, Erlangen University, Germany; H Krenn, GrazUniversity, Austria; R Würschum, Graz Technical University, Austria; H Schaefer,retired from Nycomed GmbH, Konstanz, Germany; and R Ghosh, StuttgartUniversity, Germany

The financial support of the author’s research projects by DeutscheForschungsgemeinschaft, by the European Union, Alexander von HumboldtFoundation, Deutscher Akademischer Austauschdienst, Baden-WürttembergStiftung, and NATO is highly appreciated

Parts of the book have been designed during research and teaching periods ofthe author abroad, kindly hosted by P Vargas, Universidad Técnica Federico SantaMaria, Valparaiso, Chile; by K Lu, Institute of Metal Research, Chinese Academy

of Sciences, Shenyang, China; by Y Shirai, Kyoto University; by T Kakeshitaand H Araki, Osaka University, Japan; and by A A Rempel, Institute of SolidState Chemistry, Russian Academy of Sciences, Ekaterinburg, Russia Continuoussupport by C Ascheron, Springer Verlag, Heidelberg, Germany, is gratefullyacknowledged

The efficient help by S Heldele, M Jakob, P C Li, Y Rong, and H Schatzand the financial support by H Strunk, Stuttgart University, Germany, were crucialfor the technical preparation of the manuscript Thanks are due to S Blümlein,

P Brommer, U Mergenthaler, J Roth, and H R Trebin, Stuttgart University,Germany, for most valuable technical and organizational help

Furthermore, thanks are due to many publishing houses, scientific societies,governments, companies, and individuals for kindly granting the copyright per-missions for a large number of figures: Advanced Study Center St Petersburg,Agentur-Focus, American Association for the Advancement of Sciences (AAAS),American Association of Cancer Research, American Association of PhysicsTeachers, American Cancer Society, American Chemical Society, American DentalAssociation, American Institute of Physics, American Physical Society, American

ix

1 Introduction and Some Physical Principles 1

1.1 Introduction 1

1.2 Thermal Properties of Nanostructures 7

1.2.1 Violation of the Second Law of Thermodynamics for Small Systems and Short Timescales 7

1.2.2 Surface Energy 8

1.2.3 Thermal Conductance 9

1.2.4 Melting of Nanoparticles 11

1.2.5 Lattice Parameter 12

1.2.6 Phase Transitions 13

1.3 Electronic Properties 14

1.3.1 Electron States in Dependence of Size and Dimensionality 14

1.3.2 The Electron Density of States D(E) 16

1.3.3 Luttinger Liquid Behavior of Electrons in 1D Metals 17 1.3.4 Superconductivity 17

1.4 Giant Magnetoresistance (GMR) and Spintronics 19

1.4.1 Giant Magnetoresistance (GMR) and Tunneling Magnetoresistance (TMR) 21

1.4.2 Spintronics in Semiconductors 23

1.4.3 Spin Hall Effect 26

1.5 Self-Assembly 27

1.5.1 Self-Assembly of Ni Nanoclusters on Rh (111) via Friedel Oscillations 28

1.5.2 Self-Assembly of Fe Nanoparticles by Strain Patterns 29 1.5.3 Chiral Kagomé Lattice from Molecular Bricks 29

1.5.4 Self-Assembled Monolayers (SAMs) 30

1.5.5 Magnetic Assembly of Colloidal Superstructures 31

1.5.6 Self-Assembly via DNA or Proteins 33

1.6 Casimir Forces 33

1.7 Nanoscale Measuring Techniques 35

1.7.1 Displacement Sensing 35

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xii Contents

1.7.2 Mass Sensing 35

1.7.3 Sensing of Weak Magnetic Fields at the Nanoscale 36

1.7.4 Nuclear Magnetic Resonance Imaging (MRI) at the Nanoscale 37

1.7.5 Probing Superconductivity at the Nanoscale by Scanning Tunneling Microscopy (STM) 39

1.7.6 Raman Spectroscopy on the Nanometric Scale 40

1.7.7 “Nanosized Voltmeter” for Mapping of Electric Fields in Cells 40

1.7.8 Detection of Calcium at the Nanometer Scale 41

1.8 Summary 43

References 44

2 Microscopy – Nanoscopy 49

2.1 Scanning Tunneling Microscopy (STM) 49

2.1.1 Scanning Units, Electronics, Software 50

2.1.2 Constant Current Imaging (CCI) 51

2.1.3 Constant-Height Imaging (CHI) 53

2.1.4 Synchrotron Radiation Assisted STM (SRSTM) for Nanoscale Chemical Imaging 54

2.1.5 Studying Bulk Properties and Volume Atomic Defects by STM 54

2.1.6 Radiofrequency STM 56

2.2 Atomic Force Microscopy (AFM) 56

2.2.1 Topographic Imaging by AFM in Contact Mode 57

2.2.2 Frictional Force Microscopy 59

2.2.3 Non-contact Force Microscopy 59

2.2.4 Chemical Identification of Individual Surface Atoms by AFM 60

2.2.5 AFM in Bionanotechnology 61

2.3 Scanning Near-Field Optical Microscopy (SNOM) 61

2.3.1 Scanning Near-Field Optical Microscopy (SNOM) 63

2.3.2 Near-Field Scanning Interferometric Apertureless Microscopy (SIAM) 64

2.3.3 Mapping Vector Fields in Nanoscale Near-Field Imaging 65

2.3.4 Terahertz Near-Field Nanoscopy of Mobile Carriers in Semiconductor Nanodevices 66

2.4 Far-Field Optical Microscopy Beyond the Diffraction Limit 67

2.4.1 Stimulated Emission Depletion (STED) Optical Microscopy 68

2.4.2 Stochastic Optical Reconstruction Microscopy (2D-STORM) 69

2.4.3 Three-Dimensional Far-Field Optical Nanoimaging of Cells 70

2.4.4 Video-Rate Far-Field Nanooptical

Observation of Synaptic Vesicle Movement 73

2.5 Magnetic Scanning Probe Techniques 74

2.5.1 Magnetic Force Microscopy (MFM) 74

2.5.2 Spin-Polarized Scanning Tunneling Microscopy (SP-STM) 75

2.6 Progress in Electron Microscopy 76

2.6.1 Aberration-Corrected Electron Microscopy 76

2.6.2 TEM Nanotomography and Holography 81

2.6.3 Cryoelectron Microscopy and Tomography 81

2.7 X-Ray Microscopy 84

2.7.1 Lens-Based X-Ray Microscopy 85

2.7.2 X-Ray Nanotomography 87

2.7.3 Lens-Less Coherent X-Ray Diffraction Imaging 89

2.7.4 Upcoming X-Ray Free-Electron Lasers (XFEL) and Single Biomolecule Imaging 89

2.8 Three-Dimensional Atom Probes (3DAPs) 91

2.9 Summary 95

References 95

3 Synthesis 99

3.1 Nanocrystals and Clusters 99

3.1.1 From Supersaturated Vapors 99

3.1.2 Particle Synthesis by Chemical Routes 101

3.1.3 Semiconductor Nanocrystals (Quantum Dots) 104

3.1.4 Doping of Nanocrystals 104

3.1.5 Magnetic Nanoparticles 105

3.2 Superlattices of Nanocrystals in Two (2D) and Three (3D) Dimensions 107

3.2.1 Free-Standing Nanoparticle Superlattice Sheets 107

3.2.2 3D Superlattices of Binary Nanoparticles 109

3.3 Nanowires and Nanofibers 111

3.3.1 Vapor–Liquid–Solid (VLS) Growth of Nanowires 113

3.3.2 Pine Tree Nanowires with Eshelby Twist 116

3.3.3 Ultrathin Nanowires 117

3.3.4 Electrospinning of Nanofibers 120

3.3.5 Bio-Quantum-Wires 121

3.3.6 Formation of Arsenic Sulfide Nanotubes by the Bacterium Shewanella sp Strain HN-41 122

3.4 Nanolayers and Multilayered Systems 123

3.4.1 Layered Oxide Heterostructures by Molecular Beam Epitaxy (MBE) 127

3.4.2 Atomic Layer Deposition (ALD) 128

3.5 Shape Control of Nanoparticles 132

3.6 Nanostructures with Complex Shapes 134

xiv Contents

3.7 Nanostructures by Ball Milling or Strong

Plastic Deformation 136

3.8 Carbon Nanostructures 137

3.8.1 Fullerenes 137

3.8.2 Single-Walled Carbon Nanotubes (SWNTs) – Synthesis and Characterization 139

3.8.3 Graphene 143

3.9 Nanoporous Materials 145

3.9.1 Zeolites and Mesoporous Metal Oxides 145

3.9.2 Nanostructured Germanium 150

3.9.3 Nanoporous Metals 150

3.9.4 Single Nanopores – Potentials for DNA Sequencing 152 3.10 Lithography 154

3.10.1 UV Optical Lithography 155

3.10.2 Electron Beam Lithography 157

3.10.3 Proton-Beam Writing 157

3.10.4 Nanoimprint Lithography (NIL) 158

3.10.5 Dip-Pen Nanolithography (DPN) 158

3.10.6 Block Copolymer Lithography 159

3.10.7 Protein Nanolithography 162

3.10.8 Fabrication of Nanostructures in Supercritical Fluids 163 3.10.9 Two-Photon Lithography for Microfabrication 164

3.11 Summary 165

References 165

4 Nanocrystals – Nanowires – Nanolayers 169

4.1 Nanocrystals 169

4.1.1 Synthesis of Nanocrystals 169

4.1.2 Metal Nanocrystallites – Structure and Properties 172

4.1.3 Semiconductor Quantum Dots 174

4.1.4 Colorful Nanoparticles 178

4.1.5 Double Quantum Dots for Operating Single-Electron Spins as Qubits for Quantum Computing 181

4.1.6 Quantum Dot Data Storage Devices 183

4.2 Nanowires and Metamaterials 183

4.2.1 Metallic Nanowires 183

4.2.2 Negative-Index Materials (Metamaterials) with Nanostructures 184

4.2.3 Semiconductor Nanowires 186

4.2.4 Molecular Nanowires 192

4.2.5 Conduction Through Individual Rows of Atoms and Single-Atom Contacts 193

4.3 Nanolayers and Multilayers 195

4.3.1 2D Quantum Wells 195

4.3.2 2D Quantum Wells in High Magnetic Fields 196

4.3.3 The Integral Quantum Hall Effect (IQHE) 196

4.3.4 The Fractional Quantum Hall Effect (FQHE) 198

4.3.5 2D Electron Gases (2DEG) at Oxide Interfaces 199

4.3.6 Multilayer EUV and X-Ray Mirrors with High Reflectivity 201

4.4 Summary 205

References 205

5 Carbon Nanostructures – Tubes, Graphene, Fullerenes, Wave-Particle Duality 209

5.1 Nanotubes 209

5.1.1 Synthesis of Carbon Nanotubes 209

5.1.2 Structure of Carbon Nanotubes 212

5.1.3 Electronic Properties of Carbon Nanotubes 214

5.1.4 Heteronanocontacts Between Carbon Nanotubes and Metals 219

5.1.5 Optoelectronic Properties of Carbon Nanotubes 219

5.1.6 Thermal Properties of Carbon Nanotubes 220

5.1.7 Mechanical Properties of Carbon Nanotubes 220

5.1.8 Carbon Nanotubes as Nanoprobes and Nanotweezers in Physics, Chemistry, and Biology 224

5.1.9 Other Tubular 1D Carbon Nanostructures 227

5.1.10 Filling and Functionalizing Carbon Nanotubes 230

5.1.11 Nanotubes from Materials Other than Pure Carbon 235

5.1.12 Application of Carbon Nanotubes 236

5.2 Graphene 245

5.2.1 Imaging of Graphene, Defects, and Atomic Dynamics 246 5.2.2 Electronic Structure of Graphene, Massless Relativistic Dirac Fermions, and Chirality 248

5.2.3 Quantum Hall Effect 250

5.2.4 Anomalous QHE in Bilayer Graphene 251

5.2.5 Absence of Localization 252

5.2.6 From Graphene to Graphane 252

5.2.7 Graphene Devices 252

5.3 Fullerenes, Large Carbon Molecules, and Hollow Cages of Other Materials 253

5.3.1 Fullerenes 253

5.3.2 Fullerene Compounds 254

5.3.3 Superheating and Supercooling of Metals Encapsulated in Fullerene-Like Shells 255

5.3.4 Large Carbon Molecules 257

5.3.5 Hollow Cages of Other Materials 258

5.4 Fullerenes and the Wave-Particle Duality 259

5.5 Summary 261

References 262

xvi Contents

6 Nanocrystalline Materials 267

6.1 Molecular Dynamics Simulation of the Structure of Grain Boundaries and of the Plastic Deformation of Nanocrystalline Materials 267

6.2 Grain Boundary Structure 268

6.3 Plasticity and Hall–Petch Behavior of Nanocrystalline Materials 271

6.4 Plasticity Studies by Nanoindentation 276

6.5 Ultrastrength Nanomaterials 279

6.6 Enhancement of Both Strength and Ductility 282

6.7 Superplasticity 285

6.8 Fatigue 288

6.9 Nanocomposites 290

6.9.1 Metallic Nanocomposites 290

6.9.2 Ceramic/Metal Nanocomposites with Diamond-Like Hardening 292

6.9.3 Oxide/Dye/Polymer Nanocomposites – Optical Properties 293

6.9.4 Polymer Nanocomposites 294

6.10 Nanocrystalline Ceramics 300

6.10.1 Low Thermal Expansion Nanocrystallite-Glass Ceramics 301

6.11 Atomic Diffusion in Nanocrystalline Materials 303

6.12 Surface-Controlled Actuation and Manipulation of the Properties of Nanostructures 306

6.12.1 Charge-Induced Reversible Strain in Nanocrystalline Metals 307

6.12.2 Artificial Muscles Made of Carbon Nanotubes 308

6.12.3 Electric Field-Controlled Magnetism in Nanostructured Metals 308

6.12.4 Surface Chemistry-Driven Actuation in Nanoporous Gold 310

6.13 Summary 310

References 310

7 Nanomechanics – Nanophotonics – Nanofluidics 315

7.1 Nanoelectromechanical Systems (NEMS) 315

7.1.1 High-Frequency Resonators 316

7.1.2 Nanoelectromechanical Switches 316

7.2 Putting Mechanics into Quantum Mechanics – Cooling by Laser Irradiation 319

7.3 Nanoadhesion: From Geckos to Materials 323

7.3.1 Materials with Bioinspired Adhesion 324

7.3.2 Climbing Robots and Spiderman Suit 324

7.4 Single-Photon and Entangled-Photon Sources

and Photon Detectors, Based on Quantum Dots 325

7.4.1 Single-Photon Sources 325

7.4.2 Entangled-Photon Sources 327

7.4.3 Single-Photon Detection 328

7.5 Quantum Dot Lasers 329

7.6 Plasmonics 331

7.6.1 Plasmon-Controlled Synthesis of Metallic Nanoparticles 334

7.6.2 Extinction Behavior of Nanoparticles and Arrays 335

7.6.3 Plasmonic Nanocavities 337

7.6.4 Surface-Enhanced Raman Spectroscopy (SERS) and Fluorescence 338

7.6.5 Receiver–Transmitter Nanoantenna Pairs 341

7.6.6 Electro-optical Nanotraps for Neutral Atoms 341

7.6.7 Unifying Nanophotonics and Nanomechanics 342

7.6.8 Integration of Optical Manipulation and Nanofluidics 342 7.6.9 Single-Photon Transistor 343

7.6.10 Application Prospects of Plasmonics 343

7.7 2D-Confinement of Fluids, Wetting, and Spreading 346

7.7.1 Phase Transitions Induced by Nanoconfinement of Liquid Water 347

7.7.2 Fluid Flow and Wetting 348

7.7.3 Superhydrophobic Surfaces 348

7.7.4 Liquid Spreading Under Nanoscale Confinement 349

7.8 Fast Transport of Liquids and Gases Through Carbon Nanotubes 351

7.8.1 Limits of Continuum Hydrodynamics at the Nanoscale 351

7.8.2 Water Transport in CNTs 351

7.8.3 Gas Transport in CNTs 353

7.9 Nanodroplets 353

7.9.1 Dynamics of Nanoscopic Water in Micelles 353

7.9.2 Nanoscale Double Emulsions 354

7.9.3 Zeptoliter Liquid Alloy Droplets and Surface-Induced Crystallization 354

7.9.4 Superfluid Helium Nanodroplets 356

7.10 Nanobubbles 358

7.10.1 Stable Surface Nanobubbles 358

7.10.2 Polygonal Nanopatterning of Stable Microbubbles 358

7.10.3 Bubbles for Tracking the Trajectory of an Individual Electron Immersed in Liquid Helium 359

7.11 Summary 360

References 361

xviii Contents

8 Nanomagnetism 365

8.1 Magnetic Imaging 365

8.1.1 Magnetic Force Microscopy (MFM) and Magnetic Exchange Force Microscopy (MEx FM) 366

8.1.2 Spin-Polarized Scanning Tunneling Microscopy (SP-STM) and Manipulation 370

8.1.3 Electron Microscopy 376

8.1.4 X-Ray Magnetic Circular Dichroism (XMCD) 381

8.2 Size and Dimensionality Effects in Nanomagnetism – Single Atoms, Clusters (0D), Wires (1D), Films (2D) 383

8.2.1 Single Atoms 384

8.2.2 Finite-Size Atomic Clusters 386

8.2.3 Ferromagnetic Nanowires 388

8.2.4 Magnetic Films (2D) 393

8.2.5 Curie Temperature TCin Dependence of Size, Dimensionality, and Charging 396

8.3 Soft-Magnetic Materials 397

8.4 Nanostructured Hard Magnets 399

8.5 Antiferromagnetic and Complex Magnetic Nanostructures 401

8.5.1 Spin Structure of Antiferromagnetic Domain Walls 403

8.5.2 Antiferromagnetic Monatomic Chains 403

8.5.3 Antiferromagnetic Nanoparticles 403

8.5.4 Complex Magnetic Structure of an Iron Monolayer on Ir (111) 407

8.6 Ferromagnetic Nanorings 407

8.7 Current-Induced Domain Wall Motion in Magnetic Nanostructures 410

8.8 Single Molecule Magnets 412

8.9 Multiferroic Nanostructures 412

8.10 Magnetically Tunable Photonic Crystals of Superparamagnetic Colloids 416

8.11 Nanomagnets in Bacteria 417

8.11.1 In Vivo Doping of Magnetosomes 418

8.11.2 Magnetosomes for Highly Sensitive Biomarker Detection 419

8.12 Summary 420

References 420

9 Nanotechnology for Computers, Memories, and Hard Disks 425

9.1 Transistors and Integrated Circuits 426

9.2 Extreme Ultraviolet (EUV) Lithography – The Future Technology of Chip Fabrication 431

9.3 Flash Memory 434

9.4 Emerging Solid State Memory Technologies 436

9.4.1 Phase-Change Memory Technology 437

9.4.2 Magnetoresistive Random-Access Memories (MRAM) 441

9.4.3 Ferroelectric Random-Access Memories (FeRAM) 446

9.4.4 Resistance Random Access Memories (ReRAMs) 447

9.4.5 Carbon-Nanotube (CNT)-Based Data Storage Devices (NRAM) 448

9.4.6 Magnetic Domain Wall Racetrack Memories (RM) 450

9.4.7 Single-Molecule Magnets 452

9.4.8 10 Terabit/Inch2Block Copolymer (BCP) Storage Media 452

9.5 Magnetic Hard Disks and Write/Read Heads 454

9.5.1 Extensions to Hard Disk Magnetic Recording 456

9.5.2 Magnetic Write Head and Read Back Head 457

9.6 Optical Hard Disks 462

9.6.1 Principles and Materials Considerations 463

9.6.2 Magneto-Optical Recording 466

9.6.3 Multilayer Recording 467

9.6.4 Holographic Data Storage 468

9.7 High-k Dielectrics for Replacing SiO2Insulation in Memory and Logic Devices 470

9.8 Low-k Materials as Interlayer Dielectrics (ILD) 471

9.9 Summary 474

References 474

10 Nanochemistry – From Supramolecular Chemistry to Chemistry on the Nanoscale, Catalysis, Renewable Energy, Batteries, and Environmental Protection 477

10.1 Supramolecular Chemistry 477

10.1.1 Architecture in Supramolecular Chemistry 478

10.1.2 Supramolecular Materials 480

10.1.3 Molecular Recognition, Reactivity, Catalysis, and Transport 484

10.1.4 Molecular Photonics and Electronics 486

10.1.5 Molecular Recognition and Self-Organization 489

10.1.6 DNA Self-Assembled Nanostructures 493

10.1.7 Supramolecular DNA Polyhedra 493

10.2 Large Inorganic Hollow Clusters 495

10.2.1 Nano-hedgehogs Shaped from Molybdenum Oxide Building Blocks 495

10.2.2 Vesicle-Like Structures with a Diameter of 90 nm 495

10.2.3 Nitride–Phosphate Clathrate 497

10.3 Chemistry on the Nanoscale 498

10.3.1 Nano Test Tubes 498

10.3.2 Dynamics in Water Nanodroplets 499 10.3.3 Targeted Delivery and Reaction of Single Molecules 500

xx Contents

10.4 Catalysis 502

10.4.1 Au Nanocrystals 502

10.4.2 Pt Nanocatalysts 505

10.4.3 Pd Nanocatalysts 506

10.4.4 MoS2 Nanocatalysts as Model Catalysts for Hydrodesulfurization (HDS) 507

10.4.5 In Situ Phase Analysis of a Catalyst 509

10.5 Renewable Energy 510

10.6 Solar Energy – Photovoltaics 510

10.6.1 Nitrogen-Doped Nanocrystalline TiO2Films Sensitized by CdSe Quantum Dots 511

10.6.2 Polymer-Based Solar Cells 512

10.6.3 Silicon Nanostructures 512

10.7 Solar Energy – Thermal Conversion 514

10.8 Antireflection (AR) Coating 515

10.9 Conversion of Mechanical Energy into Electricity 516

10.10 Hydrogen Storage and Fuel Cells 516

10.11 Lithium Ion Batteries and Supercapacitors 519

10.11.1 Carbon Nanotube Cathodes 519

10.11.2 Tin-Based Anodes 519

10.11.3 LiFePO4Cathodes 520

10.11.4 Supercapacitors 522

10.12 Environmental Nanotechnology 522

10.13 Summary 524

References 524

11 Biology on the Nanoscale 527

11.1 The Cell – Nanosized Components, Mechanics, and Diseases 528 11.1.1 Cell Structure 529

11.1.2 Mechanics, Motion, and Deformation of Cells 532

11.1.3 Cell Adhesion 533

11.1.4 Disease-Induced Alterations of the Mechanical Properties of Single Living Cells 534

11.1.5 Control of Cell Functions by the Size of Nanoparticles Alone 536

11.2 Nanoparticles for Bioanalysis 537

11.2.1 Various Materials of Nanoparticles 537

11.2.2 Surface Functionalization of Nanoparticles 540

11.2.3 Examples for Labeling Biosystems by Nanoparticles 540 11.2.4 In Vivo and Deep Tissue Imaging 543

11.2.5 Nanoparticle-DNA Interaction 546

11.2.6 Nanoparticle-Protein Interaction 552

11.2.7 Biodistribution of Nanoparticles 556

11.3 Nanomechanics of DNA, Proteins, and Cells 557

11.3.1 DNA Elasticity 557

11.3.2 From Elasticity to Enzymology 55711.3.3 Unzipping of DNA 55911.3.4 Protein Mechanics 56011.4 Molecular Motors and Machines 56311.4.1 Myosin 56411.4.2 Kinesin 56711.4.3 Motor–Cargo Linkage and Regulation 56811.4.4 Diseases 56911.4.5 ATP Synthase (ATPase) 56911.5 Membrane Channels 57111.5.1 The K+Channel 571

11.5.2 The Ca2 +Channel 573

11.5.3 The Chloride (Cl−) Channel 575

11.5.4 The Aquaporin Water Channel 57511.5.5 Protein Channels 57611.5.6 Pentameric Ligand-Gated Ion Channels 57911.5.7 Nuclear Pores 57911.6 Biomimetics 58011.6.1 Energy Conversion 58011.6.2 Sensing 58211.6.3 Signaling 58311.6.4 Molecular Motors 58311.6.5 Materials 58511.6.6 Artificial Cells – Prospects for Biotechnology 59011.7 Bone and Teeth 59311.7.1 Bone 59411.7.2 Tooth Structure and Restoration 59611.8 Photonic Bionanostructures – Colors of Butterflies

and Beetles 59711.8.1 Structures 59811.8.2 Formation Processes of Photonic Bionanostructures 60111.9 Lotus Leaf Effect – Hydrophobicity and Self-Cleaning 60111.10 Food Nanostructures 60411.11 Cosmetics 60511.11.1 Skin Care 60611.11.2 Encapsulating a Fragrance in Nanocapsules 60811.11.3 PbS Nanocrystals in Ancient Hair Dyeing 60911.12 Summary 610References 610

12 Nanomedicine 61512.1 Introduction 61512.2 Diagnostic Imaging and Molecular Detection Techniques 61812.2.1 Magnetic Resonance Imaging (MRI) 61812.2.2 CT Contrast Enhancement 630

xxii Contents

12.2.3 Contrast-Enhanced Ultrasound Techniques 63212.2.4 Positron Emission Tomography (PET) 63512.2.5 Raman Spectroscopy Imaging 63712.2.6 Photoacoustic Tomography 63712.2.7 Biomolecular Detection for Medical Diagnostics 63712.3 Nanoarrays and Nanofluidics for Diagnosis and Therapy 65012.3.1 Lab-on-a-Chip 65112.3.2 Microarrays and Nanoarrays 65212.3.3 Microfluidics and Nanofluidics 65312.3.4 Integration of Nanodevices in Medical Diagnostics 65612.3.5 Implanted Chips 65612.4 Targeted Drug Delivery by Nanoparticles 65812.4.1 Porous Silica Nanoparticles for Targeting

Cancer Cells 65912.4.2 Gene Therapy and Drug Delivery for

Cancer Treatment 66212.4.3 Liposomes and Micelles as Nanocarriers for

Diagnosis and Drug Delivery 66712.4.4 Drug Delivery by Magnetic Nanoparticles 67012.4.5 Nanoshells for Thermal Drug Delivery 67212.4.6 Photodynamic Therapy 67212.5 Brain Cancer Diagnosis and Therapy with Nanoplatforms 67212.5.1 General Comments 67412.5.2 MRI Contrast Enhancement with Magnetic

Nanoparticles 67412.5.3 Nanoparticles for Chemotherapy 67512.5.4 Targeted Multifunctional Polyacrylamide

(PAA) Nanoparticles for PhotodynamicTherapy (PDT) and Magnetic ResonanceImaging (MRI) 67612.6 Hyperthermia Treatment of Tumors by Using Targeted

Nanoparticles 67812.6.1 Alternating Magnetic Fields for Heating

Magnetic Nanoparticles 67912.6.2 Radiofrequency Heating of Carbon Nanotubes 68212.6.3 Light-Induced Heating of Nanoshells 68412.7 Nanoplatforms in Other Diseases and Medical Fields 68612.7.1 Heart Diseases 68612.7.2 Diabetes 68812.7.3 Lung Therapy – Targeted Delivery of

Magnetic Nanoparticles and Drug Delivery 68912.7.4 Alzheimer’s Disease (AD) 69112.7.5 Ophthalmology 69612.7.6 Viral and Bacterial Diseases 701

12.8 Nanobiomaterials for Artificial Tissues 70412.8.1 Enhancement of Osteoblast Function by

Carbon Nanotubes on Titanium Implants 70512.8.2 Nanostructured Bioceramics for Bone Restoration 70612.8.3 Fibrous Nanobiomaterials as Bone Tissue

Engineering Scaffolds 70712.8.4 Tissue Engineering of Skin 70812.8.5 Angiogenesis 70812.8.6 Promoting Neuron Adhesion and Growth 70812.8.7 Spinal Cord In Vitro Surrogate 71012.8.8 Efforts for Synthesizing Chromosomes 71212.9 Nanosurgery – Present Efforts and Future Prospects 71212.9.1 Femtosecond Laser Surgery 71212.9.2 Sentinel Lymph Node Surgery Making

Use of Quantum Dots 71312.9.3 Progress Toward Nanoneurosurgery 71312.9.4 Future Directions in Neurosurgery 71412.10 Nanodentistry 71712.10.1 Nanocomposites in Dental Restoration 71812.10.2 Nanoleakage of Adhesive Interfaces 71912.10.3 Nanostructured Bioceramics for Maxillofacial

Applications 72012.10.4 Release of Ca–PO4from Nanocomposites

for Remineralization of Tooth Lesions andInhibition of Caries 72112.10.5 Growing Replacement Bioteeth 72212.11 Risk Assessment Strategies and Toxicity Considerations 72312.11.1 Risk Assessment and Biohazard Detection 72412.11.2 Cytotoxicity Studies on Carbon, Metal, Metal

Oxide, and Semiconductor-Based Nanoparticles 72512.12 Summary 728References 728

Name Index 737

Subject Index 753

as key technology in application and business.

The term nano, derived from the Greek word nanos which means dwarf,

desig-nates a billionth fraction of a unit, e.g., of a meter Thus the science of nanostructures

is often defined as dealing with objects on a size scale of 1–100 nm

Nanostructures may be compared [1.2] to a human hair which is∼50,000 nm

thick whereas the diameters of nanostructures are∼0.3 nm for a water molecule,

1.2 nm for a single-wall carbon nanotube, and 20 nm for a small transistor DNAmolecules are 2.5 nm wide, proteins about 10 nm, and an ATPase biochemical motorabout 10 nm This is the ultimate manufacturing length scale at present with buildingblocks of atoms, molecules, and supramolecules as well as integration along severallength scales In addition, living systems work at the nanoscale

The investigation of nanostructures and the development of nanoscience startedaround 1980 when the scanning tunneling microscope (STM) was invented [1.3] andthe concept of nanostructured solids was suggested [1.4] More than 20 years earlier

R Feynman had emphasized that “ there is plenty of room at the bottom in

the science of ultra-small structures” [1.5] (see Fig.1.1)

Clearly, nanostructures were available much earlier Albert Einstein calculated inhis doctoral dissertation from the experimental diffusion data of sugar in water thesize of a single sugar molecule to about 1 nm (see [1.7]) Michael Faraday remarkedduring a lecture on the optical properties of gold in 1857 that “ a mere variation

in the size of the (nano) particles gave rise to a variety of resultant colors” [1.8] Infact, nanostructures already existed in the early solar nebula or in the presolar dust(4.5 billion years ago) as deduced from the detection of nanosized C60molecules inthe Allende meteorite [1.9]

From its early infancy the field of nanoscience has more and more grown up(see Fig.1.2) and enjoys worldwide scientific popularity and importance For thecharacterization of this research field the notations “Nanostructured Science” or

“Nanotechnology” were coined by K.E Drexler [1.11]

A huge variety of approaches are available for synthesizing nanostructures (see

Chap 3) In the top-down techniques bulk material is chiseled out of or added to

sur-faces Microchips or better nanochips with line widths of about 30 nm at present are

the most notable examples In contrast, bottom-up manufacturers use self-assembly

processes [1.12] to put together larger structures – atoms or molecules or clusters ofmany atoms – that make ordered arrangements spontaneously

1.1 Introduction 3

Fig 1.3 Gordon Moore

postulated in 1965

[ 1.13 , 1.14 ] that the computer

power will double every

elec-to Moore’s law (see Fig.1.3, [1.13, 1.14]) the sizes of transistors and data age components will exponentially shrink by a factor of 2 approximately every 18months and within the next 10 years the top-down processes for miniaturization ofelectronic components to the nanoscale (seeSect 9.1) will presumably approachphysical limits From the top ten advances in materials science [1.15] at least fiveare directly related to nanoscience Some people think that nanoscience is likely torevolutionize many areas of human activity, such as materials science, informationprocessing, biotechnology, and medicine [1.16]

stor-In chemistry nanotechnology tools such as scanning tunneling microscopy(STM) enable the study and manipulation of chemical reactions on the atomicscale, nanocatalytical processes can be initiated, and the bottom-up synthesis oforganic as well as inorganic supramolecular structures for, e.g., molecular devices

is revitalized

In biological systems nanosized structures play an important role from proteins todeoxyribonucleic acid (DNA) carrying the genetic code, ribonucleic acid (RNA, seeFig.1.4), or molecular motors Nanostructures are fundamental for the properties

Fig 1.4 Schematic of a

ribosome reading the

information from an RNA

strand (violet) and how to

synthesize a protein (golden)

from amino acid building

blocks (Reprinted with

Much progress in nanotechnology stems from the emergence of high-resolutionscanning probe microscopy tools and their many specific variants (seeChap 2) Therapid development of electron microscopy [1.20] and x-ray microscopy [1.21] alsocontributes to the exploration of nanotechnologies (seeChap 2)

From this brief outline it is evident that nanoscience is a most interdisciplinaryapproach [1.22–1.24] because all disciplines and areas converge at the nanoscale tothe same basic principles and the same basic tools so that the frontiers between thedisciplines even seem to disappear It is well recognized that for the exploitation of

Fig 1.5 Brain tumor cells

(dark) decorated by coated

magnetite nanoparticles The

tumor is sensitized for

therapy by slight heating of

the nanoparticles by means of

an alternating magnetic field.

(Reprinted with permission

from [ 1.18 ] © 2008

A Jordan)

1.1 Introduction 5

Fig 1.6 Cryo TEM of liposomes (Reprinted with permission from [1.19 ] © 2000 Materials Research Society)

nanoscience and nanotechnology one must understand the physics and chemistry

of the nanoscale and one must learn how to make materials and functional devices[1.25] This scenario centers to bring together researchers from many disciplines toprovide the tools to do nanoscale research, from work to understand the economicaland societal benefits and perils of this new field of development, and finally fromworkforce, education, and training

A large number of nanoscience research centers [1.26] and nanoscience tence centers have been established worldwide Many universities in Europe nowoffer master courses in nanotechnology [1.27] Some of the world’s largest univer-sities undergo dramatic departmental restructuring to foster interdisciplinary andnanoscience research [1.28]

compe-The role of nanotechnology in industry is rapidly growing Over 600 products inthe consumer market alone use nanomaterials with a further 1500 patented [1.29].Nanotechnology is found in applications as diverse as self-cleaning windows, high-performance paints, anti-aging products, and sunscreen (seeSect 11.11) Nanofilledresins are used to manufacture large-scale composite panels for production cars (see

Sect 6.9) Other products include a scratch-resistant topcoat used on car bodies, aswell as coatings for alloy wheels and polycarbonate headlight covers Resin-filledcarbon nanotubes (CNTs) have been used in high-performance composites in rack-ets, baseball bats, and ice hockey sticks [1.29] Energy storage is an area of greatactivity where the high surface area of nanomaterials provides an instant benefitwith a stream of new materials being developed for high charge rate batteries (see

Sect 10.11) and supercapacitors, and solid-state hydrogen storage Also of est is the generation of hydrogen by photocatalysis of water and the development

inter-of improved membranes for fuel cells [1.29] Nanotechnology solutions are also

to meet environmental challenges such as water purification and remediation, airpurification, and clean processing [1.29]

In 2006, an estimated US $50 billion in products worldwide incorporated otechnology This figure has been projected by some to reach US $1–2.6 trillion

nan-until 2011–2016 [1.30,1.31] About 10 million nano-related jobs are expected to

be created until 2014 [1.32] It is projected that the production of nanoparticles willincrease from the estimated 2,300 tons today to 58,000 tons by 2020 [1.33]

By the visions of Eric Drexler [1.11] (Fig.1.7), who brought the term nology into vogue, a discussion about the risks and benefits of this development wasinitiated Drexler envisioned an era in which factory production lines are replaced

nanotech-by self-replicating nanoscale assemblers which fabricate nanoscale medical devices(Fig.1.8), computers, etc., from atoms and molecules

The Drexlarian visions of nanotechnology gave rise to morose concern of BillJoy [1.35], the chief scientist of Sun Microsystems, who worried about the impli-cations of intelligent nanorobots that could multiply uncontrollably and that oneshould consider a moratorium for nanotechnology This has been criticized because

of the enormous difficulties to create robots at the nanoscale [1.7,1.36], because

Fig 1.7 The nanovisionary

K.E Drexler propagates the

idea of molecular machinery.

The atomic structure of such

a machine is shown in the

background (Reprinted with

permission from [ 1.34 ].

© 2010 agentur-focus)

Fig 1.8 A medical

nanorobot (top left) swims in

a blood vessel and repairs a

vessel closure by cutting the

deposits This tiny submarine

is hardly larger than the

disk-like erythrocytes Future

applications of this type of

devices are expected by

K.E Drexler (Reprinted with

permission from [ 1.34 ].

© 2010 agentur-focus)

1.2 Thermal Properties of Nanostructures 7

of incompatibilities with the laws of thermodynamics [1.37], because it belongssquarely in the realm of science fiction [1.38], or because this concern is rejected asFrankenstein nonsense on the nanoscale [1.39]

Others recommend a sober and unemotional assessment and searches for waysout of perceptible or suspected risks [1.40] or anticipate that the concerns aboutmalevolent nanoassemblers will be replaced by excitement over the field’s scientificand economic potentials [1.41]

Many scientists think that nanotechnology will eventually affect how peoplework, what they eat, how they communicate, and how long they live It will changetheir medical care, energy sources, water, and environment [1.42] Therefore, eval-uating the risks and benefits of nanotechnology, the health and environment impact

of nanotech products should be considered over their full life cycle, includingthe development, manufacture, transport, and disposal Appropriate internation-ally applied regulations should be developed to mitigate risks without stiflinginnovation [1.43] Considering the interplay with other emerging technologies,new global oversight mechanisms may be required that can handle the com-plex convergence of nanotechnology, biotechnology, and information and cognitivescience [1.42]

In the following, some physical principles of nanoscience will be discussedwithin this chapter, including thermal and electronic properties of nanostructures,giant magnetoresistance (GMR) and spintronics, self-assembly, Casimir forces,and nanoscale measuring techniques In the subsequent sections, the wider fields

of microscopy, synthesis of nanostructures, nanocrystals–nanowires–nanolayers,carbon nanostructures, bulk nanomaterials, nanomechanics–nanophotonics–nano-fluidics, nanomagnetism, nanotechnology for computers–memories–hard disks,nanochemistry, biology on the nanoscale, and nanomedicine will be presented

1.2 Thermal Properties of Nanostructures

1.2.1 Violation of the Second Law of Thermodynamics for Small Systems and Short Timescales

The second law of thermodynamics states that for large systems and over long timesthe entropy production rate is necessarily positive However, the fluctuation theorempredicts [1.44] measurable violations of the second law for small systems over shorttimescales This has been shown experimentally by following the trajectory of acolloidal particle in an optical trap An optical trap is formed when the micron-sized particle with a refraction index higher than that of the surrounding medium

is located within a focused laser beam; the refracted rays eventually exert a force

on the particle which can be resolved to 2×10−15 N From observing the

parti-cle’s position and the optical forces acting on the particle, the entropy productionrate can be determined [1.45] It turns out that entropy consuming, i.e., second-law-defying events, can be discerned for micron-sized particles on the timescales of

seconds This is particularly important to applications of nanomachines and ular motors As these nanomachines become smaller, the probability that they willrun thermodynamically in reverse inescapably increases [1.45].

molec-1.2.2 Surface Energy

The surface energy of nanoparticles has been measured to be higher than that of bulksolids This is essential for processes such as melting and evaporation This effectdepends on whether the particles are free or deposited on a substrate (see Table1.1;Fig.1.9)

The surface energyγ can, e.g., be determined according to the Kelvin equation

ps/ps0 = exp[4γ M/(ρpRTond M)]

by measuring the onset temperature Tonof evaporation in dependence of the particle

size dM(see Fig.1.9) where M is the molecular weight, ρpthe particle mass density,

R the gas constant, psthe vapor pressure of the nanoparticle, and ps0the vapor sure of a plane surface The increase ofγ with decreasing d M, which is supported

pres-Table 1.1 Surface energies of nanoparticles

[ 1.48 ] [ 1.49 ]

Fig 1.9 Onset temperature

Tonof the evaporation from

Ag nanoparticles in

dependence of the particle

size dM (Reprinted with

permission from [ 1.46 ].

© 2003 American Physical

Society)

1.2 Thermal Properties of Nanostructures 9

by molecular dynamics studies [1.50] and which is attributed to a weak dilatation ofthe nanoparticle surface, is discussed in a simple model in terms of a net increase ofthe inward cohesive force which is reduced in a particle on a substrate or in a matrixgiving rise to a reduction ofγ (see Table1.1)

1.2.3 Thermal Conductance

In nanosystems the classical picture of a diffusive heat flow mechanism is often notapplicable because the phonons or electrons that carry heat have mean free pathssimilar to or larger than the nanoscale feature size This is a challenge for heatremoval in microelectronic devices which already involve features with sizes of theorder of the mean free path

The thermal conductanceκ(Vg) of electrons in a semiconductor quantum wire at

low temperatures shows a quantized behavior in dependence of a gate voltage Vg

(Fig.1.10) This originates from the plateaus in the electrical conductance G(Vg)

quantized in units of GO = 2e2/h [1.51] In the case that charge and energy aretransported by electrons the Wiedemann–Franz relation

κ/GT = π2k2B/3e2= L o applies with LOthe Lorenz number From this relation it is expected that the elec-

trical conductance plateaus in units of GO are matched by a thermal conductance

Fig 1.10 (a) Quantized step-like behavior of the thermal conductance ˜G A(i, ii, iii) and of the trical conductance GAof a semiconductor quantum wire at 0.27 K obeying the Wiedemann–Franz

elec-relation (b) Close-up of (a) with a half plateauκ = LOT(GO/2) for G < GO (Reprinted with permission from [ 1.51 ] © 2006 American Physical Society)

quantized in units of LOTGO = π2k2B2T /3 h = 1.89· 10−12W /k2

T This is in

agreement with the data in Fig.1.10for G > GO

The temperature dependence of the phonon thermal conductivity K(T) has been

measured for individual multiwalled carbon nanotubes (MWNTs) [1.52] with a highvalue of over 3000 W/K m at room temperature (Fig.1.11) in the range of theoreticalexpectations of 3000–6000 W/K m [1.53] or of diamond or graphite (2000 W/K m[1.54])

In a simple model the phonon thermal conductivityκ = 

p

c p v p l p is given by

the specific heat cp, the group velocity vp, and the mean free path lpof the phonon

mode p The phonon mean free path consists of two contributions: l−1 = l−1st +

l−1

um where lst and lum are the static and umklapp scattering lengths, respectively

At low temperatures, the umklapp freezes out, l = lst, and therefore theκ of the

MWNT follows the temperature dependence of cp with k ∝ T2.5similar to that of3D graphite [1.55] and which therefore indicates the 3D features of MWNTs

Fig 1.11 The thermal conductance of an individual multiwalled carbon nanotube (MWNT) of

a diameter of 14 nm The solid lines represent linear fits of the data in a logarithmic scale at different temperatures with the slopes 2.50 and 2.01 Lower inset: the solid line represents κ(T) of

an individual MWNT (d = 14 nm) Broken and dotted lines represent small (d = 80 nm) and large

bundles (d = 200 nm) of MWNTs, respectively Upper inset: SEM image of suspended islands

with an individual MWNT Scale bar: 10 μm (Reprinted with permission from [ 1.52 ] © 2001 American Physical Society)

1.2 Thermal Properties of Nanostructures 11

For 50 K < T < 150 K, a κ ∝ T2 behavior is found (Fig 1.11) and athigher temperatures κ decreases due to the onset of phonon umklapp processes

and a rapidly decreasing lum From the peak value of κ(T) where lst ∼ lum a

T-independent value lst ∼ 500 nm for MWNTs can be estimated This means

that below room temperature phonon transport in the 2.5 μm long MWNT is

nearly ballistic with only a few scattering events The ratio of the thermal tancesκel/κphonof electrons and phonons increases rapidly when the temperature islowered [1.56]

conduc-The thermoelectric power of MWNTs [1.52] is found to be linearly increasing

with T which indicates a hole-like major carrier.

What happens, however, to the thermal conductivity of systems that are tively one dimensional, such as a single-wall carbon nanotube (SWNT), a nanowire,

effec-or a DNA molecule [1.57,1.58]? By analytical calculations it is estimated that thethermal conductivity diverges with a one-third power law as the length of a 1Dsystem increases This would be a very promising feature to use in the applica-tion of SWNTs, such as the design of components that dissipate heat efficiently innanocircuits

It should be mentioned here that for the investigation of heat conductivity inconfined dimensions extremely sensitive calorimeters are developed [1.59] with a

low-temperature heat capacity of c ≈ 103kB[1.59] This may lead to an energysensitivity sufficient to count individual thermal phonons at 10–100 mK and observethe particle nature of phonons [1.59]

1.2.4 Melting of Nanoparticles

According to simple theories for spherical nanoparticles the bulk melting

tempera-ture Tbis decreased in terms of the Gibbs–Thompson equation by

Tm= 4γ ν0Tb/Ld

with d the particle diameter, γ the surface energy of the solid, L the latent heat of

fusion, andν 0 the molar volume of the solid [1.60] This qualitatively applies tonanoparticles embedded in matrices or deposited on substrates (see Table1.2andFig.1.12) However, the melting behavior of embedded particles may be stronglyaffected by the characteristics of the embedding material

For free Na clusters, Tmis found to be lower than the bulk Tb(see Table1.2) butroughly independent of cluster size (Fig.1.13)

For free Ga clusters and free Sn clusters an increase of Tmcompared to the bulkmaterials is observed (see Table1.2) which is attributed to more covalent bonding

in the cluster in contrast to covalent-metallic bonding of the bulk material [1.61]

The latent heat of fusion (0.012 eV/atom) as well as the entropy of fusion (0.5 kB)

of Na clusters is lower than the bulk values (0.025 eV/atom; 0.85 kB) [1.62].The evaporation of atoms from nanoparticles is facilitated when the particle size

Table 1.2 Melting

temperatures Tm of free

nanoparticles, of particles in

matrices or on substrates, and

of the bulk materials

Fig 1.12 Melting temperatures of In particles in CPG glass or Vycor glass matrices as a function

of particle size d (Reprinted with permission from [1.68 ] © 1993 American Physical Society)

decreases This is concluded from the observation that the temperature at whichfree PbS nanoparticles start to evaporate decreases with decreasing particle size[1.63]

1.2.5 Lattice Parameter

The lattice parameter of Pd nanoparticles in a polymer matrix decreases by about3% when the particle size decreases from 10 to 1.4 nm [1.72] A similar behavior isobserved for Ag nanoparticles [1.73]

1.2 Thermal Properties of Nanostructures 13

Fig 1.13 Melting temperatures of Na+ clusters versus the cluster size The oscillations do not

cor-relate with electronic (dotted lines) or geometric (dashed lines) shell closings The smallest cluster

Na +

55 which has an icosahedral structure exhibits the highest melting temperature Tm = 290 K

where the bulk melting temperature is 371 K (Reprinted with permission from [ 1.62 ] © 2002 Elsevier)

1.2.6 Phase Transitions

Phase transitions in confined systems differ, as shown in the case of the meltingtransition above, from those of bulk materials and strongly depend on particle size,wetting, as well as the interaction of a nanoscale system with a matrix or a substrate[1.60] Phase transitions in nanosized systems have been investigated for superflu-idity where the critical behavior of superfluid He in an aerogel deviates from that ofbulk He [1.74] In superconductivity the superconducting transition temperatures of

Ga [1.75] or In nanoparticles [1.76] in vycor glass are shifted to values higher than in

the bulk materials The Curie temperature TCof ferromagnetic nanolayers decreaseswith decreasing layer thickness [1.77] Liquefaction [1.78] in nanopores or order–disorder phase transitions near surfaces [1.79] was also found to differ from that ofbulk systems Two examples of solid–solid phase transitions on the nanoscale will

be sketched in the following

Under elevated pressures PbS undergoes a B1-to-orthorhombic structural phasetransition For PbS nanoparticles in a NaCl matrix, the transition is shifted to higherpressures when the particle size is reduced [1.80]

The stability of crystal structures at ambient conditions depends on the size ofZrO2nanocrystals which exhibit a tetragonal structure for small sizes and the bulkorthorhombic structure for larger sizes [1.81,1.82] In this oxide, the lattice strainvaries with the grain sizes giving rise to a variation of the Landau free energy sothat for small grain sizes the tetragonal phase is stabilized whereas for grain sizes

d > 54 nm the bulk orthorhombic phase appears.

1.3 Electronic Properties

1.3.1 Electron States in Dependence of Size and Dimensionality

Standard quantum mechanical texts show (see [1.83]) that for an electron in an

infinitely deep square potential well of width a in one dimension, the coordinate

x has the range values−1

In addition to quantum confinement effects, quantization effects appear in

nanoparticles because the charge of the electron is quantized in units of e [1.84,

1.85] When the electron with the quantized charge tunnels to an island with the

capacitance C (see Fig.1.14a) the electrostatic potential of the island changes by

discrete values of the charging energy Ec = e2/C This can be detected at low

temperatures when the energies fluctuations

should be much higher than the resistance quantum h /e2= 25.813k.

These conditions can be met by small dots with low C values and weak tunneling

coupling For a quantum dot sphere with a 1μm diameter the value Ec ∼= 3 meV

is found [1.84] which can be easily resolved at low temperatures If a voltage Vg

is applied to the gate capacitor cg (Fig.1.14a) a charge is induced on the island

which leads to the so-called Coulomb oscillations of the source-drain conductance

as a function of Vgat a fixed source-drain voltage Vsd (Fig.1.14b) In the valleys

between the oscillations, the number of electrons on the dots is fixed to the integer N with zero conductance (Coulomb blockade) Between the two stable configurations

N and N+1 a “charge degeneracy” (see Fig.1.14b) appears where the number of

electrons can alternate between N an N+1 This produces a current flow and results

in the observed current peaks

1.3 Electronic Properties 15

Fig 1.14 (a) Schematics of a

quantum dot (island)

connected to three terminals:

source, drain, and gate The

terminals and the island are

separated by thin insulating

layers (b) Coulomb

oscillations for illustrating the

effect of single electronic

charges on the macroscopic

conductance I /Vsd The

period in the gate voltage is

about e /Cg (Reprinted with

permission from [ 1.84 ].

© 1999 Springer Verlag)

An alternative measurement with fixed gate voltage Vgand varying source-drain

voltage shows nonlinear current–voltage characteristics exhibiting a Coulomb

stair-case (see Fig.1.15) A new current step occurs at a threshold voltage e2/C at which

an extra electron is energetically allowed to enter the island This threshold age is periodic in the gate voltage (see Fig.1.15) in accordance with the Coulomboscillations of Fig.1.14(b)

volt-Quantum confinement with spacingE between the energy levels and effects of

charge quantization can be observed simultaneously Electrons in a semiconductorhetero-interface quantum dot [1.84] with a diameter of 100 nm yield a level spac-ing of∼0.03 mV which can be detected at dilution refrigeration temperatures The

change of the gate voltageVg (see Fig.1.14a) between current oscillations in aquantum dot is given by

eCg



E + e2C



where Cgis the gate capacitance (Fig.1.14a) By sweeping Vg, a peak structure in

the current is observed for kBT << E << e2/C (see Fig.1.16) where the

peak-to-peak distance is ascribed to the addition energy e2/C + E and the separation of

the minipeaks to the level spacingE.

Fig 1.15 Coulomb

staircases in the I − Vsd

characteristics of a

GaAs/AlGaAs

hetero-structure The different

curves which are shifted

vertically for clarity ( I= 0

occurs at Vsd = 0) are taken

for five different gate voltages

to illustrate the periodicity in

accordance with the

corresponds to the addition

energy, the distance between

the shoulders within a peak to

the excitation energyE

(level spacing) for a constant

number of electrons on a dot.

(Reprinted with permission

from [ 1.84 ] © 1999 Springer

Verlag)

1.3.2 The Electron Density of States D(E)

The electron density of states D(E) depends dramatically on the dimensionality

of nanostructures (see Fig.1.17) Whereas for bulk systems a square-root dence of energy prevails, a staircase behavior is characteristic for 2D-quantum wellstructures, spikes are found in 1D quantum wires, and discrete features appear in0D quantum dots Since many solid-state properties are dominated by the elec-

depen-tron density of states, these properties, such as the elecdepen-tronic specific heat Cel,the Pauli conduction electron magnetic susceptibility χel, the thermopower, thesuperconducting energy gap, etc., are sensitive to dimensional changes