Defects at Oxide Surfaces (eBook)

Preface 6
Contents 8
Contributors 15
1 Numerical Simulations of Defective Structures: The Nature of Oxygen Vacancy in Non-reducible (MgO, SiO2, ZrO2) and Reducible (TiO2, NiO, WO3) Oxides 17
Abstract 17
1.1 Introduction: The Role of Defects in Oxide Materials 17
1.2 Treating Defects in Solids: Periodic Models and Local Modes 20
1.2.1 Periodic Models 20
1.2.2 Local Cluster Models 21
1.2.3 Embedding Schemes 22
1.3 Problems of DFT in Describing Defects in Insulators: Some Instructive Examples 24
1.4 The Oxygen Vacancy in Insulating and Semiconducting Oxides 26
1.4.1 Non-reducible Oxides: SiO2, MgO, ZrO2 27
1.4.2 Reducible Oxides: TiO2, NiO and WO3 32
1.5 Conclusions 38
Acknowledgments 40
References 40
2 Atomic Scale Characterization of Defects on Oxide Surfaces 45
Abstract 45
2.1 Introduction 45
2.2 Point Defects 46
2.2.1 Non-reducible Oxides: MgO 46
2.2.1.1 Defects on MgO Surfaces---A Brief Overview 46
2.2.1.2 MgO Model Surfaces 52
2.2.1.3 Adsorption on Low-Coordinated Sites 54
Mg2+-CO Interaction 54
Metal Nucleation at MgO Step Sites 57
2.2.1.4 Identification of Color Centers on MgO Thin Films 58
2.2.1.5 Interaction of Gold with Color Centers 61
2.2.2 Reducible Oxides 64
2.2.2.1 Characterization of O-Vacancies in CeO2(111) Films 65
2.2.2.2 Adsorption at Ceria Point Defects 69
2.3 Line Defects 74
2.3.1 Dislocation Network in Alumina Thin Films 74
2.3.2 Line Defects in MgO Thin Films 79
2.3.3 Electron Trapping in MgO Line Defects 82
2.4 Summary 86
References 87
3 Defects on TiO2---Key Pathways to Important Surface Processes 97
Abstract 97
3.1 Overview of Defects in Solids 97
3.1.1 0D Defects (Point Defects) 98
3.1.2 1D Defects (Line Defects) 101
3.1.3 2D Defects (Interfacial Defects) 102
3.1.4 3D Defects (Bulk Defects) 103
3.1.5 Defects on Surfaces 103
3.2 Crystal Forms of TiO2 104
3.3 Oxygen Point Defects 106
3.3.1 Oxygen Vacancy 106
3.3.1.1 Thermal-Annealing Produced Oxygen Vacancy 108
3.3.1.2 Electron-Bombardment Produced Oxygen Vacancy 109
3.3.1.3 UV-Irradiation Produced Oxygen Vacancy 110
3.3.1.4 Reaction-Produced Oxygen Vacancy (Mars-van Krevelen Mechanism) 111
3.3.1.5 Doping-Produced Oxygen Vacancy 113
3.3.1.6 Other Methods of Making Oxygen Vacancies 113
3.3.2 Chemisorbed Ot Defect 113
3.4 Ti Point Defects 116
3.5 H Point Defects on TiO2 118
3.5.1 OH/TiO2 119
3.5.2 Atomic H/TiO2 121
3.6 Doping of TiO2 123
3.7 Summary 125
Acknowledgments 125
References 125
4 Excess Electrons at Oxide Surfaces 138
Abstract 138
4.1 Introduction 138
4.2 Rutile TiO2(110) Surface 139
4.2.1 Point Defects Versus Excess Charges 140
4.2.2 Spectroscopic Fingerprint of Excess Charges 141
4.2.3 Location of Excess Electrons 143
4.2.3.1 Surface and Subsurface 143
4.2.3.2 Interstitials Ti Ions 147
4.2.4 Excess Charges Located Independently of Their Origin 148
4.3 Adsorbates on Rutile (110) 149
4.3.1 Molecular and Dissociative Adsorption of Oxygen 149
4.3.2 Hydroxylated Sites 152
4.4 Anatase 154
4.4.1 Reduced Anatase 155
4.4.2 Defective Anatase (101) Surface 155
4.4.3 Negatively Charged Oxygen on Reduced Anatase (101) 158
4.5 Conclusion 159
Acknowledgments 159
References 159
5 Oxygen Defects at Reducible Oxide Surfaces: The Example of Ceria and Vanadia 163
Abstract 163
5.1 Introduction 163
5.2 Models and Computational Methods 165
5.2.1 Point Defect Modeling 165
5.2.2 Electronic Structure Methods 166
5.2.3 Defect Formation Energy and Statistical Thermodynamics 169
5.3 Cerium Oxide 170
5.3.1 Experimental Findings on Oxygen Defects 171
5.3.2 Structure Relaxation and Electronic Structure 172
5.3.3 Defect Stability 177
5.3.3.1 Isolated Near-Surface Defects 177
5.3.3.2 Near-Surface Defect Aggregates 180
5.3.4 Summary of Cerium Oxide 183
5.4 Vanadium Oxide 184
5.4.1 Experimental Findings on Oxygen Defects 185
5.4.2 Structure Relaxation and Electronic Structure 186
5.4.3 Defect Stability 190
5.4.3.1 Isolated Defects 191
5.4.3.2 Vanadyl Defect Aggregates 193
5.4.4 Summary Vanadium Oxide 194
5.5 Summary and Outlook 195
Acknowledgments 197
References 197
6 The Structure and Properties of Clean Steps at Oxide Surfaces 205
Abstract 205
6.1 Introduction 205
6.2 The Atomic Structure of Steps 207
6.3 Understanding and Predicting Step Stabilities 213
6.3.1 Calculation of the Step Stability 213
6.3.2 Surface Morphologies from Step Formation Energies 216
6.4 The Electronic Structure of Steps 219
6.5 Point Defects at Steps 222
6.6 Outlook 224
Acknowledgments 225
References 225
7 Defects on Bulk MgO(001) Imaged by nc-AFM 229
Abstract 229
7.1 Introduction 229
7.2 Electrostatic AFMs 229
7.2.1 Principles of EFM and KPFM 230
7.2.2 Charged Defects in EFM and KPFM 233
7.3 Magnesium Oxide Surfaces 234
7.3.1 Bulk MgO(001) Surfaces 235
7.3.1.1 Surface Preparation and Surface Charging 235
7.3.1.2 The Clean Surface and Its Defects 238
7.3.1.3 Interpretation of the Atomic Contrast 239
7.3.1.4 Kelvin Microscopy 244
7.3.2 Summary and Perspectives 246
Acknowledgments 248
References 248
8 Noncontact AFM Imaging of Atomic Defects on the Rutile TiO2(110) Surface 254
Abstract 254
8.1 Introduction 254
8.2 Noncontact Atomic Force Microscopy 255
8.2.1 Principle of Noncontact Atomic Force Microscopy 256
8.2.2 Contrast Formation in Noncontact AFM Images 257
8.2.3 Noncontact AFM Image Simulations 260
8.3 Defects on Rutile TiO2(110) Studied with Noncontact AFM 262
8.3.1 The Rutile TiO2(110) Surface and Its Surface Defects 263
8.3.2 Noncontact AFM Contrast on the Rutile TiO2(110) Surface 266
8.3.3 Tip Influence on the nc-AFM Contrast on the Rutile TiO2(110) Surface 270
8.3.4 Force Spectroscopy on the Rutile TiO2(110) Surface 272
8.3.5 Water Splitting Products, H Diffusion and Subsurface H on the TiO2(110) Surface 275
8.3.6 Summary and Perspectives 278
Acknowledgments 279
References 279
9 Defects in Metal Oxide Nanoparticle Powders 286
Abstract 286
9.1 Introduction 286
9.1.1 Particle Systems and the Hierarchy of Defects 286
9.1.2 Stoichiometry, Levels of Oxygen Deficiency and n-Type Doping 291
9.2 Experimental Probes for Defects in Nanoparticle Ensembles 293
9.2.1 Materials Characterization for Powder Systems: Inherent Challenges, Faults and the Requirement for an Integrated Approach 293
9.2.2 Point Defects and Experimental Fingerprints 293
9.2.2.1 Optical Absorption and Emission 294
9.2.2.2 Vibrational Spectroscopy 296
9.2.2.3 Magnetic Resonance Techniques 297
9.2.2.4 X-ray Absorption and X-ray Photoelectron Spectroscopy 297
9.2.2.5 X-ray and Neutron Diffraction 297
9.3 The Quest for Morphologically Defined Particle Systems 298
9.3.1 Science of Synthesis 298
9.3.2 MgO Powders as Model System for Highly Dispersed Solids 298
9.3.3 Particle Interfaces and the Microstructure of Powders 300
9.3.4 Powders of Facetted TiO2 Particles 301
9.3.4.1 Fluorinated Surface Planes on TiO2 Particles 302
9.3.4.2 Unintentional Nitrogen Incorporation in TiO2 Particles 302
9.3.4.3 Solid-Solid Interface Formation in TiO2 Nanoparticle Networks 303
9.4 Summary and Outlook 305
Acknowledgments 306
References 306
10 Point Defects in Electron Paramagnetic Resonance 315
Abstract 315
10.1 Introduction 315
10.2 The EPR Techniques: A Summary 316
10.2.1 The Spin-Hamiltonian Formalism 316
10.2.2 Single Crystal Systems 318
10.2.3 Polycrystalline Systems 319
10.3 Colour Centres in the Bulk of Ionic Solids 320
10.4 Localized Holes and V Centers in Ionic Solids 323
10.5 Surface Defects as Electron Traps: A Paradigm Shift 324
10.6 Surface Trapping Sites for Charge Carriers 330
10.6.1 Insulating Ionic Oxides 331
10.6.2 Semiconducting Oxides 335
10.7 Conclusions 336
References 336
11 Defects on Strontium Titanate 339
Abstract 339
11.1 Introduction 339
11.2 Defects in Bulk SrTiO3 340
11.2.1 Point Defects 340
11.2.2 Vacancy Clusters 341
11.2.3 Ruddlesden-Popper Phases 342
11.2.4 Dislocations and Defects 343
11.2.5 Defects Introduced by Ar-Bombardment 343
11.3 Surfaces of SrTiO3 345
11.3.1 Surface Structure of SrTiO3(001) 345
11.3.2 Polyhedral Quartet Structural Motif 346
11.3.3 Nanostructured SrTiO3 346
11.4 Surface Defects 351
11.4.1 Defects at the Surface of SrTiO3 351
11.4.2 Surface Vacancy Clusters 351
11.4.3 Polyhedral Quartet Defects 352
11.4.4 Triline Defects 353
11.5 Defect Diffusion 354
11.5.1 Point Defect Diffusion 354
11.5.2 Diffusion of Defect Clusters 354
11.6 Conclusions 358
References 358
12 Dopant and Defect Induced Electronic States at In2O3 Surfaces 362
Abstract 362
12.1 Introduction 362
12.2 The Structure of In2O3 and Its Surfaces 363
12.2.1 Bulk Structure of In2O3 363
12.2.2 Energetics of Surfaces and Surface Structures 365
12.3 Bulk Electronic Structure 369
12.3.1 Basic Features of Electronic Structure 369
12.3.2 The Bulk Bandgap 369
12.3.3 The Charge Neutrality Level in In2O3 374
12.4 Defects and Doping 376
12.4.1 Bulk Defect States 376
12.4.2 Defects at Surfaces 376
12.4.3 Bulk N-Type Doing: Chemical Aspects 378
12.4.4 Bulk N-Type Doping: The Effective Mass and the Onset of Degeneracy in Doped In2O3 379
12.4.5 Doping and Plasmons 382
12.5 Materials Preparation 385
12.6 Photoemission Studies of In2O3 and Sn-Doped In2O3 388
12.6.1 Nominally Undoped In2O3: Band Bending and Carrier Accumulation 388
12.6.2 Electronic States in the Bulk Bandgap 393
12.6.3 Dopant Induced Electronic States: Conduction Band Photoemission 395
12.6.4 Satellite Structure in Core Level Photoemission of Doped Samples 397
12.7 Summary and Outlook 399
Acknowledgements 401
References 401
13 Resistive Switching in Oxides 412
Abstract 412
13.1 Introduction 412
13.2 Classes of Resistive Switching 414
13.3 Phenomenology of Filamentary Resistive Switching 416
13.3.1 Electroforming 418
13.3.2 Extrinsic Filamentary Resistive Switching 419
13.3.3 Intrinsic Filamentary Resistive Switching 420
13.4 Memristive Systems and Memristor Model 423
13.5 Quantisation of Conductance 426
13.6 Technological Considerations 430
13.6.1 Switching Endurance 432
13.6.2 Uniformity of Switching Voltages and Currents 433
13.6.3 Data Retention 434
13.6.4 The Need for Selection Elements 434
References 435
14 Photon-, Electron-, and Scanning Tunneling Microscopy-Induced Defects on Oxide Surfaces 440
Abstract 440
14.1 Introduction 440
14.2 Photon-Induced Defects 441
14.3 Electron-Induced Defects 447
14.4 Scanning Tunneling Microscopy-Induced Defects 452
14.5 Future Outlook 457
Acknowledgments 458
References 458
Index 463