Jinbo Cao

Strain engineering and one-dimensional organization of metal-insulator domains in single-crystal VO2 beams

Correlated electron materials can undergo a variety of phase transitions, including superconductivity, the metal-insulator transition and colossal magnetoresistance. Moreover, multiple physical phases or domains with dimensions of nanometres to micrometres can coexist in these materials at temperatures where a pure phase is expected. Making use of the properties of correlated electron materials in device applications will require the ability to control domain structures and phase transitions in these materials. Lattice strain has been shown to cause the coexistence of metallic and insulating phases in the Mott insulator VO(2). Here, we show that we can nucleate and manipulate ordered arrays of metallic and insulating domains along single-crystal beams of VO(2) by continuously tuning the strain over a wide range of values. The Mott transition between a low-temperature insulating phase and a high-temperature metallic phase usually occurs at 341 K in VO(2), but the active control of strain allows us to reduce this transition temperature to room temperature. In addition to device applications, the ability to control the phase structure of VO(2) with strain could lead to a deeper understanding of the correlated electron materials in general.

Extended Mapping and Exploration of the Vanadium Dioxide Stress-Temperature Phase Diagram

Single-crystal micro- and nanomaterials often exhibit higher yield strength than their bulk counterparts. This enhancement is widely recognized in structural materials but is rarely exploited to probe fundamental physics of electronic materials. Vanadium dioxide exhibits coupled electronic and structural phase transitions that involve different structures existing at different strain states. Full understanding of the driving mechanism of these coupled transitions necessitates concurrent structural and electrical measurements over a wide phase space. Taking advantages of the superior mechanical property of micro/nanocrystals of VO(2), we map and explore its stress-temperature phase diagram over a phase space that is more than an order of magnitude broader than previously attained. New structural and electronic aspects were observed crossing phase boundaries at high-strain states. Our work shows that the actively tuning strain in micro/nanoscale electronic materials provides an effective route to investigate their fundamental properties beyond what can be accessed in their bulk counterpart.

Thermoelectric Effect across the Metal−Insulator Domain Walls in VO2 Microbeams

We report on measurements of Seebeck effect in single-crystal VO(2) microbeams across their metal-insulator phase transition. One-dimensionally aligned metal-insulator domain walls were reversibly created and eliminated along single VO(2) beams by varying temperature, which allows for accurate extraction of the net contribution to the Seebeck effect from these domain walls. We observed significantly lower Seebeck coefficient in the metal-insulator coexisting regime than predicted by a linear combination of contributions from the insulator and metal domains. This indicates that the net contribution of the domain walls has an opposite sign from that of the insulator and metal phases separately. Possible origins that may be responsible for this unexpected effect were discussed in the context of complications in this correlated electron material.

Colossal thermal-mechanical actuation via phase transition in single-crystal VO2 microcantilevers

The spontaneous strain associated with the structural change in the metal-insulator transition in VO2 is orders of magnitude higher than thermal expansion mismatch used in bimetallic strips. Here we show that this strain can be leveraged to thermally activate bending of crystalline VO2-based bilayer microcantilevers at extremely large curvatures, making them suitable for thermal sensors, energy transducers and actuators with unprecedented sensitivities. The single-crystallinity, deposition conditions, and postdeposition treatments were utilized to control the metal-insulator domain structure along the cantilever, by which we achieved bending curvatures a few hundred times higher than conventional bilayer cantilevers with the same geometry.

Functional metal oxide nanostructures

Preface 1. New Opportunities on Phase Transitions of Correlated Electron Nanostructures 1.1. Introduction 1.2. Electrical and Structural Transitions in VO2 1.3. Experimental Methods 1.4. Results and Discussions 1.4.1. Phase Inhomogeneity and Domain Organization 1.4.2. Domain Dynamics and Manipulation 1.4.3. Investigation of Phase Transition at the Single Domain Level 1.4.4. Superelasticity in Phase Transition 1.4.5. New Phase Stabilization with Strain 1.4.6. Thermoelectric Across the Metal-Insulator Domain Walls 1.5. Conclusions 2. Controlling the Conductivity in Oxide Semiconductors 2.1. Introduction 2.2. Formalism and Computational Approach 2.3. Results and Discussion 2.3.1. ZnO 2.3.2. SnO2 2.3.3. TiO2 2.4. Concluding Remarks 3. The Role of Defects in Functional Oxide Nanostructures 3.1. Introduction 3.2. Defects in Metal Oxide Nanostructures 3.2.1. Defect Structures in Metal Oxide Nanostructures3.2.2. Imaging Defects in Metal Oxide Nanostructures 3.2.3. Stability of Intrinsic Point Defects in Metal Oxide Nanostructures 3.3. Electrical Response 3.3.1. Point Defects and Charge Carriers 3.3.2. Defects and P-Type Conductivity 3.3.3. Defects and Conduction Mechanisms 3.3.4. Plasmon Response in Defect-Rich Oxide Nanostructures 3.4. Optical Response 3.4.1. Photoluminescence from Point Defects in Oxide Nanostructures 3.4.2. Raman Studies on Oxide Nanostructures 3.4.3. Magneto-Optical Properties of Oxide Nanostructures 3.5. Magnetic Response 3.5.1. Magnetism in Metal Oxide Nanoparticles 3.5.2. Ferromagnetism in Defect-Rich Semiconducting Metal Oxides 3.5.3. Spin Polarization in Defect-Rich Metal Oxide Nanostructures 3.5.4. Mechanisms for Magnetism in Metal Oxide Nanostructures 3.6. Defect Engineering in Metal Oxide Nanostructures 3.7. Conclusions 4. Emergent Metal-Insulator Transitions Associated with Electronic Inhomogeneities in Low-Dimensional Complex Oxides 4.1. Introduction 4.2. Experimental Approach 4.2.1. Fabrication of Spatially Confined Oxide Nanostructures 4.2.2. Cryogenic Four-Probe STM 4.3. Results and Discussion4.3.1. Percolative Mott Transition in Sr3(Ru1-xMnx)2O7 4.3.2. Confinement Effects and Tunable Emergent Behavior in La5/8-xPrxCa3/8MnO3 4.4. Conclusion 5. Optical Properties of Nanoscale Transition Metal Oxides 5.1. Physical, Chemical and Size-Shape Tunability in Transition Metal Oxides 5.2. Optical Spectroscopy as a Probe of Complex Oxides 5.3. Quantitative Models 5.3.1. Confinement Models 5.3.2. Descriptions of Inhomogeneous Media 5.3.3. Inhomogeneous Media and Surface Plasmons 5.3.4. Charge and Bonding Models 5.4. Charge-Structure-Function Relationships in Model Nanoscale Materials 5.4.1. Mott Transition in VO2 Revealed by Infrared Spectroscopy 5.4.2. Visualizing Charge and Orbitally Ordered Domains in La1/2Sr3/2MnO4 5.4.3. Discovery of Bound Carrier Excitation in Metal Exchanged Vanadium Oxide Nanoscrolls and Size Dependence of the Equatorial Stretching Modes 5.4.4. Classic Test Cases: Quantum Size Effects in ZnO and TiO2 5.4.5. Optical Properties of Polar Oxide Thin Films and Nanoparticles 5.4.6. Spectroscopic Determination of H2 Binding Sites and Energies in Metal-Organic Framework Materials 5.5. Summary and Outlook 6. Electronic Properties of Post-Transition Metal Oxide Semiconductor Surfaces 6.1. Introduction 6.2. Surface Space-Charge Properties 6.2.1. ZnO 6.2.2. Ga2O3 6.2.3. CdO 6.2.4. In2O3 6.2.5. SnO2 6.3. Bulk Band Structure Origin of Electron Accumulation Propensity 6.4. Conclusion 7. In Search of a Truly Two-Dimensional Metallic Oxide 7.1. Introduction 7.2. Methodology 7.3. Results and Discussion 8. Solution Phase Approach to TiO2 Nanostructures8.1. Introduction 8.2. Approaches 8.2.1. Porous Architectures Through Templated Self Assembly 8.2.2. 1-D Structures from Anodization 8.2.3. Imprinting and Molding 8.2.4. Templated Electrochemical Sythesis 8.2.5. Single Crystalline 1-D Structures by Solution Phase Hydrothermal Growth 8.3. Conclusion 9. Oxide-Based Photonic Crystals from Biological Templates 9.1. Introduction 9.2. Engineered Photonic Crystals 9.2.1. Characteristics of Photonic Band Structure Materials 9.2.2. Photonic Crystals Operating in the Infrared 9.2.3. Photonic Crystals Operating at Visible Frequencies 9.3. Natural Photonic Crystals 9.3.1. Structural Colors in Biology 9.3.2. Structure Evaluation Methods 9.3.3. Examples of Biological Photonic Structures 9.4. Bio-Templated Photonic Crystals 9.4.1. General Considerations 9.4.2. Biotemplating Techniques 9.4.2.1. Deposition and Evaporation Methods 9.4.2.2. Sol-Gel Chemistry Methods 9.4.3. Biotemplated Bandgap Crystals 9.5. Conclusions 10. Low-Dimensionality and Epitaxial Stabilization in Metal Supported Oxide Nanostructures: MnxOy on Pd(100) 10.1. Introduction 10.2. Growth of MnxOy- Layers on Pd(100) 10.2.1. Low Coverage Regime 10.2.1.1. MnO(111)-like Phases (Oxygen-Rich Regime) 10.2.1.2. MnO(100)-like Phases (Intermediate Oxygen Regime) 10.2.1.3. The Reduced Phases (Oxygen-Poor Regime) 10.2.2. High Coverage Regime 10.2.2.1. Formation of Mn3O4 on MnO(001) 10.2.2.2. Epitaxial Stabilization of MnO(111) Overlayers 11. One Dimensional Oxygen-Deficient Metal Oxides 11.1. Introduction11.2. Oxygen-Deficient 1D-Nano-Ceo2-x and its Applications in the WGS Reaction 11.2.1. Crystal Structure of Cubic-Ceria 11.2.2. Backround of the WGS Reaction 11.2.3. Synthesis of 1D-Ceria 11.2.4. Testing 1D-Ceria for the WGS Reaction 11.3. Sub-Stoichiometric Magneli Phases 1D-TinO2n-1 11.4. Sub-Stoichiometric Chromium Oxide Nanobelts with Modulation Structures 11.5. Summaries 12. Oxide Nanostructures for Energy Storage 12.1. Introduction 12.2. Nano Oxides for Li-Ion Batteries 12.2.1. Spinel LiMn2O4 12.2.2. Manganese Dioxide 12.2.3. Vanadium Pentoxide (V2O5) 12.2.4. Titanium Oxide 12.2.5. Metal Oxides with Displacement Mechanism 12.2.6. Nano-Oxide Coatings 12.3. Nano Oxide for Electrochemical Capacitors 12.3.1. Ruthenium Oxide (RuO2) 12.3.2. Manganese Oxide (MnO2) 12.3.3. Other Metal Oxides 12.3.4. Hierarchical Metal Oxide-Carbon Composites 12.4. Summary 13. Metal Oxide Resistive Switching Memory 13.1. Introduction 13.1.1. Device Operation 13.1.2. Device Characteristics 13.2. Possible Physical Mechanism for Resistive Switching 13.2.1. Conduction Mechanism 13.2.2. Electroforming/Set/Reset Process with Oxygen Migration 13.2.3. The Effect of Electrode Materials on Switching Modes 13.2.4. Summary of the Physical Mechanism for Resistive Switching in Metal Oxide Memory 13.3. Performances of Metal Oxide Memory Devices 13.4. Cell Structure of Metal Oxide Memory Arrays 13.5. Summary 14. Nano Metal Oxides for Li-Ion Batteries 14.1. Classification of Electrode Materials for Li-Ion Batteries 14.2. Advantage & Disadvantage of Nano-Electrode Materials 14.3. Nano Metal Oxide Anode Materials 14.3.1. Intercalation Metal Oxides 14.3.2. Conversion Metal Oxide Materials 14.3.3. Displacement Metal Oxide Materials 14.3.3.1. Tin Dioxides Based Anode Materials 14.4. Nano Metal Oxide Cathode Materials 14.4.1. Nanoscale Cathode Materials 14.4.2. Nanostructured Cathode Materials 14.5. Nano Metal Oxides in Electrolyte 14.6. Conclusion and Outlook

Heat transfer across the interface between nanoscale solids and gas.

When solid materials and devices scale down in size, heat transfer from the active region to the gas environment becomes increasingly significant. We show that the heat transfer coefficient across the solid-gas interface behaves very differently when the size of the solid is reduced to the nanoscale, such as that of a single nanowire. Unlike for macroscopic solids, the coefficient is strongly pressure dependent above ∼10 Torr, and at lower pressures it is much higher than predictions of the kinetic gas theory. The heat transfer coefficient was measured between a single, free-standing VO(2) nanowire and surrounding air using laser thermography, where the temperature distribution along the VO(2) nanowire was determined by imaging its domain structure of metal-insulator phase transition. The one-dimensional domain structure along the nanowire results from the balance between heat generation by the focused laser and heat dissipation to the substrate as well as to the surrounding gas, and thus serves as a nanoscale power-meter and thermometer. We quantified the heat loss rate across the nanowire-air interface, and found that it dominates over all other heat dissipation channels for small-diameter nanowires near ambient pressure. As the heat transfer across the solid-gas interface is nearly independent of the chemical identity of the solid, the results reveal a general scaling relationship for gaseous heat dissipation from nanostructures of all solid materials, which is applicable to nanoscale electronic and thermal devices exposed to gaseous environments.

Thermodynamics of strained vanadium dioxide single crystals

Vanadium dioxide undergoes a metal–insulator transition, in which the strain condition plays an important role. To investigate the strain contribution, a phenomenological thermodynamic potential for the vanadium dioxide single crystal was constructed. The transformations under the uniaxial stress, wire, and thin film boundary conditions were analyzed, and the corresponding phase diagrams were constructed. The calculated phase diagrams agree well with existing experimental data, and show that the transformation temperature (and Curie temperature) strongly depends on the strain condition.

Dense Electron System from Gate-Controlled Surface Metal–Insulator Transition

Two-dimensional electron systems offer enormous opportunities for science discoveries and technological innovations. Here we report a dense electron system on the surface of single-crystal vanadium dioxide nanobeam via electrolyte gating. The overall conductance of the nanobeam increases by nearly 100 times at a gate voltage of 3 V. A series of experiments were carried out which rule out electrochemical reaction, impurity doping, and oxygen vacancy diffusion as the dominant mechanism for the conductance modulation. A surface insulator-to-metal transition is electrostatically triggered, thereby collapsing the bandgap and unleashing an extremely high density of free electrons from the original valence band within a depth self-limited by the energetics of the system. The dense surface electron system can be reversibly tuned by the gating electric field, which provides direct evidence of the electron correlation driving mechanism of the phase transition in VO(2). It also offers a new material platform for implementing Mott transistor and novel sensors and investigating low-dimensional correlated electron behavior.