Modulation Of Metal-insulator Phase Transitions In Transition Metal Oxides And The Related Synchrotron Radiation Study

With the development of modern information technology,the core optoelectronic device units increasingly pursue lower power consumption and higher integration degree,and become more multi-functional.Oxide materials(especially transition metal oxides)show great application potentials in low-power multifunctional optoelectronic device applications due to their unique macroscopic physical or chemical properties.In fact,most transition metal oxides have typical strong correlated electronic structures and metal insulator phase transitions,thus show rich electrical,optical,magnetism and complex coupling mechanism.These characteristics have attracted tremendous attention.Among those transition metal oxides,those materials with unoccupied 3d orbitals always show strongly correlated electrons behavior,which makes them very sensitive to the external field.For example,applying a small incentive on this material can trigger significant metal-insulator phase transitions or other macroscopic mutations.So these oxide materials always have important research value and application prospects.Therefore,the current thesis focuses on the metal-insulator phase transition of transition metal oxides with unoccupied 3d orbital systems,and mainly concentrates on these two classical systems:vanadium dioxide(VO2)and perovskite rare earth metal nickel oxides(RNiO3,R=rare earth elements).The main contents and results of this paper are as follows:(1)A new method based on water vapor assisted oxidation growth of large-size VO2 crystalline film has been achieved.Due to the multi-chemmical valence state and multi-phase structure of vanadium oxide,it is always difficult to prepare pure phase VO2 film.In order to sovel this bottleneck problem,we have conducted a wet oxidation with water vapor as mild oxidant to prepare wafer-size VO2 film with high quality.The VO2 crystalline films are characterized by synchronous radiation based X-ray photoemission spectroscopy and absorption spectroscopy.Results show that the wet-oxidation method can not only produce dense and pure monoclinic VO2 films,but also prefer to form quite uniform films with wafer-size.In addition,the resistance change of the obtained VO2 film is up to four orders of magnitude before and after the phase transition.By analyzing the thermodynamic process in the chemical reaction,the chemical reaction path of vanadium oxidation is calculated based on the Gibbs free energy,which proves that water vapor can be act as a mild oxidant for the efficient preparation of VO2 films.(2)Effective regulation of VO2 film thickness and stress is achieved through a"top-down" wet thinning strategy.In the experiment,epitaxial VO2 crystal films are firslty grown on TiO2(001)crystal substrates with pronounced interfacial strain by molecular beam epitaxy.A wet etching technique is proposed to achieve the strain modulation by etching this VO2/TiO2 film.Results indicate that the interface stress will gradually relax as the etching proceeds,resulting in ultrathin VO2 films with strain release.The strains release process during the etching process is studied by using synchrotron radiation based X ray diffraction and reciprocal space imaging technologies.In this study,a new way to regulate the interfacial strains of strong stress system VO2/TiO2 epitaxial film is explored.(3)Based on the electron-proton synergistic doping effect in metal-acid solution,the controllable hydrogen doping treatments in NdNiO3 films with different crystal plane orientation are realized at room temperature.It is found that different crystal planes have direct effects on the doped hydrogen concentration.According to the first principle calculations,the energy band and density of states for the H-doped NdNiO3 are calculated and the diffusion barriers of hydrogen atoms in different crystal planes are also estimated,which well explain the experimental observations.This electron-proton synergistic doping method at room temperature provides a mild hydrogenation technique for perovskite RNiO3 oxide systems,which will be useful for perovskite RNiO3-based device applications.