Regulating The Electronic Phase Transitions Of Strongly Correlated Vanadium Dioxide

The complex interplay of the intrinsic charge,orbital,spin and lattice degrees of freedom within correlated oxides results in abundant novel physical properties,such as high-Tc superconductivity,colossal magnetoresistance effect,and metal-toinsulator transition(MIT).The 3d-orbital correlated oxides exhibiting the MIT properties have attracted extensive attentions in the field of condensed matter physics,since the abundant physical properties alter abruptly across the transition temperature(TMIT),such as electrical transportation,magnetic ground state,nearinfrared transmittance and thermal conductivity.Among the existing material systems,vanadium dioxide(VO2)exhibits the most abrupt MIT properties near room temperature(～68℃),in which the variation in the material resistivity exceeds 2-5 orders of magnitude.Therefore,this enables the promising applications of VO2 material in the field of correlated electronic devices,thermochromism and infrared camouflage.In contrast to conventional semiconductors,VO2 exhibits extremely complex electronic phase diagram and highly tunable orbital configurations that can be further triggered or manipulated by hydrogenation,interfacial strain,defect engineering and chemical doping.For example,hydrogenation can trigger multiple Mottronic transitions within VO2 toward either highly electron localized state or electron itinerant state via regulating the d-orbital occupancy and configuration.Nevertheless,the existing researches as associated to the regulations in the MIT property of VO2 are not enough,while the underneath physical mechanism and the prototypical Mottronic device applications need to be further explored and enriched.In this thesis,we mainly focus on the typical 3d-orbital correlated VO2 film and bulk materials,regulate the MIT properties of VO2 by using hydrogenation or protonation,chemical doping and strain engineering,and probe the underneath mechanisms based on the synchrotron radiation and nuclear reaction analysis.The main researches of this thesis are listed as follows:(1)A non-equilibrium spark plasma-assisted reactive sintering(SPARS)strategy was proposed for synthesizing the doped VO2 bulk pellets,which enables the precise control in the transition temperature within a broad temperature range and largely improved mechanical strength or transition abruption.Compliant variations in the electronic structure of VO2 upon chemical doping were probed via the synchrotron radiation.In addition,we further clarified the underneath correlation between the microscopic carrier transportation behaviors and macroscopic MIT properties,and investigated the MIT properties of VO2 bulk materials via introducing the rare-earth oxides as the compositing phase.(2)The valence state of vanadium and MIT properties for as-deposited VO2 films were demonstrated to be exceptionally sensitive to the deposition temperature.We further revealed the overlooked thermoelectric transport properties of VO2 films,e.g.,the thermoelectric power factor as achieved in metallic VO2 is comparable to the one for typical organic/oxide thermoelectric materials.The different status of interfacial strains was constructed for the differently oriented VO2/TiO2 heterostructures,to further manipulate the MIT properties.In addition,the strain engineering was further extended to the Mott insulator NiO that enables the resistive switch of～82%for NiO/PMN-PT heterostructure upon imparting a relatively low electric field.(3)We clarified the underneath mechanism in the hydrogen-induced opposite Mottronic transitions of VO2 toward either the highly electron localized state or electron itinerant state,by quantifying the hydrogen concentration via the nuclear reaction analysis.The critical role of hydrogen in the hydrogen-induced multiple Mottronic transitions within VO2 and the chemical stability of hydrogenated VO2 were further revealed.In addition,we demonstrated the passivation effect of intrinsic two-dimensional defects within polycrystalline VO2 films on hydrogeninduced electron localization effect.Furthermore,the hydrogenation was further extended to the NiO film that triggers the insulator-to-metal transitions with a resistive regulation of 109-1011 being observed,based on the newly found electron itinerant state of Ni(2-Δ)+.(4)We probed the hydrogen-induced electron phase transitions for doped VO2 films,and revealed the rival effect of high-valent elementary doping and heavily hydrogenation to the electron-doping-triggered orbital reconfiguration for VO2.In addition,we further demonstrated the critical impacts of the potential lattice distortion or disorder within the VO2 film as triggered by elementary substitution to the resulting hydrogenation kinetics.