Hydrogenation Induced Phase Transition Modulation And Synchrotron Radiation Characterization Of Vanadium Dioxide Thin Films

Due to the complex coupling and competition among the freedom of charge,spin,orbit and lattice,strongly correlated transition metal oxide(TMOs)materials exhibit many unique macroscopic properties,such as metal-insulator phase transition,high-temperature superconductivity,topological-insulator,etc.As a typical TMOs material,vanadium dioxide(VO2)will undergo an obvious metal-insulator transition(MIT)at 68℃,accompanied by great changes in conductivity,infrared transmittance and magnetic properties.This unique MIT behavior makes VO2 materials broad applications such as ultra-fast photoelectric switches,neuronal computing,resistive storage,microactuators and laser protection.Although the VO2 studies have been carried out for several decades,there are still many controversies on its intrinsic MIT mechanism,such as the debate between Mott and Peierls mechanism.In addition,the relatively high phase transition temperature of VO2 is also still a bottleneck problem for its practical applications.Though the MIT temperature of VO2 can be reduced by doping or other methods,its macroscopic phace-change features are greatly degraded,which seriously restricts its practical applications.Therefore,in this dissertation,we are firstly focusing on the high-quality VO2 film preparation and hydrogenation induced phase transition behavior.By using the in-situ photoelectricity measurements and advanced synchrotron radiation characterizations,the interaction between oxygen vacancies and doped hydrogen atoms in epitaxial VO2 thin films are systematically studied.The first-principles calculations agree with the experimental results quite well.Then more studies for the photoinduced hydrogenation in n-VO2/p-GaN thin-film heterojunctions are conducted.Moreover,the self-assembled growth and adsorption of organic molecules on the surface of VO2 thin film are observed,which not only induces the hydrogenation of VO2 film,but also greatly enhances the stability of metallization of VO2 thin film at room temperature.Based on the above research,the research results in this dissertation are summarized as the following:1.As a typical point defect in metal oxide materials,oxygen vacancies can seriously affect the physical and chemical properties of the materials.In order to obtain stoichiometric oxide crystals,annealing the oxide sample at high temperature in the air or in oxygen atmosphere is the most common route to reduce the oxygen vacancies.However,due to the polyvalent vanadium oxides,the annealing conditions are very sensitive.In addition,the high migration barrier of oxygen vacancy in solid lattice makes it difficult to remove the lattice oxygen vacancies with high efficiency.Thus,in the current study,we have proposed a new strategy to reduce the migration barrier of oxygen vacancies by doping hydrogen in to VO2-δcrystals.Synchrotron radiation X-ray absorption spectroscopy(XAS)has been used to characterize the change of electron orbital occupancy in VO2-δ crystals during the hydrogen doping and the oxygen vacancy removal process.Combined with first-principles calculations,the physical mechanism was explained.This effective method to reduce the migration barrier of oxygen by hydrogenation provides a feasible and general way to control the lattice defects in other metal oxides.2.The unique MIT behavior of VO2 is of great interest in the fields of ultra-fast photoelectric switches,resistive memory and neural components,etc.Therefore,how to effectively regulate the MIT process to improve the performance of these devices has been the focusing issue in this field.In this study,we have achieved a photoinduced phase transition based on VO2 heterojunction,and successfully regulated the MIT behavior of VO2 in n-VO2/p-GaN thin-film heterojunction in acid solution.In addition,by continuous UV light illumination,a continuous three-state phase transitions are observed,which undergons the insulating-metal-insulating transitions in VO2 film.The experimental results show that the phase transition of VO2 originates from the lattice doping by H+,Al3+,or Mg2+ ions from the solution.Combined with the standard photolithography process,the selective ion doping of VO2 thin films in nanometer scale has been achieved,which shed some light on the fabrication of new photosensitive detectors.3.Hydrogen atoms insertion into oxide material will induce the pronounced electron doping in correlated oxides,which can trigger the metal-insulator phase transitions.However,the hydrogen induce phase transition as well as the practical applications have been limited due to the unstable hydrogen doping.Based on the strategy of electron-proton co-doping to regulate VO2 phase transition,just we put VO2 film into AA solutions.Due to the surface-interface chelation reaction between the dissociated AA-ion and VO2,a stable hydrogen doping process is achieved.which leads to a stable metallized VO2 phase at room temperature.By using synchrotron radiation XAS technique,the evolution of electron occupancy between V 3d and O 2p hybrid orbitals has been revealed.The theoretical calculations also show that the electrons provided by AA-ion are transferred to VO2,and then H+ions in the solution will also be incorporated into VO2 due to the attraction of electrostatic force,producing the insulating-metal transition of VO2 film at room temperature.In addition,the AA molecules adsorbed on VO2 surface will inhibit the hydrogen atoms in VO2 lattice,thus greatly improving the stability of the hydrogenation induced phase transition at room temperature.