The Study Of Metal-insulator Transition Of Charge Doping VO₂ Film Via Synchrotron Radiation

As a strong electron-correlation functional material,VO₂ exhibits the reversible metal-insulator transition?MIT?behavior near room temperature,accompanied by electrical,optical,and magnetic mutations.Those special characteristics have attracted the great enthusiasm of researchers,especially in the term of phase transition mechanism.It was found that both electron doping associated electron correlation?Mott-Hubbard transition?and the lattice distortion involved Peierls transition are generally thought to be important during MIT process.However,the intrinsic microscopic mechanism of phase transition,which of them is the main role,is still debated now.In this dissertation,two minimal lattice damage methods,N-incorporation samples and surface charge transfer via molecules adsorption methods?MoO₃,F4TCNQ?,were selected to study the independent electron correlation transition.The research focused on the MIT behavior of VO₂/Al2O₃ films and the electron transport in interfaces through synchrotron radiation spectroscopy technique.The dissertation includes the following several aspects:?1?The MIT temperature of VO₂ is modulated via the low-energy N+ ion injection of VO₂ films.Comparing with the pristine VO₂ film,the critical temperature of MIT decreases about 18 ? for the VO1.9N0.1and VO1.87N0.13 samples.The SRPES and NEXAFS spectroscopy were used to detect the electronic structural properties.It was found that the N atoms substituted for the oxygen atoms in the VO₂.The injection of N+ is an effective hole doping method via occupying the d//*level and lowering the interaction of the V-V dimer,facilitating the decrease of the MIT temperature of N-incorporated VO₂?2?We systematically studied the holes doping effects on the MIT behavior of VO₂ film via MoO₃ deposition.The electronic structure of MoO₃/VO₂ interface was investigated using in situ SRPES spectroscopy,the results showed that the hole charges were doped effectively from the deposited MoO₃ into the V 3d band and the Mo5+ species were formed due to the strong chemical interactions at the interface and the stronger interaction would be formed in the interface of MoO₃/VO₂ at high temperature.The XRD test verified that the holes doping did not produce any novel structure in the VO₂ substrate,which is an independent hole carriers doping method.The phase transition of original VO₂ film and deposited samples with MoO₃ overlayers were investigated by the four-probe device and V-T Raman spectroscopy,it is identified that the hole carries doping by MoO₃ overlayers aroused earlier occurrence of the structural phase transition,especially for the thinner VO₂ sample.?3?The holes doping effect on the MIT behavior of VO₂ film is investigated via the tetrafluoro-tetracyanoquino-dimethane?F4TCNQ?molecules adsorption induced surface charge transfer.Comparing with the MIT process of pristine VO₂ film,a critical temperature decrease of about 4 ? for the F4TCNQ covered VO₂ sample is observed.The MIT depression mechanism is deeply investigated based on detailed experiments including SRPES,NEXAFS and V-T Raman spectroscopy.The results indicate that the electronic structures of F4TCNQ covered VO₂ samples are changed clearly due to the effective hole doping.In addition,the doped holes also change the V 3d orbital occupancy and weaken the electron-electron correlation as well,lowering the crystalline stability energy.Both of the above effects are in favor of triggering the earlier occurrence of MIT,resulting in the decrease of critical temperature.?4?The influence of F4TCNQ molecule on the sputtered VO₂ film was studied.Due to the electronic doping effect,oxygen defects on VO₂ surface would exert significant influence on the intrinsic electronic structure and phase transition properties of VO₂.It was found that evolution of electronic structure and chemical reactions were involved at the interface during the defects healing.The sample was detected using SRPES,NEXAFS spectroscopy.It is revealed that the V3+ ions in the sputtered VO₂ were oxidized to V4+ when the F4TCNQ molecules were adsorbed on the sputtered VO₂ film.The result can be confirmed by the NEXAFS spectra of the V-L and O-K edges.The appearance of the negatively charged F4TCNQ species implied that the electron transfer occurred at the interface from VO₂ film to F4TCNQ molecules.Restrained by the electrochemical properties,the healing of oxygen defects through F4TCNQ adsorption seems to be more reliable with respect to the annealing in oxygen by avoiding the production of V₂O₅ due to over oxidation.