Synthesis And Microstructures Of Vanadium Dioxide Thin Films With Optical And Electrical Properties

Recently,vanadium oxide thin film materials have attracted tremendous attentions due to the special metal-insulator transition（MIT）property and potential applications in photoelectric device and sensors.It is still challenge to prepare high quality pure-phase VO₂ thin films because of the multiple chemical valence of vanadium and the complexity of the phase of vanadium oxide.Therefore,it is necessary to develop a simple method to prepare the phase-controllable VO₂ thin films.The functionalization of VO₂ thin films was commonly based on adjusting the MIT property by tuning the external conditions,which would be the key point to broaden the future applications.In this thesis,the feasibility of the preparation of VO₂ by heating V2O5 was studied using thermodynamic calculation method.The results showed that VO₂ could be obtained by heating V2O5 under low pressure or vacuum condition.The VO₂ thin films with excellent MIT property were prepared by a two-step method.Firstly,V2O5 thin films with photoelectric properties were prepared by magnetron sputtering method on glass substrate,which were further annealed at the vacuum tube furnace to obtain VO₂ thin films with MIT properties.The sputtering oxygen partial pressure and sputtering time had important influence on the photoelectric properties of V2O5 thin films.In addition,the temperature & time of the heat treatment had an effective impact on the structure,composition and MIT properties of VO₂ thin film materials.The results indicated that the VO₂ thin films prepared at 450 °C for 120 min had obvious preferred orientation,and the phase transition temperature was 54 °C,the thermal hysteresis loop width was 7 °C and the visible light transmittance was 32 %.The MIT characteristics of VO₂ thin films were studied by atomic force microscope（AFM）at different temperature s,which could provide a better understanding of of the MIT mechanism of VO₂ thin films.Results showed that there was an obvious change of the atomic force amplitude before and after phase change.Low energy ion beam irradiation was used to study the impact of the ion irradiation angle,irradiation energy and irradiation time on the morphology of the VO₂ thin film materials.The results showed that VO₂ thin films with periodic rippled structure could be obtained by controlling the irradiated technological parameters.We illustrated the mechanism of the formation of VO₂ thin films with periodic rippled structure by MCB theory.The common grain structure and periodic rippled structure were induced by ion irradiation.The influence of the ion irradiation on metal-insulated phase change characteristic of VO₂ thin films was studied.These results showed that ion irradiation could bring down the phase transition temperature from 54 °C to 42.5 °C and the thermal hysteresis loop width of VO₂ thin films from 7 °C to 5 °C and improve the visible light transmittance from 32 % to 50 %,which was caused by the emergence of the surface oxygen vacancies of VO₂ thin films upon irradiation.The lattice parameters,the electronic structure and optical properties VO₂（M）and VO₂（R）under different oxygen vacancy concentration were calculated through the first principles method to analyze the influence of oxygen vacancies on VO₂ MIT characteristic.These results showed that the existence of oxygen vacancy in VO₂（M）phase made the decrease of β value,the increase of volume（V）and the bond length of V-V,thus leading to the decrease of phase transformation temperature.In addition,the existence of oxygen vacancy caused the non-isometric change between V-V bond of VO₂（R）and the transformation from the octahedron structure of VO₂（R）to the distorted VO6 octahedron.It was noteworthy that the distorted octahedron VO6 could serve as a point of nucleation in the process of phase transition point,reducing the thermal hysteresis loop width.Also,the existence of oxygen vacancy caused the smart change of V3 d and O2 p orbits in VO₂（M）and VO₂（R）,improving the visible light transmittance.Based on the phase change properties of VO₂ thin films,VO₂/glass/Yb3+/Er3+/C12A7 optical devices were prepared.The impact of the temperature and pump power on the device’s up-conversion luminescence performance was studied.These results showed that the luminous intensity of red and green of VO₂/glass/Yb3+/Er3+/C12A7 device decreased with the increase of temperature,when the temperature decreased,the luminous intensity of red and green increased reversibly.When the pump power increases,the luminous intensity of red and green of the VO₂/glass/Yb3+/Er3+/ C12A7 optical device increased.At the threshold power,the luminous intensity was significantly reduced.When the pump power reduced,the luminous intensity changed reversibly.The temperature and laser induced the phase transition of VO₂ thin film,which caused the change of transmittance around 980 nm and up-conversion luminous intensity.Thus,VO₂/glass/Yb3+/Er3+/C12A7 optical device can realize the reversible regulation for up-conversion luminescence.