Study On The Preparation And Optical Characteristics Of VO₂ Thin Films

VO2 films undergo semiconductor to metal reversible phase transition at approximately 68℃ with great changes in optical, electric and magnetic properties before and after the transition. It has been widely used in the fields of smart windows, modulators, chemical sensors and so on. Currently, Magnetron sputtering, chemical deposition and laser deposition techniques are commonly used to prepare VO2 crystal films, where vanadium targets are always needed to react with oxygen flow. However, vanadium can form many kinds of vanadium oxides. It is difficult to obtain pure VO2 without other vanadium oxides. Thus choosing suitable preparation techniques to obtain VO2 films with single chemical composition, and smooth surface is very important.In this paper, the organic sol-gel method is used to prepare VO2 crystal films. The VO2 crystalline film is prepared with vanadyl acetylacetonate methanol colloid solution, spin-coated on alumina (Al2O3) substrates, by a subsequent annealing process in N2 atmosphere. Related parameters of the process are optimized. The following works have been studied.1. Based upon the preparation of single layer VO2 crystal films with large area, small particles and good uniformity by the organic sol-gel method and the repeatded spin-coating and annealing progress, VO2 crystal film thickness can be controlled. When annealing at 550℃-600℃ for 20min in N2 atmosphere, the uniform VO2 thin film with large area can be well prepared. The thin film has the single component of VO2 (001), almost no other vanadium oxides. The grain size is about 40nm-50nm, and the film thickness is about lOnm. The prepared VO2 crystal has good properties of phase transition controlled by temperature, with the infrared transmittance rate changing 6% at 2um, and the phase transition temperature is about 70℃, close to the single VO2 crystal. The VO2 crystal thin films with different thickness can be prepared by repeating the spin-coating and annealing progress with the same parameters. Each repetition process can increase about 1 Onm while the film quality remains almost the same. Absorption and the Raman spectra indicate obviously the phase transition effect with the change of temperature.2. Utilizing the phase transition of VO2 crystal films, we develop a dual-functional sensor based on the coupling between VO2 crystal films and Ag nanoparticles, which can probe fluorescence or Raman signals on the same substrate and be switchable by changing temperature. At room temperature, the VO2 crystal films is insulator phase and the fluorescence signals of probe molecules (R6G) is detectable (Raman is in "off")-At high temperature (such as 85℃), the VO2 crystal films become metallic phase. Ag nanoparticles interact with the metal phase of VO2 crystal films to produce stronger localized electric field. The stronger electric field can excite the Raman signals of probe molecules (R6G) and the coupling structure can also emit the Raman signals out efficiently (Raman is in "on").