Effect of annealing conditions on VO2 thin films prepared by sol-gel method

Abstract

Vanadium dioxide has attracted considerable attention because of the semiconductor-metal transition at 68 °C. This reversible transition changes crystal structure from monoclinic to rutile resulting in tremendously change of transmittance and reflectance of infrared wavelength and electrical resistance making VO2 thin film potential candidate for energy-saving smart windows—automatically block heat. However, the preparation is very challenging due to the multivalence of vanadium element. In this study, we explored different annealing conditions for thin films synthesized by sol-gel method which has advantage over other methods such as lower cost, simpler process, and ability to make large scale thin films. In the experiment, vanadium (V) triisopropoxy oxide (VO(OC3H7)3) was used as precursor. The annealing conditions explored are annealing temperature and annealing time of thin films in argon rich environment. The spin-coated films were annealed at 300-500 °C for 30 minutes, while another set was annealed at 450 °C for 30, 60, 90, and 120 minutes, respectively. Then the films were characterized with Raman spectroscopy and temperature dependent resistance measurement. The results show that the longer the annealing time and the higher the annealing temperature are better for film formation—closer to VO2 (higher V:O ratio). In addition, the effect of various substrate types was also explored, another set of thin films was coated on c-plane sapphire (c-Al2O3) and soda lime glass (mostly SiO2) to compare the effect. Then the films were characterized with Raman spectroscopy and X-Ray Diffraction (XRD). The result showed that sapphire substrate yielded better film formation.