Enhanced visible transmittance and reduced transition temperature for VO thin films modulated by index-tunable SiO anti-reflection coatings.

The low visible transmission is one of the bottleneck problems for the application of vanadium dioxide films since the high refractive index (RI) of VO films results in strong reflection in the visible wavelength. To address this problem, in this paper, high-purity VO films were deposited on fused silica by DC reactive magnetron sputtering at low temperature of 320 °C. Silica sol-gel coatings with tunable refractive index (RI) coated onto VO films have been fabricated to enhance visible transmittance with the potential application in the field of smart windows. SiO coatings with tunable RI (1.16-1.42 at = 700 nm) were prepared by sol-gel dip-coating technique. The double structure SiO/VO films were characterized through several techniques, including X-ray diffraction, UV-VIS-NIR spectrophotometry and scanning electron microscopy. Compared with the single-layer VO film (Δ of 6.25% and of 38.58%), the three kinds of SiO/VO bilayer films had higher (41.93-50.44%) and larger Δ (8.15-8.51%) simultaneously due to significantly decreased reflectance. Moreover, the crystallization properties of VO films are essentially unchanged by applying a SiO top layer, while the phase transition temperature and thermal hysteresis width of sample S116 are lower than those of pure VO film. The presented RI-tunable SiO coatings, can regulate optical properties continuously for various VO substrates, paving the way for practical applications of VO films in the field of smart windows or others.