Enhanced visible transmittance and reduced transition temperature for VO2 thin films modulated by index-tunable SiO2 anti-reflection coatings

Abstract

The low visible transmission is one of the bottleneck problems for the application of vanadium dioxide films since the high refractive index (RI) of VO₂ films results in strong reflection in the visible wavelength. To address this problem, in this paper, high-purity VO₂ 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 VO₂ films have been fabricated to enhance visible transmittance with the potential application in the field of smart windows. SiO₂ coatings with tunable RI (1.16–1.42 at λ = 700 nm) were prepared by sol–gel dip-coating technique. The double structure SiO₂/VO₂ films were characterized through several techniques, including X-ray diffraction, UV-VIS-NIR spectrophotometry and scanning electron microscopy. Compared with the single-layer VO₂ film (ΔT_sol of 6.25% and T_lum of 38.58%), the three kinds of SiO₂/VO₂ bilayer films had higher T_lum (41.93–50.44%) and larger ΔT_sol (8.15–8.51%) simultaneously due to significantly decreased reflectance. Moreover, the crystallization properties of VO₂ films are essentially unchanged by applying a SiO₂ top layer, while the phase transition temperature and thermal hysteresis width of sample S116 are lower than those of pure VO₂ film. The presented RI-tunable SiO₂ coatings, can regulate optical properties continuously for various VO₂ substrates, paving the way for practical applications of VO₂ films in the field of smart windows or others.