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 (ΔTsol of 6.25% and Tlum of 38.58%), the three kinds of SiO2/VO2 bilayer films had higher Tlum (41.93–50.44%) and larger ΔTsol (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.