Sculptured thin film vanadium dioxide thermochromic coatings grown by oblique angle deposition: investigation of transmittance response and modulation enhancement by experiment and theoretical modeling

The fabrication of highly functional VO2 thermochromic coatings involves costly chemical precursors and complex fabrication procedures, one of the main constraints in the commercialization of VO2-based smart windows. Here, we have shown a simple fabrication method (low-processing temperature and environmentally friendly methodology) for VO2 sculptured thin films (STF) through industrially scalable magnetron sputtering followed by annealing in ambient air. The angular and polarization-dependent anisotropic thermochromic performance is noticed in STF VO2 with a columnar tilt of β = 20° attained at a maximum oblique angle of 85°. The optimal thermochromic performance with high luminous transmittance of Tlum(avg) = 43%, near-infrared (NIR), and solar modulation abilities of ΔTnir = 7.7% and ΔTsol = 11% respectively, is obtained when the light transmits along the inclined columns of STF VO2. The theoretical modeling of the STF VO2-based nanostructure composed of prolate spheroid VO2 nanocolumns shows a promising structural design for the optimal modulation reached by finding a balance between STF VO2 thickness and porosity. Simulation using the structural design of STF VO2 with 11% porosity, employing experimental thickness and β as simulation parameters, gives the best fitting to the experimental results, which confirms the validity of the model. The angular-dependent thermochromic response attained in this work is beneficial to maintain indoor thermal and visual comfort during the sun's varying angled rays throughout the day, different seasons, and geographic locations.