Facile color tuning method of VO₂ thin film for smart window applications

This paper presents a facile color tuning method for vanadium dioxide (VO₂) thin films via humidity control. The VO₂ thin films were deposited on an eagle glass substrate via RF magnetron sputtering using a VO₂ ceramic target. As-deposited VO_x films were amorphous in nature with yellowish color and were transformed into crystalline VO₂ upon heating at 550 °C. Generally, crystalline VO₂ films have an unfavorable brownish color. By controlling humidity before annealing, the color of crystalline VO₂ thin films was successfully modified from brownish to a light purple, which can be observed depending on the viewing direction. For the prepared humidity controlled VO₂ thin film, the optical switching efficiency (E_os) and visible transmittance (T_vis) were 49% and 41%, respectively, which is similar to the non-controlled equivalent. More dense and small grain-like morphology was noted after humidity control and this affected the light scattering on the surface of the films. The results showed that the humidity controlled VO₂ films exhibited a light purple color and showed an increase in the reflectance value.