Enhanced Optical Modulation Properties via Two-step Annealing of Sol-Gel Deposited Vanadium Dioxide Thin Films

Vanadium dioxide (VO₂) is a key coating component for smart devices, especially in smart windows, due to its excellent thermochromic properties and metal-insulating transformation. In this study, we investigate the influence of different annealing on the VO₂ thin films prepared by a sol-gel technique combined with spin-coating and compare the changes in morphology, structure, and optical properties. Of the two types of VO₂ (M) films obtained by one-step annealing at 700 °C and two-step annealing at 500 and 700 °C, the two-step annealed samples formed an intermediate phase VO₂ (B), which allowed them to exhibit higher purity and better crystallinity than those of the one-step annealed samples. An in-situ XRD study confirmed that the higher purity and better crystallinity enabled the resulting VO₂ thin films to lower the phase transition temperature, where the phase transition occurred by 5 °C earlier upon heating and about 10 °C upon cooling. As a result, the infrared and solar modulation capabilities were improved by 87 and 127%, respectively, while the high visible transmittance of the samples essentially remained unchanged. This study has demonstrated that proper annealing can refine the structures of VO₂ thin films and enhance their optical properties.