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

Vanadium dioxide (VO) 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 VO 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 VO (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 VO (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 VO 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 VO thin films and enhance their optical properties.