The Optical Behavior of VO2 Film Modulated by the Morphology and Preferred Growing Axis

摘要

We have prepared a smart VO2 film on a sapphire (0001) substrate by a direct thermal treat of a mixture of polyvinylpyrrolidone and VOCl2 solution at 803 K under an inert condition. The optical characteristics of the film near the phase transition temperature could be controlled by the surface morphology (mainly particle size) and preferred growing crystal axis of the film. The surface morphology and the preferred axis were related to the thickness of the VO2 film. The optical properties of the films could be grouped by three types: S-, M- and L-type. The S-type film, a brown colored thin film (similar to 0.3 mu m in thickness), which consists of small VO2 particles (< similar to 0.3 mu m in diameter) and highly oriented to the preferred growing orientation (020), cut off infrared (IR) radiation at the high-temperature (high-T: above the transition temperature) phase and passes IR at the low-temperature (low-T: below transition temperature) phase. The transmission gap at 1550 nm between the low-T and high-T phases of the S-type film was similar to 25%. The transition phenomena also observed in dc electrical resistance measurement at the same transition temperatures; T-H = similar to 340 K, observed in the heating process, and T-C = similar to 334 K, observed in the cooling process. However, M- and L-type films, thicker and more isotropic than S-type, have different optical behavior. Other physicochemical properties of the films were investigated to unveil the phase transition.