The optical behavior of VC₂ film modulated by the morphology and preferred growing axis

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 (⁓0.3 μm in thickness), which consists of small VO2 particles (< (⁓0.3 μm 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 (⁓25%. The transition phenomena also observed in dc electrical resistance measurement at the same transition temperatures; T_H = (⁓340 K, observed in the heating process, and T_C = (⁓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