Effect of microplate size on the semiconductor–metal transition in VO2 thin films

Abstract

VO₂ thin films composed of irregularly shaped microplates have been synthesized using V₂O₅ as a precursor via the vapor transport technique. SEM images showed that the microplate sizes in VO₂ films tended to increase (from 16 to 51 μm) with increasing quantity of V₂O₅ precursor due to the large nuclei of VO₂ crystals formed at the initial growth stage under a relatively large amount of precursor. Raman, XRD and XPS analysis collectively demonstrated that these film structures have high crystal quality of monoclinic VO₂ phase. Furthermore, the thermally driven phase transition was investigated by measuring the electrical resistance during the heating and cooling process. All the films exhibited a typical semiconductor–metal transition at 340 K, and the highest resistivity change reached 4.2 decades. More significantly, the phase transition behaviors show a noticeable variation in the amplitude of the resistivity change, which becomes more prominent as the VO₂ film microplate size grows. The relevant mechanism is considered to be predominantly associated with the electron scattering attenuation effect, related to a decrease in the grain boundary density as the microplate size of the VO₂ films grows. Additionally, a device based on a high-quality VO₂ thin film exhibits efficient IR response, with high responsivity (∼12.5 mA W⁻¹) and fast response speed (∼40 ms) under ambient conditions, which should be promising for sensitive IR detection.