Influence of annealing on the optical properties of monoclinic vanadium oxide VO2 prepared in nanoscale by hydrothermal technique

Terahertz and infrared reflectivity measurements were carried out on monoclinic vanadium oxide VO2 prepared directly in nanoscale by hydrothermal technique. The product was annealed in argon atmosphere at pressure 100 Pa for 6 h at temperatures 350 degrees C, 450 degrees C and 550 degrees C. The as-grown and annealed samples were investigated by XRD, HRTEM, VSM, DSC, THz, and FTIR techniques to explore the effect of annealing on the optical and electronic properties. The obtained results reveal the existence of monoclinic M-B-VO2 phase for the as-grown sample, while the annealed samples include two other oxidation phases V6O13 and V2O5 together with the monoclinic VO2 phase. This observation is supported by the data obtained from XRD, VSM, and DSC measurements. Both THz transmittance and infrared reflectivity measurements performed at room temperature show insulating behavior for the as-grown monoclinic VO2 sample whereas subtle modifications of monoclinic VO2 phase by annealing induces a metallic behavior via enhancement of surface area to volume ratio and increasing the grain boundaries as a result of embedded and distributed of two other phases V6O13 and V2O5 within the monoclinic VO2 matrix. Fitting of the optical conductivity deduced from both THz and infrared measurements clearly shows Drude-like behavior for the annealed sample while this behavior was not observed in case of the as-grown sample.