Two-step hydrothermal growth of a thin film of vanadium dioxide on sapphire with large terahertz modulation depth

We investigate the terahertz optical properties of VO₂ (M) films synthesized hydrothermally on a c-sapphire substrate by a two-step method. The terahertz transmittance is greater than 70% for the insulating state of monoclinic VO₂ films, and the average modulation depth approaches 94% for frequencies of 0.2–2.0 THz. A sample prepared using 450 °C postannealing undergoes a sharp insulator–metal transition around 61 °C at 1.5 THz with a 97% modulation depth. Furthermore, the effects of the postannealing temperature on the structure, morphology, and phase-transition properties of VO₂ film/c-sapphire samples are presented. The results show that the hydrothermal synthesis of VO₂ polymorphs has potential applications in the terahertz wavelength range.