A universal chemical approach to the growth of self-assembled vanadium dioxide nanostructures

Vanadium dioxide (VO2) nanocrystalline materials are of great interest as for modern electronics and photonics as well as for energy saving technologies. Nevertheless, there still remains a challenge to realize their controllable synthesis, as evidenced by the low rate of implantation of existing technologies into mass production. Here, a hydrothermal reaction in a water-ethylene glycol (EG) solution has been coupled with the post-deposition annealing, yielding a range of phase-pure vanadium dioxide nanostructures on single-crystal r-sapphire substrate. A directed change in the viscosity of the precursor solution by using a change in the EG : H2O ratio makes it possible to control the nucleation rate and thus form VO2 isolated nanocrystals, as well as their ordered ensembles with regular distribution and uniform films on the substrate. The obtained nanostructures were extensively characterized by Raman spectroscopy, scanning electron microscopy, atomic force microscopy, and transmission electron microscopy combined with energy-dispersive X-ray spectroscopy. Continuous VO2 nanostructures demonstrated a metal-insulator transition (MIT) with a jump in resistivity of about 103–104, as well as a giant terahertz (THz) modulation depth up to 86 % (in the range from 0.3 to 2.0 THz), which confirms their high-quality and the possibility of their use in the creation of functional electronic and THz optical devices.