Formation and phase transition of VO2 precipitates embedded in sapphire

Crystallographically coherent precipitates of vanadium dioxide (VO2) have been formed in the near-surface region of single crystals of sapphire (Al2O3) using a combination of ion implantation and thermal treatments. As in the case of either bulk VO2 single crystals or thin films of VO2, the thermally induced semiconductor-ro-metal phase transition of the embedded VO2 precipitates is accompanied by a large hysteretic change in the infrared optical transmission. The VO2 precipitate transition temperature (T-c, = 72 to 85 degrees C) is higher than that of bulk VO2 (T-c = 68 degrees C) and is sensitive to the implantation conditions. The present results show that the damage resulting from the coimplantation of vanadium and oxygen into an Al2O3 host lattice dictates the final microstructure of the VO2 precipitates and, consequently, affects the transition temperature, as well as the optical quality of the VO2/Al2O3 surface-nanocomposite precipitate system.