Vanadium oxide (VO_2) exhibits a very interesting semiconductor to metal transition as the crystal structure changes from tetragonal or rutile to monoclinic upon cooling close to 68℃. The characteristics of this transition are very interesting scientifically and are of immense technological importance due to a variety of sensor- and memory-type applications. We have processed high-quality films of VO_2 by pulsed laser deposition, which were grown epitaxially on (0001) sapphire substrate via domain matching epitaxy, involving matching of integral multiples of lattice planes between the film of monoclinic structure and the sapphire substrate. These films exhibit a sharp transition near 68℃, large amplitude, and very small hysteresis, similar to bulk single crystal of VO_2. The sharpness and amplitude of the transition and the hysteresis upon heating and cooling are found to be a strong function of crystal structure and microstructure (grain size, characteristics of grain boundaries, and defect content). Here, we propose a model to establish microstructure-property correlations in crystalline and amorphous phases of VO_2, which can be used to rationalize our experimental observations as well as those available in the literature. Based upon this model, we predict specific microstructures leading to properties needed for various sensor-and memory-type devices.
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