Microstructural evolution and nanograin coherence in VO₂ thin films grown by pulsed laser deposition

Vanadium dioxide (VO₂) thin films exhibit a reversible semiconductor-metal transition near room temperature, and their performance is strongly governed by microstructural and interfacial factors. In this study, VO₂ thin films were deposited on Z-cut quartz substrates by pulsed laser deposition under varying oxygen partial pressures and substrate temperatures. Plan-view transmission electron microscopy (TEM) techniques such as selected-area electron diffraction (SAD) and nanobeam diffraction (NBD) as well as confocal Raman spectroscopy were employed to examine the microstructure and crystallographic features of the films. The VO₂ films display a dense, polygonal morphology, composed of faceted grains with characteristic sizes of 50-200 nm. These polygonal grains are formed during deposition in the high-temperature metallic R phase and transform upon cooling into semi-coherent VO₂ M1 nanograins with minimal lattice mismatch. The observed morphology is attributed to surface and interface energy anisotropy acting across the amorphous SiO₂ interlayer, which promotes random in-plain orientation. Confocal Raman mapping further supports phase uniformity and orientation variation across the grains. Overall, this work provides new insight into the mechanisms governing polygonal grain formation and coherence within VO₂ thin films on quartz, highlighting how interface-energy-driven growth can stabilize ordered microstructures even in the absence of epitaxial constraint.