Thermochromic vanadium dioxide (VO₂) thin films, known for their reversible metal-insulator transition (MIT) near 68 °C, are promising candidates for energy-efficient applications such as smart window coatings. However, optimizing their structural and interfacial properties to enhance thermochromic performance remains a significant challenge. Traditional characterization techniques such as X-ray diffraction and transmission electron microscopy face inherent limitations in simultaneously providing detailed chemical information and nanoscale spatial resolution─capabilities that are essential for resolving localized structural heterogeneity and interfacial phenomena. This study employs hyperspectral tip-enhanced Raman spectroscopy (TERS) imaging to address these limitations and investigate the nanoscale structure of pristine VO₂ and VO₂/TiO₂ thin films. TERS imaging revealed nanoscale regions of lattice deformations and nanocrystallites with different orientations in VO₂ thin films, resulting in a high density of grain boundaries that elevate the MIT temperature. In VO₂/TiO₂ bilayers, TERS detected coexisting anatase and brookite phases in the TiO₂ layer, with tensile strain in the brookite phase and the VO₂/TiO₂ interface characterized by localized intermixing and strain. These novel insights underscore the polycrystalline nature of the thin films grown with pulsed layer deposition technique and highlight the critical role of nanoscale structural and interfacial properties in determining thermochromic performance of VO₂-based thin films. Furthermore, this study demonstrates the effectiveness of TERS as a robust nanoanalytical tool for advancing the design of VO₂-based smart coatings and functional materials.

The structural properties of VO₂ thin films, grown on either LSAT or Si substrates by pulsed laser deposition (PLD), are elucidated by means of transmission electron microscopy (TEM) methods. The TEM observations confirmed the successful growth of VO₂ by PLD in variable thicknesses, by optimizing the O₂ partial pressure and growth temperature. The films adopt a columnar polycrystalline morphology with narrow columns, up to the film thickness height. Four VO₂ polymorphs have been detected by electron diffraction and high-resolution TEM (HRTEM) analysis, with M1 being by far the most abundant phase. Post-experimental strain measurements in HRTEM images have revealed that the actual residual strain is minimized due to the columnar morphology of the VO₂ grains, as well as intrinsic oxide layers in the VO₂/Si epitaxy. The TEM outcomes confirmed the complementary electrical and magnetic measurements in the films, where a transition from a monoclinic M1 to a rutile VO₂ R phase has been identified, influenced by the initial percentage of phases in thick VO₂ films.

Vanadium dioxide (VO ₂ ) 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, VO ₂ 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 VO ₂ 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 VO ₂ M1 nanograins with minimal lattice mismatch. The observed morphology is attributed to surface and interface energy anisotropy acting across the amorphous SiO ₂ 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 VO ₂ thin films on quartz, highlighting how interface-energy-driven growth can stabilize ordered microstructures even in the absence of epitaxial constraint.