Nanoscale Structural Insights into Thermochromic VO Thin Films Using Tip-Enhanced Raman Spectroscopy.

Thermochromic vanadium dioxide (VO) 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 VO and VO/TiO thin films. TERS imaging revealed nanoscale regions of lattice deformations and nanocrystallites with different orientations in VO thin films, resulting in a high density of grain boundaries that elevate the MIT temperature. In VO/TiO bilayers, TERS detected coexisting anatase and brookite phases in the TiO layer, with tensile strain in the brookite phase and the VO/TiO 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 VO-based thin films. Furthermore, this study demonstrates the effectiveness of TERS as a robust nanoanalytical tool for advancing the design of VO-based smart coatings and functional materials.