Real-space Observation of Metal–Insulator Transition of Individual Anisotropic Nanodomain in VO<sub>2</sub>/TiO<sub>2</sub>(110) Thin Films

Insulator–metal phase transition in vanadium dioxide (VO2) is influenced by nanoscale morphology and defects. To gain in-depth understanding of its microscopic mechanism, it is crucial to directly characterize the correlation between structural features and phase transition behavior. In this study, we grow VO2 thin films on TiO2(110) substrates using pulsed laser deposition and investigated their phase transition using X-ray diffraction (XRD), Raman spectroscopy, and scattering-type scanning near-field optical microscopy (s-SNOM). The phase transition is confirmed by a temperature-dependent XRD measurement, indicating the structural transition from the monoclinic (insulator) to the rutile (metal) phase. Additionally, using infrared s-SNOM, we directly observe the individual anisotropic nanodomain in the VO2 thin film, enabling the visualization of the insulating and metallic phases based on the contrast in the s-SNOM contrast at the nanoscale. Our results reveal that metallic nanodomains nucleate and spread in a spatially inhomogeneous manner, strongly influenced by grain width and lattice strain along the [001] direction.