Polarized Tip-Enhanced Raman Spectroscopy in Understanding Metal-to-Insulator and Structural Phase Transition in VO2

Polarized Raman spectroscopy is capable of confirming the crystalline phase as well as the crystallographic orientation of the sample. At the same time, tip-enhanced Raman spectroscopy (TERS) is capable of enhancing the spatial resolution in the subdiffraction limit as well as enhancing the Raman scattering intensity, which is notoriously weak in the case of inorganic and particularly noncarbonaceous materials. In this context, we have carried out polarized TERS studies in the backscattering configuration to confirm the crystallographic orientation of single VO2 nanorods using 514.5 nm excitation. Moreover, as the technique is accomplished with directional field enhancement, the identification of corresponding Raman mode frequency responsible for the structural phase transition is also made possible using polarized and high-temperature TERS studies substantiated by density functional theory calculations for the phonon density of states. Detailed high-resolution transmission electron microscopic analyses were used to reconfirm the orientation of the one-dimensional VO2 nanorod. High-temperature TERS studies along with the observation of a low-frequency spin-wave mode help in understanding the metal-to-insulator phase transition of VO2.