Size effects in the structural phase transition of VO₂ nanoparticles studied by surface-enhanced Raman scattering

We report the first experimental application of surface-enhanced Raman scattering (SERS) to the study of the structural phase transition of vanadium dioxide (VO₂). Using arrays of gold-capped VO₂ nanoparticles (Au+VO₂NPs) and a VO₂film covered with Au islands, we obtained the temperature evolution of the SERS intensity with respect to the amount of accessible material across the monoclinic-tetragonal- monoclinic transformation cycle of VO₂. The smallest Au+VO ₂NPs displayed the largest deviations from the bulk transition temperatures to complete the transformation, resulting in the widest thermal hysteresis, while the Au+VO₂film exhibited the narrowest hysteresis. The observed size dependence agrees qualitatively with the model of defect-induced nucleation of the VO₂ transition, although the magnitude of the hysteresis width and its dependence on NP size were less pronounced than those in a previous study of elastic light scattering from bare VO₂NPs. The discrepancies may stem from the creation of extrinsic nucleation sites in the VO₂NPs during their high-temperature processing in the presence of the Au caps; alternatively, the hystereses of the structural and electronic transitions could each have a different dependence on size. Lastly, we correlate the size dependence of the VO₂SERS intensity with the scattering efficiency of the Au nanoparticles, within the framework of a modified Mie-theory calculation.