Size effects in the structural phase transition of VO2nanoparticles 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+VO2 NPs) and a VO2 film 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+VO2 NPs displayed the largest deviations from the bulk transition temperatures to complete the transformation, resulting in the widest thermal hysteresis, while the Au+VO2 film 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 VO2 NPs. The discrepancies may stem from the creation of extrinsic nucleation sites in the VO2 NPs 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 VO2 SERS intensity with the scattering efficiency of the Au nanoparticles, within the framework of a modified Mie-theory calculation.