From phonon confinement to phonon splitting in flat single nanostructures: A case of VO2@V2O5 core–shell nano-ribbons

Abstract

Raman spectroscopy of the VOx nano-ribbons is discussed in the framework of the Richter (1981) equation for optical phononconfinement (a) as modified for thin films by Fauchet and Campbell (1986), (b) as presented by Kim and co-workers for slabs, (c) as explained by Eklund's group for surface phonons and (d) our own modification based on the transformation from the spherical coordinates in the Richter equation to Cartesian coordinates; the latter being in keeping with the ribbon geometry. The change of coordinates also influences the profiles of the phonon dispersion curves. Phononsplitting is ascribed to the bi-layer and core–shell geometries of the ribbons and this is used to calculate the ratio of the V5+ to V4+ to the value of 0.54 ± 0.10. This is in perfect agreement with the V5+/V4+ 54.60% from X-ray photo-electron spectroscopy (XPS) measurements.