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

Raman spectroscopy of the VOx nano-ribbons is discussed in the framework of the Richter (1981) equation for optical phonon confinement (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. Phonon splitting 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.

► Preparation of advanced VO2@V2O5 core–shell nano-ribbons of typically 10 nm in thickness. ► Observation of enhanced LO and TO splitting in these ribbons. ► New theory on LO and TO splitting based on the phenomenological optical confinement model. ► Quantification of the ratio of V5+ to V4+ valance states in the core–shell by the split phonons in the Raman spectra validated by XPS for the first time.