Raman spectrum of nanocrystals: Phonon dispersion splitting and anisotropy

The effects of anisotropy and splitting of the phonon dispersions in the Raman spectrum of nanocrystals are investigated. We describe the different branches of the optical phonon dispersion curves along each high-symmetry direction by fitting a simple empirical model to experimental data. These curves are then used to calculate the Raman spectrum in the framework of the phonon confinement model and the results are compared with a wide range of available experimental data as well as with the spectra predicted by currently available models based on a single isotropic dispersion curve. We show that by considering the anisotropy and splitting of rigorously obtained optical phonon dispersions, the commonly observed deviations between experimental and theoretical data are strongly reduced. Our work enables the extraction of significantly more accurate information about relevant physical properties of nanocrystals from Raman spectroscopy.