Shape Dependence of the Scattering Processes of Optical Phonons in Colloidal Nanocrystals Detected by Raman Spectroscopy

Abstract

We investigated by Raman spectroscopy optical phonon excitations in colloidal CdTe nanocrystals of different shapes. The phonon spectra at excitation energies slightly below the absorption onset showed a strong dependence on the nanocrystal shape. For spherical nanocrystals, we detected broad longitudinal-optical phonon-scattering peaks that were blue shifted from the longitudinal-optical phonon bulk energy by the exciton resonant Stokes shift. These are reminiscent of the phonon replicas reported in fluorescence line-narrowing experiments. Tetrapod-shaped nanocrystals, on the other hand, showed a sharp LO phonon peak that was only slightly blue shifted by confinement effects on the phonon excitations. We propose that the localization of the carriers in the optical excitations is responsible for the different phonon spectra in tetrapods.