Spectral features above LO phonon frequency in resonant Raman scattering spectra of small CdSe nanoparticles

We report unusual spectral features in the resonant Raman scattering spectra of colloidal CdSe nanoparticles as small as 2-3 nm. High-frequency shoulders of the longitudinal optical phonon peak and its overtones were observed and their dependence on the excitation wavelength, temperature, nanoparticle size, and surface passivation with ZnS shell studied. As the probable origin of the uncommon spectral feature the participation of acoustic phonons and manifestation of the density of surface-related vibrational states is discussed.