Raman Spectroscopy of Exciton-Acoustic Phonon Coupling in Semiconductor Nanocrystals

While electron and hole size quantisation in spherical semiconductor nanocrystals embedded in glass matrices has been much studied in the last 15 years, the question of acoustic phonon confinement remains open. Only simple models of either bulk-like phonons [1] or vibrational modes of an elastic sphere with free or fixed surface [2–5] were applied to describe the acoustic phonon energy spectrum in nanocrystals. In these two limiting cases the acoustic phonons either do not sense the interface between the nanocrystal and the glass matrix or can not penetrate through it, i.e. the probabilities of either phonon reflection or transmission are neglected. However the low-frequency Raman spectra line shapes are expected to be very sensitive to these probabilities when the excitation is far from the excitonic resonance and thus a more accurate consideration of acoustic phonons in nanocrystals is required.