The origin of the low-frequency shoulder on the longitudinal optical (LO) phonon fundamental in the Raman spectra of CdSe quantum dots is examined. This feature has traditionally been assigned as a �surface optical� (SO) phonon, but it is only slightly affected by chemical modification of the surface. Raman excitation profile data show that the low-frequency shoulder loses intensity as the excitation is tuned to longer wavelengths, closer to resonance with the lowest-energy 1Se-1S3/2 excitonic transition. Calculations have been carried out using a fully atomistic model with an empirical force field to obtain the phonon frequencies and eigenmodes. The standard effective mass approximation envelope function model is used to calculate the electron and hole wavefunctions and, thereby, the resonance Raman activity of each mode. With a force field of the Tersoff type, which includes three-body terms, the calculated spectra closely resemble the experimental ones in showing mainly the higher-frequency LO phonon with 1Se-1S3/2 resonance but showing intensity in lower-frequency features with 1Pe-1P3/2 resonance. The calculations indicate that the sharp LO phonon peak involves mainly the interior atoms while the low-frequency shoulder is more equally distributed throughout the crystal, but not surface localized. Very different results are obtained with a widely used Coulomb plus Lennard-Jones force field, which predicts more disordered structures and more localized phonon modes for the nanocrystals compared with the Tersoff-type potential.

"Surface optical" phonons in CdSe nanocrystals

248th National Meeting (2014)

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