The “Surface Optical” Phonon in CdSe Nanocrystals

The origin of the ubiquitous low-frequency shoulder on the longitudinal optical (LO) phonon fundamental in the Raman spectra of CdSe quantum dots is examined. This feature is usually assigned as a "surface optical" (SO) phonon, but it is only slightly affected by modifying the surface through exchanging ligands or adding a semiconductor shell. Here we present excitation profile data showing 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 of the resonance Raman spectra are carried out using a fully atomistic model with an empirical force field to calculate the phonon modes and the standard effective mass approximation envelope function model to calculate the electron and hole wave functions. When a force field of the Tersoff type is used, 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. These calculations indicate that the main LO phonon peak involves largely motion of the interior atoms, while the low-frequency shoulder is more equally distributed throughout the crystal but not surface-localized. Interestingly, very different results are obtained with the widely used Coulomb plus Lennard-Jones force field developed by Rabani, which predicts far more disordered structures and more localized phonon modes for the nanocrystals compared with the Tersoff-type potential.