Size-Dependent Exciton-Phonon Coupling in CdSe Nanocrystals through Resonance Raman Excitation Profile Analysis

Resonance Raman excitation profiles, including absolute Raman cross sections, have been measured for the optical phonons of organic ligand capped spherical CdSe nanocrystals with average diameters ranging from 2.6 to 5.2 nm. The absorption spectra, fundamental excitation profiles, overtone to fundamental intensity ratios, and depolarization ratio dispersion profiles are simulated using a model for the excitonphonon coupling strengths in the various excitonic transitions that contribute to the resonance enhancement. The resonance Raman cross sections increase strongly with nanocrystal size as expected from the increased transition dipole moments (oscillator strengths) in the larger particles. However, the exciton-phonon coupling strengths, particularly for the lowest energy excitonic transition (1S(e)-1S(3/2)), are relatively independent of nanocrystal size over this range, perhaps decreasing slightly with increasing size. This result is consistent with calculations of exciton-phonon coupling using an atomistic model for the phonon modes and an effective mass approximation-envelope function model for the excitonic states if the electron and hole effective masses are adjusted to fit the experimental excitonic transition energies.