Electron-Phonon Coupling in CdSe Nanocrystals

A size dependent study of electron-phonon coupling of organically capped, wurtzite form CdSe nanocrystals dissolved in chloroform was performed using resonance Raman excitation profiles for the longitudinal optical (LO) phonon fundamental and its first overtone. CdSe nanocrystals with diameters of 2.8, 3.0, 3.2, and 5.2 nm were synthesized from variations of literature procedures and then ligand exchanged with hexadecanethiol ligands to quench the underlying fluorescence in our Raman measurements. The absolute differential Raman cross section for the fundamental is much larger when excited on the high-frequency side of the first absorption maximum whereas cross sections at shorter wavelength excitations were much smaller despite the absorption being much higher at shorter wavelengths. The optical absorption spectrum, the resonance Raman excitation profiles, and depolarization dispersion curves were reproduced using a model for the energies, oscillator strengths, electron-phonon couplings, and dephasing rates of the multiple low lying electronic excitations for each sized nanocrystals. The electron-phonon coupling for the LO phonon, expressed through the Huang-Rhys parameter, was determined to be in the range of S=0.04-0.15 while a good fit could be obtained using the same value, S=0.08, for the lowest excitonic transition for each particle size. This result shows that there is no size dependence of the electron-phonon coupling strength of the first excitonic transitions for various sized CdSe nanocrystals. Each sized nanocrystal also showed similar features in the higher energy spectral region, 2000-5000 cm-1 above the first absorption maximum. This region, typically labeled as the 1P3/2-1Pe transition, is actually a combination of at least two contributions that vary greatly in their magnitude of electron-phonon coupling strength