Ultrafast Spectroscopy of CdSe Nanocrystals: Morphological and Environmental Effects on Nonradiative and Nonadiabatic Relaxation

Ultrafast transient absorption spectroscopy was employed to investigate intraband relaxation and coherent acoustic phonons in quantum dots (QDs) and quantum rods (QRs) of various sizes. We found that the hot electrons and hot holes relaxed through a nonradiative Auger thermalization mechanism that circumvents the phonon bottleneck effect, resulting in sub-2.5 ps intraband relaxation times. The QRs showed an increased intraband relaxation time when compared with QDs as a result of the formation of a ID exciton along the axial dimension, which partially mitigates the Auger thermalization mechanism. The longer intraband relaxation times for QRs suggest that QRs would act as better sensitizers for hot electron nanocrystal solar cells. The QD samples prepared in thin-film form showed longer intraband relaxation times when compared with their colloidal counterpart; this is a result of the hydrazine treatment used, which mitigates the dominant ligand relaxation pathway for holes. Furthermore, we found that the frequencies of coherent acoustic phonon modes were reduced for both QD and QR thin-film samples as a result of neighboring NC interaction, suggesting that there is a strong dependence on environmental conditions that govern the nonadiabatic relaxation