Shape dependent ultrafast relaxation dynamics of CdSe nanocrystals: Nanorods vs nanodots

Femtosecond pump-probe transient spectroscopy is used to compare the time dependence of the bleach recovery of CdSe nanorods with that of nanodots. In the quantum dots the bleach spectrum evolved rapidly to its characteristic shape, while in the nanorods, the higher energy exciton states are first populated within the laser pulse width (100 fs) and then relax to lower states. This behavior is explained by the larger number of exciton states in the rods due to splitting of the degeneracy of the states in the dots as a result of the decrease in symmetry as predicted from existing theories.