Suppression of Biexciton Auger Recombination in CdSe/CdS Dot/Rods: Role of the Electronic Structure in the Carrier Dynamics

We studied carrier dynamics in semiconductor nanocrystals consisting of a small CdSe dot embedded in an elongated, rod-shaped CdS shell, using the ultrafast pump probe technique. We found clear evidence of a substantial suppression of the Auger nonradiative recombination in the biexciton regime. Moreover, a simple model of the dynamics in which biexcitons show no Auger recombination, and only holes are localized in the dot, fits well the differential transmission observed at all pump densities. The long biexciton lifetime results into an observed long-living gain having a peak that is red shifted with respect to the lowest energy absorption peak. We argue that the origin of the large relative gain observed at large fillings is related to the peculiar structure of the electronic levels, and in particular, to delocalization of electrons in the rod.