Photoinduced Surface Trapping and the Observed Carrier Multiplication Yields in Static CdSe Nanocrystal Samples

Abstract

Photocharging has been suggested recently as the explanation for the spread of carrier multiplication yields reported by different groups. If this hypothesis can be plausible in the case of PbSe, it is inconsistent with the reported experimental data relative to CdSe nanocrystals and cannot therefore explain the large discrepancies found in that material system between static and stirred samples. An alternative explanation, photoinduced surface trapping, is suggested here, based on the results of atomistic semiempirical pseudopotential calculations of the Auger recombination rates in a number of excitonic configurations including a variety of surface traps, which show that the photoinduced surface trapping of the hole, which leaves the core negatively charged (but the nanocrystal neutral overall), can lead to recombination rates that are indistinguishable from those of a conventional biexciton with four core-delocalized carriers and therefore result in exaggerated CM yields in static samples. In contrast, the recombination rate of a charged exciton is found to be at least a factor of 2.3 smaller than that of the biexciton and therefore easily distinguishable from it experimentally. Although increased trapping at surface states was dismissed as unlikely for PbSe nanocrystals, in the case of CdSe, this hypothesis is further supported by much experimental evidence including recent spectroscopic measurements on CdSe nanostructures, single-nanocrystal photoionization studies on CdSe core/shell nanocrystals, and state-resolved transient absorption studies of biexcitonic states, all showing increased probability of surface trapping for highly excited states. These results suggest that multicarrier processes could be mediated by different mechanisms in CdSe and PbSe nanocrystals.