Search

Using ultrafast transient absorptionand time-resolved photoluminescencespectroscopies, we studied multiple exciton generation (MEG) in quantumdots (QDs) consisting of either PbSe, PbS, or a PbSxSe1–xalloy for variousQD diameters with corresponding bandgaps (Eg) ranging from 0.6 to 1 eV. For each QD sample, we determine theMEG efficiency, ηMEG, defined in terms of the electron–holepair creation energy (εeh) such that ηMEG= Eg/εeh.In previous reports, we found that ηMEGis abouttwo times greater in PbSe QDs compared to bulk PbSe, however, littlecould be said about the QD-size dependence of MEG. In this study,we find for both PbS and PbSxSe1–xalloyed QDs that ηMEGdecreaseslineally with increasing QD diameter within the strong confinementregime. When the QD radius is normalized by a material-dependent characteristicradius, defined as the radius at which the electron–hole Coulomband confinement energies are equivalent, PbSe, PbS, and PbSxSe1–xexhibitsimilar MEG behaviors. Our results suggest that MEG increases withquantum confinement, and we discuss the interplay between a size-dependentMEG rate versus hot exciton cooling. [ABSTRACT FROM AUTHOR]

Multiple exciton generation (MEG) in PbSe quantum dots (QDs), PbSe(x)S(1-x) alloy QDs, PbSe/PbS core/shell QDs, and PbSe/PbSe(y)S(1-y) core/alloy-shell QDs was studied with time-resolved optical pump and probe spectroscopy. The optical absorption exhibits a red-shift upon the introduction of a shell around a PbSe core, which increases with the thickness of the shell. According to electronic structure calculations this can be attributed to charge delocalization into the shell. Remarkably, the measured quantum yield of MEG, the hot exciton cooling rate, and the Auger recombination rate of biexcitons are similar for pure PbSe QDs and core/shell QDs with the same core size and varying shell thickness. The higher density of states in the alloy and core/shell QDs provide a faster exciton cooling channel that likely competes with the fast MEG process due to a higher biexciton density of states. Calculations reveal only a minor asymmetric delocalization of holes and electrons over the entire core/shell volume, which may partially explain why the Auger recombination rate does not depend on the presence of a shell.