Structure-Induced Asymmetry of Auger Decay Rates Between Positive and Negative Trions

We report spectroscopic studies of nonradiative Auger decay rates of negative (X$^{-})$ and positive (X$^{+})$ trions for three QD samples having different energy-band structures. We observe that the symmetry between X$^{-}$ and X$^{+}$ Auger decay rates observed for core-only PbSe QDs (characterized by mirror-symmetric conduction and valence bands) can be substantially distorted in CdSe/ZnS QDs, where the spectral density of valence-band states is much higher than that of the conduction-band states, leading to a relative enhancement of the X$^{+}$ decay channel. The asymmetry between X$^{-}$ and X$^{+}$ Auger rates observed for CdSe/ZnS QDs is further enhanced in thick-shell CdSe/CdS QDs wherein the hole is confined within the core, but the electron is delocalized over the entire QD, which causes a considerable reduction in the X$^{-}$ Auger decay rate. In single-dot studies of thick-shell QDs, we are able to identify photoluminescence (PL) bands of X$^{-}$ and X$^{+}$ along with that of a neutral exciton (X$^{0})$. We found that, while X$^{+}$ emission shows a short lifetime (1-2 ns) and a low quantum yield (\textless 5{\%}), X$^{-}$ PL features an increased lifetime (up to $\sim$ 10ns) and a high emission efficiency (up to $\sim$ 60{\%} of that of X$^{0})$ due to strong suppression of the X$^{-}$ Auger decay pathway, which agree well with ensemble measurements.