Controlling Auger Decay Rates of CdSe/CdS Nanocrystals via Core/Shell Interfacial Alloying

We report single-dot spectroscopic studies to evaluate the effect of the core/shell interface (i.e., the shape of the confinement potential) on nonradiative Auger decay rates of CdSe/CdS quantum dots (QDs) that have either a sharp or a graded interface. Alloyed QDs with a graded potential are prepared by incorporating a CdSe$_{\mathrm{x}}$S$_{\mathrm{1-x}}$ alloy layer of a controlled composition and thickness between the core and the shell. In second-order intensity correlation (g$^{\mathrm{(2)}})$ measurements, we observed that the interfacial layer has a negligible effect on single-exciton dynamics, but leads to a systematic increase in the biexciton photoluminescence quantum yield ($Q_{\mathrm{BX}})$. We found that $Q_{\mathrm{BX}}$ of alloyed QDs can be up to $\sim$ 10 times higher than that of the reference QDs with a sharp interface. These results are further supported by independent measurements of biexciton dynamics that show a considerable elongation of biexciton lifetimes (to several ns) upon interfacial alloying. Finally, a statistical investigation of over 100 individual QDs shows that the CdS shell thickness has only a minor effect on $Q_{\mathrm{BX}}$. All of these findings point a significant role of the shape of the confinement potential in Auger recombination and should facilitate the development of ``Auger-recombination-free'' QDs.