Very low emissivity of multicarrier states (e.g., charged excitons and multi-excitons) in nanocrystal quantum dots (QDs) is usually attributed to nonradiative losses due to Auger recombination whereby the recombination energy of an electron-hole pair is not converted into a photon but is instead captured by a third carrier. Nonradiative quenching by Auger recombination complicates many applications of QDs including light-emitting diodes, single-photon and photon-pair sources, and especially lasers, as all of these applications directly rely on efficient radiation from multicarrier states. Therefore, the development of “Auger-recombination-free” QDs is an important current challenge in the field of colloidal nanostructures.
Our previous studies of so-called “giant”-QDs (g-QDs, a CdSe core surrounded with an ultra-thick CdS shell up to 19 monolayers), have shown that biexciton photoluminescence (PL) emission efficiencies (Q) of individual g-QDs can be near-unity. However, the efficiency of biexciton emission was highly nonuniform across an ensemble of nominally identical QDs. It was speculated that in addition to dot-to-dot size variation, this spread originated from variations in the structure of the core/shell interface in QDs, which determined the shape of the confinement potential.
Here we present a direct side-by-side comparison of the effect of the core/shell interface on nonradiative Auger decay rates of individual core/shell CdSe/CdS nanocrystals that have either a sharp or a graded interface. The latter type of QDs (referred to as “alloyed QDs”) comprises a CdSeS alloyed layer of controlled composition and thickness between the CdSe core and the CdS shell. We observed that, while having essentially no effect on single-exciton dynamics, the interfacial layer leads to a systematic increase in the biexciton PL quantum yield (QY), as inferred from second-order intensity correlation (g) measurements. Q in alloyed QDs was found to be up to ~10 times higher than that in the reference QDs with a sharp interface. These results are corroborated by independent measurements of biexciton dynamics that show a considerable increase in biexciton lifetimes upon interfacial alloying, leading to a remarkable quantitative agreement between the Q values derived by the g-method and those inferred from the measured lifetimes of single- and bi-exciton. Finally, a statistical investigation of over 100 individual QDs with either sharp or graded interfaces shows that the CdS shell thickness has only a minor effect on biexciton emission QY. All of these findings point towards a significant role of the shape of the confinement potential in Auger recombination and indicate the possibility of controlling this effect via appropriate engineering of QD interfaces.