  Previous single-particle spectroscopic studies of colloidal quantum dots have
indicated a significant spread in biexciton lifetimes across an ensemble of
nominally identical nanocrystals. It has been speculated that in addition to
dot-to-dot variation in physical dimensions, this spread is contributed to by
variations in the structure of the quantum dot interface, which controls the
shape of the confinement potential. Here we directly evaluate the effect of the
composition of the core-shell interface on single- and multi-exciton dynamics
via side-by-side measurements of individual core-shell CdSe-CdS nanocrystals
with a sharp vs. smooth (graded) interface. To realize the latter type of
structures, we incorporate a CdSexS1-x alloy layer of controlled composition
and thickness between the CdSe core and the CdS shell. We observe that while
having essentially no effect on single-exciton decay, the interfacial alloy
layer leads to a systematic increase in biexciton lifetimes. This observation
provides direct experimental evidence that in addition to the size of the
quantum dot, its interfacial properties also significantly affect the rate of
Auger recombination, which governs biexciton decay. These findings help
rationalize previous observations of a significant heterogeneity in the
biexciton lifetimes across similarly sized quantum dots and should facilitate
the development of Auger-recombination-free colloidal nanostructures for a
range of applications from lasers and light-emitting diodes to photodetectors
and solar cells.
