Effect of the Core/Shell Interface on Auger Recombination
Evaluated by Single-Quantum-Dot Spectroscopy
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Abstract
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 multiexciton dynamics via side-by-side measurements
of individual core–shell CdSe/CdS nanocrystals with a sharp
versus smooth (graded) interface. To realize the latter type of structures
we incorporate a CdSe<sub><i>x</i></sub>S<sub>1–<i>x</i></sub> 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, which
correlates with the increase in the biexciton emission efficiency,
as inferred from two-photon correlation measurements. These observations
provide 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