Abstract
We report the synthesis and optical characterization of core/shell CdSe/CdS quantum dots (QDs) with controlled surface composition. Using secondary phosphine chalcogenide and cadmium carboxylate precursors in an alternating layer-by-layer synthetic approach, the elemental surface composition of the quasi-type-I CdSe/CdS core/shell QDs can be repeatedly tuned from predominately cadmium to predominantly sulfur, as measured by X-ray photoelectron spectroscopy (XPS). Similar to CdS and CdSe core-only QDs, the surface composition has a significant effect on the photoluminescence (PL) quantum yield: sulfur terminated QDs exhibit quenched PL, while cadmium terminated QDs have relatively bright PL. Density-functional tight-binding calculations on CdSe/CdS core/shell clusters suggest that PL quenching for sulfur-rich surfaces is the result of a high density of hole surface states in the QD bandgap. Time-resolved PL measurements confirm the QDs’ nonradiative recombination rates are strongly sensitive to the surface composition.
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