Publication date: 15th May 2025
Many quantum dots (QDs), especially the smallest (d < 2 nm) core-only particles, exhibit broad, red-shifted emission that is commonly associated with trap states at the surface. Despite their prevalence in different QD compositions, much of the fundamental photophysics of these trap states remain ambiguous, in part due to the obscuring effect of ensemble-averaged spectroscopy. To overcome these barriers, we performed single-molecule spectroscopy on CdSe/ZnS QDs. We find that individual QDs can display both band-edge and trap-state photoluminescence, and that the absolute and relative intensities in both bands are independently intermittent. These findings show that pervasively observed trap-band emission can originate from transient trap states in photoexcited QDs. The observed kinetics of trap emission are generally monoexponential and slow (𝜏: 200-300 ns), inconsistent with rapid quenching. Band-edge photoluminescence exhibits largely biexponential decay kinetics with a long-lifetime tail that is comparable to the trap kinetics from the same QD. These observations are consistent with the reversible transfer of excitations between band-edge and dynamic trap states, matching models for delayed emission.
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