Publication date: 15th December 2025
Colloidal quantum dots (QDs) have found commercial success in light-downshifting applications on the back of incredible advances in synthesis, shelling, and engineering. However, highly insulating shells are unsuitable for many optoelectronic applications, which resurfaces the challenge of ‘trap’-state emission—observed since the dawn of the field—and associated with diminished emissivity, anomalously large Stokes’ shifts, and worsened photochemical stability. The structural and chemical identity(ies) of these trap states remain unclear, in part due to the obscuring effect of ensemble-level studies. Here, we report the kinetics and dynamics of trap emission from individual, CdSe-based QDs. We find that individual QDs can intermittently display both band-edge and trap-state photoluminescence over time, while maintaining single-photon purity. The excited-state configuration(s) that is associated with trap emission appears to be distinct from an OFF-configuration (familiar from studies of blinking), demanding an expansion of existing models for the macroscopically time-dependent photophysical behaviours of QDs. We observe that the decay kinetics of trap emission are persistently slower relative to the band-edge—phenomenologically consistent with an excited-state reservoir, rather than a quenching center. Moreover, in periods where trap emission is observed, the band-edge photoluminescence slows, consistent with delayed emission following recycling. Ultimately, we can quantitatively capture the time-averaged and macro-time-dependent photophysics of each QD with a simple kinetic model based on reversible equilibration between band-edge and trap states and a macro-time-varying energetic offset. This supports the key conclusion that the state that gives rise to this trap emission lies shallow within the gap and is energetically dynamic on individual QDs over macroscopic timescales that are consistent with photoinduced structural changes. We discuss implications with regards to trap-state identity(ies).
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