Quantum Dot Photoluminescence Activation and Decay: Dark, Bright, and Reversible Populations in ZnS-Capped CdSe Nanocrystals

Semiconductor nanocrystals or quantum dots (QDs) exhibit a number of unique optical properties including fluorescence intermittency (FI), photoluminescence (PL) enhancement, and darkening. Here we report PL activation (PLA) and darkening over populations of single colloidal ZnS-capped CdSe QDs under continuous illumination, which is described well by a simple consecutive elementary reaction ((ER) scheme in the 1 to 10 kW cm(-2) excitation intensity regime. The scheme allows determination of rate constants for both fluorescence activation and decay processes as well as the measurement of initial bright, fluorescent and dark, nonfluorescent QD fractions. The latter parameters can function as a "quality control" on the total population of detectable QDs in an imaging experiment or a synthesis. We further show reversible PLA at low intensities < 0.5 kW cm(-2) and a photoinduced conditioning of the QD that results in increased rates of PLA following repeated cycles of illumination and an induction period that precedes photoactivation upon initial exposure to light. By interrogating individual QD fluorescence trajectories, the population fractions found exclusive to each of three illumination cycles, common to any two cycles and fluorescent in all three cycles, show that only a small number of QDs (similar to 5%) remain fluorescent through multiple cycles of photoactivation and recovery.