Charging and Discharging Channels in Photoluminescence Intermittency of Single Colloidal CdSe/CdS Core/Shell Quantum Dot

Abstract

Understanding photoluminescence (PL) intermittency of single quantum dots (QDs) (intensity blinking by randomly switching between distinguishable brightness states under continuous excitation) has been a long-standing fundamental challenge and potential roadblock for their applications. Here we introduce a new analysis method for single-molecule spectroscopy that treats the blinking as photochemical/chemical processes (switching between neutral/bright and charged/dim states). It uncovers the channels for charging (bright to dim) and discharging (dim to bright) involved in PL blinking of single CdSe/CdS core/shell QDs. Both charging and discharging of the single CdSe/CdS core/shell QD possess a photochemical channel (∼10 to 10 events/photon) that linearly depends on excitation in both single- and multi-exciton regime. These two linear channels coupled to a spontaneous discharging channel (∼2 events/s) to dictate the QDs from nonblinking to gradually blinking under increasing excitation. For high-quality CdSe/CdS core/shell QDs, Auger ionization of multiexciton for both charging and discharging is negligible.

Photoluminescence Intermittency and Photo‐Bleaching of Single Colloidal Quantum Dot

“…Ensemble measurements can determine the PL quantum yield of the single‐exciton emission, which is nearly unity for the QD sample related to Figure . These results shall enable one to calculate the PL quantum yield of the trion state using the peak intensity ratio of the trion and bright state in the plot of intensity histogram …”

“…Thus, by increasing 〈 N 〉 with a fixed repetition frequency of the pulsed laser, it should result in increased occurrence of the trion state of a QD ( Figure a). In Figure a, the PL intensity trajectory of a high‐quality CdSe/CdS core/shell QD is tuned from nearly non‐blinking to blinking between two distinguishable states by increasing the average photon absorbed per excitation pulse (〈 N 〉) . Definite appearance of the trion state and its PL quantum yield relative to the bright state can be clearly observed in the intensity histogram.…”

“…In Figure b, the PL decay dynamics for both states is separately plotted and both follow nearly mono‐exponential decay dynamics. By comparing the mono‐exponential lifetime of single‐dot PL at the bright state with that of the ensemble PL, one would conclude that the bright state should be the single‐exciton state . Ensemble measurements can determine the PL quantum yield of the single‐exciton emission, which is nearly unity for the QD sample related to Figure .…”

“…b) PL decay curves of the photons from the bright (red curve) and dim state (blue curve) of the blinking trace with 〈 N 〉 = 0.25. Reproduced with permission . Copyright 2016, American Chemical Society.…”

“…The PL blinking of single colloidal QD has been actively explored with different chemical compositions, morphologies, and measurement conditions after its discovery in 1996 . The diverse results can be roughly grouped into a few types of blinking.…”