“On”/“off” fluorescence intermittency of single semiconductor quantum dots

Abstract

Single molecule confocal microscopy is used to investigate the detailed kinetics of fluorescence intermittency in colloidal II–VI (CdSe) semiconductor quantum dots. Two distinct modes of behavior are observed corresponding to (i) sustained “on” episodes (τon) of rapid laser absorption/fluorescence cycling, followed by (ii) sustained “off” episodes (τoff) where essentially no light is emitted despite continuous laser excitation. Both on-time and off-time probability densities follow an inverse power law, P(τon/off)∝1/τon/offm, over more than seven decades in probability density and five decades in time. Such inverse power law behavior is an unambiguous signature of highly distributed kinetics with rates varying over 105-fold, in contrast with models for switching between “on” and “off” configurations of the system via single rate constant processes. The unprecedented dynamic range of the current data permits several kinetic models of fluorescence intermittency to be evaluated at the single molecule level and indicate the importance of fluctuations in the quantum dot environment.

Excitation Intensity Dependence of Photoluminescence Blinking in CsPbBr3 Perovskite Nanocrystals

“…The truncation times, τ c , for all NC probability distributions that could be reliably fit to a truncated power law are summarized in Figure 3b At all intensities used, the average power law slope (α on ) is <1.2, indicating that on-state probability distributions of CsPbBr 3 NCs are observed to decay more slowly compared to chalcogenide NCs such as CdSe, where α on is consistently between 1.2 and 1.8. 19,21,38,47 On-state truncation times vary more considerably in the literature due to their demonstrated sensitivity to various experimental conditions such as photon excitation energy, excitation intensity, dielectric constant of the surrounding media, and shell thickness. An experiment in which the photon excitation energy was identical to that used here, 650 meV above the bandgap, and the sample preparation was consistent (dilute solution of NCs mixed with PMMA/ toluene), the average truncation time for CdSe/ZnS NCs was <0.1 s at an average exciton formation of ⟨N ex ⟩ = 0.28.…”

Collective fluorescence enhancement in nanoparticle clusters

“…Direct tunnelling is only likely between closepacked NRs separated by 1-2 nm. For spherical core-shell NCs, Kuno et al 12 estimate that the range of experimentally observable blinking rates (from 10 kHz to 0.01 Hz) could correspond to tunnelling between the core and trap sites on the substrate 1-2 nm away from the surface of the NC shell. This suggests direct charge tunnelling between close-packed NRs in our clusters is plausible.…”

“…In the charged core, highly efficient Auger processes lead to rapid nonradiative recombination of subsequent photoexcited excitons. The NC is 'dark' in this 'charge-separated' state; fluorescence resumes once the core regains electrical neutrality [3][4][5][10][11][12][13][14] . Recently, the Krauss and Klimov groups observed near-complete suppression of blinking in NCs synthesized with graded shells that greatly reduce the efficiency of Auger recombination, providing strong support for the proposal that dark states involve Auger recombination 10,15 .…”

Scaling Green-Kubo Relation and Application to Three Aging Systems

“…At the transition to the nonstationary phase (ν ¼ 2), the stationary result diverges. periods and remain dark during others [63][64][65]. The durations of these on periods and off periods are found to be distributed according to a power law whose exponent is such that the average on and off time is infinite (i.e., of the order of the measurement time), leading to aging in the intensity autocorrelation function [7,66].…”