Effect of the shell on the blinking statistics of core-shell quantum dots: A single-particle fluorescence study

Abstract

Fluorescence blinking of single quantum dots (QDs) under constant illumination has attracted a great deal of attention from both experimentalists and theoreticians. To explain the power-law behavior in the distribution of both “on” and “off” times, several models have been proposed, in which the charge carrier is either ejected from the QD to an external trap or localized at a trapping site within the QD. To gain insight into the blinking mechanism, we have studied the role of the shell of $\mathrm{CdSe}\text{−}\mathrm{ZnS}$ core-shell QDs. For two sets of samples varying in the fluorescence quantum yield of the core, we systematically varied the thickness of the shell and analyzed the blinking behavior of a statistically significant number of single QDs. In both cases, the distributions of on and off times were independent of the $\mathrm{ZnS}$ shell thickness. These data can be explained with a recently introduced model [Frantsuzov and Marcus, Phys. Rev. B 72, 1553211 (2005)]. After photoinduced creation of an electron-hole pair, the hole is trapped in a deep surface state of the $\mathrm{CdSe}$ core, and the excess energy is transferred to the electron in an Auger process, raising it from the $1S_e$ to the $1P_e$ state. The energy gap between these electronic states is assumed to perform stochastic diffusion so that it can be either in resonance or out of resonance with the energy gap between the hole states. This model explains the power-law behavior of the on and off times distribution, the exponential cutoff of the power-law dependence at long on times and the lacking dependence of the blinking kinetics on the shell thickness. It also explains our observation that the overall quantum yield of an ensemble is mainly governed by the fraction of nonemitting particles in the sample.