On the blinking mechanism of colloidal semiconductor quantum dots

M. Kreiter

Semiconductor quantum dots act as highly efficient and photostable fluorophores. They are known to switch upon illumination between a bright and a dark state with the probability density for the length of on- and off periods obeying a power law statistics.

In spite of the importance of the understanding of the darkening mechanism in the search for the brightest possible light sources, no satisfying explanation for this behavior has been found yet. We developed an empirical model based on two independent blinking mechanisms that quantitatively describes the observed behavior. A power dependent, photo-induced effect is evoked that accounts for the deviation of the on-time statistics from the power law (See Fig. 1). It could be shown that this photoinduced effect can be explained in a rate picture, furthermore this analysis shows that the second, power-law process is symmetric in on- and off-times and is completely insensitive to several external stimuli [1].

Figure (click to enlarge):
a) Fluorescence image of individual Quantum dots.
b,c) On- and off- time distributions for different excitation intensities

In a further study, a residual memory was discovered in the blinking in contrast to the memoryless transitions in simple quantum systems such as organic fluorescing molecules [2].

[1] F. D. Stefani, W. Knoll, M. Kreiter, X. Zhong and M. Y. Han. Quantification of photo-induced and spontaneous quantum-dot luminescence blinking. Phys. Rev. B 72, 125305 (2005)
[2] F. D. Stefani, W. Knoll, M. Kreiter, X. Zhong, M. Y. Han. Memory in quantum-dot photoluminescence blinking. New J. Phys. 7, 197 (2005)