Nonexponential “blinking” kinetics of single CdSe quantum dots: A universal power law behavior

Single molecule confocal microscopy is used to study fluorescence intermittency of individual ZnS overcoated CdSe quantum dots (QDs) excited at 488 nm. The confocal apparatus permits the distribution of “on” and “off” times (i.e., periods of sustained fluorescence emission and darkness) to be measured over an unprecedentedly large dynamic range $(109)$ of probability densities, with nonexponential behavior in $τoff$ over a $105$ range in time scales. In dramatic contrast, these same $τoff$ distributions in all QDs are described with remarkable simplicity over this $109$-fold dynamic range by a simple inverse power law, i.e., $P(τoff)∝1/τoff1+α.$ Such inverse power law behavior is a clear signature of distributed kinetics, such as predicted for (i) an exponential distribution of trap depths or (ii) a distribution of tunneling distances between QD core/interface states. This has important statistical implications for all previous studies of fluorescence intermittency in semiconductor QDs and may have broader implications for other systems such as single polymer molecules.