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

Single molecule confocal microscopy is used to study fluorescence intermitt ency 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 meas ured over an unprecedentedly large dynamic range (10(9)) of probability den sities, with nonexponential behavior in tau(off) over a 10(5) range in time scales. In dramatic contrast, these same tau(off) distributions in all QDs are described with remarkable simplicity over this 10(9)-fold dynamic rang e by a simple inverse power law, i.e., P(tau(off)) proportional to 1/tau(of f)(1 + alpha). Such inverse power law behavior is a clear signature of dist ributed kinetics, such as predicted for (i) an exponential distribution of trap depths or (ii) a distribution of tunneling distances between QD core/i nterface states. This has important statistical implications for all previo us studies of fluorescence intermittency in semiconductor QDs and may have broader implications for other systems such as single polymer molecules. (C ) 2000 American Institute of Physics. [S0021-9606(00)01007-2].