Modeling distributed kinetics in isolated semiconductor quantum dots

A detailed modeling of recently observed nonexponential fluorescence intermittency in colloidal semiconductor quantum dots (QDs) is presented. In particular, experiments have shown that both "on"-time and "off"-time probability densities generated from single-QD fluorescence trajectories follow an inverse power law, P(tau(on/off))proportional to1/tau(on/off)(1+alpha), over multiple decades in time, where the exponent 1+alpha can, in general, differ for "on" versus "off" episodes. Several models are considered and tested against their ability to predict inverse power law behavior in both P(tau(on)) and P(tau(off)). A physical picture involving electron tunneling to, and return from, traps located several nanometers away from the QD is found to be consistent with the observed P(tau(off)) but does not yield the inverse power-law behavior seen in P(tau(on)). However, a simple phenomenological model based on exponentially distributed and randomly switched on and off decay rates is analyzed in detail and shown to yield an inverse power-law behavior in both P(tau(on)) and P(tau(off)). Monte Carlo calculations are used to simulate the resulting blinking behavior, and are subsequently compared with experimental observations. Most relevantly, these comparisons indicate that the experimental on-->off blinking kinetics are independent of excitation intensity, in contradiction with previous multiphoton models of on/off intermittency based on an Auger-assisted ionization of the QD by recombination of a second electron-hole pair.