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(τ o n / o f f )1/τ 1 + α o n / o f f , over multiple decades in time, where the exponent 1 + α 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(τ o n ) and P(τ o f f ). 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(τ o f f ) but does not yield the inverse power-law behavior seen in P(τ o n ). 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(τ o n ) and P(τ o f f ). 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.

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