Modified power law behavior in quantum dot blinking: a novel role for biexcitons and auger ionization

Single photon detection methods are used to acquire fluorescence trajectories from single CdSe/ZnS colloidal quantum dots (QDs) and analyze their blinking behavior. Although the "off-time" distributions follow ideal power law behavior at all wavelengths and intensities, significant deviations from power law behavior are observed for the "on-times". Specifically, with improved time resolution, trajectory durations, and photon statistics, we report a near-exponential falloff of on-time probability distributions at long times. Investigation of this falloff behavior as a function of laser wavelength and power demonstrate that these deviations originate from multiexciton dynamics, whose formation probabilities can be very low on a "per laser pulse" basis, but become nearly unity on the time scales of the longest on-times. The near quadratic, power-dependent results indicate the predominant role of biexcitons in the long time on-to-off blinking dynamics, which can be interpreted in terms of an Auger ionization event. In conjunction with Poisson modeling of the photon statistics, the data is consistent with QD ionization efficiencies of order approximately 10(-5) and highlight a novel role for biexcitons and Auger ionization in QD blinking.