Evidence for Multiple Trapping Mechanisms in Single CdSe/ZnS Quantum Dots from Fluorescence Intermittency Measurements over a Wide Range of Excitation Intensities

Fluorescence intermittency of single CdSe/ZnS core/shell quantum dot particles is investigated over a wide range of excitation intensities and at two excitation wavelengths. Deviation from previously observed power law behavior in both the on- and off-duration probability distributions is observed at both above the band gap. Increasing the excitation intensity modifies the g I L I ilk excitation wavelengths, one near the band gap and one 350 meV off-duration probability distribution such that the probability of long off-time events decreases, an effect that is observed to saturate at an average number of excitons created per pulse of one, < N > approximate to 1. The on-duration probability distribution is well-described by a power law for short on-time events, crossing over to an exponential distribution for long on-time events. Increasing the excitation intensity induces this crossover to occur at earlier times, saturating at < N > approximate to 2. The different intensity-dependent trends for the on- and off-duration probability distributions are evidence that multiple mechanisms govern the blinking statistics, as no single mechanism can account for the different saturation behaviors of the off and on durations as a function of excitation intensity. These mechanisms are assumed to be one based on a light-induced diffusion of both the excited-state and trap state energies (i.e., diffusion-controlled electron transfer) and one that relies on the absorption of multiple photons (i.e., Auger ionization induced trapping).