Mechanisms for Fluorescence Blinking and Charge Carrier Trapping in Single Semiconductor Nanocrystals
- Author(s): Cordones, Amy Ashbrook
- Advisor(s): Leone, Stephen R
- et al.
Time-resolved fluorescence is measured for single semiconductor nanocrystals with a range of compositions and morphologies. The fluorescence is observed to blink on and off intermittently, a process known as fluorescence blinking or intermittency. Despite its wide-spread occurrence for nanocrystals, the exact cause of the blinking behavior is still highly debated. A large number of blinking models have been proposed, all of which predict charge carrier trapping to play a major role in blinking. Fluorescence blinking is measured as a function of excitation intensity for CdSe/ZnS quantum dots, and two different power-dependent behaviors are observed for the on- and off-states. While the on-state power dependence supports a multi-exciton ionization-induced charge trapping mechanism, the off-state data supports a fluctuating trapping/de-trapping rate that depends on a single exciton. This suggests that multiple charge trapping mechanisms may play a role for a single nanocrystal. Investigation of the fluctuation-based mechanism was extended to several nanocrystal compositions and morphologies through the measurement of on-state memory, or correlations of subsequent on-state durations, and distributed kinetics. The on-state memory was found to decrease for nanocrystals with less distributed blinking kinetics, indicating that both may originate from the same fluctuation-based blinking mechanism. Furthermore, reproducing this correlation using Monte-Carlo simulations required the combined effects of two blinking mechanisms. Consistent with the power-dependent results, a single rate charge trapping process and the fluctuation-based mechanism were considered. Finally, the simultaneous measurement of fluorescence blinking and decay data for CdSe/ZnS quantum dots led to the direct observation of trapping rate fluctuations occurring during the off-state. This result supports one class of blinking models that relies on rate fluctuations, and not long-lived trap states, to account for blinking. Taken together, these results help to construct a larger-scale picture of fluorescence blinking that relies on the combined effects of several charge trapping processes. Fluorescence blinking was also applied to probe the interaction of single CdSe/CdS nanocrystals and a new class of chalcogenidometalate ligand complexes. Hole trapping was identified as the dominant process affecting the optical properties of this composite system.