Amy Cordones, UC Berkeley
Charge carrier trapping at interface and surface sites of semiconductor nanomaterials decreases the efficiency of both charge transfer and carrier recombination. This process, therefore, impedes the successful incorporation of nanocrystals into optoelectronic devices and is of fundamental importance for these systems. On a single nanocrystal level, carrier trapping manifests as interruptions in the otherwise continuous fluorescence and is referred to as fluorescence intermittency or ‘blinking’. Although fluorescence blinking is observed ubiquitously for semiconductor nanocrystals, the underlying mechanism is still not fully understood. The charge carrier trapping mechanisms, as well as the non-radiative process that renders the nanocrystal non-fluorescent, are highly debated. In this talk, I will discuss evidence from our group for several charge trapping mechanisms in CdSe quantum dots based on correlated measurement of single nanocrystal fluorescence blinking, excited state decay, and photon anti-bunching. Excitation intensity dependent blinking studies highlight the role of multiple excitons in the charge trapping process, pointing to an Auger ionization mechanism. However, a high degree of memory is observed for successive on- and off-state events, highlighting the role of an energetic diffusion-based process. I will also examine the most widely accepted non-radiative decay mechanism thought to occur during the ‘dark’ durations of the quantum dot blinking and show new evidence directly contradicting such processes. Finally, I will discuss how statistical analysis of fluorescence blinking can be used to characterize the interaction and charge transfer processes at the nanocrystal/ligand interface for new materials systems.
