Simultaneous Creation and Recovery of Trap States on Quantum Dots in a Photoirradiated CdSe-ZnO System

We study changes in the steady-state absorption and ultrafast transient absorption kinetics of the photoirradiated CdSe quantum dotZnO system. The changes enable us to reconstruct kinetics of trap creation, which are analyzed with respect to three possible models: trap creation without recovery, trap creation and recovery, and trap creation with an upper limit for trap number accommodated on a quantum dot. We demonstrate that only the model of parallel trap creation and recovery can explain our experimental data. The evidence points toward oxygen generating trapping sites on QD surface and simultaneously passivating the trapping sites by their oxidation.