Decay-to-Recovery Behavior and on-off Recovery of Photoluminescence Intensity from Core/Shell Quantum Dots

We report on the intriguing phenomenon of the evolution of photoluminescence from CdSe core, CdSe/ZnS core/shell, and CdSe/Cd_1-xZn_xSe_1-yS_y core/graded shell green-emitting quantum dots (QDs). The thickness and composition profile of QDs was found to control how the optical characteristics evolve under different light exposure conditions. Unexpectedly, changes of emission intensity (decreasing and increasing) and spectral band position were observed, which can be reversible or irreversible, depending on the QD architecture and exposure conditions. It is revealed that competition between the reversible and irreversible optical changes led to unique decay-to-recovery behavior for the QD emission, metastable bright states of QDs that can be activated and deactivated numerous times, and decoupled optical changes (reversible intensity changes vs irreversible spectral shifts). We suggest that the distinct dynamic response of each QD architecture arises from how the core and shell interact with each other and the influence of environment parameters (i.e., oxygen, H₂O, and light). Furthermore, there is a large difference in the decay and recovery rates (seconds vs many minutes) to be considered for dynamic photonic systems. This work offers a general framework for evaluating dynamic photoluminescence properties of QDs where stable or variable/controlled emission is required, in situations where long device lifetimes are highly desirable (e.g., QD displays, LEDs, and lasers), or in the case of exploring dynamic properties for tunable emission patterning.