Core-Size-Dependent Trapping and Detrapping Dynamics in CdSe/CdS/ZnS Quantum Dots

The fascinating optoelectronic properties of semiconductor quantum dots (QDs) originate from the quantum confinement effect; i.e., the band gap increases as the size decreases. Another significant parameter dictating the photophysics of QD is the dynamics of trapping and detrapping from the trap-states, but it is nonetheless less understood. To understand these aspects, herein we investigate the photoluminescence (PL) fluctuations in CdSe QDs using time-resolved PL spectroscopy by systematically varying its core size, maintaining a constant shell thickness by coating with CdS followed by ZnS. The probability density distribution of ON- and OFF-events of QDs is constructed from the PL trajectories and fitted with the truncated power-law from which the trapping ( k t ) and detrapping ( k d ) rate constants are estimated. The increase in φ PL observed with the increase in core size of CdSe at the ensemble level is related to the enhanced k d / k t and charge carrier wave function localization in the core. Indeed, the band gap decreases as the core size increases, bringing the trap-states close to the band edge positions, leading to an efficient detrapping of carriers. The fluorescence lifetime-intensity distribution plots revealed the presence of a high-intensity and high-lifetime component due to neutral exciton recombination and a low-intensity and low-lifetime component due to trap-induced Auger recombination. The decay kinetics in a single QD is further modeled using a pre-equilibrium approximation.