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

Abstract

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 (kt) and detrapping (kd) rate constants are estimated. The increase in φPL observed with the increase in core size of CdSe at the ensemble level is related to the enhanced kd/kt 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.