Interplay of Multiexciton Relaxation and Carrier Trapping in Photoluminescent CdS Quantum Dots Prepared in Aqueous Medium

A comprehensive understanding of exciton dynamics of water-soluble quantum dots (QDs), especially those synthesized directly in aqueous media, is critical for devising their applications. Trapping processes are expected to play a pivotal role here, as surface trap states are abundant in such QDs. The present study, based on ultrafast transient absorption, provides insight into exciton dynamics in PEI-capped CdS QDs synthesized by a one-pot reaction in water. They are robust and strongly photoluminescent (PL), unlike many other aqueous QDs. Excitation energy dependence of PL and the underlying mechanisms have been investigated. Analysis of fluence-dependent transient absorption bleach and photoinduced absorption is performed to deconvolute contributions from multiexciton recombination and carrier trapping. A simple kinetic model is used to quantify the rate constants associated with intraband relaxation of hot excitons and hot carrier trapping. Similar magnitudes observed for the two pathways highlight the competitive kinetics between them, which is responsible for a drastic decrease in PL quantum yields for excitation above the band gap. Such efficient hot carrier trapping in these QDs may possibly foreshadow ultrafast trap-mediated electron transfer and thus render them suitable for redox-sensing and photocatalysis.