Optical control of exciton and trap-state recombination in quantum dots pairs through Forster Resonance Energy Transfer

The electron-hole recombination dynamics of colloidal quantum dots (QDs) is critically driven by charge carrier trapping. Surface traps lead to long-lived electron-hole pairs and play a pivotal role in the photochemical reactivity and the charge transfer at the interface between QDs and their medium. Here, we demonstrate that both band-edge exciton and surface trap electron-hole recombination kinetics can be optically manipulated thanks to Förster Resonance Energy Transfer (FRET) within suspensions of donor/acceptor QD pairs. To identify the specific roles of the different radiative states at play, we monitor the time evolution of their photoluminescence decays as QDs assemble, observing in real time the arising of FRET and its effects on carrier dynamics. We deduce that the time decoupling between the cooling process of photoexcited electron-hole pairs (∼
1 ps) and the FRET excitation of band-edge excitons (∼
1 ns) enables the surface trap states (of acceptor QDs) to slow down the relaxation of the FRET-excited states (induced by donor QDs): We typically observe a 400% increase in the exciton lifetime of acceptor QDs. As the rate-determining step of carrier dynamics within QD-QD pairs, FRET emerges as a regulating mechanism that can be used to optimise electron-hole recombination for targeted application.