Impact of Chalcogenide Ligands on Excited State Dynamics in CdSe Quantum Dots

The ligands that passivate the surfaces of semiconductor nanocrystals play an important role in excited state relaxation and charge transfer. Replacement of native long-chain organic ligands with chalcogenides has been shown to improve charge transfer in nanocrystal-based devices. In this report, we examine how surface-capping with S^2–, Se^2–, and Te^2– impacts photoexcited state relaxation in CdSe quantum dots (QDs). We use transient absorption spectroscopy with state-specific pumping to reveal the kinetics of electron and hole cooling, band edge electron relaxation, hole trapping, and trapped hole relaxation, all as a function of surface-capping ligand. We find that carrier cooling is not strongly dependent on the ligand. In contrast, band edge relaxation exhibits strong ligand dependence, with enhanced electron trapping in chalcogenide-capped QDs. This effect is the weakest with the S^2– ligand, but is very strong with Se^2– and Te^2–, such that the average band edge electron lifetimes for QDs capped with those ligands are under 100 ps. We conclude that, unlike the case of S^2–, improvements in electron transfer rates with Se^2– and Te^2– ligands may be overshadowed by the extreme electron lifetime shortening that may lead to low quantum yields of electron transfer