Impact of Chalcogenide Ligands on Excited State Dynamics in CdSe
Quantum Dots
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Abstract
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<sup>2–</sup>, Se<sup>2–</sup>, and Te<sup>2–</sup> 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<sup>2–</sup> ligand,
but is very strong with Se<sup>2–</sup> and Te<sup>2–</sup>, 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<sup>2–</sup>, improvements in electron transfer rates
with Se<sup>2–</sup> and Te<sup>2–</sup> ligands may
be overshadowed by the extreme electron lifetime shortening that may
lead to low quantum yields of electron transfer