posted on 2016-02-22, 00:00authored byAlexander
B. Nepomnyashchii, Rachel
D. Harris, Emily A. Weiss
This
paper describes the changes in surface chemistry that occur
in oleate-capped CdS quantum dots (QDs) upon dilution from NMR-relevant
concentrations (10 μM) to photoluminescence (PL)-relevant concentrations
(0.1 μM) and the consequences these changes have on the relative
probabilities of radiative and nonradiative decay of the QD exciton.
Characterization of the QD surface by nuclear magnetic resonance (NMR)
spectroscopy reveals that upon dilution in three solvents, C6D6, C6D12, and CDCl3,
oleate ligands, in the form of cadmium oleate and CdxOAy clusters, desorb. Changes in
the ligand coverage by 30–40% do not impact the solubility
of the QDs, do not have measurable influence on the absorption or
PL line widths, produce small (±0.05), nonmonotonic changes in
the relative PL quantum yield, and produce small, nonmonotonic changes
the relative partitioning between band-edge and “trapped”
exciton emission. Desorption of surface ligands as a result of dilution
of the QDs does, however, make the QDs more redox-active with respect
to a small-molecule photooxidant, benzoquinone (BQ), because less
dense organic adlayers allow a greater number of BQs to permeate the
ligand shell and adsorb to the QD surface. Unlike previous studies,
in which the QD concentrations used for NMR characterization were
more than a factor of 10 higher than those used for optical measurements,
this study directly correlates the surface composition of the QDs
to their photophysical properties.