Composition and Permeability of Oleate Adlayers of CdS Quantum Dots upon Dilution to Photoluminescence-Relevant Concentrations

This paper describes the changes in surface chemistry that occur in oleate-capped CdS quantum dots (QDs) upon dilution from NMR-relevant concentrations (10 mu M) to photoluminescence (PL)-relevant concentrations (0.1 mu M) 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 Cd(x)OA(y) 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.