Dependence of the Band Gap
of CdSe Quantum Dots on
the Surface Coverage and Binding Mode of an Exciton-Delocalizing Ligand,
Methylthiophenolate
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Abstract
Displacement
of native octylphosphonate (OPA) ligands for methylthiophenolate
(CH<sub>3</sub>-TP) on the surfaces of CdSe quantum dots (QDs) causes
a moderate (up to 50 meV) decrease in the band gap (<i>E</i><sub>g</sub>) of the QD. Plots of the corresponding increase in apparent
excitonic radius, Δ<i>R</i>, of the QDs versus the
surface coverage of CH<sub>3</sub>-TP, measured by <sup>1</sup>H NMR,
for several sizes of QDs reveal that this ligand adsorbs in two distinct
binding modes, (1) a tightly bound mode (<i>K</i><sub>a</sub> = 1.0 ± 0.3 × 10<sup>4</sup> M<sup>–1</sup>) capable
of exciton delocalization, and (2) a more weakly bound mode (<i>K</i><sub>a</sub> = 8.3 ± 9.9 × 10<sup>2</sup> M<sup>–1</sup>) that has no discernible effect on exciton confinement.
For tightly bound CH<sub>3</sub>-TP, the degree of delocalization
induced in the QD is approximately linearly related to the fractional
surface area occupied by the ligand for all sizes of QDs. Comparison
of the dependence of Δ<i>R</i> on surface coverage
of CH<sub>3</sub>-TP over a range of physical radii of the QDs, <i>R</i> = 1.1–2.4 nm, to analogous plots simulated using
a 3D spherical potential well model yield a value for the confinement
barrier presented to the excitonic hole by tightly bound CH<sub>3</sub>-TP of ∼1 eV
