Dependence of the Band Gap of CdSe Quantum Dots on the Surface Coverage and Binding Mode of an Exciton-Delocalizing Ligand, Methylthiophenolate

Displacement of native octylphosphonate (OPA) ligands for methylthiophenolate (CH₃-TP) on the surfaces of CdSe quantum dots (QDs) causes a moderate (up to 50 meV) decrease in the band gap (E_g) of the QD. Plots of the corresponding increase in apparent excitonic radius, ΔR, of the QDs versus the surface coverage of CH₃-TP, measured by ¹H NMR, for several sizes of QDs reveal that this ligand adsorbs in two distinct binding modes, (1) a tightly bound mode (K_a = 1.0 ± 0.3 × 10⁴ M^-1) capable of exciton delocalization, and (2) a more weakly bound mode (K_a = 8.3 ± 9.9 × 10² M^-1) that has no discernible effect on exciton confinement. For tightly bound CH₃-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 ΔR on surface coverage of CH₃-TP over a range of physical radii of the QDs, R = 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 CH₃-TP of ∼1 eV.