Role of Interligand Coupling in Determining the Interfacial Electronic Structure of Colloidal CdS Quantum Dots

Displacement of cadmium oleate (Cd­(oleate)₂) ligands for the exciton-delocalizing ligand 4-hexylphenyldithiocarbamate (C6-PTC) on the surfaces of CdS quantum dots (QDs) causes a decrease in the band gap (<i>E</i>_g) of the QD of ∼100 meV for QDs with a radius of 1.9 nm and ∼50 meV for QDs with a radius of 2.5 nm. The primary mechanism of this decrease in band gap, deduced in previous work, is a decrease in the confinement barrier for the excitonic hole. The increase in apparent excitonic radius of the QD that corresponds to this decrease in <i>E</i>_g is denoted Δ<i>R</i>. The dependence of Δ<i>R</i> on the surface coverage of C6-PTC, measured by ¹H NMR spectroscopy, appears to be nonlinear. Calculations of the excitonic energy of a CdS QD upon displacement of native insulating ligands with exciton-delocalizing ligands using a 3D spherical potential well model show that this response includes the contributions to Δ<i>R</i> from both isolated, bound C6-PTC ligands and groups of adjacent C6-PTC ligands. Fits to the experimental plots of Δ<i>R</i> <i>vs</i> surface coverage of C6-PTC with a statistical model that includes the probability of formation of clusters of bound C6-PTC on the QD surface allow for the extraction of the height of the confinement barrier presented by a single, isolated C6-PTC molecule to the excitonic hole. This barrier height is less than 0.6 eV for QDs with a radius of 1.9 nm and between 0.6 and 1.2 eV for QDs with a radius of 2.5 nm