Optical Properties of Phenylthiolate-Capped CdS Nanoparticles

Using many-body perturbation theory, we study the optical properties of phenylthiolate-capped cadmium sulfide nanoparticles to understand the origin of the experimentally observed blue shift in those properties with decreasing particle size. We show that the absorption spectra predicted by many-body perturbation theory agree well with the experimentally measured spectra. The results of our calculations demonstrate that all low-energy excited states correspond to a mixture of two fundamental types of excitations: intraligand and ligand-to-metal charge-transfer excitations. We find that for each excited state, the intraligand excitation contribution is dominant and that bright excited states, corresponding to the clear peaks in the absorption spectra, have a larger ligand-to-metal charge-transfer contribution. There are no low-energy bulk-like excitons, excited states for which both the hole and the excited electron components are predominantly delocalized over the inorganic core of the particles. Phenylthiolate-capped cadmium sulfide nanoparticles appear not to behave like the textbook cartoon picture of quantum dots. We speculate that the observed blue shift is the result of a combination of a Stark-like shift of the intraligand contribution, modulated by a change in the charge of the inorganic core and the confinement of the excited electron component of the ligand-to-metal charge-transfer contribution.