Optical Properties of Phenylthiolate Capped CdS Nanoparticles

Using many-body perturbation theory we study the optical properties of phenylthiolate capped cadmium sulifde nanoparticles to understand the origin of the experimentally observed blueshift 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 component are predominantly delocalised over the inorganic core of the particles. Phenylthiolate capped cadmium sulifde nanoparticles appear not to behave like the textbook cartoon picture of quantum dots. We speculate that the observed blueshift 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.