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.