Quantum dot or not? The effect of ligands on the nature of excitations in CdS nanoparticles

We investigate using many-body perturbation theory the optical and electronic properties of neutral cadmium sulfide nanoparticles with the zinc blende structure for which the surface is capped with bisulfide or alkylthiolate ligands, alone or in combination with phosphine, phosphine oxide or amine ligands. We study the nature of the optical excitations for these particles and how the optical and electronic properties of these particles change with the size of their inorganic core. We demonstrate that for all families of particle studied, except those where the corners are capped with aromatic ligands, the optical gap, the onset of light absorption, blue shifts with decreasing particle-core size. For the particles where the corners are capped with alkyl phosphines, phosphine oxides or amines the optical gap is predicted to correspond to an excitation where both the hole and excited electron are delocalised over the inorganic core of the particle, a core-to-core bulk-like excitation, and hence resemble the cartoon picture of a quantum dot. In contrast, we predict that for particles exclusively capped with bisulfide or alkylthiolate ligands, the optical gap corresponds to a charge-transfer excitation with the hole localised on a subset of ligands and only the excited-electron delocalised over the inorganic core of the particle. For particles where the corners are capped with aromatic ligands the optical gap is predicted to correspond to a ligand-to-ligand excitation, which because of its localised nature does not significantly shift with particle size. However for these particles with aromatic ligands, core-to-core excitation, lie only slightly higher in energy, and these excited states will shift with particle size and will likely become the optical gap for large(r) particles. Moreover, as core-to-core excitations are predicted to be much more intense than ligand-to-ligand excitations, experimentally one is likely to observe a blue shift with particle size even for particle sizes where the optical gap technically is a ligand-to-ligand excitation.