Dielectric response function for colloidal semiconductor quantum dots.

Abstract

We calculate the optical properties of InP and CdSe colloidal quantum dots (QDs) within the framework of the atomic effective pseudopotential approach and the screened configuration interaction theory. We obtain an excellent agreement with experiment with our microscopic and space-dependent screening function where the dielectric constant varies in real space with a sharp transition (width of ≈0.18 nm) from the QD material high-frequency bulk value inside the QD to the solvent or passivant high-frequency value outside. We obtain a reasonable agreement (with deviations less than 140 meV) for a computationally less demanding solvent-independent screening using the full high-frequency bulk screening, in contrast to the more commonly used reduced QD radius-dependent screening constant. We show theoretically that for QDs passivated with long-chained organic molecules, the influence of the solvent on the optical gap is in the range of 10 meV, while QDs passivated with short ligands can experience shifts in the order of 100 meV. Experiments on CdSe QDs passivated with octadecylphosphonic acid (ODPA, long-chained ligand) in two different solvents (toluene and chloroform) confirm the bandgap dependence. While the optical gap is weakly affected by the environment, the quasiparticle gap and the exciton binding energy show a strong environmental dependence. Finally, we show that the optical bandgap does not depend significantly on the crystal structure (wurtzite or zincblende) or the morphological details (faceted or "spherical" shape).