Optical transitions and nature of Stokes shift in spherical CdS quantum dots

TL;DR

This study investigates the electronic structure and optical transition characteristics of colloidal CdS quantum dots, revealing insights into the Stokes shift and the nature of dark excitons using advanced ab initio methods.

Contribution

It provides a detailed ab initio analysis of the energy spectra and optical transitions in CdS quantum dots, highlighting differences from traditional k·p models and explaining the dark exciton phenomenon.

Findings

Valence band top is bright, contrary to k·p predictions.

Electron-hole exchange splitting indicates a spin-forbidden valence state.

The dark exciton in CdS QDs may be due to spin-forbidden states.

Abstract

We study the structure of the energy spectra along with the character of the states participating in optical transitions in colloidal CdS quantum dots (QDs) using the {\sl ab initio} accuracy charge patching method combined with the %pseudopotential based folded spectrum calculations of electronic structure of thousand-atom nanostructures. In particular, attention is paid to the nature of the large resonant Stokes shift observed in CdS quantum dots. We find that the top of the valence band state is bright, in contrast with the results of numerous {\bf k$\cdot$p} calculations, and determine the limits of applicability of the {\bf k$\cdot$p} approach. The calculated electron-hole exchange splitting suggests the spin-forbidden valence state may explain the nature of the ``dark exciton'' in CdS quantum dots.