Ligand-Induced Symmetry Breaking as Origin of Multiexponential Photoluminescence Decay in CdSe Quantum Dots

The bright photoluminescence (PL) of colloidal CdSe quantum dots (QDs) makes them interesting for optical applications. For most of them well-defined PL properties, dominated by a single excitonic state, are required. However, in many PL experiments on QD ensembles multiexponential decay was observed. Based on spin-orbit density functional theory and screened configuration interaction calculations, we show that highly symmetric and defect-free CdSe QDs with diameters of 1.7 nm and 2.0 nm possess a multiexponential PL at the single-dot level. This is a consequence of a ligand-induced symmetry breaking with a subsequent rearrangement of the lowest eight excitonic states in two sets of four singly degenerate excitonic states. For each set, the lowest state is dark and the other three bright. We find that the splitting between the sets can be modified by the ligand coverage and the ligand choice, which facilitates engineering the PL properties of CdSe QDs.