Ligand-Induced Symmetry Breaking as the 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 with QD ensembles,
multiexponential decay was observed. On the basis of 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 and 2.0 nm possess a multiexponential low-temperature PL at
the single-dot level. This is a consequence of 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 are bright.
We find that the splitting between the sets can be modified by the
coverage and choice of the ligand, which facilitates the engineering
of the PL properties of CdSe QDs.