How Surface Defects Shape the Excitons and Photoluminescence of Ultrasmall CdSe Quantum Dots

Ultrasmall CdSe quantum
dots (QDs) with diameters up to 2 nm show
broad photoluminescence (PL) spectra presumably due to emission from
band-edge excitons and defect states. However, the origin of the defect
emission and the effect of defects on the band-edge excitons is not
fully understood. Based on spin–orbit density functional theory
and screened configuration interaction singles, we show that Cd-dimer
and Se defects form in-gap defect states. In comparison with experiment,
we discuss the role of deep and shallow defect states for the PL and
cover the dependence of their contributions to the PL with respect
to the QD size. Further, we observe that these defects lead to a localization
of the molecular orbitals (MOs) involved in the band-edge excitons
creating large electric dipoles in the MOs. In the excitonic states,
these dipoles cause multiexponential PL decay from the band-edge states
with a highly anisotropic polarization of the emission. The polarization
is found to be very sensitive with respect to the exact composition
of the surface.