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 and in comparison with previous PL measurements, we show that two different kinds of defects, Cd-dimer and Se defects, are likely to contribute to the defect PL. 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.