How Surface Proximity to Copper Dopants Affects Photoluminescence of CdSe Colloidal Quantum Dots

Introducing copper impurities to semicoductor quantum dots (QDs) lead to an appearance of a new photoluminescence (PL) band related to the recombination of a delocalized conduction band electron and a hole trapped at the copper d orbital. This new PL band is characterized by a large line width, large Stokes shift, and a multiexponential temporal decay, with origins not well understood. In this work, we employ density functional theory to investigate the impact of the copper dopant location inside a QD on absorption and PL properties. We find that impurities incorporated closer to the surface give rise to PL transitions at lower energies, with lower probabilities, and larger Stokes shifts than impurities at the QD center. We show that the effect arises as a result of site-dependent distortion of the dopant vicinity: larger distortions occur for impurities closer to the surface and result in stronger energy relaxations. The conclusions from the theoretical calculations are supported by measurements of PL dynamics. Our results provide a novel interpretation of the heterogeneous optical properties of Cu-doped QDs.