Surface Defects in Magic-Sized (CdSe)₁₃ Clusters: A Combined Optical and Theoretical Approach

(CdSe)₁₃ magic-sized clusters (MSCs) that consist of only 26 atoms represent an exciting class of materials at the boundary between molecules and quantum dots. At low temperatures, the characteristic photoluminescence (PL) shows signatures of the exciton bandgap emission at 3.65 eV, accompanied by a broad and redshifted emission at around 3.0 eV. Upon heating, the clusters reveal unique, energetically sharp PL features near the excitonic emission. For shedding light on the origin of these specific spectral fingerprints, density functional theory (DFT), time-dependent DFT, and screened configuration interaction singles (SCIS) calculations are performed and compared with experimental data. The SCIS calculations identify excitonic fine structure states with strongly varying oscillator strengths, in excellent agreement with emissive states observed in time-resolved PL experiments. Introducing undercoordinated Se defects to the structures used in the theoretical calculations reveals new optical states below the bandgap, which nicely fit to the temperature-dependent, energetically sharp PL features. The broadband ≈3.0 eV emission is found to be a consequence of a polaron formation of optically generated electron–hole pairs in MSCs containing undercoordinated Se defects, leading to a rearrangement of the crystal lattice and thus to a significant lowering in PL energy with respect to the excitonic emission.