Impact of Exciton Fine Structure on the Energy Transfer in (CdSe)₁₃ and Mn²⁺:(CdSe)₁₃ Magic Sized Clusters

(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. The ultrasmall size of the clusters leads to a pronounced and well separated excitonic fine structure. Besides a pronounced bandgap luminescence, these clusters show a broad emission band that is strongly Stokes shifted with respect to the bandgap and that is usually attributed to hole trapping states at the surface. The controlled incorporation of Mn²⁺ dopants into (CdSe)₁₃ MSCs does not significantly affect the excitonic structure, but adds additional optical functionality, namely an orange emission related to the ⁴T₁ – ⁶A₁ ligand field transition of the dopant. Consequently, the investigated MSCs represent an ideal model system for shedding light on the energy transfer from excited excitonic fine structure states to emitting states arising from surface traps or Mn²⁺ dopants.

In this work, we performed optical absorption, photoluminescence (PL) and photoluminescence excitation (PLE) experiments on undoped (CdSe)₁₃ and Mn²⁺-doped (CdSe)₁₃ MSCs with different doping concentrations at cryogenic temperature. Probing the Mn²⁺ luminescence in MSCs of diverse dopant concentrations allows the differentiation between the energy transfer mechanism from excitonic states to Mn²⁺ monomers and to Mn²⁺-Mn²⁺ dimers. The activation of single dopants requires a spin flip, that only dark excitonic states can provide. In contrast, the coupling between the dopants in bidoped clusters leads to the formation of so-called spin ladder states with new spin allowed transitions. As a consequence, the spin selection rules get modified and also bright excitonic states can provide an energy transfer to the coupled Mn²⁺ ions.

Our experiments on undoped (CdSe)₁₃ MSCs reveal a thermally activated energy transfer from excitonic fine-structure states to surface states that is virtually identical for different excitonic fine structure states. Upon heating, the clusters reveal unique, energetically sharp PL features near the excitonic emission. We suggest that these features are correlated to surface defects. This hypothesis is supported by a comparison of the experimental data with DFT calculations. For a defect-free (CdSe)₁₃ structure we can identify three absorption bands. The introduction of Se-surface defects leads to additional optical transitions below the bandgap that we can relate to the luminescence features found in our temperature dependent PL experiments. Therefore, the DFT calculations provide new insights into the structure of (CdSe)₁₃ MSCs and allows a theoretical description of the spectral signatures observed in PL experiments.