Co²⁺-Doping of Magic-Sized CdSe Clusters: Structural Insights via Ligand Field Transitions

Magic-sized clusters represent materials with unique properties at the border between molecules and solids and provide important insights into the nanocrystal formation process. However, synthesis, doping, and especially structural characterization become more and more challenging with decreasing cluster size. Herein, we report the successful introduction of Co²⁺ ions into extremely small-sized CdSe clusters with the intention of using internal ligand field transitions to obtain structural insights. Despite the huge mismatch between the radii of Cd²⁺ and Co²⁺ ions (>21%), CdSe clusters can be effectively synthesized with a high Co²⁺ doping concentration of ∼10%. Optical spectroscopy and mass spectrometry suggest that one or two Co²⁺ ions are substitutionally embedded into (CdSe)₁₃ clusters, which is known as one of the smallest CdSe clusters. Using magnetic circular dichroism spectroscopy on the intrinsic ligand field transitions between the different 3d orbitals of the transition metal dopants, we demonstrate that the Co²⁺ dopants are embedded on pseudotetrahedral selenium coordinated sites despite the limited number of atoms in the clusters. A significant shortening of Co-Se bond lengths compared to bulk or nanocrystals is observed, which results in the metastability of Co²⁺ doping. Our results not only extend the doping chemistry of magic-sized semiconductor nanoclusters, but also suggest an effective method to characterize the local structure of these extremely small-sized clusters.