Coalescence-Induced Growth Doping in a II-VI Magic Size Cluster

The versatility and stability of semiconductor magic size clusters (MSCs) have been exploited to synthesize unique nanostructures with well-controlled dimensionality. Strategies to incorporate dopant ions such as transition metals into II-VI MSCs typically result in substitutional doping at surface sites. In this study, we investigate the speciation of Co²⁺ in ZnS MSCs using three different cation exchange reactions at moderate temperatures. Using electronic absorption spectroscopy and ligand field theory, we confirm in every scenario that Co²⁺ either substitutes at the surface, remains as a precursor in solution, or forms a Co-rich impurity. However, upon growth of the Co²⁺-doped ZnS MSCs at higher temperatures, we observe conversion of the surface Co²⁺ to internal sites. This observation is consistent with tetrahedral Co²⁺ coordinated to μ₄-S^2- based on comparison of the same transition observed previously with internally doped Co²⁺:ZnS QDs. We propose the internalization is due to a coalescence growth mechanism involving direct attachment, interface relaxation, and reshaping of the Co²⁺-doped ZnS MSCs in contrast to typically observed stepwise MSC growth or Ostwald ripening in doped QDs.