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–S2– 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.