Revealing Au13 as Elementary Clusters During the Early Formation of Au Nanocrystals.

Understanding the formation mechanism of nanocrystals in solution is fundamental to the development of materials science. For a metal nanocrystal, the cluster-mediated formation mechanism is still poorly understood. In particular, identifying what types of clusters are dominant and how they evolve into a nanocrystal in the early nucleation stage remains a great challenge. Here, using liquid-cell transmission electron microscopy, we directly observe the formation of ultrasmall Au clusters (∼0.84 nm) in the presence of PAA-Na. These clusters, which correspond to the size of the Au13 cluster, coalesce to form nanocrystals. Our molecular dynamics simulations suggest that Au13 in an aqueous environment has greater stability when compared to other cluster sizes and provide atomistic details of growth by cluster coalescence. Collectively, our demonstration of Au13 as the dominant species with an elaboration of their coalescence kinetics sheds light on nonclassical nanocrystal formation mechanisms and offers useful guidelines for designing innovative pathways for the synthesis of nanomaterials.