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Multi-step nucleation of nanocrystals in aqueous solution

Nucleation and growth of solids from solutions impacts many natural processes and are fundamental to applications in materials engineering and medicine. For a crystalline solid, the nucleus is a nanoscale cluster of ordered atoms, which forms through mechanisms that are still poorly understood. These mechanisms have important consequences on the morphology and nucleation rates of the resultant crystals but it is unclear whether a nucleus forms spontaneously from solution in a single step or through multiple steps. Using in-situ electron microscopy, we observe and quantify how gold and silver nanocrystals nucleate from a supersaturated aqueous gold and silver solution in three distinct steps: (I) spinodal decomposition into solute-rich and solute-poor liquid phases, (II) nucleation of amorphous gold nanoclusters within the gold-rich liquid phase, followed by (III) crystallization of these amorphous clusters. Our ab-initio calculations on gold nucleation suggest that these steps might be associated with strong gold-gold atom coupling and water-mediated metastable gold complexes. The understanding of intermediate steps in nuclei formation has important implications for the formation and growth of both crystalline and amorphous materials.