(40g) Probing Nanocrystal Shape Transformations Using Parallel Tempering Molecular Dynamics and the Mold Method

Solution-phase syntheses have produced an astounding variety of metal nanocrystal shapes with unique properties that depend on shape. Studies have shown that nanocrystal shapes often depend on the shape of the “seeds” – crystallites in the 1-10 nm size range that form subsequent to nucleation. The shapes of seeds (single crystal and twinned) are highly fluxional and sensitive to the solution environment surrounding them. Though many studies have probed minimum-energy shapes for small clusters in vacuum, very few studies have probed the effect of temperature and solution environment.

Focusing on Ag nanoclusters described by an embedded-atom method potential, we use parallel-tempering molecular dynamics simulations to generate ensembles of equilibrium shapes for Ag nanoclusters in the 2-10 nm size range as a function of temperature. We apply Common Neighbor Analysis to identify the crystallographic environments of cluster atoms and we create barcodes to characterize cluster structures. These studies show that clusters fall within a few structural motifs that can change with temperature. Interestingly, a solution environment lowers the cluster interfacial free energies and alters the preferred shapes.

To quantify free-energy differences between various structures, we employ the “mold method”. In this method, we transform a nanocrystal from one shape to another by turning on and off the interactions between metal atoms and molds with initial/final shapes, while at the same time weakening/strengthening metal-atom interactions. This reversible transformation allows us to perform thermodynamic integration along the pathway, which yields the free-energy change. We create phase diagrams for clusters as a function of size, temperature, and solution environment. Our studies highlight the significant role of solution environment on nanocrystal shape.