(593b) Probing Metal Nanocrystal Shapes and Shape Transformations Using Replica Exchange Molecular Dynamics Simulations

Understanding the growth and transformations of colloidal metal nanocrystals into various shapes is both fundamentally and technologically significant. With a firmly established link between nanocrystal shapes and their properties for many applications, research into potential uses for metal nanocrystals has expanded significantly. Nanocrystal seeds in the 1-2 nm size range can play a key role in dictating the shapes of larger nanocrystals, by acting as templates for (possibly) non-equilibrium growth. Though the importance of nanocrystal seeds in determining final nanocrystal morphologies has been emphasized in many studies, control of seed evolution has been largely empirical. We use replica exchange molecular dynamics (REMD) simulations to study the temperature-dependent shapes of Ag, Cu, Pd, and Au nanocrystals in the 1-2 nm size range. Using common-neighbor analysis (CNA), we classify the nanocrystal shapes into decahedra (Dh), icosahedra (Ih), intermediate structures between Dh and Ih (Dh-Ih), single-crystal (FCC) and single-crystal with stacking faults (SCSF). We found the preferred shape at a given size depends on the material. Unlike Ag, Cu and Pd nanocrystals that exhibit highly symmetric structures, we find many Au nanocrystals possess hollow cores or cage-like structures, even at low temperatures.

We compute minimum energy pathways for Ag nanoclusters to transition between different shapes and characterize the potential-energy barriers using the climbing-image nudged-elastic band method. Such nanocrystal shape transformations can occur via either a slip-mediated twinning mechanism or a surface-reconstruction driven process. We also use an algorithm related to REMD – local REMD – to characterize the shapes of Ag nanocrystals in ethylene glycol solvent –to understand structure in solution-phase syntheses. We found Ih nanoclusters are more favored in solvent than in vacuum, because solvent changes the surface strain.