Canonical molecular dynamics simulations for crystallization of metallic nanodroplets on MgO(100)

Abstract

The crystallization of Pd, Pt, and Ag nanodroplets in the range of 100--800 atoms (1.5--3 nm diameter) supported on the MgO(100) surface is simulated by canonical molecular dynamics (MD) using tight-binding many-body potentials for the metallic interactions and a potential-energy surface approach fitted to ab initio calculations for the metal-oxide support interaction. The simulations enable to determine the equilibrium supported shapes at finite temperature. These can be compared to the quenched MD simulations or global optimization searches for equilibrium shapes at 0 K. MD simulations at finite temperature after solidification do not display any evidence of a solid-solid transition. The structure is faced-centered cubic with atomic distortions and possible stacking faults. The morphology and epitaxial relation of nanoclusters with their support depend on the size and the nature of the metal: at small sizes (less than 200 atoms), all the metals adopt the (100) epitaxy and Ag clusters keep this epitaxy all over the size range. At 201 atoms (2 nm), Pd clusters are half (111)/half (100) whereas a majority of Pt clusters keeps the (100) epitaxy. At 405 atoms (2.5--3 nm) almost all the Pd clusters change to the (111) epitaxy whereas Pt clusters are half (111)/half (100). The size at which the transition from the (100) to the (111) epitaxy occurs is correlated with the lattice parameter misfit and to the difference in adhesion energy between the two epitaxies.