Growth simulations of silver shells on copper and palladium nanoclusters

The growth of Ag-Pd and Ag-Cu core-shell nanoclusters is studied by molecular-dynamics simulations on realistic time scales. The metals are modeled by many-body potentials derived in the framework of the second-moment approximation to the tight-binding model. First, the energy barriers of the most relevant diffusion processes of Ag adatoms on Pd and Cu cores are calculated, and then growth simulations are performed for several values of the deposition flux and of the temperature. We find that well-defined Ag shells on both Pd and Cu cores are grown. In particular, single-layer shells with a few defects can be obtained in a wide range of temperatures. The main difference between the Ag-Pd and the Ag-Cu systems is that in the former better structures are obtained at low temperatures, while in the latter, the best shells are grown at high temperatures. These behaviors are explained in terms of the ability (inability) to incorporate Ag adatoms into the Pd or Cu cores.