Structural and thermodynamic properties of Ag-Co nanoclusters

摘要

The Metropolis Monte Carlo and molecular statics methods with a semiempirical embedded atom potential are used to study the structural and thermodynamic equilibrium states of Ag-Co isolated nanoparticles. The state parameters considered are size (from 200 to 3000 atoms), temperature (from 0 to 1500 K), and composition (from elemental Co to elemental Ag). A lower and an upper limit to the Co concentration is found for the occurrence of a core-shell structure. The lower limit results from a balance between Co-Co binding energy and the stress of the Ag lattice. The upper limit is a consequence of the wetting of the Co core by Ag. When the core-shell structure takes place, the Ag shell induces an expansion of the Co core of no more than 2% while the Co core induces an average contraction of the Ag lattice which is twice as large and is mainly taken over by the interfacial Ag atomic layer. Co cores melt at a temperature lower than 1500 K, which is not sensitive to the thickness of the Ag shell. Inside the Co cores, coexistence is found possible between a liquid layer surrounding a solid center and the thickness of the liquid layer is an increasing function of temperature. With increasing temperature, depending on its thickness, the Ag shell may undergo a crystal to amorphous transition followed by an amorphous to liquid transition. The former is caused by the Co core but proceeds from the Ag free surface. The melting temperature of the Ag shell is fairly lower than of the Co core, suggesting the possibility of core-shell nanoparticles with a solid core and a liquid shell.