Ordering and segregation in isolated Au-Pd icosahedral nanoclusters and nanowires and the consequences of their encapsulation inside carbon nanotubes

Metropolis Monte Carlo sampling in the semi-grand canonical ensemble with empirical potentials is used to predict equilibrium ordered structures and segregation properties of small icosahedral Au-Pd nanoclusters and helical nanowire segments over the whole range of compositions at low temperatures. The cases of free-standing clusters and wires are compared with the same systems encapsulated in carbon nanotubes. A number of chemically ordered structures and segregation states are identified and found to be consistent with the same interplay between size mismatch, mixing enthalpy and surface energies of elemental metals which determines the thermodynamic equilibrium of binary metal alloys. Encapsulation has the effect of modifying the surface energies of nanoclusters and wires, with considerable consequences on their thermodynamic states, although the metal-graphite interaction strength is low as compared with the metal cohesive energy and the carbon-carbon binding energy. (paper)