Molecular dynamics study of bimetallic nanoparticles: The case of Au _x Cu _y alloy clusters

Gold and copper nanoparticles present atomic structures which are either icosahedral (I _h ), decahedral (D _h ) or octahedral (O _h ), depending of the particle size. Some experimental results had previously reported the stabilization of the I _h phase in AuCu and AuFe clusters, being this phase predominant at 25% copper concentration in both systems. Another result reports an fcc-like and core/shell structure under similar experimental conditions and/or similar systems. In the present work, we study the possible phase conformations and thermal behavior of AuCu binary clusters using classical molecular dynamics simulations with a Sutton and Chen inter-atomic potential, addressing under which conditions the different phases mentioned above take place. To discern between the structures of bimetallic systems, the following main factors are determinant: the cluster size N, the concentration and nature of the elements in the alloy, and the annealing temperature. We choose the particles in our study closed to the sizes reported experimentally, with the cubo-octahedral symmetry as starting point. We have changed the concentration of copper from 50 to 10%, simulating an annealing process around the temperature of 750 K. We have found optimum stability of the icosahedral phase at concentrations of copper around 75 and 25% in fair agreement with experimental reports, and a trend to adopt a quasi-spherical shape with a core/shell structure at high temperatures in the cluster, just before the melting temperature.