Jérôme Creuze

Segregation and Phase Transitions in Reduced Dimension: From Bulk to Clusters via Surfaces

To describe the thermodynamics of bimetallic clusters, we use approaches that have been successfully employed for bulk alloys and their surfaces. We detail what happens for bulk and surface phase transitions when considering nanoalloys with a tendency to phase separation. A rigid-lattice approach allows us to analyze the behaviour of surface and core shells of the nanoalloys. We discuss the existence of bistabilities (or dynamical equilibrium) which are the analogous of surface and bulk phase transitions in semi-infinite alloys. Such dynamical equilibrium is susceptible to affect the cluster facets in an individual way (individual dynamical equilibrium), whereas the inner shells show a collective bistability (collective dynamical equilibrium). Then, we compare these bistabilities obtained in the semi-grand canonical ensemble with the results obtained in the canonical ensemble, before discussing the relation between experimental conditions and the two thermodynamic ensembles. Finally, a first attempt to establish generalized phase diagram for bimetallic clusters is proposed.

Superficial Phase Transitions in Nanoalloys

In order to build the phase diagram of Cu-Ag nanoalloys, we study a 405-atom nanoparticle by means of Monte Carlo simulations with relaxations using N-body interatomic potentials. We focus on a range of nominal concentrations for which the cluster core remains Cu-pure and the (001) facets of the outer shell exhibit two original phenomena. Within the (N,mAg-mCu,P,T) ensemble, a structural and chemical bistability is observed, which affects all the (001) facets together. For a nanoparticle assembly, this will result in a bimodal distribution of clusters, some of them having their (001) facets Cu-rich with the usual square shape, the other ones having their (001) facets Ag-rich with a diamond shape. This bistability is replaced in the (NAg,NCu,P,T) ensemble by a continuous evolution of both the structure and the concentration of the (001) facets from Cu-rich square-shaped to Ag-rich diamond-shaped facets as the number of Ag atoms increases. For a nanoparticle assembly, this will result in an unimodal distribution of the cluster population concerning the properties of the (001) facets. This comparison between pseudo grand canonical and isothermal-isobaric results shows that the distribution of a population of bimetallic nanoparticles depends strongly on the conditions under it is elaborated.