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The caloric and specific heat curves for the bimetallic nanoclusters Au7-xAgx (x=0,3,4,7) are obtained through a statistical determination of the configurational density of states in the evolutive ensemble obtained with a genetic algorithm. The effect of the value of x (the relative concentrations) on the thermodynamics is studied. Three peaks are observed in the specific heat curves for all values of x. This is interpreted as being due to melting, and fragmentation of the cluster into first two, and then into 3 or more parts. A fourth pre-melting peak is observed for Au4Ag3 and is attributed to a new phenomena related to the breaking of the degeneracy of the permutational isomers. The melting transition for the bimetallic clusters is significantly wider than that for the pure clusters. The boiling transition displays a larger specific heat for the bimetallic clusters.

Theoretical and experimental information on the shape and morphology of bare and passivated gold clusters is fundamental to predict and understand their electronic, optical, and other physical and chemical properties. An effective theoretical approach to determine the lowest-energy configuration (global minimum) and the structures of low energy isomers (local minima) of clusters is to combine genetic algorithms and many-body potentials (to perform global structural optimizations), and first-principles density functional theory (to confirm the stability and energy ordering of the local minima). The main trend emerging from structural optimizations of bare Au clusters in the size range of 12−212 atoms indicates that many topologically interesting low-symmetry, disordered structures exist with energy near or below the lowest-energy ordered isomer. For example, chiral structures have been obtained as the lowest- energy isomers of bare Au28 and Au55 clusters, whereas in the size-range of 75−212 atoms, defective Marks decahedral structures are nearly degenerate in energy with the ordered symmetrical isomers. For methylthiol-passivated gold nanoclusters (Au28(SCH3)16 and Au38(SCH3)24), density functional structural relaxations have shown that the ligands are not only playing the role of passivating molecules, but their effect is strong enough to distort the metal cluster structure. In this work, a theoretical approach to characterize and quantify chirality in clusters, based on the Hausdorff chirality measure, is described. After calculating the index of chirality in bare and passivated gold clusters, it is found that the thiol monolayer induces or increases the degree of chirality of the metallic core. We also report simulated high- resolution transmission electron microscopy (HRTEM) images which show that defects in decahedral gold nanoclusters, with size between 1−2 nm, can be detected using currently available experimental HRTEM techniques.

Gold nanoclusters with disordered and ordered structures are obtained as the lowest-energy configurations of the cluster potential energy surface (PES) by unconstrained dynamical and genetic/symbiotic optimization methods using an n-body Gupta potential and first-principle calculations [Phys. Rev. Lett. 81, 1600 (1998)]. In this paper, we report the distribution of lowest-energy minima which characterize the PES of the Au38 cluster, and a comparison of structural and thermal stability properties among several representative isomers is presented. Coexistence among different cluster isomeric structures is observed at temperatures around 250 K. The structure factor calculated from the most stable (lowest-energy) amorphous-like cluster configuration is in better agreement with the X-ray powder-diffraction experimental measurements than those calculated from ordered structures.