Structure and solid solution properties of Cu-Ag nanoalloys.

The nanoparticle phase diagram of an immiscible system is studied at the atomic level. Cu-Ag clusters with sizes 1000 and 2000 atoms, resulting from a global minimum search and belonging to icosahedral and crystalline structural motifs, are considered. We present the statistical analysis of the effect of temperature on the solubility of the two elements based on Metropolis Monte Carlo importance sampling. Our results suggest that the relevance of bulk phase diagrams to nanoparticles is limited to cases where the internal stress distribution does not deviate very much from uniform (e.g. sufficiently large crystalline clusters). In the general case, the principal interdependence between partial phase compositions and the overall cluster composition in nanoparticle phase diagrams need to be taken into account.

An atomistic view of the interfacial structures of AuRh and AuPd nanorods.

In this work we address the challenge of furthering our understanding of the driving forces responsible for the metal-metal interactions in industrially relevant bimetallic nanocatalysts, by taking a comparative approach to the atomic scale characterization of two core-shell nanorod systems (AuPd and AuRh). Using aberration-corrected scanning transmission electron microscopy, we show the existence of a randomly mixed alloy layer some 4-5 atomic layers thick between completely bulk immiscible Au and Rh, which facilitates fully epitaxial overgrowth for the first few atomic layers. In marked contrast in AuPd nanorods, we find atomically sharp segregation resulting in a quasi-epitaxial, strained interface between bulk miscible metals. By comparing the two systems, including molecular dynamics simulations, we are able to gain insights into the factors that may have influenced their structure and chemical ordering, which cannot be explained by the key structural and energetic parameters of either system in isolation, thus demonstrating the advantage of taking a comparative approach to the characterization of complex binary systems. This work highlights the importance of achieving a fundamental understanding of reaction kinetics in realizing the atomically controlled synthesis of bimetallic nanocatalysts.

Modelling the metal-on-top effect for Pd clusters on the MgO{100} substrate.

We introduce a novel empirical model for the adhesion of Pd clusters on the MgO{100} substrate. The new model corrects the known bias of previous models toward structures with large interfaces with the substrate due to the failure to account for the so-called "metal-on-top" effect, i.e., the enhancement of the adhesion due to the presence of other metal atoms on top of those which are directly in contact with the substrate. The new model is parametrised using density-functional theory calculations on MgO-supported Pd clusters with sizes up to 80 atoms. The proposed potential is continuous with respect to spatial coordinates and can therefore be used directly in molecular dynamics simulations.