Christopher J. Heard

Energy Landscape Exploration of Sub‐Nanometre Copper–Silver Clusters

The energy landscapes of sub-nanometre bimetallic coinage metal clusters are explored with the Threshold Algorithm coupled with the Birmingham Cluster Genetic Algorithm. Global and energetically low-lying minima along with their permutational isomers are located for the Cu(4)Ag(4) cluster with the Gupta potential and density functional theory (DFT). Statistical tools are employed to map the connectivity of the energy landscape and the growth of structural basins, while the thermodynamics of interconversion are probed, based on probability flows between minima. Asymmetric statistical weights are found for pathways across dividing states between stable geometries, while basin volumes are observed to grow independently of the depth of the minimum. The DFT landscape is found to exhibit significantly more frustration than that of the Gupta potential, including several open, pseudo-planar geometries which are energetically competitive with the global minimum. The differences in local minima and their transition barriers between the two levels of theory indicate the importance of explicit electronic structure for even simple, closed shell clusters.

Tuning the Reactivity of Ultrathin Oxides: NO Adsorption on Monolayer FeO(111)

Ultrathin metal oxides exhibit unique chemical properties and show promise for applications in heterogeneous catalysis. Monolayer FeO films supported on metal surfaces show large differences in reactivity depending on the metal substrate, potentially enabling tuning of the catalytic properties of these materials. Nitric oxide (NO) adsorption is facile on silver-supported FeO, whereas a similar film grown on platinum is inert to NO under similar conditions. Ab initio calculations link this substrate-dependent behavior to steric hindrance caused by substrate-induced rumpling of the FeO surface, which is stronger for the platinum-supported film. Calculations show that the size of the activation barrier to adsorption caused by the rumpling is dictated by the strength of the metal-oxide interaction, offering a straightforward method for tailoring the adsorption properties of ultrathin films.

Charge and Compositional Effects on the 2D–3D Transition in Octameric AgAu Clusters

Abstract The unbiased density functional-based Birmingham Cluster Genetic Algorithm is employed to locate the global minima of all neutral and mono-ionic silver-gold octamer clusters. Structural, energetic and electronic trends are determined across the series, in order to clarify the role of composition and charge on the position of the 2D–3D transition in ultrasmall coinage metal systems. Our calculations indicate a preference for three dimensional structures at high silver concentrations, which varies significantly with charge. The minimum in composition dependent mixing energies is independent of the charge, however, with a preference for the maximally mixed clusters, Ag4Au4 ν for all charge states ν. The sensitivity of isomeric preference to ν is found to be greater for electron-rich and electron-deficient clusters, implying a complexity of unambiguous determination of cluster motifs in related experiments. Vertical ionization potentials and detachment energies are calculated to probe electronic behaviour, providing numerical predictions for future spectroscopic studies.

Fe Oxides on Ag Surfaces: Structure and Reactivity

One layer thick iron oxide films are attractive from both applied and fundamental science perspectives. The structural and chemical properties of these systems can be tuned by changing the substrate, making them promising materials for heterogeneous catalysis. In the present work, we investigate the structure of FeO(111) monolayer films grown on Ag(100) and Ag(111) substrates by means of microscopy and diffraction techniques and compare it with the structure of FeO(111) grown on other substrates reported in literature. We also study the NO adsorption properties of FeO(111)/Ag(100) and FeO(111)/Ag(111) systems utilizing different spectroscopic techniques. We discuss similarities and differences in the data obtained from adsorption experiments and compare it with previous results for FeO(111)/Pt(111).

Global Optimisation Strategies for Nanoalloys

The computational prediction of thermodynamically stable metal cluster structures has developed into a sophisticated and successful field of research. To this end, research groups have developed, combined and improved algorithms for the location of energetically low-lying structures of unitary and alloy clusters containing several metallic species. In this chapter, we review the methods by which global optimisation is performed on metallic alloy clusters, with a focus on binary nanoalloys, over a broad range of cluster sizes. Case studies are presented, in particular for noble metal and coinage metal nanoalloys. The optimisation of chemical ordering patterns is discussed, including several novel strategies for locating low-energy permutational isomers of fixed cluster geometries. More advanced simulation scenarios, such as ligand-passivated, and surface-deposited clusters have been developed in recent years, in order to bridge the gap between isolated, bare clusters, and the situation observed under experimental conditions. We summarise these developments and consider the developments necessary to improve binary cluster global optimisation in the near future.