Metallic bonding and cluster structure (original) (raw)
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Structural patterns of unsupported gold clusters
2001
The structure of metal clusters is essential to predict many of their physical and chemical properties. Using ®rst principles density functional calculations it was recently found that even`magic' cluster sizes, for which very compact and symmetric structures exist, have lower-energy`disordered' structures. The origin of these structures was shown to lie in the non-pairwise metallic interactions; while the compact ordered geometries are very stable for pair potentials, they are destabilized by the tendency of metallic bonds to contract at the surface. Here we identify important patterns of the resulting`amorphous' structures, showing why they are optimal for the metallic potential, and how they can be used to predict structures for other cluster sizes.
Lowest Energy Structures of Gold Nanoclusters
Physical Review Letters, 1998
The lowest energy structures of Au n (n 38, 55, 75) nanoclusters are obtained by unconstrained dynamical and genetic-symbiotic optimization methods, using a Gupta n-body potential. A set of amorphous structures, nearly degenerate in energy, are found as the most stable configurations. Some crystalline or quasicrystalline isomers are also minima of the cluster potential energy surface with similar energy. First principles calculations using density functional theory confirm these results and give different electronic properties for the ordered and disordered gold cluster isomers.
Chirality, defects, and disorder in gold clusters
The European Physical Journal D - Atomic, Molecular and Optical Physics, 2003
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 lowestenergy 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 highresolution 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.
The European Physical Journal D, 2015
A reactive potential model and the classical molecular dynamics method (RMD) have been used to study the structure and energetics of sub-nanometre size gold clusters through well-known structural models reported in the literature for AuN, with N = 19, 20 and 21 atoms. After several simulated-annealing simulations for temperatures up to 1500 K, the AuN clusters clearly evolve to well-defined structures at room temperature. For the studied gold clusters, the low-lying structures are single-and double-icosahedra with mobile atoms on the surface, in agreement with experimental results on sub-nanometre size gold clusters exhibiting shape oscillations at room temperature and also with those involved in the design of molecules based on gold superatoms [J.-I. Nishigaki, K. Koyasu, T. Tsukuda, Chem. Rec. 14, 897 (2014)]. The evolution of the structural stability of the AuN clusters under exceptional thermal conditions is analysed by comparing the size and temperature variations of the centrosymmetry parameter and the potential energy. A key understanding of the various possible structural changes undergone by these tiny particles is thus developed. The usefulness of the RMD to study nanometre or sub-nanometre size gold clusters is shown.
Size-Dependent Structural Evolution and Chemical Reactivity of Gold Clusters
ChemPhysChem, 2007
Ground-state structures and other experimentally relevant isomers of Au 15 À to Au 24 À clusters are determined through joint firstprinciples density functional theory and photoelectron spectroscopy measurements. Subsequent calculations of molecular O 2 adsorption to the optimal cluster structures reveal a size-dependent reactivity pattern that agrees well with earlier experiments. A detailed analysis of the underlying electronic structure shows that the chemical reactivity of the gold cluster anions can be elucidated in terms of a partial-jellium picture, where delocalized electrons occupying electronic shells move over the ionic skeleton, whose geometric structure is strongly influenced by the directional bonding associated with the highly localized "d-band" electrons.
Structural properties of gold clusters at different temperatures
A series of gold clusters consisting of aggregates of from 13 to 147 atoms was studied using the Sutton-Chen type many-body potential in molecular dynamics simulations. The properties of these clusters at temperatures from 10 K to 1000 K were investigated in terms of their energy content and their respective radial distribution functions, to describe their melting behaviour. The larger clusters were deduced to melt over a range of temperatures rather than to undergo a distinctive phase change at a specific temperature as in the bulk material.
Structure and thermal stability of gold nanoclusters: The Au38 case
The European Physical Journal D, 1999
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]. In this paper, we report the distribution of lowest-energy minima which characterize the PES of the Au 38 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.
Structural Transition of Gold Nanoclusters: From the Golden Cage to the Golden Pyramid
ACS Nano, 2009
How nanoclusters transform from one structural type to another as a function of size is a critical issue in cluster science. Here we report a study of the structural transition from the golden cage Au 16 ؊ to the pyramidal Au 20 ؊ . We obtained distinct experimental evidence that the cage-to-pyramid crossover occurs at Au 18 ؊ , for which the cage and pyramidal isomers are nearly degenerate and coexist experimentally. The two isomers are observed and identified by their different interactions with O 2 and Ar. The cage isomer is observed to be more reactive with O 2 and can be preferentially "titrated" from the cluster beam, whereas the pyramidal isomer has slightly stronger interactions with Ar and is favored in the Au 18 Ar x ؊ van der Waals complexes. The current study allows the detailed structural evolution and growth routes from the hollow cage to the compact pyramid to be understood and provides information about the structure؊function relationship of the Au 18 ؊ cluster.