Binding Energies Of Small Clusters Research Articles

Ab initio study of early stage Nb growth on MgO(001)

Using density functional theory calculations we have examined the effects of cluster shape and a neutral O vacancy on the energetics and stability of Nb monomers and clusters on the MgO(001) surface. The relative stability of different monolayer structures is also examined. As found in other cases of metal adsorbates on MgO(001), our results indicate that an O site is the preferred adsorption site for a Nb atom. In addition, O-vacancy sites tend to increase the binding energy of small clusters and thus act as nucleation sites, while the effect of a nearby O vacancy on the binding energy of a Nb cluster is much weaker. In particular, we find that the binding energy for a Nb monomer at an O site (O-vacancy site) is 1.5eV (2.2eV) while the corresponding activation barriers for Nb monomer diffusion are 0.58eV (0.80eV). We also find that, in the absence of O vacancies, the preferred planar structure for tetramers and pentamers is an isotropic (100)-like structure. In contrast, the presence of an O vacancy transforms the isotropic tetramer into an anisotropic (110)-like structure. In addition, due to strain effects as well as strong Nb–Nb interactions, for large clusters (e.g. a complete monolayer) the anisotropic (110) structure is favored over the (100) structure. These results appear to explain recent experimental observations for the dependence of thin-film orientation on deposition conditions. However, we also find that for small (100)-like and (110)-like clusters, three-dimensional (rather than planar) structures are energetically preferred due to the strong Nb–Nb interaction. These results suggest that the pathway to form a coherent (100) or (110) structure during the growth of Nb films on Mg(001) may be relatively complex.

Ab initio study of water clustering in the presence of a methyl radical

The geometrical structure and binding energy of small clusters of methyl radical and water molecules (up to five water molecules) in gas phase and water media have been investigated at the MP2 level of theory using 6-311++G(2df,2p) basis set. The complexes characterized contain OH···O, CH···O, and OH···C attractive interactions with stabilization energies in the range 6–143 kJ mol−1. The solvent has an enhancing influence on the stabilities of studied clusters. The atoms in molecules theory were also applied to explain the nature of the complexes. The interaction energies have been partitioned with the natural energy decomposition analysis showing that the most important attractive term corresponds to the charge transfer one.

Comparison of interatomic potentials for UO 2. Part I: Static calculations

An improved knowledge of nuclear fuel can be gained from a better description of atomic-scale processes such as point defects behaviour under irradiation. In this perspective the different techniques involving interatomic potentials can play a major role as they permit to simulate such mechanisms at the atomic scale. In this article we will assess the range of applicability of the available interatomic potentials for UO 2 by static calculations. Lattice properties have been envisaged, together with defect properties: the formation and activation energies of vacancies and interstitials, the binding energy of small clusters of these defects and the volume change associated with them.

Influence of multiatom interactions on the shapes and energetics of two-dimensional homoepitaxial clusters on close-packed metallic surfaces.

We describe a scheme (BSM for ``bond saturation model'') for modeling the energetics of adatom clusters on close-packed metallic surfaces. In the BSM, atoms interact via coordination dependent nearest-neighbor bonds. We show that the BSM yields a detailed understanding of the relationship between shape and energy for two-dimensional (2D) homoepitaxial adatom clusters on close-packed metallic surfaces. In particular, we advance a simple rule for predicting the binding energies of small clusters based on the moments of their coordination distributions, derive a useful expression for the binding energy of a 2D cluster of arbitrary geometry, and study the role played by coordination dependent bonding in determining step energies for adatom islands. Our analysis of step energies provides insight into why the two distinct types of monatomic steps on Pt(111) have similar free energies as observed by Michely and Comsa. While our numerical results are for Pt(111), we expect our conclusions to hold for any chemically similar system. \textcopyright{} 1996 The American Physical Society.

Molecular Dynamics Simulation of Ion Mobility. 2. Alkali Metal and Halide Ions Using the SPC/E Model for Water at 25 °C

We present results of computer simulations of the mobilities of the alkali metal ions (Li+, Na+, K+, Rb+, and Cs+) and the halides (F-, Cl-, Br-, and I-) at 25 °C using the SPC/E model for water and ion−water parameters fitted to the binding energies of small clusters of ions. A simple truncation of the ion−water and water−water potentials was used, and the mobilities calculated from the mean square displacement and the velocity autocorrelation functions, respectively, were found to be in good agreement with each other. The calculations demonstrate, for the first time, cation and anion mobilities that fall on separate curves, as functions of ion size, with distinct maxima. This is in complete accord with experimental trends observed in water at 25 °C. The cation mobilities are also in better agreement with the measured values than the calculations done earlier (J. Chem. Phys. 1994, 101, 6964) using the TIP4P model. The mobilities of the halides calculated here for the SPC/E model are however slightly lowe...

Nucleation and growth of thin films: recent progress

Progress in understanding the three growth modes of crystal growth and surfaces is briefly reviewed. Island growth typically has small critical nucleus sizes (i=1 frequently) and adsorption, diffusion and cluster binding energies can be abstracted from comparison of kinetic models with electron microscope experiments. Recent work which extends this approach to include cluster mobility, defect induced nucleation and alloy films is cited. Recent work on the layer-plus-island, or Stranski-Krastanov growth mode, using an ultra-high vacuum scanning electron microscope and several related surface techniques to study the growth of silver on several metal and semiconductor surfaces, is highlighted. It is shown that typically i ∼ 10 and binding energies of small clusters on metal surfaces are much less than expected on simple pair-bond models. These ideas can be extended to layer growth, but experimental work has been hampered by lack of suitable surface sensitive microscopic techniques. Some comments on the potential use of RHEED and reflection microscopy, and predictions for the nucleation densities expected in layer growth of semiconductors and metals, are made.

LCAO HMO study of the binding energy of small clusters of lithium atoms

AbstractThe atom by atom growing of the energetically most favourable lithium clusters of 2 to 10 atoms was studied. The clusters had the structure of a body centered cubic lattice of lithium. The LCAO–HMO method was used for the energetic description of the clusters. It was assumed that there is an interaction of a given atom with its neighbours in the first six coordination spheres in the lithium lattice. All possible (two‐dimensional and three‐dimensional) positions of the n + 1st atom with respect to a cluster of n atoms were taken into account. It was shown that from 2 to 10 atoms the cluster grows twodimensionally and has the geometry of the lattice plane with the highest net density in the body centered cubic lattice of lithium, the 110 lattice plane. This was discussed in detail. It was shown that the bond energy per atom increases, whereas the energy of a bond between nearest neighbours decreases with increasing number of atoms in the cluster.