Bulk Cohesive Energy Research Articles

A density functional study of small neutral and cationic vanadium clusters Vn and Vn+ (n=2–9)

All electron density functional theory with gradient corrections to the exchange and correlation functionals has been used to investigate the properties of small neutral and cationic vanadium clusters. The energetically favorable structures of vanadium clusters are found to be those with maximized average coordinations. The binding energy is found to increase monotonically with the number of atoms in the cluster and it is predicted that a significantly large number of atoms is needed to reproduce the bulk cohesive energy. Except for the neutral vanadium tetramer, the preferred decay channels for all the neutral and cationic clusters contain the vanadium atom. The average magnetic moment per atom is found to oscillate with cluster size, with an upper bound of 0.33 μB in the range from V4 to V9. Results have been compared, in detail, with available experimental and theoretical data.

Effect of phase structure on the adhesion properties of the dimethoxysilyl-terminated polypropylene oxide/epoxy resin system

In order to investigate the effect of phase structure on the adhesion properties of the dimethoxysilyl-terminated polypropylene oxide (DMSi-PPO)/diglycidyl ether of bisphenol A (DGEBA) system, various cured resins with different phase structures obtained by varying the formulations and curing temperatures were prepared. DGEBA that formed micro-order spherical domains reinforced the DMSi-PPO matrix in the cured system. Under ambient cure conditions, the adhesion properties of the cured system decreased with the addition of more than 50 phr DGEBA, because of the large domain size distribution and coagulation in the phase structure. However, it showed good adhesion properties when cured at 80°C. SEM observations showed that the phase structure became a unimodal micro-order domain size with increasing curing temperature. The reason for the dependence of the phase structure on the curing temperature is considered to be the different degree of solubility of DGEBA in DMSi-PPO. The fine phase structure in the 80°C-cured system resulted in a higher bulk cohesive energy and higher deformability, not only because of the increase in the surface area of the DGEBA domain but also due to the fine particle size distribution. The different bulk behaviors of the cured systems were reflected in the lap shear strength and T-peel strength.

Time-resolved photofragmentation of stored silver clustersAgn+(n=8–21)

The time-resolved decay of silver clusters ${\mathrm{Ag}}_{n}^{+}$ $(n=8\ensuremath{-}21)$ has been observed after excitation by photons with energies 1.5--4 eV. Clusters were found to decay by emission of neutral atoms or dimers with lifetimes in the range 100 $\ensuremath{\mu}$s to 15 ms. Separation energies were calculated from the lifetimes assuming a statistical unimolecular decay. As a function of cluster size, the resulting values increase towards the bulk cohesive energy of silver. They show a pronounced odd-even alternation and an indication of a shell closure at $n=9$. The separation energies for $n=8,9$ are in good agreement with configuration-interaction ab initio calculations.

Ab initio study of theCoSi2(110) surface

We present results of ab initio calculations for the (110) surface of ${\mathrm{CoSi}}_{2}$. The full-potential linearized augmented plane-wave method, designed for free slabs and bulk systems, was applied to calculate energetical quantities as well as electronic-structure properties. For precise calculations of surface energies we present a computationally useful scheme based on a proper bulk reference. The surface energy of 2.19 J/${\mathrm{m}}^{2}$ for the unrelaxed surface at experimental lattice spacing was reduced to 1.92 J/${\mathrm{m}}^{2}$ at the local-density approximation equilibrium lattice parameter. The surface energies were reduced by 4% and 2%, respectively, when the surface atoms were allowed to relax. Relaxations of atomic positions in the surface and subsurface layers were calculated by force minimization, leading to a small rumpling including a small lateral relaxation of Si atoms. A model is suggested for estimating the critical cleavage stress for ideal brittle fracture by relating elastic and cleavage quantities. We found that the energy for adsorbing a complete monolayer equals the bulk cohesive energy, for which we developed a scheme for deriving the free-atom reference energy by stretching a monolayer. We combined projected bulk bands with the slab band structure, which allows the detection of surface states more clearly than the usual schemes for finite slabs. Some typical surface states are analyzed by presenting their charge density. At about 1.2 eV above the Fermi level, a peak of antibonding Co-Si surface states (originated basically from states around X-bar) arises in the density of states. The local density of states at the surface is enhanced at Fermi energy.

Molecular modelling of polymers 17. Simulation and QSPR analyses of transport behavior in amorphous polymeric materials

Quantitative structure-property relationships (QSPRs) were constructed to predict O2, N2 and CO2 diffusion in amorphous polymers. The major polymer property governing diffusion in these QSPR models is bulk modulus. Oxygen permeation and solubility QSPR models were also constructed. Cohesive energy of the polymer matrix is the polymer property most highly correlated to permeation. Oxygen solubility appears to be about equally dependent on polymer density, bulk modulus or cohesive energy. A limited QSPR exploration of aqueous diffusion in polymer matrices indicates that cohesive energy of the polymeric material governs aqueous diffusion. The QSPR models can be used to efficiently predict transport properties from molecular dynamics simulations since only bulk modulus and/or cohesive energy need to be determined from a simulation in order to estimate a transport property using the appropriate QSPR model. An original computational technique to generate close-to-equilibrium dense polymeric structures is proposed. Diffusion of small gases are studied on the equilibrated structures using massively parallel molecular dynamics simulations running on the Intel Teraflops (9200 Pentium Pro processors) and Intel Paragon (1840 processors). Compared to the current state-of-the-art equilibration methods, the new technique appears to be faster by some orders of magnitude. The main advantage of the technique is that one can circumvent the bottlenecks in configuration space that inhibit relaxation in molecular dynamics simulations. The technique is based on the fact that tetravalent atoms (such as carbon and silicon) fit in the centre of a regular tetrahedron and that regular tetrahedrons can be used to mesh three-dimensional space. Thus, the problem of polymer equilibration described by continuous equations in molecular dynamics is reduced to a discrete representation where solutions are approximated by simple algorithms.

Ab initio study of small zinc sulfide crystallites

In this work the size evolution of properties of small in vacuo zinc sulfide crystals is studied using the ab initio Hartree-Fock molecular orbital method. The calculations on finite crystallite clusters, ranging from Zn 3S 3 to Zn 240S 240, are made possible by using the transitional symmetry of two-electron integrals. The calculations show that the electronic structure of ZnS crystallites is local and is converging rapidly to those in bulk. The electron distribution on surfaces is non-uniform and a large charge fluctuation exists. Surface relaxation decreases this fluctuation somewhat. Surfaces have a strong influence on the energy spectrum. The energy bands are wide and gaps narrow if compared with the bulk system. The cohesive energy shows a linear dependence on the surface-to-volume ratio. The largest cluster has 80% of the extrapolated bulk cohesive energy. The extrapolated results are comparable with infinite systems calculated by standard methods.

Transferability of a local pseudopotential based on solid-state electron density

Local electron - ion pseudopotentials fitted to dominant density parameters of the solid state (valence, equilibrium average electron density and interstitial electron density) have been constructed and tested for sixteen simple metals. Calculated solid-state properties present little evidence of the need for pseudopotential non-locality, but this need is increasingly evident as the pseudopotentials are transferred further from their solid-state origins. Transferability is high for Na, useful for ten other simple metals (K, Rb, Cs, Mg, Al, Ga, In, Tl, Sn, and Pb), and poor for Li, Be, Ca, Sr and Ba. In the bulk solid, we define a predictor of transferability and check the convergence of second-order pseudopotential perturbation theory for bcc Na. For six-atom octahedral clusters, we find that the pseudopotential correctly predicts self-compressions or self-expansions of bond length with respect to the bulk for Li, Na, Mg, and Al, in comparison with all-electron results; dimers of these elements are also considered. For the free atom, we examine the bulk cohesive energy (which straddles the atomic and solid-state limits), the atomic excitation energies and the atomic density. For the cohesive energy, we also present the results of the simpler stabilized jellium and universal-binding-energy-curve models. The needed non-locality or angular-momentum dependence of the pseudopotential has the conventional character, and is most strongly evident in the excitation energies.

Structures and Growth Mechanisms for Heteroepitaxial fcc(111) Thin Metal Films

We investigate film morphologies of heteroepitaxial face-centered cubic (fcc) thin metal adlayers gro.wn on fcc( 1 1 1) metal substrates using a statistical-mechanical mean-field theory. In order to develop a general understanding of how various intrinsic properties of the film and substrate dictate the thermodynamicallypreferred film structures, fourteen fcc( 11 l)/fcc( 1 11) interfaces with lattice mismatches ranging from 17% to +21% and bulk cohesive energies ranging from 2 to 6 eV/atom are examined. We find that relative magnitudes of lattice mismatch, surface energy, and bulk cohesive energy all contribute to determining the film structures, in contrast to our findings for fcc(100) interfaces, where lattice mismatch played the dominant role. Pseudomorphic growth is predicted to be preferred for small-mismatched interfaces, while incommensurate films are favored for large-mismatched interfaces. When the film has a lower surface energy and bulk cohesive energy than the substrate, layer-by-layer or Stranski-Krastanov growth is favored. Threedimensional growth becomes dominant for interfaces where the relative surface and bulk cohesive energy of the film is large compared to the substrate. We also find the film structures are insensitive to temperature variations in the range 300-800 K. Finally, our theoretical calculations are in generally excellent agreement with experimentally-determined film structures, and we have gone further to predict the morphological nature of a number of heterointerfaces that have yet to be examined experimentally.

Tight-binding calculation of the elastic constants of fcc and hcp transition metals.

The elastic constants of face-centered-cubic and hexagonal-close-packed structures are calculated within an empirical tight-binding d-band model. The variation with band filling of the shear elastic constants is in contrast with the roughly parabolic behavior of the bulk modulus, cohesive energy, and atomic volume. The general trends are in very good agreement with available experimental and ab initio data. In particular, the model accounts for the anomalous stiffening of C' and ${\mathit{C}}_{44}$ observed experimentally in Rh and Ir, as well as the negative value of C' for elements of groups 5 and 6 in the hypothetical fcc structure as predicted by ab initio calculations. Using the recursion method we show that these trends are reproduced within the fifth-moment approximation of the density of states. Pronounced minima are the other main features in the elastic-constant curves. They occur when the Fermi level passes through sharp peaks in the density of states and are due to strain-splitting effects.

Stabilized jellium model for the average electron density.

Employing the stabilized jellium model [J. Perdew et al., Phys. Rev. B 42, 11 627 (1990)] without the bulk stability condition, the bulk cohesive properties and surface energy of a simple metal are calculated. In the calculations the core-radii values are taken from the pseudopotential theory. Good agreement with experimental data is achieved.

Mean-field theory of heteroepitaxial thin metal film morphologies

We develop a mean-field statistical mechanical theory to examine the growth mechanisms and structures of fcc (face-centered cubic) metal thin films grown on fcc(100) metal substrates. Eleven hetero-interfaces with bulk cohesive energy differences as large as ~ 4 eV and lattice mismatches ranging from − 21% to 16% are examined in order to develop an understanding of how these intrinsic properties of the interface may determine film morphology. For small (up to ~ 5%) mismatched interfaces, pseudomorphic Stranski-Krastanov growth is predicted to be competitive with pseudomorphic layer-by-layer growth. When the lattice mismatch is 10% or greater, the growth mechanism is invariably incommensurate three-dimensional growth, where (111) cluster formation is energetically competitive with (100) cluster growth. The overlayer structures and growth modes correlate most closely with only one property: the lattice strain between the substrate and the adlayer. By contrast, the trends in the bulk cohesive energies do not allow even any qualitative prediction of the overlayer structures. However, surface energies can dictate minor changes in the growth modes of the films. Finally, no dependence of the film morphology on temperature is found over the temperature range 300–800 K for the cases we examined, although the film morphologies can be markedly different from those predicted at lower temperatures.

On the Compact Structure of Small fcc Metal Clusters

The general structure of small fcc metal clusters was investigated through the use of site energies, i.e., interaction energy per atom as a function of coordination. We used only experimental data on the dimer binding energy, the surface energy, and the bulk cohesive energy to determine these energies for all 15 fcc metals. These showed that real fcc metal systems exhibit a slight bond weakening as the number of bonds increases. It is also demonstrated that experimental data are much closer to the limit of constant bond energy than to that of constant interaction energy per atom. The constant bond energy model leads to compact structures which maximize the number of bonds, and thus we predicted that real small cluster systems would also be compact. Using molecular dynamics/Monte Carlo corrected effective medium theory, which nearly duplicates the experimental site energy curves, we studied the structures of 13-atom clusters of fcc metals via repeated melting and quenching in computer simulations. The lowest-energy structure was found to be a compact icosahedron with very high symmetry in every case. We also performed such simulations in the presence of a SiO2 support and found that the 13-atom Pt cluster maintained its nearly icosahedral shape.

An empirical many-body potential energy function constructed from pair-interactions

A new empirical potential energy function (PEF) is proposed, which is formed from pair-interactions only, and containes the many-body contributions. The PEF satisfies bulk cohesive energy and bulk stability condition. The PEF is parameterized for copper, silver, and gold elements in fcc crystal structure. The elastic constantsC11 andC12 and the bulk modulus of the elements are calculated, and the structural stability and energetics of microclusters containing 3 to 7 atoms of the same elements are investigated.

Construction and evaluation of embedding functions

Non-self-consistent density functional theories require specification of the embedding energy for an atom in a reference system. We combine the embedding energies determined from linear muffin tin orbital (LMTO) calculations of the bulk cohesive energy curves with those determined from the experimental diatomic binding curve. These new embedding functions contain information about the variation of binding with both coordination and separation between atomic centers. These are shown to be superior to embedding functions determined solely from bulk cohesive energy curves through tests on structures and energies of small metal clusters, self-diffusion of adatoms on metal surfaces, and scattering of metal atoms from metal surfaces.

Metal-metal bonding on surfaces: electronic and chemical properties of Ag on Ru(001)

The interaction between Ag and Ru(001) has been studied using TDS, XPS and XAES. Submonolayer coverages of Ag desorbed in the temperature range from 900 to 1050 K. The desorption of the Ag monolayer followed zero-order kinetics with an activation energy of 65 kcal/mol. By comparing the desorption temperatures of Cu, Ag and Au from W(110) and Ru(001), a direct relation was found between the bulk cohesive energy of a noble metal and its adsorption energy on a transition metal. A monolayer of Ag on Ru(001) shows Ag 3d levels and Auger MVV transitions that are shifted ∼ 0.1 eV toward lower binding energy with respect to those of the bulk atoms in pure Ag. In CO/Ag/Ru(001) surfaces, the interaction between Ag and Ru enhances the strength of the AgCO bond and weakens the RuCO bond. The CO-desorption temperature from one monolayer of Ag supported on Ru(001) is ∼ 90 K higher than that found on Ag(111). This corresponds to an increase of ∼ 5 kcal/mol in the strength of the AgCO bond. The results of CO-TDS and O(1s)-XPS show that the presence of Ag induces a reduction in the charge transfer from Ru into the CO(2π ∗) orbitals, which weakens the RuCO bond by ∼ 6 kcal/mol and shifts the O(1s) peak position 0.4 eV toward higher binding energy. Oxygen molecules adsorbed on Ag atoms bonded to Ru(001) exhibit non-equivalent oxygen atoms (probably a “superoxo-like” species), a desorption temperature of 230–240 K and a very low probability for dissociation (0.05–0.15 at T < 250 K).

Thin film growth and the scattering of atoms from surface island defects

We investigate the potential energy surface and dynamics of atoms impinging upon pyramidal defects on fcc (100) and (111) surfaces. Illustrative molecular dynamics trajectory results are used to highlight the subtleties of the growth processes. Homoatomic systems of both Cu and Pd are considered in order to determine the influence of extreme differences in bonding coordination variation, i.e. the binding energy per atom decreases by a factor of 3.3 from Cu(bulk) to Cu 2 but more dramatically by a factor of 7.5 for Pd(bulk) to Pd 2. We find that stable adsorption sites on pyramidal facets only exist for three layer high (and higher) facets, but even these sites are not as energetically favorable as those at the pyramid's base. This single energetic effect can promote a Stranski-Krastanov type growth pattern, with a critical roughness of no less than three layers being required before a 3D growth mechanism can propagate in a surface temperature regime where post impact diffusion is small compared to the deposition rate. The transition to 3D growth at the minimum possible roughness would be found for Pd on the (100) face where sticking to the sides of even the three layer pyramid is a high probability event, at a low enough temperature to make post-impact diffusion negligible. However, for the Cu system, we find that the “downward funneling” mechanism smoothed the growth for the three layer pyramid and is still important for the five layer pyramid, indicating that Cu will exhibit layer-by-layer like growth even past three layers. A comparison of scattering simulations from defects show that the growth mode is diffusion limited on the fcc (100) crystal face, but becomes dominated by impact induced disruption of the original structure for a (111) surface. This implies that the (111) surface will grow more smoothly as a new atom displaces pre-adsorbed atoms more easily, an effect that is not included in the “downward funneling” model. The ratio of the diatomic molecule's binding energy to the bulk cohesive energy is shown to provide good insight into the preferred growth mechanism. As the ratio increases, at least in the range spanned by typical metals, the film grows more layer-by-layer like on the (100) surface and less layer-by-layer like on the (111) surface. On the (100) surface, stronger low coordination bonding makes the defect structures less able to adsorb the impacting atom on the sloped faces, increasing the effectiveness of “downward funneling.” On the (111) surface, stronger low coordination bonding inhibits impact disruption of the defects, decreasing this important smoothing mechanism.

Potential-energy functions for Cu, Ag and Au solids and their application to clusters of these elements

Empirical potential functions comprising two- and three-body terms have been derived for Cu, Ag and Au by fitting parameters to the phonon frequencies, elastic constants, bulk cohesive energies, vacancy formation energies and lattice distances of the f.c.c. solids. The potentials predict compact structures for small clusters of the elements with more metastable structures for Ag than for Cu and Au.

STUDIES OF BAND STRUCTURE AND STATIC PROPERTIES OF Bl MgO CRYSTAL BY LMTO-ASA METHOD

The applieation of the ab-initio LMTO-ASA method to B1 structure crystal is studied in this paper. The important function of the empty atomsphere in the calculation of static properties such as equilibrium lattic constant, bulk modulus and cohesiv energy, and the band structure on Bl MgO crystal are emphatically investigated. A simple LMTO method which is as accurate as the ab-initio pseudopotential method has been adopted. The calculated results of the lattic constant a0 ,bulk moduius B0 and cohesive energy Ec of Bl MgO are 4.21? , 1.58 Mbar and 5. 73eV/atom, respectively, which are very close to experimantal data and those obtained by more complicated ab-initio band calculating methods. The band structure calculated by our method is in good agreement with that by ab-initio pseudopotential method. The results indieate that LMTO-ASA method in which empty atom-spheres is added and L?wdin principle is introduced to raise calculation efficiency is suitable to the study of band structure and static property of Bl structure crystal.

Metal–metal bonding on surfaces: Zn–Au on Ru(001)

The properties of Zn, Au, and Zn–Au films on Ru(001) have been studied using thermal desorption mass spectroscopy (TDS) and core and valence-level photoemission. TDS spectra show desorption of Zn from Ru(001) at 450 (multilayer), 490 (second layer), and 720 K (first layer). The monolayer of Zn desorbs following zero-order kinetics with an activation energy for desorption of 36 kcal/mol. Photoemission studies show that the Zn(3s) and Zn(3d) levels of Zn bonded to Ru are shifted 0.5 eV toward lower binding energy with respect to the corresponding levels of pure metallic Zn. The direction of this binding-energy shift together with the results of CO-TDS indicate that Zn is withdrawing electrons from the Ru(001) surface. Such charge transfer is in part responsible for the large stability of the Ru–Zn bond. Au desorbs from Ru(001) at temperatures of 1175 (multilayer) and 1260 K (first layer). For a monolayer of Au deposited on Ru(001), the positions of the Au(4f ) and Au(5d) levels are very close to those of the surface atoms of Au(111), indicating that charge transfer between Au and Ru is minimal. A comparison of the desorption temperature for a monolayer of Zn or Au from Ru(001) with values previously reported for the desorption of monolayers of alkali (K and Cs), noble (Cu and Ag), and transition (Mn, Ni, and Pd) metals indicates that the strength of a metal–metal bond in a bimetallic surface depends on (1) the bulk cohesive energy of the individual metals (which gives an indication of the tendency of the element to form strong metal–metal bonds), and (2) the charge transfer within the bond. Zn and Au alloy when coadsorbed on Ru(001). Results for submonolayer coverages of Zn and Au show that the Zn–Ru and Zn–Au bonds in the trimetallic system are more stable than the corresponding bonds in Zn/Ru(001) or Zn–Au alloys. This phenomenon is probably caused by synergistic interactions in three center metal–metal bonding. The formation of ZnAu alloys induces a shift of ∼+1.0 eV in the Au(4f ) and Au(5d) levels, and −0.2 eV in the Zn(3s) and Zn(3d) levels. The shift in the core and valence levels of Au is caused by rehybridization of the Au(5d,6s,6p) levels, while the shift in the Zn levels comes from a contraction in volume.

A New Class of Empirical Many‐Body Potential Energy Functions for Bulk and Cluster Properties

AbstractA new empirical many‐body potential energy function (PEF) is proposed, which comprices two‐ and three‐body atomic interactions. The two‐body potential is a kind of hybrid function and the three‐body potential is formed by additive and nonadditive functions. The additive part is expressed in terms of two‐body interactions, and the nonadditive part is expressed as triple‐dipole function. The PEF satisfies bulk cohesive energy, bulk stability condition, and bulk modulus. The PEF is parameterized for gold, silver, and copper elements in f.c.c. crystal structure. The elastic constants of the elements are calculated, and the structural stability and energetics of microclusters containing 3 to 7 atoms of the same elements are investigated. The calculated elastic constants are in good agreement with experimental values, and the most stable microcluster geometries are qualitatively in agreement with the available literature data.