Crystal habit of fcc metal particles controlled by electrode potential in solution

The crystal habit of fcc metal particles formed on an amorphous carbon film electrode in solution at different electrode potentials is discussed. The fcc metal particles have different crystallographic habits depending on applied electrode potential; that is, icosahedral and/or decahedral particles are formed at lower potentials, and fcc single-crystalline or polycrystalline particles at higher potentials. It was found that decahedra and icosahedra of Cu-Au alloy particles are formed in the potential region of underpotential deposition (UPD) of Cu at which only fcc Au single-crystalline particles and Au polycrystalline particles appear. This is attributed to the charge transfer from the UPD Cu ions to the Au overlayer of Cu-Au alloy particles. The formation of decahedral and icosahedral Cu-Au alloy particles depends on the composition of the Cu-Au alloy. On the basis of these results it was deduced that the contraction of the surface lattice of the growing particles is responsible for the formation of icosahedral and decahedral particles. Received: 25 February 1997 / Accepted: 21 April 1997     

The minimum-energy structure of nanometer-scale gold clusters

We report results of experiments in which gold clusters with controlled diameters ranging from 1nm to 20nm are grown in a gas aggregation reactor and are subsequently melted and slowly cooled in the gas phase. These clusters are soft landed on thin carbon films and their structure determined by means of HRTEM. All of the clusters down to the smallest whose lattice fringes could be resolved (N≈405) are single fcc crystals. MD calculations using an EAM potential for gold predict that the fcc motif seen in these experiments may indeed be the minimum-energy structure for gold clusters containing more than a few hundred atoms.     

Modeling crystal growth from solution with molecular dynamics simulations: approaches to transition rate constants

The feasibility of using the molecular dynamics (MD) simulation technique to study crystal growth from solution quantitatively, as well as to obtain transition rate constants, has been studied. The dynamics of an interface between a solution of Lennard-Jones particles and the (100) face of an fcc lattice comprised of solute particles have been studied using MD simulations, showing that MD is, in principle, capable of following growth behavior over large supersaturation and temperature ranges. Using transition state theory, and a nearest-neighbor approximation growth and dissolution rate constants have been extracted from equilibrium MD simulations at a variety of temperatures. The temperature dependence of the rates agrees well with the expected transition state theory behavior.     

Novel lattice-searching method for modeling the optimal strain-free close-packed isomers of clusters

Small icosahedral, decahedral, and fcc structures have been studied by unbiased global optimization methods or Wulff construction and Northby lattice methods. Strain-free close-packed structures are not much discussed because the structures are very difficult to optimize and there is no common strain-free close-packed lattice. We propose a new strategy to construct such a lattice containing all possible strain-free close-packed isomers, and by searching the lattice with an efficient method the optimal close-packed structures were modeled. Testing with the Morse potential at rho0=14.0 for cluster size 10相似文献   

On the lattice stability of metals

A tight-binding calculation for body-centred cubic (bcc) and face-centred cubic (fcc) lithium is carried out using muffin-tin potentials which differ only in the arrangement of the muffin-tin spheres. The essential results are not restricted to lithium but also hold for other metals with similar s-p-bands. The bcc structure can be stable for the lowest valence states. The stability of fcc increases with increasing valence electron concentration. This is due to the kinetic energy which behaves as in an empty lattice case. In accordance with the behaviour of the kinetic energy, the lowest energy states have the highest distribution probability between neighbouring atoms. They are mostly delocalized. The trend in the lattice stability is explained in terms of the differences in the packing of the lattices. In real cases where the virial theorem holds an appropriate part of the kinetic energy is changed into potential energy. Hybridization plays a completely different role from that in covalent compounds. It stabilizes a compound by delocalizing the charge density.     

On fitting a gold embedded atom method potential using the force matching method

We fit a new gold embedded atom method (EAM) potential using an improved force matching methodology which included fitting to high-temperature solid lattice constants and liquid densities. The new potential shows a good overall improvement in agreement to the experimental lattice constants, elastic constants, stacking fault energy, radial distribution function, and fcc/hcp/bcc lattice energy differences over previous potentials by Foiles, Baskes, and Daw (FBD) [Phys. Rev. B 33, 7983 (1986)] Johnson [Phys. Rev. B 37, 3924 (1988)], and the glue model potential by Ercolessi et al. [Philos. Mag. A 50, 213 (1988)]. Surface energy was improved slightly as compared to potentials by FBD and Johnson but as a result vacancy formation energy is slightly inferior as compared to the same potentials. The results obtained here for gold suggest for other metal species that further overall improvements in potentials may still be possible within the EAM framework with an improved fitting methodology. On the other hand, we also explore the limitations of the EAM framework by attempting a brute force fit to all properties exactly which was found to be unsuccessful. The main conflict in such a brute force fit was between the surface energy and the liquid lattice constant where both could not be fitted identically. By intentionally using a very large number of spline sections for the pair potential, electron-density function, and embedding energy function, we eliminated a lack of functional freedom as a possible cause of this conflict and hence can conclude that it must result from a fundamental limitation in the EAM framework.     

立方面心C60晶体晶格振动的研究

采用C60分子之间相互作用势的Kihra形式,研究了立方面心C60晶体的晶格振动问题,得到了质心振动沿[111]、[110]及[100]方向的声子散射圆频率分布曲线及C60晶格振动频率的态密度分布.采用所得到的C60晶格振动频率的态密度分布,计算了晶体C60在298 K时的等压热容,所得数值与实验值相符.     

Force-field development and molecular dynamics simulations of ferrocene-peptide conjugates as a scaffold for hydrogenase mimics

The increasing importance of hydrogenase enzymes in the new energy research field has led us to examine the structure and dynamics of potential hydrogenase mimics, based on a ferrocene-peptide scaffold, using molecular dynamics (MD) simulations. To enable this MD study, a molecular mechanics force field for ferrocene-bearing peptides was developed and implemented in the CHARMM simulation package, thus extending the usefulness of the package into peptide-bioorganometallic chemistry. Using the automated frequency-matching method (AFMM), optimized intramolecular force-field parameters were generated through quantum chemical reference normal modes. The partial charges for ferrocene were derived by fitting point charges to quantum-chemically computed electrostatic potentials. The force field was tested against experimental X-ray crystal structures of dipeptide derivatives of ferrocene-1,1'-dicarboxylic acid. The calculations reproduce accurately the molecular geometries, including the characteristic C2-symmetrical intramolecular hydrogen-bonding pattern, that were stable over 0.1 micros MD simulations. The crystal packing properties of ferrocene-1-(D)alanine-(D)proline-1'-(D)alanine-(D)proline were also accurately reproduced. The lattice parameters of this crystal were conserved during a 0.1 micros MD simulation and match the experimental values almost exactly. Simulations of the peptides in dichloromethane are also in good agreement with experimental NMR and circular dichroism (CD) data in solution. The developed force field was used to perform MD simulations on novel, as yet unsynthesized peptide fragments that surround the active site of [Ni-Fe] hydrogenase. The results of this simulation lead us to propose an improved design for synthetic peptide-based hydrogenase models. The presented MD simulation results of metallocenes thereby provide a convincing validation of our proposal to use ferrocene-peptides as minimal enzyme mimics.     

Adsorption Energetics of NO and CO on Pt(111)

The adsorption energetics of NO and CO on Pt(111) are studied using an ab initio embedding theory. The Pt(111) surface is modeled as a three-layer, 28-atom cluster with the Pt atoms fixed at bulk lattice sites. Molecular NO is adsorbed at high symmetry sites on Pt(111), with the fcc threefold site energetically more favorable than the hcp threefold and bridge sites. The calculated adsorption energy at the fcc threefold site is 1.90 eV, with an N-surface distance of 1.23 Å. The NO molecular axis is perpendicular to the Pt(111) surface. Tilting the O atom away from the surface normal destablizes adsorbed NO at all adsorption sites considered. On-top Pt adsorption has been ruled out. The Pt(111) potential surface is very flat for CO adsorption, and the diffusion barriers from hcp to fcc sites are 0.03 eV and less than 0.06 eV across the bridge and the atop sites, respectively. Calculated adsorption energies are 1.67, 1.54, 1.51, and 1.60 eV at the fcc threefold, hcp threefold, bridge, and atop sites, respectively. Calculated C-surface distances are 1.24 Å at the fcc threefold site and 1.83 Å at the atop site. It is concluded that NO and CO adsorption energetics and geometries are different on Pt(111).     

Stability of two-dimensional crystalline aggregates of a protein studied by molecular dynamics

Two-dimensional protein (ferritin) aggregates with a square lattice symmetry, which were formed within a thin liquid layer on a mercury surface, were studied by molecular dynamics (MD) simulation. For the simulation, the ferritin molecule was modeled by an assembly of 49 spheres, and the intermolecular interactions were given by simple formulae. During the simulation, molecules were confined within a layer, which corresponds to the thin liquid layer. An annealing MD simulation was done starting from a random molecular configuration within the layer, and aggregates with the square lattice symmetry were also obtained. To study the stability of aggregates, dissociation processes of the aggregates were analyzed using MD simulations at room temperature. Interactions between the nearest-neighbor molecules were regarded as bonds. Mean bond energies and correlation coefficients between the bond energies were calculated from the MD trajectories. A decay profile according to the dissociation was obtained, yielding a dissociation rate constant. Buried bonds were stronger than peripheral bonds. The larger the aggregate size, the stronger the bond for each of the buried and peripheral bonds. A simple theoretical account, which is applicable to a general bonded network, was introduced to analyze the dynamics of the aggregates. © 1994 by John Wiley & Sons, Inc.     

Crystal Structure Dependent Dissolution of Non-Cubic Au Crystallites in Aqua Regia

Properties of metal crystallites are governed by their morphologies and inherent crystal structures. In this work, bipyramidal Au microcrystallites hosting non-cubic lattices, body-centered orthorhombic and tetragonal (together termed as bc(o,t)), are investigated for their stability in aqua regia. Specifically, microcrystallites comprising 92 % of bc(o,t) have been subjected to aqua regia of different concentrations and the changes in morphology and lattice phases have been monitored using scanning electron microscopy and X-ray diffraction techniques. The dissolution process was found to be crystal structure dependent and begin at the bipyramidal tips enriched with fcc lattice while retaining the bc(o,t) rich body. Interestingly, with increasing the reaction times, the remaining core was found to be highly reluctant to dissolution and instead, transformed to tetragonal lattices which with increasing treatment, exhibited lattice parameters closer to that of fcc. The study reveals the presence of a bc(o,t)-fcc core-shell structure with the tips enriched with fcc.     

Effect of polydispersity on the relative stability of hard-sphere crystals

By extending the nonequilibrium potential refinement algorithm and lattice switch method to the semigrand ensemble, the semigrand potentials of the fcc and hcp structures of polydisperse hard-sphere crystals are calculated with the bias sampling scheme. The result shows that the fcc structure is more stable than the hcp structure for polydisperse hard-sphere crystals below the terminal polydispersity.     

Atomistic Model of Uranium

The electronic state and potential data of U₂ molecules are performed by first principle calculations with B3LYP hybrid exchange-correlation functional, the valence electrons of U atom are treated with the (5s4p3d4f)/[3s3p2d2f] contraction basis sets, and the cores are approximated with the relativistic effective core potential. The results show that the ground electronic state is X⁹∑⁺_g . The pair potential data are fitted with a Murrell-Sorbie analytical potential function. The U-U embedded atom method (EAM) interatomic potential is deter-mined based on the generalized gradient approximation calculation within the framework of the density functional theory using Perdew-Burke-Ernzerhof exchange-correlation functional at the spin-polarized level. The physical properties, such as the cohesive energy, the lattice constant, the bulk modulus, the shear modulus, the sc/fcc relative energy, the hcp/fcc rela-tive energy, the shear modulus and the monovacancy formation energy are used to evaluate the EAM potential parameters. The U-U pair potential determined by the first principle calculations is in agreement with that defined by the EAM potential parameters. The EAM calculated formation energy of the monovacancy in the fcc structure is also found to be in close agreement with DFT calculation.     

Localized adsorption at solid-liquid interfaces: the sticky site-hard wall model

The interface between a structured solid and a liquid is modelled by a flat surface with sticky sites placed on a regular lattice. It is shown that this three-dimensional adsorption model can be mapped into a two-dimensional lattice-gas model with the interaction energies of the particles on the sites related to the potential of mean force in the inhomogeneous fluid. In the simple case of a liquid of hard spheres, the first layer of liquid at contact with the solid exhibits an order-disorder phase transition and the total surface excess shows a maximum as the bulk density increases.     

The chemical potential of a liquid on the liquid-vapour coexistence line and the lattice-gas model

The lattice-gas model predicts that a certain chemical potential is constant along the liquid-vapour coexistence line. It is shown that this is true to a few percent for two simulated atomic liquids and for several real liquids (argon, nitrogen and hydrogen chloride). More complex liquids (water) show substantial deviations.     

A lattice Monte Carlo simulation of the FePt alloy using a first-principles renormalized four-body interaction

Although a lattice Monte Carlo method provides an effective, simple, and fast way to study thermodynamic properties of substitutional alloys, it cannot treat by itself the off-lattice effects, such as thermal vibrations and local distortions. Therefore, even if the interaction among atoms at lattice points is calculated accurately by means of first-principles calculations, the lattice Monte Carlo simulation overestimates the order-disorder phase transition temperature. In this paper, we treat this problem in the investigation of the FePt alloy, which has recently attracted considerable interest in its magnetic properties. We apply a simple version of the potential renormalization theory to determine the interaction among atoms, including partly the off-lattice effects by means of first-principles calculations. Then, we use the interaction to perform a lattice Monte Carlo simulation of the FePt alloy on a fcc lattice. From the results, we find that the transition temperature obtained after the present renormalization procedure becomes closer to the experimental value.     

Modulating fcc and hcp Ruthenium on the Surface of Palladium–Copper Alloy through Tunable Lattice Mismatch

Herein, we report an epitaxial‐growth‐mediated method to grow face‐centered cubic (fcc) Ru, which is thermodynamically unfavorable in the bulk form, on the surface of Pd–Cu alloy. Induced by the galvanic replacement between Ru and Pd–Cu alloy, a shape transformation from a Pd–Cu@Ru core–shell to a yolk–shell structure was observed during the epitaxial growth. The successful coating of the unconventional crystallographic structure is critically dependent on the moderate lattice mismatch between the fcc Ru overlayer and PdCu₃ alloy substrate. Further, both fcc and hexagonal close packed (hcp) Ru can be selectively grown through varying the lattice spacing of the Pd–Cu substrate. The presented findings provide a new synthetic pathway to control the crystallographic structure of metal nanomaterials.     

Rapidly converging lattice sums for nonelectrostatic interactions

The relative energies of one-, two-, and three-dimensional Bravais lattice Lennard-Jones particles can be calculated by lattice sums. The expression of lattice sums over a Lennard-Jones potential can be manipulated into a form that converges rapidly. A formalism capable of calculating the lattice potential at arbitrary points of a completely general lattice has been developed. This method provides an alternative way to calculate the relative energies from the surface and the interior bulk sites of many chemical systems. The method is illustrated with application to hcp and fcc Lennard-Jonesium, both for the relative binding energy and for calculating the potential along the geometric diffusion pathway between tetrahedral and octahedral interstitial sites. Diffusion from the tetrahedral site to the octahedral site experiences a barrier of 752.600 in units of 4 epsilon. The reverse pathway experiences a barrier of 1035.614 in units of 4 epsilon.     

Diffusion of particles over dynamically disordered lattice

Using kinetic Monte Carlo simulations we have investigated the diffusion of particles over a lattice with moving atoms in the framework of a simple lattice-gas model of a reconstructive surface. The particle migration over the static and dynamic lattices differs considerably. The dynamic lattice reconstruction changes substantially the particle diffusion coefficient. The Arrhenius dependencies are quantitatively different. An interesting peculiarity of the particle migration is the formation of defectons--local lattice deformations around the adsorbed particles. In certain ranges of the system parameters (jump rates of the substrate atoms and adsorbed particles) the adsorbed particles cause local displacements of the nearest substrate atoms, resulting in deeper adsorption sites and lower jump rates. Such particle self-trapping produces a characteristic minimum on the Arrhenius dependencies.