Molecular dynamics calculation on three lithium atoms in planar cluster model C150H30 for glassy carbon

As the preliminary step to elucidate the charge-discharge mechanism in the glassy carbon cathode of lithium (Li) secondary battery, the molecular dynamics (MD) calculation was applied at molecular mechanics 2 (MM2) level for three Li atoms stabilized in the form of regular triangle in the central area of the hydrogen terminated planar cluster, C₁₅₀H₃₀. The stabilized Li sites are given on the basis of the structural optimization. Up to 10 K, the triangular Li aggregate kept at almost the same figure as that formed by three stabilization sites, rotating parallel to the cluster plane, goes around the central area of it, whereupon interatomic vibrational stretching is observed. Below 75 K, the aggregate of three Li atoms separates to a pair of two atoms and one Li atom arbitrarily, however, reformation of the pair occurs periodically among three atoms with the lapse of simulation time. Then, three Li atoms move correlatively irrespective of the long interatomic distance of 18 Å at maximum. However, at 100 K one Li atom goes out of the cluster model directly and the rest of two atoms continues the revolutional movement with the rotation as a pair in the central area of the cluster model. Thus, three Li atoms show the appreciably stable movements in the glassy carbon by forming the aggregate or the atomic pair, which will be responsible for the hysteresis in the charge-discharge cycle of lithium secondary battery.     

Large supercell molecular dynamics study of defect formation in hydrogenated amorphous silicon

We have performed molecular dynamics simulations of the defect formation associated with the Staebler-Wronski (SW) effect in a-Si:H using 224 and 231 atom supercells and employing semiempirical Si-Si and Si-H total energy functionals. The role of hydrogen in the defect formation within the bond breaking model of the SW effect has been investigated for both large supercells. The results suggest that, within this model, H can be important in weakening the normal Si-Si bonds which break to produce defects in the SW effect.     

Optical properties of methane to 288 GPa at 300 K

Optical properties of solid methane (CH₄) were studied at high pressure and room temperature using a diamond anvil cell. Reflectivity and transmission measurements were used to measure the refractive index to 288 GPa. Fabry-Perot interferometery was used to measure the sample thickness to 172 GPa. This data was fitted to the derived expression of thickness vs. pressure that was then used to calculate the thickness to 288 GPa. This in turn was combined with optical absorption experiments to obtain the absorption coefficient and hence the extinction coefficient k^*. From combined reflection and absorption experiments the refractive index n=n_s+ik^* was obtained. The index of refraction and the ratio of molar refraction to molar volume showed a large increase between 208 and 288 GPa. This behavior indicated that a phase transformation of insulator-semiconductor might have occurred in solid CH₄ by 288 GPa.     

Ag-rich phase formation in the Cu-6 wt% Al alloy with Ag additions

Ag-rich phase formation in the Cu-6 wt% Al alloy with additions of 4, 6, 8, 10, and 12 wt% Ag was studied using microhardness changes with temperature and time, differential thermal analysis (DTA), scanning electron microscopy (SEM), energy-dispersive X-ray analysis (EDX), and X-ray diffractometry (XRD). The results indicated that the Ag-rich phase formation follows two different processes depending on Ag concentration. For alloys with 4 and 6 wt% Ag the precipitate formation occurs as a second-order mechanism while for alloys with 8, 10 and 12 wt% Ag a zero-order reaction was observed.     

The direct molecular orbital dynamics study on the hydrogen species adsorbed on the surface of planar graphite cluster model

In order to investigate thermal behaviors of the hydrogen species, H⁺ ion and H atom, adsorbed on the surface of planar graphite, the direct molecular orbital dynamics (MO) calculations at AM1 level are applied to the hydrogen terminated planar cluster models included by the species, C₅₄H₁₈·H⁺ and C₅₄H₁₈·H, respectively. This is the first trial to describe the thermal behaviors of these species in terms of MO dynamics. Both hydrogen species form the covalent bonds with the surface carbon atoms through the sp³ hybrid orbitals which inhibit their dissociation at high temperature up to 2000 K. It was found that the reduction of band gap is introduced by the adsorption of H⁺ ion.     

Reconstructed (1 1 0) surfaces of FCC transition metals

The energies of the ideal, missing row (MR) and missing column (MC) (1 1 0) surfaces have been calculated by using modified embedded atom method (MEAM) for seven face centered cubic (FCC) transition metals Au, Pt, Ag, Pd, Rh, Cu and Ni. The results, that the MC reconstruction can not be formed for all metals, while the MR reconstruction can be formed naturally for Au and Pt, inductively for Ag, Pd, Rh and Cu and difficultly for Ni, are better than EAM calculated results in comparing with experimental results. In addition to the surface energy explanation, the results are also related to the surface topography and valence electron structure.     

Solution calorimetric investigation of AgCl-AgI ionic conductor composites at 298 K: observation of metastable AgI modifications

The enthalpies of solution of pure silver halides AgCl and AgI and a composite material with molar composition 0.5 AgCl-0.5 AgI were measured at 298 K in a mixture of Na₂S₂O₃ (1 M) and NH₄OH (1 M). X-ray diffraction patterns showed that the composite material contained the metastable γ-AgI phase; different mechanisms for its stabilization were discussed. The phase transition enthalpies of AgI modifications and the enthalpy of formation of the composite material were deduced from the measurements. The latter could be related to a change of interfacial enthalpies.     

The melting behaviors of the Nb(1 1 0) nanofilm: a molecular dynamics study

By using molecular dynamics (MD) and the modified analytic embedded atom method (MAEAM), we have studied the melting point, the melting mechanism and the correspondingly dynamical behaviors of a Nb(1 1 0) nanofilm. Firstly, in accordance to the MD time dependence of the potential energy, the melting point of this nanofilm has been roughly estimated. Then, the melting mechanism of the nanofilm have been analyzed in detail with the application of the structure factor. The results clearly indicate that the melting transition of the 8th, 9th, and 10th atomic layer of the nanofilm has been characterized by the exponential, polynomial and linear sequence respectively when the thickness of the quasiliquid film attains to about 1.3 nm. Thirdly, the dynamical behaviors of the nanofilm melting, such as the melting front propagation velocity and the kinetic coefficient, which have also been analyzed, demonstrate that the melting front propagation velocity has linearly increased with the incremental temperature and the evaluated kinetic coefficient has approximately equaled 1.43m/(sK). Finally, by extrapolating the melting front propagation velocity to zero, we can accurately deduce the melting point of the Nb(1 1 0) nanofilm to 2568.3 K, which is much lower than the counterpart (2740 K) of the bulk niobium.     

Analysis on experimental valence charge density in germanium at RT and 200 K

The electronic structure and hence the valence charge distribution of germanium at 296 and 200 K has been elucidated from structure factors measured by X-ray diffraction experiment using maximum entropy method (MEM) and multipole model. The methods adopted here are used to extract the fine details of the charge density distribution in the valence region. The charge density evaluated using both the models along the bond path and at the mid bond positions are compared and found to confirm the covalent bond existing in the solid. Topology of the charge density in the crystal is analysed and the critical points determined reveal unique spatial arrangement of valence charge in the direction normal to the bonding direction. The Laplacian of the charge density is also analysed for the understanding of the spatial distribution and the partitioning of the valence charge. The local charge concentration and the mapping of the electron pairs of the Lewis and valence shell electron pair repulsion (VSEPR) models have been done using electron localization function (ELF) and localized orbital locator (LOL).     

Effects of an external electromagnetic field on rutile Tio2: A molecular dynamics study

Non-equilibrium molecular dynamics simulations of the bulk rutile phase of TiO₂ have been carried out in an intense external electromagnetic field with frequency in the microwave to far-infrared range. Simulations were performed in constant-volume ensembles with and without a thermostat coupled to the atomic degrees of freedom, at 298 K and from 298 K to well above the crystal melting temperature, respectively. Fields were applied along the a and c crystallographic directions. Both the Lekner and Ewald techniques were used to handle long-range electrostatics and the impact of different choices of Ewald parameters on the results has been evaluated. It was found that the ions respond rapidly to the field. The peaks and troughs in the Ti-Ti radial distribution functions were sharpened relative to the zero-field case at the instants of maximum electric field intensity, but little evidence of this was found for the Ti-O or O-O distributions. For pure Newtonian dynamics, the 500 GHz field excited the vibrational modes to the greatest extent, raising the system temperature at the fastest rate. For temperatures above 2000-2500 K, the crystal structure was found to melt. In the canonical ensemble, the 50 GHz field led to the greatest enhancement of ionic translational mobility.     

Selective MOCVD of titanium oxide and zirconium oxide thin films using single molecular precursors on Si(1 0 0) substrates

Titanium oxide (TiO₂) and zirconium oxide (ZrO₂) thin films have been deposited on modified Si(1 0 0) substrates selectively by metal-organic chemical vapor deposition (MOCVD) method using new single molecular precursor of [M(OⁱPr)₂(tbaoac)₂] (M=Ti, Zr; tbaoac=tertiarybutyl-acetoacetate). For changing the characteristic of the Si(1 0 0) surface, micro-contact printing (μCP) method was adapted to make self-assembled monolayers (SAMs) using an octadecyltrichlorosilane (OTS) organic molecule which has -CH₃ terminal group. The single molecular precursors were prepared using metal (Ti, Zr) isopropoxide and tert-butylacetoacetate (tbaoacH) by modifying standard synthetic procedures. Selective depositions of TiO₂ and ZrO₂ were achieved in a home-built horizontal MOCVD reactor in the temperature range of 300-500 °C and deposition pressure of 1×10⁻³-3×10⁻² Torr. N₂ gas (5 sccm) was used as a carrier gas during film depositions. TiO₂ and ZrO₂ thin films were able to deposit on the hydrophilic area selectively. The difference in surface characteristics (hydrophobic/hydrophilic) between the OTS SAMs area and the SiO₂ or Si-OH layer on the Si(1 0 0) substrate led to the site-selectivity of oxide thin film growth.     

Anharmonic effects on B2-FeAl(1 1 0) surface: A molecular dynamics study

Using molecular dynamics simulations and the analytic embedded-atom method (AEAM), the surface anharmonicity of B2-FeAl(1 1 0) has been studied in the temperature range from 0 K to 1400 K. The temperature dependence of the interlayer spacing, mean square vibrational amplitudes, surface phonon frequencies and line-widths, and layer structure factor have been calculated. The obtained results indicate that the anharmonic effects are small in the temperature range from 0 K to 900 K. The temperature dependences of the interlayer spacing indicates that the rippling effect of the B2-FeAl(1 1 0) surface is exhibited by the contraction of Fe surface atoms and the expansion of Al atoms, which persists at high temperatures. The temperature dependence of the layer structure factors shows that the B2-FeAl(1 1 0) surface does not disorder until the temperature of 1300 K.     

Structural stabilities and diffusion of small Fe clusters on Fe (1 1 0) surface: A molecular dynamics study

Using Embedded-atom-method (EAM) potential of iron, structural stabilities of small Fe clusters on a Fe (1 1 0) surface have been investigated by molecular dynamics studies. It is presented that a tetramer and heptamer clusters are more stable than other sizes. These two clusters have high transition energies. They can be a critical nucleus at low and high temperature, respectively. A dimer diffuses more easily with lower energy barrier than single adatom. The trimer's rotation and dimer shearing mechanisms have been investigated in this paper.     

Surface properties of rutile TiO2(1 1 0) from molecular dynamics and lattice dynamics at 300 K: Variable-charge model results

Geometric structure, atomic vibrations and atomic charges and their thermally induced fluctuations have been calculated as a function of depth in, and thickness of, rutile TiO₂(1 1 0) slabs, within the framework of the variable-charge potential of Swamy and Gale [V. Swamy, J.D. Gale, Phys. Rev. B 62 (2000) 5406] at 300 K. Molecular dynamics simulations and lattice dynamics calculations were performed with a 2D periodic slab model for slab thicknesses between 3 and 11 triple layers (approximately 9-35 Å). Odd-even oscillations with respect to the number of slab layers are found for the surface relaxation for very thin slabs, and for the (slowly converging) rumpling in the middle of the slab. The Ti and O atomic charges in the outermost three atomic layers differ from the rest of the slab (they are less ionic); the thermal vibrations do not alter this picture. The atomic mean-square amplitudes are some 50% larger (more for O, less for Ti) at the surface than in the middle of the slab and decay rather slowly to the bulk values. Comparisons with the results of a rigid-ion potential for titania [M. Matsui, M. Akaogi, Mol. Simul. 6 (1991) 238] are presented for non-electronic properties.     

Diffusion of Li ion on graphene: A DFT study

Density functional theory investigations show that the Li⁺ ion is stabilized at the center of hexagonal carbon ring with the distance of 1.84 Å from graphene surface. The potential barrier of Li⁺ ion diffusion on the graphene surface, about 0.32 eV, is much lower than that of Li⁺ ion penetrating the carbon ring which is 10.68 eV. When a vacancy of graphene exists, potential barrier about 10.25 eV for Li⁺ ion penetrating the defect is still high, and the ability of the vacancy to sizing the Li⁺ ion is also observed. Electronic densities of states show that the formation of a localized bond between Li atom and edge carbon of vacancy is the main reason for high potential barrier when Li⁺ ion penetrate a vacancy. While Coulomb repulsion is the control factor for high potential barrier in case of Li⁺ ion penetrating a carbon ring.     

Usage of pattern recognition scheme in kinetic Monte Carlo simulations: Application to cluster diffusion on Cu(1 1 1)

We have invoked a simple pattern recognition scheme in kinetic Monte Carlo simulations of post-deposition evolution of two dimensional islands on fcc(1 1 1) surfaces. On application of the technique to the diffusion of small Cu clusters (8-100 atoms) on Cu(1 1 1) we find that, at room temperature, clusters with certain magic numbers show stick-slip type of motion with striking patterns rather than the random paths followed by the others. At higher temperatures all clusters display random motion. The calculated diffusion coefficients show dependence on size and temperature with an effective barrier ranging between 0.62 eV and 0.84 eV. Small asymmetries in diffusion barriers lead to a large difference in the frequencies of adatom diffusion along the two types of micro-facetted steps on Cu(1 1 1) leading to consequences in their shape evolution. The pattern recognition scheme revealed 49 basic periphery single atom diffusion processes whose activation energy barriers were calculated using the nudged elastic band technique and interatomic potentials from the embedded atom method.     

Topology and numerical simulation of phase separation in sodium silicate glasses

Liquid/liquid phase separation in glasses may lead to heterogeneities in the nanometer scale. The droplet phase can be nucleated from a homogeneous liquid. The chemical composition of the nanoscale phase separation changes with temperature in contrast to the nanocrystallization. The understanding of topological changes in the glass networks is of importance for the changes in viscosity and the microscopic changes in the growth rate during the course of the phase separation process. This work considers a glass system in which one of the separated phases is much more rigid than the other and the formed new phase possesses a lower viscosity in comparison to the matrix phase. The chemical composition of the matrix changes only in a thin layer around the growing droplets. A shell with increased rigidity is formed which decelerates the growth by encapsulation.     

Core-level photoelectron study of indium chains on Si(1 1 1) at 10 K

High-resolution core-level data from the prototypical In/Si(1 1 1) system have been acquired at 10 K. An asymmetric tail in the In 4d spectra reveals a metallic character of the low temperature Si(1 1 1)8 × 2 phase confined to the inner indium rows. The decoupling of the one-dimensional inner indium chains from any metallic environment at ∼10 K suggests a possible Luttinger liquid behavior. At room temperature essentially a broadening of the spectral features is noticed, which appears compatible with a fluctuation scenario.     

Sr2CeO4:Eu and Sr2CeO4:5 mol% Eu, 3 mol% Dy microphosphors: Wet chemistry synthesis from hybrid precursor and photoluminescence properties

In this article, Sr₂CeO₄:x mol% Eu³⁺ and Sr₂CeO₄:5 mol%Eu³⁺, 3 mol% Dy³⁺ phosphors were synthesized from assembling hybrid precursors by wet chemical method. As-prepared samples present uniform grain-like morphology and the particle size is about 0.2 μm. The luminescence spectra of Sr₂CeO₄:x mol% Eu³⁺ have been measured to examine the influence of the intensity of red emission lines for Eu³⁺ on the concentration of Eu³⁺, showing that the intensity of the red emission increases with an increase of the concentration from 1 to 5 mol%. Additionally, from the emission spectra of Sr₂CeO₄:5 mol%Eu³⁺, 3 mol% Dy³⁺ phosphors, the characteristic lines of Dy³⁺ have also been observed. This result indicates that there also exists an energy transfer process between Sr₂CeO₄ and Dy³⁺.