Molecular dynamics simulations of electrowetting in double-walled carbon nanotubes

On the basis of the atomistic simulations of electrowetting in single-walled carbon nanotubes, electrowetting of double-walled carbon nanotubes by mercury is studied using classical molecular dynamics simulations. Wetting of double-walled carbon nanotubes by mercury occurs above a threshold size of inner tube when the voltage is applied on the outer tube, but no wetting phenomenon appears when the voltage is applied on the inner tube. The filling rate increases greatly with enlarging the inner tube size. The space between the two walls of double-walled carbon nanotubes cannot be filled by mercury during electrowetting process.     

Molecular dynamics simulations of diffusion of carbon into iron

Molecular dynamics (MD) simulations of diffusion couple tests were conducted between carbon (diamond/graphite) and iron at three different temperatures (300, 800 and 1600 K) and contact times (0, 40 and 80 ps) to investigate the chemical interaction between carbon and iron. Two different carbon structures, namely, diamond (cubic) and graphite (hexagonal), were considered. Diffusion of carbon into iron was observed only when a graphite interlayer was added to the diamond surface. When diamond alone was used, no diffusion was observed. This result provides corroborating evidence that diamond tool wear in the machining of iron occurs via a mechanism that involves an initial graphitization of diamond followed by diffusion of the newly formed graphite into the iron workpiece.     

The far-infrared dispersion of acetonitrile in dilute solution in carbon tetrachloride

The results of refraction and absorption measurements using dispersive Fourier transform techniques are reported for dilute solutions of acetonitrile in carbon tetrachloride in the spectral region 2–200 cm⁻¹. The results are used to help to interpret the power absorption spectra in this region in terms of a model for dipole rotation in liquids developed by Evans et al.^1,2     

Thermal conductivity of multi-walled carbon nanotubes:Molecular dynamics simulations

Heat conduction in single-walled carbon nanotubes(SWCNTs) has been investigated by using various methods, while less work has been focused on multi-walled carbon nanotubes(MWCNTs). The thermal conductivities of the double-walled carbon nanotubes(DWCNTs) with two different temperature control methods are studied by using molecular dynamics(MD) simulations. One case is that the heat baths(HBs) are imposed only on the outer wall, while the other is that the HBs are imposed on both the two walls. The results show that the ratio of the thermal conductivity of DWCNTs in the first case to that in the second case is inversely proportional to the ratio of the cross-sectional area of the DWCNT to that of its outer wall. In order to interpret the results and explore the heat conduction mechanisms, the inter-wall thermal transport of DWCNTs is simulated. Analyses of the temperature profiles of a DWCNT and its two walls in the two cases and the interwall thermal resistance show that in the first case heat is almost transported only along the outer wall, while in the second case a DWCNT behaves like parallel heat transport channels in which heat is transported along each wall independently.This gives a good explanation of our results and presents the heat conduction mechanisms of MWCNTs.     

Nuclear quadrupolar relaxation and the dynamics of pairs of atoms in liquids

We have developed a model calculation for the electrical field gradient correlation function on a probe atom in the liquid, c_EFG(t)=20(0)V₂₀(t)>. In this model, symmetry of the liquid is introduced explicitly and the distribution function for therelative coordinate r_i(t) between the probe atom and particle i is calculated using Smoluchowski's diffusion equation with a mean force potential Φ(r)=k_BT In g(r). The results for c_EFG(t) can be characterized by two correlation times, , the shorter one being responsible for the small values of R_Q in pure liquid metals, the longer one producing the increase of R_Q in alloys. Also good agreement is found with recent results for c_efg(t) from molecular dynamics studies.     

Mechanical property of carbon nanotubes with intramolecular junctions: Molecular dynamics simulations

Intramolecular junctions (IMJs) of carbon nanotubes hold a promise of potential applications in nano-electromechanical systems. However, their structure-property relation is still unclear. Using the revised second-generation Tersoff-Brenner potential, molecular dynamics simulations were performed to study the mechanical properties of single-walled to four-walled carbon nanotubes with IMJs under uniaxial tension. The dependence of deformation and failure behaviors of IMJs on the geometric parameters was examined. It was found that the rupture strength of a junction is close to that of its thinner carbon nanotube segment, and the rupture strain and Young's modulus show a significant dependence on its geometry. The simulations also revealed that the damage and rupture of multi-walled carbon nanotube junctions take place first in the innermost layer and then propagate consecutively to the outer layers. This study is helpful for optimal design and safety evaluation of IMJ-based nanoelectronics.     

Molecular dynamics study of the vibrational relaxation of OCl and OCl in the bulk and the surface of water and acetonitrile

The vibrational relaxation of OCl and OCl⁻ in the bulk and the liquid/vapor interface of water and acetonitrile is studied by molecular dynamics computer simulations. Both equilibrium calculations of the vibrational friction and non-equilibrium simulation of the energy relaxation are used to elucidate the factors that influence the rate of energy relaxation in systems that represent polar ionic and non-ionic solutes in polar protic and non-protic solvents. We find that, in agreement with previous experimental and theoretical studies, the relaxation of the ionic solute is much faster than that of the non-ionic solute in both the solvents. However, while the relaxation is slowed down considerably when the non-ionic solute is transferred from the bulk to the interface, no such surface effect is found in the case of the ionic solute. This behavior can be explained by noting that the ionic solute is able to keep its first solvation shell intact upon transfer to the interface and that the main contribution to the friction is due to the Lennard-Jones part of the intermolecular potential.     

Molecular dynamics of half-integer quadrupolar nuclei studied by QCPMG solid-state NMR experiments on static and rotating samples. Theory and simulations

Simulations of QCPMG NMR type experiments have been used to explore dynamic processes of half-integer quadrupolar nuclei in solids. By setting up a theoretical approach that is well suited for efficient numerical simulations the QCPMG type experiments have been analyzed regarding the effect of the magnitude of the EFG- and CSA-tensors, the spin-quantum number, different dynamical processes and MAS. Compared to the QE experiment the QCPMG experiment offers not only intensity gain by an order of magnitude and changes in overall lineshape as a function of the kinetic rate constant but the lineshape of the individual spin-echo sidebands is also very sensitive towards dynamics. Hereby a visual identification of the dynamics is obtained. In common for all the simulations the spin-echo sidebands are narrow in the slow (k< or =10(2) Hz) and the fast (k> or =10(7) Hz) dynamic regime whereas they are broadened in the intermediate regime 10(3)< or =k< or =10(7) Hz. The maximum intensity of the spin-echo sidebands for two-site jumps is highly dependent on the type of anisotropic interactions involved and the type of QCPMG experiment. Hence, in the fast limit the maximum intensity was 140% of the initial intensity when significant CSA was present or under the QCPMG-MAS experiment compared to 89 or 71% for the static experiment influenced by the quadrupolar interaction only. For 3-, 4-, and 6-site jumps the maximum intensity in the fast limit reached up to 339% of the intensity in the static limit.     

Molecular dynamics simulations of plastoquinone in solution

Molecular dynamics simulations of plastoquinone, an important cofactor in the photosynthetic reaction in green plants, are carried out in water solution. Models of both neutral and anionic plastoquinone are built and thoroughly verified. Detailed information concerning spatial distribution of the hydrogen bonds with coordination numbers, together with rotational energetics of the tail in solution are given. The isoprenoid tail was replaced by an ethyl group, which was found to move freely between 0° (cis to the adjacent carbonyl oxygen) and 90° (perpendicular to the quinone ring plane). The results obtained should remove several inconsistencies between earlier experimental and theoretical results to yield a detailed dynamic picture of plastoquinones in solution. Neutral quinones form only a few weak hydrogen bonds to the solvent molecules, while the anionic forms show a distinct solvation structure due to several strong solute-solvent hydrogen bonds. Both the direction and strength of these hydrogen bonds agree well with recent EPR/ENDOR data.     

Temperature dependence of21Ne,83Kr,and131Xe quadrupole relaxation and of xenon diffusion in polyatomic solvents

The main aim of the present work is to contribute, by an extension of the experimental data base, to the understanding of quadrupole relaxation of nuclei of noble gases dissolved in molecular liquids. We have performed temperature dependent spin-lattice relaxation rate measurements of²¹Ne,⁸³Kr, and¹³¹Xe in various non-aqueous solvents (e.g. in methanol, ethanol, n-butanol, acetone, acetonitrile, benzene, carbon tetrachloride, N,N-dimethylformamide, dodecane, tetradecane, p-xylene, and hexafluorobenzene). In nine liquids we also measured translational diffusion coefficients of dissolved xenon as a function of temperature by the NMR spin-echo technique, obtaining partly the very first diffusion data for a number of systems. The comparison of the remarkable low activation energies for the noble gas nuclear quadrupole relaxation with that of the noble gas diffusion reveals that both seem to be closely connected. There are strong hints that the nuclear relaxation process follows an empirical “Dη ^γ =A correlation” found previously by Evans and co-workers for the tracer diffusion of noble gases in polyatomic liquids. For almost all solvents γ decreases from¹³¹Xe to²¹Ne parallel with a decrease of the corresponding activation energy for the quadrupolar relaxation. Possible physical reasons for this behavior are briefly discussed. Essential qualitative results in this paper were found to agree with two MD computer simulations for¹³¹Xe relaxation in benzene and methanol. Further MD simulations are proposed which are obviously required for the deeper understanding of the experimental results found in the present paper.     

Molecular dynamics simulations of EXAFS in germanium

Classical molecular dynamics simulations have been performed for crystalline germanium with the aim to estimate the thermal effects within the first three coordination shells and their influence on the single-scattering and multiple-scattering contributions to the Ge K-edge extended x-ray absorption fine structure (EXAFS).     

Molecular dynamics simulations of compressed hydrogen

Abstract

Molecular dynamics simulations have been performed for highly compressed fluid hydrogen in the density and temperature regime of recent shock-compression experiments. Both density functional and tight-binding electronic structure techniques have been used to describe interatomic forces. Two tight-binding models of hydrogen have been developed with a single s-type orbital on each atom that reproduce properties of the dimer, of various crystalline structures, and of the fluid. The simulations indicate that the rapid rise in the electrical conductivity observed in the gas-gun experiments depends critically on the dissociated atoms (monomers). We find that the internal structure of warm, dense hydrogen has a pronounced time-dependent nature with the continual dissociation of molecules (dimers) and association of atoms (monomers). Finally, Hugoniots derived from the equations-of-state of these models do not exhibit the large compressions predicted by the recent laser experiments.     

Molecular dynamics simulations of carbon dioxide hydrate growth in electrolyte solutions of NaCl and MgCl2

Molecular dynamics simulations are performed to study the growth of carbon dioxide (CO₂) hydrate in electrolyte solutions of NaCl and MgCl₂. The kinetic behaviour of the hydrate growth is examined in terms of cage content, density profile, and mobility of ions and water molecules, and how these properties are influenced by added NaCl and MgCl₂. Our simulation results show that both NaCl and MgCl₂ inhibit the CO₂ hydrate growth. With a same mole concentration or ion density, MgCl₂ exhibits stronger inhibition on the growth of CO₂ hydrate than NaCl does. The growth rate of the CO₂ hydrate in NaCl and MgCl₂ solutions decreases slightly with increasing pressure. During the simulations, the Na⁺, Mg²⁺, and Cl^? ions are mostly excluded by the growing interface front. We find that these ions decrease the mobility of their surrounding water molecules, and thus reduce the opportunity for these water molecules to form cage-like clusters toward hydrate formation. We also note that during the growth processes, several 5¹²6³ cages appear at the hydrate/solution interface, although they are finally transformed to tetrakaidecahedral (5¹²6²) cages. Structural defects consisting of one water molecule trapped in a cage with its hydrogen atoms being attracted by two Cl^? ions have also been observed.     

Molecular dynamics simulations of one-dimensional Lennard-Jones systems

One-dimensional Lennard-Jones systems are investigated by molecular dynamics simulations. The full Lennard-Jones potential is compared to the repulsive Lennard-Jones potential. It is found that the pair correlation function and the normalized velocity autocorrelation function agree at high densities and high temperature. However, the diffusion coefficient indicates that the attractive potential introduces additional correlations into particle dynamics which are not reflected in the statics. These results are in agreement with three-dimensional studies.     

Molecular dynamics simulations of silicon wafer bonding

Molecular dynamics simulations based on a modified Stillinger-Weber potential are used to investigate the elementary steps of bonding two Si(001) wafers. The energy dissipation and thus the dynamic bonding behaviour are controlled by the transfer rates for the kinetic energy. The applicability of the method is demonstrated by studying the interaction of perfect wafer surfaces (UHV conditions). First calculations covering the influence of surface steps, rotational misorientations and adsorbates are presented.     

Molecular dynamics simulations of lanthanum oxide surfaces

Classical molecular dynamics was used to investigate the equilibrium state of the surface region of as-grown La₂O₃. It is currently thought that bulk and epitaxial thin film La₂O₃ surfaces exhibit amorphous structures in the as-prepared state that yield bulk crystal states upon postdeposition annealing. The focus of the study is to determine if the as-prepared surface region of La₂O₃ is purely amorphous as indicated from prior experimental results. Using simulation cells sufficiently large to accommodate the formation of defects, phase segregation, compositional migration, and site defects, our results show that crystalline phases are evident from simulated X-ray diffraction patterns. Although the phase of these crystallites is unresolved, we suggest that combinations of distorted hexagonal, cubic, and nonstoichiometric phases are formed in the as-prepared state prior to annealing. These crystallites likely serve as nucleation site for long-range ordered crystal growth upon annealing.     

A study of the relaxation dynamics in a quadrupolar NMR system using Quantum State Tomography

This article reports a relaxation study in an oriented system containing spin 3/2 nuclei using quantum state tomography (QST). The use of QST allowed evaluating the time evolution of all density matrix elements starting from several initial states. Using an appropriated treatment based on the Redfield theory, the relaxation rate of each density matrix element was measured and the reduced spectral densities that describe the system relaxation were determined. All the experimental data could be well described assuming pure quadrupolar relaxation and reduced spectral densities corresponding to a superposition of slow and fast motions. The data were also analyzed in the context of Quantum Information Processing, where the coherence loss of each qubit of the system was determined using the partial trace operation.