Atomic Diameter Research Articles

Elastic properties of degenerate f.c.c. crystal of polydisperse soft dimers at zero temperature

Elastic properties of soft, three-dimensional dimers, interacting through site-site n-inverse-power potential, are determined by computer simulations at zero temperature. The degenerate crystal of dimers exhibiting (Gaussian) size distribution of atomic diameters – i.e. size polydispersity – is studied at the molecular number density 1 / 2 ; the distance between centers of atoms forming dimers is considered as a length unit. It is shown that, at the fixed number density of the dimers, increasing polydispersity causes, typically, an increase in pressure, elastic constants and Poisson's ratio; the latter is positive in most directions. A direction is found, however, in which the size polydispersity causes a substantial decrease in Poisson's ratio, down to negative values for large n. Thus, the system is partially auxetic for large polydispersity and large n.

Viscosity kernel of molecular fluids: Butane and polymer melts

The wave-vector dependent shear viscosities for butane and freely jointed chains have been determined. The transverse momentum density and stress autocorrelation functions have been determined by equilibrium molecular dynamics in both atomic and molecular hydrodynamic formalisms. The density, temperature, and chain length dependencies of the reciprocal and real-space viscosity kernels are presented. We find that the density has a major effect on the shape of the kernel. The temperature range and chain lengths considered here have by contrast less impact on the overall normalized shape. Functional forms that fit the wave-vector-dependent kernel data over a large density and wave-vector range have also been tested. Finally, a structural normalization of the kernels in physical space is considered. Overall, the real-space viscosity kernel has a width of roughly 3-6 atomic diameters, which means that generalized hydrodynamics must be applied in predicting the flow properties of molecular fluids on length scales where the strain rate varies sufficiently in the order of these dimensions (e.g., nanofluidic flows).

General trends in the structural, electronic and elastic properties of the M 3AlC 2 phases (M = transition metal): A first-principle study

In this paper, the first-principles pseudopotential total energy method is used to predict the structural, electronic and elastic properties of the M 3AlC 2 (MAX) phases, where M = 3 d, 4 d, and 5 d early transition metals. Specifically, the effects of the valence electron concentrations (VEC) of Ti, V, Cr, Zr, Nb, Mo, Hf, Ta and W were examined. The lattice constants are a linear function of the atomic diameter of the M element. In general, M d–Al p hybridizations locate just below the Fermi level and are weaker than the M d–C p bonds, which are deeper in energy. The bulk moduli of the ternary carbides are found to be proportional to the bulk moduli of the corresponding binary carbides. Because the M–Al bonds are less stiff than the M–C bonds, the latter are mainly responsible for the high bulk moduli of the M 3AlC 2 phases. The M–Al bonds, on the other hand, play a critical role in decreasing the bulk moduli compared to the binary carbides.

Lyapunov spectra and conjugate-pairing rule for confined atomic fluids

In this work we present nonequilibrium molecular dynamics simulation results for the Lyapunov spectra of atomic fluids confined in narrow channels of the order of a few atomic diameters. We show the effect that realistic walls have on the Lyapunov spectra. All the degrees of freedom of the confined system have been considered. Two different types of flow have been simulated: planar Couette flow and planar Poiseuille flow. Several studies exist on the former for homogeneous flows, so a direct comparison with previous results is performed. An important outcome of this work is the demonstration of how the spectrum reflects the presence of two different dynamics in the system: one for the unthermostatted fluid atoms and the other one for the thermostatted and tethered wall atoms. In particular the Lyapunov spectrum of the whole system does not satisfy the conjugate-pairing rule. Two regions are instead distinguishable, one with negative pairs' sum and one with a sum close to zero. To locate the different contributions to the spectrum of the system, we computed "approximate" Lyapunov exponents belonging to the phase space generated by the thermostatted area and the unthermostatted area alone. To achieve this, we evolved Lyapunov vectors projected into a reduced dimensional phase space. We finally observe that the phase-space compression due to the thermostat remains confined into the wall region and does not significantly affect the purely Newtonian fluid region.

An extended analysis of the viscosity kernel for monatomic and diatomic fluids

We present an extended analysis of the wavevector dependent shear viscosity of monatomicand diatomic (liquid chlorine) fluids over a wide range of wavevectors and for a variety ofstate points. The analysis is based on equilibrium molecular dynamics simulations, whichinvolve the evaluation of transverse momentum density and shear stress autocorrelationfunctions. For liquid chlorine we present the results in both atomic and molecularformalisms. We find that the viscosity kernel of chlorine in the atomic representation isstatistically indistinguishable from that in the molecular representation. The results furthersuggest that the real space viscosity kernels of monatomic and diatomic fluidsdepend sensitively on the density, the potential energy function and the choice offitting function in reciprocal space. It is also shown that the reciprocal spaceshear viscosity data can be fitted to two different simple functional forms overthe entire density, temperature and wavevector range: a function composed ofn-Gaussian terms and a Lorentzian-type function. Overall, the real space viscosity kernel hasa width of 3–6 atomic diameters, which means that the generalized hydrodynamicconstitutive relation is required for fluids with strain rates that vary nonlinearly overdistances of the order of atomic dimensions.

Modeling the thermodynamic properties of bimetallic nanosolids

A model has been developed to account for size, shape, surface segregation, composition and dimension dependent cohesive energy of bimetallic nanosolids, and further been extended to predict the size dependent thermodynamic properties, such as melting temperature, Curie temperatures, ordering temperature and phase diagram. The cohesive energy, melting temperature, Curie temperatures and ordering temperature of bimetallic nanosolids decrease with decreasing the particle size. The depression is dramatic in the lower range of size, while it becomes smoothly in large size. For nano phase diagram, the solidus and liquidus curves drop and the two-phase zones become small, as the size of the nanosolids decreases. The two-phase zones of the nano phase are always lower than the regions indicated in the bulk Ag–Pd alloy phase diagram, and they may deteriorate into a curve at a critical size. It is also found that the thermodynamic properties of nanosolids not only depend on the compositions, the atomic diameter and the cohesive energy of each component, but also depend on the size and the shape. The model predictions are consistent with the corresponding simulation, semi-empirical model and experimental data.

Diffusion mechanisms on amorphous silicon surfaces

Molecular dynamics simulations reveal a new picture for diffusion dynamics on the surface of amorphous silicon. The primary transport mechanism differs substantially from that observed on crystalline surfaces, in which small numbers of highly mobile adatoms or vacancies form and subsequently diffuse over substantial distances. Instead, diffusion takes place via large numbers of single atoms or dimers that move roughly one atomic diameter and are associated with highly strained rings having four atoms.

High‐Yield Synthesis of Ultrathin Metal Nanowires in Carbon Nanotubes

Filling the tube: Ultrathin metal nanowires with diameters of single atoms (ca. 1.7 nm) were synthesized in high yield by using a nanofilling reaction using the nanospace of carbon nanotubes (CNTs). As the nanowires are protected by the wall of the CNTs, they resist oxidation and structural disintegration even under ambient conditions.

First-principles study of the layering at the free liquid Sn surface

Molecular-dynamics simulations of the free surface of liquid Sn have been performed using first-principles methods. The ionic density profile shows a stratification extending several atomic diameters into the bulk. The calculated reflectivity shows a marked maximum at a wave-vector transfer of the order of the inverse nearest-neighbor distance and whose origin is related to the surface layering. Moreover, we also find another weak broad maximum at much smaller wave-vector transfers. We analyze and discuss the origin of this anomalous feature, which is also exhibited by the experimental data.

Surface Decoration at the Atomic Scale Using a Molecular Pattern: Copper Adsorption on Cyanide-Modified Pt(111) Electrodes

We report on copper adsorption through a self-ordered molecular pattern, namely cyanide-modified Pt(111), in a attempt to develop a procedure with which periodic metallic nanostructured deposits could be fabricated. We show that copper (either in ionic or metallic state) adsorbs on the surface of a cyanide-modified Pt(111) electrode following exactly the molecular pattern. Our work presents two novelties: (i) the attempt to use a molecular pattern to govern the deposition of a metal, and not the adsorption of an organic or organometallic molecule or supramolecule, and (ii) the line width of the pattern, of only one atomic diameter.

Characteristics of Cu–Zr Thin Film Metallic Glasses Fabricated Using a Carousel-Type Sputtering System

We evaluated the characteristics of Cu–Zr thin film metallic glasses (TFMGs) fabricated using a carousel-type sputtering system. In this system, it is easy to synthesize samples and control their alloy composition by controlling the radio-frequency power for each target independently. In the present study, we demonstrated the fabrication of Cu–Zr TFMGs and evaluated their thermal properties. Cu–Zr samples of various compositions showed a glass transition; however, the thermal properties of the Cu–Zr TFMGs depended on the sputter rate of each element. At the least, the sputter rate of each element during a single rotation of the substrate was set to less than the respective atomic diameter in order to obtain Cu–Zr TFMGs with a stable glass transition.

Simultaneous measurements of the release of atomic sodium, particle diameter and particle temperature for a single burning coal particle

The temporal history of the release of volatile alkali species during coal combustion is a significant, but poorly understood factor in the fouling and corrosion of heat transfer surfaces within industrial coal-fired boilers. We present new results of the simultaneous measurement of particle temperature, particle size and the atomic sodium concentration in the plume of a burning coal particle. During the char phase, the sodium concentration in the plume was found to be linearly dependent on the inverse of particle diameter, but during the ash phase the sodium concentration was found to decay exponentially with decreasing particle temperature. The centreline decay of Na within the plume above the burning particle consists of one region controlled by a first order chemical reaction and a second region controlled by diffusion.

Size-Dependent Thermodynamic Properties of Metallic Nanowires

Analytical models for size-dependent melting temperature Tm(D), melting enthalpy DeltaHm(D), and surface energy gammasv(D) of metallic nanowires have been proposed in terms of the unified nanothermodynamical model where D denotes the diameter of nanowire. As D decreases, Tm(D), DeltaHm(D), and gammasv(D) functions are found to decrease almost with the same size-dependent trend. Due to the inclusion of the effect of dimensionality, the developed model can be applied to other low-dimensional systems. It is found that the ratio of depression of these thermodynamic parameters for spherical nanoparticle, nanowire, and thin film is 3:2:1 when D is large enough (>20h with h being the atomic diameter). The validity of the model is verified by the data of experiments, molecular dynamics simulations, and other theoretical models.

One-dimensional model of strained epitaxy of a binary alloy

We discuss the self-assembly of monatomic chains composed of atoms of two kinds on a periodic substrate. We assume that there exists a small positive misfit between the average atomic diameter of the deposited atoms and the substrate lattice parameter. This may cause chain size calibration. Our model calculations show that if the interatomic interactions are of ordering type, at low temperature the distribution of chain lengths at thermal equilibrium is more strongly peaked around the optimum length than in the case of chains composed from identical atoms.

RNA polymerase II activity is located on the surface of protein-rich transcription factories

We used electron spectroscopic imaging to map nucleoplasmic transcription sites in human cells at unprecedented resolution. HeLa cells were permeabilised, nascent transcripts were extended in BrUTP by approximately 40 nucleotides and the resulting BrRNA immunolabelled with gold particles before structures were viewed. Nascent RNA is almost invariably associated with polymorphic and nitrogen-rich (but phosphorus-poor) structures with a diameter of approximately 87 nm and mass of 10 MDa (calculated by reference to nucleosomes with known numbers of phosphorus and nitrogen atoms). Structures with similar atomic signatures and diameters were observed using correlative microscopy and in unpermeabilised cells. Our results are consistent with RNA synthesis occurring on the surface of these huge protein-rich transcription factories.

Enhancing the Hydrophobic Effect in Confined Water Nanodrops

The distribution of hydrophobic solutes, such as methane, enclosed in a nanosized water droplet contained in a reverse micelle of diameter 2.82 nm is investigated using Monte Carlo simulations. The effect of the hydrophobic solute's atomic diameter on the solute-solute potential of mean force is also studied. The study reveals that confinement has a strong influence on the solute's tendency to associate. The potential of mean force exhibits only a single minimum, indicating that the contact pair is the only stable configuration between solutes. The solvent-separated pair that is universally observed for small solutes in bulk water is conspicuously absent. This enhanced hydrophobic effect is attributed to the lack of sufficient water to completely hydrate and stabilize the solvent-separated configurations. The study is expected to be important in understanding the role of hydrophobic forces during protein folding and nucleation under confinement.

Interatomic potential-based semiclassical theory for Lennard-Jones fluids

An interatomic potential based semiclassical theory is proposed to predict the concentration and potential profiles of a Lennard-Jones (LJ) fluid confined in a channel. The inputs to the semiclassical formulation are the LJ parameters of the fluid and the wall, the density of channel wall atoms, and the average concentration of the fluid inside the channel. Using the semiclassical formulation, fluid confinement in channel with widths ranging from 2sigma ff to 100sigma ff, where sigma ff is the fluid-fluid LJ distance parameter, is investigated. The concentration and potential predicted by the semiclassical formulation are found to be in good agreement with those from equilibrium molecular dynamics simulations. While atomistic simulations in large channels are computationally expensive, the proposed semiclassical formulation can rapidly and accurately predict the concentration and potential profiles. The proposed semiclassical theory is thus a robust and fast method to predict the interfacial and "bulk" fluid phenomena in channels with widths ranging from the macroscale down to the scale of a few atomic diameters.

Longitudinal fields in interactions between atoms and absorptive dielectrics

The properties of atoms close to an absorptive dielectric are studied using the phenomenological Maxwell's equations. Although radiative decay has been considered by many authors, the coupling of atoms with longitudinal modes does not seem to have been treated in detail. Here we show that there are two main effects. The first is a change in the atomic interaction potential from the Coulomb one to a static potential, i.e., one that satisfies a Poisson equation featuring the static dielectric function ${\ensuremath{\epsilon}}_{\mathrm{stat}}=\ensuremath{\epsilon}(\ensuremath{\omega}){\ensuremath{\mid}}_{\ensuremath{\omega}=0}$. The second is the decay of excited atomic states through longitudinal field interactions. We find that the corresponding decay constant is nonzero only for atom-dielectric distances in the order of an atomic diameter and that it decreases exponentially fast on an atomic scale with increasing distance. We also show that the Hamiltonian used by the Jena group [Dung et al., Phys. Rev. A 65, 043813 (2002)], featuring the Coulomb potential, is unitarily equivalent to one containing the static potential.

Parameterization of the nonlocal viscosity kernel for an atomic fluid

In this paper we present results for the wave-vector dependent shear viscosity for a model atomic fluid with short ranged repulsive interactions computed by molecular dynamics simulations. It is shown that the data can be fitted to two different simple functional forms over a large density range, namely, a function composed of two Gaussian terms and a Lorentzian type function with a variable wave-vector exponent. The parameters of both functional forms are found to obey simple density dependencies. While the first functional form has the advantage that the inverse Fourier transform can be found analytically, the Lorentzian type function fits the wave-vector dependence better over the range of wave vectors and densities studied here. The results show that the real space viscosity kernel has a width of 2 to 3 atomic diameters. This means that the generalized hydrodynamic constitutive relation is required if the strain rate varies significantly over this distance, a situation commonly encountered for nanofluidic flows.

A Survey of Control Issues in Nanopositioning

Nanotechnology is the science of understanding matter and the control of matter at dimensions of 100 nm or less. Encompassing nanoscale science, engineering, and technology, nanotechnology involves imaging, measuring, modeling, and manipulation of matter at this level of precision. An important aspect of research in nanotechnology involves precision control and manipulation of devices and materials at a nanoscale, i.e., nanopositioning. Nanopositioners are precision mechatronic systems designed to move objects over a small range with a resolution down to a fraction of an atomic diameter. The desired attributes of a nanopositioner are extremely high resolution, accuracy, stability, and fast response. The key to successful nanopositioning is accurate position sensing and feedback control of the motion. This paper presents an overview of nanopositioning technologies and devices emphasizing the key role of advanced control techniques in improving precision, accuracy, and speed of operation of these systems.