Atomistic continuum modeling of graphene membranes

The paper deals with the modeling of thin, monolayer graphene membranes, which have significant electrical and physical properties used for nano- or micro-devices, such as resonators and nanotransistors. The membrane is considered as a homogenized graphene monolayer on the macroscopic scale, and a continuum–atomistic multiscale approach is exploited, focusing the Tersoff–Brenner (TB) potential for the interaction between the carbonic bonds. The associated Representative atomistic Unit Lattice (RUL) is thereby considered as a micro-scale quasi-continuum placed in context of computational homogenization. In this development, the Cauchy–Born rule (CBN) is extended by the atomic fluctuation to allow for relaxation in the RUL. The paper discusses the handling of the TB-potential, both in the context of macro–micro homogenization, and in the context of numerical implementation perspectives. In particular, explicit expressions of the homogenized membrane forces and stiffness are expressed in terms of the first and second gradient of the potential, with due consideration to the involved “non-local” pairwise interaction in the model. In addition, the detailed resulting macroscopic non-linear and linearized finite element response is formulated in terms of the relaxed lattice level atomistic response. Numerical results are provided for the lattice response in terms of the apparent anisotropic behavior induced by the graphene atomic structure. An assessment of the convergence of RULs with respect to different deformation states of the lattice membrane is also carried out. Finally, a validation of an experiment of a circular graphene membrane, using atomic force microscopy (AFM) measurements, is provided based on standard TB-parameters available in the literature.     

Atomistic modeling of multilayered ceria nanotubes

Ceria has become a very important material for catalytic applications. Many applications take advantage of its high oxygen storage capacity (OSC). We propose a new polycrystalline multilayered nanotube structure that could go some way to further unlocking the oxygen storage capabilities of the material. We illustrate how our simulation models are constructed and further investigate the potential reactivity of the new structure, by comparing predictions of vacancy cluster segregation behavior to that predicted for the most stable flat {111} surface.     

Quantitative modeling of forces in electromagnetic tweezers

This paper discusses numerical simulations of the magnetic field produced by an electromagnet for generation of forces on superparamagnetic microspheres used in manipulation of single molecules or cells. Single molecule force spectroscopy based on magnetic tweezers can be used in applications that require parallel readout of biopolymer stretching or biomolecular binding. The magnetic tweezers exert forces on the surface-immobilized macromolecule by pulling a magnetic bead attached to the free end of the molecule in the direction of the field gradient. In a typical force spectroscopy experiment, the pulling forces can range between subpiconewton to tens of piconewtons. In order to effectively provide such forces, an understanding of the source of the magnetic field is required as the first step in the design of force spectroscopy systems. In this study, we use a numerical technique, the method of auxiliary sources, to investigate the influence of electromagnet geometry and material parameters of the magnetic core on the magnetic forces pulling the target beads in the area of interest. The close proximity of the area of interest to the magnet body results in deviations from intuitive relations between magnet size and pulling force, as well as in the force decay with distance. We discuss the benefits and drawbacks of various geometric modifications affecting the magnitude and spatial distribution of forces achievable with an electromagnet.     

Atomistic to continuum modeling of solidification microstructures

We summarize recent advances in modeling of solidification microstructures using computational methods that bridge atomistic to continuum scales. We first discuss progress in atomistic modeling of equilibrium and non-equilibrium solid–liquid interface properties influencing microstructure formation, as well as interface coalescence phenomena influencing the late stages of solidification. The latter is relevant in the context of hot tearing reviewed in the article by M. Rappaz in this issue. We then discuss progress to model microstructures on a continuum scale using phase-field methods. We focus on selected examples in which modeling of 3D cellular and dendritic microstructures has been directly linked to experimental observations. Finally, we discuss a recently introduced coarse-grained dendritic needle network approach to simulate the formation of well-developed dendritic microstructures. This approach reliably bridges the well-separated scales traditionally simulated by phase-field and grain structure models, hence opening new avenues for quantitative modeling of complex intra- and inter-grain dynamical interactions on a grain scale.     

A review on design, modeling and applications of computer experiments

In this paper, we provide a review of statistical methods that are useful in conducting computer experiments. Our focus is on the task of metamodeling, which is driven by the goal of optimizing a complex system via a deterministic simulation model. However, we also mention the case of a stochastic simulation, and examples of both cases are discussed. The organization of our review first presents several engineering applications, it then describes approaches for the two primary tasks of metamodeling: (i) selecting an experimental design; and (ii) fitting a statistical model. Seven statistical modeling methods are included. Both classical and newer experimental designs are discussed. Finally, our own computational study tests the various metamodeling options on two two-dimensional response surfaces and one ten-dimensional surface.     

Atomistic modeling of defect diffusion and interdiffusion in SiGe heterostructures

In this work we present models and simulation results for diffusion in strained SiGe heterostructures. Our approach makes a comprehensive, physically-based, treatment of defect and dopant diffusion, self-diffusion (of Si and Ge) and interdiffusion, including some effects inherent to heterostructures. The models have been implemented in the DADOS code, within the framework of the atomistic kinetic Monte Carlo approach. The parameter set has been calibrated in the whole composition range, from silicon to germanium, both for relaxed and strained conditions, and an illustrative set of experiments has been simulated.     

Atomistic modeling of materials properties by Monte Carlo Simulation

In order to optimize materials properties, in many cases a deeper understanding of the relationship between the chemical-atomistic structure and the physical properties of the solid and fluid phases of the material is necessary. Monte Carlo simulation is a tool that allows the reliable calculation of thermodynamic properties of strongly interacting many-body condensed matter systems. Given a model of effective interatomic or intermolecular interactions (drawn either from quantum-chemical-type interactions or from analysis of suitable experimental data), macroscopic bulk properties of a material can be simulated, as well as interfacial phenomena and certain kinds of slow dynamic processes (of relaxational or diffusive type). After a brief review of the foundations of this approach in statistical mechanics, the wide potential of this method is illustrated with examples taken from magnetism, metallurgy and amorphous polymeric materials. Strengths and limitations of this atomistic approach towards modeling materials properties are discussed and directions of future research are spelled out.     

Atomistic modeling of crack growth in magnesium single crystal

The analysis of crack growth in magnesium single crystal was performed using molecular dynamics simulation with Embedded Atom Method (EAM) potentials. The twinning process at the crack tip was analyzed. Four specimens with increasing sizes were used to investigate the influences of material length scale on crack growth of magnesium single crystals. Furthermore, the effects of temperature, and the loading strain rate were also verified. The specimens were subjected to uniaxial tension strain up to the total strain level of 0.2 with a constant strain rate. In the simulation of each specimen, the averaged stress strain curve was monitored. The simulation results show that the specimen size, loading strain rate, and temperature strongly influence the peak stress point at which the twin nucleated and subsequently the crack grew. The initial slope of the averaged stress strain curve is independent of the loading strain rate and temperature. Moreover, high temperatures induce increased atomic mobility, and thereby atom reorganization, which, in turn, releases the stress at the crack tip.     

Atomistic design of thermoelectric properties of silicon nanowires

We present predictions of the thermoelectric figure of merit ( ZT) of Si nanowires with diameter up to 3 nm, based upon the Boltzman transport equation and ab initio electronic structure calculations. We find that ZT depends significantly on the wire growth direction and surface reconstruction, and we discuss how these properties can be tuned to select silicon based nanostructures with combined n-type and p-type optimal ZT. Our calculations show that only by reducing the ionic thermal conductivity by about 2 or 3 orders of magnitudes with respect to bulk values, one may attain ZT larger than 1, for 1 or 3 nm wires, respectively. We also find that ZT of p-doped wires is considerably smaller than that of their n-doped counterparts with the same size and geometry.     

Atomistic simulation and modeling of localized shear deformation in metallic glasses

Since the end of 1980s, bulk metallic glasses became available for various multi-component alloys. Because bulk metallic glasses are applicable to structural materials, their mechanical properties have become a matter of great interest in these decades. A characteristic feature of plastic deformation of metallic glasses at the ambient temperature is the localized shear deformation. Since we have no appropriate experimental technique, unlike crystalline matter, to approach microscopic deformation process in amorphous materials, we have to rely on computer simulation studies by use of atomistic models to reveal the microscopic deformation processes. In this article, we review atomistic simulation studies of deformation processes in metallic glasses, i.e., local shear transformation (LST), structural characterization of the local shear transformation zones (STZs), deformation-induced softening, shear band formation and its development, by use of elemental and metal-metal alloy models. We also review representative microscopic models so far proposed for the deformation mechanism: early dislocation model, Spaepen’s free-volume model, Argons’s STZ model and recent two-state STZ models by Langer et al.     

Cryogenic transistor measurement and modeling for engineering applications

This article deals with the methodology of an electronic system design at liquid-helium temperatures. This technique includes the active device selection, characterization and simulation. Based on certain engineering criteria one commercial reference of SiGe heterojunction bipolar transistors is selected. Then, the technique of device characterization and measurement is considered. Typical output characteristics are given for this reference. All the tested devices of this reference are classified into three groups according to the presence of different low-temperature phenomena. An accurate and easy-to-use neural network model based on their experimental DC characteristics is proposed. This model is implemented in Agilent ADS Software, and the simulation results are compared with measurements in the course of the cryogenic amplifier design.     

The development of rational design criteria for brittle materials

Very brittle materials are finding increasing usage in critical engineering components. Howver, their fundamental property of brittle behaviour requires very accurate design methods if reliable applications are to result. In this paper the development of rational design criteria for these materials are described. Three major topics are discussed. The statistical techniques which enable the correlation of experimental data, and the effects of size and stress gradient to be taken into account are described, and the development of these methods to cases of non-uniform stresses through a unit volume concept are assessed. The phenomenon of static fatigue, in which failure occurs, often after a considerable time, under an applied load which is less than that required to produce instantaneous failure is considered. Recent applications of fracture mechanics techniques to the determination of the crack growth rate are described which leads to the development of methods for the estimation of component lifetimes under conditions of subcritical crack growth. Finally, the experimental results obtained from investigations into the fracture strength of various brittle materials when subjected to multiaxial stress states are compared. It is concluded that a great deal of further experimental evidence is required before fully reliable design criteria are available on which engineering decisions may be based.     

Hydrodynamic tweezers: impact of design geometry on flow and microparticle trapping

Here we explore the role of microfabricated device geometry on frequency-dependent low Reynolds number steady streaming flow and particle trapping behavior. In our system, flow and particle trapping is induced near an obstruction or cavity located in an otherwise rectilinear oscillating flow of frequency ω and amplitude s in a fluid of kinematic viscosity ν. This work expands prior studies to characterize nine distinct obstruction/cavity geometries. The imaged microeddy flows show that the device geometry affects the eddy number, shape, structure, and strength. Comparison of measured particle trap locations with the computed eddy flow structure shows that particles trap closer to the wall than the eddy core. Trapping strength and location are controlled by the geometry and the oscillation frequency. In most cases, the trapping behavior is linearly proportional to the Stokes layer thickness, δ(AC) ~ O((ν/ω)(1/2)). We show that steady streaming in microfluidic eddies can be a flexible and versatile method for noncontact microparticle trapping, and hence we call this class of devices "hydrodynamic tweezers".     

Semiconducting glass design for device applications

A method is presented by which stable amorphous semiconductors can be chosen for switching device applications. This method is based on the empirical relationship observed between the glass transition temperature, the band gap and the mean coordination number of covalent amorphous semiconductors. In particular, it is shown that tetrahedrally coordinated glasses with band gaps in the range 0.6–1.2 eV are excellent candidates for high reliability materials. One such material, a-CdAs₂, was studied in some detail and was found to be very stable and very easy to fabricate in thin film form. Both negative resistance and threshold switching devices were successfully fabricated with this material, and preliminary results from accelerated life testing are promising.     

High-efficiency collector design for extreme-ultraviolet and x-ray applications

A design of a two-reflection mirror for nested grazing-incidence optics is proposed in which maximum overall reflectivity is achieved by making the two grazing-incidence angles equal for each ray. The design is proposed mainly for application to nonimaging collector optics for extreme-ultraviolet microlithography where the radiation emitted from a hot plasma source needs to be collected and focused on the illuminator optics. For completeness, the design of a double- reflection mirror with equal reflection angles is also briefly outlined for the case of an object at infinity for possible use in x-ray applications.     

Playing with dimensions: rational design for heteroepitaxial p-n junctions

A design for a heteroepitaxial junction by the way of one-dimensional wurzite on a two-dimensional spinel structure in a low-temperature solution process was introduced, and it's capability was confirmed by successful fabrication of a diode consisting of p-type cobalt oxide (Co(3)O(4)) nanoplate/n-type zinc oxide (ZnO) nanorods, showing reasonable electrical performance. During thermal decomposition, the 30° rotated lattice orientation of Co(3)O(4) nanoplates from the orientation of β-Co(OH)(2) nanoplates was directly observed using high-resolution transmission electron microscopy. The epitaxial relations and the surface stress-induced ZnO nanowire growth on Co(3)O(4) were well supported using the first-principles calculations. Over the large area, (0001) preferred oriented ZnO nanorods epitaxially grown on the (111) plane of Co(3)O(4) nanoplates were experimentally obtained. Using this epitaxial p-n junction, a diode was fabricated. The ideality factor, turn-on voltage, and rectifying ratio of the diode were measured to be 2.38, 2.5 V and 10(4), respectively.     

Atomistic modeling of plastic deformation in B2-FeAl/Al nanolayered composites

In this work, the deformation response of the B2-FeAl/Al intermetallic composites, as a model material system for nanolayered composites comprised of intermetallic interfaces, has been explored. We use atomistic simulations to study the deformation mechanisms and the interface misfit dislocation structure of B2-FeAl/Al nanolayered composites. It is shown that two sets of dislocations are contained in the interface misfit dislocation network and are correlated with the initial dislocation nucleation from the interfaces. The effects of layer thickness on the uniaxial deformation response of the B2-FeAl/Al multilayers are investigated. We observed that under compressive loading the smaller proportion of the FeAl layers leads to the lower overall flow stress. Under tensile loading, the void formation mechanism is investigated, suggesting the interface structure and the dislocation activities in the FeAl layers playing a significant role to trigger the strain localization which leads to void nucleation commencing at the interface. It is also found that the deformation behavior in the “weak” Fe/Cu interface behaves substantially different than that of the “strong” FeAl/Al interface. The atomistic modeling study of the nanolayered composites here underpinned the mechanical response of “strong” intermetallic interface material systems. There is no void nucleation during the entire plastic deformations in the Fe/Cu simulations, which is attributed to much higher dislocation density, more slip systems activated, and relative uniformly distributed dislocation traces in the Fe phase of the Fe/Cu multilayers.