Number Of Atomic Planes Research Articles

Specifics of Thermal Transport in Graphene Composites: Effect of Lateral Dimensions of Graphene Fillers.

We report on the investigation of thermal transport in noncured silicone composites with graphene fillers of different lateral dimensions. Graphene fillers are comprised of few-layer graphene flakes with lateral sizes in the range from 400 to 1200 nm and the number of atomic planes from 1 to ∼100. The distribution of the lateral dimensions and thicknesses of graphene fillers has been determined via atomic force microscopy statistics. It was found that in the examined range of the lateral dimensions, the thermal conductivity of the composites increases with increasing size of the graphene fillers. The observed difference in thermal properties can be related to the average gray phonon mean free path in graphene, which has been estimated to be around ∼800 nm at room temperature. The thermal contact resistance of composites with graphene fillers of 1200 nm lateral dimensions was also smaller than that of composites with graphene fillers of 400 nm lateral dimensions. The effects of the filler loading fraction and the filler size on the thermal conductivity of the composites were rationalized within the Kanari model. The obtained results are important for the optimization of graphene fillers for applications in thermal interface materials for heat removal from high-power-density electronics.

Measurements of Strain and Bandgap of Coherently Epitaxially Grown Wurtzite InAsP-InP Core-Shell Nanowires.

We report on experimental determination of the strain and bandgap of InAsP in epitaxially grown InAsP-InP core-shell nanowires. The core-shell nanowires are grown via metal-organic vapor phase epitaxy. The as-grown nanowires are characterized by transmission electron microscopy, X-ray diffraction, micro-photoluminescence (μPL) spectroscopy, and micro-Raman (μ-Raman) spectroscopy measurements. We observe that the core-shell nanowires are of wurtzite (WZ) crystal phase and are coherently strained with the core and the shell having the same number of atomic planes in each nanowire. We determine the predominantly uniaxial strains formed in the core-shell nanowires along the nanowire growth axis and demonstrate that the strains can be described using an analytical expression. The bandgap energies in the strained WZ InAsP core materials are extracted from the μPL measurements of individual core-shell nanowires. The coherently strained core-shell nanowires demonstrated in this work offer the potentials for use in constructing novel optoelectronic devices and for development of piezoelectric photovoltaic devices.

Electronic band structure of carbon honeycombs

In this article, we present the electronic properties of various carbon honeycombs (C-honeycombs) with different cell sizes and junction types based on ab initio calculations. Among the three typical C-honeycombs with junctions connected via the zigzag edge, the armchair edge, or the hybrid edges of graphene nanoribbons, it is found that the zigzag C-honeycombs and the hybrid ones exhibit metallic properties. However, the armchair C-honeycombs are metallic only if the number of atomic planes N of the graphene nanoribbon in the C-honeycomb sidewall follows N = 3p+1, with p as an integer and are semiconducting for other N. For those semiconducting ones, the bandgap of the C-honeycomb is about 1.19 eV with the narrowest sidewall (5.2 Å) and monotonically decreases to about 0.15 eV when the width is increased to 34.8 Å. The semiconducting nature of such C-honeycombs with nanometer size pores may be used for lightweight semiconductor in electronic devices or supercapacitor energy storage. As a comparison, we show that all boron honeycombs are metallic, and their electronic properties are independent of sidewall widths.

Investigation of mechanical strength of 2D nanoscale structures using a molecular dynamics based computational intelligence approach

A molecular dynamics (MD) based computational intelligence (CI) approach is proposed to investigate the Young modulus of two graphene sheets: Armchair and Zigzag. In this approach, the effect of aspect ratio, the temperature, the number of atomic planes and the vacancy defects on the Young modulus of two graphene sheets are first analyzed using the MD simulation. The data obtained using the MD simulation is then fed into the paradigm of a CI cluster comprising of genetic programming, which was specifically designed to formulate the explicit relationship of Young modulus of two graphene structures. We find that the MD-based-CI model is able to model the Young modulus of two graphene structures very well, which compiles in good agreement with that of experimental results obtained from the literature. Additionally, we also conducted sensitivity and parametric analysis and found that the number of defects has the most dominating influence on the Young modulus of two graphene structures.

Application of artificial intelligence technique for modelling elastic properties of 2D nanoscale material

A novel computational approach is proposed to investigate the shear modulus of graphene nanostructures. In this approach, the factors that affect the shear modulus of graphene structures are analysed using an integrated artificial intelligence (AI) cluster comprising molecular dynamics (MD) and gene expression programming. The MD-based-AI approach has the ability to formulate the explicit relationship of shear modulus graphene nanostructure with respect to aspect ratio, temperature, number of atomic planes and vacancy defects. In addition, the shear modulus of graphene predicted using an integrated MD-based-AI model is in good agreement with that of experimental results obtained from the literature. The sensitivity and parametric analysis were further conducted to find out specific influence and variation of each of the input system parameters on the shear modulus of two graphene structures. It was found that the number of defects has the most dominating influence on the shear modulus of graphene nanostructure.

Combined CI-MD approach in formulation of engineering moduli of single layer graphene sheet

An evolutionary approach of multi-gene genetic programming (GP) is used to study the effects of aspect ratio, temperature, number of atomic planes and vacancy defects on the engineering moduli viz. tensile and shear modulus of single layer graphene sheet. MD simulation based on REBO potential is used to obtain the engineering moduli. This data is then fed into the paradigm of a GP cluster comprising of genetic programming, which was specifically designed to formulate the explicit relationship of engineering moduli of graphene sheets loaded in armchair and zigzag directions with respect to aspect ratio, temperature, number of atomic planes and vacancy defects. We find that our MGGP model is able to model the engineering moduli of armchair and zigzag oriented graphene sheets well in agreement with that of experimental results. We also conducted sensitivity and parametric analysis to find out specific influence and variation of each of the input system parameters on the engineering moduli of armchair and zigzag graphene sheets. It was found that the number of defects has the most dominating influence on the engineering moduli of graphene sheets.

Two-Dimensional Geometry Control of Graphene Nanoflakes Produced by Thermal Plasma for Catalyst Applications

The control of nanoparticle synthesis using thermal plasmas is difficult and often leads to problems of chemical and structural purity, and poor process robustness in terms of consistency of product from run to run. Good reactor design allowed to overcome these issues and to develop a new material based on graphene with a flake-like structure (labeled graphene nanoflakes, GNF) supporting nitrogen for catalytic applications, for example as platinum replacement in fuel cells. These structures showed not only to be active, but also stable in polymer electrolyte fuel cell operation. Characterization of these structures, in situ fuel cell studies and modeling analysis all indicate that achievement of stability relates on the crystalline two-dimensional graphene structure. This paper first reviews the basic aspects behind the structural objectives, describes the synthesis process design leading to this crystalline structure, and provides a two-dimensional analysis on the graphitic growth based on fundamental theory and CFD calculations. These calculations indicate that an independent control of the graphene structure thickness (number of atomic planes) and sheet lengths is possible in a thermal plasma reactor.

Nitrogen doping and vacancy effects on the mechanical properties of graphene: A molecular dynamics study

In this Letter, we used classical Molecular Dynamics (MD) simulations to investigate the tensile behavior of graphene. The validity of the proposed MD architecture is verified by comparing the simulation results with the available experimental results. By performing uniaxial tension simulations, we studied the effects of strain rate, chirality, nanoribbons width and number of atomic planes on the mechanical properties of graphene. We particularly investigated the effects of doped nitrogen atoms and point vacancies concentrations on the Youngʼs modulus and tensile strength of graphene. By plotting the deformation process of graphene at various strain levels, the failure behavior is discussed.

Coherent transport in high performance double-spacer magnetic junctions

Abstract Spin-resonant tunneling mediated by quantized interlayer states has been studied using the tight-binding model for a composite magnetic junction with a thin non-magnetic metallic spacer, coherently inserted between the tunnel barrier and one of the ferromagnetic electrodes (F/I/N/F). An explicit analytical formula for the quantum transmission coefficient is derived and used for calculation of tunnel magnetoresistance (TMR) ratio in the Landauer formalism. The existence of strong magnetoresistance peaks is shown either at varying the on-site atomic energy and the width (number of atomic planes) of the N-spacer. The strongest TMR enhancement in the shallow band regime suggests a search for new spacer materials to optimize the device performance.

Magnetization Process in Holmium: Easy Axis Spin Reorientation Induced by the Magnetostrictive Basal Plane Distortion

We report on the change of the easy axis direction in holmium, from the a to the b axis, under the application of a magnetic field in the basal plane. This spin reorientation is observed by measuring the magnetic torque in Ho(n)/Lu(15) superlattices (n and 15 are the number of atomic planes in the Ho and Lu blocks). We also observe that, at the field H0 and temperature at which the reorientation occurs, both axes are easy directions. Based on the fact that the field H0 depends on n in the same way as the field-induced magnetoelastic distortion does, we propose that this spin reorientation originates from the strong field-induced magnetoelastic deformation within the basal plane. The modulation of the alpha strains with sixfold symmetry originates a 12-fold term in the magnetic anisotropy energy.

Enhancement of the shear elastic constant in periodic and quasiperiodic Ta/Al superlattices

The Brillouin light-scattering technique has been exploited in order to reveal surface acoustic phonons in both periodic and Fibonacci quasiperiodic Ta/Al superlattices. In periodic specimens, about 0.5 μm thick, it is found that, as the superlattice period is reduced from 10 to 4 nm, the phase velocity of the Rayleigh acoustic mode exhibits an anomalous increase, corresponding to a 20% enhancement of the effective shear elastic constant c44. In 2k-component and 3k-component Fibonacci quasiperiodic superlattices, with total thickness of about 1.5 μm, an enhancement of c44 similar to that of periodic specimens has been evidenced, which is connected to the average density of the interfaces, rather than to the quasiperiodicity. This enhancement can be attributed to the presence of interfaces extended over a number of atomic planes, due to the mutual interdiffusion between Ta and Al, as inferred by x-ray and electron microscopy experiments.

Surface magnons in magnetic superlattices

Bulk and surface magnons (spin-waves) in Heisenberg ferromagnetic semi-infinite superlattices are considered theoretically within the interface response theory. Each layer is modelled by a simple-cubic lattice of spins coupled via nearest-neighbour exchange interactions. The magnons have been calculated numerically for superlattices composed of two or three different ferromagnetic materials. We address the question of surface magnons when the number of atomic planes in the material at the surface is variable.

The chemical and magnetic structures of holmium-yttrium and holmium-lutetium superlattices

We present the results of a study of the chemical and magnetic structures of Ho/Y and Ho/Lu superlattices, all grown by molecular beam epitaxy. By combining the results of high-resolution X-ray diffraction with detailed modelling we show that the superlattices have high crystallographic integrity: the average structural coherence length in the growth direction is ~2000 Å, while the interfaces between the two elements are well defined, extending over approximately four lattice planes. The magnetic structures were determined using neutron scattering techniques. In the case of the Ho/Y superlattices, the Ho 3+ moments form a basal-plane helix at all temperatures in the ordered phase, with an apparent suppression of the low-temperature cone phase. Instead spin-slip structures are formed, including some not observed in bulk Ho. For the Ho/Lu superlattices the magnetic structure was found to depend on n Ho, the number of atomic planes in the Ho block. If n Ho > 20, then the Ho 3+ moments form a basal-plane helix down to the lowest temperatures. For n Ho ≤20, the Ho 3+ moments undergo a phase transition at low temperatures from a helix to a state where they are ferromagnetically coupled within one block. In this phase, successive Ho blocks are antiferromagnetically coupled, and there is a small component of the moment along the c axis.

Subvolume embedding for interfacial electronic structure

We discuss aspects of a new method for interfacial electronic structure calculations which combines full potential accuracy and linear scaling of cpu/memory requirements with the number of atomic planes. The basic method is to partition space into subvolumes, with a separate expansion of the one-electron Green function made within each subvolume. Adjacent subvolumes are coupled via embedding potentials. We comment upon the advantages of the method, and illustrate with an application to the stepped jellium surface.

Full-potential embedding for surfaces and interfaces

The authors extend the surface-embedded Green function technique for calculating the electronic structure of surfaces and interfaces by presenting a method for determining substrate embedding potentials which makes no approximations to the substrate potential. They first present an alternative derivation of the surface-embedded Green function method, to clarify the use of a planar surface in simulating embedding on a more complicated surface and illustrate this with rigorous tests. Considering the case of a region embedded on two surfaces, they determine the conditions under which the resulting Green function may itself be used as a substrate-embedding potential, and thereby derive a procedure for obtaining an embedding potential which makes no approximation to the substrate potential. In the case of a substrate with semi-infinite periodicity this reduces to a self-consistency relation, for which they describe a first-order iterative solution. Finally, a particularly efficiency scheme for obtaining local properties within a surface or interface region is outlined. This constitutes a full-potential solution to the one-electron Schrodinger equation for systems of two-dimensional periodicity, whose calculation time scales linearly with the number of atomic planes.

Theoretical study of the onset of magnetism in (Fe)x/(Ru)x superlattices.

The electronic and magnetic structure of (Fe${)}_{\mathit{x}}$/(Ru${)}_{\mathit{x}}$ (x being the number of atomic planes in each layer) hexagonal-close-packed superlattices has been investigated by self-consistent spin-polarized linear-muffin-tin-orbital--atomic-sphere-approximation calculations for x ranging from 1 to 5. It is shown that for x=1 no magnetic order exists due to the strong hybridization between the Fe and Ru atoms. For larger thicknesses several magnetic phases appear, with a ferromagnetic, antiferromagnetic, or ferrimagnetic ordering within each layer. The total energies of these phases are very close to each other, and the ground state depends upon x. Comparison is made with the available experimental results on (Fe${)}_{\mathit{x}}$/(Ru${)}_{\mathit{y}}$ samples grown by molecular-beam epitaxy.

Interfacial Structure in Silver-Nickel Superlattices

Silver-nickel multilayers have been prepared by cathodic sputtering on glass, silicon and carbon-covered mica substrates kept at 100 K. X-ray diffraction experiments show that highly textured, polycrystalline superlattices are formed. There exists a critical number of atomic planes n c = 3, below which non-crystalline samples are obtained. This critical number can even be lowered in samples with non-equal numbers of Ag and Ni atomic planes. The simulation of the X-ray spectra assuming sharp interfaces and Ag and Ni (111) distances slightly larger than the bulk values leads to a good agreement with the experimental spectra. Transmission x-ray and electron diffraction experiments show that equivalent crystallographic directions are aligned in the Ag and Ni (111) planes, with a noticeable relaxation of the in-plane distances at the interfaces. Preliminary high-resolution electron microscopy results are also presented.

SPIN WAVE SPECTRUM IN HEISENBERG ANTIFERRO-MAGNETIC SUPERLATTICES

By means of linearized Holstein-Primakoff transformation and Green function theory, the spin wave dispersion curves are obtained for superlattices consisting of two kinds of Heisenberg antiferromagnetic layers. It is interesting that the results are different in cases that a super-cell contains odd or even number of atomic planes, implying differences in physical properties.

Integer monolayer metallic superlattices

Abstract We report the fabrication of metallic superlattices modulated by integer number of atomic planes of Mo and Ta. The set of superlattices reported here retain structural coherence and exhibit metallic resistivity behavior over the entire range of wavelengths down to the monolayer limit. These results contrast with structural and transport behavior observed in other metal-metal superlattices previously studied.

Diffraction from modulated structures II. Quasiperiodic model of the modulated structures

The present model of modulated structure assumes the variation of the number of atomic planes in the inner part of the individual zones in their array. This variation is described by the interzone correlation function. The analytical expression for the diffracted intensity from this aperiodic array of zones is also given.