Effects of pores and temperature on mechanics of gold nanowires using molecular dynamics

The deformation mechanisms of gold nanowires with different nanopores under tension were simulated by molecular dynamics (MD). The stress–strain curves varied from different porous defects, and the tension caused dislocations to take place and slip along plane (1 1 1). Moreover, the tensile strength of the nanoporous monocrystalline gold was decreased when the simulated temperature increased. The stress concentrations factors of porous nanowires were calculated, and it was found that there was a great influence of size and model effects on the stress concentration factors.     

Diverse nanowires activated self-scrolling of graphene nanoribbons

Diverse nanowires (NWs) activating the self-scrolling of planar graphene (GN) nanoribbons have been studied by using molecular dynamics (MD) simulations. Once the NWs’ radiuses reach a threshold, all the seven NWs, acting as an external force, can initiate the conformational change of the GN nanoribbons, and finally form the core/shell composite NWs. Our simulation found that van der Waals (vdW) force plays an important role in the process of forming core/shell composite NWs. This preparation method of the core/shell composite NWs will open a further development of a broad new class of metal/GN core/shell composite NWs with enhanced properties. And these core/shell structures can be the building blocks of functional nanodevices with unique mechanical, electrical, or optical properties.     

Formation of silicon oxide nanowires directly from Au/Si and Pd–Au/Si substrates

Amorphous silicon oxide (SiO_x) nanowires were directly grown by thermal processing of Si substrates. Au and Pd–Au thin films with thicknesses of 3 nm deposited on Si (0 0 1) substrates were used as catalysts for the growth of nanowires. High-yield synthesis of SiO_x nanowires was achieved by a simple heating process (1000–1150 °C) in an Ar ambient atmosphere without introducing any additional Si source materials. The as-synthesized products were characterized by field-emission scanning electron microscopy, energy-dispersive X-ray spectroscopy, and transmission electron microscopy measurements. The SiO_x nanowires with lengths of a few and tens of micrometers had an amorphous crystal structure. The solid–liquid–solid model of nanowire formation was shown to be valid.     

Size and temperature dependence of the tensile mechanical properties of zinc blende CdSe nanowires

The effect of size and temperature on the tensile mechanical properties of zinc blende CdSe nanowires is investigated by all atoms molecular dynamic simulation. We found the ultimate tensile strength and Young?s modulus will decrease as the temperature and size of the nanowire increase. The size and temperature dependence are mainly attributed to surface effect and thermally elongation effect. High reversibility of tensile behavior will make zinc blende CdSe nanowires suitable for building efficient nanodevices.     

C60、C240、C60@C240富勒烯分子压缩特性的分子动力学研究

采用Tersoff-Brenner势的分子动力学方法,研究了双石墨层作用下C60、C240及C60@C240富勒烯分子的压缩力学特性.根据计算结果,讨论了三种分子压缩过程中几何构形、能量、压缩载荷等的变化及其差异.研究表明,压缩过程中,仅C240分子出现了“塌陷“现象,塌陷时,该分子的能量及外载一度下降;相同压缩应变下,C240的体积压缩率以及C60@C240的能量吸收率最大,而C60的体积压缩率及能量吸收率均最小; C60@C240分子的最大承载能力及C240的最大承受变形能力最大,而C60分子的最大承载和最大承受变形能力均最小;在C60@C240分子的压缩中,当应变小于20%时,内笼C60的体积及其能量变化很小;C60与C240之间的范德华尔能在整个压缩C60@C240分子的能量变化中仅仅占有非常小的份额.     

Control of nucleation site density of GaN nanowires

The control of nucleation site size and density for Au catalyst-driven growth of GaN nanowires is reported. By using initial Au film thicknesses of 15-50 Å we have shown that annealing between 300 and 900 °C creates Au cluster size in the range 30-100 nm diameter with a cluster density from 300 to 3500 μm⁻².Conventional optical lithography to create parallel Au stripes shoes that a minimum separation of ∼15 μm is needed to avoid overlap of wires onto neighboring lines with our growth conditions that yield wires of this same length. The GaN nanowires exhibit strong band-edge photoluminescence and total resistances of 1.2 × 10⁸-5.5 × 10⁶ Ω in the temperature range from 240 to 400 K, as determined for the temperature-dependent current-voltage characteristics.     

Large-scale quantum mechanical simulations of carbon nanowires

The stability of quasi-one-dimensional structures of carbon is investigated using a generalized tight-binding molecular-dynamics scheme. Large-scale simulations are made possible by the parallel implementation of the diagonalization routines. Our results show that these structures can be stable provided that their geometries consist of a core of four-fold coordinated atoms, surrounded by a three-fold coordinated outer surface accommodating one of the most stable reconstructions of bulk diamond structure.     

Energetic and structural evolution of gold nanowire under heating process: A molecular dynamics study

The energetic and structural evolution of a squared gold nanowire under heating process is investigated via molecular dynamics with many-body potential. The simulations reveal that the nanowire undergoes distinct energetic and structural developments during the following four heating processes: low temperature, melting, breaking and high temperature. The cross-section of nanowire is found to change from a square to a circle shape with rising temperature at first. A neck is then found to be initiated above the overall melting point, followed by the formation of a two- to five-atom-thick chain structure before the breaking of neck. The nanowire transforms to a spherical cluster after the final breaking.     

An atomistic assessment of the impact of flaw orientation on the elastic and failure behavior of single-crystal Si nanometre-thick slabs

Abstract

Failure of nanoscale Si thin films was examined using molecular dynamics (MD) simulations that employed the modified embedded atom method (MEAM) interatomic potential. Specifically, nanometre-thick slabs of different crystallographic orientations containing asymmetric, high aspect ratio surface flaws were subjected to uniaxial tensile strains with the strain applied perpendicular to the flaw major axis. The ensuing elastic response and failure behaviour were examined as a function of variation in crystallographic orientation relative to the surface flaw. For certain flaw orientations, crack propagation was accompanied by slip along preferred directions, while in other cases, failure was purely brittle. In addition, a significant dependence of the computed elastic constants and yield stress, on the relative orientation of the surface flaw was observed. This work offers new insights into the role of surface flaws on the mechanical failure of silicon-based, nanoscale, engineered structures.     

Microstructural and magnetic properties of CoPt nanowires

In this work, the magnetic and microstructural properties of CoPt nanowires are presented as a function of the electrolyte pH and current density during electrodeposition into anodized alumina templates. CoPt nanowires of high aspect ratio have been prepared using electrolyte pH values in the range from 2 to 6. The as-made samples exhibit a face centered cubic (fcc) structure with soft magnetic properties which transform into the face centered tetragonal (fct) L10 phase after thermal treatment. Different pH values of the electrolyte during electrodeposition lead to significantly different microstructures and, therefore, different magnetic properties. The CoPt nanowires prepared at high pH value are composed of fcc nanorods of about 25 nm in length. Thermal annealing of these samples leads to a preferred (0 0 1) orientation (along the direction perpendicular to the direction of nanowires) which increases with annealing time. On the other hand, the CoPt nanowires prepared at lower pH value are composed of uniform fcc nanograins with the size ∼2−3 nm. Magnetization curves for the later sample are virtually identical in both directions indicating an isotropic behavior.     

Strain rate effects on compressive behavior of covalently bonded CNT networks

In this study, strain rate effects on the compressive mechanical properties of randomly structured carbon nanotube (CNT) networks were examined. For this purpose, three-dimensional atomistic models of CNT networks with covalently-bonded junctions were generated. After that, molecular dynamics (MD) simulations of compressive loading were performed at five different strain rates to investigate the basic deformation characteristic mechanisms of CNT networks and determine the effect of strain rate on stress–strain curves. The simulation results showed that the strain rate of compressive loading increases, so that a higher resistance of specimens to deformation is observed. Furthermore, the local deformation characteristics of CNT segments, which are mainly driven by bending and buckling modes, and their prevalence are strongly affected by the deformation rate. It was also observed that CNT networks have superior features to metal foams such as metal matrix syntactic foams (MMSFs) and porous sintered fiber metals (PSFMs) in terms of energy absorbing capabilities.     

Synthesis and magnetic properties of Mn doped CuO nanowires

Study of diluted magnetic semiconductor nanowires is one of the important topics in materials science. By using Mn-Cu alloy as the starting material, Mn doped CuO nanowire arrays have been synthesized in air at the temperature of 550 °C. X-ray diffraction measurements and scanning electron microscopic study shows that the nanowires were grown on Cu₂O substrate. Transmission electron microscopic study shows the single crystal property of the nanowires. Magnetic measurements show ferromagnetic property in the Mn doped CuO nanowires with the critical temperature higher than 80 K.     

托普利兹-循环块相位掩膜矩阵压缩成像

在托普利兹和循环矩阵的基础上,提出一种新的托普利兹-循环块相位掩膜矩阵可压缩双透镜成像方法.模拟实验结果表明:新的相位掩膜矩阵压缩成像可以在显著减少测量的同时,有效地捕获图像信息来重建原始图像|新相位掩膜矩阵的研究为确定性测量在压缩成像领域的应用提供了更多的支撑,在拥有原托普利兹和循环确定性测量优点的同时,还拥有自身的块结构特点,可以进一步减少物理实现成本,为新的照相机的设计提供若干理论、计算和技术支撑.     

First principles calculations of mechanical properties of 4,4'-bipyridine attached to Au nanowires

A first attempt is made to calculate the forces involved in the breaking of nanowires consisting of a molecule attached to nanosized metallic pieces. As a model system, we consider different Au nanowires connected by a 4,4(')-bipyridine or pyrazine molecule, for which density functional calculations were performed at different elongations. The geometry of the system was optimized for different forces applied. In all cases the calculated maximum forces were close to 1 nN, which is of the order of the experimental values, and smaller than the corresponding to the rupture of the Au-Au chain (1.5-1.6 nN). When 4,4(')-bipyridine is attached to Au monoatomic nanowire, the maximum force required to break the Au-Au bond may be lowered to values close to that obtain to break the Au-N bond, but when 4,4(')-bipyridine is attached to small Au clusters, the breaking of the nanowire takes place at the Au-N bond only.     

Magnetic coupling properties of Mn-doped AlN nanowires: First-principles calculations

Based on first-principles within the framework of the density functional theory, we have studied the magnetic coupling properties of Mn-doped AlN nanowires. By analyzing the results of different Mn-doped AlN nanowires, we found that for the passivated nanowire, ferromagnetic state is more stable, while for the unpassivated nanowire, the favorable state transits into anti-ferromagnetic state, which can be well explained by the band coupling model. The results indicate that the degree of surface passivation of dangling bonds is an important factor in the magnetic properties of doped nanowires.     

Scintillation and optical behavior of GaN nanowires in the presence of low-energy X-ray photons: A Geant4 simulation

GaN nanowires were studied as a possible candidate for use in the future development of X-ray scintillator detectors with high spatial resolution using a Monte Carlo simulation. The assessment was performed from the optical response perspective. The ability of such a nanowire to act as scintillating fibers was simulated using Geant4 code. The optimal dimensions of the nanowire (considering practical limitations) to achieve the best optical guiding effect were also determined. Moreover, the energy response of the nanowire was investigated for the design of a suitable external photodetector. To more accurately study the proposed structure, a scintillating screen containing approximately 2,000,000 nanowires was simulated in a porous anodized alumina membrane (PAAM) with a hexagonal arrangement. Using these specifications, the spatial resolution and efficiency of the detector were precisely calculated. The results showed that the spatial resolution and efficiency of the detector were <1 μm and ∼17%, respectively.     

确定性矩阵可分离压缩成像

针对可分离压缩传感使用的可分离随机正交矩阵在处理大尺度图像等高维信号感知时难度太大或成本过高的问题,引入确定性测量矩阵,提出确定性矩阵可分离压缩传感,可将如托普利兹矩阵及循环矩阵等具有确定性结构的矩阵作为可分离压缩传感的左、右可分离矩阵.该方案可以降低独立元素的数目,从而显著降低前端物理实现的难度与成本.数值模拟实验分别评估了该方法在不同采样率及不同图像尺寸下的压缩重建性能,结果表明该方法在独立元素非常少的情形下得到与原随机正交矩阵相近的重建质量,证明了其可行性.     

托普利兹-循环块相位掩膜矩阵压缩成像

在托普利兹和循环矩阵的基础上,提出一种新的托普利兹-循环块相位掩膜矩阵可压缩双透镜成像方法,模拟实验结果表明:新的相位掩膜矩阵压缩成像可以在显著减少测量的同时,有效地捕获图像信息来重建原始图像;新相位掩膜矩阵的研究为确定性测量在压缩成像领域的应用提供了更多的支撑,在拥有原托普利兹和循环确定性测量优点的同时,还拥有自身的...     

Cross-sectional geometry dependence of spontaneous phase transformation of copper nanowires

Molecular dynamics simulations have been performed to investigate the spontaneous phase transformation of copper nanowires. It is found that the spontaneous phase transformation exhibits distinct dependence on the cross-sectional geometry of the nanowires and can lead to the reconstruction of atoms into different atomistic configurations, e.g., pure hexagonal-close-packed crystals, fivefold deformation twins, and core/shell structures. For single-crystal copper nanowires, the critical cross-sectional size, above which no spontaneous phase transformation can occur, is determined. The physical mechanisms underlying the complicated transformation behavior are analyzed from the viewpoints of energy and stresses.