Molecular dynamics simulation of size and strain rate dependent mechanical response of FCC metallic nanowires

Effects of size and shape of hole defects on mechanical properties of biphenylene: a molecular dynamics study

The distinctive multi-ring structure and remarkable electrical characteristics of biphenylene render it a material of considerable interest, notably for its prospective utilization as an anode material in lithium-ion batteries. However, understanding the mechanical traits of biphenylene is essential for its application, particularly due to the volumetric fluctuations resulting from lithium ion insertion and extraction during charging and discharging cycles. In this regard, this study investigates the performance of pristine biphenylene and materials embedded with various types of hole defects under uniaxial tension utilizing molecular dynamics simulations. Specifically, from the stress‒strain curves, we obtained key mechanical properties, including toughness, strength, Young's modulus and fracture strain. It was observed that various near-circular hole (including circular, square, hexagonal, and octagonal) defects result in remarkably similar properties. A more quantitative scaling analysis revealed that, in comparison with the exact shape of the defect, the area of the defect is more critical for determining the mechanical properties of biphenylene. Our finding might be beneficial to the defect engineering of two-dimensional materials.

Surface Effect on Young's Modulus of Sub-Two-Nanometer Gold [111] Nanocontacts

The surface bond nature of face centered cubic metals has been controversial between hardening and softening theoretically because of the lack of precise measurement. Here, we precisely measured the size dependence of Young's modulus of gold [111] nanocontacts with a clean surface by our in situ TEM-frequency modulation force sensing method in ultrahigh vacuum at room temperature. Young's modulus gradually decreased from ca. 80 to 30 GPa, as the nanocontact width decreased below 2 nm, which could be explained by surface softening; Young's modulus of the outermost atomic layer was estimated to be approximately 22 GPa, while that of the other part was almost the same with the bulk.

Strain controlled fatigue response of large-scale perfect and defect nickel nanowires: A molecular dynamics study

In this study, the response of the nickel nanowire (NW) subjected to constant strain amplitudes in the range of 0.02-0.12 during cyclic deformation is investigated. The studies are carried out using molecular dynamics (MD) simulations on perfect and defect nickel NW containing ∼ 925,965 atoms. Embedded atom method (EAM) potential is used to model the interactions between nickel atoms. Initially, tensile test is conducted to determine the yield stress and yield strain of the NW at strain rate of 10⁹ s^-1 and temperature of 300 K. The yield stress is observed to be 14 GPa and the corresponding strain is 0.17. The cyclic deformation tests are carried out at a temperature of 300 K. During the cyclic deformation at strain amplitudes of 0.02 and 0.04 the plastic deformation features such as slip lines or slip bands do not appear on the surface of the NW's even after 1000 cycles of straining. These features surface when the strain amplitudes are above 0.04 and their density increase with the strain amplitude and cycles. The NWs exhibit asymmetric stress-strain (compression-tension) hysteresis loops at all the strain amplitudes and up to 1000 cycles. The compressive stress is observed to be higher than the tensile stress for all the cycles. The defect nickel NWs have also exhibited similar behavior as that observed in perfect NWs. Further, it is observed that the width of the loops increases with an increase in the strain amplitude and also with the number of cycles due to softening. The NWs did not fracture even after cyclic deformation for 1000 cycles which could be due to the lower strain amplitudes employed in the present study. Cylindrical shape NWs of different sizes are also investigated by subjecting to similar cyclic deformation conditions as above and are found to show similar behavior.

A quantitative criterion for predicting solid-state disordering during high strain rate deformation

A quantitative criterion for predicting the onset of disordering during high strain rate deformation is defined that is based on the potential energy (PE) per atom (PE/atom). The criterion is a necessary, but not sufficient condition to predict disorder. The stress state and loading direction of the crystal must allow deviatoric displacements that can induce disordering and the strain rate must be sufficiently high. The criterion is tested using molecular dynamics (MD) simulations for Ag over a range of a stress states and loading directions relative to the crystal axis. It is found that, above a minimum PE per atom of -2.70 ± 0.01 eV/atom, the crystal becomes unstable and disorders at temperatures well below the equilibrium melting temperature. This criterion is found to be independent of stress state and loading direction, and results suggest that it can be applied broadly to other material systems and to scenarios where deformation is non-uniform and time dependent. An example is given for its application to Au in shear. We show that the minimum critical PE for disordering under high strain rate loading is estimated by finding the equilibrium PE per atom at melting, which can be obtained from a single MD simulation for each material. An example is provided that illustrates how PE/atom can be used to predict where a simulated system is with respect to the disordering threshold without conducting multiple simulations.

Synthesis and modelling of the mechanical properties of Ag, Au and Cu nanowires

The recent interest to nanotechnology aims not only at device miniaturisation, but also at understanding the effects of quantised structure in materials of reduced dimensions, which exhibit different properties from their bulk counterparts. In particular, quantised metal nanowires made of silver, gold or copper have attracted much attention owing to their unique intrinsic and extrinsic length-dependent mechanical properties. Here we review the current state of art and developments in these nanowires from synthesis to mechanical properties, which make them leading contenders for next-generation nanoelectromechanical systems. We also present theories of interatomic interaction in metallic nanowires, as well as challenges in their synthesis and simulation.

Growth of single-crystalline cubic structured tin(ii) sulfide (SnS) nanowires by chemical vapor deposition

SnS nanowires with high aspect-ratio were developed by chemical vapor deposition and their physical and chemical properties were explored, along with their field effect transistor characteristics.

Tensile properties of a boron/nitrogen-doped carbon nanotube-graphene hybrid structure

Doping is an effective approach that allows for the intrinsic modification of the electrical and chemical properties of nanomaterials. Recently, a graphene and carbon nanotube hybrid structure (GNHS) has been reported, which extends the excellent properties of carbon-based materials to three dimensions. In this paper, we carried out a first-time investigation on the tensile properties of the hybrid structures with different dopants. It is found that with the presence of dopants, the hybrid structures usually exhibit lower yield strength, Young's modulus, and earlier yielding compared to that of a pristine hybrid structure. For dopant concentrations below 2.5% no significant reduction of Young's modulus or yield strength could be observed. For all considered samples, the failure is found to initiate at the region where the nanotubes and graphene sheets are connected. After failure, monatomic chains are normally observed around the failure region. Dangling graphene layers without the separation of a residual CNT wall are found to adhere to each other after failure with a distance of about 3.4 Å. This study provides a fundamental understanding of the tensile properties of the doped graphene-nanotube hybrid structures, which will benefit the design and also the applications of graphene-based hybrid materials.

Probing the Statistical Validity of the Ductile-to-Brittle Transition in Metallic Nanowires Using GPU Computing

We perform a large-scale statistical analysis (>2000 independent simulations) of the elongation and rupture of gold nanowires, probing the validity and scope of the recently proposed ductile-to-brittle transition that occurs with increasing nanowire length [Wu et al. Nano Lett. 2012, 12, 910-914]. To facilitate a high-throughput simulation approach, we implement the second-moment approximation to the tight-binding (TB-SMA) potential within HOOMD-Blue, a molecular dynamics package which runs on massively parallel graphics processing units (GPUs). In a statistical sense, we find that the nanowires obey the ductile-to-brittle model quite well; however, we observe several unexpected features from the simulations that build on our understanding of the ductile-to-brittle transition. First, occasional failure behavior is observed that qualitatively differs from that predicted by the model prediction; this is attributed to stochastic thermal motion of the Au atoms and occurs at temperatures as low as 10 K. In addition, we also find that the ductile-to-brittle model, which was developed using classical dislocation theory, holds for nanowires as small as 3 nm in diameter. Finally, we demonstrate that the nanowire critical length is higher at 298 K relative to 10 K, a result that is not predicted by the ductile-to-brittle model. These results offer practical design strategies for adjusting nanowire failure and structure and also demonstrate that GPU computing is an excellent tool for studies requiring a large number of independent trajectories in order to fully characterize a system's behavior.

Length-dependent mechanical properties of gold nanowires

The well-known "size effect" is not only related to the diameter but also to the length of the small volume materials. It is unfortunate that the length effect on the mechanical behavior of nanowires is rarely explored in contrast to the intensive studies of the diameter effect. The present paper pays attention to the length-dependent mechanical properties of 〈111〉-oriented single crystal gold nanowires employing the large-scale molecular dynamics simulation. It is discovered that the ultrashort Au nanowires exhibit a new deformation and failure regime-high elongation and high strength. The constrained dislocation nucleation and transient dislocation slipping are observed as the dominant mechanism for such unique combination of high strength and high elongation. A mechanical model based on image force theory is developed to provide an insight to dislocation nucleation and capture the yield strength and nucleation site of first partial dislocation indicated by simulation results. Increasing the length of the nanowires, the ductile-to-brittle transition is confirmed. And the new explanation is suggested in the predict model of this transition. Inspired by the superior properties, a new approach to strengthen and toughen nanowires-hard/soft/hard sandwich structured nanowires is suggested. A preliminary evidence from the molecular dynamics simulation corroborates the present opinion.

Can amorphization take place in nanoscale interconnects?

The trend of miniaturization has highlighted the problems of heat dissipation and electromigration in nanoelectronic device interconnects, but not amorphization. While amorphization is known to be a high pressure and/or temperature phenomenon, we argue that defect density is the key factor, while temperature and pressure are only the means. For nanoscale interconnects carrying modest current density, large vacancy concentrations may be generated without the necessity of high temperature or pressure due to the large fraction of grain boundaries and triple points. To investigate this hypothesis, we performed in situ transmission electron microscope (TEM) experiments on 200 nm thick (80 nm average grain size) aluminum specimens. Electron diffraction patterns indicate partial amorphization at modest current density of about 10(5) A cm(-2), which is too low to trigger electromigration. Since amorphization results in drastic decrease in mechanical ductility as well as electrical and thermal conductivity, further increase in current density to about 7 × 10(5) A cm(-2) resulted in brittle fracture failure. Our molecular dynamics (MD) simulations predict the formation of amorphous regions in response to large mechanical stresses (due to nanoscale grain size) and excess vacancies at the cathode side of the thin films. The findings of this study suggest that amorphization can precede electromigration and thereby play a vital role in the reliability of micro/nanoelectronic devices.

Thermal stabilization of thin gold nanowires by surfactant-coating: a molecular dynamics study

The thermal stabilization of thin gold nanowires with a diameter of about 2 nm by surfactants is investigated by means of classical molecular dynamics simulations. While the well-known melting point depression leads to a much lower melting of gold nanowires compared to bulk gold, coating the nanowires with surfactants can reverse this, given that the attractive interaction between surfactant molecules and gold atoms lies beyond a certain threshold. It is found that the melting process of coated nanowires is dominated by surface instability patterns, whereas the melting behaviour of gold nanowires in a vacuum is dominated by the greater mobility of atoms with lower coordination numbers that are located at edges and corners. The suppression of the melting by surfactants is explained by the isotropic pressure acting on the gold surface (due to the attractive interaction) which successfully suppresses large-amplitude thermal motions of the gold atoms.

Role of polytetrahedral structures in the elongation and rupture of gold nanowires

We report comprehensive high-accuracy molecular dynamics simulations using the ReaxFF force field to explore the structural changes that occur as Au nanowires are elongated, establishing trends as a function of both temperature and nanowire diameter. Our simulations and subsequent quantitative structural analysis reveal that polytetrahedral structures (e.g., icosahedra) form within the "amorphous" neck regions, most prominently for systems with small diameter at high temperature. We demonstrate that the formation of polytetrahedra diminishes the conductance quantization as compared to systems without this structural motif. We demonstrate that use of the ReaxFF force field, fitted to high-accuracy first-principles calculations of Au, combines the accuracy of quantum calculations with the speed of semiempirical methods.

Shock-induced breaking of the nanowire with the dependence of crystallographic orientation and strain rate

The failure of the metallic nanowire has raised concerns due to its applied reliability in nanoelectromechanical system. In this article, the breaking failure is studied for the [100], [110], and [111] single-crystal copper nanowires at different strain rates. The statistical breaking position distributions of the nanowires have been investigated to give the effects of strain rate and crystallographic orientation on micro-atomic fluctuation in the symmetric stretching of the nanowires. When the strain rate is less than 0.26% ps-1, macro-breaking position distributions exhibit the anisotropy of micro-atomic fluctuation. However, when the strain rate is larger than 3.54% ps-1, the anisotropy is not obvious because of strong symmetric shocks.

Shock-induced breaking in the gold nanowire with the influence of defects and strain rates

Defects in metallic nanowires have raised concerns about the applied reliability of the nanowires in nanoelectromechanical systems. In this paper, molecular dynamics simulations are used to study the deformation and breaking failure of the [100] single-crystal gold nanowires containing defects at different strain rates. The statistical breaking position distributions of the nanowires show mechanical shocks play a critical role in the deformation of nanowires at different strain rates, and deformation mechanism of the nanowire containing defects is based on a competition between shocks and defects in the deformation process of the nanowire. At low strain rate of 1.0% ps(-1), defect ratio of 2% has changed the deformation mechanism because micro-atomic fluctuation is in an equilibrium state. However, owing to strong symmetric shocks, the sensitivity of defects is not obvious before a defect ratio of 25% at high strain rate of 5.0% ps(-1).

Solid-solid phase transitions in Fe nanowires induced by axial strain

By means of classical molecular-dynamics simulations we investigate the solid-solid phase transition from a bcc to a close-packed crystal structure in cylindrical iron nanowires, induced by axial strain. The interatomic potential employed has been shown to be capable of describing the martensite-austenite phase transition in iron. We study the stress versus strain curves for different temperatures and show that for a range of temperatures it is possible to induce a solid-solid phase transition by axial strain before the elasticity is lost; these transition temperatures are below the bulk transition temperature. The two phases have different (non-linear) elastic behavior: the bcc phase softens, while the close-packed phase stiffens with temperature. We also consider the reversibility of the transformation in the elastic regimes, and the role of the strain rate on the critical strain necessary for phase transition.

Numerical investigations into the tensile behavior of TiO(2) nanowires: structural deformation, mechanical properties, and size effects

The mechanisms governing the tensile behavior of TiO(2) nanowires were studied by molecular dynamics simulations. Nanowires below a threshold diameter of about 10 A transformed into a completely disordered structure after thermodynamic equilibration, whereas thicker nanowires retained their crystalline core. Initial elastic tensile deformation was effected by the reconfiguration of surface atoms while larger elongations resulted in continuous cycles of Ti-O bond straightening, bond breakage, inner atomic distortion, and necking until rupture. Nanowires have much better mechanical properties than bulk TiO(2). Nanowires below the threshold diameter exhibit extraordinarily high stiffness and toughness and are more sensitive to strain rate.

Analogous mechanical behaviors in [Formula: see text] and [Formula: see text] directions of Cu nanowires under tension and compression at a high strain rate

This study analyzes the plastic deformation on the atomic scale of Cu nanowires (NWs) with [Formula: see text] and [Formula: see text] orientations during uniaxial tension and compression, using a molecular dynamic simulation. The maximum local stress (MLS) method is employed to evaluate mechanical behavior during deformation. Following yielding, the flow stress strongly depends on the variation in the degree of orientation caused by twinning. Both the tension of the [Formula: see text] NW and the compression of the [Formula: see text] NW cause twin deformation and consequent geometrical softening. In contrast, the compression of the [Formula: see text] NW and the tension of the [Formula: see text] NW form twin bands and cause geometrical hardening. These behaviors result in the stress-strain curves that reveal the pseudo-skew-symmetry characteristic. With respect to the difference between the critical resolved shear stress (τ(c)) associated with the distinct orientations, τ(c) depends strongly on the surface critical resolved stress (τ(sc)). Under tension, τ(sc) depends on the degree of lattice distortion. A larger lattice distortion (pre-tensile stress) corresponds to higher τ(sc). However, under compression, a geometrical factor can be used to describe the difference in τ(sc) between the different orientations. A larger geometrical factor corresponds to a larger τ(sc).

Where, and how, does a nanowire break?

Using molecular dynamics (MD) simulation, we studied the structural transformation and breaking mechanism of a single crystalline copper nanowire under continuous strain. At a certain strain rate, an ensemble of relaxed initial states of the nanowire can preferentially go through one or more paths of deformation. In each deformation path, disordered atoms can be generated at the specific positions of the nanowire, where necking and breaking take place afterward. Such a breaking position is not predetermined; multiple initial states lead to a strain-rate-dependent, statistical distribution of breaking positions.

Shock-induced localized amorphization in metallic nanorods with strain-rate-dependent characteristics

Classical force-field simulations were used to study the tensile shock-induced response of metallic nanorods. The nanorods were found to amorphize at strain-rate amorphization limits of 1.6% ps(-1) for a 4.0-nm-diameter platinum (Pt) nanorod and 1.2% ps(-1) for a 4.0-nm-diameter gold (Au) nanorod, with a length-to-diameter ratio of 3. Highly localized necking was observed at mid-depth of the nanorods at about 2.0% ps(-1). As the strain rates were increased further, the neck bifurcated away from the mid-depth toward the nanorod ends, resulting in multiple necks at 4.0% ps(-1) and 3.0% ps(-1) for Pt and Au, respectively. At strain rates of above 6.0% ps(-1), end delamination took place. The longitudinal wave propagation equation was adequate to explain and predict the location of the appearance of necks along the nanorod.