Surface Adatom Diffusion-Assisted Dislocation Nucleation in Metal Nanowires

L. He, G. Cheng, Y. Zhu and H.S. Park
Nano Letters 2023; 23:5779-5784

Abstract

We employ a hybrid diffusion/nucleation-based Kinetic Monte Carlo model to elucidate the significant impact of adatom diffusion on incipient surface dislocation nucleation in metal nanowires. We reveal a stress-regulated diffusion mechanism that promotes preferential accumulation of diffusing adatoms near nucleation sites, which explains the experimental observations of strong temperature but weak strain-rate dependence, as well as temperature-dependent scatter of the nucleation strength. Furthermore, the model demonstrates that decreasing adatom diffusion with increasing strain rate will lead to stress-controlled nucleation being the dominant nucleation mechanism at higher strain rates. Overall, our model offers new mechanistic insights as to how surface adatom diffusion directly impacts the incipient defect nucleation process, and resulting mechanical properties, of metal nanowires.

This paper is available in PDF form .

Exploiting Elastic Buckling of High Strength Gold Nanowire Towards Stable Electrical Probing

J-H Seo, S-G Kang, Y. Cho, H.S. Park, Y. Yoo, B. Kim, I-S Choi and J-P Ahn
iScience 2022; 25:105199

Abstract

Buckling is a loss of structural stability. It occurs in long slender structures or thin plate structures which is subjected to compressive forces. For the structural ma- terials, such a sudden change in shape has been considered to be avoided. In this study, we utilize the Au nanowire’s buckling instability for the electrical measure- ment. We confirmed that the high-strength single crystalline Au nanowire with an aspect ratio of 150 and 230-nm-diameter shows classical Euler buckling under constant compressive force without failure. The buckling instability enables sta- ble contact between the Au nanowire and the substrate without any damage. Clearly, the in situ electrical measurement shows a transition of the contact resis- tance between the nanowire and the substrate from the Sharvin (ballistic limit) mode to the Holm (Ohmic) mode during deformation, enabling reliable electrical measurements. This study suggests Au nanowire probes exhibiting structural instability to ensure stable and precise electrical measurements at the nanoscale.

This paper is available in PDF form .

Negative Thermal Expansion of Ultra-Thin Metal Nanowires: A Computational Study

D.T. Ho, S-Y Kwon, H.S. Park and S.Y. Kim
Nano Letters 2017; 17:5113-5118

Abstract

Most materials expand upon heating because the coefficient of thermal expansion (CTE), the fundamental property of materials characterizing the mechanical response of the materials to heating, is positive. There have been some reports of materials that exhibit negative thermal expansion (NTE), but most of these have been in complex alloys, where NTE originates from the transverse vibrations of the materials. Here, we show using molecular dynamics simulations that some single crystal monatomic FCC metal nanowires can exhibit NTE along the length direction due to a novel thermomechanical coupling. We develop an analytic model for the CTE in nanowires that is a function of the surface stress, elastic modulus, and nanowire size. The model suggests that the CTE of nanowires can be reduced due to elastic softening of the materials and also due to surface stress. For the nanowires, the model predicts that the CTE reduction can lead to NTE if the nanowire Young's modulus is sufficiently reduced while the nanowire surface stress remains sufficiently large, which is in excellent agreement with the molecular dynamics simulation results. Overall, we find a "smaller is smaller" trend for the CTE of nanowires, leading to this unexpected, surface-stress-driven mechanism for NTE in nanoscale materials.

This paper is available in PDF form .

Mechanical Properties of Copper Octet-Truss Nanolattices

Z.Z. He, F.C. Wang, Y.B. Zhu, H.A. Wu and H.S. Park
Journal of the Mechanics and Physics of Solids 2017; 101:133-149

Abstract

We investigate the mechanical properties of copper (Cu) octet-truss nanolattices through a combination of classical molecular dynamics (MD) simulations and theoretical analysis. The MD simulations show that Cu nanolattices with high relative density are stronger than bulk Cu, while also achieving higher strength at a lower relative density as compared to Cu meso-lattices. We demonstrate that modifying the classical octet-truss lattice model by accounting for nodal volume and bending effects through the free body diagram method is critical to obtaining good agreement between the theoretical model and the MD simulations. In particular, we find that as the relative density increases, nodal volume is the key factor governing the stiffness scaling of the nanolattices, while bending dominates the strength scaling. Most surprisingly, our analytic modeling shows that surface effects have little influence on the stiffness and strength scaling of the nanolattices, even though the cross sectional sizes of the nanowires that act as the lattice struts are on the order of 6 nm or smaller. This is because, unlike for individual nanowires, the mechanical response of the nanowire struts that form the nanolattice structure is also a function of bending and nodal volume effects, all of which depend nonlinearly on the nanolattice relative density. Overall, these results imply that nanoscale architected materials can access a new regime of architected material performance by simultaneously achieving ultrahigh strength and low density.

This paper is available in PDF form .

Origin of Size Dependency in Coherent-Twin-Propagation Mediated Tensile Deformation of Noble Metal Nanowires

J-H Seo, H.S. Park, Y. Yoo, T-Y Seong, J. Li, J-P Ahn, B. Kim and I-S Choi
Nano Letters 2013; 13:5112-5116

Abstract

Researchers have recently discovered ultra-strong and ductile behavior of Au nanowires (NWs) through long-ranged coherent-twin-propagation. An elusive but fundamentally important question arises whether the size and surface effects impact the twin propagation behavior with a decreasing diameter. In this work, we demonstrate size dependent strength behavior of ultra-strong and ductile metallic NWs. For Au, Pd, and AuPd NWs, high ductility of about 50% is observed through coherent twin propagation, which occurs by a concurrent reorientation of the bounding surfaces from {111} to {100}. Importantly, the ductility is not reduced with strength increase, while the twin propagation stress dramatically increases with decreasing NW diameter from 250 nm to 40 nm. Furthermore, we find that the power-law exponent describing the twin propagation stress is fundamentally different from the exponent describing the size-dependence of the yield strength. Specifically, the inverse diameter-dependence of the twin propagation stress is directly attributed to surface reorientation, which can be captured by a surface energy differential model. Our work further highlights the fundamental role that surface reorientations play in enhancing the size-dependent mechanical behavior and properties of metal NWs which imply the feasibility of high efficiency mechanical energy storage devices suggested before.

This paper is available in PDF form .

Polar Surface Effects on the Thermal Conductivity in ZnO Nanowires: a Shell-Like Surface Reconstruction-Induced Preserving Mechanism

J-W Jiang, H.S. Park and T. Rabczuk
Nanoscale 2013; 5:11035-11043

Abstract

We perform molecular dynamics (MD) simulations to investigate the effect of polar surfaces on the thermal transport in zinc oxide (ZnO) nanowires. We find that the thermal conductivity in nanowires with free polar (0001) surfaces is much higher than in nanowires that have been stabilized with reduced charges on the polar (0001) surfaces, and also hexagonal nanowires without any transverse polar surfaces, where the reduced charge model has been proposed as a promising stabilization mechanism for the (0001) polar surfaces for ZnO nanowires. From normal mode analysis, we show that the higher thermal conductivity is due to a shell-like reconstruction that occurs for the free polar surfaces. This shell-like reconstruction suppresses twisting motion in the nanowires such that the bending phonon modes are not scattered by the other phonon modes, and leads to substantially higher thermal conductivity in the ZnO nanowire with free polar surfaces. Furthermore, the auto-correlation function of the normal mode coordinate is utilized to extract the phonon lifetime, which leads to a concise explanation for the higher thermal conductivity in ZnO nanowires with free polar surfaces. Our work demonstrates that ZnO nanowires without polar surfaces, which exhibit low thermal conductivity, are more promising candidates for thermoelectric applications than nanowires with polar surfaces (and also high thermal conductivity).

This paper is available in PDF form .

Preserving the Q-Factors of ZnO Nanoresonators via Polar Surface Reconstruction

J-W Jiang, H.S. Park and T. Rabczuk
Nanotechnology 2013; 24:405705

Abstract

We perform molecular dynamics simulations to investigate the effect of polar surfaces on the quality (Q)-factors of zinc oxide (ZnO) nanowire-based nanoresonators. We find that the Q-factors in ZnO nanoresonators with free polar (0001) surfaces is about one order of magnitude higher than in nanoresonators that have been stabilized with reduced charges on the polar (0001) surfaces. From normal mode analysis, we show that the higher Q-factor is due to a shell-like reconstruction that occurs for the free polar surfaces. This shell-like reconstruction suppresses twisting motion in the nanowires such that the mixing of other modes with the resonant mode of oscillation is minimized, and leads to substantially higher Q-factors in the ZnO nanoresonators with free polar surfaces.

This paper is available in PDF form .

A Surface Stacking Fault Energy Approach to Predicting Defect Nucleation in Surface-Dominated Nanostructures

J-W Jiang, A.M. Leach, K. Gall, H.S. Park and T. Rabczuk
Journal of the Mechanics and Physics of Solids 2013; 61:1915-1934

Abstract

We present a surface stacking fault (SSF) energy approach to predicting defect nucleation from the surfaces of surface-dominated nanostructure such as FCC metal nanowires. The approach leads to a criteria that predicts the initial yield mechanism via either slip or twinning depending on whether the unstable twinning energy or unstable slip energy is smaller as determined from the resulting SSF energy curve. The approach is validated through a comparison between the SSF energy calculation and low-temperature classical molecular dynamics simulations of copper nanowires with different axial and transverse surface orientations, and cross sectional geometries. We focus on the effects of the geometric cross section by studying the transition from slip to twinning previously predicted in moving from a square to rectangular cross section for <100>/{100} nanowires, and also for moving from a rhombic to truncated rhombic cross sectional geometry for <110> nanowires. We also provide the important demonstration that the criteria is able to predict the correct deformation mechanism when full dislocation slip is considered concurrently with partial dislocation slip and twinning. This is done in the context of rhombic aluminum nanowires which do not show a tensile reorientation due to full dislocation slip. We show that the SSF energy criteria successfully predicts the initial mode of surface-nucleated plasticity at low temperature, while also discussing the effects of strain and temperature on the applicability of the criterion.

This paper is available in PDF form .

A Harmonic Transition State Theory Model for Defect Initiation in Crystals

T.J. Delph, P. Cao, H.S. Park and J.A. Zimmerman
Modelling and Simulation in Materials Science and Engineering 2013; 21:025010

Abstract

We outline here a model for the initiation of defects in crystals based upon harmonic transition state theory. This model combines a previously developed model for zero-temperature defect initiation with a multi-dimensional harmonic transition state theory model that is capable of accurately predicting the effects of temperature and loading rate upon defect initiation. The model has several features that set it apart from previous efforts along these lines, most notably a straightforward method of determining the energy barrier between adjacent equilibrium states that does not depend upon a priori information concerning the nature of the defect. We apply the model to two examples, triaxial stretching of a perfect FCC crystal and nanoindentation of a gold substrate. Very good agreement is found between the predictions of the model and independent molecular dynamics simulations. Among other things, the model predicts a strong dependence of the defect initiation behavior upon the loading parameter. A very attractive feature of this model is that it is valid for arbitrarily slow loading rates, in particular loading rates achievable in the laboratory, and suffers from none of the limitations in this regard inherent in molecular dynamics simulations.

This paper is available in PDF form .

Beat Phenomena in Metal Nanowires, and their Implications for Resonance-Based Elastic Property Measurements

H.F. Zhan, Y.T. Gu and H.S. Park
Nanoscale 2012; 4:6779-6785

Abstract

The elastic properties of 1D nanostructures such as nanowires are often measured experimentally through actuation of the nanowire at its resonance frequency, and then relating the resonance frequency to the elastic stiffness using elementary beam theory. In the present work, we utilize large scale molecular dynamics simulations to report a novel beat phenomenon in [110] oriented Ag nanowires. The beat phenomenon is found to arise from the asymmetry of the lattice spacing in the orthogonal elementary directions of the [110] nanowire, i.e. the [110] and [001] directions, which results in two different principal moments of inertia. Because of this, actuations imposed along any other direction are found to decompose into two orthogonal vibrational components based on the actuation angle relative to these two elementary directions, with this phenomenon being generalizable to <110> FCC nanowires of different materials (Cu, Au, Ni, Pd and Pt). The beat phenomenon is explained using a discrete moment of inertia model based on the hard sphere assumption, the model is utilized to show that surface effects enhance the beat phenomenon, while the effect is reduced with increasing nanowires cross-sectional size or aspect ratio. Most importantly, due to the existence of the beat phenomena, we demonstrate that in resonance experiments only a single frequency component is expected to be observed, particularly when the damping ratio is relatively large or very small. Furthermore, for a large range of actuation angles, the lower frequency is more likely to be detected than the higher one, which implies that experimental predictions of Young's modulus obtained from resonance may in fact be under predictions. The present study therefore has significant implications for experimental interpretations of Young's modulus as obtained via resonance testing.

This paper is available in PDF form .

On the Importance of Surface Elastic Contributions to the Flexural Rigidity of Nanowires

P.A.T. Olsson and H.S. Park
Journal of the Mechanics and Physics of Solids 2012; 60:2064-2083

Abstract

We present a theoretical model to calculate the flexural rigidity of nanowires from three-dimensional elasticity theory that incorporates the effects of surface stress and surface elasticity. The unique features of the model are that it incorporates, through the second moment, the heterogeneous nature of elasticity across the nanowire cross section, and that it accounts for transverse surface-stress-induced relaxation strains. The model is validated by comparison to benchmark atomistic calculations, existing one-dimensional surface elasticity theories based on the Young-Laplace equation, and also three-dimensional surface elasticity theories that assume homogeneous elastic properties across the nanowire cross section via three examples: surface-stress-induced axial relaxation, resonant properties of unstrained, strained and top-down nanowires, and buckling of nanowires. It is clearly demonstrated that the one-dimensional Young-Laplace models lead to errors of varying degrees for all of the boundary value problems considered because they do not account for transverse surface stress effects, and it is also shown that the Young-Laplace model results from a specific approximation of the proposed formulation. The three-dimensional surface elasticity model of Dingreville et al. (2005) is found to be more accurate than the Young-Laplace model, though both lose accuracy for ultrasmall (< 5 nm diameter) nanowires where the heterogeneous nature of the cross section elasticity becomes important. Overall, the present work demonstrates that continuum mechanics can be utilized to study the elastic and mechanical behavior and properties of ultrasmall nanowires if surface elastic contributions to the heterogeneous flexural rigidity are accounted for.

This paper is available in PDF form .

Nonlocal Instability Analysis of FCC Bulk and (100) Surfaces Under Uniaxial Stretching

G. Yun, P. Cao, J.A. Zimmerman, T.J. Delph and H.S. Park
International Journal of Solids and Structures 2011; 48:3406-3416

Abstract

The objective of this paper is to examine the instability characteristics of both a bulk FCC crystal and a (100) surface of an FCC crystal under uniaxial stretching along a <100> direction using an atomistic-based nonlocal instability criterion. By comparison to benchmark atomistic simulations, we demonstrate that for both the FCC bulk and (100) surface, about 5000-10000 atoms are required in order to obtain an accurate converged value for the instability strain and a converged instability mode. The instability modes are fundamentally different at the surface as compared to the bulk, but in both cases a strong dependence of the instability mode on the number of atoms that are allowed to participate in the instability process is observed. In addition, the nonlocal instability criterion enables us to determine the total number of atoms, and thus the total volume occupied by these atoms, that participate in the defect nucleation process for both cases. We find that this defect participation volume converges as the number of atoms increases for both the bulk and surface, and that the defect participation volume of the surface is smaller than that of the bulk. Overall, the present results demonstrate both the necessity and utility of nonlocal instability criteria in predicting instability and subsequent failure of both bulk and surface-dominated nanomaterials.

This paper is available in PDF form .

Superplastic Deformation of Defect-Free Au Nanowires via Coherent Twin Propagation

J-H Seo, Y. Yoo, N-Y Park, S-W Yoon, H Lee, S Han, S-W Lee, T-Y Seong, S-C Lee, K-B Lee, P-R Cha, H.S. Park, B. Kim and J-P Ahn
Nano Letters 2011; 11:3499-3502

Abstract

We report that defect-free Au nanowires show superplasticity on tensile deformation. Evidences from high-resolution electron microscopes indicated that the plastic deformation proceeds layer-by-layer in an atomically coherent fashion to a long distance. Furthermore, the stress-strain curve provides full interpretation of the deformation. After initial superelastic deformation, the nanowire shows superplastic deformation induced by coherent twin propagation, completely reorientating the crystal from (110) to (100). Uniquely well-disciplined and long-propagating atomic movements deduced here are ascribed to the superb crystallinity as well as the radial confinement of the Au nanowires.

This paper is available in PDF form .

Nanomechanical Resonators and Their Applications in Biological/Chemical Detection: Nanomechanics Principles

K. Eom, H.S. Park, D-S Yoon and T. Kwon
Physics Reports 2011; 503:115-163

Abstract

Recent advances in nanotechnology have led to the development of nano-electro-mechanical systems (NEMS) such as nanomechanical resonators, which have recently received significant attention from the scientific community. This has not only been for their capability for the label-free detection of bio/chemical-molecules at single-molecule (or atomic) resolution for future applications such as the early diagnostics of diseases such as cancer, but also for their unprecedented ability to detect physical quantities such as molecular weight, elastic stiffness, surface stress, and surface elastic stiffness for adsorbed molecules on the surface. Most experimental works on resonator-based molecular detection have been based on the principle that molecular adsorption onto a resonator surface increases the effective mass, and consequently decreases the resonant frequencies of the nanomechanical resonator. However, this principle is insufficient to provide fundamental insights into resonator-based molecular detection at the nanoscale; this is due to recently proposed novel nanoscale detection principles including various effects such as surface effects, nonlinear oscillations, coupled resonance, and stiffness effects. Furthermore, these effects have only recently been incorporated into existing physical models for resonators, and therefore the universal physical principles governing nanoresonator-based detection have not been completely described. Therefore, our objective in this review is to overview the current attempts to understand the underlying mechanisms in nanoresonator-based detection using physical models coupled to computational simulations and/or experiments. Specifically, we will focus on issues of special relevance to the dynamic behavior of nanoresonators and their applications in biological/chemical detection: the resonance behavior of micro/nano-resonators; resonator-based chemical/biological detection; physical models of various nanoresonators such as nanowires, carbon nanotubes, and graphene. We pay particular attention to experimental and computational approaches that have been useful in elucidating the mechanisms underlying the dynamic behavior of resonators across multiple and disparate spatial/length scales, and the resulting insight into resonator-based detection that has been obtained. We additionally provide extensive discussion regarding potentially fruitful future research directions coupling experiments and simulations in order to develop a fundamental understanding of the basic physical principles that govern NEMS and NEMS-based sensing and detection applications.

This paper is available in PDF form .

Atomistic Study of the Buckling of Gold Nanowires

P.A.T. Olsson and H.S. Park
Acta Materialia 2011; 59:3883-3894

Abstract

In this work, we present results from atomistic simulations of gold nanowires under axial compression, with a focus on examining the effects of both axial and surface orientation effects on the buckling behavior. This was accomplished by using molecular statics simulations while considering three different crystallographic systems: <100>/{100}, <100>/{110}, and <110>/{110}{100} with aspect ratios spanning from 20 to 50 and cross sectional dimensions ranging from 2.45 to 5.91 nm. The simulations indicate that there is a deviation from the inverse square length dependence of critical forces predicted from traditional linear elastic Bernoulli-Euler and Timoshenko beam theories, where the nature of the deviation from the perfect inverse square length behavior differs for different crystallographic systems. This variation is found to be strongly correlated to either stiffening or increased compliance of the tangential stiffness due to the influence of non-linear elasticity, which leads to normalized critical forces that decrease with decreasing aspect ratio for the <100>/{100} and <100>/{110} systems, but increase with decreasing aspect ratio for the <110>/{110}{100} system. In contrast, it was found that the critical strains are all lower than their bulk counterparts, and that the critical strains decrease with decreasing cross sectional dimensions; the lower strains may be an effect emanating from the presence of the surfaces, which are all more elastically compliant than the bulk and thus gives rise to a more compliant flexural rigidity.

This paper is available in PDF form .

The Influence of Shear and Rotary Inertia on the Resonant Properties of Gold Nanowires

P.A.T. Olsson, H.S. Park and P.C. Lidstrom
Journal of Applied Physics 2010; 108:104312

Abstract

In a previous publication [P. A. T. Olsson, J. Appl. Phys. 108, 034318 (2010)], molecular dynamics (MD) simulations have been performed to study the resonant properties of gold nanowires. It has been documented in the aforementioned publication that the eigenfrequencies of the fundamental mode follows the continuum mechanically predicted behavior when Bernoulli-Euler beam theory is used, whereas the higher order modes tend to be low in comparison to Bernoulli-Euler beam theory predictions. In this work, we have studied the resonant properties of unstressed and prestressed nanowires to explain why the eigenfrequencies of the fundamental mode follows the behavior predicted by Bernoulli-Euler beam theory while those of higher order modes are low in comparison. This is done by employing Timoshenko beam theory and studying the nanowire deformations for different modes. We find good agreement between the MD results and Timoshenko predictions due to the increasing importance of shearing and rotary inertia for higher order resonant modes. Furthermore, we argue that this type of behavior is merely a geometric effect stemming from low aspect ratio for the considered structures as a converging type of behavior is found when the aspect ratios fall between 15 and 20. Finally, we have found that classical Timoshenko beam theory that neglects nanoscale surface effects is able to, simply through utilization of the size-dependent Young's modulus, capture the dynamic properties of the gold nanowires as calculated through MD.

This paper is available in PDF form .

Mechanics of Crystalline Nanowires

H.S. Park, W. Cai, H.D. Espinosa and H. Huang
MRS Bulletin 2009; 34(3):178-183 (Invited Review Paper).

Abstract

Nanowires are amongst the most exiting one-dimensional nanomaterials because of their unique properties, which result primarily from their chemical composition and large surface area to volume ratio. These properties make them ideal building blocks for the development of next generation electronics, opto-electronics and sensor systems. In this article we focus on the unique mechanical properties of nanowires, which emerge from surface atoms having different electron densities and fewer bonding neighbors than atoms lying within the nanowire bulk. In this respect, atomistic simulations have revealed a plethora of novel surface-driven mechanical behavior and properties, including both increases and decreases in elastic stiffness, phase transformations, shape memory and pseudoelastic effects. This paper reviews such atomistic simulations as well as experimental data of these phenomena, while assessing future challenges and directions.

This paper is available in PDF form .

Utilizing Mechanical Strain to Mitigate the Intrinsic Loss Mechanisms in Oscillating Metal Nanowires

S.Y. Kim and H.S. Park
Physical Review Letters 2008; 101:215502.
(Also selected for publication in the Virtual Journal of Nanoscale Science and Technology, December 8, 2008).

Abstract

We utilize classical molecular dynamics to study energy dissipation (the Q-factors) of doubly clamped copper nanowire nanoresonators undergoing flexural oscillations. We find that the application of tensile strain effectively mitigates both the intrinsic surface and thermal losses, with improvements in Q by a factor of 3 to 10 across a range of operating temperatures. We also find that the nanowire Q-factors are not dependent on surface area to volume ratio, but instead their aspect ratio, and that the Q-factors exhibit a 1/T^{0.70} dependence on the temperature T that is independent of strain.

This paper is available in PDF form .

The Coupled Effects of Geometry and Surface Orientation on the Mechanical Properties of Metal Nanowires

C. Ji and H.S. Park
Nanotechnology 2007; 18:305704

Abstract

We have performed atomistic simulations of the tensile loading of <100> and <110> copper nanowires to investigate the coupled effects of geometry and surface orientation on their mechanical behavior and properties. By varying the nanowire cross section from square to rectangular, nanowires with dominant surface facets are created that exhibit distinct mechanical properties due to the different inelastic deformation mechanisms that are activated. In particular, we find that non-square nanowires generally exhibit lower yield stresses and strains, lower toughness, elevated fracture strains, and a propensity to deform via twinning; we quantify the links between the observed deformation mechanisms due to non-square cross section and the resulting mechanical properties, while illustrating that geometry can be utilized to tailor the mechanical properties of nanowires.

This paper is available in PDF form .

Molecular Dynamics Simulations of Stretched Gold Nanowires - The Relative Utility of Different Semiempirical Potentials

Q. Pu, Y. Leng, L. Tsetseris, H.S. Park, S.T. Pantelides and P.T. Cummings
Journal of Chemical Physics 2007; 126:144707

Abstract

The mechanical elongation of a finite gold nanowire has been studied by molecular dynamics (MD) simulations using different semiempirical potentials for transition metals. These potentials have been widely used to study the mechanical properties of finite metal clusters. Combining with density functional theory (DFT) calculations along several atomic-configuration trajectories predicted by different semiempirical potentials, we conclude that the second-moment approximation of the tight-binding potential (TB-SMA) is the most suitable one to describe the energetics of finite Au clusters. We find that for the selected geometries of Au wires studied in this work, the ductile elongation of Au nanowires along [001] direction predicted by the TB-SMA potential does not depend on temperature in the range of 0.01~298 K. The elongation leads to the formation of monatomic chains, as has been observed experimentally. The calculated force-versus-elongation curve is remarkably consistent with available experimental data.

This paper is available in PDF form .

Characterizing the Elasticity of Hollow Metal Nanowires

C. Ji and H.S. Park
Nanotechnology 2007; 18:115707

Abstract

We have performed atomistic simulations on solid and hollow copper nanowires to quantify the elastic properties of the hollow nanowires (nanoboxes). We analyze variations in the modulus, yield stress and strain for <100> and <110> nanoboxes by varying the amount of bulk material that is removed to create the nanoboxes. We find that while <100> nanoboxes show no improvement in elastic properties as compared to solid <100> nanowires, <110> nanoboxes can show enhanced elastic properties as compared to solid <110> nanowires. The simulations reveal that the elastic properties of the nanoboxes are strongly dependent on the relative strength of the bulk material that has been removed, as well as the the total surface area of the nanoboxes, and indicate the potential of ultralight, high-strength nanomaterials such as nanoboxes.

This paper is available in PDF form .

Surface Composition Effects on Martensitic Phase Transformations in Nickel Aluminum Nanowires

H.S. Park and V. Laohom
Philosophical Magazine 2007; 87:2159-2168.
(Invited paper: Special Issue on Nanowires).

Abstract

Atomistic simulations are utilized to quantify the effects of surface composition on stress-induced B2 to body-centered tetragonal (BCT) martensitic phase transformations in intermetallic nickel aluminum (NiAl) nanowires. The simulations show that the phase transformation is observed in all considered cases, regardless of the material composition of the transverse {100} surfaces of the initially B2 wires. The results indicate that, for <100> oriented B2 wires with {100} transverse surfaces, the {100} orientation and not the material composition of the {100} surfaces is the dominant factor in controlling the ability of NiAl alloys to undergo martensitic phase transformations at nanometer scales.

This paper is available in PDF form .

The Effect of Defects on the Mechanical Behavior of Silver Shape Memory Nanowires

C. Ji and H.S. Park
Journal of Computational and Theoretical Nanoscience 2007; 4:578-587.

Abstract

We present atomistic simulations of the uniaxial tensile deformation of silver shape memory nanowires to investigate the effects of initial defects on the resulting thermomechanical behavior. In particular, the focus of the work is on investigating the unique atomistic deformation mechanisms that are observed during the tensile loading as a result of the initial defects, while correlating that behavior to the measured mechanical properties of the shape memory nanowires. In particular, wires with initial defects show a non-constant stress state during the <110>/{111} to <100>/{100} reorientation due to the presence of multiple propagating twin boundaries, as well as reductions in transformation stresses and strains due to the presence of the initial defects. Under most circumstances, the wires with initial defects still tend to exhibit complete reversibility between the <110>/{111} and <100>/{100} orientations, and thus the shape memory effect. Comparisons are made to defect-free shape memory nanowires to illustrate the relative mechanical performance of each structure.

This paper is available in PDF form .

Geometric Effects on the Inelastic Deformation of Metal Nanowires

C. Ji and H.S. Park
Applied Physics Letters 2006; 89:181916
(Also selected for publication in the Virtual Journal of Nanoscale Science and Technology, Nov. 13, 2006.)

Abstract

This letter addresses the direct effect that geometry has in controlling the mechanisms of inelastic deformation in metal nanowires. By performing atomistic simulations of the tensile deformation of <100>{100} hollow copper nanowires (nanoboxes), we find that the nanoboxes deform in an unexpected twinning-dominated mode; the non-square wall geometries of the nanoboxes biases the deformation by allowing the larger transverse {100} surfaces to reduce their area through twinning by reorienting to a lower energy {111} surface. Additional analyses on solid nanowires with non-square cross sections confirm that geometry can be utilized to engineer the mechanical behavior and properties of nanomaterials.

This paper is available in PDF form .

Deformation of FCC Nanowires by Twinning and Slip

H.S. Park, K. Gall and J.A. Zimmerman
Journal of the Mechanics and Physics of Solids 2006; 54 (9):1862-1881.

Abstract

We present atomistic simulations of the tensile and compressive loading of single crystal FCC nanowires with <100> and <110> orientations to study the propensity of the nanowires to deform via twinning or slip. By studying the deformation characteristics of three FCC materials with disparate stacking fault energies (gold, copper and nickel), we find that the deformation mechanisms in the nanowires are a function of the intrinsic material properties, applied stress state, axial crystallographic orientation and exposed transverse surfaces. The key finding of this work is the first order effect that side surface orientation has on the operant mode of inelastic deformation in both <100> and <110> nanowires. Comparisons to expected deformation modes, as calculated using crystallographic Schmid factors for tension and compression, are provided to illustrate how transverse surface orientations can directly alter the deformation mechanisms in materials with nanometer scale dimensions.

This paper is available in PDF form .

On the Thermomechanical Deformation of Silver Shape Memory Nanowires

H.S. Park and C. Ji
Acta Materialia 2006; 54 (10): 2645-2654.

Abstract

We present an analysis of the uniaxial thermomechanical deformation of single crystal silver shape memory nanowires using atomistic simulations. We first demonstrate that silver nanowires can show both shape memory and pseudoelastic behavior, then perform uniaxial tensile loading of the shape memory nanowires at various deformation temperatures, strain rates and heat transfer conditions. The simulations show that the resulting mechanical response of the shape memory nanowires depends strongly upon the temperature during deformation, and can be fundamentally different from that observed in bulk, polycrystalline shape memory alloys. The energy and temperature signatures of uniaxially loaded silver shape memory nanowires are correlated to the observed nanowire deformation, and are further discussed in comparison to bulk, polycrystalline shape memory alloy behavior.

This paper is available in PDF form .

Stress-Induced Martensitic Phase Transformation in Intermetallic Nickel Aluminum Nanowires

H.S. Park
Nano Letters 2006; 6 (5): 958-962.

Abstract

Atomistic simulations are utilized to demonstrate a stress-induced martensitic phase transformation in intermetallic nickel aluminum (NiAl) nanowires. The martensitic phase transformation occurs by the propagation and annihilation of {101} twinning planes, and transforms the initially B2 NiAl nanowires to a body centered tetragonal (BCT) phase. The instability of the resulting BCT phase allows pseudoelastic recovery of inelastic strains on the order of 40 percent at all deformation temperatures.

This paper is available in PDF form .

Stable Nanobridge Formation in <110> Gold Nanowires under Tensile Deformation

H.S. Park and J.A. Zimmerman
Scripta Materialia 2006; 54 (6): 1127-1132.

Abstract

We present atomistic simulations of <110> oriented gold nanowires under tensile deformation. We find that <110> gold nanowires tend to form elongated, stable nanobridges upon necking, which is in agreement with previous experimental observations. In addition, the simulations reveal that the formation of a high strength multishell lattice structure during the plastic deformation of the <110> wires may account for the stability of the elongated nanobridges observed experimentally.

This paper is available in PDF form .

Shape Memory and Pseudoelasticity in Metal Nanowires

H.S. Park, K. Gall and J.A. Zimmerman
Physical Review Letters 2005; 95:255504.
(Also selected for publication in the Virtual Journal of Nanoscale Science and Technology, Dec. 25, 2005.)

Abstract

Structural reorientations in metallic fcc nanowires are controlled by a combination of size, thermal energy and the type of defects formed during inelastic deformation. By utilizing atomistic simulations, we show that certain fcc nanowires can exhibit both shape memory and pseudoelastic behavior. We also show that the formation of reversible defect free twins, a process related to the material stacking fault energy, nanometer size scale and surface stresses is the mechanism that controls the ability of fcc nanowires of different materials to show a reversible transition between two crystal orientations during loading and thus shape memory and pseudoelasticity.

This paper is available in PDF form .

Modeling Inelasticity and Failure in Gold Nanowires

H.S. Park and J.A. Zimmerman
Physical Review B 2005; 72:054106.

Abstract

We present numerical simulations of gold nanowires under tensile loading at various strain rates and wire sizes at room temperature. The simulations were performed using molecular dynamics modeling the gold nanowires using various forms of the embedded atom method (EAM), and concentrated on investigating the yield and fracture properties of the nanowires. It is clearly demonstrated that the accurate modeling of stacking fault and surface energies is critical in capturing the fundamental deformation behavior of gold nanowires. By doing so, phenomena which have been observed both experimentally and numerically in first principles calculations such as the formation of atom-thick chains (ATC) prior to fracture, zigzag, helical rotational motion of atoms within the ATC, structural reorientation of the ATC to a hexagonal crystal structure and (111) faceting of the nanowire in the yielded neck region by the ATC are accurately captured.

This paper is available in PDF form .