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Using molecular statics simulations, we specifically focus on investigating the negative Poisson's ratio of the monolayer, few-layer, van der Waals, and ribbon penta-graphene.

Among various intergranular microstructures, pore spaces in SEs play a critical role in Li-metal nucleation in SEs. To realize Li-metal nuclei-free inside the SE, removing pore spaces is essential and a commonly applicable design strategy to all SEs.

In chemistry, theory of aromaticity and π bond resonance plays a central role in intuitively understanding the stability and properties of organic molecules. Here we present an analogue theory for σ bond resonance in flat boron materials, which allows us to determine the distribution of two-center two-electron and three-center two-electron bonds without quantum calculations. Based on this theory, three rules are proposed to draw the Kekulé-like bonding configurations for flat boron materials and to explore their properties intuitively. As an application of this theory, the mystery of why neutral borophene with ~ 1/9 hole has the highest stability and the effect of charge doping on borophene’s optimal hole concentration are understood with the assumption of σ and π orbital occupation balance. Like the aromaticity theory for carbon materials, this theory greatly deepens our understanding on boron materials and paves the way for the rational design of various boron-based materials.

We gratefully acknowledge the support from the Mid-Career Researcher Support Program (Grant No. 2019R1A2C2011312) of the National Research Foundation (NRF) of Korea and from the Meta-Structure Based Seismic Shielding Research Fund (Project No. 1.200043.01) of UNIST. We also acknowledge with gratitude the supercomputing resources of the UNIST supercomputing Center.

Silicon for anodes in lithium-ion batteries has received much attention owing to its superior specific capacity. There has been a rapid increase of research related to void engineering to address the silicon failure mechanism stemming from the massive volume change during (dis)charging in the past decade. Nevertheless, conventional synthetic methods require complex synthetic procedures and toxic reagents to form a void space, so they have an obvious limitation to reach practical application. Here, we introduce SiCx consisting of nanocrystallite Si embedded in the inactive matrix of β-SiC to fabricate various types of void structures using thermal etching with a scalable one-pot CVD method. The structural features of SiCx make the carbonaceous template possible to be etched selectively without Si oxidation at high temperature with an air atmosphere. Furthermore, bottom-up gas phase synthesis of SiCx ensures atomically identical structural features (e.g., homogeneously distributed Si and β-SiC) regardless of different types of sacrificial templates. For these reasons, various types of SiCx hollow structures having shells, tubes, and sheets can be synthesized by simply employing different morphologies of the carbon template. As a result, the morphological effect of different hollow structures can be deeply investigated as well as the free volume effect originating from void engineering from both a electrochemical and computational point of view. In terms of selective thermal oxidation, the SiCx hollow shell achieves a much higher initial Coulombic efficiency (>89%) than that of the Si hollow shell (65%) because of its nonoxidative property originating from structural characteristics of SiCx during thermal etching. Moreover, the findings based on the clearly observed different electrochemical features between half-cell and full-cell configuration give insight into further Si anode research.

The coefficient of thermal expansion, which measures the change in length, area, or volume of a material upon heating, is a fundamental parameter with great relevance for many applications. Although there are various routes to design materials with targeted coefficient of thermal expansion at the macroscale, no approaches exist to achieve a wide range of values in graphene-based structures. Here, we use molecular dynamics simulations to show that graphene origami structures obtained through pattern-based surface functionalization provide tunable coefficients of thermal expansion from large negative to large positive. We show that the mechanisms giving rise to this property are exclusive to graphene origami structures, emerging from a combination of surface functionalization, large out-of-plane thermal fluctuations, and the three-dimensional geometry of origami structures.

We utilized molecular dynamics simulations to investigate the effects of single vacancy defects on the oscillation property of graphene nano-resonators. Single vacancies, representative point defects, are generated and unavoidable in the structure of graphene resonator during its synthesis process. We found that single vacancies deteriorate significantly the oscillation quality of the graphene resonator because of the softening of the local stiffness near single vacancies. The quality (Q) factor decreases monotonously as the concentration of single vacancy defects increases. We also found that thermal fluctuation at higher temperature accelerates the reduction of the Q factor of graphene resonators at the presence of single vacancies. At low temperature the softening effect due to single vacancies is dominant for the reduction of the Q factor of graphene resonators, while the thermal fluctuation in the out-of-plane direction becomes more dominant at high temperature. Moreover, the reduction rate of the Q factor with the increase of single vacancy concentration becomes smaller and thus less significant as temperature increases.

We present a novel structure for topologically protected propagation of mechanical waves in a continuous, elastic membrane using an analog of the quantum valley Hall effect. Our system involves a thin, continuous graphene monolayer lying on a pre-patterned substrate, and as such, it can be employed across multiple length scales ranging from the nano to macroscales. This enables it to support topologically-protected waves at frequencies that can be tuned from the kHz to GHz range by either selective pre-tensioning of the overlaying membrane, or by increasing the lattice parameter of the underlying substrate. We show through numerical simulations that this continuous system is robust against imperfections, is immune to backscattering losses, and supports topologically-protected wave propagation along all available paths and angles. We demonstrate the ability to support topologically-protected interface modes using monolayer graphene, which does not intrinsically support topologically non-trivial elastic waves.

The origami technique can provide inspiration for fabrication of novel three-dimensional (3D) structures with unique material properties from two-dimensional sheets. In particular, transformation of graphene sheets into complex 3D graphene structures is promising for functional nano-devices. However, practical realization of such structures is a great challenge. Here, we introduce a self-folding approach inspired by the origami technique to form complex 3D structures from graphene sheets using surface functionalization. A broad set of examples (Miura-ori, water-bomb, helix, flapping bird, dachshund dog, and saddle structure) is achieved via molecular dynamics simulations and density functional theory calculations. To illustrate the potential of the origami approach, we show that the graphene Miura-ori structure combines super-compliance, super-flexibility (both in tension and compression), and negative Poisson's ratio behavior.

With its excellent mechanical and thermal properties, bilayer graphane is a promising material for realizing future nanoelectromechanical systems. In this study, we focus on the auxetic behavior of bilayer graphane under external loading along various directions through atomistic simulations. We numerically and theoretically reveal the mechanism of the auxeticity in terms of intrinsic interactions between carbon atoms by constructing bilayer graphane. Given that the origin of the auxeticity is intrinsic rather than extrinsic, the work provides a novel technique to control the dimensions of nanoscale bilayer graphane by simply changing the external conditions without the requirement of complex structural design of the material.

Uniform pattern transformation can be observed in some structures with periodic arrays of pores at a critical compressive load because of buckling of the constituents of the structures. This pattern transformation can be exploited to design structures for various potential applications. Previous studies have focused on the instability of periodic porous structures of which the base materials were elastomers, and applications of these structures may be narrow because of the elastomer limitations of low melting temperature and stiffness. In addition, material failures such as plasticity and fracture were rarely discussed in previous studies. Here, we introduce metals as the base materials for some periodic metallic porous nanostructures (PMPNs). Our molecular dynamics simulation results show that PMPNs can exhibit pattern transformation at a critical strain because of buckling. In addition, we develop a simple formulation by incorporating the effect of surface on the Euler–Bernoulli beam theory to predict the critical load for the buckling of nanostructures. The prediction of our model is in good agreement with the molecular dynamics simulation results. When the applied strain is sufficiently large, the nanoscale metals experience dislocation-medicated plasticity. We also show that the pore shape of the PMPNs strongly affects the characteristics of the periodic metallic structures including the effective Young’s modulus, critical strain for micro-buckling, and critical strain for plasticity.

To achieve the urgent requirement for high volumetric energy density in lithium-ion batteries, alloy-based anodes have been spotlighted as next-generation alternatives. Nonetheless, for the veritable accomplishment with regards to high-energy demand, alloy-based anodes must be evaluated considering several crucial factors that determine volumetric capacity. In particular, the electrode swelling upon cycling must be contemplated if these anodes are to replace conventional graphite anodes in terms of volumetric capacity. Herein, we propose macropore-coordinated graphite-silicon composite by incorporating simulation and mathematical calculation of numerical values from experimental data. This unique structure exhibits minimized electrode swelling comparable to conventional graphite under industrial electrode fabrication conditions. Consequently, this hybrid anode, even with high specific capacity (527 mAh g−1) and initial coulombic efficiency (93%) in half-cell, achieves higher volumetric capacity (493.9 mAh cm−3) and energy density (1825.7 Wh L−1) than conventional graphite (361.4 mAh cm−3 and 1376.3 Wh L−1) after 100 cycles in the full-cell configuration., Here the authors show a pore-coordinated rational design to enable a Si hybrid anode that exhibits minimized electrode swelling comparable to conventional graphite and therefore high volumetric capacity and energy density.

We present numerical simulations and experimental results of the self-modulation of a long proton bunch in a plasma with linear density gradients along the beam path. Simulation results agree with the experimental results reported in arXiv:2007.14894v2: with negative gradients, the charge of the modulated bunch is lower than with positive gradients. In addition, the bunch modulation frequency varies with gradient. Simulation results show that dephasing of the wakefields with respect to the relativistic protons along the plasma is the main cause for the loss of charge. The study of the modulation frequency reveals details about the evolution of the self-modulation process along the plasma. In particular for negative gradients, the modulation frequency across time-resolved images of the bunch indicates the position along the plasma where protons leave the wakefields. Simulations and experimental results are in excellent agreement., Comment: 13 pages, 11 figures

We study experimentally the effect of linear plasma density gradients on the self-modulation of a 400\,GeV proton bunch. Results show that a positive/negative gradient in/decreases the number of micro-bunches and the relative charge per micro-bunch observed after 10\,m of plasma. The measured modulation frequency also in/decreases. With the largest positive gradient we observe two frequencies in the modulation power spectrum. Results are consistent with changes in wakefields' phase velocity due to plasma density gradient adding to the slow wakefields' phase velocity during self-modulation growth predicted by linear theory., Comment: 14 pages, 4 figures

The two-dimensional structure of graphene makes it difficult to realize flexibility and auxeticity (negative Poisson's ratio) in graphene-based structures. Using molecular dynamics simulations, we demonstrate for graphene origami structures effective tuning of both the flexibility and Poisson's ratio through the geometry, including the potential to combine superflexibility with a highly tunable negative Poisson's ratio in contrast to any existing graphene-based structure. Auxeticity even can be achieved under large applied strain, both tensile and compressive.

Silicon for anodes in lithium-ion batteries has received much attention owing to its superior specific capacity. There has been a rapid increase of research related to void engineering to address the silicon failure mechanism stemming from the massive volume change during (dis)charging in the past decade. Nevertheless, conventional synthetic methods require complex synthetic procedures and toxic reagents to form a void space, so they have an obvious limitation to reach practical application. Here, we introduce SiC

We study experimentally the longitudinal and transverse wakefields driven by a highly relativistic proton bunch during self-modulation in plasma. We show that the wakefields' growth and amplitude increase with increasing seed amplitude as well as with the proton bunch charge in the plasma. We study transverse wakefields using the maximum radius of the proton bunch distribution measured on a screen downstream from the plasma. We study longitudinal wakefields by externally injecting electrons and measuring their final energy. Measurements agree with trends predicted by theory and numerical simulations and validate our understanding of the development of self-modulation. Experiments were performed in the context of the Advanced Wakefield Experiment (AWAKE)., 4 figures

In this article, we briefly summarize the experiments performed during the first run of the Advanced Wakefield Experiment, AWAKE, at CERN (European Organization for Nuclear Research). The final goal of AWAKE Run 1 (2013-2018) was to demonstrate that 10-20 MeV electrons can be accelerated to GeV energies in a plasma wakefield driven by a highly relativistic self-modulated proton bunch. We describe the experiment, outline the measurement concept and present first results. Last, we outline our plans for the future. This article is part of the Theo Murphy meeting issue 'Directions in particle beam-driven plasma wakefield acceleration'.

Although continuum theory has been widely used to describe the long-range elastic behavior of dislocations, it is limited in its ability to describe mechanical behaviors that occur near dislocation cores. This limit of the continuum theory mainly stems from the discrete nature of the core region, which induces a drag force on the dislocation core during glide. Depending on external conditions, different drag mechanisms are activated that govern the dynamics of dislocations in their own way. This is revealed by the resultant speed of the dislocation. In this work, we develop a theoretical framework that generally describes the dynamic drag on dislocations and, as a result, derive a phenomenological constitutive equation. Furthermore, given that a low-angle grain boundary (LAGB) can be regarded as an array of dislocations, we extend the model to describe the mobility law of LAGBs as a function of misorientation angle. As a result, we prove that both dislocations and LAGBs follow the developed constitutive equation with the same mathematical form despite their different governing drag sources. The suggested model is also supported by molecular dynamics simulations. Therefore, this work has significance for a fundamental understanding of the dynamic drag acting on defects and facilitates a general description of various drag mechanisms., 78 pages (including 18 figures)

Electronic skins (e-skins) with high sensitivity to multidirectional mechanical stimuli are crucial for healthcare monitoring devices, robotics, and wearable sensors. In this study, we present piezoresistive e-skins with tunable force sensitivity and selectivity to multidirectional forces through the engineered microstructure geometries (i.e., dome, pyramid, and pillar). Depending on the microstructure geometry, distinct variations in contact area and localized stress distribution are observed under different mechanical forces (i.e., normal, shear, stretching, and bending), which critically affect the force sensitivity, selectivity, response/relaxation time, and mechanical stability of e-skins. Microdome structures present the best force sensitivities for normal, tensile, and bending stresses. In particular, microdome structures exhibit extremely high pressure sensitivities over broad pressure ranges (47,062 kPa−1 in the range of

Room-temperature ferromagnetism in graphene layers with defects has been experimentally measured. Despite disagreement around the intrinsic origin of carbon magnetism, experimental evidence has supported the existence of paramagnetism or ferromagnetism in carbon materials. Convincing theoretical explanations, however, have not yet been proposed. In this work, density functional theory calculations were used to suggest a plausible explanation for this phenomenon as it is observed at the zigzag grain boundaries of a mismatched single-double-single-layer graphene junction. We identified asymmetric zigzag-edge graphene nanoribbons that display ferromagnetic properties in a graphene junction structure. Two ferromagnetic asymmetric zigzag graphene nanoribbons displayed antiferromagnetic coupling in a defect-free structure at the grain boundary. The introduction of a vacancy or N-substitutional defect was found to destroy the magnetism on one side only; the nanoribbon on the other side continued to display a large ferromagnetic exchange coupling. The ferromagnetic nanoribbon in the junction was ferromagnetically correlated with other nanoribbons in the two-dimensional junction array, yielding a Curie temperature well-above room temperature. Moreover, the ferromagnetic correlation was observed regardless of the arrangement of the magnetic layers, enabling ferromagnetic ordering within the graphene junction array.

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.

Nanomaterials have recently been found to exhibit auxetic behavior, or a negative Poisson’s ratio, whereby the lateral dimensions of the material expand, rather than shrink, in response to applied tensile loading. In this brief review, we use the form of question–answer to highlight key points and outstanding issues related to the field of auxetic nanomaterials.

We present a second nearest-neighbor interatomic potential improved with the mechanical properties of the bulk B2 and B19’ phases in shape-memory NiTi alloys. The potential correctly predicts the phase transformations known so far in the temperature-stress-strain diagram of equiatomic NiTi. Bulk materials show the perfect reversibility between B2 and B19’ over many thermal cycles. The perfect reversibility showing superelasticity is also observed in the nanopillars under both tensile and compressive loadings. Most important, twinning naturally occurs in the nanopillars upon cooling below the transition temperature, and the subsequent tensile loading removes twinning irreversibly. We find that a potential based on accurate mechanics warrants the mechanical and thermomechanical behaviors in shape-memory NiTi necessary for a practical use.

The use of high-capacity anode materials to overcome the energy density limits imposed by the utilization of low-theoretical-capacity conventional graphite has recently drawn increased attention. Until now, stress management (including strategies relying on size, surface coating, and free volume control) has been achieved by addressing the critical problems originating from significant anode volume expansion upon lithiation. However, commercially viable alternatives to graphite have not yet been found. A new stress-management strategy relying on the use of a lamellar nanosphere Si anode is proposed. Specifically, nanospheres comprising ≈50 nm Si nanoparticles encapsulated by SiOx /Si/SiOx /C layers with thicknesses of

We measure the effects of transverse wakefields driven by a relativistic proton bunch in plasma with densities of 2.1×1014 and 7.7×1014 electrons/cm3. We show that these wakefields periodically defocus the proton bunch itself, consistently with the development of the seeded self-modulation process. We show that the defocusing increases both along the bunch and along the plasma by using time resolved and time-integrated measurements of the proton bunch transverse distribution. We evaluate the transverse wakefield amplitudes and show that they exceed their seed value (

Dislocation mobility, which is a fundamental unit of material plasticity, is determined by the predominant drag mechanism during dislocation motion. For edge dislocations in various FCC crystals, we first discover through a molecular dynamics simulation that the dislocation velocity abruptly changes at certain ultralow temperatures between 0.01 K and 10 K. We reveal that this sudden change is due to a change in the dislocation oscillation mode, and such a mode change is caused by the transition of the dislocation drag mechanism. Furthermore, we verify that the dislocation dynamics in FCC crystals at ultralow temperatures do not depend on material properties but are characterized by a fixed ratio of the dislocation frequency to the maximum frequency of the perfect lattice. Therefore, our findings indicate that the description of FCC crystal plasticity might be unifiable at ultralow temperatures independent of materials and suggest a possibility to further application to other materials.

According to continuum theory and the conservation theorem, the J integral represents the net translational force acting on a defect and, specifically, it is equivalent to the Peach–Koehler force for dislocation. In this study, we newly derive the J integral to quantify driving force on a uniformly moving dislocation with considering its core-induced dynamic behaviors. Using both molecular dynamics simulation and lattice dynamics theory based on atomic chain model, we prove that radiation drag and self-stress asymmetry during the dislocation motion, which are generated by lattice discreteness, make the newly derived J integral depend on the distance from the dislocation core, which is lumped into resistance to the dislocation motion. Finally, we show that the J integral converges to the Peach–Koehler force as the resistance term disappears for a stationary dislocation.

Density functional theory-based ab initio simulations were performed to investigate the bond strength of an Fe(001)/Al(001) coherent interface, its atomic configuration, oxidation at the interface, and mechanical deformation along the lateral direction of the interface. The bcc-type interface exhibited the highest bond strength in terms of the work of separation, but the bond strength decreased significantly when oxygen was introduced. The fracture of the bare interface initiated at the Al matrix under tensile loading, whereas that of the oxidized interface began at the interface because oxygen deteriorated the high bond strength between the Fe and Al atoms. Additionally, the bond strength of the interface was investigated under different biaxial strains to understand the effect of the residual stress generated during the joining process of Fe(001)/Al(001). Based on our findings, the mechanical deformation along the lateral direction does not significantly impact the bond strength.

Upon employing the conservation theorem and continuum theory, the configurational force on a singularity, or a defect, is given by a path-independent integral called the J integral. According to the continuum elasticity theory, the J integral around a steadily moving dislocation is equal to the Peach–Koehler force acting on the dislocation and is independent of the integration path. However, using a discrete lattice dynamics method, we theoretically prove that the J integral is not path-independent in practice even under uniform motion. This is because of the generation of phonons during the dislocation motion. In general, phonons are generated upon localized oscillation of the dislocation, and they dissipate energy from the dislocation core; consequently, a drag force is produced. As the drag force disturbs the dislocation motion, the J integral around the moving dislocation is smaller than that around a stationary one, and its deviation from the stationary one corresponds to the drag force. In this study, we analytically derive the drag force for each oscillation mode by adopting dislocation–phonon coordinates. We classify the oscillation mode simply as symmetric or anti-symmetric after assuming the dislocation to be a localized defect having a finite core width. Consequently, the drag force is numerically calculated upon consideration of the discrete nature of the dislocation core. In particular, our study reveals that the anti-symmetric oscillation mode mainly contributes to the drag force in the limit of high dislocation velocity. Furthermore, we show that the resulting relation between the drag force and dislocation frequency can reproduce the dislocation velocity-stress curve. This work is expected to contribute to meso- and macro-scale plasticity when the material is loaded under extreme conditions or transient dislocation motion can be assumed.

Improving the lifetime and the operational and thermal stability of organic thin-film materials while maintaining high conductivity and mechanical flexibility is critical for flexible electronics applications. Here, it is reported that highly conductive and environmentally stable organic transparent electrodes (TEs) can be fabricated by mechanically laminating poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) films containing dimethylsulfoxide and Zonyl fluorosurfactant (PDZ films) with a monolayer graphene barrier. The proposed lamination process allows graphene to be coated onto the PDZ films uniformly and conformally with tight interfacial binding, free of wrinkles and air gaps. The laminated films exhibit an outstanding room-temperature hole mobility of ≈85.1 cm2 V−1 s−1 since the graphene can serve as an effective bypass for charge carriers. The significantly improved stability of the graphene-laminated TEs against high mechanical/thermal stress, humidity, and ultraviolet irradiation is particularly promising. Furthermore, the incorporation of the graphene barrier increases the expected lifetime of the TEs by more than two orders of magnitude.

This work proves that scattering of elastic waves in the dislocation core induces an unusual behavior called stress-drop , which is defined for the first time in this study as a phenomenon where an externally applied stress drops inside a nano-sized cubic metal during dislocation motion. We develop a theoretical phonon scattering model based on discrete lattice dynamics and derive simple analytical equations to quantify the magnitude of the stress-drop. The proposed model is supported by atomistic simulations of perfect dislocations in bcc iron, where the stress-drop resulting from bond breaking is inversely proportional to the square of the dislocation speed. The derived equation is in excellent agreement with direct atomistic simulations for edge and screw dislocations. Next, we extend the equation to the edge dislocation in fcc aluminum where the dislocation exists as a stacking fault ribbon surrounded by two Shockley partial dislocations and thus the interaction between them is important. The extended equation accurately predicts the phonon scattering, resulting in the stress-drop by the oscillation of the two partials as well as bond breaking. In addition to the discussion on the validity of our model and equations at absolute zero, we investigate the temperature and size effects on our equations. As temperature increases, the magnitude of the stress-drop decreases and converges to zero even at very low temperature because of the extremely small Peierls barrier. Moreover, the magnitude of the stress-drop decreases, as the thickness of the nanoplate increases, which shows that the stress-drop is a mechanical behavior that is prominent on the nanoscale.

Indium tin oxide-free, flexible transparent electrodes (TEs) are crucial for the future commercialization of flexible and wearable electronics. While carbon-based TEs containing carbon nanotube (CNT) networks show promise, they usually exhibit poor dispersion properties, limiting their performance and practicality. In this study, we report a highly efficient and bending durable all-carbon composite TE (ac-TE) that employs uniform CNT networks on a monolayer graphene/polyethylene terephthalate (PET) substrate via a simple air spray deposition method. The air-sprayed CNT/graphene assembly was free-standing on solution, making a polymer-free transfer of carbon composites to target substrates possible. The excellent performance of the ac-TEs was attributed to the uniformly networked CNTs on the polycrystalline graphene with a well-controlled density, effectively bridging the line defects and filling the tears/voids or folds necessarily existing in the as-processed graphene. The sheet resistance of the ac-TEs was increased only 6% from its original value at a bending radius of 2.7 mm, while that of the pristine graphene/PET assembly increased 237%. Mechanical bending of the ac-TEs worsened the electrical performance by only ∼1.7% after 2000 bending cycles at a bending radius of 2.5 mm. Degradation of the performance by the bending was the result of line defects formation in the graphene, demonstrating the potential of the uniform CNT networks to achieve more efficient and flexible carbon-based TEs. Furthermore, the chemically-doped ac-TEs showed commercially suitable electronic and optical properties with much enhanced thermal stability, closer to practical TEs in flexible devices.

We conducted molecular statics simulations to investigate the negative Poisson's ratio (auxetic behavior) of periodic porous graphene structures based on the rotating rigid unit mechanism. To obtain a negative Poisson's ratio, simple voids were periodically introduced into graphene. We showed that the Poisson's ratio of the designed graphene structure is strongly dependent on the aspect ratio of the voids, and it can approach the theoretical limit of −1.0. More importantly, the graphene periodic structure maintains its auxetic behavior even under large strains (ϵ ∼ 0.20). Hence, it can be employed in a wide range of applications requiring structures that can endure large deformation. In addition, we found that the key factor in the auxeticity of the investigated structures is the deformation occurring at the void tips.

This report employed molecular statics simulation and density-functional-theory calculation to study the Poisson's ratios of face-centered-cubic materials. We provide numerical and theoretical evidences to show that cubic materials can exhibit auxetic behavior in a principal direction under proper loading conditions. When a stress perpendicular to the loading direction is applied, cubic materials can exhibit a negative Poisson's ratio at finite strain. The negative Poisson's ratio behavior, including its direction and value, is highly dependent on the direction and magnitude of the transversely applied stresses. As a result, we show that it is possible to tune the direction and magnitude of the negative Poisson's ratio behavior of cubic materials by controlling the transverse loadings.