Tunable negative Poisson's ratio in hydrogenated graphene

Prediction of 2D group-11 chalcogenides: insights into novel auxetic M2X (M = Cu, Ag, Au; X = S, Se, Te) monolayers

Two-dimensional (2D) auxetic materials have recently attracted considerable research interest due to their excellent mechanical properties and diverse applications, surpassing those of three-dimensional (3D) materials. This study focuses on the theoretical prediction of mechanical properties and auxeticity in 2D M2X (M = Cu, Ag, Au; X = S, Se, Te) monolayers using first-principles calculations. Our results indicate that the dynamically stable monolayers include low-energy α-Cu2S, α-Cu2Se, α-Cu2Te, β-Ag2S, β-Ag2Se, α-Ag2Te, β-Au2S, β-Au2Se and α-Au2Te. These M2X monolayers possess positive Poisson's ratios (PR) ranging from 0.09 to 0.52, as well as Young's moduli ranging from 19.92 to 35.42 N m-1 in x and y directions. Specially, α-Cu2S exhibits the lowest negative PR in θ = 45° × n (n = 1, 2, 3, 4) directions. The Poisson's function (PF) can be adjusted by increasing tensile strains. The β-phase monolayers exhibit positive PF with a linear change. Interestingly, the transition from positive to negative PF occurs in the α-Cu2S and α-Ag2Te monolayers at strains greater than +3% and +4%, respectively, while the α-Cu2Se, α-Cu2Te and α-Au2Te monolayers maintain positive PF within the range of 0% to +6% strains. Furthermore, taking α-Cu2S (α-Cu2Te) as an example, the mechanism underlying negative (positive) PF is demonstrated to involve increased (decreased) bond angles, decreased thickness, and weakened (enhanced) d(M)-p(X) orbital coupling. The findings of this study not only enrich the family of 2D group-11 chalcogenides but also provide insights into their mechanical properties, thereby expanding their potential applications in mechanics.

Nonlinearity induced negative Poisson's ratio of two-dimensional nanomaterials

Materials exhibiting a negative Poisson's ratio have garnered considerable attention due to the improved toughness, shear resistance, and vibration absorption properties commonly found in auxetic materials. In this work, the nonlinear effect on the Poisson's ratio was derived theoretically and verified by first-principle calculations and molecular dynamics simulations of two-dimensional nanomaterials including graphene and hexagonal boron nitride. The analytic formula explicitly shows that the Poisson's ratio depends on the applied strain and can be negative for large applied strains, owing to the nonlinear interaction. Both first-principle calculations and molecular dynamics simulations show that the nonlinear effect is highly anisotropic for graphene, where the nonlinearity-induced negative Poisson's ratio is much stronger for the strain applied along the armchair direction. These findings provide valuable insights into the behavior of materials with negative Poisson's ratios and emphasize the importance of considering nonlinear effects in the study of the Poisson's ratio of two-dimensional materials.

Structural, mechanical, electronic and optical properties of biphenylene hydrogenation: a first-principles study

Biphenylene networks typically exhibit a metallic electronic nature, while hydrogenation can open the band gap changing it to a semiconductor. This property makes hydrogenated biphenylene a promising candidate for use in semiconductor optoelectronic materials and devices. In this work, three representative configurations of hydrogenated biphenylene, denoted by α, β and γ, were investigated. The structural, mechanical, electronic, and optical properties of these hydrogenated biphenylene configurations were calculated by first-principles calculations. Band gaps with HSE correction were 4.69, 4.42 and 4.39 eV for α, β, and γ configurations, respectively. Among these three configurations, β presents the best electronic performance and special elastic properties (negative Poisson's ratio), while γ exhibits the best elastic properties. In addition, we comprehensively analyze the mechanical properties of these configurations and provide evidence that hydrogenated biphenylene possibly exhibits a negative-Poisson's-ratio along the zigzag and armchair directions when hydrogen atoms are added to biphenylene in certain ways. Furthermore, although the electronic properties of γ are weaker than those of β, they are also excellent. In addition, the binding energies of β and γ are relatively lower, which indicates that β and γ are more stable. Our findings demonstrate that the hydrogenated biphenylene is a promising material with significant application potential in optoelectronic devices.

Auxeticity of monolayer, few-layer, vdW heterostructure and ribbon penta-graphene

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. As a result, we provide evidence to show that the Poisson's ratio is the combination of bond stretching and angle rotating mechanism. The auxeticity of monolayer penta-graphene is due to the dominance of bond stretching. However, the significant effect of the angle rotating mechanism causes the enhancement of the in-plane Poisson's ratio of few-layer penta-graphene. Furthermore, the elongation of interlayer bonds results in a negative out-of-plane Poisson's ratio in few-layer penta-graphene. The strong dependence of Poisson's ratio on stacking configuration and number of layers was found. We also show that the van der Waals interaction slightly enhances the auxeticity of heterostructure penta-graphene. Finally, we discuss the significant effects of warped edges on the auxeticity of penta-graphene ribbons.

Accessing negative Poisson's ratio of graphene by machine learning interatomic potentials

The negative Poisson's ratio (NPR) is a novel property of materials, which enhances the mechanical feature and creates a wide range of application prospects in lots of fields, such as aerospace, electronics, medicine, etc. Fundamental understanding on the mechanism underlying NPR plays an important role in designing advanced mechanical functional materials. However, with different methods used, the origin of NPR is found different and conflicting with each other, for instance, in the representative graphene. In this study, based on machine learning technique, we constructed a moment tensor potential for molecular dynamics (MD) simulations of graphene. By analyzing the evolution of key geometries, the increase of bond angle is found to be responsible for the NPR of graphene instead of bond length. The results on the origin of NPR are well consistent with the start-of-art first-principles, which amend the results from MD simulations using classic empirical potentials. Our study facilitates the understanding on the origin of NPR of graphene and paves the way to improve the accuracy of MD simulations being comparable to first-principle calculations. Our study would also promote the applications of machine learning interatomic potentials in multiscale simulations of functional materials.

Negative out-of-plane Poisson's ratio of bilayer graphane

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.

From graphene to graphene ribbons: atomically precise cutting via hydrogenation pseudo-crack

The properties and applications of graphene nanoribbons (GNRs) depend heavily on their shape and size, making precise design and construction at atomic scale significantly important. Herein, we show that pseudo-cracking is a feasible method for creating atomically precise GNRs. By using molecular dynamics (MD) simulation, we find that hydrogenation can act as a pseudo-crack to trigger the fracture of graphene along the hydrogenation line and cut the graphene into a GNR. Precise GNRs with a desired width, edge type and associated properties can be realized in a controllable way by manipulating the position and dimension of the hydrogenation pseudo-crack. We also find that it is better to use hydrogenation pseudo-cracks along the armchair direction to cut graphene at lower forces into GNRs with smooth edges. Our findings suggest a promising approach to cut graphene and other two-dimensional materials into nanoribbons effectively and accurately.

Complex three-dimensional graphene structures driven by surface functionalization

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.

Unusual Sonochemical Assembly between Carbon Allotropes for High Strain-Tolerant Conductive Nanocomposites

Facile methods toward strain-tolerant graphene-based electronic components remain scarce. Although being frequently used to disperse low-dimensional carbonaceous materials, ultrasonication (US) has never been reliable for fabricating stretchable carbonaceous nanocomposite (SCNC). Inspired by the unusual sonochemical assembly between graphene oxide (GO) and carbon nanotube (CNT), we verified the roots-like GO-CNT covalent bonding, rather than just π-π conjugation, was formed during US. In addition, the shockwave-induced collision in the binary-component system enables a burst of fragmentation at the early stage, spatially homogeneous hybridization, and time-dependent restoration of graphitic domains. All of the above are distinct from extensive fragmentation of a conventional single-component system and π-π conjugative assembly. The optimized SCNC exhibits conductivity comparable to reduced monolayer GO and outperforms π-π assemblies in retaining electrical conductance at a strain of 160%-among one of the best reported stretchable conductors. Raman analysis and mechanics simulation confirm the dominant role of counterweighing between the intrinsic and external strains on the mechano-response and durability of SCNC. This work suggests the guideline of creating multiple-component sonochemical systems for various functional nanocomposites.

Chiral metallic glass nanolattices with combined lower density and improved auxeticity

Auxetic materials are promising structural and functional candidates due to their unique lateral expansion when stretched, however, bulk metallic glasses (MGs) could not show any auxeticity because of their intrinsic isotropic nature. Here we construct chiral Cu₅₀Zr₅₀ metallic glass nanolattices with cavities, and investigate their auxeticity and underlying mechanism with molecular dynamics simulations. It is found that, compared to monolithic MGs, all the chiral metallic glass nanolattices (CMGNs) exhibit improved auxeticity and lower density. For CMGNs with cavities, the negative Poisson's ratio and ultimate tensile strength (UTS) increase first and then decrease with increasing cavity radius, with the cavity radius of 2.5 nm being the most favorable for auxeticity and enhanced UTS. The auxetic mechanism is attributed to the competition between rotation behavior and non-affine deformation under tension. Our study not only reveals the mechanism of auxeticity in CMGNs having cavities but also provides a feasible method to optimize their auxetic performance and density by structure designing of MGs.

Stress-sign-tunable Poisson's ratio in monolayer blue phosphorus oxide

Negative Poisson's ratio (NPR) materials have attracted tremendous interest due to their unusual physical properties and potential applications. Certain two-dimensional (2D) monolayer materials have also been found to exhibit NPR and the corresponding deformation mechanism varies. In this study, we found, based on first-principles calculations, that the Poisson's ratio (PR) sign of monolayer blue phosphorus oxide (BPO) can be tuned by strain: the PR is positive under uniaxial strain [Formula: see text] but becomes negative under [Formula: see text]  >  0. The deformation mechanism for BPO under strain depends on the mutual competition between the P-P attraction and P-O repulsion effect, and these two factors induce two different deformation pathways (one with positive PR, and the other with NPR). Moreover, with increasing of strain, both the decreased strength of P-P attraction and the increased strength of P-O repulsion effect modulates the PR of BPO from positive to negative.

Origin of anisotropic negative Poisson's ratio in graphene

Negative Poisson's ratio (NPR) in auxetic materials is of great interest due to the typically enhanced toughness, shear resistance, and sound and vibration absorption, which enables plenty of novel applications such as aerospace and defense. Insight into the mechanism underlying NPR is significant to the design of auxetic nanomaterials and nanostructures. However, the analysis of NPR in previous studies mainly remains on the level of the evolution of geometry parameters, such as bond length and bond angle, while a thorough and fundamental understanding is lacking. In this paper, we report anisotropic differential NPR in graphene for uniaxial strains applied along both zigzag and armchair directions based on first-principles calculations. The mechanism underlying the emergence of NPR in graphene (evolution of bond length and bond angle) is found to be different from the conclusions from previous classical molecular dynamics simulations with empirical potential. We propose that the decentralized electron localization function (ELF) driven by strain leads to ELF coupling between different types of bonds, which results in the counter-intuitive anomalous increase of the bond angle and thus the emergence of NPR in graphene. Moreover, the NPR phenomenon can be anticipated to emerge in other nanomaterials or nanostructures with a similar honeycomb structure as that of graphene, where the ELF coupling would also be possible.

The negative Poisson's ratio in graphene-based carbon foams

Using molecular dynamics simulations, we find an in-plane negative Poisson's ratio intrinsically existing in the graphene-based three-dimensional (3D) carbon foams (CFs) when they are compressed uniaxially. Our study shows that the negative Poisson's ratio in the present CFs is attributed to their unique molecular structures and triggered by the buckling of the CF structures. This mechanism makes the negative Poisson's ratio of CFs strongly depend on their cell length, which offers us an efficient means to tune the negative Poisson's ratio in nanomaterials. Moreover, as the buckling modes of CFs are topographically different when they are compressed in different directions, their negative Poisson's ratio is found to be strongly anisotropic, which is in contrast to the isotropic positive Poisson's ratio observed in CFs prior to buckling. The discovery of the intrinsic negative Poisson's ratio in 3D CFs will significantly expand the family of auxetic nanomaterials. Meanwhile, the mechanism of nano-auxetics proposed here may open up a door to manufacture new auxetic materials on the nanoscale.

Theoretical discovery of novel two-dimensional VA-N binary compounds with auxiticity

Novel two-dimensional V^A-nitride binary compounds with a large negative Poisson's ratio and a suitable band-gap are predicted based on first-principles calculations.

A perspective on auxetic nanomaterials

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.

Negative Poisson's ratio in rippled graphene

In this work, we perform molecular dynamics (MD) simulations to study the effect of rippling on the Poisson's ratio of graphene. Due to the atomic scale thickness of graphene, out-of-plane ripples are generated in free standing graphene with topological defects (e.g. heptagons and pentagons) to release the in-plane deformation energy. Through MD simulations, we have found that the Poisson's ratio of rippled graphene decreases upon increasing its aspect ratio η (amplitude over wavelength). For the rippled graphene sheet η = 0.188, a negative Poisson's ratio of -0.38 is observed for a tensile strain up to 8%, while the Poisson's ratio for η = 0.066 is almost zero. During uniaxial tension, the ripples gradually become flat, thus the Poisson's ratio of rippled graphene is determined by the competing factors of the intrinsic positive Poisson's ratio of graphene and the negative Poisson's ratio due to the de-wrinkling effect. Besides, the rippled graphene exhibits excellent fracture strength and toughness. With the combination of its auxetic and excellent mechanical properties, rippled graphene may possess potential for application in nano-devices and nanomaterials.

Negative Poisson's ratio in graphene oxide

We perform molecular dynamics simulations to investigate the Poisson's ratio of graphene oxide. We find that the Poisson's ratio can be effectively tuned by increasing the degree of oxidation of graphene oxide. More specifically, the Poisson's ratio decreases linearly from positive to negative with increasing oxidation, turning negative at room temperature for a degree of oxidation of 0.27, and reaching a value of -0.567 for fully oxidized graphene. The oxidation dependence of the Poisson's ratio is attributed to the tension-induced suppression of the ripples resulting from the oxidation, whose amplitude increases with increasing oxidation. Finally, we also demonstrate the temperature dependence of the Poisson's ratio in the graphene oxide.