A theoretical analysis of the effect of the hydrogenation of graphene to graphane on its mechanical properties

Janus MoAZ3H (A = Ge, Si; Z = N, P, As) monolayers: a new class of semiconductors exhibiting excellent photovoltaic and catalytic performances

Due to the asymmetrical structure in the vertical direction, Janus two-dimensional (2D) monolayer (ML) materials possess some unique physical properties, holding great promise for nanoscale devices. In this paper, based on the newly discovered MoA2Z4 (A = Si, Ge; Z = N, P, As) ML, we propose a class of 2D Janus MoAZ3H ML materials with good stability and excellent mechanical properties using first-principles calculations. We demonstrate that the novel Janus MoAZ3H ML materials are all semiconductors with bandgaps ranging from 0.69 to 2.44 eV, giving rise to good absorption in the visible light region. Especially, both MoSiN3H and MoGeN3H MLs can be used as catalysts for producing hydrogen through water splitting. This catalytic property is much more efficient than that of the MoA2Z4 ML, attributed to the intrinsic electric field induced by the vertical asymmetry effectively separating electrons and holes. More importantly, the carrier mobility of the MoAZ3H ML is up to 103-104 cm2 V-1 s-1 due to the large elastic modulus or small effective mass. Additionally, the electronic properties of the MoAZ3H ML can be easily tuned by strain. Our results suggest a new strategy for designing novel 2D Janus materials, which not only expands the members in the 2D MA2Z4-based Janus family, but also provide candidates with excellent performances in photovoltaic and catalytic fields.

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.

Enhancing the Mechanical Stability of 2D Fullerene with a Graphene Substrate and Encapsulation

Recent advancements have led to the synthesis of novel monolayer 2D carbon structures, namely quasi-hexagonal-phase fullerene (qHPC₆₀) and quasi-tetragonal-phase fullerene (qTPC₆₀). Particularly, qHPC₆₀ exhibits a promising medium band gap of approximately 1.6 eV, making it an attractive candidate for semiconductor devices. In this study, we conducted comprehensive molecular dynamics simulations to investigate the mechanical stability of 2D fullerene when placed on a graphene substrate and encapsulated within it. Graphene, renowned for its exceptional tensile strength, was chosen as the substrate and encapsulation material. We compared the mechanical behaviors of qHPC₆₀ and qTPC₆₀, examined the influence of cracks on their mechanical properties, and analyzed the internal stress experienced during and after fracture. Our findings reveal that the mechanical reliability of 2D fullerene can be significantly improved by encapsulating it with graphene, particularly strengthening the cracked regions. The estimated elastic modulus increased from 191.6 (qHPC₆₀) and 134.7 GPa (qTPC₆₀) to 531.4 and 504.1 GPa, respectively. Moreover, we observed that defects on the C60 layer had a negligible impact on the deterioration of the mechanical properties. This research provides valuable insights into enhancing the mechanical properties of 2D fullerene through graphene substrates or encapsulation, thereby holding promising implications for future applications.

Mechanical properties of hydrogenated ψ-graphene

CONTEXT

Hydrogenation is an effective way to open a band gap of the metallic ψ-graphene, expanding its application in electronics. Evaluating the mechanical properties of hydrogenated ψ-graphene, especially the effect of hydrogen coverage, is also crucial to the application of ψ-graphene. Here, we demonstrate the mechanical properties of ψ-graphene depend closely on the hydrogen coverage and arrangement. Upon hydrogenation, Young's modulus and intrinsic strength of ψ-graphene decrease due to breaking of sp² carbon networks. Both the ψ-graphene and hydrogenated ψ-graphene exhibit mechanical anisotropy. During changing the hydrogen coverage, the variation of mechanical strength of the hydrogenated ψ-graphene relies on the tensile direction. In addition, the arrangement of hydrogen also contributes to the mechanical strength and fracture behavior of hydrogenated ψ-graphene. Our results not only present a comprehensive understanding of the mechanical properties of hydrogenated ψ-graphene, but also provide a reference to tailor the mechanical properties of other graphene allotropes, which are of potential interest in materials science.

METHODS

Vienna ab initio simulation package based on the plane-wave pseudopotential technique was employed for the calculations. The exchange-correlation interaction was described by the Perdew-Burke-Ernzerhof functional within the general gradient approximation and the ion-electron interaction was treated with the projected augmented wave pseudopotential.

First-Principles Insights on the Formation Mechanism of Innermost Layers of Solid Electrolyte Interphases on Carbon Anodes for Lithium-Ion Batteries

A solid electrolyte interphase (SEI) plays an essential role in the functionality and service life of ion batteries, where the structure and formation mechanism are still in the midst. Here, we investigate the initial decomposition and reactions of ethylene carbonate (EC) on the surface of a graphite anode using first-principles calculations. EC initially decomposes via the homolytic ring opening with the product of radical anion CH₂CH₂OCO₂^-. Bonding with Li, it forms a co-plane structure of CH₂CH₂OCO₂Li, with a binding energy of 1.35 eV. The adsorption energy is -0.91 eV and -0.24 eV on the graphite zigzag edge surface and basal surface, respectively. Two CH₂CH₂OCO₂Li molecules react to form a two-head structure of lithium ethylene dicarbonate (CH₂OCO₂Li)₂, namely LEDC, which further forms a network preferring zigzag edge surfaces. Our results suggest that the first and innermost layers of the solid electrolyte interphase are CH₂CH₂OCO₂Li sticking and networking on the zigzag edges of the surfaces of graphite anodes.

A First-Principles Study on the Multilayer Graphene Nanosheets Anode Performance for Boron-Ion Battery

Advanced battery materials are urgently desirable to meet the rapidly growing demand for portable electronics and power. The development of a high-energy-density anode is essential for the practical application of B³⁺ batteries as an alternative to Li-ion batteries. Herein, we have investigated the performance of B³⁺ on monolayer (MG), bilayer (BG), trilayer (TG), and tetralayer (TTG) graphene sheets using first-principles calculations. The findings reveal significant stabilization of the HOMO and the LUMO frontier orbitals of the graphene sheets upon adsorption of B³⁺ by shifting the energies from −5.085 and −2.242 eV in MG to −20.08 and −19.84 eV in 2B³⁺@TTG. Similarly, increasing the layers to tetralayer graphitic carbon B³⁺@TTG_asym and B³⁺@TTG_sym produced the most favorable and deeper van der Waals interactions. The cell voltages obtained were considerably enhanced, and B³⁺/B@TTG showed the highest cell voltage of 16.5 V. Our results suggest a novel avenue to engineer graphene anode performance by increasing the number of graphene layers.

MC2 (M = Y, Zr, Nb, and Mo) monolayers containing C2 dimers: prediction of anode materials for high-performance sodium ion batteries

Seeking novel high performance anode materials for sodium ion batteries (SIBs) is an attractive theme in developing energy storage devices. In this work, by means of density functional theory computations, we predicted a family of MC₂ (M = Y, Zr, Nb, and Mo) monolayers containing C₂ dimers to be promising anode materials for SIBs. The stability, electronic structure, and adsorption/diffusion/storage behavior of sodium atoms in MC₂ (M = Y, Zr, Nb, and Mo) monolayers were explored. Our computations revealed that Na adsorbed MC₂ (M = Y, Zr, Nb, and Mo) monolayers show metallic characteristics that give rise to excellent electrical conductivity and Na mobility with low activation energies for diffusion (0.21, 0.04, 0.20, and 0.22 eV, respectively) in these materials, indicative of a high charge/discharge rate. In addition, the theoretical capacities of Na-adsorbed on YC₂, ZrC₂, NbC₂, and MoC₂ monolayers are 478, 697, 687, and 675 mA h g^-1, respectively, higher than that of commercial graphite (284 mA h g^-1), and the open-circuit voltages are moderate (0.11-0.25 V). Our results suggest that MC₂ (M = Y, Zr, Nb, and Mo) monolayers have great potential to serve as anode materials for SIBs.

The Crack Angle of 60° Is the Most Vulnerable Crack Front in Graphene According to MD Simulations

Graphene is a type of 2D material with unique properties and promising applications. Fracture toughness and the tensile strength of a material with cracks are the most important parameters, as micro-cracks are inevitable in the real world. In this paper, we investigated the mechanical properties of triangular-cracked single-layer graphene via molecular dynamics (MD) simulations. The effect of the crack angle, size, temperature, and strain rate on the Young’s modulus, tensile strength, fracture toughness, and fracture strain were examined. We demonstrated that the most vulnerable triangle crack front angle is about 60°. A monitored increase in the crack angle under constant simulation conditions resulted in an enhancement of the mechanical properties. Minor effects on the mechanical properties were obtained under a constant crack shape, constant crack size, and various system sizes. Moreover, the linear elastic characteristics, including fracture toughness, were found to be remarkably influenced by the strain rate variations.

Atomic Insights into Fracture Characteristics of Twisted Tri-Layer Graphene

Graphene twistronics have recently gained significant attention due their superconductive behavior as a consequence of their tunable electronic properties. Although the electronic properties of twisted graphene have been extensively studied, the mechanical properties and integrity of twisted trilayer graphene (tTLG) under loading is still elusive. We investigated the fracture mechanics of tTLG with a twist angle of ±1.53° utilizing molecular dynamics simulation. This twist angle was chosen because it is known to exhibit highly superconductive behavior. The results indicate that tTLG does not preserve the excellent mechanical properties typically associated with graphene, with toughness and fracture strain values much lower in comparison. The Young’s modulus was an exception with values relatively close to pristine graphene, whereas the tensile strength was found to be roughly half of the intrinsic strength of graphene. The fracture toughness, fracture strain and strength converge as the crack length increases, reaching 0.26 J/m3, 0.0217 and 39.9 GPa at a crack length of 8 nm, respectively. The Griffth critical strain energy is 19.98 J/m2 and the critical stress intensity factor Kc is 4.47 MPa M1/2, in good agreement with that of monolayer graphene in the experiment. Our atomic insights might be helpful in the material design of twisted trilayer graphene-based electronics.

MX Anti-MXenes from Non-van der Waals Bulks for Electrochemical Applications: The Merit of Metallicity and Active Basal Plane

Two-dimensional transition-metal compounds (2DTMCs) are promising materials for electrochemical applications, but 2DTMCs with metallicity and active basal planes are rare. In this work, we proposed a simple and effective strategy to extract 2DTMCs from non-van der Waals bulk materials and established a material library of 79 2DTMCs, which we named as anti-MXenes since they are composed of one M atomic layer sandwiched by two X atomic layers. By means of density functional theory computations, 24 anti-MXenes were confirmed to be thermodynamically, dynamically, mechanically, and thermally stable. The metallicity and active basal plane endow these anti-MXenes with potential as excellent electrode materials, for example, as electrocatalysts for hydrogen evolution reactions (HER). Among the noble-metal free anti-MXenes with favorable H-binding, CuS can boost HER at the whole range of H coverages, while CoSi, FeB, CoB, and CoP show promise for HER at some specific H coverages. The active sites are the tetra-coordinating nonmetal atoms at the basal planes, thus rendering a very high density of active sites for these materials. CoB is also a promising anode material for lithium-ion batteries, showing low Li diffusion energy barriers, a very high capacity, and a suitable open circuit voltage. This work promotes the "computational exfoliation" of 2D materials from non-van der Waals bulks and exemplifies the applications of anti-MXenes in various electrochemical processes.

Hydrogenated Ψ-graphene as an ultraviolet optomechanical sensor

PSI (ψ)-graphene is a dynamically and thermally stable two-dimensional (2D) allotrope of carbon composed of 5-6-7 carbon rings. Herein, we study the opto/mechanical behavior of two graphene allotropes, Ψ-graphene and its hydrogenated form, Ψ-graphane under uniaxial and biaxial strain using density functional theory (DFT) calculations. We calculated the elastic constants and second Piola-Kirchhoff (PK2) stresses, in which both nanostructures indicate a similar elasticity behavior to graphene. Also, the plasmonic behavior of these structures in response to various strains has been studied. As a result, plasmonic peaks varied up to about 2 eV under strain. Our findings reveal that these two structures have a large peak in the ultraviolet (UV) region and can be tuned by different applied strain. In addition, Ψ-graphene has smaller peaks in the IR and UV regions. Therefore, both Ψ-graphene and Ψ-graphane can be used as UV optomechanical sensors, whereas Ψ-graphene could be used as an infrared (IR) and visible sensor.

PSI (ψ)-graphene is a dynamically and thermally stable two-dimensional (2D) allotrope of carbon composed of 5-6-7 carbon rings.

Graphene Adhesion Mechanics on Iron Substrates: Insight from Molecular Dynamic Simulations

The adhesion feature of graphene on metal substrates is important in graphene synthesis, transfer and applications, as well as for graphene-reinforced metal matrix composites. We investigate the adhesion energy of graphene nanosheets (GNs) on iron substrate using molecular dynamic (MD) simulations. Two Fe–C potentials are examined as Lennard–Jones (LJ) pair potential and embedded-atom method (EAM) potential. For LJ potential, the adhesion energies of monolayer GN are 0.47, 0.62, 0.70 and 0.74 J/m2 on the iron {110}, {111}, {112} and {100} surfaces, respectively, compared to the values of 26.83, 24.87, 25.13 and 25.01 J/m2 from EAM potential. When the number of GN layers increases from one to three, the adhesion energy from EAM potential increases. Such a trend is not captured by LJ potential. The iron {110} surface is the most adhesive surface for monolayer, bilayer and trilayer GNs from EAM potential. The results suggest that the LJ potential describes a weak bond of Fe–C, opposed to a hybrid chemical and strong bond from EAM potential. The average vertical distances between monolayer GN and four iron surfaces are 2.0–2.2 Å from LJ potential and 1.3–1.4 Å from EAM potential. These separations are nearly unchanged with an increasing number of layers. The ABA-stacked GN is likely to form on lower-index {110} and {100} surfaces, while the ABC-stacked GN is preferred on higher-index {111} surface. Our insights of the graphene adhesion mechanics might be beneficial in graphene growing, surface engineering and enhancement of iron using graphene sheets.

Charge-induced electromechanical actuation of two-dimensional hexagonal and pentagonal materials

Using first-principles calculations, we investigate electromechanical properties of two-dimensional (2D) hexagonal and pentagonal materials as a function of electron and hole dopings, in which 2D materials including graphene, chair-like graphane, table-like graphane, penta-graphene (PG), hydrogenated penta-graphene (HPG), and penta-CN2 are considered. We find that the actuation responses such as actuation strain, stress generated, and work area-density per cycle of the 2D materials in the case of hole doping are substantially larger than those of electron doping. Moreover, the electromechanical properties of the 2D materials can be improved by hydrogenation. In particular, the actuation strain and work area-density per cycle of graphane and HPG are much larger than those of graphene and PG for hole doping, respectively. Interestingly, both the 2D hexagonal and pentagonal materials show an asymmetric dependence of theoretical strength (a maximum value of the stress that the materials can achieve by applying the strain) on the electron and hole dopings. These results provide an important insight into the electromechanical properties of the 2D hexagonal and pentagonal materials, which are useful for artificial muscle applications.

Anisotropic and temperature dependent mechanical properties of carbon honeycomb

Carbon honeycomb (CHC) is a newly synthesized carbon allotropy with promising applications in many fields of science and engineering. In this work, we investigate the mechanical properties of CHC with focus on the anisotropicity in terms of the tilt angle θ in zigzag-armchair (x-y) plane using molecular dynamics simulations. Results show that the tensile strength of CHC ranges from 15.0 to 23.7 GPa at room temperature, which is lower than that of graphene due to the weakness on the junction. Meanwhile, except in the armchair direction, the strength of CHC reduces as the stretching direction moves away from the zigzag direction, similar to that of graphene, while the Young's modulus decreases with respect to tilt angle, opposite to that of graphene. Increasing the temperature will weaken CHC by reducing the strength, there is only a 16% reduction in the minimum strength in the x-y plane as the temperature increases from 100 to 900 K. In addition, the crack occurs first in cell axis direction then in the x-y plane, different from graphene which appears along the zigzag direction only.

Be2CO3F2 Monolayer: A Flexible Ultraviolet Nonlinear Optical Material via Rational Design

Multifunctional monolayer materials with attractive properties and novel applications are of present research interest. In this contribution, we design a new monolayer Be₂CO₃F₂ (BCF) by taking a KBe₂BO₃F₂ unique crystal that is able to produce a high energetic laser by a direct second harmonic generation method, as a parent. The cohesive energy, positive phonon modes, and elastic constants reveal that the BCF monolayer is dynamically and mechanically stable, and the appropriate cleavage energy predicts the experimental realization possibility. The property investigations demonstrated that the monolayer BCF has the significantly superiority in flexibility over the representative flexible optoelectronic material MoS₂ based on the calculation of Young's modulus. Additionally, the monolayer BCF possesses both a large band gap (5.2 eV) and a second harmonic generation response. These results demonstrate that the monolayer BCF may provide better applications as a promising multifunctional material in the flexible nonlinear optical fields. We hope that this research will pave a new way for designing new generation multifunctional devices.

Carbon Nanotubes Enhance the Radiation Resistance of bcc Iron Revealed by Atomistic Study

With extra space, a carbon nanotube (CNT) could serve as an absorber of point defects, including helium interstitials, and outgas the accumulate helium via “nano-chimneys”. The radiation resistance of CNT/Fe has still not been fully understood. Herein, we investigated the influence of CNTs on low-energy helium irradiation resistance in CNT/Fe composites by molecular dynamic simulations. CNTs reduced the small and medium He clusters in the Fe matrix. When the incident energy of the He atoms was 300 eV, the He atoms aggregated at the outer surface of CNTs. CNTs postponed the formation of He bubbles. When the irradiation energy was higher than 600 eV, He atoms could penetrate the walls of CNTs and form clusters inside the single-walled CNTs or the space in double-walled CNTs—the latter presented better performance. The reduction of Frenkel pair point defects suggested the enhancement of radiation resistance by the presentation of CNTs. Our results might be useful for the material design of advanced steels for radiation resistance.

Hydrogenation-controlled mechanical properties in graphene helicoids: exceptional distribution-dependent behavior

Mechanical properties of pristine GHs along with patterned and randomly hydrogenated GHs have been investigated for various geometries and H-coverages.

Boronated holey graphene: a case of 2D ferromagnetic metal

In search of new candidates for two-dimensional ferromagnets, we consider boronated monolayer holey graphene (C₂B), akin to recently synthesized and extensively studied nitrogenated monolayer holey graphene (C₂N).

The Mechanical Properties of Defective Graphyne

Graphyne is a two-dimensional carbon allotrope with superior one-dimensional electronic properties to the “wonder material” graphene. In this study, via molecular dynamics simulations, we investigated the mechanical properties of α-, β-, δ-, and γ-graphynes with various type of point defects and cracks with regard to their promising applications in carbon-based electronic devices. The Young’s modulus and the tensile strength of the four kinds of graphyne were remarkably high, though still lower than graphene. Their Young’s moduli were insensitive to various types of point defects, in contrast to the tensile strength. When a crack slit was present, both the Young’s modulus and tensile strength dropped significantly. Furthermore, the Young’s modulus was hardly affected by the strain rate, indicating potential applications in some contexts where the strain rate is unstable, such as the installation of membranes.

Strain-induced dimensional phase change of graphene-like boron nitride monolayers

We investigate the coupling between the electronic bandgap and mechanical loading of graphene-like boron nitride (h-BN ) monolayers up to failure strains and beyond by means of first-principles calculations. We reveal that the kinks in the bandgap-strain curve are coincident with the ultimate tensile strains, indicating a phase change. When the armchair strain is beyond the ultimate tensile strain, h-BN fails with a phase transformation from 2D honeycomb to 1D chain structure, characterized by the 'V'-shape bandgap-strain curve. Large biaxial strains can break the 2D honeycomb structures into 0D individual atoms and the bandgap closes.

Mechanical and electromechanical properties of functionalized hexagonal boron nitride nanosheet: A density functional theory study

Hydroxylation as a technique is mainly used to alter the chemical characteristics of hexagonal boron nitride (h-BN), affecting physical features as well as mechanical and electromechanical properties in the process, the extent of which remains unknown. In this study, effects of functionalization on the physical, mechanical, and electromechanical properties of h-BN, including the interlayer distance, Young's modulus, intrinsic strength, and bandgaps were investigated based on density functional theory. It was found that functionalized layers of h-BN have an average distance of about 5.48 Å. Analyzing mechanical properties of h-BN revealed great dependence on the degree of functionalization. For the amorphous hydroxylated hexagonal boron nitride nanosheets (OH-BNNS), the Young's modulus moves from 436 to 284 GPa as the coverage of -OH increases. The corresponding variations in the Young's modulus of the ordered OH-BNNS with analogous coverage are bigger at 460-290 GPa. The observed intrinsic strength suggested that mechanical properties are promising even after functionalization. Moreover, the resulted bandgap reduction drastically enhanced the electrical conductivity of this structure under imposed strains. The results from this work pave the way for future endeavors in h-BN nanocomposites research.

A Review of Current Development of Graphene Mechanics

Graphene, a two-dimensional carbon in honeycomb crystal with single-atom thickness, possesses extraordinary properties and fascinating applications. Graphene mechanics is very important, as it relates to the integrity and various nanomechanical behaviors including flexing, moving, rotating, vibrating, and even twisting of graphene. The relationship between the strain and stress plays an essential role in graphene mechanics. Strain can dramatically influence the electronic and optical properties, and could be utilized to engineering those properties. Furthermore, graphene with specific kinds of defects exhibit mechanical enhancements and thus the electronic enhancements. In this short review, we focus on the current development of graphene mechanics, including tension and compression, fracture, shearing, bending, friction, and dynamics properties of graphene from both experiments and numerical simulations. We also touch graphene derivatives, including graphane, graphone, graphyne, fluorographene, and graphene oxide, which carve some fancy mechanical properties out from graphene. Our review summarizes the current achievements of graphene mechanics, and then shows the future prospects.

Multifunctional BBF monolayer with high mechanical flexibility and strong SHG response

A newly designed Be₂BO₃F₂ (BBF) monolayer's phonon dispersion and elastic constant reveal that the BBF monolayer is dynamically and mechanically stable. The BBF monolayer is a more flexible and ductile material, with a large band gap, and an exceptional second harmonic generation (SHG) response.

The ideal strength of two-dimensional stanene may reach or exceed the Griffith strength estimate

Ideal strength is the maximum stress a material can withstand, and it is an important intrinsic property for structural applications. A Griffith strength limit of ∼E/9 is the best known upper bound of this property for a material loaded in tension. Here we report that stanene, a recently fabricated two-dimensional material, could approach and possibly exceed this limit from a theoretical perspective. Utilizing first-principles density functional theory, we investigated the nonlinear elastic behavior of stanene and found that its strength could reach ∼E/7.4 under uniaxial tension in both armchair and zigzag directions without incurring phonon instability or mechanical failure. The unique mechanical properties of stanene are appreciated by comparison with other Group-IV 2D materials.

ReaxFF Reactive Force-Field Study of Molybdenum Disulfide (MoS2)

Two-dimensional layers of molybdenum disulfide, MoS₂, have been recognized as promising materials for nanoelectronics due to their exceptional electronic and optical properties. Here we develop a new ReaxFF reactive potential that can accurately describe the thermodynamic and structural properties of MoS₂ sheets, guided by extensive density functional theory simulations. This potential is then applied to the formation energies of five different types of vacancies, various vacancy migration barriers, and the transition barrier between the semiconducting 2H and metallic 1T phases. The energetics of ripplocations, a recently observed defect in van der Waals layers, is examined, and the interplay between these defects and sulfur vacancies is studied. As strain engineering of MoS₂ sheets is an effective way to manipulate the sheets' electronic and optical properties, the new ReaxFF description can provide valuable insights into morphological changes that occur under various loading conditions and defect distributions, thus allowing one to tailor the electronic properties of these 2D crystals.

Tunable negative Poisson's ratio in hydrogenated graphene

We perform molecular dynamics simulations to investigate the effect of hydrogenation on the Poisson's ratio of graphene. It is found that the value of the Poisson's ratio of graphene can be effectively tuned from positive to negative by varying the percentage of hydrogenation. Specifically, the Poisson's ratio decreases with an increase in the percentage of hydrogenation, and reaches a minimum value of -0.04 when the percentage of hydrogenation is about 50%. The Poisson's ratio starts to increase upon a further increase of the percentage of hydrogenation. The appearance of a minimum negative Poisson's ratio in the hydrogenated graphene is attributed to the suppression of the hydrogenation-induced ripples during the stretching of graphene. Our results demonstrate that hydrogenation is a valuable approach for tuning the Poisson's ratio from positive to negative in graphene.

Thermal transport in MoS2/Graphene hybrid nanosheets

Heat dissipation is a very critical problem for designing nano-functional devices, including MoS2/graphene heterojunctions. In this paper we investigate thermal transport in MoS2/graphene hybrid nanosheets under various heating conditions, by using molecular dynamics simulation. Diverse transport processes and characteristics, depending on the conducting layers, are found in these structures. The thermal conductivities can be tuned by interlayer coupling, environment temperature, and interlayer overlap. The highest thermal conductivity at room temperature is achieved as more than 5 times of that of single-layer MoS2 when both layers are heated and 100% overlapped. Different transport mechanisms in the hybrid nanosheets are explained by phonon density of states, temperature distribution, and interlayer thermal resistance. Our results could not only provide clues to master the heat transport in functional devices based on MoS2/graphene heterojunctions, but are also useful for analyzing thermal transport in other van der Waals hybrid nanosheets.

Mechanical degradation of graphene by epoxidation: insights from first-principles calculations

Oxidation is a major cause for the degradation of materials including graphene, where epoxidation (forming the C-O-C bond) is very common. In addition, graphene oxide, in which the epoxy group is one of the two major functional groups (the other is hydroxy), is an important precursor material used for the bulk synthesis of graphene sheets. Information about the mechanical stabilities, non-linear elastic properties, and elastic limits under various strain components is invaluable for application of these nanomaterials. Here, we investigate the mechanical properties of the epoxidized graphene in ordered graphene oxide, namely C6O1, C6O2, and C6O3, representing the carbon : oxygen ratios of 6 : 1, 3 : 1, and 2 : 1, respectively, using first-principles calculations within the framework of density functional theory. We predict a reduction of Young's modulus of graphene by a factor of 20%, 23%, and 27% for C6O1, C6O2, and C6O3, respectively, indicating a monotonic degradation with respect to epoxidation. However, there is no clear trend for Poisson's ratio, implying that the local atomic configurations are dominant over oxygen concentrations in determining the Poisson ratio. Our computed high order elastic constants are good for the design of graphene oxide based flexible transparent electronics.

Peculiar pressure effect on Poisson ratio of graphone as a strain damper

Hydrogenation is an effective way to modify the electronic and magnetic properties of graphene. The semi-hydrogenated graphene, known as "graphone", has promising applications in nanoelectronics including field-effect transistors. However, the elastic limit of this two-dimensional material remains unknown despite its importance in applications as well as strain engineering to tailor functions and properties. Here we report using first-principles calculations an abnormal increase in the Poisson ratio of graphone in response to an increase in pressure. This peculiar behavior is proposed to originate from the asymmetry of hydrogenation and could be used to design a nanodevice of strain damper to reduce harmful strains in graphene-based nanoelectronics.

Mechanical properties and stabilities of g-ZnS monolayers

Planar graphene-like ZnS monolayers are mechanically stable under various large strains.

Graphane versus graphene: a computational investigation of the interaction of nucleobases, aminoacids, heterocycles, small molecules (CO2, H2O, NH3, CH4, H2), metal ions and onium ions

We compared the binding affinity of graphane and graphene with various molecules and ions.

Mechanical properties and stabilities of α-boron monolayers

α-Boron monolayers are mechanically stable under various large strains.

Elastic limit of silicane

Silicane is a fully hydrogenated silicene-a counterpart of graphene-having promising applications in hydrogen storage with capacities larger than 6 wt%. Knowledge of its elastic limit is critical in its applications as well as tailoring its electronic properties by strain. Here we investigate the mechanical response of silicane to various strains using first-principles calculations based on density functional theory. We illustrate that non-linear elastic behavior is prominent in two-dimensional nanomaterials as opposed to bulk materials. The elastic limits defined by ultimate tensile strains are 0.22, 0.28, and 0.25 along armchair, zigzag, and biaxial directions, respectively, an increase of 29%, 33%, and 24% respectively in reference to silicene. The in-plane stiffness and Poisson ratio are reduced by a factor of 16% and 26%, respectively. However, hydrogenation/dehydrogenation has little effect on its ultimate tensile strengths. We obtained high order elastic constants for a rigorous continuum description of the nonlinear elastic response. The limitation of second, third, fourth, and fifth order elastic constants are in the strain range of 0.02, 0.08, and 0.13, and 0.21, respectively. The pressure effect on the second order elastic constants and Poisson's ratio were predicted from the third order elastic constants. Our results could provide a safe guide for promising applications and strain-engineering the functions and properties of silicane monolayers.

Outstanding mechanical properties of monolayer MoS2 and its application in elastic energy storage

The structural and mechanical properties of graphene-like honeycomb monolayer structures of MoS2 (g-MoS2) under various large strains are investigated using density functional theory (DFT). g-MoS2 is mechanically stable and can sustain extra large strains: the ultimate strains are 0.24, 0.37, and 0.26 for armchair, zigzag, and biaxial deformation, respectively. The in-plane stiffness is as high as 120 N m(-1) (184 GPa equivalently). The third, fourth, and fifth order elastic constants are indispensable for accurate modeling of the mechanical properties under strains larger than 0.04, 0.07, and 0.13 respectively. The second order elastic constants, including in-plane stiffness, are predicted to monotonically increase with pressure while the Poisson ratio monotonically decreases with increasing pressure. With the prominent mechanical properties including large ultimate strains and in-plane stiffness, g-MoS2 is a promising candidate of elastic energy storage for clean energy. It possesses a theoretical energy storage capacity as high as 8.8 MJ L(-1) and 1.7 MJ kg(-1), or 476 W h kg(-1), larger than a Li-ion battery and is environmentally friendly.

New materials graphyne, graphdiyne, graphone, and graphane: review of properties, synthesis, and application in nanotechnology

Plenty of new two-dimensional materials including graphyne, graphdiyne, graphone, and graphane have been proposed and unveiled after the discovery of the “wonder material” graphene. Graphyne and graphdiyne are two-dimensional carbon allotropes of graphene with honeycomb structures. Graphone and graphane are hydrogenated derivatives of graphene. The advanced and unique properties of these new materials make them highly promising for applications in next generation nanoelectronics. Here, we briefly review their properties, including structural, mechanical, physical, and chemical properties, as well as their synthesis and applications in nanotechnology. Graphyne is better than graphene in directional electronic properties and charge carriers. With a band gap and magnetism, graphone and graphane show important applications in nanoelectronics and spintronics. Because these materials are close to graphene and will play important roles in carbon-based electronic devices, they deserve further, careful, and thorough studies for nanotechnology applications.

Writing with ring currents: selectively hydrogenated polycyclic aromatics as finite models of graphene and graphane

Alternating partial hydrogenation of the interior region of a polycyclic aromatic hydrocarbon gives a finite model system representing systems on the pathway from graphene to the graphane modification of the graphene sheet. Calculations at the DFT and coupled Hartree-Fock levels confirm that sp² cycles of bare carbon centres isolated by selective hydrogenation retain the essentially planar geometry and electron delocalization of the annulene that they mimic. Delocalization is diagnosed by the presence of ring currents, as detected by ipsocentric calculation and visualization of the current density induced in the π system by a perpendicular external magnetic field. These induced 'ring' currents have essentially the same sense, strength and orbital origin as in the free hydrocarbon. Subjected to the important experimental proviso of the need for atomic-scale control of hydrogenation, this finding predicts the possibility of writing single, multiple and concentric diatropic and/or paratropic ring currents on the graphene/graphane sheet. The implication is that pathways for free flow of ballistic current can be modelled in the same way.