Impacts of defects on the mechanical and thermal properties of SiC and GeC monolayers.

Defect engineering has been considered as an effective way for controlling the heat transport properties of two-dimensional materials. In this work, the effects of point vacancies and grain boundaries on the mechanical and thermal performances of SiC and GeC monolayers are investigated systematically by molecular dynamics calculations. The failure strength in SiC and GeC is decreased by introducing vacancies at room temperature, and the stress-strain relationship can be tuned significantly by different kinds of vacancies. When the grain boundary of 21.78° is applied, the maximal fracture strengths can be as large as 27.56% for SiC and 23.56% for GeC. Also, the thermal properties of the two monolayers show a remarkable dependence on the vacancies and grain boundaries. The high vacancy density in SiC and GeC can induce disordered heat flow and the C/Ge point defect is crucial for thermal conductivity regulation for the Si/GeC monolayer. More importantly, the SiC and GeC monolayers with a grain boundary of 5.09° show excellent interfacial thermal conductance. Our findings are of great importance in understanding SiC and GeC monolayers and seeking their potential applications.

Ultraflexible two-dimensional Janus heterostructure superlattice: a novel intrinsic wrinkled structure.

The recently reported two-dimensional Janus transition metal dichalcogenide materials present promising applications such as in transistors, photocatalysts, and thermoelectric nanodevices. In this work, using molecular dynamics simulations, the self-assembled in-plane MoSSe/WSSe heterostructure superlattice is predicted with a natural sinusoidal structure constructed by an asymmetric interface. Such a sinusoidal structure shows extraordinary mechanical behavior where the fracture strain can be enhanced up to 4.7 times than that of the symmetrical interface. Besides, the deformational structure of all these MoSSe/WSSe heterostructure superlattice are in accordance with the Fourier function curve; the fracture strength and fracture strain also demonstrate pronounced size dependence. Our investigations proposed an ultrastretchable assembled heterostructure superlattice and provided a desirable strategy to tune the mechanical properties of such an in-plane two-dimensional heterostructure.

Two-dimensional MX2Y4 systems: ultrahigh carrier transport and excellent hydrogen evolution reaction performances.

Very recently, two-dimensional MoSi2N4 has been synthetized (Y.-L. Hong, Z. Liu, L. Wang, T. Zhou, W. Ma, C. Xu, S. Feng, L. Chen, M.-L. Chen and D.-M. Sun, Chemical vapor deposition of layered two-dimensional MoSi2N4 materials, Science, 2020, 369, 670-674.). In this work, we systematically explore the mechanical, electronic, and catalytic properties of the MX2Y4 (M = Cr, Hf, Mo, Ti, W, Zr; X = Si, Ge; Y = N, P, As) monolayers by first-principles calculations. These observed monolayers exhibit an isotropic Young's moduli of 165-514 N m-1 and a Poisson's ratio of 0.26-0.33. The calculated band structures indicate that their bandgaps are in the range of 0.49-2.05 eV at the HSE06 level. In particular, a high electron mobility of about 1.04 × 104 cm2 V-1 s-1 is observed in TiSi2N4 monolayers, which shows potential for high-speed electronic devices. MX2Y4 monolayers also reveal decent performances in the hydrogen evolution reaction. More importantly, the Gibbs free energy change of the TiSi2N4 (ZrSi2N4) monolayer is as small as 0.078 eV (-0.035 eV), even being comparable with that of Pt (-0.09 eV). This investigation suggests that the MoSi2N4 family monolayers have potential advanced applications such as photocatalytic, electrocatalytic, and photovoltaic devices.

Lattice thermal conductivity of Janus MoSSe and WSSe monolayers.

In this work, the heat transport properties of Janus MoSSe and WSSe monolayers are systematically investigated using non-equilibrium molecular dynamics simulations. Strong size dependence of the thermal conductivity is found in the Janus MoSSe and WSSe monolayers. In the two-dimensional limit, the Mo-based Janus MoSSe monolayer shows a higher thermal conductivity but similar phonon mean free path as MoS2, while the W-based Janus WSSe monolayer shows a similar thermal conductivity but longer phonon mean free path than WSe2. These two Janus monolayers also present quite different temperature dependencies. With temperature increasing from 100 K to 350 K, the reduction in thermal conductivity of MoSSe is up to 28.4%, but only 12.75% in WSSe, because of the weak temperature dependence in the phonon density of states. With 2% vacancy density, the thermal conductivity of defective MoSSe is only 16.03% that of pristine MoSSe, while for defective WSSe, the thermal conductivity is 14.04% that of pristine WSSe. The strong dependence on vacancy is explained by atomic heat flux vector analysis. The present study demonstrates rich physical phenomena in the thermal transport properties of Janus transition metal dichalcogenide monolayers, which may offer a new degree of freedom for manipulating their thermal conductivity for applications including thermal management and thermoelectric devices.

Hyperbolic Graphene Framework with Optimum Efficiency for Conductive Composites.

Constructing conductive filler networks with high efficiency is essential to fabricating functional polymer composites. Although two-dimensional (2D) sheets have prevailed in nanocomposites, their efficiency in enhancing conductive functions seems to reach a limit, as if merely addressing the dispersion homogeneity. Here, we exploit the unrecognized geometric curvature of 2D sheets to break the efficiency limit of filler systems. We introduce the hyperbolic curvature concept to mediate the incompatibility between 2D planar topology and 3D filler space and hold the efficient conductive path through face-to-face contact. The hyperbolic graphene framework exhibits a record efficiency in enhancing electrically and thermally conductive functions of nanocomposites. At a volume loading of only 1.6%, the thermal and electrical conductivities reach 31.6 W/(mK) and 13 911 S/m, respectively. We demonstrate that the conductive nanocomposites with a hyperbolic graphene aerogel framework are useful for thermal management, sensing, and electromagnetic shielding. Our work provides a solution to reconcile the incompatibility between the 2D planar structure of sheets and the highly expected 3D conductive path, presenting a geometrically optimal filler system to break the efficiency limit of multifunctional nanocomposites and broaden the structural design space of 2D sheets by curvature modulation to meet more applications.

Reduced Ionic Conductivity but Enhanced Local Ionic Conductivity in Nanochannels.

The ionic transport in nanoscale channels with the critical size comparable to ions and solvents shows excellent performance on electrochemical desalination, ion separation, and supercapacitors. However, the key quantity ionic conductivity (σ) in the nanochannel that evaluates how easily the electric current is driven by an external voltage is still unknown because of the challenges in experimental measurement. In this work, we present an atomistic simulation-based study, which shows that how the ion concentration, nanoconfinement, and heterogeneous solvation modify the ionic conductivity in a two-dimensional graphene nanochannel. We find that σ in the confined channel is lower than that in the bulk (σb) at the same concentration along with enhanced ion-ion correlation. However, surprisingly, the local σ near the channel wall is more conductive than σb and is about 2-3 folds of the inner layer due to the highly concentrated charge carriers. Based on the layered feature of σ along the width of the channel, we propose a model that contains two dead (or depletion) layers, two highly conductive layers, and one inner layer to describe the ionic dynamics in the nanochannels. Our findings may open the way to unique nanofluidic functionalities, such as energy harvesting/storage and controlling transport at single-molecule and ion levels using the liquid layer near the wall.

Multi-Scale Structure-Mechanical Property Relations of Graphene-Based Layer Materials.

Pristine graphene is one of the strongest materials known in the world, and may play important roles in structural and functional materials. In order to utilize the extraordinary mechanical properties in practical engineering structures, graphene should be assembled into macroscopic structures such as graphene-based papers, fibers, foams, etc. However, the mechanical properties of graphene-based materials such as Young's modulus and strength are 1-2 orders lower than those of pristine monolayer graphene. Many efforts have been made to unveil the multi-scale structure-property relations of graphene-based materials with hierarchical structures spanning the nanoscale to macroscale, and significant achievements have been obtained to improve the mechanical performance of graphene-based materials through composition and structure optimization across multi-scale. This review aims at summarizing the currently theoretical, simulation, and experimental efforts devoted to the multi-scale structure-property relation of graphene-based layer materials including defective monolayer graphene, nacre-like and laminar nanostructures of multilayer graphene, graphene-based papers, fibers, aerogels, and graphene/polymer composites. The mechanisms of mechanical property degradation across the multi-scale are discussed, based on which some multi-scale optimization strategies are presented to further improve the mechanical properties of graphene-based layer materials. We expect that this review can provide useful insights into the continuous improvement of mechanical properties of graphene-based layer materials.

Controllable Thermal Conductivity in Twisted Homogeneous Interfaces of Graphene and Hexagonal Boron Nitride.

Thermal conductivity of homogeneous twisted stacks of graphite is found to strongly depend on the misfit angle. The underlying mechanism relies on the angle dependence of phonon-phonon couplings across the twisted interface. Excellent agreement between the calculated thermal conductivity of narrow graphitic stacks and corresponding experimental results indicates the validity of the predictions. This is attributed to the accuracy of interlayer interaction descriptions obtained by the dedicated registry-dependent interlayer potential used. Similar results for h-BN stacks indicate overall higher conductivity and reduced misfit angle variation. This opens the way for the design of tunable heterogeneous junctions with controllable heat-transport properties ranging from substrate-isolation to efficient heat evacuation.

Continuous crystalline graphene papers with gigapascal strength by intercalation modulated plasticization.

Graphene has an extremely high in-plane strength yet considerable out-of-plane softness. High crystalline order of graphene assemblies is desired to utilize their in-plane properties, however, challenged by the easy formation of chaotic wrinkles for the intrinsic softness. Here, we find an intercalation modulated plasticization phenomenon, present a continuous plasticization stretching method to regulate spontaneous wrinkles of graphene sheets into crystalline orders, and fabricate continuous graphene papers with a high Hermans' order of 0.93. The crystalline graphene paper exhibits superior mechanical (tensile strength of 1.1 GPa, stiffness of 62.8 GPa) and conductive properties (electrical conductivity of 1.1 × 105 S m-1, thermal conductivity of 109.11 W m-1 K-1). We extend the ultrastrong graphene papers to the realistic laminated composites and achieve high strength combining with attractive conductive and electromagnetic shielding performance. The intercalation modulated plasticity is revealed as a vital state of graphene assemblies, contributing to their industrial processing as metals and plastics.

Exploring the structure-property relationship of three-dimensional hexagonal boron nitride aerogels with gyroid surfaces.

Three-dimensional hexagonal boron nitride aerogels (hBNAGs) are novel porous materials with many promising applications such as energy storage, thermal insulation and sensing. However, the structure-property relationships of hBNAGs in complicated thermo-mechanical coupled environments are still not clear. In this study, we employed a binary phase-field crystal (PFC) model to construct the atomic structures of hBNAGs, upon which the mechanical and thermal behaviors of hBNAGs were systematically investigated using large-scale atomistic simulations. It is found that the hBNAG geometry and topological defects strongly affect the mechanical and thermal properties. For example, the Young's modulus and tensile strength follow the scaling laws of mass density with a power factor of about 1.4 and 1.2, respectively, indicating that the stretching and bending combine toward tensile deformation. In addition, cracks nucleate around the octagon defects, indicating that the tensile strength is also influenced by the topological defects. Under compression, complicated crumpled deformations and ridges in the entire region are observed and the compression strength follows the scaling law of mass density with a power factor above 2.0, which means that a large portion of the hBNAGs do not contribute to the compression load bearing. We find that hBNAGs have a very low thermal conductivity of about two orders of magnitude lower than that of a hBN sheet. Also, the thermal conductivity of hBNAGs increases with increasing mass density, which also follows a scaling law. The power of the scaling law is about 0.5, indicating that the thermal conductivity has a strong nonlinear dependence on the mass density. Our work provides a deep understanding of the structure-property relationships of hBNAGs, which is useful for the engineering applications of hBNAGs.

The mechanical and thermal properties of MoS2-WSe2 lateral heterostructures.

The recently fabricated monolayer MoS2-WSe2 lateral heterostructures are promising for many interesting applications, such as p-n diodes, photodetectors, transistors, sensors, light-emitting diodes and thermoelectric and flexible nanodevices. In this work, we study the mechanical and thermal properties of MoS2-WSe2 lateral heterostructures by using molecular dynamics (MD) simulations based on the recently parameterized Stillinger-Weber (SW) potential. It is found that the fracture strength and fracture strain of MoS2-WSe2 lateral heterostructures are dictated by the mechanical properties of MoS2, and are very sensitive to temperature. However, when a crack is introduced in the MoS2-WSe2 heterostructures, failure may occur either in MoS2 or WSe2, depending on the crack length and location. Interestingly, the fracture strengths obtained from our MD simulations are in agreement with those obtained from the Griffith theory. Our MD simulations further reveal that, in addition to the low thermal conductivities of MoS2 and WSe2, the MoS2-WSe2 heterojunctions exhibit a very low interfacial thermal conductance, which is about one order of magnitude lower than that of graphene-hBN heterojunctions.

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.