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Over the past decade, four distinct differences have been summarized regarding ripplocation and kink boundaries in layered solids: spontaneous reversibility, atomic sharpness, delamination, and residual stress. However, according to molecular dynamics simulations, these four distinct differences are not completely applicable to the ripplocation and kink boundaries in graphene/copper nanolaminates. Contrary to conventional understanding, ripplocation boundaries can transform into kink boundaries without triggering the collective movement of other ripplocations. In addition, ripplocation boundaries can be irreversible with out-of-plane slip, leading to coherent twins in [110] orientation and stacking faults in [111] orientation. Moreover, delamination only occurs at kink boundaries rather than ripplocation boundaries. Wide-spreading residual stresses are absent for both ripplocation and kink boundaries due to plasticity. These findings broaden the current understanding of ripplocation and kink boundaries in layered solids. [ABSTRACT FROM AUTHOR]

Recently, liquid flow over monolayer graphene has been experimentally demonstrated to generate an induced voltage in the flow direction, and various physical mechanisms have been proposed to explain the electricity-generating process between liquid and graphene. However, there are significant discrepancies in the reported results with non-ionic liquid: the observed voltage responses with deionized (DI) water vary from lab to lab under presumably similar flowing conditions. Here, a graphene-piezoelectric material heterostructure is proposed for harvesting energy from water flow; it is shown that the introduction of a piezoelectric template beneath graphene results in an obvious voltage output up to 0.1 V even with DI water. This potential arises from a continuous charging-discharging process in graphene, which is suggested to be a result of a relatively retarded screening effect of the water for the generated piezoelectric charges than that of the graphene layer, as revealed by first-principles calculations. This work considers a dynamic charge interaction among water, graphene, and the substrate, highlighting the crucial role of the underlying substrate in the electricity-generating process, which will greatly enhance understanding of the flow-induced voltage and push the graphene-water nanogenerator close to practical applications. [ABSTRACT FROM AUTHOR]

The manipulation of dynamic ripple pattern in graphene monolayer is investigated at a theory and quantitative level. High quality, stable ripple patterns are generated and maintained using interference from transverse waves in the graphene monolayer at room temperature. The dissipation of transverse waves due to edge reflections leads to low spatial and temporal quality of the graphene ripple pattern. The spatial quality of the ripple pattern is improved using oblique multi-reflection and the temporal quality is improved by overlaying dispersed waves in graphene sheet. According to our simulations and theoretical analysis, the pattern and the electronic properties of graphene ripple can be controlled by varying the interference angle and the vibrational source frequency. The dynamic ripple pattern is not compatible with devices over 3.0 THz due to chiral dispersion. Our results can provide a valuable guideline for design and implementation of surfing graphene ripples in nanoelectromechanical systems and devices. [ABSTRACT FROM AUTHOR]

The porous characteristics of disordered carbons are critical factors to their performance on hydrogen storage and electrochemical capacitors. Even though the porous information can be estimated indirectly by gas adsorption experiments, it is still hard to directly characterize the porous morphology considering the complex 3D connectivity. To this end, we construct full-atom disordered graphene networks (DGNs) by mimicking the chlorination process of carbide-derived carbons using annealing-MD simulations, which could model the structure of disordered carbons at the atomic scale. The porous characteristics, including pore volume, pore size distribution (PSD), and specific surface area (SSA), were then computed from the coordinates of carbon atoms. From the evolution of structural features, pores grow dramatically during the formation of polyaromatic fragments and sequent disordered framework. Then structure is further graphitized while the PSD shows little change. For the obtained DGNs, the porosity, pore size, and SSA increase with decreasing density. Furthermore, SSA tends to saturate in the low-density range. The DGNs annealed at low temperatures exhibit larger SSA than high-temperature DGNs because of the abundant free edges. [ABSTRACT FROM AUTHOR]

The effect of interlayer shear on bending property of multi-layer graphene is investigated using molecular dynamics simulations. Interlayer shear modulus is three orders lower than in-plane Young's modulus, thus the bending behavior is dominated by interlayer shear. Continuum theory and atomistic simulations show that bending rigidity is proportional to layer number when the layer number exceeds 5. The small difference of these two proportional constants results from the stacking pattern, and turbostratic stacking decreases the interlayer shear modulus. For layer number smaller than 4, intrinsic ripples also play an important role and further decrease the interlayer shear modulus. [ABSTRACT FROM AUTHOR]

Gas transport through nanochannels is ubiquitous in nature and also plays an important role in industry. The gas flow in this regime can be described by the Knudsen theory, which assumes that molecules diffusely reflect on the confining walls. However, with the emergence of low dimensional carbon-based materials such as graphene and carbon nanotubes, it has been evidenced that this assumption might not hold for some atomically smooth surfaces, resulting in an anomalous enhancement of gas flux. In this study, we meticulously investigated the gas transport through nanochannels constructed by two selected kinds of 2D materials, graphene and molybdenum disulfide (MoS 2), and interpreted the remarkable difference in gas flux through these two nanochannels. We revealed the underlying mechanism as the surface morphological effect on the gas collision with solid walls. Even a subtle distinction of surface roughness results in specular scattering on graphene surfaces while diffuse scattering on MoS 2 surfaces. We also found that the curvature effect could reduce the surface roughness of interaction potential surfaces, leading to an additional enhancement on the gas flow rate. These insights are expected to deepen the understanding of gas transport in nanochannels, paving a promising way in the gas permeation control. [Display omitted] [ABSTRACT FROM AUTHOR]

Transverse wave propagation in single-layer graphene sheet (SLGS) is studied via molecular dynamics simulation, continuum, and non-continuum analysis. We found that the propagation of transverse waves with frequency over 3 THz is remarkably chirality-dependent. Furthermore, the wave propagation in zigzag direction remains undistorted only when the frequency is below 16 THz, while this threshold is 10 THz in the armchair direction. The minimum permissible wavelength is proposed to explain the frequency limitation due to non-continuity. Our findings lead to an improved fundamental understanding on the vibration of graphene-based nanodevices and have potential applications in design and fabrication of nanoelectromechanical systems. [ABSTRACT FROM AUTHOR]

Experiments showed that the fracture toughness of silicon carbide (SiC) ceramics can be enhanced by adding graphene-based fillers. To understand the toughening mechanism and thoroughly explore the toughening potential, we used a multiscale approach combined with molecular dynamics (MD) simulations to theoretically study the correlation between the toughening effect and the microstructure parameters of SiC/graphene lamellar composite. MD simulations demonstrated that the pull-out force can fluctuate periodically around a constant during the pull-out process and it will drop quickly to zero when the graphene sheet is pulled out from the SiC matrix entirely. The modified crack-bridging model revealed that the toughening enhancement of graphene/SiC composites can be improved with larger graphene size and volume fraction. The macroscopic fracture toughness and the atomistic level pull-out force are linked by the proposed multiscale crack-bridging model. We also found that the fewer-layer graphene sheets can better reinforce the fracture toughness than the more layer ones. These understandings may help the design and preparation of SiC/graphene lamellar composites toward better fracture toughness. [ABSTRACT FROM AUTHOR]

Pyrolytic carbon (PyC) materials are laminar deposits formed from the high-temperature pyrolytic reaction of volatile hydrocarbons. The outstanding mechanical performance of PyC renders it attractive as interfacial materials in aerospace applications and thermonuclear fusion. But the deposition process of PyC under extreme conditions in particular makes it challenging to achieve large-sized specimens used in further cognition on processing-microstructure-property relationship for PyC materials. Toward recent molecular insights into the initial formation of PyC [Carbon 148 (2019) 307–316], we herein performed molecular dynamics simulations to investigate the micromechanical properties of PyC with the consideration of interlayer crosslinks. Under out-of-plane uniaxial tension, moderate numbers of interlayer crosslinks can significantly enhance the mechanical properties of PyC. Meanwhile, the increase of interlayer crosslinks will improve the energy dissipation through local and multilayer delamination and tearing. Interlayer crosslinks can provide strong junctions between adjacent layers to bear out-of-plane shear deformation effectively. While under in-plane tension or shearing, the mechanical properties of PyC are weakened due to intralayer pore defects introduced by interlayer crosslinks. Moreover, tension-compression asymmetry of PyC is found at nanoscale. Graphene layers with interlayer crosslinks experience kink-like puckering deformation in the in-plane compression, while they only straighten in the in-plane stretching before failure. Image 1 [ABSTRACT FROM AUTHOR]

The fatigue performance of pre-cracked graphene–copper artificial nacre (GrCu nacre) under cyclic tensile loading is investigated using theoretical analysis and molecular dynamics (MD) simulations. Mechanical models are proposed to describe the fatigue performance of GrCu nacre, which agree well with MD simulations. Compared with nanocomposites consisting of large graphene sheets, graphene fragments cannot improve the ultimate strength of GrCu nacre, but significantly improve its fatigue limit. This anti-fatigue effect is dominated by the graphene–copper interfaces with van der Waals interactions, which can be adjusted by tailoring the architectural parameters of GrCu nacre. The ultimate strength of GrCu nacre decreases as the volume fraction of graphene increases. The fatigue limit of GrCu nacre decreases as the length of graphene fragments increases, and can be approximately described by a quadratic function of interlayer repeat spacing. An anomalous weakening effect occurs when the in-plane distance between graphene fragments is less than 2 nm, while this effect can be ignored when it is higher than 2 nm. The fatigue ratio of common metals is ∼ 0.4, whereas that of GrCu nacre is generally higher than 0.9. These underlying mechanisms and theoretical models can improve the fundamental understanding of the fatigue performance of composites with incoherent interfaces, as well as the bottom-up design of graphene-metal nanocomposites. [Display omitted] [ABSTRACT FROM AUTHOR]

Covalently bonded graphene/single-walled carbon nanotube (SWCNT) hybrid material can extend the excellent properties of graphene and SWCNT to three dimensions. We perform molecular dynamics simulations to investigate the mechanical properties of pillared vertically aligned carbon nanotube-graphene (VACNT-graphene) structure under uniaxial tensile and compressive loadings. The simulation results demonstrate that VACNT-graphene structures exhibit excellent elasticity with considerable elastic compressive/tensile strain limit. The early small deformation is dominated by the bending of graphene layers. Under larger elastic strain, graphene layers will form a corrugated pattern and shrink laterally in both the compression and tension processes. In the compression process, the failure mode of VACNT-graphene structure with small pillar distance is the bond breaking and reforming on the ridges of corrugated-shaped graphene and junction areas, while the failure mode of the structure with large pillar distance is the structural buckling. The elastic compressive strain limit exhibits a local maxima at the critical point between the two failure modes. In the tensile process, the failure is due to structural damage and elasticity increases with the pillar distance increasing. The essential tendency in our simulated VACNT-graphene structure might be significant to the further design and application of CNT-graphene hybrid materials. [ABSTRACT FROM AUTHOR]

The mechanism of interfacial strengthening and self-healing effect in graphene-copper nanolayered (GCuNL) composites under shear deformation is investigated at a theory and quantitative level. It is found that the interfacial constraining effect between graphene and copper layer highly improves the shear strength and toughness of GCuNL composites. The interlayer distance between graphene monolayers and the crystal stacking orientation of copper layers plays an important role in the shear yield strength of composites. The shear toughness of composites is jointly determined by the crystal orientation of copper layer and the chirality of graphene. The shear failure strain of zigzag-based composites is remarkably higher than that of armchair-based composites, while the shear failure stress of (100)-stacking composites is larger than that of (111)-stacking composites due to rotation of slip bands. Furthermore, we find a remarkable self-healing effect in GCuNL composites by interfacial trapping dislocations. The self-healing ability is determined by the interlayer distance between graphene monolayer. Synthesizing both the strengthening and self-healing effect, the optimum distance between graphene monolayers is ranging from 5 to 15 nm. [ABSTRACT FROM AUTHOR]

• Permeation rate of ethanol through GNPs coating is much higher than that of water. • Distinctive evaporation characteristics of water and ethanol manifest on GNPs coating. • Ultrafast water permeation into GNPs nanostructures leads to filmwise evaporation. • Ethanol is retained in the GNPs nanostructures and no evaporation occurs. The mass transport of water and ethanol molecules through the oxygen-functionalized graphene nanostructures, particularly at a high temperature, is not well understood. Here, the evaporation characteristics of water and ethanol droplets on graphene nanoplatelets (GNPs) coating are experimentally investigated and compared. The ultrafast water permeation property of GNPs coatings has shown promising applications in enhancing the phase-change heat transfer attributed to the formation of ultrathin film of water, leading to the filmwise evaporation. Interestingly, the permeation rate of ethanol (and other short-chain alcohols) through the GNPs coatings is much higher than that of water, justified by our experiments and molecular dynamics simulations. However, the results of evaporation of ethanol from the GNPs at a high temperature are counterintuitive where ethanol is retained in the GNPs nanostructures and no evaporation occurs. These results are evidenced by the evaporation heat transfer experiments and thermal gravimetric analysis. Spectroscopic analyses are conducted to scrutinize the underlying mechanisms of sorption and desorption of water and ethanol through the GNPs nanostructures. It is found that ethanol absorbs the applied heat energy as the activation energy to form the molecular bonds between the ethanol molecules and the oxygenated functional groups of GNPs. The present study provides new insights into the sorption and desorption processes of water and ethanol through the GNPs nanostructures, which is beneficial to the phase-change heat transfer applications with graphene-based materials. [ABSTRACT FROM AUTHOR]