Bicrystalline Graphene Research Articles

Fracture behaviour of bicrystalline graphene characterized via freestanding indentation

Using molecular mechanics (MM) simulations, the failure stress of bicrystalline graphene with different tilt angles is investigated using their free-standing indentation behaviour, in which two types of grain boundaries, including armchair (ac) and zigzag (zz) grain boundaries, and two types of indenter tip, including cylindrical and spherical tips are considered. For reference purposes, the corresponding results under inplane stretching are also examined. It is found that the failure stress of grain boundary (GB) in bicrystalline graphene decreases with the decrease of the GB tilt angle [Formula: see text], which is similar to that determined in inplane stretching. In indentation, the stress concentration under the indenter tip will decrease the failure stress of graphene; in addition, the out-of-plane deformation of graphene in indentation can partially release the pretension induced by the GB in bicrystalline graphene. For the GB with a small prestress, the effect of the former is larger than that of the latter; but for the GB with a larger prestress, the effect of the latter is larger than that of the former. Consequently, the effect of tilt angle [Formula: see text] on the GB strength of bicrystalline graphene is lower in indentation than that given by inplane stretching.

Mechanical and fracture behaviour of pristine and defective single/bi-crystal graphene/Ti nanocomposites using molecular dynamics simulations

Metal matrix nanocomposites (MMNCs) are emerging as lightweight but strong propositions with high specific strength and elastic modulus for specific design applications. In the present paper, single-crystal (GrNSs) and bi-crystalline graphene nanosheets (5|7GrNSs) were reinforced into titanium (Ti) matrices to investigate the mechanical and fracture characteristics of Ti-based MMNCs i.e. TiGrNCs and 5|7TiGrNCs, using molecular dynamics (MD). Based on our findings it can be observed that the use of GrNSs helps improve the strength of Ti in a significant manner. The results further demonstrate that the presence of grain boundaries (GBs) did not exert a significant impact on the mechanical properties of nanocomposites (NCs). Moreover, the presence of vacancies and grain boundaries did not deteriorate the mechanical properties of NCs in a deleterious manner. These outcomes will help in the development of NCs that are both lightweight and structurally stronger for the automotive, marine, and aerospace industries.

Effect of Grain Boundaries’ Doping on the Mechanical Properties of Nitrogen-Doped Bicrystalline Graphene

Grain boundaries and doping play an important role in influencing the mechanical behaviour of materials. In this article, Tersoff potential based molecular dynamics simulations were employed to comprehensively investigate the effect of random N doping on the mechanical behaviour of pristine and bicrystalline graphene nanosheets. For bicrystalline nanosheets, three types of grain boundaries with high, transition and low tilt angles, were considered here. Each grain boundary model was doped as a whole nanosheet, only grain boundary area, and/or non-grain boundary area. Our results show that the configurations with doping of grain boundary area showed more deterioration in failure stress and failure strain values than the configurations with doping of the non-grain boundary area. Also, N doped pristine nanosheets and the configurations with non-grain boundary area doping showed a monotonic increase in Young's modulus while a monotonic decreasing-increasing trend was predicted for the rest of the configurations. These results show that the doping of the grain boundary area is more detrimental to the mechanical properties than the doping of the non-grain boundary area. Moreover, even after N doping, the high angle grain boundaries show higher strength as compared to the low angle grain boundaries.

Effect of grain boundary and defect on mechanical properties of bicrystalline graphene

The effects of grain boundary (GB) and defect on the deformation behaviors of graphene under tension or compression in directions perpendicular ([Formula: see text]-axis) or parallel ([Formula: see text]-axis) to GB are investigated by molecular dynamics (MD) simulation. The results show that the bicrystalline graphene can sustain more load under tension along [Formula: see text]-axis than along [Formula: see text]-axis, and that regardless of tensile directions, the yield strength of bicrystalline graphene with high misorientation angle is higher than that with low angle. Furthermore, we found that the effect of GB on the deformation behavior of graphene under compression strain along [Formula: see text]-axis is more obvious than that along [Formula: see text]-axis.

Sequential multiscale model to study crack tip behavior in bi-crystalline graphene

The aim of this research article is to develop a sequential multiscale model to study the inter-granular fracture behavior in bi-crystalline graphene. A multiscale model encompasses the microscale behavior at a macroscale level. A sequential multiscale model was developed by defining the traction separation law (TSL), using atomistic simulations, which was later on used to quantify damage in cohesive zone elements in continuum based finite element based simulations. Sequential multiscale models constitute developing bi-crystalline graphene configuration in the environment of molecular dynamics and capture the intergranular crack propagation in graphene. In the molecular dynamics based simulations, symmetrical and asymmetrical tilt grain boundaries were generated in conjunction with edge crack. The TSL derived from these simulations helps in improving the accuracy of continuum level models for predicting intergranular fracture. Spatial stress distribution in the vicinity of the crack tip as well as stress values were captured and compared for atomistic levels with a continuum level for the validation of sequential multiscale models.

Inter-granular fracture toughness of bi-crystalline graphene nanosheets

Aim of this research article was to study inter-granular fracture toughness of graphene containing edge crack. Molecular dynamics based simulations were performed in conjunction with AIREBO potential to study the crack tip behaviour in pristine and bi-crystalline graphene nanosheets. Higher value of linear dislocation density at the grain boundaries helps in improving the fracture toughness of bi-crystalline graphene, and trend was predicted to be independent of chirality vector in mode-I loading. Higher dislocation density imparts lower change in critical bond length with applied strain, which helps in improving resistance to crack growth. Additionally, switching between heptagon and pentagon rings positioning at the crack tip also helps in improving the fracture toughness of bi-crystalline graphene. Crack tip containing heptagon rings has positive effect on the fracture toughness of bi-crystalline graphene as compared to pentagon rings. Outcome of the analysis will provide new insights in fracture properties of bi-crystalline graphene that further helps in utilising graphene nanosheets in mechanically robust applications such as pressure barrier, nanocomposites and ion-separation.

Deformation and fracture behavior of bicrystal graphene: an atomic level simulation

In this study, extensive molecular dynamics simulations were carried out to investigate failure processes along different symmetric tilt grain boundaries (STGB) of bicrystal graphene sheet. Two different types of STGBs graphene mainly zigzag and arm-chair types were investigated. The dependence of fracture strength, strain as well as Young’s moduli on different STGBs were examined. The results clearly show that pristine graphene has the highest values of fracture strength and strain to fracture. Furthermore, bicrystal graphene with zigzag-oriented grain boundaries have improved mechanical properties in comparison to those with arm-chair oriented grain boundaries. Fracture behavior was investigated by applying mode I loadings to the outer boundary of bicrystalline graphene sheet with several misorientation angles. The critical stress intensity factors (SIFs) are calculated as a function of displacement were determined by using crack-tip opening displacements (CTOD) at the incipient bond breaking. The atomistic results show that the crack propagation along armchair-orientation grain boundaries are faster than that of zigzag-orientation grain boundaries of bicrystal graphene.

Enhanced thermal transport across a bi-crystalline graphene-polymer interface: an atomistic approach.

The objective of this investigation was to elaborate on the influence of grain boundaries on the interfacial thermal conductance between bi-crystalline graphene and polyethylene in a nanocomposite. Reverse non-equilibrium molecular dynamics simulations were implemented in combination with Lennard-Jones and reactive force field interatomic potential parameters. According to the simulation results, high-energy grain boundary atoms in bi-crystalline graphene played a substantial role in enhancing the interfacial thermal conductance values. To further illuminate the mechanisms of enhanced graphene-polyethylene interfacial thermal conductance in the presence of grain boundaries, a systematic study on the vibrational density of states and structural evolution was also performed. It was found that the vibrational coupling between bi-crystalline graphene and the polymer was enhanced; whereas a decline in the radial density profile and coordination number resulted in a shifting of the in-plane vibrational modes such that they amalgamated with those of the polyethylene matrix. Thus, bi-crystalline graphene can be considered to be a superior potential reinforcement for nanocomposites as compared to the pristine configuration for applications in thermoelectric and thermal interface materials.

Effect of grain boundaries on the interfacial behaviour of graphene-polyethylene nanocomposite

Aim of this article was to investigate the effect of grain boundaries on the interfacial properties of bi-crystalline graphene/polyethylene based nanocomposites. Molecular dynamics based atomistic simulations were performed in conjunction with the reactive force field parameters to capture atomic interactions within graphene and polyethylene atoms, whereas non-bonded interactions were considered for the interfacial properties. Atoms at the higher energy state in bi-crystalline graphene helps in improving the interaction at the nanocomposite interphase. Geometrical imperfections such as wrinkles and ripples helps the bi-crystalline graphene in increasing the number of adhesion points between the nanofiller and matrix, which eventually improves the strength and toughness of nanocomposite. These outcomes will help in opening new opportunities for defective nanofillers in the development of nanocomposites for future applications.

Reactive force field based atomistic simulations to study fracture toughness of bicrystalline graphene functionalised with oxide groups

The aim of this article was to study the effect of oxide functionalisation on the fracture toughness of bicrystalline graphene. Molecular dynamics based simulations in conjunction with reactive force field were performed to study the fracture toughness of functionalised bicrystal of graphene. Separate studies were performed with hydroxyl and epoxide functionalisation, and later on the same simulations were extended over graphene oxide (GO) as a whole. Failure morphologies depict that epoxide groups tend to boost the fracture toughness, via altering the failure path and transforming the fracture mode from mode-I to mode-II. In addition to the transformation, epoxide-to-ether conversion also played significant role in enhancing the fracture toughness of bicrystalline graphene. On the other hand, steric hindrance exhibited by the hydroxyl group mitigates the fracture toughness of GO. Overall, certain spatial sandwich configurations of epoxide groups concluded an enhanced fracture toughness for bicrystalline graphene; which further opens new avenues for the application of these graphene sheets in nanodevices, nanomembranes and nanocomposites.

Tailoring the failure morphology of 2D bicrystalline graphene oxide

The aim of this article is to study the effect of oxide functionalisation on the failure morphology of bicrystalline graphene. Molecular dynamics based simulations in conjunction with reactive force field were performed to study the mechanical properties as well as failure morphology of different configurations of bicrystalline graphene oxide. Separate simulations were performed with hydroxyl and epoxide functionalisation, and later on the same simulations were extended to study the graphene oxide as a whole. The authors have predicted that epoxide functionalisation helps in transforming the catastrophic brittle behaviour into ductile. Failure morphologies depict that epoxide groups tend to boost the ductility through altering the fracture path and not affecting the grain boundaries either. Also, the epoxide to ether transformations were found to be the decisive mechanism behind the plastic response shown by epoxide groups. Simulations help in concluding a ductile failure for bicrystalline graphene in conjunction with oxidation of selective atoms in the nanosheet, which further opens new avenues for the application of these graphene sheets in nanodevices and nanocomposites.

Dependence of the In-Plane Thermal Conductivity of Graphene on Grain Misorientation

The thermal transport across the grain boundary (GB) is inevitably encountered for large-area polycrystalline graphene. However, the influence of GB configuration on thermal transport is not well understood. Here we investigated the effect of grain misorientation angle on the in-plane thermal conductivity (κ) of suspended graphene by using the optothermal Raman technique. Graphene with well-defined grain orientation was synthesized on an electropolished, annealed, and oxygen plasma-treated single-crystalline Cu(111) substrate by low-pressure chemical vapor deposition. The κ was primarily dependent on the grain size of single-, bi-, and polycrystalline graphene, consistent with the Boltzmann transport model. Surprisingly, κ of bicrystalline graphene dramatically decreased with a slight misorientation (<4°) between two neighboring grains. This phonon-boundary scattering was successfully simulated by the GB misorientation model. The GB length or shape also affected κ as a tertiary parameter. The GB misorient...

Kapitza thermal resistance across individual grain boundaries in graphene

We study heat transport across individual grain boundaries in suspended monolayer graphene using extensive classical molecular dynamics (MD) simulations. We construct bicrystalline graphene samples containing grain boundaries with symmetric tilt angles using the two-dimensional phase field crystal method and then relax the samples with MD. The corresponding Kapitza resistances are then computed using nonequilibrium MD simulations. We find that the Kapitza resistance depends strongly on the tilt angle and shows a clear correlation with the average density of defects in a given grain boundary, but is not strongly correlated with the grain boundary line tension. We also show that quantum effects are significant in quantitative determination of the Kapitza resistance by applying the mode-by-mode quantum correction to the classical MD data. The corrected data are in good agreement with quantum mechanical Landauer-Bütticker calculations.

Molecular dynamics study of fracture toughness and trans-intergranular transition in bi-crystalline graphene

We investigate the deformation and fracture behaviors of pristine and bi-crystalline graphenes by molecular dynamics simulations. For pristine graphene with a pre-crack, fracture toughness is strongly dependent on the crack morphology and atomic configuration at the crack tip. For bi-crystalline graphene, fracture toughness becomes comparable to that of pristine graphene with increase in density of topological 5–7 defects arranged uniformly along grain boundary due to cancellation of the dipole stress field. In addition, we find that trans-intergranular transition can occur during the rupture process with increase in the slant angle of grain boundary with respect to the external loading direction. Our findings provide a fundamental understanding of failure mechanisms and fracture behaviors in graphene and other two-dimensional materials.

The influence of tilt grain boundaries on the mechanical properties of bicrystalline graphene nanoribbons

The mechanical properties of bicrystalline graphene nanoribbons with various tilt grain boundaries (GBs) which typically consist of repeating pentagon–heptagon ring defects are investigated based on the method of molecular structural mechanics. The GB models are constructed via the theory of disclinations in crystals, and the elastic properties and ultimate strength of bicrystalline graphene nanoribbons are calculated under uniaxial tensile loads in perpendicular and parallel directions to grain boundaries. The dependence of mechanical properties is analyzed on the chirality and misorientation angles of graphene nanoribbons, and the experimental phenomena that Young's modulus and ultimate strength of bicrystalline graphene nanoribbons can either increase or decrease with the grain boundary angles are further verified and discussed. In addition, the influence of GB on the size effects of graphene Young's modulus is also analyzed.

Molecular dynamics study on doping defected graphene by boron

ABSTRACTBoron doping brings remarkable features and modified properties to graphene. In this study, we used molecular dynamics simulations to investigate the influence of vacancies on the graphene boron doping process. The existence of vacancies increases the boron content of doped graphene sheet. The physical mechanisms behind this phenomenon are discussed. The vacancy concentration of graphene affects the formation energy of boron-vacancy pair, the collision frequency between boron atoms and graphene sheet and also affects the boron atom capture cross section of graphene. We have also investigated the boron doping process for bicrystalline graphene. The results reveal that boron atoms tend to be adsorbed on the grain boundary region. The mechanical properties of the boron doped bicrystalline graphene are enhanced compared with the undoped graphene.

Anomalous thermal transport along the grain boundaries of bicrystalline graphene nanoribbons from atomistic simulations

We investigate the thermal transport properties of bicrystalline graphene nanoribbons (bi-GNRs) with different symmetric tilt grain boundaries (GBs) by using the molecular dynamics (MD) simulations. It is found that the bi-GNR with the 10.98°GB (the highest dislocation density) has an anomalously enhanced thermal conductivity for the heat flux along the GB compared to other ribbons with lower dislocation densities. This is in strong contrast to the behavior of thermal conductivity across the GB, which decreases monotonically with increasing the dislocation density. We attribute this counterintuitive phenomenon to its non-folding structure and lower edge stress, which can reduce the phonon scatterings induced by GBs and rough edges. In addition, we also examine the effects of the characteristic length and temperature on the thermal conductivity of the bi-GNRs through the phonon Boltzmann transport equation. At any given temperature and characteristic length, the low-dislocation-density bi-GNR are shown to be much more efficient in suppressing the thermal conductivity, and has a higher tailoring rate. These facts reveal the bi-GNR with a lower dislocation density is more promising to be a high figure of merit thermoelectric material.