Mechanical properties of pristine and nanoporous graphene

Anthea Agius Anastasi,Konstantinos Ritos,Glenn Cassar,Matthew K Borg

doi:10.1080/08927022.2016.1209753

Anthea Agius Anastasi, Konstantinos Ritos + Show 2 more

Open Access

https://doi.org/10.1080/08927022.2016.1209753

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Abstract

We present molecular dynamics simulations of monolayer graphene under uniaxial tensile loading. The Morse, bending angle, torsion and Lennard-Jones potential functions are adopted within the mdFOAM library in the OpenFOAM software, to describe the molecular interactions in graphene. A well-validated graphene model using these set of potentials is not yet available. In this work, we investigate the accuracy of the mechanical properties of graphene when derived using these simpler potentials, compared to the more commonly used complex potentials such as the Tersoff-Brenner and AIREBO potentials. The computational speed up of our approach, which scales O(1.5N), where N is the number of carbon atoms, enabled us to vary a larger number of system parameters, including graphene sheet orientation, size, temperature and concentration of nanopores. The resultant effect on the elastic modulus, fracture stress and fracture strain is investigated. Our simulations show that graphene is anisotropic, and its mechanical properties are dependant on the sheet size. An increase in system temperature results in a significant reduction in the fracture stress and strain. Simulations of nanoporous graphene were created by distributing vacancy defects, both randomly and uniformly, across the lattice. We find that the fracture stress decreases substantially with increasing defect density. The elastic modulus was found to be constant up to around 5% vacancy defects, and decreases for higher defect densities.

Highlights

Graphene is a monolayer of sp2 hybridised carbon atoms arranged in a hexagonal crystal lattice and it is known to possess excellent mechanical,[1] electrical,[2] and chemical properties.[3]
Unlike the equivalent ZZ loaded pristine graphene simulation, where the crack propagates diagonally through covalent bonds, in this case the travel path is modified by the presence of defects, as such forming an almost perpendicular crack to the loading direction. This behaviour is similar to the fracture behaviour of an AC loaded pristine sheet. We found that this transition from a ZZ- to an AC-type fracture mode in nanoporous ZZ graphene sheets occurs at a defect density of around 4%, which is roughly equal to when the elastic modulus starts to decrease with increasing defect density
These simulations enabled the investigation of the elastic modulus, fracture stress and fracture strain of graphene with respect to the orientation, sheet-size and sheet temperature

Summary

Introduction

Graphene is a monolayer of sp hybridised carbon atoms arranged in a hexagonal crystal lattice and it is known to possess excellent mechanical,[1] electrical,[2] and chemical properties.[3]. Fabricated graphene sheets typically tend to contain a range of defects including nanopores.[8, 9] The effect that such defects have on the mechanical properties of graphene need a more thorough investigation. The mechanical properties of graphene have been investigated using both experimental and computational methods, the latter being by far the most common. Lee et al [1] used atomic force microscopy to measure the elastic modulus of graphene and the stress and strain at which pristine graphene fractures. The authors determined that graphene has an elastic modulus of 1.0 ± 0.1 TPa and a breaking stress in the zigzag (ZZ) direction of 130 ± 10 GPa with a maximum strain of 0.25. A similar result with an elastic modulus of roughly 0.95 ± 0.05 TPa was obtained by Bunch et al [11]

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