(361c) Molecular-Dynamics Analysis of the Mechanical Behavior of Graphene Nanomeshes

Graphene nanomeshes (GNMs), also known as nanoporous graphene, are ordered defect-engineered graphene nanostructures consisting of periodic arrangements of nanopores in the graphene lattice with neck widths between pores of less than 10 nm, typically fabricated by block copolymer lithography and plasma/electron etching of graphene. The unique porous structure of GNMs and their outstanding mechanical and electronic properties makes these 2D materials very appealing for numerous technological applications, ranging from field effect transistors to membranes used in water desalination. Here, we present a systematic study of the response of GNMs to mechanical loading based on molecular-dynamics (MD) simulations according to a reliable bond order interatomic potential and establish the corresponding structure-properties relationships.

We report results on the response of graphene nanomeshes (GNMs) to uniaxial tensile straining based on MD simulations of dynamic deformation tests. We examine the effects on the GNM mechanical behavior under straining along different directions of the nanomesh pore morphology and pore edge passivation by testing GNMs with elliptical pores of various aspect ratios and different extents of edge passivation through termination with H atoms of under-coordinated edge C atoms. We establish the dependences of the ultimate tensile strength, fracture strain, and toughness of the GNMs on the nanomesh porosity, derive scaling laws for GNM strength-density relations, and find the GNMs’ mechanical response to uniaxial straining to be anisotropic for pore morphologies deviating from circular. We also find that the GNM tensile strength decays exponentially with increasing GNM porosity and that pore edge termination with H atoms causes a reduction in the GNMs’ elastic stiffening, strength, deformability, and toughness; this hydrogen embrittlement effect is more pronounced at a high level of pore edge passivation that renders the edge C atoms sp³-hybridized.

We also report results on the mechanical and structural response of GNMs to indentation based on MD simulations of nanoindentation tests. We find the GNMs’ response to indentation to be nonlinearly elastic until fracture initiation, with elastic properties that depend strongly on the GNM porosity but are not sensitive to pore edge passivation, which, however, influences the GNM failure mechanism past fracture initiation. Increasing GNM porosity leads to a monotonic decrease of the 2D elastic modulus of the GNMs and the modulus-porosity dependence follows a quadratic scaling law. The maximum stress reached at the GNM breaking point is high throughout the porosity range examined, even at very high porosity. The maximum deflection of the indented GNMs at their breaking point exhibits a minimum at porosities below 20%; beyond this critical porosity, the maximum deflection increases monotonically with increasing porosity and can reach values comparable to half of the indented sample radius at high porosities. Moreover, our analysis reveals a novel inelastic, dissipative necking mechanism of GNM failure at high porosities that further enhances the excellent deformability of the GNMs. Our findings highlight the potential of graphene nanomeshes as 2D mechanical metamaterials whose mechanical response can be tuned by proper tailoring of their structural features.