(779g) Structural, Morphological, Electronic, and Mechanical Properties of Irradiated Single-Layer Graphene

Defect engineering of graphene has been studied extensively, both experimentally and computationally, aimed at approaches for tuning graphene’s electronic, optical, and mechanical properties.  Ion-beam and electron-beam irradiation have been employed to introduce in the honeycomb lattice of graphene defects, such as vacancies, reconstructions with pentagon and heptagon rings, as well as larger-scale defects such as two-dimensional (2D) holes or voids.  Recent experimental studies also have reported defect-induced amorphization of graphene after high electron-beam exposure.  High-resolution imaging of the resulting amorphized graphene demonstrated that such irradiation-induced amorphized graphene remains a coherent single-layer 2D structure that consists of a random patch of polygons and also contains 2D holes.

In this presentation, we address some fundamental questions on the structure and properties of irradiated graphene, including how the defect-induced structure and morphology depend on the inserted defect concentration, the determination of the onset of defect-induced amorphization, and the effects of the defect density and amorphization transition on the graphene’s electronic and mechanical properties.  Our study is based on isothermal-isobaric molecular-dynamics (MD) simulations according to a reliable bond-order potential for the description of interatomic interactions in graphene and using large supercells that contain thousands of C atoms.  Random distributions of single vacancies were introduced into the graphene honeycomb lattice at a given concentration and the response of the irradiated graphene sheets was explored systematically over a broad range of defect concentration based on a variant of simulated annealing.  The analysis of the simulation results for the relaxation of the irradiated graphene samples included detailed structural and morphological characterization of the relaxed configurations based on calculation of the radial distribution function, structural order parameters, and RMS surface roughness; the relaxed configurations constitute computer models of irradiated graphene.  Finally, we used density-functional tight-binding calculations to compute the electronic density of states of the structurally relaxed configurations.

We predict that the onset of the amorphization transition occurs at an inserted vacancy concentration between 5 and 10%, with the transition becoming less abrupt with vacancy concentration as temperature increases, and determine the surface roughness as a function of defect concentration.  We find that the computed amorphized configurations are in good agreement with those observed in recent experiments of high-dose exposure of graphene to 100-keV electron irradiation.  We also find that the electronic density of states of vacancy-amorphized graphene is characterized by introduction of localized states near the Fermi level of perfect single-layer graphene, consistent with those arising from random vacancy insertion in graphene at low concentrations.  Our findings set the stage for exploratory work on the functionalization of defect-amorphized graphene toward useful band-gap engineering and tuning of electronic transport and optical properties.

We have also determined the mechanical properties of irradiated single-layer graphene sheets as a function of inserted vacancy concentration, x.  Specifically, based on MD simulations of uniaxial deformation tests at constant strain rate and temperature, we have computed as a function of x the tensile strength, toughness, and failure strain of the irradiated graphene sheets. We find that the vacancy-induced crystalline-to-amorphous transition is accompanied by a brittle-to-ductile transition in the failure response of irradiated graphene sheets for x values over the range 8-12%. While point defects and larger voids appreciably degrade the strength of pristine graphene, we find that even heavily-damaged samples (~20% vacancies) exhibit tensile strengths of ~30 GPa, in significant excess of those typical of engineering materials. Our results suggest that defect engineering of graphene is feasible without losing its desirable mechanical properties.