(566c) Patterning of Defect-Engineered Graphene Sheets Driven By Pore-Pore Interactions

Defect engineering of graphene is a promising approach toward fabrication of carbon-based 2D materials with unique properties and function. Structural defects such as vacancy clusters (or nanopores) in graphene sheets, generated by electron or ion irradiation and placed accurately at preselected positions with almost atomic precision, play a key role in modifying the structure of graphene sheets and tuning their electronic, mechanical, and thermal properties. At high temperatures, under the action of thermodynamic driving forces, these defects exhibit interesting dynamics such as migration and coalescence, edge reconstruction of graphene sheets and graphene nanoribbons (GNRs), and GNR edge patterning mediated by nanopore-edge attractive interactions. These defect dynamical phenomena are of particular interest for developing graphene sheet patterning strategies. A fundamental understanding of defect interactions in graphene sheets is required to enable defect-mediated patterning strategies that can be used to tune 2D material properties and optimize their functionality.

Toward this end, we report here the results of a systematic atomic-scale computational analysis of nanopore dynamics in graphene sheets. The analysis is based on molecular-dynamics (MD) simulations according to a reliable interatomic interaction potential. The MD simulations are designed based on the results of a systematic protocol of molecular-statics (MS) computations to study the energetics of pore-pore interactions in graphene. The MS analysis reveals that when two pore edges are in close proximity (a few bond lengths) from each other, their interactions are attractive when the line segment connecting the two pore centers is aligned with the armchair direction. However, when this pore center-to-center line is along the zigzag direction, the interactions are both repulsive and attractive. Nevertheless, the repulsive interactions are very weak, while the attractive interactions become particularly strong as the pore centers approach closer than a certain distance. In both of these cases, pore-pore interactions can lead to pore coalescence and formation of a much larger pore in the graphene sheet. We have examined systematically the effects of pore sizes and other geometrical parameters on the pore-pore interaction potentials. With this understanding of pore-pore interactions, we have conducted molecular-dynamics (MD) simulations at high temperature to understand nanopore dynamics in the vicinity of other pores in graphene sheets. Our simulation results show that pore-pore interactions provide the thermodynamic driving force for a nanopore to migrate toward a larger pore, through a sequence of carbon ring reconstructions and edge atomic diffusion processes, and facilitate pore coalescence. A systematic parametric study varying pore geometrical parameters and temperature has identified four pore coalescence mechanisms. Based on analysis of MD trajectories and climbing-image nudged elastic band (NEB) calculations, we have constructed the minimum-energy pathways of these pore coalescence mechanisms and obtained a fundamental kinetic understanding of the processes underlying pore migration and coalescence in graphene.