Effect of turbostratic orientations and confined fluid on mechanical strength of bi-layer graphene: a molecular dynamics study

The rise of graphene as a reinforcement material in the last decade has been exponential owing to its superior mechanical properties. This one atom thick 2D material is applicable in many industries related to nanomechanical, nanoelectronics and optical devices. Despite its strength and superior properties, single-layer graphene tends to be unstable in a free-standing form. This led to active use of bi-layer and multilayered graphene in many of the above-stated applications. Though properties of single-layer graphene have been extensively investigated both computationally as well as experimentally for over a decade, bilayer graphene and its turbostratic form are still under research. Additionally, little is known about the effects of environmental condition such as humidity on the mechanical strength of these layered structures. Therefore, the detailed investigation of these bi-layered structures and their derivatives for real-life applications is crucial. In this study, the mechanical properties of these structures are investigated by means of Molecular Dynamics (MD) simulation. MD simulations provide a cost-effective tool to study physical and chemical interaction of atoms in such structures. Simulations have proved to be very efficient in modeling structures and predicting their mechanical properties. Herein, single-layer graphene, bilayer graphene were exposed to uniaxial tensile load in zig-zag and armchair direction. Different turbostratic orientations of bilayer graphene were also subjected to uniaxial loading in order to determine the most stable and strong bi-layer conformation. It was found that AB stacked bilayer graphene was most stable and was reported to have the highest strength of all other bilayer conformations. For further bi-layer analysis, AB stacking was preferred. The analysis was further extended to study crack propagation in single and bilayer graphene. The study was completed by understanding the effect of fluids such as water confined in bilayer graphene on its overall mechanical strength. In the past decade, several applications have come to light ranging from sensors to biomedical devices that employ such constructed nano-structures. However, the question of the mechanical stability of such structures with different water content is rarely addressed. Herein, the effect of fluid confined in bilayer graphene on its mechanical property was detailed. The results show an increased strain limit in the graphene in the presence of water content and provide an interesting insight into the surface hydrophobicity of graphene

Turbostratic Orientations, Water Confinement and Ductile-Brittle Fracture in Bi-layer Graphene

Bi-layer graphene (BLG) can be a cheaper and more stable alternative to graphene in several applications. With its mechanical strength being almost equivalent to graphene, BLG also brings advanced electronic and optical properties to the table. Furthermore, entrapment of water in graphene-based nano-channels and devices has been a recent point of interest for several applications ranging from energy to bio-physics. Therefore, it is crucial to study the over-all mechanical strength of such structures in order to prevent system failures in future applications. In the present work, Molecular Dynamics simulations have been used to study crack propagation in BLG with different orientations between the layers. There is a major thrust in analyzing how the angular orientation between the layers affect the horizontal and vertical crack propagation in individual layers of graphene. The study has been extended to BLG with confined water in interfaces. Over-all strength of graphene sheets when in contact with water content has been determined, and prominent regional conditions for crack initiation are pointed out. It was seen that in the presence of water content, graphene deviated from its characteristic brittle failure and exhibited the ductile fracture mechanism. Origin of cracks in graphenes was located at the region where the density of water dropped near the graphene surface, suggesting that the presence of hydroxyl groups decelerate the crack formation and propagation in straining graphenes.Comment: 24 pages, 10 Figure