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Scientific Reports - EML - Carbon ::: Three Papers on Graphene

Dibakar Datta's picture

 Surface hydrogenation regulated wrinkling and torque capability of hydrogenated graphene annulus under circular shearing ,   Nature Scientific Reports [ PDF ]  Wrinkles as intrinsic topological feature have been expected to affect the electrical and mechanical properties of atomically thin graphene. Molecular dynamics simulations are adopted to investigate the wrinkling characteristics in hydrogenated graphene annulus under circular shearing at the inner edge. The amplitude of wrinkles induced by in-plane rotation around the inner edge is sensitive to hydrogenation, and increases quadratically with hydrogen coverage. The effect of hydrogenation on mechanical properties is investigated by calculating the torque capability of annular graphene with varying hydrogen coverage and inner radius. Hydrogenation-enhanced wrinkles cause the aggregation of carbon atoms towards the inner edge and contribute to the critical torque strength of annulus. Based on detailed stress distribution contours, a shear-to-tension conversion mechanism is proposed for the contribution of wrinkles on torque capacity. As a result, the graphane annulus anomalously has similar torque capacity to pristine graphene annulus. The competition between hydrogenation caused bond strength deterioration and wrinkling induced local stress state conversion leads to a U-shaped evolution of torque strength relative to the increase of hydrogen coverage from 0 to 100%. Such hydrogenation tailored topological and mechanical characteristics provides an innovative mean to develop novel graphene-based devices. 

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Effect of crack length and orientation on the mixed-mode fracture behavior of graphene , Extreme Mechanics Letters [ PDF ]

 In almost all practical situations, graphene based nanodevices are subjected to complex loading i.e., combination of shear and tensile stress. Given this situation, mixed-mode fracture is inevitable during tearing of graphene. However, most of the studies on graphene fracture are based on mode-I fracture which is an idealistic situation and rarely occurs in the service conditions. We, therefore, performed classical molecular dynamics (MD) simulations on a graphene sheet with crack like flaw and investigated the complex mixedmode fracture behavior. Mode-I, mode-II, and mixed-mode stress intensity factors (KI , KII and Keff respectively) as a function ofand crack length in armchair and zigzag edges were calculated. In addition, we investigated the effect of slit length and angle on the strength of graphene sheet. Effective stress intensity factor increases with flaw size and reaches a plateau (between 3.10 and 3.80 MPapm for armchair, between 2.60 and 3.10 MPapm for zigzag) approximately at a crack length of a/b ~ 0.11 (2a and 2b are crack and model size respectively). For crack with zigzag edge surface, existence of a single bond perpendicular to crack direction facilitates bond-breaking process. While for armchair surface case, two inclined bonds at crack tip offer relatively more resistance. Finally, the effect of mixedmode loading on the crack propagation path was investigated. All the systems considered in this study mimic real service conditions. Hence, our findings will provide useful guidelines for the design of graphene-based nanodevices.

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 Anomalous mechanical characteristics of graphene with tilt grain boundaries tuned by hydrogenation,  CARBON [ PDF ]

Grain boundaries (GBs) as typical defective graphene structure have significant influence on its mechanical properties. The fracture strength of hydrogenated graphene with tilt GBs composed of pentagon–heptagon defects are systematically investigated using classical Molecular Dynamics (MD) method. Anomalous mechanical characteristics are revealed for graphene with hydrogenation either on or near the GBs. For graphene sheets with hydrogenation on tilt GBs under perpendicular pulling, the strength of hydrogenated GBs with large tilt angle is anomalously stronger than low-angle tilt boundary having fewer defects because of the interaction between polar stress fields of hydrogenated pentagon–heptagon defects. For graphene with hydrogenation near the GBs, the interaction between GBs and hydrogenated domains at different distance intervals is investigated. The strength is found to be governed by the peak normal stress in hydrogenated domain, and an exponential relationship between the normal strength and distance interval is revealed as a result of the stress field overlap between GB and hydrogenation interface. Our results gain meaningful insight into the effects of hydrogenation on the strength of graphene with tilt GBs, and provide guidelines for designing high-quality hydrogenated graphene-based nanodevices.

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