Probing the Mechanics of Crumpled Graphene Membranes under Tensile Loading

Abstract

Graphene is considered a promising material due to its unique electrical characteristics and excellent mechanical properties. These extraordinary properties make graphene a suitable material for applications like mechanical reinforcements, protective coatings, supercapacitors, sensors, etc. However, when trying to extract these properties experimentally, the challenge of producing pristine graphene prohibits researchers from attaining the required results. The challenge is primarily due to the formation of structural or surface imperfections during the fabrication, transfer or manipulation of the membrane. In addition, graphene possesses a low bending rigidity which inevitably leads to the out-of-plane crumpling of the film. So, determining the effect of these imperfections on the mechanics of graphene is critical to ensure development in the research field and application pertaining to graphene. The main focal point of this work is to experimentally determine the effects of surface corrugations, in the form of out-of-plane crumples, on the mechanical constants of the graphene membrane. In this study, two types of graphene membranes are considered, one is a flat membrane, and the other is a heavily crumpled membrane. The atomic force microscope is used to probe the membranes, and the mechanical constants are extracted from the resulting force deflection characteristics. The mechanical constants of the flat membrane, 2D Young’s modulus (E2D) and pretension (σ0) were found to be 140.42 ± 44.52 N/m and 0.119 ± 0.049 N/m respectively. The values obtained suggest a softening behaviour of the membranes, which is due to the intrinsic crumpling of the membrane in the out-of-plane direction. Surprisingly, in the case of heavily crumpled membranes, E2D and σ0 were found to be 393.34 ± 145.12 N/m and 0.203 ± 0.069 N/m respectively. The values obtained suggests that the heavily crumpled membranes provide greater resistance to deflection than the flat samples. However, it is essential to understand that the values do not depict the effective Young’s modulus or intrinsic strength of the membrane. The presence of folds in the heavily crumpled membranes is the reason for their higher resistance to deflection. The evolution of these folds, when subjected to nanoindentation, is also discussed in this work. This research helps provide insights into how the graphene membrane responds mechanically in the presence of crumples and also provides progress in realising the applications of crumpled graphene membranes.