2020 Theses Doctoral

Mechanical behavior of polycrystalline graphene

DiMarco, Christopher Samuel

Two-dimensional materials exhibit especially impressive behavior in part due to their low dimensionality and low defect density. Graphene is one such material that is of particular interest due to its immense mechanical strength. Nanoindentation experiments of suspended circular membranes have proven it to be the strongest material ever characterized - 100X stronger than steel. In an effort towards scalable production, chemical vapor deposition (CVD) techniques facilitate large-area synthesis of graphene films; however, the resulting film is polycrystalline. While subsequent nanoindentation experiments suggest the grain boundaries are still strong, some results are inconsistent and the precise mechanical strength is still unknown. Herein we seek to better understand the mechanics of grain boundaries in graphene on three fronts: computations, synthesis, and experiments. We construct a multiscale model using the finite element method and a cohesive zone model to investigate the failure modes of polycrystalline graphene. We establish a relationship between the failure load and the grain boundary distance and use it to calculate a semi-analytical probability density function (PDF) to allow for direct comparison with experiments. In parallel, we design and construct an ultra-high-purity CVD system that yields repeatable and tunable growth thanks to the significant reduction and control of oxidizing impurities. This yields a notable increase in growth rate, which we expect will lead to better-stitched grain boundaries and therefore higher quality polycrystalline films.