Rational manipulation of substrate-supported graphene by heterogeneity of substrate surface and material composition

Abstract

In many graphene-based devices graphene is adhered to a substrate that influences its performance, rather than being present in a free standing form. The interaction of graphene with these substrates can lead to deformations that give rise to out-of-plane architectures with new properties such as superhydrophobicity, opened electronic band gap, and higher in-plane rigidity. Earlier experiments and simulations with graphene-substrate interfaces demonstrating reversible and repeatable stacking of out-of-plane buckled graphene to create ridges, which are stacked protrusions of graphene, warrant a detailed understanding of the underlying mechanisms of graphene ridge formation, especially for design of tailored nanostructures. Ridges are created through substrate-mediated compression of graphene, therefore, these ridges should be related to the graphene-substrate interface. It is unknown what the direct effect of the substrate on ridge formation is besides the work done studying graphene's mechanical response to compression. It is necessary to understand how the substrate affects graphene deformation in order to fully utilize the range of accessible graphene deformation shapes. To systematically study the formation of ridges in graphene, molecular dynamics simulations are performed to characterize the deformation of graphene on substrate during and after axial compression of graphene nanoribbons, high aspect ratio (10:1) single layer sheets of graphene in this work. This is done to investigate the hypothesis that graphene deformation depends on the underlying substrate in terms of corrugation wavelength and amplitude and graphene-substrate adhesion energy. In the first part of this thesis a quantitative scheme is formulated to characterize and predict these deformations. A critical value of interfacial adhesion energy marks a transition point that separates two deformation regimes of graphene on substrate under uniaxial compression; the deformation regimes are binary featuring the stacking of graphene after buckling in one case and no stacking, otherwise. These ridges are a product of the graphene limit point buckling, where growing out-of-plane folds of graphene stack and self-adhere. In the second part of this thesis, after establishing the role of substrate and key interfacial properties, the atomistic mechanisms underlying the formation, evolution, and localization of graphene ridges are investigated using fracture mechanics theory and molecular dynamics simulations. It is shown that there is no intrinsic characteristic length scale over which to achieve certain graphene shapes or see any repeated shapes as suggested in previous experiments, but instead these shapes can be tuned by substrate selection and design, a novel approach presented in this thesis. Moreover, a major result of this work is that the location and density of surface features in graphene-substrate systems can be controlled by substrate engineering at nanoscale resolutions, which could be used for developing graphene-based devices with a more efficient use of material, or with tailored distribution of surface futures that lead to specific applications. Efficiency gains can be made through use of less material and more controlled spacing of graphene ridges. The immediate impact of this work is most clearly realized in large scale manipulation of graphene where targeted deformations of different regions of the same graphene sheet can be executed using a single rationally designed substrate. Shifting the mindset from using the substrate as a stage, but as a tool, opens up the potential for more intricate graphene deformations at the nanoscale.