Combined In Situ Experimental and Computational Study on the Intrinsic Fracture Properties and Toughening of Two-dimensional Materials

Zhang, Xu

doi:https://doi.org/10.21985/n2-584c-6q36

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Combined In Situ Experimental and Computational Study on the Intrinsic Fracture Properties and Toughening of Two-dimensional Materials

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The successful isolation of graphene marked the advent of two-dimensional (2D) materials. Their atomically thin structures enable unprecedented electrical, optical, and mechanical properties, which have triggered significant research interests in the past decade. For instance, they are promising candidates for the fabrication of flexible electronics, biological sensors, battery electrodes, and electronic interconnects, etc. Despite the intriguing properties measured in the laboratories, industrial applications of 2D materials are still in the embryo. A major reason of such lag is the difficulties in the fabrication of 2D materials-based devices, which arise from their brittle nature. Toughening of 2D materials has thus become necessary toward reliable large-scale applications of 2D materials.A better understanding of the mechanical failure of 2D materials necessitates detailed mechanistic studies at the atomic scale. Experimentally, such investigation requires the usage of in situ mechanical testing techniques inside transmission electron microscopy (TEM), with both high-fidelity mechanical testing capabilities and high-resolution characterizations. Computationally, molecular dynamics simulations enable one-to-one comparison to the atomic behaviors captured by TEM, and allow energetic and kinetic studies on lattice reconstructions and structural transitions associated to the fracture. Nevertheless, such combined study requires parametrized interatomic potentials with ab initio level accuracy on large deformation pathways, as well as a robust experimental protocol for conducting in situ fracture tests inside TEM. As such, combined in situ experimental/computational studies on the fracture of 2D materials are scarcely reported. The thesis is aimed at filling such a gap of knowledge. It describes a systematic in situ experimental/computational investigation on the fracture and toughening of 2D materials. It first presents a generally applicable framework for parameterizing interatomic potentials to accurately capture large deformation pathways with molecular dynamics simulations. The framework enables iterative definition of properties in the training and screening sets, guided by correlation relationships between properties, aiming to achieve optimal parametrizations for properties of interest. We parameterized interatomic potentials with ab initio level accuracy on large deformation pathways of monolayer MoSe2 for the sub- sequent in silico study of the fracture of monolayer MoSe2. Next, a in situ TEM investigation on the edge-mediated annihilation of vacancy clusters in monolayer MoSe2 was described. We showed that such behavior, triggered by electron beam irradiation, could be used to engineer the properties of 2D materi- als. Then, the thesis describes an integrated high-resolution TEM-numerical exploration on the intrinsic fracture properties of 2D materials. We reported the first experimental-computational measurements of the fracture toughness of 2D materials that agrees with each other, and with theoretical predictions according to the Griffith criterion. It next shows in situ TEM fracture tests conducted on monolayer MoSe2 and reveals the extrinsic toughening effect from an ultra-thin polystyrene adlayer, which enhances the energy release rate of monolayer MoSe2 by a maximum of 15 fold. Lastly, the thesis shows a system- atic, quantitative study on the nanoscale toughening of monolayer graphene oxide (GO) by an ultra-thin polymer adlayer, which impedes the propagation of cracks during intraplanar fracture. Those results are anticipated to facilitate better understanding on the fracture of the 2D materials, and offer insights toward more reliable deployment of 2D materials in large-scale applications.

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