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Title:Mechanics of interfaces in graphene-based nanostructures
Author(s):Bagchi, Soumendu
Director of Research:Chew, Huck Beng
Doctoral Committee Chair(s):Chew, Huck Beng
Doctoral Committee Member(s):Johnson, Harley T.; Geubelle, Philippe H.; Chasiotis, Ioannis
Department / Program:Aerospace Engineering
Discipline:Aerospace Engineering
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Interfaces, Graphene, Nanomaterials, Nanostructures, Atomistic simulations, Molecular dynamics, Twisted bi-layer graphene, Moire structures, nancomposites
Abstract:Interfaces are ubiquitous in graphene-based nanostructured materials. The resulting electrical, optical, chemical and mechanical properties are substantially governed by the type of interactions acting across these interfaces. For example, the interfacial interactions for an amorphous polymer matrix surrounding carbon-nanotube filaments could involve weak van der Waals forces or strong covalent interactions through the formation of cross-link bonds, thus resulting in the wide-ranging interfacial shear strength values reported by nanotube pull-out experiments. Crystalline close-packed metals such as titanium and aluminum also demonstrate contrasting adhesion properties along graphene-metal interfaces depending on the extent of thermal annealing which results in oxide formation along the interfaces. For twisted graphene bilayers, the mismatch energy resulting from interlayer van der Waals interactions in the emerging moiré structure, dictates structural stability and behavior at finite temperatures which could underpin performance in 2D graphene electronics. This dissertation research seeks to systematically investigate and elucidate the role of interfacial interactions across graphenepolymer, graphene-metal, and twisted graphene-graphene structures using atomistic simulations coupled with micromechanics modeling. By performing massively parallel molecular dynamics (MD) simulations at length-scales comparable to nanotube pull-out experiments, it is found that the pull-out forces associated with non-bonded (van der Waals) interactions between carbon nanotube (CNT) and poly-methylmethacrylate (PMMA) are generally small, and are weakly-dependent on the embedment length of the nanotube. In contrast, low-density distribution of cross-links along the CNT-PMMA interface increases the pull-out forces by an order of magnitude. At each cross-linked site, mechanical unfolding and pull-out of single or pair polymer chain(s) attached to the individual cross-link bonds result in substantial interfacial strengthening and toughening, while contributing to interfacial slip between CNT and PMMA. Unlike an amorphous polymer matrix where the weak van der Waals interactions do not significantly contribute to the pull-out forces, the bonding characteristics between a crystalline metal matrix and graphene are now much stronger, but are sensitive to the extent of thermal annealing. Density functional theory (DFT) calculations reveal that surface oxidation lowers the interfacial shear strength of graphene on Ti by two orders of magnitude, but also increases the interfacial shear strength of graphene on Al by two orders of magnitude. These dramatic changes to the interfacial properties are in good agreement with collaborative single nanotube pull-out experiments from Al and Ti-metal matrix composites (MMC) prior to, and after thermal annealing. Thermal annealing is also shown to cause stepwise untwisting of finite sized twisted bilayer graphene structures. Achieving fine control over the twist angle of these graphene bilayers is critical for scalable manufacturing of graphene electronics. Using MD simulations, we demonstrate that for finite-sized (non-periodic) graphene sheets with a twist misorientation, truncation of the moiré patterns caused by the interference patterns between the two graphene lattices results in the periodic fluctuation in the number of unstable AA versus stable AB stackings during untwisting. This in turn results in the formation of multiple local minimum energy states and associated barrier energies along the minimum potential energy pathway for untwisting to the global minimum AB state. It is shown that the number of locally stable energy states and their barrier energies scale with the flake size, allowing twisted graphene flakes of several tens of nanometers to remain thermally stable even at chemical vapor deposition temperatures. However, the magnitude of these barrier energies can be significantly lowered by the presence of lattice mismatch strains, due to preferential strain transfer across the AB versus AA stacking. These competing effects enable strain-controlled rotation of macroscopic-size twisted 2D sheets.
Issue Date:2020-08-31
Rights Information:Copyright 2020 Soumendu Bagchi
Date Available in IDEALS:2021-03-05
Date Deposited:2020-12

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