The University of Massachusetts Amherst
University of Massachusetts Amherst

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PhD Defense, Mengxi Chen, Structure-Properties Relations in Graphene Nanomeshes and Interlayerbonded Twisted Bilayer Graphene Nanocomposite Superstructures Obtained by Atomic-Scale Modeling


Thursday, May 21, 2020 - 2:00pm


(will be held via Zoom, contact for a link)


Committee Chair:  Dimitrios Maroudas



Graphene-based metamaterials are engineered two-dimensional (2D) carbon-based materials characterized by
 structural features with tunable length scales that give rise to remarkable properties. This dissertation focuses on  establishing structure-properties relations in two such 2D materials classes, namely, graphene nanomeshes or nanoporous graphene, consisting of regular arrays of nanopores in single-layer graphene, and superstructures of diamond nanodomains embedded between chemically functionalized graphene planes in twisted bilayer graphene (graphene-diamond nanocomposites). 

We have analyzed the response of graphene nanomeshes (GNMs) to uniaxial tensile straining and to nanoindentation based on molecular-dynamics (MD) simulations of deformation tests. We have examined the effects on the GNM mechanical behavior under straining along different directions of the nanomesh pore morphology and pore edge passivation, established the dependences of the ultimate tensile strength, fracture strain, and toughness of the GNMs on the nanomesh porosity, and derived scaling laws for GNM strength-density relations. We also studied the mechanical and structural response of GNMs to nanoindentation by a rigid spherical indenter. The GNMs’ response to indentation is nonlinearly elastic until fracture initiation, with elastic properties that depend strongly on the GNM porosity but are not sensitive to pore edge passivation, which, however, influences the GNM failure mechanism past fracture initiation. The elastic modulus decreases monotonically with increasing porosity according to a quadratic scaling law and the maximum deflection of the indented GNMs at their breaking point exhibits a minimum at porosities below 20%; beyond this critical porosity, the maximum deflection increases monotonically with increasing porosity and can reach remarkably high values at high porosities. 

We have also conducted a systematic computational study on the mechanical behavior of a class of two dimensional (2D) graphene-diamond nanocomposite superstructures, formed through patterned hydrogenation-induced interlayer covalent bonding of twisted bilayer graphene with commensurate bilayers. These superstructures are fully characterized by the commensurate bilayer’s twist angle, the interlayer bond pattern, and the concentration of sp3- bonded C atoms in the nanocomposite material. We studied the mechanical behavior of such carbon nanocomposite superstructures based on MD simulations of uniaxial straining tests and indentation tests and established the dependence of the superstructures’ mechanical properties on the concentration of sp3-bonded C atoms. We have discovered that a brittle-to-ductile transition occurs in these superstructures under tensile straining with increasing concentration of sp3-bonded C atoms beyond a critical level and characterized the underlying ductile fracture mechanism, which is mediated by void formation, growth, and coalescence. The resulting structural responses, deformation mechanisms, and fracture mechanisms of the superstructures under nanoindentation testing also have been characterized in detail. Our findings highlight the vast potential of graphene nanomeshes and graphene-diamond nanocomposites as 2D mechanical metamaterials whose mechanical response can be tuned by proper tailoring of their structural features.

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