(666d) Formation and Thermomechanical Behavior of Nanocomposite Superstructures from Interlayer Bonding in Twisted Bilayer Graphene

Graphene-based nanomaterials have exceptional electronic, mechanical, and thermal properties that can be tuned by precise control of their nanostructural features. Such tunable properties are responsible for the unique function of these nanomaterials that has potential to enable numerous technological applications. Toward this end, we report here a systematic computational analysis of the formation and mechanical behavior of a class of two-dimensional (2D) carbon-based nanostructures, namely, graphene-diamond nanocomposites formed through interlayer covalent bonding of twisted bilayer graphene with commensurate bilayers. The interlayer bonding is induced by patterned hydrogenation that leads to formation of superlattices of 2D nanodiamond domains embedded between the two graphene layers with the periodicity of the underlying Moiré pattern. We have conducted a comprehensive classification of all such superstructures that can be formed with respect to the corresponding structural parameters, including the commensurate bilayer’s twist angle, the stacking type of the nanodomains where the interlayer bonds are formed, the interlayer bond pattern and density, and the concentration of sp³-bonded C atoms in these superstructures.

We also report results of a systematic study of the mechanical behavior of such carbon nanocomposite superstructures based on molecular-dynamics (MD) simulations of uniaxial straining tests according to a reliable interatomic bond-order potential. The mechanical properties of these 2D materials are investigated over the full range of structural parameters. We introduce a ductility metric, demonstrate its direct dependence on the concentration of sp³-bonded C atoms, and show that increasing the concentration of sp³-bonded C atoms beyond a critical level induces ductile mechanical response. We analyze the ductile fracture mechanisms, mediated by void formation, growth, and coalescence, in contrast to the typical brittle fracture of graphene and probe the brittle-to-ductile transition. In addition, we report results of MD simulations of nanoindentation tests that we have performed on these superstructures, including mechanical properties of these 2D materials, such as elastic modulus, strength, and toughness, and demonstrate their direct dependence on the superstructures’ structural parameters as well as indentation parameters. The underlying fracture mechanism of such superstructures under nanoindentation testing also is characterized in detail. Finally, we report results of non-equilibrium MD simulations of thermal transport in these superstructures, aimed at determining the dependence of the lattice thermal conductivity of the superstructures on the key structural parameters, such as the concentration of covalent interlayer C bonds. Our study sets the stage for designing few-layer-graphene-based nanocomposites with unique thermomechanical properties and functionality over a broad range of applications.