(236b) Thermomechanical Properties of Nanodiamond Superstructures in Interlayer-Bonded Twisted Bilayer Graphene

Graphene derivatives and graphene-based metamaterials and nanocomposites have been studied extensively due to their exceptional properties. These properties can be tuned precisely by tailoring structural features introduced in these 2D materials by means of chemical functionalization and defect engineering. Toward this end, here, we report results of a systematic computational study of a new class of graphene-based nanomaterials, namely, nanodiamond superstructures embedded in interlayer-bonded twisted bilayer graphene (IB-TBG). As a result of interlayer bonding within twisted bilayer graphene, induced by chemical functionalization due to patterned hydrogenation, superstructures of nanodiamond domains are formed between the two graphene layers. We have found that such IB-TBG nanodiamond superstructures with an interlayer bond density higher than a critical value undergo ductile failure upon uniaxial tensile straining, which makes these structures very appealing toward developing mechanically superior graphene-based metamaterials.

Based on molecular-dynamics (MD) simulations according to a reliable interatomic bond-order potential, we determine a broad range of thermomechanical properties of IB-TBG nanodiamond superstructures. Specifically, we study the mechanical response of these superstructures to indentation loading and shear straining based on MD simulations of nanoindentation tests and dynamic shear straining tests. We establish the dependences of the elastic modulus, maximum stress sustained upon indentation, and shear strength of the superstructures on the full range of their structural parameters, especially on the diamond fraction in the nanocomposite superstructure as measured by the concentration of sp³-hybridized interlayer-bonded C atoms, f_sp3. The resulting structural responses and fracture mechanisms of the superstructures under nanoindentation testing and shear straining also are characterized in detail. We have found that superstructures characterized by a less-than-critical f_sp3 exhibit a strongly nonlinear inelastic response to indentation up to the onset of fracture. A non-dissipative and non-recoverable stiffening effect caused by the relative twisting between the top and the bottom layer of the nanodiamond clusters in the central region of the indented superstructure is found to induce a kink-like discontinuity and oscillations in the corresponding force-displacement indentation loading curve. In addition, we have found that the 2D elastic modulus and the deformability of this type of superstructure, as measured by the vertical deflection at the breaking point, are relatively insensitive to the interlayer bond density, while the maximum sustained stress decreases monotonically with increasing f_sp3 but remains very high even at high f_sp3.

Finally, we report results for the lattice thermal conductivity of these superstructures based on non-equilibrium MD (NEMD) simulations of thermal transport. We find that the lattice thermal conductivity is reduced significantly with increasing diamond fraction in these nanocomposite superstructures, which also makes these 2D carbon nanomaterials very promising for thermal management applications.