(393ac) Electronic and Mechanical Properties of Superlattices of Crystalline Domains Embedded in Twisted Bilayer Graphene

We report results of a systematic computational study on mechanical and electronic properties of carbon nanostructures generated upon covalent interlayer C-C bonding in twisted bilayer graphene (TBG); such interlayer bonding is the result of chemical functionalization, namely, hydrogenation of the graphene layers in TBG by controlling the hydrogen coverage and hydrogenation pattern.  The structural features of these configurations are determined by the twist angle, i.e., the angle of rotation of the graphene planes with respect to each other.  Interlayer covalent bonding generates superlattices of diamond-like nanocrystalline domains and of caged fullerene-like configurations embedded within the graphene layers for small twist angles (near 0 degrees) and for large twist angles (near 30 degrees), respectively.  The analysis is based on a combination of first-principles density functional theory (DFT) calculations, density-functional tight-binding (DFTB+) computations, and classical molecular-dynamics simulations based on a reliable reactive bond-order potential (AIREBO).

We have calculated the electronic band structure of these superlattice structures and we show that it depends strongly on the hydrogenation pattern and density of interlayer bonds; the predicted band gaps span a broad spectrum from ~0 eV, i.e., in the band-gap limit of single-layer graphene, to ~3 eV, i.e., in the band-gap limit of bulk diamond phases.  We have also carried out MD simulations of the dynamic deformation of the superlattice structures at constant strain rate and temperature.  We have obtained stress-strain curves and determined the Young modulus, tensile strength, and fracture strain of all of the generated nanostructures.  These physical properties are determined over the entire range of twist angles and density of interlayer C-C bonds; this allows for a fundamental understanding of how these structural parameters affect material properties and for the development of criteria for choosing such parameters optimally to fine tune electrical and mechanical response, aiming at specific technological applications.