(779f) Structural, Electronic, and Mechanical Properties of Superlattices of Interlayer-Bonded Domains in Twisted Bilayer Graphene

We present a comprehensive computational analysis of the atomic and electronic structure and properties of a novel class of carbon nanostructures, formed due to interlayer covalent sp³ C-C bonding in twisted bilayer graphene as a result of controlled chemical functionalization (hydrogenation or fluorination).   Depending on the twist angle and local stacking of layers, these nanostructures are superlattices of diamond-like nanocrystals or caged fullerene-like configurations embedded within the bilayer.  According to first-principles density functional theory calculations, the electronic behavior of these sp²/sp³ hybrid configurations ranges from semi-metallic, characterized by linear dispersion around the K point in the first Brillouin zone (Dirac cones), to semi-conducting with electronic band gaps from a few meV to ~1.2 eV, to insulating with band gaps up to ~4 eV; the band gap of the semiconducting configurations can be tuned by controlling the density of interlayer C-C bonds and the chemical functionalization pattern.  The electronic character of the superstructures depends on the symmetry and periodicity of the superlattices and on the type of chemisorbed species (H or F atoms).

We have also conducted a systematic study of the mechanical response of these superlattices based on molecular-dynamics simulations of tensile deformation and shear loading tests according to a reliable interatomic bond-order potential.  We have found that the mechanical properties of these superstructures can be precisely tuned by controlling the fraction of sp³-hybridized C-C bonds in the material through the extent of chemical functionalization.  Their Young modulus and tensile strength decrease moderately compared to those of pristine bilayer graphene, but remain superior to those of most conventional engineering materials.  However, their interlayer shear modulus increases strongly compared to that of pristine bilayer graphene and monotonically with the fraction of sp³-hybridized C-C bonds.