(383b) Mechanical Behavior of Interlayer-Bonded Nanostructures Obtained from Bilayer Graphene

Based on first-principles density functional theory calculations, we have recently introduced 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 embedded within the bilayer. The electronic behavior of these sp²/sp³ hybrid configurations ranges from metallic to semiconducting with tunable electronic band gaps from a few meV to about 1.2 eV, to insulating with band gaps up to about 4 eV.  This behavior makes this class of nanostructures very attractive for enabling technologies in nanoelectronics.

In this presentation, we report a comprehensive computational study of the mechanical behavior 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 tuned precisely 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. On the contrary, their interlayer shear modulus increases strongly compared to that of pristine bilayer graphene and monotonically with the fraction of sp³-hybridized C-C bonds. We have analyzed in detail the fracture mechanisms of the superstructures under tension as a function of the extent of interlayer bonding through chemical functionalization. In most cases, fracture is initiated at the interface between pristine graphene and nanodiamond domains in the superstructure and subsequently propagates across the superstructure leading to brittle failure. However, beyond a certain extent of interlayer bonding or a certain separation of nanodiamond domains within the unit cell of the superstructure, there is a transition to ductile failure with a structural response that is characterized by void formation and coalescence.  In addition to the analysis of the fracture mechanisms, the predicted stress-strain curves are presented and discussed and the fracture toughness of all the nanostructures examined is determined.