Graphyne Better Than Graphene?

Carbon nanotubes and graphene have paved the way for the next step in the evolution of carbon materials. Among the novel forms of carbon allotropes is graphyne – a two-dimensional lattice of sp–sp2-hybridized carbon atoms similar to graphene for which recent progress has been made in synthesizing dehydrobenzoannulene precursors that form subunits of graphyne. Here, we characterize the mechanical properties of single-atomic-layer graphyne sheets by full atomistic first-principles-based ReaxFF molecular dynamics. Atomistic modeling is carried out to determine its mechanical properties for both in-plane and bending deformation including material failure, as well as intersheet adhesion. Unlike graphene, the fracture strain and stress of graphyne depends strongly on the direction of the applied strain and the alignment with carbon triple-bond linkages, ranging from 48.2 to 107.5 GPa with ultimate strains of 8.2–13.2%. The intersheet adhesion and out-of-plane bending stiffnesses are comparable to graphene, despite the density of graphyne being only one-half of that of graphene. Unlike graphene, the sparser carbon arrangement in graphyne combined with the directional dependence on the acetylenic groups results in internal stiffening dependent on the direction of applied loading, leading to a nonlinear stress–strain behavior.

The existence of Dirac cones in the band structure of two-dimensional materials accompanied by unprecedented electronic properties is considered to be a unique feature of graphene related to its hexagonal symmetry. Here, we present other two-dimensional carbon materials, graphynes, that also possess Dirac cones according to first-principles electronic structure calculations. One of these materials, 6,6,12-graphyne, does not have hexagonal symmetry and features two self-doped nonequivalent distorted Dirac cones suggesting electronic properties even more amazing than that of graphene.