First-principles prediction of the transition from graphdiyne to a superlattice of carbon nanotubes and graphene nanoribbons

Graphdiyne is a recently-synthesized carbon allotrope with a framework of sp- and sp2-hybridized carbon atoms. From first-principles calculations, we propose a possible transition of graphdiyne to a novel carbon allotrope (h-carbon) whose structure is a superlattice of carbon nanotubes and graphene nanoribbons. The energy barrier of this endothermic transition was estimated to be 4.30 kcal/mol at zero pressure, which is much lower than that of the graphite-diamond transition at high pressure. First-principles calculations on the phonon spectrum and the elastic constants of the h-carbon revealed that it is kinetically and mechanically stable. This unique framework of sp2- and sp3-hybridized carbon atoms is energetically neutral versus diamond. The hardness of the h-carbon (35.52 GPa) is 1/3 that of diamond and very close to β-SiC crystal. Accurate electronic structure calculations based on the Heyd, Scuseria, and Ernzerhof approach and GW approximation indicate that the h-carbon is a semiconducting material with a band gap of 2.20-2.56 eV.