Prediction of an ultrasoft graphene allotrope with Dirac cones

Searching for the Dirac materials with ultrasoftness is crucial for flexible electronics applications. Based on first-principles calculations, we propose a new carbon allotrope (named as ph-graphene) with a penta-hexagonal framework, which is energetically more favorable than the penta-graphene composing surely of pentagons and some of already-synthesized carbon allotropes. Ph-graphene has an in-plane stiffness of 27.75 GPa·nm, smaller than those of graphene and penta-graphene by one order. The famous isotropic Dirac cones are well preserved in the ultrasoft ph-graphene, exhibiting delocalized feature of pz orbits with the Fermi velocity of 2.8 × 105 m/s. Additionally, surface hydrogenation alters drastically the electronic and mechanical properties of ph-graphene, resulting in electronic spin-polarization and anisotropic negative Poisson's ratios sequentially with the increase of hydrogenation concentrations.