Auxetic Tetrahex Carbon with Ultrahigh Strength and a Direct Band Gap

Tetrahex carbon is a recently predicted two-dimensional (2D) carbon allotrope that is composed of tetragonal and hexagonal rings. Unlike flat graphene, this new 2D carbon structure is buckled and possesses a direct band gap of approximately 2.6 eV and a high carrier mobility of approximately 104cm2/(Vs) with anisotropic features. In this work, we employ first-principles density-functional theory calculations to explore the mechanical properties of tetrahex C under uniaxial tensile strain. We find that tetrahex C demonstrates ultrahigh ideal strength, outperforming both graphene and pentagraphene. It shows superior ductility and sustains uniaxial tensile strain up to 20% (16%) until phonon instability occurs and the corresponding maximal strength is 38.3 N/m (37.8 N/m) in the zigzag (armchair) direction. It shows an intrinsically negative Poisson ratio. This exotic in-plane Poisson ratio takes place when the axial strain reaches a threshold value of 7% (5%) in the zigzag (armchair) direction. We also find that tetrahex C maintains a direct band gap of 2.64 eV at the center of the Brillouin zone. This direct-gap feature remains intact upon strain application, with no direct-indirect gap transition. The ultrahigh ideal strength, the negative Poisson ratio, and the integrity of the direct gap under strain in tetrahex C suggest that it may have potential applications in nanomechanics and nanoelectronics.