Beyond Graphitic Carbon Nitride: Nitrogen-Rich Penta-CN₂ Sheet

Using first-principles calculations combined with ab initio molecular dynamics and tight binding model, we predict the existence of a kinetically stable two-dimensional (2D) penta-CN₂ sheet, which is isostructural to the recently discovered penta-graphene. The concentration of N in this new carbon nitride sheet exceeds the maximum N content, namely 21.66%, that has been achieved experimentally in honeycomb geometry. It even exceeds the N content found recently in hole-doped carbon nitride C_0.5N_0.5 as well as in porous graphitic C₃N₄. The penta-CN₂ sheet contains N-N single bonds with an energy density of 4.41 kJ/g, higher than that predicted recently in nitrogen-rich B-N compound. Remarkably, penta-CN₂ has an in-plane axial Young's modulus of 319 N/m, even stiffer than the h-BN monolayer. The electronic band structure of penta-CN₂ exhibits an interesting double degeneracy at the first Brillouin zone edges which is protected by the nonsymmorphic symmetry and can be found in other isostructural chemical analogues. The band gap of penta-CN₂ calculated using HSE06 functional is 6.53 eV, suggesting its insulating nature. The prediction of a stable penta-CN₂ implies that puckering might be more effective than porosity in holding nitrogen in 2D carbon nitrides. This sheds new light on how to design nitrogen-rich C-N compounds beyond N-doped graphene.