Non-regular hexagonal 2D carbon, an allotrope of graphene: a first-principles computational study

In a first principle computational study, using density functional theory, we have identified four types of 2D carbon sheets, similar to graphene, made entirely of non-regular hexagons. In one case, we get a structure where the non-regular hexagons have four sides of length d₁ = 1.416 Å and two sides of length d₂ = 1.68 Å. Next case, in the non-regular hexagons the side d₁ (two times) and d₂ (four times) are exchanged. In two other cases, the non-regular hexagons have three pairs (opposite sides) of different lengths (d₁ = 1.529 Å, d₂ = 1.567 Å, and d₃ = 1.612 Å; d₁ = 1.387 Å, d₂ = 1.348 Å, and d₃ = 1.387 Å). By propper choice of the non-regular hexagon sides, one could arrive at a 2D carbon system like graphene, but with a tunable band gap. The structure is more stable when the system has more number of regular C-C bonds than the longer C-C bonds. Due to its non-regular hexagons, special atomic configuration, this system may have, like graphene, unusual properties. It is semiconducting, and there is no need to functionalize it for opening the band gap as is the case with graphene.