Prediction And Electronic Properties Study Of Two-dimensional Dirac Semi-metallic Carbon Allotropes

Carbon is an important element in nature,and can form sp,sp~2and sp~3hybridization modes,which makes it have numerous allotropes from zero dimension,one dimension,two dimension to three dimension carbon materials.In particular,two-dimensional carbon materials possess many exotic properties due to the limitation of dimension and size,and have attracted extensive experimental and theoretical researches.Through experiments and theoretical studies,people have discovered and predicted many two-dimensional carbon materials with different structural and electronic properties,including insulators,conductors,semiconductors,metals,semi-metals,etc.Among them,Dirac semi-metallic materials possess fascinating physical properties,such as high carrier fermi velocity and massless dirac fermions,which have great potential applications in high-speed nano-devices and provide ideal platform to study related physical phenomena.Therefore,exploration of two-dimensional novel Dirac semi-metallic carbon allotropes has attracted enormous attentions.In this paper,based on materials genetic engineering and high-throughput structure search methods,we have searched two-dimensional Dirac semi-metallic carbon and studied their electronic properties systematically.1.Based on the experimentally fabricated 5-7-ring carbon chain and its possible assembling configurations,we proposed a new planar carbon structure(PHH-graphene)connecting the 5-7 ring carbon chains with 5-7-6 carbon ring,which is different from the previous configurations(TPH-graphene and Phagraphene).Through molecular dynamics and first-principles calculations,it is found that PHH-graphene is thermally and dynamically stable,especially that the total energy of PHH-graphene is lower than that of TPH-graphene synthesized in the experiment,indicating that the structure is likely to be synthesized by assembling 5-7chains.By comparing the electronic properties of these three allotropes and their nanoribbons,it is found that PHH-graphene and Phagraphene are type-I semimetal with a highly anisotropic dirac cone,while TPH-graphene is a metal.Their nanoribbons exhibit different properties with the increase of the width N(the number of 5-7 chains),and the TPH-graphene nanoribbons become metal at N≥2.The band gap of Phagraphene and PHH-graphene nanoribbons decreases with the increase of width N,but the band gap of PHH-graphene shows oscillation phenomenon,which is affected by the symmetry determined by its width.2.1114 new stable planar carbon allotropes were obtained by using the structure search software RG2 and first-principles calculations.In order to calculate the band structure efficiently,we fit the HSE06 band structure of six known structures,optimize the tight-binding parameters with good generality,and quickly calculate the electronic properties of these allotropes and classify them.It found 683 metals,241semiconductors and 190 semi-metals.Many stable structures with exotic electronic properties have been found in these semi-metallic structures,such as type-III,type-I/II mixed,and type-I/III mixed dirac cones,which are very rare in two-dimensional carbon materials.In particular,a stable nodal-line semimetal,127-11-84-r568-1,was found,which is the first semimetal with node-line in sp~2planar carbon systems.Our study not only adds some new members to the two-dimensional carbon family,but also provides a good research platform and object for exploring some strange physical properties of two-dimensional materials.