Pentadiamond-like Carbides:A Theoretical Design And Study

Carbon is one of the most important elements in the universe and an essential component for the existence of life.Because of its unique outer electron structure,carbon has more ways to form bonds than other elements,which leads to a wide variety of molecular structures.Up to now,hundreds of carbon structures have been discovered,and many of them,such as graphene and carbon nanotube,are already well known.Although carbon materials have excellent properties,they are still not sufficient for certain applications,such as photovoltaic cells,which require a specific band gap width,redox for wastewater treatment,and catalysts for synthesis.Based on the needs of these applications,the properties of the original carbon material have been modified by substituting the original carbon atoms in the carbon structural framework,or by reintroducing other different atoms.Pentadiamond is a new type of carbon structure predicted in recent years,named after its structural framework consisting of a pentagonal covalent network of carbon atoms.Pentadiamond is an indirect bandgap semiconductor with a bandgap of 2.52e V（PBE level）and a high carrier mobility.In this paper,a first-principles approach is used to systematically explore the pentadiamond carbon materials that have emerged in recent years,considering the effects of silicon and boron doping on the structure and properties of the materials,and to design and construct new carbon nitride materials,the main contents are as follows.Two newly designed penta-silicon carbides,namely p-Si C₁₀and p-Si₄C₇,are proposed in this work with the aid of density functional theory（DFT）simulations.Their structural stabilities are verified from energetic,lattice dynamic,and mechanical aspects.These systems possess low mass densities and relatively high mechanical strengthes owing to the big interior spaces inherited from the host pentadiamond.The substitutions of the sp³ carbon atoms by silicon in pentadiamond results in the narrowing of the energy band gaps from 3.78 e V to 3.18 e V and 2.81 e V in p-Si C₁₀and p-Si₄C₇lattices,respectively.Moreover,the band gap is changed from indirect in pentadiamond into quasi-direct in p-Si C₁₀.These advantages make p-Si C₁₀and p-Si₄C₇ materials to be promising candidates in semiconductor industry and aerospace field.Two B-doped pentadiamond systems p-B₆C₅ and p-B₇C₄ are designed and carefully studied.As an additional reference system,the N-doped pentadiamond p-C₁₀N is also taken into account in this study.Their structural stabilities are verified from different aspects with the aid of DFT calculations.These systems can be considered as heavily doped semiconductors.Especially,the p-B₆C₅ lattice possesses an inherent indirect band gap of 2.01 e V estimated at HSE06 level.Doping with B-atoms in pentadiamond expand its absorption region into the visible and near infrared regions.These excellent electronic and optical properties,coupled with their significantly low bulk densities around 2.0 g/cm^-2 and fairish mechanical strengthes,make the p-B₆C₅ and p-B₇C₄systems to be promising candidates in semiconductor and solar cell fields.A newFm（?）mphase carbon nitride C₃N₂ has been found by replacing and removing part of the carbon atoms in the pentadiamond with nitrogen atoms and its structural stability is verified from energetic,lattice dynamic and mechanical aspects.The cavity structure of theFm（?）m-C₃N₂ results in a low bulk modulus compared to other known carbon nitrides,as well as a low bulk density of 2.422 g/cm³.Electronic property calculations at the HSE06 level show thatFm（?）m-C₃N₂ has a quasi-direct band gap of 2.78 e V,which is narrower than the pristine pentadiamond.An extra carbon nitride Immm-C₄N is also obtained by substituting half of the N atoms inFm（?）m-C₃N₂ lattice with C atoms.resulting in a narrower band gap of 2.47 e V.These advantages make these new carbon nitride candidates for aerospace and energy applications.