Theoretical Prediction And Physical Properties Study On Novel Two-dimensional Carbon Nitride Porous Materials

The successful preparation of graphene in 2004 prompts the research upsurge of two-dimensional（2D）materials.The unique crystal structure and excellent electronic properties of graphene show broad application prospects in many fields,which boosts the research on graphene and its derivatives in recent years.Among them,2D carbon nitride materials have attracted much attention in the fields of semiconductor devices,catalysis,batteries,gas adsorption and separation due to their abundance of raw material,high physicochemical stabilities and appealing electronic properties.Up to now,most of the 2D carbon nitride materials synthesized experimentally and predicted theoretically are semiconductors,while the 2D carbon nitride metals or semimetals with characteristics suitable for applications in high performance nano-electronics devices are very rare.By combing the RG² structural search method with the first-principles calculations,a series of holey 2D carbon nitride materials with unique structures and physical properties are predicted in this thesis.Specifically,the predicted 2D carbon nitride materials are classified into carbon-rich 2D carbon nitride,2D carbon nitride with equal mole fraction of carbon and nitrogen atoms,and nitrogen-rich 2D carbon nitride.The main conclusions of the work can be summarized as following three parts:（1）For carbon-rich 2D carbon nitride,forty two novel carbon-rich 2D carbon nitride were found,and all of them show porous structures.Among them,there are six 2D carbon nitride semimetals named as QPH-C₅N₃,PHO-C₅N₄,and C_xN₃（x=7,10,13,19）,respectively.Moreover,there are eight 2D carbon nitride semiconductors and twenty eight 2D carbon nitride metals.First-principles calculations indicate that C_xN₃（x=7,10,13,19）,PHO-C₅N₄ and QPH-C₅N₃ exhibit excellently energetic,dynamic,thermal and mechanical stabilities that indicates their experimental feasibility.The calculated electronic structures of C_xN₃（x=7,10,13,19）,PHO-C₅N₄ and QPH-C₅N₃ show that all of the six 2D carbon nitrides are Dirac semimetals.The Dirac cones in all the2D carbon nitrides originate from the p_z orbitals of C and N atoms.Dirac cones in C_xN₃ are robust when considering SOC effect and external strains.The constructed tight-binding model shows that the Dirac states in C_xN₃ come from the protection of time reversal symmetry and C₃ rotation symmetry.In addition,the nontrivial edge states of C_xN₃,PHO-C₅N₄ and QPH-C₅N₃ further clarify their topological properties.In addition,the carbon-rich 2D carbon nitride semiconductor S-C₃N₂ and metal M-C₅N₃ are the most stable ones with lowest Gibbs free energies and dynamic stabilities.S-C₃N₂ is an indirect semiconductor and its band gap is 1.54 e V.（2）As for the 2D carbon nitride with equal mole fraction o f carbon and nitrogen atoms,nine novel 2D carbon nitride were predicted,and all of them show porous structures.Among them,PH-CN and S-CN are defined as semimetal and semiconductor,respectively.Moreover,there are seven metals.First-principles calculations indicate that PH-CN shows dynamic,thermal and mechanical stabilities that indicate its experimental feasibility.Furthermore,the ELF shows that PH-CN has strong covalent bond.The calculated band structure,3D energy bands as well as contour image of the energy band gap between highest occupied band and lowest unoccupied band near the Fermi level show that PH-CN is a Dirac nodal line semimetal.The nodal line surrounds the high symmetryΓpoint and forms a closed ring.The calculated topological edge states in PH-CN shows that it is topological nontrival.In addition,we confirmed that S-CN shows dynamic and thermal stabilities,S-CN is an indirect semiconductor and its band gap is 1.18 e V.The 2D carbon nitride metal M-CN are the most stable ones with the lowest Gibbs free energies and dynamic stabilities.（3）For the nitrogen-rich 2D carbon nitride,nine novel nitrogen-rich 2D carbon nitride were predicted,and all of them show porous structures.Two of them are semiconductor and other seven systems are metals.We confirmed that semiconductor S-C₆N₇ and metal M-C₆N₇ are the most stable systems with the lowest Gibbs free energies and dynamic,thermal and mechanical stabilities.Moreover,the electron localization function reveals their strong covalent electron states of S-C₆N₇ and M-C₆N₇.S-C₆N₇ is a direct semiconductor whose band gap is 1.21 e V.Furthermore,the S-C₆N₇ is expected to be prepared from the polymerization of 3-s-triazine and melem by hydrothermal treatment in experiment.