Theoretical Design And Properties Study Of Two-dimensional Carbon And C-N Materials

Two-dimensional materials have been attracting attentions of researchers due to their large specific surface area and unique electronic structure.The design of novel functional two-dimensional materials has been a hot topic in condensed matter physics,chemistry and materials science.With the rapid development of computer technology,first-principles calculation has become a powerful tool for the design and study of crystal materials,which is of great significance for the study of basic properties of materials and the development and design of new functional materials.In the family of two-dimensional materials,carbon materials and carbon-nitrogen based monolayer nanomaterials both play an important role and have been the focus of research in the field of materials,physics and chemistry.The crystal structure of a material is a fundamental factor in determining its physical properties,therefore,it is necessary to design the crystal structure of novel two-dimensional carbon materials and carbon-nitrogen materials in theory for the development of new functional two-dimensional materials.In this paper,we use the global structure prediction method in combination with the first-principles calculation method of density functional theory,and select the typical two dimensional system?carbon materials and carbon-nitrogen based materials?as research objects to carry out theoretical research of crystal structure.The main results of the thesis are as follows:1.Two-dimensional materials have broad application prospects in many fields of nanotechnology due to their excellent optical,mechanical and magnetic properties.In this work,we performed structure prediction search of two-dimensional carbon material using the CALYPSO methodology at ambient pressure,and found a metastable two-dimensional carbon allotrope composed of pentagons and heptagons,which has the similar topology as that of superhard M-carbon,so it is denoted as M-graphene.The phonon spectrum and molecular dynamics computations show that M-graphene is not only dynamically stable,but also thermodynamic stable.The results of electrochemical performance study show that it is a promising candidate as an anode material for Li-ion batteries?LIBs?,because of the relatively low diffusion energy barrier of Li?0.338 eV?,open-circuit potential?0.029 V?and high lithium-storage capacity(279 mAh g^-1),M-graphene is expected to become a promising high-performance carbon anode material for Li-ion batteries.This research has played an important role in promoting and guiding the theoretical and experimental research of novel 2D carbon materials.2.The realization of experimental synthesis and the photocatalysis and photovoltaic applications of two-dimensional carbon-nitrogen materials have greatly stimulated the exploration of novel 2D carbon-nitrogen materials.We systematically predict the structure of the C-N system via the CALYPSO crystal structure method at ambient pressure.As a result of structure searches,four porous carbon nitride monolayers are predicted with stable stoichiometries of C₅N₂,C₂N,C₃N₂ and CN.It was found that the C₅N₂,C₂N,C₃N₂ and CN monolayer with planar porous structure had dynamical and thermodynamic stability.These predicted structures have direct band gap and good light absorption characteristics as indicated in our calculation simulations.Our current results also reveal the high tensile strengths of the predicted C₂N.The results of this study confirmed the stability of two-dimensional carbon nitrogen porous materials C₅N₂,C₂N,C₃N₂ and CN,and highlighted their promising electrical,optical and mechanical properties.3.The effect of substitution of silicon atoms and the adsorption of alkali metal atoms?Li,Na and K?on the structure and electronic properties of two-dimensional porous C₃N₂ proposed by us are investigated using first-principles calculations.Firstly,the different topologies are obtained by replacing C atoms with Si atoms,resulting in the transition of electronic properties from semiconductor to metal.At the same time,we learned from the calculation of band structure and density of states that different substitution ratios can regulate the size of the direct band gap by changing the ratio of Si atoms to C atoms in the two-dimensional C₃N₂ crystal structure.Secondly,the calculation results of adsorption energy show that the most stable site for the adsorption of alkali metal atoms is the center of the hole of C₃N₂.As the atomic number of adsorbed alkali metal atoms increases,the adsorption energy becomes smaller,the adsorption strength becomes weaker,and the charge transfer becomes smaller.Phonon spectrum and molecular dynamics simulation show that the adsorption of Li on C₃N₂ has dynamical stability and thermal stability.The energy band structure and density of states show that the adsorption of Li on C₃N₂ has the greatest influence on the electronic properties of C₃N₂.The results of this study confirmed that atomic substitution and alkali metal adsorption are effective methods to regulate the electronic structure and physicochemical properties of 2D carbon-nitrogen materials,which is of fundamental significance in the study of 2D carbon-nitrogen materials,and provide theoretical guidance for future research on how to regulate the physicochemical properties of 2D carbon-nitrogen materials.