Study On The First - Principles Of Two - Dimensional BC ₂ N Materials

Due to the quantum size effect, two-dimensional（2D） materials show lots of superior electronic, optical, mechanical properties which are different from the traditional three-dimensional materials. As a result, two-dimensional materials have promising applications in sensors, transistors, energy storage and other areas. Graphene and hexagonal boron nitride（h-BN） are two kinds of two-dimensional materials which have been studied most widely. The structures of graphene and h-BN are very similar, whereas the electronic properties between them are so different. Graphene has extremely high electron mobility and conductivity, but its applications in devices and spintronics are limited because of its zero band gap. By contrast, h-BN has excellent thermal and chemical stability but too wide band-gap, which is inconvenient to modulate in practical applications. Notably, researchers have discovered new ternary boron carbon nitrogen 2D materials with excellent performance, which can be applied to nano photoelectric devices more effectually. Therefore, the 2D boron carbon nitrogen materials are becoming increasingly attractive.This paper has investigated the structural stability, electronic structure, optical properties and elasticity of sixteen kinds of monolayer BC₂ N using the first principles based on density functional theory. Furthermore, the effects of stress and vacancy defects on the electronic properties of monolayer BC₂ N were studied. These research results could provide reliable theoretical basis for experimental synthesis and potential applications of 2D boron carbon nitrogen materials.Firstly, the structural properties of sixteen kinds of monolayer BC₂ N were calculated. We discussed the dependence of the structural stability of BC₂ N on atomic arrangements, the type and number of chemical bonds. The results showed that the structural stability of BC₂ N enhanced with the number of C-C and B-N bonds increased, while the less stable structure of BC₂ N contained more N-N and B-B bonds. Then the electronic properties of monolayer BC₂ N were investigated systematically. The calculation results showed that most structures of BC₂ N were semiconductors except the structure A with highest symmetry which was half-metallic. Similar to graphene, BC₂ N was sp2 hybridization and showed strong covalent bonding characteristics. By changing the external stress on monolayer BC₂ N, we investigated the influence of stress on the electronic properties of BC₂ N. It was found that stress could modulate the band gap of BC₂ N without changing the band gap properties（direct or indirect band-gap） of it. Moreover, the effect of single vacancy on the structure and electronic properties of the most stable structure B of BC₂ N were discussed. It was shown that the introduction of vacancies resulted in the band gap reduced, originating from some new energy levels appeared between the valence band maximum and conduction band minimum in the defective structures.Based on the complex dielectric function, the other optic constants such as the absorption coefficient, reflectivity, complex refractive index, complex conduction and loss function of monolayer BC₂ N were obtained. Combining with the electronic properties, we found that the peaks of the imaginary part of the dielectric function for monolayer BC₂ N were contributed by the electron transition from C-2p occupied state to B-2p unoccupied state. The results of the static dielectric constants showed that the excitons were formed due to the electron-hole interactions. This phenomenon was more obvious in two-dimensional materials since the low dimensionality reduced the dielectric screening in the material. From the absorption spectrum, refractive index and the energy loss spectra of BC₂ N, we found that BC₂ N had high absorption coefficient, high refractive index and strong energy loss peaks in the ultraviolet light region, which indicated that BC₂ N had superior photopermeability in the ultraviolet region. The real part of the optical conductivity tended to zero in the high-energy ultraviolet light region and the low-energy visible region. By analyzing reflectance spectrum of BC₂ N, we found that the reflectivity of BC₂ N was low.The elastic properties of monolayer BC₂ N with different configurations were studied by calculating the elastic constants, elastic modulus and Poisson’s ration. The results showed that, the elastic modulus of the most configurations for BC₂ N were between that of graphene and hexagonal boron nitride. Thus, the mechanical property of BC₂ N was between that of graphene and h-BN as we had expected. The calculated layer modulus, shear modulus and Young’s modulus showed that, in general, they ranked according to the structural stability order of BC₂ N. By calculating the shear anisotropy factor, we found that BC₂ N showed weak elastic anisotropy. The tensile stress-strain curves showed that the most stable structure had high ideal strength while the tensile strength of the most unstable structure was low.