Theoretical Design And Investigation Of Biphenyl Network-like Boron Nitride

With the rapid development of social science and technology,miniaturization of applied devices has gradually become a new goal for researchers.Low-dimensional materials show many physical and chemical characteristics different from traditional three-dimensional materials because of their structural specificity.For example,the movement of electrons in two-dimensional materials is limited in the plane,so its conductivity in the plane will be higher than that in other directions,so its application in nano devices has a good prospect.With the successful preparation of graphene,two-dimensional materials have been widely concerned by scientists all over the world.In recent years,researchers have turned their research focus into the exploration of planar sp~2 hybrid carbon allotypes other than graphene,such as kind of graphene and biphenyl networks.Both experimental synthesis and theoretical prediction have proposed a large number of new two-dimensional materials.Theoretical design of new materials can predict their structural,mechanical and electronic structure properties,as well as their applications according to the calculation results.However,due to the peculiar hybridization and bonding mode of carbon atoms,it is difficult to form a stable ring structure except hexagon in the plane.In2021,Science reported that a two-dimensional planar biphenyl network compound（BPN）with 4-,6-,and 8-membered rings periodically arranged by sp~2 hybrid carbon atoms was successfully synthesized by surface copolymerization dehydrofluorination（HF-zipping）.This work has guiding significance for the preparation of planar carbon allotropic materials.Although the B-N pair is isoelectronic of the C-C pair,the B-N bond has obvious polarity due to the different electron configuration.Therefore,researchers also commonly use B-N to doped carbon materials,through its own polarity to generate built-in electric field in the material,and then control the physical and chemical properties of the material,or even completely replace the carbon element to design a new boron nitrogen compound.Therefore,we use B-N to replace C-C in BPN,and obtain a new B-N counterpart of biphenyl network with 4-,6-,and 8-membered rings,which is named bi-BN.Firstly,the cohesive energy of bi-BN is calculated.Compared with other two-dimensional boron nitride materials,its cohesive energy is about 0.21 e V higher than h-BN,indicating that it has a high possibility of synthesis;AIMD simulation is carried out at 500 K and 1000 K,and the results show that its thermodynamic stability is good;the phonon dispersion relationship is simulated to prove the lattice dynamic stability of bi-BN.Based on the calculation results of energy,thermodynamics and lattice dynamics,it is verified that the structure of bi-BN can exist stably in theory.Then we study its mechanical and electronic structure properties.It has a large two-dimensional Young’s modulus and shows strong mechanical strength.Although the electron band gap of single-layer bi-BN materials is still large（PBE level is 3.20 e V and HSE06 level is 4.49 e V）,it has been significantly lower than that of single-layer h-BN materials and most other two-dimensional boron nitride materials.Four superposition forms of three-dimensional block bi-BN lattice are also considered in this paper.According to our calculation,the most stable stacking form is the so-called AB stacking form,that is,the upper B and N atoms face the lower N and B atoms respectively.But the energy difference between the different forms of stacking is only on the order of a few millielectronford per atom,this result shows that the three-dimensional stacked bi-BN is similar to graphite and three-dimensional h-BN,and there may be interlayer sliding.Not only the stacking form,its physical properties can be controlled by applying external force.For example,the applied hydrostatic pressure will reduce the band gap width of small bi-BN lattice to a certain extent.We hope that the new two-dimensional boron nitride materials predicted in this work and the research results of this work can provide new ideas for the prediction and synthesis of new low-dimensional boron nitride materials and even other low-dimensional materials,and extend application of low-dimensional materials to family members.