Theoretical Design And Investigation Of Novel Low-Dimensional Boron Nitride Materials With Narrow Band Gap

Low-dimensional nanomaterials exhibit many unique electronic,optical,me-chanical and chemical properties due to their size effects,surface effects and quantum properties,which make them have wide application potential in optoelectronic devices,catalysts,batteries,photovoltaic,sensors and other fields.The successful synthesis of graphene has set off a research climax of low-dimensional materials,and a series of low-dimensional materials have been successfully synthesized and theoretically pre-dicted.Due to the various hybridization forms of carbon atoms,there are abundant allo-tropes in carbon materials.Recent years,many carbon materials with unique proper-ties have been theoretically predicted and experimentally prepared by researchers.As the nearest two elements of carbon atoms,boron nitride materials composed of boron（B）atoms and nitrogen（N）atoms have also received extensive attention.In particular,two-dimensional monolayer hexagonal boron nitride（h-BN）has outstanding mechan-ical properties,high thermal stability and thermal conductivity,low density,large sur-face area,low dielectric constant,low capacitance,good insulation performance,ex-cellent chemical inertia,high thermal shock resistance,and excellent corrosion re-sistance,which are favored by scientific researchers.Boron nitride nanotubes（BNNTs）and boron nitride nanoribbons（BNNRs）corresponding to carbon nanotubes（CNTs）and graphene nanoribbons（GNRs）also play an important role in many fields due to their excellent properties.In addition,several novel boron nitride materials have also been prepared by researchers through theoretical prediction and experimental synthe-sis,such as t-B₂N₃,th-BN,Z-BN,bi-BN,etc.These novel boron nitride materials not only greatly enrich the boron nitride materials family and provide new ideas for reasearchers to explore new materials,but also show some novel properties.Most boron nitride materials are semiconductors with wide band gaps,which makes it difficult to play its role in the field of photoelectric conversion.In order to improve the application potential of boron nitride materials in optoelectronic devices,we have designed several different types of low-dimensional boron nitride materials,and have carefully investigated their properties using density functional theory（DFT）simulation.They not only have good stability,but also exhibit excellent mechanical and electronic properties.Different from the traditional boron nitride materials with wide band gap characteristics,these materials we designed have relatively narrow band gap,which shows great application potential in the field of electronic and photo-electric conversion.Our research not only provides a new design idea for the experi-mental synthesis of boron nitride materials,enriches the types of boron nitride materi-als,provides some help for the exploration and experimental preparation of novel low-dimensional materials,and obtains some research achievements,but also clarifies some scientific issues,which has very important theoretical value.The main research results are as follows:（1）A novel two-dimensional（2D）boron nitride material,named di-BN,com-posed of B-B bond and N-N bond,was designed.Its structural stability has been veri-fied from multiple perspectives,and its geometric and electronic configuration,me-chanical properties,and electronic properties have been discussed in detail with the aid of DFT simulation.The monolayer di-BN not only has excellent mechanical strength very close to monolayer h-BN,but also exhabits a moderate direct band gap in the visible light range.The band gap is 1.622 e V at the HSE06 level and 2.446 e V at the G₀W₀ level.In addition,the detailed band edge arrangement of di-BN indicates that it can meet the hydrogen evolution reaction（HER）in photocatalysis of water spliting,indicating that it has an application potential in photocatalysis of water split-ing.Its carrier mobility of di-BN in the zigzag direction and the armchair direction are significantly anisotropic,and the carrier mobility in the zigzag direction is much higher.The carrier mobilities of the monolayer di-BN in the zigzag direction is up to 1×10⁴ cm²V^-1s^-1,which is much higher than some popular 2D materials.The partial density of states projection（p DOS）also shows that the CBM and VBM of di-BN are respectively contributed by the p-orbitals of B and N atoms,which is conducive to the separation of excitons.（2）Two kinds of one-dimensional（1D）nanowires,a-BNnw and d-BNnw com-posed of N-N and B-B bonds,were designed based on monolayer di-BN.Their struc-tural stability has been verified by the cohesive energy,phonon dispersion and ab inint molecular dynamics（AIMD）.The band gap of a-BNnw and d-BNnw at HSE06 level is 3.256 e V and 4.631 e V,respectively,which is much narrower than that of h-BNnw.The projected DOS patterns point out that the CBM and VBM are respectively com-posed of the p-orbitals of B-and N-atoms.The strains in the elastic range can also significantly affect their electronic properties.Unlike in the case of h-BNnw,their band gaps monotonically reduce with increasing strain applied in the axial direction.The change of the band gaps under strains can be explained with the aid of the Heit-ler-London model.Moreover,their carrier mobilities of electrons and electron holes are comparable to those of many other reported 1D materials,and shows obvious ani-sotropy.Especially for a-BNnw,the carrier mobility of the electron hole even ap-proaches 4.5×10³ cm²V^-1s^-1,which is two orders of magnitude higher than the elec-tron mobility.（3）According to strict mathematical derivation,it has been clearly pointed out that monolayer di-BN with a rectangular unit cell can only be rolled into zigzag（n,0）and armchair（0,n）nanotubes,but can not form chiral（n,m）di-BNNTs with n≠m≠0that are not rigorous in mathematics and physics.The results of phonon dispersion and AIMD show that the（6,0）and（0,4）systems are the thinnest and structurally sta-ble di-BNNTs of the corresponding types,which can even exist stably at 1000 K.By comparing their Young’s moduli with single-wall carbon nanotubes（SWCNTs）and single-wall boron nitride nanotubes（SWBNNTs）,di-BNNTs has very outstanding mechanical strength.Moreover,owing to the bending strian,the band gap of di-BNNTs will increase or decrease to a certain extent according to its rolling vector.However,with the increase of the radius,the band gap of both types of nanotubes will be close to the value of 2D monolayer di-BN.In addition,there is an even-odd rela-tion of the band gaps due to the band folding of zigzag（n,0）di-BNNTs.With the in-crease of tube size,the band gap of di-BNNTs can cover all visible light regions.More importantly,the arrangements of their band edges are suitable for photocatalytic hydrogen evolution reaction（HER）and/or oxygen evolution reaction（OER）in water splitting process at appropriate p H value,especially the hydrogen evolution ability will not be lost with the change of the pH change.