First Principles Investigation Of Mechanical Property And Electrical Structure Of Bi₂O₂Se

A new material field-two dimensional(2D)nanomaterials have attracted extensive attention since the discovery of graphene.Graphene has aroused scientists' great interest because of its high conductivity,good thermal conductivity and high mechanical strength.Despite these excellent properties,there is no gap existing in the intrinsic graphene,which severely limits its application in electronic and optoelectronic devices.Therefore,researchers have to try their best to look for new two-dimensional material to take the place of graphene.Currently,High-mobility atomically thin Bi₂O₂Se,a typical non-neutral layered crystal,is synthetized via a facial chemical vapor deposition(CVD)method with high charge-carrier mobility and.air-stable quality,thus it can be a promising material for fundamental investigations and.high-speed low-power electronic applications.In addition,the research on this material in the literature is mainly focused on thermoelectric properties,transport characteristics and native.point defects,but little is known about its important mechanical property.In view of this,First-principles calculation based on the density functional theory are used in this paper to systematically study the mechanical property and electrical structure of Bi₂O₂Se,and we draw the following main conclusions:First,this paper uses stress and strain perturbation method to calculate the four independent elastic constants of 2D Bi₂O₂Se.The results show that monolayer Bi₂O₂Se satisfy the mechanical stability criteria.Therefore,the 2D Bi₂O₂Se material is mechanically stable.Then,the mechanical property of it is further investigated based on these four elastic constants,mainly the modulus of elasticity(Young's modulus,Poisson's ratio,shear modulus).It is found that the young's modulus and Poisson's ratio of this two-dimensional material are highly anisotropic and the maximum values for young's modulus and Poisson's ratio are 73.24 GPa-nm and 0.32,respectively.What is most prominent is that our study finds that the shear modulus reaches 42.97 GPa·nm,which is about 2 times larger than that of graphene,far beyond the other common two-dimensional materials such as black phosphorus.Next,in order to study the effect of quantum size effect on the mechanical property of Bi₂O₂Se materials,we also calculate the elastic constants of bulk Bi₂O₂Se with stress and strain perturbation method.Then using the calculated independent elastic constants combined with Voigt,Reuss and Hill methods,we further study the modulus of elasticity in mechanical properties,namely,bulk modulus,shear modulus,Young's modulus and Poisson's ratio.We find that the calculated results of elastic modulus are generally smaller than 100 GPa·nm,which may prove that bulk Bi₂O₂Se material is essentially a soft material.However,it may be not enough to conclude that bulk Bi₂O₂Se is soft only from elastic modulus.This paper also discusses the intrinsic hardness and Debye temperature,for these two parameters can also reflect the hardness of a material.It turns out to be that all the intrinsic hardness results are less than 8 GPa,and the Debye temperature is below 20 K.All these three aspects can firmly confirm that the bulk Bi₂O₂Se is essentially a soft material with good ductility,besides,the mechanical anisotropy shows that the anisotropy of shear modulus is greater than that of bulk modulus.Last,Electric structure of two-dimensional monolayer Bi₂O₂Se is investigated,and it is known to all that standard density functional theory fails to predict the accurate bandgap of semiconductor material.For example,the bandgap of 2D Bi₂O₂Se is only 0.12 eV through generalized gradient approximation theory and the hybrid density functional theory gives a value of 0.51 eV.Obviously,both of these two results are far less than the experimental gap value 1.90 eV.The reason lies in the fact that traditional density functional theory can not describe the material system containing heavy metal element such as bismuth.Finally,we pay attention to the highly accurate quasiparticle GW method,and get the result of 1.99 eV,which is in good agreement with the experimental value.Furthermore,strain engineering can have an important effect on the electronic structure of 2D material,so the relationship between strain and bandgap has been studied in this paper.It is found that biaxial strain is more effective in modulating bandgap of 2D Bi₂O₂Se,unfortunately,direct-to-indirect transition has not been found.The reason may lie in the weak electrostatic interactions in Bi₂O₂Se material.Besides,It is found that the bandgap changes from 1.99 eV to 0.99 eV when 2D monolayer Bi₂O₂Se turns to its bulk form,which implies that the quantum size effect has a significant effect on the electronic structure of Bi₂O₂Se.