Structure Prediction And Property Investigation Of Two-dimensional Materials On Theory

In this thesis,using the first-principles method within the framework of the density functional theory and the genetic algorithm,we have studied in detail three different kinds of two-dimensional materials and their properties.Firstly,we studied the two-dimensional nanomaterials with Cairo pentagonal tiling lattice,which have geometrical structures fully composed of pentagons.By arranging B,N and Al,N atoms at nonequivalent positions in the primitive cell,we obtained all the possible structures and carefully optimized them.Finally,there are two kinds of stable structures,including penta-BN,penta-BN2 and penta-AlN2 nanosheets.For all the new structures,the dynamic stabilities,thermal stabilities,thermodynamic stabilities and mechanical stabilities were carefully estimated.Then,we investigated their electronic properties.Due to the unique geometrical configuration of penta-BN2,its electronic properties can be turned from conducting to semiconducting by the means of ionizing and functional group adsorption.Besides,it has remarkable mechanical properties such as the negative Poisson's ratio and the comparable in-plane stiffness as to that of h-BN sheet.For the penta-AlN2 nanosheet,we find it to be a ferromagnetic semiconductor.Additionally,certain tensile strains can be applied to manipulate the electronic properties as well as structural configurations of penta-AlN2.What's more,the magnetic coupling strength could also be continuously enhanced by applying strain,rendering it as a robust ferromagnetic material,calling for further studies on both theory and experiment.Secondly,for the transition metal-borophene complex two-dimensional materials,we began our research with the global minimum structural searches by using the genetic algorithm.The lowest-energy structures were carefully optimized by the first-principles method.They consist of borophene-like layers and an TM-layer with the TM-layer being sandwiched between boron planes,opening the door for tailoring unique properties by manipulating TM atoms.For MnB6 nanostructure,we have verified their stabilities.It is found to be magnetic semiconductor.Furthermore,the gap of spin-down electrons is entirely located inside the forbidden zone of the gap of the spin-up electrons,showing the promising for the spin-filtering by controlling the gate voltage.The semioxidized and oxidized MnB6 sheets also show good stabilities and electronic properties.For two-dimensional FeB6 nanostructure,we have found a global minimum ground state structure,which is obviously lower in energy than the previous reported structure configurations.It was named as tri-FeB6 to account for the structural characteristics.We have also carefully examined its stabilities.The tri-FeB6 shows remarkable stiffness comparable to that of graphene.Oxidization would not harm its structure stabilities.Interestingly,the oxidization would induce negative Poisson's ratio.Furthermore,tensile strain can be used to continuously tune the band gap over a wide range,making FeB6 and its oxide material being attractive for nanoelectronics.Finally,using systematic evolutionary structure searching method and the first-principles calculations,a new planar nanostructure composed of 5-6-8 structural rings is proposed for boron nitride,which is referred as pho-B2N3.We have examined its stabilities and estimated the possibility in synthesizing.The studies of electronic bandstructure show that it is a semiconductor,whose gap width could be continuously tuned by applying strain,rendering it as a promising material for the field effect transistors.