Simulation And Design Of Two-Dimensional Pentagonal N-containing Materials And Their Heterostructures

The successful synthesis of graphene and its novel properties have greatly stimulated the study of other two-dimensional（2D）materials with different structural morphology and chemical composition.Among them,penta-graphene is one of such typical representatives,which has become a new structural paradigm for 2D materials.Based on the atomic configuration of penta-graphene,dozens of new stable 2D materials with pentagonal structural units have been theoretically predicted,pentasilicene nanoribbons and monolayer and multilayer penta-Pd Se2 have been experimentally synthesized.These advances have further promoted extensive research on the applications of 2D pentagonal materials.In this context,considering the important applications of nitrogen-containing 2D materials in high energy density materials,piezoelectricity,ferromagnetism,etc,in this thesis,on the basis of density functional theory,the stability and physical properties of 2D pentagonal N-containing materials,including Penta-MN₂（M = Mn,Pd,and Pt）,penta-BCN,and the heterostructure composed of penta-BN₂ and penta-graphene have been systematically studied.The main contents are as follows:（1）2D ferromagnetic penta-MnN₂ sheet.Since Mn has high magnetic moment and N can effectively mediate the magnetic coupling between Mn atoms,we proposed a pentagonal planar structure,penta-MnN₂,which containsN₂ dimers.First priciples calculated results show that the ground state of the structure is ferromagnetic halfmetallic.Using the Monte-Carlo simulation,we found that the Curie temperature（TC）is 913 K,which can be explained with the super-exchange interactions between the Mn and N atoms.Moreover,the TC can be effectively tuned by applying stress.From the hexagonal Mn N to penta-MnN₂,TC increases significantly.Penta-MnN₂ can be expected to have applications in spintronics devices.（2）Stable planar penta-PdN₂ and penta-PtN₂ sheets.Due to the existence of sp2 and sp3 hybrid bonds in 2D pentagonal structures,2D materials composed of pure pentagons usually have buckled configurations.Using global structure search,we identified two stable pentagonal planar structures: penta-PdN₂ and penta-PtN₂,which exhibit metallic and semiconducting properties,respectively.Our calculated results show that pentaMN₂（M = Pd,Pt）has a high binding energy,suggesting that they are more stable than that of synthesized pyrite-MN₂.In addition,they have the characteristics of high melting point,especially the melting point of penta-PtN₂ is higher than 2000 K.Due to the high elastic constants and small effective carrier mass,penta-PtN₂ exhibits extremely high carrier mobility.Penta-MN₂ may have applications in high heatresistant materials and electronic devices.（3）2D ternary piezoelectric penta-BCN sheet.B and N are located on the left and right sides of C in the periodic table,respectively,and have very similar atomic radius,but different electronegativity to C.We found that the rational combination of these three elements can form a stable penta-graphene-like structure,penta-BCN.Penta-BCN is a semiconductor with a direct bandgap of 2.81 e V and has high mechanical strength and anisotropic properties.The special arrangement of B-N bonds induces spontaneous polarization along the（010）direction.Due to the shorter bond length and stronger covalent bond of B-N,as compared with those in h-BN,its spontaneous polarization is greater than that of h-BN.Because of the characteristic of non-centrosymmetric and semiconductor,penta-BCN exhibits excellent piezoelectric properties,showing the great potential as a piezoelectric material at the nanoscale.（4）Penta-graphene/penta-BN₂ heterostructure.We constructed a heterostructure by vertically stacking penta-graphene and penta-BN₂ sheets,and systematically studied its stability,electronic properties and device performance.Using first principles calculations,the system has negligible lattice stress,and the electronic structure shows that it has n-type Schottky barrier.By applying in-plain uniaxial or biaxial stress or vertical stress,the Schottky barrier height of electrons and holes can be effectively tuned,and even the type of Schottky barrier can be transformed from n-type to p-type.This work expands the application of 2D pentagonal N-containing materials in field effect transistors.These studies provide important theoretical understanding and information on novel 2D N-containing pentagonal sheets with many promising applications.