Design And Electronic Structures Of Neotype Two-Dimensional Borophenes Based On First-Principle Study

With the successful preparation of graphene,two-dimensional?2D?materials have been a hot research topic in condensed matter physics,and borophene has attracted tremendous research interest for its abundant crystal structures and properties.In this thesis,several new kinds of 2D borophene materials with triangular and hexagonal boron lattices are proposed,and some electronic properties of these 2D materials are studied based on the density functional theory.The main conclusions are listed as follows:?1?A hexagonal boron lattice can not be stable by itself.By introducing the modification of transition metal atoms,the stability of the system may be improved.In this paper,three kinds of metal atoms?Ti,Fe and Hf?are used to modify the hexagonal boron lattice,and stable hexagonal borophene monolayes?FeB₂,TiB₂ and HfB₂?are obtained:.The electronic structure calculations show that all of them are Dirac materials,in which the Dirac cones are attributed dominantly not to the electronic states of boron atoms on the hexagonal sublattice,but to the d orbit states of the transition metal atoms.This is quite different from the case in many other Dirac materials.In most 2D Dirac materials,as reported up to date,the Dirac cones stems from the p orbit states of the main group elements,while the Dirac cones in the modified hexagonal borophenes studies in this paper have a completely different origin.?2?The idea on the design of the modified hexagonal borophene monolayers has been generalized to design borophene sandwiches,in which a transition metal layer is placed between two hexagonal boron lattices and the upper and lower boron layers are modified at the same time,so as to guarantee the stability of the configuration of the bilayes.In this these,we find that only part of transition metal atoms?Cr,Mo and W?can be used as modification atoms to stablize the sandwiched hexagonal borophene systems.The electronic structure calculation results of CrB₄,MoB_4,and WB₄ show that all of their energy bands have the characteristic with two inequivalent Dirac cones.Unlike the monolayer hexagonal borophene system,the Dirac cones in sandwiching hexagonal borophene bilayers are attributed not only to the d-orbit states of the transition metal atoms,but also to the p-orbit states of boron sublattices.?3?A triangular boron lattice is also not stable by itself,but the introduction of proper nonmetallic modifying atoms may stabilize it.In this thesis,we use hydrogen atoms to modify part of the boron atoms on the triangular lattice and obtain a stable triangular borophene system:B₃H;we also use oxygen atoms as bridging atoms to modify the upper and lower triangular boron lattices,and obtain a stable bridged triangular borophene bilayers:the B₆O system.Calculations of their electronic structures show that there is one Dirac cone at the K point in the energy band of the B₃H system,while the B₆O system has two Dirac cones at the K point.In B₆O bilayer system,the two Dirac cones are located on both sides of the Fermi surface,and they intersect with each other just near the Fermi surface,forming a closed continuous curve on the Fermi surface.This characteristic indicates that the B₆O system has the properties of socalled node-line semimetal.The Dirac cones in both B₃H and B₆O systems are attributed not only to the pz orbit states of the unmodified boron atoms,but also to the px and py orbit states of the modified boron atoms.?4?In order to guide for experiments,we calculated the electronic structure of FeB₂,TiB₂,HfB₂,MoB₄ and WB₄ grown on SiC substrate,as well as CrB₄,B₃H and B₆O grown on AlN substrate.For all borophene systems,calculation results show that substrate does not destroy their intrinsic band structure,their original Dirac cone still exist.Combine the results of formation energy and atomic distance,we can judge that borophene and substrate are combined by van der Waals forces.