Theoretical Studies On NiB:A Novel Two-Dimensional Transition Metal Boride Monolayer

Two-dimensional(2D)materials have developed rapidly in recent decades.So far,through experimental synthesis and theoretical prediction,thousands of 2D materials databases have been established.Boron has a variety of chemical coordination,so it can bond with transition metals to form a family of 2D transition metal borides(MBenes)with rich electronic properties.In this paper,the first-principles calculation method based on density functional theory(DFT)combined with adaptive genetic algorithm(AGA)is used to study the new type of 2D transition metal boride monolayer NiB based on the 3D LiNiB layered crystal made by recent experiments.The structural stability,electronic property,the existence of quantum spin Hall(QSH)effect and the physical mechanisms involved of monolayer NiB are discussed and analyzed.The main contents of this paper are as follows:(1)The 2D planar monolayer NiB(named αα-Ni4O4)with topological non-trivial electronic structure is studied as the new member of the MBenes family.The NiB monolayer exhibits metallic behavior with two Dirac cones residing slightly below the Fermi level in the absence of spin orbit coupling(SOC).When a tensile strain of 11%is applied,the NiB monolayer transforms into a semimetal.A gap opening of 18 meV is observed at the Dirac points when SOC is considered.Based on the results of Z2 invariant(Z2=1)and the helical edge states,we identify the pristine NiB as a topological metal while the stretched monolayer under 11%strain as a QSH insulator.In addition,the NiB monolayer undergoes a phase transition from topological metal to QSH insulator and then to topological metal with the increasing of strain from 1%to 14%.Our findings demonstrate that the electronic properites of NiB,the new member of MBenes families,can be modulated by strain.(2)The monolayer NiB of different molecular formulas searched by AGA is studied,named after B-B bond arrangement:αα-Ni4B4、αβ-Ni4B4、αββ-Ni6B6、αβββNi8B8 and ααββ-Mi8B8,respectively.The focus is on the difference in properties between the lowest energy phase αα-Ni4B4 and the highest energy phase αβ-Ni4B4 with dynamic,thermodynamic and mechanical stability.The contribution of the Fermi level from the different orbitals of the B atom of metallic αα-Ni4B4 and αβ-Ni4B4 are due to their large differences in the arrangement of the B-B bond.In addition,after rotating the B-B bond in αα-Ni4B4 by 90°,the phase transition of a monolayer of αα-Ni4B4 from a topological metal to a QSH insulator is realized.The analysis results show that the arrangement of B-B bonds is indispensable to the properties of the 2D monolayer NiB.