First-principles Study On The Structure And Properties Of Two-dimensional Conjoined Sandwich CrB₇ And MnB₇ Nanosheets

Since the discovery of graphene,two-dimensional materials have received wide attention due to their novel properties and great prospects.Many two-dimensional materials have been prepared experimentally,but each one has its own limitations,the corresponding application field and scope are greatly restricted.Therefore,it is urgent to design a new type of multifunctional two-dimensional material with unique structure,high stability and excellent performance.Based on the first principles calculations,we systematically studied the structures and properties of novel two-dimensional transition metal borophene CrB₇and MnB₇nanosheets.The main points we got are listed as follows:(1)Through evolutionary algorithm crystal structure search and density functional theory calculations,we have discovered five new two-dimensional CrB₇nanosheets,namely?,?,?,?and?,according to the energy level.Their dynamical and thermal stability were verified by the phonon dispersion spectrums and ab initio molecular dynamics simulations.In particular,?,?and?contain conjoined sandwich motifs,which have not been reported.Detailed analyses regarding the chemical bonding,electronic structures and mechanical properties of these predicted two-dimensional CrB₇nanosheets were performed.The?,?,?and?nanosheets are metallic,while?nanosheet is semiconductor with direct band gap(?0.8 e V),all of them are magnetic.The strong bonding between the constituents of two-dimensional CrB₇nanosheets lead to very high mechanical strength.In-plane Young's modulus(371.7 N/m)and Poisson's ratio(0.147)of?are superior to that of graphene(340 N/m,0.173),while the mechanical properties of?,?,?and?are comparable to that of graphene.Note that,?shows unusual negative Poisson's ratio in specific directions.After applying biaxial strains on the?(-4%?(10)9%),?(-3%?(10)7%)and?(-2%?(10)7%)nanosheets,there were no imaginary modes in the phonon dispersion spectrums.Furthermore,the electronic properties of?,?and?can be modulated by the strain engineering.The present study open a new branch of two-dimensional materials.(2)Comprehensive crystal structure search method was used to explore the allotropes of two-dimensional MnB₇nanosheets,many diverse structures with various motifs have been discovered.We mainly discussed MnB₇-n(n=1,2,3,...,10)thin films in this work.There exist 3-,4-,7-,8-,10-,12-,13-,14-coordinate Mn in two-dimensional MnB₇thin films.MnB₇-n(n=1,3,4,6,8,10)thin films share similar features,all of them have conjoined configurations.They are a brand new two-dimensional material with high thermal stability,which framework can be survived simulated annealing above 1000 K for10 ps.The spin-polarrized calculations showed that two-dimensional MnB₇-n(n=1,3,4,7,9)thin films are antiferromagnetic metal,while MnB₇-n(n=2,5,6,8,10)are ferromagnetic metal.Mechanical properties of MnB₇-n(n=1,2,3,...,10)are comparable to that of graphene.These novel properties render the two-dimensional MnB₇thin films a promising material for spintronic nanodevices.This work is the first report of the two-dimensional transition metal borophene with conjoined sandwich configurations.High stability,high melting temperature and remarkable mechanical properties may pave the way for two-dimensional CrB₇and MnB₇nanosheets to be used in nanoelectronics.