First-principles Study Of New Two-dimensional Materials Design And Properties Tuning

Two-dimensional(2D)materials research,as ignited bythe fabrication of graphene,has experienced an explo-sive increase in recent years due to their unique and exceptional properties that can be exploited for electronic,pho-tonic,energy,and environmental device applications.It has become a research focus to search new 2D materials and explore the special physical and chemical properties in the field of low dimensional materials in recent years.In this paper,a series of novel 2D monolayer materials have been predicted using the first-principles calculations based on density functional theory.Furthermore,we further investigated the electronic structure and magnetic properties tuning of black phosphorus.The research content of this paper is divided into four parts:1)Using the particle-swarm optimization method in combination with firstprinciples calculations,here we predict a new category of 2D monolayers named tellurene,composed of the metalloid element Te,with stable 1T-MoS2-like(α-Te)and metastable tetragonal(β-Te)and 2H-MoS2-like(γ-Te)structures.The underlying formation mechanism of such tri-layer arrangements is uniquely rooted in the multivalent nature of Te,with the central-layer Te behaving more metal-like(e.g.,Mo),and the two outer layers more semiconductor-like(e.g.,S).In particular,the α-Te phase can be spontaneously obtained from the magic thicknesses truncated along the [001] direction of the trigonal structure of bulk Te.Furthermore,both the α-and β-Te phases possess electron and hole mobilities much higher than MoS2,as well as salient optical absorption properties.These findings effectively extend the realm of 2D materials to group-VI monolayers,and provide a new and generic formation mechanism for designing 2D materials.2)We propose previously unrecognized allotropes of monolayer boron phosphorus(BP)based on ab initio density functional calculations.In addition to the hexagonal structure of h-BP,four new types of boron phosphide compounds were predicted to be stable as a monolayer.They can form sp2 hybridized planar structures composed of 6-membered rings,and buckled geometries including 4-8 or 3-9 membered rings with sp3 like bonding for P atoms.The calculated Bader charges illustrate their ionic characters with charge transfers from B to P atoms.The competing between the electrostatic energy and the bonding energy of sp2 and sp3 hybridizations reflected in P atoms results in the multiple structures of BP.These new 2D-BP structures can be semiconducting or metallic depending on their geometric structures.Our findings significantly broaden the diversity of monolayer BP allotropes and provide valuable guidance to other 2D group-III-V allotropes.3)We have investigated,by means of ab initio calculations,the electronic and magnetic structures of zigzag edge phosphorene nanoribbons(ZPNRs)with various widths.The stable magnetic state was found in pristine ZPNRs by allowing the systems to be spin-polarized.The ground state of pristine ZPNRs prefers ferromagnetic(FM)order in the same edge but antiferromagnetic(AFM)order between two opposite edges.The magnetism arises from the dangling band states as well as edge localized ?-orbital states.The presence of a dangling band is crucial to the formation of the magnetism of ZPNRs.The hydrogenated ZPNRs get nonmagnetic semiconductors with a direct band gap.While,the O-saturated ZPNRs show magnetic ground states due to the weak P-O bond in the ribbon plane between the pz-orbitals of the edge O and P atoms.4)First-principles calculations are performed to determine the structures of grain boundary(GB)in 2D phosphorene and two typical GBs have been predicted:A-GB and Z-GB defects.The effects of a single substitutional C(O)dopant atom on the energetics and electronic properties were further investigated.Our results indicate that the grain boundary region is reactive and C or O impurity atoms prefer to be incorporated into the GB region atoms instead of the phosphorene bulk region.Particularly,it was found that the formation of OP along the GBs is an exothermic process.Furthermore,both C and O doping inside the grain boundary defects give rise to magnetism in phosphorene.The band structures are also dramatically tuned by the C(O)doping.The study suggests that GBs in 2D phosphorene provide an accessible structure such that the electronic and magnetic properties can be effectively tailored by C or O doping.Four typical GB structures with particularly low formation energies were also found in stanene.A single H adsorption poses a drastic effect on the electronic behavior of GB defects,and the band structures can be tuned by the coverages of H adsorption at these GB defects in stanene.