First Principles Study On Two Kinds Of New Two-dimensional Chalcogenides

With the advent of graphene,more and more two-dimensional materials have been found,which has greatly promoted the vigorous development of two-dimensional materials in various fields.Because of its atomic thickness,two-dimensional materials usually show electronic and mechanical properties completely different from their corresponding structures.These unique physical and chemical properties can well meet the urgent needs of electronic device miniaturization technology in the field of microelectronics applications.Among them,two-dimensional ferroelastic materials and new two-dimensional materials with negative Poisson’s ratio have become one of the research hotspots.In this paper,based on the previous research and the first principle calculation of density functional theory,we predicted two kinds of new chalcogenide compounds Yb X（X=S,Se,Te）and Janus Mo XB₂（X=S,Se）,and systematically studied the ferroelasticity and negative Poisson’s ratio properties in Yb X monolayer and effects of biaxial strain on the electronic and optical properties of Janus Mo XB₂ monolayer.（1）A class of monolayer chalcogenide materials Yb X（X=S,Se,Te）are predicted by first principle calculation.Their dynamic stability,thermal stability and mechanical stability are proved by calculating phonon spectrum and molecular dynamics simulation.The energy band structure of Yb X（X=S,Se,Te）calculated by HSE06functional shows that it is an indirect broad band gap semiconductor.The mechanical analysis shows that the anisotropic low Young’s modulus and high critical strain of the three materials show excellent mechanical flexibility.They all have a large negative Poisson’s ratio in the plane,which mainly comes from the rotation of tetrahedral structure in monolayer Yb X.At the same time,the ferroelasticity of monolayer Yb X has medium energy barrier and strong ferroelastic signal,which is conducive to its application in shape memory devices.These interesting properties make monolayer Yb X a promising candidate material in nanoelectronics and nanomechanics.（2）By means of comprehensive first-principles calculations,we studied the geometric structure,the stability and electronic properties of the new two-dimensional Janus Mo XB₂（X=S,Se）monolayers.Our calculations demonstrated that the predicted Janus Mo XB₂ monolayers are all stable semiconductors with direct band gap.In this paper,we focus on impacts upon the electronic and optical properties of the Mo XB₂monolayers under the different biaxial strains.Under compressive strain,Mo XB₂monolayer will become an indirect band gap semiconductor,and its conduction band bottom remains in the middle of the high symmetry pointГand K,but the top of the valence band gradually moves toГPoint.With the increase of compressive strain,the valence band top and conduction band bottom of monolayer Mo XB₂ will gradually cross through the Fermi surface and behave as semi-metal.While under tensile strain,Mo Se B₂ always maintains a direct band gap,and its valence band top and conduction band bottom move in the same direction.In addition,the optical calculation shows that biaxial strain leads to blue shifts in the optical absorption and reflectivity.The result indicates that Mo XB₂ may be promised nano candidate materials in optoelectronic devices.