First-Principles Study On The New Two-Dimensional Semiconductors

Recent years have witnessed a booming development in two-dimensional materi-als since the experimental discovery of graphene.Besides graphene,two-dimensional materials such as phosphorene,borophene,and transition-metal dichalcogenides exhibit many novel physical properties which do not exist in their bulk analogues.In this disser-tation,we discuss about the ferroelectric property,ferromagnetism,mobility and spin-orbit coupling effect in the GaTeCl monolayer,PdSe₂monolayer,Mo₆Br₆S₃monolayer and BiTeI monolayer.We propose through first-principles investigation that the GaTeCl monolayer is an excellent two-dimensional multiferroic.The calculated phonon spectrum,molecular dynamic simulations,and elastic moduli confirm its dynamic and mechanical stability,and our cleavage energy analysis shows that exfoliating one GaTeCl monolayer from the existing GaTeCl bulk is feasible.The calculated in-plane ferroelectric polarization reaches 578 p C m^-1.The energy barriers per formula unit of the ferroelastic 90^°ro-tational and ferroelectric reversal transitions are 476 me V and 754 me V,respectively,being the greatest in the 2D multiferroics family so far.Importantly,on the other hand,a tensile stress of 4.7 N m^-1perpendicular to the polarization can drive the polarization to rotate by 90^°.These can make the GaTeCl monolayer have not only robust ferroe-lasticity and ferroelectricity but also easy mechanical controllability.Furthermore,the GaTeCl monolayer has giant piezoelectricity and optical second harmonic generation,especially in the range of visible light,and a tensile stress of 0.3 N m^-1along the polar-ization can make the indirect gap transit to the direct gap.These interesting mechanical,electronic,and optical properties of the GaTeCl monolayer show its great potential in high-performance multi-functional applications.Recently,a highly-air-stable PdSe₂ monolayer semiconductor has been made through exfoliation from the layered bulk material.It is very highly desirable to realize ro-bust ferromagnetism,even half-metallic ferromagnetism?100%spin polarization?,in such excellent nonmagnetic monolayer semiconductors.Here,our first-principles in-vestigation shows that the hole-doping PdSe₂monolayer can be made to attain high-temperature Stoner ferromagnetism,and the hole concentration threshold for ferromag-netism decreases with applied uniaxial stress.Furthermore,half-metallicity can be achieved in some hole concentration regions.Under a strain of 10%,the monolayer can attain half-metallic ferromagnetism.The highly-air-stable PdSe₂monolayer,with these advantages,should be promising for spintronic applications.Two-dimensional materials with direct semiconductor gaps and high mobilities can play an important role in future electronic applications.Here we propose that Mo₆Br₆S₃monolayer as a new two-dimensional material is stable and can be exfoliated from corre-sponding layered bulk.Our first-principles results show that the monolayer has a direct semiconductor gap beyond 1 e V and a very high electron mobility(6880 cm²V^-1s^-1),and these can be tuned through in-plane strain by applying uniaxial stress.Furthermore,we show that the Mo₆Br₆S₃/graphene heterostructure makes a p-type Schottky barrier and the amplitude of band bending?0.03 e V?is extremely low compared to other simi-lar junctions because the Mo₆Br₆S₃monolayer has a close work function to graphene.With all these useful properties and functions,the Mo₆Br₆S₃monolayer can be very promising for nanoelectronic applications.Combining a first-principles investigation and model analysis,we use uniaxial stress to control monolayer BiTeI as a Rashba 2D semiconductor.We find that the stress-driven electron system can be described by an effective anisotropic Rashba model including all three Pauli matrices,and uniaxial stress allows an out-of-plane spin com-ponent because of rotational symmetry breaking.When appropriate electron carriers are introduced into the monolayer,an in-plane electric field can induce a charge current and three spin current components?including that based on the out-of-plane spin?be-cause of the reduced symmetry.Therefore,uniaxial stress can be used to control Rashba2D electron systems such as monolayer BiTeI for promising spintronic devices.