Theoretical Study On Structural Design And Physical Properties Of Several New Two-dimensional Materials

Since the successful preparation of graphene,two-dimensional?2D?materials have attracted wide attention due to their unique physical and chemical properties and potential applications in the fields of electronics and optoelectronics.As a typical two-dimensional material,graphene is famous for its excellent carrier mobility and ultra-high mechanical strength.Molybdenum disulfide has suitable band gap and can be used in field effect transistor.Monolayer black phosphorus also has suitable band gap and high carrier mobility,making it an ideal semiconductor device.But they also have the drawbacks:the zero band gap of graphene cannot be used in electronic devices,the carrier mobility of molybdenum disulfide is too low as electronic devices,the environmental stability of black phosphorus is poor.These shortcomings limit their application in high performance devices.Therefore,the search and design of new two-dimensional materials with excellent physical and chemical properties is still one of the research hotspots.With the improvement of computer performance and the development of first-principle calculation method,it is of great scientific significance to design the structure of materials theoretically and guide the synthesis of experiments.Using density functional theory and global search method,we design a Dirac material?Be₃C₂ monolayer?with ultra-high carrier mobility,ultra-thin semiconductors Bi₂Te₂S and Bi₂Te₂Se with good light absorption,two-dimensional ferromagnetic materials?ScCl,MnX,and GdI₂ monolayers?with high Curie temperature.The main research contents include the following aspects:?1?Structural prediction and properties of a Dirac material?Be₃C₂ monolayer?.Two-dimensional materials with Dirac cones exhibit rich physics and many intriguing properties and the seeking of new 2D Dirac materials is still current hotspot.Using global particle-swarm optimization method and density functional theory,we predict a new stable graphene-like 2D Dirac material:Be₃C₂ monolayer with a hexagonal honeycomb structure.The Dirac point occurs exactly at the Fermi level and arises from the merging of the hybridized p_z bands of Be and C atom.Most interestingly,this monolayer exhibits high Fermi velocity in the same order of graphene.Moreover,the Dirac cone is very robust and retains even included spin-orbit coupling or external strain.These outstanding properties render Be₃C₂ monolayer a promising 2D material for special electronics applications.?2?Ultrathin semiconducting Bi₂Te₂S and Bi₂Te₂Se with high electron mobilities.High carrier mobility and moderate band gap are two key properties of electronic device applications.Two ultrathin two-dimensional semiconductors,namely Bi₂Te₂S and Bi₂Te₂Se nanosheets,with novel electronic and optical properties are predicted based on first-principles calculations.The Bi₂Te₂S and Bi₂Te₂Se monolayers own moderate band gaps?⁰.7 eV?and high electron mobilities(²0,000 cm²V^-1s^-1),and they can absorb sunlight efficiently through the whole incident solar spectrum.Meanwhile,layer-dependent exponential decay band gaps are also unveiled.The relatively low interlayer binding energies suggest that these monolayers can be easily exfoliated from bulk structures.Their high dynamical and thermal stabilities are further verified by phonon dispersion calculations and ab initio molecular dynamics simulations.The exceptional properties render Bi₂Te₂X?X=S,Se?monolayers promising candidates in future high-speed?opto?electronic devices.?3?Theoretical study of a two-dimensional high temperature ferromagnetic metal material.Two-dimensional ferromagnetic materials provide new platforms for spintronic applications,but most of the reported 2D magnetic orderings can only be maintained at low temperature.The search for high Curie temperature intrinsic ferromagnetism is still a current hotspot.Herein through comprehensive first-principles calculations and Monte Carlo simulation,we predict a promising 2D scandium chlorine?ScCl?monolayer with intrinsic ferromagnetism and a Curie temperature of 185 K,which is much higher than that of the reported CrI₃ monolayer?45 K?and the boiling point of liquid nitrogen?77 K?.Moreover,a small amount of hole doping can induce a transition from ferromagnetic metal to half-metal.Furthermore,ScCl monolayer possesses excellent thermal and dynamical stabilities as well as feasibility of experimental exfoliation from its layered bulk.These intriguing electronic and magnetic properties endow ScCl monolayer a promising candidate for spintronic applications.?4?A class of two dimensional ferromagnetic Mn based compounds with half-metal and room-temperature Curie temperature.Recent experimentally demonstrated intrinsic two-dimensional magnetism has sparked intense interest for advanced spintronic applications.However,the rather low Curie temperature and small magnetic anisotropic energy?MAE?greatly limit their application scopes.Here,by using density functional theory calculations,we predict a series of stable 2D MnX?X=P,As,Sb?monolayers,among which MnP and MnAs monolayers exhibit intrinsic ferromagnetic?FM?ordering and considerably large MAE of 166 and 281?eV per Mn atom,respectively.More interestingly,the 2D MnP and MnAs monolayers expose highly desired half-metallicity with wide spin gaps of about 3 eV.Monte Carlo simulations suggest markedly high Curie temperature of MnP and MnAs monolayers,⁴95 K and 711 K,respectively.Besides,these monolayers are global minima in the 2D space with excellent dynamical and thermal stabilities.A viable experimental synthesis route is also proposed to produce MnX monolayer via selectively chemical etching method.The outstanding attributes of MnP and MnAs monolayers would substantially broaden the applicability of 2D magnetism for a wide range of applications.?5?Theoretical prediction of two-dimensional intrinsic ferromagnetic semiconductor GdI₂ monolayer with near room-temperature Curie temperature.Monolayer van der Waals?vdW?ferromagnetic semiconductor?FMS?opens up exciting opportunities for nanoscale device applications,but the intrinsic FMS is rather scaled and has only been observed at low temperatures,greatly restrict their practical applications.Based on the first-principles calculations,we predict a stable 2D semiconductor,GdI₂ monolayer,which shows robust ferromagnetic ordering with nearly room-temperature Curie temperature?T_c?,large spin polarization,large magnetic moments,and sizable magnetic anisotropic energy?MAE?.Interestingly,this monolayer exhibits highly desired bipolar magnetic semiconductor?BMS?with a appropriate band gap?1.21 eV?.The coexistence of direct exchange and superexchange interactions results in its FM coupling greatly and improves Curie temperature?T_c?up to 241 K,which exceeds the record?¹55 K?of the most-studied diluted magnetic semiconductor?Ca,Mn?As material.Besides,the FM state is quite robust tunder external strain and carrier doping,and the T_c is well maintained under this external perturbations.Meanwhile,the 2D crystal promises great dynamical and thermal stabilities as well as easy experimental fabrication from its layered van der Waals?vdW?bulk ferromagnet.The coexistence of long-range ferromagnetic ordering with near room temperature Tc and semiconducting behavior enables GdI₂ monolayer to be a promising candidate for spintronics applications...