The Design And Theoretical Studies On Phyical Properties Of Several Two-Dimensional Atomic Crystals

The physical properties of materials depend on the arranging atoms and the electronic structures.In the two-dimensional materials,the behaviors of the atoms and electrons are different from those of the three-dimensional systems.The unique behaviors of the atoms and electrons lead to interesting properties in the two-dimensional materials,which is absent in the three-dimensional ones,i.e.the massless Dirac fermions in the graphene,the valley polarization in the monolayer MoS₂.These unique properties in the materials stimulate the discovery and studies of new two-dimensional materials.Graphene which is made of carbon atoms arranged on a honeycomb structure,has many novel properties,such as linear dispersion and massless Dirac fermions as low-energy excitations.The unique properties in the graphene stimulate the research on other two-dimensional materials,i.e.silence,germanene,borophene,transition metal chalcogenides（TMD）,transition metal carbides and carbonitrides（MXenes）,blue phosphorus,etc.The two-dimensional materials range from insulators to semiconductors to metals.These lead to a new world of the quantum materials and stimulate the research on the new materials.The thesis focuses on the structures and properties of two-dimensional quantum materials.The main contents are design of two-dimensional graphene-like Dirac materials from borophene,the study of the Dirac states from p_x,y,y orbitals in the buckled honeycomb structures and the structures and electronic properties of several transition metal dichalcogenides.The main research methods used in the thesis are the tight-binding and density functional theory（DFT）methods.The main results are,as follow:1.A new design scheme of Dirac materials is proposed.The Dirac states can be obtained by adding to electrons to each hexagonal ring,which can be achieved by embedding the electron-accepting atoms like H,F,and Cl and the electron-donating atoms like Be.Several new isolated 2D Dirac materialsβ₁₂-XBeB₅（X=H,F,Cl）are identified,by rationally designing the non-graphene-likeβ₁₂ boron sheets.Usingβ₁₂-HBeB₅ as an example,the newly designed Dirac material is confirmed to be energetically,dynamically,and thermally stable by DFT calculations.The designedβ₁₂-HBeB₅ not only has a high Fermi velocity of 0.73×10⁶ m/s,but also has very robust Dirac state against unsuual large tensile strains.Based on the recently reported successful synthesis ofβ₁₂ boron sheets,the experimental feasibility of this new 2D Dirac material is expected as well.The designedβ₁₂-FBeB₅ andβ₁₂-ClBeB₅ show similar electronic properties.2.The four-orbital tight-binding model for the p_x,y,y orbital Dirac states is constructed to study the effects of the buckling of the honeycomb structure on the electronic structures.The analytical expression for the dispersion relation is obtained,with a linear relationship between energy and momentum vectors near the Dirac cones.The linear dispersion persists,even the structure is buckled.The Fermi velocity is determined not only by the hopping throughπbonding but also by the hopping throughσbonding of p_x,y─orbitals.Two additional narrow bands which are flat are obtained in the limit of vanishingπbonding.The Fermi velocity is more sensitive to the changes of lattice constants and buckling angles,if strain is applied,according to the DFT calculation.This work provides a deeper understanding of p_x,y─orbital Dirac states in honeycomb lattices,and useful clues for the applications of this family in nanoelectronics by using this family of materials.3.The configurations and electronic properties of monolayer PtSe₂,NiSe₂,as new members of the TMDs family,PtSe₂/Pt（111）,NiSe₂/Ni（111）and multilayer MoS₂ were calculated by the DFT based method.The DFT calculations combined with characterizations of low-energy electron diffraction（LEED）,scanning tunneling microscopy（STM）,scanning transmission electron microscopy（STEM）,elucidated both in-plane and vertical monolayer structures with atomic resolution,which confirmed the monolayer PtSe₂,NiSe₂ on the Pt（111）,Ni（111）substrate,respectively.The calculations of electronic monolayer PtSe₂ and their agreement with the angle-resolved photoemission spectroscopy（ARPES）measurements revealed that the PtSe₂is a monolayer on Pt（111）and is in 1T phase.The as grown PtSe₂ shows semiconducting electronic structure of the single-layer PtSe₂,which is not affected by the substrate significantly.The circular polarization calculations indicate that the valley-selective circular dichroism can occur in monolayer PtSe₂.The surface confined quantum well state is found in the surface of a thin film of MoS₂.This quantum well state is confined around the surface of the thin film of MoS₂,having an inverse parabolic decay into the film.