The Study Of Peculiar Electronic Properties Of Two-dimensional Carbon Dirac Materials

Two-dimensional(2D)carbon materials have various configurations and rich properties,which can be widely used in nanoelectronics,optics,sensing and other fields.In particular,the appearance of the Dirac point gives carbon materials excellent properties.According to the tilt of the dispersion,Dirac points can be classified into three types: type-I,II and III.Three kinds of Dirac points have different topological properties.The unique electronic band structure of type-III Dirac cone offers a solidstate platform for the study of Hawking radiation in high-energy physics.But so far,other types of Dirac points appear in a few materials in addition to type-I Dirac points.It is necessary to search for such lattices or materials in order to explore the rich properties of Dirac points.Due to the rich hybrid properties of carbon,a large number of allotropes are formed,covering a wealth of electronic properties.Based on this,we predict a series of carbon structures with different types of Dirac points.The paper is divided into the following five parts:In chapter 1,we briefly introduce the two-dimensional carbon material system and Dirac material system.In chapter 2,the computational methods used in this paper are introduced,such as density functional theory and tight-bonding model.In chapter 3,we propose that ideal type-Ⅲ nodal points can be obtained in a diamond-like lattice.The flat bands in the lattice originate from destructive interference of wavefunctions,and thus are intrinsic and robust.Moreover,the specific lattice can be realized in some 2D carbon networks,such as T-graphene and its derivatives.All the carbon structures possess type-Ⅲ Dirac points.In two of the structures,made of triangular carbon rings,the type-Ⅲ Dirac points just locate on the Fermi level and the Fermi surface are very clean.These two-dimensional carbon materials verify that rhomboid lattices have the third type of Dirac points and provide a platform for studying the properties of the third type of Dirac points.In chapter 3,We predict a new 2D carbon material.By first principles calculation,we obtain the energy bands of the structure.A flat band appears below the Fermi level.The breaking interference of the wave function at the adjacent points of the C8 ring results in flat bands.In addition,there is a small band gap at the high-symmetry point X,which can be adjusted by the external stress.By applying compression stress of-1.1% along the y-axis or tensile stress of 2% along the x-axis,the small band gap disappears,forming exactly a semi-Dirac point.Then the crossing point becomes the type-II Dirac point with the increase of stress.In chapter 5,we make a brief summary of this paper and look forward to materials with different types of Dirac points.