First-principles Calculation Of Novel Two-dimensional Materials

In the past two decades,two-dimensional(2D)materials have played a very important role in con-densed matter physics.As the first successfully prepared 2D material in experiments,graphene has become the focus of research in condensed matter physics and materials due to its unique electronic properties.Since the dicovery of graphene,many new 2D materials have been successfully synthe-sized experimentally,such as Mxene,Mo S₂,black phosphorus,BN,and numerous transition metal sulfides,etc.In these materials,people found many excellent physical properties,such as 2D topol-ogy,2D magnetism,2D ferroelectricity and so on.Considering the advantage of the small size,2D material could shine in the post moore's law era,especially in the next generation of electronic devices and storage field.In this thesis,we first introduce the research background of 2D materials in the introduction part,and then introduce the experiemental progress of magnetic 2D materials.Finally,we introduce the 2D Kagome lattice and its band characteristics.In chapter 2,we introduce density functional theory and Stoner magnetism theory.In chapter 3,we discuss the mechanism of a ferromagnetic 2D material VSe₂.By using the first-principles calculations,we unveil that the ferromagnetism in monolayer 1T-VSe₂is originated from its intrinsic huge Stoner instability enhanced by the confinement effect,which can eliminate the in-terlayer coupling,and lead to a drastic increase of the density of states at the Fermi level due to the presence of Van Hove singularity.Our calculations also demonstrate that the Stoner instability is very sensitive to the interlayer distance.These results provide a useful route to modulate the nonmagnetic to ferromagnetic transition in few-layers or bulk 1T-VSe₂,which also shed light on the enhancement of its Curie temperature by enlarging the interlayer distance.In chapter 4,three new Kagome 2D materials Pd3P2Se8,Pd3As2S8,Pd3Sb2S8are theoretically predicted.We first established the structure of the three materials.Then we calculated the phonon spectrum,which confirmed that the crystal structre of Pd₃P₂Se₈,Pd₃As₂S₈are stable.We further cal-culated the cleavage energy of the materials,which show that these materials are easily to be cleaved,and can well form 2D materials.Then we analyzed the electronic structure and found that these ma-terials have typical Kagome band characteristics both in the bulk and slab.Finally,we calculated the energy gap and light absorption coefficient and found that this kind of material has the potential to be used in photoelectronic devices and solar cell.