Research On The Structure And Magnetism Of Ultrathin Two-dimensional Material

Two-dimensional materials exhibit unique layered structures and excellent physical,chemical,electronic,and optical properties,and have broad application prospects in spintronics,energy storage,catalysis,medicine,and chemical engineering.Based on the thickness,two-dimensional materials can be divided into two categories: one is single-atomic-layer two-dimensional materials such as graphene and BN,and the other is multi-atomic-layer two-dimensional materials such as phosphorene and Mo S2.Two-dimensional materials with a single atomic thickness and containing 3d transition metals,which have the advantages of both two-dimensional materials and transition metal compounds,have been rarely reported in previous studies.In this article,we use first-principles calculation methods to focus on the structural design of two-dimensional materials containing transition metals,and study the geometric structure and electronic properties of single-atomic-layer transition metal compounds,aiming to provide theoretical support for the discovery and design of new two-dimensional transition metal materials.Firstly,based on first-principles methods,we predict that Co N4C10,Co2N8C6,and Co2N6C6 are a new type of two-dimensional material composed of "Co N4 unit" and "graphene fragment".By forming energy,phonon spectra,and molecular dynamics simulations,we demonstrate the stability of the single-layer Co N4C10,Co2N8C6,and Co2N6C6.Spin polarization calculations show that these three compounds are magnetic metalsand the electronic states near the Fermi level are mainly dominated by Co 3d electronic states.In addition,due to the relatively isolated dz2 orbital of Co atoms,there is a clear density of states peak near the Fermi level.Thanks to the addition of transition metals,this new type of single-atomic-layer two-dimensional material,such as graphene-like transition metal carbides and nitrides,has rich electronic properties.Next,inspired by the above "Co N4-graphene" composite materials,we further predict through first-principles calculations that Co N4C2 is a single-atom-layer 2D material composed of "Co N4 units" and "carbon dimers C2." Surprisingly,the electronic states near its Fermi level are still dominated by Co 3d orbitals,further indicating that these carbohydrates benefit from the incorporation of transition metals,giving a richer electronic nature to the conventional single-atom layer.Similarly,Co N4C2 exhibits flat bands near the Fermi level and undergoes a non-magnetic to magnetic phase transition with increasing tensile strain.When the biaxial tensile strain reaches 8.6 per cent,the flat band is pushed to the Fermi level.It has been shown that tensile strain is an effective means of controlling the flat band at the Fermi level,similar to the twist angle in recently reported twisted bilayer graphene.Thus,this class of Co N4C2 monolayers containing transition metal elements undergo another important non-magnetic to magnetic phase transition under tensile strain.Lastly,since the unique electronic properties of the above two classes of 2D materials come from the transition metal-containing "Co N4 units," we simplify the problem and discover through atomic substitution calculations that the transition metal nitride Cr N4 single layer is a stable square-planar lattice-type 2D material.The structural stability of Cr N4 is attributed to the cooperative effects of N= N double bonds,Cr-N coordination bonds,and π-d conjugation,with the double π-d conjugation mechanism not reported in previous studies.Moreover,this mechanism gives Cr N4 monolayers a lower formation energy than g-C3N4.Moreover,the Cr N4 single layer exhibits a ferromagnetic(FM)ground state,with FM coupling between two Cr atoms mediated by electrons in the half-filled large π orbitals.The results obtained by solving the Heisenberg model using the Monte Carlo method indicate that the Cr N4 monolayer exhibits high temperature ferromagnetism.This study provides researchers with more candidates and design ideas to explore new 2D transition metal materials that may have potential applications in future 2D magnetic materials and other fields.