First Principles Study Of Two-dimensional Transition Metal Nitride MN₄

Currently,many transition metal nitrides have been discovered and characterized,but their number is still limited compared to transition metal oxides and sulfides.After the successful preparation of graphene,an increasing number of novel two-dimensional materials have been discovered and synthesized.Among them,two-dimensional transition metal nitrides（MXenes）have attracted significant attention due to their excellent conductivity and chemical stability.Although these nitride MXenes are referred to as two-dimensional materials,the transition metal atoms are actually situated in a three-dimensional crystal field because MXenes are composed of three or more atomic layers.Binary transition metal nitrides with a planar structure and strong magnetism are relatively rare.In the exploration of two-dimensional materials,it is crucial not only to synthesize new materials through experiments but also to predict them through theoretical calculations.Theoretical prediction and design of two-dimensional structures can provide references and directions for experimental synthesis,accelerating the discovery and synthesis of two-dimensional materials.Therefore,we designed two magnetic transition metal nitride monolayers through theoretical calculations.This paper is divided into five chapters.Chapter 1 mainly introduces the development and research status of transition metal nitrides and two-dimensional transition metal nitrides,and proposes the research objectives and ideas of this paper.Chapter 2 mainly outlines the basics of first-principles calculations,the background of density functional theory,and relevant calculation software.Chapter 3proposes the concept of two new transition metal nitride monolayers and analyzes the stability of the two structures through various methods.Chapter 4 focuses on the electronic structure and magnetism of the designed transition metal nitride monolayers and calculates the Curie temperature of the ferromagnetic structure.Chapter 5 summarizes and looks forward to the paper.The research content of this paper is based on first-principles calculations,and is as follows:First,we designed two types of MN₄（M=transition metal V,Cr,Mn,Fe,Co）monolayers,which are two-dimensional transition metal nitrides with planar structures,composed of alternating and vertically stacked MN₄units,named r-MN₄and s-MN₄according to their cell shapes.The MN₄units in both monolayers have arrangements similar to the H-type carbon atoms in graphene and biphenyl monolayers,respectively.Secondly,using first-principles calculations of formation energy,phonon spectra,and molecular dynamics,we demonstrated that both types of MN₄monolayers are energetically,dynamically,and thermodynamically stable,and have the possibility of experimental synthesis.The stability mechanism was analyzed through projected density of states and differential charge density analysis.The lattice of these two structures can form a planar structure and possess strong stability and good compatibility with different transition metal atoms.The basic stability mechanism involves the sp²hybridization of nitrogen atoms,the coordination bond between transition metal atoms and nitrogen atoms,andπ-conjugation.Finally,we investigated the electronic structure and magnetism of the two types of MN₄monolayers.Due to their similar crystal structures,the total charge of the two systems increases by one electron as the transition metal atoms change from V,Cr,Mn,Fe,to Co,leading to continuously adjustable electronic structure and magnetism characteristics.The planar crystal field is another feature of the two MN₄lattices,which splits the five d orbitals of the transition metal atoms and generates strong magnetism.With the change of transition metal atoms,the two types of MN₄monolayers also show rich magnetism,including the transition from antiferromagnetism to ferromagnetism and the change of magnetic moment.In addition,by using the Heisenberg model combined with the Monte Carlo method,the room-temperature ferromagnetism of a single-layer s-CoN4 with a Curie temperature of 321 K has been determined.