First-principles Study On The Electronic And Magnetic Properties Of 2D Transition Metal Carbides Ti_n+1C_n

As a kind of new 2D material,two-dimensional transition metal carbides or nitrides（MXene）are widely used in various fields such as energy storage,spintronics devices,and catalysts due to their excellent physical and chemical properties which are directly related to the electronic structures.Band engineering is the key step in designing of new materials.By properly introducing defects,stresses,heterojunctions or superlattices,not only can the performances of existing materials be improved or regulated,but also the methods for designing new materials are broaden.Based on these understandings,this work comprehensively studied the band engineering of MXene materials and its impact on the performances from the first-principles calculations.The specific research contents are as follows:Firstly,the effects of the intrinsic point defects on electronic properties are systematically studied.Our results show that the Ti atoms in both the surface layer and the inner layer in pristine Ti₃C₂ see C_3v crystal fields.Their 3d oribitals split into three parts:two degenerate3d_xy,3d²²_x-y,two degenerate 3d_xz,3d_yz and single 3d_z².The introducd single Ti/C vacancy deforms the lattice structure,which in turn changes the crystal field and further splits the original degenerate orbitals.The positive formation energy of a single vacancy can be attributed to two competitive factors,one is the introduction of the dangling bonds leads to an increase in the total energy,and the other is the further spilitting of the original degenerate 3d orbitals of the Ti atoms induced by the deformation of the structure,leading to a decrease in the total energy.The formation energy and configuration entropy together determine the final vacancy concentration.We introduced the relationship between the equibrilum concentration of the clusters and the temperature,implying the existence of vacancy clusters.We also studied the diffusion of Ti/C vacancies and found that the diffusion of a single Ti vacancy from the surface Ti layer to the inner Ti layer is more difficult than the versed path,and it is difficult for C vacancy to diffuse from one C layer to another C layer.Secondly,MXene materials may possess rich magnetic properties because of the presences of transition metal atoms.However,the introduction of surface functional groups during their preparation procsses destroy the intrinsic magnetic properties,which limits the application of MXene materials in spintronic devices.Therefore,it has become a hot topic to develop the ferromagnetic MXene materials with the high Curie temperature.In this paper,we proposed a new method to regulate the magnetic properties of Ti₂C by asymmetrically adsorbing atoms on both surfaces of the monolayer.On the one hand,the introduced electron donators or acceptors cause the different valence states of transition metal ions on both sides of the surface sublayers,which will make the magnetic coupling of the system tends to double-exchange interaction and lead to the ferromagnetic（FM）ground state.On the other hand,the electron donators,such as-Li,push the electron bands upward while the electron acceptors,such as-O,pull the bands downward,and the combination of them adjust the materials to be a half-matal.Based on this discovery,two different kinds of groups（electron acceptors or electron donators）are choosed to study their effects on the electronic and magnetic properties of the Ti₂C monolayer whose ground state is antiferromagnetic（AFM）,and finally achieving the transition from AFM to FM.The Curie temperature of the FM MXene was estimated by Monte Carlo simulation based on the Ising model,which is approximately 303 K,above the room temperature.This study provides a new method for the design and regulation of 2D FM materials.