Structural Prediction And Property Analysis Of Novel Two-Dimensional Transition Metal Compounds

Since graphene was successfully separated from graphite,two-dimensional materials have been widely used in micro-nano electronics and lithium ion batteries.However,in the experimental preparation of two-dimensional materials,the synthesis cycle is too long,and the two-dimensional materials prepared have the disadvantages of many defects,poor universality and low yield,etc.Two-dimensional materials cannot keep up with the development requirements in the field of materials science,and many new two-dimensional materials need to be discovered.Therefore,the theoretical prediction of the structure and properties of two-dimensional materials will provide an important theoretical basis for the synthesis of two-dimensional materials in experiments,and promote the further development of two-dimensional materials in spintronics and lithium ion batteries.In this dissertation,we explored new structures formed by some transition metal compounds,and the feasibility of their application in lithium ion batteries and spintronics was predicted theoretically.The main research contents are as follows:（1）The feasibility of a new two-dimensional transition metal carbide XC₃（X=Fe,Mn）as electrode materials for lithium ion batteries was studied.Firstly,first principles methods and Crystal structure Analysis by Particle Swarm Optimization（CALYPSO）structure search software were used to predict two two-dimensional XC₃ structures named d-XC₃and m-XC₃.Secondly,the binding energy,phonon spectrum and molecular dynamics simulation were performed to verify the thermodynamic and dynamic stability of the XC₃ structure.At the same time,Young’s modulus was calculated to prove the high strength stability of XC₃ from the point of view of mechanical properties.Then the electromagnetic properties of XC₃ were studied by first principles.It was found that d-XC₃ and m-XC₃ are ferromagnetic metals.Subsequently,the feasibility of two-dimensional XC₃ as electrode material for lithium ion batteries was studied,and the adsorption energy,diffusion energy barrier,open-circuit voltage and storage capacity of d-XC₃ and m-XC₃ structures after lithium adsorption were calculated respectively.The results were as follows:the lithium diffusion barrier of XC₃ structures were low:d-FeC₃ structure was0.16 e V,m-FeC₃ structure was 0.19 e V,d-MnC₃ monolayer was 0.16 e V,m-MnC₃ monolayer was 0.17 eV,indicating excellent charging and discharging rate;High theoretical storage capacity:the theoretical capacity of FeC₃ structure was 874 mAhg^-1,and the theoretical capacity of MnC₃ monolayer was 885 mAhg^-1.Therefore,the new two-dimensional XC₃（X=Fe,Mn）structure can be used as the cathode material for fast charging and discharging and large capacity lithium ion batteries.（2）The feasibility of a new two-dimensional transition metal chalcogenide FeY₄（Y=S,Se,Te）as spintronics materials in devices was discussed.Firstly,using the first principles method and CLAYPSO structure search software,we successfully predicted two two-dimensional FeY₄structures named o-FeY₄ and t-FeY₄.The thermodynamic and dynamic stability of FeY₄structures were predicted by binding energy,phonon spectrum and molecular dynamics simulations.Next,Young’s modulus was calculated to prove the high strength stability of FeY₄structure from the point of view of mechanical properties.The results showed that the FeS₄ and FeSe₄ exhibited ferromagnetic half-metal properties,and the FeTe₄ monolayers exhibited ferromagnetic metal properties.The Curie temperature of t-FeS₄,o-FeS₄ and o-FeSe₄ monolayers with ferromagnetic half-metallic properties calculated by Monte Carlo simulation was 580 K,1110 K and 880 K,respectively,which were all above room temperature.Finally,the magnetic anisotropy energy of t-FeS₄,o-FeS₄ and o-FeSe₄monolayer was 0.14 meV/atom,0.90 meV/atom and 2.11 meV/atom,respectively,having large numbers,which can be used as high-density magnetic information storage materials.These results indicate that the two-dimensional room-temperature ferromagnetic half-metallic FeY₄（Y=S,Se）materials can prepare a reference for the fabrication of spintronics devices.