Properties Regulation Of The Two-dimensional Ferromagnetic Semiconductor CrI₃:A First Principle Study

Two-dimensional(2D)materials with atomic layer thickness have been widely used in optoelectronic devices,spin electronic devices,energy storage devices and biosensors.Because it is difficult to find 2D materials with intrinsic ferromagnetism,the application of 2D materials in spintronics is limited.Recently,experiments have confirmed the successful exfoliation of two-dimensional intrinsic ferromagnetic semiconductor materials,Cr I₃ monolayers,which opens up a new prospect for low-dimensional spintronics.To widen its applications,we should modify the performance of the materials.For layered materials,the physical properties can be regulated by applying strain and electric field,adsorbing atom,and introducing defect.In this thesis,therefore,using first-principles calculations based on density functional theory(DFT),we systematically studied the effects of iodine vacancy defects,surface atomic adsorption and non-mirror symmetrical structure on the electronic and magnetic properties of Cr I₃ monolayer.Our study provides a theoretical basis for the practical application of 2D Cr I₃ monolayer.Firstly,we introduced iodine vacancy defects in the Cr I₃ monolayer.Our study show that the introduction of iodine vacancy defects reduces the stability of the FM state and can separate the flat bands around the Fermi level.In addition,the concentration of iodine vacancies does not change the semiconducting characteristic of Cr I₃,while affects the total magnetic moment of the material.Based on the iodine vacancies,carrier doping can further regulate the electronic and magnetic properties of Cr I₃monolayer.Secondly,single atom adsorption was performed on the surface of Cr I₃ monolayer.The adsorbed atoms include alkali(earth)metal atoms(Li,Mg,K),transition metal atoms(Ti,V,Mn,Fe,Co,Ni)and non-metal atoms(N,P,O,S).Our result show that the intrinsic ferromagnetic semiconductor remains for the Cr I₃ monolayer when the external atoms are adsorbed except for Ti and Mn atoms.However,the adsorption of Ti and Mn atoms change the Cr I₃ monolayer to antiferromagnetic semiconductor.The adsorption of metal atoms increases the local magnetic moments of Cr atoms,but not exceeding 4?_B.In contrast,the adsorption of non-metallic atoms makes the local magnetic moments of Cr atoms diversification.Combined with the density of states,we analyzed in detail the mechanism of Cr local magnetic moment and reveal the mechanism of the increase of Cr local magnetic moments.We found that the increase of magnetic moment of Cr atoms are closely related to the charge transfer between the adsorbed atoms and Cr I₃ substrates.Finally,we constructed the 2D Janus Cr₂I₃X₃(X=F,Cl,Br)monolayers by replacing a layer of I atoms with X elements.Through investigating the mechanical,electronic and magnetic properties of the Janus Cr₂I₃X₃ monolayer,we found that the properties of Cr₂I₃X₃do not range between Cr I₃ and Cr X₃,but exceeding this range.The exit of this phenomenon mainly attribute to the significantly change of the charge distribution and structural parameters of Janus Cr₂I₃X₃ monolayers induced by the element substitution,thus resulting in the obvious variation of properties.