Theoretical Study On Effectively Modulate Electronic Structure And Magnetism Of 2D VI₃ Monolayer And Janus ReSSe Monolayer

Since the discovery of graphene in 2004,two-dimensional（2D）materials with atomic thickness have received extensive attention from interdisciplinary subjects due to their fascinating properties and potential applications.In recent years,the CrI3 monolayer and Cr2Ge2Te6 double layer with intrinsic ferromagnetism have opened a door for seeking the two-dimensional materials with inherent magnetism.However,the Curie temperature is relatively low,which stimulates us to look for 2D intrinsic magnetic materials with high Curie temperature.On the other hand,realizing materials with spin polarization is also an urgent problem to be solved.Recently,the successful preparation of VI₃ crystal,which is the materials like-CrI3 monolayer,provide researchers with an alternative direction.At the same time,the synthesized ReSSe monolayer has the advantages of suitable band gap,high stability,and large carrier mobility.It has been widely used in optoelectronic equipment,thermoelectronic devices,transistors and other fields,but its intrinsic non-magnetic greatly inhibits its application in the spintronics.Previous studies have found that alternative methods such as doping,surface adsorption,defect design,biaxial strain,and application of electric fields can effectively control the electronic and magnetic properties of low-dimensional materials,and obtain two-dimensional magnetic materials with high spin polarization.Moreover,materials with large magnetic anisotropy also play an important role in magnetic memory cells.Therefore,in this study,the electronic structure and magnetic properties of the VI₃ monolayer with intrinsic magnetism are systematically studied,and these properties are manipulated by biaxial strain,carrier doping and alloy compound.The 3d transition metal atoms doping can introduce magnetism into the intrinsic non-magnetic ReSSe monolayer,explore the physical origin of its magnetism,and expand their applications in spintronics devices.The specific research contents are as follows:（1）Theoretical research on effective modulating the electronic and magnetic properties of VI₃ monolayerA series of studies on the electronic and magnetic properties of VI₃ monolayer are carried out by using first-principles calculation based on density functional theory.The results indicate that the most stable structure of VI₃ monolayer is particularly susceptible to the calculation parameter such as Hubbard U and the spin-orbit coupling（SOC）.Ultimately,under density functional theory（DFT）+U+SOC（U=3 eV）,the most stable structure of VI₃ monolayer is a ferromagnetic semiconductor with a direct band gap of 0.51 eV,and the predicted Curie temperature（Tc）is 29 K.Biaxial strain and carrier doping could not only induce VI₃ monolayer FM-AFM-FM transition,but also effectively increase TC to 123 K under 0.5 electron doping.Meanwhile,carrier doping realizes the potential ferromagnetic half-metal.Fortunately,in the constructing alloy monolayer,the VTaI6 monolayer is found a ferromagnetic half-metal under DFT+U（U=3,1 eV for V and Ta）,and the Tc is increased to 74 K.Moreover,VTaI6 monolayer possesses a large in-plane magnetic anisotropy energy（-6.505 meV）.Through the analysis of the second-order perturbation theory and the orbital-resolved MAE,it is found that the large MAE is mainly derived from the（dz2 ↑|Lx|dxz ↑）of Ta,（px ↑ |Lx|pz ↑）and（pz ↓ |Lx|px ↓）of I in the same spin channel.These excellent electronic and magnetic properties make the VI₃ monolayer expected to be a candidate material for the next generation of spintronic devices.（2）Theoretical study on the electronic and magnetic properties of Janus ReSSe monolayer with 3d transition metal atom dopingBased on the first-principles density functional theory,we systematacially investigate the electronic,magnetism and magnetic anisotropy energy of 3d transition metal doped Janus ReSSe monolayer.The calculation results show that 3d transition metal atom doping can induce magnetism in the Janus ReSSe monolayer,except for Cu.The magnetic properties of the doped system are mainly derived from the transition metal atoms and the Re,S and Se atoms around them.Among them,Sc,V,Cr,Co and Ni doping exhibit half-metallic characteristics,while Ti,Mn,Fe and Zn doping systems retain semiconductor properties.In addition,the V-doped system exhibits large in-plane magnetic anisotropy（-4609 μeV）,which is mainly derived from the coupling between（dx2-y2 |Lz|dxy）from the different spin channels and（dz2|Lx|dyz）from the same spin channel of Re.However,the Mn-doped system has a large out-of-plane magnetic anisotropy（2228 μeV）,which is mainly derived from the coupling between（dx2-y2|Lz|dxy）from the same spin channels and（dz2|Lx|dyz）from the different spin channel of Re.The research results show that the 3d transition metal atom doping can adjust the electronic and magnetic properties of the Janus ReSSe monolayer,and promote its functional design and application in the field of spintronic devices.