Magnetic Properties In Non-mangnetic Elements Doped Monolayer MoS₂ From First Principles Study

In recent years, in scientific circles, an increasing attention was paid to how to prepare practical diluted magnetic semiconductors in spintronics. However, much more efforts have focused on the three-dimensional bulk materials doped with 3d or 4f magnetic elements. After finding graphene, graphene-like two-dimensional material MoS2 has raised in particular attention recently, due to its geometry structure, wide band-gap, and stability. Therefore, this is in favor of trendy to nanocrystallization, and facilitates substitutional doping, which means a promising way toward 2D nono-electronic devices based on MoS2 monolayer. Although we have made some achievements after a lot of investment in exploring in MoS2 materials doped with magnetic elements, we still have a long way to go for practical applications. In this work, on the base of previous studies and density functional theory we try to regulate and control the electronic structures and magnetic properties via nonmagnetic elements sbustitutional doping with the aim to obtain a new insight ino the origin of d0 magnetism indueced by nonmagnetic elements in MoS2 maxtrix and to search for suitable MoS2-based materials for nono-electronic spintronc devices. The mainly contents and results are stated as follows:In chapter 1, we briefly introduce the concepts of spintronics, magnetic semiconductors, and bipolar magnetic semiconductors, and then review the geometry structure and electronic property of two-dimensional material MoS2, as well as magnetic properties. The chapter is aslo a survey of the advances in MoS2-based magnetic materials.The second chapter briefly introduces the mechanism of magnetic interations in solid substance, various magnetic ordering structures, and theory of crystalline field. We turn back to the previous Hartree-Fock approximation, the relevant concept of quantum mechanics, density functional theory, and supercell method in this chapter.In the third chapter, based on density functional theory using ab initio calculations, the electronic structure and magnetic properties of the B-, C-, N-, O-, F-, Cl-, Br-, and I-doped MoS2 monolayer are investigated. The B-, C-, and N-doped systems are p-type doping, while the halogen-doped systems are n-type doping. C-doping leads fairly deep and highly localized states in the gap of MoS2 without spin-splitting. O-doping result in a narrower band-gap for MoS2. At the PBE level, results show that all the doped systems, expect for C-doping and O-doping, exhibit half-metallic behaviors with a total magnetic moment of ～1.0μB.At the HSE level, the Br-doped case remains half-metallic, while the other doped cases revert to magnetic semiconductor. The magnetic moment mainly comes from the unpaired Mo-4d oribitals and the p orbital of the dopant, as well as the S-3p states. The formation of local magnetic moment depend on the local structure around the dopant, the localization of Mo-4d, variable valency of Mo cation, and the partially filled anion-p orbitals of the dopant itself. The long-range half-metallic ferromagnetic order is attributed to the double-exchange interactions.In the fouth chapter, based on density functional theory, the electronic structures and magnetic properties have been studied in MoS2 monolayer via substitutional doping of nonmagnetic elements （IA, IIA, and IIIA elements）. The magnetic moment of those doped systems origins form the interplay between the crystal-field of MoS2 matrix and localized Mo 4d states. On the whole, these doped MoS2 monolayers exhibit a half-metalâ†'spin gapless （or narrow gap） semiconductorâ†' ferromagnetic semiconductor transition as the dopants change from IA to IIIA groups. Electron and hole doping by a potential gate can realize a transition from ferromagnetic semiconductor to half metal. In important, the spin-polarization direction is switchable depending on the doped carrier’s type.In the fifth chapter, we explore the electronic structures and magnetic properties in Cu-doped MX2 （=MoS2, MoSe2, MoTe2, and WS2） based on density functional theory. A Cu dopant leads to a net moment of 5.0 or 1.0 μB in MX2, which mainly depend on the size of crystal-field splitting relative to that of the spin splitting. No magnetism is observed in Cu-doped MoTe2. The local distortion around the Cu atom reduces the total magnetic moment in two-Cu-doped MX2. The magnetic coupling between the nearest neighboring Cu atoms is ferromagnetic for all the cases, but they demonstrate various magnetic ground states with the increasing distance between Cu atoms:the Cu-doped MoS2 and WS2 exhibit anti-ferromagnetic and nonmagnetic ground state, respectively. A long-range ferromagnetic or ferrimagnetic coupling is attributed to double-exchange interaction in Cu-doped MoSe2. Half-metallic ferromagnetism with Curie temperature above room temperature in Cu-doped MoSe2 provides a useful guidance to engineer the magnetic properties of MoSe₂ in experiments.