The First-principles Study On The Magnetic Properties Of Two-dimensional Semiconductors

The magnetic semiconductor has the ferromagnetic and semiconducting properties, and can be very good compatible with fabrication process for semiconductor. Thus, it was supposed to be a promising materials that can manipulate the spin of electrons to make the principle and function of spintronics to achieve in the semiconductor. The two-dimensional (2D) materials represented by graphene, have attracted a surge of attention due to their unique properties and promising applications. In recent years, the researches to use graphene as the carbon basic spintronic materials have gained high attention and development. However, the intrinsic zero band gap of graphene limit its applications in nanoelectronic devices. Some fabricated 2D materials, such as monolayer MoS2, monolayer phosphorene and monolayer TiS3 are semiconductors with a band gap. Thus, the monolayer MoS2, monolayer phosphorene and monolayer TiS3 may be more suitable materials for applications in nano-spintronics than graphene. It is well known that the monolayer MoS2, monolayer phosphorene and monolayer TiS3 are all intrinsically nonmagnetic. For their prospective applications in nano-spintronic devices, it is required to induce and manipulate the magnetism in these 2D materials. Therefore, developing approaches to effectively induce and manipulate the magnetism in monolayer 2D materials is a research hotspot in the field of the magnetism of condensed matter. A number of researches showed that substitutional doping, defect, adsorption, and strain can induce the magnetism in the nonmagnetic semiconducting materials. In this doctoral thesis, first-principles calculations based density functional theory are performed to investigate the influences of vacancies, antisite defects and strain, the substitutional doping of nonmetal atoms, and the substitutional doping of transition metal atoms on the geometrical structure, electronic structure and magnetic properties of the monolayer MoS2, monolayer phosphorene and monolayer TiS3, respectively. The main works are as follows:1. We systematically investigate the influences of the observed vacancies (Vs. Vs2,VmoS3 and VmoS6) and antisite defects (S2mo and MoS2) on the geometrical structure, electronic structure and magnetic properties of the monolayer MoS2, and the effect of the strain on the geometrical structure, electronic structure and magnetic properties of the above-mentioned vacancies and antisite defects doped monolayer MoS2 systems. Our calculation results indicate that the Vs,Vs2,VMoS3, S2Mo and MoS2 doped monolayer MoS2 systems are nonmagnetic due to the formation of Mo-Mo metallic bonds, S-S covalent bonds and Mo-S covalent bonds, or the strengthening of the Mo-S covalent bonds around the defects or at the defects. In contrast, the VMoS6 in the monolayer MoS2 can induce a local magnetic moment of 6.0μB and the magnetic moment mainly arise from the 4d orbitals of Mo atoms around VMoS6; When the tensile strain is applied for Vs, VS2, VMoS3, S2Mo and Mos2 doped monolayer MoS2 systems, the bonds around these defects or at these defects are weaken. When the strain is increased to certain value, these bonds are broken or obviousaly weaken, which leads to the formation of the localized nonbonding electrons of Mo or S atoms around these defects or at these defects, thus inducing magnetism in Vs, Vs2, VMoS3, S2Mo and Mos2 doped systems. For VMoS6 doped monolayer MoS2 system, the tensile strain can lead to the Mo-Mo metallic bonds around VMoS6 are weaken and even broken, and then resulting in the increase of the magnetic moment. On the contrary, the compressive strain can lead to the the decrease of the magnetic moment. When the strain decreases to-12%, the magnetic moment of the VMoS6 doped system decreases to zero.2. We have systematically investigated the influences of the substitutional doping of nonmetal atoms on the geometrical structure, electronic structure and magnetic properties of the monolayer phosphorene and blue phosphorene. Our calculation results indicate that neutral H, F, Cl, Br, I, B, N, As, C, Si, O, S and Se doped monolayer phosphorene and F, Cl, B, N, C, Si and O doped blue phosphorene are stable. Due to the saturation or pairing of the valence electron of the dopant and its neighboring P atoms, the H, F, Cl, Br, I, B, N and As or stable C", Si", S+and Se+ doped monolayer phosphorene systems and F, Cl, B, and N doped blue phosphorene systems are nonmagnetic. In contrast, due to the appearance of one unpaired valence electron of the dopant or its neighboring P atoms, the substitution of C, Si, O, S, and Se in monolayer phosphorene and the substitution of C, Si, O in blue phosphorene can induce a local magnetic moment of 1.0μB, which mainly arise from the p orbitals of the dopant or its neighboring P atoms. Furthermore, the magnetic coupling between magnetic moments induced by two Si, O, S and Se in monolayer phosphorene and two C, Si, and O in blue phosphorene are found to be long-range antiferromagnetic and the origin of the coupling can be attributed to the p-p hybridization interaction involving polarized electrons, whereas the coupling between the moments induced by two C in monolayer phosphorene is very weak.3. The geometrical structure, electronic structure and magnetic properties of doped monolayer TiS3 with some TM atoms, such as Sc, V, Cr and Mn, are studied by using first-principles calculations. The calculation results show that the substitutional doping of Sc, V, Cr and Mn atoms in monolayer TiS3 are stable and the S-rich condition is more energetically favorable for the substitutional doping at the Ti site in monolayer TiS3 compared to the Ti-rich one. For Sc doped system, due to nonlocalized character of the band crossed by the Fermi level, the substitutional doping of Sc can not produce the magnetism in monolayer TiS3. Different from Sc doped monolayer TiS3, one, two and three additional valence electrons of V, Cr and Mn with respect to Ti are unpaired, respectively, thus a doping V, Cr and Mn atom in doped doped TiS3 monolayer produce the magnetic moment of 1.0,2.0 and 3.0μB, respectively. The magnetic moments are mainly come from the 3d orbitals of the dopant with a partial contribution from the 3p orbitals of the neighboring S atoms around the dopant and the 3d orbitals of the neighboring Ti atoms around the dopant. More importantly, the magnetic coupling between the moments induced by two V and Mn are found to be long-range ferromagnetic, whereas the coupling between the moments induced by two Cr is long-range antiferromagnetic. Thus, our results suggest that room temperature ferromagnetism is likely induced in monolayer TiS3 by doping V and Mn.