Nitrogen/Oxygen Doping Enhances Room Temperature Of MoS₂

As one of the important members of the two-dimensional layered transition metal dichalcogenides family,the molybdenum disulfide has been widely studied and successfully applied in various fields,such as field effect transistor(FET),gas sensors,photocatalytic hydrogen evolution,digital circuits,supercapacitor electrode,dye-sensitized solar cells,etc.which is associated with its unusual electronic structure and intriguing physical properties.Additionally,due to the lack of spatial inversion symmetry and the existence of strong spin-orbit coupling(SOC),the monolayer MoS₂ has become an ideal platform for the frontier research field of spintronics.However,the pristine MoS₂ does not have room temperature ferromagnetism.Thus,how to efficiently and conveniently realize the long range ferromagnetic order and charge carrier spin polarization in the two-dimensional layered MoS₂semiconductor is the most critical issue,which is of great significance for promoting the frontier research of spintronics field and the practical application of spintronics nanodevices.In this thesis,radio frequency plasma is introduced to achieve non-metallic doping of MoS₂nanosheets,and the magnetic properties of modified MoS₂ films are systematically studied at room temperature.The main research contents and results are listed as follows:1.The high crystalline MoS₂ nanosheets are prepared by one-step hydrothermal method.Then,the prepared films are treated by nitrogen plasma for various durations to improve the magnetic properties at room temperature.Weak ferromagnetism with saturated ferromagnetic magnetization of 0.64 memu/g has been observed in the as-synthesized MoS₂.However,with the introduction of nitrogen element,the ferromagnetic behavior of the nitrogen-doped MoS₂nanosheets is significantly enhanced,and its saturation magnetization is increased to 3.67memu/g.X-ray photoelectron spectroscopy(XPS)confirms the adsorption of nitrogen elements and higher valence state of Mo than+4,which directly demonstrates the feasibility of non-metallic element doping via radio frequency plasma.Based on density functional theory,we further study the intrinsic mechanism of enhanced ferromagnetism in the nitrogen-doped MoS₂ system.The calculations indicate that the chemically adsorbed nitrogen ion will form a conjugated?bond with the adjacent molybdenum ion,which will cause the hybridization of the N 2p and Mo 4d orbitals to have the net magnetic moment.2.MoS₂ powders are synthesized by low temperature hydrothermal method,and then exposed to oxygen plasma atmosphere for oxidation treatment for 0-40 minutes,to achieve various oxidation degrees.The characteristic peaks of molybdenum trioxide(?-Mo O3)can be clearly observed in X-ray diffraction(XRD)and Raman spectra.In addition,the existence of the observation of O 1s XPS spectrum further indicate that MoS₂ powders are partially oxidized.For MoS₂ samples with different oxidation degrees,the hysteresis loops at T=300K show that the maximum saturation magnetization increases from 2×10^-3 emu/g to 16×10^-3 emu/g.Through first-principles calculations,we find that the room temperature ferromagnetic properties of oxygen-doped molybdenum disulfide are mainly derived from the contribution of oxygen atoms.Due to the doping of oxygen atoms,the density of states of the spin down electrons are very high,whereas the spin-up electrons state is negligible near the Fermi level,which induces the oxidized MoS₂ to have net magnetic moments.We have successfully introduced room temperature ferromagnetism in MoS₂,and this novel two-dimensional ferromagnetic semiconductor has potential important applications in the frontier research field of spintronics.