Study On The Manipulation Of Room Temperature Magnctism Of Transition Metal Dichalcogenides

Magnetic semiconductor is a type of materials which exhibits both semiconducting characteristics and magnetism for information processing and storage.It can be potentially applied to a new generation of electronic devices for its unique physical properties.Transition metal chalcogenides,as excellent two-dimensional semiconductors,have attracted intensive attention.The introduction and manipulation of room temperature ferromagnetism of transition metal dichalcogenides have been widely studied.In previous studies,theoretical calculations were the main approaches to look into the magnetic properties of transition metal chalcogenides.However,the relationship between the microstructure(such as amorphous state,defects)and the magnetic properties remains unclear.In this thesis,MoSe2 has been prepared by a facial hydrothermal method.The crystallinity and phase structure of the samples have been adjusted by changing the annealing temperature.In addition,non-stoichiometric MoSe2-x samples have been prepared by controlling the ratio of the precursors during the hydrothermal process.The relationship between magnetic properties and microstructure,as well as the origin of magnetism have been investigated.Furthermore,one-step method was used to parepare MoSSe while the two-step method was used to prepare the MoSe2/MoS2 composite.Furthermore,and the effect of the structure on the magnetism was investigated.The main research work includes the following:1.Amorphous MoSe2 has been fabricated via hydrothermal method.Post-annealing at varied temperatures from 200 ?-600? gives rise to the change in the degree of amorphousness and the transition from the 1T to 2H phase.The amorphousness and the 1T phase play important roles in the room temperature magnetism.Room temperature magnetization of 0.0027 emu/g has been achieved for the amorphous MoSe2 which weakens during the crystallization and phase transition.Between the two factors,the amorphousness contributes to the ferromagnetism of the MoSe2 dominantly.2.By controlling the ratio of the Se/Mo during the hydrothermal process,non-stoichiometric MoSe2-x samples have been prepared.The basic structure of MoSe2 remains with decreased ratio the Se/Mo from 2.00 to 1.70.The magnetization of the samples experiences a trend of increasing,decreasing follow by increasing again.The 1T phase and Se vacancies play important roles in this process.The saturation magnetization of the MoSe2-x improves with the ratio of the 1T phase and the increment of the concentration of Se vacancies.Variation in the magnetic properties of the mobe2-x may be caused by the spin of Mo4+ ion in the 1T phase.3.MoSSe alloy and MoSe2/MoS2 composites have been prepared by one-step and two-step hydrothermal method,respectively.Effects of preparation methods on the magnetic properties have been investigated.Experimental results indicate that the origin of room temperature ferromagnetism in the samples with different structure differs from each other.For the sample prepared by the one-step method,the ferromagnetism origins from defects in MoSSe,while the magnetism origins from the boundaries of MoSe2/MoS2 for the sample prepared via the two-step method.In comparion,the edges of the phase boundary in MoSe2/MoS2 contribute to larger Ms than the-defects in MoSSe.