Synchrontron Radiation Study On The Two Dimensional Magnetic Materials

In recent years,two-dimensional layered materials have been a focus of condensed matter physics and materials science due to their outstanding magnetism,mechanical,electronic and optical properties.Thus,it is the core issue and challenge in future to effectively regulate the electronic and band structures of two-dimensional materials at atomic scale and endow them with novel physical phenomena and properties as in such a way to promote the materials',application in optical&electrical devices.In this thesis,we aimed to explore the structure design of two-demensional materials and study they related physical and chemical properties.We used supercritical hydrothermal reaction and liquid exfoliation to prepare sanples,and combine the synchrotron-based radiation technique and density fuctional calculation to study the structure and properties of two-demensional materials.It provide a new train of thought and theoretical basis for controllable design,development and application of two-dimensibnal nanomaterials.The main contents of this dissertation are as follows:1.Robust ferromagnetism in Mn doped MoS2 nanosheetsMoS2 is one of the most studied 2D materials due to their fascinating properties including highly tunablecharge-carrier type and concentration,medium intrinsic band gap and novel valley polarization properties.However,the use of MoS2 in spintronics is highly proh:ibited by the inherently non-magnetic nature of normal MoS2 nanosheets.Doping of transition-metal(TM)elements provide a way to obtain ferromagnetic interactions,but it's difficut because of substitutional TM atoms can be easily expelled out as metallic clusters by the"self-purification"effect.Here,a breakthrough has been made by doping Mn atoms into MoS2 nanosheets in a sulfur-rich supercritical environment.The Mn-substituted MoS2 nanosheets show robust intrinsic ferromagnetic response with a saturation magnetic moment of 0.05?B/Mn at room temperature.The intrinsic ferromagnetism is further confirmed by the reversibility of the magnetic behavior during the cycle of incorporating/removing Li codopants,showing the critical role of Mn 3d electronic states in mediating the magnetic interactions in MoS2 nanosheets.2.Reversible tuning of the ferromagnetic behavior in Mn-doped MoS2 nanosheets.Reversible manipulation of the magnetic behavior of 2D van der Waals crystals is crucial for promoting their applications in spintronics or other spin-based information processing technologies.However,to date,most approaches have limited potential applications in spintronics since they are single way in altering the magnetic properties.In this work,we demonstrate an interface charge-transfer method to reversibly tune ferromagnetic behaviors of Mn-doped MoS2 nanosheets.By adsorption of Benzyl Viologen(BV)molecules as the charge donor,the saturation magnetic moment of Mn-doped MoS2 nanosheets increased from 0.05 to 0.08 emu/g,and then reversibly restored to the original value of 0.05 emu/g after BV desorption.Details from experimental characterizations and first-princples calculations sugrest that the enhanced magnetic moment can be attributed to the increase of Mn magnetic moment due to the enriched electrons transferred from BV molecules.This work shows that interface charge transfer may open up a new pathway for reversibly tuning the exchange interactions in 2D nanostructures.3.Intrinsic Ferromagnetism in Two-Dimensional V2C MXenes NanosheetsTwo-dimensional(2D)materials with intrinsic magnetism are essential and highly sought due to their potential applications in spintronics,logic and memory operations,and other quantum information devices.To date,only a handful of 2D materials with stable intrinsic magnetic properties have been reported.In this work,we reported a ferromagnetic behavior in 2D V2C nanosheets.The V2C MXenes nanosheets show robust intrinsic ferromagnetic response with a saturation magnetic moment of 0.013emu/g at room temperature.The robust ferromagnetism in V2C MXenes nanosheets can be attributed to a small twist angle of 2.5° between the two similar lattices.This work highlights the V2C MXenes as a promising material with intrinsic ferromagnetism by extending their potential applications in spintronics or other spin-based electronics.