Study On Ferromagnetism Of Two-dimensional Materials Based On Synchrotron Radiation Spectroscopy

In recent years,two-dimensional(2D)materials have attracted extensive attention of physicists and material scientists due to their unique spin dependent physical properties(such as the ultra long spin relaxation time of graphene and the spin Valley locking effect in transition metal chalcogenides).However,most of the 2D materials discovered so far are inherently nonmagnetic,which limits their practical applications in the next-generation of spintronics devices.Effective means to endow and modulate 2D thin-layer materials with room temperature ferromagnetism,or to develop new intrinsic 2D ferromagnetic materials,are key issues and challenges to promote their practical applications in low-dimensional spintronics in the future.In this paper,we modulate the band structure characteristics of 2D materials by doping and other strategies to endow diamagnetic/non-magnetic 2D materials with room temperature ferromagnetism:(1)With the aid of co-doped N atoms,Co atoms are stably embedded into the graphene lattices and the room-temperature intrinsic ferromagnetism of graphene is activated;(2)By introducing substitutional vanadium(V)dopants and S vacancy(Vs)into the MoS2 nanosheets,the ferromagnetism of the MoS2 nanosheets is endowed and modulated synergically.In addition,?-Mo2N phase nanosheets with intrinsic room temperature ferromagnetism were prepared for the first time by molten salt-directed catalytic synthesis.On this basis,combined with synchrotron radiation X-ray absorption fine structure(XAFS)spectroscopy,wavelet transform(WT)analysis and first-principles calculation,the local structure,bonding,electronic states and magnetic mechanism of 2D room-temperature ferromagnetic materials were analyzed,and the internal relationship between microstructure(local environment and electronic structure)and ferromagnetism was established.The specific research content of this paper is as follows:1.Embedding atomic cobalt into graphene lattices to activate room-temperature ferromagnetismAlthough graphene is a promising material for the next generation of spintronics applications,its non-magnetic properties limit its practical applications.We used a two-step impregnation and pyrolysis method to monodispersed-doped Co atoms into the graphene lattices with the aid of N atoms,and obtained a saturation magnetization of 0.11 emu g-1 at room temperature and a Curie temperature of 400 K.Through synchrotron radiation soft and hard X-ray spectroscopy technology and a variety of analytical methods of X-ray spectroscopy(real-space multiple scattering theoretical calculation,extended edge quantitative fitting,multi-configuration calculation and wavelet transform),it is confirmed that Co atoms exist as isolated square-planar Co-N4 moieties in the graphene lattices,eliminating the possibility that the magnetism originates from the Co-related second phases.Ddensity functional theory(DFT)electronic structure calculations further show that the CoN4-graphene system has a metallic band structure,and the density of states at the Fermi plane is significantly enhanced,and there are hybridization between the Co-3d and C/N-2p orbitals.Combined with the experimental and theoretical results,it is proposed that the Co-N4 moieties are the main carriers providing magnetic moment in the Co doped graphene,and the RKKY-like long-range ferromagnetic exchange mechanism is the internal mechanism leading to its room-temperature ferromagnetism.2.Synergetic effect of substitutional dopants and sulfur vacancy in modulating the ferromagnetism of MoS2 nanosheetsActivating and modulating the magnetism of MoS2 nanosheets is the key to promote its application in the next generation of spintronics.In this work,a two-step method was used to simultaneously intrdduce Hubstitutional-V dopants and Vs into the MoS2 nanosheet host,and a stable and adjustable ferromagnetic response at room temperature was obtained.The ferromagnetism can be modulated by changing the content of Vs through different periods of Ar plasma irradiation.When the irradiation time is 6 s,the maximum saturation magnetization is 0.011 emu g-1.Experimental characterization such as X-ray absorption near-edge structure(XANES)and first-principles calculations show that the adjustable ferromagnetism comes from the modulation of the band structure of MoS2 nanosheets by strong hybridization between the V 3d state and the Vs-induced impurity bands.3.Directional catalytic synthesis of two dimensional ferromagnetic transition metal nitrides by molten saltFinding and preparing novel 2D ferromagnetic materials is an important content in the research field of 2D materials,and also a key step to promote the application of spintronic devices.Based on the theoretical prediction that transition metal nitrides(TMNs)may have ferromagnetism,the intrinsic room temperature ferromagnetism of?-Mo2N phase nanosheets with a lateral dimensions of about 500 nm were prepared for the first time by molten salt-directed catalysis.The magnetic measurement results show that Mo2N nanosheets have a saturation magnetization of 0.002 emu g-1 at room temperature.The Curie temperature is 350 K,and the Mo2N nanosheets show a transition from antiferromagnetic to ferromagnetic at 38 K.Combining the results of XRD,XPS,XAFS and ICP-AES,it can be seen that only y-Mo2N exist in the prepared samples,and there are no impurity phases(Fe,Co and Ni,etc.)that can cause room-temperature ferromagnetism,which means that the ferromagnetism observed in y-Mo2N nanosheets is intrinsic.