Controllable Preparation Of Two-dimensional Magnetic Nanomaterials And Manipulation Of Valley Polarizations In Heterostructure

Two-dimensional(2D)nanomaterials show great application prospect in new optoelectronic information devices,due to their excellent optoelectronic properties and multiple integration advantages.For example,layered transition-metal chalcogenides(TMDs)as a kind of typical 2D nanomaterials,of which the crystal inversion symmetry breaking and the spin-orbit coupling(SOC)lead to their valley information can be read and detected by the external approaches of optical,electric and magnetic.It is further to provides an important research direction for the development and application of the next generation of new high-speed and low-power optoelectronic devices.In addition,as a unique 2D material,2D magnetic nanomaterials provide an ideal research platform for developing and investigating of spintronic devices due to the introduction of“magnetic”property.The van der Waals(vd W)heterostructures based on2D magnetic materials can be used to manipulate of spin states via its intrinsic magnetic proximity effect(MPE)without external magnetic filed,thus opening up a novel view for the simple manipulation of spin and valley pseudospin.At present,the related researches mainly focus on 2D vd W layered magnetic materials,the controllable preparation of 2D non-layered magnetic materials and their related photophysics have rarely been explored.Remarkably,there are much fewer studies on the manipulation of spin and valley polarizations based on 2D non-layered magnetic materials vd W heterostructures.Herein,this paper focus on the controllable preparation of 2D non-layered magnetic materials Cr_nSe(0<n?1),and by constructing vd W heterostructure of 2D non-layered magnetic materials/monolayer TMDs(Cr₂Se₃-WS₂)to manipulate the spin and valley polarizations physical property based on the MPE.The major research results are summarized as follows:(1)We have exploited vd W epitaxy to synthesize few layers of high crystallinity non-layered CrSe crystals and vertical heterostructure WS₂-CrSe on Si O₂/Si substrates via CVD and two-step(PVD+CVD)approaches,which has established foundation for the controllable preparation of non-layered magnetic materials and heterostructures.(2)By precise controlling the growth temperature(820-900?)and thus changing the concentration of metal precursor Cr,we have realized the synthesis of thickness-tunable and air-stable rhombohedral single-crystal Cr₂Se₃nanosheet on the mica via the modified CVD method.The atomic force microscopy(AFM)characterization shows that the thinnest of synthesized Cr₂Se₃nanosheet can down to one-unit-cell thickness(?1.8 nm).Time-dependent AFM characterization and Raman spectroscopy measurements demonstrated that the stability of the synthesized Cr₂Se₃nanosheets in the atmospheric environment.We have explored the growth parameters of Cr₂Se₃nanosheets on different substrates(Si O₂/Si substrates,glassy silica)as well;(3)We have constructed high quality vd W vertical heterostructure Cr₂Se₃-WS₂on three substrates(mica,Si O₂/Si substrates and glassy silica)by two-step approaches(mechanical exfoliation and artificial transfer)and vacuum annealing method.The manipulation of spin and valley polarizations in heterostructure on different substrates has been investigated under high vacuum and low temperature based on the MPE.The results show that both the thermal conductivity of difference substrate and the interficial interaction have greatly influence on the valley polarization property of the heterostructure.We have further analyzed the temperature-dependent polarization mechanism to ensure the reliability and validity of the manipulation of spin and valley polarizations based on the MPE.Moreover,an elementary demonstration of carrier dynamics was given by the time-resolved polarized photoluminescence(PL)spectra.Our work can be applied to a variety of material systems as a universal method,which will promote the research and application of low-dimensional semiconductor materials in the field of spin photonics and spintronic devices.