| Since the successful exfoliation of atom-thin graphene,two-dimensional(2D)van der Waals materials have received widespread attention.Among them,two-dimensional transition metal sulfides possess excellent optical,electrical and magnetic properties,showing important research significance and wide application prospects.Compared with bulk or film materials,the reduced dimension in 2D materials highlights the influence of atomic defects on their properties.On the one hand,intrinsic defects could act as charge trap centers or scattering source of carriers thus harm some intrinsic performance of materials,on the other hand,controllable introduction of defects could modulate physical properties and even endow materials with completely new phenomenon.Therefore,it is highly important to realize the precise control of defects in 2D materials and fully clarify their structure-activity relationship with the corresponding physical properties.In view of the above key scientific problems,this paper selects binary transition metal-sulfur dicogenides(TMDs)and ternary transition metal phosphorus sulfides(MPS3)as research objects,and systematically explores the influence of various defects on the basic physical properties of materials such as optics,electricity and magnetism.Combined with advanced characterization methods such as synchrotron radiation Xray absorption fine structure spectroscopy(XAFS),synchrotron radiation photoelectron spectroscopy(SRPES),synchrotron radiation X-ray magnetic dichroism spectroscopy(XMLD)and first-principles calculations,we tried reveal the complete relationship between the microstructure of materials and macroscopic physical properties.We further pursue the device application by constructing new logic devices based on defectregulated 2D materials.This work deepens the understanding of defects in 2D materials and promotes the practical application of 2D materials in future electronic devices.The specific research contents of this paper are showed as follows:1.In-situ repair of sulfur vacancies in monolayer WS2 to improve the photoluminescence(PL)performance.The monolayer tungsten disulfide(WS2)with low defect density was synthesized by chemical vapor deposition(CVD)method.Appling synchrotron radiation XAFS,SRPES and other characterization techniques,the crystal structure of the sample and the fine coordination information of the repair atom were systemically investigated.It is demonstrated that the in-situ introduction of oxygen atoms could effectively repair the intrinsic sulfur vacancy.Further optical spectroscopic tests determined that the photoluminescence quantum yield(PLQY)of low-defect density WS2 was 9.3%,which was 90 times higher than that of the untreated sample,and the high PLQY could persist with an endurance of up to 3 months under ambient conditions without any protection.The above results strongly proved that this defect engineering could significantly improve the PLQY and environmental stability of the WS2.More in-depth insights from the first-principle calculations illustrate that the enhancement mechanism is the synthetic action of the suppression of nonradiative recombination and conversion from trion to neutral,and the excellent stability arises from repaired saturated coordination bonds at sulfur vacancy sites.This method opens up more possibilities for both fundamental exciton physics and optoelectronics applications.2.Modulate the defect types of sulfur atoms to obtain atom-thin WS2 lateral homojunctionsThe intrinsic WS2 lateral homojunctions(WS2-HMJs)defined by different sulfur defects were synthesized by two-step CVD method.Combining electron microscopy techniques and other characterizations,defects in different domains were determined as the sulfur single-vacancy(Vs)and sulfur antisites(Sw and S2w),respectively.Besides,perfect lattice arrangements are maintained at the interface.The Kelvin probe force microscope(KPFM)and other test methods were used to explore the electronic band structure in different domains,and the band offset of about 190 meV was determined at the junction region.Field-effect transistor(FET),p-n junction and logic inverter based on WS2-HMJs were further constructed by microfabrication techniques,and the basic electrical characteristics of the two types of domains,the rectification ratio of the p-n junction and the gain value of the logic inverter were measured.Finally,theoretical calculations show that this growth method is universal,the HMJs defined by chalcogenide defects could exist in other TMDs,and can form different band alignment situations.This work realizes the direct growth of single-layer 2D lateral HMJs,which provides more possibility for using 2D materials to construct in-plane electronic devices in the post-Moore era.3.Introduction of metal substitutional defects to regulate the magnetic anisotropy of MPS3 systemTernary transition metal phosphorus sulfides single crystals(Fe1-xNixPS3)with different contents of nickel metal substitution were synthesized by chemical vapor transport(CVT)method.Combining X-ray diffraction,XAFS and electron microscopy characterizations confirms that the substitution of metal defect could induce lattice distortion rather than electron transfer.The magnetic test showed that with the increase of substituted nickel atom,the magnetic anisotropy of the material changed from Ising type to XXZ type.Furthermore,using synchrotron radiation X-ray line dichroic absorption spectroscopy(XMLD)and theoretical calculation,the evolution of electron orbital distribution and spin structure of different component materials was further explored.It is revealed that the physical essence of magnetic phase transition is the reconstruction of electronic structure between split orbitals induced by lattice distortion.This work reveals the spin-state transition as a function of atomic composition in MPS3 system,deepen the understanding of magnetic order in antiferromagnetic vdW materials and hence opens a path for developing antiferromagnet-based quantum information technologies. |
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