Synchrotron Radiation Study On Atom-doping Modification Of Two Dimensional Layered Chalcogenides

Two-dimensional layered chalcogenides have attracted more and more interests for their unique structure as well as many novel physical and chemical properties.Taking MoS2,MoTe2 and WSe2 as examples,when their thickness decreases to a single layer,the corresponding band gap becomes direct.When the electrons in the valence band jumps to the conduction band,it only needs to absorb the energy of the photon but not phonon.This unique energy band structure has endowed them great advantages in the fields of energy conversion and optoelectronic devices.Recently,with the continuous research of two-dimensional layered chalcogenides,lots of progress has been made in various fields.However,the practical application of two-dimensional layered chalcogenides still remains a challenge,for their low energy conversion efficiency,high process cost,complex device structure and unknown structure-activity relationship,etc.Hence,to realize the leap from theoretical study to application,it is imperative to modify two-dimensional layered chalcogenides to optimize their properties and verify their microscopic mechanisms.Among the modification methods of two-dimensional layered chalcogenides,atom doping is the most simple and efficient strategy,which dose not involve complicated micro-nano operations and has universal applicability.In this thesis,we take two-dimensional GaS and MoS2 as examples,to display the feasibility of the intercalation and substitutional doping method for two-dimensional layered chalcogenides,which prominently promote catalytic efficiency of GaS nanosheets for water splitting,endow bilayer MoS2 with ferromagnetism,regulate it via creating sulphur vacancy subsequently,and induce intrinsic valley polarization in monolayer MoS2.The main contents of this dissertation are as follows:(1)Ru?-intercalated GaS nanosheets for photocatalytic water splittingPhotocatalytic water splitting over layered nanosheet catalysts has caught lots of attention on the part of renewable hydrogen fuel production.However,the weak van der Waals interlayer interactions make it a great challenge to realize effective dissociation of photogenerated excitons and efficient charge transfer across the interior of layered catalysts during the photocatalysis process.Here we propose an intercalation strategy of high-valence Ru? atoms to render two-dimensional GaS nanosheet photocatalysts with rapid electron-hole dissociation and long photocarrier lifetime in visible-light driven water splitting.XAFS measurements and theoretical results unravel that the intercalated single-site Ru,confined in interlayer of GaS NSs,with a hexagonal structural configuration of"Ru1-S6",can serve as an electron-trapped high-speed channel towards simultaneously accelerating electron-hole pairs dissociation and promoting photoelectron transportation through the Van der Waals interlayer.Consequently,the as-developed Ru-intercalated GaS NSs can give a notable H2 production rate of 340 ?mol·g-1·h-1 under visible-light irradiation and an apparent yield of 7%at 420 nm,38 times that of pure GaS NSs.This study opens up a feasible way for new design of high-activity layered photocatalysts towards high-efficiency solar energy conversion.(2)Ferromagnetism in large area Co-doped bilayer MoS2Endowing bilayer transition-metal dichalcogenides(TMDs)with tunable magnetism,thus breaking the symmetry of time inversion,is significant to investigate the coupling of multiple electron degrees of freedom,such as valley-spin and layer-spin coupling,ect.However,effectively inducing and tuning the magnetic interaction of bilayer TMDs are still challenges.Herein,we report a strategy to tune the interlayer exchange interaction of centimeter-scale MoS2 bilayer with substitutional doping of Co-ion,by introducing sulfur vacancy(Vs)to modulate the interlayer electronic coupling.This strategy could transform the interlayer exchange interaction from antiferromagnetism to ferromagnetism,as revealed by the magnetic measurements.XAFS studies and theoretical calculations indicate that the enhanced magnetization is mainly because the hybridization of Co 3d band and Vs-induced impurity band alters the forms of interlayer orbital hybridization between the partial Co atoms in upper and lower layers,and also enhances the intralayer ferromagnetism.Our work paves the way for tuning the interlayer exchange interaction with defects and could be extended to other two-dimensional magnetic materials.(3)Ferrovalley property of Co-doped monolayer MoS2The emergence of valley degree of freedom(DOF)in two-dimensional layered materials with hexagonal lattice offers new opportunity for electronic information technology.The realization of the intrinsic valley polarization in valley materials without application of external field is the key to fabricate valley electronic devices.We experimentally achieved intrinsic valley polarization in monolayer MoS2 by magnetic Co-atom doping strategy.The polarization resolved photoluminescence results show that Co-MoS2 can achieve about 60%polarizability at room temperature.In order to explore the application potential of Co-MoS2 in valley electronic devices,we constructed circularly polarized LED device based on Co-MoS2 with highly doped p-type silicon as a hole injection substrate.Electroluminescence results show that the circularly polarized LED device can give an intrinsic polarization helicity of 15%at room temperature.XAFS measurements and magnetic characterizations indicate that the intrinsic valley polarization results from the exchange interactions between the d orbitals of Mo and Co atoms.This work provides a new method for the effective utilization of valley DOF in the next generation of electronic and computing equipment.