Synchrotron Radiation Study On Microstructure And Novel Physical Properties Of Monolayer Molybdenum Disulfide

Two-dimensional thin-layer semiconductor materials have great application prospects in spintronics,flexible electronics,field-effect transistors,energy storage and catalysis due to their excellent optical,electrical,magnetic,mechanical and thermal properties.It is currently a hot research topic in many disciplines such as physics,information,materials and chemistry.In particular,monolayer transition metal dichalcogenides(TMDs)show a new "energy" degree of freedom(DOF)due to the breaken inversion symmetry,making them an ideal platform for studying novel phenomena of physics and chemistry.It is the core contents and central challenges in this research field that endowing it excellent physical or chemical properties and promoting its application in the future optoelectronic devices and energy storage by band-structure engineering.In this thesis,taking monolayer MoS2 as an example,by introducing doping or Moire superlattice and other strategies to tailor its band structure or electronic structure,monolayer MoS2 has the coexistence of ferromagnetism and liminescence,excellent hydrogen production and high temperature ferromagnetic properties.Combining characterization techniques such as synchrotron radiation and first-principle calculations,we establish the internal relationship between microstructure and macroscopic physical properties at the atomic and electronic levels.This provides new opportunities and theoretical basis for the preparation and modification of two-dimensional nanomaterials.The main contents of this dissertation are as follows:1.Beating the exclusion rule against the coexistence of robust luminescence and ferromagnetism in chalcogenide monolayersMonolayer chalcogenide semiconductors with both luminescent and ferromagnetic properties are dreamed for simultaneous polarization and detection of the valley degree of freedom in valleytronics.However,a conventional chalcogenide monolayer lacks these coexisting properties due to their mutually exclusive origins.Herein we demonstrate that robust ferromagnetism and photoluminescence(PL)could be achieved in a(Co,Cr)-incorporated monolayer MoS2,where the ferromagnetic interaction is activated by Co ions,and the nonradiative recombination channels of excitons is cut off by Cr ions.This strategy brings a 90-fold enhancement of saturation magnetization and 35-fold enhancement of PL intensity than the pristine MoS2 monolayer,which result from the electronic interactions between the impurity bands of atop Cr adatoms and substitutional Co atoms,as well as the increased content of neutral exciton.Our findings could extend the applications of two-dimensional chalcogenides into spintronics,valleytronic and photoelectric devices.2.Spontaneous valley polarization activating single-atom-layer catalysis in monolayer MoS2:beyond the single-atom catalysisMonolayer catalysts with fully activated basal-atoms will provide an ultimate solution to the low loading-density bottleneck of single-atom catalysts;however,the active sites of most of the state-of-the-art monolayers are limited to the edge sites.Herein,we propose a strategy to activate the basal-plane sulfur atoms in monolayer MoS2 via spontaneous valley polarization induced by magnetic cobalt ion doping,which induces a high interior magnetic field of 32.5 T and dramatically increases the S-p state density around the Fermi level.On-chip microcell measurements of electrochemical and operando synchrotron radiation microscopic infrared spectroscopy reveal that 52%of the originally inert basal-plane sulfur atoms are active for the hydrogen evolution reaction.This brings an unprecedentedly ultrahigh mass activity of 28,200 A g-1 at 10 mA cm-2 and exchange current density of 65 ?A cm-2,110 and 500 times higher than those originated from the edge sites,respectively.3.Ultrahigh-temperature ferromagnetism in MoS2 Moire superlatticeRealizing high-temperature ferromagnetism in two-dimensional(2D)semiconductor nanosheets is significant for their applications in next-generation magnetic and electronic nanodevices.Herein,this goal could be achieved on a MoS2 Moire superlattice grown on the reduced graphene oxide(RGO)substrate by a hydrothermal approach.The as-synthesized bilayer MoS2 superlattice structure with a thickness of 2 nm and a 14.50 rotating angle of two hexagonal MoS2 lattices,possesses outstanding ferromagnetic property and an ultrahigh Curie temperature of 990 K.The X-ray absorption near-edge structure and ultraviolet photoelectron spectroscopies combined with density functional theory calculation indicate that the covalent interactions between MoS2 Moire superlattice and RGO substrate lead to the formation of interfacial Mo-S-C bonds and complete spin polarization of Mo 4d electrons near the Fermi level.This design could be generalized and may open up a possibility for tailoring the magnetism of other 2D materials.