Investigation Of Electronic Properties In Heterostructures Based On Two-Dimensional Transition-Metal Dichalcogenides

Recently,two-dimensional(2D)transition-metal dichalcogenides(TMD)have aroused general concern in the past decade owing to their novel physical properties.Group Ⅵ TMD,such as WS2 and WSSe,have great potential for the applications of spintronics because of their strong spin-orbit coupling and large spin splitting.However,most of pristine group Ⅵ TMD lack magnetic properties and spin polarization,which greatly limits its application for spintronic device.In addition,the modulation of band structure is of significant importance for promoting the performance of device.Hence,in this thesis,we fabricate mixed-dimensional heterostructures with 2D TMD to modulate their magnetic properties,band structure and spin polarization by interfacial interactions.The research contents and main results are briefly summarized as follows:(1)The fabrication of identically sized Co quantum dots(QD)/WS2 heterostructures and the investigation of electronic and magnetic properties.We fabricate identically sized Co QDs on monolayer tungsten disulfide(WS2)by molecular beam epitaxy(MBE)and investigate their electronic properties by scanning tunneling microscopy(STM)and first-principles calculations.The results show that the Co QDs possess a unique magic number with a tetrahedral CO4 configuration,and a ptype doping and an Ohmic contact property exist in the Co4/WS2 system.Firstprinciples-calculated results reveal that the work function of Co QDs is smaller than that of bulk,and covalent interaction is formed between CO4 and monolayer WS2,which are responsible for Ohmic transport and conduction properties of the coupled heterostructures.Meanwhile,we design two kinds of Co-adsorbed monolayer WS2 frameworks,i.e.Co1/WS2 and Co2/WS2,and comprehensively explore the dependences of their magnetic properties on injected charge by using first-principles calculations.The magnetic moment and magnetic anisotropy can be tuned almost through injecting charge.The transition from ferromagnetism to non-ferromagnetism and the change of magnetic easy axis from in-plane to out-ofplane direction occur in Co/WS2 system.The differential charge densities and orbital decomposed densities reveal that modulated interaction and the changes of Co-3d states are responsible for the tunable magnetic moment and magnetic anisotropy,respectively.This work opens up a great prospect for the design and fabrication of 2D spintronics devices.(2)Layer-dependent interfacial interaction in WS2/GaN heterostructure.Here,we synthesize multilayer 2D WS2 on GaN substrate by chemical vapor deposition(CVD)and analyze the relationship between the modulation of interfacial interaction and the thickness of 2D WS2 by combining STM and first-principles calculations.The results show that the moderate fluctuations of the dielectric environment induced by the local corrugation of GaN surface give rise to large variations of the Coulomb interaction of monolayer WS2,and thus dramatically modulate the electronic structure(the valence band maximum show a huge decrease of 0.85 eV).However,under the screening effect of the sublayer WS2,the dielectric environment shows a weak modulation effect on second-layer WS2,and even can be neglected for third and fourth WS2.The relationship between dielectric environment and interlayer distance is further simulated by first-principles calculations.When increasing interlayer distance,weaker interfacial interaction gives rise to the downshift of VBM energy and the increase of band gap.When decreasing the interlayer distance,all the above evolution tendencies reverse.The planar average electrostatic potential energy,differential charge densities and charge transfer of WS2/GaN heterojunction under different interlayer distances reveal the physical mechanism of the layer-dependent dielectric modulation.This work offers some references for the design and fabrication of novel 2D optoelectronic device.(3)Controllable enormous valley splitting in Janus WSSe on CrN monolayer.We design two kinds of WSSe/CrN van der Waals heterostructures(S-and Seterminated stacking patterns),and investigate their spin-valley polarization properties,anomalous Hall effect and the modulation of valley splitting based on the first-principles calculations.The results show that intrinsic enormous valley splitting of 103 meV and 144 meV are generated in S-and Se-terminated WSSe/CrN heterostructures,respectively,due to the internal electric field and magnetic proximity effect.The Berry curvatures at K and K’valleys have opposite signs and slightly different absolute values,which predicts the intriguing valleycontrasting physics in WSSe/CrN heterostructure.Furthermore,valley splitting can be manipulated continually by the in-plane strain and interlayer distance.The largest valley splitting of 272(240)meV and the effective Zeeman magnetic field of 2560(2275)T are achieved for S(Se)-terminated WSSe/CrN heterostructures,respectively,under the compressive interlayer distance.It is found that the changes of induced spin charges around S and W atom are responsible for the modulated valley splitting.This work opens new vistas for the design of valleytronic devices.