MBE Growth And STM Study Of The Transition Metal Dichalcogenides 1T'-WTe₂

Since the 21st century,when the graphene were prepared by mechanical exfoliation method,two-dimensional materials have attracted the attention of science researchers.Because of its promising application in fields of science and electron industry,there was a stirring of interest in graphene.However,the zero energy gap problems have limited the graphene application scope.Therefore,discovering neotype materials with large energy gap,low dimension and easy preparation has become the hotspots of post-grapheme era.Transition metal chalcogenides(TMDs)is a family of materials with various structures.The 2H phase generally has a large energy gap.As a new generation of low-dimensional materials,TMDs has gradually become a research hotspots.For the past few years,theoretical studies have predicted that the monolayer 1T' phase TMDs have the Quantum Spin Hall Effect,which drawing our attention on the two-dimensional topological insulators once again.The emergence of 1T' phase TMDs has inspired a new wave of research.For the two-dimensional topological insulators,which electronic states have band gaps as well as topologically protected boundary states,because of this,two-dimensional topological insulators are likely to be used in spintronic devices.In earlier research,due to the structural features,it is impossible to directly characterize the topological boundary states,and didn't acquire single-layer TMDs with mechanical exfoliation method.Therefore,searching for the ideal two-dimensional topological insulators has become a research hotspot.In this thesis,based on the prediction that single layer 1T' phase TMDs have Quantum Spin Hall Effect,1T'-WTe2 was selected as the research object.Transport properties and microscopic electronic structure were the keystone of this thesis.In this thesis,we successfully growth single-layer 1T'-WTe2 films on the Bilayer Graphene/SiC(0001)substrate by molecular beam epitaxy(MBE).Scanning tunneling microscopy/spectroscopy(STM/S)were used to characterize film's structural and electronic states.When Bilayer Graphene/SiC(0001)were used as the substrate,the epitaxial monolayer IT'-WTe2 presented a weak coupling state with the substrate,and there were incommensurate charge density modulation under low temperature.We found that monolayer 1T'-WTe2 has an energy gap of 20-40mV,with the substrate of Bilayer Graphene/SiC(0001),topological boundary state were also found by means of STS.For the evolution of the electronic state of different layers WTe2,we found that the electronic state of WTe2 gradually changes from semiconductor to semi-metal as the layer thickness increase.MBE growth monolayer 1T'-WTe2/Bilayer Graphenen/SiC(0001)samples were used for the electrical transport measurement.Physical Property Measurement System(PPMS)and a standard four-electrode test method was used in the electrical transport measurement and magnetoresistance measurement.It was found that the system appears as a strong localization system,the magnetic resistance has strong anisotropy.By means of adjusting the angle between the magnetic field and the current directions,the magnetoresistance show a transition between negative and positive,providing a possibility for the practical application of the system.