Transport Properties Of Graphene And MoS₂ Investigated By Four-probe Scanning Tunneling Microscope

Two-dimensional（2D）materials have attracted much interest since the discovery of graphene.The advantages of the two-dimensional materials over bulk materials are both their atomic-scale thickness and the gate-tunable transport properties,and the unconventional magnetism,superconductivity,and charge density wave.2D materials have been regarded as a good candidate to replace silicon after the failure of the Moore’s Law.Hence,the research on the transport properties of the 2D materials has been a hot topic for a long time.The four-probe scanning tunneling microscope（4P-STM）can not only conduct the in-situ transport measurement,but also identify the atomic structure of the materials.Combining the two measurements,the relationship between the micro-scale structure and the transport properties can be verified.Therefore,4P-STM has become an important implement to study the transport properties of 2D materials.The main work of the thesis is to study the transport properties of graphene and Mo S₂ based on a 4P-STM system.The thesis consists of five parts.1.We designed the time-shared switching system（TSS）in the 4P-STM to make it a suitable platform to determine the tip position and obtain the STM images from macro-scale to micro-scale by upgrading the related circuits.Simultaneously,the TSS could combine the STM imaging with in-situ transport measurements by approaching the tips in the micro-scale.The TSS could also conduct the scanning tunneling potentiometry measurement.The upgraded 4P-STM could do the STM imaging and local transport measurements precisely,which is of great significant to the researches about the low-dimension materials.2.We studied the electrical degradation effects induced by multilayered islands,grain boundaries,wrinkles,cracks,holes,and adsorbates in the CVD-grown millimeter-scale graphene on the Si O₂ substrate using the 4P-STM.By comparing the local measurement results near and far away from the imperfections within square configuration,we found that except for the multilayered islands,the presence of the imperfections could lead to an increase of the local sheet resistance up to 650%while the carrier mobility decreases by up to 95%.As for the multilayered islands,the decrease of the sheet resistance near the Dirac-point voltage was attributed to the additional conducting channels from the multilayered structure while the reduction of the carrier mobility was mainly due to the interlayer scattering and grain boundary-induced scattering.Owing to the large variation of the width and the amount of the curved structures,the electrical performance of the graphene wrinkles varied greatly.The angle-dependent van der Pauw measurements have also verified that these local imperfections could not lead to electrical inhomogeneity of the whole graphene flake unless they are quite extended.The present study can pave the way toward practical application of graphene.3.We conducted the direct probing of conductivity enhancement in the quasi-one-dimensional wrinkles within millimeter scale graphene on Si O₂ substrate.The micro-scale wrinkles are composed of many nano-scale wrinkles,whose main orientations are nearly in parallel.According to the variable-spacing four-probe measurements results both away from and along the wrinkles,it was confirmed that the introduction of the wrinkles strongly alters the 2D transport behaviors of pristine graphene.And the collinear four-probe measurements showed that the conductivity measured along the wrinkles is distinctly higher than those at the flat monolayer area.Combined with the resistor network simulation,the conductivity inside the wrinkled region can be 33.6times higher than that of the non-wrinkled monolayer region,which can be attributed to its multilayered structure and the relatively large strain gradient at its boundary.These results illustrated that the graphene wrinkle is a structure with high conductivity and may be adopted in the electric devices.4.We designed a special two-step CVD method to synthesize large size single crystal Mo S₂ monolayer and measured its transport properties using the two-terminal measurement and the 4P-STM.The topography information from the direct STM characterization explains the threshold voltage variation and N-type behaviors in the two-terminal transport measurements.Simultaneously,direct four-probe transport measurement via van der Pauw geometry illustrates its relatively high carrier mobility.The synthesis of high quality Mo S₂ and the direct 4P-STM characterization methods could also be applied in the other 2D materials grown on the Si O₂ substrate.5.We reported the local density of states modulated by strain in marginally twisted bilayer graphene（TBG）using low-temperature STM.After determining the target area in the 4P-STM,the sample is transferred into the gate-tunable low-temperature STM to do further STM/STS characterization.Four peaks near the Fermi level are identified in the STS at the center of the AA stacking regions.Density Functional Theory calculations indicate that the V1 and C1 peaks possibly originate from the strain introduced during the preparation process and the shift of these four peaks as a function of the back-gate voltage may derive from the vertical electric field.d I/d V map across Moirépatterns with different strain strengths exhibits the different helical edge states,implying the large strain in the graphene lattice might change the band structure of marginally TBG,hence making the edge states unable to be detected in the d I/d V map.Our results indicate that the strain caused by sample preparation can tune the electronic states in the TBG system.The work is useful for other strain-related researches in the TBG system.In summary,we have designed the time-shared switching system and developed several experiment methods within the 4P-STM.Then we utilized the 4P-STM to study the electrical transport properties of graphene with different imperfections,including grain boundary,multilayered islands,wrinkles,cracks,holes,and adsorbates,Mo S₂single crystal and marginally TBG.These works could be the reference for the utilization of 4P-STM in the measurements about the surface-related and structure-related transport properties in the future.