Upgrade Of A Four-probe Scanning Tunneling Microscopy System And Electrical Transport Properties Of Graphene

Graphene,a two-dimensional?2D?crystal with a honeycomb structure,has novel mechanical,electrical,optical properties and potential applications,such as field-effect transistors,solar cells and flexible transparent electrodes,etc.Due to its unique properties,it is of great importance both in fundamental science and industrial applications.In the past decades,researchers have developed a variety of ways to synthesize high-quality and large-area graphene.However,an obstacle in practical electronic applications of large-area graphene is its polycrystalline nature and the presence of defects such as grain boundaries?GBs?would strongly scatter charge carriers.Therefore,the conductivity and mobility are reduced,which impact its large-scale technological applications in electronics.To study the transport properties of graphene,the synthesized graphene should be transferred to the insulating substrate and lithographically defined electrodes are fabricated in order to connect with the macroscopic measuring instrument.The microfabrication process would inevitably contaminate graphene surface and thus changes its intrinsic transport properties.The four-probe scanning tunneling microscope?STM?is ideal for utilizing the ultra-high spatial resolution of STM in standard four-point transport characterization and can effectively characterize the morphology and transport properties of graphene and other2D crystals in situ.This PhD thesis is focused on the upgrade of a commercial ultra-high vacuum?UHV?four-probe STM system,morphology and transport properties of single-crystal and polycrystalline graphene grown by chemical vapor deposition?CVD?.The first part of the thesis is the upgrade of a commercial UHV four-probe STM system.The first generation of commercial UHV four-probe STM system was manufactured by OMICRON company.As the world's earliest commercial multi-probe system?model:UHV Nanoprobe?,it cannot effectively characterize the morphology and transport propertie of nano systems in situ as proposed.In views of the problems in large noise,obvious thermal drift and low resolution,the upgrade is mainly focused on several aspects,including vibration isolation and damping,scanning structure,thermal links and thermal shielding,time-shared control unit,and replacement of the scanning electron microscope by an optical microscope.Meanwhile,modifications in sample pretreatment and wire shielding are also introduced.After the upgrade,the performance of the four-probe STM system has been fundamentally improved in signal to noise ratio,mechanical and temperature stability,imaging resolution,cooling,etc.The second part of the thesis is direct four-probe measurements of conductivity and mobility in millimeter-sized single-crystalline graphene.The STM characterization verifies the continuity of CVD-grown graphene on Cu foil.Direct four-probe measurements of millimeter-sized single-crystalline graphene on SiO₂/Si are performed by the upgraded system.The extracted conductivity and mobility of the overall graphene flake from three setups via van der Pauw geometry confirm homogenous nature of the whole graphene flake.However,the local transport properties measured via square and collinear configurations show great deviations from the overall properties.These differences are due to the local distributions of graphene wrinkles,residues,and multilayered graphene islands on the monocrystalline graphene sheet,affecting the local transport properties of graphene,but the presence of these factors does not destroy uniform nature of the whole graphene flake.The third part is direct four-probe measurement of grain-boundary resistivity and mobility in millimeter-sized graphene.The work was focused on graphene bicrystals transferred onto SiO₂/Si substrate,ensuring single GBs in the measurements.Non-destructive four-probe measurements of intra-grain and inter-grain sheet resistances are performed on graphene bicrystals.To extract GB resistivity,an extending model based on Ohm's law is proposed,and accordingly,GB resistivities under different carrier densities can be obtained.In addition,this model can be applied to the measurements of graphene wrinkles.Based on the Drude transport model,the carrier mobility of graphene GBs or wrinkles can be extracted.The results show that the mobility of GBs is three to four orders of magnitude lower than that of intrinsic graphene,and the mobility at the wrinkles is about 1/6 to 1/5 of intrinsic graphene.This work extends the understanding of the intrinsic electron transport properties of graphene GBs/wrinkles,and shows the unique advantages of four-probe STM systems in studying the effects of microstructures such as defects on the transport properties of materials.Moreover,it provides a feasible method to characterize the transport properties of GBs on other 2D materials.