Construction And Structural Properties Of Several Graphene Nanoribbons

Graphene has peculiar physical properties.Further reducing the size,onedimensional graphene nanoribbon and zero-dimensional nanographene can be obtained,which are called graphene nanostructures.Graphene nanostructures have the opening band gap due to the quantum confinement effect and zigzag edges further host spinordered edge states,which enable the potential applications in conventional electronics and spintronics.Additionally,the properties of graphene nanostructures are directly determined by their size,dopants and edge configurations.Therefore,exploring novel atomically precise structures is significantly meaningful for investigating the properties and applications of graphene nanostructures.Atomically well-defined customerdesigned graphene nanostructures can be obtained through bottom-up on-surface synthesis approach.Combining molecular beam epitaxy,scanning tunneling microscopy and atomic force microscopy,the in-situ fabrication and characterization of samples can be achieved,which are powerful tools to deeply study structural configurations and physical properties of graphene nanostructures.The innovative works of this thesis are shown as below:1.Synthesize two types of NBN-doped graphene nanoribbons with zigzag edges(NBN-ZGNR1 and NBN-ZGNR2)and investigate their electronic structures.Zigzag edge proportions of NBN-ZGNR1 and NBN-ZGNR2 are 36% and 57%,respectively.Scanning tunneling microscopy(STM)images demonstrate the length of NBN-ZGNR1 is larger than that of NBN-ZGNR2,which is due to the larger steric hindrance of the precursor molecules of the latter in the reaction process.High-resolution noncontact atomic force microscopy(nc-AFM)images resolve the bond information of the ribbons,showing their zigzag edge structures.d I/d V spectra and theoretical calculations reveal the NBN-ZGNR1 and NBN-ZGNR2 have the band gap of 1.50 e V and 0.9 e V,respectively.2.Fabricate and characterize sulfur-doped cove-edged graphene nanoribbons(SCGNRs).The reaction process of precursor monomers on Au(111)surface and the topography and electronic structures of final S-CGNRs are investigated by STM and nc-AFM.Nc-AFM image shows the thiophen rings on the ribbon edges,indicating the successful introduction of sulfur atoms into the ribbons.First-principles calculations point out freestanding well-defined S-CGNR has a band gap of 1.17 e V,obviously smaller than that of pristine cove-edged graphene nanoribbons(1.7 e V),showing S atoms on the cove edges could effectively tune the electronic structures of the ribbons.Moreover,the S-CGNRs exhibit branched structures at the low coverage.Increasing the coverage of precursor molecules to one monolayer,the ribbons topography can be tuned and some ribbons display linear structures.3.Synthesize and characterize two types of NBN-doped nanographenes(NBN-575-NGs and NBN-PT).STM measurements study the structure variation of two types of precursor monomers(NBN-hepa and NBN-BT)on Au(111)substrate at cyclodehydrogenation reactions.Nc-AFM images shows the bond information of NBN-575-NGs and NBN-PT.There are two five-membered rings and a sevenmembered ring embedding in the NBN-575-NG,while NBN-PT has two groups of zigzag and armchair edges.d I/d V spectra show the conduction band of NBN-575-NG on Au(111)substrate locates at 2.6 e V.Additionally,calculations results display NBN units and the oxidation process of NBN-PT evidently tune the bandgap of the nanographene.Furthermore,participate in developing a self-designed ultrahigh-vacuum lowtemperature scanning tunneling microscopy system.The system consists of samplepreparation chamber,STM-characterization chamber and load-lock chamber,and can perform in-situ sample preparation and STM characterization.The development process includes designing the mechanical structures,fabricating the chamber and components,assembling the parts and testing all functions.The three important parts in the process contains the cable arrangement of cryostat,optimization of low temperature quality and scanner test.The holding time of the system with 4 L liquid helium eventually reaches 74 h by selecting proper cable materials,optimizing methods of cable arrangements,changing design of low temperature shields and so on.For scanner test,structure design and the electrical screen are optimized to eliminate various mechanical and electrical noise.Eventually,the clear atomic resolution STM images of graphite were obtained at room,liquid nitrogen and liquid helium temperatures by using the system.