Stm/Sts Study Of The Electronic Properties Of Defective Structures In Graphene

After first time being isolated experimentally in2004, graphene, a two-dimensional (2D) honeycomb structure of pure sp2carbon, with a linear dispersion near the Dirac cones, has attracted tremendous attention. Graphene has been considered to possibly replace Si as the next generation super material, with wide applications. Defects, such as dislocations, grain boundaries (GBs), wrinkles and point defects, could significantly impact the structure and electronic properties of graphene. In this thesis, we focus on the structural and electronic behaviors of the defective structures in graphene, which are both spontaneously or artificially induced in graphene, studied by scanning tunneling microscope (STM).In chapter1, we give a brief introduction of the structure, properties, preparation method and characterization of graphene. Also, the main characterization method used in this thesis, scanning tunneling microscope (STM), is simply mentioned from this invention, improvement and working principle.In chapter2, using STM, we find the relative disordered GBs and two types of ordered GBs in single layer graphene on the300nm SiO2/Si substrate prepared by chemical vapor deposition (CVD). Two types of ordered GBs, named (3,1)|(1,3) GB and (2,0)|(2,0) GB, formed by successive pentagon-heptagon rings and pentagon-octagon-pentagon rings, respectively, are found and detailed studies on the electronic properties with-atomic precision. Joint with the first-principles calculation, for the first time, we present the direct experimental evidence of the existence of the van Hove singularities (VHSs) in ordered graphene, which can greatly enhance the conductivity of graphene. Then, we propose a promising structure of graphene nanoribbons (GNRs) embedding with a proper ordered GB to fabricate functional devices with enhanced conductivity. The relative disordered GBs are shown quite opposite results, which are detrimental to the conductance of graphene. Our experimental results shed light to understanding the contradictory transport measurement results about GBs if the order degree of GBs is taken into account.In chapter3, we study two types of strained structures and their electronic properties. Firstly, the (1,0) dislocation, a pentagon-heptagon pair, is observed in graphene by STM. The (1,0) dislocation shows intrinsic out-of-plane distortion, with a3D size of2.1nm×2.4nm×3.3A. The electronic properties of VHS states are determined, combined with theoretical calculations. Gap opening due to the strain in the (1,0) dislocations is observed. The other strained structure is a triple-folded graphene. Landau quantization is found, with a linear relation between the energies of the LLs and sgn(n)(|n|(|n|+1))1/2. Combined with the apparent height of the structure, a model of a triple-folded graphene structure is proposed.In chapter4, by controlling the ratio of the source gases, we obtain N-doped graphene with different chemical forms and concentrations of the N dopants prepared by low pressure-CVD. Characterized by XPS, the chemical forms of the N dopants are determined to be graphitic, pyrrolic and pyridinic. After STM studies combined with the theoretical calculations, we confirm the structures, electronic properties and doping effects of the different N dopants. Typically, the graphitic N is with n-type doping effect, while the pyridinic N with p-type doping effect. After the Hall Effect measurements of the N-doped graphene with different N dopant ratios, we obtain both n-type graphene and p-type graphene. Therefore, we controllably tune the graphene doping level with only single element of N. The graphitic and pyridinic N defects are formed isolated domains by each, with n-type or p-type doping level. With liquid pyridine by atmospheric pressure-CVD, we obtained single layer N-doped graphene, too. Using STM/STS, the graphitic and pyridinic N defects are also found, which can locate very close to each other in the graphene sheet, without forming isolated domains with one kind of pure N defects.In chapter5, after Ar+sputtering, we obtain monovacancies, divacancies, tetravacancies and other point defects in graphene on an insulator substrate. During the local probe of STM, we find most the defects are monovacancies with our optimized sputtering parameters. Most interestingly, more than80%of the monovacancies are located at the same sublattices. The electronic properties of the monovacancies are also resolved.