【作者】 马传许；

【导师】 王兵； 侯建国；

【作者基本信息】 中国科学技术大学 ， 单分子科学， 2014， 博士

【摘要】 自2004年石墨烯被解理获得以来,其独特的二维结构和电子学性质,引起了世界范围内广泛的关注。石墨烯具有广阔的应用前景,如量子计算机,燃料电池,传感器,等等。缺陷的存在,比如畴界,皱褶以及点缺陷等,会从结构上以及电子学性质上对石墨烯产生巨大的影响。本文中,我们将从结构和电子学行为两方面,来研究石墨烯中自发产生的和人为引入的缺陷结构。第一章是石墨烯简介,简单介绍了石墨烯的结构及其独特的电子学性质,以及制备方法,常用的表征手段。这里,我们还简要地介绍了实验中使用的扫描隧道显微镜/谱(scanning tunneling microscopy/spectroscopy, STM/STS)的发明,发展和工作原理。第二章,利用STM/STS研究了化学气象沉积(chemical vapor deposition, CVD)方法制备并转移到SiO2/Si衬底上的单层石墨烯中有序和无序畴界(grain boundaries, GBs)。研究中发现了两种有序畴界结构,分别为(3,1)|(1,3)畴界(由五元环和七元环交替连接而成)和(2,0)|(2,0)畴界(由五元环对和八元环交替连接而成),并结合第一性原理计算,首次从实验上证明了在有序的石墨烯畴界中存在范霍夫奇异性(van Hove singularities, VHSs)。我们的研究还表明,由于VHSs的存在,有序畴界结构能显著的提高石墨烯的电导。据此,提出了一种基于石墨烯的器件结构,就是内嵌有序石墨烯畴界的纳米条带,可以极大地改善其电输运行为。另外,我们还从实验上证明了,无序的畴界与有序畴界是恰恰相反的,前者的存在对输运是有害的。主要原因在于有序畴界中具有沿着畴界方向没有位置依赖关系的VHS态,而无序畴界中存在的局域的缺陷态是具有位置依赖性的,他们的存在会对电子产生散射,阻碍其传输。我们的结果表明,如果将畴界的有序度考虑进来,就能解释前人输运测量实验中得到的关于畴界的电导的矛盾结果了。第三章,研究了石墨烯中存在应力的两种结构,并研究了应力对其电子学性质的调制效应。通过STM/STS的研究,发现了石墨烯中的(1,0)位错(dislocation)和皱褶结构,分别为本征的和衬底引起的应力结构。结合第一性原理的计算,(1,0)位错的存在已经使得石墨烯的能带结构发生了巨大的变化,导致了VHS态的出现。并且由于应力的存在,实验观测到在畴界中打开的能隙。在另外一种石墨烯的皱褶结构中,通过对其结构和电子态的研究,发现了朗道能量化,同时根据其表观高度特征,我们提出了三重折叠皱褶的结构模型。第四章中,采用低压CVD方法,调制生长过程的各气体组分的比例,得到了具有不同化学存在形式和不同掺杂原子浓度的N掺杂的单层石墨烯。通过XPS研究,证明了N原子的存在形式有石墨型,吡咯型以及吡啶型三种构型。利用STM/STS的微观研究,确定了石墨型和吡啶型N缺陷的存在及其原子结构,电子学性质以及掺杂效果。石墨型N和吡啶型N缺陷分别具有电子型和空穴型的掺杂效应。通过对不同N掺杂类型的石墨烯样品的霍尔效应(Hall effect)测量,得到了电子型掺杂和空穴型掺杂的石墨烯,并且与微观测量结果一致。通过单一元素掺杂得到了不同掺杂类型的石墨烯。采用这种方法得到样品中,石墨型与吡啶型N缺陷会各自形成独立地畴。同时,采用液态吡啶源,使用常压CVD方法也制备出了单层氮掺杂石墨烯,并观察到了石墨型和吡啶型N缺陷的存在。不同的是,二者是可以距离得很近,而不是形成独立地畴。第五章,采用氩离子溅射的方法,在绝缘体衬底上的石墨烯表面成功制备出了单空位、双空位、四空位及其他一些点缺陷结构。基于STM研究我们发现,经过优化溅射参数以后,得到的大多数的缺陷结构都是单空位,而其中超过80%的单空位都是具有相同的取向,也就是说,缺失原子都是来源于同一套子格子的。通过STS研究,我们确定了空位结构的电子学性质。更多还原

【Abstract】 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.更多还原