Triphenylene adsorption on Cu(111) and relevant graphene self-assembly

Investigations on adsorption behavior of triphenylene(TP) and subsequent graphene self-assembly on Cu(111) were carried out mainly by using scanning tunneling microscopy(STM).At monolayer coverage,TP molecules formed a longrange ordered adsorption structure on Cu(111) with an uniform orientation.Graphene self-assembly on the Cu(111) substrate with TP molecules as precursor was achieved by annealing the sample,and a large-scale graphene overlayer was successfully captured after the sample annealing up to 1000 K.Three different Moire patterns generated from relative rotational disorders between the graphene overlayer and the Cu(111) substrate were observed,one with 40 rotation between the graphene overlayer and the Cu(111) substrate with a periodicity of 2.93 nm,another with 70 rotation and 2.15 nm of the size of the Moire supercell,and the third with 100 rotation with a periodicity of 1.35 nm.     

Adsorption of L-Alanine on Cu（111） Studied by Scanning Tunnelling Microscopy

The adsorption of L-alanine on Cu（111） surface is studied by means of scanning tunnelling microscopy under ultra-high vacuum conditions. The results show that the adsorbates are chemisorbed on the surface, and can form a two-dimensional gas phase, chain phase and solid phase, depending on deposition rate and amount. The adsorbed molecules can be imaged as individual protrusions and parallel chains in gas and chain phases respectively. It is also found that alanine can form （2 × 2） superstructure on Cu（111） and copper step facet to （110） directions in solid phase. On the basis of our scanning tunnelling microscopic images, a model is proposed for the Cu（111）（2 ×2）-alanine superstructure. In the model, we point out the close link between （110）-direction hydrogen bond chains with the same direction copper step faceting.     

高质量大面积石墨烯的化学气相沉积制备方法研究

石墨烯因其奇特的能带结构和优异的物理性能而成为近年来大家研究的热点, 但是目前单层石墨烯的质量与尺寸制约了其实际应用的发展. 本文采用常压化学气相沉积(CVD)方法, 基于铜箔衬底, 利用甲烷作为碳源制备了高质量大面积的单层与多层石墨烯. 研究发现: 高温度、稀薄的甲烷浓度、较短的生长时间以及合适的气体流速是制备高质量、大面积石墨烯的关键. Raman光谱, 扫描电子显微镜、透射电子显微镜等表征结果表明: 制备的石墨烯主要为单层, 仅铜箔晶界处有少量多层石墨烯. 电学测试表明CVD制备的石墨烯在低温时呈现出较明显的类半导体特性; 薄膜电阻随外界磁场的增大而减小.     

Epitaxial growth of graphene on transition metal surfaces: chemical vapor deposition versus liquid phase deposition

The epitaxial growth of graphene on transition metal surfaces by ex situ deposition of liquid precursors (LPD, liquid phase deposition) is compared to the standard method of chemical vapor deposition (CVD). The performance of LPD strongly depends on the particular transition metal surface. For Pt(111), Ir(111) and Rh(111), the formation of a graphene monolayer is hardly affected by the way the precursor is provided. In the case of Ni(111), the growth of graphene strongly depends on the applied synthesis method. For CVD of propene on Ni(111), a 1 × 1 structure as expected from the vanishing lattice mismatch is observed. However, in spite of the nearly perfect lattice match, a multi-domain structure with 1 × 1 and two additional rotated domains is obtained when an oxygen-containing precursor (acetone) is provided ex situ.     

Effect of copper surface morphology on grain size uniformity of graphene grown by chemical vapor deposition

The graphene grain boundaries (GGBs) of polycrystalline graphene grown by chemical vapor deposition (CVD) typically constitute a major reason of deterioration of the electrical properties of graphene-based devices. To reduce the density of GGB by increasing the grain size, CVD growth conditions with a reduced CH₄ flow rate have been widely applied and, recently, electropolishing of copper (Cu) foil substrates to flatten the surface has been undertaken prior to graphene growth. In this study, we show that polycrystalline graphene layer grown on typical Cu foil features two heterogeneous regions with different average grain sizes: small-grain regions (SGRs) and large-grain regions (LGRs). Statistical analysis of the grains of the graphene layers grown under different process conditions showed that SGRs (which form on Cu striations) limit the average grain size, the ability to control the grain size through adjustment of growth conditions, and global grain-size uniformity. Analysis showed that the surface-flattening process significantly improves grain-size uniformity, and monolayer coverage, as well as the average grain size. These results suggest that a process for flattening the surfaces of Cu substrates is critical to controlling the quality and uniformity of CVD-grown graphene layers for practical device applications.     

化学气相沉积法制备石墨烯的铜衬底预处理研究

铜作为一种在化学气相沉积法制备石墨烯中被广泛采用的衬底材料,其表面形貌和质量对石墨烯的品质有较大的影响.提出了一种简单有效的铜衬底预处理方法,在生长石墨烯前,将铜衬底在浓度为1 mol/L的硝酸铁水溶液中进行预刻蚀,研究了不同刻蚀时间的影响.发现当预刻蚀时间为90 s时,经石墨烯生长后得到了相对平整且无杂质颗粒的表面;与盐酸预刻蚀及电化学抛光方法进行了比较,实验结果表明,硝酸铁溶液预刻蚀的效果优于盐酸处理,可与电化学抛光效果比拟,且操作更为简单快捷.经过不同型号铜衬底实验验证,此方法具有普遍适用性.     

不同衬底条件下石墨烯结构形核过程的晶体相场法研究

利用可描述气-固转变的三模晶体相场模型,在原子尺度上研究了不同衬底条件下石墨烯结构的形核过程.结果表明:无论衬底存在与否,气态原子均是先聚集为无定形过渡态团簇,随着气态原子的不断堆积和固相团簇中原子位置的不断调整,过渡态团簇逐渐转变为有序的石墨烯晶核,在此过程中,五元环结构具有重要的过渡作用;石墨烯在结构匹配较好的衬底(如面心立方(face-centered cubic,FCC)结构(111)和(110))上生长时,可形成几乎没有结构缺陷单晶石墨烯岛;在无衬底或结构匹配性较差的衬底(如FCC结构(100)面)上生长时,形成的石墨烯岛结构缺陷和晶界较多,不利于高质量石墨烯的制备.     

Formation of nanoislands in the course of copper deposition on a Cu(111)-(9 × 9)-Ag surface

The process of copper deposition on a structured Cu(111)-(9 × 9)-Ag surface, which represents a (9 × 9) loop dislocation network, is studied by scanning tunneling microscopy. It is found that, when the substrate temperature is 100 K and the copper coverage is 0.1–0.4 of a monolayer, islands of a size no greater than 50 Å are formed at the Ag/Cu(111) interface. The islands remain stable as the sample is heated to room temperature. The shape and boundaries of the nanoislands follow the initial surface superstructure and are determined by the nonuniformity of the interaction of the upper silver layer with the copper substrate. The mechanism of island formation and the origin of their stability are explained in terms of the atom exchange between the adsorbate and substrate.     

Scanning tunneling microscopy investigation of leucine and asparagine adsorbed on Cu(1 1 1)

Adsorption of leucine and asparagine on Cu(1 1 1) surface has been studied by means of scanning tunneling microscopy (STM) in ultra-high vacuum (UHV) conditions. It has been found that leucine can form (3 × 2) superstructure on Cu(1 1 1) and copper steps facet to 〈1 1 0〉 directions. On the basis of our STM images, model has been proposed for the Cu(1 1 1)(3 × 2)-leucine superstructure. The model explains quite naturally the motivation behind the step faceting process. On the other hand, asparagine cannot form any ordered superstructure on Cu(1 1 1) and no copper step faceting can be observed either. The role of amino group in the side chain played in asparagine adsorption geometry is discussed in this paper.     

Synthesis of high-quality monolayer graphene by low-power plasma

The growth of high-quality graphene on copper substrates has been intensively investigated using chemical vapor deposition (CVD). It, however, has been considered that the growth mechanism is different when graphene is synthesized using a plasma CVD. In this study, we demonstrate a dual role of hydrogen for the graphene growth on copper using an inductively coupled plasma (ICP) CVD. Hydrogen activates surface-bound carbon for the growth of high-quality monolayer graphene. In contrast, the role of an etchant is to manipulate the distribution of the graphene grains, which significantly depends on the plasma power. Atomic-resolution transmission electron microscopy study enables the mapping of graphene grains, which uncovers the distribution of grains and the number of graphene layers depending on the plasma power. In addition, the variation of electronic properties of the synthesized graphene relies on the plasma power.     

Structural determinations of the Rh(100) and Cu(111) surfaces using the reliability factor proposed for LEED by Zanazzi and Jona

The reliability factor (R) proposed for LEED by Zanazzi and Jona has been applied to experimental and calculated intensities for the (100) surface of rhodium and for the (111) surface of copper. The calculations used the dynamical perturbation programs of Van Hove and Tong. For each metal, phase shifts were calculated both from a band structure potential and from a potential calculated with a 13 atom cluster. For Cu(111) the I(E) curves from the two potentials were indistinguishable visually and gave similar minimum R values (0.132 and 0.136); the two potentials used for rhodium showed somewhat greater differences. The approach described by Zanazzi and Jona has been supplemented by a simple statistical analysis of the errors involved in the predicted geometries. This study indicates that the topmost interlayer spacing in Cu(111) is contracted by 4.1 ± 0.6% from the bulk value; in Rh(100) the top spacing equals the bulk value to within 3%.     

Epitaxial growth of semiconductor thin films on metals in the halogenation process. Atomic structure of copper iodide on the Cu(110) surface

The atomic structure of thin (7–20 Å) copper iodide layers formed on the Cu(110) surface during a chemical reaction with molecular iodine in ultrahigh vacuum has been studied with scanning tunneling microscopy. A double stripe superstructure with an average period of 90–100 Å was found on the surface of CuI. The structural model is proposed for the copper iodide surface taking into account the contraction of the CuI lattice and the formation of striped domain walls.     

甘氨酸在Cu(111)表面吸附的扫描隧道显微镜研究

用超高真空扫描隧道显微镜(UHV-STM)研究了室温下甘氨酸在Cu(111)表面的吸附行为.实验发现,在低覆盖度下甘氨酸分子在表面表现为二维气体.当覆盖度足够高时,甘氨酸分子最终会形成二维固相结构,为(4×8)超结构.针对这种结构提出了两种可能的结构模型,模型能够很好地解释STM图.当覆盖度介于气相和固相之间时,根据蒸镀条件和退火条件的不同,表面可能出现两种不同的中间相,一种为条纹结构,另一种为六角结构,对于中间相有待于进一步的研究 关键词： 表面吸附 甘氨酸 铜 扫描隧道显微术     

石墨烯-六方氮化硼面内异质结构的扫描隧道显微学研究

石墨烯-六方氮化硼面内异质结构因可调控石墨烯的能带结构而受到广泛关注. 本文介绍了在超高真空体系内, 利用两步生长法在两类对石墨烯分别有强和弱电子掺杂的基底, 即Rh(111)和Ir(111)上制备石墨烯-六方氮化硼单原子层异质结构. 通过扫描隧道显微镜及扫描隧道谱对这两种材料的形貌和电子结构进行研究发现: 石墨烯和六方氮化硼倾向于拼接生长形成单层的异质结构, 而非形成各自分立的畴区; 在拼接边界处, 石墨烯和六方氮化硼原子结构连续无缺陷; 拼接边界多为锯齿形型, 该实验结果与密度泛函理论计算结果相符合; 拼接界面处的石墨烯和六方氮化硼分别具有各自本征的电子结构, 六方氮化硼对石墨烯未产生电子掺杂效应.     

Phonon dispersion of quasi-freestanding graphene on Pt(111)

High-resolution electron energy loss spectroscopy has been used to probe phonon dispersion in quasi-freestanding graphene epitaxially grown on Pt(111). Loss spectra clearly show different dispersing features related to both acoustic and optical phonons. The present results have been compared with graphene systems which strongly interact with the substrate, i.e. the nearly-flat monolayer graphene (MLG)/Ni(111) and the corrugated MLG/Ru(0001). We found that the phonon dispersion of graphene/Pt(111) reproduces well the behavior of pristine graphite. This could be taken as an indication of the negligible interaction between the graphene sheet and the underlying Pt substrate. The softening of out-of-plane modes observed for interacting graphene/metal interfaces does not occur for the nearly-free-standing graphene/Pt(111).     

Electronic structure of the Au-intercalated graphene/Ni(111) surface

We report the electronic structure of the Au-intercalated graphene/Ni(111) surface using angle-resolved photoemission spectroscopy and low energy electron diffraction. The graphene/Ni(111) shows no Dirac cone near the Fermi level and a relatively broad C 1s core level spectrum probably due to the broken sublattice symmetry in the graphene on the Ni(111) substrate. When Au atoms are intercalated between them, the characteristic Dirac cone is completely recovered near the Fermi level and the C 1s spectrum becomes sharper with the appearance of a 10?×?10 superstructure. The fully Au-intercalated graphene/Ni(111) surface shows a p-type character with a hole pocket of ～0.034?Å^?1 diameter at the Fermi level. When the surface is doped with Na and K, a clear energy gap of ～0.4?eV is visible irrespective of alkali metal.     

Multi-oriented moiré superstructures of graphene on Ir(111): experimental observations and theoretical models

Six types of moiré superstructures of graphene on Ir(111) with different orientations (labeled as R0, R14, R19, R23, R26 and R30) are investigated by low-energy electron diffraction, scanning tunneling microscopy and first-principles calculations. The moiré superstructure of R0 graphene has remarkable diffraction spots and deeper corrugation than that of the other superstructures. A high-order commensurate (HOC) method is applied to produce a list of all possible graphene moiré superstructures on Ir(111). Several useful structural data including the precise matrices of the moiré patterns are revealed. Density functional theory based first-principles calculations that include van der Waals interactions reveal the differences of the geometric environment and electronic structures of carbon atoms with respect to the underlying Ir(111) lattices for all the observed moiré patterns. The further calculations of electronic properties at the graphene-Ir interfaces show that the electron transfers for all superstructures are small and of the same order of magnitude, which demonstrates a weak interaction between graphene and the Ir(111) substrate, leading to the coexistence of multi-oriented moiré superstructures.     

Xenon monolayer structures on copper and silver

LEED studies of xenon monolayers at 77K on (111), (100) and (110) faces of copper and (111), (110) and (211) faces of silver show that the xenon atoms are hexagonally close-packed (or nearly so) on each surface, and that the surface area per adatom is about 17Ų. The adsorbate layer is epitaxially related to the substrate but is in full registry only on Cu (111). Surface potential values are consistent with those already reported for annealed polycrystalline films of copper and silver indicating that the latter are not specifically related to surface roughness.     

Study of chemisorption on copper low-index and stepped surfaces

Adsorption of CHCl₃, O₂, and hydrocarbons has been studied on Cu(111) and stepped surfaces using LEED, AES, and UPS at room temperature. We find that ordered Cl overlayers form upon Cu(111), Cu[3(111) × (100)], and Cu[5(111) × (100)] surfaces upon exposure to CHCl₃. Exposure to O₂ results in rearrangement of the Cu[5(111) × (100)] surface to hill-and-valley regions with large (111) areas, whereas Cu[2(111) × (100)] is stable for the same exposure. The photoemission spectra show new energy levels due to C1 above and below the Cu d band region and a small splitting of the halogen p orbitals. Effects consistent with interaction with the Cu d band are observed. Similar effects are observed with oxygen adsorption. The initial rate of Cl or O₂ chemisorption as measured by photoemission is proportional to the density of steps on these surfaces. Apparently, structural effects play an important role in chemisorption on metals (such as copper) with low density of states at the Fermi energy.     

硅晶体表面石墨烯褶皱形貌的分子动力学模拟研究

本文利用分子动力学的方法和模拟退火技术从原子尺度分析研究了Si (100), Si (111)和Si (211)表面单原子层石墨烯的褶皱形貌及其演化特点. 研究表明, 分别置于Si晶体的三种不同原子表面的石墨烯都展现出原子尺度的褶皱形貌. 石墨烯与Si晶体表面原子的晶格失配是引起石墨烯褶皱的主要原因. 研究发现, Si晶体表面石墨烯的褶皱形貌强烈的依赖于退火温度. 石墨烯的褶皱形貌还将直接影响其在Si晶体表面的吸附稳定性. 这些研究结果有助于人们认识基于Si晶体衬底的石墨烯的结构形貌及其稳定性, 为石墨烯的进一步应用提供理论参考.