Showing papers on "Graphene published in 1995"

Unraveling Nanotubes: Field Emission from an Atomic Wire

Abstract: Field emission of electrons from individually mounted carbon nanotubes has been found to be dramatically enhanced when the nanotube tips are opened by laser evaporation or oxidative etching. Emission currents of 0.1 to 1 microampere were readily obtained at room temperature with bias voltages of less than 80 volts. The emitting structures are concluded to be linear chains of carbon atoms, Cn, (n = 10 to 100), pulled out from the open edges of the graphene wall layers of the nanotube by the force of the electric field, in a process that resembles unraveling the sleeve of a sweater.

A structure model and growth mechanism for multishell carbon nanotubes.

Abstract: A model that postulates a mixture of scroll-shaped and concentric, cylindrical graphene sheets is proposed to explain the microstructure of graphite multishell nanotubes grown by arc discharge. The model is consistent with the observed occurrence of a relatively small number of different chiral angles within the same tubule. The model explains clustering in a natural way and is consistent with the observation of asymmetric (0002) lattice fringe patterns and with the occurrence of singular fringe spacings larger than c/2 (c is the c parameter of graphite) in such patterns. Anisotropic thermal contraction accounts for the 2 to 3 percent increase in the c parameter of nanotubes, compared with bulk graphite, but is too small to explain the singular fringe spacings. The model also explains the formation of multishell closure domes. Nucleation is attributed to the initial formation of a fullerene "dome."

Nanodispersed silicon in pregraphitic carbons

Abstract: Using chemical‐vapor deposition nanodispersed silicon has been prepared in carbon at temperatures between 850 and 1050 °C. Samples with up to 11% atomic silicon in carbon show the same pregraphitic x‐ray‐diffraction pattern as those without silicon. X‐ray‐absorption spectroscopy shows that the silicon is bonded mostly to carbon neighbors and that large clusters of silicon are not found. It is believed that silicon atoms, or small clusters of a few silicon atoms, are located in regions of ‘‘unorganized carbon’’ which separate small regions of organized graphene layers. These materials may have application as electrode materials in advanced rechargeable lithium batteries.

Electron microscopy study of coiled carbon tubules

Abstract: Carbon fibres prepared by the catalytic decomposition of acetylene over finely dispersed cobalt, supported on amorphous silica, are often helix shaped. The structure of such carbon fibres was studied by means of various electron diffraction techniques and by electron microscopy. It was shown that the tubules are hollow and consist of concentric cylindrical graphene sheets. The diffraction patterns are analysed in detail and compared with theoretically predicted patterns. Good semi-quantitative agreement is found. It is shown that the helices are polygonized; their atomic structure is consistent with models predicted on the basis of molecular dynamics simulations.

Efficient cleavage of carbon graphene layers by oxidants

Abstract: The graphene layers at carbon nanotube cap regions are efficiently cleaved by several oxidants (e.g., KMnO4, OsO4 and RuO4) in acidic solution at 100 °C, but not by stronger oxidants, such as K2Cr2O7–H+ and H2O2–H+.

Evidence for glide and rotation defects observed in well-ordered graphite fibers

Abstract: New structural features observed in heat-treated vapor-grown carbon fibers (VGCF’s), produced by the thermal decomposition of hydrocarbon vapor, are reported using image analysis of the lattice plane structure observed by transmission electron microscopy (TEM) and atomic force microscopy (AFM). The TEM lattice image of well-ordered graphite fibers (heat-treated VGCF’s at 2800 °C) was treated by a two-dimensional fast Fourier transform, showing sharp bright spots associated with the 002 and 100 lattice planes. The heat-treated VGCF’s consist of a polygonally shaped shell, and the long and short fringe structures in the TEM lattice image reflect the 002 and 100 lattice planes, respectively. From this analysis, new facts about the lattice structure are obtained visually and quantitatively. The 002 lattice planes remain and are highly parallel to each other along the fiber axis, maintaining a uniform interlayer spacing of 3.36 A. The 100 lattice planes are observed to make several inclined angles with the 002 lattice planes relative to the plane normals, caused by the gliding of adjacent graphene layers. This work visually demonstrates coexistence of the graphitic stacking, as well as the gliding of the adjacent graphene layers, with a gliding angle of about 3–20°. These glide planes are one of the dominant stacking defects in heat-treated VGCF’s. On the other hand, turbostratic structural evidence was suggested by AFM observations. The structural model of coexisting graphitic, glide, and turbostratic structures is proposed as a transitional stage to perfect three-dimensional stacking in the graphitization process. These structural features could also occur in common carbons and in carbon nanotubes.

Cross-sectional high-resolution transmission electron microscopy study of the structures of carbon nanotubes

Abstract: The structures of carbon nanotubes are investigated by cross-sectional high-resolution transmission electron microscopy. Some results for longitudinal sections are also included. No examples of perfect seamless tubes or of “scroll-type” Archimedean spirals are found; in each case examined, papier m[acaron]he-like aggregates of graphene sheets occur, each of finite extent. There is a strong tendency for the structures to relax, locally at least, towards the three-dimensional structure of graphite. Thus polygonal rather than circular or spherical geometries predominate. Some comments are offered concerning a realistic model for the structures, as well as for understanding the electronic properties of multilayer carbon nanotubules. It is proposed that our specimens grew in an atmosphere rich in single graphene sheets, rather than C60 cages.

Graphite: Flat, Fibrous and Spherical

Abstract: The paradigm for the structure of graphite is that of a staggered stacking of flat layers of carbon atoms (Figure 6-1). Individual layers, sometimes referred to as graphene sheets,1 are weakly bonded to each other and are composed of strongly bonded carbon atoms at the vertices of a network of regular hexagons in a honeycomb pattern.2 Both the properties and the morphology of graphite reflect its highly anisotropic structure. Due to the strong bonding within layers and the weak bonding between layers, the growth of graphite takes place predominantly along the edges of the layers (perpendicular to the c axis) and only very slowly normal to the layers (parallel to the c axis). As a result of the growth rate anisotropy, the anisotropic surface energy, and the crystallographic symmetry, the expected morphology for graphite crystals is that of tabular hexagonal prisms.3 However, well-formed natural crystals, such as shown in Figure 6-2, are rare,4 and it has been said that near-ideal crystals of graphite may be rarer than diamonds.5 Well-formed, laboratory-grown crystals of graphite are also uncommon. During the 1960s, graphite crystals from Ticonderoga, New York, and Sterling Hill, New Jersey, became a standard of perfection for experiments and for comparison with laboratory-grown crystals.6,7

Scanning tunneling microscopy current-voltage characteristics of carbon nanotubes

Abstract: Scanning tunneling microscopy (STM) has been used to obtain images and current–voltage (I–V) curves of carbon nanotubes produced by arc discharge of carbon electrodes. The STM I–V curves indicate that carbon nanotubes with diameters from 2.0 to 5.1 nm have a metallic density of states. Using STM, we also observe nanometer‐size graphene sheets which are four graphite layers thick. The STM images of carbon nanotubes are in good agreement with transmission electron microscope images.

Quantitative Elemental Distribution Image of a Carbon Nanotube

Abstract: Energy-filtering transmission electron microscopy was applied to carbon nanotubes in order to investigate quantitative property of elemental maps obtained by inelastically scattered electrons corresponding to the carbon K-edge. An 1 MeV high-resolution electron microscope (JEOL, ARM-1000) equipped with a GATAN imaging filter was employed. Because of a cylindrical structure of nanotubes the number of carbon atoms contributing to the image changes across the tube axis. We could detect the contrast difference due to 20 carbon atoms in the carbon distribution image of 6 layers tube. Furthermore, we examined the carbon mapping from a conical tip region with progressive closure of carbon layers, where an intensity profile clearly distinguishes the difference of 6 graphene sheets. From the consideration of signal-to-noise ratio, the detection limit is concluded to be less than 22 carbon atoms in the present experimental conditions.