Showing papers on "Graphene published in 1996"

Edge state in graphene ribbons: Nanometer size effect and edge shape dependence.

Abstract: Finite graphite systems having a zigzag edge exhibit a special edge state. The corresponding energy bands are almost flat at the Fermi level and thereby give a sharp peak in the density of states. The charge density in the edge state is strongly localized on the zigzag edge sites. No such localized state appears in graphite systems having an armchair edge. By utilizing the graphene ribbon model, we discuss the effect of the system size and edge shape on the special edge state. By varying the width of the graphene ribbons, we find that the nanometer size effect is crucial for determining the relative importance of the edge state. We also have extended the graphene ribbon to have edges of a general shape, which is defined as a mixture of zigzag and armchair sites. Examining the relative importance of the edge state for graphene ribbons with general edges, we find that a non-negligible edge state survives even in graphene ribbons with less developed zigzag edges. We demonstrate that such an edge shape with three or four zigzag sites per sequence is sufficient to show an edge state, when the system size is on a nanometer scale. The special characteristics of the edge state play a large role in determining the density of states near the Fermi level for graphite networks on a nanometer scale.

Hard elastic carbon thin films from linking of carbon nanoparticles

Abstract: HARD carbon thin films find many technological applications—as protective or biocompatible coatings, for instance. A very hard and elastic form of carbon nitride, in which curved graphene sheets are interlinked owing to the presence of small amounts of nitrogen, has recently been reported1. The hardness of these films is thought to arise from the presence of sp3-like bonds that introduce curvature into and bind together the sp2-bonded graphitic planes, rather as they do in hard, highly tetrahedrally bonded amorphous carbon films2–4. Here we show that hard, elastic thin films of pure carbon can be created by depositing closed, hollow graphitic carbon nanoparticles—nanotubes5 and carbon onions6—onto a substrate at high velocity. The particles are apparently disrupted on impact, causing them to link up. Electron-energy-loss spectra reveal a reduction in π (sp2) bonding in the intersecting regions of the nanoparticles, supporting the idea that they are covalently linked by tetrahedral sp3 bonds.

Correlation Between Lithium Intercalation Capacity and Microstructure in Hard Carbons

Abstract: Hard carbons prepared from organic precursors by pyrolysis near 1000°C generally have poorly developed (002) Bragg peaks and also show evidence for microporosity in small angle x‐ray scattering experiments. When the peak‐to‐background ratio of the (002) peak is small, it indicates that hard carbons contain significant amounts of single, bilayer, or trilayer graphene sheets which are arranged at arbitrary angles. These carbons also contain the micropores implied by such a stacking arrangement. With electrochemical measurements on Li/carbon electrochemical cells, we show that those carbons with the largest capacity for lithium are those with the largest fraction of single graphene sheets and with the smallest average micropore size. Over 80 samples prepared from a variety of precursors at a selection of temperatures have been studied. The trends observed may help point the way to better anode materials for advanced Li‐ion batteries.

Synthesis, Extraction, and Purification of Fullerenes

Abstract: This chapter describes the laboratory methods commonly used to synthesize, extract, and purify fullerenes. Fullerene molecules are formed in the laboratory from carbon-rich vapors which can be obtained in a variety of ways, e.g., resistive heating of carbon rods in a vacuum, ac or dc plasma discharge between carbon electrodes in He gas, laser ablation of carbon electrodes in He gas, and oxidative combustion of benzene/argon gas mixtures. Most methods for the production of large quantities of fullerenes simultaneously generate a mixture of stable fullerenes (C 60 , C 70 , …), impurity molecules such as polyaromatic hydrocarbons, and carbon-rich soot. Therefore, the synthesis of fullerenes must be followed by procedures to extract and separate fullerenes from these impurities according to mass, and for the higher fullerenes, separation according to specific isomeric forms may also be required. Fullerenes can be synthesized in the laboratory in a wide variety of ways, all involving the generation of a carbon-rich vapor or plasma. All current methods of fullerene synthesis produce primarily C 60 and C 70 , and these molecules are now routinely isolated in gram quantities and are commercially available. Higher-mass fullerenes and endohedral complexes can also be made and isolated, albeit in substantially reduced amounts. At present the most efficient method of producing fullerenes involves an electric discharge between graphite electrodes in ∼200 torr of He gas. Fullerenes are embedded in the emitted carbon soot and must then be extracted and subsequently purified. A variation of the arc technique is used to synthesize graphene tubules. However, it appears that it will be difficult to extend the chemical methods now used to isolate particular fullerene isomers to separate the carbon tubules according to diameter and chiral angle.

Density of States Calculations of Small Diameter Single Graphene Sheets

Abstract: The densities of states for the {pi}-band of single graphene sheets with small diameters were calculated by employing a linear combination of atomic orbital approach using as the basis set the carbon p{sub z} atomic orbitals together with a modified Hueckel approximation wherein the overlap integrals out to the fourth nearest neighbors set were included. These densities of states were used to predict the voltage of lithiated carbon vs. lithium metal, an important characteristic for disordered carbon used as the negative electrode in rechargeable lithium-ion batteries. Calculations were made for isolated single graphene sheets, C{sub n}, with n = 24, 54, 96, 150, and 216. The results suggested that the lowest voltage should occur for lithiated carbon electrodes composed of single graphene sheets with the smallest diameter ({approx} 0.7 nm for C{sub 24}).

Observation of potassium‐intercalated carbon nanotubes and their valence‐band excitation spectra

Abstract: Second‐stage potassium‐intercalated carbon nanotubes were synthesized in a specially designed ultrahigh vacuum analytical electron microscope and their valence‐band excitation spectra in the region of the π+σ plasmon were measured by electron energy loss spectroscopy. The carbon nanostructures consisted of graphene sheets. Potassium was deposited in an ultrahigh vacuum at room temperature. As a result, a second stage of intercalated nanotubes was found to be formed close to the surface. The energy loss spectra of the intercalated nanotubes showed humps at about 16, 19, and 22 eV, in addition to those of unintercalated tubes. This suggests that intercalation modified the band structure of the interlayer bands and/or the σ(σ*) bands.

Collective plasmon excitations in graphene tubules.

Abstract: Due to the inherent simplicity of the graphene tubule systems, it is expected that these systems will become model systems for the calculation of the mechanical and electronic properties of idealized carbon fibers. In this paper the collective electronic excitations on graphene tubules are discussed. The frequencies of the plasmons on graphene tubules are calculated using an empirical infinitely thin cylindrical-shell model within the framework of a two-fluid hydrodynamic description. There are two parameters involved in our calculation which are calibrated on graphite. The variations of plasmon frequencies and oscillator strength with the size of tube and longitudinal plasmon wavelength are discussed. \textcopyright{} 1996 The American Physical Society.

Local-density-functional study of the fullerenes, graphene and graphite

Abstract: Large basis set local-density-functional calculations on fullerenes, graphene and graphite suggest that fullerene stability increases monotonically with the number of carbon atoms (N), and is nearly linear in 1/N. is less stable energetically than and higher fullerenes, despite the apparent preference for in nature. The heat of formation found here for relative to graphite is in good agreement with experiment, while the heat of formation for graphene relative to graphite is overestimated by a factor of two.

Carbon nanotubes with single-layer walls

Abstract: Macroscopic quantities of single-layer carbon nanotubes have recently been synthesized by co-condensing atomic carbon and iron group or lanthanide metal vapors in an inert gas atmosphere. The nanotubes consist solely of carbon, sp2-bonded as in graphene strips rolled to form closed cylinders. The structure of the nanotubes has been studied using high-resolution transmission electron microscopy. Iron group catalysts, such as Co, Fe, and Ni, produce single-layer nanotubes with diameters typically between 1 and 2 nm and lengths on the order of micrometers. Groups of shorter nanotubes with similar diameters can grow radially from the surfaces of lanthanide carbide nanoparticles that condense from the gas phase. If the elements S, Bi, or Pb (which by themselves do not catalyze nanotube production) are used together with Co, the yield of nanotubes is greatly increased and tubules with diameters as large as 6 nm are produced. Single-layer nanotubes are anticipated to have novel mechanical and electrical properties, including very high tensile strength and one-dimensional conductivity. Theoretical calculations indicate that the properties of single-layer tubes will depend sensitively on their detailed structure. Other novel structures, including metallic crystallites encapsulated in graphitic polyhedra, are produced under the conditions that lead to nanotube growth.

Vibrational modes of carbon nanotubes; spectroscopy and theory

Abstract: Experimental and theoretical studies of the vibrational modes of carbon nanotubes are reviewed. The closing of a 2D graphene sheet into a tubule is found to lead to several new infrared (IR)- and Raman-active modes. The number of these modes is found to depend on the tubule symmetry and not on the diameter. Their diameter-dependent frequencies are calculated using a zone-folding model. Results of Raman scattering studies on arc-derived carbons containing nested or single-wall nanotubes are discussed. They are compared to theory and to that observed for other sp2 carbons also present in the sample.

Vibrational Spectrum of Carbon Nanotubes

Abstract: We calculate the vibrational densities of states (VDOS) for the two kinds of carbon nanotubes (armchair- and zigzag-type), using the recursion method and compare them with that for a single layer of graphite. The results show that the VDOS for the carbon nanotubes have more peaks than that for the graphene layer, but retain the strong peaks corresponded to those of the graphene layer. And the spectrums of the two kinds of nanotubes contain some feature peaks independent of the diameter.

Hemi-toroidal networks in pyrolytic carbon nanotubes

Abstract: Evidence for the formation of an archetypal hemi-toroidal link structure between adjacent concentric walls in pyrolytic carbon nanotubes is presented. The observed and simulated TEM images for such structures are in excellent agreement. This study suggests that double-walled carbon nanotubes, in which the inner and outer tubes are linked by such hemi-toroidal seals, may be one viable way of overcoming the reactivity at the graphene edges of open-ended tubes to engineer stable and useful graphene nanostructures.