Showing papers on "Graphene published in 2001"

Energy Gaps in "Metallic" Single-Walled Carbon Nanotubes

Abstract: Metallic single-walled carbon nanotubes have been proposed to be good one-dimensional conductors. However, the finite curvature of the graphene sheet that forms the nanotubes and the broken symmetry due to the local environment may modify their electronic properties. We used low-temperature atomically resolved scanning tunneling microscopy to investigate zigzag and armchair nanotubes, both thought to be metallic. “Metallic” zigzag nanotubes were found to have energy gaps with magnitudes that depend inversely on the square of the tube radius, whereas isolated armchair tubes do not have energy gaps. Additionally, armchair nanotubes packed in bundles have pseudogaps, which exhibit an inverse dependence on tube radius. These observed energy gaps suggest that most “metallic” single-walled nanotubes are not true metals, and they have implications for our understanding of the electronic properties and potential applications of carbon nanotubes.

Very High Surface Area Microporous Carbon with a Three-Dimensional Nano-Array Structure: Synthesis and Its Molecular Structure

Abstract: By using zeolite Y as a template, a novel microporous carbon with a three-dimensional nano-array structure was prepared. The arrangement and periodicity of this array are identical to those of supercages in zeolite Y crystal. The carbon has a surprisingly high BET surface area (3600 m2/g) with almost no mesoporosity. This carbon may be comprised of very curved graphene sheets whose faceting and curvature are accommodated to the curved inner surface of the zeolite nanochannels.

Ghost Excitonic Insulator Transition in Layered Graphite

Abstract: Some unusual properties of layered graphite, including a linear energy dependence of the quasiparticle damping and weak ferromagnetism at low doping, are explained as a result of the proximity of a single graphene sheet to the excitonic insulator phase which can be further stabilized in a doped system of many layers stacked in the staggered ( $\mathrm{ABAB}\dots{}$) configuration.

Phonons and Thermal Properties of Carbon Nanotubes

Abstract: The thermal properties of carbon nanotubes display a wide range of behaviors which are related both to their graphitic nature and their unique structure and size. The specific heat of individual nanotubes should be similar to that of two-dimensional graphene at high temperatures, with the effects of phonon quantization becoming apparent at lower temperatures. Inter-tube coupling in SWNT ropes, and interlayer coupling in MWNTs, should cause their low-temperature specific heat to resemble that of three-dimensional graphite. Experimental data on SWNTs show relatively weak inter-tube coupling, and are in good agreement with theoretical models. The specific heat of MWNTs has not been examined theoretically in detail. Experimental results on MWNTs show a temperature dependent specific heat which is consistent with weak inter-layer coupling, although different measurements show slightly different temperature dependences. The thermal conductivity of both SWNTs and MWNTs should reflect the on-tube phonon structure, regardless of tube-tube coupling. Measurements of the thermal conductivity of bulk samples show graphite-like behavior for MWNTs but quite different behavior for SWNTs, specifically a linear temperature dependence at low T which is consistent with one-dimensional phonons. The room-temperature thermal conductivity of highly aligned SWNT samples is over 200 W/mK, and the thermal conductivity of individual nanotubes is likely to be higher still.

Electron-electron interactions in graphene sheets

Abstract: The effects of the electron-electron interactions in a graphene layer are investigated. It is shown that short-range couplings are irrelevant and scale towards zero at low energies, due to the vanishing of density of states at the Fermi level. Topological disorder enhances the density of states and can lead to instabilities. In the presence of sufficiently strong repulsive interactions, p-wave superconductivity can emerge.

Theory of thermal conduction in thin ceramic films

Abstract: The theory of heat conduction in ceramics by phonons, and at high temperatures also by infrared radiation, is reviewed. The phonon mean free path is limited by three-phonon interactions and by scattering of various imperfections. Point defects scatter high-frequency phonons; extended imperfections, such as inclusions, pores, and grain boundaries, affect mainly low-frequency phonons. Thermal radiation is also scattered by imperfections, but of a larger size, such as splat boundaries and large pores. Porosity also reduces the effective index of refraction. For films there are also external boundaries, cracks, and splat boundaries, depending on the method of deposition. Examples discussed are cubic zirconia, titanium oxide, and uranium oxide. Graphite and graphene sheets, with two-dimensional phonon gas, are discussed briefly.

Electronic Properties, Junctions, and Defects of Carbon Nanotubes

Abstract: The nanometer dimensions of the carbon nanotubes together with the unique electronic structure of a graphene sheet make the electronic properties of these one-dimensional structures highly unusual. This chapter reviews some theoretical work on the relation between the atomic structure and the electronic and transport properties of single-walled carbon nanotubes. In addition to the ideal tubes, results on the quantum conductance of nanotube junctions and tubes with defects will be discussed. On-tube metal-semiconductor, semiconductor-semiconductor, and metal-metal junctions have been studied. Other defects such as substitutional impurities and pentagon-heptagon defect pairs on tube walls are shown to produce interesting effects on the conductance. The effects of static external perturbations on the transport properties of metallic nanotubes and doped semiconducting nanotubes are examined, with the metallic tubes being much less affected by long-range disorder. The structure and properties of crossed nanotube junctions and ropes of nanotubes have also been studied. The rich interplay between the structural and the electronic properties of carbon nanotubes gives rise to new phenomena and the possibility of nanoscale device applications.

Scanning tunneling microscope study of boron-doped highly oriented pyrolytic graphite

Abstract: Atomically resolved scanning tunneling microscopy (STM) results are shown for substitutionally doped boron atoms in the hexagonal carbon network of highly oriented pyrolytic graphite (HOPG). STM images of boron-doped HOPG reveal not only a clear change in the electronic structure of the surface graphene network, but also one that directly affects the electronic structure of the graphene layer from the HOPG surface which is very exceptional for STM measurements. The boron atom site in the graphene network appears as the brightest area in the image including the six adjacent carbon atoms which have relatively higher intensity than normal carbon atoms in the STM image. The average boron-to-boron distance in the basal plane is consistent with Raman spectroscopy results. These structural results suggest that the graphite planes can be tailor made both atomically and electronically, and that boron doping can contribute to controlling the properties of the hexagonal carbon network in order to modify its properti...

Comparison Between the Electrochemical Behavior of Disordered Carbons and Graphite Electrodes in Connection with Their Structure

Abstract: This work relates to a rigorous study of the surface chemistry (Fourier transform infrared, X-ray photoelectron spectroscopy), crystal structure (X-ray diffraction), galvanostatic, cyclic voltammetric, and impedance behavior of lithiated carbon electrodes in commonly used liquid electrolyte solutions. Two different types of disordered carbons and graphite as a reference system, were explored in a single study. All three types of carbons develop a similar surface chemistry in alkyl carbonate solutions, which are dominated by reduction of solvent molecules and anions from the electrolyte. The differences in the crystal structure of these carbons lead to pronounced differences in the mechanisms of Li insertion into them Whereas Li-ion intercalation into graphite is a staged process, Li-ion insertion into the disordered carbons occurs in the form of adsorption on both sides of the elementary graphene flakes and on their edges. The electroanalytical behavior of the disordered carbons was found to correlate well with their unique structure described in terms of the butterfly model. Both types of the disordered carbons reveal exceptionally good cyclability in coin-type cells (vs Li counter electrodes), with only moderate capacity fading. Highly resolved plots of the chemical diffusion coefficient of Li-ions. D vs. potential E, for the disordered carbon electrodes were obtained. Surprisingly, a maximum in D appears on these plots at intermediate levels of Li-ion insertion corresponding to ca. 0.4-0.5 V (vs. Li/Li + ). We propose that these maxima may originate from a combination of two effects, (i) repulsive interactions between the inserted species, and (ii) pronounced heterogeneity of Li insertion sites in terms of carbon-Li interactions and Li-ion mobility.

Molecular simulation of xenon adsorption on single-walled carbon nanotubes

Abstract: Adsorption of xenon on single-walled (10,10) carbon nanotubes at a temperature of 95 K has been studied by molecular simulation and the results have been compared with recent experiments [A. Kuznetsova, J. T. Yates, Jr., J. Liu, and R. E. Smalley, J. Chem. Phys. 112, 9590 (2000)]. Simulations indicate that adsorption takes place primarily on the inside of the nanotubes at the experimental conditions. Interstitial and external adsorption were found to be negligible in comparison with adsorption inside the nanotubes. The coverage computed from simulation of 0.06 Xe–C is in good agreement with the experimentally measured value of 0.042 Xe–C. The isosteric heat of adsorption from simulation ranges from about 3000 to 4500 K as a function of coverage, which is consistent with the experimental desorption activation energy of 3220 K. Adsorption on the external surfaces of the nanotubes is observed to take place at Xe pressures that are larger than those probed in the experiments. The good agreement between simulations and experiments for the coverage and heat of adsorption indicate that the curvature of the nanotube does not substantially perturb the adsorption potential from that of a graphene sheet.

Thermochemistry of fluorinated single wall carbon nanotubes.

Abstract: The gradient corrected Perdew−Burke−Ernzerhof density functional in conjunction with a 3-21G basis set and periodic boundary conditions was employed to investigate the geometries and energies of C2F fluorinated armchair single wall carbon nanotubes (F−SWNT's) with diameters ranging from 16.4 to 4.2 A [(12,12) to (3,3)] as well as a C2F graphene sheet fluorinated on one side only. Using an isodesmic equation, we find that the thermodynamic stability of F−SWNT's increases with decreasing tube diameter. On the other hand, the mean bond dissociation energies of the C−F bonds increase as the tubes become thinner. The C−F bonds in the (5,5) F−SWNT's are about as strong as those in graphite fluoride (CF)n and are also covalent albeit slightly (<0.04 A) stretched. Whereas a fluorine atom is found not to bind covalently to the concave surface of [60]fullerene, endohedral covalent binding is possible inside a (5,5) SWNT despite a diameter similar to that of the C60 cage.

Nanoporous Metal Oxides Synthesized by the Nanoscale Casting Process Using Supercritical Fluids

Abstract: Nanoporous metal oxides (TiO2, Al2O3) have been synthesized using activated carbon templates with supercritical fluid solvents by using the nanoscale casting (NC) process. The precursors were dissolved in supercritical CO2 and attached to activated carbon fibers or powders as templates. After removal of the activated carbon templates by calcination in air at 873 K or by treatment in oxygen plasma, the nanoporous TiO2 or Al2O3 replicating the macroscopic shapes of the activated carbon templates was obtained. The surface area of the titania sample was 387 m2/g. The titania sample crystallized in the anatase form. The alumina samples have mesopores corresponding to the graphene crystallite size of the activated carbon. The alumina samples crystallized in the γ-alumina form.

Electronic interwall interactions and charge redistribution in multiwall nanotubes

Abstract: Using density functional theory, we calculate the charge redistribution incurred upon forming multiwall carbon nanotubes, or by sandwiching initially isolated single-wall nanotubes between graphene layers. In these systems, we observe a significant charge transfer between the \ensuremath{\pi} electron system of the tube walls and a newly formed interlayer state. We discuss the direction of charge flow in terms of the interlayer hybridization and work function differences in the composite systems.

Study of the activation process of Mg-based hydrogen storage materials modified by graphite and other carbonaceous compounds

Abstract: A nanocomposite (Mg–V)nano made of 90 wt% Mg and 10 wt% V was prepared by high-energy ball-milling during 40 h. The activation characteristics of (Mg–V)nano are rather poor, the hydrogen content [H] reaching 4 wt% after more than 100 h (t4wt%) following the initial exposure of the material to H2. Adding 9 wt% graphite to (Mg–V)nano and resuming the milling operation for 30 min leads to the formation of (Mg–V)nano /G, which exhibits a t4wt% value of only 10 min. The addition of more than 9 wt% graphite to (Mg–V)nano does not lead to any significant reduction of the t4wt% value. However, extending the milling period with graphite over 30 min leads to a steady increase in t4wt% and, thus, to a deterioration of the activation characteristics. Comparison of the behavior of graphite with other C-based compounds revealed that perylene (C20H12) and pentacene (C22H14), which are made of linked benzene rings, and thus have a 2D structure similar to that of the graphene sheet, are as effective as graphite in improving the activation characteristics of (Mg–V)nano. A structural investigation of (Mg–V)nano /G as a function of the milling time through both C 1s core-level x-ray photoelectron spectroscopy and C K edge x-ray absorption near-edge spectroscopy has shown that the integrity of graphite is progressively lost as the milling period is extended over 30 min. On the basis of these results, it is hypothesized that the adsorption of graphene layer on freshly created Mg surfaces and the formation of highly reactive C species during milling prevents the re-formation of the surface oxide layer responsible for the poor activation characteristics of untreated (Mg–V)nano

Electron energy-loss spectroscopy of electron states in isolated carbon nanostructures

Abstract: Electronic states of an isolated nano-object made of single graphene layer have been successfully explored by means of electron energy-loss spectroscopy with high sensitivity and high spatial resolution. In further systematic study of various carbon nanostructures, the curvature and/or the interlayer coupling between adjacent graphene layers have been proved to govern the electronic states of carbon nanostructures. Using density functional theory, experimentally observed variations of the carbon $K(1s)$ near-edge fine structure have been ascribed mostly to the increasing curvature of the graphene layers.

Lithium storage in polymerized carbon nitride nanobells

Abstract: Polymerized carbon nitride nanobells (CNNBs) have been intercalated by a large amount of Li ions using an electrochemical method. Li nanocrystals are observed on the inside surface of CNNB walls, which is direct evidence that a Li nanocrystal can exist at the micropore structure at a heavy intercalating level. Graphene layers are expanded and become partly disordered by Li intercalation, while after deintercalation, they are reordered to a certain degree, and Li nanocrystals disappear. The samples show a reversible Li storage capacity of 480 mAhg−1, much higher than 330 mAhg−1 of commercial carbon materials used for Li ion batteries.

A study of the reaction of oxygen with graphite: Model chemistry

Abstract: A considerable amount of research has been directed towards the mechanism of oxidation of graphite as a model reaction system and because of its industrial importance A number of recent studies have been concerned with ab initio molecular orbital calculations on graphite including model chemistry and the reactions with molecular oxygen This study is concerned with oxidation steps involving the attachment of molecular oxygen to the graphene, the formation of carbon monoxide and, in particular, the subsequent oxidation reactions

Lithium Doping of Multiwalled Carbon Nanotubes Produced by Catalytic Decomposition

Abstract: Electrochemical lithium doping into carbon electrodes consisting of multiwalled carbon nanotubes produced by catalytic decomposition of acetylene has been carried out in button cells. Direct observation of the sample by low- and high-resolution transmission electron microscopy reveals that the lithium species are not intercalated between the graphene shells of nanotubes and that the electrochemical process is reversible. 7Li NMR and Raman spectroscopies allow us to study the nature of interactions between lithium atoms and the host material and to propose an insertion mechanism that only implies a storage of lithium at the surface of nanotubes.

Structural and electronic properties of carbon nanotube tapers

Abstract: Since their initial discovery in 1990 by Iijima, 1 carbon nanotubes have come under ever increasing scientific scrutiny. They not only have outstanding mechanical and electrical properties, but also show considerable technological potential as field emitters and electrochemical storage devices. 2 In the emerging field of nanotechnology, carbon nanotubes are playing a crucial role by providing a suitable ‘‘test bed’’ or ‘‘laboratory’’ for materials properties at the nanometer length scale. Single-wall carbon nanotubes are formed when a graphene sheet is curled up into a cylinder and the carbon atoms are joined seamlessly to each other. Nanotubes are therefore characterized by their length, diameter, and helicity. The latter is a measure of the orientation of the graphene sheet as it is folded to form nanotubes. Following the notation of Hamada et al., 3 the structure of a nanotube is described by a pair of integers ( l,m), which give the coordinates of its circumference vector in the basis of the primitive lattice vector of graphene. The helicity is important because it determines both the mechanical and electrical properties of

Low-temperature resistance of individual single-walled carbon nanotubes: A theoretical estimation

Abstract: The resistances of individual single-walled carbon nanotubes (SWNTs) are calculated based on Boltzmann transport equation, graphene approximation relation of electronic energy for SWNTs and weak localization (WL) theory. The theoretical results of resistance for SWNTs are compared with experimental measurements. It has been found that, for nearly metallic and semiconducting SWNTs the theoretical result is consistent with experimental data roughly, whereas for truly metallic SWNTs (armchair ones) the theoretical result is in good agreement with measurement.

The Structure of Lithium Intercalated Graphite Using an Effective Atomic Charge of Lithium

Abstract: Using the experimental graphene layer spacing of the stage I model as a constraint, the effective atomic charge of lithium, δ Li , in lithium-intercalated graphite (LIG) was determined. In order to confirm that lithium in LIG exists in a partially ionic state. quantum mechanical calculations were also carried out for several lithium-carbon systems. Using a fixed δ Li , the graphene layer spacing and structures for hexagonal graphite, stage 3, stage 2, and stage I models, were obtained. The more lithium is intercalated into the graphite, the wider the layer spacing becomes. The distortion of structures due to lithium intercalation was not observed until the stage I model was formed. In stage I and stage 2 models, the graphene layers shifted from ABAB to AAAA stacking as lithium was intercalated to the hexagonal graphite. However, the stage 3 model showed a shift of layers from ABABAB to AB'AABA stacking, where B' and B represent the graphene layers which have shifted slightly from B. Only the graphene layers that have the intercalated lithium layers between them shifted to AA stacking.

More spherical large fullerenes and multi-layer fullerene cages

Abstract: According to the experimental investigation, the carbon nano-particles have spherical multi-layer structure (also called onion-like carbon structure). Theoretically, the optimum structures of these large fullerenes contain highly faceted shapes with icosahedral symmetry. This discrepancy in structure may be attributed to the formation mechanism. Thus, a method is devised to construct spherical large fullerenes (C 240 , C 540 , C 960 , C 2160 , C 2940 , C 3840 , C 4860 ) by using the triangular motif. The 5–7–5–7 shape defect is applied in this method for assembling the large spherical fullerenes which could transform the graphene sheet to a spherical motif via SW rearrangement. The geometry-optimized structures of large spherical fullerenes have been generated by molecular mechanics calculation. Then, the average radius and standard deviation of these large fullerenes were obtained to verify the spherical shape. The multi-layer fullerene with spherical shape was confirmed by the TEM observation. According to the structure analysis, the distance between two neighboring encapsulating carbons is about 3.5 A, which approximately coincides with the distance between two layers of graphite. The van der Waals force per carbon atom and of multi-layer fullerene with the spherical shape generated by force field calculation, predict their relative stability.