Showing papers on "Carbon nanotube published in 1994"

Theory of graphitic boron nitride nanotubes

Abstract: Based upon the similarities in properties between carbon- and BN-based (BN=boron nitride) materials, we propose that BN-based nanotubes can be stable and study their electronic structure. A simple Slater-Koster tight-binding scheme has been applied. All the BN nanotubes are found to be semiconducting materials. The band gaps are larger than 2 eV for most tubes. Depending on the helicity, the calculated band gap can be direct at [Gamma] or indirect. In general, the larger the diameter of the nanotube the larger the band gap, with a saturation value corresponding to the calculated local-density-approximation band gap of hexagonal BN. The higher ionicity of BN is important in explaining the electronic differences between these tubes and similar carbon nanotubes.

Aligned Carbon Nanotube Arrays Formed by Cutting a Polymer Resin—Nanotube Composite

Abstract: A simple technique is described here that produces aligned arrays of carbon nanotubes. The alignment method is based on cutting thin slices (50 to 200 nanometers) of a nanotube-polymer composite. With this parallel and well-separated configuration of nanotubes it should be possible to measure individual tube properties and to demonstrate applications. The results demonstrate the nature of rheology, on nanometer scales, in composite media and flow-induced anisotropy produced by the cutting process. The fact that nanotubes do not break and are straightened after the cutting process also suggests that they have excellent mechanical properties.

Stability and Band Gap Constancy of Boron Nitride Nanotubes

Abstract: Extensive LDA and quasi-particle calculations have been performed on boron nitride (BN) single-wall and multi-wall nanotubes. Strain energies are found to be smaller for BN nanotubes than for carbon nanotubes of the same radius, owing to a buckling effect which stabilizes the BN tubular structure. For tubes larger than 9.5 A in diameter, the lowest conduction band is predicted to be free-electron-like with electronic charge density localized inside the tube. For these tubes, this band is at constant energy above the top of the valence band. Consequently, contrarily to carbon nanotubes, single- and multi-wall BN nanotubes are constant-band-gap materials, independent of their radius and helicity. In addition, we expect them to exhibit remarkable properties under n-type doping.

A simple chemical method of opening and filling carbon nanotubes

Abstract: SINCE carbon nanotubes1 were first synthesized in macroscopic quantities2, it has become possible to explore their physical and chemical characteristics. There has been much speculation3 about the properties of materials encapsulated within the tubes, but experimental studies of this issue require a reliable means of open-ing and filling the tubes. Various approaches have been developed for opening up4–6 the tube ends and encapsulating material4,6,7, but these work only for a limited range of materials or in low yield. Here we describe a general method that allows carbon nanotubes to be opened at the end and filled with a variety of metal oxides using wet chemical techniques. We anticipate that this method will lead to extensive study of the chemistry and physics of filled nanotubes, which might find applications in catalysis, separation and storage technology and in the development of materials with new magnetic and electrical properties.

Capillarity and Wetting of Carbon Nanotubes

Abstract: The wetting and capillarity of carbon nanotubes were studied in detail here. Nanotubes are not "super-straws," although they can be wet and filled by substances having low surface tension, such as sulfur, selenium, and cesium, with an upper limit to this tension less than 200 millinewtons per meter. This limit implies that typical pure metals will not be drawn into the inner cavity of nanotubes through capillarity, whereas water and organic solvents will. These results have important implications for the further use of carbon nanotubes in experiments on a nanometer scale.

Doping Graphitic and Carbon Nanotube Structures with Boron and Nitrogen

Abstract: Composite sheets and nanotubes of different morphologies containing carbon, boron, and nitrogen were grown in the electric arc discharge between graphite cathodes and amorphous boron-filled graphite anodes in a nitrogen atmosphere. Concentration profiles derived from electron energy-loss line spectra show that boron and nitrogen are correlated in a one-to-one ratio; core energy-loss fine structures reveal small differences compared to pure hexagonal boron nitride. Boron and carbon are anticorrelated, suggesting the substitution of boron and nitrogen into the carbon network. Results indicate that singlephaase CyBxNx as well as separated domains (nanosize) of boron nitride in carbon networks may exist.

Relation between metal electronic structure and morphology of metal compounds inside carbon nanotubes

Abstract: SEVERAL attempts have been made to fill carbon nanotubes1 with metals or metallic compounds to obtain nanocomposite materials with potentially interesting properties. Capillary action, predicted2 to be a filling mechanism, has been used3'4 to encapsulate lead and bismuth in open tubes. Compounds of yttrium5, manganese6 and gadolinium7 have also been encapsulated by formation of the nano-tubes in an arc discharge with the metals present in situ. Very recently, Tsang et al.8 showed that oxides of nickel, cobalt, iron and uranium can be encapsulated by opening the tubes and deposit-ing the filling material using wet chemical techniques. Here we report a search for general principles relating to the nature and structure of the filling material, using the arc-discharge method to fill tubes with fifteen metals and/or their compounds: Ti, Cr, Fe, Co, Ni, Cu, Zn, Mo, Pd, Sn, Ta, W, Gd, Dy and Yb. We find that the propensity for forming continuous 'nanowires' throughout the length of the tubes seems to be strongly correlated with the existence of an incomplete electronic shell in the most stable ionic state of the metal. We also find that the interplay between growth of the nanotube and growth of the filling results, in one case, in the formation of an unusual helical filling morphology.

Defects in carbon nanostructures.

Abstract: Previous high-resolution electron microscopy (HREM) observations of the carbon nanotubes have led to a "Russian doll" structural model that is based on hollow concentric cylinders capped at both ends. The structures of the carbon nanotubes and particles were characterized here by bulk physical and chemical property measurements. The individual nanostructure is as compressible as graphite in the c axis, and such nanostructures can be intercalated with potassium and rubidium, leading to a saturation composition of "MC 8 ." These results are counter to expectations that are based on a Russian doll structure. HREM after intercalation with potassium and deintercalation indicates that individual nanoparticles are a "paper-mache" of smaller graphite layers. Direct current magnetization and electron spin resonance measurements indicate that the electronic properties of the nanostructures are distinctly different from those of graphite. Although the nanostructures have distinct morphologies and electronic properties, they are highly defective and have a local structure similar to turbostratic graphite.

Aharonov-Bohm effect in carbon nanotubes

Abstract: Optical absorption spectra are calculated in carbon nanotubes in the presence of a magnetic flux parallel to the tube axis. A drastic change in the band gap manifests itself in optical spectra for light polarization parallel to the axis. In the case of perpendicular polarization, the absorption is suppressed by a large depolarization effect.

Structural properties of a carbon-nanotube crystal.

Abstract: Carbon nanotubes of uniform size may soon be available in macroscopic quantities. Here we examine the ordered condensed phase of these tubes. As the tube diameter varies, the structural properties show a clear transition between two regimes with qualitatively different behavior. Tubes 10 \AA{} and less in diameter behave as rigid cylinders. For diameters over 25 \AA{}, the tubes flatten against each other under the van der Waals attraction, forming a honeycomb structure. This structure exhibits an anomalous rigidity, which does not decrease as expected with increasing tube diameter. Based on reported tube sizes, both regimes should be experimentally accessible.

Growth and sintering of fullerene nanotubes.

Abstract: Carbon nanotubes produced in arcs have been found to have the form of multiwalled fullerenes, at least over short lengths. Sintering of the tubes to each other is the predominant source of defects that limit the utility of these otherwise perfect fullerene structures. The use of a water-cooled copper cathode minimized such defects, permitting nanotubes longer than 40 micrometers to be attached to macroscopic electrodes and extracted from the bulk deposit. A detailed mechanism that features the high electric field at (and field-emission from) open nanotube tips exposed to the arc plasma, and consequent positive feedback effects from the neutral gas and plasma, is proposed for tube growth in such arcs.

Role of sp3 defect structures in graphite and carbon nanotubes

Abstract: THE discovery of fullerenes1,2 and carbon nanotubes3,4 has led to the realization that it should be possible to tailor the properties of graphitic sheets if their geometry can be controlled5–7. In exploring these possibilities, we have found that the folding and tearing of graphitic sheets follow well defined patterns which seem to be governed by the formation of sp3-like line defects in the sp2 graphitic network. Our studies with the atomic force microscope and scanning tunnelling microscope reveal that these folds and tears occur preferentially along the symmetry axes of graphite, and that 'ripples' are observed in the curved portions of the folds. We also see ripples in deformed carbon nanotubes. They lie along the directions for which sp3-like line defects can form most easily to relieve strain. Our ab initio molecular orbital calculations indicate that the ripples stabilize the π-electronic energy in the bent structures with the total energy balance being determined by the amount of nuclear repulsion. These results provide insight into the geometries that graphitic structures will preferentially accommodate, and the properties that might ensue.

Plasmons and optical properties of carbon nanotubes

Abstract: The optical properties of carbon nanotubes are studied within the gradient approximation. The calculated dielectric function exhibits many divergent structures, due to the divergencies in its density of states. As a result, the electron-energy-loss spectrum has many peak structures, including a prominent one at \ensuremath{\sim}6 eV, which is identified as the collective excitations of the \ensuremath{\pi}-band electrons. This plasmon is found to be insensitive to both radius and chiral angle, due to the unique one-dimensional band structure of carbon nanotubes. The result is consistent with the experimental measurements. The reflectance also exhibits many interesting features, including a weak but sharp plasmon edge at \ensuremath{\sim}6 eV.

Single-walled carbon nanotubes produced at high yield by mixed catalysts

Abstract: We report here on the high‐density preparation of single‐wall tubes in the presence of mixed catalysts of the types Fe/Ni and Co/Ni, in the soot as well as in the weblike deposits forming in the chamber. The yield is much higher than previously reported, and gram quantities can be obtained. Diameters cover the range from 0.9 to 3.1 nm, larger than previously reported, with the histogram showing only one peak at 1.7 nm. Evidence of an epitaxial action between C60 and single‐walled nanotubes is presented. Results from the mixed catalysts Co/Cu, Ni/Mg, and Ni/Ti are also reported.

Electrical-resistance of a Carbon Nanotube Bundle

Abstract: The first direct electrical resistance measurements performed on a single carbon nanotube bundle from room temperature down to 0.3 K and in magnetic fields up to 14 T are reported. From the temperature dependence of the resistance above 2 K, it is shown that some nanotubes exhibit a semimetallic behavior akin to rolled graphene sheets with a similar band structure, except that the band overlap, DELTA almost-equal-to 3.7 meV, is about 10 times smaller than for crystalline graphite. In contrast to graphite which shows a constant low-temperature resistivity, the nanotubes exhibit a striking increase of the resistance followed by a broad maximum at very low temperatures. A magnetic field applied perpendicular to the sample axis decreases the resistance. Above 1 K, this behavior is consistent with the formation of Landau levels. At lower temperatures, the resistance shows an unexpected drop at a critical temperature which increases linearly with magnetic field. These striking features could be related to the unique quasi-one-dimensional structure of the carbon nanotubes.

Method of purifying carbon nanotubes

Abstract: A mixture of carbon nanotubes and impurity carbon materials, which include carbon nanoparticles and may possibly include amorphous carbon, is purified into carbon nanotubes of high purity by utilizing a significant difference in oxidizability between the nanotubes and the nanoparticles. The mixture is pulverized and heated in the presence of an oxidizing agent at a temperature in the range from 600° to 1000° C. until the impurity carbon materials are oxidized and dissipated into gas phase. The nanotubes remain almost unoxidized except for loss of some lengths from the tube tips. It is suitable to perform the heating in air or oxygen.

Magnetic Susceptibility of Molecular Carbon: Nanotubes and Fullerite

Abstract: Elemental carbon can be synthesized in a variety of geometrical forms, from three-dimensional extended structures (diamond) to finite molecules (C(60) fullerite). Results are presented here on the magnetic susceptibility of the least well-understood members of this family, nanotubes and C(60) fullerite. (i) Nanotubes represent the cylindrical form of carbon, intermediate between graphite and fullerite. They are found to have significantly larger orientation-averaged susceptibility, on a per carbon basis, than any other form of elemental carbon. This susceptibility implies an average band structure among nanotubes similar to that of graphite. (ii) High-resolution magnetic susceptibility data on C(60) fullerite near the molecular orientational-ordering transition at 259 K show a sharp jump corresponding to 2.5 centimeter-gram-second parts per million per mole of C(60). This jump directly demonstrates the effect of an intermolecular cooperative transition on an intramolecular electronic property, where the susceptibility jump may be ascribed to a change in the shape of the molecule due to lattice forces.

Catalytic Synthesis of Single-Layer Carbon Nanotubes with a Wide Range of Diameters

Abstract: We have synthesized single-layer carbon nanotubes by co-vaporizing cobalt with carbon in an arc fullerene generator and have identified conditions that lead to high yields. The diameter distribution of the tubes and their morphologies are studied using transmission electron microscopy. For nanotubes produced using cobalt and carbon, the tube diameters range from 1 to 2 nm with distribution peaks at 1.3 and 1.5 nm. When sulfur is added to the carbon and cobalt, production of single-layer nanotubes is enhanced and the tubes have a wider range of diameters (from 1 to 6 nm). The diameter distribution for these nanotubes shows prominent peaks at 1.3 and 1.5 nm and additional maxima at 2.7 and 3.6 nm. Cobalt-containing crystallites, some encapsulated in graphitic polyhedra, are produced with the nanotubes and are found in the soot away from the cathode.

Growth energetics of carbon nanotubes.

Abstract: The growth energetics of carbon nanotubes during arc discharge conditions are investigated. Ab initio molecular dynamics calcualtions show that the electric field alone cannot stabilize the growth of open metallic tubes. The addition of atoms and small clusters to tubes were studied using realistic atomic potentials. Deposition on tubes narrower than \AA{}3 nm leads to nucleation of curved defects (adjacent pentagon pairs) and eventual tube closure, while deposition on wider tubes favors the formation of hexagons and isolated pentagons, thereby promoting open-ended growth.

Single‐walled carbon nanotubes growing radially from YC2 particles

Abstract: In the primary soot produced by arc discharge using an yttrium carbide loaded anode, bundles of single‐walled carbon nanotubes (SWT) are observed, protruding radially from YC2 particles coated with graphitic multilayers. The graphitic cages separating YC2 particle and SWT bundles fall into the narrow range of 10–20 layers. The morphology of the clusters suggests a two‐step growth model: The radial SWT growth pattern is first initiated by catalytic action between the YC2 droplet and the carbon in the gas phase. Second, and upon cooling, the graphitic cage starts by segregating excess carbon from the YC2 bulk, arresting further growth of SWT.

Process of isolating carbon nanotubes from a mixture containing carbon nanotubes and graphite particles

Abstract: Carbon nanotubes are isolated from a mixture containing the carbon nanotubes and graphite particles by a process including the steps of: finely pulverizing the mixture; dispersing the pulverized product in a liquid medium; centrifuging the resulting dispersion to obtain a supernatant containing carbon nanotubes and graphite particles having a particle size of 0.3 μm or less; separating the supernatant into a solid phase and a liquid phase; and calcining the solid phase in an oxygen-containing atmosphere at a temperature sufficient to burn the graphite particles and to leave the nanotubes.

Single-Wall Carbon Nanotubes Growing Radially from Ni Fine Particles Formed by Arc Evaporation

Abstract: DC arc evaporation of nickel-graphite composite in helium atmosphere produces single-wall (SW) nanotubes. Electron microscopy examination of carbonaceous soot synthesized by this method revealed that the SW tubes grew radially from ultrafine nickel particles, and that they were found exclusively in soot material formed on the surface of a sluglike deposit grown on the end of a cathode.

Scanning tunneling spectroscopy of carbon nanotubes

Abstract: Calculations predict that carbon nanotubes may exist as either semimetals or semiconductors, depending on diameter and degree of helicity. This communication presents experimental evidence supporting the calculations. Scanning tunneling microscopy and spectroscopy (STM-S) data taken in air on nanotubes with outer diameters from 17 to 90 A show evidence of one-dimensional behavior; the current-voltage (I-V) characteristics are consistent with a density of states containing Van Hove type singularities for which the energies vary linearly with inverse nanotube diameter.

Surface plasmon observed for carbon nanotubes

Abstract: Electron energy loss spectroscopy and high-resolution images of carbon nanotubes, obtained using a 2 nm probe and energy resolution of 0.5 eV, revealed a 15 eV surface plasmon for tubes containing [le]12 cylindrical graphitic sheets. As the number of sheets increases further the 24 eV bulk plasmon quickly dominates the low-loss spectra. It is concluded that carbon nanotubes should be semimetallic, as is graphite. The first few graphitic sheets of nanotube structures act as if they are effectively decoupled electronically. As the radius of curvature increases then the interlayer structures approach the structure of graphite, as do the electronic properties. The present results do not exclude the possibility that the electronic structure of carbon nanotubes is the same as that of graphite, due to the operation of a conformal invariance principle.