Showing papers on "Nanotube published in 1996"

Electrical-Conductivity of Individual Carbon Nanotubes

Abstract: THE interest in carbon nanotubes has been greatly stimulated by theoretical predictions that their electronic properties are strongly modulated by small structural variations1–8. In particular, the diameter and the helicity of carbon atoms in the nanotube shell are believed to determine whether the nanotube is metallic or a semiconductor. Because of the enormous technical challenge of making measurements on individual nanotubes, however, experimental studies have been limited mainly to bulk measurements9, which indicate only that a fraction of the nanotubes are metallic or narrow-band semiconductors10. Recently, measurements of the magneto-conductance of a single multi-shell nanotube in a two-probe configuration showed that the transport is characterized by disorder and localization phenomena11. To avoid possible ambiguities due to poor sample contacts, four-probe measurements are needed. Here we report four-probe measurements on single nanotubes made by lithographic deposition of tungsten leads across the tubes. We find that each multi-shell nanotube has unique conductivity properties. Both metallic and non-metallic behaviour are observed, as well as abrupt jumps in conductivity as the temperature is varied. The differences between the electrical properties of different nanotubes are far greater than expected. Our results suggest that differences in geometry play a profound part in determining the electronic behaviour.

Probing Electrical Transport in Nanomaterials: Conductivity of Individual Carbon Nanotubes

Abstract: A general approach has been developed to determine the conductivity of individual nanostructures while simultaneously recording their structure. Conventional lithography has been used to contact electrically single ends of nanomaterials, and a force microscope equipped with a conducting probe tip has been used to map simultaneously the structure and resistance of the portion of the material protruding from the macroscopic contact. Studies of individual carbon nanotubes demonstrate that the structurally most perfect nanotubes have resistivities an order of magnitude lower than those found previously and that defects in the nanotube structure cause substantial increases in the resistivity.

Nanocapillarity and Chemistry in Carbon Nanotubes

Abstract: Open carbon nanotubes were filled with molten silver nitrate by capillary forces. Only those tubes with inner diameters of 4 nanometers or more were filled, suggesting a capillarity size dependence as a result of the lowering of the nanotube-salt interface energy with increasing curvature of the nanotube walls. Nanotube cavities should also be less chemically reactive than graphite and may serve as nanosize test tubes. This property has been illustrated by monitoring the decomposition of silver nitrate within nanotubes in situ in an electron microscope, which produced chains of silver nanobeads separated by high-pressure gas pockets.

Quantum transport in a multiwalled carbon nanotube

Abstract: We report on electrical resistance measurements of an individual carbon nanotube down to a temperature T = 20 mK. The conductance exhibits a lnT dependence and saturates at low temperature. A magnetic field applied perpendicular to the tube axis increases the conductance and produces aperiodic fluctuations. The data find a global and coherent interpretation in terms of two-dimensional weak localization and universal conductance fluctuations in mesoscopic conductors. The dimensionality of the electronic system is discussed in terms of the peculiar structure of carbon nanotubes.

Biomembrane Templates for Nanoscale Conduits and Networks

Abstract: Long nanotubes of fluid-lipid bilayers can be used to create templates for photochemical polymerization into solid-phase conduits and networks. Each nanotube is pulled from a micropipette-held feeder vesicle by mechanical retraction of the vesicle after molecular bonding to a rigid substrate. The caliber of the tube is controlled precisely in a range from 20 to 200 nanometers merely by setting the suction pressure in the micropipette. Branched conduits can be formed by coalescing separate nanotubes drawn serially from the feeder vesicle surface. Single nanotubes and nanotube junctions can be linked together between bonding sites on a surface to create a functionalized network. After assembly, the templates can be stabilized by photoinitiated radical cross-linking of macromonomers contained in the aqueous solution confined by the lipid bilayer boundary.

Electrical resistance of a single carbon nanotube

Abstract: A single multiwalled carbon nanotube with an outside diameter of about 12 nm and an inside diameter of about 4 nm has been isolated in order to measure its electrical properties (resistivity, transverse magnetoresistance and temperature dependence of resistance). The semi-metallic character of the conductance of the selected nanotube is concluded from these measurements.

Carbon nanotube materials for hydrogen storage

Abstract: Carbon single-wall nanotubes (SWNTs) are essentially elongated pores of molecular dimensions and are capable of adsorbing hydrogen at relatively high temperatures and low pressures. This behavior is unique to these materials and indicates that SWNTs are the ideal building block for constructing safe, efficient, and high energy density adsorbents for hydrogen storage applications. In past work we developed methods for preparing and opening SWNTs, discovered the unique adsorption properties of these new materials, confirmed that hydrogen is stabilized by physical rather than chemical interactions, measured the strength of interaction to be ~ 5 times higher than for adsorption on planar graphite, and performed infrared absorption spectroscopy to determine the chemical nature of the surface terminations before, during, and after oxidation. We also made significant advances in the synthesis of SWNT materials by turning to a laser-vaporization method rather than the arc-generation method employed previously. In addition, we began to develop methods to purify nanotubes and cut nanotubes into shorter segments. This year we have made further advances in the development of our laser synthesis technique, and we now generate crude material containing 20-30 wt% SWNTs at a rate of ~150 mg / hr or ~ 1.5 g / day. In addition we have perfected a very simple 3-step purification process which results in material that is > 98 wt% pure which is the purest reported to date. In conjunction with the purification method we pioneered a thermal gravimetric analysis (TGA) technique which enables accurate determination of SWNT wt% contents in carbon soot. Finally, we have simplified our previous nanotube cutting technique and have developed a process that allows for highly reproducible cutting of our purified laser-generated materials. The new cutting method enables the opening of laser-produced tubes which were unreactive to the oxidation methods that successfully opened our previously synthesized arc-generated tubes, and offers a path towards organizing nanotube segments to enable high volumetric hydrogen storage densities. Most importantly, this year we have demonstrated that purified cut SWNTs adsorb between 3.5 ‐ 4.5 wt% hydrogen under ambient conditions in several minutes and that the adsorbed hydrogen is effectively “capped” by CO 2 making it stable for weeks in atmospheric conditions.

Collective excitations in a single-layer carbon nanotube.

Abstract: The inter-\ensuremath{\pi}-band response function of a single-layer carbon nanotube is calculated within the random-phase approximation. Such a cylindrical system exhibits rich inter-\ensuremath{\pi}-band excitations. The decoupled \ensuremath{\pi} plasmons of different angular momenta (L's) strongly depend on the transferred momenta (q's). Furthermore, they exhibit a dimensionality crossover as the nanotube radius is increased. For small q's and L's, the \ensuremath{\pi} plasmon frequency is \ensuremath{\sim}6--8 eV, which is insensitive to the nanotube geometry such as the radius and chiral angle. Comparison with recent experiments is discussed. \textcopyright{} 1996 The American Physical Society.

Lattice Distortion with Spatial Variation of Carbon Nanotubes in Magnetic Fields

Abstract: A lattice instability of carbon nanotubes induced by a magnetic field perpendicular to the tube axis is studied in a k · p scheme. Both in-plane Kekule and out-of-plane distortions are enhanced drastically with increase of a magnetic field independent of whether a nanotube is metallic or semiconducting and magnetic flux passing through the cross section of a nanotube. The distortions become dependent on the position in the circumference direction.

Influence of external factors on electron field emission from thin‐film nanofilament carbon structures

Abstract: The influence of temperature and residual gas pressure on the field emission of thin‐film nanotube carbon structures has been studied. These structures are shown to exhibit low work functions. They are promising for fabrication of effective low‐voltage field emitters and low‐temperature thermal emitters.

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.

Carbon Nanotubes and Aligned Carbon Nanotube Films

Abstract: Carbon nanotube are slowly coming to the forefront of fullerene materials especially since mass quantities can be produced rather easily. A new methods to make aligned nanotube films hold many promises, not only for basic research but also for eventual applications. With this method the tubes can be aligned either parallel or perpendicular to the surface. Here we give a short review of the field with several examples of the microscopic properties of single tubes and the bulk properties of the aligned films.

Work function estimate for electrons emitted from nanotube carbon cluster films

Abstract: Relying on the obtained theoretical and experimental results the electron work function from the nanotube carbon film was estimated. It was shown that these structures have a low work function, substantially lower than for graphite. Under estimation we regarded the influence of the film surface relief on its emission ability.