Showing papers on "Nanotube published in 1997"

Controlled production of aligned-nanotube bundles

Abstract: Carbon nanotubes1, 2 might be usefully employed in nanometre-scale engineering and electronics. Electrical conductivity measurements on the bulk material3, 4, on individual multi-walled5, 6 and single-walled7 nanotubes and on bundles of single-walled nanotubes8, 9 have revealed that they may behave as metallic, insulating or semiconducting nanowires, depending on the method of production—which controls the degree of graphitization, the helicity and the diameter. Measurements of Young's modulus show10 that single nanotubes are stiffer than commercial carbon fibres. Methods commonly used to generate nanotubes—carbon-arc discharge techniques1, 2, 4, catalytic pyrolysis of hydrocarbons11, 12 and condensed-phase electrolysis13, 14—generally suffer from the drawbacks that polyhedral particles are also formed and that the dimensions of the nanotubes are highly variable. Here we describe a method for generating aligned carbon nanotubes by pyrolysis of 2-amino-4,6-dichloro-s-triazine over thin films of a cobalt catalyst patterned on a silica substrate by laser etching. The use of a patterned catalyst apparently encourages the formation of aligned nanotubes. The method offers control over length (up to about 50 mum) and fairly uniform diameters (30–50 nm), as well as producing nanotubes in high yield, uncontaminated by polyhedral particles.

Conductivity enhancement in single-walled carbon nanotube bundles doped with K and Br

Abstract: Single-walled carbon nanotubes (SWNTs), prepared by metal-catalysed laser ablation of graphite, form close-packed bundles or ‘ropes;1. These rope crystallites exhibit metallic behaviour above 50K (ref. 2), and individual tubes behave as molecular wires, exhibiting quantum effects at low temperatures3,4. They offer an all-carbon host lattice that, by analogy with graphite5 and solid C60 (ref. 6), might form intercalation compounds with interesting electronic properties, such as enhanced electrical conductivity and superconductivity. Multi-walled nanotube materials have been doped with alkali metals7 and FeCl3 (ref. 8). Here we report the doping of bulk samples of SWNTs by vapour-phase reactions with bromine and potassium—a prototypical electron acceptor and donor respectively. Doping decreases the resistivity at 300K by up to a factor of 30, and enlarges the region where the temperature coefficient of resistance is positive (the signature of metallic behaviour). These results suggest that doped SWNTs represent a new family of synthetic metals.

Coulomb Interactions and Mesoscopic Effects in Carbon Nanotubes

Abstract: We argue that long-range Coulomb forces convert an isolated sN, Nd armchair carbon nanotube into a strongly renormalized Luttinger liquid At high temperatures, we find anomalous temperature dependences for the interaction and impurity contributions to the resistivity, and similar power-law dependences for the local tunneling density of states At low temperatures, the nanotube exhibits spin-charge separation, visible as an extra energy scale in the discrete tunneling density of states (for which we give an analytic form), signaling a departure from the orthodox theory of Coulomb blockade [S0031-9007(97)04651-6]

Synthesis of Nanoparticles and Nanotubes with Well-Separated Layers of Boron Nitride and Carbon

Abstract: Polyhedral and tubular graphitic nanoparticles made of carbon layers and boron nitride (BN) layers have been synthesized. These particles were observed in the soot collected on the anode deposit formed by arcing a hafnium diboride rod with graphite in a nitrogen atmosphere. Elemental profiles with subnanometer-scale resolution revealed a strong phase separation between BN layers and carbon layers along the radial direction. Most of these tubes have a sandwich structure with carbon layers both in the center and at the periphery, separated by a few BN layers. This structure provides insight into the atomistic mechanism of nanotube growth in the boron-carbon-nitrogen ternary system and may lead to the creation of nanostructured electronic devices relying on the controlled production of heteroatomic nanotubes.

Field emission from nanotube bundle emitters at low fields

Abstract: The fabrication of nanotube field emitters with an onset field as low as 0.8 V/μm is described and the low-field electron emission mechanism is discussed. These emitters are made using nanotube cathode deposit with the addition of epoxy resin. The preferred orientation of nanotubes in nanotube bundles of the deposit is preserved. The nanotube tips are sharpened by exposing the nanotube bundle surface to a microwave oxygen plasma. The local-field enhancement factor is estimated to be 8000 by using the Fowler–Nordheim equation. The low onset field is attributed to the well-distributed, highly orientated sharp tips at the sample surface.

Catalytic Growth of Single-Wall Carbon Nanotubes: An Ab Initio Study

Abstract: We propose a catalytic growth mechanism of single-wall carbon nanotubes based on density functional total energy calculations Our results indicate nanotubes with an “armchair” edge to be energetically favored over “zigzag” nanotubes We also suggest that highly mobile Ni catalyst atoms adsorb at the growing edge of the nanotube, where they catalyze the continuing assembly of hexagons from carbon feedstock diffusing along the nanotube wall In a concerted exchange mechanism, Ni atoms anneal carbon pentagons that would initiate a dome closure of the nanotube [S0031-9007(97)02791-9]

Microscopic growth mechanisms for carbon nanotubes

Abstract: The uncatalyzed edge growth of carbon nanotubes was investigated by first-principles molecular dynamics simulations. At experimental temperatures the open end of single-walled nanotubes closed spontaneously into a graphitic dome, which may explain why these nanotubes do not grow in the absence of transition metal catalysts, On the other hand, chemical bonding between the edges of adjacent coaxial tubes (''lip-lip'' interactions) trapped the end oi a double-walled nanotube in a metastable energy minimum, thus preventing dome closure. These calculations show that this end geometry exhibits a high degree of chemical activity and easily accommodates incoming carbon fragments, supporting a model of growth by chemisorption from the vapor phase.

Theory of composite BxCyNz nanotube heterojunctions

Abstract: Examines the stability and electronic properties of composite B[sub x]C[sub y]N[sub z] nanotube heterojunctions. Use of ab initio density functional calculations and semi-empirical approaches; Advantage of nanotubes; Independence of junction characteristics from nanotube factors.

Macroscopically manipulable nanoscale devices made from nanotube assemblies

Abstract: Macroscopically manipulable nanoscale devices made from nanotube assemblies are disclosed. The article of manufacture comprises a macroscopic mounting element capable of being manipulated or observed in a macroscale environment, and a nanoscale nanotube assembly attached to the mounting element. The article permits macroscale information to be provided to or obtained from a nanoscale environment. A method for making a macroscopically manipulable nanoscale devices comprises the steps of (1) providing a nanotube-containing material; (2) preparing a nanotube assembly device having at least one carbon nanotube for attachment; and (3) attaching said nanotube assembly to a surface of a mounting element.

Length control of individual carbon nanotubes by nanostructuring with a scanning tunneling microscope

Abstract: We present a technique to control the length of carbon nanotubes. Individual carbon nanotubes can be locally cut by applying a voltage pulse to the tip of a scanning tunneling microscope (STM). Topographic imaging and STM spectroscopy are subsequently used to investigate the result. The electronic properties of a nanotube can be strongly changed by reducing the size. Current-voltage curves obtained by STM spectroscopy on a 30 nm short tube created from a longer nanotube show a stepwise increase of the current, which is attributed to quantum size effects.

Kinetics of metal-catalyzed growth of single-walled carbon nanotubes

Abstract: Molecular-dynamics and total-energy calculations using a realistic three-body potential for carbon reveal the basic atomic processes by which single-shelled nanotubes can grow out of metal-carbide particles by the root growth mechanism. We find that nanometer-sized protrusions on the metal-particle surface lead to the nucleation of very narrow tubes. Wide bumps lead to a strained graphene sheet and no nanotube growth. The results also explain the absence of multishelled tubes in metal-catalyzed growth.

Synthesis of carbon nanotubes containing metal oxides and metals ofthe d-block and f-block transition metals and related studies

Abstract: The filling of carbon nanotubes with metals and metal oxidesvia one- and two-step processes is presented. Both molten media and wet chemistry solution methods have been used to introduce foreign materials into the hollow nanotube cavities. Chemical reactions inside the tubes have been carried out, including the reduction of encapsulated materials to the metals. The nature of the crystalline filling has been found to be highly dependent on the techniques used. Wet chemical methods tend to result in filling which consists of discrete crystallites, whereas molten media methods tend to give long, continuous single crystals.

Quantitative theory of diffraction by carbon nanotubes

Abstract: A quantitative theory of the kinematical diffraction of a plane wave by a carbon nanotube is developed. The formalism is based on the Cochran, Crick, and Vand theory of the diffraction by helical molecules. This leads to a closed-form expression of the diffracted amplitude produced by a single-wall tubule of arbitrary helicity, applicable to both X rays and high-energy electrons. The theory, which can be used to simulate the diffraction pattern of any multilayer nanotube, is illustrated on the case of a crystalline rope of carbon nanotubes.

Carbon nanotubes are coherent electron sources

Abstract: The observation of electron interference effects provides direct evidence of the coherence of the electrons emitted from a carbon nanotube. To demonstrate this, the low-energy electron point source microscope has been used to mount an individual carbon nanotube onto a tungsten tip. In subsequent experiments, the electrons emitted from the nanotube were used to generate holograms. Comparison with a standard tungsten atomic point source emitter establishes a high degree of coherence for a nanotube emitter.

Nanotube size-dependent melting of single crystals in carbon nanotubes

Abstract: A model for the dependence of the melting tem- perature of nanocrystals on the size of carbon nanotubes is developed. The model is applied to investigate the nano- tube size-dependent melting of TaC and Pb single crystals encapsulated in carbon nanotubes. It is shown that the melt- ing temperature for these single crystals in carbon nanotubes can be strongly suppressed. The results also imply that nano- tubes may provide an effective means for investigating the supercooled state of liquids and the liquid-glass transition.

Theoretical Investigation of the Cyclic Peptide System Cyclo(( D-Ala-Glu-D-Ala-Gln)m)1-4)

Abstract: Using newly developed theoretical methods, we present preliminary results for some calculated properties of the cyclic peptide system cyclo[(d-Ala-Glu-d-Ala-Gln)m=1-4]. These calculations are motivated by the fact that the cyclo[(d-Ala-Glu-d-Ala-Gln)2] cyclic peptide structure was the first cyclic peptide structure synthesized by an experimental group through a self-assembly process. In this paper, we calculate the electronic structure and vibrational mode properties of the isolated ring structures for cyclo[(d-Ala-Glu-d-Ala-Gln)m=1-4] and of the cyclic peptide nanotube system cyclo[(d-Ala-Glu-d-Ala-Gln)2]. The HOMO−LUMO gap is wide (∼5.0 eV) yielding a transparent material with possibly unique bioelectronic device applications. In addition, we find that the C−O carbonyl stretch modes and the N−H amide-I stretch modes of these isolated ring structures to be highly localized at around 1773 and 3192 cm-1, respectively.

Nonlinear Electron Transport Effects in a Chiral Carbon Nanotube

Abstract: We present a novel, general, semiclassical theory of electron transport in a carbon nanotube exposed to an external electric field. The charge carriers are treated in the framework of the simplified tight-binding model. Simultaneous exposure to rapidly oscillating (ac) and constant (dc) electric fields is considered to exemplify our theory. Nonlinear and chiral effects are found, and their interaction is delineated. We predict the effect of an ac electric field on the magnitude and the direction of the total time-averaged current. {copyright} {ital 1997} {ital The American Physical Society}

Dynamics of flow inside carbon nanotubes

Abstract: Future nanotechnology applications are likely to involve reactive or non-reactive species carried along a fluid stream. We have performed several molecular dynamics simulations of a buckyball, , cage or idealized atom in a helium fluid flowing axially inside a carbon nanotube. The fluid was started at some initial velocity and both the fluid and buckyball allowed to recycle axially via minimum image boundary conditions. A buckyball introduced into the feedstream (started at zero velocity) usually reached fluid velocity within 5 ps. Leakage rates of helium past the depended on the nanotube diameter and fluid velocity. These leakage rates and other important features of the dynamics changed significantly when was modelled as an idealized atom or when the nanotube was held rigid, suggesting that simulations of fluid dynamics inside nanomachines should be fully dynamic and atomistic.

Scanning tunneling microscope investigation of carbon nanotubes produced by catalytic decomposition of acetylene

Abstract: A detailed scanning tunneling microscopy (STM) study of carbon nanotubes prepared by catalytic decomposition of acetylene over a supported transition metal catalyst is reported. Nanotubes with diameters in the 10-nm range and others with diameters as small as 1 nm were found. The mechanism of STM image formation and image deconvolution with respect to the tip shape are discussed, as well as the corrections which have to be applied when measuring by STM nanotube diameters in the ranges of 10 nm and of 1 nm, respectively. The behavior of a nanotube crossing steps on the graphite surface is analyzed. Modified height values measured on a single-wall nanotube in regions where it was subjected to strong mechanical stresses indicate changes in the electrical conductivity by several orders of magnitude. Carbon nanotubes are found to be good candidates to be used for STM tip shape characterization in the nm range.

Thin films consisting of carbon nanotubes as a new material for emission electronics

Abstract: The investigation results for the emission characteristics of thin films composed of nanotube carbon structures are described. These films are shown to provide high field electron emission and substantial thermal electron emission even at low temperatures. They exhibit low electron work functions and ensure stable field emission under conditions of operating vacuum.