Ropes Of Single-walled Carbon Nanotubes Research Articles

Some problems in understanding the electronic transport properties of carbon nanotube ropes

Abstract The resistance of single-wall carbon nanotube (SWCN) ropes or mats, and some individual tubes, typically shows a crossover from non-metallic to metallic temperature dependence as temperature increases. This systematic pattern is consistent with a series heterogeneous model involving metallic resistance and tunnelling through barriers such as defects and inter-rope contacts. The metallic resistivity term increases linearly with temperature for the ropes or mats, but faster for the individual nanotubes. In contrast to the almost vanishing thermoelectric power expected from electronic band structure calculations, the measured values for mats or films (including recent measurements in a vacuum) are even larger than for typical metals. The thermopower increases with temperature as for metals, but has a characteristic non-linear shape. This temperature dependence can be modelled, for example, with parallel conduction in metallic and semiconducting tubes, but the size of the metallic thermopower required is anomalously large.

Tensile strength of single-walled carbon nanotubes directly measured from their macroscopic ropes

20 mm long ropes consisting of soundly aligned single-walled carbon nanotube (SWNT) ropes, synthesized by the catalytic decomposition of hydrocarbons, were employed for direct tensile strength measurements. The average tensile strength of SWNT rope composites is as high as 3.6±0.4 GPa, similar to that of carbon fibers. The tensile strength of SWNT bundles was extrapolated from the strength of the composites to be 2.3±0.2 to 14.2±1.4 GPa after simply taking into account the volume fraction of SWNT bundles in the minicomposite, and the tensile strength of single SWNTs was estimated to be as high as 22.2±2.2 GPa. The excellent mechanical properties of SWNTs will make them an ideal reinforcement agent for high performance composite materials.

Kinetics of alkali insertion in single wall carbon nanotubes: an electrochemical impedance spectroscopy study

Electrochemical impedance spectroscopy (EIS) was used to study the kinetics and insertion mechanism of alkali metals in single wall carbon nanotubes (SWNT). Three distinct processes with very different time constants were identified: charge transfer across the macroscopic electrolyte/electrode interface, diffusion through the mesoscale porosity of the SWNT electrode, and nanoscale diffusion in individual SWNT ropes. Electrode resistance and capacitance from EIS compare well with direct dc measurements. We propose that alkalis decorate the internal and external surfaces of the SWNT ropes.

Acoustoelectric effects in carbon nanotubes

We report observations of acoustoelectric effects in carbon nanotubes. We excite sound in &mgr;m long ropes of single walled carbon nanotubes suspended between two metallic contacts by applying radio-frequency electric field. The sound is detected by measuring either the dc resistance of the tubes in a region of strong temperature dependence (in the vicinity of superconducting or metal-insulator transition), or their critical current. We show that, depending on the excitation power, the vibrations produce either electron heating or phase coherence breaking.

Filled and mixed nanotubes: from TEM studies to the growth mechanism within a phase-diagram approach

Two types of complex nanotubes produced by the arc-discharge method are investigated in this study: multi-walled carbon nanotubes filled with metallic nanowires and composite BN–C nanotubes. Their multi-element character yields specific spatial chemical arrangements — deduced from transmission electron microscopy (TEM) studies — which give precious information about the growth mechanism of nanotubes. Concerning filled nanotubes, if the metal–graphite cathode is carbon free, no filling is obtained, whereas if it contains sulfur — either as an impurity of graphite or when added under controlled quantities — complete or very long fillings are achieved, even for metals with very high melting temperatures. The chemical analyses revealed various types of fillings: pure sulfides, grains of sulfides alternating with grains of pure metals, or in some specific cases, pure metals. As for the B–C–N tubes, a total phase separation is observed between BN and C, and these two phases form concentric shells typically of the C/BN/C type. In this paper, we show that an approach combining the vapor–liquid–solid (VLS) scheme and the characteristics of the solidification given by phase diagrams account very well for the observed structures. The contrast between the absence of filling, when a sulfur-free carbon–metal rod is used for the cathode, and the successful fillings, when sulfur is added, is explained by metal–sulfur phase diagrams: adding sulfur to a liquid metal decreases the solidification temperature. The different types of fillings are also explained by the nature of the sulfur–metal phase diagram. An eutectic solidification, such as in Ni–S, yields two phases — the pure metal and the first sulfide — within a given tube, whereas the existence of a miscibility gap in the liquid, such as in Cr–S, leads to two separate liquids and, therefore, to two different fillings. In the same way, the eutectic-like pseudo-binary C–BN phase diagram explains not only the complete phase separation between BN and C, but also the observed organisation between layers: we propose that the latter is due to a sequential solidification of the two phases. As a perspective, this phase diagram approach is also discussed in the context of the formation of ropes of single-walled carbon nanotubes from the solidification of a metal–carbon liquid particle.

Synthesis of Macroscopically Long Ropes of Well-Aligned Single-Walled Carbon Nanotubes

chinese acad sci, inst met res, int ctr mat phys, shenyang 110015, peoples r china. mit, dept elect engn & comp sci, cambridge, ma 02139 usa. mit, dept phys, cambridge, ma 02139 usa.;cheng, hm (reprint author), chinese acad sci, inst met res, int ctr mat phys, 72 wenhua rd, shenyang 110015, peoples r china

Chemical doping of individual semiconducting carbon-nanotube ropes

We report the effects of potassium doping on the conductance of individual semiconducting single-walled carbon nanotube ropes. We are able to control the level of doping by reversibly intercalating and de-intercalating potassium. Potassium doping changes the carriers in the ropes from holes to electrons. Typical values for the carrier density are found to be $\ensuremath{\sim}100$--1000 electrons/$\ensuremath{\mu}\mathrm{m}.$ The effective mobility for the electrons is ${\ensuremath{\mu}}_{\mathrm{eff}}\ensuremath{\sim}20$--60 ${\mathrm{cm}}^{2}{\mathrm{V}}^{\ensuremath{-}1}{\mathrm{s}}^{\ensuremath{-}1},$ a value similar to that reported for the hole effective mobility in nanotubes [R. Martel et al., Appl. Phys. Lett. 73, 2447 (1998)].

Pressure dependence of the resistivity of single-wall carbon nanotube ropes

We have performed dc transport measurements on purified thick films of single-walled carbon nanotubes under hydrostatic pressures up to 2 GPa in the temperature range 4--300 K. At room temperature we have found nonmonotonic variation of the resistance with applied pressure. The resistance first drops with increasing pressure, but in the range where hexagonal deformation of the tubes is evidenced by the Raman experiments of Venkateswaran et al. [Phys. Rev. B 59, 10 928 (1999)], we see a fast increase in sample resistivity acompanied by very long time-scale relaxations. The temperature dependence of the resistivity does not change much with pressure, and we find an $\mathrm{exp}[{(T}_{0}{/T)}^{1/4}]$ temperature dependence indicating hoppinglike conduction.

Electron emission and structural characterization of a rope of single-walled carbon nanotubes

The electron emission and structural properties of an isolated ‘‘rope’’ comprised of ;70 individual singlewalled nanotubes ~SWNT’s! were investigated by measuring the field-emission energy distributions and by using field-ion microscopy~FIM!. Field-emission energy distributions, obtained under ultrahigh-vacuum conditions, revealed that the emitting nanotube has a large density of states near the Fermi energy, with an energy distribution of emitted electrons close to that predicted by the free-electron theory. Two small features located on the trailing edge of the energy distributions are attributed to localized features in the density of states of a SWNT. FIM studies were also performed on the same rope in an attempt to provide structural information about the emitting nanotube. Initial FIM micrographs showed an uneven distribution of atoms. Eventually, rings of atoms were imaged. The atom placement around an individual ring structure is analyzed and found to be consistent with that expected from a single ~19,13! SWNT. I. INTRODUCTION Much work has been done toward an understanding of the structural and electronic properties of carbon nanotubes. Many calculations of the electronic structure of single-walled nanotubes ~SWNT’s! have been reported. 1‐4 SWNT’s of the armchair ~n,n! variety are expected to be metallic; nanotubes of the zigzag ( n,0) family can be either metallic or semiconducting; and nanotubes with an inherent ~n,m! helicity are expected to be either semiconducting or metallic, depending on the exact values of n and m. Calculations illustrating how the density of states ~DOS! varies near the capped end of a nanotube have also been reported. 5‐7 Nanotubes can be imaged in the transmission electron microscope ~TEM! relatively easily which has provided an understanding of the structural properties and the growth mechanisms involved in nanotube formation. 3,8‐10 Scanning

Purification of single-walled carbon nanotubes synthesized by the catalytic decomposition of hydrocarbons

A procedure for purifying single-walled carbon nanotubes (SWNTs) synthesized by the catalytic decomposition of hydrocarbons has been developed. Based on the results from SEM observations, EDS analysis and Raman measurements, it was found that amorphous carbon, catalyst particles, vapor-grown carbon nanofibers and multi-walled carbon nanotubes were removed from the ropes of SWNTs without damaging the SWNT bundles, and a 40% yield of the SWNTs with a purity of about 95% was achieved after purification.

Experimental observation of individual single-wall nanotube species by Raman microscopy

Abstract Individual single-wall carbon nanotubes (SWNT) or small ropes of SWNTs with the same diameter have been characterised by Raman spectroscopy. Highly pure, length-selected SWNTs adsorbed onto substrates designed for surface-enhanced Raman spectroscopy were investigated with a Raman microscope. Various spectra, each with a distinct sharp peak in the radial breathing mode area and four C–C stretching modes could be observed. The modes also showed a strong variation in the relative intensities. In correlation with theoretical predictions this should establish Raman microscopy as a non-destructive nano-technological tool, capable of determining the diameter, symmetry and position of individual SWNTs.

Elastic strain of freely suspended single-wall carbon nanotube ropes

We have induced large elastic strains in ropes of single-wall carbon nanotubes, using an atomic force microscope in lateral force mode. Freely suspended ropes were observed to deform as elastic strings with tension proportional to elongation. Ropes were elastically deformed over >10 cycles without showing signs of plastic deformation. The maximum strain observed, 5.8±0.9%, gives a lower bound of 45±7 GPa for the tensile strength (specifically, yield stress) of single-wall nanotube ropes.

A Simple Method to Make Field Emitter Using Ropes of Single-Walled Carbon Nanotubes

Field emission characteristics of single-walled carbon nanotubes were studied by using a simple method in a field emission microscope. The nanotube emission gun works effectively at the room temperature under a threshold field as low as 3.9 mV/μm. A typical I-V relationship of field emission was obtained with a high current density. The observed stable bright spots on the fluorescent screen originate from an ensemble emission from micro-ropes of the single-walled carbon nanotubes.

Luttinger-liquid behaviour in carbon nanotubes

An interacting one-dimensional (1D) electron system is predicted to behave very differently than its higher-dimensional counterparts. Coulomb interactions strongly modify the properties away from those of a Fermi liquid, resulting in a Luttinger liquid (LL) characterized by a power-law vanishing of the density of states at the Fermi level. Experiments on one-dimensional semiconductor wires and fractional quantum Hall conductors have been interpreted using this picture, but questions remain about the connection between theory and experiment. Recently, single-walled carbon nanotubes (SWNTs) have emerged as a new type of 1D conductor that may exhibit LL behavior. Here we present measurements of the conductance of individual ropes of such SWNTs as a function of temperature and voltage. Power law behavior as a function of temperature or bias voltage is observed: G~ T^a and dI/dV ~ V^a. Both the power-law functional forms and the inferred exponents are in good agreement with theoretical predictions for tunneling into a LL.

Elastic and Shear Moduli of Single-Walled Carbon Nanotube Ropes

Reference LNNME-ARTICLE-1999-004doi:10.1103/PhysRevLett.82.944View record in Web of Science Record created on 2007-04-23, modified on 2016-08-08

Thermal conductivity of single-walled carbon nanotubes

We have measured the temperature-dependent thermal conductivity $\ensuremath{\kappa}(T)$ of crystalline ropes of single-walled carbon nanotubes from 350 K to 8 K. $\ensuremath{\kappa}(T)$ decreases smoothly with decreasing temperature, and displays linear temperature dependence below 30 K. Comparison with electrical conductivity experiments indicates that the room-temperature thermal conductivity of a single nanotube may be comparable to that of diamond or in-plane graphite, while the ratio of thermal to electrical conductance for a given sample indicates that the thermal conductivity is dominated by phonons at all temperatures. Below 30 K, the linear temperature dependence and estimated magnitude of $\ensuremath{\kappa}(T)$ imply an energy-independent phonon mean free path of \ensuremath{\sim}0.5--1.5 \ensuremath{\mu}m.

Spin Splitting and Even-Odd Effects in Carbon Nanotubes

The level spectrum of a single-walled carbon nanotube rope, studied by transport spectroscopy, shows Zeeman splitting in a magnetic field parallel to the tube axis. The pattern of splittings implies that the spin of the ground state alternates by 1/2 as consecutive electrons are added. Other aspects of the Coulomb blockade characteristics, including the current-voltage traces and peak heights, also show corresponding even-odd effects.

STM morphology study of ropes of single-wall carbon nanotubes

Scanning tunneling microscopy has been performed on ropes of single-wall carbon nanotubes (SWNTs) under ambient conditions. Results show that ropes are generally polycrystalline with grain size ranging from less than 10 nm to more than 100 nm and typical grain size between 10 and 20 nm. Grains remain distinct over the entire rope length. A study of rope shapes reveals that their cross section is not circular. A histogram of the width-to-height ratio presents a marked peak at three with a long tail extending up to twenty. Bundles of SWNTs often have a ribbon-like rather than a rope-like shape.

Chromatographic size separation of single-wall carbon nanotubes

The efficient purification of single-wall carbon nanotubes (SWNTs) is reported. Carbon nanospheres, metal particles, and amorphous carbon could be successfully removed by size exclusion chromatography (SEC) applied to surfactant stabilised dispersions of SWNT raw material. In addition, length separation of the tubes was achieved. The SWNTs obtained can be adsorbed in high densities onto chemically modified substrates. As determined by AFM investigations. the purified material consists of about equal fractions of both individual SWNTS and ropes of SWNTs.

Transport spectroscopy of single-walled carbon nanotubes

We have performed transport spectroscopy on individual ropes of single-walled carbon nanotubes. We find that the levels are Zeeman split in a magnetic field, with a g-factor of 2.04±0.05. The observed pattern of peak splittings indicates the parity of the number of electrons on the dot. In one device there are also signs of the presence of a second dot. We observe features which resemble anticrossings between quantum levels in the two dots, which may be formed from separate conducting nanotubes within the rope.