Causes of different catalytic activities of metals in formation of single-wall carbon nanotubes

When single-wall carbon nanotubes (SWNTs) were formed by pulsed Nd:YAG laser ablation or arc discharge, the yield depended on the metal catalyst: NiCo> Ni∼NiFe≫Co∼Fe>Pd∼Pt. It appears that an effective catalyst for SWNT growth must satisfy three conditions: it must be a good graphitization catalyst, have low solubility in carbon, and have a stable crystallographic orientation on graphite. NiCo, Ni, and NiFe satisfy these three conditions. The poor catalytic activities of Co, Fe, Pd, and Pt for SWNT formation would be explained by the ineffectiveness of Pt and Pd as graphitization catalysts, crystallographic orientation instability of Co crystals on graphite, and high solubility of Fe in graphite. Received: 29 October 2001 / Accepted: 7 November 2001 / Published online: 23 January 2002     

Thermal properties of carbon nanotubes and nanotube-based materials

The thermal properties of carbon nanotubes are directly related to their unique structure and small size. Because of these properties, nanotubes may prove to be an ideal material for the study of low-dimensional phonon physics, and for thermal management, both on the macro- and the micro-scale. We have begun to explore the thermal properties of nanotubes by measuring the specific heat and thermal conductivity of bulk SWNT samples. In addition, we have synthesized nanotube-based composite materials and measured their thermal conductivity. The measured specific heat of single-walled nanotubes differs from that of both 2D graphene and 3D graphite, especially at low temperatures, where 1D quantization of the phonon bandstructure is observed. The measured specific heat shows only weak effects of intertube coupling in nanotube bundling, suggesting that this coupling is weaker than expected. The thermal conductivity of nanotubes is large, even in bulk samples: aligned bundles of SWNTs show a thermal conductivity of >200 W/m K at room temperature. A linear K(T) up to approximately 40 K may be due to 1D quantization; measurement of K(T) of samples with different average nanotube diameters supports this interpretation. Nanotube–epoxy blends show significantly enhanced thermal conductivity, showing that nanotube-based composites may be useful not only for their potentially high strength, but also for their potentially high thermal conductivity. Received: 17 October 2001 / Accepted: 3 December 2001 / Published online: 4 March 2002     

N-doping and coalescence of carbon nanotubes: synthesis and electronic properties

Self-assembly pyrolytic routes to large arrays (<2.5 cm²) of aligned CN_x nanotubes (15–80 nm OD and <100 μm in length) are presented. The method involves the thermolysis of ferrocene/melamine mixtures (5:95) at 900–1000 °C in the presence of Ar. Electron energy loss spectroscopy (EELS) reveals that the N content varies from 2–10%, and can be bonded to C in two different fashions (double-bonded and triple-bonded nitrogen). The electronic densities of states (DOS) of these CN_x nanotubes, using scanning tunneling spectroscopy (STS), are presented. The doped nanotubes exhibit strong features in the conduction band close to the Fermi level (0.18 eV). Using tight-binding and ab initio calculations, we confirm that pyridine-like (double-bonded) N is responsible for introducing donor states close to the Fermi Level. These electron-rich structures are the first example of n-type nanotubes. Finally, it will be shown that moderate electron irradiation at 700–800 °C is capable of coalescing single-walled nanotubes (SWNTs). The process has also been studied using tight-binding molecular dynamics (TBMD). Vacancies induce the coalescence via a zipper-like mechanism, which has also been observed experimentally. These vacancies trigger the organization of atoms on the tube lattices within adjacent tubes. These results pave the way to the fabrication of nanotube heterojunctions, robust composites, contacts, nanocircuits and strong 3D composites using N-doped tubes as well as SWNTs. Received: 10 October 2001 / Accepted: 3 December 2001 / Published online: 4 March 2002     

Synthesis of aligned carbon nanotubes

Carbon nanomaterials seem to be most attractive because of their fascinating features. Carbon nanotubes emerged recently as unique nanostructures with remarkable mechanical and electronic properties. Future applications will require a fabrication method capable of producing uniform carbon nanotubes with well-defined and controllable reproducibility of their properties. In this review, recent results addressing rational and efficient methods to obtain aligned arrays of these one-dimensional carbon nanomaterials will be discussed. Received: 3 November 2000 / Accepted: 30 May 2001 / Published online: 30 August 2001     

Field-induced evaporation of carbon nanotubes

We report here an experimental observation of field emission from arrays of multiwall carbon nanotubes. Current densities in the range 10–30 mA/cm² with excellent long-term stability were recorded. A detailed study of the destruction of nanotubes at extreme operation conditions is performed. We established that field evaporation of nanotubes accompanies field emission from a cold cathode at electric fields higher than 2 V/?. Electron microscopy of the evaporation products reveals irregularly shaped carbon nanoparticles with a hollow core. The diameter of the particles is ∼20 nm. A mechanism of the process is proposed and discussed. Received: 6 October 2000 / Accepted: 28 April 2001 / Published online: 27 June 2001     

High yield incorporation and washing properties of halides incorporated into single walled carbon nanotubes

We describe here the high yield filling (i.e. >50%) of single walled nanotubes (SWNTs) with a variety of halides, achieved according to various modified filling procedures. Both bundles and discrete SWNTs can be filled continuously up to lengths of several hundred nm, often with filling yields approaching 60–70% or better. In addition some high yield filled SWNTs were subjected to long-term washing in either boiling or room temperature. aqueous media, which does not remove the filling from the tubules, but enables effective removal of water-soluble extraneous materials . Received: 10 May 2002 / Accepted: 25 October 2002 / Published online: 10 March 2003 RID="*" ID="*"Corresponding author. Fax: +44-1865/272-690, E-mail: jeremy.sloan@chem.ox.ac.uk     

The electronic structure of polymerized fullerenes and dimerized heterofullerenes

\chem{Rb_1C_{60}} and dimerised using electron energy-loss spectroscopy in transmission. From the excitation spectra a reduced density of states is observed for polymerized . This is in contrast to and can be explained by the different type of \squt{doping} and by the different bonding between the fullerene molecules in the two systems. Additional information about the optical properties was obtained from the low energy loss function. Using a Kramers-Kronig analysis, the dielectric function, (), and the optical conductivity, (), have been derived. and the onset of the spectral weight have been compared between the polymer, the dimer and . This onset of spectral weight is found to be at and for o- and for , respectively. Received: 28 October 1996/Accepted: 13 December 1996     

Neutron scattering studies of the structure and dynamics of nanobundles of single-wall carbon nanotubes

Elastic and inelastic neutron scattering are used to study the structure and dynamics of single-wall carbon nanotubes (SWNT) self-assembled into nanobundles (NBSWNT). Suspensions of NBSWNT are characterized by small-angle neutron scattering. Neutron diffraction is used as a useful tool to study the structure of both the SWNT and NBSWNT. Calculations on finite-size bundles are compared to the data in order to estimate the distribution of tube diameters. Finally, we present time of flight inelastic scattering measurements of the phonon density of states and discuss the main features of the spectra in comparison with calculations. Received: 9 June 1999 / Accepted: 3 August 1999 / Published online: 27 October 1999     

Work function of single-walled carbon nanotubes determined by field emission microscopy

The field emission characteristics of a single micro-bundle of single-walled carbon nanotubes (SWCNTs) were investigated using field emission microscopy (FEM). Fowler–Nordheim plots revealed that the work function of the SWCNTs was reduced with increasing heating temperature, and reached a minimum value around 1000 °C, assuming that the β factor was constant during the heating process. Field emission patterns also demonstrated fine structures that were believed to be images of the cap of a SWCNT, which was in a clean state. The radius of the SWCNT micro-bundle was measured by transmission electron microscopy (TEM), and the β factor was calculated using two empirical formulae. Then, the work function of the SWCNT was determined from the slope, K, of its Fowler–Nordheim plot. The work function values were Φ₁=4.76 eV and Φ₂=4.88 eV, respectively. Received: 26 October 2001 / Revised version: 19 February 2002 / Published online: 6 June 2002     

Structural and electronic properties of composite BxCyNz nanotubes and heterojunctions

x C_yN_z nanotubes and related heterojunctions have been studied using both ab initio and semi-empirical approaches. Pure BN nanotubes present a very stable quasiparticle band gap around 5.5–6.0 eV independent of the tube radius and helicity. The bottom of the conduction bands is controlled by a nearly-free-electronn state localized inside the nanotube, suggesting interesting properties under doping. In the case of nanotubes with BC₂N stoichiometry, we show that in the thermodynamic limit the system is driven towards segregation of pure C and BN sections. This demixing significantly affects the electronic properties of such materials. The same process of segregation into BC₃ islands is evidenced in the case of B-doped carbon nanotubes. These spontaneous segregation processes lead to the formation of quantum dots or nanotube heterojunctions. In particular, C/BN superlattices or isolated junctions have been investigated as specific examples of the wide variety of electronic devices that can be realized using such nanotubes. Received: 27 November 1998 / Accepted: 14 December 1998     

Carbon nanotubes formed in graphite after mechanical grinding and thermal annealing

Multi-walled carbon nanotubes with cylindrical and bamboo-type structures are produced in a graphite sample after mechanical milling at ambient temperature and subsequent thermal annealing up to 1400 °C. The ball milling produces a precursor structure and the thermal annealing activates the nanotube growth. Different nanotubular structures indicate different formation mechanisms: multi-wall cylindrical carbon nanotubes are probably formed upon micropores and the bamboo tubes are produced because of the metal catalysts. A two-dimensional growth governed by surface diffusion is believed to be one important factor for the nanotube growth. A potential industrial production method is demonstrated with advantages of large production quantity and low cost. Received: 17 May 2002 / Accepted: 12 September 2002 / Published online: 4 December 2002 RID="*" ID="*"Corresponding author. Fax: +61-2/6125-8338, E-mail: ying.chen@anu.edu.au     

Elastic properties of single-wall nanotubes

3 , BC₂N, and C₃N₄. These studies have been carried out using a total-energy, non-orthogonal, tight-binding parametrisation which is shown to provide results in good agreement both with calculations using higher levels of theory and the available experimental data. Our results predict that of all types of nanotubes considered, carbon nanotubes have the highest Young’s modulus. We have considered tubes of different diameters, ranging from 0.5 to 2 nm, and find that in the limit of large diameters the mechanical properties of nanotubes approach those of the corresponding flat graphene-like sheets. Received: 30 November 1998 / Accepted: 14 December 1998     

Structure of single-wall carbon nanotubes purified and cut using polymer

Following on from our previous report that a monochlorobenzene solution of polymethylmethacrylate is useful for purifying and cutting single-wall carbon nanotubes (SWNTs) and thinning SWNT bundles, we show in this report that polymer and residual amorphous carbon can be removed by burning in oxygen gas. The SWNTs thus obtained had many holes (giving them a worm-eaten look) and were thermally unstable. Such severe damage caused by oxidation is unusual for SWNTs; we think that they were chemically damaged during ultrasonication in the monochlorobenzene solution of polymethylmethacrylate. Received: 28 March 2001 / Accepted: 2 August 2001 / Published online: 17 October 2001     

Filling carbon nanotubes

Received: 2 March 1998     

Dispersion and alignment of carbon nanotubes in polycarbonate

Dispersion and alignment of carbon nanotubes in thermoplastic polymers such as polycarbonate have been studied. Dispersion was accomplished by mixing in a conical twin-screw extruder and alignment was carried out using a fiber-spinning apparatus. The effects of mixing time and fiber draw rates on dispersion and alignment were investigated. Uniform dispersions were produced with relatively short residence times in the extruder. Excellent alignment of carbon nanotubes in nanocomposite filaments was obtained when the fiber draw rate was greater than 70 m/min. The ability to closely control the dispersion and alignment of carbon nanotubes in polymers is expected to lead to the development of nanocomposites with desirable electronic and structural properties. Received: 7 January 2002 / Accepted: 11 April 2002 / Published online: 10 September 2002 RID="*" ID="*"Corresponding author. Fax: +1-508/233-5521, E-mail: Michael.Sennett@natick.army.mil     

Growth of single-wall carbon nanotubes dependent on laser power density and ambient gas pressure during room-temperature CO2 laser vaporization

Single-wall carbon nanotubes (SWNTs) were synthesized by the irradiation of 20-ms CO₂ laser pulses onto a graphite–Co/Ni target at room temperature. We investigated the effect of laser power density (10–150 kW/cm²) and ambient Ar gas pressure (150–760 Torr) on the abundance of SWNTs with lengths of up to about 200 nm in soot-like carbonaceous deposits. For a constant power density (30 kW/cm²), depending on the Ar gas pressure, SWNTs with diameters of 1.2–1.4 nm were synthesized. Expansion behavior and temperature-fall rates of clusters and/or particles in laser plumes were also analyzed by high-speed video imaging and temporally and spatially resolved emission spectroscopy. The temperature-fall rates were estimated to be 171–427 K/ms. The SWNT growth on the time scale of a few milliseconds appeared to be related to some features of condensing clusters and/or particles, including resident densities, collision frequencies and temperatures. Received: 16 July 2001 / Accepted: 23 July 2001 / Published online: 30 August 2001     

Microscopic growth mechanisms for carbon and boron-nitride nanotubes

Received: 27 November 1998 / Accepted: 18 December 1998     

Catalysed growth of novel aluminium oxide nanorods

Micron-scale coral-like aluminium oxide structures have been generated by heating a mixture of AlB₂ and Co powders in a quartz boat at ca. 1050 °C under N₂. Upon sonication in acetone, the structures break down into elongated single-crystal aluminium oxide nanorods ranging from 20 to 200 nm in diameter and up to 5 μm in length. Single Co particles are often found attached to nanorod tips. A vapour–liquid–solid (V–L–S) mechanism appears to be responsible for the aluminium oxide nanorod growth. Received: 21 January 2003 / Accepted: 22 January 2003 / Published online: 28 March 2003 RID="*" ID="*"Corresponding author. E-mail: d.walton@sussex.ac.uk     

Large-scale synthesis of GaN nanorods and their photoluminescence

Large quantities of gallium nitride (GaN) nanorods have been synthesized via direct reaction of metallic gallium vapor with flowing ammonia at 970 °C in a quartz tube. The nanorods have been confirmed as crystalline wurzite GaN by powder X-ray diffraction, selected-area electron diffraction and X-ray photoelectron spectrometry. Transmission electron microscopy and scanning electron microscopy reveal that the nanorods are straight and uniform, with diameters ranging from 40 nm to 150 nm and lengths up to hundreds of micrometers. The growth mechanism is discussed briefly. Photoluminescence measurements on bulk GaN nanorods at room temperature show two strong peaks at 377 nm (3.28 eV) and 360 nm (3.44 eV) attributed to the zero-phonon donor-acceptor pair transition and the donor-bound exciton, respectively. Received: 19 April 2001 / Accepted: 10 May 2001 / Published online: 20 June 2001     

NMR study of the magnetic properties of the polymerized phase of Cs1C60

\chem{Cs_{1}C_{60}} . The NMR spectrum allows, when the Magic Angle Spinning (MAS) technique is used, to get information about the electronic structure of this phase, which points out that the spin density is not uniformly distributed on the ball. An estimation of the strength of the quadrupolar effects on is then given, which shows that it is possible to analyse the spin-lattice relaxation time in terms of magnetic effects. A comparison with the of is then justified and allows us to point out the importance of low dimensional antiferromagnetic fluctuations in this compound. We also investigate the nature of the magnetic ground state in : the broadening of the spectra states unambiguously its magnetic nature and a detailed analysis of their shape gives further indications on the antiferromagnetic nature of the magnetic order, which seems characterized by a large amount of disorder. Received: 19 November 1996/Accepted: 19 December 1996