Towards processing of carbon nanotubes for technical applications

Production methods for carbon nanotubes are now well established and allow their synthesis on a scale of grams per day. For many potential applications of this unique material, its purification still remains a crucial problem. In this article various purification methods for single- and multi-wall carbon nanotubes are reviewed. These methods are compared in terms of their capacity, efficiency, and effects on the tubes. In addition, the use of Raman spectroscopy for monitoring the chromatographic purification of single-wall nanotubes is described. Received: 17 May 1999 / Accepted: 18 May 1999 / Published online: 29 July 1999     

Synthesis and Raman characterization of mono-sized single-wall carbon nanotubes in one-dimensional channels of AlPO4-5 crystals

Mono-sized single-wall carbon nanotubes were formed in one-dimensional channels of AlPO₄-5 single crystal (AFI) by pyrolysis of tripropylamine (TPA). Raman spectra have been measured for the TPA-AFI crystals thermally processed at different conditions. TPA molecules are carbonized at 400 °C, and carbon nanotubes were formed at 500 °C or above. The radial-breathing mode, which is special for carbon nanotube geometry, was observed. Three Raman-active modes with symmetry A _1g, E _1g, and E _2g were identified by detailed symmetrical analysis for the polarized-Raman spectra. Received: 29 October 1998 / Accepted: 29 March 1999 / Published online: 24 June 1999     

Multishell conduction in multiwalled carbon nanotubes

The full electronic complexity of multiwalled carbon nanotubes may be explored by sequentially removing individual carbon shells. This technique is employed to directly measure the number of shells contributing to conduction at room temperature, as well as the contribution of each shell to the overall conductance. By exploring the gate dependence of the conductance, the random alternation between semiconducting and metallic shells can also be observed. Received: 31 August 2001 / Accepted: 3 December 2001 / Published online: 4 March 2002     

Analysis of the concentration of C60 fullerenes in single wall carbon nanotubes

Single wall carbon nanotubes filled with C₆₀ were analyzed using resonance Raman scattering and electron energy loss spectroscopy. In order to obtain concentrations of the fullerene molecules inside the tubes, the scattering intensity from the fullerenes relative to that from the tubes was used. Since the scattering intensity from the tubes is subject to strong fluctuations, the determination of the concentrations is shown to require averaging of results from different lasers and from all observable Raman lines. The fluctuations are shown to be intrinsic and a consequence of photoselective resonance scattering. Calibration of absolute concentrations can be obtained from electron energy loss spectroscopy performed on the same samples. Samples with three different diameters were analyzed and good agreement between the fullerene concentrations measured by the two methods was obtained. Received: 20 September 2002 / Accepted: 4 November 2002 / Published online: 10 March 2003 RID="*" ID="*"Corresponding author. Fax: +43-1/4277-51375, E-mail: kuzman@ap.univie.ac.at     

A new purification method for single-wall carbon nanotubes (SWNTs)

A new purification procedure is introduced, which uses the advantages of column chromatography and vacuum filtration. Potassium polyacrylate is used as stationary phase. This method is based on the idea that the size of the existing cavities in the polymer increases during a swelling process in distilled water. The cavities are big enough to entrap the nanoparticles, but allow for a free movement of nanotubes and bundles. Received: 6 March 2000 / Accepted: 7 March 2000 / Published online: 5 April 2000     

Raman spectroscopy on single- and multi-walled nanotubes under high pressure

The pressure dependence of the high-energy Raman modes in single- and multi-walled carbon nanotubes was measured in the range 0–10 GPa. We found the pressure coefficient to be linear in both materials but 25% smaller in MWNT. Given that the curvature effects on vibrational properties of the rolled-up graphene sheets are small, we can explain this difference simply with elasticity theory. Received: 17 May 1999 / Accepted 18 May 1999 / Published online: 4 August 1999     

Laser heating method for estimation of carbon nanotube purity

A new method of a carbon nanotube purity estimation has been developed on the basis of Raman spectroscopy. The spectra of carbon soot containing different amounts of nanotubes were registered under heating from a probing laser beam with a step-by-step increased power density. The material temperature in the laser spot was estimated from a position of the tangential Raman mode demonstrating a linear thermal shift (-0.012 cm^-1/K) from the position 1592 cm^-1 (at room temperature). The rate of the material temperature rise versus the laser power density (determining the slope of a corresponding graph) appeared to correlate strongly with the nanotube content in the soot. The influence of the experimental conditions on the slope value has been excluded via a simultaneous measurement of a reference sample with a high nanotube content (95 vol. %). After the calibration (done by a comparison of the Raman and the transmission electron microscopy data for the nanotube percentage in the same samples) the Raman-based method is able to provide a quantitative purity estimation for any nanotube-containing material. Received: 11 December 2001 / Accepted: 12 December 2001 / Published online: 4 March 2002     

Analysis of semiconductor surface phonons by Raman spectroscopy

Only recently Raman spectroscopy (RS) has advanced into the study of surface phonons from clean and adsorbate-covered semiconductor surfaces. RS allows the determination of eigenfrequencies as well as symmetry selection rules of surface phonons, by k-conservation limited to the Brillouin zone-center, and offers a significantly higher spectral resolution than standard surface science techniques such as high-resolution electron energy loss spectroscopy. Moreover, surface electronic states become accessible via electron–phonon coupling. In this article the fundamentals of Raman scattering from surface phonons are discussed and its potential illustrated by considering two examples, namely Sb-monolayer-terminated and clean InP(110) surfaces. Both are very well understood with respect to their atomic and electronic structure and thus may be regarded as model systems for heteroterminated and clean semiconductor surfaces. In both cases, localized surface phonons as well as surface resonances are detected by Raman spectroscopy. The experimental results are compared with surface modes predicted by theoretical calculations. On InP(110), due to the high spectral resolution of Raman spectroscopy, several surface modes predicted by theory can be experimentally verified. Surface electronic transitions are detected by changing the energy of the exciting laser light indicating resonances in the RS cross section. Received: 7 April 1999 / Accepted: 25 June 1999 / Published online: 16 September 1999     

A symmetry-adapted force-constant lattice-dynamical model for single-walled carbon nanotubes

We develop a lattice-dynamical model based on the screw symmetry of single-walled carbon nanotubes that allows for reducing the size of the dynamical matrix to six, for all tube chiralities. The model uses force constants derived from fitting to the phonon dispersion of 2D graphite. We present the calculation procedure in a clear and transparent way, making the model easier to follow. We calculate the phonon dispersions of a number of nanotubes of different chiralities. The splitting of two highest Raman active modes and the radial breathing mode frequency are studied by changing the tube diameter and chirality.     

Radial breathing modes of single-walled carbon nanotubes in resonance Raman spectra at high temperature and their chiral index assignment

Radial breathing modes (RBMs) in resonance Raman spectra from single-walled carbon nanotubes (SWCNTs) on a SiO₂/Si (0 0 1) substrate are studied between 25 and 720 °C. A change in the relative intensity of each RBM peak with temperature is observed, which originates from the temperature dependence of the resonance condition of nanotubes. For 25 °C, each RBM peak is reasonably assigned on the basis of data in the literature [J. Maultzsch, H. Telg, S. Reich, F. Hennrich, C. Thomsen, Phys. Rev. B 72 (2005) 205438]. By taking into account the temperature-dependent behavior of the relative intensity of the RBM peaks, each RBM peak is successfully assigned even for 720 °C. It is found that most of the observed RBM peaks for a laser excitation energy of E_exc = 1.96 eV are from chiral SWCNTs. These results make it possible to discuss further details of the chirality-dependent growth behavior observed for in situ Raman spectroscopy.     

Pulsed laser deposition of multiwalled carbon nanotubes thin films

The physical/chemical properties of multiwalled carbon nanotubes have attracted much interest for applications in different fields, from micro-electronic to coating technology due, in particular, to their peculiar conductivity properties, to their hardness and high resistance to thermal stress. The technology to produce carbon nanotubes thin films with the desired properties, however, is still under development. In this work, we report on multiwalled carbon nanotubes thin films deposited by pulsed laser deposition techniques ablating commercially polystyrene-nanotubes pellets on alumina substrates. MicroRaman spectroscopy and high resolution Transmission Electron Microscopy provide the experimental confirmation that carbon nanotubes-like structures are present on the alumina surface with both minimal morphological damage of the tubes and structural changes induced by laser beam.     

Raman spectroscopy of pentyl‐functionalized carbon nanotubes

We present Raman scattering on carbon nanotubes functionalized with pentyl groups. Studies of the intermediate frequency region and the C–H bond stretching signal along with the D mode show evidence of the addition reaction by Raman spectroscopy. From the resonance profiles of the radial breathing mode (RBM) we assign the chiral indices of the tubes and study the influence of the functionalization on the transition energies, shift and intensity of the RBM signal. The largest effect we observe is on the Raman intensity of the radial breathing mode. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

     

Solid-phase production of carbon nanotubes

4 C₃) are heated to 1000–1100 °C for 2–4 hr in an argon flow. Carbon nanotubes are formed when metal powders, e.g. Fe, Co, Y and Ni, are also present. Received: 2 February 1999 / Accepted: 3 February 1999     

Intermediate frequency modes in Raman spectra of Ar+‐irradiated single‐wall carbon nanotubes

We study the effect of Ar⁺ irradiation on the intermediate frequency modes (IFM) in Raman spectra of single‐wall carbon nanotubes. We observe new features in the intermediate frequency region from 386 to 635 cm^–1 as the defect concentration increases, whereas at the same time there is a decrease of other IFM modes. After annealing in vacuum, the IFM modes recover and become similar to those of the nanotubes before irradiation. We interpret the new features in the Raman spectra as originating from the phonon density of states that becomes visible in the Raman process due to the presence of defects. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Raman spectroscopy and field-emission properties of CVD-grown carbon-nanotube films

Carbon-nanotube films are very efficient cathodes for field-emission devices. This study presents a comprehensive comparison between structural, spectroscopic and field-emission properties of films of aligned and non-aligned multi-wall nanotubes (MWNTs) which are grown by thermal chemical vapour deposition. Three types of films are investigated: vertically aligned MWNTs with clean and coated nanotube side walls as well as non-aligned MWNT films. Raman spectra taken on the aligned MWNT films consist of many lines of first-, second- and third-order signals. Several lines are reported here for the first time for MWNTs. The presence of the surface coating leads to a decrease and broadening of the higher-order signals as well as an increase in the disorder-induced contributions in the first-order regime. The aligned MWNT films have excellent field-emission properties with very high emission current densities and low turn-on and threshold fields. The presence of a surface coating has no impact on the efficiency of the field-emission process. Films of non-aligned MWNTs show considerably reduced electron-emission current densities and larger critical fields. Received: 25 April 2001 / Accepted: 30 May 2001 / Published online: 25 July 2001     

Raman spectroscopy of optical phonons as a probe of GaN epitaxial layer structural quality

We report on Raman scattering measurements of all Raman-active phonons in wurtzite and zinc blende structure GaN epilayers grown on GaAs (001), GaAs (111)A, and GaAs (111)B oriented substrates by means of molecular beam epitaxy (MBE). Raman spectra are taken from these epilayers at room temperature and 77 K in backscattering geometry. The measured values of the phonon frequencies are in agreement with other studies and with lattice dynamic calculations of phonon modes in GaN zinc blende and wurtzite structures. We show that crystal quality is much better in samples grown on GaAs (111) substrates than in samples grown on GaAs (001) substrates. The observation of disorder-activated modes gives information about sample quality. Comparison of the spectra from different thickness epilayers shows that the GaN is more highly disordered close to the substrate, particularly for the (001) substrates. Received 16 July 1999     

Filling carbon nanotubes

Received: 2 March 1998     

Ester-functionalized soluble single-walled carbon nanotubes

We report the preparation of soluble ester- functionalized single-wall carbon nanotubes (sSWNT-COO(CH₂)₁₇CH₃). By use of solution phase IR spectroscopy we are able to compare the ratio of the carbon atoms in the SWNT backbone to the carbon atoms in the ester and amide functionalities of s-SWNTs. Received: 16 July 2001 / Accepted: 3 December 2001 / Published online: 4 March 2002     

Production of carbon nanotubes

Received: 25 February 1998     

CVD synthesis and purification of single-walled carbon nanotubes on aerogel-supported catalyst

Single-walled carbon nanotubes (SWNTs) were synthesized by disproportionation of carbon monoxide on an aerogel-supported Fe/Mo catalyst. A simple acidic treatment followed by an oxidation process produced a high purity (>99%) of SWNTs. The nanotubes obtained are bundled SWNTs and free of amorphous-carbon coating. Several factors that affect the yield and the quality of the SWNTs were also studied. This method shows great promise for large-scale production of SWNTs. Received: 30 August 2001 / Accepted: 3 December 2001 / Published online: 4 March 2002