Laboratoire Charles Coulomb UMR 5221 CNRS/UM2 (L2C)


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Preparation of individual carbon nanotubes by CCVD

par Sébastien LAYSSAC - publié le , mis à jour le

Involved researchers : V. Jourdain, D.-Y. Kim, M. Picher, X. T. Than
Collaborations : B. Lounis, L. Cognet (CPMOH, Bordeaux), P. Poncharal, A. Ayari, C. Journet, J.-M. Benoît, S. Purcell, A. San Miguel (LPMCN, Lyon), P. Roussignol (ENS Paris), A. Loiseau, R. Arenal (ONERA, Châtillon), C. Amiens, B. Chaudret (LCC Toulouse), J. Dijon (LITEN, CEA Grenoble), C. Bichara (CINAM, Marseille)

Many synthesis techniques were developed to produce carbon nanotubes. To study the physical properties of individual carbon nanotubes, the most convenient approach is to grow nanotubes that are already individual. Up to now, only the Catalytic Chemical Vapour Deposition (CCVD) method allows to do that. Hereunder is a summary of our work toward the controlled synthesis of samples of individual carbon nanotubes to allow the study of their physical properties.
Our group has a CCVD reactor devoted to the synthesis of carbon nanotubes. Carbon nanotubes are commonly synthesized by catalytic decomposition of gaseous carbon precursors at the surface of nanoparticles (usually transition metals such as nickel, iron or cobalt). Nanoparticles play two crucial roles in the formation of carbon nanotubes : they favour a spatially-localized decomposition of the carbon precursor at their surface and they act as a template for controlling the nanotube diameter. The particles are usually deposited on a substrate (e.g. silicon with a thermal oxide layer).
The density of grown nanotubes depends on the density of active catalyst particles while the nanotube structure is dependent on the nature and size of the catalyst particle and on the growth conditions (pre-treatment conditions, synthesis temperature and precursor supply). The latter control the activation or the deactivation of the nanoparticles. Depending on the density of grown nanotubes, samples of individual nanotubes, crossing nanotubes or bundled nanotubes can be preferentially prepared.
To prepare partially-suspended nanotubes, we used perforated silicon oxide or silicon nitride membranes as substrates for nanotube growth (Figure 2). In the absence of substrate, the structure and the optical properties of the nanotubes can be easily studied. An additional advantage is that the nanotubes can be directly characterized without any special preparation.

Figure 1 : Left and middle : AFM images of individual carbon nanotubes on a SiO2/Si substrate. Right : TEM image of partially-suspended carbon nanotubes over a perforated silicon oxide membrane - © L2C

During the CCVD growth, nanotubes with one end suspended in the gas phase and their other end still attached to the substrate are frequently observed. The suspended end of the nanotube displays random motion whose amplitude intensifies with the increasing nanotube length. If the free part of the nanotube contacts a surface, i twill stick to it via van der Waals interactions. This phenomenon can be employed to align the nanotubes along the gas flow (Figure 3a) by using growth conditions enabling a so-called "kite-growth". This requires special and well-controlled aerodynamic conditions (laminar flow, adequate balance between forced convection and natural buoyancy). Freed from the deactivation processes occurring on the substrate, these nanotubes can reach lengths of several centimetres, i.e. a single molecule with an aspect ratio of 107. By combining two successive kite-growths on the same substrate, one can obtain a crossed network of carbon nanotubes with a high density of connections (Figure 3b).

Figure 2. SEM images of (a) ultralong flow-aligned nanotubes and of (b) a crossed network of ultralong carbon nanotubes - © L2C

M. Paillet, et al., J. Phys. Chem. B 108, 17112 (2004), M. Paillet, Thèse de l’Université Montpellier 2 (2005), M.-F. Fiawoo et al., Surf. Sc. 603, 1115 (2009).

Fundings : ANR PNANO Nanotubes suspendus and T-NICE, ANR P3N Excitubes and SOS Nanotube.