Abstract Submitted to the    NANOTUBE'04 Conference:

We have compared many CVD processes, involving classical thermal and activated CVD processes, to grow films of aligned carbon nanotubes and other carbon nanostructures (nanofibers, nanocones, ...) with a high selectivity. Activation of the gas phase was carried out by hot filaments (HF CVD), by a gas discharge (DC CVD) or by the use of both activation pathways (DC HF CCVD). Prior to the CVD growth, a dispersion of either Co or Fe catalytic particles on SiO2 (8nm)/Si(100) was in situ performed through an effusive cell in ultra high vacuum directly connected to the CVD reactor (limiting pressure 10-9 mbar). This ensures a high reproducibility and a narrow size distribution down to 2 nm of the catalytic particles. Samples before and after CVD were characterized by SEM, TEM, HRTEM, X ray absorption (XAS)and Raman spectroscopies and in situ XPS, AES, ELS.
It is found that the DC HF CVD process is by far the most efficient process to get aligned nanotubes as it combines the activation of the gas mixture (C2H2:H2) both by a plasma created between two electrodes and by hot filaments. Thus both ions and highly reactive neutral radicals are formed with a high density and they are focussed onto the substrate which is slightly negatively biased relative to the cathode. Therefore we propose a mechanism of nanotubes nucleation and growth that account for the occurrence of ions and highly reactive radicals. The nucleation occurs through a nest of carbon that cannot further develop due to etching and sputtering. We also found that the occurrence of carbon nanotubes, relative to other carbon nanostructures, is highly sensitive to the ratio of the power input into the hot filaments and into the DC discharge, respectively. This ratio governs the density of highly reactive neutral and ions, respectively. Finally with highly dispersed Fe nanoparticles, individual and aligned single or double wall nanotubes were grown with density higher than 1012 cm-2. The alignment of the films of carbon nanotubes has been quantitatively studied by angular X-ray absorption spectroscopy (XAS) on the C K edge. It is found that the p -----> p* transition at 285.2 eV is highly sensitive to the mutual orientation of the carbon nanotubes. However the XAS spectra are strongly surface sensitive and the molecules adsorbed on the tube must be preliminary removed by a thermal treatment.