Catalytic Growth Mechanism for Nanotube Bundles

Both in the vaporization [5] and in the arc-discharge [6] catalytic techniques, nearly perfect single-wall nanotubes of uniform diameter 1.4 nm) are produced in high yield (70-90%), and are arranged in bundles or ropes (2D triangular lattice) of a few tens of tubes with similar spacing between adjacent neighbors 3.2 A [42]). Only a few isolated carbon single-shell tubes are observed. Additionally, the use of a second element beside the catalyst (bi-metal mixtures : Ni-Co, Ni-Y, ...) is also found to produce the greatest yield, sometimes 10-100 times that for single metals alone.

Actually no coherent explanation of the growth mechanism has clearly emerged yet to explain the formation of ropes of single-shell tubes. However, some crucial information can be extracted from TEM observations. Larger-diameter bundles consist mostly of an assembly of smaller bundles separated by twin-like boundaries [6], suggesting for their formation an alignment of the pre-formed smaller bundles due to van der Waals forces. In contrast, these pre-formed smaller bundles are probably the result of a correlated growth between single-wall nanotubes which constitute this rope.

Such a correlated growth could be explained by a modified "spot-weld" model proposed for the stabilization of the open growing edge of multishell tubes [21]. During the growth of ropes, bridging metal catalyst adatoms could connect the open edges of two neighboring single-wall nanotubes (see Fig. 15), preventing the spontaneous closure of each single-shell tube. The chemical activity of the open end would still be very important due to the presence of the metal catalyst as a surfactant, favoring the formation of long ropes.

The feed stock for the rope arrives at the growing end of the nanotube mostly by the diffusion of carbon clusters along the sides of the tubes and also from direct incorporation of carbon from the vapor phase. While only a few

Fig. 15. Schematic ball-and-stick representation of a nanotube bundle composed of seven (6,6) armchair carbon nanotubes (white spheres). Several transition metal catalyst Ni and/or Co atoms (in black) are shown, occupying sites between the growing edge of adjacent single-shell nanotubes, thus stabilizing the open edge configuration of the nanotube bundle catalytic bridges need to be involved in the growth of nearest-neighboring single-wall tubes, other metal atoms will soon congregate at the growing edge, forming other links or small metal clusters. The metal catalyst that exists at the live end of the growing rope, needs to be sufficiently efficient to incorporate the large amount of available carbon feed-stock in the perfect cylindrical network of the tubes. If the cluster is too large, it can be ejected from the growing edge of the rope, explaining its observation along the walls of nanotube bundles [38]. Some static and dynamical ab initio calculations are now being performed [35] to check the validity of the proposed mechanism.

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