Conclusion

Since their discovery in 1991, great progress has been made in the understanding of carbon nanotube growth. There has been a constant fruitful interplay between theoretical calculations and experimental measurements which has enhanced our insight into the formation processes of these ultimate carbon fibers.

Fig. 18. The atomistics of the addition of a single hexagon at the nanotube base by bond formation between a pair of handle atoms (black dots) at the opposite sides of a heptagon [45]. (a) for an isolated heptagon, a 5-7 pair forms in addition to a hexagon; (b) for a 5-7-6 complex, only an additional hexagon forms. (c) Shows the annihilation of two adjacent 5-7 pairs into four hexagons by a Stone-Wales switch, but retaining the same number of carbon atoms [46]

Fig. 18. The atomistics of the addition of a single hexagon at the nanotube base by bond formation between a pair of handle atoms (black dots) at the opposite sides of a heptagon [45]. (a) for an isolated heptagon, a 5-7 pair forms in addition to a hexagon; (b) for a 5-7-6 complex, only an additional hexagon forms. (c) Shows the annihilation of two adjacent 5-7 pairs into four hexagons by a Stone-Wales switch, but retaining the same number of carbon atoms [46]

Several mechanisms, as described above, have been proposed to account for the growth mechanisms of single-wall, multi-wall and ropes of carbon nanotubes with or without the presence of any catalyst. The key role played by the metal catalyst is crucial for understanding the growth of single-shell tubes at the microscopic level. However, the actual role of the metal or alloy is still very confusing, although some interesting models have been proposed. At the present stage, experimental observations of quenched-growth singlewall structures are required to validate one or another of these models. In-situ experimental studies have just started [47,48], and are very promising for a direct determination of important characteristics, such as the temperature gradient, time evolution of the matter aggregation after the initial vaporization of the different chemical species, ... Actually, experimental observations to date have relied only on the study of the soot after the synthesis has occurred.

Probably the most intriguing problem is to understand the microscopic mechanism and optimum conditions for the formation of well-designed singlewall nanotubes. Although it has been argued that the armchair nanotube structure is favored energetically [5], experimental conditions under which these tubules would be grown with good control are still not yet well known. Nonetheless, as more experimental data become available to correlate the atomic structure and the synthesis conditions, and more is known about the growth at the atomic level, it is hoped that controlled growth of single-walled carbon nanotubes with designed structures will be achieved soon. In addition, with further experimental confirmation of their unique properties, there will be a great incentive to develop industrial-scale production methods.

Acknowledgments

J.C.C. acknowledges the National Fund for Scientific Research (FNRS) of Belgium for financial support. This paper presents research results of the Belgian Program on Inter-university Attraction Poles initiated by the Belgian State-Prime Minister's Office-Science Policy Programming. This work is carried out within the framework of the EU Human Potential - Research Training Network project under contract N° HPRN-CT-2000-00128 and within the framework of the specific research and technological development European Union (EU) programme "Competitive and Sustainable Growth" under contract G5RD-CT-1999-00173. J.C.C. is indebted to X. Blase, A. De Vita and R. Car for their collaborative efforts and to the Institut de Recherche Numérique en Physique des Matériaux (IRRMA-EPFL, Lausanne, Switzerland) for hosting part of the research presented above.

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