Is Uncatalyzed Growth Possible for Single Shell Nanotubes

The growth of single-shell nanotubes, which have a narrow diameter distribution (0.7-2 nm), differs from that of multishell tubes insofar as catalysts are necessary for their formation. This experimental fact is consequently an indirect proof of the existence of covalent lip-lip interactions which are postulated to be indispensable in a pure carbon atmosphere and imply that all nanotubes should have multiple walls. Single-wall tubes with unsaturated carbon dangling bonds at the growing edge are prone to be etched away in the aggressive atmosphere that is operative under typical synthesis conditions, which again explains the absence of single wall tubes in a pure carbon environment. However, the growth process of single-walled tubes differs also from the growth of conventional catalyst-grown carbon fibers in that no "observable" catalyst particle is seen at the tips of single-shell tubes, which appear to be closed by domes of half-fullerene hemispheres.

There have been several works based on classical, semi-empirical and quantum molecular dynamics simulations attempting to understand the growth process of single-shell tubes [22,23,26,27,28]. Most importantly, these studies have tried to look at the critical factors that determine the kinetics of open-ended tube growth, as well as studies that determine the relative stability of local-energy minimum structures that contain six-, five-, and seven-membered carbon rings in the lattice. Classical molecular dynamics simulations show that wide tubes which are initially open can continue to grow straight and maintain an all-hexagonal structure [27,28]. However, tubes narrower than a critical diameter, estimated to be about ~ 3 nm, readily nucleate curved, pentagonal structures that lead to tube closure with further addition of carbon atoms, thus inhibiting further growth. Continued carbon deposition on the top of a closed tube yields a highly disordered cap structure, where only a finite number of carbon atoms can be incorporated, implying that uncatalyzed defect-free growth cannot occur on single-shell tubes.

First-principles molecular dynamics simulations [22] show that the open end of single-walled nanotubes closes spontaneously, at experimental temperatures (2000K-3000K), into a graphitic dome with no residual dangling bonds (see Fig. 9). Similar self-closing processes should also occur for other nanotubes in the same diameter range, as is the case for most single walled nanotubes synthesized so far [3,5,6]. The reactivity of closed nanotube tips was also found to be considerably reduced compared to that of open end nanotubes. It is therefore unlikely that single-walled nanotubes could grow by sustained incorporation of C atoms on the closed hemifullerene-like tip. This is in agreement with the theoretical finding that C atoms are not incorporated into Ceo [30].

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