NUCLEATION OF CARBON NANOTUBES WITHOUT PENTAGONAL RINGS

The unique one-dimensional structure of single-walled and multiwalled carbon nanotubes [1] depends critically upon a nucleation mechanism which somehow restricts graphitic growth to a single axis. The transient and extreme growth conditions of carbon nanotubes have obscured the mechanism of nucleation. Most hypothetical nucleation mechanisms invoke pentagon formation to produce a hemispherical graphitic cap, with the cap’s edges either open [2–4], attached to a substrate via a tubular segment [5–7], or mated with a second cap into a closed fullerene [8–10]. These models all require an abrupt transition from a regime favoring pentagon formation (when the cap is forming) to one that favors exclusively hexagon formation (when the tube is lengthening) once exactly six pentagons have formed [11]. Other models nucleate a nanotube from a 40-atom polyyne ring [12,13]. Unfortunately, such a large carbon ring is unlikely: carbon clusters change from simple rings to double rings and cages above 20 atoms [14]. Very recently, Bandow et al. have discovered that the temperature during synthesis controls the diameter of the nanotubes [15]. Since the diameter of a nanotube is fixed by its size at nucleation, these results provide rare experimental insight into nucleation itself. Here we propose a new nucleation model for carbon nanotubes which (1) contains only hexagonal rings within the tube nucleus and therefore does not require a transition from pentagon to exclusively hexagon formation, (2) can explain the temperature dependence of the nanotube diameter distribution, (3) accounts for the narrow diameter distribution of single-walled nanotubes, and (4) can explain the wider diameter distribution of multiwalled nanotubes. Carbon nanotube synthesis apparently requires a surface on either an electrode or a metallic particle [16]. Surfaces favor the growth of small flat graphitic patches. (Such graphitic patches can form even in much higher density environments [17].) Although it may be difficult to imagine a means by which a single graphitic patch could curl into the nucleus of a nanotube without the incorporation of pentagons to produce the needed curvature [18], we herein describe a natural kinetic pathway by which a two-layered graphitic patch can transform into the nucleus of a nanotube.