A FORMATION MECHANISM FOR CATALYTICALLY GROWN HELIX-SHAPED GRAPHITE NANOTUBES

The concept of a spatial-velocity hodograph is introduced to describe quantitatively the extrusion of a carbon tubule from a catalytic parti cle. The conditions under which a continuous tubular surface can be ge nerated are discussed in terms of this hodograph, the shape of which d etermines the geometry of the initial nanotube. The model is consisten t with all observed tubular shapes and explains why the formation proc ess induces stresses that may lead to ''spontaneous'' plastic deformat ion of the tubule. This result is due to the violation of the continui ty condition, that is, to the mismatch between the extrusion velocity by the catalytic particle, required to generate a continuous tubular s urface, and the rate of carbon deposition.