| While fullerenes form in yields exceeding 30% by mass when a laser generated pure carbon vapor is allowed to condense under annealing conditions, more than 70% of all vaporized carbon assembles in the form of single-wall fullerene nanotubes (SWNTs) in the presence of certain transition metal SWNT catalysts. Due to the high carbon to metal ratio in the vapor will the early stages of condensation, even in the presence of a SWNT catalyst, be characterized by an abundance of all-carbon fullerene-precursors. Metal atoms will then condense onto these clusters as condensation proceeds, and SWNTs are nucleated from fullerene-precursors interacting with a metal vapor as an off-shoot of the road leading to C{dollar}sb{lcub}60{rcub}{dollar}-fullerene. Electron nano-diffraction performed on crystalline strands of aligned SWNTs demonstrates that samples are dominated by tubes of "armchair" geometry, while X-ray diffraction shows a narrow diameter distribution, centered around 1.36 nm. In conclusion, the particular armchair tube matching the observed diameter, labeled the (10,10) tube, is the most prominent individual tube in the investigated samples. The astonishing efficiency with which armchair tubes form suggests that carbon clusters tend to anneal towards armchair geometries even in early stages of condensation, eventually rearranging into bowl-shaped fullerene-precursors with their open edge energy reduced by formation of triply-bonded pairs of 2-coordinated carbon atoms. In a pure carbon environment, such fullerene-precursors are predetermined to close into C{dollar}sb{lcub}60{rcub}{dollar}-fullerene. In the presence of a SWNT catalyst however, metal atoms diffusing (scooting) along the growing edges of fullerene-precursors prevent their closure, forcing them to grow into tubelets with an open, growing armchair edge instead. The optimum diameter of such tubelets results from competition between strain energy in their cylinders (favoring tubelets of large diameters) and open edge energy (favoring tubelets of small diameters). At annealing temperatures, tubelets will initially widen their diameters as they add more carbon to reduce strain. Beyond a certain size however, clusters will finally have too many atoms to rearrange on the relevant time scale. At this time, their diameters get kinetically frozen due to shear size, and a diameter distribution centered around the (10,10) tube diameter is locked in place. |
