Nature 512, 61–64 (2014)

Credit: NATURE PUBLISHING GROUP

The electronic properties of single-walled carbon nanotubes are dictated by their diameters and also by how the graphene lattice from which they are notionally made has been rolled up. In 'armchair' structures, polyacene chains run parallel to the long axis of the tube, whereas it is poly-p-phenylenes that do this in 'zigzig' nanotubes. 'Chiral' nanotubes are those in which neither of these substructures run straight from one end of the tubes to the other, but rather twist along their length. Carbon nanotubes are typically made as a mixture of different tube types, although samples with narrow diameter distributions can be obtained using various synthesis techniques or post-synthetic separation methods.

Now, a team of researchers in Germany and Switzerland, led by Konstantin Amsharov and Roman Fasel, have shown that by using a defined molecular precursor it is possible to very selectively produce carbon nanotubes of just a single type. Starting from 2-acetonaphthone, a three-fold symmetric polycyclic hydrocarbon (C96H54, pictured) was prepared using traditional solution-phase organic synthesis. When this precursor was deposited on to a platinum surface and heated to 770 K under ultrahigh-vacuum conditions, a surface-catalysed cyclodehydrogenation (CDH) reaction resulted in more than half of the polycyclic hydrocarbons forming atomically precise nanotube caps. Although other structures were also created through alternative CDH reactions, none of these products could act as seeds for nanotube growth.

When the nanotube caps were exposed to carbon sources such as ethanol or ethene at temperatures from 670–770 K they grew away from the metal substrate, which was confirmed by scanning tunnelling microscopy (STM). Characterization of the surface-bound nanotubes with Raman spectroscopy suggested that only nanotubes with a (6,6) chiral index had been formed, and high-resolution STM images of nanotubes lying flat on the surface were also consistent with the hexagonal lattice expected in such a structure. Taken together, the evidence suggests that an atomically precise precursor leads to a single cap structure, which, in turn, controls the subsequent nanotube growth.