With their unusual mechanical, thermal, and electronic properties, carbon nanotubes (CNTs) promise the ability to construct the next generation of smaller and faster electronic and electro-optical components. To achieve this goal, however, the CNTs must exhibit specific properties that depend on their structures. However, current production methods lead to a mixture of different CNTs, as characterized by a “chiral index” that describes the way the graphene sheet is wrapped.
As described in the August 7 issue of Nature (DOI: 10.1038/nature13607; p. 61), researchers have now developed a new method that can be used to produce single-walled carbon nanotubes (SWCNTs) with a single, pre-specified structure. Led by Martin Jansen, Director Emeritus at the Max Planck Institute for Solid State Research, and Roman Fasel, head of Empa’s Nanotechnology Department and titular professor at the Department of Chemistry and Biochemistry of the University of Bern, the research team also confirmed that these nanotubes have identical electronic properties.
By depositing the precursor molecule C96H54 on a Pt(111) surface, and using surface-catalyzed cyclodehydrogenation, the researchers formed ultrashort singly capped (6,6) “armchair” nanotube seeds. They then used ethanol as a carbon feedstock gas to epitaxially elongate the seeds up to a few hundred nanometers. Out of 100 precursor monomers, more than 50% adopted the desired configurations. “Most importantly,” the researchers reported, “the condensation products of precursor molecules exhibiting ‘wrong’ conformations cannot act as seeds for the subsequent CNT growth process via epitaxial elongation, and thus will not affect the selectivity of SWCNT formation.”
The researchers have thus proven that they can unambiguously specify the growth and thus the structure of long SWCNTs using custom-made molecular seeds. The SWCNTs synthesized in this study can exist in two forms, which correspond to an object and its mirror image. By choosing the precursor molecule appropriately, the researchers were able to influence which of the two variants forms. Depending on how the honeycomb atomic lattice is derived from the original molecule—straight or oblique with respect to the CNT axis—it is also possible for helically wound tubes, that is, with right- or left-handed rotation, and with non-mirror symmetry to form. And it is precisely this structure that then determines the electronic, thermoelectric, and optical properties of the material. In principle, the researchers can therefore specifically produce materials with different properties through their choice of the precursor molecule.
An end cap is produced from planar carbon, which forms the seed for the growth of a carbon nanotube. The calculated computer images on the left were supplemented with images taken with a scanning tunneling microscope, as seen on the right. © Empa/Juan-Ramon Sanchez.
