Single-walled carbon nanotubes (SWNTs) are poised to play central roles in future nanoelectronic devices, but it remains necessary to develop methods to reliably control their structures. Even small changes in the width of the tubes or the orientation of carbon atoms can alter the conductivity of SWNTs, determining whether they behave as a semiconductor or metal. Now, in a collaboration between Peking University in China and the Samsung Advanced Institute of Technology in Korea, Jin Zhang, Jae-Young Choi, Zhongfan Liu and co-workers1 have developed a ‘cap engineering’ method that generates SWNTs with well-defined diameters.

Typical SWNT syntheses rely on the formation of hemispherical carbon-based nanostructures, or ‘caps’, on metal nanoparticle catalysts at high temperature. These caps initiate the growth of nanotubes, and also define their final structure. The extreme conditions required for synthesis, however, make it difficult to generate caps of uniform size. “We think that if the SWNTs can be grown from existing carbon caps through a vapor–solid or open-ended growth mechanism, the structure of SWNTs could be controlled,” says Zhang.

Fig. 1: Schematic representation of the growth of a single-walled carbon nanotube from an opened fullerene cap (inset), and a scanning electron microscopy image of the prepared nanotubes.From Ref. 1. Reproduced with permission. © 2010 ACS

The team developed a method using fullerenes (C60) and derivatives called fullerendiones, which can open to form caps with predetermined diameter. After depositing these ball-shaped carbon structures on a quartz substrate, the team cracked opened the carbon–carbon bonds by high-temperature oxidation. Treating the caps with water eliminated any amorphous carbon from the structure, and subsequent heating removed oxygen-containing groups. The activated caps were finally exposed to ethanol at 900 °C, which induced the growth of uniform SWNTs (Fig. 1). Ethanol releases carbon radicals that are added directly to the cap ends, resulting in the growth of nanotubes by an open-ended growth mechanism.

The researchers found that the oxidation temperature was crucial in determining the diameter of the fullerene-derived caps, and therefore for controlling the SWNT structure. Higher temperatures caused the breakage of more carbon–carbon bonds, giving smaller caps and resulting in thinner SWNTs. The team also observed that at lower temperatures, the oxidation produced oxygen-bridged ‘opened fullerenes’ that further coalesced into larger caps, leading to larger-diameter nanotubes.

“We are now working on controlling the chirality of SWNTs during growth using this cap engineering approach,” says Liu.