Two of the most intriguing materials used in organic electronic devices — single-walled carbon nanotubes and graphene sheets — share the same hexagonally arranged carbon crystal structure. However, while carbon nanotubes can be synthesized with relative ease, effective techniques to produce graphene are still in their infancy.

To many scientists, this raises the question: can graphene sheets be formed directly from carbon nanotubes? The answer, according to Lunhui Guan and Jiaxin Li from the Fujian Institute of Research on the Structure of Matter in China,1 is yes — but only if the carbon nanotubes encapsulate foreign particles.

The researchers introduced the molecule ferrocene — composed of an iron atom sandwiched between two five-membered cyclic carbon rings — into single-walled carbon nanotubes by heating the ferrocene and nanotubes together at 300 °C for 48 hours. Transmission electron microscopy (TEM) revealed that the ferrocene particles were trapped inside the hollow nanotube cavities.

Fig. 1: Transmission electron microscopy image of few-layer graphene formed from single-walled carbon nanotubes.

Next, the researchers annealed the ferrocene–carbon nanotube material by heating at 1200 °C. After 24 hours, the nanotubes had spontaneously transformed into sheets of graphene just 2–10 atomic layers thick. TEM imaging and Raman spectroscopy confirmed the purity of the newly formed graphene (Fig. 1).

According to Guan, the encapsulated ferrocene molecules modify the electronic properties of the carbon nanotubes in a way that reduces the energy barriers for the transformation into graphene. The researchers hypothesize that iron nanoparticles and hydrogen/hydrocarbon vapor, formed during the high-temperature annealing process, etch away the nanotubes to yield the graphene sheets.

The structural transformation reported by Guan is also highly dependent on the diameter of the host single-walled carbon nanotubes. Whereas graphene sheets were successfully obtained from nanotubes with diameters of 0.9 nm and 1.1 nm, tubes with diameters of 1.4 nm were found to form double-walled carbon nanotubes upon annealing. Fortunately, Guan and colleagues are world-leaders in creating nanotubes with exceptionally small diameters.

“The reactivity and stability of single-walled carbon nanotubes are inversely proportional to their diameters,” says Guan. “And so, the tubes with smaller diameters collapsed first.”

Guan has a range of uses in mind for this technique. “These findings should allow single-walled carbon nanotubes to be used as starting materials for new substances, such as graphene nanoribbons, in a controlled manner.”