Single-walled carbon nanotubes (SWNTs) show promise for a wide range of electrical and optical devices. But their properties are highly dependent on how exactly single atomic sheets of carbon are rolled into nanotubes − what is known as chirality. Chirality determines whether a nanotube is metallic or semiconducting, the size of its bandgap, its electron mobility, and other properties, but controlling or predetermining chirality during growth has remained a challenge. Now researchers think that they have found an approach that could lead to on-demand synthesis of SWNTs of specific chiralities [Xu et al., Scientific Reports 7 (2017) 11149].

“This critical issue, chirality controlled synthesis, has not been solved in over 25 years since the discovery of SWNTs,” points out Toshiaki Kato of Tohoku University.

There are hundreds of different chirality SWNTs, but only a handful can be selectively synthesized. But now, together with colleagues from the University of Tokyo, Kato has developed a way of controlling the chirality of SWNTs by tuning the degree of oxidation of the Co catalyst during growth by plasma chemical vapor deposition (CVD). A pre-heating process, whereby a small amount of a reactive gas or mixture of gases is introduced into the growth chamber before synthesis begins, changes the degree of oxidation of the Co catalyst.

“We found that the chirality selectivity of SWNTs grown by plasma CVD is very sensitive to the surface state of the Co catalyst,” Kato explains.

The researchers report that, after the pre-treatment process, small-diameter (6,4) SWNTs, which are otherwise difficult to grow, were preferentially produced. Theoretical calculations and computation studies indicate that the key is the difference in binding energy between the nanotube and the catalyst. At the start of the growth process, a fullerene-like semi-spherical cap-like structure forms on the catalyst surface. To allow the nanotube to grow up from the catalyst surface, the binding energy between the cap and the catalyst must to be overcome. By changing the degree of oxidation of the catalyst, the binding energy can be varied and, therefore, the selectivity.

“Since the degree of oxidation of the catalyst can be precisely tuned, we think that our method has the potential to be applied to other kinds of chirality species. This concept, surface state control of a catalyst for selective synthesis of specific chirality species, is novel,” says Kato.

The approach is a also simple one, which is a major advantage, the researchers believe. The only potential downside is that a catalyst-support material, such as zeolite, is needed to keep the size of the catalyst small, but the team think that this can be overcome in the future.

“We are now trying to expand this method for other species to realize on-demand synthesis of chirality-controlled SWNTs,” adds Kato.

Boris I. Yakobson from Rice University believes that the work is an interesting and useful advance towards producing a specific helicity type of nanotube by manipulating the catalyst’s oxidation degree.

“This study is novel in going beyond ‘passively’ observing the marvels of single-helicity growth, but rather actively changing the catalyst composition in order to tune the outcome of synthetic process,” he comments. “There is definitely more to be learned in this direction, yet this step looks quite encouraging.”

This article was originally published in Nano Today (2017), doi: 10.1016/j.nantod.2017.10.004