Finding practical applications of carbon nanotubes (CNTs) depends on being able to control their physical properties, such as chirality — the way in which the tubes are rolled. Although methods have been developed to determine the chirality of CNTs, it has proved difficult to measure their chirality-dependent redox properties, or the amount of energy needed to add or take away an electron.

Now, Naotoshi Nakashima from Kyushu University and colleagues in Japan1 describe a simple spectroscopic method for measuring the redox properties of CNTs. The researchers employed photoluminescence spectroelectrochemistry to measure the redox properties of individual single-walled nanotubes (SWNTs), and then correlated their results with the chirality of the nanotubes. An understanding of the relationship between these properties allows the chirality of nanotubes to be matched with the desired applications.

The method involved embedding individual SWNTs into a polymer film, which was then cast over a thin electrochemical electrode. The researchers then used photoluminescence spectroscopy to produce a map of nanotubes of different chirality across the film based on the fact that the photoluminescence of nanotubes is chirality-dependent.

Fig. 1: Sample data for three different chiralities of SWNTs.

The polymer/nanotube composite film was then electrochemically cycled, and Nakashima and his colleagues monitored how the photoluminescence from each tube changed with applied potential. From the photoluminescence response, the researchers deduced the addition or withdrawal of an electron from each tube, thereby determining the redox properties of individual tubes (Fig. 1).

Nakashima's team also used their results to determine the work functions of the SWNTs based on the values of the Fermi levels. Their results correlated well with theoretical predictions; in particular, they found that the work functions of SWNTs with diameter greater than 0.85 nm changed significantly with chirality. Furthermore, for SWNTs of smaller than 0.85 nm in diameter, the work functions increased with decreasing tube diameter.

“These basic results are important for the design and fabrication of electronic nanodevices using single-walled nanotubes,” says Nakashima. “They are also useful for control of the desired electronic states of isolated nanotubes, since this is possible by fine-tuning of the external applied potential.”