Carbon nanotubes are pristine electronic systems, but a less pristine connection to the outside world, such as a physical contact to a metallic electrode, is necessary for device applications. The interface between nanotubes and metals is also important for carbon nanotube growth, which occurs with the help of metal catalyst particles. Now, Ming-Sheng Wang, Dmitri Golberg and Yoshio Bando at the National Institute for Materials Science in Japan1 describe the structural changes that this interface undergoes when it is heated in an environment similar to that for nanotube growth.

Fig. 1: A carbon nanotube (CNT) adjacent to the head of a tungsten (W) tip forms tungsten carbide (WC) as an intermediate, which serves as a catalyst for the formation of graphitic shells on its surface.

The researchers began by making contact between a single multi-walled carbon nanotube and a sharpened tungsten electrode inside a high-resolution transmission electron microscope (TEM). They then used a large current to heat the tungsten, causing it to absorb portions of the nanotube, which reappeared as graphitic shells on the tungsten surface. While the catalytic growth processes of carbon nanotubes have been studied in the past, its observation in real time using a TEM allowed for several important conclusions.

First, the researchers found that the diffusion of carbon atoms occurs through the bulk of the tungsten, rather than on its surface. Second, an intermediate material, tungsten carbide, plays the role of the catalyst, rather than the pure tungsten metal itself. Finally, the tungsten carbide catalyst remains crystalline, rather than becoming a liquid, during growth. Each of these observations may have direct relevance to nanotube synthesis, even for catalyst metals other than tungsten.

In addition to elucidating the nanotube growth process, the junction formed during the procedure showed a lower resistance than typically achieved by directly contacting a multi-walled nanotube to a metal. This is partly because the usual connection serves only to contact the outer shells of the tube. The nanotube/tungsten-carbide/tungsten junction investigated by Wang's team may have technological relevance. “We believe that such a heterojunction, with its excellent transport properties and mechanical robustness, could find applications in carbon nanotube-based electronic devices, as well as nanoelectromechanical systems,” says Wang.

In the near future, the researchers plan to use nanotubes as point-emitters of electrons, and the junction they’ve created could provide a rigid and low-resistance mechanical connection to a substrate.