Researchers at the National Institute of Advanced Industrial Science and Technology in Tsukuba, Japan1 have managed to connect together carbon nanotubes like water pipes. This achievement could allow bottom-up engineering of nanotube structures for tiny devices.

Fig. 1: ‘Plumbing’ together of two carbon nanotubes. One single-walled nanotube (SWNT) is held on a palladium (Pd) electrode, and another is moved towards it using the tungsten (W) tip of a transmission electron microscope (a). On applying an electric current (b) the nanotubes spontaneously join together (c).Copyright © Chuanhong Jin and Kazu Suenaga 2008.

In their technique, the researchers used the tungsten tip of a transmission electron microscope to move a single-walled nanotube towards another nanotube of the same diameter held on a palladium electrode (Fig. 1). When a voltage was applied between the electrode and the TEM tip, the two halves spontaneously fused together to form a new nanotube.

The whole process was repeated several times on the same nanotube and also successfully applied to a double-walled nanotube. The researchers even managed to create a Y-junction by joining the end of one nanotube to the wall of another.

Along the walls of a carbon nanotube, atoms are arranged in a hexagon lattice, but at the hemispherical ends they form pentagons. One consequence of joining the nanotubes was that the resulting product had some bulges down its length, caused by pentagon structures left over from the original tube ends.

“The pentagonal defects lead to imperfections in the carbon nanotubes,” says contributing scientist Chuanhong Jin. “This may affect electronic or fluid delivery applications. However, we could realize a functional nanotube device thanks to these pentagons, for example metallic and semiconducting tubes could be seamlessly joined.”

However, it proved more difficult to join nanotubes of different diameters. When this was attempted, both nanotubes became slightly squashed and deformed, before quickly jumping away from one another without joining.

To overcome this problem, the Jin and colleagues added tungsten nanoparticles, which acted as catalysts to assist the rearrangement of carbon atoms. By moving the tungsten particles back and forth between the tips of the nanotubes, it was possible to join two tubes of different diameters.

“With the tungsten particles stuck inside, the nanotube inner channel cannot be continuous,” says Kazu Suenaga, senior scientist on the project. “However applications using nanotubes as heat or charge carriers should rely on the tubular layers and so will find fewer problems.”