Quantum wells are artificial semiconductor structures with important applications in optics, such as laser diodes. Typically, to make a quantum well, a semiconductor with a small band gap is sandwiched between two semiconductors with larger band gaps so that electrons become trapped in the central semiconductor or 'well'. The trapped electrons then enhance the optical properties of the central semiconductor. Engineering similar quantum confined regions in carbon nanotubes, which are promising electronic elements for nanoscale circuits, is therefore of considerable interest. Scientists at RIKEN in Wako, Japan1 have now shown a clean and simple method of achieving this essentially by simply orienting the nanotube in a particular direction on a silver surface.

“Our findings suggest that doping of single-walled carbon nanotubes can be applied at scales of smaller than ten nanometers,” says Hyung-Joon Shin, one of the researchers on the team.

Although there are other ways of producing quantum confined regions for electrons, referred to as ‘quantum dots’, in a nanotube, the advantage of the method of Shin and his team is that it does not require any chemical or mechanical treatment of the nanotube or substrate.

Fig. 1: Electrons from a silver surface migrate to a carbon nanotube via discrete points where the carbon comes into direct contact with silver atoms.

The researchers placed a 0.6 nm-diameter nanotube on a silver surface such that it was orientated about 16° away from the principle crystallographic direction of the silver lattice. Electrons in the silver can lower their energy by migrating to the carbon nanotube. However, because of the orientation of the nanotube, carbon atoms are in direct contact with silver atoms only once every 6 nm, and it is at these points that the transferred electronic charge collects, effectively forming quantum dots periodically along the nanotube (Fig. 1).

Given the simplicity of their method, it is perhaps surprising that this effect was not seen before. The authors suggest that using a nanotube with a small diameter, which has fewer contact points with silver compared to longer tubes, and carefully placing the nanotube on the silver to preserve its orientation, may be important reasons for their success.

“This method could be used to construct arrays of quantum dots and p–n junctions,” says Shin. “We also expect that this strategy can be applied to other kinds of nanowire systems as well.”