Carbon nanotubes are promising materials for electronic devices thanks to their excellent mechanical and electrical properties. However, before real-world devices see the light of day, researchers need to be able to modify the electronic structure of the tubes so that different functionalities can be incorporated into the materials. This is difficult to do with pristine tubes because the sidewalls in these structures are extremely stable, which makes them difficult to chemically dope.

Now, Kim and colleagues have come up with a simple method to overcome this problem. The scientists have produced arrays of quantum dots inside single-walled carbon nanotubes by producing a misalignment between the tube and an underlying silver substrate. The good thing about the technique is that it does not require any physical or chemical treatment on the tubes.

Mismatch
The team, which includes researchers from the University of Tokyo and Aix-Marseille University in France, found that the electronic properties of carbon nanotubes are strongly influenced by the way the tubes are registered on metal substrates. Quantum confinements in the form of periodic quantum-well (QW) structures are produced over the whole length of the nanotube and the size of the confined regions can be controlled by changing the mismatch between the tube and substrate.

The band-structure of the nanotubes can also be manipulated depending on the degree of mismatch between the nanotube and substrate so that is resembles a superlattice in which the bandgap energy is periodically modulated. In turn, this produces periodic modulations of the nanotube's electronic structure, which then appears as 1D multiple quantum dots. "These dots are analogous to multiple quantum wells in a 3D superlattice, one of the types of QW proposed very early on in the history of semiconductor bandgap engineering," team leader Maki Kawai told nanotechweb.org.

"This is the first report of a periodic QW structure realized inside a carbon nanotube," he added, "and the technique will allow us to construct nanoscale p-n junctions in a carbon nanotube."

And that's not all: thanks to the extremely small size of the confined region, the energy level splittings observed for the nanotubes are very large (around 260 meV), which means that the structure could operate as a quantum-dot system at room temperature rather than just at ultra-low temperatures.

The team will now study other nanosystems, such as nanowires and even graphene.

The work was published in Nature Nanotechnology.