In previous work with gated carbon nanotubes, the nanofabrication techniques employed produced contaminated nanotubes that are unsuitable for use in high quality electronic devices. Moreover, scientists cannot control electron confinement in such dirty tubes because it gets trapped in the random potential created by the contamination. It is therefore impossible to reduce the number of electrons in the tubes down to the single-electron regime important for fundamental studies.

"We got around this problem in our devices by 'flipping' the whole fabrication procedure upside down," team member Gary Steele told nanotechweb.org. "We now perform all nanofabrication before the nanotube is grown, and grow nanotubes on the chip in the very last step. This approach keeps the nanotubes clean and allows us to consistently trap and control a single electron."

Single or double quantum dot
The researchers can make either a single quantum dot or double quantum dot in their devices by changing the voltages on three gates present. These gates can push the electrostatic potential of the nanotube up and down in three different places.

To make a single quantum dot, the same voltage is applied to the three gates. For example, to make a single quantum dot containing one electron, positive voltages are applied to all gates, which creates an attractive potential across the whole nanotube, explains Steele. "And to make a double quantum dot, we can then change the voltage on the middle gate so that it repels electrons while the left and right gates are still attracting them. The electron then 'sees' a double-well potential."

Klein-like tunnelling
The nanotubes produced this way may also be used to study new physics that was previously hidden by the disorder from the contamination in earlier devices. Indeed, the Delft team has already seen Klein-like tunnelling in its devices by studying the strength of tunnelling in a single-electron double quantum dot. The researchers observed that as the middle barrier is made taller, the tunnelling is suppressed but an even bigger barrier makes the tunnelling stronger again. "This happens in the nanotube because electrons tunnel through the barrier making use of valence band states – just as how electrons tunnel using negative energy states in the Klein paradox," said Steele.

The team is now working on new double quantum dot devices for quantum computing applications. The goal here is to make a "spin quantum-bit" (qubit) using a single spin confined in a carbon nanotube, reveals Steele. "We have already demonstrated making p-n junctions by electrostatic doping using our gates. This could open up a road to carbon nanotube optoelectronics, something we are also actively pursuing."

The researchers reported their work in Nature Nanotechnology.