Conventional quantum dots are often too large to show single-electron effects at room temperature. But researchers at Stanford University, US, have proved that a carbon-nanotube cross structure could theoretically produce quantum dots small enough for the job.

"Traditionally, most quantum dots are created in two-dimensional electron gases at heterojunction interfaces using lithographical techniques," explained researcher Alireza Nojeh. "Their sizes are much bigger than a few nanometres and single-electron effects are easily screened by the thermal energy of electrons. Nanotubes are quantum wires, and by imposing only one additional constraint on them, you can turn them into quantum dots."

According to Nojeh, previous work at Stanford showed that the mechanical deformation of a nanotube could change its electronic band structure. So the team looked for an object that could deform a nanometre-sized region along the length of a tube. "A natural choice for this object was another nanotube," said Nojeh.

Such a nanotube cross structure has the advantage that it can form by self-assembly, rather than needing precise manipulation of the nanotubes using scanning-probe microscopy techniques. "In other words, if you just manage to make two nanotubes cross each other, their natural relaxed configuration should lead to the creation of a quantum dot that operates at room temperature," explained Nojeh.

The scientists proved the technique would work by modelling the structure of the device. They compared the results of their calculations with an actual nanotube cross structure that they produced using chemical vapour deposition.

The team believes that, since nanotubes are very strong structures and can carry high densities of electric current without losing their properties, the crossed nanotube structure could be a good candidate for devices such as single-electron transistors. The structure should also provide an opportunity to study the physics of quantum dots at both low and room temperature.

The researchers reported their work in Nano Letters.

About the author:
Liz Kalaugher is editor of nanotechweb.org