"People have developed sorting methods based on both chemical and electrical properties, but ours is the first that's capable of sorting semiconducting nanotubes based upon their dielectric constant, which is determined by their diameter," said Howard Schmidt, executive director of Rice's Carbon Nanotechnology Laboratory.

The team used nanotubes produced using the HiPco process, which has been found to produce more than 50 individual (n, m) types of nanotube. Typically, around two-thirds of these are direct band gap semiconductors and the remaining third are metallic. The nanotubes used in this study had diameters in the range 1.01-1.15 nm.

The DEP-FFF method pumped carbon nanotubes suspended in solution through a chamber with an array of electrodes at the bottom. The electrodes were alternately grounded or driven with a 1 MHz, 10 V peak-to-peak alternating current signal. A planar electrode at the top of the chamber was set to -0.5 V relative to the bottom electrodes. This mimicked gravity sedimentation for the neutrally-buoyant surfactant-coated nanotubes.

The team flowed nanotube solution through the chamber at a rate of 0.02 cubic cm per minute. This preferentially removed around 70% of the metallic nanotubes, which tended to stick to the bottom electrode array.

Semiconducting nanotubes began to elute from the chamber after a retention time of around 90-160 minutes. The team found that there were fewer smaller nanotubes as the retention time increased. Smaller diameter semiconducting nanotubes have a larger band gap, which also affects their dielectric constant.

In turn, dielectric constant affects the dielectrophoretic force, changing the height of the nanotubes in the chamber. In this system, fluid in the middle of the chamber flows faster than the top and bottom fluid layers that are next to the electrodes. A change in height thus affects the speed of flow.

The researchers reported their work in the Journal of the American Chemical Society.