A SWCNT is a sheet of carbon just one atom thick that has been rolled up into a tube with a diameter of about 1 nm. The atoms in the sheet are arranged in a hexagonal lattice and the relative orientation of the lattice to the axis of the tube (its chirality) determines whether the tube is a metal or a semiconductor. SWCNTs are ideal for use in a host of applications, such as sensors and transistors, thanks to their extremely high surface area and excellent charge transport properties but to make such devices, the tubes all need to be produced with the same chirality – something that is costly and difficult to achieve.

Ralph Krupke and colleagues of the Karlsruhe Institute of Technology have now obtained single-chirality nanotubes by using a polymer (poly(9,9-di-n-octylfluorenyl-2,7-diyl)), which selectively disperses certain SWCNT chiralities in solution. The sorted nanotubes are then assembled into arrays by dielectrophoresis. Here, a force is exerted on a dielectric particle when a non-uniform electric field is applied to it.

Making devices
Krupke and colleagues put a drop of the sorted solution onto a chip and then applied an alternating electric field between the chip electrodes. They took advantage of a technique called "capacitive coupling" to simultaneously apply a bias field across a large array of electrodes without having to contact them individually first, explained team member author Aravind Vijayaraghavan, who is also lead author of the ACS Nano paper in which the work appears. When applying the electric field, the dielectrophoretic force attracts the sorted nanotubes so that they deposit between the electrodes, so bridging them to form devices.

The arrays consist of up to 90% of nanotubes with the same chirality. "This figure might be increased to almost 100% by improving the purity of the pre-sorting process," Vijayaraghavan told nanotechweb.org.

The nanotube devices in the array all have identical physical and electronic properties -- something that the researchers confirmed by Raman and photoluminescence spectroscopy on a statistically large number of devices.

According to the team, the arrays could be used directly in sensing applications. And, the same sorting and assembly process might come in useful for the large-scale integration of functional devices for nanoelectronics applications.

The researchers are now studying nanotube assembly sorted by other methods, such as density-gradient ultra-centrifugation. "This technique has advantages for certain applications, like high performance electronics," said Vijayaraghavan.