A single-walled carbon nanotube (SWNT) 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 – or its "chirality" – dictates whether the tube is a metal or a semiconductor and so the type of application that the tube can be used in. Semiconducting tubes can be used to build transistors, for example, and some scientists even believe that these structures could replace silicon in future electronic devices because they are tiny, but can still carry huge amounts of current.

Although SWNTs are relatively easy to grow, sorting them according to whether they are metallic or semiconducting is difficult, costly and time consuming. Now, a team led by Zhenan Bao has developed a way of isolating different types of nanotube by mixing them with regioregular poly(3-alkylthiophenes) polymers. The sorting process is quite simple, say the researchers, and involves putting the polymer and SWNT mixture in an ultrasonic bath, followed by centrifuging.

“The major improvement compared to previous such work is that we used the polythiophene with longer side-chains to produce a higher sorting yield of small-diameter semiconducting nanotubes,” team member Huiliang Wang told nanotechweb.org. “Molecular dynamics simulations performed by our colleagues Peng Liu and Gonzalo Jimenez-Oses at UCLA showed that the higher yield comes thanks to an increased surface interaction area between the longer side-chain polymers and the SWNTs.”

Larger bandgaps ideal for efficient electron and hole separation

As well as being able to segregate large numbers of semiconducting tubes, the technique also isolates more smaller-diameter SWNTs. These tubes have larger bandgaps – which is ideal for efficient electron and hole separation in solar cells, especially in single-junction devices, says team member Ghada Koleilat.

The researchers looked at CoMoCAT SWNTs in their study. In comparison with previous work on semiconducting HiPco SWNTs, the sorted large bandgap CoMoCAT tubes have a higher open-circuit voltage and so-called infrared external quantum efficiency, which measures the ratio of generated charge carriers (electrons and holes) to the incident photons falling on the structures. Indeed, the open circuit voltage of 0.44 V in the CoMoCAT tubes is the highest ever reported for solar cells that exploit SWNTs as light absorbers.

The team now is busy trying to better understand how the structure of the polymer employed affects SWNT sorting. They would also like to find the best type of polymer for separating nanotubes. “We hope to do this by designing new polymers and with detailed molecular dynamics simulations,” revealed Wang.

The present work is reported in ACS Nano 10.1021/nn406256y.

Further reading

Making nanotube devices chiral (May 2010)
Nanotubes get sorted (Jul 2008)
Monolayers improve CNT transistors (Oct 2010)
Polymer helps separate CNT mix (Apr 2012)