When single-walled carbon nanotubes are made, a mixture of both metallic and semiconducting nanotubes is produced. This is a problem when trying to make electronic devices from these nanomaterials because, ideally, either semiconducting or metallic tubes are needed depending on the application and not both. Now, researchers in the US have found that a polymer sorting technique in solvents can not only be used to separate the two types of tubes but that some solvents are more compatible with ink-jet processes for making printed electronics devices, such as transistors.
A single-walled carbon nanotube (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 – 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 SWCNTs are relatively easy to grow, sorting them according to whether they are metallic or semiconducting is difficult, costly and time consuming. Last year, a team of researchers led by Zhenan Bao at Stanford University developed a way of isolating different types of nanotube by mixing them with regioregular poly(3-alkylthiophenes) polymers in the solvent toluene. The sorting process is quite simple, say the researchers, and involves putting the polymer and SWCNT mixture in an ultrasonic bath, followed by centrifuging.
Four new solvents
Now, the researchers have gone a step further and have found four new different solvents (decalin, tetralin, m-xylene, and o-xylene) for dispersing SWCNTs by mixing them with poly(3-dodecylthiophene) P3DDT. These solvents were chosen from a much longer list of potential solvent candidates because they can dissolve the sorting polymer.
The original list was whittled down by choosing only the solvents in which SWCNTs were sparingly soluble, and which had a lower density than the tubes themselves, explains team member Huiliang Wang, also of Stanford. “A final sorting process, involving sonification and a spin in the centrifuge, narrowed the solvents down to the four that ultimately worked.”
Of these four, decalin or o-xylene produced the highest sorting yield (up to 40%), he says, a result likely due to the higher intrinsic solubility of SWCNTs in these solvents.
Non-polar solvents disperse semiconducting SWCNTs
“Our sorting results indicate that only non-polar solvents can disperse semiconducting SWCNTs,” he tells nanotechweb.org. “These findings provide another ‘design rule’ for choosing the best solvents for polymer sorting of SWCNTs.”
These solvents can be employed as the “ink” containing the nanotubes in ink-jet printing of electronics devices and means that we can now choose inks that are more compatible with these printing processes, he says. These solvents have viscosities, surface tensions and boiling points that better match the equipment and nanotube properties, and they can also be chosen to be less toxic.
Printing devices from sorted SWCNTs
Benjamin Flavel of the Karlsruhe Institute of Technology in Germany, who was not involved in this work, says that the new technique is “inspiring and thought provoking” and provides new insights into the importance of selecting solvents for polymer sorting of semiconducting carbon nanotubes.
Bao and colleagues reveal that they are now collaborating with Bsing Hsieh at the Palo Alto Research Institute in California for printing devices made from their sorted SWCNTs.
The team, which also includes scientists from the University of California at Los Angeles and the Stanford Synchrotron Radiation Lightsource at the SLAC National Accelerator Laboratory, reports its work in Small DOI: 10.1002/smll.201401890.
About the author
Belle Dumé is contributing editor at nanotechweb.org