Nanotube Ink

Despite all their useful electronic properties, as well as their flexibility and strength, carbon nanotubes have proven very difficult to manipulate and arrange into patterns. Scientists have tried everything from growing them on wafers to depositing them on a substrate from a chemical solution. But all these methods are complicated and time consuming.

Now researchers at Rensselaer Polytechnic Institute in Troy, NY and the University of Oulu in Oulu, Finland, have come up with a simple way to make carbon nanotube patterns on a flexible substrate: they disperse multi-walled carbon nanotubes in water and use a commercial desktop inkjet printer to lay down a design on paper and plastic surfaces.

The method, say the researchers, could be used to mass-produce flexible conductive circuits, and one day lead to low-cost, roll-up displays, radio-frequency identification tags for tracking goods, and gas sensors.

“You can design patterns on the computer and if you want to change the pattern, just print out a new circuit,” says Robert Vajtai, a researcher at the Rensselaer Nanotechnology Center. The ink preparation was the trickiest part, he says. Carbon nanotubes are normally hydrophobic, which means they repel water, making them hard to disperse evenly in an aqueous solution. The researchers added carboxyl groups, which are attracted to water, to the multi-walled carbon nanotubes. The resulting carbon nanotube-water dispersion can be used as printer ink; the researchers used it to print designs with lines as narrow as 70 micrometers. The nanotube patterns conduct electricity after multiple runs of the paper or plastic sheet through the printer.

The method holds promise for tiny gas sensors. The researchers found that the conductivity of the printed patterns changes when they are exposed to vapors of water, ammonia, and methanol. “The carbon nanotubes could ‘smell’ the material and the resistance changed…[the response] was different for different materials,” Vajtai says.

The RPI group have previously developed a way to create patterns of carbon nanotubes in polymers that could form the basis of flat-panel screens. Isolated nanotubes would emit electrons, stimulating phosphors to light up pixels (see “Flexible CRT Displays”). To address each pixel, Vajtai says “you can [inkjet] print the wires on the back of the screen” using nanotubes, giving you the flexible electronics needed for a roll-up display.

Commercial applications are still a ways off, though. “The concept demonstration is excellent…but there is a lot more work to be done,” says Bingquing Wei, who does carbon nanotube research in the department of electrical and computer engineering at Louisiana State University. For commercial applications, says Wei, the ink dispersion would have to have a shelf life of months; so far it lasts for only days.

Vajtai says his colleagues are working on improving the ink preparation and conductivity of the images. He believes that the technology could be used in RFID tags in about a year. RFID tags cost a few cents and are silicon-based, consisting of an integrated circuit chip and a simple antenna circuit. With the new method, one could make more durable antennas with carbon nanotubes, printed out in bulk, Vajtai says.

Conductive inks for flexible substrates are currently made of metal nanoparticles, which, unlike the carbon nanotubes, need to be annealed, a process that requires time and special chemicals. “But the final result pays off because you have features that are as conductive as conventional metals,” says Ana Arias, a research associate at the Palo Alto Research Center (PARC), where researchers use polymer thin-film transistors and silver nanoparticle ink to make large flexible displays.

According to Arias, Eikos, a company based in Franklin, MA, already makes transparent carbon nanotube inks for flat-panel displays. That company has already used the inks to make organic solar cells, and claims the coatings could also be used in flexible displays.