Flexible electronic circuits would make possible radical new kinds of devices, like water-resistant tablet computers that can be rolled or folded. A group of academic and industry researchers has now demonstrated one of the most important components for this fully flexible future: graphene radio-frequency electronics that are speedy enough to produce, receive, and process telecommunication signals.
There are many different candidate materials for flexible circuits, each with its own set of problems. Some perform calculations too slowly for practical applications; others excel at speedy signal processing, but researchers know there’s no hope of manufacturing them at reasonable prices. That’s now starting to change, says Deji Akinwande, an electrical and computer engineer at the University of Texas at Austin, who leads the work on printed graphene transistors. “I think we can realistically envision flexible smartphones, tablets, and other communication devices,” he says.
Akinwande’s group is focused on practical applications for graphene, one-atom-thick sheets of carbon with exceptional mechanical and electrical properties.
Graphene transistors and circuits, made on rigid surfaces using conventional chip-making techniques, have broken electronic speed records. But when researchers have tried to take advantage of graphene’s toughness and extreme flexibility in bendable devices built on plastic, the switching speed takes a dive. That’s a problem because for flexible electronics to become economical, says Akinwande, they must be printed over larger areas, like newspaper.
This week in the journal ACS Nano, Akinwande and University of Texas materials scientist Rodney Ruoff describe record-breaking 25-gigahertz graphene transistors printed on flexible plastic. Communications circuits have to be able to switch on and off billions of times per second—2.4 gigahertz for Bluetooth, and about 1 gigahertz for cellular communications. To really work for practical applications, the transistors in these circuits have to be rated about 10 times faster than that, says Akinwande. The University of Texas graphene transistors make the cut.
Other researchers have worked with different materials to make flexible circuits that reach even higher frequencies, but the materials and methods they use are not likely to be practical for large-scale manufacturing. University of California, Berkeley, materials scientist Ali Javey, for example, has made fast radio-frequency electronics by transferring thin slices of crystalline materials from rigid wafers onto flexible plastics. This method results in speedy circuits, but it requires expensive materials; the need to transfer lots of little slices of the crystal would make it difficult to pull off at large scale, according to Javey. “Graphene is very practical and low cost,” he says.
Indeed, says Akinwande, his group is very focused on keeping costs down by making graphene from inexpensive starting materials and producing the devices over large areas. To make the transistors, the researchers first fabricate all the non-graphene-containing structures—the electrodes and gates that will be used to switch the transistors on and off—on sheets of plastic. Separately, they grow large sheets of graphene on metal, then peel it off and transfer it to complete the devices. Akinwande says they use this graphene-last approach because the material is very sensitive to all the processing needed to make the other components. Finally, they top the sheet with a waterproof layer.
The graphene transistors are not only speedy but robust. The devices still work after being soaked in water, and they’re flexible enough to be folded up. “As you make [electronics] thinner, the mechanical properties get better and better,” says Javey. “And graphene is the thinnest material you can have.”
Akinwande is now working with industry partners, including glass maker Corning of New York and 3M of Saint Paul, Minnesota, to demonstrate printed graphene circuits on a larger, more practical scale. And the group is currently designing a printer for continuously manufacturing graphene circuits. “All the building blocks are done,” says Akinwande. He says the circuits could be manufacturable in five to 10 years.
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