Clear Transistors Printed

Using a low-temperature process, engineers have printed a lattice of more than 20,000 n-type and p-type nanotube transistors capable of high-performance electronics on a clear, colorless, 5-in. disk that's as flexible as a playing card.

The University of Southern California (USC) creators of the see-through electronics believe the prototype points the way to such long sought after applications as affordable head-up displays in car windshields.

The lattices could also be used to create cheap, ultrathin, low-power "e-paper" displays. They might even be incorporated into fabric that would change color or pattern as desired for clothing or even wall covering, into nametags, signage and other applications, or allow LEDs to be embedded in materials.

See-through circuit makers: Hsaioh-Kang Chang (left) and Fumiaki Ishikawa with their transparent, flexible transistor array. (Images: USC Viterbi School of Engineering)

A team at the USC Viterbi School of Engineering created the new device, described in a just-published paper on "Transparent Electronics Based on Printed Aligned Nanotubes on Rigid and Flexible Structures" in the journal ACS Nano.

Graduate students Fumiaki Ishikawa and Hsiaoh-Kang Chang worked on the project under associate professor Chongwu Zhou of the School's Ming Hsieh Department of Electrical Engineering, solving the problems of attaching dense matrices of carbon nanotubes not just to heat-resistant glass but also to flexible but highly heat-vulnerable transparent plastic substrates.

The researchers not only created printed circuit lattices of nanotube-based transistors to the transparent plastic but also additionally connected them to commercial gallium nitrate (GaN) LEDs, which change their luminosity by a factor of 1000 as they are energized.

"Our results suggest that aligned nanotubes have great potential to work as building blocks for future transparent electronics," the researchers said.

The thin, transparent thin-film transistor technology developed employs carbon nanotubes -- tubes with walls 1 carbon atom thick -- as the active channels for the circuits, controlled by iridium-tin oxide electrodes which function as sources, gates and drains.

Earlier attempts at transparent devices used other semiconductor materials with disappointing electronic results, enabling n-type but not p-type transistors; both types are needed for most applications.

Fabrication steps leading to regular arrays of single-wall nanotubes (bottom).

The critical improvement in performance came from the ability to produce extremely dense, highly patterned lattices of nanotubes, rather than random tangles and clumps of the material. The Zhou lab has pioneered this technique over the past three years.

The nanotubes were first grown on quartz substrates and then transferred to glass or PET substrates with prepatterned indium-tin oxide (ITO) gate electrodes, followed by patterning of transparent source and drain electrodes. In contrast to random networked nanotubes, the use of massively aligned nanotubes enabled the devices to exhibit high performance, including high mobility, good transparency, and mechanical flexibility, according to the research paper.

The researchers said the ability to manufacture large quantities of the devices have to be addressed before practical applications are considered.

Ishikawa and Chang are the principal authors of the paper; Viterbi School graduate students Koungmin Ryu, Pochiang Chen, Alexander Badmaev, Lewis Gomez De Arco, and Guozhen Shen also participated in the project.

The Focus Center Research Program (FCRP FENA) and the National Science Foundation supported the research.

For more information, visit: http://viterbi.usc.edu