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Viterbi Nanowires Shine in Active Matrix Displays

USC-Purdue-Northwestern team uses USC-made transparent transistors to build Organic Light Emitting Diodes
Emil Venere, (765) 494-4709, venere@purdue.edu
March 31, 2008 — Chongwu Zhou, associate professor in the Ming Hsieh Department of Electrical Engineering, is part of a team that has created a brilliant, luminous active matrix display, the first ever made using transparent transistors and circuits. The demonstration is a major step toward realizing such long-envisioned applications such as e-paper, flexible color monitors and "heads-up" displays in car windshields or even eyeglasses.

The  transparent transistors are made of "nanowires," tiny cylindrical structures that are assembled on glass or thin films of flexible plastic in Zhou's laboratory. USC-fabricated nanowires as small as 20 nanometers, a thousand times thinner than a human hair, when energized create a display containing organic light emitting diodes, or OLEDS. Even in their first-draft pilot stage, they are nearly as bright as the flat-panel television sets, computer monitors and displays in consumer electronics.

Last year, the same team announced the creation of the transparent crystals used in the system. At the time, Zhou, the first holder of the Jack Munushian Early Career Chair noted that transistor nanowires also provide a way to embed reliable displays and computing power in thin “smart cards,”

"This is a step toward demonstrating the practical potential of nanowire transistors in displays and for other applications," said David Janes, a researcher at Purdue University's Birck Nanotechnology Center and a professor in the School of Electrical and Computer Engineering, whose post-doctoral researcher, Sanghyun Ju, was lead author of the project.

The nanowires were used to create an active matrix display, a system able to precisely direct the flow of electricity to produce video because each picture element, or pixel, possesses its own control circuitry.

Findings are detailed in a research paper featured on the cover of the April issue of the journal Nano Letters. In addition to Zhou, Janes, and Ju, Tobin J. Marks, the Vladimir N. Ipatieff Research Professor in Chemistry in Northwestern's Weinberg College of Arts and Sciences and a professor of materials science and engineering was a co-author. "We've shown how to fabricate nanowire electronics at room temperature in a simple process that might be practical for commercial manufacturing," he said.

Zhou creates the nanowires using a process he helped develop, which uses laser beams to blast metal atoms off targets made of indium and other metal alloys. The process condenses the high temperature (700 degrees C) vapors on a nest of nanoscale gold particles, where they oxidize and self-assemble into nanowires.

"Purdue had the nanowire idea," said Zhou last year. "We provided and optimized the material; they assembled it into a device."

OLEDS are now used in cell phones and MP3 displays and prototype television sets, but their production requires a complex process, and it is difficult to manufacture OLEDs that are small enough for high-resolution displays.

Unlike conventional computer chips - called CMOS, for complementary metal oxide semiconductor chips - the nanowire thin-film transistors could be produced less expensively under low temperatures, making them ideal to incorporate into flexible plastic s that would melt under high-temperature processing.

Conventional liquid crystal displays in flat-panel televisions and monitors are backlit by a white light, and each pixel acts as a filter that turns on and off to create images. OLEDS, however, emit light directly, eliminating the need to backlight the screen and making it possible to create more vivid displays that are thin and flexible.

The technology also could be used to create antennas that aim microwave and radio signals more precisely than current antennas.
Early transparent transistor. The array regions now light up and display brilliant images.
Such antennas might improve cell phone reception and make it more difficult to eavesdrop on military transmissions on the battlefield.

Electronic displays such as television screens contain millions of pixels located at the intersections of rows and columns that crisscross each other. In the new findings, the researchers showed that they were able to selectively illuminate a specific row of active-matrix OLEDS in a display about the size of a fingernail.

Future research is expected to include work to design displays that can control individual OLEDs to generate images." The nanowire transistors are made of a transparent semiconductor called indium oxide, a potential replacement for silicon in future transparent circuits. The OLEDS consist of the transistors, electrodes made of a material called indium tin oxide and plastic capacitors that store electricity. All of the materials are transparent until activated to emit light.

"This could enable applications such as GPS navigational displays right on the windshield of your car," Janes said. "Imagine having a local map displayed on your windshield so that you didn't have to take your eyes off the road."

The new OLEDs have a brightness nearly comparable to that of the pixels in commercial flat -panel television sets. The OLEDS have an average brightness of more than 300 candelas per square meter, compared with 400 to 500 candelas per square meter for commercially available liquid-crystal- display televisions.

The researchers also demonstrated they could create OLEDS of the proper size for commercial displays, about 176 by 54 microns, or millionths of a meter. OLEDS that size would be ideal for small displays in cell phones, personal digital assistants and other portable electronics.

The research has been funded by NASA through the Institute for Nanoelectronics and Computing, based at Purdue's Discovery Park.

Besides Zhou,  Marks,  Janes, and Ju, other members of the team included doctoral students  Jianfeng Li and Jun Liu at Northwestern; doctoral students Po-Chiang Chen, Hsiao-kang Chang and Fumiaki Ishikawa at USC Viterbi, and at Northwestern research associate professor of chemistry Antonio Facchetti and graduate student Young-geun Ha.

reported by
Emil Venere, Purdue University