Logo: University of Southern California

Viterbi Nanotechnologist Creates Transparent Transistors

July 04, 2007 —

The Viterbi School's Chongwu Zhou and a team of researchers have created the first prototype devices in which transparent electronics are built on top of a flexible transparent base.  These devices have the potential of serving as new kinds of displays, including “e-paper” and heads-up displays in automobile windshields or even eyeglasses.

The transistor nanowires also provide a way to embed reliable displays and computing power in thin “smart cards,” according to Zhou, an associate professor in the Ming Hsieh Department of Electrical Engineering and in the USC College Department of Chemistry.  Zhou was recently named the first holder of the Viterbi School's Jack Munushian Early Career Chair.

His research was reported in the June issue of Nature Nanotechnology.

The fabrication of the prototype nanowire transistors was done in Zhou's laboratory, working from designs co-created by David Janes of the Purdue University School of Electrical Engineering and Computer Engineering, who works in  Purdue's Birck Nanotechnology Center; and by Tobin J. Marks, holder of the Vladimir N. Ipatieff chair of chemistry at Northwestern University, who has a joint appointment in Northwestern’s Department of Materials Science and Engineering. (See Purdue's press announcement.)

While some semiconductors are transparent, they've needed metal wires -- which are not -- for connection. The new nanowire designs are made of metal oxides, including two oxides of indium (InO and In2O3); tin oxide (SnO2); zinc oxide (ZnO) and cadmium oxide (CdO) They don't require metallic connectors.

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. "We provided and optimized the material; they assembled it into a device."
Chongwu Zhou holds up a piece of plastic substrate with transparent contacts. The material is used to build nanoscale transistors and circuits.

The result, said Janes, is that "the nanowires themselves are transparent, the contacts we put on then are transparent, and the glass or plastic substrate is transparent."

Marks said the research "opens the door to entirely new technologies for high-performance transparent flexible displays."

Three early priorities include:

  • Transparent displays for uses such as heads-up displays on windshields and information displays on eyeglasses and visors. The displays enable drivers to see information without looking down at the dashboard and could project information on visors for workers without obstructing their view. Potential applications also include sports goggles for spectators to follow a particular player while having relevant statistics displayed and real-time interactive information for soldiers and surgeons.
  • Flexible displays for future "e-paper," promising to allow full-motion video. E-paper is a technology designed to mimic regular ink on paper. Unlike conventional flat-panel displays, which use a backlight to illuminate pixels, e-paper reflects light like ordinary paper and is capable of holding text and images indefinitely without drawing electricity while allowing the image to be changed later. Potential uses of e-paper include low-cost, energy efficient ways of displaying information and video as a replacement for paper in magazines, newspapers, books, electronic signs and billboards.
  • Transparent and flexible electronics for radio frequency identification tags, electronic bar codes and smart credit cards, which resemble ordinary credit cards but contain an embedded microprocessor. This microprocessor replaces the usual magnetic strip on a credit or debit card, increasing the security of data stored on the card and enabling computers to "talk" to the microprocessor. Such a technology could be used to display balances on cards and could be used for the free flow of people through transportation systems, avoiding the need of ticketing machines or validation gates. The cards could contain encryption software, secure data for use in pay phones and banking, and to contain health-care data for patients and allow tamper-proof identification information for workers.

Fumiaki Ishikawa, an Ming Hsieh Department doctoral student, is part of the transparent transisotor research team. 

Zhou is looking to add carbon nanowire transistors to the metal oxide variety, noting that the electronic properties of carbon have the potential to create devices that provide better performance with less power.

The possibilities don't end with these short-term projects. Unlike conventional computer chips- - called CMOS, for complementary metal oxide semiconductor chips -- the thin-film transistors could be produced less expensively under low temperatures, making them ideal to incorporate into plastic films, which melt under high-temperature processing.

Liquid crystal displays now used in applications such as color cell phone screens are made with thin-film electronics. This thin-film technology makes it possible to lay down electronic devices in large sheets containing individual pixels. Current thin-film electronics use technologies known as amorphous silicon and poly-silicon.

"These approaches work fine if you have a flat, rigid display that's going to be opaque," Janes said. "They require fairly high-temperature processing, so they are not good on plastic, although industry is working really hard to get them on plastic and make them lightweight, flexible and transparent."

An alternative, emerging technology uses so-called "organic" or "plastic" transistors to replace the conventional silicon that has been a mainstay of microelectronics for decades. While this technology enables transistors to be embedded in or printed on flexible plastic, it has lower performance, although major advances are being made, Marks said.

The new research represents the best of both worlds.

"You can get high performance because the nanowires themselves give you some unique performance advantages, and you could still think of dispersing them down over large areas for displays, smart credit cards and other applications," Janes said.
New nanowire design, which can't be seen with the naked eye.

Zhou's nanotechnology laboratory has many firsts in its portfolio, including production of a wide variety of nanotubes and other structures for a wide variety of applications.

Besides Zhou, Janes and Tobin, the Nature Nanotechnology paper authors include Ming Hsieh Department Ph.D. candidate Fumiaki Ishikawa;  Sanghyun Ju, a postdoctoral research associate in Purdue's School of Electrical and Computer Engineering; Antonio Facchetti, a research associate professor in the Department of Chemistry at Northwestern University; Yi Xuan, a postdoctoral research associate in electrical and computer engineering at Purdue; Jun Liu, a doctoral student in chemistry at Northwestern; and Peide Ye, an associate professor of electrical and computer engineering at Purdue;

The research was funded by the NASA Institute for Nanoelectronics and Computing and by Northwestern University.