Nov. 6, 1998 BUFFALO, N.Y. -- The pictures that accompany a University at Buffalo paper in this week's issue of Science show what looks like an exquisitely uniform field of wheat, or a close-up of a neatly trimmed "buzz cut."
In fact, the perfectly even rows of tall, skinny, carbon nanotubes represent a major advance that brings researchers much closer to developing the flat panel displays that one day will make it possible to hang your TV or computer monitor on the wall like a picture.
Flat panel products currently on the market, such as laptop computers, are based on technologies that cannot produce the excellent viewing angles and high resolution that carbon nanotubes will make possible.
The technical advances made by the UB team are precisely those that will help make flat panel displays made with carbon nanotubes affordable.
"We have made three major achievements," said Zhifang Ren, Ph.D., UB research associate professor of physics and chemistry, and first author. "Our nanotubes are beautifully aligned, they grow at relatively low temperatures and they are grown on glass."
Glass is the preferred material for monitors, costing only a few dollars as compared to several hundred dollars for silicon-based materials, which would make the cost of flat panel display products prohibitive.
Carbon nanotubes are actually tiny, elongated, tubular versions of C60, the soccer ball-shaped molecule also known as the "buckyball."
What makes them so tantalizing is their incredible strength, at least 100-1,000 times stronger than the strongest steel available, Ren explained, along with their very-high stability and excellent electron-emission capabilities. The combination makes them ideal for use in flat panel displays.
"In a conventional television, a high-voltage electron gun is constantly in motion, bombarding each pixel on the screen, and that's what gives you your picture," Ren explained. "But in order to have enough room for the gun to scan the whole length and breadth of the screen, you need about a foot." That gives televisions and computer monitors their unwieldy bulk. Flat panel displays, on the other hand, need less than a millimeter of space between the carbon nanotubes, which act as the electron emitters, and the phosphor screen.
"With these displays, because each pixel is an electron source, there is no need for scanning and therefore no need for that distance between the electron source and the screen."
But technical problems have prevented flat panel displays from advancing to the development stage.
"We know from earlier work on carbon nanotubes that electrons come out only from the tip of each tube, not from the sides," said Ren. "Therefore, it is necessary to have all the nanotubes positioned exactly perpendicular to the substrate on which they are grown. If the alignment is not good, then you cannot obtain good electron-emission properties."
Previously published work on carbon nanotubes has shown poor alignment, with nanotubes, in some cases, resembling jumbled strands of spaghetti. Previous work also involved growing carbon nanotubes on materials other than glass, which was necessary because of the high temperatures required for the synthesis of the nanotubes.
To use glass as the substrate, synthesis temperatures have to be below 650 degrees Centigrade, the point at which glass begins to deform.
"Our work shows that large arrays of well-aligned carbon nanotubes can be grown on anything, so long as the substrate can take temperatures of 650 C," explained Ren.
He believes that the reason the nanotubes produced by the UB researchers grew at such comparatively low temperatures is the use of ammonia, instead of nitrogen, during the synthesis.
"For the first time, we found ammonia acting as a catalyst," he said. "I think that this helps the disassociation of acetylene, which is necessary during the synthesis of the carbon nanotubes."
Carbon nanotubes have many other applications, from components in energy-storage devices to super-strong cables.
The UB researchers also are investigating the possibility of using nanotubes in scanning tunneling microscopes to enhance resolution.
The paper's co-authors are visiting scholar Zhongping Huang; Jui H. Wang, Ph.D., Einstein Professor of Science, and Jianwei Xu, doctoral candidate in the UB Department of Chemistry, all in the UB Materials Synthesis Laboratory, and Peter J. Bush, director of the UB Instrumentation Center. Other co-authors are Michael Siegal and Paula Provencio at Sandia National Laboratory, who provided transmission electron microscope characterization.
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