From: John Clark (jonkc@worldnet.att.net)
Date: Sat Nov 07 1998 - 23:52:14 MST
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- From www.sciencedaily.com posted Oct 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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