grown in place
Technology Research News
Research in recent years has generated
a lot of excitement about carbon nanotubes, those infinitesimal tubes
formed of carbon sheets a mere one atom thick. The more researchers find
out about them, it seems, the more potential uses they gain.
Nanotubes are extraordinarily strong, conduct electricity, vibrate at
high frequencies, emit light, and are sensitive to the presence of minute
amounts of substances.
But there's a lot of hard work to be done before the average person routinely
uses devices that contain nanotubes. Getting the thread-like molecules
to form in specific places, for instance, is a formidable challenge.
Stanford University researchers have made this easier with a method for
growing individual tubes directly between pairs of electrodes. The researchers
built arrays of metal electrodes, then coaxed nanotubes to form, suspended,
in the gaps between them, said Hongjie Dai, an assistant professor of
chemistry at Stanford.
The technique could be used to produce several types of electronic components,
said Dai. Nanotubes' microscopic size and very long, thin shape mean they
are able to vibrate extremely rapidly, and rapidly vibrating nanotubes
are sensitive to subtle changes.
This ability could make them useful as sensors that measure forces like
acceleration, and for tuning radio waves in communications devices. They
could be used in chemical sensors; strain gauges; electromechanical transducers,
which convert sound, pressure or light to electrical signals; and high
frequency mechanical resonators for telecommunications applications like
cell phones, according to Dai.
A common laboratory method for producing nanotubes, which occur naturally
in small amounts in soot, is to cause carbon to condense out of a hot
vapor of chemicals. It is difficult, however, to use this method to get
nanotubes to grow in the specific locations and orientations needed to
form electrical components.
In contrast, the Stanford method allowed the nanotubes to form between
metal electrodes the researchers had added to a silicon wafer using photolithography,
the process of etching materials with light and chemicals that is used
to make computer chips.
The resulting nanotubes were 2.5 nanometers in diameter and spanned electrode
gaps ranging from 3,000 to 10,000 nanometers. A nanometer is one millionth
of a millimeter. The longest of the nanotubes were as long as the width
of two red blood cells, but were 2,000 times narrower than a single red
The researchers used the metal molybdenum for the electrodes and added
small amounts of iron to encourage the nanotube growth process. They found
that molybdenum both survived the high-temperature nanotube growth process
and did not inhibit nanotube growth, according to Dai. Using the metals
gold, titanium, tantalum or tungsten resulted either in faulty electrodes
or no nanotubes.
The researchers were able to get single nanotubes to form between electrodes
30 percent of the time, and were able to get multiple nanotubes to span
the gaps 90 percent of the time, according to Dai.
Some of the suspended nanotubes worked as transistors, which control the
flow of electricity in computer chips. The suspended nanotube transistors
were comparable to those grown on flat surfaces using more common techniques,
according to Dai.
The researchers also used a nanotube to electrically connect a cantilever
to a silicon surface. Making a precise connection between a nanotube and
a moving part allowed the researchers to test the nanotubes' electromechanical
properties, said Dai. For instance, the researchers found that a nanotube's
resistance to the flow of electricity increased when they stretched it,
The work is important because it allows researchers to grow nanotubes
directly in electrical and electromechanical devices, said Paul McEuen,
a physics professor at Cornell University. "You can grow them where you
want them." Other approaches to integrating nanotubes and electronics
involve processing steps that occur after the nanotube are grown, which
can damage the nanotubes, he said.
The researchers' technique could be used in practical applications in
two to five years, said Dai. The next steps in their research are carrying
out studies of the tubes' basic electromechanical properties studies and
making nanoelectromechanical devices, he said.
Dai's research colleagues were Nathan Franklin, Qian Wang, Thomas Tombler,
Ali Javey and Moonsub Shim. They published the research in the July 29,
2002 issue of Applied Physics Letters. The research was funded by the
Defense Advanced Research Projects Agency (DARPA), the Semiconductor Industry
Association, The David and Lucile Packard Foundation, and the Alfred P.
Timeline: 2-5 years
Funding: Government, Corporate, Private
TRN Categories: Chemistry; Integrated Circuits; Materials
Science and Engineering; Nanotechnology
Story Type: News
Related Elements: Technical paper, "Integration of suspended
carbon nanotube arrays into electronic devices and electromechanical systems,"
Applied Physics Letters, July 29, 2002
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