Cousins
of the 1996 Nobel Prize-winning buckyball, carbon
nanotubes have taken the nanotechnology industry
by storm. Exhibiting extraordinary strength, flexibility
and unique electrical, mechanical and optical properties,
these hollow microscopic fibers are being integrated
into numerous electronic and biological products—high-performance
computer chips, combat jackets, bomb detectors and
drug delivery devices for the treatment of diseases.
Pushing
the field one step further, scientists at Stanford
University have devised a novel method for growing
vertical single-walled carbon nanotubes (SWNTs)
on a large scale, a feat that has eluded researchers
until now. By modifying the industry's standard approach
to producing carbon-based materials—plasma-enhanced
chemical vapor deposition (PECVD)—they achieved ultra-high-yield
growth of SWNTs, thus increasing their application
into commercial products. They report their research
in the Oct. 26 issue of Proceedings of the National
Academy of Sciences .
Carbon
nanotubes are cylindrical molecules 2 nanometers
in diameter—more than 10,000 times smaller than the
width of a human hair. Since their discovery in 1991,
multi-walled carbon nanotubes have been easily synthesized
using several methods. Yet, large-scale production
of smaller single-walled nanotubes into ordered films
has remained intangible.
Given widespread commercial use of the PECVD method
for economical, robust production of various materials
by the semiconductor industry, scientists hoped to
harness this same method for generating high-quality
single-walled nanotubes. PECVD works by exposing
substrates densely seeded with catalytic particles
to a hydrocarbon gas such as methane, which should
theoretically produce a plush carpet of carbon nanotubes.
Previous attempts, however, have generated only sparse
and inefficient synthesis of SWNTs.
Hongjie
Dai, associate professor of chemistry, and his
colleagues discovered the key component to attaining
single-walled fibers—adding oxygen to the reaction.
"There is a dilemma here," Dai said. "What
we found is that the carbon atoms are good and
needed for nanotube growth, but the hydrogen atoms
are bad. The carbon atoms try to form the nanotube's
planar structure, while at the same time the hydrogen
radicals are eating the carbon tube away. This
was never realized before in nanotube synthesis."
Adding
oxygen remedies the problem. By scavenging up the
hydrogen radicals—creating a carbon-rich and
hydrogen-deficient environment—growth is jumpstarted,
spawning a vertical forest of nanotubes.
Using
this method, Dai and his colleagues were able to
create 4-inch wafers blanketed with SWNTs. In addition,
they devised a method for lifting the nanotubes
off their original growth substrate and transferring
them onto a variety of more desirable mediums such
as plastics and metals—materials incompatible with
the high temperatures required for nanotube growth.
These planted plastics and metals further expand
the nanotubes' commercial utility.
Testing already has begun to determine the effectiveness
of single-walled carbon nanotube wafers as a thermal
interface material, conducting and dissipating heat
away from computer chips. The researchers are pursuing
additional applications as well.
Postdoctoral fellow Guangyu Zhang is lead author
of the study. Other co-authors are chemistry graduate
students David Mann, Li Zhang, Ali Javey, Yiming
Li and Erhan Yenilmez, research associate Qian Wang
and staff scientist James McVittie. James Gibbons,
former dean of the School of Engineering, and Yoshio
Nishi, director of the Stanford Nanofabrication Facility,
both professors of electrical engineering, also contributed
to the work.
The study was supported in part by the Global Climate
and Energy Project at Stanford. The synthesized nanotubes
may be used for hydrogen storage.
Anne Strehlow is a science-writing intern at
the Stanford News Service.
This release was written by
science-writing intern Anne Strehlow.
Relevant Web URLs:
Hongjie
Dai Lab
Proceedings of the National Academy of Sciences
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