The
engineers first created a "thin film" containing
two layers of aluminum sandwiching one ultra-thin
layer of iron using electron-beam evaporation, a
standard process employed in the semiconductor industry.
The engineers then used "anodization," a process
that causes metals to oxidize — like rusting — to
selectively create tiny cylindrical cavities and
turn the film into a "porous anodic alumina template" less
than 1/100th the width of a human hair in thickness.
During the process, an electric field was used to
form a precisely aligned array of nanoscopic holes,
turning aluminum into porous alumina, the oxidized
form of aluminum also known as aluminum oxide.
A mixture of hydrogen and methane gas was then flowed
into the template's holes, and microwave energy was
applied to break down the methane, which contains
carbon. The iron layer acted as a catalyst that prompted
the carbon nanotubes to assemble from carbon originating
in the methane, and the tubes then grew vertically
out of the cavities.
"You get a single nanotube in each pore, and that's
important because we can start to think about controlling
how and where to put nanotubes to vertically integrate
them for future electronic devices and sensing technologies," Sands
said.
Findings are detailed in a research paper that appeared
July 11 in the journal Nanotechnology. The paper
was written by graduate students Matthew R. Maschmann
and Aaron D. Franklin; postdoctoral research associate
Placidus Amama; Dmitri Zakharov, a staff scientist
at Purdue's Birck Nanotechnology Center; Eric Stach,
an associate professor of materials engineering;
Sands and Fisher.
"The pores in the template and the nanotubes that
grow in the pores really self-assemble once you set
the process in motion," said Stach, who used two
types of electron microscopes to take images of the
nanotubes emerging vertically from the cavities.
The research is based at the Birck
Nanotechnology Center in Purdue's Discovery
Park , the university's hub for interdisciplinary
research.
The cavities form within seconds, and the nanotubes
take several minutes to finish growing. The holes
vary in width from 30-50 nanometers. A nanometer,
or billionth of a meter, is about as long as 10 atoms
strung together.
Carbon nanotubes, which were discovered in the early
1990s, might enable industry to create new types
of transistors and more powerful, energy-efficient
computers, as well as ultra-thin "nanowires" for
electronic circuits. Reaching that potential promise,
however, won't be possible unless carbon nanotubes
can be integrated with other parts of circuitry and
devices, Sands said.
The experiments at Purdue yielded both single- and
double-walled nanotubes, meaning they are made of
either one or two single sheets of carbon atoms,
yielding tubes about one nanometer in diameter.
Researchers will continue the work in efforts to
understand which conditions are needed to produce
single-wall tubes versus the double-wall variety
and to learn how to produce more of one or the other.
Other researchers previously have made the templates,
but the Purdue researchers are the first to add a
layer of iron, which was Maschmann's idea, Fisher
said.
"He was told by many people, including me, that
it probably wouldn't work," Fisher said. "We were
surprised to see that the nanotubes grow from the
sidewall of the hole and then extend vertically."
Early applications are most likely in wireless computer
networks and radar technology. Long-term uses are
possible in new types of transistors, other electronic
devices and circuits. |
