A
team of researchers has developed a new process to
make flexible, conducting 'nano skins' for a variety
of applications, from electronic paper to sensors
for detecting chemical and biological agents. The
materials, which are described in the March issue
of the journal Nano Letters, combine the strength
and conductivity of carbon nanotubes with the flexibility
of traditional polymers.
"Researchers have long been interested in making
composites of nanotubes and polymers, but it can
be difficult to engineer the interfaces between the
two materials," says Pulickel Ajayan, the Henry Burlage
Professor of Materials Science and Engineering at
Rensselaer Polytechnic Institute. "We have found
a way to get arrays of nanotubes into a soft polymer
matrix without disturbing the shape, size, or alignment
of the nanotubes."
Nanotube arrays typically don't maintain their shape
when transferred because they are held together by
weak forces. But the team has developed a new procedure
that allows them to grow an array of nanotubes on
a separate platform and then fill the array with
a soft polymer. When the polymer hardens, it is essentially
peeled back from the platform, leaving a flexible
skin with organized arrays of nanotubes embedded
throughout.
The skins can be bent, flexed, and rolled up like
a scroll, all while maintaining their ability to
conduct electricity, which makes them ideal materials
for electronic paper and other flexible electronics,
according to Ajayan.
"The general concept (growing nanotubes on a stiff
platform in various organizations, and then transferring
them to a flexible platform without losing this organization)
could have many other applications, all the way from
adhesive structures and Velcro-like materials to
nanotube interconnects for electronics," says Swastik
Kar, a postdoctoral researcher in materials science
and engineering at Rensselaer and lead author of
the paper, along with Yung Joon Jung, assistant professor
of mechanical and industrial engineering at Northeastern
University and a recent doctoral student in Ajayan's
Rensselaer lab.
For example, with researchers at the University
of Akron, Ajayan is using a similar process to mimic
the agile gecko, with its uncanny ability to run
up walls and across ceilings. The team recently reported
a process for creating artificial gecko feet with
200 times the sticking power of the real thing, using
nanotubes to imitate the thousands of microscopic
hairs on a gecko's footpad. Ajayan's team is also
working with Ali Dhinojwala, associate professor
of polymer science at Akron, to develop a range of
products with nanotubes and flexible substrates.
The researchers also envision using the process
to build miniature pressure sensors and gas detectors. "There
are a lot of possibilities if you have an easy way
to transfer the nanotubes to any platform, and that
is what we have developed," Ajayan says.
The team has shown that the flexible materials demonstrate
an extremely useful physical property called "field
emission." When a voltage is applied to certain materials,
electrons are pulled out from the surface, which
can be used to produce high-resolution electronic
displays. "Nanotubes are very good field emitters
because they have a low threshold for emission and
they produce high currents," Kar says. "But when
you lay nanotubes very close to each other, each
tube tends to shield its neighbor from the electric
field."
This effect has limited the development of field
emission devices based on densely packed, aligned
nanotubes, but it seems to go away when the nanotubes
are embedded in a polymer, according to Kar. Tests
showed that the team's "nano skins" are excellent
field emitters when compared to some of the best
values obtained by other research groups.
Several other Rensselaer researchers also collaborated
on the project, along with colleagues from New Mexico
State University. Funding for this research was provided
by two National Science Foundation Nanoscale Science
and Engineering Centers: Rensselaer's Center for
Directed Assembly of Nanostructures and Northeastern's
Center for High-rate Nanomanufacturing. Additional
funding came from the Focus Center-New York, which
is part of the Interconnect Focus Center.
Nanotechnology at Rensselaer
In September 2001, the National Science Foundation selected Rensselaer as one
of the six original sites for a new Nanoscale Science and Engineering Center
(NSEC). As part of the U.S. National Nanotechnology Initiative, the program
is housed within the Rensselaer Nanotechnology Center and forms a partnership
between Rensselaer, the University of Illinois at Urbana-Champaign, and Los
Alamos National Laboratory. The mission of Rensselaer's Center for Directed
Assembly of Nanostructures is to integrate research, education, and technology
dissemination, and to serve as a national resource for fundamental knowledge
in directed assembly of nanostructures.
About Rensselaer
Rensselaer Polytechnic Institute, founded in 1824, is the nation's oldest technological
university. The university offers bachelor's, master's, and doctoral degrees
in engineering, the sciences, information technology, architecture, management,
and the humanities and social sciences. Institute programs serve undergraduates,
graduate students, and working professionals around the world. Rensselaer
faculty are known for pre-eminence in research conducted in a wide range
of fields, with particular emphasis in biotechnology, nanotechnology, information
technology, and the media arts and technology. The Institute is well known
for its success in the transfer of technology from the laboratory to the
marketplace so that new discoveries and inventions benefit human life, protect
the environment, and strengthen economic development.
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