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With carbon nanotubes, a path to flexible, low-cost sensors
Researchers at the Technische Universität München (TUM) are showing the way toward low-cost, industrial-scale manufacturing of a new family of electronic devices. A leading example is a gas sensor that could be integrated into food packaging to gauge freshness, or into compact wireless air-quality monitors. New types of solar cells and flexible transistors are also in the works, as well as pressure and temperature sensors that could be built into electronic skin for robotic or bionic applications. All can be made with carbon nanotubes, sprayed like ink onto flexible plastic sheets or other substrates.
Carbon nanotube-based gas sensors created at TUM offer a unique
combination of characteristics that can't be matched by any of the
alternative technologies. They rapidly detect and continuously respond
to extremely small changes in the concentrations of gases including
ammonia, carbon dioxide, and nitrogen oxide. They operate at room
temperature and consume very little power. Furthermore, as the TUM
researchers report in their latest papers, such devices can be
fabricated on flexible backing materials through large-area, low-cost
processes.
Thus it becomes realistic to envision plastic food wrap that
incorporates flexible, disposable gas sensors, providing a more
meaningful indicator of food freshness than the sell-by date. Measuring
carbon dioxide, for example, can help predict the shelf life of meat.
"Smart packaging" – assuming consumers find it acceptable and the
devices' non-toxic nature can be demonstrated – could enhance food
safety and might also vastly reduce the amount of food that is wasted.
Used in a different setting, the same sort of gas sensor could make it
less expensive and more practical to monitor indoor air quality in real
time.
Not so easy – but "really simple"
Postdoctoral researcher Alaa Abdellah and colleagues at the TUM
Institute for Nanoelectronics have demonstrated that high-performance
gas sensors can be, in effect, sprayed onto flexible plastic substrates.
With that, they may have opened the way to commercial viability for
carbon nanotube-based sensors and their applications. "This really is
simple, once you know how to do it," says Prof. Paolo Lugli, director of
the institute.
The most basic building block for this technology is a single
cylindrical molecule, a rolled-up sheet of carbon atoms that are linked
in a honeycomb pattern. This so-called carbon nanotube could be likened
to an unimaginably long garden hose: a hollow tube just a nanometer or
so in diameter but perhaps millions of times as long as it is wide.
Individual carbon nanotubes exhibit amazing and useful properties, but
in this case the researchers are more interested in what can be done
with them en masse.
Laid down in thin films, randomly oriented carbon nanotubes form
conductive networks that can serve as electrodes; patterned and layered
films can function as sensors or transistors. "In fact," Prof. Lugli
explains, "the electrical resistivity of such films can be modulated by
either an applied voltage (to provide a transistor action) or by the
adsorption of gas molecules, which in turn is a signature of the gas
concentration for sensor applications."
And as a basis for gas sensors in particular, carbon nanotubes
combine advantages (and avoid shortcomings) of more established
materials, such as polymer-based organic electronics and solid-state
metal-oxide semiconductors. What has been lacking until now is a
reliable, reproducible, low-cost fabrication method.
Spray deposition, supplemented if necessary by transfer printing, meets
that need. An aqueous solution of carbon nanotubes looks like a bottle
of black ink and can be handled in similar ways. Thus devices can be
sprayed – from a computer-controlled robotic nozzle – onto virtually any
kind of substrate, including large-area sheets of flexible plastic.
There is no need for expensive clean-room facilities.
"To us it was important to develop an easily scalable technology
platform for manufacturing large-area printed and flexible electronics
based on organic semiconductors and nanomaterials," Abdellah says. "To
that end, spray deposition forms the core of our processing technology."
Remaining technical challenges arise largely from application-specific
requirements, such as the need for gas sensors to be selective as well
as sensitive.
This research was partially supported by the German Research Foundation
(DFG) through the Cluster of Excellence Nanosystems Initiative Munich
(NIM), and by the Bavarian State Ministry for Science, Research and the
Arts under the initiative Solar Technologies Go Hybrid.
News Release; TUM, Sept 24, 2013
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