With carbon nanotubes, a path to flexible, low-cost sensors

(Nanowerk News) Researchers at the Technische  Universitaet Muenchen (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.

Flexible, high-performance gas sensors (left) were made by spraying  a solution of carbon nanotubes (right) onto a plastic backing.

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,” Dr. 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.

Publications

Fabrication of carbon nanotube thin films on  flexible substrates by spray deposition and transfer printing. Ahmed  Abdelhalim, Alaa Abdellah, Giuseppe Scarpa, Paolo Lugli. Carbon, Vol.  61, September 2013, 72-79.

Flexible carbon nanotube-based gas sensors  fabricated by large-scale spray deposition. Alaa Abdellah, Zubair Ahmad,  Philipp Köhler, Florin Loghin, Alexander Weise, Giuseppe Scarpa, Paolo Lugli.  IEEE Sensors Journal, Vol. 13 Issue 10, October 2013, 4014-4021.

Scalable spray deposition process for high  performance carbon nanotube gas sensors. Alaa Abdellah, Ahmed Abdelhalim,  Markus Horn, Giuseppe Scarpa, and Paolo Lugli. IEEE Transactions on  Nanotechnology 12, 174-181, 2013.

Source: Technische Universität München

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