March 2, 2012 feature
All-carbon-nanotube transistor can be crumpled like a piece of paper
The researchers, Shinya Aikawa and coauthors from the University of Tokyo and the Tokyo University of Science, have published their study in a recent issue of Applied Physics Letters.
The most important thing is that electronics might now be usable in places or situations that were previously not possible, coauthor Shigeo Maruyama, a mechanical engineering professor at the University of Tokyo, told PhysOrg.com. Since our device is so flexible and deformable it could potentially be stuck anywhere. This could lead to active electronic devices that are applied like a sticker or an adhesive bandage, as well as to wearable electronics.
Unlike other field-effect transistors (FETs), the new FET is unique in that all channels and electrodes are made of carbon nanotubes (CNTs), while the substrate is made of highly flexible and transparent poly(vinyl alcohol) (PVA). Previously, the majority of flexible, transparent FETs have used gold or indium tin oxide as electrodes. However, gold decreases the devices transparency while brittle indium tin oxide limits the flexibility. A few recent FETs have been made that consist entirely of CNTs, but so far these devices have been built on thick plastic substrates, limiting their flexibility.
After patterning the components using standard photolithography and laminating the device with the PVA, the final thickness of the new all-CNT-FET was approximately 15 µm. This thinness made the device highly pliable, with tests showing that the finished transistor could withstand a 1-mm bending radius with almost no changes in electrical properties. Although other transistors have been developed with bendable radii as low as 0.1 mm, the new transistor is the most bendable that experiences no performance degradation.
After subjecting the transistor to 100 wrinkling cycles, the researchers observed a slight decrease in maximum drain current, which may be due to some broken connections in the CNT network. However, the minimal decrease in maximum drain current, which stabilizes after about 30 cycles, does not affect the overall transconductance, which was not affected by the repeated bending.
In addition to its flexibility, the all-CNT-FET also has an optical transmittance of more than 80%, which is sufficient to clearly see through the device. The researchers attribute the high flexibility to the inherent robustness of carbon nanotubes, and predict that they could increase the flexibility even further by optimizing the positions of the channels. Overall, the results demonstrate that flexible, transparent all-carbon electronics are coming closer to commercial reality.
Ongoing topics are to control device properties and to integrate them, Maruyama said. If these issues can be resolved, we would like to realize flexible and transparent all-carbon working circuits.
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