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Transparent, flexible electronics with greater performance possible with nanotubes

Traditional materials used for transparent electronics consist of InGaO3 (ZnO5) films, indium tin oxide films and indium oxide nano wires. Researchers have turned to single-walled carbon nanotubes for better mobility and better performance. Single-walled carbon nanotubes (SWNTs) conduct electricity well, are mechanically strong and chemically stable. Research groups are investigating how to use them to create new electronics, particularly bendable electronics applications such as flexible roll-up display screens and electronic “skin.” These applications require thin semiconducting films - thin transistors, essentially uniformly deposited onto large, flexible substrates, such as polymers, using methods that are compatible with large-scale manufacturing.
New research work at the University Of Southern California (USC) has demonstrated the great potential of massively aligned single-walled carbon nanotubes for high-performance transparent electronics. Aligned nanotubes are established as viable active material for transparent transistors and they are shown to offer higher mobility than traditional materials for transparent electronics. The USC team has achieved the highest device mobility among transparent transistors (mobility is a number related to how fast electrons and holes can move inside a semiconductor). The critical improvement in performance came from the ability to produce extremely dense, highly patterned lattices of nanotubes, rather than random tangles and clumps of the material. The researchers first grew the nanotubes on quartz substrates and then transferred them to glass or PET substrates with pre-patterned indium-tin oxide gate electrodes, followed by patterning of transparent source and drain electrodes. The use of massively aligned nanotubes enables the devices to exhibit high performance, including high mobility, good transparency and mechanical flexibility. The aligned nanotube transistors are easy to fabricate and integrate, as compared to individual nanotube devices. The transfer printing process allows the devices to be fabricated through a low temperature process, which is particularly important for realizing transparent electronics on flexible substrates.
As a proof-of-concept demonstration, the researchers constructed a fully transparent and flexible logic inverter on a plastic substrate and used it to control commercial gallium nitrate light-emitting diodes, which changed their luminosity by a factor of 1,000 as they were energized. One of the challenges that the team is struggling with is the difficulty of achieving a highly separated sample of nanotubes. Current production methods for carbon nanotubes result in units with different diameter, length, chirality and electronic properties, all packed together in bundles. These mixtures are of little practical use since especially nanoelectronics applications are sensitively dependent on tube structures. In conventional synthesis processes, a significant proportion of the nanotubes (20-30%) are metallic and a highly reliable and effective separation of metallic and semiconducting CNTs is essential for large-scale commercial manufacturing processes for future nanoelectronic devices. Developing novel and efficient ways to remove metallic nanotubes is one of the tasks the researchers are now working on.
The emerging market of transparent and flexible electronics, that could be more environmentally green, has good potential. Ability to print transparent circuits on low-cost and flexible plastic substrates also opens up the possibility of a wide range of new applications such as windshield displays, flexible solar cells, clear toys, artificial skin, sensor implants. Three broad application areas for this technology are likely to emerge:
Transparent displays : For applications such as heads-up displays on windshields and informational displays on eyeglasses orn contact lenses.
Flexible displays: Emerging applications such as electronic paper require flexible electronics to be integrated within the pixel array of the display area. Thin-film transistors (TFTs) on flexible substrates represent an important step toward the required circuitry.
Transparent/flexible electronics: Electronic bar codes, RFID tags and smart credit cards would be advanced by the availability of relatively high performance electronics that could be integrated on a variety of substrates. Flexible circuitry would allow integration on curved and non-rigid surfaces. Transparency will permit integration into multi-layer packaging. The product information could be seen beneath the electronics.
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