(Nanowerk Spotlight) The emerging field of transparent and flexible electronics not only holds the promise of a new class of device components that would be more environmentally benign than current electronics; being able to print transparent circuits on low-cost and flexible plastic substrates also opens up the possibility of a wide range of new applications, ranging from windshield displays and flexible solar cells to clear toys and artificial skins and even sensor implants. Three broad application areas for this technology are taking shape:
Transparent displays. These are of interest for applications such as heads-up displays on windshields and informational displays on eyeglasses or even contact lenses.
Flexible displays. Emerging applications such as electronic paper require flexible electronics to be integrated within the pixel array of the display area. Reliable thin-film transistors (TFTs) on flexible substrates represent an important step toward the required circuitry. While TFTs suitable for static images have already been demonstrated, the required transistors will have to operate at much higher speeds to allow full-motion video.
Transparent/flexible electronics. Applications such as 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 would allow integration into multi-layer packaging, in a fashion such that product information could be seen beneath the electronics.
New work at the University of Southern California (USC) has now demonstrated the great potential of massively aligned single-walled carbon nanotubes for high-performance transparent electronics.
"We fabricated transparent thin-film transistors on both rigid and flexible substrates with transfer printed aligned carbon nanotubes as the active channel and indium-tin oxide as the source, drain, and gate electrodes," Chongwu Zhou, Jack Munushian Associate Professor in USC's Department of Electrical Engineering, tells Nanowerk. "We have fabricated these transistors through low-temperature processing, which allowed device fabrication even on flexible substrates."
A fully transparent aligned single-walled carbon nanotube transistors on a 4 inch glass wafer. (Reprinted with permission from American Chemical Society)
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. As a matter of fact, the USC team has achieved the highest device mobility among transparent transistors reported so far (mobility is a number related to how fast electrons and holes can move inside a semiconductor).
Zhou explains that earlier attempts at transparent devices used other semiconductor materials with disappointing electronic results, enabling one kind of transistor (n-type), but not p-types; both types are needed for most applications. 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 Zhou lab has pioneered this technique over the past three years.
Zhou's team 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.
"In contrast to random networked nanotubes, the use of massively aligned nanotubes enables the devices to exhibit high performance, including high mobility, good transparency, and mechanical flexibility," says Zhou. "In addition, our 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 Zhou's 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%) is 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 Zhou's team is now working on.