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A newly reported, low-cost, low-temperature process for fabricating thin-film transistors on flexible substrates could pave the way for the manufacture of wearable and disposable electronic products, according to scientists at Nanosys Inc., an advanced technology company based in Palo Alto, Calif. FLEXIBLE Plastic sheet is substrate for nanowire electronic circuitry. Insets show nanowire thin-film transistors on plastic (bottom) and a close-up image of one of the transistors (top). Photos courtesy of Nanosys Nanosys researcher Xiangfeng Duan and coworkers have used thin films of oriented silicon nanowires or cadmium sulfide nanoribbons as the semiconducting channels in high-performance thin-film transistors they have fabricated [Nature, 425, 274 (2003)]. "We have made a general conceptual breakthrough by taking nanoelectronics in a new direction--exploiting nanomaterials not for electronic miniaturization, but for better and cheaper electronics over large areas," Duan tells C&EN. "We have shown that nanowires can be assembled into densely packed, oriented nanowire thin films that can be subjected to conventional electronic fabrication processes to produce thin-film transistors for a broad range of applications in macroelectronics." The work shows that high-performance, low-cost macroelectronics could be "just around the corner," comments Ed Gerstner, Nature associate editor for physical sciences, in the same issue of Nature. The charge-carrier paths in the Nanosys transistors are single-crystal nanowires aligned in parallel, like a log bridge, between the source and drain electrodes. "Unlike amorphous silicon or polycrystalline silicon thin-film transistors, in which charge carriers have to travel across multiple grain boundaries, nanowire thin-film transistors have perfect conducting channels that ensure high carrier mobility," Duan explains. Carrier mobility is a crucial parameter that characterizes a transistor. It is a measure of the velocity of the charge carriers in a device, and it determines the magnitude of the transistor current and the switching speed of the device. The Nanosys team demonstrated that the carrier mobility of their silicon nanowire thin-film transistor on a plastic substrate is around 100 times better than current technologies based on amorphous silicon or organic semiconductors. SILICON Single-crystal nanowires bridge source and drain electrodes. "We also demonstrated that cadmium sulfide nanoribbons, which are equivalent to multiple nanowire thin films, produce thin-film transistors with nearly single-crystal performance," Duan says. The team prepared the single-crystal nanowire materials by catalytic chemical vapor deposition at high temperature. They dispersed the crystals into ethanol solution by ultrasonication and assembled them on silicon or polyether ether ketone substrates using a flow-directed alignment method. The source and drain electrodes were defined on the thin films using lithography techniques and then metalized with titanium/gold films. Duan and coworkers anticipate that they will be able to enhance the performance of their transistors by increasing the surface coverage density of the nanowires and nanoribbons between the source and drain electrodes and developing advanced core-shell nanowire structures with an integrated dielectric shell. They also suggest that their approach offers a general technology platform for the fabrication of high-performance thin-film transistors from materials with higher mobilities, such as indium phosphide and indium arsenide. The approach may also be useful for the development of novel large-area optoelectronic devices from optically active nanowire materials on flexible substrates. Duan comments that performance improvements coupled with the ability to process active materials over large areas on plastic, paper, or other flexible substrates using processes such as microcontact stamping or ink-jet printing might provide unique technologies and generate new applications. |
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