Left, photograph of a large-scale silver nanowire-coated flexible film. Right, silver nanowire particles viewed under the microscope. Credit: S.K. Yoon, Korea University
A new, ultrathin film that is both transparent and highly conductive to electric current has been produced by a cheap and simple method devised by an international team of nanomaterials researchers from University of Illinois at Chicago and Korea University. The film is also bendable and stretchable, offering potential applications in roll-up touchscreen displays, wearable electronics, flexible solar cells and electronic skin.
The new film is made of fused silver nanowires, and is produced by spraying the nanowire particles through a tiny jet nozzle at supersonic speed. The result is a film with nearly the electrical conductivity of silver-plate – and the transparency of glass. “The silver nanowire is a particle, but very long and thin,” Prof. Yarin said. The nanowires measure about 20 microns long, so four laid end-to-end would span the width of a human hair. But their diameter is a 1000X smaller – and significantly smaller than the wavelength of visible light, which minimizes light scattering.
The researchers suspended the nanowire particles in water and propelled them by air through a de Laval nozzle, which has the same geometry as a jet engine, but is only a few millimeters in diameter. “The liquid needs to be atomized so it evaporates in flight,” Yarin said. When the nanowires strike the surface they are being applied to at supersonic speed, they fuse together, as their kinetic energy is converted to heat.
“The ideal speed is 400m/s,” Yarin said. “If the energy is too high, say 600m/s, it cuts the wires. If too low, as at 200m/s, there’s not enough heat to fuse the wires.” The researchers applied the nanowires to flexible plastic films and to 3D objects. “The surface shape doesn’t matter,” Yarin said. The transparent flexible film can be bent repeatedly and stretched to 7X its original length and still work, said Sam Yoon, professor of mechanical engineering at Korea University.
Earlier this year, Yarin and Yoon and their colleagues produced a transparent conducting film by electroplating a mat of tangled nanofiber with copper. Compared to that film, the self-fused silver nanowire film offers better scalability and production rate, Yoon said. “It should be easier and cheaper to fabricate, as it’s a one-step versus a two-step process,” said Yarin. “You can do it roll-to-roll on an industrial line, continuously.” https://news.uic.edu/supersonic-spray-yields-new-nanomaterial-for-bendable-wearable-electronics
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