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Welding nanowires using light

Posted: 22 Feb 2012 ?? ?Print Version ?Bookmark and Share

Keywords:welding? plasmonics? metal nanowire?

A team of engineers from Stanford University has developed a nanowire welding technique that stands as a solution to creating electrically conductive meshes made of metal nanowires. Currently, creating these meshes involve heating or pressing to unite the crisscross pattern of nanowires that form the mesh, damaging the structures in the process. However, with the newly discovered technique, the wires can be fused damage-free by using light.

At the heart of the technique is the physics of plasmonics, the interaction of light and metal in which the light flows across the surface of the metal in waves, like water on the beach. "When two nanowires lie crisscrossed, we know that light will generate plasmon waves at the place where the two nanowires meet, creating a hot spot. The beauty is that the hot spots exist only when the nanowires touch, not after they have fused. The welding stops itself. It's self-limiting," explained Mark Brongersma, senior author of the study and associate professor of materials science engineering at Stanford.

"The rest of the wires and, just as importantly, the underlying material are unaffected," added Michael McGehee, a Stanford materials engineer and also senior author of the paper. "This ability to heat with precision greatly increases the control, speed and energy efficiency of nanoscale welding."

nanowire welding

The nanowire welding technique uses plasmonics to fuse the wires with a simple blast of light.

In before-and-after electron-microscope images, individual nanowires are visually distinct prior to illumination. They lie atop one another, like fallen trees in the forest. When illuminated, the top nanowire acts like an antenna of sorts, directing the plasmon waves of light into the bottom wire and creating heat that welds the wires together. Post-illumination images show X-like nanowires lying flat against the substrate with fused joints.

In addition to making it easier to produce stronger and better performing nanowire meshes, the researchers stated that the new technique could open the possibility of mesh electrodes bound to flexible or transparent plastics and polymers.

To demonstrate the possibilities, they applied their mesh on Saran wrap. They sprayed a solution containing silver nanowires in suspension on the plastic and dried it. After illumination, what was left was an ultrathin layer of welded nanowires.

"Then we balled it up like a piece of paper. When we unfurled the wrap, it maintained its electrical properties," said co-author Yi Cui, an associate professor materials science and engineering at Stanford. "And when you hold it up, it's virtually transparent."

This could lead to inexpensive window coatings that generate solar power while reducing glare for those inside, the researchers indicated.

"In previous welding techniques that used a hotplate, this would never have been possible," noted lead author, Erik C. Garnett, post-doctoral scholar in materials science at Stanford. "The Saran wrap would have melted far sooner than the silver, destroying the device instantly."

"There are many possible applications that would not even be possible in older annealing techniques," said Brongersma. "This opens some interesting, simple and large-area processing schemes for electronic devicessolar, LEDs and touch-screen displays, especially."

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