One area of intensive research at the nanoscale is the
creation of electrically conductive meshes made of metal nanowires. Promising
exceptional electrical throughput, low cost, and easy processing, engineers
foresee a day when such meshes are common in new generations of touch-screens,
video displays, light-emitting diodes and thin-film solar cells.
Standing in the way, however, is a major engineering
hurdle: In processing, these delicate meshes must be heated or pressed to unite
the crisscross pattern of nanowires that form the mesh, damaging them in the
process.
In a paper published in Nature Materials, a team of engineers at Stanford University
has demonstrated a promising new nanowire welding technique that harnesses
plasmonics to fuse the wires with a simple blast of light.
Self-limiting
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 lay 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, an
associate professor of materials science engineering at Stanford and an expert
in plasmonics. Brongersma is one of the study's senior authors.
"The rest of the wires and, just as importantly, the
underlying material are unaffected," noted Michael McGehee, a
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."
In before-and-after electron-microscope images,
individual nanowires are visually distinct prior to illumination. They lay 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.
Transparency
In addition to making it easier to produce stronger and better performing
nanowire meshes, the researchers say 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. "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
said.
"In previous welding techniques that used a hotplate,
this would never have been possible," said lead author, Erik C. Garnett, PhD, a
post-doctoral scholar in materials science who works with Brongersma, McGehee
and Cui. "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 devices—solar,
LEDs, and touchscreen displays, especially."
This research was supported by the Center for Advanced Molecular Photovoltaics (CAMP)
at Stanford University funded by King Abdullah University of Science
and Technology (KAUST).
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