Boosting solar power with Ultra-thin metal coating

Most
people probably don’t think of a coating of paint as being a particularly major
component of a manufactured item. If the object is quite large, however, or if
a lot of them are being made, paint can add considerably to its weight and/or
production costs.
With
that in mind, researchers from Harvard University’s Laboratory for Integrated
Science and Engineering have created a new lightweight, low-cost coloring
technology for both rough and smooth surfaces. Researchers have created an
ultra-thin conductive metal coating that could be used with almost any material
to make flexible smart clothes or efficient solar cells or as light weight
industrial paint. In a sub-basement deep below the Laboratory for Integrated
Science and Engineering at Harvard University, Mikhail Kats gets dressed. Mesh
shoe covers, a face mask a hair net, jumpsuit and safety goggles with clasps at
the collar – these are not to protect him, Kats explains, but to protect the
delicate equipment and materials inside the cleanroom.
  

Mikhail
Kats displays a piece of paper that has been colored using the new system
(Photo: Eliza Grinnell)
 

Kats
places the paper inside the electron-beam evaporator (Photo: Eliza Grinnell)
Due
to the nature in which that coating scatters reflected light, it appears to the
human eye as a given color – exactly which color depends upon the metals used,
and the ratios in which they’re applied (Photo: Eliza Grinnell)
While
earning his PhD at Harvard, Kats has spent countless hours in this cutting-edge
facility. With his adviser, Federico Capasso, Kats has contributed to some
stunning advances. One is a metamaterial that absorbs 99.75 per cent of
infrared light – useful for thermal imaging. Another is a thin, flat lens that
focuses light without distortions. The team has also produced light beams that
resemble a corkscrew that could help transmit more data. The most colorful
innovation, however, is a technique that coats a metallic object with a thin
layer of semiconductor, just a few nanometers thick. Although the semiconductor
is a steely gray color, the object ends up shining in vibrant hues. That’s
because the coating exploits interference effects in the thin films; Kats
compares it to the rainbows that are visible when oil floats on water.
Carefully tuned in the lab, these coatings can produce a bright, solid pink –
or a vivid blue – using the same two metals, applied with only a few atoms’
difference in thickness.

The
ultrathin coatings can be applied to essentially any rough or flexible
material, from wearable fabrics to stretchable electronics. “This can be viewed
as a way of coloring almost any object while using just a tiny amount of
material,” Capasso says. Demonstrating the tech in the cleanroom facility at
Harvard, Kats uses a machine called an electron beam evaporator to apply the
gold and germanium coating. He seals the paper sample inside the machine’s
chamber, and a pump sucks out the air until the pressure drops to a staggering
billionth of an atmosphere. A stream of electrons strikes a piece of gold held
in a carbon crucible, and the metal vaporizes, traveling upward through the
vacuum until it hits the paper.

Repeating
the process, Kats adds the second layer. A little more or a little less
germanium makes the difference between indigo and crimson. This particular
technique is unidirectional, so to the naked eye very subtle differences in the
color are visible at different angles, where slightly less of the metal has
landed on the sides of the paper’s ridges and valleys. “You can imagine
decorative applications where you might want something that has a little bit of
this pearlescent look, where you look from different angles and see a different
shade,” he notes. Many different pairings of metal are possible, too.
“Germanium’s cheap. Gold is expensive, of course we’re not using much of it,”
Kats explains. Capasso’s team has also demonstrated the technique using
aluminum.

“This
is a way of coloring something with a very thin layer of material, so in principle,
if it’s a metal to begin with, you can just use 10 nanometers to color it, and
if it’s not, you can deposit a metal that’s 30 nm thick and then another 10 nm.
That’s a lot thinner than a conventional paint coating that might be between a
micron and 10 microns thick.” In situations where the weight of the paint
matters, this could be significant. For example, that the external fuel tank of
NASA’s space shuttle used to be painted white. After the first two missions,
engineers stopped painting it and saved 600 pounds of weight. Because the metal
coatings absorb a lot of light, reflecting only a narrow set of wavelengths,
Capasso suggests that they could also be used in optoelectronic devices like
photodetectors and solar cells. “The fact that these can be deposited on
flexible substrates has implications for flexible and maybe even stretchable
optoelectronics that could be part of your clothing or could be rolled up or
folded,” Capasso says. The study appears in Applied Physics Letters. 

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