Researchers at Harvard University have developed an ultra-thin, low-cost and lightweight alternative to paint, which can be used on smooth as well as rough surfaces.

Mikhail Kats, a graduate student in applied physics at the Harvard School of Engineering and Applied Sciences, along with his advisor Federico Capasso, developed a process that involves the use of an electron-beam evaporator to vaporize metal pieces by hitting them with electron streams.

The vapor moves via a vacuum compartment inside the evaporator and gets collected on a metallic surface at the top of the machine. Repeating the process deposits multiple layers on the metallic surface that results in ultra-thin paint coating.

Scientists explain that the ultra-thin coating distributes reflected light and makes it appear as color to human eyes. The exact color depends on the metals and the ratio in which they are being used in the evaporator.

Kats' experiment involved coating a piece of paper with a film that was made from germanium and gold. Previous studies have determined that the method worked properly on surfaces that were smooth. However, Kats suggests that it is the first time the color was applied successfully to a rough surface.

The scientists learned that the paper with germanium and gold coating remained flexible. The color appeared to be the same when viewed from various angles. However, the "hills and valleys" in its microstructure added subtle differences to the light-scattering process, which resulted in a pearlescent appearance that may be desirable in some applications. A different method to apply the color made it look uniform when viewed from any angle.

Even though gold is very expensive, only a small quantity was used to create the color. A number of other metals, such as aluminum, can also be used in the technique.

"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 [nm] to color it, and if it's not, you can deposit a metal that's 30 nm thick and then another 10 nm," said Kats. "That's a lot thinner than a conventional paint coating that might be between a micron and 10 microns thick."

Researchers claim that the latest study is important since the technology can be used in coloring any type of material. As the color coatings absorb lots of light, it may be used in optoelectronic devices like solar cells and photo detectors.

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