Researchers Craft Vivid Colors on Ultra-Thin Coatings

Tech Briefs INSIDER

A new coating exploits interference effects in thin films, creating a range of vivid colors.

Behind the scenes in the Center for Nanoscale Systems, Mikhail Kats (Ph.D. ’14) demonstrates the fabrication process for ultrathin coatings that shine in vivid colors. Kats and Prof. Federico Capasso have shown that these interference effects work on rough materials like paper.

The new technique coats a metallic object with an extremely 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.

The ultrathin coatings could be applied to essentially any rough or flexible material, from wearable fabrics to stretchable electronics.

A machine called an electron-beam evaporator applies the gold and germanium coating. The paper sample is sealed inside the machine's chamber, and a pump sucks out the air until the pressure drops to a staggering 10-6 Torr (a 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, a second layer is added. A little more or a little less germanium makes the difference between indigo and crimson.

"This is a way of coloring something with a very thin layer of material, said Mikhail Kats, postdoctoral researcher at the Harvard School of Engineering and Applied Sciences, "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.”

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