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At Harvard, Color by Vapor

Wednesday, January 7, 2015

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Coating thickness has typically been measured in mils or, at its thinnest, microns. Now, a team of Harvard researchers is measuring it in nanometers—one 24,500th of a mil.

The team's new lightweight, ultra-thin coating technology breathes color onto smooth and rough surfaces without adding weight.

The technology has potential anywhere the weight of paint matters: electronics, delicate instrumentation or the external fuel tank of NASA’s space shuttle, according to researchers at Harvard’s School of Engineering and Applied Sciences.

Mikhail Kats
Eliza Grinnell / SEAS Communications / Harvard University

Student Mikhail Kats and his team at Harvard's School of Engineering and Applied Sciences have discovered new ways to create colorful coatings using minimal materials.

The team first demonstrated the coloring technique on smooth surfaces in 2012, spray painting an ultra-thin germanium coating onto gold.

Recently, the group took the technology a step further, coating a rough surface.

“I cut a piece of paper out of my notebook and deposited gold and germanium on it,” according to Ph.D. student Mikhail Kats, “and it worked just the same.”

The findings have been published in the journal Applied Physics Letters.

Wide Range of Applications

While it sounds simple, the finding has big implications. It suggests that ultra-thin coatings could be applied to essentially any rough or flexible material, from wearable fabrics to stretchable electronics, according to the university.

gold films
Mikhail Kats, Romain Blanchard, and Patrice Genevet / Harvard University

Gold films colored with nanometer-thick layers of germanium show a range of color possibilities.

“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,” said Federico Capasso, the Robert L. Wallace professor of applied physics and Vinton Hayes research fellow in electrical engineering.  

“That's a lot thinner than a conventional paint coating that might be between a micron and 10 microns thick.”

Coating Technique

In the lab, Kats uses an electron-beam evaporator to apply the gold and germanium coating to the paper.

He seals the paper sample 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).

paper
Eliza Grinnell / SEAS Communications / Harvard University

Kats cut a piece of paper from his notebook to test the coating technique on rough surfaces. The paper was coated using about 30 nanometers of gold. The germanium (at right) was applied to the paper using an electron-beam evaporator. Where deposited on top of the gold, it turned violet.

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 less germanium makes the difference between indigo and crimson.

The school’s Office of Technology Development is seeking commercial opportunities for the new color coating technology and welcomes contact from interested parties.

   

Tagged categories: Coating Materials; Coatings technology; Color; Design; Metal coatings; Nanotechnology; North America; Research

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