A minor miracle takes place every day in the sub-basement high-tech clean room at Harvard University—at least as far as folks involved in metal fabrication might be concerned.
Here, researchers in face masks and safety goggles work out the finer details of a technique they’ve developed that coats substrates with an infinitesimally thin layer of metallic semiconductor.
The work unfolding in this cutting-edge facility is meaningful because it offers an entirely unprecedented method for introducing colour to metal.
Just a few nanometres thick and superficially grey, the coating exploits interference effects in the thin films. Therefore, it can be carefully tuned to produce bright, solid colours, unlike any possible on metals before.
Here’s how it works: A machine called an electron beam evaporator applies the coating and then seals the material inside its chamber. Next, a pump sucks the air out until the pressure drops to a staggering 10-6 Torr (a billionth of an atmosphere). A stream of electrons strikes a piece of the coating held in a carbon crucible, and it vaporizes, travelling upward through the vacuum until it hits the material for which it’s bound. Scientists add subsequent layers, and the colour changes dramatically with each microscopic addition. A little more or less germanium makes the difference between, say, indigo and crimson.
Just 10 nanometres will add a brilliant hue to any metal, significantly thinner than the micron-10-micron-thick conventional paint coating. The significance of the coating’s weight becomes apparent in places like the external fuel tank of NASA’s space shuttle. Painted white for the first two missions, engineers stopped painting it and saved a remarkable 600 pounds of weight.
Because the metal coatings absorb a lot of light, reflecting only a narrow set of wavelengths, their innovators suggest they could also be incorporated into optoelectronic devices such as photodetectors and solar cells.
Harvard’s research group published its findings on its new colour-coating technology in Applied Physics Letters this past fall. The university’s Office of Technology Development is in the process of exploring commercial opportunities for it.