Thickness of new coating changes its color

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New materials with a network-like structure create a full spectrum of intense colors and highly scratchproof coating for metals.

The lower layer of the designer material is a metallic network perforated by tiny cavities and made of an alloy of platinum, yttrium, and aluminum. The researchers used a simple etching process to create the cavities and then placed a very thin oxide layer on top of this “nano-sponge network.”

The impression of the color depends on the thickness of the aluminum oxide coating: a 12-nanometer layer makes the material appear green, a 24-nanometer layer yellow, a 28-nanomater layer orange, a 48-nanometer layer blue, and a 53-nanometer layer purple.

“The color arises from the interaction between the ambient light and both layers of the material, and in particular with the randomly organized boundary layer between the two materials,” explains physicist Henning Galinski of ETH Zurich. “We are able to capture and concentrate particular wavelengths of light in a very targeted manner.”

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The principle has long existed in nature: for example, in the colorful plumage of the South American bird, the plum-throated cotinga. Keratin networks are responsible for the color of the plumage in such bird species. “However, we are the first to demonstrate that these networked materials can be used technically as structural colors and thus influence which color is perceived,” says Galinski.

Embracing chaos

Until now, structural colors have generally possessed a repetitive structure that determines the color we perceive. The disadvantage is that even tiny flaws can cause huge changes in the optical properties.

In contrast, the networks developed by Galinski and his colleagues do not follow a clear structure: the cavities in the networks are a similar size, but not exactly the same. The physical characteristics are determined by the average cavity size, not by the size of each individual cavity.

“Our approach is based on disorder, rather than on the precise production of millions of repeating sub-units. This makes it extremely error-tolerant,” says Galinski. “The etching and coating process can also be used on a large scale over surfaces of several square meters.” Until now, structural colors have been restricted to a smaller scale due to their difficult and expensive manufacture.

Money, cars, and planes

The new structural colors could be used to create very thin security features in banknotes, for example, or to color vehicles and aircraft, also for camouflage paint in military applications.

The new metamaterial—human-made materials with optical, electrical, or magnetic properties that do not occur in nature—could also prove interesting for energy systems, such as thin-film solar cells. “We have developed an extremely thin material that concentrates and perfectly absorbs light at individual points,” says Galinski. This effect could be used to develop highly efficient light-harvesting systems. In addition, the light concentration is largely independent of its angle of incidence—another advantage for applications in solar cells.

Galinski is co-lead author of the paper, which the journal Light: Science and Applications has accepted for publication. Galinski works in the laboratory of ETH Zurich professor Ralph Spolenak and Harvard University professor Federico Capasso.

Andrea Fratalocchi, professor at King Abdullah University of Science and Technology (KAUST) in Saudi Arabia, and his group contributed to the theoretical explanation of the functional principle through their comprehensive computer simulations.

Source: ETH Zurich