Researchers develop ultrathin colour coatings

Harvard researchers have developed ultrathin coatings that shine in vivid colours and can be applied to any rough or flexible material, from wearable fabrics to stretchable electronics.
Mikhail Kats at Harvard School of Engineering and Applied Sciences has developed a technique that coats a metallic object with an extremely thin layer of semiconductor, just a few nanometres thick.
Although the semiconductor is a steely gray colour, the object ends up shining in vibrant hues. This is because the coating exploits interference effects in the thin films; Kats compares it to the iridescent rainbows that are visible when oil floats on water.

Carefully tuned in the laboratory, these coatings can produce a bright, solid pink-or, say, a vivid blue-using the same two metals, applied with only a few atoms' difference in thickness.
Kats worked with his adviser, Federico Capasso, the Robert L Wallace Professor of Applied Physics and Vinton Hayes Senior Research Fellow in Electrical Engineering.
The research group announced the finding in 2012, but at that time, they had only demonstrated the coating on relatively smooth, flat surfaces like silicon.

The group has now published a second paper, in the journal Applied Physics Letters, taking the work much further.
"I cut a piece of paper out of my notebook and deposited gold and germanium on it and it worked just the same," Kats said.

The finding suggests that the ultrathin coatings could be applied to essentially any rough or flexible material, from wearable fabrics to stretchable electronics.
"This can be viewed as a way of colouring almost any object while using just a tiny amount of material," Capasso said.
The applied films are so extremely thin that they interact with light almost instantaneously, so looking at the coating straight on or from the side - or looking at the rough imperfections in the paper - doesn't make much difference to the colour. And the paper remains flexible, as usual.
Because the metal coatings absorb a lot of light, reflecting only a narrow set of wavelengths, Capasso suggests that they could also be incorporated into optoelectronic devices like photodetectors and solar cells.

"The fact that these can be deposited on flexible substrates has implications for flexible and maybe even stretchable optoelectronics that could be part of your clothing or could be rolled up or folded," Capasso said.