UV LEDs promise cheap and safe drinking water Tuesday, 22 November 2016

American engineers have developing foil based light-emitting diodes (LEDs) for portable ultraviolet (UV) lights that soldiers and others can use to purify drinking water and sterilise medical equipment.

Associate professor of materials science and engineering at The Ohio State University, Roberto Myers, said they designed the LEDs to shine in the high-energy 'deep' end of the UV spectrum, which is already used by the military, humanitarian organisations and industry for applications ranging from detection of biological agents to curing plastics.

“Right now, if you want to make deep ultraviolet light, you’ve got to use mercury lamps,” said Myers. “Mercury is toxic and the lamps are bulky and electrically inefficient. LEDs, on the other hand, are really efficient, so if we could make UV LEDs that are safe and portable and cheap, we could make safe drinking water wherever we need it.”

He explained other research groups have fabricated deep-UV LEDs at the laboratory scale, but only by using extremely pure, rigid single-crystal semiconductors as substrates—a strategy that imposes an enormous cost barrier for industry.

The technology they are using relies on the well-established semiconductor growth technique, molecular beam epitaxy, in which vaporised elemental materials settle on a surface and self-organise into layers or nanostructures. The Ohio State researchers used this technique to grow a carpet of tightly packed aluminum gallium nitride wires on pieces of metal foil such as titanium and tantalum.

The individual wires measure about 200 nm tall and about 20-50 nm in diameter — thousands of times narrower than a human hair and invisible to the naked eye.

In laboratory tests, the nanowires grown on metal foils lit up nearly as brightly as those manufactured on the more expensive and less flexible single-crystal silicon.

Myers is also hopeful that they might turn a niche research field known as nanophotonics into a viable industry.

“People always said that nanophotonics will never be commercially important, because you can’t scale them up. Well, now we can. We can make a sheet of them if we want,” he said. “That means we can consider nanophotonics for large-scale manufacturing.”

The researchers are working to make the nanowire LEDs even brighter, and will next try to grow the wires on foils made from more common metals, including steel and aluminum.

[Ohio State University engineers have developed a technique to create light-emitting diodes on metal foil. Image by Brelon May/OSU]