LED foil developed for ultra-portable water purification

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The researchers say that their technique could one day allow commercial-scale production of less expensive, lighter, environmentally friendly deep-UV LEDs(Credit: Brelon J. May/Ohio State University)

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Deep-ultraviolet (deep-UV) lights are used for the heavy-duty purification of drinking water and the rigorous sterilization of medical equipment. Unfortunately, they are also usually made using large and heavy mercury lamps not particularly suited to portable use in battlefield or emergency field situations. In an effort to overcome this problem, engineers at The Ohio State University have created a flexible, lightweight, LED-based, deep-UV foil prototype that can be wrapped around items and energized to kill harmful microorganisms.

"Right now, if you want to make deep ultraviolet light, you've got to use mercury lamps," said Roberto Myers, associate professor of materials science and engineering at Ohio State. "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."

UV light of any type is generally pretty good at killing off a good deal of harmful microbes, such as in the UV system proposed for Boeing airliners or on some vacuum cleaners, and the specific LEDs used in these instances are perfectly capable of producing it. Deep-UV light (that is, UV light emitted at wavelengths shorter than 300 nm and particularly effective at rendering bacteria and other pathogens inert), on the other hand, has only ever been produced using LEDs at laboratory prototype scales before, and with semiconductor substrates created from exceptionally purified, inflexible single-crystals that are expensive and largely impractical for flexible devices.

The Ohio State research, however, uses new foil-based nanotechnology that uses a semiconductor growth method known as molecular beam epitaxy, where beams of molecules are used to vaporize materials and then allow them to land on a surface where they self-organize into nanostructure layers. In this case, the researchers employed this technique to propagate a field of tightly packed aluminum gallium nitride wires (some 200 nanometers tall and 20-50 nanometers in diameter) on foils such as titanium and tantalum, to produce a flexible mat of conductive fibers.

In tests conducted by Ohio State researchers, when an electrical current was applied to the nanowires, they claim that they emitted light almost as bright as the LEDs created on the inflexible single-crystal silicon.

The researchers say that their technique could one day allow commercial-scale production of less expensive, lighter, environmentally friendly deep-UV LEDs and help turn the rarefied area of nanophotonics research into a profitable and practical industry.

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

The Ohio State engineers are conducting further research to improve the brightness of the nanowire LEDs, and intend to attempt growing the wires on more common metal foils such as those made from steel and aluminum.

The results of this research were recently published in the journal Applied Physics Letters.

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