LEDs offer new way to kill germs in water

New devices also could lead to bendable computer displays and more
Jan 11, 2017 — 7:10 am EST
hand water

Clean water is necessary for life. A new technology could make it easier to disinfect water and make it safer to drink.

greenaperture/iStockphoto

Tiny wires grown on thin, flexible metal sheets can now give off an ultraviolet (UV) glow. One day, such wires might find use in disinfecting water or killing germs on instruments that had been used in surgery. These wires might even lead to bendable computer displays, scientists say.

The nano-scale wires are light-emitting diodes, or LEDs, — devices that are very energy efficient. LEDs typically glow most strongly at only a few wavelengths. Those wavelengths will depend on what the LEDs had been made from, notes Roberto Myers. He’s a materials scientist at Ohio State University in Columbus. His team developed the new LED nanowires.

flexible LEDs
Tiny light-emitting diodes (LEDs) grown on this flexible metal foil emit ultraviolet light. That light could be used to disinfect water in developing nations.
Brelon J. May/Ohio State University

The ones his group made emit UV light. Overall, UV wavelengths range from about 10 to 380 nanometers. This part of the light spectrum can kill bacteria. That makes the light useful for tasks such as disinfecting water and sterilizing surgical scalpels. (Manufacturers also use UV light to harden some plastics and dentists use it to harden dental fillings.)

The higher the energy of the UV light, the better job it does at killing germs. But it is difficult to generate light at such short wavelengths in the ultraviolet spectrum. (Those shorter wavelengths correspond to higher frequencies and thus higher energies.) The only way to generate high-energy UV radiation now is by using light bulbs that contain mercury vapor, Myers says. Large and heavy, these bulbs also need a lot of power. Moreover, mercury is toxic. So if a bulb breaks, it could poison the environment. “But if we could make UV LEDs that are safe and portable and cheap, we could make safe drinking water wherever we need it,” argues Myers.

His team built its LEDs out of a semiconductor. As that term name suggests, a semiconductor conducts electricity only under certain conditions. Myers’ team used a semiconductor called aluminum gallium nitride. The scientists sprayed a hot mix of aluminum vapor, gallium vapor and nitrogen atoms onto a thin, flexible metal surface. Everything was done in a low-pressure chamber so the elements could spread out evenly, sort of like hot spray paint.

On the surface of the film, the elements reacted with each other. As they combined, they made aluminum gallium nitride. The substance didn’t make an even coating, however. It formed as a forest of nanowires. Under a microscope, this nanowire forest looked like a miniature shag carpet.

The metal onto which the forest grew is important. A thin, flexible film., it resembles the aluminum foil found in any kitchen.  It is, however, about six times thicker. It also is made from a different metal. In one test, the team grew their LED forest on a film of titanium. In another, they grew those nanowires on a film of tantalum. Tantalum is a metal that doesn’t corrode easily. Myers and his team described their research in the April 4, 2016 issue of Applied Physics Letters.

nanowire LEDs
Each vertical spike in this image is a LED nanowire that emits ultraviolet light. The light can be used for such purposes as sterilizing surgical instruments.
Brelon J. May/Ohio State University

“It’s very exciting to be able to grow LEDs on a flexible metallic foil,” says Lutz Geelhaar. He’s a physicist at the Paul Drude Institute for Solid State Electronics in Berlin, Germany. If scientists could figure out how to grow LEDs that glow at different wavelengths of visible light, he says, then it might be possible to create tiny bendable computer displays. He cautions, however, that “those applications are still a long way down the road.”

And flexibility might not be the system’s only benefit, notes Pallab Bhattacharya. He’s an electrical engineer at the University of Michigan in Ann Arbor. Semiconductors don’t conduct heat well. So heat released by LEDs on the surface of a semiconductor can’t easily flow away. If they overheated, that could shorten the lifespan of the LEDs. But the metal floor in these nanowire forests could shed any heat that built up from glowing LEDs, he says. That could help keep the nanowires cool and long-lasting.

Power Words

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aluminum     A metallic element, the third most abundant in Earth’s crust. It is light and soft, and used in many items from bicycles to spacecraft.

atom     The basic unit of a chemical element. Atoms are made up of a dense nucleus that contains positively charged protons and neutrally charged neutrons. The nucleus is orbited by a cloud of negatively charged electrons.

chemical     A substance formed from two or more atoms that unite (become bonded together) in a fixed proportion and structure. For example, water is a chemical made of two hydrogen atoms bonded to one oxygen atom. Its chemical symbol is H2O. Chemical can also be an adjective that describes properties of materials that are the result of various reactions between different compounds.

disinfect     To clean an area by killing dangerous infectious organisms, such as bacteria.

electrical engineer     An engineer who designs, builds or analyzes electrical equipment.

electricity     A flow of charge, usually from the movement of negatively charged particles, called electrons.

electronics     Devices that are powered by electricity but whose properties are controlled by the semiconductors or other circuitry that channel or gate the movement of electric charges.

element     (in chemistry) Each of more than one hundred substances for which the smallest unit of each is a single atom. Examples include hydrogen, oxygen, carbon, lithium and uranium.

engineer     A person who uses science to solve problems. As a verb, to engineer means to design a device, material or process that will solve some problem or unmet need.

infection     A disease that can spread from one organism to another. It’s usually caused by some sort of germ.

light emitting diodes (LEDs)        Electronic components that, as their name suggests, emit light when electricity flows through them. LEDs are very energy-efficient and often can be very bright. They have lately been replacing conventional lights in auto taillights and in some bulbs used for home lighting.

mercury     Sometimes called quicksilver, mercury is an element with the atomic number 80. At room temperature, this silvery metal is a liquid. Mercury is also very toxic.

microscope     An instrument used to view objects, like bacteria, or the single cells of plants or animals, that are too small to be visible to the unaided eye.

nanowire     A wire or rod on the order of a billionth of a meter in cross-section or in circumference. It is usually made from some type of semiconducting material. However some bacteria make string-like anchoring structures on the same size scale. Like the semiconductor wire, the bacterial ones also can transport electrons.

nitrogen     A colorless, odorless and nonreactive gaseous element that forms about 78 percent of Earth's atmosphere. Its scientific symbol is N. Nitrogen is released in the form of nitrogen oxides as fossil fuels burn.

physics     The scientific study of the nature and properties of matter and energy. Classical physics is an explanation of the nature and properties of matter and energy that relies on descriptions such as Newton’s laws of motion. Quantum physics, a field of study which emerged later, is a more accurate way of explaining the motions and behavior of matter. A scientist who works in that field is known as a physicist.

plastic     Any of a series of materials that are easily deformable; or synthetic materials that have been made from polymers (long strings of some building-block molecule) that tend to be lightweight, inexpensive and resistant to degradation.

pressure     Force applied uniformly over a surface, measured as force per unit of area.

semiconductor     A material that sometimes conducts electricity. Semiconductors are important parts of computer chips and certain new electronic technologies, such as light-emitting diodes.

solid state     A term for electronics technologies that create circuitry or devices from solid materials known as semiconductors. As they work, their electrons (or other charge carriers) remain confined entirely within the solid material.

tantalum     A metallic element used in electronics, such as computers and DVD players.

toxic     Poisonous or able to harm or kill cells, tissues or whole organisms. The measure of risk posed by such a poison is its toxicity.

ultraviolet     A portion of the light spectrum that is close to violet but invisible to the human eye.

ultraviolet light     A type of electromagnetic radiation with a wavelength from 10 nanometers to 380 nanometers. The wavelengths are shorter than that of visible light but longer than X-rays.

wavelength     The distance between one peak and the next in a series of waves, or the distance between one trough and the next. Visible light — which, like all electromagnetic radiation, travels in waves — includes wavelengths between about 380 nanometers (violet) and about 740 nanometers (red). Radiation with wavelengths shorter than visible light includes gamma rays, X-rays and ultraviolet light. Longer-wavelength radiation includes infrared light, microwaves and radio waves.

Citation

Journal:   B.J. May, A.T.M. Golam Sarwar and R.C. Myers. Nanowire LEDs grown directly on flexible metal foil. Applied Physics Letters. April 4, 2016, p. 141103. doi: 10.1063/1.4945419.