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Your local traffic light may not use a bulb. Instead, it could be relying on a new source of light that may find its way soon into your TV, your computer, your cell phone, and your lamps at home. As this ScienCentral News video reportrs, some nanotechnologists want to replace every light bulb with something they say is far better.
Brilliant Solution
At Sandia National Laboratories, one research team looks to a wall map of the earth for their motivation. The map shows lights that can be seen from space at night. To the Sandia team, the map shows the “tremendous amount of illumination that escapes into outer space,” says the group’s director, physicist Jerry Simmons . “Our vision is to replace every single light that you see on that map” with something he believes will be inexpensive and much brighter, but will use far less energy.
As anyone who tries to change a conventional light
bulbs immediately finds out, bulbs give off high heat. In fact, they generate more heat than light, so they waste a great deal of energy. Simmons thinks that with help from nanotechnology, lighting could consume only half as much energy as it requires now.
The Sandia team wants to replace traditional light bulbs with semiconductor crystals, which can conduct electricity and emit light. These crystals are called LEDs, or light emitting diodes. Much brighter and longer lasting than light bulbs, colored LEDs are already part of traffic lights, exit signs, and outdoor displays in stadiums They flash stock prices across the NASDAQ sign in New York’s Times Square, and illuminate the Thomas Jefferson Memorial in Washington, D.C.
But apart from flashlights and indicators on computers and cell phones, LEDs haven’t lit up homes and offices yet. Growing LEDs that can produce white light is very difficult; the crystals end up with flaws that waste energy and cut down on their brilliance. Before LEDs can be effective, inexpensive and widely available, researchers must find better ways to manufacture them.
To grow an LED, Simmons says, “you make a kind of sandwich out of two semi-conducting layers.” One layer carries positive electrical charges; the other, negative charges. When the two charges meet, they send out a particle of light–but only if atoms in the two slabs match up perfectly. Otherwise, Simmons says, the crystal “ends up having a lot of defects in it that reduce the efficiency of the LED.” To ensure that every electrical charge comes out as light, the Sandia team is growing LEDs atom by atom–on the nanoscale.
To grow an LED, Simmons says, “you make a kind of sandwich out of two semi-conducting layers.” One layer carries positive electrical charges; the other, negative charges. When the two charges meet, they send out a particle of light–but only if atoms in the two slabs match up perfectly. Otherwise, Simmons says, the crystal “ends up having a lot of defects in it that reduce the efficiency of the LED.” To ensure that every electrical charge comes out as light, the Sandia team is growing LEDs atom by atom–on the nanoscale.
How to grow an LED that emits white light: "diving boards" over a sapphire base. image: Sandia National Laboratories
Simmons’ group has developed an innovative approach called cantilever epitaxy. A cantilever is a rectangular board attached to a base at one end; epitaxy is a way of growing crystals. In Simmons’ lab, cantilever epitaxy begins with a thick bottom slab, or base, made of sapphire. The researchers cut miniscule trenches into the sapphire base, and then use a semiconductor material, gallium nitride, to create the top slab. Along the tops of the trenches, the material grows upwards as tiny posts. Then, when the temperature changes, the posts start growing sideways over the trenches. The result is what Simmons calls tiny “diving boards, suspended over the trenches” without touching the lower sapphire base. “Those diving board regions have virtually no flaws in them,” resulting in a pristine surface on which to grow LEDs that have dramatically fewer defects.
In the long run, Simmons and his researchers, who have patented their new process, aim to see LEDs replace light bulbs as rapidly as semiconductors replaced vacuum tubes in transistors and computers.
The Sandia team’s work has been published in Applied Physics Letters, and has been presented most recently at the 15th American Conference on Crystal Growth and Epitaxy, and at the 2003 Electronic Materials Conference. This research is funded by the U.S. Department of Energy (DOE) and the Defense Advanced Research Projects Agency (DARPA).
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