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Tiny Light Bulbs

Ultrasmall light-emitting fibers deposited on electrodes can serve as nanometer-scale light sources.

By depositing narrow light-emitting fibers on a silicon substrate patterned with gold electrodes, researchers at Cornell University have created extremely small light sources with dimensions of only a few hundred nanometers. The fibers are made of a polymer that is embedded with light-emitting molecules, which light up when exposed to an electric field. When the researchers apply voltage to the electrodes, the fiber glows orange at different points, much like Christmas lights, says Hector Abruna, a chemistry and chemical-biology professor at Cornell who is one of the project’s leaders.

Nano lamps: A light-emitting nanofiber spans gold electrodes that are 500 nanometers apart. When a voltage is applied to the electrodes, a tiny spot 240 by 325 square nanometers in area lights up.

The researchers use a straightforward technique called electrospinning to lay down the fibers directly on the substrate. Because the method is relatively simple, the light sources should be easy to integrate into lab-on-a-chip devices, where light can be used to detect chemical and biological molecules, such as drugs and proteins, which could be tagged with fluorescent dyes or might absorb a portion of the light. And because the fibers are made of polymers, they could find use in flexible displays. “You can imagine these [fibers] integrated into clothing,” says George Malliaras, a Cornell materials-science and engineering professor who is collaborating on the work with Abruna and Harold Craighead at Cornell’s Center for Nanobiotechnology.

The extremely small size of the light sources could also lead to novel approaches to doing microscopy, Malliaras says. The fibers range from 150 nanometers to 5 micrometers in diameter. But the light-emitting spots on the fibers measure 240 and 325 nanometers or less. This makes the light sources smaller than the 600-nanometer wavelength of the light that they emit, a property that could be harnessed to develop new microscopy methods.

To electrospin the fibers, the researchers place a tiny droplet of polymer solution on a metal needle tip. Then they apply a voltage difference between the tip and the silicon substrate, which is etched with gold electrodes and is placed a few millimeters away. The voltage causes the droplet to elongate and form a jet that flows down to the substrate. As it moves down, the solvent evaporates, and hardened polymer fibers get deposited on the electrode-covered substrate.

The polymer in this case contains ruthenium-based molecules, which emit light when subjected to an electrical current. When the researchers apply a voltage to the gold electrodes, tiny spots on the stretches of fiber spanning adjacent electrodes glow orange. At high voltages of 100 volts, the light is bright enough that the researchers can see it in the dark in spite of the emitters’ small size. “I would say [this] is a breakthrough in the way nanosize light sources are made,” says Stefan Bernhard, a chemistry professor at Princeton University.

The electrospinning technique offers multiple advantages. Using the method, one should be able to make fibers with diameters of 50 nanometers or less, which could lead to even smaller light sources, Malliaras says. Plus, the technique should make fabricating nanoscale light emitters on practical lab-on-a-chip devices relatively easy, although one would still need to etch the gold electrodes.

“The distinguishing and extremely interesting aspect of this work is the minute size of the light sources they describe,” says John de Mello, who researches nanoscale organic light-emitting devices at Imperial College London. Until now, organic light-emitting devices have typically been about one square millimeter in size, he says, which is ideal for standard lab-on-a-chip applications, such as detecting bacteria or proteins. But the nanometer-sized light sources would be important for niche applications requiring speed and a very small resolution–for example, monitoring how a chemical reaction is proceeding as chemicals flow through microfluidic channels. “This approach offers a means of dramatically improving the resolution of such measurements,” de Mello says.

Much research remains to be done, however. For any practical application, the researchers would need to precisely control the arrangement of the fibers on the silicon substrate. But the work is a first step in making nanoscale light sources using a straightforward method, Malliaras says.

Says de Mello, “Once it’s known there’s a low-cost route to making sub-wavelength light sources, you can be sure somebody will find a use for them. That’s the real excitement of this kind of work.”

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