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  • Nanoantennas Hold Promise for IR Photovoltaics
May 2011
HOUSTON, May 11, 2011 — A new device that can act as an optical antenna by collecting and focusing light, and as a photodiode by converting light into a current of electrons, has been developed by Rice University researchers. This nanoscale device, dubbed nanoantenna, has the potential to improve photosensing, energy harvesting, imaging and light-detection technologies.

By attaching nanoscale antennas to silicon semiconductors, Rice researchers showed they could harvest infrared light and turn it into electricity. (Images: Science/AAAS)

"We're merging the optics of nanoscale antennas with the electronics of semiconductors," said lead researcher Naomi Halas, the university's Stanley C. Moore professor in electrical and computer engineering. "There's no practical way to directly detect infrared light with silicon, but we've shown that it is possible if you marry the semiconductor to a nanoantenna. We expect this technique will be used in new scientific instruments for infrared-light detection and for higher-efficiency solar cells."

Artist’s rendering of a nanoantenna array under laser illumination. The nanorods (gold) are covered with a transparent indium tin oxide layer (purple) and surrounded by an insulating layer of silica (blue).

The majority of today’s solar panels use silicon to convert sunlight into electricity, but silicon cannot capture infrared light’s energy. By attaching a metal nanoantenna to the silicon, where the tiny antenna is specially tuned to interact with infrared light, the Rice team showed that it could extend the frequency range for electricity generation into the infrared. When infrared light hits the antenna, it creates a "plasmon," a wave of energy that sloshes through the antenna's ocean of free electrons.

Plasmons decay and give up their energy in two ways: They either emit a photon of light or they convert the light energy into heat. The heating process begins when the plasmon transfers its energy to a single electron - a "hot" electron. Rice graduate student Mark Knight, lead author on the paper published in the journal Science, together with Rice theoretical physicist Peter Nordlander, his graduate student Heidar Sobhani, and Halas set out to design an experiment to directly detect the hot electrons resulting from plasmon decay.

A representation of a single Au resonant antenna on an n-type silicon substrate.

Patterning a metallic nanoantenna directly onto a semiconductor to create a "Schottky barrier," Knight showed that the infrared light striking the antenna would result in a hot electron that could jump the barrier, creating an electrical current. This works for infrared light at frequencies that otherwise would pass directly through the device.

"The nanoantenna diodes we created to detect plasmon-generated hot electrons are already pretty good at harvesting infrared light and turning it directly into electricity," Knight said. "We are eager to see whether this expansion of light-harvesting to infrared frequencies will directly result in higher-efficiency solar cells."

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Calculated quantity of the entire longitudinal wave of a solid substance's electron gas.
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