Imagine mobile phones charged by their own power generating skins, buildings cooled without the need for external power, or solar collectors that produced energy around the clock. Sounds fanciful doesn't it? Well, scientists at the U.S. Department of Energy's Idaho National Laboratory have made a breakthrough that may see such startling advances being made within just a few years.

By embossing thin sheets of plastic with tiny nanoantennas, the team of researchers have been able to capture electromagnetic energy at the mid-infrared range and convert it to electricity. Similar antennas have been created in the past to harvest lower frequency electromagnetic radiation such as microwaves, but capturing the higher frequency infrared radiation has, until now, been difficult. Infrared radiation is the means by which heat is transmitted between objects. And therein lies the problem because when objects heat up, their shape and other characteristics change.

The size and shape of the nanoantennas is crucial to their ability to capture energy. Even small amounts of heat could have a huge effect on such small objects. The team have overcome the difficulty of heat absorption by surrounding the nanoantennas with specially treated polyethylene, a material typically found in plastic shopping bags. Each nanoantenna is a gold spiral approximately 1/25 the width of a human hair. Plastic sheets containing billions of interconnected nanoantennas collect heat energy emitted by the sun and other sources and release it as electricity.

The breakthrough came after the team had constructed several computer models to examine the reaction of various materials to infrared radiation. Gold, copper and manganese were tested. The simulations confirmed that nanoantennas of the correct shape, size and material could capture over 92% of the infrared energy to which they were exposed. Then, using a technique usually used in the manufacture of electronics, actual gold nanoantennas were etched into thin wafers of silicon. These prototypes converted over 80% of the infrared energy to electrical energy. Finally a stamp-and-repeat method was used to emboss billions of the nanoantennas onto thin plastic sheets. These, too, demonstrated a more than 80% conversion rate while traditional solar cells convert roughly 20% of the solar energy that strikes their surface into electricity.

The beauty of the discovery lies in the abundance of infrared energy all around us. Unlike visible light, on which typical solar cells rely, infrared energy is not only given off by the sun. "Every process in our industrial world creates waste heat. It's energy that we just throw away," says Steven Novack, the leader of the team responsible for the discovery. The heat radiated from electronics, industrial processes and even the Earth itself could be captured by arrays of nanoantennas.

The nanoantennas can also be tweaked to capture different wavelengths of electromagnetic energy.  This gives them a distinct advantage over traditional solar cells that focus on a relatively small range of energies within the electromagnetic spectrum. In the future, specialised double-sided solar panels could capture solar energy during the day on one side and infrared energy emitted by the earth at night. In addition, because infrared energy is the mechanism used to transmit heat between objects, arrays of nanoantennas could be used to cool buildings and other structures, transforming the once wasted heat into electrical energy.

Some problems remain to be solved, however. The infrared radiation causes the nanoantennas to oscillate trillions of times per second, producing alternating current (AC). A device known as a rectifier is needed to convert the current into usable direct current (DC). However, modern rectifiers cannot handle AC at such high frequencies. "We need to design nanorectifiers to go with our nanoantennas," says Dale Cotter, a member of the research team. However, the team are confident that they can overcome the remaining barriers. One possibility is that a diode could be placed at the centre of each nanoantenna to slow down the current.

Team leader Steven Novack believes that using their computer modelling techniques, commercially viable nanoantenna arrays will be possible within a few years. The stamp-and-repeat process used to develop the prototypes could be extended to a large scale roll-to-roll manufacturing process. Novack believes that such a process could produce sheets of nanoarrays at a rate of several yards per minute at a cost of a mere few dollars per yard.

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