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"Near-perfect" broadband absorber with potential in solar cells

A new flexible material developed by engineers at the University of California, San Diego (UCSD) is claimed to be able to tune out various portions of the electromagnetic spectrum while allowing others to pass through, such as being opaque to infra-red but transparent to visible light, for example. This material has the potential to vastly improve the efficiencies of solar cells, or create window coatings that not only let in visible light and keep out heat, but also stop electronic eavesdropping by blocking electromagnetic signals.

There are a number of materials that are able to almost perfectly absorb various frequencies of light (such as Harvard's use of vanadium dioxide), but these are not normally suitable for everyday use because they are costly, bulky, or simply impractical. In addition, they are only applicable to a very select array of frequencies, further restricting their broader use. 

However, the UCSD researchers responsible for creating a new material that is thin, flexible and transparent to visible light claim it is a near-perfect broadband absorber that soaks up more than 87 percent of light at near-infrared frequencies (wavelengths of between 1,200 to 2,200 nanometers), while being flexible and thin. It also has a claimed 98 percent absorption at the wavelength primarily used for fiber optic communication, at around 1,550 nanometers. The material also absorbs light at any angle.

According to the researchers, these specifications are for the material as it stands today, and they believe it could eventually be transformed to absorb specific light frequencies whilst allowing others to pass through.

"This material offers broadband, yet selective absorption that could be tuned to distinct parts of the electromagnetic spectrum," says Professor Zhaowei Liu at the UCSD Jacobs School of Engineering.

The material was created by depositing arrays of nanotubes made from alternating layers of zinc oxide and aluminum-doped zinc oxide some 1,730 nanometers tall and around 650-770 in diameter. Initially created on a solid silicon substrate, the nanotubes were transferred to a thin, transparent elastic polymer to create the final version of the material.

Each of the nanotubes acts as a semiconductor in the resulting nanostructure, with varying degrees of "doping" (the introduction of impurities into a semiconductor to alter its electrical properties) applied to control the flow of free electrons.

In this case, the combination of zinc oxide, which has a medium quantity of free electrons, with aluminum-doped zinc oxide, which possesses a large quantity of free electrons, provided sufficient movement of free electrons to generate a surface plasmon resonance – this is the resonant oscillation of free electrons occurring on the surface of metal nanoparticles in response to certain wavelengths of light – sufficient to block particular frequencies of light.

Altering the proportions of free electrons by changing the physical and chemical properties of the nanotubes, and other wavelengths should be able to be tuned in or out, according to the researchers.

"Make this number lower, and we can push the plasmon resonance to the infrared," says Professor Donald Sirbuly, also from UCSD Jacobs School of Engineering. "Make the number higher, with more electrons, and we can push the plasmon resonance to the ultraviolet region."

Conor Riley, a recent nanoengineering Ph.D. graduate from UCSD, adds, "There are different parameters that we can alter in this design to tailor the material's absorption band: the gap size between tubes, the ratio of the materials, the types of materials, and the electron carrier concentration. Our simulations show that this is possible."

If these properties are realized, the material may also assist in focusing specific wavelengths of light onto solar cells to provide better photon absorption, whilst blocking the effects of potentially destructive infra-red heat.

The researchers believe that their arrays of nanotubes may also be transferable to many different surfaces, and could potentially be applied to large surface areas, such as windows, to act as broadband absorption devices. It's early days, but the team is hopeful.

"Nanomaterials normally aren't fabricated at scales larger than a couple centimeters, so this would be a big step in that direction," says Sirbuly.

Though still very much at the working prototype stage, the researchers intend to further their research by analyzing the effects of different materials, physical arrangements, and semiconductor properties in an attempt to create materials that absorb light at different wavelengths for use in a variety of applications. 

The results of this research were recently published in the journal Proceedings of the National Academy of Sciences.



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