The near-perfect broadband absorber developed by engineers at UC San Diego is thin, flexible and transparent in visible light. Photo: UC San Diego Jacobs School of Engineering.
The near-perfect broadband absorber developed by engineers at UC San Diego is thin, flexible and transparent in visible light. Photo: UC San Diego Jacobs School of Engineering.

Transparent window coatings that keep buildings and cars cool on sunny days. Devices that could more than triple solar cell efficiencies. Thin, lightweight shields that block thermal detection. These are some of the potential applications for a thin, flexible, light-absorbing material developed by engineers at the University of California (UC), San Diego.

The material, termed a near-perfect broadband absorber, absorbs more than 87% of near-infrared light (1200–2200nm wavelengths), with 98% absorption at 1550nm, the wavelength for fiber optic communication. The material is capable of absorbing light from every angle and can also theoretically be customized to absorb certain wavelengths of light while letting others pass through.

Materials that ‘perfectly’ absorb light already exist, but they are bulky and can break when bent. They also cannot be tailored to absorb only a selected range of wavelengths, which is a disadvantage for certain applications. Imagine if a window coating used for cooling not only blocked infrared radiation but also normal light and radio waves that transmit television and radio programs.

Utilizing a novel nanoparticle-based design, a team led by Zhaowei Liu and Donald Sirbuly at the UC San Diego Jacobs School of Engineering has now created a broadband absorber that's thin, flexible and tunable. The work was reported in a paper in the Proceedings of the National Academy of Sciences.

"This material offers broadband, yet selective absorption that could be tuned to distinct parts of the electromagnetic spectrum," Liu said.

The absorber relies on an optical phenomenon known as surface plasmon resonance, which is the collective movement of free electrons that occurs on the surface of metal nanoparticles upon interaction with certain wavelengths of light. Metal nanoparticles can carry a lot of free electrons, so they exhibit strong surface plasmon resonance – but mainly in visible light, not in the infrared.

The UC San Diego engineers reasoned that if they could change the number of free electron carriers, they could tune the material's surface plasmon resonance to different wavelengths of light. "Make this number lower, and we can push the plasmon resonance to the infrared. Make the number higher, with more electrons, and we can push the plasmon resonance to the ultraviolet region," Sirbuly explained. The problem with this approach is that it is difficult to do in metals.

To address this challenge, the engineers designed and built an absorber made from semiconducting materials, which can be modified, or doped, to carry a different amount of free electrons. They took a semiconductor called zinc oxide, which has a moderate number of free electrons, and combined it with its metallic version, aluminum-doped zinc oxide. This metallic version houses a high number of free electrons – not as much as an actual metal, but enough to give it plasmonic properties in the infrared.

The materials were combined and structured in a precise fashion using advanced nanofabrication technologies in the Nano3 cleanroom facility at the Qualcomm Institute at UC San Diego. The materials were deposited one atomic layer at a time on a silicon substrate to create an array of standing nanotubes, each made of alternating concentric rings of zinc oxide and aluminum-doped zinc oxide. The tubes are 1730nm tall, 650–770nm in diameter, and spaced less than a hundred nanometers apart. This nanotube array was then transferred from the silicon substrate to a thin, elastic polymer, producing a material that is thin, flexible and transparent in the visible.

"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," said Conor Riley, a recent nanoengineering PhD graduate from UC San Diego and first author of the paper. Riley is currently a postdoctoral researcher in Sirbuly's group.

Those are just a few of the exciting features of this particle-based design, the engineers said. It's also potentially transferrable to any type of substrate and can be scaled up to make large surface area devices, like broadband absorbers for large windows. "Nanomaterials normally aren't fabricated at scales larger than a couple centimeters, so this would be a big step in that direction," Sirbuly said.

The technology is still at the developmental stage. Liu and Sirbuly's teams are continuing to work together to explore different materials, geometries and designs, with the aim of developing absorbers that work at different wavelengths of light for various applications.

This story is adapted from material from the University of California, San Diego, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.