Caption: Schematic of the nanotube array
A new thin, flexible, light-absorbing material may be a boon for advancements in energy and stealth applications.
Engineers at the University of California-San Diego Jacobs School of Engineering, led by professors Zhaowei Liu and Donald Sirbuly, have created the material, called a near-perfect broadband absorber, that can absorb more than 87 percent of near-infrared light at 1,200 to 2,200 nanometer wavelengths, with 98 percent absorption at 1,550 nanometers, the wavelength for fiber optic communication.
The material can be theoretically customized to absorb certain wavelengths of light while letting others pass through.
The absorber uses an optical phenomena known as surface plasmon resonances, which are collective movements of free electrons that occur on the surface of metal nanoparticles upon interaction with certain wavelengths of light.
Metal nanoparticles carry a lot of free electrons, which enables them to exhibit strong surface plasmon resonance but mainly in visible light and not in the infrared.
The research team was able to change the number of free electron carriers so 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,” Sirbuly said in a statement. “Make the number higher, with more electrons, and we can push the plasmon resonance to the ultraviolet region.
“The problem with this approach is that it is difficult to do in metals.”
However, the engineers were able to design and build an absorber from materials that could be modified or doped to carry a different amount of free electrons.
The researchers used a zinc oxide semiconductor, which has a moderate number of free electrons and combined it with its metallic version—aluminum-doped zinc oxide, which houses a high number of free electrons, enough to give it plasmonic properties in the infrared but not as much as an actual metal.
The materials were combined and structured in a precise manner using advanced nanofabrication technologies. 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 1,730 nanometers tall, 650 to 770 nanometers in diameter, and spaced less than a hundred nanometers apart and the nanotube array was then transferred from the silicon substrate to a thin, elastic polymer.
“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,” Conor Riley, a recent nanoengineering Ph.D. graduate from UC San Diego and the first author of this work, said in a statement. “Our simulations show that this is possible.”
While materials that perfectly absorb light already exist, they are often bulky and can break when they are bent. These materials also can’t be controlled to absorb only a selected range of wavelengths, which is a disadvantage for certain applications.
The material has a number of applications including transparent window coatings that keep buildings and cars cool on sunny days.
For example, the material could be used as a window coating for cooling not only blocked infrared radiation but also normal light and radio waves that transmit television and radio programs.
“This material offers broadband, yet selective absorption that could be tuned to distinct parts of the electromagnetic spectrum,” Liu said in a statement.
The material can also be potentially transferrable to any type of substrate and can be scaled up to make large surface area devices including 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 research team will continue to work on exploring different materials, geometries and design to develop absorbers that work at different wavelengths of light for various applications.
The study was published in the Proceedings of the National Academy of Sciences of the United States of America.
