Researchers at Rensselaer Polytechnic Institute (RPI) and Rice University recently created the darkest manmade material out of vertically aligned carbon nanotubes. The new material has an incredibly low level of reflection and a high level of absorption, making it as dark as it is with a total reflective index of 0.045 percent.
“You have to see it to believe it; it’s very, very dark,” says Shawn-Yu Lin, professor of physics at RPI.
The darkest material before this latest discovery made by Lin’s group used a film deposition of nickel-phosphorous alloy and had a reflective index of 0.16 percent. The new material composition is more than three times darker than the previous “darkest material,” according to an RPI release.
This new material composition has potential applications for anything needing high absorption, but specifically photovoltaics and thermophotovoltaics. According to Lin the material must have three criteria in order to improve solar applications: It must exhibit the same absorption/reflection properties across all angles, all polarizations and all wavelengths of interest. “If you can satisfy all three criteria, then you have a near 100-percent absorption; this material does that,” he says. “There is no other material we know that can do the same trick.”
The material is made by heating up an ion film, turning it into nanoparticles, which act as a catalyst and vertically grow the nanotubes between 10 microns and 1mm long. “Once they grow vertically it almost looks like a forest,” says Lin. The nanotubes have a diameter of between 9 and 10 microns and are spaced between 15 and 16 microns apart. At the end of the nanotubes, furthest from the substrate, the tips curl and curve and end up touching each other, creating a dynamic surface.
The next step for Lin and researchers is to determine what the properties of these materials are in the infrared and microwave range. “We so far have measured very, very low refraction or very, very dark material only in the visible range,” says Lin. “We need to extend it into infrared wavelengths — there are a lot of applications for infrared detection — and even further to terahertz wavelengths and even microwave wavelengths.”
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