Really Black
by Jade Boyd
Blacker than night. Blacker than pitch. Blacker than ... well, blacker than anything. Researchers from Rice University and Rensselaer Polytechnic Institute weren’t searching for a new metaphor when they created the darkest material ever measured, but they may have set a new standard with a thin carpet of carbon nanotubes that’s four times darker than the previous record holder.
By comparison, the darkest conventional black paint reflects about 100 times more light than the nanotube carpet. The researchers who developed the material have received a Guinness World Record. The previous record holder for darkest human-made material was a nickel-phosphorus alloy whose surface was pitted with tiny, light-trapping craters. The researchers say the new material could find use in super-efficient solar converters, telescopes and special coatings.
“The low-density arrangement of the nanotubes is the key,” said study co-author Pulickel Ajayan, Rice’s Benjamin M. and Mary Greenwood Anderson Professor in Mechanical Engineering and Materials Science. “We engineered the super-dark optical properties by tightly controlling the dimensions and spacing of the nanotubes.”
“The low-density arrangement of the nanotubes is the key. We engineered the super-dark optical properties by tightly controlling the dimensions and spacing of the nanotubes.”
-Pulickel Ajayan
The carpet, which looks like a thin sheet of black paper, is made of billions of tiny hollow tubes of pure carbon that are vertically aligned like packaged spaghetti. The material’s darkness lies in the loose packing of the tubes. Because the tips of each nanotube measure just one-billionth of a meter across, most light striking the surface is absorbed. As a result, the material reflects only 0.045 percent of the light that strikes it.
“It is a fascinating technology, and this discovery will allow us to increase the absorption efficiency of light as well as the overall radiation-to-electricity efficiency of solar energy conservation,” said Rensselaer physicist Shawn-Yu Lin, the lead co-author on the study. “The key to this discovery was finding how to create a long, extremely porous, vertically aligned carbon nanotube array with certain surface randomness, simultaneously minimizing reflection and maximizing absorption.”
The team’s results appeared in the January issue of the journal Nano Letters.