Black is Back

CHICAGO (Reuters) - U.S. researchers said on Tuesday they have made the darkest material on Earth, a substance so black it absorbs more than 99.9 percent of light.

Made from tiny tubes of carbon standing on end, this material is almost 30 times darker than a carbon substance used by the U.S. National Institute of Standards and Technology as the current benchmark of blackness.

And the material is close to the long-sought ideal black, which could absorb all colors of light and reflect none.

"All the light that goes in is basically absorbed," Pulickel Ajayan, who led the research team at Rice University in Houston, said in a telephone interview. "It is almost pushing the limit of how much light can be absorbed into one material."

The substance has a total reflective index of 0.045 percent -- which is more than three times darker than the nickel-phosphorous alloy that now holds the record as the world's darkest material.

Basic black paint, by comparison, has a reflective index of 5 percent to 10 percent.

The researchers are seeking a world's darkest material designation by Guinness World Records. But their work will likely yield more than just bragging rights.

Ajayan said the material could be used in solar energy conversion. "You could think of a material that basically collects all the light that falls into it," he said.

It could also could be used in infrared detection or astronomical observation.

THREE-FOLD BLACKNESS

Ajayan, who worked with a team at Rensselaer Polytechnic Institute in Troy, New York, said the material gets its blackness from three things.

It is composed of carbon nano-tubes, tiny tubes of tightly rolled carbon that are 400 hundred times smaller than the diameter of a strand of hair. The carbon helps absorb some of the light.

These tubes are standing on end, much like a patch of grass. This arrangement traps light in the tiny gaps between the "blades."

The researchers have also made the surface of this carbon nano-tube carpet irregular and rough to cut down on reflectivity.

"Such a nano-tube array not only reflects light weakly, but also absorbs light strongly," said Shawn-Yu Lin, a professor of physics at Rensselaer, who helped make the substance.

The researchers have tested the material on visible light only. Now they want to see how it fares against infrared and ultraviolet light, and other wavelengths such as radiation used in communications systems.

"If you could make materials that would block these radiations, it could have serious applications for stealth and defense," Ajayan said.

The work was released online last week and will be published in an upcoming issue of the journal Nano Letters. The Indian-born Ajayan holds the 2006 Guinness World Record as co-inventor of the smallest brush in the world.

Julie Steenhuysen