News in Science

Nanotubes yield the blackest black

Scientists have created the blackest material on Earth that absorbs more than 99.9% of light and say it could one day be used to block defence signals or create highly efficient solar panels.

The substance was created from carbon nanotubes by Professor Pulickel Ajayan, of Rice University in Houston, and team whose work will be published in an upcoming issue of the journal Nano Letters.

"All the light that goes in is basically absorbed," says Ajayan. "It is almost pushing the limit of how much light can be absorbed into one material."

The material is almost 30 times darker than a carbon substance used by the US 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 colours of light and reflect none.

The substance has a total reflective index of 0.045%, 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% to 10%.

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 says 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 says.

It could also could be used in infrared detection or astronomical observation, the researchers say.

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

First, the carbon nanotubes - tiny tubes of tightly rolled carbon that are 400 hundred times smaller than the diameter of a strand of hair - help absorb some of the light.

The nanotubes are standing on end, much like a patch of grass and light is trapped in the tiny gaps between the "blades".

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

"Such a nanotube array not only reflects light weakly, but also absorbs light strongly," says 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 defence," Ajayan says.

The Indian-born Ajayan holds the 2006 Guinness World Record as co-inventor of the smallest brush in the world.