Very black is the new black

Engineers from the Massachusetts Institute of Technology, US, say they have created a material that is 10 times blacker than anything previously reported. 

Made from vertically aligned carbon nanotubes (CNTs) – microscopic filaments of carbon grown on a surface of chlorine-etched aluminium foil – it captures more than 99.96% percent of any incoming light.

The researchers report their findings in the journal ACS-Applied Materials and Interfaces and are showcasing the achievement in an exhibit at the New York Stock Exchange.

The Redemption of Vanity – a collaboration with MIT artist-in-residence Diemut Strebe – features a 16.78-carat natural yellow diamond coated with the new material, which makes the brilliantly faceted gem appears as a flat, black void.

“There are optical and space science applications for very black materials, and of course, artists have been interested in black, going back well before the Renaissance,” says research leader Brian Wardle, a professor of aeronautics and astronautics. 

“Our material is 10 times blacker than anything that’s ever been reported, but I think the blackest black is a constantly moving target. 

“Someone will find a blacker material, and eventually we’ll understand all the underlying mechanisms, and will be able to properly engineer the ultimate black.”

In this case a new black was not the aim. Wardle and then MIT postdoc Kehang Cui were experimenting with ways to grow carbon nanotubes on electrically conducting materials such as aluminium to boost their electrical and thermal properties.

However, they ran into problems with oxide which forms on aluminium when it’s exposed to air, acting as an insulator, and Cui turned to sodium chloride as a possible solution. He found he could remove the oxide layer by soaking the aluminium foil in saltwater.

With the oxide layer removed, the researchers were able to grow carbon nanotubes on the aluminium at much lower temperatures and, as expected, the combination of CNTs on aluminium significantly enhanced the material’s thermal and electrical properties.

What surprised them was the colour. “I remember noticing how black it was before growing carbon nanotubes on it, and then after growth, it looked even darker,” Cui says. “So I thought I should measure the optical reflectance of the sample.”

The rest, as they say, is history.

The researchers aren’t entirely sure of the mechanism contributing to the material’s opacity, but suspect it has something to do with the combination of etched aluminium, which is somewhat blackened, with the carbon nanotubes.

Wardle says scientists believe that forests of carbon nanotubes can trap and convert most incoming light to heat, reflecting very little of it back out as light, thereby giving CNTs a particularly black shade.