A forest of carbon nanotubes has given researchers at Massachusetts Institute of Technology the blackest material ever made, one that absorbs 99.995% of any light incident upon it. [Cui and Wardle, ACS Appl Mater Interfac (2019); DOI: 10.1021/acsami.9b08290]
Brian Wardle's team grew the vertically aligned carbon nanotubes on chlorine-etched aluminum foil. Such a blacker than black material could be used in optical devices as a "blinder" that absorbs and reduces unwanted glare perhaps allowing space telescopes to get a clearer view of planets orbiting distant stars.
"There are optical and space science applications for very black materials," Wardle says. "Our material is ten 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."
The original aim of the work involved experimenting with ways to grow carbon nanotubes on electrically conducting materials such as aluminum, to boost their electrical and thermal properties, rather than searching for a void-like black material. Team member Kehang Cui hit a stumbling block in that work though - the ever-present oxide layer that coats an aluminum surface.
Some of the experiments involved common salt, sodium chloride, and Cui noticed that the chloride ions were eating away the oxide continuously from the aluminum surface. This gave him the idea to deliberately etching the surface and protecting it from oxygen once it was clear of oxide ions. The carbon nanotubes could then be grown on a pristine surface using chemical vapor deposition. This meant that the nanotube forest could be cultivated at a mere 100 Celsius rather than the usual high temperatures for that process.
Cui said the blackness of the resulting material piqued his interest. Of course, carbon allotropes are commonly black, diamond being the exception. But, this particular product seemed different and measurements showed it to absorb virtually all of the light hitting it regardless of the angle of the light or the angle at which the team attempted to measure any reflections. Moreover, it is a tougher material than previous deposited carbon nanotube materials.
"CNT forests of different varieties [were already] known to be extremely black, but there is a lack of mechanistic understanding as to why this material is the blackest. That needs further study," Wardle says.
David Bradley blogs at Sciencebase Science Blog and tweets @sciencebase. His popular science book Deceived Wisdom is still available.