I’ll stop wearing black when they invent a darker colour
– Wednesday Addams
Even with our fairly narrow perspective on the electromagnetic spectrum, most people would say that they know their colours. A red car is red, a green plant is green and a black shirt is going to be black regardless of the circumstances. Right?
Well your classic wardrobe essentials aren’t quite...black enough for astronomers and astrophysicists. When you’re trying to look at the cosmos and track Earth-like planets or stars in the universe, light can become a pollutant – bouncing off mirrors and contaminating measurements from telescopes and other instruments. So to reduce this, you need something that is (literally) blacker than black. This isn’t to do with shades of colour – this is a combination of physics, chemistry, and physically holding light captive.
So what’s blacker than black? A Belgian designer, Frederik de Wilde has created a material known only as Nanoblack. Having partnered with NASA to develop a novel technique, De Wilde has utilised carbon nanotubes and arranged them in vertical patterning to create one of the blackest materials in history. According to Leroy Sparr, who is giving the material a test run on the ORCA (Ocean Radiometer for Carbon Assessment) the material is “about 10 times better than [the] black paint” which is currently used by NASA’s engineers and designers to coat surfaces and control stray light. In an interview with Wired magazine, De Wilde discussed the use of Nanoblack in the years to come; “NASA is using the blackest black to coat the inside of a space telescope so stray light inside the lens is eliminated. And you could use it for stealth technology such as aircraft.”
Now the big question – what are carbon nanotubes?
Carbon nanotubes (CNTs hereafter) are members of the Fullerene family (pretty much any hollow carbon-based structure) and are over 10,000 times thinner than the average human hair. They form these kind of rope-like structure that you see in the GIF image above, and are formed from one-atom thick sheets of graphene. Graphene is closely related to the graphite in pencil lead, and has been hailed as a wonder material in recent years. It is impeccably strong, with the reputation of being 200 times stronger than structural steel. As James Hone, mechanical engineering professor at Columbia University put it, “It would take an elephant, balanced on a pencil, to break through a sheet of graphene the thickness of Saran Wrap.”
In terms of the CNTs structure and bonding, the individual ‘tubes’ are held together by Van der Waals forces and pi-stacking (attractive, non-covalent interactions between aromatic rings). The whole structure contains only sp2 bonding (seen in graphite) which is far stronger than the sp3 bonding in diamond. This basically means that all of the bonds are pi bonds (a type of covalent bond). A question remains however; how do they cut out so much light?
The reason that this material is so black is because it sucks in almost 100% of the light that hits it. The ‘gaps’ between these arrays of CNTs are so small that 99.5% of the light photons that hit the material are absorbed and ‘trapped’ between the molecules. Furthermore, De Wilde’s material captures light from the extremes of both the infrared and UV spectra and stuffs it deep into the chasms of the structure. Accordingly, De Wilde created a painting using the CNTs, nicknamed “the blackest black nano engineered painting in the world” and aptly named it ‘Hostage’ to reflect the photons that are being held captive in the molecules.
In terms of the actual chemistry and optical properties, carbon nanotubes are approaching the rank of black body. A black body is a idealized physical body/substance which absorbs 100% of incident (incoming) electromagnetic radiation from the far-UV (200 nm) to the far-infrared (200 μm) wavelengths. Furthermore black bodies should have emissivity or ‘absorbance’ of 1.0. With respect to this property a recent study showed that vertically aligned arrays or ‘forests’ of CNTs have emissivity of 0.98-0.99. Basically, just over 0.5% of the light hitting Frederik De Wilde magic material will be reflected, so that it may be interpreted by the eye. This substance absorbs so much light that when people first see the structure, they often think they are staring into a huge black hole.
Where do CNTs come from?
Although discovered in the mid 1900s, CNTs have been synthesised on Earth by man made devices since the early 1800s! The first practical electrical light called the carbon arc light was invented by Humphry Davy in the early 19th century, and consisted of two carbon electrodes and a current passing through air. Huge voltage are present in the system and when the rods are touched and slowly drawn apart, the air is ionised and a huge burst of light and heat is emitted. This system was originally used in cinema projectors, even though they were a huge fire hazard. This process generates members of the Fullerene family, including ‘buckyballs’ (buckminsterfullerene, or C60) and carbon nanotubes. Next time you’re in an old cinema, remember that the place is probably loaded with a few of these little graphene ropes! There are of course much easier/safer/cost effective methods of synthesis nowadays, such as laser ablation of graphite and the most widely used method, chemical vapour deposition or ‘CVD’.
Nanoblack will be on display in Science Gallery Dublin in the Naughton Institute on Pearse Street as part of the upcoming exhibition SECRET, which will run from August 7th 2015 to November 1st 2015.
GIF of rotating CNTs; “Kohlenstoffnanoroehre Animation” by Original hochgeladen von Schwarzm am 30. Aug 2004; Selbst gemacht mit C4D/Cartoonrenderer, GNU FDL – German Wikipedia, original upload 29. Dez 2004 by APPER. Licensed under CC BY-SA 3.0 via Wikimedia Commons – https://commons.wikimedia.org/wiki/File:Kohlenstoffnanoroehre_Animation.gif#/media/File:Kohlenstoffnanoroehre_Animation.gif
Featured Image; “CSIRO ScienceImage 1074 Carbon nanotubes being spun to form a yarn” by CSIRO. Licensed under CC BY 3.0 via Wikimedia Commons – https://commons.wikimedia.org/wiki/File:CSIRO_ScienceImage_1074_Carbon_nanotubes_being_spun_to_form_a_yarn.jpg#/media/File:CSIRO_ScienceImage_1074_Carbon_nanotubes_being_spun_to_form_a_yarn.jpg
Nanoblack; http://we-make-money-not-art.com/archives/2014/02/frederik-de-wilde.php#.VbtgCflViko. Credit: Frederik de Wilde, NASABlck-Crcl #1, 2013