Lateral Thoughts Physics World  August 2016
(Surrey NanoSystems)

Paint it nanoblack

Ever since our ancestors painted images on the walls of caves, artists have sought pigments to represent the 10 million tints that humans can differentiate. Some artists, however, choose to portray the world in black and white, a radical simplification that pulls us in with surprising force. Over the centuries, these artists have employed substances from India ink to titanium white to portray shadow, light and contrast. Now they have a new ally: researchers who are using optical design principles, nanotechnology and inspiration from nature to create deeper blacks and purer whites.

Black coatings and materials that absorb photons at visible and longer wavelengths have long been used to reduce unwanted stray light within optical equipment such as cameras and telescopes. These materials also have uses in military stealth operations and efficient solar energy conversion. Many are based on paints that include carbon or graphite, and they typically reflect only a few percent of incoming light while absorbing the rest. But now we have graphene, a 2D form of carbon that can be rolled up into nanotubes to offer a way to create even deeper blacks.

Since 2007 scientists have competed in a “blackest black” arms race, developing methods to create nano-forests of aligned carbon nanotubes that both scatter and absorb incoming light (November 2015 Features). One of the resulting products, Vantablack, reflects only 0.035% of visible light, and looks qualitatively different from “normal” blacks even to the naked eye. A display at London’s Science Museum in February 2016 showed how Vantablack’s negligible reflectivity obliterates an object’s surface features, creating the eerie look of a hole in space or a tunnel to another dimension.

Vantablack reflects only 0.035% of visible light, and looks qualitatively different from “normal” blacks even to the naked eye

Artists quickly grasped the value of this darkest black. National Geographic photographer Peter Essick, known for his striking black and white images, notes that current digital printers do not reproduce the full black-to-white range that digital cameras capture. He speculates that Vantablack, or something like it, will soon be incorporated into digital printer ink. Vantablack has also inspired the prize-winning UK-based sculptor Anish Kapoor. In a December 2015 talk at the Hirshhorn Museum in Washington, DC, Kapoor spoke about creating a huge hollow steel construction coated internally with the material. This would, he said, give viewers inside it the sense that “what’s inside the object is bigger than what’s outside. That’s how we are. What’s inside us has a completely bigger imaginative reality”.

Vantablack is expensive and hard to make, and its military potential makes it subject to export controls. The main restriction on its broad use, though, is that its creator, Surrey NanoSystems, has given Kapoor exclusive artistic rights to the material, angering artists who spoke against arbitrarily limiting access to an artistic medium.

Fortunately, nature’s own nanoscale experiments provide alternative paths to better blacks and whites. Take the black areas seen on butterfly wings. Here, scales containing the pigment melanin absorb sunlight to regulate the insect’s temperature. Pete Vukusic and colleagues at the University of Exeter, UK, have shown that in the Papilio ulysses butterfly, such absorption is augmented by a nanometre-sized fine structure within the scales, a honeycomb-like network of ridges and struts made of chitin, a natural polymer. This structure gives light a longer optical path through the melanin and increases absorption from 52% for the pigment alone to 95% at a wavelength of 600 nm. In China, Qibin Zhao of Shanghai Jiao Tong University and co-workers showed that nanoscale enhancement makes carbon itself a better absorber. When a sheet of amorphous carbon is patterned with a ridged nanostructure that is found in a different butterfly, the carbon becomes blacker as its reflectivity drops from over 12% to under 1%.

White compounds can also be complicated to create. Unless they are “true whites”, with the same reflectivity at all visible wavelengths, they carry a subtle tinge that the human eye can perceive, which affects calibration standards and paper made for printing and artistic use. But here, too, natural nanostructures produce whites that provide models for human-made versions. One example is found in Cyphochilus, a genus of beetle whose white body is thought to provide protective colouration in environments containing white fungi. As determined by Vukusic, Lorenzo Cortese at the University of Firenze in Italy and others, its whiteness comes from highly efficient scattering within chitin scales 250 μm long that contain random networks of filaments 250 nm across. This structure amounts to a natural photonic solid optimized to produce an exceptionally bright true white with nearly constant reflectivity, from a layer only 5 μm thick. This is far thinner than synthetic layers that produce a comparable white. With its desirable optical and physical properties, this natural geometry could be used for ultrawhite coatings, enhanced LED sources and for camouflage.

Nanotechnology is still new. We are barely beginning to exploit it and to use lessons from nature to create novel materials. It may not be long before a digital printer uses ink derived from butterflies to produce exceptional images on paper derived from beetles, or before artists everywhere can freely extend their imaginations with the blackest blacks and whitest whites.