Light As a Super-Black Feather

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Materials science index , 1 (2018) 9 . COMMUN ET AL. NATURE E. MCCOY D. A gold sputter-coated normal black feather (left) and a super-black feather with the same treatment. Light as a super-black feather

At the interface of biology, photonics and materials science, a paper inspired by bird species colouration has struck a chord. By Katharine Sanderson

ody McCoy was an undergraduate materials could also be used to make solar pan- conglomeration of vertically aligned nanotube at Yale University, Connecticut, dis- els concentrate power more efficiently. arrays, absorbs up to 99.965% of visible light. cussing potential projects with the The team, including McCoy, Prum and evo- The striking feature of the barbule struc- lecturer in her ornithology class, Rick lutionary biologists Todd Harvey of Yale and tures, though, is that they are on the micro- Prum, when the pair walked to the Pea- Teresa Feo from the Smithsonian National metre rather than the nanometre scale, Cbody Museum of Natural History and looked Museum of Natural History in Washington DC, comparable to human hair. “This shows that at the bird of paradise specimens. The males measured the plumage darkness of five bird of you can use micro- rather than nanoscale of some of these birds have extremely dark paradise specimens using spectral reflectance structures to very efficiently focus scattered velvety feathers against intensely coloured spectroscopy, which tracks the amount of light light and iteratively absorb it,” McCoy says. bright plumage. “They looked so black, you felt reflected across different wavelengths. The wings of butterflies and other insects, by like you were looking at a shadow,” McCoy says. Next they used scanning electron micros- contrast, rely on nanoscale structural features McCoy was hooked. Her Harvard PhD copy (SEM) to delve deeper. To prepare the that make them appear black. research on the darkness of those ‘super-black’ sample, the feathers, including the darkest The curved, 3D projecting, elaborate bar- feathers, published in a 2018 Nature Commu- ones, were coated in a thin layer of gold. “When bules in the super-black birds of paradise nications paper, revealed the mechanism that we coated them in gold, they still looked black, feathers are completely different from typical places bird of paradise super-black feathers which is strong, direct evidence that this is not barbules and feathers. Their extra-matt quality among the blackest of known materials: they just a pigmentary colour,” McCoy says. helps attract a mate. By absorbing most light absorb light through a unique microstructure The SEM images revealed stark differences and having no gloss, the feathers trick the eye of tiny ‘barbules’, that branch off the main between the coloured and super-black bird into thinking that the colourful feathers are barbs from the feathers’ central shaft (D. E. of paradise feathers. Rather than flat, smooth brighter than they are. McCoy et al. Nature Commun. 9, 1; 2018). barbules, the super-black feathers had small, McCoy speculates that her paper engaged The paper has found its niche in the inter- deeply curved spiky barbules, like curled oak materials scientists as well as evolutionary face between biology, photonics and materi- leaves. These structures trap the light, mak- biologists, because it’s cheaper and easier to als science. Scientists are interested in black ing the feathers appear extremely dark and produce features on the microscale than on colouration because of the range of potential matt. The researchers calculated that the the nanoscale, where scientists working on uses for super-black materials. In space, super- feathers absorb up to 99.95% of all the light manipulating the effects of light usually focus. black coatings can be used to give clearer that hits them, approaching the darkness of vision through telescopes by stopping errant synthetic super-black materials. For exam- Katharine Sanderson is a freelance writer light from entering the aperture. Super-black ple, Vantablack, a material developed using a based in Cornwall, UK.