A unique property of carbon nanotubes – the low refractive index of low-density aligned nanotubes – also can camouflage 3-D objects, making them look like nothing more than a flat black sheet. The tiny cylinders, composed of one-atom-thick carbon lattices, are one of the strongest materials known to science. Carbon nanotube “forests” have a low index of refraction very close to that of air. Because the two materials affect the passage of light in similar ways, there is little reflection or scattering of the light as it passes from air into a layer of nanotubes. Researchers at the University of Michigan realized that they could use this property to visually hide the structure of objects. They etched a 3-D image of a tank in silicon. When the image was illuminated with white light, its reflection revealed the tank’s contours; however, after the researchers grew a forest of carbon nanotubes on top of the tank, the coating soaked up the light, so nothing more than a black sheet was visible. These scanning electron microscope images show a tank etched out of silicon, with and without a carbon nanotube coating (top row). When the tank structure is viewed under white light with an optical microscope (bottom row), the nanotube coating camouflages it against a black background. Courtesy of L. Jay Guo, University of Michigan. “This work may inspire researchers to use impedance-matched low-density and absorbing material to develop stealth technology,” said L. Jay Guo, a professor at the university. By absorbing light instead of scattering it, the coating could cloak an object against a black background, such as that of deep space. In such cases, the carbon nanotube forest acts as a magic black cloth that conceals the 3-D object’s structure. “We would be interested in searching to see if low-density carbon nanomaterials exist in outer space, as such materials could form a ‘dark veil’ that can render large objects undetectable by our current instruments,” Guo said. The research appeared online in Applied Physics Letters (doi: 10.1063/1.3663873).