One of the unique properties of carbon nanotubes — the low refractive index of low-density aligned nanotubes — also has a unique application: making 3-D objects appear as nothing more than a flat, black sheet.

Researchers from the University of Michigan have written a paper on this topic titled “Low density carbon nanotube forest as an index-matched and near perfect absorption coating.” It was published online yesterday in Applied Physics Letters.

---

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. (Credit: L.J. Guo et al, University of Michigan/<i>Applied Physics Letters</i>)

---

Carbon nanotubes, tiny cylinders composed of one-atom-thick carbon lattices, are also one of the strongest materials known to science.

Carbon nanotube “forests” have a low index of refraction very close to that of air. Since the two materials affect the passage of light in similar ways, there is little reflection or scattering of light as it passes from air into a layer of nanotubes.

The scientists realized they could use this property to visually hide the structure of objects.

They etched a 3-D image of a tank out of silicon. When the image was illuminated with white light, reflections revealed the tank's contours; however, after the researchers grew a forest of carbon nanotubes on top of the tank, the light was soaked up by the tank's coating, revealing nothing more than a black sheet.

By absorbing instead of scattering light, carbon nanotube coatings could cloak an object against a black background, such as that of deep space, the researchers said.

In such cases, the carbon nanotube forest “acts as a perfect magic black cloth that can completely conceal the 3-D structure of the object.”

For more information, visit: www.umich.edu