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Thin and Black

August 2, 2013

One interesting experiment nearly every child has done is to see what color you get when you mix all your paints together. The thrill of science quickly subsides when he or she gets a gray, murky mess. Paint works the way it does because its pigments reflect light from specific colors and absorb the others. The child's mixture has absorbers at nearly all color wavelengths, but there's still an overall broadband reflection. That's why the product is gray and not black.

Black materials are useful for many applications. Black coatings will suppress stray light in sensitive optical detectors, and black will give you maximum absorption of solar energy. You must remember, however, that the black must extend to even those parts of the solar spectrum you can't see, as shown in the figure. I wrote about the quest for super black materials in a previous article (Very White and Very Black, November 23, 2011)

Solar energy incident at Earth's atmosphere and surface. The yellow curve is the radiation incident at the top of the atmosphere, while the red curve is the radiation at Earth's surface, diminished by the atmospheric absorbers shown. The radiation approximates a blackbody curve for 5250 kelvin. These data are from the American Society for Testing and Materials (ASTM) Terrestrial Reference Spectra, which is used as a test condition for solar panels in North America. (Modified image from Wikimedia Commons.)

The optical properties of nanomaterials are usually different from that of the bulk materials. One example of this is black gold. When gold and some other metals are evaporated under an argon or nitrogen pressure of about 750 millitorr, the gold atoms cool and cluster before attaching to a substrate to form a black layer.[1-3] Intense bursts of femtosecond laser light on a gold layer will also produce black gold. The laser irradiation transforms the surface into a convoluted nanoscale topography that's optically absorbent.[4]

When people think about black, soot comes immediately to mind. Since soot is mostly carbon, when carbon nanotubes were discovered, scientists had the idea that these could be a constituent of a super-black material. In 2008, scientists from Rensselaer Polytechnic Institute created a carbon nanotube material with a lower reflectance than any other material to date. Their super-black, a low-density array of vertically aligned carbon nanotubes, absorbed more than 99.955% of incident light at visible wavelengths. Light incident on this forest of vertically aligned carbon nanotubes must make many multiple reflections to exit, losing energy at each reflection.[5-6]

Scientists from the Japanese National Institute of Advanced Industrial Science and Technology and Nagoya University prepared a similar array of vertically-aligned single-walled carbon nanotubes in 2009. Their material was a nearly perfect absorber over the extremely wide wavelength range from 0.2 - 200 μm.[7] NASA prepared similar multi-walled carbon nanotubes structures on a variety of substrates in 2011 (see photograph).[8]

Iron-catalyzed multi-walled carbon nanotubes on silicon. A patch was removed to show the structure.

(Image: Stephanie Getty, NASA Goddard.)

The NASA material was prepared by using a thin iron layer as a catalyst for nanotube growth. This was achieved by heating them at 750 °C in the presence of carbon feedstock gases.[8] The NASA material didn't qualify as super-black, since it absorbed just 99.5% of visible and ultraviolet light and only 98% of far-infrared light. Still, such numbers are useful because of the complementary emissivity which would allow efficient radiation of heat from electronic components in spacecraft.[8]

NASA continues to work on black materials with a goal of being able to coat the complex three-dimensional components found on spacecraft. For this reason, they've turned to atomic layer deposition (ALD) of the iron oxide catalyst that's placed on surfaces to induce the carbon nanotube growth. ALD is not a line-of-sight deposition technique, so it coats the bottom of deep notches as well as flat surfaces. The resultant black surfaces are very thin, just a few tens of nanometers thick [9].

NASA relied on the expertise of the Melbourne Centre for Nanofabrication for the atomic layer deposition. The laboratory coated a number of NASA components, including an intricately-shaped optical occulter intended to block stellar light to allow observation of planets around other stars (see figure).[9]

Occulter mask in an ALD deposition fixture at the Melbourne Centre for Nanofabrication.

The mask is coated with an iron oxide catalyst prior to growth of a carbon nanotube layer.

(NASA image.)[9)]

Scientists at Stanford University are also using atomic layer deposition to create thin, black absorbers, albeit near a particular wavelength of light, 600 nm.[10-11] Says Stacey F. Bent, a Professor of Chemical Engineering at Stanford and a member of the research team, "Our results show that it is possible for an extremely thin layer of material to absorb almost 100 percent of incident light of a specific wavelength."[11]

The Stanford absorber is made from a planar sea of gold dots, about 14 nanometers tall and 17 nanometers wide (see figure).[11] These particles optically resonate at 600 nm because of plasmonic waves. The fabrication process used block-copolymer lithography, which produced about 520 billion nanodots per square inch in an hexagonal array on wafers. The gold dots are subsequently coated with tin sulfide by atomic layer deposition to tune the resonance.[11]

An electron micrograph showing a cross-section of the Stanford absorber layer. Three gold nanodots are seen, each about 14 nm by 7 nm in size, coated with tin sulfide.

(Image: Carl Hagglund, Stanford University.)[11]

These coatings, which were equivalent to a continuous gold coating just 1.6 nm thick, were still capable of absorbing 99% of the 600 nm light. The effective absorption coefficient was 1.7 × 10⁷ cm^-1.[10] Such a coating is three orders of magnitude thinner than the absorbers used on commercial solar cells.[11] The next step is to demonstrate an improvement in solar cell efficiency with this technology.[11]

Although gold was used in these layers, less-expensive silver may perform better. Tin sulfide was used as a coating material, but zinc oxide or aluminum oxide may work as well. The research was supported by Stanford's Center on Nanostructuring for Efficient Energy Conversion, an Energy Frontier Research Center funded by the U.S. Department of Energy.[11]

I often wear gloves like this.

Experimental samples of the black absorber.

(Image: Mark Shwartz, Stanford University.)[11]

References:

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