Friday, November 15, 2019

Black to the Future Redux

A bit more than a decade ago, Hue Angles presented an article called “Black to the Future” [1]. To further darken a black carbon surface, investigators in Rensselaer Polytechnic Institute and at Rice University roughened the surface by a carpet-like arrangement of carbon nanotubes (.01" long, 1/30,000 as wide) standing on their ends. The result was a surface with a reflectance as low as 0.045 percent (three times darker than any previous material) and a refractive index that could theoretically be as low as 1.01 [2]. We proposed possible uses for such a material in spectrophotometry:
1. Black surfaces for minimizing stray light in optical instruments

2. Light traps for suppressing unwanted diffraction orders

3. Gloss traps for removing specular reflection

4. Black calibration standards
Now, after more than a decade, how well did our wish-list work out? Not well, at least for commercial applications. At the ISCC topical meeting on black and white, held that very same year, it became clear that the carbon nanotube technology was too delicate and too expensive for our purposes. But the technology evolved and improved anyway, and new uses were found.
Beginning in 2014, Surrey NanoSystems issued a product called Vantablack, which reflects 0.04 percent of UV, visible, and IR radiation. Vantablack had the same mechanical vulnerability as its predecessors, so it did not find many applications on Earth. However, in space the substance could be undisturbed, and starting in 2015 helped capture stray light to enhance spaceborne imagery (e.g., tracking stars) without a large payload penalty. Also, back on Earth, it achieved an effect that was coveted by artists: three-dimensional objects covered with Vantablack would appear to be flat surfaces because not enough light was reflected to reveal the 3D topography. BMW even painted a car with Vantablack. By 2017, a version of Vantablack (S-VIS) became available in a spray-on form. The reflectance was understandably not quite so low in this form: 0.2 percent. But the material still served its various functions. Although Vantablack is not commercially available, Surrey NanoSystems has licensed the product.
Now there is a material that is still blacker. It emerged from laboratories in Shanghai and at Massachusetts Institute of Technology[3], and has a reflectance of 0.004 percent. The discovery was accidental, during attempts to grow carbon nanotubes on aluminum foil. To avoid the formation of oxides between the nanotubes and the foil, the investigators soaked the foil in salt water and moved it into a small oven where the nanotubes could grow without oxygen interference.
A popular article by Brandon Specktor [4] describes two implications of the new black technology. There is a $2 million diamond on exhibit in the New York Stock Exchange that has been covered with the material and is invisible on a background of similar black material. Specktor speaks poetically of the black material “eating” the diamond and that it is a “veritable black hole.” Indeed, as he suggests, we may soon be able to see real black holes if the new black material is deployed to optical instruments in space. But I don’t expect to see signs with the words “Schwarzschild radius” on any photos, though we have seen cartoons of other human-created follies (such as Pluto bedecked with the sign “am too a planet.”) Enough about black holes…
In summary, the last ten years have brought a factor of 10 reflectance decrease in the blackest black. We’ve achieved a decade in a decade. Stay tuned for the next decade.
[1] M H Brill, A Ingleson, and C McLellan, Black to the future. ISCC News # 434 (2008), 3-4.

[2] Z-P Yang, L. Ci, JA Bur, S-Y Lin, PM Ajayan, Experimental observation of an extremely dark material made by a low-density nanotube array. Nano Letters 8, No. 2 (Feb. 2008), 446-451.

[3] K Cui, B L Wardle, Breakdown of native oxide enables multifunctional, free-form carbon nanotube-metal hierarchical architectures. CS Appl. Mater. Interfaces 2019 XXXXXXXXXX-XXX:September 12, 2019;

[4] B Specktor, There’s a new blackest material ever, and it’s eating a diamond as we speak. Live Science, Sept 16, 2019,… 

Michael H. Brill