Thursday, July 31, 2008

Black to the Future

by Michael H. Brill, Alan Ingleson, Chuck McLellan

New technology prompts anticipatory thoughts about how we can use it. Now, some adaptations of carbon nanotubes seem ready to be great light absorbers and others should make great detectors. Let’s take a look, then…

Black to the Future

A bit over a year ago, one of us (MHB) discussed parallel development of similar innovations for power engineering and color-measurement technology (“Power to the pupil: color vision, cameras, and the energy crisis,” ISCC News Issue 427). Now another example of the phenomenon is emerging---the interaction of nanotubes with light. Carbon nanotubes are strong enough so that, even when very thin, they can be vertically grown to considerable length on a horizontal surface. Growing carbon nanotubes on a surface produces a region of very low refractive index at that surface, from which very little light is reflected over a wide range of angles. Depending on the dimensions of the nanotubes and on materials with which they might be coated, the light incident on the surface is efficiently absorbed and transduced into either heat or electricity.

Consider the low-reflectance objective [1]. To further darken a black carbon surface, investigators at RPI and Rice roughened the surface by a carpet-like arrangement of carbon nanotubes (.01" long, 1/30,000 as wide) standing on their ends. The result is 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. We’ll hear more about the nanotube absorber at the November ISCC topical meeting on black and white.

The efficient-electricity objective [2] uses carbon nanotubes in a different geometry and context to obtain an efficient solar-cell design. Jud Ready at Georgia Tech Research Institute (GTRI) developed photovoltaic cells that trap light between their tower structures, which are about 100 microns tall, 40 microns by 40 microns square, 10 microns apart-and grown from arrays containing millions of vertically aligned carbon nanotubes. Conventional flat solar cells reflect a significant portion of the light that strikes them, reducing the energy they absorb. Light incident in the new design is turned efficiently into electricity through semiconductor layers (cadmium telluride and cadmium sulfide) deposited on the nanotube “towers.” In a solar cell so designed, the carbon nanotubes serve not only to support the structure in three dimensions, but also to conduct the charge carriers quickly away from the absorption site before they can recombine and waste energy.

For spectrophotometry (a subject near and dear to many ISCC members), one is prompted by both these technologies to envision a spectrophotometer in which one or more of the following is incorporated:

Following [1], using vertically aligned nanotubes that are thousands of times longer than they are wide, one could comprise

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.

Using nanotubes only a few times longer than they are wide (as suggested by [2]) it may be possible to comprise:

5. An array of nanotubes to guide light to a detector very efficiently. The longer the nanotubes, the more they can guide light in a collimated way and eliminate unwanted diffraction orders. If the nanotubes have different diameters (commensurate with wavelengths of light), they can wavelength-select, perhaps enough to make gratings unnecessary.

The originators of [1] have cross-claimed into the solar-power arena, saying “The observed reflectance from the nanotube arrays is the lowest-ever reported reflectance from any material and could have applications from solar energy conversion to pyroelectric detectors.” For such adaptations, however, there remains the daunting problem of transducing the absorbed light efficiently. For any applications, the long nanotubes also apparently pose health risks similar to those of asbestos once they get in our lungs [3]. The future is once again obscure, then, as perhaps it should be.

[1] 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.
[2] J. Toon, NanoManhattan: 3-D solar cells boost efficiency while reducing size, weight, and complexity, Georgia Tech News, 11 April 2007. See this web link.
[3] C-C Chou, et al., Single-Walled Carbon nanotubes can induce pulmonary injury in mouse model, Nano Letters 8 No. 2 (Feb 2008) pp 437 – 445