SCIENCE/TECHNOLOGY
Fullerenes Produced By Harnessing Sunlight • Work at Rice University and national laboratory indicate new technique might he advantageous for large-scale synthesis
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wo independent groups of researchers have demonstrated that fullerenes can be produced by harnessing focused sunlight to vaporize carbon. Adapted to a large scale, generation of the carbon-cage molecules in solar furnaces might overcome yield-limiting problems associated with other fullerene production techniques, the researchers suggest. At Rice University, Houston, chemistry professor Richard E. Smalley and graduate students L. P. Felipe Chibante, Andreas Thess, J. Michael Alford, and Michael D. Diener used a parabolic mirror to focus sunlight on a graphite target to produce what appears to be a high yield of fullerenes. At the National Renewable Energy Laboratory (NREL), Golden, Colo., Roland R. Pitts, Mary Jane Hale, Carl Bingham, Allan Lewandowski, and David E. King, working in collaboration with Clark L. Fields, a chemistry professor at the University of Northern Colorado, Greeley, used NREL's high-flux solar furnace to produce soot that contains C60 and C70. Papers describing the Rice and NREL results appeared together in last week's Journal of Physical Chemistry [97, 8696 and 8701 (1993)]. The experiment is "something of a stunt/7 Smalley admits. ''We don't have access to a large enough facility to thoroughly test our ideas about generating fullerenes using sunlight." Nevertheless, the intellectual basis for pursuing such a strategy is solid, Smalley argues. Since the original dis-
covery in 1985 by Smalley and coworkers that fullerenes are produced spontaneously in laser-generated carbon vapors, extensive research has been devoted to finding ways to make the molecules in large amounts at high yield. The first breakthrough in that quest came as the discovery by Wolfgang Kràtschmer, a physicist at Max Planck Institute for Nuclear Physics, Heidelberg, Germany, and Donald R. Huffman, a physics professor at the University of Arizona, Tucson, that resistive heating of graphite in a helium atmosphere produced milligram quantities of fullerenes, predominantly C^ and C70. "However, even with subsequent improvements using carbon arcs, production of fullerenes has remained a highly expensive process that has resisted all attempts to scale past levels of several tens of grams per hour," Smalley points out. Smalley now thinks that he and his
Rice University's solar C60 generator
coworkers have discovered the primary mechanism that prevents efficient scaling of the carbon arc technique to large-scale production of fullerenes. Their research suggests that intense ultraviolet radiation generated by the arc destroys many of the newly formed molecules before they can migrate away from the light source. "After considering ways this problem could be overcome in a fashion that scales well to large rod sizes, we decided that the best answer may also be the simplest," Smalley says. "Use sunlight." Smalley explains that the HuffmanKratschmer technique for producing fullerenes obtains rather good yields, with 20 to 30% of the vaporized carbon forming fullerenes. Unfortunately, this approach is limited to graphite rods with diameters of 3 mm or smaller, making it impractical for large-scale synthesis of fullerenes. In an early attempt to reproduce the Huffman-Kratschmer technique with larger graphite rods, Smalley and coworkers discovered what they called the "contact arc" method of fullerene production. The term is still in use, although the Rice chemists subsequently discovered that contact between the two graphite electrodes does not have to be maintained during fullerene production. The yield of fullerenes from 6-mmdiameter rods in a contact arc apparatus is about 15%. The yield drops off linearly with increasing rod diameter, an observation that has never been adequately explained, Smalley notes. Nevertheless, "this method rapidly became the method of choice for commercial fullerene producers worldwide," he says. Although a wide variety of designs have been tried in efforts to improve the yield from a contact arc apparatus, none has been successful. And, Smalley points out, the problem of low yields with large rods is AUGUST 30,1993 C&EN
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SCIENCE/TECHNOLOGY now the single most important factor keeping the cost of fullerenes high and their availability low. Curiously, in another attempt to im prove the yield of fullerenes from a con tact arc, Smalley and coworkers discov ered a way to make the yield dramati cally worse, and the discovery provided the clue that led to their hypothesis con cerning photochemical destruction of fullerenes produced in contact arcs. The Rice chemists designed a fullerene generator that consisted of a quartz tube in which two 6-mm-diameter graphite rods met at a 30° angle. In the apparatus, the rods were preheated to about 1200 °C. A flow of inert gas down the tube swept the fullerenes away from the arc between the rods. Instead of im proving the 15% yield typically seen for 6-mm-diameter rods, this arrangement produced fullerenes at about 3% yield, and the yield remained poor regardless of gas flow rate, oven temperature, and arcing current. The only significant difference be tween this design and previous designs was in the flux of UV radiation from the acute contact arc. Fullerenes pro duced in the arc were fully exposed to intense UV radiation coming from the central portion of the arc plasma, which has a temperature of about 10,000 °C. Smalley thinks this radiation limits the yield of fullerenes. Generating fullerenes by using sun light to vaporize graphite could miti gate this problem in a number of ways, Smalley suggests. "In order to mini mize the photochemistry, it appears necessary to transport the carbon vapor into a relatively dark zone before the fullerenes have begun to form/' he says. He also thinks it is important to allow the carbon vapor to expand so that the concentration of reactive Cx species is low in the region in which fullerenes are forming. Under these conditions, a fullerene in an excited state would be unlikely to suffer a col lision with a reactive species. "Ironically, the answer to how to min imize cluster photochemistry may in volve more light," Smalley says. His idea is to overwhelm the clustering by photolysis induced by intense sunlight. That is, in addition to vaporizing the carbon, use sunlight to maintain the carbon as atoms or very small fragments until the vapor is carried away from the vaporization region into a shadow where clustering is permitted to occur. 22
AUGUST 30,1993 C&EN
The Rice chemists demonstrated the feasibility of producing fullerenes by us ing solar radiation to vaporize carbon in an apparatus that uses a parabolic mir ror to focus sunlight onto the tip of a thin graphite rod. They operated the ap paratus for three hours in the Franklin Mountains near El Paso, Tex., where the solar flux was 800 to 900 watts per square meter. During the experiment, 5 mg of carbon evaporated from the tar get. Most of this soot dissolved in tolu ene at room temperature, and most of the soluble fraction was C^ and C70. Although the result is impressive, "a much larger scale test will be necessary before it is clear whether the advantag es of solar-furnace generation of fuller enes are as substantial as envisioned," Smalley says. One problem with per forming such a test is the lack of large solar furnaces that generate the requi site power flux—about 1000 watts per sq cm—needed to vaporize carbon. The NREL 10-kW high-flux solar fur nace generates such fluxes by using a reflective secondary concentrator at the focal point of the furnace. In fact, Lewandowski says, the Rice and NREL groups learned of each other's indepen dent efforts to use sunlight to produce fullerenes when Smalley contacted him
about the capabilities of the NREL solar furnace. According to Lewandowski, the NREL researchers realized that fullerene pro duction required high temperatures to vaporize graphite, and that those tem peratures were obtainable in the highflux solar furnace. The researchers pro posed a small-scale test of the idea, and found that C^ and C70 can, in fact, be produced in the device. In the NREL furnace, milligram quantities of carbon are vaporized in a matter of minutes. In the NREL experiment, soot depos its form only in zones of the apparatus that are not directly irradiated. Although the amount of UV radiation in the solar furnace is at least two orders of magni tude less than that from the plasma of a contact arc, this finding is at least consis tent with Smalley^ idea that transport ing the carbon vapor into a shadow is important for fullerene formation. Lewandowski says "We hope the ex periment will generate enough interest for us to pursue further funding. We would like to put together a collabora tive team that includes the Rice chem ists and potential industrial end users to explore this approach to producing fullerenes." Rudy Baum
light microscope rivals electron microscope By combining a simple, low-power la ser with a standard light microscope, scientists at the University of Califor nia, Berkeley, have created a "laserfeedback microscope" with a resolution that rivals that of the scanning electron microscope, currently the most com monly used device for obtaining de tailed images of the surface of objects. According to Alan J. Bearden, pro fessor emeritus in Berkeley's depart ment of molecular and cell biology, the laser-feedback microscope (LFM), even in its early stages of development, can resolve details as fine as 100 nm, on the scale of a virus or the components of even the smallest integrated circuits. The LFM is even better at resolving depth: It can detect depth details as small as 1 nm, demonstrating 20 to 30 times better resolution than other light or electron microscopes. Bearden, who developed the LFM with Berkeley biophysics graduate stu dents Michael O'Neill and Terrence L. Wong, described the device and its ap-
plications at the Microscopy Society of America's annual meeting held earlier this month in Cincinnati. The LFM is based on a phenomenon discovered early in the development of lasers. Called laser feedback interfer ence, it occurs when light from a laser bounces off an object and reflects back
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Laser-feedback micrograph of silicon wafer with 40-nm depressions
