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
ο
Laser-feedback micrograph of silicon wafer with 40-nm depressions
American Chemical Society Presents into the laser, interfering with the coherent light beam. "After we rediscovered the effect, several well-known laser physicists warned us, 'Never let light go back into the laser/" Bearden says. 'To them, it was something to avoid. To us, it seemed highly useful." Bearden and O'Neill first observed the effect in 1989 when they built a laser interferometer to measure the motion of small structures in the ear as they vibrate in response to sound. The scientists worked out the physics behind the interference effect and realized that it could be the basis of a microscope. O'Neill, and later Wong, built versions of the device. The microscope works by detecting light reflected off the sample as the laser beam scans it. The reflected light remains coherent, and thus can interfere with the laser light. The interference signal is amplified and converted into a small piece of the image. The pieces are reconstructed into a full image on a computer screen through software developed by Berkeley graduate students Leslie C. Osborne and Blaise Frederick. The most immediate application of the LFM may be in the semiconductor industry, Bearden says. As integrated circuits shrink in the effort to build smaller and faster computers, chip designers are pushing the limits of light microscopes used to detect defects in the chips. Although an electron microscope can resolve details at about the same resolution as the LFM, the electron beam also can damage the chip. Ultrapointe Corp., San Jose, Calif., has an exclusive license from UC Berkeley for semiconductor applications of the LFM. Perhaps more exciting are potential biological applications of the LFM, however. Whereas scanning electron microscopes require a dead and fixed sample, the LFM can image live tissue. Bearden suggests that his microscope could achieve resolutions that would allow the imaging of features on the order of 10 nm, about the size of the pores in cell membranes or the large proteins that regulate genes. UC Berkeley is currently seeking licensees for biological applications of the LFM. "The optical microscope is not dead," Bearden says. "I think we're at the next level." Rudy Baum
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