Sandia's Dawson works with molecular beam epitaxy machine slightly different positions than they would take in the bulk material. "What comes out of this," Chaffin says, "is that, for example, two ma terials such as gallium arsenide and indium gallium arsenide arranged in layers, looked at macroscopically, can be viewed as semiconductor X. Semiconductor X will have a bandgap somewhere in between the bandgaps of gallium arsenide and indium gallium arsenide, even though it is made up of those mate rials and they have two distinct bandgaps." The macroscopic bandgap is determined by the mate rials used, the thickness of the layers of material, and by the strain in the lattice mismatch. "What this means is that I can custom design a semiconductor," Chaffin says. The research indicates that many elements from Groups III and V of the periodic table can be used in SLS semiconductors, intro ducing yet another variable for cus tom designing the semiconductors. "We've examined only a fraction of the material systems that could be used," he says. "Essentially, there are an infinite number of possible com binations." Central to the development of the current SLS structures has been the ability to grow the superlattices in a very precise fashion. Sandia crystal growers Robert Biefeld and L. Ralph Dawson use two techniques, molec
ular beam epitaxy and metal organic chemical vapor deposition. Neither is generally used to mass-produce semiconductor devices, but progress in producing the SLS materials with the techniques has convinced the Sandia researchers that the semi conductors can be produced in com mercial quantities. In the molecular beam epitaxy technique, an element is evaporated and focused in a molecular beam onto the SLS substrate. One beam is used for each element being depos ited. In the chemical vapor deposi tion technique, the substrate is first heated at about 800 °C. To grow a layer of gallium phosphide, for ex ample, phosphine gas (PH3) and trimethylgallium in a hydrogen carrier gas then contact the heated substrate. At the 800 °C temperature of the substrate, the compounds break down, leaving pure gallium and phosphorus, which then react. Both processes are under computer con trol. The crystal growth rates are about 1 μην per hour, allowing changes in the molecular beams or in
the gas mixture to change layer components. So far, only simple diodes have been produced by the Sandia re searchers. "We still have a lot of theoretical work to do to optimize these [processes]," Chaffin says. "The work we are doing is primarily de signing, analyzing structures, and measuring properties. We are still very much in the materials phase of research." Patent considerations prevent the researchers from discussing details of potential new devices that may be developed from SLS materials. Many such devices are likely, they say, evolving first along lines currently not well served by conventional materials such as silicon. The initial emphasis is likely to be on opto electronic devices such as lightemitting diodes. The researchers believe that the technology will produce much brighter LEDs in a variety of colors than are currently available. The color will be pre-es tablished by the SLS structure. Rudy Baum, San Francisco
Tritium labeling facility to start up Compounds labeled with radioactive nuclides have proved invaluable for tracing reaction pathways in living systems. One of the most important such nuclides is tritium. Although many tritium-labeled compounds can be obtained commercially, many others cannot be, and concocting them from scratch can be tricky. Within the next few weeks, a Na tional Tritium Labeling Facility will open at Lawrence Berkeley Labora tory in California to provide bio medical researchers with the hard ware and technical expertise to label any compound with tritium. The new facility is an outgrowth of a tritiation laboratory that has existed at LBL for about 20 years, says Rich ard M. Lemmon, associate director of LBL's Laboratory of Chemical Biodynamics and principal investigator at the new tritium facility. The orig inal laboratory was built up slowly, Lemmon says, and was designed only to meet the needs of LBL re searchers. The tritium was supplied by Lawrence Livermore National Laboratory, which uses large quan-
tities of the element in weapons re search. "That lab was fine for our pur poses," Lemmon says, "but as the years went by, many of our col leagues here at Berkeley and at the University of California, San Fran cisco, asked us to help them label compounds for their research." The growing demand for such assistance outstripped the capabilities of the laboratory and the scientists working there. In 1982, the National Institutes of Health agreed to fund work to upgrade the laboratory to the status of a national facility. Lemmon points out that research on increasingly complex reaction pathways in living systems has ex panded the need for labeled com pounds of high specific activity; that is, a large amount of radioactivity per unit mass of compound. It also has increased the need to be able to label complex molecules such as enzymes, hormones, and nucleotides. Produc tion of such labeled compounds has been hampered by a lack of labora tories capable of handling large June 27, 1983 C&EN
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Heterogeneous Catalysis Selected American Histories
Burtron H. Davis, Editor University of Kentucky William P. Hettinger, Jr., Editor Ashland Petroleum Company Surveys 50 years of progress in U.S. heterogeneous catalysis research. Records personal accounts of pioneers in the field and reports their scientific achievements. Discusses industrial and academic research including instrumental analysis techniques, development of synthetic rubber production, high grade fuels, heavy water, and automobile catalytic converters. CONTENTS Heterogeneous Catalysis Before 1934 · Irving Langmuir's Contribution · V.N. Ipatieff: As I Knew Him · Sir Hugh Taylor: The Man · Paul H. Emmett: Six Decades of Contributions · Eugene J. Houdry's Contributions to Catalytic Cracking · Herman Pines and Organic Heterogeneous Catalysis · Research with Para-Hydrogen and Heavy Hydrogen · Allan T. Gwathmey and His Contributions to Catalysis · P.W. Selwood's Contributions to Catalysis · Development of the Platforming Process · Otto Beeck and His Colleagues in Catalysis · Ernest W. I Theile: Defining Diffusion in Catalysis · Ahlborn| Wheeler—Catalytic Scientist · IR Studies of Adsorbed Molecules at Beacon · Fixed Nitrogen Research Lab · Einstein in the U.S. Navy · History of the BET Paper · Carbonium Ion Mechanism of Catalytic Cracking · Catalytic Cracking Research at Universal Oil Products · Development of Theory of Catalytic Cracking · Invention of Zeolite Cracking Catalysts · Development of Fluid Catalytic Cracking · Hydrocracking Development · Selective Oxidation by Heterogeneous Catalysis · Methanol: Past and Future · Magnetic Resonance Applications · Chain Growth and Iron Nitrides in FischerTropsch Synthesis · Olefin D i s p r o p o r t i o n a t e (Metathesis) · Development of Automotive Exhaust Catalysts · Measuring the Number of Active Sites · Leonard C. Drake—Mercury Porosimetry for Catalyst Characterization · Catalysis at Princeton's Chemistry Department · Murray Raney—Pioneer Catalyst Producer · Frank G. Ciapetta—Pioneer · A Society of Catalytic Chemists and Engineers
Based on a symposium sponsored by the Division of History of Chemistry of the American Chemical Society ACS Symposium Series No. 222 532 pages (1983) Clothbound LC 83-8745 ISBN 0-8412-0778-X US & Canada $49.95 Export $59.95 Order from: American Chemical Society Distribution Office Dept. 33 1155 Sixteenth St., N.W. Washington, DC 20036 or CALL TOLL FREE 800-424-6747 and use your VISA or MasterCard.
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June 27, 1983 C&EN
Science quantities of tritium and insufficient research on new labeling techniques to be applied in cases where conventional routes fail. The new facility consists of two laboratories. One is the upgraded version of LBL/s original tritium labeling lab. When completed, researchers will be able to conduct two separate labeling reactions simultaneously in the lab. Two large glove boxes are connected to a central source of tritium. In one glove box, conventional labeling techniques such as exchange labeling can be carried out. In the other glove box, the newer microwave discharge method—in which reactive tritium atoms are generated from diatomic tritium in a microwave furnace—can be carried out. The other laboratory, for radiopurification, is completely new. "A lot of the literature on tracer work in biochemistry and biorelated sciences is full of bad data because people have done tracer experiments using labeled compounds that were contaminated with radioactive side products of the tritium labeling reaction/' Lemmon says. The lab will make use of gas, liquid, and affinity chromatography and gel electrophoresis for purification of the labeled products. It also will contain a tritium nuclear magnetic resonance spectrometer, which allows a researcher to determine which hydrogens on a labeled molecule have been replaced by tritium. Lemmon points out that the facility also will play an instructional role. "People will not simply send us a sample to label," he says. "They [often] can do that through a commercial firm. We won't do anything that can be done commercially." Generally, there are two reasons why a commercial firm cannot supply a particular tritium-labeled compound: The firm cannot incorporate enough tritium for the researcher's purposes and/or the firm cannot supply the compound at a sufficiently high level of radiopurity. In such a case, Lemmon says, the researcher will come to the tritium facility and under the guidance of the staff there carry out the required labeling procedure. "We expect that before a researcher comes here, he
Lemmon: literature full of bad data will have carried out in his own laboratory on the cold compound the hydrogénation/tritiation reaction . . . or whatever reaction he intends to use to incorporate the tritium. Also, that he will have done the chromatographic work so that he knows what is a likely way to radiopurify the compound." The facility's staff scientists will be carrying out basic research on tritium labeling techniques. Lemmon cites studies of microwave discharge labeling as an example. In that technique, diatomic tritium is passed through a microwave discharge to produce what is generally regarded as two tritium atoms. "These very reactive atoms easily replace hydrogen on any organic molecule," he says. However, he points out that in reality the microwave discharge also produces mono-, di-, and triatomic tritium cations and, possibly, the triatomic radical. Those species may affect, either positively or negatively, the course of the labeling reaction. "So what we want to do is put an electric field on the discharge to remove all the cations, and see what that does to the product mix of the labeling reaction," Lemmon says. NIH and the Department of Energy are supporting the tritium labeling facility. According to Lemmon, after the first couple of years of operation, a system of user fees will make the facility self-supporting. Rudy Baum, San Francisco
