Fullerene helps synthetic diamonds grow.
In an upcoming report in APPLIED PHYSICS LETTERS, scientists at Northwestern University in Evanston, Ill., describe a technique for making diamond films on silicon -- an approach in which a thin layer of fullerenes increases diamnd formation by almost 10 orders of magnitude over untreated silicon surfaces. In addition, the researchers suggest that the arrangements of carbon atoms in any starting material may determine how well the material promotes diamond growth.
"[The results] will throw some light on how diamonds nucleate," comments John C. Angus, a chemical engineer at Case Western Reserve University in Cleveland.
Despite considerable progress in making diamond films during the past five years (SN: 8/4/90, p.72), scientists lack good method for covering large surfaces cheaply and completely, says Robert P.H. Chang of Northwestern. Until now, diamond-makers had to rub diamond powder or paste onto a surface first. Scientists have tried using graphite or organic molecules, but nothing worked as well as bits of diamond. So "up until now, there was no way to massively nucleate diamonds," Chang says.
It turns out, however, that the 70-carbon fullerene works as well as diamond paste, says Manfred M. Kappes, who works with Chang in making the diamond films. Also, one can "make patterns of diamonds because you can put [[C.sub.70]] exactly where you want [on a surface]," he adds.
To make their film, the Northwestern researchers first coat silicon with the fullerene. Then they nick these carbon cages with fast-moving, positively charged carbon and hydrogen ions. "We've converted the [C.sub.70] so it has parts of its surface that look like little pieces of diamond," Kappes explains. As a result, the ragged fullerene carbons that hang free can link up with free-floating carbon atoms and prompt deposition of the diamond crystal.
"Fullerenes might be a way of getting a lot of very closely spaced nucleation sites," Angus says. Chang hopes to identify carbon-based molecules with the geometry necessary to produce single-crystal diamond layers.
Japanese materials scientists have focused on a different sort of carbon molecule. Sumio Iijima of Fundamental Research Laboratories at NEC Corp. in Tsukuba, Japan, examined the carbon material that stayed stuck to the negative electrode typically used in making fullerene-filled soot. With a transmission electron microscope, he discovered that those needlelike specks consist of nested graphite tubes. The needles grew to a length of 1 micrometer and contained up to 50 tubes, Iijima reports in the Nov. 7 NATURE. The tubes grow so that they exhibit the same spacing as exists between the carbon layers in graphite, Iijima notes.
"It's not a scroll; it's straws inside straws," comments Mildred Dresselhaus, a physicist at the Massachusetts Institute of Technology in Cambridge. In August, she described a theoretical fullerene fiber similar to the ones now observed by Iijima. The fibers probably start off as buckyball spheres that develop a defect as they form and so grow into cylinders, she says. However, no one yet knows the exact arrangement of atoms in the fullerene-like tubes and, consequently, whether these tubes fit the definition of a true fullerene.
Scientists expect that fullerene fibers will be stronger than other carbon fibers and that these tubes might make good containers for holding other atoms. "[The fiber] ought to have very few defects, so it ought to have good mechanical properties," says Dresselhaus.
|Printer friendly Cite/link Email Feedback|
|Date:||Nov 16, 1991|
|Previous Article:||Stress puts squeeze on clogged vessels.|
|Next Article:||Marked questions on elderly depression.|