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Fullerenes: stacked, squeezed, polymerized.

No one can say that speculation about practical applications for those hollow all-carbon molecules called fullerenes is all talk and no action.

Several research groups have now demonstrated that they can use fullerenes as lubricants and can coax these molecules to form pure polymers and three-dimensional structures.

Not only do fullerene molecules work as well as other lubricants, but they also can protect surfaces under conditions present in space, says Bharat Bhushan, a mechanical engineer at Ohio State University in Columbus.

To study this new lubricant, he and his colleagues first deposit a thin fullerene film onto a silicon surface. They then roll a steel ball back and forth along that surface and measure the amount of friction generated under different temperatures and humidities. The engineers have also evaluated these friction forces in a vacuum and in a nitrogen environment. After each test, the Ohio State group examines the surfaces of the steel ball and the fullerene film with a scanning electron microscope and a scanning tunneling microscope (STM).

As with other solid lubricants, the film breaks down slightly because of friction. As a result, clusters of carbon molecules bond loosely into larger balls that resemble raspberries. These clusters then roll, like ball bearings, between the silicon and the steel ball, Bhushan says. The fullerenes worked best at 110 [degree] C and in low humidity and inert environments, the researchers will report this spring in APPLIED PHYSICS LETTERS.

Two other research groups have created new materials out of the 60-carbon buckyballs.

Zapping buckyball films with visible or ultraviolet light causes their molecules to bond together into a polymer, says Peter C. Eklund of the University of Kentucky in Lexington. Other researchers had created fullerene polymers by attaching the carbon molecules via hydrocarbon bridges, but his polymer is pure fullerene, Eklund adds.

First he and his colleagues deposit a thin [C.SUB.60] film onto quartz, silicon, or stainless steel, at the same time minimizing the film's exposure to oxygen. After exposing the film to light, they boil the sample a few minutes in a hydrocarbon solvent. The film, looking a little like molecular plastic wrap, breaks free and, in places, folds onto itself, the researchers report in the Feb. 12 ScIENcE.

Based on their chemical analyses, they suggest that adjacent buckyballs link up when one of the double bonds between a pair of carbon atoms in a molecule breaks and those carbon atoms reach across to neighboring carbon atoms.

A different research team uses "guest" molecules to get buckyballs to assemble into a film or even a three-dimensional structure. The process involves building up the buckyball structure by alternately coating a substrate with guest molecules and then dipping the substrate into fullerene solutions, says Chad A. Mirkin, a chemist at Northwestern University in Evanston, Ill.

Initially, they use guest molecules whose bottom ends bond to an oxide surface and whose tops attract buckyballs, explains Mirkin. Thus, a single, tightly packed layer of carbon molecules assembles on top, the Northwestern team reports in the Feb. 10 JOURNAL OF THE AMERICAN CHEMICAL SOCIETY

To add more layers of buckyballs, the researchers coat that carbon layer with piperazine, a ringed molecule both of whose ends attach to buckyballs. Consequently, scientists can vary the fullerenes or the type of guest molecules to control the chemical identity of each layer. "It allows greater versatility in terms of properties," Mirkin says.
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Title Annotation:fullerene research
Author:Pennisi, Elizabeth
Publication:Science News
Date:Feb 20, 1993
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