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Friction reveals chemical composition.

A new type of atomic force micro-scope now lets scientists tell in molecular detail not only what a surface looks like, but also the distribution of its chemical components.

The technique, which takes advantage of differences in the friction forces exerted by various materials, may prove useful for chemical analysis, characterization of mixtures of molecules, and studies of wettability and friction, says Jane Frommer, a chemist with the IBM Almaden Research Center in San Jose, Calif.

Typically, scientists monitor the vertical shifting of the very fine tip of an atomic force microscope (AFM) to image the surface of a sample. Then in 1987, IBM researchers discovered that the sideways deflection of the tip was indicative of friction between the tip and the surface, and they modified an AFM to make a "friction force" microscope, Frommer says.

Now, working with physicist Hans-Jorg Guntherodt and his colleagues at the University of Basel in Switzerland, Frommer has demonstrated that this microscope can distinguish materials based on the friction forces exerted by clusters of molecules. "We use the lateral response [of the tip] to differentiate between species in surfaces with more than one compound," she says.

To explore this potential, the Swiss group obtained special films from Masamichi Fujihira, a chemist at the Tokyo Institute of Technology. Fujihira's group makes these films by dipping a silicon plate into a solution of hydrocarbon and fluorocarbon molecules. The resulting film consists of discrete islands of hydrocarbon in a sea of fluorocarbon, Frommer notes.

Until now, scientists have mapped the chemical makeup of such a film by adding fluorescing dyes that some components in the sample absorb more readily than others. Then the researchers examine the distribution of dye with fluorescence microscopy. But the resolution of the friction force microscope is hundreds of times better; it sees features on the scale of angstroms as opposed to microns, says Frommer.

The friction force microscope revealed that "there are indeed subdomains that are down under the micron scale," says Frommer. In one supposedly single-layer sample, the researchers detected that hydrocarbon clumps actually floated on top of the fluorocarbon molecules.

The fluorocarbon molecules exert four times the friction force of the hydrocarbon clumps, and the silicon exerts 10 times as much, Basel graduate students Rene M. Overney and Ernst Meyer and their colleagues report in the Sept. 10 NATURE.

"We aren't claiming to have absolute measurements of friction," Frommer cautions. "What's important is the contrast between the areas." The researchers do not know what property of the molecular clusters creates this side-ways deflection in an AFM tip, but they expect to use the technique to study friction and lubrication on a molecular scale. Also, friction force microscopy may have commercial application for studies of heterogeneous materials such as analyses of the distribution of the chemical elements in membrane filters or the degree of mixing of new additives in petroleum products, says Frommer.
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Title Annotation:using atomic force microscope
Author:Pennisi, Elizabeth
Publication:Science News
Date:Sep 19, 1992
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