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Scientists develop technique to determine molecular structure of heterogeneous surfaces

DOE/Brookhaven National Laboratory : 21 May, 2007  (Technical Article)
Scientists have refined a technique that uses the very intense light emitted by the National Synchrotron Light Source at the U.S. Department of Energy
“Surfaces with gradually varying structures are being investigated by academia and industry for their potential uses in creating cleaner energy sources, designing chemical and biological sensors, and creating molecular patterns,” said Jan Genzer, a chemical engineer at North Carolina State University in Raleigh and the lead author of the study. “By determining the chemical structure of surfaces covered with films as thin as a few billionths of a meter, scientists and engineers can improve their properties and performance.”

Genzer added, “A limited number of techniques can be used to study the physical and chemical properties of chemically heterogeneous materials at the millimeter scale. Most techniques are limited in sensitivity, can damage the samples under study, or require special preparations protocols. This new technique is non-invasive, does not require transparent samples, and provides simultaneous information about the chemical nature and orientation of the molecules on the surface.”

In the original, non-refined technique, called near-edge x-ray absorption fine structure spectroscopy, intense ultraviolet light produced by the NSLS interacts with a target material. Electrons emitted by the material are collected with a detector to provide information about the concentration and orientation of the molecules on the surface. When applied to a chemically homogeneous surface, NEXAFS provides the same information from any area on the surface. But for a chemically heterogeneous surface, the technique needs to be applied sequentially at regular small distance intervals to scan the surface from one end to the other.

“Because the size of a typical sample is about 50 millimeters, it would be tedious to apply the technique manually every half-a-millimeter, for example,” said Kirill Efimenko, a senior research associate at North Carolina State and a coauthor of the study. “So we combined the NEXAFS technique with a device called a goniometer, which allows us to automatically move the sample in a vacuum chamber and probe points separated by half-a-millimeter along the surface of the sample where the molecular densities vary.”

The researchers applied the technique, called combinatorial NEXAFS, to a rectangular silica surface covered with a layer of molecules, called organosilanes, their concentration being the highest on the edges of the surface, and decreasing towards the middle. After probing about 100 points along one length of the sample, the scientists successfully reconstructed the expected molecular density profile.

Cross section of structure investigated with combinatorial NEXAFS. The molecules of organosilanes stand up straight on the left and right edges of the sample and then start to tilt toward the surface as they move away from the edges to the middle.

The scientists also looked at the molecular orientation of the organosilanes on the sample surface. “On the surface edges, as the molecules are very concentrated, they stand straight up like soldiers at attention all squashed together,” explained Daniel Fischer, a physicist from the U.S. Department of Commerce’s National Institute of Standards and Technology and another coauthor of the study. “Then, as the molecules become less and less populated toward the middle, they become less aligned. We are now studying the unique geometry of this region of the surface to learn more about the nature of the self-assembly of organosilane molecules on a surface.”

Genzer and his colleagues expect that the technique will be used to probe a large diversity of heterogeneous materials, such as catalysts, which are used to speed up chemical reactions. “Combinatorial NEXAFS could be used to probe, say, 100 different catalysts at the same time,” Fischer said. “You could align these catalysts on a surface, pass reactive chemicals over them, and compare the amount of final products on the 100. Then you could use combinatorial NEXAFS to probe the sites with the largest number of final products, which reveal which catalysts are the most efficient.”
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