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Uncharted atomic landscapes

1:46pm, August 14, 2007
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Electron microscopes can now not only image single atoms but also map the locations of different chemical elements in a sample.

A scanning-transmission electron microscope (STEM) operates by sending an atom-thin beam of electrons through a sample. Those electrons lose energy as they kick up the energy of some of the sample's electrons. The energy losses depend on the characteristic energies of electron states in an atom. Exploiting that fact, scientists have in recent years learned to identify which elements the beam has encountered.

So far, however, researchers haven't been able to determine the locations of those elements on the two-dimensional image that the microscope produces. That's because the STEM detector varies somewhat in sensitivity across its area, and because it's hard to keep a sample still.

Michel Bosman, then of the University of Sydney in Australia, and his collaborators scanned a crystalline material at the SuperSTEM facility in Daresbury, England. They then slightly shifted the sample and scanned it again. Comparing data from different scans greatly reduced uncertainties due to motion of the sample and to nonuniform detector sensitivity, Bosman says. The researchers describe their results in an upcoming Physical Review Letters.

"These are unquestionably the best two-dimensional compositional maps so far obtained," says Steve Pennycook, of the Oak Ridge (Tenn.) National Laboratory.

"It's exciting to be able to do chemical analysis at the atomic level," Bosman says. The technique could prove useful in the development of new materials, he adds. For example, the way small amounts of elements are scattered in a material can dramatically affect an alloy's toughness or a semiconductor's reaction to changes in a voltage.

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