June 20, 2006

STEM annular dark field imaging, sees atoms more clearly

Mkhoyan's team at Cornell have created an improved view of atoms. They used a scanning transmission electron microscope (STEM) at IBM on samples of aluminum nitride, gallium nitride and other crystals with particular significance in nanotechnology research, in a chamber padded and shielded to reduce potentially atom-jiggling acoustic noise and electromagnetic radiation. Fitting the STEM with an aberration corrector (a focusing device) developed at Nion Co., they directed a 0.9 angstroms-wide electron beam at tiny crystal samples, collecting the scattered electrons on a ring-shaped detector and forming an image based on the resulting scatter pattern. (An angstrom is one hundred-millionth of a centimeter). Because larger atoms deflect electrons at a larger angle than small ones, the resulting data is relatively simple to interpret.

Used on a sample of aluminum nitride, the technique, called annular dark imaging, shows pear-shaped molecule columns with the larger aluminum atoms at the thicker end and the smaller nitrogen atoms at the narrower end. It is the first time the smaller atoms in such a structure have been caught in an image.

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