STEM imaging reveals atoms within crystal molecules

June 26, 2006
June 26, 2006, Ithaca, NY--Researchers at Cornell have modified a scanning transmission electron microscope (STEM) to get a closer-than-ever look at individual atoms within crystal molecules -- allowing them for the first time to see the polarity of those constituent atoms and to get a view of the smaller atoms.

June 26, 2006, Ithaca, NY--Researchers at Cornell have modified a scanning transmission electron microscope (STEM) to get a closer-than-ever look at individual atoms within crystal molecules -- allowing them for the first time to see the polarity of those constituent atoms and to get a view of the smaller atoms. The research -- by Cornell postdoctoral associate K. Andre Mkhoyan, John Silcox, and colleagues at Cornell and Philip Batson of IBM -- is described in the June 2 issue of Science.

With the new technique, researchers can better predict the physical properties of a crystal at every point -- an advance that offers potential improvements in lasers and other devices, particularly at the nanoscale, where the structure of an individual molecule can determine a device's behavior.

Mkhoyan's team used a 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 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. 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.

The key, said Silcox, is the narrowness of the scanning electron beam.

"We're down to the atom size, as opposed to the atom spacing," he said. "We can start to see the light atom columns; we can characterize the crystal very nicely and precisely, at every place on the structure."

Mkhoyan said the inability to capture such images in the past has been a huge hurdle for nanotechnology researchers.

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