Focusing on the tiniest things

Tuesday, 20 June, 2006

Researchers at Cornell University have developed a technique to get closer look at individual atoms within crystal molecules.

For the first time, this technique will allow the study of the polarity, or physical alignment, of those constituent atoms and to get a view of the smaller atoms.

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 behaviour.

The research team includes: Cornell postdoctoral associate K. Andre Mkhoyan, Silcox and colleagues at Cornell, and Philip Batson of IBM.

The team used a scanning transmission electron microscope (STEM) at IBM on samples of aluminium 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 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 aluminium nitride, the technique, called annular dark imaging, shows pear-shaped molecule columns with the larger aluminium 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," said Silcox. "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.

"The study and application of these lattice crystals are at the core of nanotechnology. Many papers are dedicated to synthesis and application of the nanoparticles -- quantum dots, rods, wires, you name it -- based on these materials," he said. "However, the performance of the devices is highly dependent on the structural quality of these nanoparticles."

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