The AC-STEM delivers electron beams accelerated at voltages from 80 kV to 200 kV, allowing researchers to study properties of structures at the nanoscale — crucial for materials scientists working on everything from microelectronics to nuclear weapons.
The instrument’s unique combination of X-ray detectors and very high resolution offers magnification Kotula compares to a telescope powerful enough to show two peas side by side on the moon. High-clarity slides of microstructures analyzed with the AC-STEM and fuzzy images taken by Sandia’s older analytical microscope highlight the new capabilities. An analysis that took seven minutes on the AC-STEM took two hours on the older instrument, he said.
The image on the left was captured in seven minutes at 0.5nm/pixel with Sandia's new AC-STEM; the image on the right was captured in 120 minutes at 2nm/pixel with the old microscope. The analytical power of the AC-STEM is at least 70 times better than the older analytical microscope at Sandia. These high-resolution chemical images are confirming predictions from the 1970s regarding the atomic-scale characteristics of electrical contact materials. (Image courtesy of Sandia National Laboratories)
A researcher sits in front of a computer screen showing an image of a 50-nanometer-thick specimen inside the AC-STEM, a sample 2,000 times thinner than a human hair.
What looks like a close-up of mesh or lattice on the screen is really an image of 3-angstrom atomic spacing between titanium and strontium. An angstrom equals one-tenth of a billionth of a meter.
The microscope uses a unique in-lens design in which four X-ray detectors surround a sample placed in the center, increasing collection efficiency, Lu said.
Older instruments were limited by lens aberrations, particularly spherical aberration that prevents sharp focus because electrons off the optical axis are focused more strongly than ones near the optical axis, Kotula said. The AC-STEM’s additional lenses and computational elements eliminate such problems, he said.
“With the aberration-correction technology, you can open the aperture up and keep all those electrons focused to a nice point on your sample,” he said.
Principal investigators Paul Kotula, left, and Ping Lu of Sandia National Laboratories show off the Labs’ new aberration-corrected scanning transmission microscope, which has a unique combination of X-ray detectors and very high resolution and is capable of doing analyses in far less time than its predecessor. (Photo by Randy Montoya)
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