ORNL's record-breaking electron microscopy and spectroscopy techniques will help researchers explore and control nanoscale materials.
In 1959 Richard Feynman, renowned American physicist at the California Institute of Technology, first mentioned nanoscience in one of his famous lectures. He predicted that nanoscience would be greatly accelerated if the resolution of electron microscopes could be enhanced by a factor of one hundred.
That leap in resolution occurred 45 years later with the achievement of one ORNL world record by an aberration-corrected, Z-contrast scanning transmission electron microscope (STEM) housed in ORNL's new vibration-free Advanced Microscopy Laboratory. The instrument obtained incredibly sharp images of single lanthanum atoms at a world-record resolution of 0.6 angstrom (Å).
"Our microscope allows us to see heavy atoms, such as bismuth, on a substrate of light atoms, such as silicon, giving a direct, real space image of single atoms," Pennycook says. "The bright atoms in the image have a much higher atomic, or Z, number than the lighter atoms in the substrate, which show up dark and provide contrast."
In 1988 Pennycook worked with VG Microscopes to develop the first Z-contrast STEM for ORNL research. In 2001, after sharpening the instrument's electron beam probe, Pennycook "saw" silicon atoms with a world-record resolution of 0.8 Å. In 2002 the Nion Company of Kirkland, Washington, built and delivered a spherical aberration corrector for ORNL's 300-kilovolt Z-contrast STEM, again breaking the world record.
Yet another world record was achieved when CMSD's Maria Varela used the electron energy loss spectroscopy capability of a Z-contrast STEM to carry out a spectroscopic analysis of a single lanthanum atom embedded in calcium titanate. She identified and located the atom based on the amount of energy lost as the microscope's electron beam as passed through the lanthanum atom.
Pennycook and his colleagues have used the Z-contrast STEM to help explain mechanisms behind materials phenomena. Researchers, for example, have observed blockages of current flow in a high-temperature superconductor containing yttrium-barium-copper oxide (YBCO) that they attribute to misoriented grain boundaries low in oxygen.
"When oxygen is missing from grain boundaries, part of the superconductor becomes an insulator instead of a conductor," Pennycook says. "Several scientists observed that, if YBCO is doped with calcium, more current passes across the boundary, They thought calcium replaces yttrium and supplies additional current carriers."
Pennycook and colleagues from Brookhaven National Laboratory studied YBCO samples with and without calcium doping using the Z-contrast STEM. Their objective was to figure out why a perfect-crystal YBCO superconductor carries very little current. Pennycook's studies convinced him that the sources of the problem were the very high strains in the YBCO grain boundaries, which caused oxygen to be expelled. Pennycook found that the introduction of calcium atoms can heal the strains placed on the superconductor's grain boundaries by the perfect YBCO crystalline film. Surprisingly, the calcium atoms healed the strains by replacing copper and barium atoms normally in the boundary.
"The scientists who advocated calcium doping of superconductors were right for the wrong reasons," Pennycook says, adding that he believes nanoscale grain boundary engineering can prevent areas of YBCO superconductors from becoming insulators. "To keep the oxygen from escaping, the superconductor could be doped with big atoms to plug grain boundaries that are stretched or with little atoms when the boundaries are compressed. The aberration-corrected Z-contrast STEM helps scientists see which dopant atoms are the right size for healing misoriented grain boundaries."
Pennycook's group has also determined why nickel aluminide for turbine blades is less brittle when doped with boron and why silicon nitride for more efficient car engines becomes even stronger when doped with lanthanum atoms. His insights stem from microscope images that are crucial to the exploration and control of the nanoworld.
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