Determination of interface structure and bonding at atomic resolution in the stem

The scanning transmission electron microscope now routinely produces probes of atomic dimensions, 2.2 A at 100 kV and 1.3 A at 300 kV. This capability opens up a new strategy for determining the structure and bonding of materials at the atomic scale, which is particularly advantageous for complex interfaces such as grain boundaries. The Z-contrast image allows the high-Z column locations to be determined by direct inspection, and quantified through maximum entropy methods to a positional accuracy of currently approximately 0.2 A. With the same electron probe, located to sub-Angstrom precision with reference to the Z-contrast image, parallel detection EELS can determine light element coordination from core edge fine structure, in principle around a single atomic column. There are several attractive features to combining Z-contrast imaging with EELS. First, the incoherent nature of Z-contrast imaging provides higher resolution, compared with 0.66 for phase contrast imaging, freedom from contrast reversals and a highly local image. This allows a greatly extended regime of intuitive image interpretation. For example, the presence of dumbbells, with a spacing of 1.36 A, in a Si group of zone axes (110) image is seen directly at 2.2 A resolution from the elongation of the image features. For microanalysis, the Z-contrast image facilitates the accurate placement of the probe over selected atomic columns, either a single atomic column or using a line scan to reduce the effects of beam damage. Column-by-column analysis by EELS is possible, without beam broadening, through the columnar channeling effect that occurs when a highly convergent coherent probe is incident along a low order crystal zone axis.