Direct Imaging of the Atomic Structure and Chemistry of Defects and Interfaces by Z-Contrast Stem
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DIRECT IMAGING OF THE ATOMIC STRUCTURE AND CHEMISTRY OF DEFECTS AND INTERFACES BY Z-CONTRAST STEM S. J. PENNYCOOK, M. F. CHISHOLM, D. E. JESSON, D. P. NORTON, J. W. McCAMY, and D. H. LOWNDES Solid State Division, Oak Ridge National Laboratory, P. 0. Box 2008, Oak Ridge, TN 37831-6024 ABSTRACT Z-contrast scanning transmission electron microscopy (STEM) is a fundamentally new approach to high-resolution imaging which provides unambiguous, compositionally sensitive images on the atomic scale. Such images are intuitively interpretable, even in thick regions of the sample, tremendously simplifying determination of the structure and chemistry of defects and interfaces. To illustrate this, examples are presented of commonly observed planar defects in laser-ablated thin films of YBa2Cu307_x. Film/substrate interfaces are shown to be chemically diffuse on the atomic scale and steps or undulations in the substrate need not result in defects in the film. Low-angle grain boundaries are found to be chemically clean, the drastic reductions in critical currents with tilt angle being due to the array of intrinsic structural defects comprising the boundary. Z-CONTRAST STEM A new approach to high-resolution electron microscopy has been described recently based on detecting electrons scattered through high angles with an annular detector in a STEM (Fig. 1) [1-3]. As the electron probe scans across the sample a map is built up of its scattering power, which for the large-angle Rutherford scattering reaching the annular detector is proportional to Z2 , where Z is the atomic number of the species under the probe. This remains true even at atomic resolution, where the probe is below the unit cell size of a crystal viewed along the major zone axis. This surprising result is borne out by detailed calculations of the dynamical diffraction of the STEM probe as it propagates through the crystal, which indicates that the image is almost exactly what would be predicted on the simple classical picture of an incident probe intensity profile convoluted with the scattering power of the crystal [4-6]. For a zone-axis orientation, the scattering power can be considered to be a delta function at each atomic column with a strength dependent on its composition. Such images can therefore be predicted and interpreted by intuition, and can be simulated by straightforward convolution. This contrasts markedly with the situation for conventional high-resolution imaging based on phase contrast between diffracted beams, where contrast reversals can occur with objective lens defocus or with sample thickness, and the chemical sensitivity is in general very weak. The situation is even more complex at interfaces where the diffracted beams from one material can interfere with those from the other, and in general extensive computer simulation of a number of imaging conditions are necessary to determine a structure with confidence. The Z-contrast method, being essentially a "column-by-column" image has no problems with interfaces or defects. It provides a high-resolut
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