Effects of Composition and Strain on Image Contrast in Atomic-Resolution Transmission Electron Microscopy
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EFFECTS OF COMPOSITION AND STRAIN ON IMAGE CONTRAST IN ATOMIC-RESOLUTION TRANSMISSION ELECTRON MICROSCOPY JAMES M. HOWE AND DUANE SUNDO Department of Metallurgical Engineering and Materials Science, Carnegie Mellon University, Pittsburgh, PA 15213 ABSTRACT In this investigation, multislice image calculations were used to determine the effects of point defects on image contrast in ARTEM. It is shown that point defects with an atomic number different from the matrix produce a regular change in image contrast. The minimum detectable defect concentration for any system of matrix and solute can be predicted from a simple formula based on a rule of mixtures and the atomic numbers of the matrix and solute. The effects of lattice distortions associated with point defects appears to have minimal effect on image contrast in ARTEM. Optimum specimen and microscope conditions for observing point defects in crystals and the possibility of extending the analyses to interpret atomic-resolution images of larger defects are discussed. INTRODUCTION It is necessary to understand how composition and strain affect contrast in atomicresolution transmission electron microscope (ARTEM) images in order to quantitatively interpret experimental structures. The usual procedure for simulating a ARTEM image is to assume a model atomic structure, often based on an experimental image in the case of defects [1,2], and then to refine the model structure intuitively until a convincing match is obtained between the calculated and experimental images under the same conditions. Although this has method has been quite successful for determining atomic structures in many cases [3,4], it is very time consuming since each situation must be treated uniquely. The effects of composition and strain on amplitude-contrast imaging in the TEM are well understood and the theory is highly developed [5-7]. In the case of phase-contrast imaging, only a few specific situations have been treated [8-11] and a general theory and understanding are not available. The purpose of the present research is to develop a quantitative understanding of the effects of composition and strain on phase-contrast imaging at atomic resolution. The simplest defect which can cause a local change in composition or atomic positions is a point defect, in particular a substitutional atom, and this serves as a starting point for analysis. It will be shown that the results for substitutional atoms can be extended to account for image contrast from larger defects in many cases because of the column approximation [12]. THEORETICAL BASIS Although the contrast arising from substitutional atoms in ARTEM has not been fully treated theoretically, x-ray scattering theory indicates that the diffuse scattering from point defects due to composition and strain is linearly additive [13]. For small defect concentrations, the scattered intensity Sdiff(k) is given by: Sdiff(k) = cN IF(k)12 with Mat. Res. Soc. Symp. Proc. Vol. 139. C1989 Materials Research Society
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F(k) = fD(k) + f(k) Z exp (ik- rn) [exp
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