On the possibility of the direct imaging of point defects in crystals using transmission electron microscopy

Abstract

Images with dimensions of 3–5 A were observed in the tilted beam dark field mode electron microscope using the diffuse scattering from b.c.c. Zr-Nb alloys. In an attempt to identify the origin of the image contrast, computer calculations were made of the dark field images from various crystalline defects with atomic dimensions. For the experimental conditions of this study, the calculated image profile for a vacancy had a width at half maximum of ∼ 3 Å and a peak intensity of 6 × 10⁻⁴ of the incident intensity. The measured intensities from the observed images were in the range of 4 × 10⁻³ to 1·2 × 10⁻² and a comparison of the calculated and experimental values showed that the observed images could not have resulted from the simple defects used for the contrast calculations. The present status of this work is that the observed contrast results either from another type of crystalline defect, possibly associated with a phase change in this alloy system, or from a surface film on the thin foil specimen. Some experimental evidence exists to reduce the possibility of the latter, while preliminary image calculations demonstrate that higher intensities can occur for a newly proposed defect structure. It is suggested that the best chance for success in the direct imaging of point defects would be achieved by using the dark field mode with a heavy element specimen having a defect with a large strain field.