Problems in interpreting sub-Å-resolution images studied by simulated evolution based digital image matching

Abstract

1. Introduction:Experimentally determined CTF-data caused predictions that atomic resolution microscopy (JEM ARM1250) of sapphire should become possible with single atomic columns resolved (Fig. 1). The smallest projected bond-length (ca. 0.85 Å) lies beyond the point-resolution of 1.05 Å but within the information limit of this instrument (around 0.85 Å). After extensive trials such images could not be verified experimentally. Instead, images like Fig. 3 b typically appear, containing still high frequencies but different from predictions.2. Ideal and realistic image simulations:Reasons for this deviations have been examined following with special respect to sub-Å-resolution, including specimen tilt, beam tilt, achromatic anisotropic damping (MTF, vibration, drift), non-ideal columns and amorphous contamination. The stepwise EMS-contrast generation from the crystal single slice over the specimen exit plane into the image plane is compared in Fig. 1 (ideal alignment) and Fig. 2 (realistic simulation). Predominant factors lie in specimen misalignment or other disturbancies (e.g. strain fields) during multislice followed by trade-off effects during fitting of imaging parameters and achromatic damping. The single slice potential is of less influence in the present case.