Seeing atoms with aberration-corrected sub-Angström electron microscopy.

High-resolution electron microscopy is able to provide atomic-level characterization of many materials in low-index orientations. To achieve the same level of characterization in more complex orientations requires that instrumental resolution be improved to values corresponding to the sub-Angström separations of atom positions projected into these orientations. Sub-Angström resolution in the high-resolution transmission electron microscope has been achieved in the last few years by software aberration correction, electron holography, and hardware aberration correction; the so-called "one-Angström barrier" has been left behind. Aberration correction of the objective lens currently allows atomic-resolution imaging at the sub-0.8A level and is advancing towards resolutions in the deep sub-Angström range (near 0.5A). At current resolution levels, images with sub-Rayleigh resolution require calibration in order to pinpoint atom positions correctly. As resolution levels approach the "sizes" of atoms, the atoms themselves will produce a limit to resolution, no matter how much the instrumental resolution is improved. By arranging imaging conditions suitably, each atom peak in the image can be narrower, so atoms are imaged smaller and may be resolved at finer separations.