Towards atomic column-by-column spectroscopy

The optical arrangement of the scanning transmission electron microscope (STEM) is ideally suited for performing analysis of individual atomic columns in materials. Using the incoherent Z-contrast image as a reference, and arranging incoherent conditions also for the spectroscopy, a precise correspondence is ensured between features in the inelastic image and elastic signals. In this way the exact probe position needed to maximise the inelastic signal from a selected column can be located and monitored during the analysis using the much higher intensity elastic signal. Although object functions for EELS are typically less than 1 Angstrom full-width at half-maximum, this is still an order of magnitude larger than the corresponding object functions for elastic (or diffuse) scattering used to form the Z-contrast image. Therefore, the analysis is performed with an effective probe that is significantly broader than that used for the reference Z-contrast image. For a 2.2 Angstrom probe the effective probe is of the order of 2.5 Angstrom, while for a 1.3 Angstrom probe the effective probe is 1.6 Angstrom. Such increases in effective probe size can significantly reduce or even eliminate contrast between atomic columns that are visible in the image. However, this is only true if we consider circular collector apertures. Calculations based upon the theory of Maslen and Rossouw [Maslen and Rossouw, Philos. Mag. 49 (6) (1984) 735-742; Rossouw and Maslen, Philos. Mag. 49 (6) (1984) 743-757] show that employing an annular collector aperture can reduce the FWHM of the inelastic object function down to values close 0.1 Angstrom. With practical collector aperture sizes it should be possible to achieve this increased spatial resolution without losing too much signal. (C) 1999 Elsevier Science B.V. All rights reserved.