Towards atomic column-by-column spectroscopy

The optical arrangement of the scanning transmission electron microscope (S TEM) is ideally suited for performing analysis of individual atomic columns in materials. Using the incoherent Z-contrast image as a reference, and ar ranging incoherent conditions also for the spectroscopy, a precise correspo ndence is ensured between features in the inelastic image and elastic signa ls. In this way the exact probe position needed to maximise the inelastic s ignal from a selected column can be located and monitored during the analys is using the much higher intensity elastic signal. Although object function s for EELS are typically less than 1 Angstrom full-width at half-maximum, t his is still an order of magnitude larger than the corresponding object fun ctions for elastic (or diffuse) scattering used to form the Z-contrast imag e. Therefore, the analysis is performed with an effective probe that is sig nificantly 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, wh ile for a 1.3 Angstrom probe the effective probe is 1.6 Angstrom. Such incr eases in effective probe size can significantly reduce or even eliminate co ntrast between atomic columns that are visible in the image. However, this is only true if we consider circular collector apertures. Calculations base d upon the theory of Maslen and Rossouw [Maslen and Rossouw, Philos. Mag. 4 9 (6) (1984) 735-742; Rossouw and Maslen, Philos. Mag. 49 (6) (1984) 743-75 7] show that employing an annular collector aperture can reduce the FWHM of the inelastic object function down to values close 0.1 Angstrom. With prac tical collector aperture sizes it should be possible to achieve this increa sed spatial resolution without losing too much signal. (C) 1999 Elsevier Sc ience B.V. All rights reserved.