In contrast to conventional and high-resolution TEM in STEM mode the image is sequentially recorded using a finely focussed electron beam, which scans the object line per line. Similar to conventional TEM the intensity distribution in STEM bright field images is governed by the mass thickness and diffraction contrast mechanisms. Here the electrons scattered in forward direction are registered on a detector.
If instead those electrons are detected that have been incoherently scattered under large angles, dark field images are obtained, whose contrast is dominated by differences of the mean atomic number, so-called high-angle annular dark-field STEM (HAADF-STEM) or Z-contrast images. This method allows to image local variations of the chemical composition on the nanometer scale. Given a high mechanical stability of the specimen lattice images can dbe directly interpreted, thanks to the high degree of information localisation. Combined with energy-dispersive X-ray spectroscopy and electron energy-loss spectroscopy STEM enables quantitative analyses of the composition and the electronic structure at atomic resolution.