What is the physical reason for the Z-contrast in HAADF-STEM images?
When using STEM, depends on your detector range, you can get High angle annular dark field (HAADF)-STEM image and Annular bright field (ABF)-STEM image. If image with the high angle diffracted electrons (50-200mrad), i.e. HAADF-STEM, the contrast is approximately proportional to square of atomic number (Z), which is so called Z contrast. The high angle diffracted electron is caused by the strong coulomb interaction between electron and atom core (Rutherford scattering).
When using STEM, depends on your detector range, you can get High angle annular dark field (HAADF)-STEM image and Annular bright field (ABF)-STEM image. If image with the high angle diffracted electrons (50-200mrad), i.e. HAADF-STEM, the contrast is approximately proportional to square of atomic number (Z), which is so called Z contrast. The high angle diffracted electron is caused by the strong coulomb interaction between electron and atom core (Rutherford scattering).
Hi Stefan,
the concise principle of Z-contrast is the HAADF detector collect incoherently scattered electrons (under angles mentioned by Xinfu Gu), so no interference typical for diffraction doesn´t come into being. For this reason the atomic visualisation is direct and we don´t need any simulations to resolve atomic arrangement.
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