Analytical Review of Direct Stem Imaging Techniques for Thin Samples

Abstract

Abstract Scanning transmission electron microscopy (STEM) imaging, which has been in use for many decades, is analyzed mathematically for thin nonmagnetic samples. The result is a closed-form description of a general STEM image, showing that STEM imaging is, in general, nonlinear (contrast transfer is sample dependent), except when an ideal first moment detector is used. The closed-form description is subsequently used to optimize STEM imaging. We distinguish between STEM techniques using symmetric scalar detectors and antisymmetric vector detectors and show that for both cases practical experimental techniques can be defined that are approximately linear. The case of antisymmetric vector detectors yields the newly introduced integrated differential phase contrast (iDPC-STEM) technique. For this technique we show experimental results, showing that it is capable of imaging light and heavy elements together as well as giving full low-frequency transfer. We demonstrate that it can be used under low-dose conditions.