Momentum-resolved scanning transmission electron microscopy

Momentum-resolved scanning transmission electron microscopy (MR-STEM) refers to the recording of a full diffraction pattern for each scan point of a scanning electron probe. The resulting four-dimensional datasets not only overcome the limited flexibility of established detector geometries determined by hardware but also transform formerly challenging methods into widespread imaging modes. After briefly reviewing multidimensionality aspects in STEM, a unique terminology for MR-STEM is introduced. The concept of first-moment STEM (FM-STEM) is derived robustly from fundamental quantum mechanics, followed by a detailed description of practical aspects related to ultrafast pixelated detectors. FM-STEM delivers the average lateral momentum of the electron beam. The article works out how atomic electric fields can be measured in thin specimens, and outlines methodologies for measuring polarization-induced electric fields in thick ones. By angular multirange analysis, it is elucidated how MR-STEM can yield specimen thickness and composition simultaneously. A brief assessment of inelastic scattering effects on MR-STEM closes this article.