What is 4D-STEM?

Scanning Transmission Electron Micropscopy (STEM) is a powerful tool for materials characterization. In a traditional STEM experiment, a beam of high energy electrons is focused to a very fine probe - on the order of, or even smaller than, the spacing between atoms - and rastered across the surface of the sample. A conventional two-dimensional STEM image is formed by populating the value of each pixel with the electron flux through a detector at the corresponding beam position. In a high resolution tool, this enables imaging at the level of atoms.

Four-dimensional scanning transmission electron microscopy (4D-STEM) uses a fast, pixelated electron detector to collect far more information than a traditional STEM experiment. In 4D-STEM, a pixelated detector is used to record a 2D diffraction image at every raster position of the beam. A 4D-STEM scan thus results in a 4D data array: two dimensions in diffraction space (i.e. the detector pixels), and two dimensions in real space (i.e. the rastering of the beam).

4D-STEM data is information rich. A 4D datacube can be collapsed in real space to yield information comparable to nanobeam electron diffraction experiment, or in diffraction space to yield a variety of virtual images, corresponding to both traditional STEM imaging modes as well as more exotic virtual imaging modalities. The structure, symmetries, and spacings of Bragg disks can be used to extract spatially resolved maps of crystallinity, grain orientations, and lattice strain. Redundant information in overlapping Bragg disks can be leveraged to calculate the sample potential. Structure in the diffracted halos of amorphous systems can be used to describe the short and medium range order.

py4DSTEM supports many different modes of 4D-STEM analysis.