4D STEM involves acquiring a 2D convergent beam electron diffraction (CBED) pattern, at every pixel of a 2D STEM raster, generating a 4D dataset containing momentum and spatial information about the specimen. The key advantage of 4D STEM compared to conventional 2D STEM imaging is that, in a single scan, one can acquire a 4D dataset that contains all possible 2D STEM images of the sample, as well as a variety of novel signals that cannot be derived from conventional 2D STEM imaging.

4D STEM enables virtual detector imaging using simple masks, the reconstruction of differential phase contrast (DPC) images, which were previously only available by using segmented detectors, and “Centre of Mass” (COM) images also been referred to as “first moment” or COM DPC images in the literature. DPC and COM both essentially measure the magnitude and direction of the average shift in the 2D momentum of the electron probe for each CBED pattern in the 4D STEM dataset. The DPC and COM imaging modes are sensitive to electric fields in the specimen, which can cause a shift in the average momentum of the electron probe either by displacing the entire diffraction pattern, or altering the distribution of intensity within the diffraction pattern, depending on the relative length scales of the probe and the source of the field. Magnetic fields can also be detected using DPC and COM when operating the microscope in a Lorentz mode.

In scanning nanobeam diffraction experiments 4D STEM data can map crystallographic phases, their orientation and distortion across a field of view. 4D STEM is an elegant way to record position-averaged CBED or Kossel-Moellenstedt data for a precise  determination of  lattice parameters or space-group symmetry.

Figure: 4D-STEM virtual detector and diffraction deflection images. The figures show data derived from a single 4D-STEM data set of a CsPbBr3 crystal in <100> viewing direction taken at an acceleration voltage of 80 kV.  Upper row: Virtual detector images exposing heavy and light atom columns in different balance depending on the section of scattering angles. Lower row: COM deflection of the electron probe around atomic columns in the principal image dimensions, magnitude and in angle. The angular deflection map is plotted in a circular color map spanning from 0 to 2𝜋. White dots mark column positions.

Further reading

• Ophus, C. (2019). Microscopy and Microanalysis, 25(3), 563-582. doi:10.1017/S1431927619000497