4D scanning transmission electron microscopy

4D scanning transmission electron microscopy (4D STEM) is a subset of scanning transmission electron microscopy (STEM) which utilizes a pixelated electron detector to capture a convergent beam electron diffraction (CBED) pattern at each scan location. This technique captures a 2 dimensional reciprocal space image associated with each scan point as the beam rasters across a 2 dimensional region in real space, hence the name 4D STEM. Its development was enabled by evolution in STEM detectors and improvements computational power. The technique has applications in visual diffraction imaging, phase orientation and strain mapping, phase contrast analysis, among others. The name 4D STEM is common in literature, however it is known by other names: 4D STEM EELS, ND STEM (N- since the number of dimensions could be higher than 4), position resolved diffraction (PRD), spatial resolved diffractometry, momentum-resolved STEM, "nanobeam precision electron diffraction", scanning electron nano diffraction, nanobeam electron diffraction, or pixelated STEM. One of the earliest instances where the analysis of diffraction patterns as a function of probe position was considered took place in 1995, where Peter Nellist, B.C. McCallum and John Rodenburg attempted electron Ptychography analysis of crystalline Silicon. The fluctuation electron microscopy (FEM) technique, proposed in 1996 by Treacy and Gibson, also included quantitative analysis of the differences in images or diffraction patterns taken at different locations on a given sample. The field of 4D STEM remained underdeveloped due to the limited capabilities of detectors available at the time. CCD detectors commonly used in transmission electron microscopy (TEM) had limited data acquisition rates, could not distinguish where on the detector an electron strikes with high accuracy, and had low dynamic range which made them undesirable for use in 4D STEM. In the late 2010s, the development of hybrid pixel array detectors (PAD) with single electron sensitivity, high dynamic range, and fast readout speeds allowed for practical 4D STEM experiments.