In conventional Transmission Electron Microscopy (CTEM, left), the flood-beam illumination traverses the sample, and is focused with the help of lenses onto (or slightly below) the image plane, at which a high-quality direct electron detector camera records the real-space images.
In iDPC STEM mode (center), the focused beam is scanned over the sample in a 2D array, while for each beam position the number of scattered electrons is counted on four quadrants of a segmented dark field detector. The difference of the counts between the North and South quadrants, or between the East and West quadrants, then give two differential phase contrast images (X and Y), the integration of which results in the ïntegrated differential phase contrast" image. Such an iDPC image contains a strong phase contrast signal that can be used to study biological cryo-EM specimens, and recently pioneered by the group of Carsten Sachse at the ER-C Jülich in a collaboration with the company TFS, see their milestone publication at
Our group collaborates with the team of Carsten Sachse.
In 4D STEM in the variant of Ptychography, the concentrated electron beam is slightly widened and stepped over the sample in a coarser 2D pattern, while the electron diffraction pattern from each beam position is recorded with a pixelated 2D camera. For this, a very high-speed hybrid pixel detector is required, which should at least record the patterns at 4000 frames per second, if not much faster. Alternatively, a detector can be used that records each single electron and its time of arrival, resulting in a long stream of pixel coordinates. Either way, 4D STEM rapidly results in massive amounts of data, which have to be processed as quickly as possible (ideally in real-time) to reduce the amount of data on the harddrive.
Ptychography is a special application of 4D STEM, in which the beam is stepping over the sample in a tighter raster, so that the wider illumination areas are recorded from overlapping positions. The electron diffraction patterns are a Fourier space recording of the diffraction intensities of the electron beam. However, the overlap of beam positions, or more precisely the fact that electron diffraction patterns from neighboring beam positions contain a high amount of identical sample, can be used with certain software algorithms to reconstruct the real-space structure of the specimen, even to a certain extent in 3D. Our group collaborates with the team of Knut Müller-Caspary and Carsten Sachse in this area.