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4D STEM

Modern fast framing electron detectors like MerlinEM allow practical imaging of a full distribution of electrons for each probe position in a scanning transmission electron microscope. The most common name of the technique is 4D-STEM, however, scanned diffraction or momentum resolved STEM are also used.

What is 4D STEM?

4D-STEM is a cutting-edge technique that uses the richness of information available in STEM for exploring the structure of materials. Rather than a single averaged signal, 4D-STEM cameras capture the full electron diffraction pattern so that researchers can peer into the subtle details of data, visualizing atoms that are difficult to see in conventional STEM and extracting information about crystal orientation, strain, electric and magnetic fields, and other important features.

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What is a STEM image & the differences between TEM and STEM

Scanning Transmission Electron Microscopy uses a converged beam that is rastered across a sample to produce an image from the localized interactions between the beam and the sample.

TEM illuminates samples with a broad beam of electrons. STEM utilizes a condensed beam that is rastered across the sample.

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Where should STEM be used?

Since 4D-STEM collects the full distribution of electrons for each point of the scan, there are many uses for this technique.

Virtual STEM detection – reconstruction of traditional STEM signals in post-processing but with full freedom of detector positioning (annular, disk, DPC). In traditional STEM, once a dataset is collected the geometry is fixed and very few corrections are possible.

Strain Mapping – changes in positions of diffraction peaks can be directly related to the strain present in the sample.

Orientation Mapping – complex polycrystalline specimens can contain various phases and types of crystals. By scanning the sample with a fine probe, the data can be indexed and related to the orientation and type of the given grain or part of the sample.

Electron Ptychography – 4D information is key to this technique. Ptychography can be used to quantitatively image the phase of the sample. This can be done with a focused beam allowing direct, one-step, reconstruction or with a defocused beam where iterative algorithms have to be deployed.

Differential Phase Contrast – this technique can be used on two main length scales – with high-resolution probe (atomic length scales) DPC can visualise atomic electric fields and with nano-resolution, it can be used to show quantitative information from local electric or magnetic fields present in the sample.

Center of Mass – closely related to DPC, the technique expands what is possible with DPC and allows more quantitative information to be extracted.

Cross-Correlation and Computer Vision techniques – in general, computer vision techniques can be used to track specific structures within diffraction patterns or TEM images. This could either be a DPC style of an image where shifts of the beam can be tracked with much higher precision by tracking the edge of the beam or even more advanced techniques which track symmetry changes within the probe or diffracted beams.

[Obtained under CC v4.0 licence from Nord, M. et all, Small 2019, 15, 1904738.]
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