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 (STEM). 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, visualising atoms that are difficult to see in conventional STEM and extracting information about crystal orientation, strain, electric and magnetic fields, and other important features.

What is the difference between 4D STEM and conventional STEM?

A conventional scanning transmission electron microscope (STEM) image is formed by scanning a converged electron beam across a sample. The single pixel bright field and annular detectors below the sample integrate the electrons scattered within a specific angular range, and a PC displays the integrated signal in sync with the scan direction.

In 4D STEM, the conventional detectors are replaced by a pixelated detector, such as MerlinEM, enabling the measurement of the entire diffraction pattern at each probe position. The 4D STEM dataset can be processed and analysed by a host of different algorithms, from the application of masks to form a virtual image, to ptychographic reconstructions.

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 al., Small 2019, 15, 1904738.]

Supporting new science all over the world

Discover the undiscovered with Quantum Detectors

Get in touch with Quantum Detectors to find out how we can support your research. Working together we’ll discover more.

Contact Us