Four-Dimensional Scanning Transmission Electron Microscopy (4D STEM)

Imagine that you could flexibly change your STEM detector’s size and shape to cover different parts of the scattering distribution. And that you could do this after the experiment, without scanning the electron beam over the same sample region again.

With 4D STEM datasets, it is possible to synthesize STEM images by selecting your desired regions from the recorded diffraction patterns. The use of virtual masks to select regions of interest flexibly allows emulation of customized detector geometries, equivalent to the traditional STEM modes and beyond.

Electron scattering from a polycrystalline gold sample, and reconstructed virtual BF and ADF after a 4D STEM acquisition. (a) The sum of diffraction patterns contained in the 4D STEM acquisition. (b) and (c) Virtual BF and ADF images, respectively, using the indicated masks. Acquired with DECTRIS ARINA.

DPC has proven to be a great way to measure a sample’s electromagnetic fields. It aids the visualization of magnetic domains, or the mapping of local electrostatic fields down to the atomic columns. 

Coupled with smart data analysis, DPC can be used to reveal light atomic species. And the flexibility provided by a 4D STEM acquisition allows for the most precise data analysis.

Comparing the visibility of light atoms in a SmB6 crystal sample. (a) Virtual BF with a SmB6 atomic structure overlay. (b) A DPC vector field with color coding to indicate direction and modulus. (c) A calculated iCoM image from the previous DPC. All three images were calculated from a single 4D STEM acquisition. Acquired with DECTRIS ARINA. Acknowledgment: Elisabeth Mueller (PSI) and Mingjian Wu (FAU).

This advanced imaging technique can uncover the most detailed view of the specimen under investigation. Ptychography is a computational image reconstruction technique that explores the interference between the direct electron beam and its diffracted components. 

The most recent electron ptychography results allow for a posteriori correction of optical aberrations, permitting microscopists to achieve an unmatched spatial resolution.

Also relevant to engineering materials, particularly when mechanical and electronic properties are key, is the local strain at interfaces or structural defects. 

The existing compression, or stretching, of interatomic spacing is also encoded in the diffraction patterns that are recorded in a 4D STEM measurement. Given its potential to map properties with nm-scale resolution, this approach is promising. It will support the strain-engineering approach that is currently used in the semiconductor and optoelectronic industries – just more straightforward and faster than other high-resolution characterization approaches. 

Crystalline phase and orientation mapping is extremely important to the development of engineering materials. Besides the method’s quotidian use for characterizing polycrystalline materials, it plays a decisive role in differentiating compounds that may present allotropes or polymorphs.

While the diffraction map implementation on the TEM enhances the spatial resolution in comparison to its SEM counterpart EBSD down to the few-nanometer scale, 4D STEM acquisition with a pixelated detector delivers improved yield and sensitivity when mapping crystalline domains and conducting relative orientation/texture analyses. 

Orientation Mapping applied to an AlN polycrystalline sample with a columnar structure. (a) and (b) A virtual BF image and a sample localized diffraction pattern, respectively, referring to AlN along a [0001] zone axis. (c) and (d) Orientation Mapping, split via in-plane rotation and out-of-plane tilt with respect to the main columns axis. Acquired with DECTRIS ARINA. Acknowledgment: Erdmann Spiecker and Mingjian Wu (FAU).