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Cabral et al. Microstructures 2023;3:2023040 https://dx.doi.org/10.20517/microstructures.2023.39 Page 7 of 17

collects electrons at each pixel position, converting them to a charge pulse. This charge pulse is integrated
over time to produce a signal that is directly proportional to the number of electrons collected. This
configuration results in a detection system that is sensitive to a single electron and yields a dynamic range of
1,000,000:1 with fast readout speeds [19,43,44] . Although the number of pixels (128 × 128) in the EMPAD is
comparatively small, collecting diffraction at each probe position enables extensive opportunities for
analysis, including crystallographic orientation measurements, strain mapping, phase mapping, and other
diffraction-based analyses. Moreover, the collection of the total electron diffraction pattern allows for the
use of “virtual detectors”, which can be used to reconstruct any imaging mode, including HAADF, BF,
DPC, etc., through post processing.

Direct electron and pixelated detectors have enabled the development of advanced techniques such as
electron ptychography. Although the resolution of conventional STEM imaging is limited by diffraction and
the optics of the microscope, electron ptychography presents a way of overcoming these limits. By using a
high-speed pixelated detector or direct electron detector that is sensitive to single electrons, the interference
patterns of the scattered beams can be resolved and analyzed to determine the phase of the object. These
patterns can then be used to reconstruct the image through post processing, thereby overcoming
conventional diffraction limitations [14,45] . Electron ptychography is a rapidly advancing technique that holds
great promise due to its numerous applications, including applicability for biological imaging, imaging
beam-sensitive materials, characterizing magnetic materials, and measuring strain in materials at sub-
Angstrom length scales [46,47] .

Four-dimensional STEM presents numerous opportunities for advanced characterization of ferroic and
other functional materials. The combined real space image with a corresponding diffraction pattern opens
the opportunity to measure strain at material interfaces at atomic resolution by analyzing the distances
between the diffraction disks [48,49] . Further, diffraction patterns can be analyzed at each pixel in a 4D-STEM
dataset to identify order/disorder nanostructures in ferroic materials. Nanobeam diffraction is highly
sensitive to subtle changes in microstructure, enabling the identification of rhombohedral nanostructures in
[50]
the tetragonal and orthorhombic phases of BaTiO . This study demonstrates the unique ability for local
3
diffraction observations in an otherwise average technique as for neutron and x-ray diffraction. Four-
dimensional STEM continues to see numerous applications, including many in piezoelectric and other
functional materials, but data analysis remains a critical step to obtaining useful information.

In-situ S/TEM
The structure, chemical distribution, and properties of ferroic materials, including ferroelectricity and
ferromagnetism, are strongly linked to external stimuli such as temperature, electrical biasing, and strain.
Understanding how ferroic materials respond to these stimuli is critical for understanding the fundamental
mechanisms that result in their properties, including domain switching kinetics, domain growth, and phase
[51]
transitions . To this end, the evolution of in-situ electron microscopy technology provides an extensive
opportunity to study the dynamic processes of ferroic materials. While in-situ capabilities have been
available in electron microscopes for decades, they have been limited by factors such as sample drift due to
environmental changes. The combined improved stability of S/TEMs with innovative in-situ holder
technology now enables direct observation of materials kinetics at sub-Angstrom resolution .
[52]

The opportunities for in-situ S/TEMs are extensive and include liquid cell electron microscopy [53,54] ,
heating [55,56] , cooling [51,57] , biasing [58,59] , and mechanical stressing [60-62] . While behaviors, such as domain wall
motion and domain switching, can be observed using conventional TEM, the emergence of sub-Angstrom
resolution, combined ADF/iDPC imaging, and 4D-STEM offer the opportunity for more in-depth