Identifying and manipulating single atoms with scanning transmission electron microscopy

Quantifying phase magnitudes of open-source focused-probe 4D-STEM ptychography reconstructions

Accurate computational ptychographic phase reconstructions are enabled by fast direct-electron cameras with high dynamic ranges used for four-dimensional scanning transmission electron microscopy (4D-STEM). The availability of open software packages is making such analyses widely accessible, and especially when implemented in Python, easy to compare in terms of computational efficiency and reconstruction quality. In this contribution, I reconstruct atomic phase shifts from convergent-beam electron diffraction maps of pristine monolayer graphene, which is an ideal dose-robust uniform phase object, acquired on a Dectris ARINA detector installed in a Nion UltraSTEM 100 operated at 60 keV with a focused-probe convergence semi-angle of 34 mrad. For two different recorded maximum scattering angle settings, I compare a range of direct and iterative open-source phase reconstruction algorithms, evaluating their computational efficiency and tolerance to reciprocal-space binning and real-space thinning of the data. The quality of the phase images is assessed by quantifying the variation of atomic phase shifts using a robust parameter-based method, revealing an overall agreement with some notable differences in the absolute magnitudes and the variation of the phases. Although such variation is not a major issue when analysing data with many identical atoms, it does put limits on what level of precision can be relied upon for unique sites such as defects or dopants, which also tend to be more dose-sensitive. Overall, these findings and the accompanying open data and code provide useful guidance for the sampling required for desired levels of phase precision, and suggest particular care is required when relying on electron ptychography for quantitative analyses of atomic-scale electromagnetic properties.

Direct Visualization of Defect-Controlled Diffusion in van der Waals Gaps

Diffusion processes govern fundamental phenomena such as phase transformations, doping, and intercalation in van der Waals (vdW) bonded materials. Here, the diffusion dynamics of W atoms by visualizing the motion of individual atoms at three different vdW interfaces: hexagonal boron nitride (BN)/vacuum, BN/BN, and BN/WSe2, by recording scanning transmission electron microscopy movies is quantified. Supported by density functional theory (DFT) calculations, it is inferred that in all cases diffusion is governed by intermittent trapping at electron beam-generated defect sites. This leads to diffusion properties that depend strongly on the number of defects. These results suggest that diffusion and intercalation processes in vdW materials are highly tunable and sensitive to crystal quality. The demonstration of imaging, with high spatial and temporal resolution, of layers and individual atoms inside vdW heterostructures offers possibilities for direct visualization of diffusion and atomic interactions, as well as for experiments exploring atomic structures, their in situ modification, and electrical property measurements of active devices combined with atomic resolution imaging.

Simultaneous secondary electron microscopy in the scanning transmission electron microscope with applications for in situ studies

Scanning/transmission electron microscopy (STEM) is a powerful characterization tool for a wide range of materials. Over the years, STEMs have been extensively used for in situ studies of structural evolution and dynamic processes. A limited number of STEM instruments are equipped with a secondary electron (SE) detector in addition to the conventional transmitted electron detectors, i.e. the bright-field (BF) and annular dark-field (ADF) detectors. Such instruments are capable of simultaneous BF-STEM, ADF-STEM and SE-STEM imaging. These methods can reveal the 'bulk' information from BF and ADF signals and the surface information from SE signals for materials <200 nm thick. This review first summarizes the field of in situ STEM research, followed by the generation of SE signals, SE-STEM instrumentation and applications of SE-STEM analysis. Combining with various in situ heating, gas reaction and mechanical testing stages based on microelectromechanical systems (MEMS), we show that simultaneous SE-STEM imaging has found applications in studying the dynamics and transient phenomena of surface reconstructions, exsolution of catalysts, lunar and planetary materials and mechanical properties of 2D thin films. Finally, we provide an outlook on the potential advancements in SE-STEM from the perspective of sample-related factors, instrument-related factors and data acquisition and processing.