The electron microscope provides numerous insights into physics, from demonstrations of fundamental quantum mechanical principles to the physics of imaging and materials. It reveals the atomic and electronic structure of key regions such as defects and interfaces. We can learn the underlying physics governing properties, and gain insight into how to synthesize new materials with improved properties. Some recent advances and possible future directions are discussed.

I had the privilege and the pleasure to work closely with Stephen J. Pennycook for about twenty years, having a group of post-docs and Vanderbilt-University graduate students embedded in his electron microscopy group at Oak Ridge National Laboratory, spending on average a day per week there. We combined atomic-resolution imaging of materials, electron-energy-loss spectroscopy, and density-functional-theory calculations to explore and elucidate diverse materials phenomena, often resolving long-standing issues. This paper is a personal perspective of that journey, highlighting a few examples to illustrate the power of combining theory and microscopy and closing with an assessment of future prospects.

One of the major innovations awaiting in electron microscopy is full three-dimensional imaging at atomic resolution. Despite the success of aberration correction to deep sub-ångström lateral resolution, spatial resolution in depth is still far from atomic resolution. In scanning transmission electron microscopy (STEM), this poor depth resolution is due to the limitation of the illumination angle. To overcome this physical limitation, it is essential to implement a next-generation aberration corrector in STEM that can significantly improve the depth resolution. This review discusses the capability of depth sectioning for three-dimensional imaging combined with large-angle illumination STEM. Furthermore, the statistical analysis approach remarkably improves the depth resolution, making it possible to achieve three-dimensional atomic resolution imaging at oxide surfaces. We will also discuss the future prospects of three-dimensional imaging at atomic resolution by STEM depth sectioning.

The stable sub-angstrom resolution of the aberration-corrected scanning transmission electron microscope (AC-STEM) makes it an advanced and practical characterization technique for all materials. Owing to the prosperous advancement in computational technology, specialized software and programs have emerged as potent facilitators across the entirety of electron microscopy characterization process. Utilizing advanced image processing algorithms promotes the rectification of image distortions, concurrently elevating the overall image quality to superior standards. Extracting high-resolution, pixel-level discrete information and converting it into atomic-scale, followed by performing statistical calculations on the physical matters of interest through quantitative analysis, represent an effective strategy to maximize the value of electron microscope images. The efficacious utilization of quantitative analysis of electron microscope images has become a progressively prominent consideration for materials scientists and electron microscopy researchers. This article offers a concise overview of the pivotal procedures in quantitative analysis and summarizes the computational methodologies involved from three perspectives: contrast, lattice and strain, as well as atomic displacements and polarization. It further elaborates on practical applications of these methods in electronic functional materials, notably in piezoelectrics/ferroelectrics and thermoelectrics. It emphasizes the indispensable role of quantitative analysis in fundamental theoretical research, elucidating the structure-property correlations in high-performance systems, and guiding synthesis strategies.

Electrocatalytic water splitting for hydrogen production is an appealing strategy to reduce carbon emissions and generate renewable fuels. This promising process, however, is limited by its sluggish reaction kinetics and high-cost catalysts. The two-dimensional (2D) transition metal dichalcogenides (TMDCs) have presented great potential as electrocatalytic materials due to their tunable bandgaps, abundant defective active sites, and good chemical stability. Consequently, phase engineering, defect engineering and interface engineering have been adopted to manipulate the electronic structure of TMDCs for boosting their exceptional catalytic performance. Particularly, it is essential to clarify the local structure of catalytically active sites of TMDCs and their structural evolution in catalytic reactions using atomic resolution electron microscopy and the booming in situ technologies, which is beneficial for exploring the underlying reaction mechanism. In this review, the growth regulation, characterization, particularly atomic configurations of active sites in TMDCs are summarized. The significant role of electron microscopy in the understanding of the growth mechanism, the controlled synthesis and functional optimization of 2D TMDCs are discussed. This review will shed light on the design and synthesis of novel electrocatalysts with high performance, as well as prompt the application of advanced electron microscopy in the research of materials science.

The discovery of nickelate superconductors, including doped infinite-layer (IL) nickelates $R$NiO$_{2}$ ($R= {\rm La}$, Pr, Nd), layered square-planar nickelate Nd$_{6}$Ni$_{5}$O$_{12}$, and the Ruddlesden-Popper (RP) phase La$_{3}$Ni$_{2}$O$_{7}$, has spurred immense interest in fundamental research and potential applications. Scanning transmission electron microscopy (STEM) has proven crucial for understanding structure-property correlations in these diverse nickelate superconducting systems. In this review, we summarize the key findings from various modes of STEM, elucidating the mechanism of different nickelate superconductors. We also discuss future perspectives on emerging STEM techniques for unraveling the pairing mechanism in the “nickel age” of superconductivity.

With the explosive exploration of two-dimensional (2D) semiconductors for device applications, ensuring effective electrical contacts has become critical for optimizing device performance. Here, we demonstrate a universal resist-assisted metal transfer method for creating nearly free-standing metal electrodes on the SiO$_{2}$/Si substrate, which can be easily transferred onto 2D semiconductors to form van der Waals (vdW) contacts. In this method, polymethyl methacrylate (PMMA) serves both as an electron resist for electrode patterning and as a sacrificial layer. Contacted with our transferred electrodes, MoS$_{2}$ exhibits tunable Schottky barrier heights and a transition from n-type dominated to ambipolar conduction with increasing metal work functions, while InSe shows pronounced ambipolarity. Additionally, using $\alpha$-In$_{2}$Se$_{3}$ as an example, we demonstrate that our transferred electrodes enhance resistance switching in ferroelectric memristors. Finally, the universality of our method is evidenced by the successful transfer of various metals with different adhesion forces and complex patterns.

Steve Pennycook is a pioneer in the application of high-resolution scanning transmission electron microscopy (STEM) and in particular the use of annular dark-field (ADF) imaging. Here we show how a general framework for 4D STEM allows clear links to be made between ADF imaging and the emerging methods for reconstructing images from 4D STEM data sets. We show that both ADF imaging and ptychographical reconstruction can be thought of in terms of integrating over the overlap regions of diffracted discs in the detector plane. This approach allows the similarities in parts of their transfer functions to be understood, though we note that the transfer functions for ptychographic imaging cannot be used as a measure of information transfer. We also show that conditions of partial spatial and temporal coherence affect ADF imaging and ptychography similarly, showing that achromatic interference can always contribute to the image in both cases, leading to a robustness to partial temporal coherence that has enabled high-resolution imaging.

Recent advances in scanning transmission electron microscopy (STEM) have led to increased development of multi-dimensional STEM imaging modalities and novel image reconstruction methods. This interest arises because the main electron lens in a modern transmission electron microscope usually has a diffraction-space information limit that is significantly better than the real-space resolution of the same lens. This state-of-affairs is sometimes shared by other scattering methods in modern physics and contributes to a broader excitement surrounding multidimensional techniques that scan a probe while recording diffraction-space images, such as ptychography and scanning nano-beam diffraction. However, the contrasting resolution in the two spaces raises the question as to what is limiting their effective performance. Here, we examine this paradox by considering the effects of aberrations in both image and diffraction planes, and likewise separate the contributions of pre- and post-sample aberrations. This consideration provides insight into aberration-measurement techniques and might also indicate improvements for super-resolution techniques.

The functionalities and diverse metastable phases of multiferroic BiFeO$_{3}$ (BFO) thin films depend on the misfit strain. Although mixed phase-induced strain relaxation in multiphase BFO thin films is well known, it is unclear whether a single-crystalline BFO thin film can accommodate misfit strain without the involvement of its polymorphs. Thus, understanding the strain relaxation behavior is key to elucidating the lattice strain-property relationship. In this study, a correlative strain analysis based on dark-field inline electron holography (DIH) and quantitative scanning transmission electron microscopy (STEM) was performed to reveal the structural mechanism for strain accommodation of a single-crystalline BFO thin film. The nanoscale DIH strain analysis results indicated a random combination of multiple strain states that acted as a primary strain relief, forming irregularly strained nanodomains. The STEM-based bond length measurement of the corresponding strained nanodomains revealed a unique strain accommodation behavior achieved by a statistical combination of multiple modes of distorted structures on the unit-cell scale. The globally integrated strain for each nanodomain was estimated to be close to $-1.5%$, irrespective of the nanoscale strain states, which was consistent with the fully strained BFO film on the SrTiO$_{3}$ substrate. Density functional theory calculations suggested that strain accommodation by the combination of metastable phases was energetically favored compared to single-phase-mediated relaxation. This discovery allows a comprehensive understanding of strain accommodation behavior in ferroelectric oxide films, such as BFO, with various low-symmetry polymorphs.

The microstructure significantly influences the superconducting properties. Herein, the defect structures and atomic arrangements in high-temperature Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta }$ (Bi-2212) superconducting wire are directly characterized via state-of-the-art scanning transmission electron microscopy. Interstitial oxygen atoms are observed in both the charge reservoir layers and grain boundaries in the doped superconductor. Inclusion phases with varied numbers of CuO$_{2}$ layers are found, and twist interfaces with different angles are identified. This study provides insights into the structures of Bi-2212 wire and lays the groundwork for guiding the design of microstructures and optimizing the production methods to enhance superconducting performance.

The switching characteristics of ferroelectrics and multiferroics are influenced by the interaction of topological defects with domain walls. We report on the pinning of polarization due to antiphase boundaries in thin films of the multiferroic hexagonal YbFeO$_{3}$. We have directly resolved the atomic structure of a sharp antiphase boundary (APB) in YbFeO$_{3}$ thin films using a combination of aberration-corrected scanning transmission electron microscopy (STEM) and total energy calculations based on density-functional theory (DFT). We find the presence of a layer of FeO$_{6}$ octahedra at the APB that bridges the adjacent domains. STEM imaging shows a reversal in the direction of polarization on moving across the APB, which DFT calculations confirm is structural in nature as the polarization reversal reduces the distortion of the FeO$_{6}$ octahedral layer at the APB. Such APBs in hexagonal perovskites are expected to serve as domain-wall pinning sites and hinder ferroelectric switching of the domains.

We present a method using Zernike moments for quantifying rotational and reflectional symmetries in scanning transmission electron microscopy (STEM) images, aimed at improving structural analysis of materials at the atomic scale. This technique is effective against common imaging noises and is potentially suited for low-dose imaging and identifying quantum defects. We showcase its utility in the unsupervised segmentation of polytypes in a twisted bilayer TaS$_2$, enabling accurate differentiation of structural phases and monitoring transitions caused by electron beam effects. This approach enhances the analysis of structural variations in crystalline materials, marking a notable advancement in the characterization of structures in materials science.

Minimizing disorder and defects is crucial for realizing the full potential of two-dimensional transition metal dichalcogenides (TMDs) materials and improving device performance to desired properties. However, the methods in defect control currently face challenges with overly large operational areas and a lack of precision in targeting specific defects. Therefore, we propose a new method for the precise and universal defect healing of TMD materials, integrating real-time imaging with scanning transmission electron microscopy (STEM). This method employs electron beam irradiation to stimulate the diffusion migration of surface-adsorbed adatoms on TMD materials grown by low-temperature molecular beam epitaxy (MBE), and heal defects within the diffusion range. This approach covers defect repairs ranging from zero-dimensional vacancy defects to two-dimensional grain orientation alignment, demonstrating its universality in terms of the types of samples and defects. These findings offer insights into the use of atomic-level focused electron beams at appropriate voltages in STEM for defect healing, providing valuable experience for achieving atomic-level precise fabrication of TMD materials.

High-angle annular dark field (HAADF) imaging in scanning transmission electron microscopy (STEM) has become an indispensable tool in materials science due to its ability to offer sub-Å resolution and provide chemical information through Z-contrast. This study leverages large language models (LLMs) to conduct a comprehensive bibliometric analysis of a large amount of HAADF-related literature (more than 41000 papers). By using LLMs, specifically ChatGPT, we were able to extract detailed information on applications, sample preparation methods, instruments used, and study conclusions. The findings highlight the capability of LLMs to provide a new perspective into HAADF imaging, underscoring its increasingly important role in materials science. Moreover, the rich information extracted from these publications can be harnessed to develop AI models that enhance the automation and intelligence of electron microscopes.

Four-dimensional scanning transmission electron microscopy (4-D STEM) is a state-of-the-art image acquisition mode used to reveal high and low mass elements at atomic resolution. The acquisition of the electron momenta at each real space probe location allows for various analyses to be performed from a single dataset, including virtual imaging, electric field analysis, as well as analytical or iterative extraction of the object induced phase shift. However, the limiting factor in 4-D STEM is the speed of acquisition which is bottlenecked by the read-out speed of the camera, which must capture a convergent beam electron diffraction (CBED) pattern at each probe position in the scan. Recent developments in sparse sampling and image inpainting (a branch of compressive sensing) for STEM have allowed for real-time recovery of sparsely acquired data from fixed monolithic detectors, Further developments in compressive sensing for 4-D STEM have also demonstrated that acquisition speeds can be increased, i.e., live video rate 4-D imaging is now possible. In this work, we demonstrate the first practical implementations of compressive 4-D STEM for real-time inference on two different scanning transmission electron microscopes.

The design and preparation of novel quantum materials with atomic precision are crucial for exploring new physics and for device applications. Electron irradiation has been demonstrated as an effective method for preparing novel quantum materials and quantum structures that could be challenging to obtain otherwise. It features the advantages of precise control over the patterning of such new materials and their integration with other materials with different functionalities. Here, we present a new strategy for fabricating freestanding monolayer SiC within nanopores of a graphene membrane. By regulating the energy of the incident electron beam and the in-situ heating temperature in a scanning transmission electron microscope (STEM), we can effectively control the patterning of nanopores and subsequent growth of monolayer SiC within the graphene lattice. The resultant SiC monolayers seamlessly connect with the graphene lattice, forming a planar structure distinct by a wide direct bandgap. Our in-situ STEM observations further uncover that the growth of monolayer SiC within the graphene nanopore is driven by a combination of bond rotation and atom extrusion, providing new insights into the atom-by-atom self-assembly of freestanding two-dimensional (2D) monolayers.

Metal halide perovskites (MHPs) are excellent semiconductors that have led to breakthroughs in applications in thin-film solar cells, detectors, and light-emitting diodes due to their remarkable optoelectronic properties and defect tolerance. However, the performance and stability of MHP-based devices are significantly influenced by their microstructures including the formation of defects, composition fluctuations, structural inhomogeneity, etc. Transmission electron microscopy (TEM) is a powerful tool for direct observation of microstructure at the atomic-scale resolution and has been used to correlate the microstructure and performance of MHP-based devices. In this review, we highlight the application of TEM techniques in revealing the microstructures of MHP thin films at the atomic scale. The results provide critical understanding of the performance of MHP devices and guide the design of strategies for improving the performance and stability of MHP devices.

Transmission electron microscopy (TEM) offers unparalleled atomic-resolution imaging of complex materials and heterogeneous structures. However, high-energy imaging electrons can induce structural damage, posing a challenge for electron-beam-sensitive materials. Cryogenic TEM (Cryo-TEM) has revolutionized structural biology, enabling the visualization of biomolecules in their near-native states at unprecedented detail. The low electron dose imaging and stable cryogenic environment in Cryo-TEM are now being harnessed for the investigation of electron-beam-sensitive materials and low-temperature quantum phenomena. Here, we present a systematic review of the interaction mechanisms between imaging electrons and atomic structures, illustrating the electron beam-induced damage and the mitigating role of Cryo-TEM. This review then explores the advancements in low-dose Cryo-TEM imaging for elucidating the structures of organic-based materials. Furthermore, we showcase the application of Cryo-TEM in the study of strongly correlated quantum materials, including the detection of charge order and novel topological spin textures. Finally, we discuss the future prospects of Cryo-TEM, emphasizing its transformative potential in unraveling the complexities of materials and phenomena across diverse scientific disciplines.