In situ transmission electron microscopy (TEM) has become an increasingly important tool for materials characterization. It provides key information on the structural dynamics of a material during transformations and the ability to correlate a material’s structure and properties. With the recent advances in instrumentation, including aberration-corrected optics, sample environment control, the sample stage, and fast and sensitive data acquisition, in situ TEM characterization has become more powerful. In this article, a brief review of the current status and future opportunities of in situ TEM is provided. The article also introduces the six articles in this issue of MRS Bulletin exploring the frontiers of in situ electron microscopy, including liquid and gas environmental TEM, dynamic four-dimensional TEM, studies on nanomechanics and ferroelectric domain switching, and state-of-the-art atomic imaging of light elements (i.e., carbon atoms) and individual defects.

The need to understand fast, complex physical phenomena through direct in situ observation has spurred the development of high-time-resolution transmission electron microscopy (TEM). Two complementary approaches have emerged: the single-shot and stroboscopic techniques. Single-shot TEM has advanced through the development of dynamic transmission electron microscopy (DTEM) and, more recently, by the advent of movie-mode DTEM, which enables high-frame-rate in situ TEM experimentation by capturing nanosecond-scale sequences of images or diffraction patterns. Previous DTEM studies produced only single snapshots of fast material processes. Movie-mode DTEM provides the ability to track the creation, motion, and interaction of individual defects, phase fronts, and chemical reaction fronts, providing invaluable information on the chemical, microstructural, and atomic-level features that govern rapid material processes. This article discusses movie-mode DTEM technology, its application in the study of reaction dynamics in Ti–B-based reactive nanolaminates, and future instrumentation.

This article reviews atomic-resolution in situ electron microscopy studies of two-dimensional materials such as graphene, hexagonal boron nitride, and metal dichalcogenides with a focus on defect structures. Electron irradiation allows defect formation and atomic-resolution imaging at the same time by the same electron beam. Two-dimensional hexagonal lattices show unique mechanisms of defect reconstruction that do not appear in other materials. The combination of thermal annealing and irradiation, both adjustable in the electron microscope, sets a balance between equilibrium and nonequilibrium and allows for the generation of new structures and morphologies.

Catalytic nanomaterials play a major role in chemical conversions and energy transformations. Understanding how materials control and regulate surface reactions is a major objective for fundamental research on heterogeneous catalysts. In situ environmental transmission electron microscopy (ETEM) is a powerful technique for revealing the atomic structures of materials at elevated temperatures in the presence of reactive gases. This approach can allow the structure–reactivity relations underlying catalyst functionality to be investigated. Thus far, ETEM has been limited by the absence of in situ measurements of gas-phase catalytic products. To overcome this deficiency, operando TEM techniques are being developed that combine atomic characterization with the simultaneous measurement of catalytic products. This article provides a short review of the current status and major developments in the application of ETEM to gas-phase catalysis over the past 10 years.

Materials synthesis and the functioning of devices often involve liquid media. However, direct visualization of dynamic processes in liquids, especially with high spatial and temporal resolution, has been challenging. For solid materials, advances in aberration-corrected electron microscopy have made observations of atomic-level features a routine practice. Here, we discuss the extent to which one can take advantage of the resolution of modern electron microscopes to image phenomena occurring in liquids. We describe the fundamentals of two different experimental approaches that use closed and open liquid cells. We illustrate the capabilities of each approach by considering processes in batteries and nucleation and growth of nanoparticles from solution. Liquid-cell electron microscopy appears to be duly fulfilling its role and promise for in situ studies of nanoscale processes in liquids, revealing physical and chemical processes that are otherwise difficult to observe.

This article reviews current progress in research in ferroelectric switching phenomena using in situ electron microscopy. We focus on state-of-the-art instrumentation, analytical methods, experimental procedures, and image contrast mechanisms. Particular emphasis is on ferroelectric domain and domain wall structures that determine ferroelectric behaviors. The applicability of in situ microscopy to studying a wide range of switching phenomena, such as domain nucleation, domain wall motion, and domain wall pinning by various types of defects, in ferroelectric thin films is demonstrated. The underlying physics of these dynamic processes is also discussed.

In situ transmission electron microscopy (TEM) nanomechanical testing has benefited from a number of recent technical developments related to both how deformation is imaged and how deformation is induced and measured inside a TEM instrument. These developments have led to new insights into the deformation mechanisms of a wide range of metals and alloys, as well as measurements of the unusual mechanical properties of small-scale objects such as whiskers and nanocrystals. Herein, we describe this recent progress through selected highlights of recent findings on the dynamic behavior of defects such as dislocations, twins, and grain boundaries.

In his famous 1959 lecture “There’s plenty of room at the bottom,” Richard Feynman put out this challenge: “Is there no way to make the electron microscope more powerful?” He called for “improving the electron microscope by a hundred times,” which, given that the resolution then was about 10 Å, meant he was calling for a resolution in the range of 0.1 Å. Today’s aberration-corrected microscopes have come a long way, achieving a resolution of around 0.5 Å. This has enormously improved our ability to see atomic arrangements in crystals, measure ferroelectric displacements, and even determine valence and spin states with electron energy-loss spectroscopy. However, there remain many structures crucial to materials properties that we cannot yet see. Continuing the road toward Feynman’s goal would bring these structures to light, with yet more dramatic impacts on the entire field of materials science.