Liquid phase (also called “liquid cell”) transmission electron microscopy (TEM) is a powerful platform for nanoscale imaging and characterization of physical and chemical processes of materials in liquids. It is a direct approach to address critical scientific questions on how materials form or transform in response to external stimuli, such as changes in chemical potential, applied electric bias, and interactions with other materials or their environment. Answers to these questions are essential for understanding and controlling nanoscale materials properties and advancing their applications. With the recent technical advances in TEM, such as the development of sample stages, detectors, and image processing toolkits, liquid phase TEM is transforming our ability to characterize materials and revolutionizing our understanding of many fundamental processes in materials science and other fields. In this article, we briefly review the current status, challenges, and opportunities in liquid phase TEM. More details of the development and applications of liquid cell TEM are discussed in the articles in this issue of MRS Bulletin.

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.

Self-assembled vertical heteroepitaxial nanostructures (VHN) in the complex oxide field have fascinated scientists for decades because they provide degrees of freedom to explore in condensed matter physics and design-coupled multifunctionlities. Recently, of particular interest is the perovskite-spinel-based VHN, covering a wide spectrum of promising applications. In this review, fabrication of VHN, their growth mechanism, control, and resulting novel multifunctionalities are discussed thoroughly, providing researchers a comprehensive blueprint to construct promising VHN. Following the fabrication section, the state-of-the-art design concepts for multifunctionalities are proposed and reviewed by suitable examples. By summarizing the outlook of this field, we are excitedly expecting this field to rise with significant contributions ranging from scientific value to practical applications in the foreseeable future.

Coalescence is a significant pathway for the growth of nanostructures. Here we studied the coalescence of Bi nanoparticles in situ by liquid cell transmission electron microscopy (TEM). The growth of Bi nanoparticles was initiated from a bismuth neodecanoate precursor solution by electron beam irradiation inside a liquid cell under the TEM. A significant number of coalescence events occurred from the as-grown Bi nanodots. Both symmetric coalescence of two equal-sized nanoparticles and asymmetric coalescence of two or more unequal-sized nanoparticles were analyzed along their growth trajectories. Our observation suggests that two mass transport mechanisms, i.e., surface diffusion and grain boundary diffusion, are responsible for the shape evolution of nanoparticles after a coalescence event.