In Situ Nanometer-Resolution Strain and Orientation Mapping for Gas-Solid Reactions via Precession-Assisted Four-dimensional Scanning Transmission Electron Microscopy

Abstract: Chemomechanical interactions in gas or liquid environments are crucial for the functionality and longevity of various materials used in sustainable energy technologies, such as rechargeable batteries, water-splitting catalysts, and next-generation nuclear reactors. A comprehensive understanding of nanoscale strain evolution involved in these processes can advance our knowledge of underlying mechanisms and facilitate material design improvements. However, traditional microscopy workflows face challenges due to trade-offs between field of view (FOV), spatial resolution, temporal resolution, and electron beam damage, particularly in gas or liquid environments. Here, we demonstrate in situ nanometer-resolution strain and orientation mapping in a temperature-controlled gas environment with a large FOV. This is achieved by integrating a microelectromechanical system (MEMS)-based closed-cell TEM holder, precession-assisted four-dimensional scanning transmission electron microscopy (4D-STEM), and a direct electron detector (DED). Using the strain evolution during zirconium initial oxidation as a case study, we first outline critical strategies for focused ion beam gas-cell sample preparation and gas-phase TEM workflows to enhance experimental success. We then show that integrating DED with precession electron diffraction and optimizing gas pressure substantially improve the quantity and quality of the detected Bragg peaks in nano-beam electron diffraction patterns, enabling more precise strain measurements. Furthermore, we introduce a practical protocol to pause the reactions, allowing sufficient time for 4D-STEM data collection while ensuring the temporal resolution needed to resolve material dynamics. Our methodology and workflow provide a robust framework for quantitative analysis of chemomechanical evolutions in materials exposed to gas or liquid environments.