Four-Dimensional Scanning Transmission Electron Microscopy: From Material Microstructures to Physicochemical Properties

The resolution limit of scanning transmission electron microscopy (STEM) has now reached atomic resolution. Further, owing to its flexible multi-channel imaging and powerful spectral characterization abilities, STEM has shown immense promise for microscale characterization in materials sciences, life sciences, and other fields. However, the traditional STEM detector is limited by its single-pixel integral detection mechanism, due to which it can only collect scattered electrons at a specific angle. This not only results in a loss of angle -resolved information of the scattered electrons, but also reduces the dose efficiency of the incident electrons. Therefore, it is imperative that new imaging techniques are developed to achieve high-throughput, high-electron-dose-efficiency imaging. Recent advances in electron direct detection techniques and detectors with partitioned or pixelated configurations, as well as the rapidly increasing computing power and disk storage, have contributed to the rapid development of four-dimensional STEM (4D -STEM) technology. Uniquely, 4D-STEM allows one to acquire structural information associated with scattered electrons. During the acquisition of 4D-STEM data, the convergent electron beam performs two-dimensional scanning on the sample plane, while a pixelated array detector with a high frame rate, wide dynamic range, and high signal-to-noise ratio collects two-dimensional diffraction data in the far field. Because these diffraction data are angle-resolved, they can be used for conventional STEM imaging as well as phase contrast imaging at the leading edge. For example, electron ptychography is used to reconstruct the sample object function from a series of diffraction patterns measured at different spatial locations. In addition, 4D-STEM technology can be explored to obtain more information about the internal structure of materials, providing opportunities for the multi-scale characterization of materials. This paper introduces the basic principles of 4D -STEM imaging and summarizes a series of 4D-STEM applications ranging from the microstructural characterization of materials to the analysis of their physicochemical properties. Typical applications include virtual detector imaging as well as measurements of micro-electromagnetic fields, micro-crystal orientations, micro-strain distributions, and the local specimen thickness. In addition, electron ptychography imaging technology realized using 4D-STEM data is highly promising for low-electron-dose applications owing to its high utilization efficiency of scattered electrons. The application of 4D-STEM technology in low-electron-dose applications is discussed. Overall, with the rapid development of electron detectors and post-processing analysis software for 4D-STEM data, it is believed that the novel 4D-STEM technology will eventually completely replace traditional STEM technology.