Introduction to electron ptychography for materials scientists

Even when aberration-corrected electron microscopy is in the ascendant, the microstructural analysis of materials faces the problems of residual aberrations, zone-axis deviation, surface damage, and irradiation damage, which not only have kept the spatial resolution of the electron microscope at about 0.5 Å for years, but also limited the reliability and accuracy of the microstructural analysis. Electron ptychography is a computational imaging method that utilizes the rich information in four-dimensional scanning transmission electron microscopy datasets to obtain high spatial resolution, phase accuracy, and dose efficiency. Recently, the information limit of microscopic imaging has reached 0.14? by local-orbital ptychography. Electron ptychography consists of two stages: data acquisition and numerical reconstruction. In the data acquisition stage, a four-dimensional scanning transmission electron microscopy dataset is formed by recording the two-dimensional diffraction patterns at each position by means of a pixelated detector during the two-dimensional scanning of the electron beam on the sample. In the numerical reconstruction stage, the amplitude and phase of the sample are retrieved from these datasets by means of suitable algorithms. In contrast to aberration correction, which reduces the effect of lens aberrations on the imaging quality, electron ptychography eliminates this effect and achieves aberration-free imaging, reproducing the true structure of the sample nearly perfectly. We briefly introduce the basic principles and algorithms of electron ptychography from the perspective of the main problems faced by the microstructure analysis of materials, and introduce the main features of electron ptychography through application examples.