STEM High Angle Annular Dark-Field Imaging

High-angle annular dark field (HAADF) imaging in the aberration-corrected scanning transmission electron microscope (STEM) has become an invaluable technique for characterizing catalysts. Its powerful combination of good atomic number (Z) contrast and ultrahigh spatial resolution allows us to capture atomic resolution images whose intensity variations include important compositional information on the catalyst being examined. The physical principles and experimental limitations of the STEM-HAADF imaging technique are presented in this chapter, followed by some representative examples of applications to supported metals, supported oxides, and bulk mixed oxide catalyst systems. The challenges and progress in developing a “gentler” more benign form of STEM-HAADF imaging for analyzing highly beam-sensitive materials such as two dimensional (2D) materials and zeolites are then described. We also explain how three dimensional (3D) depth information from the catalyst can be extracted by quantifying STEM-HAADF image intensities, by performing electron tomography or through-focal imaging in HAADF mode. We will also introduce some applications of HAADF imaging of catalysts in a gaseous or liquid environment in order to understand their structural evolution under more realistic conditions. Finally, we stress the benefits of correlating HAADF imaging results with information obtained from complementary characterization methods.