the late 1980s and early 1990s, improvements in STEM technology allowed for samples to be imaged with better than 2 Å resolution, meaning that atomic structure could be imaged in some materials. Aberration correction The addition of an aberration corrector to STEMs enables electron probes to be focused to sub-angstrom diameters, allowing images with sub-angstrom resolution to be acquired. This has made it possible to identify individual atomic columns with unprecedented clarity. Aberration-corrected STEM was demonstrated with 1.9 Å resolution in 1997 and soon after in 2000 with roughly 1.36 Å resolution. Advanced aberration-corrected STEMs have since been developed with sub-50 pm resolution. Aberration-corrected STEM provides the added resolution and beam current critical to the implementation of atomic resolution chemical and elemental spectroscopic mapping. Applications Scanning transmission electron microscopes are used to characterize the nanoscale and atomic-scale structure of specimens, providing important insights into the properties and behaviour of materials and biological cells. Materials science Scanning transmission electron microscopy has been applied to characterize the structure of a wide range of material specimens, including solar cells, semiconductor devices, complex oxides, batteries, fuel cells, catalysts, and 2D materials. Biology The first application of STEM to the imaging of biological molecules was demonstrated in 1971. The advantage of STEM imaging of biological samples is the high contrast of annular dark-field images, which can allow imaging of biological samples without the need for staining. STEM has been widely used to solve a number of structural problems in molecular biology. STEM detectors and imaging modes Annular dark-field In annular dark-field mode, images are formed by fore-scattered electrons incident on an annular detector, which lies outside of the path of the directly transmitted beam. By using a high-angle ADF detector, it is possible to form atomic resolution images where the contrast of an atomic column is directly related to the atomic number (Z-contrast image). Directly interpretable Z-contrast imaging makes STEM imaging with a high-angle detector an appealing technique in contrast to conventional high-resolution electron microscopy, in which phase-contrast effects mean that atomic resolution images must be compared to simulations to aid interpretation. Bright-field In STEM, bright-field detectors are located in the path of the transmitted electron beam. Axial bright-field detectors are located in the centre of the cone of illumination of the transmitted beam, and are often used to provide complementary images to those obtained by ADF imaging. Annular bright-field detectors, located within the cone