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Enhancing the imaging power of microscopy to identify all chemical types of atom, from low- to high-atomic-number elements,would significantly contribute for a direct determination of material structures. Electron microscopes have successfully provided images of heavy-atom positions, particularly by the annular dark-field method, but detection of light atoms was difficult owing to their weak scattering power. Recent developments of aberration-correction electron optics have significantly advanced the microscope performance, enabling identification of individual light atoms such as oxygen, nitrogen, carbon, boron and lithium. However, the lightest hydrogen atom has not yet been observed directly, except in the specific condition of hydrogen adatoms on a graphene membrane. Here we show the first direct imaging of the hydrogen atom in a crystalline solid YH(2), based on a classic 'hollow-cone' illumination theory combined with state-of-the-art scanning transmission electronmicroscopy. The optimized hollow-cone condition derived from the aberration-corrected microscope parameters confirms that the information transfer can be extended to 22.5 nm(-1), which corresponds to a spatial resolution of about 44.4 pm. These experimental conditions can be readily realized with the annular bright-field imaging in scanning transmission electron microscopy according to reciprocity, revealing successfully the hydrogen-atom columns as dark dots, as anticipated from phase contrast of a weak-phase object.

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▪ Abstract  In the nanoscience era, the properties of many exciting new materials and devices will depend on the details of their composition down to the level of single atoms. Thus the characterization of the structure and electronic properties of matter at the atomic scale is becoming ever more vital for economic and technological as well as for scientific reasons. The combination of atomic-resolution Z-contrast scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS) represents a powerful method to link the atomic and electronic structure to macroscopic properties, allowing materials, nanoscale systems, and interfaces to be probed in unprecedented detail. Z-contrast STEM uses electrons that have been scattered to large angles for imaging. The relative intensity of each atomic column is roughly proportional to Z2, where Z is the atomic number. Recent developments in correcting the aberrations of the lenses in the electron microscope have pushed the achievable spatial resolution and the sensitivity for imaging and spectroscopy in the STEM into the sub-Ångstrom (sub-Å) regime, providing a new level of insight into the structure/property relations of complex materials. Images acquired with an aberration-corrected instrument show greatly improved contrast. The signal-to-noise ratio is sufficiently high to allow sensitivity even to single atoms in both imaging and spectroscopy. This is a key achievement because the detection and measurement of the response of individual atoms has become a challenging issue to provide new insight into many fields, such as catalysis, ceramic materials, complex oxide interfaces, or grain boundaries. In this article, the state-of-the-art for the characterization of all of these different types of materials by means of aberration-corrected STEM and EELS are reviewed.