Abstract
Modern electron microscopes are now capable of imaging individual atoms with extraordinary spatial resolution. Furthermore, by combining electron energy-loss spectroscopy (EELS) with electron microscopy, it is possible to analyze not only atomic structures but also elemental distributions and electronic states with atomic resolution. However, two fundamental limitations have long been accepted. First, whether electronic orbitals themselves can be directly visualized in real space has been a subject of both experimental and theoretical debate. Second, the spatial resolution of EELS has been considered fundamentally limited because inelastic scattering originates from long-range Coulomb interactions, causing electronic excitations to be spatially delocalized.
In this seminar, I will present our recent studies that challenge these two long-standing concepts. First, I will demonstrate that EELS can reveal anisotropic atomic contrast reflecting the directionality of electronic states associated with chemical bonding, going beyond conventional elemental mapping. I will then discuss the physical origin of this anisotropic contrast and show that it does not directly represent the shape of atomic orbitals, but instead reflects the directionality of the selected electronic transitions. Finally, I will introduce a new annular electron energy-loss spectroscopy (AEELS) technique, which selectively detects high-angle inelastically scattered electrons and enables atomic-resolution imaging beyond the conventional spatial-resolution limit imposed by delocalization.
These studies demonstrate that the contrast and spatial resolution of EELS imaging are determined not only by the excitation process itself, but also by which inelastically scattered electrons are detected. This new perspective opens exciting opportunities for visualizing electronic structures with electron microscopy and expands the future capabilities of EELS imaging.