Unraveling atomic-resolution valence electron energy-loss spectroscopic imaging in a single-crystal CaNb2O6

Abstract

Despite advancements in electron optics and spectrometer design over the past twenty years, atomic-resolution valence-electron energy-loss spectroscopy imaging remains challenging due to the delocalization of inelastic electron scattering. In this study, we used an energy-filtered spectrometer equipped with a hybrid-pixel direct electron detector and spherical aberration-corrected scanning transmission electron microscopy to analyze many-electron excitations and interband transitions in a single-crystal calcium niobate, CaNb2O6, with spatial resolution ranging from the nanometers to the atomic scale. In the low-loss region above the bandgap at about 3.8 eV, we observed volume plasmons, around 6 eV and 15 eV energy loss, as well as a mix of strongly correlated plasmons and excitons, known as plexcitons, at approximately 7.3 eV energy loss. Additionally, we employed an on-axis EELS setup for atomic-resolution zero-loss peak (ZLP) imaging and visualized energy- and atom-resolved images of plexcitons and VPs, which showed contrast reversal relative to high-angle annular dark-field images. To investigate elastic contrast preservation, we also analyzed the effect of the collection angle and minimized its influence to produce delocalized VP images. In fact, the ZLP and VEELS images obtained using the weak-beam setup demonstrate that, in both cases, the contrast resembles Z-contrast. Moreover, we found that [NbO6] octahedra directly contributed to the lateral maps of interband transitions in the range from 3.2 eV to 3.5 eV energy loss. These findings demonstrate that Cs-STEM-EELS, which examines atomic-scale contrast associated with low-energy losses, can be a powerful tool for visualizing the structure, bonding, and electronic properties of complex crystalline nanostructures, including individual atomic sites, interstitial sites, and point defects.