Nanoscale Localized Phonons at Al₂O₃ Grain Boundaries

Abstract

Nanoscale defects like grain boundaries (GBs) would introduce local phonon modes and affect the bulk materials’ thermal, electrical, optical, and mechanical properties. It is highly desirable to correlate the phonon modes and atomic arrangements for individual defects to precisely understand the structure–property relation. Here we investigated the localized phonon modes of Al2O3 GBs by combination of the vibrational electron energy loss spectroscopy (EELS) in scanning transmission electron microscope and density functional perturbation theory (DFPT). The differences between GB and bulk obtained from the vibrational EELS show that the GB exhibited more active vibration at the energy range of <50 meV and >80 meV, and further DFPT results proved the wide distribution of bond lengths at GB are the main factor for the emergence of local phonon modes. This research provides insights into the phonon-defect relation and would be of importance in the design and application of polycrystalline materials.

Current opinion on the prospect of mapping electronic orbitals in the transmission electron microscope: State of the art, challenges and perspectives

“…The proposed approaches to map electronic states at high spatial resolution could be complemented with the plethora of signals now accessible within the aberration-corrected transmission electron microscope, such as phonons through vibrational EELS 103,104 and photons emitted from point defects through cathodoluminescence, 105 to refine even further the nature of atomic bonding and the properties of individual defects. Spatially resolved vibrational excitations using EELS, which have been evidenced down to atomic resolution, 106,107 at defects and interfaces [108][109][110][111][112][113] and single atoms, 114,115 could enable access to electron-phonon interactions 116 and to the directionality of phonon modes. Specifically, the vibrational anisotropy of oxygen atoms in SrTiO 3 , recently measured by momentum-selective darkfield vibrational EELS, 117 suggests that the directionality of phonon modes mapped at atomic resolution may contain information about the directionality of bonding between atoms, which relates to electronic orbitals.…”