Revealing the Intricacies of Vibrations in Complex Structures Using Polarization Selective Electron Energy-Loss Spectroscopy

Vibrational electron energy-loss spectroscopy (EELS) in a monochromated scanning transmission electron microscope (STEM) has proven to be a useful tool to understand how local heterogeneity impacts atomic vibrations. Such vibrations are typically measured with optical spectroscopies that have superior spectral resolution. However, the power of STEM-EELS is to enable high-spatial resolution while maintaining spectral resolvability, which provides a local understanding of defect vibrations.<br/><br/>Much effort has been put into decreasing the energy resolution gap between optical spectroscopies and monochromated STEM in an effort to observe more detailed information about material’s vibrational responses. Combined with the advent of off-axis EELS, where the delocalized dipole excitations of the optic axis are deflected away from the EELS collection aperture, this has enabled vibrational excitations to be mapped with atomic-resolution, far beyond current optical techniques.¹<br/><br/>However, optical spectroscopies also offer the ability to polarize the incident and collected light, which provides details about vibration eigenvectors. While directional polarization selectivity has been examined in aloof EELS² it has been overlooked in off-axis EELS. Recent efforts have demonstrated that the direction of the off-axis deflection in reciprocal space directly enables sensitivity to anisotropies in the vibrational eigenvectors due to their projective property in the scattering probability.^3,4 Here we demonstrate high-spatial-resolution polarization selectivity in vibrational EELS and its application to spatially varying anisotropic vibrations in nitride interfaces and complex oxides heterostructures. By operating at nanometer-scale resolution, we gain mixed-space insights into the behavior of unique vibrations. We also demonstrate using the polarization selective off-axis geometry and high-momentum-resolution EELS to study the intricacies of unique modes from materials symmetry-vibration relations.⁵<br/><br/>1. Hage, F. S., Kepaptsoglou, D. M., Ramasse, Q. M. & Allen, L. J. Phonon Spectroscopy at Atomic Resolution. <i>Phys. Rev. Lett.</i> <b>122</b>, 016103–5 (2019).<br/>2. Radtke, G. <i>et al.</i> Polarization Selectivity in Vibrational Electron-Energy-Loss Spectroscopy. <i>Phys. Rev. Lett.</i> <b>123</b>, 256001 (2019).<br/>3. Hoglund, E. R. <i>et al.</i> Non-equivalent Atomic Vibrations at Interfaces in a Polar Superlattice. <i>Advanced Materials</i> 2402925 (2024) doi:10.1002/adma.202402925.<br/>4. Yan, X. <i>et al.</i> Real-Space Visualization of Frequency-Dependent Anisotropy of Atomic Vibrations. Preprint at https://doi.org/10.48550/arXiv.2312.01694 (2023).<br/>5. Vibrational EELS experiments were supported by the U.S. Department of Energy, Office of Basic Energy Sciences (DOE-BES), Division of Materials Sciences and Engineering, and were performed at the Center for Nanophase Materials Sciences, (CNMS), which is a DOE Office of Science User Facility.