Revealing the Intricacies of Vibrations in Complex Structures using Monochromated Electron Energy-loss Spectroscopy
Abstract: 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.
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 are suppressed, this has enabled vibrational excitations to be mapped with atomic-resolution, far beyond current optical techniques. [1] With such resolution we have unveiled the intricacies of oxide interfaces and grain boudnaries. [2,3]
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 EELS [4] 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. [5,6] 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. [7]
[1.] Hage, F. S., Kepaptsoglou, D. M., Ramasse, Q. M. & Allen, L. J. Phonon Spectroscopy at Atomic Resolution. Phys. Rev. Lett. 122, 016103–5 (2019).
[2.] Hoglund, E. R. et al. Emergent interface vibrational structure of oxide superlattices. Nature 601, 556–561 (2022).
[3.] Hoglund, E. R. et al. Direct visualization of localized vibrations at complex grain boundaries. Adv. Mater. 35, 2208920 (2023).
[4.] Radtke, G. et al. Polarization Selectivity in Vibrational Electron-Energy-Loss Spectroscopy. Phys. Rev. Lett. 123, 256001 (2019).
[5.] Hoglund, E. R. et al. Non-equivalent Atomic Vibrations at Interfaces in a Polar Superlattice. Advanced Materials 2402925 (2024) doi:10.1002/adma.202402925.
[6.] Yan, X. et al. Real-Space Visualization of Frequency-Dependent Anisotropy of Atomic Vibrations. Preprint at https://doi.org/10.48550/arXiv.2312.01694 (2023).
[7.] 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.
Bio: Eric obtained his PhD from University of Virginia where he focused on investigating interfaces and phase transformations using imaging, diffraction, and electron energy-loss spectroscopy (EELS) in a transmission electron microscope (TEM) with high-spatial resolution. He continued with a postdoctoral position at UVA and ORNL where he used time-domain optical spectroscopies like Brillouin zone scattering and time-domain thermal reflectance, along with TEM to understand phonon propagation in materials. Eric is now joining as a Staff Scientist at ORNL where he will continue using vibrational EELS and 4D-STEM techniques to understand the complex interplay between atomic arrangements, bonding, and phonons at heterogeneities in quantum materials.
About the QSQM: The EFRC-QSQM center aims to develop and apply nontrivial quantum sensing to measure and correlate local and nonlocal quantum observables in exotic superconductors, topological crystalline insulators, and strange metals. The center is led by the University of Illinois at Urbana-Champaign in partnership with the University of Illinois at Chicago and the SLAC National Accelerator Laboratory.