MSE Seminar: Eric Hoglund, University of Virginia
Wednesday, February 1, 2023
Room 2110 Chemical and Nuclear Engineering Building
Understanding Local Interface Vibrations Through Structure and Bonding
As the length-scales of materials decrease, heterogeneities associated with interfaces approach the importance of the surrounding materials. The importance of interfaces, or interface-like planes, has led to extensive studies combining electron energy-loss spectroscopy (EELS) in a scanning transmission electron microscope (STEM) that relate local atomic structure to emergent electronic and magnetic properties of superlattices. Previous studies have focused on electronic characterization because the energy resolution limitations of STEM-EELS precluded losses of tens to hundreds of meV. The energy resolution of monochromated STEM is now less limiting, allowing for local characterization of lower-energy-loss electronic excitations and atomic vibrations in materials with local heterogeneities.
In this seminar I will show how high-spatial and high-spectral resolution of STEM-EELS has been used to correlate local atomic arrangements, local bonding, and local nonstoichiometry to local atomic vibrations. Understanding these relations has elucidated the cross-over from incoherent to coherent phonon transport in superlattices where the description of materials separated by closely spaced interfaces loses physical meaning.1Such crossovers span different length scales of phonon quantization based on hierarchical lattices, which are becoming increasingly relevant in the phononic state of functional topological oxides2and the optical properties in quantum applications.3We can also explore the inverse relation where some phonons behave as if there are interfaces in a crystal from which phonons are fundamentally derived. With knowledge of local interface phenomena we can then design interfaces and leverage local structure-chemistry-phonon physics to present properties that benefit the performance of materials.
Lastly, we will investigate the structure – chemistry – vibrational relations found in grain boundaries.4I will show that with the resolving power of a STEM we can differentiate building blocks of a grain boundary, like dislocations, and understand their unique contributions to the overall boundary.
1. Hoglund, E. R. et al. Emergent interface vibrational structure of oxide superlattices. Nature 601, 556–561 (2022).
2. Li, Q. et al. Subterahertz collective dynamics of polar vortices. Nature 592, 376–380 (2021).
3. Kavrik, M. S. et al. Emergence of distinct electronic states in epitaxially-fused PbSe quantum dot superlattices. Nat Commun 13, 6802 (2022).
4. Hoglund, E. R. et al. Direct Visualization of Localized Vibrations At Complex Grain Boundaries. Advanced Materials 2208920 (2023) doi:10.1002/adma.202208920.