Center for Functional Nanomaterials Seminar

The past decades have witnessed transformative advances in scanning transmission electron microscopy (STEM) instrumentation, including aberration correctors, cold field-emission guns, meV-resolution monochromators, pixelated detectors, and in-situ capabilities. Together, these developments have enabled nanoscale characterisation of quasiparticle excitations such as phonons¹, magnons², and plasmons³ with unprecedented spatial, energy, and momentum resolution. In particular, momentum-resolved mapping of vibrational modes¹,4, analogous to neutron scattering, has become accessible within the STEM-EELS framework. Despite these advances, most measurements are still performed under static conditions, limiting insight into the non-equilibrium behaviour of functional materials and devices. In this talk, I will demonstrate how the capabilities of the monochromated and aberration-corrected Nion UltraSTEM 100MC "HERMES", equipped with a high-stability Nion IRIS spectrometer and a Dectris ELA direct electron detector for EELS, can be exploited to perform quantitative analytical experiments that reveal the operating mechanisms of functional electronic devices. I will first present an on- and off-axis high-resolution EELS study of a yttrium iron garnet (Y3Fe5O12, YIG) based spintronic device under thermal stimulus. Using a cross-sectional lamella mounted on a heating chip, we investigated the temperature-dependent magnon–phonon coupling below Curie temperature. The evidence of magnon-phonon coupling below Curie temperature is consistent with previous Raman spectroscopy studies5, demonstrating the ability of monochromated STEM-EELS to probe collective excitations under operando thermal conditions. I will then discuss recent results on memristive devices obtained using atomically resolved STEM-EELS together with in-situ biasing experiments performed in a scanning electron microscope operated in transmission mode, referred to as transmission SEM ((T)SEM)6. Compared with conventional TEM and STEM, (T)SEM enables imaging at lower accelerating voltages (30 kV) and reduced electron doses (≤ 50 e?/Ų), thereby minimising beam-induced effects that can influence phase transformations and switching behaviour. Two case studies will be presented. First, vertical electrochemical metallisation (ECM) memristors comprising a two-dimensional MoSe2 solid electrolyte layer sandwiched between an electrochemically active Cu electrode and an inert Au electrode. Devices were investigated before and after multiple resistive switching cycles to assess device integrity and to directly image conductive filament formation and metal breakdown regions using high-resolution STEM-EELS. Second, I will present combined STEM imaging and in-situ biasing (T)SEM studies of perovskite SrFeO3 oxide memristors. These experiments reveal that resistive switching is governed by the formation and annihilation of oxygen-vacancy channels, which locally modify the oxygen stoichiometry and drive a reversible topotactic phase transformation. Specifically, the transition between the metallic SrFeO3 perovskite phase and the insulating SrFeO2.5 brownmillerite phase accounts for the bipolar switching behavior observed in the electrical I–V characteristics of these devices7. References: 1. Krivanek, O. L. et al. Vibrational spectroscopy in the electron microscope. Nature 514, 209–212 (2014). 2. Kepaptsoglou, D. et al. Magnon spectroscopy in the electron microscope. Nature 644, 83–88 (2025). 3. Ruthemann, G. Diskrete Energieverluste schneller Elektronen in Festkörpern. Naturwissenschaften 29, 648 (1941). 4. Hage, F. S., Radtke, G., Kepaptsoglou, D. M., Lazzeri, M. & Ramasse, Q. M. Single-atom vibrational spectroscopy in the scanning transmission electron microscope. Science (80-. ). 367, 11