Methodological Developments for Spectroscopy at Low Primary Energies and High Resolution

The implementation of aberration correction stands as an era-defining advancement in nanoscience, allowing electron microscopy to move beyond previous limits to routinely achieve sub-angstrom spatial resolution [1]. More recently, the integration of high-resolution monochromators has pushed electron energy loss spectroscopy (EELS) into the meV regime [2]. This transition has essentially transformed the microscope into a nanoscale synchrotron, enabling a series of unprecedented experimental milestones. These include the detection of atomic-level contrasts in vibrational signals [3] and the identification of spectral signatures from individual impurity atoms [4], alongside spatially resolved measurements of point and line defects in crystals [5]. Most recently, this synchrotron-in-a-microscope capability has extended to momentum-resolved measurements using nanoscale beams, ultimately allowing for the precise detection of magnons with a nanoscale probe [6]. Beyond these defining developments, low-voltage operation (even down to 1 kV) is highly sought-after for specific applications and to reduce knock-on damage in sensitive materials. Crucially, lowering the primary beam energy significantly increases inelastic scattering cross-sections, enhancing the signal-to-noise ratio for spectroscopy. This regime also provides a unique window into scattering physics such as resonant excitations, surface modes etc. Here, we discuss experimental methodologies for ultra-low-loss EELS at high spatial and momentum resolution, spanning recent developments in STEM-EELS and their supporting theoretical frameworks. We further explore the prospects of using high-resolution spectroscopy beyond phonon detection, proposing a combined experimental and theoretical approach for probing spin waves (magnons). Furthermore, we outline the conditions required to excite and detect these quasiparticles within the electron microscope including their energy-momentum dispersion, and detail acquisition geometries to achieve atomic-scale magnon EELS. Furthermore, we demonstrate how high-resolution (T)SEM instrumentation can be pushed toward the near-atomic limit for EELS mapping, proving that such performance can belie an otherwise simple and cost-effective platform. We extend these capabilities into the ultra-low voltage regime, down to 1 kV and below, enabling experiments that may take advantage of the very strong coupling between electrons and optical modes at these reduced primary energies [7] but also potentially paving the way to resonant EELS experiments, among other prospects [8].