Local bandgap and optoelectronic measurement using monochromated STEM-VEELS: fundamentals, challenges, and recent...

Optoelectronic characterization of local bandgap, dielectric response, and plasmon at the nanometer scale has become increasingly important for understanding and optimizing modern electronic, optoelectronic, and quantum materials. Conventional optical techniques generally provide spatially averaged information and are therefore limited in their ability to resolve localized electronic variations arising from defects, interfaces, strain fields, and quantum confinement. To overcome these limits, monochromated scanning transmission electron microscopy-valence electron energy loss spectroscopy (STEM-VEELS) has evolved from a qualitative low-loss spectroscopy technique into a quantitative nanoscale bandgap metrology platform. By analyzing low-loss inelastic scattering signals with a sub-nanometer electron probe and sub-100 meV energy resolution, STEM-VEELS enables the direct investigation of bandgap onsets, local density of states (LDOS), and interband transitions at the nanoscale, although the effective spatial resolution of these measurements can be substantially broader than the probe size because of inelastic-scattering delocalization.This review summarizes the fundamental principles and recent advances in monochromated STEM-VEELS for local bandgap measurement. Particular emphasis is placed on the physical origin of low-loss excitations, addressing critical challenges such as momentum transfer and spatial delocalization. Practical experimental strategies for rigorous zero-loss peak (ZLP) subtraction and bandgap onset determination are systematically discussed. Furthermore, major analytical artifacts-such as Cherenkov radiation, guided optical modes, and beam-induced damage-are critically reviewed alongside state-of-the-art mitigation strategies.Recent developments in off-axis and Bessel-aperture STEM-VEELS geometries, machine-learning-assisted spectral analysis, automated onset extraction, and uncertainty-aware bandgap mapping are highlighted. Representative applications involving quantum dots, wide-bandgap oxides, heterointerfaces, and two-dimensional materials demonstrate the unparalleled capability of STEM-VEELS to resolve spatially varying electronic structures. Finally, future perspectives, including operando spectroscopy, ultrafast VEELS, artificial intelligence-driven analysis, and SEM-REELS, are discussed, providing a robust framework for standardization and quantitative nanoscale bandgap metrology.