Energy-Resolved Localized Surface Plasmon Resonances With Non-Monochromated STEM-EELS Assisted by Multivariate Analysis.

Localized surface plasmon resonances (LSPRs) govern the optical response of plasmonic nanomaterials and are central to applications in sensing, spectroscopy, therapeutics, and photocatalysis. Nanoscale mapping of low-energy plasmon modes can be characterized using scanning transmission electron microscopy/electron energy-loss spectroscopy (STEM-EELS). However, achieving sufficient energy resolution to extract low-energy LSPR signals typically requires advanced instrumentation, such as a monochromator and/or a cold-field emission gun. Here, an accessible STEM-EELS approach is developed for resolving LSPRs using a conventional non-monochromated TEM. Silica-coated gold nanorods (Au@SiO2 NRs) are investigated using low-dose, low-keV STEM-EELS combined with Richardson-Lucy deconvolution and principal component analysis, reducing the average full width at half maximum of the zero-loss peak to 0.46 eV and enabling the resolution of closely spaced LSPRs without a monochromator or other specialized hardware. Successful resolution of the longitudinal and transverse LSPR modes enables the visualization of the electromagnetic field distribution around the Au@SiO2 NRs. Three-dimensional electron tomography and finite element method simulations further validate the experimentally resolved plasmon distributions and field localization. This work establishes a broadly accessible strategy for high energy resolution plasmon mapping using standard STEM instrumentation, extending the capabilities of conventional electron microscopy platforms.