Cathodoluminescence, light injection and EELS in STEM: From comparative to coincidence experiments

TL;DR

This article surveys temporal coincidence approaches in SEM/STEM electron spectroscopies to access ultrafast nano-optical dynamics with nanometer spatial resolution. It outlines how coordinated schemes across CL, EELS, PINEM/EEGS, and related photon/electron detections reveal radiative versus non-radiative pathways and enable nanothermometry, by aligning energies and timing through monochromation, fast detectors, and engineered light delivery. The review details experimental setups, key synchronization modalities (photon-photon, photon-electron, and injection-EELS), and advances that allow direct, time-resolved correlations between different excitations at the nanoscale. These methods promise deeper insight into exciton dynamics, plasmonic interactions, and temperature-dependent processes, with potential for standardization and wider adoption in nano-optics research.

Abstract

Electron spectroscopy implemented in electron microscopes provides high spatial resolution, down to the atomic scale, of the chemical, electronic, vibrational and optical properties of materials. In this review, we will describe how temporal coincidence experiments in the nanosecond to femtosecond range between different electron spectroscopies involving photons, inelastic electrons and secondary electrons can provide information bits not accessible to independent spectroscopies. In particular, we will focus on nano-optics applications. The instrumental modifications necessary for these experiments are discussed, as well as the perspectives for these coincidence techniques.

Figures (11)

• Figure 1: Sketch of SEM/STEM for spectroscopies linked to inelastic electron light scattering. The fundamental parts of the system are an electron source, focusing, scanning and projection optics, a light collection/injection system and an electron spectrometer. Nano-optics synchronized experiments can be performed in both STEM or SEM microscopes.
• Figure 2: Correlative EELS/CL experiments on plasmonic gold nanotriangles. (a) Spectra and fitted maps for the dipolar, tip (T) mode. (b) Same for the hexapolar, side (S) mode. Note the absence of fittable S peak in CL. Adapted from Losquin2015.
• Figure 3: Comparison between EEGS, CL, EELS for whispering gallery modes in a silica sphere of diameter 4 µm. Data have been acquired at the same point close to the sphere surface (marked $R_e$ in the ADF image in the inset). Adapted with permission from Auad2023.
• Figure 4: Comparison between CL and EELS for a WS$_2$ monolayer encapsulated in hBN (a) An EELS and CL spectrum of a hBN/Wse2/hBN heterostructure. In the EELS spectrum, the exciton $X_A$, $X_B$ and $X_C$ and the excited exciton $X_A^*$ are seen. In the CL spectrum the neutral exciton $X_A$ and the charged exciton (trion) $X^-$ emission are observed. (b) Filtered CL map at the trion energy. (c) line profiles along the arrows marked “c” (upper) and “e” (lower). Adapted with permission from Bonnet2021.
• Figure 5: Experimental setup for time-resolved and synchronized experiments in electron microscopes and details about the necessary instrumentation for uses with a continuous or a pulsed electron source.