In Situ Electron Energy-Loss Spectroscopy in Liquids

TL;DR

This paper investigates the capabilities of electron energy-loss spectroscopy (EELS) in liquids during in situ STEM, demonstrating its potential for real-time chemical analysis and understanding nanomaterial reactions in native environments.

Contribution

It demonstrates the use of valence EELS to determine local electronic properties and monitor chemical reactions in liquids during in situ STEM, expanding analytical options in liquid environments.

Findings

Valence EELS can determine local electron density, optical gap, and liquid thickness.

Liquids follow the free-electron model similar to solids.

EELS signals below optical gap effectively detect metallic copper clusters.

Abstract

In situ scanning transmission electron microscopy (STEM) through liquids is a promising approach for exploring biological and materials processes. However, options for in situ chemical identification are limited: X-ray analysis is precluded because the liquid cell holder shadows the detector, and electron energy-loss spectroscopy (EELS) is degraded by multiple scattering events in thick layers. Here, we explore the limits of EELS for studying chemical reactions in their native environments in real time and on the nanometer scale. The determination of the local electron density, optical gap and thickness of the liquid layer by valence EELS is demonstrated. By comparing theoretical and experimental plasmon energies, we find that liquids appear to follow the free-electron model that has been previously established for solids. Signals at energies below the optical gap and plasmon energy of the liquid provide a high signal-to-background ratio regime as demonstrated for LiFePO4 in aqueous solution. The potential for using valence EELS to understand in situ STEM reactions is demonstrated for beam-induced deposition of metallic copper: as copper clusters grow, EELS develops low-loss peaks corresponding to metallic copper. From these techniques, in situ imaging and valence EELS offer insight into the local electronic structure of nanoparticles and chemical reactions.