Investigating Surface Plasmon‐Enhanced Local Electric Fields by EELS with tunable <60 meV Energy Resolution

Abstract

Recent improvements in energy resolution, enabled by the use of electron energy monochromators, have the potential to turn EELS into a tool able to provide quantitative information of localized surface plasmons (LSPs), such as damping effects in single particles and electron kinetics of single plasmon modes.[1] Crucial to the prospect of quantitative analysis of LSPs is the requirement that the experimental energy resolution must be better than the natural line width of the plasmon resonances, all the while retaining high enough signal‐to‐noise ratio to enable an accurate determination of the properties of interest.[1] The energy resolution of EELS is customarily determined by the full‐width at half‐maximum (FWHM) of the zero‐loss (ZL) peak. The plasmon resonances, lying in the low‐loss regime, often overlap with the broad tail of the ZL peak, blurring many spectral signatures of interest. Until now, several processing techniques had to be applied to overcome these issues, relying for instance on deconvolution algorithms [2] which can introduce artifacts [3] A new generation of electron monochromators now allows for high signal‐to‐noise ratios while varying the energy resolution controllably, down to the 10meV regime [4]. Here we present recent results aimed at spatially and spectrally resolving the plasmon resonances of individual plasmonic nanostructures and of functional plasmonic devices using a Cs‐corrected and monochromated Nion UltraSTEM 100MC (‘Hermes’) microscope with a nominal energy resolution of 10meV. Figure 1 shows spatially resolved LSP modes of two individual Ag particles with different shapes and different number of crystalline domains for the energy ranges indicated in the figure. Both shape and crystallinity appear to affect the plasmonic response. We show how the energy resolution, which also affects the attainable signal‐to‐noise ratio and dictates the required integration (exposure) time, can be conveniently set and tuned, depending on the inherent properties of the system of interest. Figure 2A shows as‐recorded EEL spectra taken from the centre of a Ag nanowire (inset) and showing a narrow bulk plasmon resonance at 3.85eV. We note that the FWHM of the peak does not decrease with decreasing energy resolution from 40meV to 16meV, meaning that 40meV must be below the natural line width of the resonance. In this context, we will discuss the prospect of not only characterizing bare metallic nanostructures, but of also interrogating chemically functionalized plasmonic nanostructures using EELS. We note that the accessible energy ranges (sub‐40 meV) also allows us to probe molecules adsorbed onto metal nanostructures. In the raw spectra in Fig. 2B taken from different locations within a sample, spectroscopic signatures of aromatic thiols chemisorbed onto multiple Ag nanoparticles are shown.[5]