Exploring the capabilities of monochromated electron energy loss spectroscopy in the infrared regime

Jordan A Hachtel,Juan Carlos Idrobo,Andrew R Lupini

doi:10.1038/s41598-018-23805-5

Jordan A Hachtel, Juan Carlos Idrobo + Show 1 more

Open Access

https://doi.org/10.1038/s41598-018-23805-5

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Journal: Scientific Reports	Publication Date: Apr 4, 2018
Citations: 70	License type: open-access

Affiliation: Oak Ridge National Laboratory

Abstract

Monochromated electron energy loss spectroscopy (EELS) is one of the leading techniques to study materials properties that correspond to low (<5 eV) energy losses (i.e. band-gaps, plasmons, and excitons) with nanoscale spatial resolution. Recently a new generation of monochromators have become available, opening regimes and unlocking excitations that were previously unobservable in the electron microscope. The capabilities of these new instruments are still being explored, and here we study the effect of monochromation on various aspects of EELS analysis in the infrared (<1 eV) regime. We investigate the effect of varying levels of monochromation on energy resolution, zero-loss peak (ZLP) tail reduction, ZLP tail shape, signal-to-noise-ratio, and spatial resolution. From these experiments, the new capabilities of monochromated EELS are shown to be highly promising for the future of localized spectroscopic analysis.

Highlights

The full-width half-maximum (FWHM) of the zero-loss peak (ZLP) is generally taken as the figure of merit for describing the energy resolution of an electron microscope
Thermionic emission guns or Schottky guns have a ZLP with a larger FWHM, usually around 700 meV11,26,27. While these widths are small compared to the operating voltage of the STEM, they set a limit for the energy resolution of energy loss spectroscopy (EELS), and hinder the ability to distinguish between peaks separated by less than those values – hundreds of meV
The FWHM is measured for each spectrum, and in this example, monochromation improves the energy resolution from 287 meV for the non-monochromated beam down to 22 meV

Summary

Introduction

Since the ZLP background in both spectra clearly possess a positive second derivative on the logarithmic scale, a standard exponential fit, I(∆E) = e−a⋅∆E−b, cannot accurately fit the background. For a second order exponential to have a positive second derivative in log scale the coefficient of the second order term in the exponent would need to be positive, which would result in the expression diverging to infinity at high energies. A third order exponential is the lowest order exponential fit suitable for fitting the ZLP background in this energy regime.

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Results

Discussion

Conclusion