Enabling electron-energy-loss spectroscopy at very high energy losses: An opportunity to obtain x-ray absorption spectroscopy–like information using an electron microscope

Abstract

Electron-energy-loss spectroscopy (EELS) with an electron microscope and X-ray absorption spectroscopy (XAS) with a synchrotron are techniques for material characterization, both of which are based on exciting core electrons. Both techniques have a similar energy resolution, but while the spatial resolution of EELS can drop to atomic scales, the spatial resolution of XAS is typically limited to micrometer scales. Yet, XAS is commonly the preferred technique for analysis of the extended fine structure of ionization edges, mainly thanks to the excellent signal-to-noise ratio and the large range of ionization energies (from ∼5 to ∼40 keV) that can be probed at synchrotron end stations. In contrast, EELS is traditionally limited to ionization energies of <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"> <a:mo>≲</a:mo> </a:math> 2 keV because electrons in the beam that lose more than 2 keV will be too distant from the operating energy of the electron microscope. Chromatic effects in the postsample lenses allow only some of such energy-loss electrons to reach the EELS detector, as the latter electrons will either suffer from being strongly defocused or will not make it to the detector at all. In this paper, we present results from our novel Iliad EELS spectrometer, which offers a greatly increased range of ionization energies up to 30 keV. We achieve this vast increase by carefully controlling the optics of our electron microscope and carefully matching the optics of our EELS spectrometer to it, such that all the chromatic effects are removed or compensated. We exemplify its performance by recording EELS near-edge fine structure (ELNES) of the <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"> <c:mi>Zr</c:mi> </c:math> L-edges at 2.3 keV, extended fine structure EELS (EXELFS) of <e:math xmlns:e="http://www.w3.org/1998/Math/MathML" display="inline"> <e:mi>Cu</e:mi> </e:math> K-edge at ∼9 keV, and EELS of the Mo K-edge at 20 keV and <g:math xmlns:g="http://www.w3.org/1998/Math/MathML" display="inline"> <g:mi>Sb</g:mi> </g:math> K-edge at ∼30 keV. We benchmark our data against near-edge and extended fine structure X-ray absorption (XANES and EXAFS) data, and we quantitatively analyze the <i:math xmlns:i="http://www.w3.org/1998/Math/MathML" display="inline"> <i:mi>Cu</i:mi> </i:math> K-edge EXELFS, demonstrating the capability to determine element-specific bond lengths and to distinguish different oxidation states such as metallic <k:math xmlns:k="http://www.w3.org/1998/Math/MathML" display="inline"> <k:mi>Cu</k:mi> </k:math> , <m:math xmlns:m="http://www.w3.org/1998/Math/MathML" display="inline"> <m:msub> <m:mi>Cu</m:mi> <m:mn>2</m:mn> </m:msub> <m:mrow> <m:mrow> <m:mi mathvariant="normal">O</m:mi> </m:mrow> </m:mrow> </m:math> , or <p:math xmlns:p="http://www.w3.org/1998/Math/MathML" display="inline"> <p:mrow> <p:mi>Cu</p:mi> <p:mi mathvariant="normal">O</p:mi> </p:mrow> </p:math> on a submicrometer scale.

Topics & Concepts