A 10-minute read
Electron Energy Loss Spectroscopy (EELS)* answers the question of how much energy electrons lose after interacting with the sample. These insights are then used for the characterization of a vast class of materials. The technique was developed in the 1940s by J. Hillier and R.F. Baker. However, it took another 50 years and a significant step forward in microscopy instrumentation for it to claim its rightful place today. A lot has happened over the almost century-long history of EELS. We decided to sieve through the extensive literature on the subject and select some of the most interesting publications. Take a look at our picks!
An electron energy loss (EEL) spectrum can be seen as the fingerprint of a material. Spanning an energy range from millielectronvolts to kiloelectronvolts, it contains information on the nature of chemical bonds, structural configurations, dielectric properties and band structures, electrical, and even thermal properties of the sample. Nowadays the method is so advanced that a whole zoo of quasiparticles routinely leaves its footprints on the EEL spectra, and a lot of effort is being dedicated to improve the visibility of these features.
Scientists have been developing the technique for almost a century now, leveraging advancements in electron optics and detector technologies. For example, hybrid-pixel direct electron detectors perform exceptionally well when combined with EEL spectrometers, enabling higher speed and experimental flexibility. We took a look at the immense body of work on the subject, selecting some of the early seminal works on the technique development, some recent articles that describe the state-of-the-art instrumentation, and one exotic publication with a "precious" turn-up.
O.L. Krivanek, C.C. Ahn, R.B. Keeney, 1987
Recent Kavli Prize winner Ondrej Krivanek, contributed significantly to the development of electron energy loss spectrometers during his years working at Gatan, supporting the decade-long success of post-column spectrometers.
This work describes how the introduction of three quadrupoles lenses in a post-column spectrometer, a linear array of photodiodes with a YAG scintillator, and a computer-controlled data transfer interface, dramatically increased the operating efficiency and the flexibility of the instrument.
In times when photographic films were still a reasonable option, this work enabled acquisition times of fractions of a second, much appreciated by many Ph.D. students.
O.L. Krivanek, A.J. Gubbens, N. Dellby, 1991
An EEL spectrum surely offers a lot of information, but the expectation, when using a microscope, is to focus on images. By 1991 parallel EELS was an established technique, so Gatan devoted its R&D resources to combining spectroscopic and imaging information. This work presents the concepts behind their first energy filter (GIF, in the future). For both TEM and STEM modes, it unlocked the possibility of selecting an energy window and generating images using only electrons that have undergone a defined interaction with the sample. Suddenly, elemental mapping became available, and (S)TEM images had to be printed in colors!
The need to correct several orders of aberrations to obtain undistorted and sharp images caused a considerable increase in the complexity of the electron-optics design and with it the complexity of the routines to optimize the spectrometer alignments. Luckily the user was assisted by powerful software that did its best to optimize the spectrometer's performance.
F. Kahl, V. Gerheim, M. Linck, et al., 2019
From an electron optical point of view, the design of an energy filter requires similar analytical and mathematical methods with respect to the design of a multipole aberration corrector.
Given the extensive experience in the field, and their prestigious academic background, the team of CEOS GmbH undertook the task of designing a novel post-column imaging filter, with the aim of improving the performances of the products commercially available.
Stability, reproducibility, and ease of use were as well primary goals of this project, as presented in this article from 2019, showcasing the complete characterization of the CEOS Energy-Filtering and Imaging Device (CEFID).
T. Nakane, A. Kotecha, A. Sente, et al., 2020
Not only Materials Science but also Life Science benefits from the use of imaging filters, even if this technique transcended the osmotic barrier that separates the two communities only in recent years. In presence of thick amorphous substrates, the image degradation caused by inelastic scattering can be mitigated by zero-loss energy-loss filtering, and this, together with other improvements in their setup, allowed the group from the MRC Laboratory of Molecular Biology in Cambridge, and collaborators, to push the boundaries for single-particle cryoEM.
Using a cold field emission gun, a next-generation direct electron camera, and a Selectris energy filter, developed by Thermo Fisher Scientific, using the experience that led to the energy filter described in the previous article, the group managed to obtain a reconstruction for Apoferritin with an impressive resolution of 1.22 Å.
F. Carbone, B. Barwick, O. Kwon, et al., 2009
Nature is not always in its ground state: materials are subject to phase transitions, chemical reactions are not static, and heat and sound propagate. To really study physical systems in their natural form, time-resolved techniques are necessary, where samples are driven into excited states by an impulsive stimulus, and observed while they relax.
In the laboratories of Nobel Prize winner Ahmed Zewail, known as "Femtoland", a couple of microscopes had been modified integrating ultrafast lasers, with the intent of studying reversible physical phenomena in a stroboscopic manner, and several standard TEM techniques have been extended to the 4th dimension.
In this article, by Carbone et al., Ultrafast EELS is used to study the behavior of a sample of graphite, after being hit by a short laser pulse. Monitoring the intensity of the plasmon peak corresponding to the interaction across layers, it’s possible to observe the signature of an ultrafast phase transition from graphite towards the most famous of the carbon allotropes, diamond.
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*Everybody who’s interested in the details of this topic, should read the “bible” in the field: Electron energy-loss spectroscopy in the electron microscope,
R.F. Egerton, 2011