Electron Energy Loss Spectroscopy Spectrometer Research Articles

Advanced Analytical Electron Microscopy Techniques for Probing Individual PGM-Free Catalyst Sites

Hydrogen fuel cells currently utilize expensive platinum group metal nanoparticles as catalysts [1]. To reduce the cost of catalyst materials, large efforts to develop viable platinum group metal-free (PGM-free) catalyst materials, which often consist of single transition metal atoms embedded in nitrogen-doped graphitic carbon structures, are being undertaken [2]. Given the atomic-scale nature of the metal sites in these materials, their performance and stability are highly dependent on the local coordination environment and consequently electronic structure of the sites [3]. A detailed understanding of the local elemental composition and lattice structure of individual sites, and how these properties vary between sites, is therefore needed to further develop improved PGM-free catalyst materials. While many bulk techniques are available that can provide average properties of these sites, such as Mössbauer and X-ray absorption spectroscopies, techniques with high spatial resolution are needed to measure the properties of individual sites. As a result, scanning transmission electron microscopy (STEM) combined with electron energy-loss spectroscopy (EELS) and energy-dispersive X-ray spectroscopy (EDS) has proven an invaluable tool for characterization of these materials due to its ability to probe structural and electronic properties at the atomic scale [4]. However, combined with the instability of many metal sites under the electron probe, the quantity and type of information accessible by conventional STEM instrumentation and acquisition methods is limited. To better understand the local coordination environment and electronic structure of individual sites, as well as how these properties vary site to site, new techniques and hardware are therefore needed.Here, we demonstrate advancements in STEM-EELS and -EDS that allow the quality and quantity of spectroscopic information obtained from individual sites to be significantly improved over conventional method. First, we demonstrate direct real-space mapping of metal sites by using a direct electron detector in the EELS spectrometer. This increases the data acquisition rate and signal-to-noise ratio sufficiently to map metal atom positions at the atomic scale. In addition, we show how automated metal atom position identification and subsequent probe positioning allow the rate at which metal sites are probed to be increased by another order of magnitude while simultaneously increasing the quantity of data obtained from each site. Using EELS and EDS signals acquired in parallel, this automated acquisition method enables a wide range of metal site elements to be identified, as well as the local coordination environment around the sites to be determined. Combined with electronic structure measurements of the metal atoms themselves, this technique provides significantly more information about individual sites than conventional methods. The techniques presented here are therefore poised to significantly improve our understanding of the local coordination environment and electronic structure of individual PGM-free metal sites, as well how these properties vary site to site, accelerating development of these low-cost hydrogen fuel cell catalyst materials [5].

Challenges of Nanoscale Characterisation of Litihum-Based Energy Materials

The electric energy revolution is driven by battery technology. New chemistries and control systems enables the widespread use of portable electric devices, such as phones, computers, power tools and automobiles. The upward trend in battery performance relies on continuous research[1]. Scanning transmission electron microscopy (STEM) is a commonly used tool for nano-scale characterisation, as it allows for imaging as well as elemental and chemical analysis, when combined with energy dispersive spectroscopy (EDS) and electron energy loss spectroscopy (EELS)[2], [3]. EELS is a particularly useful technique, as the near-edge structure of the ionization edges often give chemical bonding information. However, the high energy electron beam used in STEM can damage materials[4]. This is especially problematic in the field of battery research. Low-Z elements such as lithium, carbon and oxygen, are particularly susceptible to knock-on damage[4]. Since these elements are common in lithium-ion batteries, understanding how the beam affects materials is critical to the analysis of the materials. Commercially pure chemical compounds (LiF, LiCl, Li2CO3), acquired from MilliporeSigma, are exposed to the microscope beam, under condition favorable for EELS analysis of the Lithium K-edge. An EELS spectrometer is used to serially acquire energy loss spectra of the materials. These time-sensing series show how the near edge structure of lithium evolve as the material is damaged by the beam. In this work, the abherrent effects of the electron microscope on lithium samples is made evident.[1] J. B. Goodenough and K.-S. Park, “The Li-Ion Rechargeable Battery: A Perspective,” Journal of the American Chemical Society, vol. 135, no. 4, pp. 1167–1176, Jan. 2013.[2] D. B. Williams, Transmission electron microscopy: a textbook for materials science, 2nd ed. New York: Springer, 2008.[3] J. I. Goldstein et al., Scanning Electron Microscopy and X-Ray Microanalysis a Text for Biologists, Materials Scientists, and Geologists. Boston, MA: Springer US, 1992.[4] R. F. Egerton, “Mechanisms of radiation damage in beam-sensitive specimens, for TEM accelerating voltages between 10 and 300 kV,” Microsc. Res. Tech., vol. 75, no. 11, pp. 1550–1556, Nov. 2012. Figure 1

Spectroscopic coincidence experiments in transmission electron microscopy

We demonstrate the feasibility of coincidence measurements on a conventional transmission electron microscope, revealing the temporal correlation between electron energy loss spectroscopy (EELS) and energy dispersive X-ray (EDX) spectroscopy events. We make use of a delay line detector with ps-range time resolution attached to a modified EELS spectrometer. We demonstrate that coincidence between both events, related to the excitation and deexcitation of atoms in a crystal, provides added information not present in the individual EELS or EDX spectra. In particular, the method provides EELS with a significantly suppressed or even removed background, overcoming the many difficulties with conventional parametric background fitting as it uses no assumptions on the shape of the background, requires no user input and does not suffer from counting noise originating from the background signal. This is highly attractive, especially when low concentrations of elements need to be detected in a matrix of other elements.

From a physicist's toy to an indispensable analytical tool in many fields of science: A personal view of the leading contribution of Ondrej Krivanek to the spectacular successes of EELS spectroscopy in the electron microscope

This contribution aims at reporting, from the subjective point of view of a witness based in Orsay, the fundamental role of Ondrej Krivanek in the spectacular emergence of EELS (Electron Energy-Loss Spectroscopy) as a key tool in analytical electron microscopy. In this regard, he has successively designed and built while he was at Gatan, serial EELS spectrometers, parallel EELS spectrometers and post-column energy filters which have been fitted to many different (S)TEM columns installed around the world. More recently the implementation of monochromators on the NION dedicated STEM together with the realization and performance of aberration correctors (which lie out of the scope of the present review), have placed the most advanced instrumental tool in the hands of continuously increasing populations of users in many domains of materials science and in life sciences. Furthermore, the impact of Ondrej Krivanek has spread widely beyond his technical achievements into that of a highly respected organizer of workshops, bringing together at regular intervals, all the experts from around the world and building up a real community of scientists.

Cs-corrected scanning transmission electron microscope investigations of an ODS ferritic steel for fusion applications

Over the last few years, oxide dispersion strengthened (ODS) ferritic steels have emerged as one of the major materials for fusion reactors. Despite the progress made in their technology, currently available properties still do not meet all the expectations. As a result these steels remain the subject of extensive research, recently centred on a possible improvement due to their nanometric engineering. In particular, technologies are developed for grain size refinement down to nanometres and strengthening by nanooxides. This, in turn, calls for nanoscale investigations of the microstructures, which can be efficiently carried out only with the use of high-resolution transmission electron microscopy and spectroscopy. The present paper illustrates the results of investigations of a model alloy of ODS steel using a recent high-resolution scanning transmission electron microscope equipped with a spherical abberation Cs-correction system, energy-dispersive x-ray spectroscopy and electron energy loss spectroscopy spectrometers. It allows bright field on transmitted electron imaging and secondary electron imaging, as well as high-angle annular dark field imaging, the so-called Z-contrast imaging. The results obtained in dark field prove that modern microscopy techniques are a necessity to provide new information on the nanostructure of the ODS steels relevant to the understanding and shaping of their properties.

HREELS characterisation of polymer surfaces using new generation spectrometers

Spectra of polystyrene film surfaces achieving a resolution of 2.55 meV (20.4 cm −1) were recorded using new generation high resolution electron energy loss spectroscopy spectrometers. Spectra show a high number of characteristic energy losses corresponding to infrared- and Raman-active modes. Mechanisms involved in the electron interaction with the film surface were unraveled through their energy dependence. Impact is the predominant mechanism revealing an analysis depth of some angstroms. Absence of dipole mechanisms is related to a surface conformation where phenyl rings are mostly perpendicular to the substrate.

First Results of a Monochromized 200KV TEM

FEI is currently finishing the construction of a monochromized 200kV (S)TEM which aims at 0.1 eV resolution in Electron Energy Loss Spectroscopy (EELS). This requires improvement of the 200kV supply, improvement of the EELS spectrometer, and the incorporation of an electron beam monochromator. This microscope will be delivered to the National Centre for High Resolution Electron Microscope at the University of Delft.The present 200kV supply contributes 0.2 eV to the energy resolution because of its instabilities, drift and high frequency ripple. in order to reduce this, mechanical as well as electrical damping elements are being added, and sources of cross-talk between the high frequency generator and the high tension are eliminated. in this paper, we present preliminary results obtained with a standard 200kV supply; we expect to present results of the improved 200kV supply at the conference.The resolution of a present-day standard 200kV spectrometer is now typically 0.4 eV. in order to improve on this, GATAN has developed a HR-EELS spectrometer for the monochromized TEM.

Comparison of STEM EELS Spectrum Imaging vs EFTEM Spectrum Imaging

Abstract Electron energy-loss spectroscopy (EELS) in the transmission electron microscope (TEM) analyzes the energy distribution of the probe electrons after they have lost energy within the sample. The resultant energy-losses are characteristic of elemental, chemical, and dielectric properties and are typically measured in one of two ways. Parallel-detection EELS spectrometers (PEELS) acquire large energy ranges of the energy-loss spectrum simultaneously for rapid acquisition of spectral data at a single area. In contrast, the energy filtering TEM (EFTEM) acquires only a single energy band at once, but does so for thousands or even millions of image pixels simultaneously. Spectrum imaging (SI) involves acquisition of detailed spectroscopic data sufficient for rigorous analysis at each pixel in a digital image. (Fig. la) A STEM EELS spectrum image “data cube” can be acquired by stepping a focused electron probe to each pixel and filling the spectrum image one spectrum at a time.

EFTEM and STEM EELS Spectrum Imaging

Abstract Electron energy-loss spectroscopy (EELS) in the transmission electron microscope (TEM) is a powerful technique that analyzes the inelastic scattering distribution of the fast TEM electrons after they have lost energy within the sample. The resultant energy-losses are characteristic of elemental, chemical, and dielectric properties and are typically measured in one of two ways. Parallel-detection EELS spectrometers (PEELS) acquire spectral data over a large range of energy-loss simultaneously for rapid acquisition of spectral data at a single point. In contrast, the energy filtering TEM (EFTEM) acquires only a single energy band at once, but does so for thousands or even millions of image pixels simultaneously. Spectrum-imaging concerns the acquisition of spectroscopic data of sufficient detail for rigorous analysis at each pixel in a digital image. (Fig. 1) A STEM EELS spectrum image “data cube” can be acquired by stepping a focused electron probe to each pixel and filling the spectrum image one spectrum at a time.

Electron Energy-Loss Spectroscopy (EELS) of Cerium and Uranium Oxidation States

Abstract EELS is a powerful analytical tool for exploring the local solid state environment in materials. There is an extensive literature on EELS analysis of the transition elements and light elements, yet few EELS data exist for heavy elements with multiple oxidation states. We report here on EELS studies of oxidation state influences on near-edge structure in cerium and uranium compounds. All materials analyzed for this study were embedded in low viscosity epoxy and TEM specimens prepared using ultramicrotomy. The thin sections (∼50 nm thick) were analyzed using a JEOL 2010 (200kV) TEM operated with a LaB6 filament and equipped with a GAT AN 666 parallel EELS spectrometer. EELS data were collected in image mode at 105X magnification with a collection semi-angle of ∼ 100 milliradians at a dispersion of 0.1 eV/channel. The full width at half maximum of the raw (unprocessed) zero-loss peak was 0.7 eV. Peak positions were calibrated by applying a known voltage to a drift tube.

A study of the interfacial microstructure and composition of CVD diamond films — effect of neon

High quality and less defective diamond films grown by microwave plasma chemical vapor deposition (CVD) on silicon were prepared and characterized by transmission electron microscopy (TEM), X-ray diffraction, Auger electron spectroscopy (AES) and Raman spectroscopy. Both plan-view and cross-sectional TEM specimens were made from diamond films deposited in ethane/neon/hydrogen plasma mixtures. It is found that the addition of neon can change the surface morphology of the diamond film from triangular {111} to square {100} facets. The results of TEM, electron energy loss spectroscopy (EELS) and AES indicate that a high quality of diamond film can be achieved through an ionization process of neon in methane/hydrogen plasma. The EELS and energy dispersive spectrometer, which in-situ combined with TEM equipment, were used to analyze the chemistry of interfacial layer. The diamond films prepared in this study were frequently grown on an amorphous carbon layer which initially formed on the surface of silicon substrate. The interfacial layers may be strongly dependent on substrate pretreatments. For instance, the diamond nucleation rate could be enhanced by modifying the surface of the substrate material using a process that ultrasonically stimulated cavitation-erosion in an aqueous suspension of diamond dusts. The diamond crystallites growing on this surface were of a much more uniform size than was achieved after the more conventional lapping process.

Core excitation studies of poly(ethylene terephthalate) and molecular analogues by EELS

The near-edge region of core ionization edges can be used as a probe of the local bonding and structure of atoms excited by incident radiation, such as in electron energy loss spectroscopy (EELS) or x-ray absorption techniques. Recent installation of efficient parallel EELS spectrometers in ATEMs promotes study of these transitions in radiation sensitive materials such as polymers. To study relationships between poly(ethylene terephthalate) [PET] structure and EELS transitions, we compared polymer spectra with molecular analogues.The PET examined (from Scientific Polymer Products) was cut using a diamond knife and a Reichert-Jung Ultracut E ultramicrotome with FC4E cryoattachment. A JEOL 2000FX TEM with a LaB6 filament and a Gatan parallel-EELS system was used to examine PET sections on holey carbon. The gas phase EELS of molecules (from Aldrich, Matheson) was performed under inelastic scattering conditions as described previously.

Radiation damage characteristics of silicon oxynitride ceramic

Silicon Oxynitride is a ceramic that often forms during sintering of Si3N4, as a result of reactions with the sintering aid or from oxygen adsorbed on the surface of the powder particles. It is an orthorhombic crystal in which Si is tetrahedrally coordinated, and it is quite susceptible to electron irradiation effects. We present here the first quantitative observations of radiation effects.Time-resolved Electron Energy Loss Spectroscopy (EELS) was used to follow the changes in elemental concentration with irradiation time in crystals of Si2N2O during electron-irradiation at 100kV. The experiments were performed in a Philips EM400ST microscope fitted with a field emission source using a focussed probe and a Gatan 666 Parallel EELS spectrometer. An LN2 cooled specimen holder was used to minimize contamination. Specimens were made from a ceramic containing plate-type Si2N2O microcrystals with long edges parallel to [010], described earlier. The special software for time resolved spectrum acquisition was developed in this laboratory. The Si-L, O-K and N-K edges were acquired as a function of irradiation time at known current density and then individually processed and numerically integrated over a 100eV interval, beginning at edge onset.

High resolution electron energy loss spectroscopy: Explored regions and the frontier

High-resolution electron energy loss spectroscopy (EELS) is rapidly being developed into one of the more useful techniques for probing physical and chemical phenomena at surfaces. Key elements of the technique include high surface sensitivity, the fact that it is a molecular sensitive rather than an atom sensitive spectroscopy, and the direct relationship that exists between experimental data and important surface parameters. New applications of EELS are currently being explored; these applications will be the primary focus of this paper. Recent work which combines EELS measurements with modeling of surface vibrations based on lattice dynamical calculations has shown that this approach can provide useful insight into structural parameters at surfaces. Other experiments aimed at refining our understanding of EELS scattering mechanisms and the selection rules that govern them have yielded some new and unexpected results which will be discussed. Finally, development of a new generation of EELS spectrometers should eventually produce improvements of two to three orders of magnitude in signal levels as well as more precise control of instrument response. These instruments will open up new applications of the EELS technique in areas including the study of technical surfaces and real time studies of surface reaction kinetics. The new spectrometers will also permit more precise studies of fundamental surface phenomena including electron scattering mechanisms.

New lens system for surface vibrational spectroscopy at high impact energies

The design, characterization, and performance of a new lens system which permits high-resolution electron energy-loss spectroscopy (EELS) studies at high energies is described. The lens system can be adapted to most existing EELS instruments and extends the impact energy range to 300 eV. Electron ray-tracing techniques are used to characterize operating modes of one of the more common EELS optics designs as well as the new lens configuration. The results presented in this paper demonstrate some limitations of matrix optics approaches and the applicability of electron ray-tracing techniques for characterizing the transmission properties, angular profiles, image sizes, and operating modes of EELS spectrometers.