Electron Energy Loss Spectroscopy Spectra Research Articles

Quasinormal mode theory and modelling of electron energy loss spectroscopy for plasmonic nanostructures

Understanding light–matter interactions using localized surface plasmons (LSPs) is of fundamental interest in classical and quantum plasmonics and has a wide range of applications. In order to understand the spatial properties of LSPs, electron energy loss spectroscopy (EELS) is a common and powerful method of spatially resolving the extreme localized fields that can be obtained with metal resonators. However, modelling EELS for general shaped resonators presents a major challenge in computational electrodynamics, requiring the full photon Green function as a function of two space points and frequency. Here we present an intuitive and computationally simple method for computing EELS maps of plasmonic resonators using a quasinormal mode (QNM) expansion technique. By separating the contribution of the QNM and the bulk material, we give closed-form analytical formulas for the plasmonic QNM contribution to the EELS maps. We exemplify our technique for a split ring resonator, a gold nanorod, and a nanorod dimer structure. The method is accurate, intuitive, and gives orders of magnitude improvements over direct dipole simulations that numerically solve the full 3D Maxwell equations. We also show how the same QNM Green function can be used to obtain the Purcell factor (and projected local density of optical states) from quantum dipole emitters or two level atoms, and we demonstrate how the spectral features differ in general to the EELS spectrum.

Correlative theoretical and experimental investigation of the formation of AlYB14 and competing phases

The phase formation in the boron-rich section of the Al-Y-B system has been explored by a correlative theoretical and experimental research approach. The structure of coatings deposited via high power pulsed magnetron sputtering from a compound target was studied using elastic recoil detection analysis, electron energy loss spectroscopy spectrum imaging, as well as X-ray and electron diffraction data. The formation of AlYB14 together with the (Y,Al)B6 impurity phase, containing 1.8 at. % less B than AlYB14, was observed at a growth temperature of 800 °C and hence 600 °C below the bulk synthesis temperature. Based on quantum mechanical calculations, we infer that minute compositional variations within the film may be responsible for the formation of both icosahedrally bonded AlYB14 and cubic (Y,Al)B6 phases. These findings are relevant for synthesis attempts of all boron rich icosahedrally bonded compounds with the space group: Imma that form ternary phases at similar compositions.

A Complete Overhaul of the Electron Energy-Loss Spectroscopy and X-Ray Absorption Spectroscopy Database: eelsdb.eu.

The electron energy-loss spectroscopy (EELS) and X-ray absorption spectroscopy (XAS) database has been completely rewritten, with an improved design, user interface, and a number of new tools. The database is accessible at https://eelsdb.eu/ and can now be used without registration. The submission process has been streamlined to encourage spectrum submissions and the new design gives greater emphasis on contributors' original work by highlighting their papers. With numerous new filters and a powerful search function, it is now simple to explore the database of several hundred EELS and XAS spectra. Interactive plots allow spectra to be overlaid, facilitating online comparison. An application-programming interface has been created, allowing external tools and software to easily access the information held within the database. In addition to the database itself, users can post and manage job adverts and read the latest news and events regarding the EELS and XAS communities. In accordance with the ongoing drive toward open access data increasingly demanded by funding bodies, the database will facilitate open access data sharing of EELS and XAS spectra.

Maghemite-like regions at the crossing of two antiphase boundaries in doped BiFeO3

We report the observation of a novel structure at the point where two antiphase boundaries cross in a doped bismuth ferrite of composition (Bi0.85Nd0.15)(Fe0.9Ti0.1)O0.3. The structure was investigated using a combination of high angle annular dark field imaging and electron energy loss spectroscopy spectrum imaging in the scanning transmission electron microscope. A three-dimensional model was constructed by combining the position and chemistry data with previous results and assuming octahedral coordination of all Fe and Ti atoms. The resulting structure shows some novel L-shaped arrangements of iron columns, which are coordinated in a similar manner to FeO6 octahedra in maghemite. It is suggested that this may lead to local ferromagnetic orderings similar to those in maghemite.

Nitrogen Nanobubbles in a-SiOxNy Coatings: Evaluation of Its Physical Properties and Chemical Bonding State by Spatially Resolved Electron Energy-Loss Spectroscopy

Nanoporous silicon-based materials with closed porosity filled with the sputtering gas have been recently developed by magnetron sputtering. In this work the physical properties (density and pressure) of molecular nitrogen inside closed pores in a SiOxNy coating are investigated for the first time using spatially resolved electron energy-loss spectroscopy (EELS) in a scanning transmission electron microscope. The paper offers a detailed methodology to record and process multiple EELS spectrum images (SIs) acquired at different energy ranges and with different dwell times. An adequate extraction and quantification of the N–K edge contribution due to the molecular nitrogen inside nanopores is demonstrated. Core-loss intensity and N chemical bond state were evaluated to retrieve 2D maps revealing the stable high density of molecular nitrogen (from 40 to 70 at./nm3) in nanopores of different size (20–11 nm). This work provides new insights into the quantification of molecular N2 trapped in porous nitride matr...

Quantitative parameters for the examination of InGaN QW multilayers by low-loss EELS.

We present a detailed examination of a multiple InxGa1-xN quantum well (QW) structure for optoelectronic applications. The characterization is carried out using scanning transmission electron microscopy (STEM), combining high-angle annular dark field (HAADF) imaging and electron energy loss spectroscopy (EELS). Fluctuations in the QW thickness and composition are observed in atomic resolution images. The impact of these small changes on the electronic properties of the semiconductor material is measured through spatially localized low-loss EELS, obtaining band gap and plasmon energy values. Because of the small size of the InGaN QW layers additional effects hinder the analysis. Hence, additional parameters were explored, which can be assessed using the same EELS data and give further information. For instance, plasmon width was studied using a model-based fit approach to the plasmon peak; observing a broadening of this peak can be related to the chemical and structural inhomogeneity in the InGaN QW layers. Additionally, Kramers-Kronig analysis (KKA) was used to calculate the complex dielectric function (CDF) from the EELS spectrum images (SIs). After this analysis, the electron effective mass and the sample absolute thickness were obtained, and an alternative method for the assessment of plasmon energy was demonstrated. Also after KKA, the normalization of the energy-loss spectrum allows us to analyze the Ga 3d transition, which provides additional chemical information at great spatial resolution. Each one of these methods is presented in this work together with a critical discussion of their advantages and drawbacks.

Controlling the Active Sites of Sulfur‐Doped Carbon Nanotube–Graphene Nanolobes for Highly Efficient Oxygen Evolution and Reduction Catalysis

Controlling active sites of metal‐free catalysts is an important strategy to enhance activity of the oxygen evolution reaction (OER). Many attempts have been made to develop metal‐free catalysts, but the lack of understanding of active‐sites at the atomic‐level has slowed the design of highly active and stable metal‐free catalysts. A sequential two‐step strategy to dope sulfur into carbon nanotube–graphene nanolobes is developed. This bidoping strategy introduces stable sulfur–carbon active‐sites. Fluorescence emission of the sulfur K‐edge by X‐ray absorption near edge spectroscopy (XANES) and scanning transmission electron microscopy electron energy loss spectroscopy (STEM‐EELS) mapping and spectra confirm that increasing the incorporation of heterocyclic sulfur into the carbon ring of CNTs not only enhances OER activity with an overpotential of 350 mV at a current density of 10 mA cm−2, but also retains 100% of stability after 75 h. The bidoped sulfur carbon nanotube–graphene nanolobes behave like the state‐of‐the‐art catalysts for OER but outperform those systems in terms of turnover frequency (TOF) which is two orders of magnitude greater than (20% Ir/C) at 400 mV overpotential with very high mass activity 1000 mA cm−2 at 570 mV. Moreover, the sulfur bidoping strategy shows high catalytic activity for the oxygen reduction reaction (ORR). Stable bifunctional (ORR and OER) catalysts are low cost, and light‐weight bidoped sulfur carbon nanotubes are potential candidates for next‐generation metal‐free regenerative fuel cells.

Quantification of the boron speciation in alkali borosilicate glasses by electron energy loss spectroscopy.

Transmission electron microscopy and related analytical techniques have been widely used to study the microstructure of different materials. However, few research works have been performed in the field of glasses, possibly due to the electron-beam irradiation damage. In this paper, we have developed a method based on electron energy loss spectroscopy (EELS) data acquisition and analyses, which enables determination of the boron speciation in a series of ternary alkali borosilicate glasses with constant molar ratios. A script for the fast acquisition of EELS has been designed, from which the fraction of BO4 tetrahedra can be obtained by fitting the experimental data with linear combinations of the reference spectra. The BO4 fractions (N4) obtained by EELS are consistent with those from 11B MAS NMR spectra, suggesting that EELS can be an alternative and convenient way to determine the N4 fraction in glasses. In addition, the boron speciation of a CeO2 doped potassium borosilicate glass has been analyzed by using the time-resolved EELS spectra. The results clearly demonstrate that the BO4 to BO3 transformation induced by the electron beam irradiation can be efficiently suppressed by doping CeO2 to the borosilicate glasses.

Importance of Time Scale and Local Environment in Electron-Driven Proton Transfer. The Anion of Acetoacetic Acid

Anion photoelectron spectroscopy (PES) and electron energy-loss spectroscopy (EELS) probe different regions of the anionic potential energy surface. These complementary techniques provided information about anionic states of acetoacetic acid (AA). Electronic structure calculations facilitated the identification of the most stable tautomers and conformers for both neutral and anionic AA and determined their relative stabilities and excess electron binding energies. The most stable conformers of the neutral keto and enol tautomers differ by less than 1 kcal/mol in terms of electronic energies corrected for zero-point vibrations. Thermal effects favor these conformers of the keto tautomer, which do not support an intramolecular hydrogen bond between the keto and the carboxylic groups. The valence anion displays a distinct minimum which results from proton transfer from the carboxylic to the keto group; thus, we name it an ol structure. The minimum is characterized by a short intramolecular hydrogen bond, a significant electron vertical detachment energy of 2.38 eV, but a modest adiabatic electron affinity of 0.33 eV. The valence anion was identified in the anion PES experiments, and the measured electron vertical detachment energy of 2.30 eV is in good agreement with our computational prediction. We conclude that binding an excess electron in a π* valence orbital changes the localization of a proton in the fully relaxed structure of the AA(-) anion. The results of EELS experiments do not provide evidence for an ultrarapid proton transfer in the lowest π* resonance of AA(-), which would be capable of competing with electron autodetachment. This observation is consistent with our computational results, indicating that major gas-phase conformers and tautomers of neutral AA do not support the intramolecular hydrogen bond that would facilitate ultrarapid proton transfer and formation of the ol valence anion. This is confirmed by our vibrational EELS spectrum. Anions formed by vertical electron attachment to dominant neutrals undergo electron autodetachment with or without vibrational excitations but are unable to relax to the ol structure on a time scale fast enough to compete with autodetachment.

Extended reverse Monte Carlo method for extracting optical constants of thin Ni film from reflection electron energy-loss spectroscopy

SynopsisAn extended reverse Monte Carlo (RMC) method for obtaining optical constants of ultrathin films and solids is presented. Our results show that this method works very well in extracting some exquisite information from experimentally measured reflection electron energy loss spectroscopy (REELS) spectra. The energy loss function (ELF) of 100 nm nickel film in a relatively larger energy loss range of 0-200 eV has been obtained, which agrees well with other computational data. Employing this ELF, individual contributions to REELS intensity due to surface and bulk plasmon excitations are quantitatively separated for further study.

(Invited) Capabilities of Analytical Transmission Electron Microscopy for the Analysis of Structural, Chemical and Electronic Properties Exemplified by the Study of Y-Doped (Ba,Sr)(Co,Fe)O3-δ

Transmission electron microscopy (TEM) is a standard technique for microstructure studies with a resolution that meanwhile has reached values better than 0.1 nm. Essential is the capability of obtaining images and diffraction patterns from the same small specimen region which facilitates crystal structure determination of small precipitates. Particularly unique is the possibility to perform atomic-scale chemical analyses by energy dispersive X-ray spectroscopy and electron energy loss spectroscopy (EELS). Information on the electronic material properties like, e.g., valence states or bond configurations between neighbor atoms is obtained by the electron near edge structure of ionization edges in EELS spectra. In addition to these well-known capabilities, many more imaging modes and evaluation techniques are available which yield additional material properties and render electron microscopy an extremely powerful technique. We will illustrate the capabilities of analytical TEM for the example of (Ba0.5Sr0.5)(Co0.8Fe0.2)O3-δ (BSCF) which is a promising and highly complex material for oxygen separation membranes. BSCF suffers from degradation of the oxygen diffusivity at temperatures below 840 oC which is attributed to the decomposition of the cubic phase into several secondary phases [1,2]. Composition and crystal structure analyses of the different phases combined with Co-valence state determination sheds light on the origin of the instability of the cubic phase. Indispensable is the determination Co-valence state in the cubic and hexagonal BSCF phases which required the introduction of a new evaluation technique due to the overlap of the Co-L2,3ionization edges with ionization edges of other elements in BSCF [3]. Doping with Yttrium has been used to stabilize the cubic Sr1−x Y x Co1−y Y y O3−δ phase [4] which also appears to be a promising strategy for BSCF. Substitution of B-site Co- and Fe-cations in BSCF by Y3+-cations with a fixed valence state should lead to the stabilization of the Co-valence and, hence, the stabilization of the cubic BSCF phase. The volume fraction of secondary phases is indeed substantially reduced in Y-doped BSCF. Only the hexagonal BSCF phase is observed which is confined to grain boundaries (Fig. 1). In contrast, a large volume fraction of different secondary phases is found in undoped BSCF (Fig. 2). A closer inspection suggests, however, that the suppression of secondary phases in Y-doped samples could be partially related to the lack of nucleation sites rather than the stabilizing effect of Y-doping. To achieve an improved understanding of the effect of Y-doping, the occupation sites of Y-cations were analyzed by atom location by channeling enhanced microanalysis (ALCHEMI) as proposed by Spence and Taftø [5]. ALCHEMI analyses demonstrate that Y-cations occupy A- and B-sites. Despite substitution of B-site cations, 25 % to 40 % of the Y-cations occupy A-sites depending on the doping level. This indicates the complex effect of Y-doping in BSCF which will be discussed.

Direct observation of the crystal structure changes in the MgxZn1 − xO alloy system

We directly observed the crystal structure changes of the MgxZn1−xO alloy thin film deposited on Si (111) substrates. Through the in situ heating transmission electron microscopy study, it was determined that the crystal structure changes did not occur up to at 400°C, whereas the disappearance of the hexagonal structure was observed at 500°C in the layer of nanosized grains. Additionally, the decreased intensity of the Zn L-edge was analyzed in the comparison of the core loss electron energy loss spectroscopy spectra of the Zn L-edge and the Mg K-edge obtained at room temperature and 500°C. Based on these experimental results, the process of crystal structure changes could be explained by the evaporation of Zn atoms in the MgxZn1−xO alloy system. This phenomenon is prominent in the improvement of the microstructure of the MgxZn1−xO alloy thin film by controlling the thermal annealing temperature.

Experimental verification of the van Vleck nature of long-range ferromagnetic order in the vanadium-doped three-dimensional topological insulator Sb(2)Te(3).

We demonstrate by high resolution low temperature electron energy loss spectroscopy (EELS) measurements that the long range ferromagnetic (FM) order in the vanadium- (V-)doped topological insulator Sb_{2}Te_{3} has the nature of van Vleck-type ferromagnetism. The positions and the relative amplitudes of two core-level peaks (L_{3} and L_{2}) of the V EELS spectrum show unambiguous change when the sample is cooled from room temperature to T=10 K. Magnetotransport and comparison of the measured and simulated EELS spectra confirm that these changes originate from the onset of FM order. Crystal field analysis indicates that in V-doped Sb_{2}Te_{3}, partially filled core states contribute to the FM order. Since van Vleck magnetism is a result of summing over all states, this magnetization of core level verifies the van Vleck-type ferromagnetism in a direct manner.

Electron Energy-Loss Spectroscopy Calculation in Finite-Difference Time-Domain Package

Electron energy-loss spectroscopy (EELS) is a unique tool that is extensively used to investigate the plasmonic response of metallic nanostructures. We present here a novel approach for EELS calculations using the finite-difference time-domain (FDTD) method (EELS-FDTD). We benchmark our approach by direct comparison with results from the well-established boundary element method (BEM) and published experimental results. In particular, we compute EELS spectra for spherical nanoparticles, nanoparticle dimers, nanodisks supported by various substrates, and a gold bowtie antenna on a silicon nitride substrate. Our EELS-FDTD method can be easily extended to more complex geometries and configurations. This implementation can also be directly exported beyond the FDTD framework and implemented in other Maxwell’s equation solvers.

Investigation of the oxidation states of Cu additive in colored borosilicate glasses by electron energy loss spectroscopy

Three optically transparent colorful (red, green, and blue) glasses were synthesized by the sol-gel method. Nano-sized precipitates were found in scanning electron microscopy images. The precipitates were analyzed by transmission electron microscopy (TEM) and high resolution TEM. The measured lattice parameters of these precipitates were found to fit the metallic copper in red glass but deviate from single valenced Cu oxides in green and blue glasses. The chemistry of these nano-sized particles was confirmed by electron energy loss spectroscopy (EELS). By fitting the EELS spectra obtained from the precipitates with the linear combination of reference spectra from Cu reference compounds, the oxidation states of Cu in the precipitates have been derived. First principle calculations suggested that the Cu nano-particles, which are in the similar oxidation states as our measurement, would show green color in the visible light range.

A review of advantages of high-efficiency X-ray spectrum imaging for analysis of nanostructured ferritic alloys

Nanostructured ferritic alloys (NFAs) exhibit complex microstructures consisting of 100–500nm ferrite grains, grain boundary solute enrichment, and multiple populations of precipitates and nanoclusters (NCs). Understanding these materials’ excellent creep and radiation-tolerance properties requires a combination of multiple atomic-scale experimental techniques. Recent advances in scanning transmission electron microscopy (STEM) hardware and data analysis methods have the potential to revolutionize nanometer-to micrometer-scale materials analysis. Modern high-brightness, high-X-ray collection STEM instruments are capable of enabling advanced experiments, such as simultaneous energy dispersive X-ray spectroscopy and electron energy loss spectroscopy spectrum imaging at nm to sub-nm resolution, that are now well-established for the study of nuclear materials. In this paper, we review past results and present new results illustrating the effectiveness of latest-generation STEM instrumentation and data analysis.

Effect of multipole excitations in electron energy-loss spectroscopy of surface plasmon modes in silver nanowires

We have characterized the surface plasmon resonance (SPR) in silver nanowires using spatially resolved electron energy loss spectroscopy (EELS) in the scanning transmission electron microscope. Non-symmetric EELS spectra due to high-k SPR propagation along the nanowire and spectral shifts due to higher-order mode excitation are observed when the beam is positioned near the tip of the nanowire. When the beam is far from the tip region and on the side of nanowire, no spectral shifts are observed as the beam is scanned in the radial direction of the nanowire. The experimental spectra are compared with three different theoretical approaches: direct numerical calculation of the energy loss, analytical models for energy loss, and numerical simulations using an optical model. All three models reproduce the spectral shifts as the electron beam approaches the cap of the nanowire. The analytical model reveals the origin of the shifts in high-order plasmon mode excitation.

Electron impact induced collective and atomic-like single-electron Ga3d → εl excitations in GaAsN alloy

Nitride nanolayer of about 4nm thickness formed on the GaAs (100) surface by N2+ ion bombardment with energy Ei=2500eV has been studied in situ by electron energy loss spectroscopy (EELS). Collective (plasmonic) and single-electron excitations were resolved in the EELS spectrum. Analysis of the plasmonic contributions from different chemical phases showed domination of the phase of GaAsN alloy in the nitrated layer. Single-electron excitations were shown to be related to the transitions from core- and valence band states into unoccupied local atomic-like states Ga3d→εl and “GaAs4p”→εl. The model of the atomic-like state formation in a temporary quantum dot near Ga atom was assumed. It has been shown that analysis of collective and single-electron excitations enables synchronous EELS diagnostics of both the chemical composition and unoccupied states of GaAs nitrides.

Atomically resolved EELS mapping of the interfacial structure of epitaxially strainedLaNiO3/LaAlO3superlattices

The interfacial atomic structure of a metallic ${\mathrm{LaNiO}}_{3}/{\mathrm{LaAlO}}_{3}$ superlattice grown on a ${\mathrm{LaSrAlO}}_{4}$ substrate was investigated using a combination of atomically resolved electron energy loss spectroscopy (EELS) at the Al $K$, Al ${L}_{2,3}$, Sr ${L}_{2,3}$, Ni ${L}_{2,3}$, La ${M}_{4,5}$, and O $K$ edges as well as hybridization mapping of selected features of the O $K$-edge fine structure. We observe an additional ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{Al}}_{1\ensuremath{-}y}{\mathrm{Ni}}_{y}{\mathrm{O}}_{3}$ layer at the substrate-superlattice interface, possibly linked to diffusion of Al and Sr into the growing film or a surface reconstruction due to Sr segregation. The roughness of the ${\mathrm{LaNiO}}_{3}/{\mathrm{LaAlO}}_{3}$ interfaces is found to be on average around one pseudocubic unit cell. The O $K$-edge EELS spectra revealed reduced spectral weight of the prepeak derived from Ni-O hybridized states in the ${\mathrm{LaNiO}}_{3}$ layers. We rule out oxygen nonstoichiometry of the ${\mathrm{LaNiO}}_{3}$ layers and discuss changes in the Ni-O hybridization due to heterostructuring as possible origin.

Direct observations of local electronic states in an Al-based quasicrystal by STEM-EELS.

Most quasicrystals (QCs) reveal pseudogaps in their density of states around Fermi level, and hence the stability of QCs have been discussed in terms of energetic gains in electron systems. In fact, many QCs have been discovered by tuning valence electron density based on Hume-Rothery rule. Therefore, understanding electronic structures in QCs may provide an important clue for their stabilization mechanism. Generally, it has been frequently discussed based on an interaction between Fermi surface and Brillouin zone boundary within the framework of nearly free electron model, which is believed to be an underlying physics of a Hume-Rothery's empirical criteria. However the hybridization effect also stabilize electron system, particularly in Al-transition metal system, in which a lot of quasicrystalline phases were discovered. Therefore, the electronic structures of QCs have not yet been fully understood, whereas their atomic structures have been studied well in terms of configuration entropy by scanning transmission electron microscopy (STEM) [1]. In the present work, we investigate local electronic states in Al-based QCs using electron energy loss spectroscopy (EELS) combined with STEM, by which EELS spectra with sub-Å probe and atomic structure can be obtained simultaneously. We report STEM-EELS results on AlCuIr decagonal phases [2].jmicro;63/suppl_1/i17-a/DFU069F1F1DFU069F1Fig. 1.Core-loss edges obtained from cluster-centers and cluster-edges. Al L1 (left) Ir O23, Ir N67 (center) and Cu L23 (right). Principal components analysis clearly shows up the atomic-site dependence of plasmon loss spectra in a two-dimensional map. Qualitatively, there seems to be certain correlations between the plasmon peaks and the core-loss edges, Al L1, Ir O23, Ir N67 and Cu L23, all of which reveal different behaviors at the cluster centers and the edges (Fig. 1). All results indicate the cluster centers have metallic states and the cluster edges have covalent states in comparison. First-principles calculations confirm the unusual electronic state. On the basis of the calculated DOS and charge-density map, we conclude Al-Ir pairs, which are mainly located at the cluster edges, hybridize their orbitals and Cu atoms are localized at cluster center without hybridization. We analyze a distribution of the hybridized orbitals by Fourier transformation of electron localization function. The distribution seems like a 10-fold standing wave with Fermi wave length. It suggests that the Hume-Rothery mechanism works even when hybridization effect mainly contributes to pseudogap formation. In other words, global distribution of hybridized orbitals is important to stabilize structures whereas usually only local atomic configurations are discussed. Moreover, in the 10-fold standing wave, no distinct peak appears at the cluster centers. It means hybridized orbitals located at cluster centers are unable to contribute to the Hume-Rothery mechanism. It may be the reason why the metallic regions appear at the cluster centers.