Energy-loss Near-edge Structure Research Articles

Local electronic structure analysis for brownmillerite Ca(Sr)FeO2.5 using site-resolved energy-loss near-edge structures

Oxygen K-edge and Fe L2,3-edge electron energy-loss near-edge structures (ELNES) were measured for FeO6 octahedra and FeO4 tetrahedra in the brownmillerite Ca(Sr)FeO2.5 by focusing an electron probe at individual Fe sites using scanning transmission electron microscopy combined with electron energy-loss spectroscopy. The observed site-resolved oxygen K-ELNES showed different features reflecting the local chemical bonding around the FeO6 octahedra and FeO4 tetrahedra. A pre-peak in the O K-edge spectra, which is attributed to a transition to an unoccupied O 2p band hybridized with the Fe-3d band, shows splitting in the spectrum of the FeO6 octahedral site. Additionally, for the oxygen linking the octahedral and tetrahedral Fe sites in CaFeO2.5, charge transfer was found to preferentially occur toward the tetrahedral Fe ions. In the case of SrFeO2.5, charge transfer from the oxygen located in the ac plane was biased toward the tetrahedral Fe atoms. Based upon an analysis of the pre-peak intensity of the O K-ELNES, it was concluded that bonding between the oxygen and iron atoms at the tetrahedral site was more covalent in character than at the octahedral site. The strong covalent character of the tetrahedral sites would be one of the reasons for distortion in the FeO6 octahedra in Ca(Sr)FeO2.5, as exhibited by an extension along the b-axis.

The valence state and partitioning of iron in the Earth's lowermost mantle

[1] While iron occurs predominantly in the form of Fe2+ in the Earth's upper mantle and transition zone, Al-bearing (Mg,Fe)SiO3 perovskite (Pv), a primary mineral in the lower mantle, contains a large proportion of ferric iron (Fe3+). It has been demonstrated that such Fe3+ strongly affects physical and chemical properties of Pv. On the other hand, the iron substitution mechanism and valence state of iron in postperovskite (PPv) are still unclear. Here we determined the valence state of iron in PPv with changing Al3+ content, on the basis of electron energy-loss near-edge structure spectroscopy measurements. The results show that PPv includes a small amount of Fe3+, which is independent from the Al3+ content. This indicates that Fe3+ in Pv and coexisting metallic iron recombine to form Fe2+ upon phase transition from Pv to PPv in pyrolitic mantle. Such Fe2+ partitions preferentially into (Mg,Fe)O ferropericlase (Fp), and therefore PPv is depleted in iron compared to Pv. Such a marked change in the valence state of iron and resulting iron depletion in PPv have broad implications for seismic and transport properties in the lowermost mantle. Phase transition from Pv to Fe-poor PPv occurs in a much narrower pressure range than that for a fixed iron content.

Boron-Assisted Transformation to Rod-Like Graphitic Carbons from Multi-Walled Carbon Nanotubes in Boron-Mixed Multi-Walled Carbon Nanotube Solids

We produced boron-mixed multi-walled carbon nanotube solids (B-mixed MWCNT solids) by heating and pressing the powder of purified MWCNTs mixed with 1, 5, and 10 wt % boron in the temperature range 1400-1800 °C every 200 °C under a constant pressure of 20 MPa in vacuo, and investigated the influence of boron addition on nanotube structure and the mechanical and electrical properties of the resulting B-mixed MWCNT solids. The structure of the prepared material was characterized by scanning electron microscopy equipped with energy-dispersive X-ray spectroscopy, high-resolution transmission electron microscopy-electron energy loss spectroscopy, Raman scattering spectroscopy, and X-ray diffraction, and their mechanical properties and conductivity were measured using a mechanical and Vickers indentation tester and an electric resistor, respectively. It is notable that part of the nanotubes in the B-mixed MWCNT solids solidified at 1800 °C had dramatically changed into rod-like graphitic carbons (RLGCs). The occupancy distribution of RLGCs increased with increasing boron contents. However, boron was not detected in the energy-loss near-edge structure spectrum of RLGCs. Furthermore, RLGCs were not observed in the boron-unmixed sample treated with the same solidified condition, indicating that adding boron causes a remarkable ability to transform the phase of MWCNT. Transformation from MWCNTs to RLGCs resulted in increased specific bending strength and modulus, Vickers hardness, and electrical conductivity of B-mixed MWCNT solids with increasing boron content and solidified temperature.

Calculated and experimental electron energy-loss spectra of La2O3, La(OH)3, and LaOF nanophases in high permittivity lanthanum-based oxide layers

Using first principles methods, the O K energy-loss near-edge structure of cubic and hexagonal La2O3, La(OH)3, and LaOF phases have been calculated. These calculations support the identification of nanocrystalline phases evidenced experimentally by electron energy-loss spectroscopy (EELS) performed in a transmission electron microscope. The nanocrystals have been observed in atomic layer deposited La2O3 thin films developed for advanced metal-oxide-semiconductor field effect transistor applications. The presence of the nanophases can be explained by the hygroscopicity and the particular reactivity with fluorine of La2O3. These calculations provide a set of EELS fingerprints which will be useful for local phase identification in La2O3-based materials.

A strategy for simulating Electron Energy-Loss Near-Edge Structures of nanoparticles: application to size effects in Gd2O3

Nanoparticles (NPs) have attracted a considerable interest in the last decades, owing to their remarkable physical and chemical properties. The most important characteristic of NPs is the size effect, that is, their properties differ from that of the corresponding bulk material. We will focus here on Electron Energy-Loss Spectroscopy (EELS) investigations of Gd 2 O 3 NPs of different and controlled sizes. EELS spectra near the O-K edge of Gd 2 O 3 were recorded and compared with feff8.2 ELNES simulations. The calculation of the EELS response from small particles by the feff code raises some particular problems which have been carefully examined and partially solved. The simulations are in fair agreement with experiment and reveal the existence of size effects.

Theoretical Fingerprints of Transition Metal L2,3 XANES and ELNES for Lithium Transition Metal Oxides by ab Initio Multiplet Calculations

With the ab initio multiplet method, transition metal (M) L2,3-edge X-ray absorption near-edge structures (XANES) and electron energy loss near-edge structures (ELNES) can be predicted in detail. I...

AlL2,3edge x-ray absorption spectra in III-V semiconductors: Many-body perturbation theory in comparison with experiment

We investigate core excitations of the Al $2p$ edge in the III-V semiconductors AlP, AlAs, AlSb, and AlN. For the latter, we consider the wurtzite, zinc-blende, and rock-salt polymorphs. First-principles calculations are performed utilizing two different approaches, which are the solution of the Bethe-Salpeter equation (BSE) as well as the supercell technique employing the core-hole approximation. In addition, measurements of the electron energy-loss near-edge structure of the metastable AlN phase are presented. We find that the relative intensities of the spectral features are better described by the BSE than by the supercell method. We analyze the character of the near-edge peaks and trace back their origin to strongly bound core excitons in the case of AlSb and rock-salt AlN.

Phase separation in the iron chalcogenide superconductor Fe1+yTexSe1−x

We present direct evidence for phase separation and chemical inhomogeneity in Fe1+yTexSe1−x single crystals from scanning transmission electron microscopy (STEM) and electron energy-loss spectroscopy (EELS) data. In STEM, images recorded using a high-angle annular dark field (HAADF) detector show characteristic nanometer-scale patterns of phase separation from the Z-dependent image contrast. The separation was observed both in non-superconducting samples with excess iron and in superconducting samples. Using EELS, we determined ∼20% (or lower) fluctuation in Te concentration from the average local compositions by integrating the intensity of the Te-M4,5 edge. The energy-loss near-edge structure (ELNES) of the Fe-L2,3 edge changes with varying composition, especially the L3/L2 white-line intensity ratio, which is sensitive to the d-state occupancy of the Fe. The results suggest a miscibility gap in the Fe1+yTexSe1−x system and changes in the d-electron states at the nanometer scale from the separated phases.

Towards predictive modeling of near-edge structures in electron energy-loss spectra of AlN-based ternary alloys

Although the analysis of electron energy loss near-edge structure provides a tool for experimentally probing the density of unoccupied states, a detailed comparison with simulations is necessary in order to understand the origin of individual peaks. This paper presents a density functional theory based technique for predicting the N K edge for ternary (quasibinary) nitrogen alloys by adopting a core hole approach, a methodology that has been successful for binary nitride compounds. It is demonstrated that by using the spectra of binary compounds for optimizing the core hole charge ($0.35 e$ for cubic Ti${}_{1\ensuremath{-}x}$Al${}_{x}$N and $0.45 e$ for wurtzite Al${}_{x}$Ga${}_{1\ensuremath{-}x}$N), the predicted spectra evolutions of the ternary alloys agree well with the experiments. The spectral features are subsequently discussed in terms of the electronic structure and bonding of the alloys.

Ab initiocharge transfer multiplet calculations on theL2,3XANES and ELNES of3dtransition metal oxides

The ${L}_{2,3}$ x-ray absorption near-edge structures (XANES) and electron energy loss near-edge structures (ELNES) of $3d$ transition metal (TM) oxides are systematically calculated by the ab initio charge transfer multiplet (CTM) method using fully relativistic molecular spinors on the basis of density-functional theory. The electronic excitation from molecular spinors mainly composed of O-$2p$ to those of TM-$3d$, that is, charge transfer, is included by considering additional electronic configurations in the configuration interactions. The effects of the covalency and charge transfer on the TM-${L}_{2,3}$ XANES are investigated in detail. The power of the ab initio CTM method to quantitatively reproduce the spectra is demonstrated. Meanwhile, limitations of the application of the method are discussed.

Electronic structure and physical properties of the spinel-type phase of BeP2N4from all-electron density functional calculations

Using density-functional-theory-based ab initio methods, the electronic structure and physical properties of the newly synthesized nitride BeP{sub 2}N{sub 4} with a phenakite-type structure and the predicted high-pressure spinel phase of BeP{sub 2}N{sub 4} are studied in detail. It is shown that both polymorphs are wide band-gap semiconductors with relatively small electron effective masses at the conduction-band minima. The spinel-type phase is more covalently bonded due to the increased number of P-N bonds for P at the octahedral sites. Calculations of mechanical properties indicate that the spinel-type polymorph is a promising superhard material with notably large bulk, shear, and Young's moduli. Also calculated are the Be K, P K, P L{sub 3}, and N K edges of the electron energy-loss near-edge structure for both phases. They show marked differences because of the different local environments of the atoms in the two crystalline polymorphs. These differences will be very useful for the experimental identification of the products of high-pressure syntheses targeting the predicted spinel-type phase of BeP{sub 2}N{sub 4}.

O K-energy loss near-edge structure change induced by tantalum impurity in monoclinic hafnium oxide

The present paper reports the energy loss near-edge structure (ELNES) study of monoclinic HfO2 (m-HfO2) and tantalum doped m-HfO2 (Ta0.1Hf0.9O2) thin films prepared by radio frequency magnetron co-sputtering method. A change in the O K-ELNES spectra was observed as the amount of dopant increases. In order to precise the common features and the differences as a function of Ta defect nature (substitutional or interstitial) in HfO2, the O K-ELNES were commented with respect to density functional theory calculations implemented in Vienna ab initio simulation package code. The calculated Ta doped HfO2 band structure showed that substitutional tantalum is the dominant defect and the spectral differences between doped and non-doped HfO2 are mainly originated from the change in the local cation distribution around the oxygen atoms.

Comparison of the surface changes on cathode during long term storage testing of high energy density cylindrical lithium-ion cells

Nickel-based oxide cathode material taking out from lithium-ion cell after storage for 2 years at 45 °C is analyzed by electron energy-loss spectroscopy in a scanning transmission electron microscope (STEM-EELS) and the result of STEM-EELS is compared with cobalt-based oxide cathode material which is treated as same manor as nickel-based oxide cathode material. The Ni-L 2,3 energy-loss near-edge structure (ELNES) spectra of nickel-based oxide cathode material show peak positions similar to original material before storage. This result indicates that nickel-based oxide material has no significant change in the surface structure. On the other hand, a remarkable shift to low energy is observed in the Co-L 2,3 ELNES spectra of the cobalt-based oxide cathode material after storage. The cycle test at 60 °C under the conditions of aggressive driving cycle (US06) mode for the nickel-based oxide cathode/graphite cell is also carried out. It is clear that cycle performance of the nickel-based oxide cathode/graphite cell is dependent on the depth of discharge (DOD).

Characterization of chemical composition and electronic structure of Pt/YSZ interfaces by analytical transmission electron microscopy

Platinum/yttria stabilized zirconia (Pt/YSZ) interfaces with two different orientation relationships were characterized using advanced analytical transmission electron microscopy methods. Quantitative X-ray energy dispersive spectroscopy (XEDS) was performed by the recently developed ζ-factor method. Neither interdiffusion nor segregation was detected across the Pt/YSZ interfaces of both orientation conditions within the resolution limit of the technique. The interface specific components of the electron energy-loss near-edge structures (ELNES) were extracted by employing the spatial difference technique. Features of the O–K ELNES at the interface are distinctly different from that at bulk YSZ indicating Pt–O bonding. The experimentally observed changes in the O–K ELNES are in good agreement with the results of ab-initio full multiple scattering calculation, based on structure modeling at the interfaces.

Characterization of nickel silicides using EELS‐based methods

The characterization of Ni-silicides using electron energy loss spectroscopy (EELS) based methods is discussed. A series of Ni-silicide phases is examined: Ni₃Si, Ni₃₁Si₁₂, Ni₂Si, NiSi and NiSi₂. The composition of these phases is determined by quantitative core-loss EELS. A study of the low loss part of the EELS spectrum shows that both the energy and the shape of the plasmon peak are characteristic for each phase. Examination of the Ni-L edge energy loss near edge structure (ELNES) shows that the ratio and the sum of the L₂ and L₃ white line intensities are also characteristic for each phase. The sum of the white line intensities is used to determine the trend in electron occupation of the 3d states of the phases. The dependence of the plasmon energy on the electron occupation of the 3d states is demonstrated.

Characterizing CA 2 and CA 6 using ELNES

Calcium aluminates, compounds in the CaO–Al 2O 3 phase system, are used in high-temperature cements and refractory oxides and have wide range of potential technological applications due to their interesting optical, electrical, thermal, and mechanical properties. They are used in both crystalline and glassy form; the glass is an isotropic material while the crystalline materials may be highly anisotropic. This paper will consider two particular crystalline materials, CA 2 and CA 6, but the results should be applicable to all calcium aluminates. Although CA 2 and CA 6 crystals contain the same chemical species, Ca, Al, and O, the coordination and local environments of these species are different in the two structures and hence they show very different energy-loss near-edge structures (ELNES) when examined by electron energy-loss spectroscopy (EELS) in the TEM. The data obtained using ELNES can effectively provide a fingerprint for each compound and a map for their electronic structure. Once such fingerprints are obtained, they can be used to identify nano-sized particles/grains or material at interfaces and grain boundaries. In the present study, the local symmetry fingerprints for CA 2 and CA 6 structures are reported combining experimental spectra with electronic-structure calculations that allow the different features in the spectra to be interpreted. Al-L 2,3 and O-K edge core-loss spectra from CA 2 and CA 6 were measured experimentally using electron energy-loss spectroscopy in a monochromated scanning transmission electron microscope. The near-edge structures were calculated for the different phases using the orthogonalized linear combination of atomic-orbitals method, and took account of core–hole interactions. It is shown that CA 2 and CA 6 structures exhibit distinctive experimental ELNES fingerprints so that these two phases can be separately identified even when present in small volumes.

Quantification of the Ti oxidation state in BaTi 1− xNb xO 3 compounds

The Ti oxidation state of a series mixed-valence BaTi 1− x Nb x O 3 compound (where x=0.002, 0.004, 0.02, 0.10, 0.20 and 0.50) is investigated using high resolution electron energy loss spectroscopy (EELS). The energy loss near edge structure (ELNES) of the Ti- L 2,3 and O- K edges was recorded with high energy resolution. The fraction of Ti 4+ and Ti 3+ components is determined in each compound by linear profile fitting with Ti 4+ and Ti 3+ standard spectra obtained from reference compounds within the series. The fitting results indicate an increase in the fraction of the Ti 3+ component as the Nb content increases. A deviation from the expected Ti 3+ valence fraction based on the charge balance across the series was detected and discussed. By considering all detailed features on the spectra obtained with high energy resolution, this linear fitting method can be used to determine the oxidation state of transition metal oxides, especially for the early transition metals where conventional methods based on the L 2,3 edge ratio have shown to fail. The potential of this method to provide insight to mixed valence systems, vacancies and properties of oxides is discussed.

Combining FIB milling and conventional Argon ion milling techniques to prepare high‐quality site‐specific TEM samples for quantitative EELS analysis of oxygen in molten iron

This paper reports a procedure to combine the focused ion beam micro-sampling method with conventional Ar-milling to prepare high-quality site-specific transmission electron microscopy cross-section samples. The advantage is to enable chemical and structural evaluations of oxygen dissolved in a molten iron sample to be made after quenching and recovery from high-pressure experiments in a laser-heated diamond anvil cell. The evaluations were performed by using electron energy-loss spectroscopy and high-resolution transmission electron microscopy. The high signal to noise ratios of electron energy-loss spectroscopy core-loss spectra from the transmission electron microscopy thin foil, re-thinned down to 40 nm in thickness by conventional Argon ion milling, provided us with oxygen quantitative analyses of the quenched molten iron phase. In addition, we could obtain lattice-fringe images using high-resolution transmission electron microscopy. The electron energy-loss spectroscopy analysis of oxygen in Fe(0.94)O has been carried out with a relative accuracy of 2%, using an analytical procedure proposed for foils thinner than 80 nm. Oxygen K-edge energy-loss near-edge structure also allows us to identify the specific phase that results from quenching and its electronic structure by the technique of fingerprinting of the spectrum with reference spectra in the Fe-O system.

Ab initio calculation of the energy-loss near-edge structure of some carbon allotropes: Comparison with n-diamond

The energy-loss near-edge structure (ELNES) spectra of several carbon allotropes (non-hydrogenated and hydrogenated face-centered cubic (FCC) carbon, rhombohedral carbon, glitter, hexagonite and lonsdaleite) are calculated within the supercell-core-excited density functional theory approach. In particular an experimental ELNES spectrum of ‘new diamond’ (‘n-diamond’) [Konyashin et al., Diamond Relat. Mater. 10, (2001) 99–102] is compared with the ELNES spectra of FCC carbon, rhombohedral carbon and the so-called glitter structure. Our calculations show that the ELNES spectrum considered in that publication cannot be that of FCC carbon.

EELS of Niobium and Stoichiometric Niobium-Oxide Phases—Part I: Plasmon and Near-Edges Fine Structure

A comprehensive electron energy-loss spectroscopy study of niobium (Nb) and stable Nb-oxide phases (NbO, NbO2, Nb2O5) was carried out. In this work (Part I), the plasmons and energy-loss near-edge structures (ELNES) of all relevant Nb edges (Nb-N2,3, Nb-M4,5, Nb-M2,3, Nb-M1, and Nb-L2,3) up to energy losses of about 2600 eV and the O-K edge are analyzed with respect to achieving characteristic fingerprints of Nb in different formal oxidation states (0 for metallic Nb, +2 for NbO, +4 for NbO2, and +5 for Nb2O5). Chemical shifts of the Nb-N2,3, Nb-M4,5, Nb-M2,3, and Nb-L2,3 edges are extracted from the spectra that amount to about 4 eV as the oxidation state increases from 0 for Nb to +5 for Nb2O5. Four different microscopes, including a 200 keV ZEISS Libra with monochromator, were used. The corresponding wide range of experimental parameters with respect to the primary electron energy, convergence, and collection semi-angles as well as energy resolution allows an assessment of the influence of the experimental setup on the ELNES of the different edges. Finally, the intensity of the Nb-L2,3 white-line edges is correlated with niobium 4d-state occupancy in the different reference materials.