Energy-loss Near-edge Structure Research Articles

Ferric iron in Al-bearing akimotoite coexisting with iron-nickel metal in a shock-melt vein in an L-6 chondrite

We report evidence for high ferric iron to total iron (Fe 3+ /∑Fe) ratios in Al-bearing akimotoite coexisting with other high-pressure silicates and Fe-Ni metal from shock melt-veins in the Sixiangkou (L-6) chondrite. The measurements were made using electron energy-loss near-edge structure (ELNES) spectroscopy. The results demonstrate that akimotoite in shock-melt veins of this meteorite has high proportions of Fe 3+ , with a Fe 3+ /∑Fe ratio of 0.67(3). In contrast, the coexisting majoritic garnet and ringwoodite, which are the typical Fe-bearing phases in shock veins in this meteorite, are enriched in Fe 2+ rather than Fe 3+ , with Fe 3+ /∑Fe ratios of 0.10(5) and 0.15(5), respectively. We conclude that the higher affinity of Fe 3+ for akimotoite, rather than for the other dense silicate phases, is related strongly to the substitution mechanism of trivalent cations. This mechanism is described as VI(A) Fe 3+ + VI(B) Al 3+ = VI(A) Mg 2+ + VI(B) Si 4+ in the ABO 3 structural formula of MgSiO 3 -ilmenite under high pressures and temperatures, and operates even at a low oxygen fugacity where Fe-Ni metal is stable.

Trichroism in energy-loss near-edge structure spectroscopy: Polarization dependence of near-edge fine structures

The goal of this paper is to relate the current of inelastically scattered electrons collected in a transmission electron microscope (TEM) to the double differential electron energy-loss cross section. Up to now, this relationship, which depends on the point symmetry around the probed atom site, has been essentially studied in a situation called dichroism. This situation can be encountered when a principal threefold, fourfold, or sixfold rotation axis through the probed atom site exists. The electron energy-loss cross section is then a linear combination of longitudinal and transversal cross sections, and the weights of these components are cos{sup 2} {theta}{sub q} and sin{sup 2} {theta}{sub q}, where {theta}{sub q} is the angle between the scattering wave vector q and the principal rotation axis. The first aim of this paper is to find the dependence on q of the cross section in all other cases, that is to say, when the symmetry around the probed atom site is described with one of the eight low symmetry point groups C{sub 1}, S{sub 2}, C{sub 1h}, C{sub 2}, C{sub 2h}, C{sub 2v}, D{sub 2}, and D{sub 2h}. In these eight cases of low symmetry, three distinct situations called trichroism can be distinguished. Inmore » these situations, the cross section is expressed in terms of the cross sections obtained for six, four, or three particular orientations of the scattering wave vector. The second aim of this paper is to provide an expression of the inelastically scattered electron current collected in a TEM for these three situations of trichroism. This current is expressed in terms of experimental parameters, such as the incident beam convergence, the collector acceptance, the electron beam kinetic energy, and the sample orientation. As in the case of dichroism, magic conditions can be found, for which the collected current becomes independent of the single-crystal sample orientation. The case of the C K edge in the nonstoichiometric V{sub 6}C{sub 5} metallic carbide with a trigonal symmetry is given as an illustration.« less

Oxidation Mapping by Postpeak Technique

Department of Physics, University of Alberta, Edmonton T6G 2G7, CanadaSpatially resolved energy -loss near-edge structure (ELNES) reveals geometric and electronic structure of materials at high spatial resolution [1]. In scanning transmission ele ctron microscope (STEM), ELNES allows to study the valence state of atoms at interfaces of a nanocomposite system. The obtainable signal-to-noise ratio (SNR), set by specimen drift and radiation damage, is an obstacle in obtaining interpretable ELNES data at high spatial resolution. The white line ratio (WLR) is often used as a “fingerprinting” technique in identifying the oxidation of transition metals (TMs) [2], but hard to extract when the SNR is low. We recently found that a broad postpeak following the L-edge, observed in thin specimens where plural scattering is negligible, is characteristic of TMs in an oxidized state [3]. The postpeak was applied to identify surface oxidation of nanoparticles (NP) by examining average oxidation state of many NPs [4]. Here we show that the postpeak can be a convenient and sensitive indicator of oxidation of single iron NP by forming point analysis using a STEM with a small (0.5 nm) probe. The measurements were performed on isolated Fe NPs embedded in silica, shown in the high-angle annular dark-field (HAADF) image (Fig. 1a). The data were obtained using a JEOL 2200 FS (a 200 kV Schottky field emission instrument equipped with an W filter). Fig. 1b shows the ELNES of Fe L-edges recorded at different sites of a selected particle, as labelled in HAADF image in Fig. 1a. The signal comes mostly from the surface of the NP when the beam is focused at its edge, and mainly from the interior when the beam is focused at the center. A broad postpeak, about 40 eV above the onset of Fe L

Experimental electron energy loss spectroscopy of clays

The behaviour of clays is not still well understood. Most information concerning clays has been obtained by techniques which give statistical information on the structure and chemistry. However papers have reported results from scanning and high resolution electron microscopy. This work presents a nanoscopic approach, using electron energy loss spectroscopy (EELS), of different purified clays. EELS permitted to detect all the elements found by classical chemistry at a macroscopic level. In particular it made it possible to determine the Si/Al ratio in kaolinite (Si/Al ∼ 1), smectite and illite (Si/Al ∼ 2). In all the cases, the K edge oxygen energy loss near edge structure (ELNES) is often similar to that of other clays we studied, but a strong heterogeneity has been observed. It was also possible to highlight an influence of the presence of iron on the profile of the oxygen peak.

Energy-loss near-edge structure (ELNES) and first-principles calculation of electronic structure of nickel silicide systems

The electronic structures of nanometre-sized nickel silicide systems, Ni 2Si and NiSi, have been studied by energy-loss near-edge structure (ELNES) and first-principles band structure calculations. Experimental ELNES of Ni L 3- and Si L 2,3-edges could be explained well using theoretical spectra calculated for the ground state without the core hole, suggesting metallic properties for both silicides. It was shown that a slight difference in ELNES spectra of Ni 2Si and NiSi comes from the coupling among the Ni d and Si p, d states in the unoccupied bands. The density of states and the contour plots of all the valence electron densities for Ni 2Si, NiSi together with NiSi 2 show that Ni 2Si has the bond with the strongest covalent character between Ni and Si atoms and the most transition metal-like character of the Ni 3d band among the three silicides.

Manifestation of incommensurate structural modulations in the Ti- L 2,3 electron energy-loss near-edge structure of Sr2TiSi2O8

Structural modulations hosted by complex oxides are–beyond relative orientation changes of ‘rigid units’–accompanied by characteristic distortions of coordination polyhedra. Electronic structure calculations in the local density approximation, performed at model clusters derived from positionally modulated structures, proved that the molecular–orbital structure also sensitively reflects such distortions. The latter crystal-field splitting of unoccupied states is accessible by high-resolution electron energy-loss near-edge structure (ELNES) spectroscopy. However, since ELNES spectra average over many different individual target-ion environments, the resulting ELNES possesses significantly broadened spectral features. In this contribution, such broadening is shown to dramatically influence the spectral shape of the Ti-L 2,3 ELNES acquired at the strongly modulated fresnoite framework compound Sr2TiSi2O8.

The effect of valence state and site geometry on Ti L3,2 and O K electron energy-loss spectra of TixOy phases

Titanium L 3,2 and O K electron energy loss near-edge structures (ELNES) of seven Ti oxides have been measured in a transmission electron microscope to obtain information on the valence state and site geometry of Ti. The coordination of Ti in all phases studied is octahedral, whereas the valence states occurring are Ti2+, Ti3+, and Ti4+. Effects of polyhedra distortions are particularly observed for two oxides with mixed Ti3+-Ti4+ valence state, i.e., the Magneli phases Ti4O7 and Ti5O9. A prominent pre-peak in the Ti L 3 edge is attributed to the orthorhombic polyhedra distortions in these compounds, leading to complex crystal field splitting. The effect of valence state manifests itself in a systematic chemical shift of Ti white lines by 2 eV per valence state. On the basis of collected Ti L 3,2 ELNES spectra we propose a new quantification technique for the determination of Ti4+/Ti3+ ratios. Complementary O K ELNES spectra were well reproduced by Density Functional Theory calculation, revealing that the O K -edge is sensitive to the covalent bonding in all analyzed oxides.

Stability Due to Peripheral Halogenation in Phthalocyanine Complexes

The effect of peripheral halogenation is examined based on analytical transmission electron microscopy and thermal analyses of two chemical family structures, specifically the vanadyl-phthalocyanine family (VOPcX: X = H16, F14.5) and the copper-phthalocyanine family (CuPcX: X = H16, F16, Cl16, Cl8Br8), focusing on the process of molecular changes and crystalline disintegrations. To clarify the molecular transformations, electron energy-loss spectroscopy (EELS) is applied to two fluorinated phthalocyanines (VOPcF14.5 and CuPcF16), by monitoring mass changes as well as energy loss near edge structures (ELNES). The elemental mass of both VOPcF14.5 and CuPcF16 remain constant up to 0.5 C x cm(-2), except in the case of mass reduction attributed to oxygen loss occurring in VOPcF14.5. It is expected that the released oxygen will induce higher radiation damage in VOPcF14.5. Although mass variation is not observed in CuPcF16, it is found from ELNES that the pi resonant system of nitrogen is more radiation sensitive than that of carbon. These results imply that the electron sensitivity in VOPcX is triggered by eliminated oxygen or, thus, an induced larger empty space, whereas the sensitivity of CuPcX is dominated only by a large intermolecular empty space resulting in the following bond alterations. It is also found that the decomposition temperature (Td) measured by thermal analyses and the characteristic dose (D1/e) are exponentially correlated to the "effective molecular occupancy" (Oe) evaluated as a volume function of molecules in unit cells. By measuring Td and/or Oe, we discuss the durability of peripheral halogenation with respect to the radiation damage.

Microstructure of Mn-doped γ-Ga2O3 epitaxial film on sapphire (0001) with room temperature ferromagnetism

Mn-doped Ga2O3 thin film showing room temperature ferromagnetism has been grown on a sapphire (0001) plane by using a pulsed-laser deposition technique. The microstructure of the Mn-doped film is investigated in detail using selected-area electron diffraction, high-resolution transmission electron microscopy (HRTEM), x-ray energy-dispersive spectroscopy, and electron energy-loss spectroscopy, in comparison with an undoped film. Careful diffraction analysis with the [21¯1¯0]Al2O3 and [101¯0]Al2O3 zone axes of the substrates reveals that the Mn-doped film shows the γ-Ga2O3 phase with a defective spinel structure, while the undoped film shows the β-Ga2O3 phase. The orientation relationship between the film and substrate is determined by electron diffraction and HRTEM from the interface region to be (2¯01)β-Ga2O3∕∕(0001)Al2O3 and [102]β-Ga2O3∕∕[21¯1¯0]Al2O3 or [1¯02¯]β-Ga2O3∕∕[21¯1¯0]Al2O3 for the undoped film, and (111)γ-Ga2O3∕∕(0001)Al2O3 and [21¯1¯]γ-Ga2O3∕∕[21¯1¯0]Al2O3 or [2¯11]γ-Ga2O3∕∕[21¯1¯0]Al2O3 for the Mn-doped film. Mn ions are uniformly dissolved in the film with 7.8 cation % and no detectable precipitates are found. Mn-L2, 3 energy-loss near-edge structure reveals that Mn ions take the valency of 2+, which is consistent with Mn-L2, 3 near edge x-ray absorption results in our previous report.

High spatially resolved morphological, structural and spectroscopical studies on copperoxide nanocrystals

Copper oxide nanocrystals decorated on multi-wall carbon nanotubes have been prepared.Comprehensive morphological, structural and spectroscopical studies have been carried outon the nanometre/atomic scale by the combination of high-resolution transmission electronmicroscopy and electron energy-loss near-edge structure in electron energy-lossspectroscopy, which has a high spatially resolved capacity advantage over the normallyused analytical techniques such as x-ray powder diffraction (XRD) and x-rayphotoelectron spectroscopy (XPS). The result reveals that highly crystalline cubicCu2O nanocrystals with highly uniform dispersion, homogeneous size of about 5.3 nm and nearlyspherical morphology are synthesized as the predominant phase, while rare individualmonoclinic CuO nanocrystals with irregular shape are still present as the minor phase. Theanalysis based on the survey result and the structural symmetry difference betweenCu2O and CuO demonstrates that XRD underestimates the presence of the CuO phase withmuch lower structural symmetry while XPS overestimates the proportion of CuO phase.

Ab initio study of electronic densities of states at copper–alumina interfaces

The electronic structure of Cu(1 1 1)/α-Al 2O 3(0 0 0 1) interfaces with two experimentally observed orientation relationships were studied in terms of local site and angular-momentum-projected electronic densities of states. The electronic band structures were calculated using density functional theory in the local density approximation. The local densities of unoccupied states are compared with experimentally determined electron energy loss near-edge structures. A very good coincidence was found for the interfacial oxygen and aluminum atoms. Interpretation guidelines for the electron energy-loss near-edge structures (ELNES) are suggested for further experimental measurements. The local bonding across the interface is analyzed in terms of local densities of occupied states.

Surface effects in the energy loss near edge structure of different cobalt oxides

The energy loss near edge structure (ELNES) of the O-K and Co-L edges of both Co 3O 4 and CoO particles were studied. Significant changes between the edge and nearby thicker regions are found for the O-K edge, the Co-L edge, the O/Co atomic ratio, the separation between the prepeaks in the O-K edge, as well as the L 3/L 2 intensity ratios in the Co-L edge. These reflect the changes in the average oxidation state of the specimen from the edge to nearby thicker regions. These variations originate from a surface layer which dominates the results near the edge of the particles. These variations allow us to infer the surface structure, which indicates that both particles are encapsulated by different non-stoichiometric oxides.

Investigation of Local Coordination and Electronic Structure of Dielectric Thin Films from Theoretical Energy-Loss Spectra

AbstractQuantum mechanical simulations were performed to calculate the valence electron energy-loss spectra (VEELS) for hafnium oxide, hafnium silicate, silicon oxide and silicon systems using the full potential Linearized Augmented Plane Wave (LAPW) formalism within the Density Functional Theory (DFT) framework. The needed energy-loss function (ELF) was derived from the calculation of the complex dielectric tensor within the random phase approximation (RPA). The calculated spectra were compared with experimental scanning transmission electron microscopy (STEM)/EELS of atomic layer deposited (ALD) HfO2 on Si(100) to evaluate their use as a “fingerprint” method that can be used to distinguish among various polymorphs of HfO2 thin films and relate the fine structure to the electronic structure and local bonding environment. Calculated low-loss spectra are found to be in satisfactory agreement with experimental data. Also, the combination of such theoretical calculations and experimental data could be of key importance in our understanding of fundamental issues of these systems. Compared to energy-loss near edge structure (ELNES) or core energy-loss spectra, the ELF calculated for low-loss spectra is computationally less expensive and can prove useful for prompt analysis.

Internal strain formed in oxide ceramics upon spark-plasma sintering

Spark-plasma sintering of oxide ceramics is shown to be capable of provoking internal strain. This finding, which in terms of materials design is invaluable, is based on an in-depth analysis of the Ti-L 2,3and O-Kelectron energy-loss near-edge structure of Ba2TiSi2O8acquired in a dedicated scanning transmission electron microscope. The electronic structure of Ba2TiSi2O8as probed by electron energy loss spectroscopy proved particularly sensitive to distortions of the titanium and oxygen environment.

The effect of hydrogen on the electronic and bonding properties of amorphouscarbon

A series of amorphous carbon and hydrogen-containing amorphous carbon structures hasbeen generated, by using classical Monte Carlo and molecular dynamics simulationtechniques, respectively. The Brenner empirical bond order potential is used to generatehydrogenated amorphous carbon while that by Tersoff is used for amorphous carbon. Theresulting structures are relaxed further using the density functional theory approach.Structures containing 15 at.% hydrogen are generated for various mass densitiesin order to investigate the effect of pressure changes on the properties of thehydrogenated amorphous carbon. The structures are analysed in terms of theirsp3/sp2 ratio. The density of states (DOS), energy-loss near-edge structure (ELNES)and x-ray photoelectron spectroscopy (XPS) calculations are performedwithin the first-principles methodology on the generated carbon systems. TheELNES and the C 1s energy calculations are performed taking into accountthe core–hole effect. Our calculations show that depending on the densityρ,hydrogenated amorphous carbon can be classified into three categories: polymeric at low densities(ρ≤2.0 g cm−3), graphitic at intermediatedensities (2.0 g cm−3<ρ≤2.4 g cm−3) and diamond-likeat high densities (ρ>2.4 g cm−3).

High-resolution EELS investigation of the electronic structure of ilmenites

The electronic structure of a series of compounds belonging to the ilmenite family is investigated using high resolution electron energy loss spectroscopy (EELS). The energy loss near edge structure (ELNES) of the O-K, $\mathrm{Ti}\text{\ensuremath{-}}{\mathrm{L}}_{23}$ and transition metal ${\mathrm{L}}_{23}$ edges have been recorded in $\mathrm{Mn}\mathrm{Ti}{\mathrm{O}}_{3}$, $\mathrm{Fe}\mathrm{Ti}{\mathrm{O}}_{3}$, $\mathrm{Co}\mathrm{Ti}{\mathrm{O}}_{3}$, and $\mathrm{Ni}\mathrm{Ti}{\mathrm{O}}_{3}$ with an energy resolution of $0.20--0.25\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. The formal valency and spin-state of the transition metal are determined unambiguously from the analysis of the ${\mathrm{L}}_{23}$ edges with theoretical ligand field multiplet calculations. The results confirm the evolution of the interatomic distances observed along this series. The O-K edge is analyzed by comparison to the experimental signatures observed in the structurally related ${\mathrm{Ti}}_{2}{\mathrm{O}}_{3}$ compound and using theoretical molecular orbital results.

Magnetic origin and oxydation states of cations in the TC∼440 K ferrimagnet Sr 2Cu(Re 0.69Ca 0.31)O 6

We have studied crystal structure, ionic-model energy calculation, and electron energy-loss spectroscopy (EELS) for a new magnetic ordered-perovskite Sr 2Cu(Re 0.69Ca 0.31)O 6 in order to determine oxidation states of the Re and Cu ions. All of the experimental results of the bond-valence sum, the static energies, and the energy-loss near-edge structure (ELNES) in EEL spectra consistently indicated that the Re 7+, Cu 2+ and Cu 3+ valence states are realized in the compound. This is evident that the Cu spins are responsible for the magnetism in the phase. By considering the Cu 3d and O 2p orbital shapes, we concluded that the unusually high Curie temperature, T C∼440 K, originates in ferrimagnetism with strongly coupled Cu spins in a three dimensional (3-D) network of the Cu 2+–O–Cu 3+–O–Cu 2+ bond in structure.

Experimental Assessment of Structural Differences between Amorphous and Amorphized Matter

The atomic structure of amorphous matter as well as its relation to properties constitute key materials-science challenges of tomorrow. High-resolution transition-metal L 2,3 electron energy-loss near-edge structure spectroscopy in the transmission electron microscope at a barium titanium silicate glass and its amorphized counterpart, prepared by ion irradiation of a single crystal of identical composition, is shown to be capable of distinguishing different cation coordination. The introduced methodology is able to separate polyamorphous phases at nanometer spatial resolution and hence of greatest relevance for the experimental assessment of structural preconditions for sintering of ceramics and controlled devitrification of glasses.

EELS Investigations of Different Niobium Oxide Phases

Electron energy loss spectra in conjunction with near-edge fine structures of purely stoichiometric niobium monoxide (NbO) and niobium pentoxide (Nb2O5) reference materials were recorded. The structures of the niobium oxide reference materials were checked by selected area electron diffraction to ensure a proper assignment of the fine structures. NbO and Nb2O5 show clearly different energy loss near-edge fine structures of the Nb-M4,5 and -M2,3 edges and of the O-K edge, reflecting the specific local environments of the ionized atoms. To distinguish the two oxides in a quantitative manner, the intensities under the Nb-M4,5 as well as Nb-M2,3 edges and the O-K edge were measured and their ratios calculated. k-factors were also derived from these measurements.

Ab initiosimulation of the electron energy-loss near-edge structures at the LiKedge in Li,Li2O, andLiMn2O4

Ab initio simulations of the electron energy-loss near edge structures (ELNES) at the Li $K$ edge were obtained with the WIEN2k code, which is based on the density functional theory (DFT). Since the $1s$ states of the lithium atoms must be considered as semicore states, the standard procedure to calculate the dynamic form factor (DFF) has to be modified. The difficulties raised by the delocalized nature of these states and their influence on the simulated spectra are discussed. Unphysical monopolar transitions are shown to be extremely large and the description of the Li $1s$ orbital as a core or valence state for the self-consistent determination of potentials is proven to have little influence on the calculated spectrum shape. Comparisons between simulated and experimental spectra for three different compounds are presented: Li, ${\mathrm{Li}}_{2}\mathrm{O}$, and $\mathrm{Li}{\mathrm{Mn}}_{2}{\mathrm{O}}_{4}$ (a metal, an insulator, and a transition metal oxide). All simulations fit well the experiments, thus demonstrating that this approach can be applied to a wide range of lithiated compounds. In the case of the Li $K$ edge in ${\mathrm{Li}}_{2}\mathrm{O}$, we also show that the recently proposed partition of the spectrum into two energy range regions, with different core hole strengths, improves the concordance between the simulation and the experiment. Finally, the influence of the real part of the dielectric function, omitted when using the DFF, is assessed. A comparison of Li $K$ edges calculated using either the loss function $[\mathrm{Im}(\ensuremath{-}1∕\ensuremath{\epsilon})]$ or only the DFF (imaginary part of $\ensuremath{\epsilon}$) is presented in detail. We show that polarization effects are particularly strong in the Li $K$ edge of $\mathrm{Li}{\mathrm{Mn}}_{2}{\mathrm{O}}_{4}$ due to the presence of the Mn ${M}_{2,3}$ edge. Other calculations including the GW approximation and the local field effects would, however, be necessary to precisely quantify these effects.