Electron Energy-loss Near-edge Spectra Research Articles

Core electron spectroscopic studies for 11 iron-based superconductors

Using density functional theory (DFT)-based first principal calculations we have investigated the electron energy loss near edge spectra (ELNES) of two different iron-based superconducting material from 11 family (FeSe and FeTe). The origin of spectral features in Fe K-edge absorption spectra are thoroughly described in light of Fe/Se/Te atom projected partial density of states (PDOS). When include the “core–hole effect”, we found drastic change in absorption spectra. Our theoretically calculated results including core–hole effect agrees well with experimentally observed X-ray Absorption Near Edge Structure (XANES) data available in literature.

The sp2 and sp3 fractions of unknown carbon materials: electron energy-loss near-edge structure analysis of C-K spectra acquired under the magic-angle condition by the electron nanobeam technique

A method is described for the quantification of the sp2, sp3 and intermediate hybridizations in several carbon (C) material samples. Electron energy-loss near-edge spectra were acquired using fast electrons (120keV) in an electron microscope in nanobeam configuration under the so-called ”magic-angle” condition, and were analysed to extract the sp2 and sp3 fractions, and identify the possible mixed sp2+ε hybridizations. The method consists in projecting the unknown spectra on a basis made up of pure sp2 and sp3 spectra, obtained under the same experimental conditions from graphite and diamond crystals, respectively. The residual spectra contain information about the intermediate hybridizations sp2+ε occurring in the samples. The method was successfully tested on “ab initio” numerically generated spectra relative to amorphous C materials. Finally, it was applied to actual C amorphous and pyrolytic samples, and results were compared to those obtained by the most commonly used, conventional ”three-Gaussian” method. The combined application of electron diffraction and spectroscopy, in the nanobeam configuration, yielded useful information about the atomic and electronic structure from very small volumes of the unknown C material.

Determination of manganese valence states in (Mn3+, Mn4+) minerals by electron energy-loss spectroscopy

Various manganese valence quantification methods using manganese L2,3 and oxygen K electron energy loss near-edge spectra (ELNES) were applied to determine the relative portions of individual valence state in a mixed (Mn3+, Mn4+) valence system. Multiple linear least-squares (MLLS) fitting of Mn L2,3 ELNES using reference spectra and Gaussian peak fitting of Mn L3 edge are newly developed and the feasibility of these methods was tested on a set of cryptomelane minerals with different valence states. The selection of appropriate standards is crucial to the success of the MLLS method. The O K-edge structures for manganese oxides can provide valuable guidance in the selection of appropriate reference spectra for quantitative determination of Mn valence state. Gaussian peak fitting, however, failed to determine the Mn valence for (Mn3+, Mn4+) minerals due to the small separation between the primary L3 peaks from Mn4+ and Mn3+ valence. As to the methods based on calibration curves, the energy difference between Mn L3 and oxygen K, i.e., ΔE (L3-O K) vs. valence, is the most valence-sensitive method in the range of Mn3+ and Mn4+ and yields good agreement with the actual values.

Quantification of Manganese Valence States Using Mn L2,3 Electron Energy-Loss Near-Edge Spectra

Extended abstract of a paper presented at Microscopy and Microanalysis 2009 in Richmond, Virginia, USA, July 26 – July 30, 2009

Density functional theory calculations of energy-loss carbon near-edge spectra of small diameter armchair and zigzag nanotubes: Core-hole, curvature, and momentum-transfer orientation effects

We perform density functional theory calculations on a series of armchair and zigzag nanotubes of diameters less than 1nm using the all-electron Full-Potential(-Linearised)-Augmented-Plane-Wave (FPLAPW) method. Emphasis is laid on the effects of curvature, the electron beam orientation and the inclusion of the core-hole on the carbon electron energy loss K-edge. The electron energy loss near-edge spectra of all the studied tubes show strong curvature effects compared to that of flat graphene. The curvature induced $\pi-\sigma$ hybridisation is shown to have a more drastic effect on the electronic properties of zigzag tubes than on those of armchair tubes. We show that the core-hole effect must be accounted for in order to correctly reproduce electron energy loss measurements. We also find that, the energy loss near edge spectra of these carbon systems are dominantly dipole selected and that they can be expressed simply as a proportionality with the local momentum projected density of states, thus portraying the weak energy dependence of the transition matrix elements. Compared to graphite, the ELNES of carbon nanotubes show a reduced anisotropy.

Quantification of ferrous/ferric ratios in minerals: new evaluation schemes of Fe L 23 electron energy-loss near-edge spectra

Determination of Fe3+/ΣFe in minerals at submicrometre scale has been a long-standing objective in analytical mineralogy. Detailed analysis of energy-loss near-edge structures (ELNES) of the Fe L 23 core-loss edges recorded in a transmission electron microscope (TEM) provides chemical information about the iron oxidation state. The valence-specific multiplet structures are used as valence fingerprints. Systematic investigations on the Fe L 23 ELNES of mono and mixed-valence Fe-bearing natural minerals and synthetic solid solutions of garnets (almandine-skiagite and andradite-skiagite), pyroxenes (acmite-hedenbergite) and spinels (magnetite-hercynite) are presented where the presence of multiple valence states is distinguished by a splitting of the Fe L 3 edge. We demonstrate the feasibility of quantification of the ferrous/ferric ratio in minerals by analyzing the Fe L 23 ELNES as a function of the ferric iron concentration resulting in three independent methods: (1) The method of the modified integral intensity ratio of the Fe L 23 white lines employs two 2-eV-wide integration windows centring around both the Fe L 3 maximum for Fe3+ and the Fe L 2 maximum for Fe2+. This refined routine, compared to the previously published quantification method of the ferrous/ferric ratio in minerals, leads to an improved universal curve with acceptable absolute errors of about ±0.03 to ±0.04 for Fe3+/ΣFe ratios. (2) The second method uses a simple mathematical description of the valence-dependent splitting of Fe L 3 ELNES by fitting several Gaussian functions and an arctan function. The systematic analysis of the integral portions of the individual Gaussian curves for different mineral groups provides a further Fe3+/ΣFe quantification method with an absolute error of about ±0.02 to ±0.03. (3) The Fe L 3 ELNES can also be modelled with the help of reference spectra, whereby the Fe3+/ΣFe ratio can be determined with an absolute error of ca. ±0.02.

Density-functional modelling of core-hole effects in electron energy-loss near-edge spectra

A series of (MgO) n supercells ( n=1,4,8,16,32) with three-dimensional periodic boundary conditions is investigated by density-functional band-structure calculations. The influence of supercell size and shape on calculated electron energy-loss near-edge spectra is assessed quantitatively, employing the Z+1 approximation for the representation of final-state effects. Relevant convergence criteria are the length scale set by the spatial extension of the valence-electron screening cloud around the core hole and the interaction energy of neighbouring core-hole centres. A sufficient supercell size provided, the Z+1 approximation yields a highly satisfactory description of excitations from the 1s shell of light elements, such as Mg and O, compared to experimental data. For comparison, pseudopotentials for excited states were generated for Mg, both with a large core (1s, 2s, 2p orbitals) and a small core (1s orbital only) included into the pseudopotential. The corresponding calculations with frozen core holes lead to very good agreement with the results from the Z+1 calculation for the 1s excitations. The explicit treatment of the subvalence shell (2s, 2p), however, is mandatory for the proper modelling of excitations from orbitals higher than 1s. This indicates that the core polarisability plays an important role in excitations from more extended shells.

Electronic Structure and Bonding of β‐SiAlON

The α‐ and β‐SiAlONs are ceramic solid solutions with charge‐neutral substitutions in α‐ and β‐Si3N4. They have high potential for applications as structural materials. We have calculated the electronic structure and bonding of β‐Si6—zAlzOzN8—z for z= 0, 1, 2, 3, 4 using a first‐principles method. Total energy calculations show that the bulk modulus of β‐Si6—zAlzOzN8—z decreases as z increases, in general agreement with experimental trends. Simultaneous substitution of the (Si,N) pair by (Al,O) results in impurity‐like states in the upper portion of the bandgap of β‐Si3N4. As z increases, more and more states are introduced into the gap, forming a new conduction band (CB) edge for SiAlON. At z= 4, the calculated bandgap is ∼1.3 eV. Density of states (DOS) calculations show the top of the valence band remains steep for all z, and the bottom of the CB is formed predominately by Si—O antibonding states. Orbitally resolved partial DOS calculations in the CB region are used to predict the trends of the electron‐energy‐loss near‐edge spectra (ELNES) of Si‐L2,3, Al‐L2,3, Si‐K, Al‐K, O‐K, and N‐K edges in β‐SiAlON. The impurity‐like states near the CB edge result in pre‐edge structures in all ELNES spectra. Effective charge and bond order calculations show that the overall bond strength in β‐SiAlON decreases only slightly as z increases. Although the stronger Si—N bonds are replaced by weaker Al—O bonds, the remaining Si—N and Al—O bonds actually strengthen as z increases because of the effective charge redistribution after substitution. This is a very interesting finding that may partly explain the superior mechanical properties of the SiAlON system that render them suitable for structural applications.