Successful application of spatial difference technique to electron energy-loss spectroscopy studies of Mo/SrTiO3 interfaces

The electron energy‐loss near‐edge structure (ELNES) of Mo/SrTiO₃ interfaces has been studied using high spatial resolution electron energy‐loss spectroscopy (EELS) in a dedicated scanning transmission electron microscope. Thin films of Mo with a thickness of 50 nm were grown on (001)‐orientated SrTiO₃ surfaces by molecular beam epitaxy at 600 °C. High‐resolution transmission electron microscopy revealed that the interfaces were atomically abrupt with the (110)_Mo plane parallel to the substrate surface. Ti‐L_2,3 (～460 eV), O‐K (～530 eV), Sr‐L_2,3 (～1950 eV) and Mo‐L_2,3 (～2500 eV) absorption edges were acquired by using the Gatan Enfina parallel EELS system with a CCD detector. The interface‐specific components of the ELNES were extracted by employing the spatial difference method. The interfacial Ti‐L_2,3 edge shifted to lower energy values and the splitting due to crystal field became less pronounced compared to bulk SrTiO₃, which indicated that the Ti atoms at the interface were in a reduced oxidation state and that the symmetry of the TiO₆ octahedra was disturbed. No interfacial Sr‐L_2,3 edge was observed, which may demonstrate that Sr atoms do not participate in the interfacial bonding. An evident interface‐specific O‐K edge was found, which differs from that of the bulk in both position (0.8 ± 0.2 eV positive shift) and shape. In addition, a positive shift (0.9 ± 0.3 eV) occurred for the interfacial Mo‐L_2,3, revealing an oxidized state of Mo at the interface. Our results indicated that at the interface SrTiO₃ was terminated with TiO₂. The validity of the spatial difference technique is discussed and examined by introducing subchannel drift intentionally.     

Electron energy-loss spectroscopy study of thin film hafnium aluminates for novel gate dielectrics

We have used conventional high‐resolution transmission electron microscopy and electron energy‐loss spectroscopy (EELS) in scanning transmission electron microscopy to investigate the microstructure and electronic structure of hafnia‐based thin films doped with small amounts (6.8 at.%) of Al grown on (001) Si. The as‐deposited film is amorphous with a very thin (～0.5 nm) interfacial SiO_x layer. The film partially crystallizes after annealing at 700 °C and the interfacial SiO₂‐like layer increases in thickness by oxygen diffusion through the Hf‐aluminate layer and oxidation of the silicon substrate. Oxygen K‐edge EELS fine‐structures are analysed for both films and interpreted in the context of the films’ microstructure. We also discuss valence electron energy‐loss spectra of these ultrathin films.     

Extended energy-loss fine structure spectroscopy of structural modifications in Nd2CuO4−δ at the oxygen K edge

The discovery of the superconducting electron-doped compound Nd₁₈₅Ce₀₁₅CuO_4?δ has stimulated great interest in its micro- and crystal structure, since the superconducting properties depend on parameters such as nonstoichiometry, phase composition, heat treatment and microstructure. The work presented herein is focused on the determination of the oxygen environment in the undoped parent compound Nd₂CuO₄ and in the structural modification Nd₂CuO₃₅ The analysis of the oxygen K (O 1s) edge extended electron energy-loss fine structure (EXELFS) of the tetragonal parent compound Nd₂CuO₄ and of the orthorhombic modification Nd₂CuO₃₅ is reported by using electron energy-loss spectroscopy in combination with transmission electron microscopy. Nd₂CuO₃₅ is produced by in situ heating and reduction of Nd₂CuO₄ in the transmission electron microscope. The EXELFS of the O 1s electron energy-loss edges is analysed with the classical extended X-ray absorption fine structure (EXAFS) treatment and compared with ab initio multiple scattering EXAFS calculations for both structural modifications. Highly accurate information on the local atomic environment of the oxygen atoms in Nd₂CuO₃₅ is obtained from EXELFS analysis using Nd₂CuO₄ as a standard. The results are in accordance with the structural data gained from X-ray diffraction analysis. This applies especially to the more complicated structure of Nd₂CuO₃₅ determined recently.     

Electron spectroscopic study (ESI,EELS) of Nanoplast-embedded mammalian lung

The potential of Nanoplast melamine resin embedding for the study of mammalian lung parenchyma was examined by means of electron spectroscopic imaging (ESI) and electron energy-loss spectroscopy (EELS). Samples were either fixed with glutaralde-hyde-paraformaldehyde or glutaraldehyde-tannic acid, or were directly transferred to the embedding medium without prior fixation. Organic dehydrants, as well as fixatives containing heavy metals and stains, were omitted. A very high level of ultrastructural detail of chromatin, ribosomes, mitochondria and plasma membranes was achieved by ESI from the Nanoplast-embedded samples. The most prominent gain in ultrastructural detail was achieved when moving from an energy loss just below the L_2,3 edge of phosphorus at 132 eV to an energy loss just beyond this edge. This reflects the prominent P L_2,3 edge observed by EELS of Nanoplast-embedded samples in comparison with conventionally processed samples. Thus, taking into account possible sectioning artefacts, excellent heterochromatin images which rely on the phosphorus distribution can be obtained from Nanoplast-embedded samples by computer-assisted analysis of electron spectroscopic images. In this respect glutaraldehyde-paraformaldehyde fixation is preferable to glutaraldehyde-tannic acid fixation because the presence of silicon, revealed by EELS, in tannic-acid-fixed samples may introduce artefacts in phosphorus distribution images obtained by the three-window method because of the close proximity of the L_2,3 edges of silicon and phosphorus.     

The electron energy-loss near-edge structure (ELNES) on the N K-edges from the transition metal mononitrides with the rock-salt structure and its comparison with that on the C K-edges from the corresponding transition metal monocarbides

This paper presents the shapes of the electron energy-loss near-edges structure (ELNES) on the N K-edge of the group IV_A (Ti, Zr, Hf) and group V_A (V, Nb, Ta) transition metal mononitrides close to stoichiometry. With the exceptions of NbN and TaN, these compounds have the rock-salt (B1) structure when close to stoichiometry. NbN exists with both the rock-salt structure and a hexagonal structure. Two distinct ELNES shapes were observed from it, one of which corresponds closely with previously published data from the rock-salt structure. Under normal conditions, TaN is considered to exist only in the hexagonal form, the rock-salt form being a high-temperature/high-pressure phase although it has been reported as the result of plasma jet heating of the hexagonal form. Again two distinct ELNES shapes were observed, one of which appeared to fit into the pattern of the shapes from the other compounds with the rock-salt structure. The systematic changes of shape observed are very similar to those observed in the equivalent carbides and qualitatively follow the behaviour expected from theoretical band structures. The change in the chemical shift of the N K-edge on going from a group IV_A nitride to a group V_A nitride is ～-0·8 eV while that on going from a group IV_A carbide to a group V_A carbide is ～+0·8 eV. This difference in behaviour is explained as the result of differences in the densities of states at the Fermi levels of the compounds. The position of the first peak in the ELNES also shows a systematic change in its energy relative to the core state as the number of valence electrons in the compound increases and also as the transition series of the metal species changes. The energies, E_r, of the peaks in the ELNES relative to the threshold follow a relationship similar to that predicted by Natoli, i.e. (E_r - V)a = const. where V is the ‘muffin tin’ potential and a is the lattice parameter. The first peak gives a negative constant in the relationship. The value of constant increases for each subsequent peak up to the sixth becoming positive for the fourth and higher peaks but drops slightly on going from the sixth to the seventh peak. Each peak gives a different value of V in the relationship. The data sets for the carbides and the nitrides are systematically different in a similar way for each peak and there are deviations from linearity within each set. The systematic difference is minimized and the linearity significantly improved if the difference in the energies of two prominent peaks is used instead of E_r. This systematic variation of peak energy with lattice parameter can be used to predict the lattice parameter. If both the nitride and the carbide data for the energy of a prominent peak relative to the threshold are used, this results in a maximum deviation of 4% (or ～0·02 nm). However, if the differences in the energies of two prominent peaks are used and the data for the carbides and the nitrides are treated independently, the maximum deviation drops to 0·4% (or ～0·002 nm). At this level, uncertainties in the lattice parameters themselves come into play and better characterized materials are required to set true limits to the accuracy of the predictions. Finally some applications in the microanalysis of materials are outlined briefly.     

Quantification of the Ti oxidation state in BaTi1−xNbxO3 compounds

The Ti oxidation state of a series mixed-valence BaTi_1−xNb_xO₃ 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 Ti⁴⁺ and Ti³⁺ components is determined in each compound by linear profile fitting with Ti⁴⁺ and Ti³⁺ standard spectra obtained from reference compounds within the series. The fitting results indicate an increase in the fraction of the Ti³⁺ component as the Nb content increases. A deviation from the expected Ti³⁺ 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.     

Contribution of energy-filtering TEM to the detection of calcium: Application to mast cells

The ultrastructural distribution and quantification of calcium in mast cells prepared by anhydrous processing was investigated by energy-filtering transmission electron microscopy (EFTEM) using a Zeiss 902 electron microscope. Optimal conditions for calcium detection were determined using inorganic (calcium phosphate) and organic (calcium-loaded chelex beads) standards with known amounts of calcium. Electron energy-loss spectroscopy (EELS) revealed calcium at the L_2,3 edge and also at the M_2,3 edge for all specimens examined. Comparison with X-ray microanalysis confirmed the results obtained with EELS. Electron spectroscopic imaging (ESI) was applied for mapping calcium both in standards and in cells and we showed that mast cell granules were the main site of calcium localization. Although, results have shown that a combination of analytical techniques is required to obtain reliable results.     

Study of changes in L32 EELS ionization edges upon formation of Ni-based intermetallic compounds

EELS L₃₂ ionization edges in several Ni‐based intermetallic compounds have been studied and interpreted in terms of the distribution of electrons in the valence d‐bands. It is demonstrated that the integral EELS cross‐sections change only slightly upon the formation of intermetallic compounds and therefore the charge transfer between atoms is negligible. On the other hand, the changes in the fine energy‐loss near‐edge structure (ELNES) of the Ni L₃ edge can be readily detected indicating an important redistribution of d‐electrons at the Ni site with alloying. These features are well reproduced by ab‐initio calculations with a FLAPW method in its WIEN97 implementation. In contrast to the drastic effect of chemical environment, structural transformations in the investigated intermetallics result in smaller ELNES changes, which can be detected by only exceptional instruments with a higher energy resolution.     

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_0.94O 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.     

An EELS-based study of the effects of pyrolysis on natural carbonaceous materials used for activated charcoal preparation

Electron energy‐loss spectroscopy (EELS) has been used to characterize the electronic structure of charcoal phases at the nanoscale, thus demonstrating that the technique can be applied to environmental science. Activated charcoal is extensively used to remove pollutants from liquid and gaseous sewage. It is mainly obtained by activation of coke or charcoal produced from ligneous precursors. The present study concerns the use of by‐products of local Caribbean agriculture, such as sugar cane bagasse, fruit stones and seeds, for use as activated charcoal precursors. Charcoal phases are prepared by high‐temperature pyrolysis of lignocellulosic raw materials under a nitrogen gas flow. With the aim of optimizing the pyrolysis temperature and duration and oxygen content, the concentration of carbon sp² hybridized chemical bonds and structural ordering have been followed by EELS for different treatment temperatures. To quantify the carbon sp² content, near edge structure (NES) at the carbon K edge has been measured to determine the strength of π → π* and 1s → π* transitions. Three precursors of plant origin, shells of Terminalia catappa and Acrocomia karukerana and seeds of Psidium guajava, with the pyrolysis temperatures between 600 and 900 °C, were investigated. The fraction of carbon sp² bonding is found to increase when the temperature rises from 600 °C to the range 700–750 °C and becomes stable at higher temperatures. For temperatures in excess of 700 °C, structural ordering probably occurs and well‐defined 1s → σ* NES is present, whose intensity increases with increasing preparation temperature. For the highest temperature of around 900 °C, the structure of the final product is less well organized than graphitized carbon but a few per cent of a highly ordered phase is found.     

Momentum-resolved EELS and EMCD spectra from the atomic multiplet theory: Application to magnetite

While the energy loss near edge structures of metallic crystals can be calculated with a good accuracy using density functional theory based codes, core-level spectra of transition metal oxides show pronounced multiplet effects which are better described by atomic multiplet codes. We describe the formalism which allows to calculate momentum-resolved electron energy loss spectra in the electric dipole approximation from the atomic multiplet theory, and we apply this formalism to the calculation of energy loss magnetic chiral dichroic spectra of magnetic transition metal oxides. Explicit results are given for magnetite Fe₃O₄.     

Elemental composition of pyroantimonate precipitates analysed by electron spectroscopic imaging (ESI) and electron energy-loss spectroscopy (EELS) in vitellogenic ovarian follicles of Drosophila

Ca²⁺ was precipitated with potassium antimonate in vitellogenic follicles of the fruit fly Drosophila melanogaster and the distribution of the precipitates formed was studied by electron microscopy. The microvilli of the oolemma in mid- and late vitellogenic follicles were lined with precipitates. The chemical composition of the precipitates was analysed by electron spectroscopic imaging (ESI). The images produced by inelastically scattered electrons at specific ionization edges were compared, and the nonspecific background signals were subtracted by an image processing system. The presence of Ca²⁺, antimony and oxygen in the precipitates formed could be demonstrated. The elemental composition of the precipitates and of yolk spheres was also analysed by electron energy-loss spectroscopy (EELS). With respect to the precipitates, signals at the calcium L_2,3-edge, the oxygen K-edge and the antimony M_4,5-edge were recorded without deconvolution and background subtraction. The yolk spheres, which were free of precipitates, gave the characteristic signal of the nitrogen K-edge. The applied techniques combine good ultrastructural resolution with the possibility of analysing the elemental composition of histochemical reaction products and cellular structures.     

Linear least-squares fit evaluation of series of analytical spectra from planar defects: extension and possible implementations in scanning transmission electron microscopy

In a previous paper, a new technique was introduced to determine the chemistry of crystallographically well‐defined planar defects (such as straight interfaces, grain boundaries, twins, inversion or antiphase domain boundaries) in the presence of homogeneous solute segregation or selective doping. The technique is based on a linear least‐squares fit using series of analytical (electron energy‐loss or energy‐dispersive X‐ray) spectra acquired in a transmission electron microscope that is operated in nano‐probe mode with the planar defect centred edge‐on. First, additional notes on the use of proper k‐factors and determination of Gibbsian excess segregation are given in this note. Using simulated data sets, it is shown that the linear least‐squares fit improves both the accuracy and the robustness to noise beyond that obtainable by independently repeated measurements. It is then shown how the method originally developed for a stationary nano‐probe mode in transmission electron microscopy can be extended to a focused electron beam that scans a square region in scanning transmission electron microscopy. The necessary modifications to scan geometry and corresponding numerical evaluation are described, and three different practical implementations are proposed.     

Electron energy-loss near edge structure (ELNES) of InGaN quantum wells

Lasers and light‐emitting diodes (LEDs) that emit in the blue to green region are often based on In_xGa_1–xN quantum well structures. Ionization edges in the electron energy‐loss spectrum contain fine structures (called the energy‐loss near edge structure (ELNES)) and provide information about the electronic structure. In this paper we compare the experimental and calculated ELNES for the N‐K ionization edge of In_xGa_1–xN quantum wells. When the effects of the core‐hole are included in the calculations, agreement between experimental and calculated spectra is very good. Strain has been shown to accentuate the effects of In on the ELNES and moves the ionization edge onset down in energy, relative to the other features. These results suggest that ELNES may provide an alternative method to lattice imaging to determine the presence of strain in this system.     

Microscopy and microanalysis of crystalline glazes

Crystalline glazes on ceramic plates produced commercially in the U.K. and on ceramic pots produced commercially in Taiwan and Spain have been examined by X‐ray diffraction, conventional and polarized light microscopy, and scanning electron microscopy in order to identify the crystalline phases present in the glazes and to ascertain through X‐ray microanalysis the partitioning behaviour of the transition metal ions used to colour the glazes and the crystals within them. In each case examined, the macroscopic two‐dimensional spherulites within the glazes clearly seen by the naked eye were found to consist of large numbers of radially orientated acicular crystals each 5 µm or less in width embedded within the silica‐rich glaze. Energy dispersive X‐ray microanalysis and X‐ray diffraction of these crystals identified these crystals as willemite, α‐Zn₂SiO₄. The strong [001] texture of these crystals within the glaze evident from the X‐ray diffraction patterns was consistent with polarized light microscopy observations of the willemite crystals. In addition to willemite, small iron‐doped gahnite (ZnAl₂O₄) crystals were found in a honey‐coloured crystalline glaze and acicular rutile (TiO₂) crystals were found in the Portmeirion Pottery plates examined. Transition metal ions with a preference for tetrahedral coordination were observed to substitute for Zn²⁺ ions in willemite and to partition preferentially to the willemite crystals, whereas ions preferring octahedral coordination preferred to remain in the glaze.     

Microscopy and microanalysis of complex nanosized strengthening precipitates in new generation commercial Al–Cu–Li alloys

Precipitates (ppts) in new generation aluminum–lithium alloys (AA2099 and AA2199) were characterised using scanning and transmission electron microscopy and atom probe tomography. Results obtained on the following ppts are reported: Guinier–Preston zones, T₁ (Al₂CuLi), β’ (Al₃Zr) and δ’ (Al₃Li). The focus was placed on their composition and the presence of minor elements. X‐ray energy‐dispersive spectrometry in the electron microscopes and mass spectrometry in the atom probe microscope showed that T₁ ppts were enriched in zinc (Zn) and magnesium up to about 1.9 and 3.5 at.%, respectively. A concentration of 2.5 at.% Zn in the δ’ ppts was also measured. Unlike Li and copper, Zn in the T₁ ppts could not be detected using electron energy‐loss spectroscopy in the transmission electron microscope because of its too low concentration and the small sizes of these ppts. Indeed, Monte Carlo simulations of EEL spectra for the Zn L_2,3 edge showed that the signal‐to‐noise ratio was not high enough and that the detection limit was at least 2.5 at.%, depending on the probe current. Also, the simulation of X‐ray spectra confirmed that the detection limit was exceeded for the Zn K_α X‐ray line because the signal‐to‐noise ratio was high enough in that case, which is in agreement with our observations.     

Dislocations in nanostructured two‐phase Fe30Ni20Mn20Al30

In a previous study, the dislocations in Fe₃₀Ni₂₀Mn₂₅Al₂₅ (at. %), which consist of 50 nm wide alternating b.c.c. and B2 phases, were shown to have a/2<111> Burgers vectors after room temperature deformation. The dislocations were found to glide in pairs on both {110} and {112} slip planes and were relatively widely separated in the b.c.c. phase, where the dislocations were uncoupled, and closely spaced in the B2 phase, where the dislocations were connected by an anti‐phase boundary. In this article, we analyze the dislocations in the two ～5 nm‐wide B2 phases in a related two‐phase alloy Fe₃₀Ni₂₀Mn₂₀Al₃₀, with compositions Fe‐23Ni‐21Mn‐24Al and Fe‐39Ni‐12Mn‐34Al, compressed to ～3% strain at a strain rate 5 × 10^?4 s^?1 at 873 K (the lowest temperature at which substantial plastic flow was observed). It is shown that slip occursby the glide of a<100> dislocations. A review of the literature suggests that the differences in the observed slip vector between these B2 phases could be due to the differences in composition, differences in deformation temperature, or possibly both. Microsc. Res. Tech. 76:263–267, 2013. © 2013 Wiley Periodicals, Inc.     

In situ high temperature microscopy study of the surface oxidation and phase transformations in titanium alloys

Two popular commercial titanium alloys, Ti‐6Al‐4V and Ti‐6Al‐2Sn‐4Zr‐2Mo‐0.08Si, were used for in situ high temperature microscopy study. The experiments were performed on an optical microscope equipped with high temperature stage using both normal and florescence lights. Two kinds of experiments were performed, at continuous heating/cooling with different rates and in isothermal conditions at different temperatures. The changes taking place on the sample surface during the experiments were monitored. The morphology of the α?β?α phase transformation was recorded at different heat treatment conditions using the effect of thermal etching. An effect of sample surface oxidation and deoxidation was observed during continuous heating. The appearance and disappearance of ordered titanium oxides Ti₃O and Ti₂O are discussed based on the phase equilibrium diagram. The kinetics of the surface oxidation was monitored in both isothermal and continuous cooling conditions.     

Quantitative and qualitative characterization of aquatic iron oxyhydroxide particles by EF-TEM

Electron energy-loss spectroscopy (EELS) and elemental imaging under the energy-filtered transmission electron microscope are powerful tools for the characterization of iron-rich particles present in natural waters. Features present in EEL spectra (Fe-M_2,3 Fe-L_2,3 and O-K ionization edges) of goethite (α-FeOOH) have been studied with an energy filter operated at 80 keV to determine optimal quantification and elemental imaging of Fe-rich natural aquatic particles in the 30–200 nm range of thickness. For quantitative aims, the Fe-M_2,3 ionization edge cannot be used easily, but the Fe-L_2,3 edge provides more accurate results owing to a better background extrapolation. The partial cross-section of the Fe(III) M shell has been determined for iron oxide. The use of two-windows (jump-ratio) and three-windows (background stripping) imaging methods is discussed in relation to the specimen thickness.     

New examples for near-edge fine structures in electron energy loss spectroscopy

The electron energy loss near-edge structures of the K and L edges of elements occurring in sulfides and oxides of copper and zinc have been investigated. Furthermore, the K edge of carbon in some carbonates and the L edges of sulfur and phosphorus in sulfates and phosphates have been measured and the near-edge fine structures of these edges have been found to be characteristic for the complex ions CO^2-₃, SO^2-₄. Thus, near-edge fine structures of EELS edges can be very useful as a fingerprint for rapid identification of chemical compounds such as carbonates, sulfates, phosphates and some oxides in the TEM.