Vibrational excitations and structure of C2H4 adsorbed on Cu(100)

The adsorption of C₂H₄ on Cu(100) at 80 K has been investigated by angle-dependent high-resolution electron energy loss spectroscopy (EELS) and low-energy electron diffraction (LEED). Our observations are consistent with a model where the ethene molecule is adsorbed in a configuration parallel to the Cu(100) surface. The molecule-metal interaction is weak and presumably of π character.     

Structural Investigation of Mixed Nitrided Galloaluminophosphates “AlGaPON” by EELS, XAS, and XPS Spectroscopies

Amorphous nitrided galloaluminophosphates “AlGaPON” catalysts with nitrogen contents varying from 0 to 23.3 wt% N were obtained by nitriding an Al_0.5Ga_0.5PO₄ precursor under ammonia flow at 750°C in a tubular furnace. The structural changes induced by this treatment were investigated by electron energy loss spectroscopy (EELS), X-ray absorption spectroscopy (XAS), and X-ray photoelecton spectroscopy (XPS). XANES and XPS results indicate that the first-coordination spheres of P, Ga, and Al atoms are modified by nitridation. In particular, the comparison of the P XANES spectra recorded on “AlGaPON” and on a PON phosphorus oxynitride (reference of mixed PO₂N₂ tetrahedra) reveals that mainly PO₂N₂ tetrahedra are present in highly nitrided samples. Moreover, the replacement of oxygen by nitrogen probably concerned P-O-Ga bonds rather than P-O-Al. EELS investigation reveals that the precursor is homogeneous at the used probe scale, but indicates that nitridation is accompanied by a loss of homogeneity of the material.     

Application of EELS to the microanalysis of materials

Electron energy loss spectroscopy (EELS) is used in analytical electron microscopy (AEM) because it can provide results on the chemical composition and structure of a small volume of material. The practical application of EELS was demonstrated by the investigation of a refractory hard metal of the type WC-TiC-Co and by the investigation of a BaTiO₃ ceramic material. To demonstrate the present status of quantitative analysis by EELS, the spectra of Be₂SiO₄, TiB₂ and BaTiO₃ were quantified and the results indicate that quantitative analysis is feasible for major concentrations of light elements and also of heavier elements even in the presence of severe edge overlap.     

Niobium nitride films formed by rapid thermal processing (RTP): a study of depth profiles and interface reactions by complementary analytical techniques

The nitridation of niobium films approximately 250 and 650 nm thick by rapid thermal processing (RTP) at 800 °C in molecular nitrogen or ammonia was investigated. The niobium films were deposited by electron beam evaporation on silicon substrates covered by a 100 or 300 nm thick thermally grown SiO₂ layer. In these investigations the reactivity of ammonia and molecular nitrogen was compared with regard to nitride formation and reaction with the SiO₂ substrate layer. The phases formed were characterized by X-ray diffraction (XRD). Depth profiles of the elements in the films were recorded by use of secondary neutral mass spectrometry (SNMS). Microstructure and spatial distribution of the elements were imaged by transmission electron microscopy (TEM) and energy-filtered TEM (EFTEM). Electron energy loss spectra (EELS) were taken at selected positions to discriminate between different nitride, oxynitride, and oxide phases. The results provide clear evidence of the expected higher reactivity of ammonia in nitride formation and reaction with the SiO₂ substrate layer. Outdiffusion of oxygen into the niobium film and indiffusion of nitrogen from the surface of the film result in the formation of oxynitride in a zone adjacent to the Nb/SiO₂ interface. SNMS profiles of nitrogen reveal a distinct tail which is attributed to enhanced diffusion of nitrogen along grain boundaries.     

Direct measurement of the nitrogen content by XPS in self‐passivated TaNx thin films

The nitrogen content in tantalum nitride (TaN_x) thin films, where x indicates that TaN_x is not generally stoechiometric, can be measured directly by XPS. This is the purpose of the present study. However, the XPS spectra of TaN_x present electron energy loss spectroscopy (EELS) peaks that lead to a complex peak fitting, particularly for self‐passivated thin films. A complete peak fitting procedure based upon Tougaard's background, the Doniach‐Sunjic Function and EELS peaks, is presented. It is applied to two self‐passivated TaN_x thin films elaborated by reactive sputtering and presenting a different nitrogen content. The physical properties of these surfaces are interpreted in terms of Ta 4f_7/2 chemical states directly dependent on the nitrogen content. The main results are discussed and improvements are proposed to the method. Copyright © 2008 John Wiley & Sons, Ltd.     

TEM-induced structural evolution in amorphous Fe oxide nanoparticles

Exposure to the high energy electron beam of a TEM changes the morphology of amorphous Fe oxide nanoparticles from solid spheres to hollow shells. Amorphous Fe oxide nanoparticles prepared via high-temperature methods using hexadecylamine and trioctylphosphine oxide surfactants were compared to crystalline gamma-Fe2O3 particles of similar size. Both sets of particles are fully characterized via SQUID magnetometry, X-ray powder diffraction, BET surface analysis, EPR spectroscopy, high-resolution transmission electron microscopy (TEM), and electron energy loss spectroscopy (EELS). Time-resolved TEM images reveal that the amorphous Fe oxide particles evolve from solid spheres into hollow shells in <2 min, whereas crystalline gamma-Fe2O3 are unaffected by the electron beam. The resulting nanocrystalline Fe oxide shells bear striking resemblance to core-shell nanocrystals, but are a result of a morphology change attributed to restructuring of particle voids and defects induced by quasi-melting in the TEM. These results thus imply that caution is necessary when using TEM to analyze nanoparticle core-shell and heterostructured nanoparticles.     

EELS nanoanalysis for investigating both chemical composition and bonding of interlayers in composites

The paper is concerned with the application of analytical transmission electron microscopy (TEM) to characterize both chemical composition and bond state of the elements detected in interlayers in C- and SiC-fibre reinforced composites. The chemical bond state of nanometre-sized regions is characterized by means of electron energy loss spectroscopy (EELS), where respective information is gained by analysing energy loss near edge structures (ELNES). In this context results of Si-L₂₃ ELNES investigations are presented concerning the chemical bonding of silicon with carbon, nitrogen and oxygen. The specific bond state of silicon is revealed by recording series of EEL spectra at high energy resolution across the fibre/ matrix interlayers of interest. Moreover, the element distribution is imaged by energy-filtered TEM.Dedicated to Professor Dr. rer.nat. Dr. h.c. Hubertus Nickel on the occasion of his 65th birthday     

Nanostructure investigations with the analytical transmission electron microscope

Concerning the conventional TEM-imaging as well as the analytical procedures the capabilities are pointed out: electron diffraction, energy dispersive X-ray spectroscopy (EDXS) and electron energy loss spectroscopy (EELS). The possibilities of investigation of both nanocrystalline materials and multilayers are discussed, accompanied by examples of current investigations: At alloys, produced by intense milling, at single nanocrystals the imaging by diffraction contrast was successful, the analysis has failed because of the sample thickness. By means of energy spectroscopic imaging multilayers from Fe-SiB/NbCu and Fe/Cr as well as Al₂O₃/TiN have been characterized. Received: 15 July 1997 / Revised: 16 February 1998 / Accepted: 21 February 1998     

Electron Beam Induced Transformation of MoO3 to MoO2 and a New Phase MoO

The transformation of MoO₃ induced by electron beam irradiation was studied by electron energy‐loss spectroscopy (EELS) in combination with electron diffraction and high‐resolution transmission electron microscopy (HRTEM) techniques. The routes of structure transformation were dependent on the applied electron current density. In case of low current density, MoO₂ was obtained. In case of high current density, MoO with a rock‐salt structure is suggested to be the final phase. The change in oxidation states of the Mo oxides was deduced from the features in energy‐loss near edge structure (ELNES) of the O K‐edge. Quantitative analysis was successfully employed on Mo M₃‐edge and O K‐edge to obtain the O/Mo ratio of the reduced phases. The mechanisms of different structure transformation behaviors were suggested in the frame of radiolysis enhanced diffusion.     

Direct View of Cr Atoms Doped in Anatase TiO2(001) Thin Film

Imaging the doping elements is critical for understanding the photocatalytic activity of doped TiO₂ thin film. But it is still a challenge to characterize the interactions between the dopants and the TiO₂ lattice at the atomic level. Here, we use high angle annular dark-field/annular bright-field scanning transmission electron microscope (HAADF/ABF-STEM) combined with electron energy loss spectroscopy (EELS) to directly image the individual Cr atoms doped in anatase TiO₂(001) thin film from [100] direction. The Cr dopants, which are clearly imaged through the atomic-resolution EELS mappings while can not be seen by HADDF/ABF-STEM, occupy both the substitutional sites of Ti atoms and the interstitial sites of TiO₂ matrix. Most of them preferentially locate at the substitutional sites of Ti atoms. These results provide the direct evidence for the doping structure of Cr-doped A-TiO₂ thin film at the atomic level and also prove the EELS mapping is an excellent technique for characterizing the doped materials.     

Analysis of light elements in superposed layers by Monte Carlo simulation of EELS spectra

A Monte Carlo code, previously set up to simulate electron energy loss spectra of carbon films on silicon at 100 kV, has been extended to the analysis, at 300 kV, of a Si/SiO₂/Si structure; the final goal is the determination of the oxygen concentration in SiO_x precipitates embedded in a Si matrix. The upgrading of the programme has required the introduction of relativistic kinematics and relativistic corrections to elastic and inelastic cross sections.The Si/SiO₂/Si samples have been prepared by CVD deposition of a 16 nm thick silicon film onto a silicon wafer covered with a 11 nm thick thermal oxide. The thickness of both films has been checked by transmission electron microscopy on cross sections. The EELS experiments have been performed on planar sections, in regions of different thickness; the EELS spectra have been acquired with a parallel 666 Gatan spectrometer, fitted to a Philips CM 30 TEM/STEM, operating at 300 kV. The stoichiometry of the SiO_x can be obtained by the ratioing of the areas under the OK and SiK edges, taking advantage of the possibility given by the Monte Carlo simulation to separate the background electrons from the one suffering the characteristic energy loss. The agreement between experiments and calculations in the case examined is satisfactory, so that the application of this procedure to SiO_x precipitates is promising.     

On the defect center electron energy loss structures from MgO surfaces

Electron energy loss spectroscopy (EELS) was applied to surfaces of (1) clean MgO(100), (2) ultrathin (0.5 Å average thickness) Cu layers deposited on MgO(100) by an electron beam evaporation technique, and (3) a carbon-contaminated MgO(100). The surface-defect-related energy loss peak was ascribed to the presence of surface states arising from the V_s⁻ centers rather than from the F_s⁺ centers. The copper deposit is supposed to be trapped by the magnesium ion vacancies and bonded to the oxygen ligands as ions. The new electronic structures caused by the Cu deposit aie explained in terms of Cu impurity levels.     

X‐ray microanalysis in STEM of short‐term physicochemical reactions at bioactive glass particle/biological fluid interface. Determination of O/Si atomic ratios

Short‐term physicochemical reactions at the interface between bioactive glass particles and biological fluids are studied and we focus our attention on the measurements of O/Si atomic ratio. The studied bioactive glass is in the SiO₂? Na₂O? CaO? P₂O₅? K₂O? Al₂O₃? MgO system. The elemental analysis is performed at the submicrometre scale by scanning transmission electron microscopy associated with energy‐dispersive x‐ray spectroscopy (EDXS) and electron energy‐loss spectroscopy (EELS). We previously developed an EDXS quantification method based on the ratio method and taking into account local absorption corrections. In this way, we use EELS data to determine, by an iterative process, the local mass thickness, which is an essential parameter for correcting absorption in EDXS spectra. After different immersion times of bioactive glass particles in a simulated biological solution, results show the formation of different surface layers at the bioactive glass periphery. Before 1 day of immersion, we observe the presence of an already shown (Si,O,Al)‐rich layer at the periphery. In this paper, we demonstrate that a thin ‘electron dense’ (Si,O)‐layer is formed on top of the (Si,O,Al)‐layer. In this (Si,O)‐layer, depleted in aluminium, we point out an increase of oxygen weight concentration that can be interpreted by the presence of Si(OH)₄ groups, which permit the formation of a (Ca,P)‐layer. Aluminium plays a role in the glass solubility and may inhibit apatite nucleation. After the beginning of the (Ca,P)‐layer formation, the size of the ‘electron dense’ (Si,O)‐layer decreases and tends to disappear. After 2 days of immersion, the (Ca,P)‐layer grows in thickness and leads to apatite precipitation. Copyright © 2004 John Wiley & Sons, Ltd.     

The effect of variable electronic transition moment on the EELS intensity distribution within a vibrational progression: use of the R-centroid approximation to analyze results for the CO B-X and O2B′-X transitions

The CO B-X and O₂ B′-X transitions are analyzed with the help of ab initio MRD-CI calculations. In both cases a violation of a well-known rule in electron energy loss spectra (EELS) is observed, namely that the intensity distribution of vibrational bands is normally independent of both the scattering angle and the incident electron energy. In both cases strongly avoided crossings are shown to be responsible for the unusual appearance of the EELS. The analysis is simplified by making use of the R-centroid approximation, which was originally developed to organize and systematize optical absorption and emission data. It is concluded that a breakdown of the intensity distribution invariance in EELS should occur for a wide class of transitions in which avoided crossings lead to rapid variations in the electronic transition moment as a function of internuclear separation.     

Treatment of citrate-capped Au colloids with NaCl, NaBr and Na2SO4: a TEM, EAS and EPR study of the accompanying changes

In the present study, effects of the treatment of citrate-reduced Au sols with NaCl, NaBr and Na₂SO₄ are described. The particles are characterized by transmission electron microscopy (TEM) and spectroscopic methods, suggesting an exchange of the citrate capping by the anions of the Na salts. Under electron beam exposure the capping of the NaCl- and NaBr-treated particles disappears. The specific electronic properties of the Na salt-treated particles are studied by electron absorption spectroscopy (EAS) and electron paramagnetic resonance (EPR). A discussion of the results in comparison with the spectroscopic responses from the original particles is given. A correlation between the data of EAS and EPR is found. The respective electron-withdrawing effect of the capping anions towards the Au core seems to be of considerable significance regarding the orbital situation around the Fermi level.     

Characterization of the chemical bonding in inner layers of composite materials

The efficiency of near edge structure investigations in electron energy loss spectroscopy (EELS) is discussed for characterizing the chemical bonding of elements present in the interfacial zone in fibre/matrix composites at nanometre resolution. Two different examples of corresponding analyses are given for a SiC-fibre reinforced borosilicate glass. In particular, the chemical bonding between silicon and carbon or oxygen (e.g. SiC, SiO₂ and SiO_xC_y), respectively, is characterized. The results have been attained in a fingerprint manner by comparing the fine structure measured from a material of unknown stoichiometry to that of a standard specimen. In addition, a possibility is demonstrated to image the chemical bonding by energy-filtered microscopy using energy loss near edge structures (ELNES).     

Vanadium Nitride Films Formed by Rapid Thermal Processing (RTP): Depth Profiles and Interface Reactions Studied by Complementary Analytical Techniques

The nitridation of vanadium films in molecular nitrogen and ammonia using a RTP‐system was investigated. The V films were deposited on silicon substrates covered by 100 nm thermal SiO₂. For a few experiments sapphire substrates were used. Nitride formation at high temperatures (900 and 1100 °C) and interface reactions and diffusion of oxygen out of the SiO₂‐layer into the metal lattice at moderate temperatures (600 and 700 °C) were studied. For characterisation complementary analytical methods were used: X‐ray diffraction (XRD) for phase analysis, secondary neutral mass spectrometry (SNMS) and Rutherford Backscattering (RBS) for acquisition of depth profiles of V, N, O, C and Si, transmission electron microscopy (TEM) in combination with electron energy filtering for imaging element distributions (EFTEM) and recording electron energy loss spectra (EELS) to obtain detailed information about the initial stages of nitride, oxide and oxynitride formation, respectively, and the microstructure and element distributions of the films. In these experiments the SiO₂‐layer acts as diffusion barrier for nitrogen and source for oxygen causing the formation of substoichiometric vanadium oxides and oxynitrides near the V/SiO₂‐interface primarily at temperatures ≤ 900 °C. At a temperature of 1100 °C just a small amount of oxynitride forms near the interface because rapid diffusion of nitrogen and fast formation of VN (diffusion barrier for oxygen) inhibit the outdiffusion of oxygen into the metal layer. In the 600 °C regime, in argon atmosphere oxynitride phases observed in the surface region of these films originate from reaction of residual oxygen in the argon gas, whereas NH₃ as process gas does not lead to oxide or oxynitride formation at the surface (apart from the oxidation caused by storage). NH₃ seems to support the diffusion of oxygen out of the SiO₂‐layer. During the decomposition of ammonia at higher temperatures hydrogen is formed, which could attack the SiO₂. In contrast, sapphire substrates do not act as oxygen source in the 600 °C regime and change the nitridation behaviour of the vanadium films.     

Risk assessment of TiO2 photocatalyst by individual micrometer-size particle analysis with on-site combination of SEM-EDX and SR-XANES microscope

Applications of synchrotron radiation X-ray fluorescence (SR-XRF) microscopy combined with scanning electron microscopy (SEM) are reported. Electron beam excited and synchrotron radiation induced X-ray emission spectra of the same yellow sand single particles are reported and compared. The Ti-K edge absorption fine structure of single microparticles of TiO₂ (rutile, anatase, and a photocatalyst aerosol) are recorded by using monochromatic synchrotron radiation of tunable energy. It is shown that the discrimination between rutile and anatase is possible. Based on the single particle speciation, the toxicity of photocatalyst aerosol powder is discussed.     

Tracking the structural evolution at atomic-scale in the spinel Mn3O4 induced by electrochemical cycling

Mn₃O₄ is a possible candidate for use as an electrode material and has been found to undergo structural transformation during electrochemical cycling. Clarifying the transformation process is important in developing methods for improving electrochemical performance. Here, using scanning transmission electron microscopy (STEM) combined with the electron energy loss spectroscopy (EELS) technique in aberration-corrected TEM, we succeeded in tracking the structural evolution at an atomic-scale and identified the intermediate stage as rock-salt-structured MnO. A reasonable route was deduced via which the spinel Mn₃O₄ was transformed firstly into MnO and then into MnO₂.     

Study on electron beam cross‐linked ethylene‐tetrafluoroethylene copolymer‐insulated cables for aerospace applications: Mechanical performance,crystallization kinetics,and fluoride precipitation

The cross‐linking effects of cross‐linked ethylene‐tetrafluoroethylene copolymer (XETFE)‐insulated cables at different electron beam radiation doses were analyzed in this paper. Evaluation of the mechanical performance of the cables revealed that the highest tensile and breaking elongation was achieved at a radiation dose of 8 × 10⁴ Gy and that XETFE had a good resistance to extreme electron beam irradiation. This is attributed to the cross‐linking effects induced by electron beam irradiation, and this takes full advantage of the strength of molecular chain crosslink to each other. The crystallization kinetics of XETFE at different electron beam radiation doses were studied in detail in terms of the non‐isothermal and isothermal crystallization processes. The results indicated that the crystallinity of the XETFE domain increased with an increase in the radiation dose as a result of heterogeneous nucleation. Moreover, the highest ΔE_a was obtained, indicating that XETFE absorbed some energy at a radiation dose of 8 × 10⁴ Gy. These kinetic parameters had help in carrying out a comprehensive evaluation of the performance of XETFE‐insulated cables for aerospace applications. Moreover, the fluoride precipitation observed in this study indicated that upon electron beam irradiation, XETFE could internally produce hydrogen fluoride, which is corrosive to metals. Thus, optimizing the radiation dose was necessary to achieve the desired performance. We could believe that the improvement for properties of electron beam XETFE insulation cables would expand their range of applications in the aerospace field.