Electron Energy Loss Spectra of Carbodiimides

Compounds with carbodiimide bonds are of special interest to organic syntheses, materials science and biochemistry. The chemical reactivity of carbodiimides and their utilization ensue from their electronic structure, which can be studied via electron spectroscopic methods. Electron energy loss spectra of dicyclohexylcarbodiimide, polysilyl‐ and polytitanylcarbodiimides have been recorded. The energy loss near edge structures (ELNES) of the C–K, N–K, Si–L_2,3 and Ti–L_2,3‐ionization edges and their onsets as well as the features of the low‐loss region have been interpreted by ab‐initio quantumchemical calculations using density functional theory (DFT).     

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.     

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.     

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).     

Chemical-bond analysis of nanostructured materials using near-edge fine structures (ELNES)

The paper is concerned with the application of combined electron energy loss spectroscopy (EELS) and transmission electron microscopy (TEM) to the characterization of the chemical-bond state of elements at nanometer resolution. Respective information is contained in the energy loss near-edge fine structure (ELNES) usually appearing in the energy range up to about 30 eV above the edge onset. The practical way of chemical-bond analysis was chosen, viz. to compare the ELNES of the feature under investigation with those of known stoichiometry. The efficiency of this special technique is demonstrated for different material systems, particularly fiber-reinforced composites and functional ceramics, having structural details of some ten nanometres, or smaller.     

Study of the Interface Microstructures of CVD Diamond Films by TEM

 The characteristics of the interface microstructures between a CVD diamond film and the silicon substrate have been studied by transmission electron microscopy and electron energy loss spectroscopy. The investigations are performed on plan-view TEM specimens which were intentionally thinned only from the film surface side allowing the overall microstructural features of the interface to be studied. A prominent interfacial layer with amorphous-like features has been directly observed for CVD diamond films that shows a highly twinned defective diamond surface morphology. Similar interfacial layers have also been observed on films with a <100> growth texture but having the {100} crystal faces randomly oriented on the silicon substrate. These interfacial layers have been unambiguously identified as diamond phase carbon by both electron diffraction and electron energy loss spectroscopy. For the CVD diamond films that exhibit heteroepitaxial growth features, with the {100} crystal faces aligned crystallographically on the silicon substrate, such an interfacial layer was not observed. This is consistent with the expectation that the epitaxial growth of CVD diamond films requires diamond crystals to directly nucleate and grow on the substrate surface or on an epitaxial interface layer that has a small lattice misfit to both the substrate and the thin film material.     

The role of Mn in the electronic structure of Ba3Ti2MnO9

The energy loss near edge structure (ELNES) of the O-K, Ti-L₂₃ and Mn-L₂₃ edges have been recorded in hexagonal Ba₃Ti₂MnO₉ with an energy resolution of 0.10-0.20 eV using a monochromator on a commercial transmission electron microscope (TEM) and compared with a tetragonal BaTiO₃ reference sample. The formal valency and symmetry of Mn have been determined using atomic multiplets calculations and its effect on the electronic structure of BaTiO₃ has been interpreted through a molecular-orbital model.     

Electron microscopic phase analysis of BN-thin films

A series of BN films was deposited by means of r.f. magnetron sputtering of a h-BN target onto Si(1OO) surfaces. Hereby, the substrate bias voltage was varied. Special interest is focussed to the influence of the deposition parameters on the orientation of the growing hexagonal BN film with respect to the substrate. For structural investigation, cross section samples were prepared. In addition to HRTEM and diffraction investigations, especially electron energy loss spectroscopy (EELS) was applied successfully for phase identification. For negative bias voltages of U _B =–300 V and U _B =–350V, we found a phase system consisting of a first-grown 25 nm thick layer of hexagonal structure with the c axis parallel to the substrate surface followed by the cubic phase.Dedicated to Professor Dr. rer. nat. Dr. h.c. Hubertus Nickel on the occasion of his 65th birthday     

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, SiO(2) and SiO(x)C(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).     

Probing for Structural Features of Boron‐rich Solids with EELS

Crystal structures of boron‐rich solids are characterized by boron atom arrangements that are quite diverse: chains, sheets, and a variety of polyhedra like octahedra, pentagonal bipyramids, cuboctahedra, and icosahedra are observed. Probing by electron energy‐loss spectroscopy (EELS), these different structural features are mirrored by a pronounced variation of the energy loss near‐edge fine structure (ELNES) of the B_K ionization edges. For identification, characteristics of these fine structures can be used as so‐called “coordination fingerprints”, which is shown for solids like MgB₂, TaB₂, ZrB₂, CaB₆, SrB₆, BaB₆, NaB₅C, KB₅C, Na₃B₂₀, Na₂B₂₉, UB₁₂, ZrB₁₂, LaB₂C₂, CeB₂C₂, and CaB₂C₂. In addition, theoretical calculations of ELNES based on the density functional theory (FLAPW method) are presented for an example of boron‐rich solids.     

Application of analytical transmission electron microscopy to the characterization of interlayers in fibre-reinforced composites with ceramic and glass matrices

The microchemistry of interfaces and corresponding interlayers in different fibre-reinforced ceramic and glass composite systems has been investigated by using a dedicated scanning transmission electron microscope demonstrating the potential applicabilities of such an instrument to this large field of materials science. Energy-dispersive X-ray spectroscopy and electron energy loss spectroscopy were used to determine the materials composition on a nanometre scale. Besides analyses performed in the spot mode of the electron probe the distributions of the elements present in the interface region were measured as line profiles across the relevant interface structure by X-ray spectroscopy with a lateral resolution of about 5 nm, even for the detection of a light element as carbon. Moreover, in the composite systems under investigation the two-dimensional element distribution was also attained by energy-filtered imaging. In addition, first results of energy loss near edge structure analyses are presented indicating variations of the chemical bonding of silicon at the interface in a Nicalon fibre/Duran glass composite.     

Local and surface analysis within an analytical electron microscope

Analytical transmission electron microscopes have the ability to display at very high spatial resolution both the structural information of solid specimens prepared as thin films and the spectroscopic information related either to electronic properties or to the elemental composition. An example of study, by electron energy loss spectroscopy, of small spherical silicon particles is given as an illustration of the performances of the technique.     

Quantitative chemical phase imaging by means of energy filtering transmission electron microscopy

Electron spectroscopic imaging (ESI) in the transmission electron microscope (TEM) is a powerful method to produce 2-dimensional elemental distribution maps. These maps show in a clear way the chemical situation of a small specimen region. In this work we used a Gatan Imaging Filter (GIF) attached to a 200 kV TEM to investigate a Ba-Nd-titanate ceramic. The three phases occuring in this material could be visualized using inner-shell ionization edges (Ba M₄₅, Nd M₄₅ and Ti L₂₃). We applied different image correlation techniques to the ESI elemental maps for direct visualization of the chemical phases. First we simply overlaid the elemental maps assigning each element one colour to form an RGB image. Secondly we used the technique of scatter diagrams to classify the different phases. Finally we quantified the elemental maps by dividing them and multiplying them by the appropriate inner-shell ionization cross-sections which gave atomic ratio images. By using these methods we could clearly identify and quantify the various phases in the Ba-Nd-titanate specimen.Dedicated to Professor Dr. rer. nat. Dr. h.c. Hubertus Nickel on the occasion of his 65th birthday     

Interface characterization of nickel contacts to bulk bismuth tellurium selenide

Transmission electron microscopy (TEM) characterization of nickel contacts to bulk bismuth tellurium selenide [Bi₂(Te,Se)₃] is reported. Samples were prepared in a dual column focused ion beam/scanning electron microscope (FIB/SEM) system using a lift‐out technique, with ion beam energy and exposure times carefully optimized to minimize sample damage. Diffusion of Ni into Bi₂(Te,Se)₃ was observed and the formation of a nickel telluride (NiTe) interfacial region confirmed after heat treatment at 200 °C. Selected area diffraction patterns provided evidence of a modified bismuth telluride–like structure at the interface, identified by analytical electron microscopy to be composed of Ni and Bi₂(Te,Se)₃. Copyright © 2009 John Wiley & Sons, Ltd.     

Analytical electron microscopy of materials

Analytical electron microscopy enables combined crystallographic and chemical information with a high spatial resolution to be gained from microregions of electron-transparent specimens. This is reached by the combined application of imaging, diffraction and spectroscopic methods, using either a dedicated scanning transmission electron microscope or a conventional high-resolution electron microscope (having a strong objective lens) equipped with suitable X-ray or electron spectrometers. Of the diffraction methods especially the technique of convergent beam diffraction is used, yielding valuable information on crystal structures, lattice parameter changes, symmetry variations and crystal perfection, respectively. For chemical analysis, either energy-dispersive X-ray spectroscopy (EDX) is used or electron energy loss spectroscopy (EELS). Finally, high-resolution electron microscopy in the lateral resolution range of some 0.1 nm allows the reliable geometrical inspection of extreme microregions.     

Local A‐Site Layering in Rare‐Earth Orthochromite Perovskites by Solution Synthesis

Cation size effects were examined in the mixed A‐site perovskites La_0.5Sm_0.5CrO₃ and La_0.5Tb_0.5CrO₃ prepared through both hydrothermal and solid‐state methods. Atomically resolved electron energy loss spectroscopy (EELS) in the transmission electron microscope shows that while the La and Sm cations are randomly distributed, increased cation‐radius variance in La_0.5Tb_0.5CrO₃ results in regions of localised La and Tb layers, an atomic arrangement exclusive to the hydrothermally prepared material. Solid‐state preparation gives lower homogeneity resulting in separate nanoscale regions rich in La³⁺ and Tb³⁺. The A‐site layering in hydrothermal La_0.5Tb_0.5CrO₃ is randomised upon annealing at high temperature, resulting in magnetic behaviour that is dependent on synthesis route.     

Structural characterization of modern and fossilized charcoal produced in natural fires as determined by using electron energy loss spectroscopy

Charcoal produced in natural fires is widespread, but surprisingly little is known about its structure and stability. TEM and electron energy loss spectroscopy (EELS) were used to characterize the organized graphite-like microcrystallites and amorphous nonorganized phases of modern charcoal that had been produced in natural fires. In addition, a semiordered structure was identified in two modern charcoal samples. Fossilized charcoal contains fewer graphite-like microcrystallites than modern samples. EELS spectra confirmed that the dominant structure in fossilized charcoal is amorphous carbon. EELS measurements also revealed that only the nonorganized phase contains oxygen, which indicates that the degradation of the fossilized charcoal structure occurs mainly through oxidation processes. The few graphite-like microcrystallites found in fossilized charcoal were composed of onion-like structures that are probably less prone to oxidation owing to their rounded structures.     

Structural Study of Micro and Nanotubes Synthesized by Rapid Thermal Chemical Vapor Deposition

The structures of micro and nanotubes obtained by pyrolysis of hydrocarbons, hold onto silicon (Si) substrates, are reported in this work. The tubes fabrication experiments were carried out by Rapid Thermal Chemical Vapor Deposition (RTCVD) using propane (C₃H₈) as carbon (C) precursor. Selection of parameters such as temperature of deposition, vacuum conditions or surface cleaning leads to the creation of tubular structures. Transmission electron microscopy (TEM), scanning electron microscopy (SEM), selected area electron diffraction (SAED) and energy dispersive X-ray measurements (EDX) are the microbeam techniques that allow to characterize the tubes found in the studied specimens. Different tube configurations such as isolated nanorods, Y-type junctions or fiber-like layers are evidenced. Metallic catalysis seems to be the mechanism involved in the wires formation since Fe particles are present inside the CNT tubes. Other poly-crystalline inclusions are also evidenced by SAED. The composition of the nanotubes changes from tip to tail in an amorphous matrix. The growth mechanisms leading to tube formation are described.     

Construction of database of effective energy‐loss functions

Effective energy‐loss functions for Al, Cu, Ag and Au were derived from the reflection electron energy‐loss spectroscopy (REELS) spectra for 1 keV electrons using extended Landau theory. Features of the obtained effective energy‐loss functions are close to those of optical surface energy‐loss functions, revealing the significant contribution of the low energy loss below a few tens of electron‐volts in the REELS spectrum for Cu, Ag and Au. The REELS spectra were reproduced using the newly derived effective energy‐loss functions, leading to the confirmation that this type of database of the effective energy‐loss function is very useful not only for more comprehensive understanding of the measured spectrum of surface electron spectroscopies but also for practical background subtraction in surface electron spectroscopy. Copyright © 2003 John Wiley & Sons, Ltd.     

Highly Efficient Singlet–Singlet Energy Transfer in Light‐Harvesting [60,70]Fullerene–4‐Amino‐1,8‐naphthalimide Dyads

New C₆₀ and C₇₀ fullerene dyads formed with 4‐amino‐1,8‐naphthalimide chromophores have been prepared by the Bingel cyclopropanation reaction. The resulting monoadducts were investigated with respect to their fluorescence properties (quantum yields and lifetimes) to unravel the role of the charge‐transfer naphthalimide chromophore as a light‐absorbing antenna and excited‐singlet‐state sensitizer of fullerenes. The underlying intramolecular singlet–singlet energy transfer (EnT) process was fully characterized and found to proceed quantitatively (Φ_EnT≈1) for all dyads. Thus, these conjugates are of considerable interest for applications in which fullerene excited states have to be created and photonic energy loss should be minimized. In polar solvents (tetrahydrofuran and benzonitrile), fluorescence quenching of the fullerene by electron transfer from the ground‐state aminonaphthalimide was postulated as an additional path.