Extended Energy Loss Fine Structure Research Articles

The structure of a metallic glass system using EXELFS and EXAFS as complementary probes

The short-range atomic order around all three constituent atoms in a prototypical bulk metallic glass (BMG) system was probed in a complementary way, using extended X-ray absorption fine structure for neighborhood of the higher atomic number elements, and extended energy loss fine structure (EXELFS) for the lower atomic number ones. The Pd x Ni (80− x) P (20) system is a prototype for a whole class of BMG formers which are 80% transition metal and 20% metalloid. We find that the structure of these BMGs could be explained in terms of those of glasses at the end of the BMG range, namely, Pd 60Ni 20P 20 and Pd 30Ni 50P 20. The binary phosphide crystals near x=0 and 80 are found to be simulate very well the local atomic structure of Pd 30Ni 50P 20 and Pd 60Ni 20P 20 glasses, respectively. The best glass former in this series, Pd 40Ni 40P 20, is best described by a weighted average of Pd 30Ni 50P 20 and Pd 60Ni 20P 20 structures.

Fine determination of interatomic distances on surface using extended energy‐loss fine structure (EELFS) data: peculiarities of the technique

AbstractFor determination of interatomic distances on a surface we use spectra below the lines of backscattered electrons and below the Auger lines. In contrast to other authors, who record spectra in dN/dE mode for extended energy‐loss fine structure (EELFS) analysis, the integral N(E) mode is used with further filtering of the structure‐dependent oscillations by original software. The sequence of structure‐dependent oscillation extraction from the energy‐loss spectrum is presented, including the following main stages: synthesis of peak and inelastic background and detection of inelastic oscillations; normalization of the inelastic spectrum; recalculation of the spectrum to K‐space; and Fourier transformation.Good agreement between theoretical and experimentally obtained interatomic distances for various systems (multicomponent grain boundary segregations in steels, hard diamond‐like coatings and films generated on the surface of cutting tools with engineered coatings) is observed. Detailed results of application of EELFS for investigation of the atomic structure of these systems are presented.In particular, the results of EELFS analysis of hard amorphous hydrogenated carbon films show the ability of this method to resolve interatomic bonds of different types (sp, sp2, sp3, aromatic bonds). The ‘carbon–hydrogen’ distances are also determined. Copyright © 2002 John Wiley & Sons, Ltd.

Investigation of the Structure of γ-Al2O3-Supported MgO by Surface Extended Energy Loss Fine Structure

This study for the first time investigated the structure of a supported oxide system by SEELFS, the research of which had significance not only in the preparation of advanced solid base catalyst but also in the application of SEELFS technique onto a supported system. We present here the experimental results of a supported oxide system by SEELFS through the structural analysis of magnesia on a γ-alumina carrier. Using a fitting procedure based on an EXAFS-like formula and the phase shifts of a model compound of MgO, the coordination distances for Mg-O and Mg-Mg shells were obtained semiquantitatively and the coordination numbers obtained qualitatively. In addition, the contribution terms in the SEELFS oscillation of Mg K-edge of magnesia were also analyzed by the back Fourier transform of a corresponding Fourier function.

Studies on Surface and Interface with X-ray Absorption Fine Structure (XAFS). Analysis of Local Structures in Light-Element Materials by Core-Excitation Spectra in Transmission Electron Energy-Loss Spectroscopy.

Analysis of core-loss spectra in electron energy-loss spectroscopy associated with transmission electron microscopy (TEM-EELS) is introduced as a tool giving information equivalent to X-ray absorption fine structure (XAFS) on nano-scale areas. After briefly describing the principle of TEM-EELS, including extended energy-loss fine structure (EX-ELFS) and energy-loss near edge structure (ELNES), merits and demerits of TEM-EELS are compared to those of XAFS analysis. A new analysis method applying the ‘wavelet transformation’ is introduced. This method is effective particularly to isolate the signals from the original EXELFS spectrum containing noises and artifacts. Furthermore, information on higher-order coordination shells and many-body correlations can be extracted from the EXELFS interference function, using the wavelet transformation. As an application example of this method, we show characterization of local structural relaxation of amorphous silicon by intense X-ray illumination. Finally some future prospects of TEM-EELS are briefly mentioned.

Extended energy-loss fine structure analysis of 3d transition metals using L ionization edges

The extended energy-loss fine structure (EXELFS) in electron-energy-loss spectroscopy has been compared experimentally with the extended X-ray absorption fine structure (EXAFS), for determining local structure around 3d transition metals. Since the EXELFS spectrum is acquired in an analytical transmission electron microscope, the probing beam can be focused to offer a lateral spatial resolution in the nanometre range, which is several orders of magnitude better than that of X-rays. Also, the microscope allows the area of interest to be imaged and analysed by other methods. However, difficulties in the analysis of EXELFS of 3d transition metals arise from the overlap of the edges in the accessible L series. EXELFS of L ionization edges of crystalline Ni and Cu were examined as test specimens. The overlapped L1,L2 and L3 edges were separated and compared with the K-edge EXAFS of the same samples. The first- and second-nearest-neighbour distances from EXELFS are in agreement with those measured from EXAFS and X-ray diffraction. As a structural probe, however, the accuracy of quantitative analysis of EXAFS is still superior since the L-edge EXELFS decays more rapidly than K-edge EXAFS.

EELFS study of the cementite local atomic structure.

The changes in the nearest - neighbour surroundings of carbon atoms in the cementite Fe3C lattice due to the heat treatment are analysed by the Extended Energy Loss Fine Structure (EELFS) method. To study the local atomic structure of the carbon steel U15 after different heat treatment regimes the electron energy loss spectra above the C K edge and Fe L2,3 edge were measured. The measured experimental EELFS spectra were treated by the standard for the Extended X - ray Absorption Fine Structure method procedure using Fourier transformation to obtain the parameters of the nearest - neighbour atomic surroundings. Qualitatively the assumption was made that the number of the iron atoms in the carbon atom nearest - neighbour surroundings changes due to the heat treatment, the evidence of which was the change in the ratio of the first two feature intensities of the atomic radial distribution function. This assumption is consistent well with the previous Mössbauer data.

Formation of Si clusters in electron-irradiated SiC studied by electron energy-loss spectroscopy

Abstract Electron-irradiation damage in 6H-SiC was examined at room temperature by means of transmission electron microscopy and electron energy-loss spectroscopy (EELS). A detailed analysis of extended energy-loss fine structure (EXELFS) gave unambiguous evidence of the formation of direct Si–Si bonding after prolonged irradiation, where the sample still maintained the well-defined 6H structure, while no appreciable indication of direct C–C bonding was observed. These results suggest that C atoms should be predominantly displaced away compared to Si atoms and the residual Si atoms form small clusters. The changes in the energy-loss near-edge structure (ELNES) were consistent with the appearance of Si clusters.

エネルギー損失広域微細構造による短距離構造相関の解析—二体分布から多体分布導出へ—

エネルギー損失広域微細構造による短距離構造相関の解析—二体分布から多体分布導出へ—

Exafs and Exelfs Study of the Structure of Pd-Ni-P Bulk Metallic Glasses

AbstractWe have explored the short-range order around all three constituent atoms in (Pd-Ni)80P20 bulk metallic glasses (BMGs), a system that is a prototype for a whole class of BMG formers containing 80% transition metal and 20% metalloid. We have examined the changes in the nearest neighbor environments around the transition metals in (Pd-Ni)80P20 glasses using extended X-ray absorption fine structure (EXAFS) in comparison to their binary counterparts. We have done similar studies around the coordination of P using extended energy loss fine structure (EXELFS). The environment around the all the atoms in Pd60Ni20P20 and Pd30Ni50P20are very similar to those of the binary phosphides at the ends of the composition range. However, the (Pd-Ni)80P20 glasses are not simply solid solutions of the phases. The nearest neighbor distances of the metals are reduced in the ternary alloys with respect to those of the binary phosphides. The best glass former in this series, Pd40Ni40P20 is nearly isostructural to Pd30Ni50P20 but shows shorter distances at the 2nd and 3rd coordination shells, which we believe is due to more efficient packing in this glass. The metal environments in Pd40Ni40P20, on the other hand, are better described by a weighted average of those of Pd30Ni50P20 and Pd60Ni20P20.

EELS characterization of TiN grown by the DC sputtering technique

Abstract Titanium nitride thin films were deposited on monocrystalline silicon (mc-Si) substrates by direct current reactive magnetron sputtering. Auger electron spectra (AES) of deposited films at different nitrogen partial pressures, show the typical N KL23L23 and Ti L3M23M23 Auger transition overlapping. Also, changes in the Ti L3M23M45 Auger transition peak are observed. X-ray diffraction and high resolution electron microscopy (HRTEM) of a golden color TiN/mc-Si sample, reveal a preferential polycrystalline columnar growth in the 〈111〉 orientation. This sample was also analyzed by electron energy-loss spectroscopy (EELS). The N/Ti elemental ratio is slightly different to the value determined by AES. Atomic distribution around the N atoms is in agreement with that expected from the N atom in the fcc unit cell of TiN. This distribution was obtained via an extended energy-loss fine structure (EXELFS) analysis from EELS spectra.

Analysis of Extended Energy-Loss Fine Structure of Nanometerscale Clusters

Small atomic clusters are of great importance for applications such as catalysts whose activity depends on the surface of the cluster. Attempts to determine the atomic short-range order and size of clusters have been made by analyzing the extended X-ray absorption fine structure (EXAFS). However, the analysis was made on an average of many small clusters. Analysis of extended energy-loss fine structure (EXELFS) in an electron energy-loss spectrum (EELS) has developed to the point where in some cases, the quality of the results is comparable to its X-ray analogue, EXAFS. No other technique provides nanometer-scale spatial resolution of the analyzed area for determining the atomic structure. Most EXELFS analysis has been performed on the K-ionization edge of lighter elements. For heavier elements, a more complex ionization edge such as the L-edge has to be used, due to the inefficiency of collecting high quality EEL spectra at higher energy-losses (Z > 18).

Towards nanoscale EXELFS analysis: limitation due to radiation damage

Spatial resolution limitations due to radiation damage were studied in the extended energy loss fine structure (EXELFS) spectroscopy technique using a transmission electron microscope (TEM). Three types of samples were used for testing that represented a wide variety of practical materials: a metal (Al) for metallic, a semiconductor (SiC) for covalent, and a ceramic (MgO) for ionic-bonding. A TEM was used that was operated under various experimental conditions, with emphasis in electron probe currents, for the EXELFS experiments. The radiation damage was monitored using the fine details of energy loss K-edge structures of various elements contained in the samples using the newly-developed AcqLong software for EXELFS acquisition and analysis. It was found that it is possible to obtain an EXELFS signal with excellent counting statistics allowing quantitative analysis of short range atomic order from local regions as small as 20–30 nm in Al, 30–40 nm in SiC, and about 200 nm in MgO. Further improvements of spatial resolution toward nanoscale EXELFS analysis are discussed.

Change in Chemical Bonding States in Electron-Irradiated Graphite

The damage process in electron-irradiated graphite was investigated by electron energy-loss spectroscopy (EELS). An analysis of the extended energy-loss fine structure (EXELFS) revealed that new components with slightly different interatomic distances grew with irradiation time in the first-nearest neighbor coordination shell. In addition, the energy-loss near edge structure (ELNES) exhibited features characteristic of higher fullerenes. The consistency of these results can be explained by the formation of non-sixfold atomic rings by electron irradiation, similar to fullerenes.

The local atomic structure of Cu(111): An extended energy loss fine structure investigation

Extended energy loss fine structures spectra (EELFS) above the M 2,3 ionisation edge of a Cu(111) single crystal have been obtained at primary electron energies of 1500 and 560 eV, corresponding to a studied layer depth of ∼15 and ∼8 Å. For experimental EELFS spectra the solution of an inverse problem has been performed and pair correlation functions (PCFs) have been obtained by Tikhonov's regularisation method with allowance made for normalisation of the atomic intensity of ionisation losses. Parameters of the nearest atomic environment, namely, the bond length, the co-ordination number, the Debye–Waller factor and the parameter of asymmetry of the first peak of the pair correlation function, have been determined for both large (∼15 Å) and small (∼8 Å) depths of the studied layer.

Study of the local atomic structure of a silver aluminum alloy by the method of extended electron energy loss fine structure (EELFS)

Extended energy loss fine structure spectra are obtained for electrons at the Al K and Ag M4,5 edges for an Al-20 wt %Ag solid solution after high-temperature aging, as well as for the pure alloy components. The analysis layer depth was ∼20 A. Radial distribution functions for the atoms are determined by Fourier transforming these spectra with a correction for the phase shift and the method of regularization. For pure aluminum and silver it is found that the position of the first coordinate spheres does not differ from bulk interatomic distances. For the binary alloy it was shown that the state of the sample corresponds to a decomposed solid solution with inclusions of a phase enriched with silver against an aluminum host background. The interatomic Al-Al distances in the binary alloy correspond to the length of a pure aluminum bond. The partial distances to the first two coordination spheres for the pairs Ag-Ag, Ag-Al are the same and equal the interatomic distances of the γ-phase Ag2Al.

Electron-beam induced amorphization of stishovite: Silicon-coordination change observed using Si K -edge extended electron energy-loss fine structure

The analysis of the extended energy-loss fine structure (EXELFS) of the Si K-edge for sixfold-coordinated Si in synthetic stishovite and fourfold-coordinated Si in natural α-quartz is reported by using electron energy-loss spectroscopy (EELS) in combination with transmission electron microscopy (TEM). The stishovite Si K-edge EXELFS spectra were measured as a time-dependent series to document irradiation-induced amorphization. The amorphization was also investigated through the change in Si K- and O K-edge energy-loss near edge structure (ELNES). For α-quartz, in contrast to stishovite, electron irradiation-induced vitrification, verified by selected area electron diffraction (SAED), produced no detectable changes of the EXELFS. The Si K-edge EXELFS were analysed with the classical extended X-ray absorption fine structure (EXAFS) treatment and compared to ab initio curve-waved multiple-scattering (MS) calculations of EXAFS spectra for stishovite and α-quartz. Highly accurate information on the local atomic environment of the silicon atoms during the irradiation-induced amorphization of stishovite is obtained from the EXELFS structure parameters (Si−O bond distances, coordination numbers and Debye-Waller factors). The mean Si−O bond distance R and mean Si coordination number N changes from R=0.1775 nm and N=6 for stishovite through a disordered intermediate state (R≈0.172 nm and N≈5) to R≈0.167 nm and N≈4.5 for a nearly amorphous state similar to α-quartz (R=0.1609 nm and N=4). During the amorphization process, the Debye-Waller factor (DWF) passes through a maximum value of \(\) as it changes from \(\) for sixfold to \(\) for fourfold coordination of Si. This increase in Debye-Waller factor indicates an increase in mean-square relative displacement (MSRD) between the central silicon atom and its oxygen neighbours that is consistent with the presence of an intermediate structural state with fivefold coordination of Si. The distribution of coordination states can be estimated by modelling the amorphization as a decay process. Using the EXELFS data for amorphization, a new method is developed to derive the relative amounts of Si coordinations in high-pressure minerals with mixed coordination. For the radiation-induced amorphization process of stishovite the formation of a transitory structure with Si largely in fivefold coordination is deduced.

Exelfs of Metallic Glasses

AbstractThe feasibility of using extended energy-loss fine structure (EXELFS) obtained from ∼1 nm regions of metallic glasses to study their short-range order has been examined. Ionization edges of the metallic glasses in the electron energy-loss spectrum (EELS) have been obtained from PdNiP bulk metallic glass and Ni2P polycrystalline powder in a transmission electron microscope. The complexity of EXELFS analysis of L- and M-ionization edges of heavy elements (Z>22, i.e. Ni and Pd) is addressed by theoretical caluculations using an ab initio computer code, and its results are compared with the experimental data.

Application of extended energy loss fine structure in determining the structure of amorphous SiO 2

The application of extended energy loss fine structure (EXELFS) in an electron energy loss spectrum is demonstrated for the determination of the element-specific local structure. EXELFS, the electron analogue of extended X-ray absorption fine structure, is particularly appropriate for lighter elements. EXELFS spectra are obtained by using a magnetic-prism spectrometer and a parallel electron energy loss detector attached to a dedicated scanning transmission electron microscope. The specimen is illuminated by a 100 keV electron beam from a cold field-emission gun. Electron-energy-loss spectra are acquired using a defocused electron probe to minimize beam damage. Oxygen and silicon K-edges of a magnetron-sputtered amorphous SiO 2 film were analyzed with in-house software, which extracts oscillatory components and calculates the partial radial distribution functions by a Fourier-transform method. The results are in good agreement with those obtained by X-ray methods.

EXELFS χ-data renormalization

In this paper, a procedure developed for accurate modeling of the background of core edges in electron energy loss spectroscopy (EELS) is described. This procedure is necessary for improving data analysis technique used in extended energy loss fine structure (EXELFS) spectroscopy via the adaptation of X-ray absorption fine structure (XAFS) analysis programs. With the improved background modeling, it now becomes possible to renormalize the EXELFS χ-data and to ensure the correct use of the XAFS programs for EXELFS analysis.

Extended Energy Loss Fine Structure above Sub Valence Edges of the First-Transition Row Atoms and Germanium

Extended energy loss fine structure (EELFS) above the M 2,3 -edge of amorphous Ge was measured and evaluated in the previously developed calculational scheme taking into account the 3p- and 3d-ionization channels. The 3d-ionization contribution is prevalent in the EELFS spectrum of Ge in contrast with the case of EELFS for 3d-transition metals. The influence of both plasmon excitations and 3p-spin-orbit splitting on EELFS was considered. The excellent agreement of the experimental and theoretical results confirms the possibility of the nearest neighbour distance determination with high accuracy by shallow level EELFS technique.