Electron Energy-loss Near-edge Structure Research Articles

Electronic structure and physical properties of the spinel-type phase of BeP2N4from all-electron density functional calculations

Using density-functional-theory-based ab initio methods, the electronic structure and physical properties of the newly synthesized nitride BeP{sub 2}N{sub 4} with a phenakite-type structure and the predicted high-pressure spinel phase of BeP{sub 2}N{sub 4} are studied in detail. It is shown that both polymorphs are wide band-gap semiconductors with relatively small electron effective masses at the conduction-band minima. The spinel-type phase is more covalently bonded due to the increased number of P-N bonds for P at the octahedral sites. Calculations of mechanical properties indicate that the spinel-type polymorph is a promising superhard material with notably large bulk, shear, and Young's moduli. Also calculated are the Be K, P K, P L{sub 3}, and N K edges of the electron energy-loss near-edge structure for both phases. They show marked differences because of the different local environments of the atoms in the two crystalline polymorphs. These differences will be very useful for the experimental identification of the products of high-pressure syntheses targeting the predicted spinel-type phase of BeP{sub 2}N{sub 4}.

Theoretical ELNES spectra of Si-K, Si-L, N-K, and O-K edges of an intergranular glassy film model in β-Si3N4

A realistic atomic model of an intergranular glassy film (IGF) between the prismatic faces of crystalline β-Si3N4 has been used for ab initio exploration of variations in electron energy loss near edge structure (ELNES) spectral response from each of the component elements and available edges. The computed ELNES spectra within the IGF region show substantial variability that sometimes defy expectation. Upon careful analysis, we have devised guidelines for comparison and interpretation of the spectra in terms of the local coordination and bonding environment. The bond lengths, bond angles, coordination number, and elemental types of the bonded atoms cannot be relied upon to systematically explain the computed variations. It is anticipated that the rise of ELNES spectral measurements with enhanced spatial and energy resolution that are performed in conjunction with atomic scale imaging techniques will require new methods to interpret observed results. The intent of this work is to make progress and stimulate others in that direction.

Study on atomic and electronic structures of ceramic materials using spectroscopy, microscopy, and first principles calculation

In this review, following two topics are introduced: 1) experimental and theoretical electron energy loss (EEL) near edge structures (ELNES) and X-ray absorption near edge structures (XANES), and 2) atomic and electronic structure analysis of ceramic interface by combing spectroscopy, microscopy, and first principles calculation. In the ELNES/XANES calculation, it is concluded that inclusion of core-hole effect in the calculation is essential. By combining high energy resolution observation and theoretical calculation, detailed analysis of the electronic structure is achieved. In addition, overlap population (OP) diagram is used to interpret the spectrum. In the case of AlN, sharp and intense first peak of N-K edge is found to reflect narrow dispersion of the conduction band bottom. By applying ELNES and the OP diagram to Cu/Al2O3 heterointerface, it is revealed that intensity of prepeak in O-K edge is inverse proportional to interface strength. The relationships between atomic structure and defect energetics at SrTiO3 grain boundary are also investigated, and reveal that the formation behavior of Ti vacancy is sensitive to the structural distortion. In addition, by using state-of-the-art spectroscopy, microscopy, and first principles calculations, atomic scale visualization of fluorine dopant in LaFeOAs and first principles calculation of HfO2 phase transformation are demonstrated.

Characterization of chemical composition and electronic structure of Pt/YSZ interfaces by analytical transmission electron microscopy

Platinum/yttria stabilized zirconia (Pt/YSZ) interfaces with two different orientation relationships were characterized using advanced analytical transmission electron microscopy methods. Quantitative X-ray energy dispersive spectroscopy (XEDS) was performed by the recently developed ζ-factor method. Neither interdiffusion nor segregation was detected across the Pt/YSZ interfaces of both orientation conditions within the resolution limit of the technique. The interface specific components of the electron energy-loss near-edge structures (ELNES) were extracted by employing the spatial difference technique. Features of the O–K ELNES at the interface are distinctly different from that at bulk YSZ indicating Pt–O bonding. The experimentally observed changes in the O–K ELNES are in good agreement with the results of ab-initio full multiple scattering calculation, based on structure modeling at the interfaces.

ELNES study of chemical solution deposited [formula omitted] Ruddlesden–Popper films: Experiment and simulation

This article analyzes electron energy-loss near-edge fine structures of the SrO(SrTiO3)n=1 Ruddlesden–Popper system and of the parent compounds SrTiO3 and SrO by comparison with first principles calculations. For that, the fine structures of chemical solution deposited Ruddlesden–Popper films have been experimentally recorded by means of transmission electron microscopy. Moreover, density of states computations using an all-electron density-functional code have been performed. It is shown that the appearance and shape of the experimental O–K and Ti–L2,3 fine structure features result from the crystallography-dependent electronic structure of the investigated oxides, which display technologically interesting dielectric as well as lattice–structural properties.

Spectroscopic imaging of electron energy loss spectra using ab initio data and function field visualization

We have devised a technique for spectral imaging using accurate ab initio electron energy loss near edge structure (ELNES) data and function field visualization. The technique is initially applied to a planar defect model in Si with different ring structures and no broken bonds where experimental probes are severely limited. The same model with B doping is also considered. It is shown that specific deviations in different energy ranges of the ELNES spectra are correlated with different structural components of the models.

Prediction of γ‐B28 ELNES with comparison to α‐B12

AbstractUsing ab initio techniques we have calculated the electron energy loss near edge structure (ELNES) of a new high pressure phase of boron (γ‐B28) and the structurally similar allotrope, α‐B12. The total ELNES spectra are presented as weighted sums of the site specific spectra of the constituent non‐equivalent B atoms. The five different non‐equivalent B sites in γ‐B28 all show rich ELNES spectra and their similarities and differences to the simpler α‐B12 case are detailed. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Density-functional theory calculations of the electron energy-loss near-edge structure of Li-intercalated graphite

We have studied the structural and electronic properties of lithium-intercalated graphite (LIG) for various Li content. Atomic relaxation shows that Li above the center of the carbon hexagon in a AAAA stacked graphite is the only stable Li configuration in stage 1 intercalated graphite. Lithium and Carbon 1 s energy-loss near-edge structure (ELNES) calculations are performed on the Li-intercalated graphite using the core-excited density-functional theory formulation. Several features of the Li 1 s ELNES are correlated with reported experimental features. The ELNES spectra of Li is found to be electron beam orientation sensitive and this property is used to assign the origin of the various Li 1 s ELNES features. Information about core-hole screening by the valence electrons and charge transfer in the LIG systems is obtained from the C 1 s ELNES and valence charge density difference calculations, respectively.

Phase-transition mechanism of h-BN [formula omitted]w-BN from first principles

We present ab initio calculations of the electron energy-loss near edge structure (ELNES) on hexagonal boron nitride to reveal the mechanism of its high pressure transition to wurtzite phase. Our simulated ELNES with the proposed phase-transition path of a 1/8 shift along [0001] for N atoms in h -BN is in excellent agreement with experimental data. Subsequent enthalpy calculation further confirmed the validity of this transition path. Our results support the experimental suggestion by Meng et al. [Y. Meng, H.-K. Mao, P.J. Eng, T.P. Trainor, M. Newville, M.Y. Hu, C.-c. Kao, J.F. Shu, D. Hausermann, R.J. Hemley, Nature Mater. 3 (2004) 111].

Chemical Shift of Electron Energy-Loss Near-Edge Structure on the Nitrogen K-Edge and Titanium L3-Edge at TiN/Ti Interface

We investigated the chemical shift of the electron energy-loss near-edge structure (ELNES) for the nitrogen K-edge and titanium L3-edge measured from the interface region between a titanium nitride layer and a titanium layer. Both the titanium nitride and titanium layers were prepared by a sputtering method. Elemental analysis for nitride and titanium in the vicinity of the interface region was performed using a standard technique in electron energy-loss spectroscopy. It was demonstrated that both the ELNES of nitrogen K-edge and titanium L3-edge presented the chemical shift, more or less, depending on the composition of TiNx. The experimental findings were interpreted using a first-principles band structure calculation. The chemical shifts of nitrogen K-edge and titanium L3-edge can be used as fingerprinting for readily distinguishing the composition of TiNx.

Beam damage suppression of low- porous Si-O-C films by cryo-electron-energy loss spectroscopy (EELS)

Porous Si-O-C films with lower dielectric constant (kappa) relative to silicon dioxide have been widely studied as inter-layer dielectrics in new-generation microelectronic devices. On the analysis of the film by transmission electron microscopy (TEM), it is susceptible of beam damage during both sample preparation by a focused ion beam (FIB) technique and TEM observation. We use electron energy loss spectroscopy (EELS) to quantify the magnitude of the beam damage during these processes. The intensity of the 285-eV peak in C-K electron energy loss near edge structures (ELNES) is enhanced by the damage, which can be ascribed to the formation of the C=C double bonds as a result of the decomposition of the methyl groups by the beam. The use of cryo-holder for TEM at 100 K is found to be essential to reduce the damage of the low-kappa layers. The lowering of the acceleration voltage of FIB down to 5 keV does not change the spectra. Since the FIB damage is localized at the surface, the use of thick regions in the TEM foil such as 130 nm is preferred to reduce the superposition of EELS of the damaged region on those from the sample of interest.

Overlap population diagram for ELNES and XANES: peak assignment and interpretation

This article reviews overlap population (OP) diagrams for electron energy loss near-edgestructure (ELNES) and x-ray absorption near-edge structure (XANES). By using theOP diagrams, peaks in ELNES and XANES of MgO, ZnO, AlN, GaN, InN, andYBa2Cu3O7−x are interpreted in terms of cation–anion and cation–cation interactions. Commonfeatures are found in the OP diagrams for different edges. A reconstruction of theunoccupied electronic structure is demonstrated by aligning the different edges with thecommon features in the OP diagrams. The OP diagram is also applied to theCu/Al2O3 hetero-interface to find the relationships among ELNES, atomic and electronic structures,and properties.

X-ray absorption near edge structure/electron energy loss near edge structurecalculation using the supercell orthogonalized linear combination of atomic orbitalsmethod

Over the last eight years, a large number of x-ray absorption near edge structure (XANES)and/or electron energy loss near edge structure (ELNES) spectroscopic calculations forcomplex oxides and nitrides have been performed using the supercell-OLCAO (orthogonalizedlinear combination of atomic orbitals) method, obtaining results in very good agreementwith experiments. The method takes into account the core-hole effect and includes thedipole matrix elements calculated from ab initio wavefunctions. In this paper, we describethe method in considerable detail, emphasizing the special advantages of this method forlarge complex systems. Selected results are reviewed and several hitherto unpublishedresults are also presented. These include the Y K edge of Y ions segregated to the core of aΣ31 grain boundary in alumina, O K edges of water molecules, C K edges in different types ofsingle walled carbon nanotubes, and the Co K edge in the cyanocobalamin (vitaminB12) molecule. On the basis of these results, it is argued that the interpretation of specificfeatures of the calculated XANES/ELNES edges is not simple for complex material systemsbecause of the delocalized nature of the conduction band states. The long-standing notionof the ‘fingerprinting’ technique for spectral interpretation of experimental data is nottenable. A better approach is to fully characterize the structure under study, using eithercrystalline data or accurate ab initio modeling. Comparison between calculatedXANES/ELNES spectra and available measurements enables us to ascertain the validity ofthe modeled structure. For complex crystals or structures, it is necessary to use theweighted sum of the spectra from structurally nonequivalent sites for comparison with themeasured data. Future application of the supercell-OLCAO method to complexbiomolecular systems is also discussed.

Local electronic structure analysis by site-selective ELNES using electron channeling andfirst-principles calculations

In this paper, we review our recent analyses of electron energy loss near edge structure(ELNES) of particular crystalline sites, exploiting dynamical electron diffractioneffects, or electron channeling, whereby the excitation weights of the Bloch wavespropagating in a crystal can be controlled systematically by adjusting the diffractionconditions. A state-of-the-art data processing technique, multivariate curve resolution(MCR), can restore purely site-specific spectral profiles and their compositionsfrom the experimental data set. Another technique, the Pixon deconvolutionmethod, effectively removes the statistical noise, which enables us to compare thespectral fine structures with those calculated by first principles and discuss thesite-specific local atomic and electronic structures. We demonstrate typical casestudies in model materials and then an advanced chemical state analysis in a realmaterial. Finally, some remarks toward further refinement of the method are made.

First-principles calculation of spectral features, chemical shift and absolute threshold ofELNES and XANES using a plane wave pseudopotential method

Spectral features, chemical shifts, and absolute thresholds of electron energy loss near-edgestructure (ELNES) and x-ray absorption near-edge structure (XANES) for selected compounds,i.e. TiO2 (rutile), TiO2 (anatase), SrTiO3, Ti2O3,Al2O3, AlNand β-Ga2O3, were calculated by a plane wave pseudopotential method. Experimental ELNES/XANESof those compounds were well reproduced when an excited pseudopotential, whichincludes a core hole, was used. In addition to the spectral features, it was found thatchemical shifts among different compounds were also reproduced by correcting thecontribution of the excited pseudopotentials to the energy of the core orbital.

Local Electronic and Atomic Structure of Ce<SUP>3+</SUP>-Containing Fluoride/Oxide Determined by TEM-EELS and First-Principles Calculations

We investigated the local atomic and electronic structure around Ce in CeF3 and Ce2O3 by the combination analysis of F- and O-K shell electron energy loss near edge structures (ELNES) and first-principles calculations. The energy width of the main edge structure depended on the interaction between Ce5d orbitals and the neighboring F/O atoms. Not only ELNES but also the reported emission and excitation spectra were qualitatively consistent with the electronic structures of density functional theory (DFT) calculations with Hubbard U for the Ce4 f energy correction. Main factors determining the emission wavelength of the fluoride and Ce-doped oxides were discussed. [doi:10.2320/matertrans.MC200828]

Ferric iron content in (Mg,Fe)SiO3 perovskite and post-perovskite at deep lower mantle conditions

We have determined the Fe 3+ /∑Fe ratio of Al-free (Mg,Fe)SiO 3 perovskite, post-perovskite, and (Mg,Fe)O ferropericlase synthesized at 99 to 187 GPa and 1830 to 3500 K based on the electron energy-loss near-edge structure (ELNES) spectroscopy. The results demonstrate that post-perovskite includes minor amounts of ferric iron with Fe 3+ /∑Fe ratios of 0.11 to 0.21. These values are substantially lower than those of Al-rich post-perovskite (Fe 3+ /∑Fe = 0.59 to 0.69) reported in a previous study, suggesting that the Fe 3+ -Al 3+ coupled substitution is important in post-perovskite, as in the case of perovskite. The Al-bearing post-perovskite in a pyrolitic mantle composition likely contains a considerable amount of ferric iron, which affects various physical properties in the lowermost mantle.

Comparison of Mg L 23 edges in MgO and Mg(OH) 2—Importance of medium-range structure

Mg L 23-edge electron energy-loss near-edge structure in Mg(OH) 2 was studied experimentally and theoretically for comparison with that of MgO. The differences in the near-edge fine structure between Mg(OH) 2 and MgO were interpreted as a result of differences in medium-range structure.

Application of a fully relativistic theory to the EELS investigation of anisotropy effects at the oxygen K edge in rutile andZrO2

The electron energy-loss near-edge structures (ELNES) measured using a transmission electron microscope (TEM) generally depend on the orientation and the convergence of the electron beam and on the collector aperture. Within the dipole approximation, the current of inelastic electrons emitted from a particular atomic core level can be expressed as a linear combination of some particular electron inelastic-scattering cross sections (the intrinsic components) calculated for particular orientations of the transferred wave vector. In the case of a very low symmetry around the probed atom (cases of trichroism), the collected current can be expressed as a combination of three, four or six intrinsic components, a situation more complicated than the well-known situation of dichroism where only two intrinsic components exist. This paper aims at investigating situations of very low symmetry. At first, we examine the case of the $\text{O}\text{ }K$-edge in the rutile form of ${\text{TiO}}_{2}$, where the local symmetry around the oxygen site is described with the ${\text{C}}_{2v}$ point group. We are in a situation of (a) trichroism, where the collected current depends on three intrinsic components. These three components are evaluated using either the FEFF8.2 code or the TELNES subroutine of the WIEN2K code. From these components, the collected current can be calculated for any beam orientation and any collector aperture. These simulations are compared with experimental $\text{O}\text{ }K$ near-edge structures recorded in a TEM for different orientations and collection conditions. The simulations of the collected current are in good agreement with the experimental results, and orientation as well as collection angle effects on the ELNES are evidenced. Moreover, it is clearly shown that, in such a case, the relativistic velocity of the incident electrons cannot be neglected. It clearly appears that, neglecting these relativistic effects induces significant changes in the ELNES, leading to a marked deterioration of the agreement between experiment and theory. Such changes are also confirmed in another case of trichroism: the $\text{O}\text{ }K$-edge in monoclinic zirconia.

Martensitic transformation ofNi2FeGaferromagnetic shape-memory alloy studied via transmission electron microscopy and electron energy-loss spectroscopy

The structural properties of ${\text{Ni}}_{2}\text{FeGa}$ Heusler alloy synthesized by melt-spinning technique have been systematically studied by means of in situ heating and cooling transmission electron microscopy. It was found that the ${\text{Ni}}_{2}\text{FeGa}$ alloy was annealed into a well-defined $L{2}_{1}$ structure at around 980 K, and complex microstructural domains appeared along with lowering temperature. At room temperature (293 K), a rich variety of micromodulated domains were observed. The domain structures were aligned along the $⟨110⟩$ or $⟨100⟩$ directions resulting to complex tweed structures. Below martensitic transformation (MT) temperature $({M}_{s},\ensuremath{\sim}142\text{ }\text{K})$, the cubic parent phase transformed into unmodulated martensitic variants and modulated martensitic variants. The variants were alternated along the $⟨100⟩$ direction with various arrangements and steplike incommensurate boundaries. The modulated martensitic variants were composed of lamellar structures that have predominately a $5M$ modulation structure along the $⟨110⟩$ directions. The electron energy-loss spectroscopy analysis of the low-loss region and the electron energy-loss near-edge fine structure revealed a visible change of the electronic structure along with MT, which can be well interpreted by means of intra-atomic or intraband charge redistribution due to $spd$ orbital hybridization among the Ni-Fe-Ga atoms.