Edge Onset Research Articles

Experimental probing of the anisotropy of the empty p states near the Fermi level in MgB2

We have studied the boron K edge in the superconductor MgB2 by electron energy loss spectroscopy (EELS) and experimentally resolved the empty p states at the Fermi level that have previously been observed within an energy window of 0.8 eV by soft x-ray absorption spectroscopy. Using angular-resolved EELS, we find that these states at the immediate edge onset have pxy character in agreement with predictions from first-principle electronic structure calculations.

Metal-induced gap states at well defined alkali-halide/metal interfaces.

In order to search for states specific to insulator/metal interfaces, we have studied epitaxially grown interfaces with element-selective near edge x-ray absorption fine structure. An extra peak is observed below the bulk edge onset for LiCl films on Cu and Ag substrates. The nature of chemical bonds as probed by x-ray photoemission spectroscopy and Auger electron spectroscopy remains unchanged, so we regard this as evidence for metal-induced gap states (MIGS) formed by the proximity to a metal, rather than local bonds at the interface. The dependence on the film thickness shows that the MIGS are as thin as one monolayer. An ab initio electronic structure calculation supports the existence of the MIGS that are strongly localized at the interface.

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 InxGa1-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 InxGa1-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.

Metal induced gap states at LiCl–Cu(0 0 1) interface studied by X-ray absorption fine structure

A single crystalline LiCl film grows heteroepitaxially on Cu(001) in a layer-by-layer fashion at 300K. The LiCl–Cu(001) system provides a well-defined insulator–metal interface. Electronic structure of the LiCl film grown on Cu(001) was studied using Cl K-edge near edge X-ray absorption fine structure (NEXAFS). A pre-peak was observed on the bulk edge onset for the thin LiCl films. The pre-peak showed formation of metal-induced gap states (MIGS) in the band gap of LiCl. The characteristic decay length of the MIGS was determined to be 2.6Å by analyzing the thickness dependence on the intensity of the pre-peak. The present result has revealed the MIGS at the well-defined insulator–metal interface for the first time.

Synthesis and characterization of mixed-valence barium titanates

A single-crystal barium oxotitanate(III, IV) of approximate composition , containing mixed-valence Ti, was grown from a borate flux. The crystal structure was identified as hollandite type by single-crystal X-ray diffractometry. Electron-energy-loss spectroscopy of Ti L 2,3 and O K edges was used to determine chemical shifts related to the presence of mixed-valence Ti in the crystal. Comparison of Ti L 2,3 and O K energy-loss near-edge structure (ELNES) of with those obtained from a K 1.54 Mg 0.77 Ti 7.23 O 16 single crystal with hollandite structure, containing only Ti 4+ , revealed a shift in the Ti L 2,3 edge by 0.4-0.5 eV towards lower energy losses whereas only slight intensity variations without a detectable energy shift of the edge onset occur at the O K ELNES. In addition, valence-specific multiplet structures of the Ti L 23 ELNES are used as valence fingerprints. The observed fine structures of O K and Ti L 2,3 edges can be used to interpret coordination and bonding in related compounds.

ELNES investigations of the oxygen K-edge in spinels

The results of a systematic study of the oxygen K-edge electron energy-loss spectroscopy (ELNES) from a series of aluminium- and chromium-containing spinels are presented. Extra fine structure in the region up to 10 eV above the edge onset is observed for the chromium-containing compounds and is assigned to transitions to states created by mixing of oxygen 2p and metal 3d orbitals. The experimental data has been simulated using the multiple scattering code, FEFF8. Good agreement was obtained in the case of magnesium aluminate, but relatively poor agreement was obtained in the case of the chromites. The possible fingerprints in the oxygen K-edge ELNES corresponding to a high degree of inversion in the spinel structure and to a tetragonal distortion of the cubic structure are discussed.

Success and limits of common final-state approximations

Several current models for the treatment of final-state effects in the calculation of electron energy loss near edge structures are studied. Ab-initio calculations for the corresponding electronic reference configurations were carried out for the atoms Li to Kr. Edge onset energies can be directly obtained as energy differences between ground and excited state only if the excitation is explicitly accounted for; implicit schemes such as the Z+1 or Slater's transition state approximation yield satisfactory results only for the K-edge of atoms up to phosphorous. Concerning the intensity modulation of the edge, however, the Z+1 approximation exhibits the closest overall agreement with the exact K-shell excitation. Other approaches model the electron–hole interaction more effectively for the d-elements, but show pronounced, non-continuous discrepancies for the noble gases and the early p-elements.

Electron Energy-Loss Near-Edge Structure Studies of a Cu/(11-20)?-Al2O3 Interface

Electron energy-loss near-edge structure (ELNES) studies were employed to determine the bonding mechanisms and electronic structure of a Cu/(1120)α-Al 2 O 3 interface prepared by molecular beam epitaxy. The existence of an interfacial ELNES component at the Al-L 2,3 edge reveals that the Al atoms participate in the bonding and change their local coordination compared to bulk Al 2 O 3 . At the interfacial O-K ELNES some pre-edge intensity appears, indicating the presence of deep-level O-2p states hybridised with Cu-3d and/or Al-3p states. The interfacial Cu-L 2,3 ELNES shows a chemical shift of the edge onset to higher energy-loss values and the existence of unoccupied Cu-3d states. The shape and edge onset are similar to the Cu-L 2,3 edge measured in an intermetallic CuAl 2 compound. Image simulations of the experimental high-resolution transmission electron microscopic images were carried out assuming Cu-Al bonds at the Cu/(1120)Al 2 O 3 interface as found by the ELNES studies. The derived structural model for the atomistic arrangement at the interface contains a mixed monolayer of Cu and Al atoms that exhibits a projected bonding distance of (0.15 ± 0.02) nm to the first O-layer of the Al 2 O 3 substrate.

The Effect of Local Symmetry on Atomic Resolution EELS Near-Edge Structures: Predictions for Grain Boundaries In NiAl

Abstract It has been demonstrated that electron energy loss spectrometry (EELS) can be used to probe the electronic structure of materials on the near-atomic scale. The electron energy loss near edge structure (ELNES) observed after the onset of a core edge reflects a weighted local density of final states to which core electrons are excited by fast incident electrons. Lately ‘atomic resolution EELS’ and ‘column-by-column spectroscopy’ have become familiar themes amongst the EELS community. The next generation of STEMs, equipped with spherical aberration (Cs) correctors and electron beam monochromators, will have sufficient spatial and energy resolution, along with the superior signal to noise required, to detect small changes in the ELNES from atomic column to atomic column. Core loss ELNES provides information about unoccupied states, but the structure observed in spectra is sensitive to changes in the underlying occupied states, and thus to the bonding in the material.

The role of spatiotemporal edges in visibility and visual masking.

What parts of a visual stimulus produce the greatest neural signal? Previous studies have explored this question and found that the onset of a stimulus's edge is what excites early visual neurons most strongly. The role of inhibition at the edges of stimuli has remained less clear, however, and the importance of neural responses associated with the termination of stimuli has only recently been examined. Understanding all of these spatiotemporal parameters (the excitation and inhibition evoked by the stimulus's onset and termination, as well as its spatial edges) is crucial if we are to develop a general principle concerning the relationship between neural signals and the parts of the stimulus that generate them. Here, we use visual masking illusions to explore this issue, in combination with human psychophysics, awake behaving primate neurophysiology in the lateral geniculate nucleus of the thalamus, and optical recording in the primary visual cortex of anesthetized monkeys. The edges of the stimulus, rather than its interior, generate the strongest excitatory and inhibitory responses both perceptually and physiologically. These edges can be imaged directly by using optical recording techniques. Excitation and inhibition are moreover most powerful when the stimulus turns both on and off (what might be thought of as the stimulus's temporal edges). We thus conclude that there is a general principle that relates the generation of neural signals (excitatory and inhibitory) to the spatiotemporal edges of stimuli in the early visual system.

Bonding in alpha-quartz (SiO2): A view of the unoccupied states

High-resolution core-loss and low-loss spectra of α-quartz were acquired by electron energyloss spectroscopy (EELS) with a transmission electron microscope (TEM). Spectra contain the Si L 1 , L 2,3 , K, and O K core-loss edges, and the surface and bulk low-loss spectra. The core-loss edges represent the atom-projected partial densities of states of the excited atoms and provide information on the unoccupied s, p, and d states as a function of energy above the edge onset. The band structure and total density of states were calculated for α-quartz using a self-consistent pseudopotential method. Projected local densities of Si and O s, p, and d states (LDOS) were calculated and compared with the EELS core-loss edges. These LDOS successfully reproduce the dominant Si and O core-loss edge shapes up to ca. 15 eV above the conduction-band onset. In addition, the calculations provide evidence for considerable charge transfer from Si to O and suggest a marked ionicity of the Si-O bond. The experimental and calculated data indicate that O 2p-Si d π-type bonding is minimal. The low-loss spectra exhibit four peaks that are assigned to transitions from maxima in the valence-band density of states to the conduction band. A band gap of 9.65 eV is measured from the low-loss spectrum. The structures of the surface low-loss spectrum are reproduced by the joint density of states derived from the band-structure calculation. This study provides a detailed description of the unoccupied DOS of α-quartz by comparing the core-loss edges and low-loss spectrum, on a relative energy scale and relating the spectral features to the atom- and angular-momentum-resolved components of a pseudopotential band-structure calculation.

Methods for ELNES-quantification: characterization of the degree of inversion of Mg–Al-spinels

The energy loss near-edge structures in electron energy-loss spectra contain information about bonding characteristics, the electronic structure and coordinations of the excited atoms. We have calculated sets of reference spectra for the normal and for the inverse Mg–Al-spinel using a full multiple scattering approach. By a quantitative comparison of these reference spectra with experimental data ELNES-quantification becomes possible. We characterized the degree of inversion λ by the analysis of the relative peak-intensities and the relative peak-positions within 35 eV beyond the edge onset. The results demonstrate that by using the provided methods ELNES-quantification will become possible when uncertainties in the experiment are reduced and a better fit of the simulations to the experiment is achieved.

Direct experimental observation of the local electronic structure at threading dislocations in metalorganic vapor phase epitaxy grown wurtzite GaN thin films

The electronic structure of pure edge threading dislocations in metalorganic vapor phase epitaxy grown wurtzite GaN thin films has been studied directly by atomic resolution Z-contrast imaging and electron energy loss spectroscopy in a scanning transmission electron microscope. Dislocation cores in n-type samples grown in N-rich conditions show no evidence for the high concentration of Ga vacancies predicted by previous theoretical calculations. Nitrogen K-edge spectra collected from edge dislocation cores show a sudden and significant increase in the intensity of the first fine-structure peak immediately above the edge onset compared to the bulk spectra. The origin of this increase is discussed.

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.

ANALYSIS OF NEAR EDGE FINE STRUCTURE AND ELECTRONIC POPULATIONS IN SAPPHIRE

AlL and OK near edge fine structures in sapphire are studied using electron energy loss microspectroscopy in a transmission electron microscope;an analysis of the electronic populations of atoms Al and O,and a comparison between different partial cross sections are given.We consider the ionization of an atom in a homogeneous solid to interpret the elemental effects in the ionization region near edges.The chemical effects in the region near edge onset are interpreted by using extended Hückel molecular orbital theory and Bloch's theorem including the effects of translation symmetry in crystals to calculate the electronic transitions from core-shell to unoccupied valence-shell.We also consider the additional chemical effects, which arise from the elastic backscattering of the atomic ionization waves by the atoms neighboring the exited atom,giving rise to the so-called extended fine structure in the ionization region.The agreement between the calculated results and the experimental energy loss spectra of sapphire from single-phase region is good.

Study on the transition process of diamond grain shape in stone grinding

The edge fracture of a diamond grain plays an important role in heavy grinding such as stone grinding, in which the self-dressing action of the diamond grain keep up the edge sharpness and the stock removal. In order to analyze the variation of the edge shape distribution of a diamond grain during grinding, a new analysis method applying the theory of the Marcov Process is proposed. The results are summarized as follows: (i) the diamond grains have large probabilities of transition to a sharp edge and small fracture onset probabilities, hence a high cutting ability is maintained in steady state; (ii) the disappearance onset probabilities of diamond grains don't depend on the edge shape; and (iii) these probabilities made it possible to evaluate transition process of the shapes of diamond grains.

Rheological scaling of montmorillonite suspensions: the effect of electrolytes and polyelectrolytes

This paper presents the results of investigations on flocculated alkaline montmorillonite aqueous dispersions based on the scaling correlation between rheological parameters and the clay weight fraction. Two types of montmorillonite flocculation, induced by the presence of electrolytes, may be monitored from the scaling analysis of the rheological data: one produced by edge—face interactions occurring at a critical concentration of electrolyte and one derived from face—face interactions occurring only in the presence of high concentrations of polyvalent cations. At the onset of edge—face flocculation the exponent of the scaling function decreases from a value in the range 8–11, associated with a stable montmorillonite dispersion, to a value in the range 2–3. The face—face coagulation produces, instead, a shift of the entire scaling line towards higher montmorillonite concentrations without changes in the exponent. Anionic polyelectrolytes adsorbed on the clay surface weaken the network structure of the flocculated particles and thereafter deflocculate the dispersion, producing a partial recovery of the scaling exponent. It is suggested that the selective adsorption of polyelectrolyte on the edges of the particles restores more effectively the negative surface charge of the edges, thereby assisting the peptisation of the clay dispersion. Scaling analysis of the Bingham yield stress of both flocculated and stabilised montmorillonite dispersions permitted the evaluation of energy variations of the particle separation.

Nature of the absorption bands in Zn 1−xCo xS

To study the nature of the absorption bands in Zn 1− x Co x S single crystals, optical absorption measurements were made on samples with cobalt composition x = 0.03 and 0.05 as a function of temperature in the wavelength region between 400 and 3000 nm. Each spectrum showed three well-defined absorption bands below the fundamental absorption gap, corresponding to transitions from the fundamental 4A 2( 4F) ground state to the 4T 1( 4F), 4T 2( 4F) and 4T 1(P) excited states. Besides these main bands, the infrared region has a series of small peaks. In the visible region, just below the onset of the band edge, there is a well-defined structure related to electronic transitions from the 3d shell of the cobalt ion.

Optical constants of YBaCuO from polarized reflectance measurements

We report results of polarized reflection spectra of YBa 2Cu 3O 7 films and a calculation of the in-plane optical constants. We find a typical metallic behavior up to a photon energy of 3 eV. In particular, the imaginary part of the dielectric function is small above the onset of the plasma edge, in contrast to previous results. A plasma enery of 1.2-1.3 eV is estimated from the results.

VUV and UV fluorescence and absorption studies of Pr^3+-doped LiLuF_4 single crystals

The laser-induced fluorescence spectrum of LiLuF(4):Pr(3+) single crystals, pumped by an F(2) pulsed-discharge molecular laser at 157 nm, was obtained in the VUV and UV regions of the spectrum at room temperature. A number of new fluorescence peaks were observed for the first time to our knowledge. They were assigned to the dipole-allowed transitions 4f5d ? 4f(2) of Pr(3+) ion. In addition, the absorption spectrum of the crystal samples was recorded. The positions of the bands with the 4f5d configuration were found to be 46 412, 53 267, and 63 397 cm(-1) from the ground state, (3)H(4), of the Pr(3+) ion. The edge (onset) of the 4f5d bands was at 45 100 cm(-1).