Combined X-ray Absorption Spectroscopy Research Articles

Insights into the Lithiation of Nano-Sized α-Fe2O3: A Combined X-ray Absorption Spectroscopy and Pair Distribution Function Study

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Defect states in epitaxial HfO2 films induced by atomic transport from n-GaAs (100) substrate

We investigated the chemical states and nature of the defect states below the conduction band edge of HfO2 films grown on GaAs (100) substrates using high-resolution x-ray photoelectron spectroscopy (HRXPS), x-ray absorption spectroscopy (XAS), and density functional theory calculations. O K1-edge absorption spectra of the HfO2/GaAs film revealed two distinct conduction band edge defect states, located at 1.6 ± 0.2 eV and 3.0 ± 0.2 eV below the conduction band edge in HfO2. The combined XAS and HRXPS results as a function of post-deposition annealing temperature indicated that the changes in defect states below the conduction band edge of HfO2 were correlated with the extent of interfacial chemical reactions between the HfO2 film and the GaAs substrate. Spectroscopic and theoretical results revealed that the two conduction band defect states are caused by (i) diffused Ga–O states, Hf3+ states, and (ii) an O divacancy related to the As–O states, respectively.

Multiplet Theory for Conduction Band Edge and O-Vacancy Defect States in SiO2, Si3N4, and Si Oxynitride Alloy Thin Films

This article, the second of a two-part sequence, combines X-ray absorption spectroscopy (XAS) and many-electron theory to develop a model for intrinsic bonding defects in non-crystalline and nanocrystalline thin films used primarily as gate dielectrics for field effect transistors and related devices. In SiO2 these defects are O-atom vacancies and their spectral features in O K pre-edge spectra are assigned to multiplet transitions based on a d2 model for the electrons in a neutral O-vacancy site. This model is applied to (i) non-crystalline (nc-) SiO2, and extended to (ii) Si oxynitride alloys, (nc-SiO2)1-x(Si3N4)x, and nc-Hf Si oxynitrides. As in Part I, this relies on Tanabe–Sugano (TS) energy diagrams to identify spin-allowed X-ray transitions and negative ion states. Differences between band edge states in (i) nc-SiO2, and (ii) nc-Si3N4 explain quantitative differences in trapping and trap-assisted tunneling. Preliminary results for nc-GeO2 films prepared by remote-plasma-enhanced chemical vapor deposition shows these films exhibit surface stability similar to nc-SiO2, suggesting that local nc-GeO2 bonding involves the same 4-fold Ge, and 2-fold O coordination as Si and O in nc-SiO2.

Humidity driven nanoscale chemical separation in complex organic matter

Environmental contextThe absorption of water by mixtures of organic matter in aerosols influences various atmospheric processes, such as scattering of solar radiation and cloud formation. We use X-ray techniques to elucidate the swelling behaviour of representative organic matter under humid, sub-saturated conditions, and show chemical separation according to the functional group composition in the organic mixtures upon water uptake. The results will further our understanding of the complex role of aerosol organic matter in atmospheric processes. AbstractHumic-like substances (HULIS) represent an important fraction of particulate organic matter in the atmosphere. Understanding the water uptake by HULIS and the associated morphology evolution will improve the assessment of their ability to act as cloud condensation nuclei as well as their light scattering properties. The water uptake properties of Suwannee River Fulvic Acid and of tannic acid used as proxy for atmospheric HULIS, were investigated using X-ray absorption spectroscopy in combination with a scanning transmission X-ray microscope. For both compounds, continuous water uptake was observed, whereby in fulvic acid phase separation occurred, resulting in an inhomogeneous organic matrix. Within the inhomogeneous mixture, different regions with different amounts of water uptake could be differentiated based on their spectral signatures in near-edge X-ray absorption fine structure (NEXAFS) spectra, thus based on carbon functional group signatures, indicating that carboxyl-poor compounds separated from carboxyl-rich compounds upon water uptake. The differentiation into fractions with high/low water uptake ability is further refined by considering phenols, aromatic groups, and O-alkylic groups.

A chemical model of intermediate states implied in the switching properties of CoFe Prussian blue analogues: how a cell parameter lengthening can cause a crystal field parameter increase

A series of CoFe Prussian blue analogues of chemical formula Rb2Co4−xZnx[Fe(CN)6]3.3·11H2O (x = 0, 1, 1.95 and 2.7) has been synthesized along which the MII/CoIII ions ratio at the Co site has been tuned. The long range order and the electronic structure of the Co ions have been investigated by combined powder X-ray diffraction and X-ray absorption spectroscopy (XAS) measurements. The cell parameter of the face-centered cubic structure lengthens as the MII/CoIII ions ratio increases without phase demixing. The study of the electronic structure of the Co ions by XAS shows that the coordination polyhedra of the CoII(HS) ions play an important role in the flexibility of the cubic structure. The variation of the cell parameter in the series of compounds is accompanied by the variation of the CoII–NC bond angle which allows the expansion or contraction of the cubic structure accompanying the electronic switch without phase demixing. Due to this structural re-arrangement, a lengthening of the cell parameter unusually produces an increase of the Co ion crystal field. Such a re-arrangement occurs in the course of the photo-induced electron transfer.

Spectroscopic detection of hopping induced mixed valence for Ti and Sc in GdSc1−xTixO3 for x greater than the percolation threshold of ∼0.16

Only two of the first row transition metals have elemental oxides that are either ferromagnetic or ferrimagnetic; these are CrO2 and Fe3O4. The electron spin alignment enabling ferromagnetism and/or ferrimagnetism in these oxides is associated with a double exchange mechanism that requires mixed valence and metallic conductivity. This article describes a novel way to realize these two necessary, but insufficient conditions for double exchange magnetism. These are mixed valence and a hopping conductivity that can force intraplane electron spin alignment in a complex oxide host perovskite, A(B,C)O3, where A is an ordinary metal or d0 lanthanide, B is a d0 transition metal, and C is a dn transition metal with n≥1 as, for example, in GdS1−xTixO3. This article combines x-ray absorption spectroscopy, multiplet theory, charge transfer multiplet theory, and degeneracy removal by Jahn–Teller effect mechanisms to demonstrate mixed valence for both Sc and Ti above a percolation limit, x>0.16, in which hopping transport gives rise to a metal to insulator transition. In this alloy, ferromagnetism/ferrimagnetism is not observed due to alternating spin alignment in sequenced (Sc,Ti)O2 planes.

Formation and vanishing of short range ordering inGaAs1−xBixthin films

We address the compositional evolution of the structure of ${\text{GaAs}}_{1\ensuremath{-}x}{\text{Bi}}_{x}$ thin films at different scale lengths by combining x-ray absorption spectroscopy, atomic force microscopy, and x-ray diffraction. We find that Bi short range ordering, observed for $x<3\mathrm{%}$, drastically vanishes for $x\ensuremath{\ge}5.4\mathrm{%}$. The recovery of random anion distribution goes along with the formation of Bi droplets at the sample surface while bulk maintains high crystalline quality. These structural changes go with the anomalous behavior of optical and electronic properties. In particular, we find that the decrease in emission intensity in the samples corresponds to the concentration point where Bi short range ordering is lost, hence the preservation of nonrandom distribution turns to be a key point in devices design.

Structure and properties of PVMo 11O 40 heteropolyoxomolybdate supported on silica SBA-15 as selective oxidation catalyst

Investigations into structure and properties of heteropolyoxomolybdates (HPOM) supported on nanostructured silica SBA-15 for the selective oxidation of propene are presented. H 4[PVMo 11O 40] Keggin ions supported on SBA-15 (PVMo 11-SBA-15) were prepared by incipient wetness of silica SBA-15 with suitable precursor solutions. Structure and properties of the resulting material were studied ex situ by X-ray diffraction and nitrogen physisorption measurements and in situ by combined X-ray absorption spectroscopy (XAS) and mass spectrometry under various reaction conditions. The characteristic structure of PVMo 11-SBA-15 and its evolution under reactive gas atmosphere are compared to that of bulk H 4[PVMo 11O 40]⋅ xH 2O. Structural investigations of as-prepared PVMo 11-SBA-15 have shown that the respective Keggin ions can be readily supported on silica SBA-15. In situ XAS measurements under reducing or oxidizing conditions revealed a pronounced support interaction effect. This effect resulted in a further decreased thermal stability of the supported Keggin ions compared to bulk H 4[PVMo 11O 40]⋅ xH 2O. Apparently, no stable HPOM Keggin ions could be obtained on silica SBA-15 under reaction conditions. However, in spite of their low thermal stability, HPOM supported on SBA-15 remain suitable well-defined precursors for preparing mixed oxide model catalysts for selective oxidation reactions.

Combination of optical and X-ray techniques in the study of amorphous semiconductors under high pressure: an upgrade setup for combined XAS and XRD measurements

X-ray absorption spectroscopy (XAS) and X-ray diffraction (XRD) are complementary techniques whose combination is a powerful tool of investigation of matter under extreme conditions (high pressure, high temperature). The standard setup at the ODE beamline (Soleil Synchrotron) has been modified in order to allow the performance of simultaneous XAS/XRD measurements on the same sample. These techniques, together with Raman spectroscopy measurements, were applied to the study of pressure-induced phase transitions of amorphous silicon-germanium alloy (a-Si x Ge1−x , with x=0.75) using a membrane diamond anvil cell as the pressure device.

The role of Cu on the reduction behavior and surface properties of Fe-based Fischer–Tropsch catalysts

The effect of Cu on the reduction behavior and surface properties of supported and unsupported Fe-based Fischer-Tropsch synthesis (FTS) catalysts was investigated using in situ X-ray photoelectron spectroscopy (XPS) and in situ X-ray absorption spectroscopy (XAS) in combination with ex situ bulk characterization. During exposure to 0.4 mbar CO-H(2) above 180 degrees C, the reduction of CuO to Cu(0) marked the onset of the reduction of Fe(3)O(4) to alpha-Fe. The promotion effects of Cu are explained by a combination of spillover of H(2) and/or CO molecules from metallic Cu(0) nuclei to closely associated iron oxide species and textural promotion. XAS showed that in the supported catalyst, Cu(+) and Fe(2+) species were stabilized by SiO(2) and, as a result, Fe species were not reduced significantly beyond Fe(3)O(4) and Fe(2+), even after treatment at 350 degrees C. After the reduction treatment, XPS showed that the concentration of oxygen and carbon surface species was higher in the presence of Cu. Furthermore, it was observed that the unsupported, Cu-containing catalyst showed higher CO(2) concentration in the product gas stream during and after reduction and Fe surface species were slightly oxidized after prolonged exposure to CO-H(2). These observations suggest that, in addition to facilitating the reduction of the iron oxide phase, Cu also plays a direct role in altering the surface chemistry of Fe-based FTS catalysts.

Spin and Orbital Ground State of Co in Cobalt Phthalocyanine

The 3d orbital ground state of transition-metal ions that are incorporated in a molecular matrix determines the total spin of the transition-metal ion as well as the spin anisotropy and thus the essential magnetic properties of the corresponding molecule. However, there is little known to date on the exact 3d ground state of many molecular systems, including quite complex molecular magnets as well as relatively simple systems such as, for instance, cobalt phthalocyanine (CoPc). For the latter, there are contradictory theoretical predictions with respect to the occupation of the various Co 3d electronic levels. We demonstrate that polarization-dependent X-ray absorption spectroscopy in combination with a simulation of the spectra is able to shed a brighter light on the spin and orbital ground state of the transition-metal ion in CoPc. Our results reveal a temperature-dependent ground state and emphasize the importance of taking 3d correlation effects properly into account.

Pressure-induced structural distortion ofTbMnO3: A combined x-ray diffraction and x-ray absorption spectroscopy study

The variation of electronic states and structural distortion in highly compressed multiferroic ${\text{TbMnO}}_{3}$ was probed by x-ray diffraction (XRD) and x-ray absorption spectroscopy (XAS) using synchrotron radiation. Refined XRD data enabled observation of a reduced local Jahn-Teller (JT) distortion of Mn sites within ${\text{MnO}}_{6}$ octahedra in ${\text{TbMnO}}_{3}$ with increasing hydrostatic pressure. A progressively increasing intensity of the white line in $\text{Mn}\text{ }K$-edge x-ray absorption spectra of ${\text{TbMnO}}_{3}$ was detected with increasing pressure. The absorption threshold of $\text{Mn}\text{ }K$-edge spectra of ${\text{TbMnO}}_{3}$ is shifted toward higher energy, whereas the pre-edge peak is slightly shifted to lower energy with increasing hydrostatic pressure. We provide spectral evidence for pressure-induced bandwidth broadening for mangnites. The enhanced intensity of the white line and the shifted absorption threshold of $\text{Mn}\text{ }K$-edge spectra are explained in terms of a reduced JT distortion of ${\text{MnO}}_{6}$ octahedra in compressed ${\text{TbMnO}}_{3}$. Comparison of XAS data with full-multiple-scattering calculations using code FDMNES shows satisfactory agreement between experimental and calculated $\text{Mn}\text{ }K$-edge spectra.

EXAFS and DFT: Evidence for the [TcO]2+ core

A structure elucidation strategy combining X-ray Absorption Spectroscopy (XAS) and Density Functional Theory (DFT) is demonstrated and reveals the existence of the [Tc=O](2+) moiety.

XAS and XMCD study of the influence of annealing on the atomic ordering and magnetismin an NiMnGa alloy

The proper annealing of Ni51Mn28Ga21 ribbon alloy gives rise to an increase of the saturation magnetization and of the magnetic orderTC (up to 20 K) andmartensitic transition TM (up to 10 K) temperatures. The combined x-ray absorption spectroscopy (XAS) and x-raymagnetic circular dichroism (XMCD) studies indicate that the annealing treatment drivesthe alloy to a more ordered structure without significantly affecting the local structure interms of interatomic distances and bonding geometry. By contrast, the annealing stronglyaffects the near-edge absorption at the Mn K-edge while no effect is observed at either theNi or Ga K-edge. These results suggest that annealing leads to a modification of theelectronic structure of the Mn atoms while that of Ni and Ga atoms remains unvaried.However, strong XMCD signals are detected at both Ni and Ga K-edges whoseamplitude increases after annealing. These results point out that despite thechange of the magnetic properties of the system being mainly associated withthe modification of the electronic properties of the Mn atoms, both Ni and Gamay play a non-negligible role through the polarization of the conduction band.

Local environment in Ba 2In 2−xW xO 5+3x/2 oxide ion conductors

The actual oxygen environment of the tungsten dopant in the Ba 2In 2− x W x O 5+3 x /2 solid solution was revealed by combining X-ray absorption spectroscopy at the tungsten L I and L III edges and at the indium L I edge. Whatever the substitution ratio, the tungsten atoms exhibit a regular octahedral environment. When the substitution ratio increases, the oxygen vacancies are progressively filled until their total occupancy for x=2/3. For x⩾0.3, the perovskite structure is stabilised; the tungsten atoms are randomly distributed in the structure. Although X-ray diffraction revealed a cubic symmetry for these compositions, a local distortion of the indium environment is observed when a tungsten atom is in its surrounding.

The Structure of the Amyloid- β Peptide High-Affinity Copper II Binding Site in Alzheimer Disease

Neurodegeneration observed in Alzheimer disease (AD) is believed to be related to the toxicity from reactive oxygen species (ROS) produced in the brain by the amyloid- β (A β) protein bound primarily to copper ions. The evidence for an oxidative stress role of A β-Cu redox chemistry is still incomplete. Details of the copper binding site in A β may be critical to the etiology of AD. Here we present the structure determined by combining x-ray absorption spectroscopy (XAS) and density functional theory analysis of A β peptides complexed with Cu 2+ in solution under a range of buffer conditions. Phosphate-buffered saline buffer salt (NaCl) concentration does not affect the high-affinity copper binding mode but alters the second coordination sphere. The XAS spectra for truncated and full-length A β-Cu 2+ peptides are similar. The novel distorted six-coordinated (3N3O) geometry around copper in the A β-Cu 2+ complexes include three histidines: glutamic, or/and aspartic acid, and axial water. The structure of the high-affinity Cu 2+ binding site is consistent with the hypothesis that the redox activity of the metal ion bound to A β can lead to the formation of dityrosine-linked dimers found in AD.

XAS and XMCD Investigation of Mn12 Monolayers on Gold

The deposition of Mn(12) single molecule magnets on gold surfaces was studied for the first time using combined X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD) methods at low temperature. The ability of the proposed approach to probe the electronic structure and magnetism of Mn(12) complexes without significant sample damage was successfully checked on bulk samples. Detailed information on the oxidation state and magnetic polarization of manganese ions in the adsorbates was obtained from XAS and XMCD spectra, respectively. Partial reduction of metal ions to Mn(II) was clearly observed upon deposition on Au(111) of two different Mn(12) derivatives bearing 16-acetylthio-hexadecanoate and 4-(methylthio)benzoate ligands. The average oxidation state, as well as the relative proportions of Mn(II), Mn(III) and Mn(IV) species, are strongly influenced by the deposition protocol. Furthermore, the local magnetic polarizations are significantly decreased as compared with bulk Mn(12) samples. The results highlight an utmost redox instability of Mn(12) complexes at gold surfaces, presumably accompanied by structural rearrangements, which cannot be easily revealed by standard surface analysis based on X-ray photoelectron spectroscopy and scanning tunnelling microscopy.

Interface electronic states and molecular structure of a triarylamine based hole conductor on rutile TiO2(110)

The molecular and electronic surface structure of a triarylamine based hole-conductor (HC) molecule evaporated onto rutile TiO2(110) single crystal is investigated by means of synchrotron light based photoelectron spectroscopy and x-ray absorption spectroscopy in combination with calculations based on density functional theory. Different amounts of the HC molecule was evaporated spanning the monolayer to multilayer region. The molecular surface structure is investigated and the results indicate that no specific covalent chemical bonding is formed and that the plane formed by the different nitrogens in the HC molecules has a rather small angle versus the TiO2 substrate surface plane. Some molecular ordering also persists in the multilayer region. The experimental core level spectra, valence level spectra, and the N 1s x-ray absorption spectroscopy spectra are well modeled by calculations on an individual molecule. Interestingly, the formation of the TiO2HC interface results in significant binding energy shifts in core levels and valence levels shifting all peaks of a the HC material to the same extent. Smaller shifts were also observed in the substrate core level peaks. The shift is discussed in terms of nanoscale energy level bending and final state hole screening. With respect to electronic applications, specifically in a solid state dye-sensitized solar cell, it is argued that the observed energy level alignment at the TiO2HC interface can act as a hole trap.

Structure and properties of a supported MoO 3–SBA-15 catalyst for selective oxidation of propene

Local structure and catalytic properties of molybdenum oxide supported on nanostructured SiO 2 (SBA-15) were studied under reaction conditions by combined X-ray absorption spectroscopy (XAFS) and mass spectrometry. MoO 3 supported on SBA-15 exhibits stability and catalytic properties different from those of binary bulk oxides. The interaction between support and molybdenum oxide stabilizes particular hexagonal MoO 3 structure that is highly active and selective. The hex-MoO 3–SBA-15 is stable under reducing (propene) and oxidizing reaction conditions in the temperature range from 20 to 500 °C. In contrast to bulk hex-MoO 3, the onset of activity at about 250 °C is not accompanied by a transformation to α-MoO 3. Moreover, in contrast to α-MoO 3, hex-MoO 3–SBA-15 is capable of directly oxidizing propene to acrylic acid without additional metal sites.

Magnetic properties of coupled ultrathinNiO∕Fe3O4(001) films

A strong reduction in the magnetic spin and orbital moments as well as large magnetocrystalline anisotropies are observed in ultrathin $\mathrm{Ni}\mathrm{O}∕{\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$ (001) films compared to ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$ (001). Combining x-ray absorption spectroscopy, x-ray magnetic circular and linear dichroism (XMCD-XMLD), and high-resolution transmission electron microscopy (HRTEM) the magnetic anisotropy of ultrathin $\mathrm{Ni}\mathrm{O}∕{\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$ (001) films was studied and compared to structural anisotropy at the interface. The evolution of the XMCD signal at the Fe octahedral sites allows a quantitative description of the interface. A simple model is proposed where uncompensated spins are located in a $1.5\text{\ensuremath{-}}\mathrm{nm}$-thick interfacial $\mathrm{Ni}{\mathrm{Fe}}_{2}{\mathrm{O}}_{4}$ compound. Angular-dependent measurements of the effective spin and orbital magnetic moments at the Fe ${L}_{2,3}$ edges reveal the presence of a large magnetocrystalline anisotropy, appearing for a particular growth temperature of the films. The magnetocrystalline anisotropy measured at the Fe ${L}_{2,3}$ edges is attributed to the specific anisotropic structure developed at the interface and to crystalline ordering of a strained compound. HRTEM confirms the presence of an ordered anisotropic structure at the $\mathrm{Ni}\mathrm{O}∕{\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$ (001) interface. The in-plane strained NiO(001) films promote antiferromagnetic order where the spins are oriented parallel to the ⟨100⟩ in-plane direction, collinear to the magnetization direction in the ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$ (001) substrate.