Oxygen K-edge X-ray Absorption Spectra Research Articles

Role of Oxygen Vacancy Sites on the Temperature-Dependent Photoluminescence of SnO2 Nanowires

The role of oxygen vacancies in temperature-dependent photoluminescence of SnO2 nanowires was investigated by X-ray absorption spectroscopy. Two types of oxygen vacancies are present in the nanowires: at out-of-plane sites and at in-plane sites; both play crucial roles in the temperature dependence of the photoluminescence. Oxygen vacancies at in-plane sites participate in photon emission at low temperature, whereas those at out-of-plane sites result in photoluminescence at room temperature. Accordingly, the luminescence color changes from orange (630 nm, 1.93 eV) to green (515 nm, 2.4 eV) at 100 K. The color change is accompanied with a notable change in the oxygen K-edge X-ray absorption spectra. The scanning transmission X-ray microscopy results indicate that more oxygen vacancies at in-plane sites are present in the surface region than in the bulk region, whereas more oxygen vacancies at out-of-plane sites are present in the bulk region than in the surface region. Overall, the results demonstrate that oxygen-vacancy-mediated fluorescence properties of SnO2 nanowires are temperature-dependent; i.e., the photoluminescence mechanisms of the nanowires are mediated by oxygen vacancies at different sites, and the bicolor fluorescence originates from charge transfer between the states.

Oxygen K-edge X-ray Absorption Spectra.

We review oxygen K-edge X-ray absorption spectra of both molecules and solids. We start with an overview of the main experimental aspects of oxygen K-edge X-ray absorption measurements including X-ray sources, monochromators, and detection schemes. Many recent oxygen K-edge studies combine X-ray absorption with time and spatially resolved measurements and/or operando conditions. The main theoretical and conceptual approximations for the simulation of oxygen K-edges are discussed in the Theory section. We subsequently discuss oxygen atoms and ions, binary molecules, water, and larger molecules containing oxygen, including biomolecular systems. The largest part of the review deals with the experimental results for solid oxides, starting from s- and p-electron oxides. Examples of theoretical simulations for these oxides are introduced in order to show how accurate a DFT description can be in the case of s and p electron overlap. We discuss the general analysis of the 3d transition metal oxides including discussions of the crystal field effect and the effects and trends in oxidation state and covalency. In addition to the general concepts, we give a systematic overview of the oxygen K-edges element by element, for the s-, p-, d-, and f-electron systems.

Charge States of Cations in Lithium–Nickel Phosphates LiNiPO4 Doped with Manganese and Cobalt

Using Ni, Mn, and Co L2,3-edge soft X-ray absorption spectra supported by calculations of atomic multiplets with allowance for the crystal field and charge transfer, the charge states of cations in doped phosphates LiNiPO4, LiNi0.9Co0.1PO4, and LiNi0.9Mn0.1PO4 are determined. It is established that nickel, manganese, and cobalt ions are in the divalent high-spin state and have an octahedral oxygen environment. Oxygen K-edge X-ray absorption spectra showed the absence of impurity states in the energy gap of manganese- or cobalt-doped lithium nickel phosphates.

Identifying the Chemical Origin of Oxygen Redox Activity in Li-Rich Anti-Fluorite Lithium Iron Oxide by Experimental and Theoretical X-ray Absorption Spectroscopy.

Harnessing oxygen redox reactions is an intriguing route to increasing capacity in Li-ion batteries (LIBs). Despite numerous experimental and theoretical attempts to unravel the mechanism of oxygen redox behavior, the electronic origin of oxygen activities in energy storage of Li-rich LIB materials remains under intense debate. In this work, the onset of oxygen activity was examined using a Li-rich material that has been reported to exhibit oxygen redox, namely, Li5FeO4. By comparing experimental measurements and first-principles Bethe-Salpeter equation calculations of oxygen K-edge X-ray absorption spectra (XAS), it was found that experimentally-observed changes in XAS originate from the nonbonding oxygen states in cation-disordered delithiated Li5FeO4, and the spectral features of oxygen dimers were also determined. This combined experimental and theoretical study offers an effective approach to disentangle the intertwined signals in XAS and can be further utilized in broader contexts for characterizing other energy storage and conversion materials.

X-ray magnetic circular dichroism and near-edge X-ray absorption fine structure of buried interfacial magnetism measured by using a scanning tunneling microscope tip

Magnetism at buried interfaces plays a crucial role in many emerging phenomena, but detection of interfacial magnetism in close proximity to a surface with elemental and chemical sensitivity is a challenging task. Here, we use low temperature synchrotron x-ray scanning tunneling microscopy to investigate x-ray magnetic circular dichroism and the near edge x-ray absorption fine structure of La0.67Sr0.33MnO3-LaNiO3 superlattices. In stark contrast to the weak magnetic signal of Mn when the La0.67Sr0.33MnO3 layers are located on top, a robust x-ray magnetic circular dichroism signal is detected when they are buried underneath the LaNiO3 layers. The near edge x-ray absorption fine structure reveals the valence states of manganese, while the oxygen K-edge x-ray absorption spectra show an increase in hole formation, indicating a cogent charge transfer at the LaNiO3/La0.67Sr0.33MnO3 interface. This work demonstrates that scanning tunneling microscopy can be extended to the synchrotron X-ray study of buried interfaces by controlling the tip-sample separation in the nanometer regime.

X-ray absorption of liquid water by advanced ab initio methods

Oxygen K-edge X-ray absorption spectra of liquid water are computed based on the configurations from advanced ab initio molecular dynamics simulations, as well as an electron excitation theory from the GW method. One one hand, the molecular structures of liquid water are accurately predicted by including both van der Waals interactions and hybrid functional (PBE0). On the other hand, the dynamic screening effects on electron excitation are approximately described by the recently developed enhanced static Coulomb hole and screened exchange approximation by Kang and Hybertsen [Phys. Rev. B 82, 195108 (2010)]. The resulting spectra of liquid water are in better quantitative agreement with the experimental spectra due to the softened hydrogen bonds and the slightly broadened spectra originating from the better screening model.

Quantum simulation of thermally-driven phase transition and oxygen K-edge x-ray absorption of high-pressure ice

The structure and phase transition of high-pressure ice are of long-standing interest and challenge, and there is still a huge gap between theoretical and experimental understanding. The quantum nature of protons such as delocalization, quantum tunneling and zero-point motion is crucial to the comprehension of the properties of high-pressure ice. Here we investigated the temperature-induced phase transition and oxygen K-edge x-ray absorption spectra of ice VII, VIII and X using ab initio path-integral molecular dynamics simulations. The tremendous difference between experiments and the previous theoretical predictions is closed for the phase diagram of ice below 300 K at pressures up to 110 GPa. Proton tunneling assists the proton-ordered ice VIII to transform into proton-disordered ice VII where only thermal activated proton-transfer cannot occur. The oxygen K edge with its shift is sensitive to the order-disorder transition, and therefore can be applied to diagnose the dynamics of ice structures.

On the Origin of the Hydrogen‐Bond‐Network Nature of Water: X‐Ray Absorption and Emission Spectra of Water–Acetonitrile Mixtures

A liquid microjet was used to obtain oxygen K-edge X-ray absorption and emission spectra of water–acetonitrile mixtures of various compositions. The observed spectral changes are unambiguously related to the increasing number of broken hydrogen bonds with decreasing water concentration, and the hydrogen-bond network of liquid water can thus be addressed on purely experimental grounds without the need for theoretical modeling.

Origin of the pre-peak features in the oxygen K-edge x-ray absorption spectra ofLaFeO3 andLaMnO3 studied by Ga substitution of the transition metal ion

We report on experimental oxygen K-edge x-ray absorption near edge structure (XANES) spectra ofthe LaFe1 − xGaxO3 and LaMn1 − xGaxO3 series. Transition metal substitution by the 3d full shell Ga atom is mainly reflected in asystematic decrease of the pre-edge structures in the XANES spectra of the two series.This result shows that the associated states originate from the hybridization ofoxygen 2p and unoccupied Fe (or Mn) 3d states. In order to gain insight intothe states associated with the pre-edge spectral features, we have performed abinitio theoretical calculations based on multiple scattering theory. Simulationswith variable cluster size and composition around the absorber oxygen in theLaFeO3 andLaMnO3 crystal structures were carried out. We find that the low-energy pre-peak is reproducedonce the absorbing oxygen and the two nearest neighbour Fe (or Mn) ions are considered inthe cluster. Conversely, higher energy pre-peaks only arise when the full oxygencoordination geometry around the two metal sites is taken into account, implyingthat their energy distance is a reflection of the strength of the oxygen ligandfield. Substitutions of the two nearest neighbours by Ga atoms in the cluster ofcalculation lead to changes in the theoretical spectra that reasonably agree with theevolution of the pre-peaks in the experimental XANES spectra of both the series.

Theory of oxygenKedge x-ray absorption spectra of cuprates

The dynamical mean-field theory of the three-band model of copper-oxide superconductors is used to calculate the doping dependence of the intensity of the oxygen K-edge x-ray absorption spectra of high-$T_c$ copper oxide superconductors. The model is found not to reproduce the results of a recent experiment, suggesting that at sufficiently high doping the physics beyond the conventional three-band model becomes important.

Structure and dynamics in liquid water from x-ray absorption spectroscopy

Oxygen K-edge x-ray absorption spectra of water are discussed. The spectra of gas-phase water, liquid water and ice illustrate the sensitivity of oxygen K-edge x-ray absorption spectroscopy to hydrogen bonding in water. Transmission mode spectra of amorphous and crystalline ice are compared to x-ray Raman spectra of ice. The good agreement consolidates the experimental spectrum of crystalline ice and represents an incentive for theoretical calculations of the oxygen K-edge absorption spectrum of crystalline ice. Time-resolved infrared-pump and x-ray absorption probe results are finally discussed in the light of this structural interpretation.

Correlation of high temperature x-ray photoemission valence band spectra and conductivity in strained LaSrFeNi oxide on SrTiO3(110)

Reversible and irreversible discontinuities at around 573 and 823 K in the electric conductivity of a strained 175 nm thin film of (La0.8Sr0.2)0.95Ni0.2Fe0.8O3−δ grown by pulsed laser deposition on SrTiO3 (110) are reflected by valence band changes as monitored in photoemission and oxygen K-edge x-ray absorption spectra (XAS). The irreversible jump at 823 K is attributed to depletion of doped electron holes concomitant with reduction of Fe3+ toward Fe2+, as evidenced by oxygen and iron core level soft XAS, and possibly of a chemical origin, whereas the reversible jump at 573 K possibly originates from structural changes.

X-Ray Raman Spectroscopic Study of Water in the Condensed Phases

Oxygen K-edge x-ray absorption spectra of high-density amorphous (HDA) ice, low-density amorphous ice Ic, ice Ih, normal and deuterated liquid water were measured with the synchrotron x-ray Raman scattering method under almost identical experimental conditions by in situ heating of an HDA ice sample. The distinct preedge structure previously reported in water was observed in all the spectra. The results show that core-hole excitations are localized and not strongly affected by the local environment. Therefore, the existence of the preedge feature is not a concise indicator of the magnitude of local disorder within the hydrogen bonded network. The intensity of the near-edge absorption shifts into the postedge region when the hydrogen bond network becomes more ordered. This observation is interpreted as an enhancement of Wannier over Frenkel excitations in an ordered crystal.

Effects of Cations on the Hydrogen Bond Network of Liquid Water: New Results from X-ray Absorption Spectroscopy of Liquid Microjets

Oxygen K-edge X-ray absorption spectra (XAS) of aqueous chloride solutions have been measured for Li(+), Na(+), K(+), NH(4)(+), C(NH(2))(3)(+), Mg(2+), and Ca(2+) at 2 and 4 M cation concentrations. Marked changes in the liquid water XAS are observed upon addition of the various monovalent cation chlorides that are nearly independent of the identity of the cation. This indicates that interactions with the dissolved monovalent cations do not significantly perturb the unoccupied molecular orbitals of water molecules in the vicinity of the cations and that water-chloride interactions are primarily responsible for the observed spectral changes. In contrast, the addition of the divalent cations engenders changes unique from the case of the monovalent cations, as well as from each other. Density functional theory calculations suggest that the ion-specific spectral variations arise primarily from direct electronic perturbation of the unoccupied orbitals due to the presence of the ions, probably as a result of differences in charge transfer from the water molecules onto the divalent cations.

Electronic structure of the mixed valence system(YM)2BaNiO5 (M=Ca,Sr)

Electronic structure of the system Y{sub 2{minus}x}M{sub x}BaNiO{sub 5} (M=Ca,Sr) was calculated for x=0, 0.2, and 0.5 using the full-potential linearized augmented-plane-wave method. To describe the exchange and correlation the local spin-density approximation (LSDA), generalized-gradient approximation (GGA), and two versions of the LSDA+U method were employed. Independently of the method used, the ground state of the parent compound corresponds to an antiferromagnetic insulator. The gap as obtained by LSDA and GGA is smaller than the experimental gap, while LSDA+U methods yield the correct value. To calculate the electronic structure in the mixed-valence region (x{ne}0) the virtual-crystal approach was used. The ground state of the system is still antiferromagnetic, but a finite density of states appears at the Fermi level. The oxygen K-edge x-ray absorption spectra calculated using the LSDA+U methods agree well with the experiment, in particular, the appearance of an additional peak when Y{sup 3+} is substituted by the M{sup 2+} cation is correctly described.

Hole states in Eu0.9 − xPrxCa0.1BaSrCu3O7 − δ studied by X-ray absorption spectroscopy

The hole states located on different oxygen sites have been investigated in Eu0.9 – xPrxCa0.1BaSrCu3O7 – δ with increasing Pr3+ substitution by high-resolution oxygen K-edge and copper L-edge X-ray absorption spectra. The results reveal that the hole states are depleted systematically by the substitution and in turn lead to a reduction in Tc. However, the rate at which the depletion occurs is lower in Eu0.9 – xPrxCa0.1BaSrCu3O7 – δ compared to that in Y1 – xPrxBa2Cu3O7 – δ and Dy1 – xPrxBa2Cu3O7 – δ. This is due to the partial substitution of Ba2+ by Sr2+ ions and R (rare earth element) by Ca2+ ions in the present case. The depletion of hole states gives evidence in support of the hole-depletion models based on Pr 4f–O 2pπ hybridization. The possible reasons for the anomalous behavior of Pr3+ ions in RBa2Cu3O7 – δ are also discussed.

Study of the electronic structure of rhenium and tungsten oxides on the O K-edge

Oxygen K-edge x-ray absorption spectra were studied on the electrochromic amorphous thin film a-WO3 in the comparison with crystalline oxides having variable electronic (d0, d1, d2) and atomic structure: monoclinic m-WO3 (insulator - d0), cubic Na0.6WO3 (metal - d1), cubic ReO3 (metal - d1), layered-type hexagonal h-WO3, WO3H2O and with intercalated HxReO3 (metal - d2), HxWO3 oxides having a metal-isulating transition. The changes in the XANES range 10–15 eV above the absorption edge are interpreted based on the known band-structure calculations. The high-energy features are related to the multiple-scattering processes (EXAFS) on the nearest atoms. The intensity of the feature at 550–560 eV is attributed for the first time to the value of the metal-oxygen-metal bond angle.

Electronic Structure of LnFeO3(Ln = Rare Earth) Compounds: XANES Analysis

Oxygen Kedge x-ray absorption spectra of LnFeO 3 (Ln=Sm, Eu, Nd) have been measured and theoretical calculations of O K-XANES and partial O p-, Fe s- and Fe d density of states on the basis of full multiple scattering formalism in the real space have been made. Unoccupied O p-states are found to be hybridized with both Fe 3d and RE 4f states in the conduction band of LaFeO 3 and there are some indications that unoccupied oxygen p-states interact with Fe 3d not in simple manner of states admixture but in a specific resonance manner when localized d states push oxygen p states out of the energy interval where these d states are localized. It was found that the observed shift of O K-XANES pre-edge can be explained by the changes in a value of indirect exchange interaction.

Electronic structure of EuFeO 3: X-ray absorption fine structure analysis

Oxygen K-edge X-ray absorption spectra of EuFeO 3 have been measured and theoretical analyses of these data on the basis of full multiple scattering formalism in the real space have been made. Oxygen K-edge XANES is shown to be a useful tool for the study of complex oxides (like RE orthoferrites) electronic structure. The full multiple scattering approach is very successful in order to analyze experimental XANES in EuFeO 3, while the single scattering approximation is not valid in this case. Unoccupied oxygen p-states hybridize with both Fe 3 d and RE 4 f states in the conduction band of EuFeO 3. A breakdown of the oxygen phase shifts transferability for the scattering photoelectrons has been found in the compounds investigated.