O1s Edge Research Articles

Role of Oxygen Vacancies for Mediating Ferromagnetic Ordering in La-Doped MgO Nanoparticles

In present work, we have reported the room temperature ferromagnetism in lanthanum (La)-doped magnesium oxide (MgO) nanoparticles. It has been found that the magnetic character increases with dilute concentration of La up to 4% and decreases afterwards. The conception of ferromagnetic ordering in such systems is understood on the basis of oxygen intrinsic defects in the host matrix which was elucidated through X-ray photoelectron spectroscopy (XPS) measurements. XPS studies infer that Mg 2p edge is not altered with the incorporation of La ions in the host lattice. However, effective variation in the intensity of O1s edge confirms the presence of defects in the material. EPR (electron paramagnetic resonance) signal peak to peak line width increases with increase in La content and supports the defects present in the material which in turn is related with the change in particle size and saturation magnetisation. These results demonstrate the key role played by defects in stabilising the room temperature ferromagnetic ordering in MgO nanoparticles.

X-ray Absorption Spectroscopy and Magnetism of Synthetic Greigite and Greigite Magnetosomes in Magnetotactic Bacteria

ABSTRACTScanning transmission X-ray microscopy at the Fe 2p (L2,3), O1s, C1s, and S2p edges was used to study greigite magnetosomes and other cellular content of a magnetotactic bacterium known as a multicellular magnetotactic prokaryote (MMP). X-ray absorption spectrum (XAS) and X-ray magnetic circular dichroism (XMCD) spectra of greigite (Fe3S4) nanoparticles, synthesized via a hydrothermal method, were measured. Although XAS of the synthetic greigite nanoparticles and biotic magnetosome crystals in MMPs are slightly different due to partial oxidation of the MMP greigite, the XMCD spectra of the two materials are in good agreement. The Fe 2p XAS and XMCD spectra of Fe3S4 are quite different from those of its oxygen analog, magnetite (Fe3O4), suggesting Fe3S4 has a different electronic and magnetic structure than Fe3O4 despite having the same crystal structure. Sulfate and sulfide species were also identified in MMPs, both of which are likely involved in sulfur metabolism.

INFLUENCE OF THE SKELETON HIERARCHICAL ORGANIZATION ON ELECTRONIC STATE OF IONS IN BONE MATRIX

The authors suggested the 3D-superlattice (3DSL) model to describe the effect of coplanar assembly of the hydroxyapatite (HA) nanocrystallites on local electronic state of ions in mineralized bone. This model is based on the main structural and functional relationships between adjacent levels of the hierarchical organization of bone tissue. In the framework of the 3DSL model the authors predicted the distinct assembly-to-crystal red shift of the unoccupied electronic states located near the bottom of the conduction band in HA and dependence of this shift on the ratio of the thickness of the hydrated layer to the crystallite size. To check these predictions the experimental X-ray absorption studies of native bone are performed near the Ca2р1/2,3/2-, P2р1/2,3/2- и O1s edges. Comparison of the measured spectra with the known spectra of the reference compounds has confirmed appearance of the distinct assembly-to-crystal red shift. The observed effect is the ground for development of new diagnostic methods for bone status and imaging changes in the local electronic structure of bone tissue by using ultrasoft X-ray absorption spectroscopy and measuring the assembly-tocrystal shifts. The experimental data analysis proved the applicability of the 3DSL model for better understanding of the hierarchical organization of bone at nanolevel.

Gap states in the electronic structure of SnO2 single crystals and amorphous SnOx thin films

The electronic structure of a SnO2 single crystal is determined by employing resonant photoelectron spectroscopy. We determine the core level, valence band, and X-ray absorption (XAS) data and compare these with those of amorphous SnOx thin films. We find similar properties concerning the data of the core levels, the valence band features, and the absorption data at the O1s edge. We find strong signals arising from intrinsic in-gap states and discuss their origin in terms of polaronic and charge-transfer defects. We deduce from the XAS data recorded at the Sn3d edge that the Sn4d10 ground state has contributions of 4d9 and 4d8 states due to configuration interaction. We identify localized electronic states depending on the strength of the 4d-5s5p interaction and of the O2p-to-Sn4d charge-transfer processes, both appear separated from the extended band-like states of the conduction band. For the amorphous SnOx thin films, significant differences are found only in the absorption data at the Sn3d-edge due to a stronger localization of the in-gap states.

Selectivity in fragmentation of N-methylacetamide after resonant K-shell excitation.

The fragmentation pattern of the peptide model system, N-methylacetamide, is investigated using ion time-of-flight (TOF) spectroscopy after resonant K-shell excitation. Corresponding near-edge X-ray absorption fine structure (NEXAFS) spectra recorded at high resolution at the C1s, N1s and O1s edges are presented. Analysis of the ion TOF data reveals a multitude of fragmentation channels and dissociation pathways. Comparison between the excitation of six different resonances in the vicinity of the C1s, N1s and O1s edges suggests evidence for site-selective bond breaking. In particular the breaking of the peptide bond and the N-C(α) bond show a clear correlation with resonant excitation at the N1s edge. Also, stronger tendencies towards site-selective bond breaking are found for the generation of single ions compared with ion pairs. Analysis of angular distributions of ions from breakage of the peptide bond yields a fragmentation time of <400 fs.

Spin states in Co-PI catalysts

Cobalt based oxygen evolution catalysts (Co-PI, PI = electrodeposited from pH 7 phosphate) are studied by resonant photoemission spectroscopy at the Co L3 and O1s edge. For the individual Co3d states, we determine their configuration, their spin, and their energy level relative to the Fermi energy. We identify the Co divalent high spin state and the trivalent low spin state. In addition, we also find the corresponding oxygen-to-metal charge transfer states. We attribute the trivalent charge transfer state to be the active state for the oxygen evolution reaction.

Experimental evidence for molecular ultrafast dissociation in O2 clusters

Resonant Auger spectra of O2 clusters excited at the O1s edge are reported. After excitation to the repulsive 1s-13σ* state, the resulting resonant Auger spectrum displays features that remain constant in kinetic energy as the photon energy is detuned. The shift between known atomic fragment features and these features is consistent with that observed for atoms and clusters in singly charged states in direct photoemission. These findings are strong evidence for the existence of molecular ultrafast dissociation processes within the clusters or on their surface.

Silicate formation at the interface of Hf-oxide as a high- k dielectrics and Si(0 0 1) surfaces

The composition and chemical bonding of the first atoms across the interface between Si(0 0 1) and the gate dielectrics determine the quality of gate stacks. An analysis of that hidden interface is a challenge as it requires high sensitivity in both elemental and chemical state information. We used synchrotron radiation (SR) based photoelectron spectroscopy and, in particular, X-ray absorption spectroscopy in total electron yield and total fluorescence yield at the Si2p and the O1s edges to address this issue. We report on results for Hf oxide prepared by ALD and compare to Pr 2O 3/Si(0 0 1). For Hf oxide thin films we find evidence for the silicate formation at the interface as derived from the characteristic features in the X-ray absorption spectra at the Si2p and the O1s edges. Resonant photoelectron spectroscopy is used to analyze the absorption band in detail. Following the resonant profiles of initial and final states we deduce from the resonant behaviour a charge donation via a Si-induced charge transfer.

Silicate Formation at the Interface of high-k dielectrics and Si(001) Surfaces

AbstractThe composition and chemical bonding of the first atoms across the interface between Si(001) and the dielectric determine the quality of dielectric gate stacks. An analysis of that hidden interface is a challenge as it requires both, high sensitivity and elemental and chemical state information. We used SR based photoelectron spectroscopies and, in particular, X-ray absorption spectroscopy in total electron yield and total fluorescence yield at the Si2p and the O1s edges to address that issue. We report on results of Hf-oxide prepared by ALD and compare to Pr2O3 / Si(001), and compare the two to the SiO2 / Si(001) system as a reference. For both, Hf-oxide and Pr-oxide thin films we find evidence for the silicate formation at the interface as derived from the characteristic features at the Si2p and the O1s edges.

Silicate formation at the interface of Pr-oxide as a high-K dielectric and Si(001) surfaces

The composition and chemical bonding of the first atoms across the interface between Si(001) and the dielectric determine the quality of dielectric gate stacks. An analysis of that hidden interface is a challenge as it requires both, high sensitivity and elemental and chemical state information. We used X-ray absorption spectroscopy in total electron yield and total fluorescence yield at the Si2p and the O1s edges to address that issue. We report on results of Pr 2O 3/Si(001) as prepared by both, epitaxial growth and metal organic chemical vapor deposition (MOCVD), and compare to the SiO 2/Si(001) system as a reference. We find evidence for the silicate formation at the interface as derived from the characteristic features at the Si2p and the O1s edges. The results are in line with model experiments in which films of increasing film thickness are deposited in situ on bare Si(001) surfaces.

Covalent and localized contributions of Pr4f states at the interface of Pr 2O 3 to Si(001)

Resonant photoelectron spectroscopy (PES) at the Pr4d and O1 s absorption edges is used to study the electronic properties at the interface of epitaxially grown Pr 2O 3 on Si(001). In the electronic structure of bulk Pr 2O 3, the valence band (VB) states are predominant of Pr6s and O2p atomic parentage. The contribution of Pr4f states is identified from the strong increase of the VB features at the Pr4d resonances. The data at the O1s edge are caused by Raman scattering and resonant Auger decay and reflect the existence of charge transfer (CT) complexes. These complexes are the consequence of a mixed valency caused by ligand-to-Pr4f charge transfer states. The decrease of their intensity is attributed to an increase in covalent bandwidth between the ligand (O2p, Si3p) and Pr4f states. The CT complexes, originally localized now, become broadened and form gap states which fill the gap towards a metallic density of states. The metallic phase may be hindered upon alloying with SiO 2 or other oxides.

Pr4f occupancy and VB/CB band offsets of Pr2O3 at the interface to Si(001) and SiC(0001) surfaces

AbstractResonant photoelectron spectroscopy (PES) at the Pr4d and O1s absorption edges is used to study the electronic properties at the interface of epitaxial grown Pr2O3 on Si(001).In the electronic structure of bulk Pr2O3 the valence band (VB) states are predominantly of Pr6s and O2p atomic parentage. The contribution of Pr4f states are identified from the strong increase the VB features at the Pr4d resonances. The data at the O1s edge are caused by Raman scattering and resonant Auger decay and reflect the existence of CT complexes. They are the consequence of mixed valency caused by ligand-to-Pr4f charge transfer states. The decrease of their intensity is attributed to an increase in covalent bandwidth between the Ligand (O2p, Si3p) and Pr4f states. The CT complexes originally localized now become broadened and form gap states which fill the gap towards a metallic density of states. The metallic phase may be hindered upon alloying with SiO2 or other oxides

Core shell excitation of furan at the O1s and C1s edges: An experimental and ab initio study

The K-shell spectra of gaseous furan have been measured using the inner-shell electron energy loss spectroscopy (ISEELS) method at the carbon and oxygen thresholds. Large-scale ab initio configuration interaction calculations have been carried out in order to help in the assignments of the observed bands. The spectra are close to previous low resolution ones obtained using ISEELS in the gas phase and photoabsorption with the synchrotron radiation in gaseous and condensed phases. The presence of a new feature located at 287.3 eV in the C1s spectrum and recently detected by photoabsorption with synchrotron radiation is confirmed. At both edges, the calculations confirm the assignments proposed in earlier experimental works, with the exception of several C1s pre-edge features for which a new interpretation is given.

Electronic structure of ReO3Me by variable photon energy photoelectron spectroscopy, absorption spectroscopy and density functional calculations.

Valence photoelectron (PE) spectra have been measured for ReO(3)Me using a synchrotron source for photon energies ranging between 20 and 110 eV. Derived branching ratios (BR) and relative partial photoionization cross sections (RPPICS) are interpreted in the context of a bonding model calculated using density functional theory (DFT). Agreement between calculated and observed ionization energies (IE) is excellent. The 5d character of the orbitals correlates with the 5p --> 5d resonances of the associated RPPICS; these resonances commence around 47 eV. Bands with 5d character also show a RPPICS maximum at 35 eV. The RPPICS associated with the totally symmetric 4a(1) orbital, which has s-like character, shows an additional shape resonance with an onset of 43 eV. The PE spectrum of the inner valence and core region measured with photon energies of 108 and 210 eV shows ionization associated with C 2s, O 2s, and Re 4f and 5p electrons. Absorption spectra measured in the region of the O1s edge showed structure assignable to excitation to the low lying empty "d" orbitals of this d(0) molecule. The separation of the absorption bands corresponded with the calculated orbital splitting and their intensity with the calculated O 2p character. Broad bands associated with Re 4d absorption were assigned to (2)D(5/2) and (2)D(3/2) hole states. Structure was observed associated with the C1s edge but instrumental factors prevented firm assignment. At the Re 5p edge, structure was observed on the (2)P(3/2) absorption band resulting from excitation to the empty "d" levels. The intensity ratios differed from that of the O 1s edge structure but were in good agreement with the calculated 5d character of these orbitals. An absorption was observed at 45 eV, which, in the light of the resonance in the 4a(1) RPPICS, is assigned to a 4a(1) --> ne, na(2) transition. The electronic structure established for ReO(3)Me differs substantially from that of TiCl(3)Me and accounts for the difference in chemical behavior found for the two complexes.

Memories of excited femtoseconds: effects of core–hole localization after Auger decay in the fragmentation of ozone

We present the first photoelectron–photoion–photoion coincidence (PEPIPICO) spectra of ozone, O 3, excited at the O1s edge. From symmetry considerations the oxygen atoms at the end of the V-shaped molecule are chemically in-equivalent to the central oxygen atom. For the excitation from one of the terminal O1s orbitals to 7a 1( σ ∗ ), experiments combined with simulations yielded that photo-induced breaking of the bond closest to the core-excited terminal atom occurs in at least 70±10% of the cases. We interpret this as due to a `memory' effect of the localized core–hole after the Auger decay. Consequently the sequential two-step model, i.e. core excitation-and-decay resulting in a state with delocalized valence holes followed by dissociation, is not adequate in this case.

Identification of Mono- and Bidentate Carboxylate Surface Species on Cu(111) using X-Ray Absorption Spectroscopy

In order to identify the nature of the bond which is formed when anhydride containing molecules such as pyromellitic dianhydride (PMDA) react with a Cu surface we have carried out measurements using x-ray absorption spectroscopy (NEXAFS) at the C1s and O1s edges. By comparing to reference spectra for benzoic acid bound to Cu surfaces the position of the resonances in NEXAFS-spectra for monolayers of PMDA on a Cu(111)-substrate can be shown to be fully consistent with the presence of a bidentate surface species. Evidence is provided that monodentate species show a characteristic shift of the C1s-π CO -excitation to 287.6 eV (from 288.05 eV for the bidentate species).

Site specific and doping dependent electronic structure of YBa 2Cu 3O x probed by O1s and Cu2p x-ray absorption spectroscopy

The electronic structure of the CuO 2 planes and CuO 3 chains in single domain crystals of YBa 2Cu 3O x has been investigated as a function of oxygen concentration (6≤×≤7) using polarization-dependent x-ray absorption spectroscopy at the O1s and Cu2p core absorption edges. The polarization-dependent observation of unoccupied states with O2p and with Cu3d orbital character parallel to the crystallographic a, b, and c axes of the crystals allows the determination of hole states near the Fermi level in Cu3d x 2−y 2 and O2p x,y orbitals in the planes and in Cu3d y 2−z 2 and O2p y,z orbitals in the chains. The onset of superconductivity does not significantly change the hole occupation.

The electronic structure of KMnO4 investigated by resonant photoemission

The valence band of potassium permanganate (KMnO4) was investigated by means of resonant photoemission spectroscopy (ResPES) at the Mn2p, Mn3p and O1s edge. These data confirm the previous conclusions of a strong deviation from a purely ionic charge distribution in this compound. The ResPES data are in agreement with previous results about the character of the individual valence band states. A simple cluster model is used to explain qualitatively the different structures seen in the valence band spectra and their dependence on the photon energy.

The electronic structure of KMnO4 investigated by resonant photoemission

The valence band of potassium permanganate (KMnO4) was investigated by means of resonant photoemission spectroscopy (ResPES) at the Mn2p, Mn3p and O1s edge. These data confirm the previous conclusions of a strong deviation from a purely ionic charge distribution in this compound. The ResPES data are in agreement with previous results about the character of the individual valence band states. A simple cluster model is used to explain qualitatively the different structures seen in the valence band spectra and their dependence on the photon energy.

Electron energy-loss and x-ray absorption spectroscopy of cuprate superconductors and related compounds

O1s and Cu2p absorption edges of cuprate high- T c superconductors (HTSC) and related compounds measured by electron energy-loss spectroscopy (EELS) and X-ray absorption spectroscopy (XAS) are reviewed. Since these edges are related to the site-selective unoccupied density of states, they provide important information on the normal-state electronic structure of HTSCs. In particular they can determine the character of the charge carriers of both the p- and n-type doped cuprates and they indicate that correlation effects are important in these systems. Orientation dependent EELS and polarization dependent XAS measurements on cuprate single crystals provide information on the symmetry of the hole states in the CuO 2 planes and block layers of the HTSCs. The results impose strong constraints on theories of high- T c superconductivity which are based on charge carriers in out-of-plane orbitals.