Oxygen K-edge Research Articles

Microstructure evolution of yttria-doped ceria in reducing atmosphere

The evolution of microstructures of yttria-doped ceria (YDC) upon the heating at 500 °C in a reducing atmosphere has been characterized. Even though obvious cracks will not appear at such a low temperature, local microstructures will change in terms of superstructure formation. Electron energy-loss near-edge structure (ELNES) analysis reveals that newly appeared superstructures formation is mainly attributed to the reduction of Ce 4+ to Ce 3+ . Furthermore, the ELNES at the oxygen K-edge illustrates that such superstructures have enhanced oxygen vacancy ordering level, compared to non-H 2 -treated YDC samples. This type of long-range ordered structure may act as trap/sink centers for mobile oxygen vacancies, the charge carrier of oxide ionic conductors, which is detrimental to solid oxide fuel cell performance. In the light of this study, it elucidates that microstructural evolution under an operating environment may be one possible reason for the degradation of the fuel cells. ► Yttria-doped-ceria can be partially reduced as low as 500 °C. ► Superstructures will be formed after Ce 4+ reduction. ► Long-range ordered structure may be one reason for IT-SOFC degradation.

Interactions between glycine and amorphous solid water nanoscale films

The interactions of glycine (Gly) with amorphous solid water (ASW) nanolayers (≤100ML), vapor-deposited on single crystalline AlOx surfaces at 100K, have been investigated by near-edge X-ray absorption fine structure spectroscopy (NEXAFS) at the oxygen K-edge, temperature-programmed thermal desorption (TPD), X-ray photoelectron spectroscopy (XPS), and temperature-dependent work function measurements. Gly-on-ASW, ASW-on-Gly, and Gly on top of ASW-on-Gly ultrathin films have been fabricated. In contrast to the uniform ASW films grown directly on the hydrophilic AlOx, water molecules adsorb on the hydrophobic Gly films in the form of 3D ASW clusters. This leads to significant differences in the NEXAFS and work function data obtained from ASW-on-AlOx and ASW-on-Gly films, respectively. Furthermore, these structural differences influence the chemical state of Gly molecules (neutral vs. zwitterionic) adsorbed on top of ASW films. N1s XPS measurements revealed an increased amount of neutral Gly molecules in the film top-deposited on the ASW-on-Gly structure in comparison to the neutral Gly in the films directly condensed on AlOx or grown on the ASW substrate. H2O TPD spectra demonstrate that the crystallization and desorption processes of ASW are affected in a different way by the Gly layers, top-deposited on to ASW-on-AlOx and ASW-on-Gly films. At the same time, Gly adlayers sink into the ASW film during crystallization/desorption of the latter and land softly on the alumina surface in the form of zwitterionic clusters.

Development of a simulation method for dynamics of electrons ejected from DNA molecules irradiated with X-rays

Purpose: To develop a method for simulating the dynamics of the photoelectrons and Auger electrons ejected from DNA molecules irradiated with pulsed monochromatic X-rays.Materials and methods: A 30-base-pair (bp) DNA molecule was used as the target model, and the X-rays were assumed to have a Gaussian-shaped time distribution. Photoionization and Auger decay were considered as the atomic processes. The atoms from which the photoelectrons or Auger electrons were emitted were specified in the DNA molecule (or DNA ion) using the Monte Carlo method, and the trajectory of each electron in the electric field formed around the positively charged DNA molecule was calculated with a Newtonian equation. The kinetics of the electrons produced by irradiation with X-rays at an intensity ranging from 1 × 1012 to 1 × 1016 photons/mm2 and energies of 380 eV (below the carbon K-edge), 435 eV (above the nitrogen K-edge), and 560 eV (above the oxygen K-edge) were evaluated.Results: It was found that at an X-ray intensity of 1 × 1014 photons/mm2 or less, all the produced electrons escaped from the target. However, above an X-ray intensity of 1 × 1015 photons/mm2 and an energy of 560 eV, some photoelectrons that were ejected from the oxygen atoms were trapped near the target DNA.Conclusions: A simulation method for studying the trajectories of electrons ejected from a 30-bp DNA molecule irradiated with pulsed monochromatic X-rays has been developed. The present results show that electron dynamics are strongly dependent on the charged density induced in DNA by pulsed X-ray irradiation.

Observation of cleavage in DNA and nucleotides following oxygen K-shell ionization by measuring X-ray absorption near edge structure

Purpose: To investigate which type of bond is more likely to be cleaved in functional groups in DNA including the nucleobases by the K-shell ionization of oxygen of DNA, and to determine whether the production of propenal is specific to the oxygen resonant excitation. To investigate the degradation pattern which depends on the type of nucleobase in the DNA monomer.Materials and methods: Calf thymus DNA film and four nucleotides (dAMP, TMP, dGMP, and dCMP) films were used as samples. Soft X-rays with energy of 560 eV were used to irradiate the samples, and the changes in the X-ray absorption near edge structure (XANES) spectra during the irradiation were measured. The XANES measurements were performed by using a 0.02 eV scanning photon energy step.Results: The difference spectra for DNA and nucleotides were similar to those for pyridine deprotonation. The oxygen K-edge regions in the difference spectra were all similar apart from the spectrum obtained at the resonant excitation energy of oxygen in DNA. The spectral change did not depend on the type of nucleotide.Conclusion: (1) Deprotonation of the nucleobase -NH is usually induced by core ionization of carbon, nitrogen, and oxygen; (2) propenal production is specific to the oxygen K-shell resonant excitation; and (3) the pattern of XANES spectral changes does not significantly depend on the type of nucleobase.

Lifetime of the unpaired electron species in calf thymus DNA thin films induced by nitrogen and oxygen K-shell photoabsorption

Purpose: To study the lifetime of the short-lived unpaired electron species induced in a calf thymus DNA film by soft X-rays in the energy regions around nitrogen and oxygen K-edge.Materials and methods: Dry calf thymus DNA films were measured by electron paramagnetic resonance (EPR) spectrometer during the monochromatic soft X-ray irradiation provided from a synchrotron accelerator (SPring-8, Super Photon ring-8 GeV). Two operation modes of SPring-8 with different electron bunch structures were used to explore dependence of EPR intensity on pulse interval of soft X-rays.Results: Observed EPR spectra did not show significant difference, in the spectral shape, intensity, g-factor and photon energy dependence, between the two bunch modes.Conclusions: The total yield of the unpaired electron species were estimated by assuming the exponential relaxation of the unpaired electrons with two different bunch modes of the synchrotron. The lifetime of the unpaired electron species is estimated to be in the range of a few tens to several hundreds of μs.

X-ray Spectroscopy of Heterocyclic Biochemicals: Xanthine, Hypoxanthine, and Caffeine

The electronic structures of the purine derivatives xanthine, hypoxanthine and caffeine have been investigated in the gas phase using C, N, and O 1s X-ray photoemission (XPS) and near edge X-ray absorption fine structure (NEXAFS) spectroscopy. The results have been interpreted by means of ab initio calculations using the third-order algebraic-diagrammatic construction (ADC(3)) method for the one-particle Green's function and the second-order ADC method (ADC(2)) for the polarization propagator. The carbon, nitrogen and oxygen K-edge NEXAFS spectra of xanthine and caffeine are very similar, since the molecules differ only by substitution of three hydrogen atoms by methyl groups. For hypoxanthine, the electronic structure and spectra differ considerably from xanthine as the purine ring is more highly conjugated, and there is one less oxo group. Effects due to oxo-hydroxy tautomerism were not observed. However, the two oxo tautomeric forms of hypoxanthine oxo-N(9)-H and oxo-N(7)-H are populated in the gas phase, and the C 1s spectra can be simulated only by taking account of these two tautomers, with appropriate Boltzmann population ratios which we have also calculated. For xanthine and caffeine, single tautomeric forms were observed.

Near edge X-ray absorption fine structure spectroscopic and infrared reflection absorption spectroscopic studies of surface modification of poly(butylene terephthalate) induced by UV irradiation

Surface modification of poly(butylene terephthalate) (PBT) induced by ultraviolet (UV) irradiation was investigated using near edge X-ray absorption fine structure spectroscopy (NEXAFS) and infrared reflection absorption spectroscopy (IRAS). UV irradiation drastically altered carbon and oxygen K-edge NEXAFS spectra of PBT. NEXAFS spectra obtained after UV irradiation were compared with those of various molecules containing carbonyl, carboxylic, ester, and anhydride groups. UV-irradiated PBT surfaces had similar spectral features to pyromellitic dianhydride, which contains anhydride groups. IRAS spectra of PBT surfaces were also changed by UV irradiation. The changes indicate that UV irradiation destroyed alkyl chains and changed the ester bond structure. These results indicate that UV irradiation removes alkyl chains and ester bonds in PBT and forms anhydride groups on PBT surfaces. It was also suggested that anhydrides might be formed near PBT surfaces by UV irradiation.

Two-dimensional stimulated resonance Raman spectroscopy of molecules with broadband x-ray pulses.

Expressions for the two-dimensional stimulated x-ray Raman spectroscopy (2D-SXRS) signal obtained using attosecond x-ray pulses are derived. The 1D- and 2D-SXRS signals are calculated for trans-N-methyl acetamide (NMA) with broad bandwidth (181 as, 14.2 eV FWHM) pulses tuned to the oxygen and nitrogen K-edges. Crosspeaks in 2D signals reveal electronic Franck-Condon overlaps between valence orbitals and relaxed orbitals in the presence of the core-hole.

NEXAFS Spectroscopic Study of Surface Modification on Poly(butylene terephthalate) Induced by UV Irradiation

Surface modification of poly(butylene terephthalate) (PBT) induced by ultraviolet (UV) irradiation was investigated using near-edge X-ray absorption fine structure spectroscopy (NEXAFS). UV irradiation drastically altered carbon and oxygen K-edge NEXAFS spectra of PBT. NEXAFS spectra obtained after UV irradiation were compared with those of various molecules containing carbonyl, carboxylic, ester, and anhydride groups. UV-irradiated PBT surfaces had spectral features similar to those of pyromellitic dianhydride, which contains anhydride groups. These results indicate that UV irradiation removes alkyl chains and ester bonds in PBT and forms anhydride groups on PBT surfaces. It was also suggested that anhydrides might be formed near PBT surfaces by UV irradiation.

Orbital-Resolved Partial Charge Transfer from the Methoxy Groups of Substituted Pyrenes in Complexes with Tetracyanoquinodimethane—A NEXAFS Study

It is demonstrated that the near-edge X-ray absorption fine structure (NEXAFS) provides a powerful local probe of functional groups in novel charge transfer (CT) compounds and their electronic properties. Microcrystals of tetra-/hexamethoxypyrene as donors with the strong acceptor tetracyano-p-quinodimethane (TMP/HMP-TCNQ) were grown by vapor diffusion. The oxygen and nitrogen K-edge spectra are spectroscopic fingerprints of the functional groups in the donor and acceptor moieties, respectively. The orbital selectivity of the NEXAFS pre-edge resonances allows us to precisely elucidate the participation of specific orbitals in the charge transfer process. Upon complex formation, the intensities of several resonances change substantially and a new resonance occurs in the oxygen K-edge spectrum. This gives evidence of a corresponding change of hybridization of specific orbitals in the functional groups of the donor (those derived from the frontier orbitals 2e and 6a(1) of the isolated methoxy group) and acceptor (orbitals b(3g), a(u), b(1g), and b(2u), all located at the cyano group) with π*-orbitals of the ring systems. Along with this intensity effect, the resonance positions associated with the oxygen K-edge (donor) and nitrogen K-edge (acceptor) shift to higher and lower photon energies in the complex, respectively. A calculation based on density functional theory qualitatively explains the experimental results. NEXAFS measurements shine light on the action of the functional groups and elucidate charge transfer on a submolecular level.

Atomic structure of cobalt-oxide nanoparticles active in light-driven catalysis of water oxidation

The atomic structure of water-oxidizing nanoparticles (10–60nm) formed from cobalt(II) salts and methylenediphosphonate (M2P) is investigated. These amorphous nanoparticles are of high interest for production of solar fuels. They facilitate water oxidation in a directly light-driven process using [Ru(bpy)3]2+ (bpy=2,2’-bipyridine) as a photosensitizer and persulfate (S2O82−) as an electron acceptor. By X-ray absorption spectroscopy (XAS) at the cobalt K-edge, cobalt L-edge and oxygen K-edge, we investigate the light-driven transition from the CoII/M2P precursor to the active catalyst, which is a layered cobalt(III) oxide with structural similarities to water-oxidizing electrocatalysts. The M2P ligand likely binds at the periphery of the nanoparticles, preventing their further agglomeration during the catalytic reaction. This system opens a possibility to link the catalytically active nanoparticles via a covalent bridge to a photosensitizer and build an artificial photosynthetic system for direct utilization of solar energy for fuel production without production of electricity as an intermediate step.This article is part of a Special Issue entitled: Photosynthetic and Biomimetic Hydrogen Production.

Temperature-Induced Structural Changes of Tetrahydrofuran Clathrate and of the Liquid Water/Tetrahydrofuran Mixture

We present two complementary inelastic X-ray scattering studies on the structure of tetrahydrofuran (THF) clathrate hydrate and the supercooled stoichiometric liquid mixture of water and THF. Compton scattering experiments of the liquid mixture show that formation of hydrate precursors is unlikely. By comparing experimental spectra of THF hydrate and water/THF mixtures at temperatures above 250 K with density functional theory calculations, structural changes that manifest in OH bond length changes are observed. X-ray Raman scattering measurements of the oxygen K-edge in the same temperature range corroborate these results. The experimental results of THF hydrate at temperatures between 20 and 244 K can be modeled best by assuming thermal expansion only. Therefore, dependency on the system’s temperature different structural behavior of THF hydrate is reported.

Impact of Thermally Induced Structural Changes on the Electrical Properties of TiN/HfLaSiO Gate Stacks

Thermally induced structural changes in TiN/Hf(La)SiO gate stacks were investigated by back-side X-ray photoelectron spectroscopy (XPS) and near edge X-ray absorption fine structure (NEXAFS). A distinct correlation between bottom oxide growth and an increase in equivalent oxide thickness (EOT) was confirmed under high-temperature annealing at over 850 °C regardless of La content. Back-side XPS also revealed that oxygen and nitrogen diffusion occurs, forming partially oxidized TiON layers at a metal/high-k interface under moderate annealing temperatures of approximately 600 °C, and that annealing at over 750 °C leads to the reduction of the oxide phase and produces a thinner inter-layer with a clear Ti–N bond feature. Moreover, with an increase in annealing temperature, a change in the local atomic configuration in the HfLaSiO dielectric layer was identified from oxygen K-edge spectra. This structural change induced by thermal reaction can be considered as a possible cause of the Vth instability of La-incorporated high-k gate stacks. On the basis of these findings on structural changes, the physical origins of the effective work function modulation of the gate stacks are discussed in detail.

Impact of Thermally Induced Structural Changes on the Electrical Properties of TiN/HfLaSiO Gate Stacks

Thermally induced structural changes in TiN/Hf(La)SiO gate stacks were investigated by back-side X-ray photoelectron spectroscopy (XPS) and near edge X-ray absorption fine structure (NEXAFS). A distinct correlation between bottom oxide growth and an increase in equivalent oxide thickness (EOT) was confirmed under high-temperature annealing at over 850 °C regardless of La content. Back-side XPS also revealed that oxygen and nitrogen diffusion occurs, forming partially oxidized TiON layers at a metal/high-k interface under moderate annealing temperatures of approximately 600 °C, and that annealing at over 750 °C leads to the reduction of the oxide phase and produces a thinner inter-layer with a clear Ti–N bond feature. Moreover, with an increase in annealing temperature, a change in the local atomic configuration in the HfLaSiO dielectric layer was identified from oxygen K-edge spectra. This structural change induced by thermal reaction can be considered as a possible cause of the Vth instability of La-incorporated high-k gate stacks. On the basis of these findings on structural changes, the physical origins of the effective work function modulation of the gate stacks are discussed in detail.

Heterogeneous formation of ferric oxide nanoparticles on chlorite surfaces studied by x-ray absorption spectromicroscopy (STXM)

The homogeneous and heterogeneous formation of ferric iron oxides from acidic solution (pH 2.06–2.50) has been studied in the presence of chlorite under constant pH conditions using titration experiments. Significantly shorter induction times for the ferrihydrite formation have been macroscopically observed in the presence of chlorite particles, thus indicating heterogeneous nucleation as a major precipitation mechanism. A combination of Scanning Electron Microscopy (SEM) and Scanning Transmission X-Ray microscopy (STXM) on the O K-edge and Fe L-edge was used to clearly distinguish ferric iron precipitates from chlorite and to obtain the spatial distribution of iron precipitates on the chlorite surface under conditions of homogenous and heterogeneous formation.Oxygen K-edge results show the precipitation of a highly hydrolyzed chlorite surface associated ferric iron phase under heterogeneous conditions. Precipitates with different oxygen K-edge features are observed on the chlorite basal faces compared to the edge surfaces which can be attributed to cluster size effects and differences in the O/OH ratio. The results obtained from iron L-edge XANES corroborate the observations of the oxygen K-edge, indicating a thin surface layer of ferric iron precipitates and preferential precipitation at the chlorite edge surfaces. Under conditions where homogeneous ferrihydrite formation dominates, beside surface coatings separate iron colloids/nanoparticles are observed. Iron L-edge analysis and the pre-edge splitting on the oxygen K-edge indicate a higher long-range order/lower hydration comparable to ferrihydrite reference spectra. The results show that the Fe(III) secondary phase formation as surface coating or nanoparticles and their spatial distribution depends on the degree of supersaturation. These observations may have direct consequences for the retention and mobility of various contaminants in aquifer systems and potential remediation strategies.

Magnetic structure and electric field effects in multiferroic YMn2O5

The magnetic structure of multiferroic RMn${}_{2}$O${}_{5}$ (R $=$ Y, Er) has been investigated by means of resonant soft x-ray diffraction. Energy, temperature, and azimuthal angle scans were performed in addition to reciprocal space maps on the magnetic reflection in the different magnetic phases of YMn${}_{2}$O${}_{5}$. We also investigated the orbital magnetic moment at the oxygen K-edge for RMn${}_{2}$O${}_{5}$ with both, R $=$ Y and R $=$ Er compositions. These moments reflect the strong hybridization between Mn 3$d$ and oxygen 2$p$ states. Experiments with applied electric fields are additionally presented, showing that the helical component of the magnetic structure in the CM phase of YMn${}_{2}$O${}_{5}$ can be reversed by the application of an electric field. However, the incommensurate magnetic reflection in the high-temperature phase is unaffected. Interestingly, this is observed only in the presence of a small electrical current, indicative of a current-induced/-enhanced switching of magnetic domains.

Local electronic structure analysis for brownmillerite Ca(Sr)FeO2.5 using site-resolved energy-loss near-edge structures

Oxygen K-edge and Fe L2,3-edge electron energy-loss near-edge structures (ELNES) were measured for FeO6 octahedra and FeO4 tetrahedra in the brownmillerite Ca(Sr)FeO2.5 by focusing an electron probe at individual Fe sites using scanning transmission electron microscopy combined with electron energy-loss spectroscopy. The observed site-resolved oxygen K-ELNES showed different features reflecting the local chemical bonding around the FeO6 octahedra and FeO4 tetrahedra. A pre-peak in the O K-edge spectra, which is attributed to a transition to an unoccupied O 2p band hybridized with the Fe-3d band, shows splitting in the spectrum of the FeO6 octahedral site. Additionally, for the oxygen linking the octahedral and tetrahedral Fe sites in CaFeO2.5, charge transfer was found to preferentially occur toward the tetrahedral Fe ions. In the case of SrFeO2.5, charge transfer from the oxygen located in the ac plane was biased toward the tetrahedral Fe atoms. Based upon an analysis of the pre-peak intensity of the O K-ELNES, it was concluded that bonding between the oxygen and iron atoms at the tetrahedral site was more covalent in character than at the octahedral site. The strong covalent character of the tetrahedral sites would be one of the reasons for distortion in the FeO6 octahedra in Ca(Sr)FeO2.5, as exhibited by an extension along the b-axis.

The relationship between microstructure and electrical breakdown in cathodic arc deposited hafnium oxide films

Hafnium oxide films were deposited with a range of substrate temperatures using a filtered cathodic vacuum arc deposition system. The microstructure, electronic structure, and electrical breakdown of the films were characterized. In films deposited at temperatures above 200 °C, the microstructure became more ordered and x-ray diffraction indicated that the dominant phase was monoclinic hafnium oxide. Evidence for the presence of the tetragonal phase was also found in the films deposited at temperatures above 400 °C. The near edge structure of the oxygen K-edge measured using x-ray absorption spectroscopy, provided further evidence that films prepared at high temperatures contained a combination of the monoclinic and tetragonal phases. Films deposited at room temperature were disordered and exhibited the best electrical breakdown characteristics. The electrical breakdown of the films deteriorated as the crystallinity increased with increasing deposition temperature. These results support the proposition that electrical breakdown paths may occur along grain boundaries in polycrystalline hafnium oxide films and therefore a disordered microstructure is preferable.

Spectroscopic Evidence of Keto−Enol Tautomerism in Deliquesced Malonic Acid Particles

Scanning transmission X-ray microscopy combined with near-edge X-ray absorption fine structure spectroscopy (STXM/NEXAFS) and optical microscopy coupled with Fourier transform infrared spectroscopy (micro-FTIR) have been applied to observe hygroscopic growth and chemical changes in malonic acid particles deposited on substrates. The extent of the hygroscopic growth of particles has been quantified in terms of the corresponding water-to-solute ratios (WSR) based on STXM/NEXAFS and micro-FTIR data sets. WSR values derived separately from two applied methods displayed a remarkable agreement with previous data reported in the literature. Comparison of NEXAFS and FTIR spectra acquired at different relative humidity (RH) shows efficient keto-enol tautomerization of malonic acid, with the enol form dominating at higher RH. The keto-enol equilibrium constants were calculated using relevant peak intensities in the carbon and oxygen K-edge NEXAFS spectra as a function of RH. We report strong dependence of the equilibrium constant on RH, with measured values of 0.18 ± 0.03, 1.11 ± 0.14, and 2.33 ± 0.37 corresponding to 2, 50, and 90% RH, respectively. Enols are important intermediates in aldol condensation reactions pertaining to formation and atmospheric aging of secondary organic aerosol (SOA). The present knowledge assumes that constituents of atmospheric deliquesced particles undergo aqueous chemistry with kinetic and equilibrium constants analogous to reactions in bulk solutions, which would estimate absolute dominance of the keto form of carboxylic acids. For instance, the keto-enol equilibrium constant of malonic acid in diluted aqueous solution is <10(-4). Our results suggest that in deliquesced micrometer-size particles, carboxylic acids may exist in predominantly enol forms that need to be explicitly considered in atmospheric aerosol chemistry.

Electronic Structure Characterization of La Incorporated Hf-Based High-&lt;I&gt;k&lt;/I&gt; Gate Dielectrics by NEXAFS

The electronic structures of lanthunum (La) incorporated hafnium (Hf)-based oxides (HfLaO) and their silicate (HfLaSiO) films were investigated by the Near Edge X-ray Absorption Fine Structure (NEXAFS) technique. The oxygen (O) K-edge spectra, which reflected the hybridized Hf 5d state with the O 2p orbital, were found to reveal features of the unoccupied state of the metal oxides, as well as the conduction-band edge. We also found that, while La incorporation into the Hf-based oxides simply changed the features of the conduction-band structure, subsequent thermal annealing of the La-incorporated films led to a conduction-band edge shift due to an interface silicate reaction and/or local bond rearrangement depending on the La concentration and annealing temperature. The impact of La incorporation into the Hf-based high-k materials on the electronic structure is discussed by taking into account the intrinsic nature of these metal oxides.