Sensitive X-ray Absorption Spectroscopy Research Articles

The formation of Ti–O tetrahedra and band gap reduction in SiO2 via pulsed ion implantation

Titanium ions are implanted into amorphous SiO2 at two different fluences using pulsed ion implantation, and the resulting samples are annealed. Bulk sensitive soft X-ray absorption spectroscopy of the Ti L2,3 edge reveal strikingly different spectra for the two fluences. Spectral simulations using multiplet crystal field theory show clearly that for low fluence the Ti ions have a local octahedral coordination, while at higher fluence the formation of Ti4+–O tetrahedra dominates. Using O K-edge X-ray absorption and emission, the effect of the Ti states on the valence and conduction bands of the host SiO2 is revealed. With the introduction of Ti tetrahedra, the band gap reduces from about 8 eV to just over 4 eV, due entirely to the Ti 3d conduction band states. These results demonstrate the possibility to obtain Ti–O tetrahedra in silica by Ti ion implantation and a suitable thermal treatment, clarify the mechanism of band gap reduction with Ti doping in SiO2, and demonstrate the sensitivity of L-edge X-ray absorption with a multiplet crystal field theory analysis to the Ti coordination of novel materials.

Quantitative Analysis of Modulated Excitation X-ray Absorption Spectra: Enhanced Precision of EXAFS Fitting

The sensitivity of X-ray absorption spectroscopy (XAS) can be increased by using the modulated excitation approach, where the sample is excited with a periodic external stimulation (e.g., changes in temperature, gas/liquid concentration, etc.) and the corresponding spectra are filtered with the frequency of the excitation. The resulting demodulated spectra contain similar spectral features as difference spectra but to much higher k values because of the isolation of the structural change that occurs with the same frequency as that of the excitation. Furthermore, multiple intermediate species that occur with a phase delay can uniquely be differentiated. We present a robust approach for the quantitative analysis of demodulated extended X-ray absorption fine structure (EXAFS) spectra. Simulations show that the proposed fitting approach resolves small changes in the sample’s structure with greatly enhanced precision. Experimentally, this is demonstrated by the formation of ruthenium oxide species upon partial oxidation of a Ru metal particle, which cannot be detected in a standard EXAFS experiment.

Publisher’s Note: Bulk sensitive x-ray absorption spectroscopy free of self-absorption effects [Phys. Rev. B83, 081106(R) (2011)

Publisher’s Note: Bulk sensitive x-ray absorption spectroscopy free of self-absorption effects [Phys. Rev. B<b>83</b>, 081106(R) (2011)

Bulk sensitive x-ray absorption spectroscopy free of self-absorption effects

We demonstrate a new method of x-ray absorption spectroscopy (XAS) that is bulk sensitive, like traditional fluorescence yield measurements, but is not affected by self-absorption or saturation effects. This measure of XAS is achieved by scanning the incident photon energy through an absorption edge and using an energy sensitive photon detector to measure the partial fluorescence yield (PFY). The x-ray emission from any element or core-hole excitation that is not resonant with the absorption edge under investigation is selected from the PFY. It is found that the inverse of this PFY spectrum, which we term inverse partial fluorescence yield (IPFY), is linearly proportional to the x-ray absorption cross-section without any corrections due to saturation or self-absorption effects. We demonstrate this technique on the Cu L and Nd M absorption edges of the high-Tc cuprate LNSCO by measuring the O K PFY and comparing the total electron yield, total fluorescence yield and IPFY spectra.

Detecting oxygen vacancies inSrTiO3by3dtransition-metal tracer ions

X-ray absorption experiments on $3d$ transition-metal tracer ions in ${\text{SrTiO}}_{3}$ are presented. The absorption spectra of the tracer-ion changed upon reduction in the ${\text{SrTiO}}_{3}$. This change is due to an oxygen vacancy created at the tracer-ion site. This finding is supported by density-functional theory calculations, which prove that the oxygen vacancies preferentially are created at the tracer-ion sites. Using the chemical sensitivity of x-ray absorption spectroscopy, tracer ions can be used to detect oxygen vacancies in ${\text{SrTiO}}_{3}$ and possibly in other oxide systems.

Sensitivity of x-ray absorption spectroscopy to hydrogen bond topology

We demonstrate the sensitivity of x-ray absorption spectroscopy to hydrogen bonding using as experimental model system water on Ru(0001). We stepwise go from fully broken to complete H-bond network by varying the morphology from isolated monomers via two-dimensional clusters to a saturated monolayer as probed by scanning tunneling microscopy. The sensitivity of x-ray absorption to the symmetry of H bonding is further elucidated for the amino $({\text{-NH}}_{2})$ group in glycine adsorbed on Cu(110) where the $E$ vector is parallel either to the NH donating an H bond or to the non-H-bonded NH. We show that the pre-edge in the x-ray absorption spectrum is associated with an asymmetric hydrogen-bonding situation while the postedge is directly associated with hydrogen bond formation. The results give further evidence for the much debated interpretation of the various spectral features of liquid water and demonstrate the general applicability of x-ray absorption spectroscopy to analyze H-bonded systems.

In-situ investigation of Bi thin film condensation by surface sensitive X-ray absorption spectroscopy at cryogenic temperatures

Reflection mode grazing incidence X-ray absorption spectroscopy at the Bi L3-edge and simultaneous electrical resistivity measurements were used to investigate the microstructure of quench condensed thin Bi metal films on float-glass substrates at temperatures from 20 K to 300 K. While thin films of ≈ 6 nm thickness appear to be amorphous after deposition at 20 K, thicker films of about 18 nm show the well-known rhombohedral structure of bulk Bi. During a subsequent heat treatment, the amorphous structure of the thin films transforms irreversibly into the crystalline form for temperatures at (42 ± 2) K. This crystallisation is accompanied by an irreversible increase of the film resistivity by a factor of more than 2.7. The film density and roughness do not change within detection limits during the heat treatment.

Growth and characterization of superconducting spinel oxide [formula omitted] thin films

Epitaxial films of LiTi 2 O 4 on single crystalline substrates of MgAl 2 O 4 , MgO, and SrTiO 3 provide model systems to systematically explore the effects of lattice strain and microstructural disorder on the superconducting state. Lattice strain that affects bandwidth gives rise to variations in the superconducting and normal state properties. Microstructural disorder, such as antiphase boundaries that give rise to Ti network disorder, reduces the critical temperature, and Ti network disorder combined with Mg interdiffusion lead to a much more dramatic effect on the superconducting state. Surface sensitive X-ray absorption spectroscopy has identified Ti to retain site symmetry and average valence of the bulk material regardless of film thickness.

In-situ X-ray investigations of quench-condensed thin gold films

Thin gold films were deposited on float glass substrates held at cryogenic temperatures down to 77 K and investigated in-situ using X-ray reflectometry and surface sensitive reflection mode X-ray absorption spectroscopy (XAFS). The combination of these in-situ X-ray methods with simultaneous electrical resistivity measurements yields information about the surface and volume microstructure of the deposited films as a function of the deposition temperature and their changes induced by a subsequent annealing treatment. The surface sensitive XAFS experiments clearly proved that the films exhibit a polycrystalline structure throughout the temperature range studied here. The data were fitted using a correlated Debye-model. The results show that for film deposition at low substrate temperatures < 130 K, a significantly decreasing Debye-temperature was found, reaching values of about 100 K in comparison to 165 K for the polycrystalline bulk material. This decrease was interpreted to be predominantly related to defective film regions with an increased static disorder.

Electronic structure of halogen doped CuCr2Se4

We have employed element and chemically sensitive x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) in order to address a long standing controversy regarding the electronic and magnetic states of CuCr2Se4 via halogen doping of the Se anion site in CuCr2Se4−xYx (Y=Cl and Br). Long range magnetic order is observed above room temperature for all samples. The Cr L2,3 XAS spectra show a prevalent 3+ valence for the Cr ions independent of doping concentration and doping agent. The Cu L2,3 XAS spectra show a combination of 1+ and 2+ valence states for all samples. XMCD spectra indicate the presence of a magnetic moment associated with the Cu ions that is aligned antiparallel to the Cr moment.

XANES and EXAFS modelling of configurational disorder in silver borate glasses

The sensitivity of x-ray absorption spectroscopy (XAS) to the local structure around a selected atomic species symmetry, distances and angles is commonly exploited to quantitatively describe systems where all the configurations around the absorbing atoms may be approximated by the mean one. However, in many cases of disordered systems a simple description of the local structure with single-configuration models cannot be reconciled with experimental spectra.In this paper the necessity to analyse the XAS spectra by considering a multiplicity of atomic structural configurations is demonstrated in the case of silver borate glasses.The local coordination around Ag cations in (Ag2O.nB2O3) glasses has been studied by comparing the XAS experimental spectra at the Ag K edge with theoretical XANES and EXAFS spectra obtained by a configurational average of model structures.In order to provide a reasonable agreement with experiment, these models have to take into account that there are many geometrically distinct Ag neighbourhoods present in the system, and that the measured XANES and EXAFS signals are in fact averages over spectra originated at all the non-equivalent photoabsorbing sites (multi-configuration approach).

X-ray absorption spectroscopy of the cubic and hexagonal polytypes of zinc sulfide

We investigate the sensitivity of x-ray absorption spectroscopy to cubic-hexagonal polytypism in which nearest-neighbor positions are virtually unchanged. Experimental spectra and multiple-scattering calculations are reported at the major absorption edges in the zinc-blende and wurtzite modifications of ZnS. We demonstrate that d-like unoccupied bands are preferentially affected, as only L-edge absorption is sensitive to this structural transition. The results allow near-edge x-ray absorption spectroscopy to be evaluated as a detection method for crystal structure modifications in nano-scale systems.

Method for position sensitive X-ray absorption spectroscopy

Method for position sensitive X-ray absorption spectroscopy

Probing the electronic structure of an active catalyst surface under high-pressure reaction conditions: the oxidation of methanol over copper

The active phase of a bulk metallic copper catalyst is investigated by surface sensitive X-ray absorption spectroscopy at the oxygen K-edge and the Cu L-edges in the total electron yield mode under practical steady state flow-through conditions. The active catalyst surface contains oxygen atoms revealing significant spectral differences compared to those of known copper oxides. The partial oxidation of methanol to formaldehyde is correlated to the abundance of this copper suboxide. These oxygen atoms probe defects of the copper lattice, which represent catalytically active sites. The suboxide is undetectable under UHV conditions. The total oxidation of methanol is catalysed by a conventional copper(I) oxide species and the abundance of carbon dioxide in the gas phase is increasing with decreasing integrated intensity of the oxide species.

Surface analysis of DC oxygen plasma treated or chromium evaporated poly(ethylene terephthalate) foils by soft X-ray absorption spectroscopy (NEXAFS)

Surface modified biaxially stretched poly(ethylene terephthalate) (PET) foils have been analysed by surface sensitive soft X-ray absorption (NEXAFS) spectroscopy. Generally, an orientation of polymer chain segments can be detected by a variation of the angle between the linearly polarized synchrotron light beam and the surface normal of the investigated sample (angle resolved NEXAFS spectroscopy). Biaxially stretched PET foils have been found to be characterized by angle dependent NEXAFS C and O K-edge spectra. This angle dependence was interpreted to be a result of a preferential orientation of the PET chain segments at the surface wherein the plane defined by the benzene ring is parallel to the surface plane. In the course of chromium deposition or low pressure O 2 DC plasma treatment that preferential orientation is continuously diminished. This is interpreted to be a structural effect of the respective treatment.

The initial stages of the growth of copper on a (1 × 1) and a (√31 × √31) R ± 9° α-Al 2O 3(0001) surface

We have studied the first stages of the growth of copper on two different α-Al 2O 3 (0001) alumina substrates, presenting respectively a (1 × 1) and a (√31 × √31) R ± 9° LEED (low energy electron diffraction) pattern. The study was realised by means of surface sensitive X-ray absorption spectroscopy (SEXAFS) at the CuK edge. For equal amounts of deposited copper (50% of an equivalent monolayer), we found quite different copper/alumina interfaces as a function of the substrate reconstruction. In the case of the (1 × 1) surface, we noticed the coexistence of copper dimers and very small copper nanoclusters (about 1 shell) weakly bonded to the substrate via the Al surface atoms. In the case of the (√31 × √31) R ± 9° reconstructed substrate, we detected slightly bigger Cu nanoclusters than those present on the (1 × 1) surface. We also observe that some Cu atoms are incorporated to the substrate, occupying the vacancies of the reconstructed surface. No CuO bonds are detected in both interfaces. This result can be understood in the case of the aluminium rich (√31 × √31) R ± 9° surface. However, it is more surprising in the case of the (1 × 1) surface and supports the hypothesis of an Al-terminated (1 × 1) alumina substrate.

X-ray Absorption Spectroscopic Evidence on the Partial Formation of Copper(III) in the Superconducting La2CuO4.08

In the present work, we suggest for the first time the contribution of the unoccupied Cu 3d states to the newly formed holes using the bulk sensitive X-ray absorption spectroscopy at the Cu K edge for nonsuperconducting La{sub 2}CuO{sub 4} and two superconducting La{sub 2}CuO{sub 4-{delta}} phases prepared electrochemically and chemically, respectively, on the basis of the previous XAS study for the Cu{sup III} compounds. In summary, it has been clearly confirmed by XAS that the same doping effect on the stoichiometric La{sub 2}CuO{sub 4} could be induced by chemical and electrochemical methods. Comparing the Cu K edge XANES spectra between the present compounds and the chemically well defined Cu{sup III} references, it is concluded that a Cu{sup III} contribution is present in the ground state of the oxidized phase, La{sub 2}CuO{sub 4.08}. 19 refs., 2 figs., 1 tab.

Anomalous scattering and DAFS measurements of small single crystal of barium platinum oxide

Anomalous diffraction and DAFS (diffraction anomalous fine structure) are X-ray techniques combining the short range structure sensitivity of X-ray absorption spectroscopy with the long range periodicity of X-ray diffraction. These techniques are demonstrated on a small single crystal of barium platinum oxide. Anomalous scattering factors f′ and f″ are calculated and an accurate absorption correction procedure for a small concentrated single crystal is presented.

A SEXAFS study of the Cu/Al 2O 3 interface

Surface sensitive X-ray absorption spectroscopy at the Cu K edge has been used for studying the Cu/Al 2O 3 interface. We have prepared in situ both oxidised and reduced α-alumina substrates and have evaporated equal amounts of copper (≈ 80% monolayer) on each. From the analysis of the data we can infer that: (a) The Cu/Al 2O 3 interface grows via the Volmer-Weber mode whatever the α-alumina surface oxidation state. (b) For equal amounts of copper, much larger clusters are found on the reduced α-Al 2O 3 than on the oxidised substrates.

Influence of the substrate oxidation state in the growth of copper clusters on Al 2O 3(0001) surface: a XANES and EXAFS study

Surface sensitive X-ray absorption spectroscopy has been used for investigating the differences in the environment of copper atoms at the Cu/Al 2O 3 interface for different oxidation states of the α-alumina substrate. We have prepared in situ both oxidised and reduced α-alumina surfaces and have evaporated equal amounts of copper (∼ 80% monolayer) on each. Under our experimental conditions, we infer that: (a) the interface grows via the Volmer-Weber mode rather than the Stranski-Krastanov mode suggested in the literature. (b) The morphology of supported Cu clusters depends strongly on the oxidation state of the alumina. For equal amounts of copper, much larger clusters are found on reduced Al 2O 3(0001) substrates than on oxidised substrates.