X-ray Absorption Spectroscopy Methods Research Articles

Theoretical description of X-ray absorption spectroscopy of the graphene-metal interfaces

The present paper considers the application of the method of the near-edge X-ray absorption spectroscopy (NEXAFS) for the investigation of the graphene-based systems (from free-standing graphene to the metal-intercalation-like systems). The NEXAFS spectra for the selected systems are calculated in the framework of the approach, which includes the effects of the dynamic core-hole screening. The presented spectral changes from system to system are analysed with the help of the corresponding band-structure calculations. The obtained results are compared with available experimental data demonstrating the excellent agreement between theory and experiment. The direct correlation between the strength of the graphene interaction with the metallic substrate and the spectral distributions (shape and intensities of π* and σ* features in the C K NEXAFS spectra) is found that can be taken as a fingerprint for the description of interaction at the graphene/metal interface.

An in situ quick-EXAFS and redox potential study of the Fe(II)-catalysed transformation of ferrihydrite

In this study we examine the kinetics of the Fe(II)-accelerated conversion of ferrihydrite to goethite using quick-scanning X-ray absorption spectroscopy (XAS) and measure the associated changes in oxidation–reduction potential (ORP). The quick-scanning method, where full scans up to k=15Å−1 were achieved in around 20s, provided improvements over traditional XAS methods, particularly with regard to the ability to conduct in situ experiments. The method allowed us to quantify the kinetics of the conversion of ferrihydrite to goethite at two different Fe(II) concentrations (0.25M and 0.05M) and at two different pH values (5.85 and 6.20). The higher Fe(II) concentration resulted in a five-fold higher first order rate constant for the conversion of ferrihydrite to goethite than at the lower Fe(II) concentration despite surface complexation modelling indicating that similar concentrations of surface-adsorbed Fe(II) species were present under both conditions. These results suggest that bulk solution Fe(II) concentrations rather than surface-adsorbed Fe(II) concentrations are controlling the crystallisation kinetics of ferrihydrite to goethite under the conditions used in these quick-XAS studies, most likely through direct injection of electrons from aqueous Fe(II) into the conduction band of ferrihydrite. The ORP decreased monotonically over time with this behaviour reasonably described using a simple model in which bulk solution Fe3+ activity was assumed to be determined by the extent of transformation of ferrihydrite to goethite and the associated Fe3+ activity in local equilibrium with each iron oxide phase.

Damage Analysis of n-GaN Crystal Etched with He and N2 Plasmas

To understand the details of etching-induced damage on a GaN surface, n-GaN crystals were plasma-etched with He and N2 gases. The etched surfaces were analyzed by X-ray photoelectron spectroscopy (XPS) and soft X-ray absorption spectroscopy (XAS) methods. The composition of the surface etched with He plasma changed significantly to being Ga-rich with the N/Ga ratio nearly equaling 0.4–0.5. The ratio of the surface etched with N2 plasma was about 0.6. The shape of the near-edge X-ray absorption fine structure (NEXAFS) of the N-K edge deformed with increasing gas pressure and processing time. The deformation can be explained by the increase in the band widths of a number of peaks in the NEXAFS spectra owing to the increase in the degree of structural disorder in the crystal. The increase in band width for the surface etched with N2 plasma was larger than that for the surface etched with He plasma. The above results can be explained with the model of the elastic energy transfer ratio of He+ and N2+ ions incident on the solid surface.

X-ray absorption spectroscopy of biological samples. A tutorial

X-ray absorption spectroscopy (XAS) is an element specific spectroscopy sensitive to the local chemical and structural order of the absorber element. XAS is nowadays increasingly used for the speciation analysis of chemical elements owing to the development of new synchrotron radiation facilities worldwide. XAS can be divided into X-ray absorption near edge structure (XANES), which provides information primarily about the geometry and oxidation state, and extended X-ray absorption fine structure (EXAFS), which provides information about metal site ligation. The main advantages of the XAS method are its subatomic (angstrom) resolution, the ability to analyze almost any type of samples including amorphous (non-crystalline) materials, the possibility to analyze such materials in situ requiring minor or no sample preparation. The main limitations of XAS are its sensitivity in the mM (or μg g−1) range, the difficulty to deconvolute the bulk data when the sample is composed of a mixture of structures of the absorber element, and the limited chemical selectivity of ligands to within one row of the periodic table. This tutorial will discuss the strengths and limitations of XAS and compare them to those of alternative or complementary methods such as X-ray diffraction and X-ray photoelectron spectroscopy. The tutorial will also present and discuss the specific needs in terms of sample preparation and preservation all along the process of storage and analysis, and discuss the importance of the use of cryogenic methods when XAS is applied to biological samples. Applications in life sciences are reviewed, not exhaustively, with a special emphasis on some characteristic examples. The article ends with some perspectives on future trends of XAS: micro- and nano-XAS, time-resolved XAS, and high energy resolution XAS.

Structure of the dense amorphous carbon phase synthesized in a mixture with diamond as a result of shock compression of carbon black

Soft X-ray emission and absorption spectroscopy methods were used in the present work to study experimentally the electronic structure of the dense amorphous carbon phase synthesized in a mixture with diamond as a result of shock compression of carbon black. The X-ray emission C Kα bands, representing the energy distribution of the C 2p-like states, have been recorded for the amorphous carbon phase and, for comparison, for diamond, lonsdaleite, graphite, and carbon black. For the amorphous carbon phase, diamond and lonsdaleite the spectra of quantum yields of the X-ray photoeffect in the area of the C K absorption edge have been recorded as well. Total and partial densities of states of the carbon H-6 and bct-4 phases, diamond and lonsdaleite have been calculated using the first-principles self-consistent full potential linearized augmented plane wave (FP-LAPW) method. Comparison on a common energy scale of the X-ray emission C Kα bands and C K quantum yields of the dense amorphous carbon phase as well as the theoretical curves of partial C 2p-like states of the carbon H-6 and bct-4 phases reveals that, the electronic structure of the amorphous carbon phase under consideration is well described by that of the carbon bct-4 phase. The experimental X-ray emission and absorption spectroscopy data indicate that the amorphous carbon phase studied is metallic, being in agreement with the results of the theoretical FP-LAPW calculations.

X-ray absorption spectroscopy characterization of the interface in capped ZnO nanoparticles

We report here an extensive study of the ZnO nanoparticles (NPs) capped with different organic molecules by X-ray absorption spectroscopy (XAS) methods to proceed in the understanding of their unusual magnetism. We have performed a detailed ab initio calculation of the Zn K-edge XAS aimed at determining the influence of the different coordination induced by the capping. The results suggest that the observed differences of the XAS spectra are due to the formation of a well-defined interface between the oxide NPs and the capping molecules. The appearance of magnetism should be related to the modifications induced in the NPs by the surface bonds between Zn and the capping molecule.

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.

Local structure of photoexcited bimetallic complexes refined by quantitative XANES analysis

Photoexcited states of a pyrazolate bridged PtPt dimer and a supramolecular RuCo complex with bipyridine-type ligands were studied using the pump-and-probe X-ray absorption spectroscopy method. The local structure refinement based on the fitting of XANES difference spectra has been performed. The theoretical analysis included the full multiple scattering calculations and the multidimensional interpolation of spectra as a function of structural parameters. The influence of possible correlations between the fraction of molecules in the photoexcited state and structural parameters is discussed.

X-ray absorption spectroscopy studies of the adducts formed between cytotoxic gold compounds and two major serum proteins

Gold metallodrugs form a class of promising antiproliferative agents showing a high propensity to react with proteins. We exploit here X-ray absorption spectroscopy (XAS) methods [both X-ray absorption near-edge spectroscopy (XANES) and extended X-ray absorption fine structure (EXAFS)] to gain insight into the nature of the adducts formed between three representative gold(I, III) metallodrugs (i.e., auranofin, [Au(2,2'-bipyridine)(OH)(2)](PF(6)), Aubipy, and dinuclear [Au(2)(6,6'-dimethyl-2,2'-bipyridine)(2)(μ-O)(2)](PF(6))(2), Auoxo6) and two major plasma proteins, namely, bovine serum albumin (BSA) and human serum apotransferrin (apoTf). The following metallodrug-protein systems were investigated in depth: auranofin/apoTf, Aubipy/BSA, and Auoxo6/apoTf. XANES spectra revealed that auranofin, upon protein binding, conserves its gold(I) oxidation state. Protein binding most probably takes place through release of the thiosugar ligand and its subsequent replacement by a thiol (or a thioether) from the protein. This hypothesis is independently supported by EXAFS results. In contrast, the reactions of Aubipy with serum albumin and of Auoxo6 with serum apoTf invariantly result in gold(III) to gold(I) reduction. Gold(III) reduction, clearly documented by XANES, is accompanied, in both cases, by release of the bipyridyl ligands; for Auoxo6 cleavage of the gold-gold dioxo bridge is also observed. Gold(III) reduction leads to formation of protein-bound gold(I) species, with deeply modified metal coordination environments, as evidenced by EXAFS. In these adducts, the gold(I) centers are probably anchored to the protein through nitrogen donors. In general, these two XAS methods, i.e., XANES and EXAFS, used here jointly, allowed us to gain independent structural information on metallodrug/protein systems; detailed insight into the gold oxidation state and the local environment of protein-bound metal atoms was achieved in the various cases.

Temperature-dependent local structure of LaFeAsO1−xFx: Probing the atomic correlations

The local structure of the parent and doped LaFeAsO1−xFx (pnictide) compounds were studied by x-ray absorption spectroscopy and density functional methods. In the doped system, the Fe–As and Fe–Fe correlations are both well modeled by an Einstein model. For the Fe–As bonds, the Einstein temperatures are identical for the doped (11%) and undoped samples but the doped sample is found to have a lower level of static disorder. Doping is found to increases the effective Einstein temperature of Fe–Fe atomic correlation. The results suggest that the onset of superconductivity in the F doped system may be related to enhanced magnetic interactions. Density functional calculations of the total charge density reveal strong bonding between neighboring As ions but metal-like behavior in the Fe layers. It is also seen directly that the replacement of oxygen by fluorine modifies the electron charge density mainly on the Fe sites.

EXAFS as Powerful Analytical Tool for the Investigation of Organic–Inorganic Hybrid Materials

AbstractThe potential and application of X‐ray absorption spectroscopy (XAS) for structural investigations of organic–inorganic hybrid materials, with a special emphasis on systems consisting of inorganic building blocks (clusters) embedded into polymer backbones, is extensively reviewed. In the first part of the paper, the main features of organic–inorganic hybrid materials, their classification, the synthetic approaches for their preparation, and their applications are concisely presented, whereas the particular issues related to their characterization are discussed in more detail. In the second section of the paper, the principles and the theoretical background of the XAS method, including experimental design, data reduction, evaluation, analysis, and interpretation are described and discussed. Examples of potentialities of the method for the short‐range structural investigation of inorganic nanostructures in hybrids are provided, and the state‐of‐the‐art in the field of hybrid materials is reviewed. In the third part, six different case studies belonging to our past and present experience in this field are presented and discussed, with a particular focus on their XAS investigation.

X-ray absorption measurements on nanoparticle systems: self-assembled arrays and dispersions

X-ray absorption spectroscopy methods are presented as a useful tool to determine local structure, composition and magnetic moments as well as to estimate the effective anisotropy of substrate supported self-assembled arrays of wet-chemically synthesized FePt nanoparticles. A compositional inhomogeneity within the nanoparticles yields reduced magnetic moments with respect to the corresponding bulk material and may also hinder the formation of the chemically ordered L10 phase in FePt nanoparticles. The latter is indicated by a reduced effective anisotropy, which is one order of magnitude smaller than expected from the known value of the corresponding bulk material.As a new approach, measurements of the x-ray absorption near-edge structure of Fe-oxide nanoparticles in dispersion are presented and ageing effects are discussed on the basis of multiplet calculations.

Pressure-inducedFe↔Cucationic valence exchange and its structural consequences: High-pressure studies of delafossiteCuFeO2

The present high-pressure studies of ${\text{CuFeO}}_{2}$ to 30 GPa using x-ray diffraction, along with $^{57}\text{F}\text{e}$ M\"ossbauer and Fe and $\text{Cu}\text{ }K$-edge x-ray absorption spectroscopy methods, reveal a sequence of intricate structural/electronic-magnetic pressure-induced transitions. The low-pressure $R\overline{3}m$ structure (0--18 GPa) is composed of sheets of ${\text{Fe}}_{S=5/2}^{3+}$ ions alternating with layers of ${\text{O-Cu}}_{S=0}^{1+}\text{-O}$ dumbbells, the latter oriented along the $c$ axis. This structure is characterized by an unusual positive $d(c/a)/dP$. At 18 GPa a structural transition takes place to a more isotropic $C2/c$ structure with the ${\text{O-Cu}}_{S=0}^{1+}\text{-O}$ axis tilted $28\ifmmode^\circ\else\textdegree\fi{}$ from the $c$ axis and with negative $d(c/a)/dP$. This transition corroborates with the onset of long-range antiferromagnetic order. Starting at $\ensuremath{\sim}23\text{ }\text{GPa}$, with an initial volume reduction in $\ensuremath{\sim}|\ensuremath{\Delta}V/{V}_{0}|=0.16$, the Cu-Fe bands overlap and this leads to a $({\text{Cu}}_{S=0}^{1+}{\text{Fe}}_{S=5/2}^{3+})\ensuremath{\rightarrow}({\text{Cu}}_{S=1/2}^{2+}{\text{Fe}}_{S=2}^{2+})$ interionic valence exchange in about 1/3 of the $C2/c{\text{-CuFeO}}_{2}$ at 27 GPa. As a result: (i) the ${\text{Cu}}^{2+}\text{-O}$ becomes fourfold coordinated and is in a new crystallographic structure with space group $P\overline{3}m$, and (ii) the N\'eel temperature increases above twofold $[{T}_{\text{N}}({\text{Cu}}_{S=1/2}^{2+}{\text{Fe}}_{S=2}^{2+})\ensuremath{\approx}2.2{T}_{\text{N}}({\text{Cu}}_{S=0}^{1+}{\text{Fe}}_{S=5/2}^{3+})]$. This sequence of transitions is reversible with minimal hysteresis.

Spectroscopic Determination of HOMO and LUMO Energies of Retinal in Bacteriorhodopsin for Solar Cell Applications

Bacteriorhodopsin (bR) is a potential sensitizer for bio-sensitized solar cells(Fig. 1). In this study, the energies of the Highest Occupied Molecular Orbital (HOMO) and the Lowest Unoccupied Molecular Orbital (LUMO) of retinal in bR are investigated using X-ray Photoemission Spectroscopy (XPS), X-ray Absorption Spectroscopy (XAS), and Ultraviolet Photoemission Spectroscopy (UPS). With the combination of XPS, XAS and UPS methods, the absolute energies of the HOMO and LUMO can be determined for comparison to the valence and conduction band energies of the biosensitized semiconductor. The HOMO-LUMO gap of retinal was spectroscopically determined to be 2.49 eV. For comparison, we also test the feasibility of DFT calculations in determining the HOMO-LUMO gap of free retinal. Using the G-311G basis set, the calculated HOMO-LUMO gap was 2.69 eV. The results shows that the DFT method overestimates the experimentally found band gap; consequently, higher level calculations are required.View Large Image | View Hi-Res Image | Download PowerPoint Slide

Dielectric properties of single crystalline PrO2(111)/Si(111) heterostructures: Amorphous interface and electrical instabilities

Single crystalline PrO2(111)/Si(111) heterostructures are flexible buffers for global Ge integration on Si. A combined materials science–electrical characterization is carried out to study the influence of postdeposition annealing in 1 bar oxygen at 300–600 °C on the dielectric properties of PrO2(111)/Si(111). The materials science transmission electron microscopy and x-ray reflectometry studies reveal that postdeposition oxidation of the PrO2(111)/Si(111) boundary results in an amorphous interface (IF) layer, which grows in thickness with temperature. Nondestructive depth profiling synchrotron radiation-based x-ray photoelectron spectroscopy and x-ray absorption spectroscopy methods demonstrate that this amorphous IF layer is composed of two Pr-silicate phases, namely, with increasing distance from Si, a SiO2-rich and a SiO2-poor Pr silicate. The electronic band offset diagram shows that the wide band gap dielectric Pr silicate results in higher band offsets with respect to Si than the medium band gap dielectric PrO2. The electrical characterization studies by C-V measurements show that (a) well-behaved dielectric properties of the PrO2(111)/IF/Si(111) are achieved in a narrow postdeposition oxidation window of 400–450 °C and that (b) defects are distributed over the Pr-silicate IF layer. Temperature-dependent J-V studies report furthermore that the formation of the single crystalline PrO2/amorphous Pr-silicate bilayer structure on Si(111) results in (a) improved insulating properties and (b) strong electrical instability phenomena in the form of a Maxwell–Wagner instability and dielectric relaxation.

A study of the hydrogen sorption properties, thermal stability and the character of the chemical bonds of Ho and Lu Me–H hydrides through the use of thermodesorption and X-ray absorption spectroscopy

Abstract Hydrogensorption properties and thermal stability of HoH 2 , HoH 3 and LuH 3 hydrides have been studied with the use of thermodesorption and X-ray absorption spectroscopy methods. A charge transfer from a metal to hydrogen was determined through the L III -absorption spectra shift in these hydrides. The experiments conducted has demonstrated that in the case of REM hydrides as in the case of early studied hydrides of III and IV groups of the periodical table metals the correlation exists between value of charge transferred from metal to hydrogen (or ionicity of metal–hydrogen bonds) in metalhydrides and their thermal stability and decomposition temperature.

Electronic structure of monoclinic α-KY(WO 4) 2 tungstate as determined from first-principles FP-LAPW calculations and X-ray spectroscopy studies

Total and partial densities of states of the constituent atoms of potassium yttrium double tungstate, KY(WO 4) 2 (KYW), have been calculated using the first-principles self-consistent full potential linearized augmented plane wave (FP-LAPW) method. The results derived reveal that the O 2p-like states are the dominant contributors into the valence band of KYW, while the conduction band of the compound is dominated by contributions of the W 5d-like states. The FP-LAPW calculations render that KYW is an indirect-gap material with the band-gap value of 3.645 eV. The X-ray emission spectroscopy (XES) and X-ray absorption spectroscopy (XAS) methods were also used in the present work to study experimentally electronic properties of potassium yttrium double tungstate. For the above compound, the XES bands reflecting the energy distribution of the valence W d-, O p- and K p-like states were derived and compared on a common energy scale with the X-ray photoelectron valence-band spectrum. Measurements of the energy shift of the XAS W L III edge of KYW reveal that tungsten atoms are in the formal valence state 6+. A rather good agreement of the experimental XES and theoretical FP-LAPW data concerning electronic properties of KYW has been obtained in the present paper.

First-principles calculations and X-ray spectroscopy studies of the electronic structure of CuWO 4

First-principles self-consistent band-structure calculations of copper tungstate, CuWO 4, have been made using the full potential linearized augmented plane wave (FP-LAPW) method. Total and partial densities of states of the constituent atoms of CuWO 4 have been derived. The results obtained reveal that the valence band of CuWO 4 is dominated by contributions of the O 2p-like states, while the W 5d-like states are the main contributors into the conduction band of the tungstate. Additionally, the FP-LAPW calculations render that the W 5d- and Cu 3d-like states contribute mainly at the bottom and at the top of the valence band of CuWO 4, respectively. In the present work, the X-ray emission spectroscopy (XES) and X-ray absorption spectroscopy (XAS) methods were also employed to investigate experimentally the electronic structure of copper tungstate. For the mentioned compound, the XES bands reflecting the energy distribution of mainly the W 5d-, Cu 3d- and O 2p-like states were derived and compared on a common energy scale with the X-ray photoelectron valence-band spectrum. A rather good agreement of the experimental and theoretical data concerning electronic properties of CuWO 4 has been obtained in the present paper. Measurements of the energy shift of the XAS W L III edge of CuWO 4 clearly demonstrate that tungsten atoms in copper tungstate are in the formal valence state +6.

The determination of Ti2O3 in titania slags: a comparison of different methods of analysis

Nine titania slags, consisting mostly of ferropseudobrookite (denoted as M3O5), were investigated with the aim of evaluating various methods to estimate their Ti2O3 content. Direct methods include the determination of the Ti3+ content using chemical titration and X-ray absorption spectroscopy (XAS) at the Ti K edge. Indirect determinations of the Ti2O3 content are based on the bulk chemical composition analysed by inductively coupled plasma optical emission spectrometry and the analysis of pseudobrookite cell parameters using X-ray diffraction. It was found that the use of bulk chemistry, the FeO contents, as well as the a unit cell dimensions of the M3O5 phase give acceptable estimates, provided that the slags are not oxidised on cooling to room temperature. Only the direct XAS and titration methods determine the actual Ti3+ or Ti2O3 contents and are not influenced by the degree of oxidation of the slags. The XAS results provide a valuable independent validation of the titration method. It is concluded that the titration method is still the most practical and reliable method for estimating Ti3+ contents.

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.