Capacitance XAFS method: X-ray absorption spectroscopy of low-dimensional structures.

The generation of X-rays is discussed in the first section of this chapter. X-rays are generated by (1) the bombardment of a target by electrons, (2) the decay of certain radio isotopes, (3) as part of the synchrotron-radiation spectrum and (4) in plasmas produced by bombarding targets with high-energy laser beams. The spectra produced by the first method consist of a line spectrum characteristic of the target material accompanied by a continuum of white radiation (Bremsstrahlung). The intensities and wavelengths of these components are discussed and the nomenclatures of the lines in the characteristic spectrum are compared. Synchrotron radiation and plasma generation of X-rays are discussed very briefly. X-ray wavelengths are discussed in the second section of the chapter. Tables of K- and L-series reference wavelengths and K- and L-emission lines and absorption edges are provided. X-ray absorption spectra are discussed in the third section of the chapter. A detailed discussion of the problems associated with the measurement of X-ray absorption and the requirements for the absolute measurement of the X-ray attenuation coefficients is followed by a description of the problems which are encountered in making measurements at X-ray absorption edges of atomic species in materials. The popular, and extremely valuable, experimental technique of X-ray absorption fine structure (XAFS) is discussed. This is a relative, rather than an absolute measurement of X-ray absorption, and the theoretical analysis of XAFS spectra depends on the deviation of the data points from a cubic spline fit to the data rather that the deviation from the extrapolation of the free-atom absorption. A brief description of the origin and use of X-ray absorption near edge structure (XANES) is given. In the fourth section of the chapter, X-ray absorption (or attenuation) coefficients are considered. A detailed description of the theoretical and experimental techniques for the determination of X-ray absorption (and attenuation coefficients) is given. The tables supercede the existing experimental and theoretical tables that were available up to 2001. The theoretical values given here are restricted to the characteristic radiations commonly available from laboratory X-ray sources (Ti Kβ to Ag Kα). Computational problems become significant for elements in the lanthanide and actinide series of the periodic table. These tables include a significant recalculation of the absorption and attenuation coefficients for the lanthanide series. Note that although many theory-based tables exist, few accurate experimental measurements have been made on atomic species with atomic numbers greater than 50, especially in the region of absorption edges. A comparison is given of the extent to which theoretical and experimental data agree. A recent tabulation discusses soft X-ray absorption in the XANES region. Filters and monochromators are discussed in the fifth section of the chapter. This section serves as an introduction to the use of filters and monochromators in experimental apparatus. It discusses, using synchrotron-radiation experimental configurations as examples, the use of X-ray reflectivity, X-ray refraction and X-ray Bragg (and Laue) or Fresnel scattering for the production of monochromatic or quasi- monochromatic (pink) beams from polychromatic sources. Amongst the techniques discussed are: bent and curved mirrors, capillaries, quasi-Bragg (multilayer) mirrors, multiple filters, crystal monochromators, polarization and polarization-producing systems, and focusing using a Bragg–Fresnel optical system. The development of monochromators for synchrotron-radiation research is a rapidly evolving field, but in practice all depend on either the processes of reflection, refraction or scattering, or combinations of these processes. In the final section of the chapter, X-ray dispersion corrections are considered. The various theoretical methods for the computation of the X-ray dispersion corrections and the experimental techniques used for their determination are described in some detail. The theoretical values given here are restricted to the characteristic radiations commonly available from laboratory X-ray sources (Ti Kβ to Ag Kα). An artificial distinction is made between the relativistic Dirac–Hartree–Fock–Slater (RDHFS) formalism used to calculate the data in these tables and the S-matrix method used by a number of theoreticians. Formally, the two techniques are equivalent: in terms of the computations the two approaches are different, and the outcomes are different. Comparison with experimental data shows that the RDHFS formalism gives a better fit to experimental results. Keywords: absorption; anomalous dispersion; atomic form factor; atomic scattering factors; attenuation coefficients; Auger shifts; Bijvoet-pair techniques; black-body radiation in X-ray region; characteristic line spectrum; continuous spectrum; Dirac–Fock method; dispersion corrections; EXAFS; extended X-ray absorption fine structure; filters; Friedel-pair techniques; generation of X-rays; intensity; monochromators; normal attenuation; photo-effect data; QED corrections; Rayleigh scattering; scattering; synchrotron radiation; theoretical Rayleigh scattering data; theoretical photo-effect data; wavelengths; X-ray absorption coefficients; X-ray absorption spectra; X-ray attenuation coefficients; X-ray dispersion corrections; X-ray sources; X-ray tubes; X-ray wavelengths; XAFS; X-ray absorption near-edge structure; XANES

Cathodoluminescence (CL) is a very powerful technique for studying the optical properties of semiconductor structures, especially low-dimensional structures (structures with sub-micron features). We will give a number of examples of studies, where CL has been used to study a variety of low-dimensional semiconductor structures. We will also compare the CL technique with other techniques for studies of luminescence with high spatial resolution.

As the global energy structure evolves and clean energy technologies advance, electrocatalysis has become a focal point as a critical conversion pathway in the new energy sector. Transitional metal electrocatalysts (TMEs) with their distinctive electronic structures and redox properties show great potential in electrocatalytic reactions. However, complex reaction mechanisms and kinetic limitations hinder the improvement of energy conversion efficiency, highlighting the necessity for comprehensive studies on structure and performance of electrocatalysts. X-ray Absorption Fine Structure (XAFS) spectra stand out as a robust tool for examining the electrocatalyst's structures and performance due to its atomic selectivity and sensitivity to local environments. This review delves into the application of XAFS technology in characterizing TMEs, providing in-depth analyses of X-ray Absorption Near-Edge Structure (XANES) spectra, and Extended XAFS (EXAFS) spectra in both R-space and k-space. These analyses reveal intrinsic structural information, electronic interactions, catalyst stability, and aggregation morphology. Furthermore, the paper examines advancements in in-situ XAFS techniques for real-time monitoring of active site changes, capturing critical intermediate and transitional states, and elucidating the evolution of active species during electrocatalytic reactions. These insights deepen our understanding on structure-activity relationship of electrocatalysts and offer valuable guidance for designing and developing highly active and stable electrocatalysts.

An X-ray absorption fine structure (XAFS) study has been conducted to reveal the local structure and chemical state of the copper in the complex of an acetylacetonate-based ligand (L1) and copper ion in acetonitrile solution. The copper ion in the complex was found to be divalent from the Cu K-edge X-ray absorption near-edge structure (XANES) spectrum. The FEFF (ab initio multiple scattering calculations of XAFS) were performed with the model compounds, whose structures were optimized by using MOPAC program with AM1 Hamiltonian. The comparative study of the experimental XAFS spectra and theoretical calculations from FEFF gave the perspectives for clarifying the coordination structure of the complex of L1 and copper ion.

A critical assessment of the evidence from XAFS and crystallography for the breakage of the imidazolate bridge during catalysis in CuZn superoxide dismutase.

Hydrothermal synthesis is an important route to novel materials. Hydrothermal chemistry is also an important aspect of geochemistry and a variety of waste remediation technologies. There is a significant lack of information about the speciation of inorganic compounds under hydrothermal conditions. For these reasons we describe a high-temperature, high-pressure cell that allows one to acquire both x-ray absorption fine structure (XAFS) spectra and x-ray transmission and absorption images of heterogeneous hydrothermal mixtures. We demonstrate the utility of the method by measuring the Cu(I) speciation in a solution containing both solid and dissolved Cu phases at temperatures up to 325 °C. X-ray imaging of the various hydrothermal phases allows micro-XAFS to be collected from different phases within the heterogeneous mixture. The complete structural characterization of a soluble bichloro-cuprous species was determined. In situ XAFS measurements were used to define the oxidation state and the first-shell coordination structure. The Cu–Cl distance was determined to be 2.12 Å for the CuCl2− species and the complete loss of tightly bound waters of hydration in the first shell was observed. The microreactor cell described here can be used to test thermodynamic models of solubility and redox chemistry of a variety of different hydrothermal mixtures.

In order to discuss the local structure of deep level carrier traps, the site-selective x-ray absorption fine structure (XAFS) by measuring the x-ray photon-energy dependence of the capacitance of a Schottky barrier diode is proposed. Dropping of the localized electron into a core hole arising from x-ray absorption of the atom in the carrier trap, not in bulk, increases the capacitance. The site-selective XAFS is adopted for the local structure analysis of DX center in Al0.33Ga0.67As:Se. The Ga K-edge site-selective XAFS is different from the conventional XAFS, suggesting that Ga in the DX center with a large lattice relaxation is selectively observed.

A new technique by XAFS (X-ray Absorption Fine Structures) and XANES (X-ray Absorption Near Edge Structures) has been developed for in situ observation of metal corrosion. XAFS and XANES spectra were obtained with a special electrochemical cell to elucidate pitting for stainless steel. A sheet of stainless steel was attached below a reservoir of a specific aqueous environment with thin films. The solution in the reservoir attached to the metal thorough the film, and the solution inside the film corresponds to an artificial pit. X-ray beams pass through the film containing the solution, and XAFS and XANES measurements were performed in a transparent geometry with keeping the specimen at a pitting potential. Measurements were performed for Cr-K, Mo-K, Cl-K and Br-K edges, and changes of concentrations and coordination states of ions were successfully obtained as a function of the distance (d(z)) from the metal/solution interface. Concentrations of chromium and bromide ions inside the artificial crevice of Fe-18Cr-12Ni-2Mo (mass%) alloy shows a linear dependence on the distance d(z). Structures of bromide ion were changed with positions; the distance between bromide ion and the nearest-neighboring ion at a position near the interface was shorter than that at a position far from the interface. This indicates the formation of hydrobromo-complex near the metal/solution interface. The state of molybdenum inside the artificial crevice of Fe-18Cr-20Ni-5Mo (mass%) alloy was investigated using two solutions: LiCI and LiBr. The formation of the [MoO 4 (H 2 O) 2 ] 2- octahedra was observed in both solutions, but there was observed a significant difference in networking of the octahedral. In LiCl solutions it was similar to that of molybdate ions (MoO 2- 4 ), but in LiBr it was rather different form that of molybdate ions. This shows that the favorable effects of molybdenum can be attributed to the formation of MoO 2- 4 network near the interface.

Selective X-ray absorption spectroscopy of self-assembled atom in InAs quantum dot

Recent high-accuracy X-ray absorption measurements of the sandwich organometallics ferrocene (Fc) and decamethylferrocene (DmFc) at temperatures close to liquid helium are compared with new full-potential modeling of X-ray absorption fine structure (XAFS) covering the near-edge region (XANES) and above up to k = 7 Å(-1). The implementation of optimized calculations of the oscillatory part of the spectrum from the package FDMX allows detailed study of the spectra in regions of the photoelectron momentum most sensitive to differences in the molecular stereochemistry. For Fc and DmFc, this corresponds to the relative rotation of the cyclopentadienyl rings. When applied to high-accuracy XAFS of Fc and DmFc, the FDMX theory gives clear evidence for the eclipsed conformation for Fc and the staggered conformation for DmFc for frozen solutions at ca. 15 K. This represents the first clear experimental assignment of the solution structures of Fc and DmFc and reveals the potential of high-accuracy XAFS for structural analysis.

The aim of this project is the calculation of the absorption coefficient μ of x-ray absorption spectra as a function of energy and a comparison with experimental data. A characteristic fine structure can be found in x-ray absorption spectra (XAS) consisting of the XANES (X-Ray Absorption Near Edge Structure) and the EXAFS (Extended X-Ray Absorption Fine Structure) region. XANES is characterized by multiple scattering and provides information about coordination chemistry and bonding angles of the irradiated sample. The program FEFF 8.4 was used for the calculations of the absorption K-edge spectra. FEFF was generated for ab initio multiple scattering calculations of X-ray Absorption Fine Structure (XAFS) of atom-clusters. The code yields scattering amplitudes, phases and other quantities. We computed μ at the K-edge of several elements like Ti, S and Fe. For this purpose, clusters of Na2SO4, Ba2TiO4, FeS2, CaSO2 2(H 2O) were used, working with several space groups. Some of the calculations are consistent with the results of the experiments, but others show energy shifts in the range of some eV. In summary, the FEFF calculations and the experimental data exhibit similarities as well as deviations. By using trimming parameters, deviations could be eliminated to a certain extent, which will be presented.

The X-ray absorption fine structure (XAFS) spectra at the K-edge of the copper complexes Cu(PDC)(Mim)3 H2O (1) and Cu(PDC)2(EA)2H2O (2) (where PDC – Pyridine-2,3-dicarboxylic acid, Mim – 2-methylimidazole, and EA – ethyl acetate) have been investigated. The experimental extended X-ray absorption fine structure data of complex 1 have been analyzed by fitting the theoretical model generated from its own crystallographic data. The crystallographic data for complex 2 are not available. It has been found by comparing the intensity of the pre-edge peaks and X-ray absorption near edge structure features of complexes 1 and 2 that both complexes possess square pyramidal geometry around the copper centers and thus complex 2 is analogous to complex 1. Hence, the theoretical model generated for complex 1 has been fitted to the experimental EXAFS data of complex 2 to determine the structural parameters of complex 2. The coordination geometry of both complexes has been depicted. Further, the chemical shifts have been used to determine the oxidation state as well as to estimate the effective nuclear charge on the copper atom.

Solvent effects on cobalt(III) and chromium(III) oxalato complexes by x-ray absorption fine structure

Impurity phases of CaMgSi2O6:Eu (CMS) phosphor have been investigated in detail by X-ray absorption fine structure (XAFS), ESR, X-ray diffraction (XRD) and photoluminescence (PL) analyses. XRD studies of CMS phosphor powders clarify the volume fraction of CMS, Ca2MgSi2O7, (Ca,Eu)10(SiO4)6O2, and crystalline SiO2. XAFS and ESR studies clarify that the soluble limit of Eu ions in CMS crystals is observed at about 0.01 mol. Excess Eu ions yield an impurity (Ca,Eu)10(SiO4)6O2 and some of them are captured by another impurity Ca2MgSi2O7. From the results of XAFS, ESR, XRD, and PL measurements, it was revealed that the PL peak intensities under 147 and 360 nm excitation correlate closely with the amount of these impurity phases containing divalent and trivalent Eu ions. Variation of X-ray absorption near edge structure and ESR spectra indicate that the optimum Eu concentration of the CMS phosphor is at approximately 0.01 molar versus Ca.

The oxidation state and coordination environment of antimony (Sb) incorporated into polyethylene terephthalate (PET) bottles were estimated based on X-ray absorption fine structure (XAFS) at Sb K-edge. Prior to XAFS analyses, Sb concentrations in 177 PET bottles collected in Japan and China were determined, showing that 30.5% and 100% of Japanese and Chinese PET bottles, respectively, contained more than 10 mg/kg of Sb. Most of the bottles used for aseptic cold filling and carbonated drinks contained a larger amount of Sb. Extended X-ray absorption fine structure (EXAFS) showed that the first neighboring atom of Sb in PET was estimated to be oxygen with a coordination number of about three. In addition, the contribution of Sb to Sb shell was discounted in the EXAFS, showing that Sb was not present as Sb2O3 in PET, although Sb was initially added as Sb2O3 in the production of PET. This information is consistent with the coordination environment estimated from the polycondensation reaction catalyzed by Sb, where Sb can be present as either Sb glycolate or Sb glycolate binding to the end group of the PET polymer. X-ray absorption near-edge structure (XANES) showed that Sb(III) initially added as Sb2O3 into PET was partially oxidized and the Sb(V) fractions reached approximately 50% in some samples. However, the oxidation state and coordination environment of Sb in PET had no relationship with the concentrations of Sb that leached into water from PET. Based on the present XAFS results and previous studies on the effects of temperature and others, it was concluded that the leaching behavior of Sb into water is primarily due to the degradation of PET itself, but is not related to the Sb species in the PET bottles.