Extended X‐ray absorption fine structure and multiple‐scattering simulation of nickel dithiolene complexes Ni[S2C2(CF3)2]2n (n = −2, −1, 0) and an olefin adduct Ni[S2C2(CF3)2]2(1‐hexene)

A series of Ni dithiolene complexes Ni[S₂C₂(CF₃)]₂ⁿ (n = ?2, ?1, 0) ( 1 , 2 , 3 ) and a 1‐hexene adduct Ni[S₂C₂(CF₃)₂]₂(C₆H₁₂) ( 4 ) have been examined by Ni K‐edge X‐ray absorption near‐edge structure (XANES) and extended X‐ray absorption fine‐structure (EXAFS) spectroscopies. Ni XANES for 1 – 3 reveals clear pre‐edge features and approximately +0.7 eV shift in the Ni K‐edge position for `one‐electron' oxidation. EXAFS simulation shows that the Ni—S bond distances for 1 , 2 and 3 (2.11–2.16 Å) are within the typical values for square planar complexes and decrease by ～0.022 Å for each `one‐electron' oxidation. The changes in Ni K‐edge energy positions and Ni—S distances are consistent with the `non‐innocent' character of the dithiolene ligand. The Ni—C interactions at ～3.0 Å are analyzed and the multiple‐scattering parameters are also determined, leading to a better simulation for the overall EXAFS spectra. The 1‐hexene adduct 4 presents no pre‐edge feature, and its Ni K‐edge position shifts by ?0.8 eV in comparison with its starting dithiolene complex 3 . Consistently, EXAFS also showed that the Ni—S distances in 4 elongate by ～0.046 Å in comparison with 3 . The evidence confirms that the neutral complex is `reduced' upon addition of olefin, presumably by olefin donating the π‐electron density to the LUMO of 3 as suggested by UV/visible spectroscopy in the literature.     

Titanium local structure in tektite probed by X‐ray absorption fine structure spectroscopy

The local structure of titanium in tektites from six strewn fields was studied by Ti K‐edge X‐ray absorption near edge structure (XANES) and extended X‐ray absorption fine structure (EXAFS) in order to provide quantitative data on Ti—O distance and Ti coordination number. The titanium in tektites possessed different coordination environment types. XANES spectra patterns revealed resemblance to high‐temperature TiO₂–SiO₂ glass and TiO₂ anatase. All samples showed that the valence of Ti is 4+. Based on the Ti—O distances, coordination numbers and radial distribution function determined by EXAFS analyses, the tektites were classified into three types: type I, Ti occupies a four‐coordinated tetrahedral site with Ti—O distances of 1.84–1.79 Å; type II, Ti occupies a five‐coordinated trigonal bipyramidal or tetragonal pyramidal site with Ti—O distances of 1.92–1.89 Å; type III, Ti occupies a six‐coordinated octahedral site with Ti—O distances of 2.00–1.96 Å. Although Ti occupies the TiO₆ octahedral site in most titanium minerals under ambient conditions, some tektites have four‐ and five‐coordinated Ti. This study indicated that the local structure of Ti might change in impact events and the following stages.     

SUT‐NANOTEC‐SLRI beamline for X‐ray absorption spectroscopy

The SUT‐NANOTEC‐SLRI beamline was constructed in 2012 as the flagship of the SUT‐NANOTEC‐SLRI Joint Research Facility for Synchrotron Utilization, co‐established by Suranaree University of Technology (SUT), National Nanotechnology Center (NANOTEC) and Synchrotron Light Research Institute (SLRI). It is an intermediate‐energy X‐ray absorption spectroscopy (XAS) beamline at SLRI. The beamline delivers an unfocused monochromatic X‐ray beam of tunable photon energy (1.25–10 keV). The maximum normal incident beam size is 13 mm (width) × 1 mm (height) with a photon flux of 3 × 10⁸ to 2 × 10¹⁰ photons s^?1 (100 mA)^?1 varying across photon energies. Details of the beamline and XAS instrumentation are described. To demonstrate the beamline performance, K‐edge XANES spectra of MgO, Al₂O₃, S₈, FeS, FeSO₄, Cu, Cu₂O and CuO, and EXAFS spectra of Cu and CuO are presented.     

Insight into growth of Au–Pt bimetallic nanoparticles: an in situ XAS study

Au–Pt bimetallic nanoparticles have been synthesized through a one‐pot synthesis route from their respective chloride precursors using block copolymer as a stabilizer. Growth of the nanoparticles has been studied by simultaneous in situ measurement of X‐ray absorption spectroscopy (XAS) and UV–Vis spectroscopy at the energy‐dispersive EXAFS beamline (BL‐08) at Indus‐2 SRS at RRCAT, Indore, India. In situ XAS spectra, comprising both X‐ray near‐edge structure (XANES) and extended X‐ray absorption fine‐structure (EXAFS) parts, have been measured simultaneously at the Au and Pt L₃‐edges. While the XANES spectra of the precursors provide real‐time information on the reduction process, the EXAFS spectra reveal the structure of the clusters formed in the intermediate stages of growth. This insight into the formation process throws light on how the difference in the reduction potential of the two precursors could be used to obtain the core–shell‐type configuration of a bimetallic alloy in a one‐pot synthesis method. The core–shell‐type structure of the nanoparticles has also been confirmed by ex situ energy‐dispersive spectroscopy line‐scan and X‐ray photoelectron spectroscopy measurements with in situ ion etching on fully formed nanoparticles.     

Extraction of local coordination structure in a low‐concentration uranyl system by XANES

Obtaining structural information of uranyl species at an atomic/molecular scale is a critical step to control and predict their physical and chemical properties. To obtain such information, experimental and theoretical L₃‐edge X‐ray absorption near‐edge structure (XANES) spectra of uranium were studied systematically for uranyl complexes. It was demonstrated that the bond lengths (R) in the uranyl species and relative energy positions (ΔE) of the XANES were determined as follows: ΔE₁ = 168.3/R(U—O_ax)² ? 38.5 (for the axial plane) and ΔE₂ = 428.4/R(U—O_eq)² ? 37.1 (for the equatorial plane). These formulae could be used to directly extract the distances between the uranium absorber and oxygen ligand atoms in the axial and equatorial planes of uranyl ions based on the U L₃‐edge XANES experimental data. In addition, the relative weights were estimated for each configuration derived from the water molecule and nitrate ligand based on the obtained average equatorial coordination bond lengths in a series of uranyl nitrate complexes with progressively varied nitrate concentrations. Results obtained from XANES analysis were identical to that from extended X‐ray absorption fine‐structure (EXAFS) analysis. XANES analysis is applicable to ubiquitous uranyl–ligand complexes, such as the uranyl–carbonate complex. Most importantly, the XANES research method could be extended to low‐concentration uranyl systems, as indicated by the results of the uranyl–amidoximate complex (～40 p.p.m. uranium). Quantitative XANES analysis, a reliable and straightforward method, provides a simplified approach applied to the structural chemistry of actinides.     

Augmentation of the step‐by‐step Energy‐Scanning EXAFS beamline BL‐09 to continuous‐scan EXAFS mode at INDUS‐2 SRS

An innovative scheme to carry out continuous‐scan X‐ray absorption spectroscopy (XAS) measurements similar to quick‐EXAFS mode at the Energy‐Scanning EXAFS beamline BL‐09 at INDUS‐2 synchrotron source (Indore, India), which is generally operated in step‐by‐step scanning mode, is presented. The continuous XAS mode has been implemented by adopting a continuous‐scan scheme of the double‐crystal monochromator and on‐the‐fly measurement of incident and transmitted intensities. This enabled a high signal‐to‐noise ratio to be maintained and the acquisition time was reduced to a few seconds from tens of minutes or hours. The quality of the spectra (signal‐to‐noise level, resolution and energy calibration) was checked by measuring and analysing XAS spectra of standard metal foils. To demonstrate the energy range covered in a single scan, a continuous‐mode XAS spectrum of copper nickel alloy covering both Cu and Ni K‐edges was recorded. The implementation of continuous‐scan XAS mode at BL‐09 would expand the use of this beamline in in situ time‐resolved XAS studies of various important systems of current technological importance. The feasibility of employing this mode of measurement for time‐resolved probing of reaction kinetics has been demonstrated by in situ XAS measurement on the growth of Ag nanoparticles from a solution phase.     

Alternative difference analysis scheme combining R‐space EXAFS fit with global optimization XANES fit for X‐ray transient absorption spectroscopy

Time‐resolved X‐ray absorption spectroscopy (TR‐XAS), based on the laser‐pump/X‐ray‐probe method, is powerful in capturing the change of the geometrical and electronic structure of the absorbing atom upon excitation. TR‐XAS data analysis is generally performed on the laser‐on minus laser‐off difference spectrum. Here, a new analysis scheme is presented for the TR‐XAS difference fitting in both the extended X‐ray absorption fine‐structure (EXAFS) and the X‐ray absorption near‐edge structure (XANES) regions. R‐space EXAFS difference fitting could quickly provide the main quantitative structure change of the first shell. The XANES fitting part introduces a global non‐derivative optimization algorithm and optimizes the local structure change in a flexible way where both the core XAS calculation package and the search method in the fitting shell are changeable. The scheme was applied to the TR‐XAS difference analysis of Fe(phen)₃ spin crossover complex and yielded reliable distance change and excitation population.     

In situ XAFS experiments using a microfluidic cell: application to initial growth of CdSe nanocrystals

The design and performance of a compact fluorescense XAFS apparatus equipped with a microfluidic cell for in situ studies of nanoparticles are described. CdSe nanoparticles were prepared by solution reaction starting from trioctylphosphine‐Se. Time‐resolved experiments were performed by precisely controlling the reactor coordinates (x,y), allowing the synchrotron X‐ray beam to travel along a reactor channel, covering nucleation and initial growth of nanoparticles. Detailed analysis of EXAFS data combined with UV–vis spectra allow reliable estimation of particle size and density in the initial growth that cannot be accessible by conventional optical techniques based on a long‐range order. The Se K‐XANES spectra are interpreted by multi‐scattering calculations providing bond formation kinetics consistent with the EXAFS data.     

X‐ray K absorption spectral studies of copper (II) hydroxamic acid mixed ligand complexes

X‐ray absorption spectra at the K‐edge of copper have been studied in copper mixed ligand complexes having hydroxamic acid as one of the ligands. The X‐ray absorption spectra have been recorded at BL‐8 Dispersive Extended X‐ray Absorption Fine Structure (EXAFS) beamline at the 2.5‐GeV INDUS‐2 Synchrotron Source, RRCAT, Indore, India. The data obtained has been processed using EXAFS data analysis program Athena. The energies of the K absorption edge, chemical shifts, edge‐widths and shifts of the principal absorption maximum in the complexes have been determined and discussed. The chemical shift data have been utilized to estimate effective nuclear charge on the absorbing atom. The normalized EXAFS spectra have been Fourier transformed. The position of the first peak in the Fourier transform gives the value of first shell bond length, which is shorter than the actual bond length as a result of energy dependence of the phase factors in the sine function of the EXAFS equation. This distance is thus the phase‐uncorrected bond length and has also been determined by Lytle, Sayers and Stern's (LSS) graphical method. The results obtained from LSS and the Fourier transformation methods are comparable with each other. The first shell bond length has also been estimated by Lytle's and Levy's methods from the EXAFS data. Copyright © 2014 John Wiley & Sons, Ltd.     

Surface EXAFS via differential electron yield

Surface‐sensitive analysis via extended X‐ray absorption fine‐structure (EXAFS) spectroscopy is demonstrated using a thickness‐defined SiO₂ (12.4 nm)/Si sample. The proposed method exploits the differential electron yield (DEY) method wherein Auger electrons escaping from a sample surface are detected by an electron analyzer. The DEY method removes local intensity changes in the EXAFS spectra caused by photoelectrons crossing the Auger peak during X‐ray energy sweeps, enabling EXAFS analysis through Fourier transformation of wide‐energy‐range spectral oscillations. The Si K‐edge DEY X‐ray absorption near‐edge structure (XANES) spectrum appears to comprise high amounts of SiO₂ and low Si content, suggesting an analysis depth, as expressed using the inelastic mean free path of electrons in general electron spectroscopy, of approximately 4.2 nm. The first nearest neighbor (Si—O) distance derived from the Fourier transform of the Si K‐edge DEY‐EXAFS oscillation is 1.63 Å. This value is within the reported values of bulk SiO₂, showing that DEY can be used to detect a surface layer of 12.4 nm thickness with an analysis depth of approximately 4.2 nm and enable `surface EXAFS' analysis using Fourier transformation.     

Local structure of uncapped and capped InGaN/GaN quantum dots

The local structure around the indium atoms in uncapped and capped In_xGa_1?xN quantum dots has been studied by In K‐edge extended X‐ray absorption fine structure (EXAFS) spectroscopy. The samples were grown by metal organic vapour phase epitaxy. The EXAFS was successfully applied to study the structural properties of buried quantum dots which are not optically active. The analysis revealed that capping the quantum dots with GaN does not affect the bond distances of the In—N and In—Ga, but makes the In—In distance shorter by 0.04 Å.     

An attempt to analyze the bark disease in Havea brasiliensis using X‐ray absorption near‐edge spectroscopy

The X‐ray absorption near‐edge spectroscopy (XANES) technique has been used to determine the chemical change of elements induced by bark diseases in Havea brasiliensis (rubber latex tree). The results show the good sensitivity of in situ XANES for characterizing the chemical structure of phosphorus, sulfur, potassium and calcium in healthy and diseased Havea brasiliensis. Important information for understanding the bark disease involved in the sulfur metabolism of plants was also obtained from XANES.     

X‐ray absorption spectroscopy analysis of Ru in La2RuO5

The compound La₂RuO₅ was examined by the x‐ray absorption spectroscopy (XAS) methods, x‐ray absorption near edge structure (XANES) and extended x‐ray absorption fine structure (EXAFS). XANES technique was used to probe directly the average valence of Ru atoms in the compound. The energy shift of the Ru K‐edge in the XANES signal gave the average Ru valence state as 4.0 ± 0.1. EXAFS analysis provided, by yielding directly the interatomic distances and coordination numbers, the first information on the Ru atom neighborhood, on which the model for the Rietveld refinement of the unit cell of the new compound was devised. Finally, the local structure around the Ru atoms from the refinement was used in the FEFF6 code for a model EXAFS spectrum. The very good quantitative agreement with the measured spectrum proves that the refined crystal structure contains no systematic defects in the vicinity of Ru atoms. This result, together with the valence obtained from XANES, strongly confirms the proposed La₂RuO₅ stoichiometry. Copyright © 2007 John Wiley & Sons, Ltd.     

Data analysis method to achieve sub‐10 pm spatial resolution using extended X‐ray absorption fine‐structure spectroscopy

Obtaining sub‐10 pm spatial resolution by extended X‐ray absorption fine structure (EXAFS) spectroscopy is required in many important fields of research, such as lattice distortion studies in colossal magnetic resistance materials, high‐temperature superconductivity materials etc. However, based on the existing EXAFS data analysis methods, EXAFS has a spatial resolution limit of π/2Δk which is larger than 0.1 Å. In this paper a new data analysis method which can easily achieve sub‐10 pm resolution is introduced. Theoretically, the resolution limit of the method is three times better than that normally available. The method is examined by numerical simulation and experimental data. As a demonstration, the LaFe_1–xCr_xO₃ system (x = 0, 1/3, 2/3) is studied and the structural information of FeO₆ octahedral distortion as a function of Cr doping is resolved directly from EXAFS, where a resolution better than 0.074 Å is achieved.     

XAFS investigations of copper(II) complexes with tetradentate Schiff base ligands

X‐ray absorption fine structure spectra have been investigated at the K‐edge of copper in copper(II) salen/salophen complexes: [Cu(salen)] (1), [Cu(salen)CuCl₂].H₂O (2), [Cu(salophen)] (3) and [Cu(salophen) CuCl₂].H₂O (4), where salen^2? = N,N′‐ethylenebis (salicylidenaminato); salophen^2? = o‐phenylenediaminebis(salicylidenaminato). Complexes 1 and 3 are supposed to have one type of copper centers (called (Cu1)) and complexes 2 and 4 two types of copper centers (called (Cu1) and (Cu2)) having different coordination environments and geometries. A theoretical model has been generated using the available crystallographic data of complex 1 and it has been used for analysis of the extended X‐ray absorption fine structure (EXAFS) data of the four complexes to obtain the structural parameters for (Cu1) center. For this center, the obtained Cu–Cu distance (3.2 Å) verifies the binuclear nature of all the complexes. For determining the coordination geometry around (Cu2) center in 2 and 4, a theoretical model has been generated using the crystal structure of a Cu(II) complex, [Cu(C₁₆H₁₂N₂O₂Cl₂)]. This theoretical model has been fitted to the EXAFS data of 2 and 4 to obtain the structural parameters for (Cu2) center. The present analysis shows that (Cu1) center has square pyramidal geometry involving 2N and 3O donor atoms, whereas (Cu2) center has distorted tetrahedral geometry with 2O and 2Cl donor atoms. The values of the chemical shifts and presence of typical Cu(II) X‐ray absorption near‐edge spectroscopy features suggest that copper is in the +2 oxidation state in all these complexes. The intensity of ls → 3d pre‐edge feature has been used to investigate the geometry and binuclear nature of the complexes. Copyright © 2012 John Wiley & Sons, Ltd.     

Investigating structural aspects to understand the putative/claimed non‐toxicity of the Hg‐based Ayurvedic drug Rasasindura using XAFS

XANES‐ and EXAFS‐based analysis of the Ayurvedic Hg‐based nano‐drug Rasasindura has been performed to seek evidence of its non‐toxicity. Rasasindura is determined to be composed of single‐phase α‐HgS nanoparticles (size ～24 nm), free of Hg⁰ or organic molecules; its structure is determined to be robust (<3% defects). The non‐existence of Hg⁰ implies the absence of Hg‐based toxicity and establishes that chemical form, rather than content of heavy metals, is the correct parameter for evaluating the toxicity in these drugs. The stable α‐HgS form (strong Hg—S covalent bond and robust particle character) ensures the integrity of the drug during delivery and prevention of its reduction to Hg⁰ within the human body. Further, these comparative studies establish that structural parameters (size dispersion, coordination configuration) are better controlled in Rasasindura. This places the Ayurvedic synthesis method on par with contemporary techniques of nanoparticle synthesis.     

Scanning transmission X‐ray microscopy probe for in situ mechanism study of graphene‐oxide‐based resistive random access memory

Here, an in situ probe for scanning transmission X‐ray microscopy (STXM) has been developed and applied to the study of the bipolar resistive switching (BRS) mechanism in an Al/graphene oxide (GO)/Al resistive random access memory (RRAM) device. To perform in situ STXM studies at the C K‐ and O K‐edges, both the RRAM junctions and the I₀ junction were fabricated on a single Si₃N₄ membrane to obtain local XANES spectra at these absorption edges with more delicate I₀ normalization. Using this probe combined with the synchrotron‐based STXM technique, it was possible to observe unique chemical changes involved in the BRS process of the Al/GO/Al RRAM device. Reversible oxidation and reduction of GO induced by the externally applied bias voltages were observed at the O K‐edge XANES feature located at 538.2 eV, which strongly supported the oxygen ion drift model that was recently proposed from ex situ transmission electron microscope studies.     

Early commissioning results for spectroscopic X‐ray Nano‐Imaging Beamline BL 7C sXNI at PLS‐II

For spectral imaging of chemical distributions using X‐ray absorption near‐edge structure (XANES) spectra, a modified double‐crystal monochromator, a focusing plane mirrors system and a newly developed fluorescence‐type X‐ray beam‐position monitoring and feedback system have been implemented. This major hardware upgrade provides a sufficiently stable X‐ray source during energy scanning of more than hundreds of eV for acquisition of reliable XANES spectra in two‐dimensional and three‐dimensional images. In recent pilot studies discussed in this paper, heavy‐metal uptake by plant roots in vivo and iron's phase distribution in the lithium–iron–phosphate cathode of a lithium‐ion battery have been imaged. Also, the spatial resolution of computed tomography has been improved from 70 nm to 55 nm by means of run‐out correction and application of a reconstruction algorithm.     

Polycapillary‐optics‐based micro‐XANES and micro‐EXAFS at a third‐generation bending‐magnet beamline

A focusing system based on a polycapillary half‐lens optic has been successfully tested for transmission and fluorescence µ‐X‐ray absorption spectroscopy at a third‐generation bending‐magnet beamline equipped with a non‐fixed‐exit Si(111) monochromator. The vertical positional variations of the X‐ray beam owing to the use of a non‐fixed‐exit monochromator were shown to pose only a limited problem by using the polycapillary optic. The expected height variation for an EXAFS scan around the Fe K‐edge is approximately 200 µm on the lens input side and this was reduced to ～1 µm for the focused beam. Beam sizes (FWHM) of 12–16 µm, transmission efficiencies of 25–45% and intensity gain factors, compared with the non‐focused beam, of about 2000 were obtained in the 7–14 keV energy range for an incoming beam of 0.5 × 2 mm (vertical × horizontal). As a practical application, an As K‐edge µ‐XANES study of cucumber root and hypocotyl was performed to determine the As oxidation state in the different plant parts and to identify a possible metabolic conversion by the plant.     

XAFS study of aqua (diethylenetriamine)(isonicotinato)‐copper(II) complex – inference of square‐pyramidal geometry

X‐ray absorption fine structure (XAFS) of aqua (diethylenetriamine) (isonicotinato) copper(II) complex has been investigated. The crystal structure of the complex is unavailable. The X‐ray absorption near edge structure (XANES) spectrum of the complex has been compared with those of the standard compounds to estimate the coordination geometry and oxidation state of copper. This study indicates that the copper center has +2 oxidation state and that it may be penta or hexa‐coordinated. The spectrum has been further compared with those of the earlier studied complexes having square‐pyramidal geometry. Similarity has been observed between them indicating that the complex should have penta‐coordinated square‐pyramidal geometry. The same inference has been obtained from the study of the characteristic pre‐edge and XANES features. This inference has been further examined by analysis of the extended X‐ray absorption fine structure (EXAFS) spectrum. The experimental EXAFS data has been fitted with three different theoretical models for tetra‐coordination, penta‐coordination, and hexa‐coordination. The tetra model is unable to provide a satisfactory fit so it has been ignored. Although the penta and hexa models give comparable results, the parameters obtained indicate that the penta‐coordinated model fits better to the experimental data than the hexa‐coordinated model. This penta‐coordinated model has been further fitted to the experimental data by allowing the coordination number to vary. Such fitting also shows that the penta‐coordinated model is a satisfactory model for the studied complex. The obtained values of mean‐square disorder indicate that the force constants are different for the equatorial and axial bonds. Copyright © 2014 John Wiley & Sons, Ltd.