Employed X-ray Absorption Spectroscopy Research Articles

Direct Observation of Orbital Driven Strong Interlayer Coupling in Puckered Two-Dimensional PdSe2.

Interlayer coupling between individual unit layers is known to be critical in manipulating the layer-dependent properties of two-dimensional (2D) materials. While recent studies have revealed that several 2D materials with significant degrees of interlayer interaction (such as black phosphorus) show strongly layer-dependent properties, the origin based on the electronic structure is drawing intensive attention along with 2D materials exploration. Here, the direct observation of a highly dispersive single electronic band along the interlayer direction in puckered 2D PdSe2 as an experimental hallmark of strong interlayer couplings is reported. Remarkably large band dispersion along the kz -direction near Fermi level, which is even wider than the in-plane one, is observed by the angle-resolved photoemission spectroscopy measurement. Employing X-ray absorption spectroscopy and density functional theory calculations, it is revealed that the strong interlayer coupling in 2D PdSe2 originates from the unique directional bonding of Pd d orbitals associated with unexpected Pd 4d9 configuration, which consequently plays a decisive role for the strong layer-dependency of the band gap.

X-ray absorption spectroscopy and neutron-diffraction study of persistent luminescent Sr2MgSi2O7 glass-ceramics

Glass-ceramics based on the Eu2O3/Dy2O3-doped Sr2MgSi2O7 phosphor have been obtained from sintering and crystallization of glass powders. The doped parent glasses show red emission under excitation of UV light. After thermal treatment, the corresponding glass-ceramics show persistent blue emission which emanates from Eu2+ incorporated in the crystals; red emission also occurs from Eu3+ in the residual vitreous phase.The aim of this work is to investigate the structure of defects such as oxygen and strontium vacancies in the Sr2MgSi2O7 crystals and the environment, oxidation state and coordination of the Eu and Dy ions in the glass-ceramics, their partitioning between crystalline and glassy phase and their influence on luminescence properties, employing X-ray absorption spectroscopy (XAS) and neutron diffraction (ND) as investigative structural tools. We suggest that the luminescence mechanism involves the presence of shallow electron traps that are abruptly emptied for temperatures above 100 K. The most persistent luminescence is likely due to an effective concentration of Eu2+ in the dendritic-shaped crystals in one of the particular compositions, and with a suitable level of Sr vacancies, even though Sr vacancies are most likely formed in all the investigated samples.

Strain healing of spin–orbit coupling:a cause for enhanced magnetic moment in epitaxial SrRuO3 thin films

Enhanced magnetic moment and coercivity in SrRuO3 thin films are significant issues for advanced technological usages and hence are researched extensively in recent times. Most of the previous reports on thin films with enhanced magnetic moment attributed it to the high spin state. Our magnetization results show high magnetic moment of 3.3 μB/Ru ion in the epitaxial thin films grown on LSAT substrate against 1.2 μB/Ru ion observed in bulk compound. Contrary to the previous reports the Ru ions are found to be in low spin state and the orbital moment is shown to be contributing significantly in the enhancement of magnetic moment. We employed x-ray absorption spectroscopy and resonant valance band spectroscopy to probe the spin state and orbital contributions in these films. The existence of strong spin–orbit coupling responsible for the de-quenching of the 4d orbitals is confirmed by the observation of the non-statistical large branching ratio at the Ru M2,3 absorption edges. X-ray magnetic circular dichroism studies performed at the Ru M2,3 edges provided direct evidence of significant contribution of orbital moment in the film grown on LSAT. The relaxation of orbital quenching by strain engineering provides a new tool for enhancing magnetic moment and strain disorder is shown to be an efficient mean to control the spin–orbit coupling.

Visible light responsive La and Fe co-doped NaTaO3 photocatalysts: Local structure around dopants

NaTaO3 doubly doped with La and transition metal cations has been of interest for photocatalytic water splitting under visible light. However, there is not yet convincing evidence to propose a plausible structure for the dopant pair incorporated in the NaTaO3 host. In this study, efforts were made to reveal the local structure of La and Fe cations doped in NaTaO3. For this purpose, we employed X-ray absorption spectroscopy, including extended X-ray absorption fine structure and X-ray absorption near-edge structure. It was revealed that upon double doping, La cations occupied the Na-sites, whereas Fe cations occupied the Ta-sites. A LaFeO3-NaTaO3 solid solution was formed accordingly, together with a trace amount of iron oxide impurities. The solid solution was able to absorb visible light to produce electrons. A volcano-shaped pattern of the electron population with respect to the dopant concentration was present, in which 2 + 2 mol% gave the highest electron population.

Magnetic dichroism in the Kondo insulator SmB6

Samarium hexaboride is a topological Kondo insulator, with metallic surface states manifesting from its insulating band structure. Since the insulating bulk itself is driven by strong correlations, both the bulk and surface host compelling magnetic and electronic phenomena. We employed X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD) at the Sm M$_{4,5}$ edges to measure surface and bulk magnetic properties of Sm$^{2+}$ and Sm$^{3+}$ within SmB$_6$. We observed anti-alignment to the applied field of the Sm$^{3+}$ magnetic dipole moment below $T = 75$ K and of the total orbital moment of samarium below 30 K. The induced Sm$^{3+}$ moment at the cleaved surface at 8 K and 6 T implies 1.5% of the total Sm as magnetized Sm$^{3+}$. The field dependence of the Sm$^{3+}$ XMCD dichorism at 8 K is diamagnetic and approximately linear. The bulk magnetization at 2 K is however driven by Sm$^{2+}$ Van Vleck susceptibility as well as 1% paramagnetic impurities with $\mu_{\rm Eff} = 5.2(1)~\mu_{\rm B}$. This indicates diamagnetic Sm$^{3+}$ is compensated within the bulk. The XAS and XMCD spectra are weakly affected by Sm vacancies and carbon doping while XAS is strongly affected by polishing.

Magnetic properties of single rare-earth atoms on graphene/Ir(111)

We employed x-ray absorption spectroscopy and x-ray magnetic circular dichroism to study the magnetic properties of single rare-earth (RE) atoms (Nd, Tb, Dy, Ho, and Er) adsorbed on the graphene/Ir(111) surface. The interaction of RE atoms with graphene results for Tb in a trivalent state with $4{f}^{n\ensuremath{-}1}$ occupancy, and in a divalent state with $4{f}^{n}$ occupancy for all other studied RE atoms ($n$ corresponds to the $4f$ occupancy of free atoms). Among the studied RE on graphene/Ir(111), Dy is the only one that shows magnetic hysteresis and remanence at 2.5 K. By comparing measured spectra and magnetization curves with multiplet calculations, we determine the energy diagram of the magnetic states and show for each element the magnetization reversal process that determines the timescale of its magnetic bistability.

Heteroepitaxial growth of self-assembled BaTiO3-Fe2O3 nanostructures

The self-assembled growth of vertically aligned nanostructures of BaTiO3 in the matrix of Fe2O3 is described. The arrays of well controlled and systematically ordered nanostructures of composite phases were grown on MgO and SrTiO3 single crystalline substrate by using the pulse laser deposition technique under optimized conditions. The phase purities of the composite structures were characterized by using x-ray diffraction and Raman spectroscopy. The surface topographical and morphological measurements were carried out for confirming the growth on nanostructures in the composite thin films. The ferroelectric properties of these composite films were probed by electrical polarization versus electric field (P-E) measurements. The electronic and magnetic properties of the composite were studied by employing x-ray absorption spectroscopy and magnetization measurements. The presence of strain state in nanostructures is found to play an important role in modifying the crystal field effects as well as the magnetic properties of the composite compound. It is shown that the vertical self-assembly of nanorods of BaTiO3 can be grown in the matrix layer of Fe2O3 on the MgO substrate by coablation.

Cysteine Addition Promotes Sulfide Production and 4-Fold Hg(II)-S Coordination in Actively Metabolizing Escherichia coli.

The bacterial uptake of mercury(II), Hg(II), is believed to be energy-dependent and is enhanced by cysteine in diverse species of bacteria under aerobic and anaerobic conditions. To gain insight into this Hg(II) biouptake pathway, we have employed X-ray absorption spectroscopy (XAS) to investigate the relationship between exogenous cysteine, cellular metabolism, cellular localization, and Hg(II) coordination in aerobically respiring Escherichia coli (E. coli). We show that cells harvested in exponential growth phase consistently display mixtures of 2-fold and 4-fold Hg(II) coordination to sulfur (Hg-S2 and Hg-S4), with added cysteine enhancing Hg-S4 formation. In contrast, cells in stationary growth phase or cells treated with a protonophore causing a decrease in cellular ATP predominantly contain Hg-S2, regardless of cysteine addition. Our XAS results favor metacinnabar (β-HgS) as the Hg-S4 species, which we show is associated with both the cell envelope and cytoplasm. Additionally, we observe that added cysteine abiotically oxidizes to cystine and exponentially growing E. coli degrade high cysteine concentrations (100-1000 μM) into sulfide. Thermodynamic calculations confirm that cysteine-induced sulfide biosynthesis can promote the formation of dissolved and particulate Hg(II)-sulfide species. This report reveals new complexities arising in Hg(II) bioassays with cysteine and emphasizes the need for considering changes in chemical speciation as well as growth stage.

Interfacial properties ofLaMnO3/LaNiO3superlattices grown along (001) and (111) orientations

We have employed x-ray absorption spectroscopy and x-ray magnetic circular dichroism (XMCD) at the Ni and Mn ${L}_{3,2}$ edges in ${\mathrm{LaMnO}}_{3}/{\mathrm{LaNiO}}_{3}$ superlattices grown along (001) and (111) orientations. Our results show a significant XMCD signal and thus the presence of magnetic moments localized on both Ni and Mn, which are ferromagnetically coupled to each other. X-ray absorption experiments reveal a charge transfer between Ni and Mn which is larger for (111) than for (001) superlattices. These results are compared to theoretical predictions for such superlattices.

Structure and speciation of chromium ions in chromium doped Fe2O3catalysts

In this paper, we report a detailed characterisation of chromium doped iron oxide catalysts using a range of techniques to establish the nature of chromium species in the near surface and bulk of iron oxide, high-temperature shift (HTS) catalysts. In particular we have employed X-ray absorption spectroscopy Cr K-edge near edge and extended fine structure data for comparison with chemical and X-ray photoelectron spectroscopy. There was excellent agreement between the techniques in terms of identification and quantification of Cr(6+) and Cr(3+) species as a function of calcination temperatures between 100 and 500 °C.

XAFS investigation of the role of orientational disorder in the stabilization of the ferromagnetic metallic phase in nanoparticles of La0.5Ca0.5MnO3

The inclusion of the contribution of Jahn–Teller distortion of MnO6 units, in addition to double-exchange, has been largely successful in explaining the magneto-transport behavior of manganites. However, our recent experiments on La0.5Ca0.5MnO3 demonstrated the limitation of these factors in explaining the radical difference between the magneto-transport properties of bulk and nanocrystalline forms. While bulk La0.5Ca0.5MnO3 exhibits insulator character (4–300 K) and an anti-ferromagnetic–ferromagnetic transition at 200 K, the nanocrystalline form stabilizes in a metallic ferromagnetic phase (4–300 K). This is counter-intuitive since large Jahn–Teller distortion, which promotes anti-ferromagnetism or insulator character, exists in the nanocrystals too (as indicated by x-ray diffraction results). In this work, we resolve this paradox by considering the role of structural disorder. Employing x-ray absorption spectroscopy, we establish that the disorder in inter-octahedral coupling is enhanced by 57% in the nanocrystals, as the octahedral units are randomly oriented with respect to each other. This orientational disorder promotes metallic ferromagnetism by destroying the stringent orbital ordering that is needed for anti-ferromagnetism and the co-operative nature of the orbital order.

Geochemical weathering increases lead bioaccessibility in semi-arid mine tailings.

Mine tailings can host elevated concentrations of toxic metal(loid)s that represent a significant hazard to surrounding communities and ecosystems. Eolian transport, capable of translocating small (micrometer-sized) particles, can be the dominant mechanism of toxic metal dispersion in arid or semiarid landscapes. Human exposure to metals can then occur via direct inhalation or ingestion of particulates. The fact that measured doses of total lead (Pb) in geomedia correlate poorly with blood Pb levels highlights a need to better resolve the precise distribution of molecularly speciated metal-bearing phases in the complex particle mixtures. Species distribution controls bioaccessibility, thereby directly impacting health risk. This study seeks to correlate Pb-containing particle size and mineral composition with lability and bioaccessibility in mine tailings subjected to weathering in a semiarid environment. We employed X-ray absorption spectroscopy (XAS) and X-ray fluorescence (XRF), coupled with sequential chemical extractions, to study Pb speciation in tailings from the semiarid Arizona Klondyke State Superfund Site. Representative samples ranging in pH from 2.6 to 5.4 were selected for in-depth study of Pb solid-phase speciation. The principle lead-bearing phase was plumbojarosite (PbFe(6)(SO(4))(4)(OH)(12)), but anglesite (PbSO(4)) and iron oxide-sorbed Pb were also observed. Anglesite, the most bioavailable mineral species of lead identified in this study, was enriched in surficial tailings samples, where Pb concentrations in the clay size fraction were 2-3 times higher by mass relative to bulk. A mobile and bioaccessible Pb phase accumulates in surficial tailings, with a corresponding increase in risk of human exposure to atmospheric particles.

Local structure of fluorescent platinum nanoclusters

Fluorescent nanoclusters synthesized from a variety of noble metals have recently been reported in the scientific literature. Various characterization methods have been employed in their study, but uncertainty still remains regarding the local structure of such materials. Herein we present the results of employing X-ray absorption spectroscopy to probe the local structure and electronic properties of platinum nanoclusters reduced and stabilized using N,N-dimethylformamide. Most notable amongst these results is an apparent lack of metallic bonding in the samples studied; in light of these findings, it is proposed that a non-metallic platinum nanocluster is instead responsible for the observed fluorescence behaviour.

Influence of Polyethyleneimine and Ammonium on the Growth of ZnO Nanowires by Hydrothermal Method

In this study, ZnO nanowire arrays were prepared using a hydrothermal method. During growth, polyethyleneimine (PEI) and ammonia were added to adjust the structure and optical properties of the ZnO nanowires. Emission analysis revealed visible photoluminescence emissions from ZnO nanowires produced under various growth conditions. To correlate the relationship between visible emissions and structural defects in ZnO nanowires, we employed X-ray absorption spectroscopy (XAS) to characterize the coordination number and bond length of the ZnO nanowires. On the basis of analytical results, we determined that the red emission is attributed to interstitial zinc defects (Zni) and the yellow emission is attributed to interstitial oxygen defects (Oi).

Mechanistic study of arsenate adsorption on lithium/aluminum layered double hydroxide

Lithium/aluminum layered double hydroxide intercalated by chloride (Li/Al LDH-Cl) was considered as a superior adsorbent for anionic compounds. This study aimed to establish the adsorption behavior of arsenate on Li/Al LDH-Cl by employing X-ray absorption spectroscopy (XAS) and adsorption kinetics. The XAS analysis demonstrated that inner-sphere complexes were the dominant arsenate adsorption configurations on the planar surfaces and edges of Li/Al LDH-Cl. Based on a kinetic study, arsenate adsorption on Li/Al LDH-Cl could be separated into fast and slow reactions. This biphasic arsenate adsorption behavior was partially attributable to: (i) two different adsorption sites associated with Li, exposing on planar surfaces, and Al, existing on the edges of double hydroxyl layers, and (ii) micropore adsorption sites within the Li/Al LDH-Cl surfaces. Activation energies derived by the Arrhenius equation indicated that the diffusion process was the rate-limiting step of arsenate adsorption on Li/Al LDH-Cl.

Structural Investigation of Lanthanoid Coordination: a Combined XANES and Molecular Dynamics Study

This is the first systematic study exploring the potentiality of the X-ray absorption near edge structure (XANES) technique as a structural tool for systems containing lanthanoid(III) ions. A quantitative analysis of the XANES spectra at the K- and L(3)-edges has been carried out for three hydrated lanthanoid(III) ions, namely, Yb, Nd, and Gd, in aqueous solution and in the isostructural trifluoromethanesulfonate salts. The structural and dynamic properties of the hydrated lanthanoid(III) ions in aqueous solution have been investigated by a combined experimental-theoretical approach employing X-ray absorption spectroscopy and molecular dynamics (MD) simulations. This method allows one to perform a quantitative analysis of the XANES spectra of ionic solutions using a proper description of the thermal and structural fluctuations. XANES spectra have been computed starting from the MD trajectory, without carrying out any minimization in the structural parameter space. A comparative K- and L(3)-edge XANES data analysis is presented, demonstrating the clear advantages of the L(3)-edge XANES analysis over the K-edge studies for structural investigations of lanthanoid compounds. The second hydration shells provide a detectable contribution to the L(3)-edge spectra while the K-edge data are insensitive to the more distant coordination spheres because of the strong damping and broadening of the signal caused by the extremely large core hole widths. The XANES technique has been found to be a new valuable tool for the structural characterization of metal complexes both in the solid and in the liquid state, especially in the presence of low symmetry.

X-ray absorption spectroscopy measured in resonant X-ray scattering mode: How unnatural is the resolution beyond the natural width?

The aim of this work is to give a clarifying view of the ‘super-high resolution’ scheme employing X-ray absorption spectroscopy in the resonant scattering mode. If the idea is not applied carefully, unrealistic results, as ‘perfect’ resolution, can be produced. The background of the problem is that the technique of extracting lifetime-broadening-free spectra assumes a known location of the resonance in the two-dimensional map of X-ray scattering spectra. The accuracy of the latter is, however, related to the lifetime broadening of the core-excited state. Thus, a compromise must be found between the uncertainty in the position of the X-ray absorption peak and the width of the desired absorption profile.

Integrated experimental and theoretical approach for the structural characterization of Hg2+ aqueous solutions

The structural and dynamic properties of the solvated Hg2+ ion in aqueous solution have been investigated by a combined experimental-theoretical approach employing x-ray absorption spectroscopy and molecular dynamics (MD) simulations. This method allows one to perform a quantitative analysis of the x-ray absorption near-edge structure (XANES) spectra of ionic solutions using a proper description of the thermal and structural fluctuations. XANES spectra have been computed starting from the MD trajectory, without carrying out any minimization in the structural parameter space. The XANES experimental data are accurately reproduced by a first-shell heptacoordinated cluster only if the second hydration shell is included in the calculations. These results confirm at the same time the existence of a sevenfold first hydration shell for the Hg2+ ion in aqueous solution and the reliability of the potentials used in the MD simulations. The combination of MD and XANES is found to be very helpful to get important new insights into the quantitative estimation of structural properties of disordered systems.

Combined Experimental and Theoretical Approach to the Study of Structure and Dynamics of the Most Inert Aqua Ion [Ir(H2O)6]3+ in Aqueous Solution

Quantitative determination of the hydration structure of hexaaquairidium(III), [Ir(H2O)6]3+, in aqueous solution, the most inert aqua ion known, has been achieved for the first time by a combined experimental-theoretical approach employing X-ray absorption spectroscopy and molecular dynamics (MD) simulations. The Ir LIII-edge extended X-ray absorption fine structure (EXAFS) spectrum and LI-, LII-, and LIII-edge X-ray absorption near-edge structure (XANES) spectra of three concentrations of [Ir(H2O)6]3+ in perchloric acid media were measured. To carry out classical MD simulations of the aqua ion in water, a new set of first-principles Ir-H2O intermolecular potentials, based on the hydrated ion concept, has been developed. Structural, dynamics, and energetic properties have been obtained from the analysis of the statistical trajectories generated. The Ir-O radial distribution function shows two well-defined peaks at 2.04 +/- 0.01 and 4.05 +/- 0.05 A corresponding to the first and second hydration shell, respectively; the fundamental frequencies for the aqua ion in water are well reproduced by the MD simulation, and its dynamic properties are similar to the experimental values corresponding to other hexahydrated trivalent ions. Particular attention has been devoted to the experimental determination of the second hydration shell. It has been found that contrarily to what expected on the basis of the inertness of the Ir3+ aquaion, the detection of the second hydration shell by EXAFS for this cation is more difficult than for others less inert aqua ions such as Cr3+ or Rh3+. But when combined with MD simulations it is possible to confirm the coordination distance for this shell at 4.1 +/- 0.1 A. In addition, the computation of LI, LII and LIII XANES spectra were carried out using the structural information obtained from MD. These computations allowed the assignment of special features of the spectra to the second hydration shell on a quantitative basis. Therefore, interestingly XANES spectra have given a stronger support to the second hydration shell than EXAFS. The fit of the LIII-edge EXAFS gives an accurate description of the first hydration shell structure in aqueous solution. The value for Ir-O first shell is 2.04 +/- 0.01 A. The statistical information available with the MD results has allowed the analysis of the standard deviation associated with the computation of the XANES spectrum. It is shown that the standard deviation increases with the number of hydration shells and this increase is nonuniform along the average spectrum.

Annealing temperature dependence of the electronic structure of La0.7Ca0.3MnO3−δ perovskite

The local atomic and electronic structures of La0.7Ca0.3MnO3 (LCMO) prepared by solid-state reaction was investigated at different annealing temperatures (TA). We employed x-ray absorption spectroscopy and x-ray photoemission spectroscopy to understand the local atomic and electronic structures of the LCMO system. Both the atomic and electronic structures due to the oxygen deficiency in LCMO turn out to change with increasing TA. Especially, the eg ↑ states, leading to the ferromagnetic interaction, are reduced by the diminution of Mn 3d–O 2p hybridization with increasing TA.