X-ray Absorption Spectroscopy Spectra Research Articles

Significant role of substrate temperature on the morphology, electronic structure and thermoelectric properties of SrTiO3 films deposited by pulsed laser deposition

The effect of substrate temperature on the surface morphology, electronic structure, and thermoelectric properties of SrTiO3 (STO) films were investigated using the atomic force microscopy, X-ray diffraction, X-ray absorption spectroscopy (XAS). The STO films were grown by pulsed laser deposition by varying the substrate temperatures from room temperature to 850 °C. A strong dependence of the lattice parameters and the surface morphology lead to the 3D island growth process with substrate temperature. The variation in the substrate temperature results in the evolution of the microstructure and the enhancement of electrical and thermoelectric properties. The large power factor of 10 μWm−1K−2 at (400 K) is evident for the thin films deposited at the substrate temperature of 650 °C and is highly suitable for thermoelectric applications. The XAS spectra reveal the reduction in valence state and off-center displacement of Ti ions in TiO6 octahedron with substrate temperatures. These spectra shift to the lower energy side indicating the presence of oxygen vacancies that are responsible for the observed transport behaviour. These results show that the substrate temperature plays a significant role in tuning the electrical and thermoelectric properties due to O vacancies. Thus, the optimum growth temperature required for the deposition of the SrTiO3 films is 650 °C for thermoelectric applications.

Electronic and crystal structures of (Na1−xCax)Cr2O4 with anomalous colossal magnetoresistance

The electronic and crystal structures of $({\mathrm{Na}}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}){\mathrm{Cr}}_{2}{\mathrm{O}}_{4}$ have been studied in detail by combining photoelectron spectroscopy (PES), x-ray absorption spectroscopy (XAS), and x-ray diffraction (XRD). The PES results suggest a gap opening at the Fermi level with decreasing temperature and/or increasing the Ca concentration. The XAS spectra at the O $K$-absorption edge suggest a slight increase of the O $2p$--Cr $3d$ hybridization at low temperatures in ${\mathrm{NaCr}}_{2}{\mathrm{O}}_{4}$ and ${\mathrm{Na}}_{0.8}{\mathrm{Ca}}_{0.2}{\mathrm{Cr}}_{2}{\mathrm{O}}_{4}$, which corresponds to the appearance of the antiferromagnetic order. However, XRD showed no corresponding structural transition. The experimental results were compared with spin-resolved density functional theory (DFT) calculations. In ${\mathrm{Na}}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}{\mathrm{Cr}}_{2}{\mathrm{O}}_{4}$ the pre-edge intensity of the XAS spectra at the O $K$-absorption edge is strongly suppressed with increasing $x$, in accordance with the gap opening by Ca doping. This observation is consistent with the DFT calculation, where the density of states just above and below the Fermi level diminishes as the electron is doped into ${\mathrm{NaCr}}_{2}{\mathrm{O}}_{4}$.

Spin transition in SrFeO2 under pressure by x-ray spectroscopy

We have investigated the local Fe electronic changes in the square planar Fe(II) compound ${\mathrm{SrFeO}}_{2}$ by x-ray emission (XES) and x-ray absorption spectroscopy (XAS) through the pressure-induced spin transition up to 60 GPa. The analysis of Fe $K\ensuremath{\beta}$ XES confirms the transition under pressure from a high spin to intermediate spin state at ${P}_{c}=43$ GPa while XAS at the Fe K edge reveals the spectral signatures of the lattice compression under pressure. Simulations of the XAS spectra point to significant changes in Fe-$p$ states across ${P}_{c}$ and highlight the active role of the Sr-$p$ states at the transition.

(Invited) Direct Observation of the Sulfur-Based Chemical Species in Li-S Batteries By Spatially-Resolved X-Ray Fluorescence Microscopy and X-Ray Absorption Spectroscopy

The demands on low cost and high energy density rechargeable batteries for both transportation and large-scale stationary energy storage are attracting more and more research attention toward new battery systems such as metal, metal-sulfur, metal-air, and multivalent batteries. Since sulfur is an earth-abundant material with low cost and has a high theoretical capacity, Li-S battery chemistry has attracted significant interest during the past decade.The Li-S battery utilizes electrochemical conversion of sulfur (S8) to lithium sulfide (Li2S), going through multiple electron transfer processes associated with long- and short-chain polysulfide (Li2Sx) intermediates. It is well known that the long-chain polysulfides can be dissolved into electrolyte with aprotic organic solvents and migrated to the Li anode side. This so-called “shuttle effect” is considered as the main reason for the capacity loss and low coulombic efficiency of the Li-S system. A lot of efforts have been made on suppressing the polysulfide dissolution through new sulfur-based material and electrolyte, as well as cell engineering. Although the Li-S battery performance has been improved over the past decades, the long‐term cycling stability is still one of the major barriers for the practical application of Li–S batteries. More in‐depth studies on the fundamental understanding of the sulfur reaction mechanism and interactions among the different polysulfide species, the electrolyte, and the electrodes are still greatly needed.In this study, we utilize the spatially-resolved X-ray fluorescence (XRF) microscopy combined with X-ray absorption spectroscopy (XAS) to directly probe the inhomogeneous electrochemical/chemical reactions of the sulfur cathode, and chemical reactions of sulfur contained chemical species. The morphology changes and the redistribution of sulfur and polysulfides in both the S cathode and lithium metal anode were monitored through the XRF images, while the oxidation state of sulfur and changes of the sulfur-containing interfacial layer (i.e., SEI) were characterized through the XAS spectra. Our characterization studies on the sulfur-based cathode materials in Li-S batteries using spatially resolved XRF/XAS techniques provides critical information enabling us to understand the reaction processes and their correlation to cell cycling behavior and failure mechanisms. More details of the result will be discussed at the meeting. Acknowledgement The work done at Brookhaven National Laboratory was supported by Assistant Secretary for Energy Efficiency and Renewable Energy, Vehicle Technology Office of the U.S. DOE, through the Advanced Battery Materials Research (BMR) Program, including Battery500 Consortium under contract DE-SC0012704. This research used beamline 8-BM (TES) of the National Synchrotron Light Source II, U.S. DOE Office of Science User Facilities operated for the DOE Office of Science by Brookhaven National Laboratory under Contract No. DE-SC0012704.

Direct Evidence of Electronic Interaction at the Atomic-Layer-Deposited MoS2 Monolayer/SiO2 Interface.

The electronic structure of an atomic-layer-deposited MoS2 monolayer on SiO2 was investigated using X-ray absorption spectroscopy (XAS) and synchrotron X-ray photoelectron spectroscopy (XPS). The angle-dependent evolution of the XAS spectra and the photon-energy-dependent evolution of the XPS spectra were analyzed in detail using an ab initio electronic structure simulation. Although similar to the theoretical spectra of an ideal free-standing MoS2 ML, the experimental spectra exhibit features that are distinct from those of an ideal ML, which can be interpreted as a consequence of S-O van der Waals (vdW) interactions. The strong consensus among the experimental and theoretical spectra suggests that the vdW interactions between MoS2 and adjacent SiO2 layers can influence the electronic structure of the system, manifesting a substantial electronic interaction at the MoS2-SiO2 interface.

A low-pass filtering Fresnel zone plate for soft x-ray microscopic analysis down to the lithium K-edge region.

We have designed a new low-pass Fresnel zone plate (LPFZP) to extend soft x-ray absorption spectroscopy (XAS) to the lithium K absorption edge in a scanning transmission x-ray microscope (STXM). The performance of the LPFZP was evaluated in the STXM beamline at the UVSOR-III Synchrotron (Okazaki, Japan); the contribution of the higher-order harmonics is successfully suppressed to 0.1% of the fundamental energy, and a spatial resolution of 72 nm and an energy resolution (E/∆E) above 1000 are achieved as expected. XAS spectra of lithium are measured successfully in an electrode of a lithium-ion battery.

Regulating Electronic Status of Platinum Nanoparticles by Metal–Organic Frameworks for Selective Catalysis

Selective hydrogenation of alkynes to alkenes remains challenging in the field of catalysis due to the ease of over-hydrogenated of alkynes to alkanes. Favorably, the incorporation of metal nanopar...

Scrutinizing "Ligand Bands" via Polarized Single-Crystal X-ray Absorption Spectra of Copper(I) and Copper(II) Bis-2,2'-bipyridine Species.

High-energy resolution fluorescence-detected Cu K-edge X-ray absorption spectroscopy (XAS) and single-crystal polarized XAS data are presented toward refining the assignments of bands assigned as excitations from Cu 1s to ligand-localized molecular orbitals. These have been previously dubbed "XAS-metal-ligand charge transfer" (XAS-MLCT) bands. Data are presented for a series of [Cu(xbpy)2]n+ complexes (xbpy = 2,2'-bipyridine (1n+), 4,4'-bisamino-2,2'-bipyridine (2n+), and 4,4'-dimethoxy-2,2'-bipyridine (3n+); n = 1 and 2). Dipolar dependencies of these "XAS-MLCT" bands in both Cu1+ and Cu2+ species lead to reassignment of these features as owing their intensities primarily to Cu 1s → Cu 4p excitations. The transition densities are Cu-localized, highlighting that XAS-MLCT features in Cu XAS spectra are not "charge transfer" transitions but rather quasi-atomic transitions. Although scrutiny of the acceptor orbitals supports assignment as Cu 1s → ligand π* transitions, it ultimately appears that while the ligand orbital energetics govern the positions of these bands the intensity is conferred through a small degree of metal 4p mixing into otherwise ligand-dominated acceptor molecular orbitals.

Unveiling spin-dependent unoccupied electronic states of Co2MnGe (Ga) film via Ge (Ga) L2,3 absorption spectroscopy

X-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) spectroscopy were applied at the Ge (Ga) ${L}_{2,3}$ edge to unravel the spin-resolved unoccupied electronic states of ${\mathrm{Co}}_{2}\mathrm{Mn}\mathrm{Ge}$ (Ga). Complicated spectral features were observed in both XAS and XMCD spectra. For their interpretation, we compared the experimental XAS and XMCD spectra with the calculated Ge (Ga) $4s$ and $4d$ orbital partial density of states. The comparison enabled a qualitative explanation of the XMCD spectra as the difference between the majority- and minority-spin unoccupied density of states summed over the $4s$ and $4d$ orbitals. Our finding provides a new approach to uncover the spin-split partial density of states above the Fermi level.

Hybridization effect on the x-ray absorption spectra for actinide materials: Application to PuB4

Studying the local moment and 5$f$-electron occupations sheds insight into the electronic behavior in actinide materials. X-ray absorption spectroscopy (XAS) has been a powerful tool to reveal the valence electronic structure when assisted with theoretical calculations. However, the analysis currently taken in the community on the branching ratio of the XAS spectra generally does not account for the hybridization effects between local $f$-orbitals and conduction states. In this paper, we discuss an approach which employs the DFT+Gutzwiller rotationally-invariant slave boson (DFT+GRISB) method to obtain a local Hamiltonian for the single-impurity Anderson model (SIAM), and calculates the XAS spectra by the exact diagonalization (ED) method. A customized numerical routine was implemented for the ED XAS part of the calculation. By applying this technique to the recently discovered 5$f$-electron topological Kondo insulator PuB$_4$, we determined the signature of 5$f$-electronic correlation effects in the theoretical X-ray spectra. We found that the Pu 5$f$-6$d$ hybridization effect provides an extra channel to mix the $j=5/2$ and $7/2$ orbitals in the 5$f$ valence. As a consequence, the resulting electron occupation number and spin-orbit coupling strength deviate from the intermediate coupling regime.

Spectral Decomposition of X-ray Absorption Spectroscopy Datasets: Methods and Applications

X-ray absorption spectroscopy (XAS) today represents a widespread and powerful technique, able to monitor complex systems under in situ and operando conditions, while external variables, such us sampling time, sample temperature or even beam position over the analysed sample, are varied. X-ray absorption spectroscopy is an element-selective but bulk-averaging technique. Each measured XAS spectrum can be seen as an average signal arising from all the absorber-containing species/configurations present in the sample under study. The acquired XAS data are thus represented by a spectroscopic mixture composed of superimposed spectral profiles associated to well-defined components, characterised by concentration values evolving in the course of the experiment. The decomposition of an experimental XAS dataset in a set of pure spectral and concentration values is a typical example of an inverse problem and it goes, usually, under the name of multivariate curve resolution (MCR). In the present work, we present an overview on the major techniques developed to realize the MCR decomposition together with a selection of related results, with an emphasis on applications in catalysis. Therein, we will highlight the great potential of these methods which are imposing as an essential tool for quantitative analysis of large XAS datasets as well as the directions for further development in synergy with the continuous instrumental progresses at synchrotron sources.

A new internally heated diamond anvil cell system for time-resolved optical and x-ray measurements

We have developed a new internally heated diamond anvil cell (DAC) system for in situ high-pressure and high-temperature x-ray and optical experiments. We have adopted a self-heating W/Re gasket design allowing for both sample confinement and heating. This solution has been seldom used in the past but proved to be very efficient to reduce the size of the heating spot near the sample region, improving heating and cooling rates as compared to other resistive heating strategies. The system has been widely tested under high-temperature conditions by performing several thermal emission measurements. A robust relationship between electric power and average sample temperature inside the DAC has been established up to about 1500 K by a measurement campaign on different simple substances. A micro-Raman spectrometer was used for various in situ optical measurements and allowed us to map the temperature distribution of the sample. The distribution resulted to be uniform within the typical uncertainty of these measurements (5% at 1000 K). The high-temperature performances of the DAC were also verified in a series of XAS (x-ray absorption spectroscopy) experiments using both nano-polycrystalline and single-crystal diamond anvils. XAS measurements of germanium at 3.5 GPa were obtained in the 300 K-1300 K range, studying the melting transition and nucleation to the crystal phase. The achievable heating and cooling rates of the DAC were studied exploiting a XAS dispersive setup, collecting series of near-edge XAS spectra with sub-second time resolution. An original XAS-based dynamical temperature calibration procedure was developed and used to monitor the sample and diamond temperatures during the application of constant power cycles, indicating that heating and cooling rates in the 100 K/s range can be easily achieved using this device.

Photoelectron based soft x-ray detector for removing high order x rays.

Soft x-ray absorption spectroscopy (XAS) in the low energy region below 200 eV is important to investigate chemical and biological phenomena under an atmospheric condition since it covers K-edges of Li and B and L-edges of Si, P, S, and Cl. Monochromatic soft x rays usually include not only first order x rays but also high order x rays due to the high order diffraction of a plane grating monochromator. It is difficult to measure XAS in the low energy region under an atmospheric helium condition since the transmitted soft x rays mostly consist of the high order x rays due to the low transmission of the first order x rays. In this study, we have developed a photoelectron based soft x-ray (PBSX) detector, where the Au 4f photoelectrons emitted by the first order x rays are separated from those by the high order x rays using a difference in kinetic energies of photoelectrons. By using the PBSX detector, we have successfully obtained Si L-edge XAS spectra of the SiC and polymer/SiC films that mainly include the first order x rays by removing the major contributions of the second order x rays at the C K-edge and the fifth order x rays at the O K-edge. In the future, several physical, chemical, and biological phenomena in solution will be investigated by XAS in the low energy region with the PBSX detector.

Random forest machine learning models for interpretable X-ray absorption near-edge structure spectrum-property relationships

X-ray absorption spectroscopy (XAS) produces a wealth of information about the local structure of materials, but interpretation of spectra often relies on easily accessible trends and prior assumptions about the structure. Recently, researchers have demonstrated that machine learning models can automate this process to predict the coordinating environments of absorbing atoms from their XAS spectra. However, machine learning models are often difficult to interpret, making it challenging to determine when they are valid and whether they are consistent with physical theories. In this work, we present three main advances to the data-driven analysis of XAS spectra: we demonstrate the efficacy of random forests in solving two new property determination tasks (predicting Bader charge and mean nearest neighbor distance), we address how choices in data representation affect model interpretability and accuracy, and we show that multiscale featurization can elucidate the regions and trends in spectra that encode various local properties. The multiscale featurization transforms the spectrum into a vector of polynomial-fit features, and is contrasted with the commonly-used “pointwise” featurization that directly uses the entire spectrum as input. We find that across thousands of transition metal oxide spectra, the relative importance of features describing the curvature of the spectrum can be localized to individual energy ranges, and we can separate the importance of constant, linear, quadratic, and cubic trends, as well as the white line energy. This work has the potential to assist rigorous theoretical interpretations, expedite experimental data collection, and automate analysis of XAS spectra, thus accelerating the discovery of new functional materials.

Surfactant-modified siliceous zeolite Y for pertechnetate remediation

Separation of pertechnetate (99TcO4−) from contaminated groundwater and nuclear waste streams remains a technical challenge for the long-term management of 99TcO4− released to the environment. Herein, a siliceous zeolite Y phases(s) were modified with cationic surfactant and subsequently used to sequester TcO4− from acidic aqueous solution containing competing ions. Pristine and modified zeolite Y phase(s) were characterized with X-ray diffraction, X-ray absorption spectroscopy (XAS), sorption isotherms, zeta potential, and BET surface area measurements. Measured TcO4− sorption capacity of the modified zeolite Y (MSAV) was 337 mg/g, which was in good agreement with modeled estimates, 357 mg/g. The Langmuir and the pseudo-second-order models adequately described the sorption of TcO4−on the modified zeolite Y phases. Kinetic data suggest that liquid film diffusion and intraparticle diffusion were the rate-limiting steps governing TcO4− sorption. Analyses of the XAS spectra data revealed that ReO4−, a chemical analogue for TcO4−, was associated with the modified zeolite Y phase(s) as an inner-sphere complex of ReO4− possibly incorporated into a structural cavity, and there was no evidence of the reduced form of Re(IV). The modified zeolite Y exhibited fast sorption kinetics, significant sorption capacity, and robust uptake performance in acidic conditions (pH 3–5) and in the presence of high concentrations of competing anions. An important attribute of this remediation technology is that it sequesters TcO4− (Tc oxidized form) that exists naturally in most surface environments and waste streams, and it provides an effective alternative to the existing reductive precipitation technologies that have limited long-term environmental applications.

Iron Speciation in Different Saharan Dust Advections and Effect of the Procedural Blank on the Results From X-ray Absorption Spectroscopy and Selective Leaching Experiments

In this work, we applied X-ray Absorption Spectroscopy (XAS) and selective leaching experiments for investigating iron speciation in different dust advections collected on different unwashed quartz fiber filters. XAS analysis evidenced a predominance of Fe(III) in 6-fold coordination for Saharan dust and a trend towards Fe(II) and 4-fold coordination in the order: Saharan dust, mixed Saharan, and non-Saharan aerosol samples. The role of the sampling substrate was evaluated explicitly, including in the analysis a set of blank filters. We were able to pinpoint the possible contribution to the overall XAS spectrum of the residual Fe on quartz as the concentration decrease towards the blank value. In particular, the filter substrate showed a negligible effect on the structural trend mentioned above. Furthermore, selective leaching experiments evidenced a predominance of the residual fraction on Fe speciation and indicated the lowest Fe concentrations for which the blank contribution is <20% are 1 μ g for the first three steps of the procedure (releasing the acid-labile, reducible and oxidizable phases, respectively) and 10 μ g for the last step (dissolving the insoluble residuals).

X-ray imaging and spectroscopy of nitrogen in the SiO2/SiC interface of the 4H–SiC MOSFET trench sidewalls

The nitrogen found in the SiO2/SiC interface of SiC–MOSFET trench sidewalls, which had been introduced by annealing nitric oxide, was investigated via X-ray absorption spectroscopy (XAS) with the aid of a photoemission electron microscope. The trench sidewall, which had an m-face configuration of a micrometer order, was compared with the planar configuration of an m-face SiC wafer. The XAS spectra of the trench sidewall showed no agreement with the planar configuration, meaning that there was an observable difference in the atomic structure and environment of the nitrogen in the trench sidewall and the one in the planar SiC wafer.

Interplay of structural, optical, and magnetic properties of Ce1-xNdxO2-δ nanoparticles with electronic structure probed using X-ray absorption spectroscopy

The CeO2-δ and Ce1-xNdxO2-δ (x = 0.02, 0.04 and 0.06) nanoparticles, synthesized by microwave assisted co-precipitation method have been investigated for structural, optical, electronic structure and magnetic properties using X-ray diffraction, transmission electron microscopy, Absorption spectroscopy, surface enhanced Raman spectroscopy, X-ray absorption spectroscopy (XAS), and SQUID magnetometer. TEM and XRD measurements indicate face-centred cubic fluorite structure of CeO2 with Fm-3m space group. The Raman spectroscopy indicates that all the Nd3+-doped CeO2 samples downward shifting of F2g band with introduction of oxygen vacancies. Further, band gap decrement has been observed with incorporation of Nd3+ ions in CeO2 lattice. The XAS spectra of Ce M − edge clearly indicate changed valence state of Ce ions from Ce4+ to Ce3+ with Nd3+ doping concentrations. The formation of oxygen vacancies is also confirmed with O K- and Nd M-edges XAS analysis. It is revealed that new complexes Ce3+-VO- Nd3+ and Ce4+- VO –Ce3+ have been introduced to support FM ordering through the development of F+-centres. Therefore, it can be concluded that oxygen vacancies play a prominent role in improving the ferromagnetic and various properties of these nanomaterials, to make them suitable for many applications as UV blocker, Photocatalyst, Spintronics and Optoelectronics applications.

Vanadium 3d charge and orbital states in V2OPO4 probed by x-ray absorption spectroscopy

V 3$d$ charge and orbital states in V$_2$OPO$_4$ have been investigated by means of x-ray absorption spectroscopy (XAS). The electronic structure of V$_2$OPO$_4$ is very unique in that the charge transfer between V$^{2+}$ and V$^{3+}$ in face sharing VO$_6$ chains provides negative thermal expansion as reported by Pachoud {\it et al.} [J. Am. Chem. Soc. {\bf 140}, 636 (2018).] The near edge region of O 1$s$ XAS exhibits the three features which can be assigned to transitions to O 2$p$ mixed into the unoccupied V 3$d$ $t_{2g}$ and $e_{g}$ orbitals of V$^{2+}$ and V$^{3+}$. The V 2$p$ XAS line shape can be reproduced by multiplet calculations for a mixed valence state with V$^{2+}$ and V$^{3+}$. The polarization dependence of the O 1$s$ and V 2$p$ XAS spectra indicates V 3$d$ orbital order in which $xy$ and $yz$ (or $zx$) orbitals are occupied at the V$^{3+}$ site in the face sharing chains. The occupied $xy$ orbital is essential for the antiferromagnetic coupling between the V$^{2+}$ and V$^{3+}$ sites along the chains while the occupied $yz$ (or $zx$) orbital provides the antiferromagnetic coupling between the V$^{2+}$ and V$^{3+}$ sites between the chains.

Application of microprobe soft X-ray fluorescence and absorption spectroscopic analyses to characterize the buried multi-layered micro-structure

We have applied a recently developed scanning soft X-ray (SX) spectromicroscope to characterize the buried multi-layered micro-structure by means of microprobe X-ray fluorescence (μXRF) analyses. We could successfully visualize single element maps of O and Fe contained in a buried 500 μm square test-pattern made of SiO2 and Ni81Fe19 (permalloy) beneath 30 nm thick Au coating. For their chemical-state analyses, we measured O Kα and Fe Lα X-rays emitted from the buried micro-structure using a SX microprobe to explore the X-ray absorption spectroscopy spectra of O and Fe contained in a local area by partial fluorescence yield spectroscopy.