X-ray Absorption Fine-structure Spectroscopy Data Research Articles

Beagle: a near-edge X-ray absorption fine-structure spectroscopy data processing solution for beamline experiments at Pohang Accelerator Laboratory.

Near-edge X-ray absorption fine-structure (NEXAFS) spectroscopy is a powerful tool for identifying chemical bonding states at synchrotron radiation facilities. Advances in new materials require researchers in both academia and industry to measure tens to hundreds of samples during the available beam time on a synchrotron beamline, which is typically allocated to users. Automated measurement methods, along with analysis software, have been developed for beamlines. Automated measurements facilitate high-throughput experiments and accumulate vast amounts of measured spectral data. The analysis software supports various functions for analyzing the experimental data; however, these analysis methods are complicated, and learning them can be time-consuming. To process large amounts of spectral data, a new analysis software, dedicated to NEXAFS spectroscopy, that is easy to use and can provide results in a short time is desired. Herein, the development of Beagle is described, software calculating molecular orientation from NEXAFS spectroscopy data that can report results in a short time comparable with that required to measure one sample at the beamline. It was designed to progress in a single sequence from data loading to the printing of the results with a `click of a button'. The functions of the software include recognizing the dataset, correcting the background, normalizing the plot, calculating the electron yield and determining the molecular orientation. The analysis results can be saved as {\tt{.txt}} files (spectral data), {\tt{.pdf}} files (graphic images) and Origin files (spectral data and graphic images).

Active Sites in Ni2P/USY Catalysts for the Hydrodeoxygenation of 2-Methyltetrahydrofuran

A series of nickel phosphide catalysts supported on ultrastable Y zeolites USY (Si/Al = 40) with microporous/mesoporous structure were prepared by impregnation and temperature-programmed reduction and were studied for the hydrodeoxygenation (HDO) of 2-methyltetrahydrofuran (2-MTHF). The loading of the active phase was varied from 0.58, 1.16, and 1.74 to 2.3 mmol (g support)−1 and the corresponding samples were denoted as 0.5, 1.0, 1.5, and 2.0 Ni2P/USY. The two lowest loading supports did not show X-ray diffraction (XRD) lines, but X-ray absorption fine-structure spectroscopy (XAFS) indicated the formation of a Ni2P phase, with low Ni–Ni coordination, consistent with high dispersion. The two highest loading supports showed XRD patterns typical of Ni2P, and XAFS indicated similar bond distances to bulk Ni2P and high Ni–Ni coordination. Furthermore, the XAFS data indicated that the low-loading samples had shorter bond distances and more Ni in square-pyramidal coordination compared to the high loading samples and the reference Ni2P material, suggesting that there were differences in structural properties in the samples. This likely was due to preferred termination of the small crystallites with pyramidal Ni sites. The HDO of 2-MTHF was studied at 0.5 MPa and 513–593 K and the main products were n-pentane and n-butane for all catalysts. The low-loading samples showed higher turnover frequency (based on sites titrated by CO chemisorption), and this was attributed to the higher intrinsic activity of the pyramidal Ni sites. In addition, the low-loading samples showed higher selectivity to n-pentane, and this was attributed to lower C–C hydrogenolysis type reactions, which are favored by metallic ensembles as found in the high-loading samples.

EXAFS analysis of cadmium(II) adsorption to kaolinite

The local molecular structure of Cd sorbed to kaolinite was examined with X-ray absorption fine structure spectroscopy (XAFS) over a range of pH and initial Cd solution concentrations, ([Cd aq] in). Quantitative analysis of kaolinite samples with [Cd aq] in of 100 μM and pH 7, shows a single hydration sphere around Cd, consistent with an outer-sphere adsorption complex. At pH 9, a Cd–M coordination (where M stands for Si or Al) at second shell distance indicates the formation of inner-sphere adsorption complex(es) on edge sites. Although adsorption to Al edge sites is probably preferred, the similarity in atomic numbers between Al and Si and the small difference in bond lengths prevents us from distinguishing complexation to Si- versus Al-edge sites based on the XAFS data. The possibility of precipitation of Cd solid phases was ruled out due to similar local environment around Cd in samples prepared under air (where carbonate species would be present) and N 2 atmospheres, and the absence of a Cd–Cd coordination characterizing a Cd-bearing solid phase. Finally, analysis of kaolinite samples with [Cd aq] in of 3 μM, 10 μM, and 100 μM at pH 9 shows a consistent change in Cd–M distance and Debye–Waller factor as a function of [Cd aq] in, suggesting a change in the type of inner-sphere complex being formed as a function of loading. Whether this means that there is a change in the relative importance of Al versus Si edge sites with loading, or whether adsorption of Cd to edge sites alters binding to other nearby sites, or some other factor, has not been determined at this time.

The local structural characterization of the inactive clusters in B, BF2 and BF3 implanted Si wafers using X-ray techniques

Characterization of the inactive clusters formed by high dose implantation silicon are one of the crucial topics in the semiconductor industry. Analytical techniques, which could provide quantitative information on the detailed description of the complexes that are formed at dopant concentrations above solid solubility levels, are valuable to post-implantation researchers. Previously, X-ray absorption fine-structure spectroscopy (XAFS) studies of the As and Sb implanted Si wafers revealed not only that of the chemical nature of clusters, but also the arsenic case, the ratio of the precipitated versus the substitutional form of the As in the system [1,2]. In this study, we have used F K-edge and Ge K-edge (for the Ge pre-amorphized wafers) XAFS in order to probe the nature and of the clusters in B, BF2 and BF3 implanted Si wafers. Implants with various doses and implant energies were subjected to various annealing techniques such as laser annealing, spike annealing, solid state phase epitaxy, and flash annealing and the evolution of the spectral features of the clusters were followed upon annealing. The theoretical multiple scattering XAFS calculations were performed in order to correlate the spectral features in the XAFS data with the near-neighbor configuration around the main absorbing atom. The identification and the relative weight of the clusters after different annealing conditions will be presented in order to provide insight to the optimum annealing conditions in these systems.

Determining the Ratio of the Precipitated versus Substituted Arsenic by XAFS and SIMS in Heavy Dose Arsenic Implants in Silicon

ABSTRACTDoping silicon with arsenic by ion implantation above the solid solubility level leads to As clusters and/or precipitates in the form of monoclinic SiAs causing electrical deactivation of the dopant. Information on the local structure around the As atom, and the As concentration depth profiles is important for the implantation and annealing process in order to reduce the precipitated As and maximize the electrically activated As. In this study, we determined the local As structure and the precipitated versus substituted As for As implants in CZ (001) Si wafers, with implant energies between 20 keV and 100 keV, and implant doses ranging from 1 × 1015/cm2 to 1 × 1018/cm2. The samples were subjected to different thermal annealing conditions. We used secondary ion mass spectrometry (SIMS) and UT- MARLOWE simulations to determine the region where the As-concentration is above the solid solubility level. By x-ray absorption fine structure spectroscopy (XAFS), we probed the structure of the local environment around As. XAFS being capable of probing the short-range order in crystalline and amorphous materials provides information on the number, distance and chemical identity of the neighbors of the main absorbing atom. Using Fourier analysis, the coordination numbers (N) and the nearest-neighbor distances (R) to As atoms in the first shell were extracted from the XAFS data. When As precipitates as monoclinic SiAs, the nearest-neighbor distances and coordination numbers are ∼2.37 Å and ∼3, as opposed to ∼2.40 Å and ∼4 when As is substitutional. Based on this information, the critical implant dose where the precipitation/clustering of As starts, and the ratio of the substitutional versus cluster/precipitate form As in the samples were determined.

In Situ XAFS of the LixNi0.8Co0.2O2 Cathode for Lithium-Ion Batteries

ABSTRACTThe layered LiNi0.8AsCo0.2O2 system is being considered as a new cathode material for the lithium-ion battery. Compared with LiCoO2, the standard cathode formulation, it possesses improved electrochemical performance at a projected lower cost. In situ x-ray absorption fine-structure spectroscopy (XAFS) measurements were conducted on a cell cycled at a moderate rate and normal Li-ion operating voltages (3.0-4.1 V). The XAFS data collected at the Ni and Co edges approximately every 30 min. revealed details about the response of the cathode to Li insertion and extraction. These measurements on the LixNi0.8AsCo0.2O2 cathode (0.29<×<0.78) demonstrated the excellent reversibility of the cathode's short-range structure. However, the Co and Ni atoms behaved differently in response to Li insertion. This study corroborates previous work that explains the XAFS of the Ni atoms in terms of a Ni3+ Jahn-Teller ion. An analysis of the metal-metal distances suggests, contrary to a qualitative analysis of the x-ray absorption near-edge structure (XANES), that the Co3+ is oxidized to the maximum extent possible (within the Li content range of this experiment) at x = 0.47 ± 0.04, and further oxidation occurs at the Ni site.

X-ray study of atomic correlations inZn0.5Cd0.5Se0.5Te0.5epitaxial thin films

X-ray absorption fine-structure spectroscopy (XAFS) measurements have been performed at approximately 90 K to study the local structure of II-VI quaternary alloy ${\mathrm{Zn}}_{0.5}$${\mathrm{Cd}}_{0.5}$${\mathrm{Se}}_{0.5}$${\mathrm{Te}}_{0.5}$. Samples were grown by molecular beam epitaxy on (100) GaAs substrates, 4\ifmmode^\circ\else\textdegree\fi{} miscut towards the (111)B direction with a ZnTe buffer layer. The XAFS data indicate that there are more Zn-Se and Cd-Te bonds in the alloy than expected for a random arrangement. The results imply the formation of high-strain local structure and indicate that electronic pairing energies dominate over strain energy. These results are similar to the earlier observation of interlayer switching in ZnTe/CdSe superlattices. The preference of Zn-Se and Cd-Te bonding also implies a tendency to spontaneously form a composition-modulated microstructure.