Extended X-ray Absorption Fine Structure Oscillations Research Articles

Analysis of Anharmonic EXAFS Spectra of Crystalline Nickel Using High-order Debye-Waller Factors

The extended X-ray absorption fine structure (EXAFS) has been developed into a powerful technique and is widely applied to determine many structural parameters and dynamic properties of materials. The EXAFS technique is now the technique of choice in many materials science investigations, and the EXAFS data analysis is being performed in many laboratories spread around the world. In this work, the anharmonic EXAFS spectra of crystalline nickel (Ni) has been analyzed based on the quantum anharmonic correlated Einstein model. The anharmonic EXAFS oscillation presented in terms of the Debye-Waller factors using the cumulant expansion approach up to the fourth-order. This calculation model has been developed from the high-order anharmonic effective potential that described the contribution of their nearest-neighbor atoms to the pair interaction potential. The analytical expressions of the anharmonic EXAFS cumulants are not only explicit forms but also satisfy all of their fundamental properties in temperature dependence. The analysis of the anharmonic EXAFS spectra was performed by evaluating the contributions of the cumulants to the amplitude reduction and the phase shift of the anharmonic EXAFS oscillation. The numerical results for Ni were in good agreement with those obtained using the other theoretical methods and experiment at various temperatures, which are useful for analyzing the experimental EXAFS data of the metal crystals.

Local atomic structure analysis of GaN surfaces via X-ray absorption spectroscopy by detecting Auger electrons with low energies.

GaN is a promising material for power semiconductor devices used in next-generation vehicles. Its electrical properties such as carrier mobility and threshold voltage are affected by the interface between the oxide and the semiconductor, and identifying the interface states is important to improve these properties. A surface-sensitive measurement of Ga K-edge extended X-ray absorption fine structure (EXAFS) by detecting Ga LMM Auger electrons that originate from Ga K-shell absorption is proposed for GaN. LMM Auger electrons with low energies were detected and the EXAFS oscillation was confirmed, providing information on the Ga atoms at the surface. Investigation of thermally oxidized GaN with an oxide film of defined thickness showed that the analysis depth was less than 10 nm, which is consistent with the inelastic mean free path of 2.3 nm estimated for LMM Auger electrons in GaN.

Relationship between electrical properties and interface structures of SiO2/4H-SiC prepared by dry and wet oxidation

We have investigated the relationship between the electrical properties and interfacial atomic structure of SiO2/4H-SiC interfaces, prepared by dry and wet thermal oxidation procedures with 4H-SiC (0001) and 4H-SiC (000-1) substrates, using extended x-ray absorption fine structure (EXAFS) spectroscopy and electrical methods. From the current–voltage (I–V) and capacitance–voltage (C–V) measurements, the gate leakage current onset and density of interface states were shown to depend on the thermal oxidation procedure used for creating SiO2/4H-SiC (000-1) interfaces. This dependence was not observed for SiO2/4H-SiC (0001) interfaces. From EXAFS oscillations, we found that C and Si vacancies formed at the SiC side of SiO2/4H-SiC (0001) and SiO2/4H-SiC (000-1) interfaces, respectively. Compressive stress at the SiC sides of SiO2/4H-SiC (0001) and SiO2/4H-SiC (000-1) interfaces prepared with the dry thermal oxidation procedure caused decreases in bond lengths. Stress at the interface was smaller in the case of wet oxidation. Wet oxidation released the interface stress more effectively for 4H-SiC (000-1) substrates than for 4H-SiC (0001) substrates. A high gate leakage current onset related to a high interface charge, while a high density of interface states related to a high interface stress and a high interface roughness.

Structural investigations on an amorphous Se90Te10 alloy produced by mechanical alloying using EXAFS, cumulant expansion and RMC simulations

We investigated an amorphous Se90Te10 alloy produced by mechanical alloying using two different approaches. First, we used extended x-ray absorption fine structure (EXAFS) spectroscopy and the cumulant expansion method using the Einstein model for the temperature dependence of the cumulants to obtain the cumulants , and . From these, we found information about the structure of the alloy as well as the thermal and structural disorder, anharmonicity of the effective interatomic pair potentials, thermal expansion of the Se–Se and Se–Te bonds and asymmetry of the partial distribution functions gSe−Se(r) and gSe−Te(r). The cumulants , and also allowed us to reconstruct the functions from EXAFS. Then, we made reverse Monte Carlo (RMC) simulations using the total structure factor \U0001d4ae(K) obtained from synchrotron x-ray diffraction and the EXAFS oscillations χ(k) on the Se K edge as input data to obtain the functions. Both methods furnished very similar gij(r) functions, and the structural data obtained from them were also very similar. The results obtained from both methods showed the presence of Se–Te pairs indicating that there is alloying at the atomic level. In addition, we could not find any evidence of the presence of Te clusters in the alloy.

X-ray-Raman-scattering-based EXAFS beyond the dipole limit

X-ray Raman scattering (XRS) provides a bulk-sensitive method of measuring the extended X-ray absorption fine structure (EXAFS) of soft X-ray absorption edges. Accurate measurements and data analysis procedures for the determination of XRS-EXAFS of polycrystalline diamond are described. The contributions of various angular-momentum components beyond the dipole limit to the atomic background and the EXAFS oscillations are incorporated using self-consistent real-space multiple-scattering calculations. The properly extracted XRS-EXAFS oscillations are in good agreement with calculations and earlier soft X-ray EXAFS results. It is shown, however, that under certain conditions multiple-scattering contributions to XRS-EXAFS deviate from those in standard EXAFS, leading to noticeable changes in the real-space signal at higher momentum transfers owing to non-dipole contributions. These results pave the way for the accurate application of XRS-EXAFS to previously inaccessible light-element systems.

Synthesis of a bowl-type dodecavanadate by the coupling reaction of alkoxohexavanadate and discovery of a chiral octadecavanadate

A new route to an alkoxohexavanadate species leads to the isolation of a bowl-type dodecavanadate without a guest molecule in the cavity. The 1D tetravanadate, [V4O11]2-, is a good precursor for the alkoxohexavanadate, [V6O13(OCH3)6]2-, which is then utilized for the [V6 + V6] coupling reaction to form the dodecavanadate, [V12O42]4-, with a capped dichloromethane molecule at the cavity entrance. We also obtained structural information on a newly discovered octadecavanadate, [V18O46(NO3)]5-,in both the solid and solution states through extended X-ray absorption fine structure (EXAFS) studies. The existence of a chiral inorganic species in acetonitrile with double-stranded V8 chains was confirmed through EXAFS oscillations.

Correlation between local vibrations and metal mass in AlB2-type transition-metal diborides

Lattice vibrations have been investigated in TiB2, ZrB2 and HfB2 by temperature-dependent extended X-ray absorption fine structure (EXAFS) experiments. Data clearly show that the EXAFS oscillations are characterized by an anomalous behavior of the Debye-Waller factor of the transition-metal-boron pair, which is suggested to be associated with a superposition of an optical mode corresponding to phonon vibrations induced by the B sublattice and an acoustic mode corresponding to the transition-metal (TM) sublattice. Data can be interpreted as a decoupling of the metal and boron vibrations observed in these transition-metal diborides (TMB2), a mechanism that may be responsible for the significant reduction of the superconducting transition temperature observed in these systems with respect to the parent MgB2 compound. The vibrational behavior of TM-TM bonds has also been investigated to study the occurrence of anisotropy and anomalies in the lattice vibrational behavior of TM-TM bonds.

X-ray Excited Optical Luminescence Studies of InGaN and Rare-Earth Doped GaN Epilayers

ABSTRACTA successful attempt to use X-ray Excited Optical Luminescence (XEOL) for the detection of Extended X-ray Absorption Fine Structure (EXAFS) in III-nitrides is reported. The samples studied were InGaN and rare-earth (RE) doped GaN epilayers. For the first time Ga K-edge EXAFS oscillations were measured by monitoring the well-known “yellow” emission of GaN at 560 nm. The analysis of Optically Detected (OD) EXAFS data confirmed the expected local structure for Ga in GaN. The intensity oscillation of the “yellow” band when X-ray energy was scanned across the Ga K-edge indicates that core excitation of Ga atom has a high probability of transfer to defects responsible for “yellow” emission.

Tails of near-edges x-ray absorption spectra as a fingerprint of magnetic and structural phase transitions. Application to metallic 3d ultra thin films

Tails of x-ray absorption near-edge structure (XANES)/x-ray magnetic circular dichroism spectroscopy (XMCD) spectra (50–100 eV above the edges) are characterized by the beginning of the extended x-ray absorption fine-structure/magnetic extended x-ray absorption fine-structure oscillations. We present a systematic study of these tails for three different thin films: Fe and Fe0.65Ni0.35 on stepped Cu(111) and Fe/ZnSe(001). The dependance of the detected oscillations on the film thickness leads us to conclude that XANES/XMCD tails are a useful in situ probe of the structural and magnetic phase of the sample.

Extended X-ray absorption fine structure and time differential perturbed angular correlation study of hafnium(IV) sorbed onto amorphous silica

Sorption of Hf(IV) onto amorphous silica at pH values between 1 and 7 is investigated by extended X-ray absorption fine structure (EXAFS) and time differential perturbed angular correlation (TDPAC) spectroscopies. Hafnium(IV) is studied as a chemical analogue for understanding the speciation of tetravalent actinides. In addition, 181Hf is a frequently investigated TDPAC probe nuclide. Spectra obtained from both techniques show that there is a change in Hf-species with sample preparation pH. Results from TDPAC measurements show that two similar surface binding sites exist for Hf(IV) on the silica surface at low pH values. At a pH above 3, a third, unspecific surface binding site for Hf(IV) begins to dominate. EXAFS investigations do not detect the presence of polynuclear surface species. Both TDPAC and Hf L3 edge EXAFS spectra of the sorption samples differ from that of a Hf(IV) solution or a Hf(IV) hydrolysis product. This indicates that the sorbed species are different from solution or simple hydrolysis species. Short Hf-O distances are obtained from theoretical fits to the EXAFS oscillations for all samples. This suggests that the primary interaction of Hf(IV) with the silica surface occurs via silanol groups. Differences in the electric field gradient surrounding sorbed Hf(IV) cations as a function of pH, observed in the TDPAC spectra, must result from changes in overall species geometry rather than a change in the primary Hf-silica surface interaction.

Structure of Mn films grown on (111) and (001) fcc Ir determined by EXAFS and the multiple-scattering approach.

Mn films were grown on (001) and (111) fcc Ir lattices by molecular beam epitaxy, extended x-ray absorption fine structure (EXAFS) experiments were performed on these films in order to determine the Mn structures. As Mn can adopt different crystalline structures, it is first shown that the usual Fourier treatment of EXAFS oscillations is not an appropriate technique for this purpose. On the contrary, the application of multiple scattering theory of x-ray-absorption fine structure allows us to identify the structures. For relaxed (polycrystalline) thick Mn deposits, Mn crystallizes in the Mn\ensuremath{\alpha} structure. For unrelaxed (single-crystalline) thick Mn films, the Mn EXAFS oscillations are well simulated assuming a mixing of fcc and Mn\ensuremath{\alpha} structure. For thin Mn films in Mn/Ir (111) and (001) superlattices, this multiple scattering approach allows us to show that Mn is in a trigonal structure when it is grown on Ir(111) and a tetragonal structure when it is grown on Ir(001). Finally, the (\ensuremath{\surd}3\ifmmode\times\else\texttimes\fi{}\ensuremath{\surd}3)R30\ifmmode^\circ\else\textdegree\fi{} superstructure of Mn films grown on (111) Ir is shown to come from the epitaxy of a mixed fcc and Mn\ensuremath{\alpha} structures and not from the occurence of Mn Laves phases like ${\mathrm{Cu}}_{2}$Mg or ${\mathrm{Zn}}_{2}$Mg as assumed by several workers. \textcopyright{} 1996 The American Physical Society.

EXAFS study of the carbon monoxide adsorption-induced morphology change in ruthenium clusters supported on alumina

Highly dispersed Ru/Al{sub 2}O{sub 3} catalysts were prepared from Ru{sub 3}(CO){sub 12}. EXAFS (extended X-ray absorption fine structure) studies show that all the Ru atoms in the catalyst are in small metal clusters. The Ru clusters are so small that the oxygen atoms at the cluster-support interface have a significant contribution to the EXAFS signal. Adsorption of CO completely changes the structure of this small metal cluster as is evidenced from EXAFS oscillation; Ru-Ru bonds are disrupted and Al-O-Ru-(CO){sub n} (n = 1-2) is formed. When the temperature is raised, Ru atoms agglomerate and small metal clusters are recovered. Infrared spectroscopic observation supports the conclusions obtained from EXAFS.

Pb L3 EXAFS and near-edge studies of lead metal and lead oxides

The Pb L 3 extended X-ray absorption fine structure (EXAFS) spectra of polycrystalline foil and powder specimens of Pb metal; polycrystalline PbO 2, and both red and yellow allotropes of PbO have been recorded. Although the EXAFS oscillations in the spectrum of Pb metal are rapidly damped with increasing temperature, they are easily detected, contrary to recent suggestions (Samant et al., J. Am. chem. Soc. 109, 5970, 1987) that the large thermal motion in Pb completely damps the EXAFS signal at room temperature. We suggest that Samant et al. may have failed to observe EXAFS because of large structural disorder in their sample. There are indications that anharmonicity of thermal motion affects the EXAFS signal since, with the standard EXAFS analysis, there is an apparent decrease in the nearest neighbour (NN) Pb-Pb distance of metallic Pb with increased temperature. The near-edge and EXAFS signals of the PB oxides are analysed. As expected from their much more rigid ionic structure the Pb L 3 EXAFS of the lead oxides is not greatly changed between 77 and 300 K.

Optical and EXAFS Investigations of ZnS:Tm3+ Powders

AbstractThe EXAFS (extended X‐ray absorption fine structure) analysis at the thulium LIII edge is applied to ZnS:Tm3+ luminescent powders. EXAFS oscillations are observed only for higher initial dopant concentrations of 1% and are attributed to oxysulfide (TmO)2S. This parasitic phase is not detected in the fluorescence spectra. By site selective excitation the emission spectrum of the same ZnS:Tm3+ (1%) powder is separated into two components. One of them is attributed to substitutional Tm on Zn sites. The nature of the other one could not be established, it is tentatively interpreted as due to a TmO complex.

A time-resolved x-ray absorption study of amorphous Si during pulsed laser irradiation

We report the first time-resolved x-ray absorption measurements on pulsed laser irradiated Si in the photon energy range from 90 to 300 eV, at irradiation energy densities up to 4 J/cm2, and at several delay times between irradiation pulse and x-ray probe pulse. The absorption spectra recorded at a delay time of 12 ns can be classified in three categories. Below ≊0.17 J/cm2 only thermal damping of extended x-ray absorption fine-structure (EXAFS) oscillations is observed. Between ≊0.17 J/cm2 and ≊1 J/cm2 annealing takes place. Several changes are observed, among which a ≊7-eV LII,III edge shifts towards higher energy, while the LI edge remains approximately at the same position. This edge shift behavior is explained by a largely reduced 3s density of states of the conduction band. This supports the idea that pulsed laser molten Si has a metalliclike structure. Thermal damping in this region is so strong that EXAFS oscillations are no longer visible. This is consistent with a lattice temperature above or equal to the melting temperature of Si. Above 1 J/cm2, the damage region, further edge shifts and the development of several peaks are observed. Clear cooling effects are visible at long delay times (60 ns) between irradiation and x-ray probe pulse. Cooling on this time scale can only be explained by evaporation of Si.