High-resolution X-ray Absorption Spectroscopy Research Articles

Electronic structure of Ni(II) porphyrins and phthalocyanine studied by soft X-ray absorption spectroscopy

Investigations of chemical bonding and electronic structure features for polycrystalline Ni(II) phthalocyanine (NiPc), Ni(II) tetrabenzoporphyrin (NiTBP), Ni(II) 2,3,7,8,12,13,17,18-octaethylporphyrin (NiOEP), Ni(II) 5,10,15,20-tetraisobutylporphyrin (NiT iBuP), Ni(II) 5,15-diphenylporphyrin (NidPP) and Ni(II) 5,15-diisobutylporphyrin (Nid iBuP) have been performed by means of high-resolution soft X-ray absorption spectroscopy. It has been found that the low-lying unoccupied electron states in nickel porphyrins are mainly generated by the NiN 4 quasi-molecule: the lowest empty state is the weakly antibonding molecular orbital (MO) of σb 1g symmetry with nearly pure Ni 3 d x 2 - y 2 character, while the next empty state is the e g MO, which has a hybridized Ni 3dπ–N 2pπ character and reflects π chemical bonding (π-back-donation) between nickel and neighbouring nitrogen atoms of the pyrrole rings. The differences observed in the Ni 2p spectra of nickel porphyrins and phthalocyanines indicate a limited usefulness of the NiN 4 quasi-molecule and a need for its extension to the whole porphyrin (phthalocyanine) macrocycle. As a result, the differences in the Ni 2p spectra of the complexes and in their electronic structure are associated with a torsion of the opposite pyrrole ring planes (NiOEP, NiT iBuP) and the presence of N atoms in meso positions (NiPc). These conclusions are further confirmed by a comparative analysis of the Ni 2p and N 1s absorption spectra for NiPc and NiOEP.

Observation of molecular nitrogen in N-doped Ge2Sb2Te5

Ge 2 Sb 2 Te 5 (GST) film in the crystalline state was nitrogen doped using the reactive sputtering method in order to increase sheet resistance. High-resolution x-ray absorption spectroscopy revealed that molecular nitrogen (N2) existed in the N-doped GST film. This finding implies that both molecular nitrogen and atomic-state nitrogen should be taken into account in understanding the structures of N-doped GST film. The molecular nitrogen is believed to exist at interstitial and vacancy sites, and more likely at grain boundaries.

F-Level occupancy in TmTe under pressure investigated by high-resolution X-ray absorption spectroscopy

We report on the electronic structure and the valence of Tm in TmTe studied under high pressure by X-ray absorption spectroscopy in the partial fluorescence yield mode (PFY-XAS). The pressure-dependent behavior of the Tm valence v can be divided into three regimes: (i) in the 0–3 GPa range Tm remains mostly divalent; (ii) in the 3–4.5 GPa range v rapidly increases up to 2.45; (iii) in the 4.5–10.6 GPa range v increases more gently up to 2.85. The variations of v in these regimes are consistent with previous compressibility data, though the pressure ranges slightly differ between both studies. More interestingly, our results shed new light on the highest pressure regime, where Tm still has not reached a full trivalent state unlike commonly accepted ideas.

Direct observation of interstitial molecular N2 in Si oxynitrides

High-resolution near edge x-ray absorption spectroscopy and x-ray photoelectron spectroscopy were used to characterize ultrathin plasma-nitrided silicon oxides. The direct observation of interstitial molecular N2 was made by vibrationally resolved N K-edge absorption spectroscopy. The N2 molecules were trapped during the plasma nitridation at the near surface and could be eliminated by annealing via molecular out-diffusion.

High-resolution x-ray absorption spectroscopy studies of metal compounds in neurodegenerative brain tissue

Fluorescence mapping and microfocus X-ray absorption spectroscopy are used to detect, locate and identify iron biominerals and other inorganic metal accumulations in neurodegenerative brain tissue at sub-cellular resolution (<5 microns). Recent progress in developing the technique is reviewed. Synchrotron X-rays are used to map tissue sections for metals of interest, and XANES and XAFS are used to characterise anomalous concentrations of the metals in-situ so that they can be correlated with tissue structures and disease pathology. Iron anomalies associated with biogenic magnetite, ferritin and haemoglobin are located and identified in an avian tissue model with a pixel resolution ∼5 microns. Subsequent studies include brain tissue sections from transgenic Huntington's mice, and the first high-resolution mapping and identification of iron biominerals in human Alzheimer's and control autopsy brain tissue. Technical developments include use of microfocus diffraction to obtain structural information about biominerals in-situ, and depositing sample location grids by lithography for the location of anomalies by conventional microscopy. The combined techniques provide a breakthrough in the study of both intra- and extra-cellular iron compounds and related metals in tissue. The information to be gained from this approach has implications for future diagnosis and treatment of neurodegeneration, and for our understanding of the mechanisms involved.

High resolution x-ray absorption spectroscopy using a laser plasma radiation source

In this article we present a setup of a spectrometer for near edge x-ray absorption spectroscopy at the carbon K-edge (284 eV). The spectrometer works with a laser produced plasma as x-ray source and a single x-ray optical element in grazing incidence configuration called off-axis reflection zone plate. Using different sample foils we were able to demonstrate a spectral resolution of E/ΔE≈600. The positions of specific absorption peaks are in agreement with data measured at synchrotron sources. Since the off-axis zone plate is a focusing system, the x-ray flux in the detector plane is high enough to record an absorption spectrum on a charge-coupled device detector with a single shot. In the single shot modus a spectrum can be recorded before a sample degradation takes place. This will enable time-resolved experiments using pump-probe techniques.

Characterization of Ultrathin Electroless Barriers Grown by Self-Aligned Deposition on Silicon-Based Dielectric Films

A self-aligned, electrochemically integrated seeding/plating approach is developed for fabricating patterns of cobalt- (or nickel-) based metallic barriers and copper films selectively on silicon-based dielectric (hybrid siloxane-organic polymer and films using electroless plating. High-resolution X-ray absorption spectroscopy, transmission electron microscopy, atomic force microscopy, and grazing-incidence X-ray diffractometry indicate that, after they have been appropriately pretreated by a gaseous plasma or and basic aqueous solutions that contain sufficient amounts of peroxide hydrogen the dielectric films can adsorb highly populated metallic (nickel or cobalt) precipitates of sizes between 2 and 4 nm, which catalyze the deposition of ultrathin (⩽20 nm) barriers. The barriers are initially highly resistive and contain ultrafine (3-5 nm) crystallites embedded in amorphous-like, 20 nm grains, but become highly conductive (50-80 μΩ-cm) following optimal annealing at temperatures ⩾470°C because of crystallization, grain growth, and precipitation of Finally, the capacity of this method to fabricate “self-aligned” patterns of barrier and copper is established, and the adhesion strength and effectiveness of the barriers against copper’s diffusion/drift are quantified. The importance of the plasma pretreatment and the use of hydrogen peroxide (in the SC-1 solution) is also addressed. © 2004 The Electrochemical Society. All rights reserved.

Chemistry and band offsets of HfO 2 thin films on Si revealed by photoelectron spectroscopy and x-ray absorption spectroscopy

High-resolution photoelectron spectroscopy and x-ray absorption spectroscopy (XAS) measurements have been performed on HfO 2 films grown on Si(0 0 1) for ULSI ultra-thin gate insulators to elucidate the chemistry and band offsets of interfacial silicate layers. The valence-band discontinuity is determined by subtracting valence-band spectra of H-terminated Si(0 0 1) from those of the HfO 2 films on Si which have a HfO 2/Hf 1− x Si x O 2/Si double layer structure. On the other hand, the conduction-band discontinuity is clearly deduced by oxygen K-edge absorption spectra and O 1s photoelectron spectra. Consequently, two kinds of energy-band offsets of HfO 2/Hf 1− x Si x O 2 and Hf 1− x Si x O 2/Si are determined separately.

Self-Aligned Deposition Process for Ultrathin Electroless Barriers and Copper Films on Low-k Dielectric Films

A self-aligned, integrated plating technique based on plasma physics and colloidal-related chemistry is proposed to fabricate patterns of ultrathin (≤20 nm) Co-based barriers and copper films in a selective manner on dielectric (HOSP and SiO 2 ) films using electroless plating. High-resolution X-ray absorption spectroscopy, transmission electron microscopy, and atomic force microscopy reveal that, once properly pretreated by a gaseous plasma (O 2 or H 2 /N 2 ) and hydrogen peroxide (H 2 O 2 ) in a basic aqueous solution, the dielectric films can adsorb highly populated metallic (Ni) precipitates of sizes approximately from 2 to 4 nm to catalyze the deposition of electroless Co-based barriers. Finally, the capability of this technique to fabricate self-aligned patterns of barrier and copper is demonstrated and the importance of the plasma pretreatment and hydrogen peroxide (in SC-1 solution) is discussed.

Resonant Auger spectroscopy in solid alkali nitrates as a probe of nuclear motion in the core-excited NO 3− anion

We investigate the interplay between electronic and nuclear relaxation of the core-excited NO 3 − anion in solid LiNO 3 and NaNO 3 by means of the high-resolution X-ray absorption and resonant Auger spectroscopy. It is shown how the relative energy distances between the potential energy surfaces of the intermediate and final states can be mapped using the information on the dispersion of individual participator Auger features upon scanning the photon energy across the resonance. The influence of the cation nature (Li + or Na +) on the potential surfaces is traced and found to be minimal as a consequence of the rather ionic cation–anion interaction.

Oxidized Derivatives of Octopus vulgaris and Carcinus aestuarii Hemocyanins at pH 7.5 and Related Models by X-ray Absorption Spectroscopy

The binuclear copper sites of the met and met-azido derivatives of Octopus vulgaris and Carcinus aestuarii hemocyanins at pH 7.5 were characterized by high-resolution x-ray absorption spectroscopy in the low energy region (XANES) and in the higher region (EXAFS). The accuracy of the analysis of the data was tested with two mononuclear and six binuclear copper(II) complexes of the poly(benzimidazole) ligand systems 2-BB, L-5,5 and L-6,6 (Casella et al., 1993, Inorg. Chem. 32:2056–2067; 1996, Inorg. Chem. 35:1101–1113). Their structural and reactivity properties are related to those of the protein’s derivatives. The results obtained for those models with resolved x-ray structure (the 2-BB-aquo and azido mononuclear complexes, and the binuclear L-5,5 Cu(II)-bis(hydroxo) (Casella et al., unpublished)), extends the validity of our approach to the other poly(benzimidazole)-containing complexes and to the hemocyanin derivatives. Comparison between the protein’s and the complexes’ data, support a description of the met-derivatives as a five-coordinated O-bridged binuclear copper(II) center and favors, for both species, a bis(hydroxo) structure with a 3-Å Cu Cu distance. For O. vulgaris met-azido derivative a μ-1,3 bridging mode for the ligand appears the most likely. The structural situation of C. aestuarii met-azido-derivative is less clear: a μ-1,1 mode is favored, but a terminal mode cannot be excluded.

Chemical effects at metal/oxide interfaces studied by x-ray-absorption spectroscopy

A chemical and magnetic characterization of ferromagnet/antiferromagnet interfaces is essential to understand the microscopic origins of exchange anisotropy and other magnetic phenomena. We have used high-resolution L-edge x-ray absorption spectroscopy (XAS), which is element specific and sensitive to chemical environment and spin orientation, to investigate the interface of antiferromagnetic oxides with ferromagnetic metals. Clear quantitative evidence of oxidation/reduction reactions at the as-grown metal/oxide interface is presented. In situ-- and ex situ--grown samples of the form oxide $(5--30 \AA{})/\mathrm{metal}$ $(1--10 \AA{}),$ where oxide is either NiO or CoO and metal is either Fe, Co, or Ni, were studied by high-resolution XAS. For all samples, a metal(oxide) layer adjacent to an oxide(metal) layer was partially oxidized(reduced). Quantitative analysis of the spectra showed that one to two atomic layers on either side of the interface were oxidized/reduced. An elemental series of samples showed that the amount of oxidation/reduction was in accord with the difference in oxidation potentials of the adjacent cations, e.g., oxide layers were more strongly reduced by an iron metal layer than by cobalt or nickel metal layers. Annealing to temperatures, typically used to bias devices, was shown to significantly increase the amount of oxidation/reduction. The oxidation behavior of iron was shown to depend on the amount of oxygen available. Our results are believed to provide important information for the improved understanding of exchange anisotropy.

High-Resolution X-ray Emission and X-ray Absorption Spectroscopy

In this review, high-resolution X-ray emission and X-ray absorption spectroscopy will be discussed. The focus is on the 3d transition-metal systems. To understand high-resolution X-ray emission and reso-nant X-ray emission, it is first necessary to spend some time discussing the X-ray absorption process. Section II discusses 1s X-ray absorption, i.e., the K edges, and section III deals with 2p X-ray absorption, the L edges. X-ray emission is discussed in, respectively, the L edges. X-ray emission is discussed in, respec-tively, and section V on 2p3s and 2p3d X-ray emission. Section VI focuses on magnetic dichroism effects, and in section VII selective X-ray absorption experiments are discussed. To limit the scope of this review paper, many related topics (for example, EELS, XPS, and resonant photoemission, phonon-oriented inelastic X-ray scat-tering, and X-ray microscopy) will not be discussed. In addition, many aspects of X-ray absorption, such as reflection experiments, diffraction absorption fine structure, and related experiments, will remain untouched. EXAFS will be discussed very briefly, and its X-ray emission analogue EXEFS 69,71 will not be discussed.

Electronic structure effects from hydrogen bonding in the liquid phase and in chemisorption: an integrated theory and experimental effort.

A closely integrated theoretical and experimental effort to understand chemical bonding using X-ray spectroscopic probes is presented. Theoretical techniques to simulate XAS (X-ray absorption spectroscopy), XES (X-ray emission spectroscopy), RIXS (resonant inelastic X-ray scattering) and XPS (X-ray photoelectron spectroscopy) spectra have been developed and implemented within a density functional theory (DFT) framework. In combination with new experimental techniques, such as high-resolution XAS on liquid water under ambient conditions and XES on complicated surface adsorbates, new insight into e.g. hydrogen-bonded systems is obtained. For the (3x2) overlayer structure of glycine/Cu(110), earlier work has been extended to include adsorbate-adsorbate interactions. Structures are optimized for large cluster models and for periodic boundary conditions. It is found that specific features in the spectra arise from hydrogen-bonding interactions, which thus have important effects at the molecular-orbital level. XAS on liquid water shows a pronounced pre-edge feature with significant intensity, while the spectrum of ice shows only little intensity in this region. Theoretical spectrum calculations, based on instantaneous structures obtained from molecular-dynamics (MD) simulations, show that the pre-edge feature in the liquid is caused by water molecules with unsaturated hydrogen bonding. Some aspects of the theoretical simulations will be briefly discussed.

High-resolution soft X-ray absorption spectroscopy of solids.

The present research deals with the high-resolution soft x-ray absorption spectra (XAS) of Si, Ce- and Sm- compounds measured at BL25SU of SPring-8. The spectra are compared with theoretical results. The Si 1s spectrum shows good agreement with the calculated empty density of states so far reported. The Ce 3d spectra are very sensitive to the local electronic structure. The Sm 3d XAS of SmS shows clear temperature dependence, which is partly explained by a calculation for Sm2+ ion.

Hole-state and defect structure of proton conductor SrTiO 3 observed by high-resolution X-ray absorption spectroscopy

The hole-state and defect structure of proton conductor SrTi 1− x Sc x O 3 have been studied by high-resolution X-ray absorption spectroscopy. The proton-induced level is observed in proton-doped SrTi 1− x Sc x O 3. The energy separation between the top of the valence band and the bottom of the acceptor or proton level is considered to be the activation energy determined by electronic or proton conductivity.

Calcium Carbonate Modifications in the Mineralized Shell of the Freshwater SnailBiomphalaria glabrata

The mineralized shell (consisting of calcium carbonate) of the tropical freshwater snail Biomphalaria glabrata was investigated with high resolution synchrotron X-ray powder diffractometry and X-ray absorption spectroscopy (EXAFS). Parts from different locations of the snail shell were taken from animals of different age grown under various keeping conditions. Additionally, eggs with ages of 60, 72, 120, and 140 hours were examined. Traces of aragonite were found as first crystalline phase in 120 h old eggs, however, Ca K-edge EXAFS indicated the presence of aragonitic structures already in the X-ray amorphous sample of 72 h age. The main component of the shell of adult animals was aragonite in all cases, but in some cases minor amounts of vaterite (below 1.5%) are formed. The content of vaterite is generally low in the oldest part of the shell (the center) and increases towards the mineralizing zone (the shell margin). In juvenile snails, almost no vaterite was detectable in any part of the shell.

Calcium Carbonate Modifications in the Mineralized Shell of the Freshwater SnailBiomphalaria glabrata

The mineralized shell (consisting of calcium carbonate) of the tropical freshwater snail Biomphalaria glabrata was investigated with high resolution synchrotron X-ray powder diffractometry and X-ray absorption spectroscopy (EXAFS). Parts from different locations of the snail shell were taken from animals of different age grown under various keeping conditions. Additionally, eggs with ages of 60, 72, 120, and 140 hours were examined. Traces of aragonite were found as first crystalline phase in 120 h old eggs, however, Ca K-edge EXAFS indicated the presence of aragonitic structures already in the X-ray amorphous sample of 72 h age. The main component of the shell of adult animals was aragonite in all cases, but in some cases minor amounts of vaterite (below 1.5%) are formed. The content of vaterite is generally low in the oldest part of the shell (the center) and increases towards the mineralizing zone (the shell margin). In juvenile snails, almost no vaterite was detectable in any part of the shell.

Reinterpretation of the MolecularO2Chemisorbate in the Initial Oxidation of theSi(111)7×7Surface

The initial oxygen adsorption on the Si(111)7 x 7 surface was investigated by high-resolution x-ray absorption spectroscopy. Below 220 K, a molecular adsorption species is identified by distinctive absorption resonances due to the 1 pi(g) molecular orbitals. The molecular species is metastabilized to have a lifetime of 15-35 min at 135 K only with the presence of atomic adsorbates of more than 0. 1 ML (monolayer). It is thus clearly evidenced that the very initial adsorption is dissociative even at 100 K and the molecular species is not a precursor state. The molecular adsorption structures with the coadsorbed oxygen atoms are suggested.

Electronic structure in the band gap of lightly dopedSrTiO3by high-resolution x-ray absorption spectroscopy

The electronic structure of lightly Nb-doped ${\mathrm{SrTiO}}_{3}$ has been investigated using high resolution x-ray absorption spectroscopy (XAS). Below the O $1s$ threshold, XAS spectra show two features due to empty states whose energy positions match with those of the $3d$ photoemission spectra in the band gap energy region of the parent undoped ${\mathrm{SrTiO}}_{3}.$ Similar features are also observed in lightly La-doped ${\mathrm{SrTiO}}_{3}.$ These features exhibit systematic temperature dependence, which is well explained by the Fermi-Dirac distribution function. This indicates that the two features in the band gap are real bulk states such as simple donor states.