Polarization-dependent X-ray Absorption Spectroscopy Research Articles

Growth of NiO on Ag(001): Atomic Environment, Strain, and Interface Relaxations Studied by Polarization Dependent Extended X-ray Absorption Fine Structure

This paper reports a structural study of 3 and 10 monolayer (ML) thick NiO films grown on Ag(001). Polarization-dependent X-ray absorption spectroscopy at the Ni K edge allowed us to obtain an accurate description of the local atomic environment of the Ni atoms up to the seventh coordination shell, including the determination of the in-plane and out-of-plane strains for the NiO films (determined with an accuracy better than 1%, resulting from the determination of Ni-O bond distances with a statistical error lower than 0.01 A). Multishell multiple scattering calculations have been used to fit the data. Within the experimental errors, the 3 ML film has both in and out-of-plane Ni-O distances compatible with those expected in the case of perfect pseudomorphism on Ag(001). A rough film morphology, possibly including also the presence of NiO islands, is also suggested by the optimized coordination numbers. Conversely the 10 ML film is partially relaxed without significant deviation from ideal bidimensional film. The present EXAFS study allows us to rule out any significant atomic interdiffusion process between the substrate and the NiO film. Comparison with previous literature data allows us to make the hypothesis of a two step growth mechanism for NiO films on Ag(001). The NiO-Ag(001) interface distance determined in this EXAFS study (d i n t e r f a c e = 2.36 ′ 0.05 A) is in remarkable agreement with theoretical calculations (d i n t e r f a c e = 2.38 A).

Unusual electronic structure of the pseudoladder compoundCaCu2O3

Experimental and theoretical studies of the unoccupied electronic structure of ${\mathrm{CaCu}}_{2}{\mathrm{O}}_{3}$ single crystals have been performed using polarization-dependent x-ray absorption spectroscopy and band-structure calculations. The measured hole distribution shows an unusual large number of holes in orbitals parallel to the interlayer direction which is in agreement with the theoretical analysis. ${\mathrm{CaCu}}_{2}{\mathrm{O}}_{3}$ deviates significantly from the standard $\mathrm{pd}\ensuremath{\sigma}$ cuprate picture. The corresponding strong interlayer exchange is responsible for the missing spin gap which is generic for other two-leg ladder cuprates.

Ni atomic environment in epitaxial NiO layers on Ag(0 0 1)

This paper reports a structural study of 3 and 10 ML thick NiO films grown on Ag(0 0 1). Polarization-dependent X-ray absorption spectroscopy at the Ni K-edge allowed us to obtain an accurate description of the local atomic environment of the Ni atoms, including the determination of the in-plane and out-of-plane strains for the NiO films. Multi-shell multiple scattering calculations have been used to fit the data. The local structure of Ni is always very similar to that of bulk NiO, with no detectable interdiffusion of Ag from the substrate. A compressive in-plane epitaxial strain has been found at 3 and 10 ML, indicating that the relaxation is not complete at 10 ML.

Polarization-Dependent X-ray Absorption Spectroscopic Study of [Cu(cyclam)]2+-Intercalated Saponite

The [Cu(cyclam)]2+ (copper-1,4,8,11-tetraazacyclotetradecane)−saponite complex has been obtained by an ion exchange reaction, and a thin film for the [Cu(cyclam)]2+−saponite complex has been prepared by spin-coating onto glass slides. The orientation and the structural change of the guest complex ion in the interlayer space of saponite have been investigated using X-ray diffraction (XRD) and polarization-dependent X-ray absorption spectroscopy (XAS). The interlayer thickness of 3.75 A from XRD analysis suggests that the copper complex ions are intercalated as a monolayer with the square planes of (CuN4) parallel to the silicate layers. Angle-resolved Cu K-edge XAS from 10 to 80° for the incident beam shows the well-resolved peak intensity variation of the xy (in-plane) and z (out-of-plane) components. From the polarized Cu K-edge X-ray near-edge spectroscopic (XANES) analysis, all the peaks can be assigned to the in-plane and out-of-plane components. The change of the bonding character also can be explain...

X-Ray Absorption Studies of Ge Layers Buried in Silicon Crystal

Polarization-dependent X-ray absorption spectroscopy was used to study the local microstructure of Ge layers buried in silicon. The layers with thickness from 6 to 20 monolayers of Ge were grown by molecular beam epitaxy on Si substrate and were covered by Si (20 nm). To investigate the morphology of grown structures, X-ray absorption near edge structure and extended X-ray absorption fine structure analysis of the Ge K-edge was done. The performed qualitative analysis proves that X-ray absorption spectra are very sensitive to the local order in the formed structures and are sources of unique information about morphology of the buried Ge layers. Using these techniques we were able to observe the changes in atomic order around the Ge atoms in investigated buried layers and compare the formed atomic order with that in crystalline Ge. A substantial increase in intensity, broadening and chemical shift of the X-ray absorption near edge structure spectrum for 8 ML were observed. It can be related to the increase in density of electron states caused by increase in the localization of the states due to potential appearing at the Ge island boundaries and indicated the formation of quantum dots. The observed in-plane modulations of radial distribution and out-of-plane for different layers were discussed.

Polarization dependent X-ray absorption studies of the chemical bonds anisotropy in wurtzite GaN grown at different conditions

Polarization-dependent X-ray absorption spectroscopy was used to examine the influence of crystal growth techniques and substrates type on the bond lengths and the bond structure of the single crystalline, wurtzite GaN in a form of bulk materials and epitaxial layers. The layers were grown by molecular beam epitaxy (MBE) and metalorganic chemical vapor deposition (MOCVD) on different substrates such as SiC, sapphire and GaN. From the observed X-ray absorption near edge structure (XANES) of the Ga K-edges, it was found that MOCVD introduces a stronger disorder around Ga atoms than MBE. Comparing the Ga and N K-edges of the epilayers and the bulk crystal, we found a prevailing contribution of N-vacancies in the layers and dominance of Ga-vacancies in the bulk crystal. The bonds along the c-axis are less perfect than the bonds in the c-plane for all investigated epilayers. The performed standard extended X-ray absorption fine structure analysis (EXAFS) resulted in a direct estimate of the bond lengths in the c-plane and along the c-axis.

Growth at GaN/AlN heterostructures: a local view

The local structure at GaN on AlN(0001) heterostructures is studied using polarization dependent X-ray absorption spectroscopy with synchrotron radiation; the Ga K edge has been used and the analysis has been extended up to the third coordination shell. Samples have been deposited using molecular beam epitaxy with deposition temperatures in the range 620–790 °C and are approximately 6 nm thick. We find that the GaN/AlN interface is abrupt (no interdiffusion) and that a partially pseudomorphically strained growth occurs, the in-plane strain increasing as the deposition temperature decreases.

Order on disorder: Copper phthalocyanine thin films on technical substrates

We have studied the molecular orientation of the commonly used organic semiconductor copper phthalocyanine (CuPC) grown as thin films on the technically relevant substrates indium tin oxide, oxidized Si, and polycrystalline gold using polarization-dependent x-ray absorption spectroscopy, and compare the results with those obtained from single crystalline substrates [Au(110) and GeS(001)]. Surprisingly, the 20–50 nm thick CuPC films on the technical substrates are as highly ordered as on the single crystals. Importantly, however, the molecular orientation in the two cases is radically different: the CuPC molecules stand on the technical substrates and lie on the single crystalline substrates. The reasons for this and its consequences for our understanding of the behavior of CuPC films in devices are discussed.

X-ray absorption spectroscopy of single-crystalline(VO)2P2O7:Electronic structure and possible exchange paths

Using polarization-dependent V2p and O1s near-edge x-ray absorption spectroscopy, we studied the unoccupied electronic structure of single-crystalline (VO){sub 2}P{sub 2}O{sub 7}. It is highly anisotropic, and shows similarities to vanadium oxides like VO{sub 2} and V{sub 2}O{sub 5} at the V2p edge and at the O1s threshold. The contributions from V-O and P-O orbitals could be identified. The results rule out the spin ladder model for the magnetic behavior of (VO){sub 2}P{sub 2}O{sub 7}, but are consistent with the alternating chain scenario.

3D Highly Oriented Nanoparticulate and Microparticulate Array of Metal Oxide Materials

AbstractAdvanced nano and micro particulate thin films of 3d transition and post-transition metal oxides consisting of nanorods and microrods with parallel and perpendicular orientation with respect to the substrate normal, have been successfully grown onto various substrates by heteronucleation, without template and/or surfactant, from the aqueous condensation of solution of metal salts or metal complexes (aqueous chemical growth). Three-dimensional arrays of iron oxide nanorods and zinc oxide nanorods with parallel and perpendicular orientation are presented as well as the oxygen K-edge polarization dependent x-ray absorption spectroscopy (XAS) study of anisotropic perpendicularly oriented microrod array of ZnO performed at synchrotron radiation source facility.

Hole distribution in(Sr,Ca,Y,La)14Cu24O41ladder compounds studied by x-ray absorption spectroscopy

The unoccupied electronic structure for the ${\mathrm{Sr}}_{14}{\mathrm{Cu}}_{24}{\mathrm{O}}_{41}$ family of two-leg ladder compounds was investigated for different partial substitutions of ${\mathrm{Sr}}^{2+}$ by ${\mathrm{Ca}}^{2+},$ leaving the nominal hole count constant, and by ${\mathrm{Y}}^{3+}$ or ${\mathrm{La}}^{3+},$ reducing the nominal hole count from its full value of 6 per formula unit. Using polarization-dependent x-ray absorption spectroscopy on single crystals, hole states on both the chain and ladder sites could be studied. While for intermediate hole counts all holes reside on O sites of the chains, a partial hole occupation on the ladder sites in orbitals oriented along the legs is observed for the fully doped compound ${\mathrm{Sr}}_{14}{\mathrm{Cu}}_{24}{\mathrm{O}}_{41}.$ On substitution of Ca for Sr orbitals within the ladder planes but perpendicular to the legs receive some hole occupation as well.

Orbital character ofO−2punoccupied states near the Fermi level inCrO2

The orbital character, orientation, and magnetic polarization of the O 2$p$ unoccupied states near the Fermi level ($E_F$) in CrO$_2$ was determined using polarization-dependent X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD) from high-quality, single-crystal films. A sharp peak observed just above $E_F$ is excited only by the electric field vector ($\bf E$) normal to the tetragonal $c$-axis, characteristic of a narrow band ($\approx$ 0.7 eV bandwidth) constituted from O 2$p$ orbitals perpendicular to $c$ (O 2$p_y$) hybridized with Cr 3$d_{xz-yz}$ $t_{2g}$ states. By comparison with band-structure and configuration-interaction (CI) cluster calculations our results support a model of CrO$_2$ as a half-metallic ferromagnet with large exchange-splitting energy ($\Delta_{exch-split}$ $\approx$ 3.0 eV) and substantial correlation effects.

Unoccupied electronic structure ofLi2CuO2

We have investigated the unoccupied electronic structure of quasi-one-dimensional ${\mathrm{Li}}_{2}{\mathrm{CuO}}_{2}$ using polarization-dependent x-ray absorption spectroscopy. Although an isotropic ``in-plane'' situation is expected for the O $1s$ absorption edge, due to the single oxygen site in the edge-sharing chain geometry, we observe a clear energy shift of peaks derived from the upper Hubbard band, dependent on the direction of the light polarization. For the possible explanation of this observed shift, alternative scenarios are discussed, based upon the electronic structure as obtained from calculations within the local-density approximation, the effects of coupling to phonons during the core-level excitation process, as well as on the polarization-dependent charge distribution in the final state resulting in varying contributions from intersite electron-electron interactions.

Electronic structure of metallicK0.3MoO3and insulatingMoO3from high-energy spectroscopy

We present high-energy spectroscopy results on the occupied and unoccupied electronic structures of the strongly anisotropic compounds ${\mathrm{MoO}}_{3}$ and ${\mathrm{K}}_{0.3}{\mathrm{MoO}}_{3}$ near the Fermi level. X-ray photoemission reveals a close similarity of the valence-band spectra in both materials. The metal ${\mathrm{K}}_{0.3}{\mathrm{MoO}}_{3}$ shows an additional Mo $4d$-derived conduction band whose population is achieved by electron transfer from K to Mo. This is concluded from studies on intentional surface doping of insulating ${\mathrm{MoO}}_{3}$ which leads to the occurrence of Mo $4d$-like defect states near the Fermi level. Information on the unoccupied states and their orbital symmetries was obtained by polarization-dependent x-ray absorption spectroscopy. These experiments confirm previous descriptions of the electronic structure in terms of a molecular-orbital picture. For ${\mathrm{K}}_{0.3}{\mathrm{MoO}}_{3},$ strong local Mo $4d\ensuremath{-}\mathrm{O}$ $2p$ bonding is not only found along the chain direction but also perpendicular to it. This finding is reminiscent of earlier band calculations, which found the chains to form pairs by pronounced transverse hybridization, with the overall bandwidth remaining one dimensionally anisotropic. This adds an important aspect to the discussion on possible Luttinger-liquid physics in the low-dimensional Mo bronzes.

1998 Alcan Award Lecture Surface electrochemistry - surface science with a joy stick

This lecture gives a review of thermodynamic, spectroscopic, STM and AFM imaging, and X-ray diffraction studies of molecular and ionic adsorption at Au(111) electrodes. In the first part, thermodynamics of adsorption of simple ions such as sulfate, chloride, bromide, and iodide will be discussed. At high coverages, anions adsorbed at a single crystal surface form ordered 2D adlayers. We show that the structure of these adlayers can be studied by STM and surface X-ray diffraction techniques. Next, the information concerning adsorption of simple ions is used to describe mixed adlayers formed by coadsorption of anions and metal adatoms. We demonstrate how to combine electrochemical experiments with in situ polarization-dependent Cu K-edge X-ray absorption spectroscopy to determine the composition and the structure of mixed films formed by deposition of Cu on Au(111) in the presence of SO42-. In the last part we review our efforts to describe coordination of organic molecules to gold electrodes. First, we will discuss adsorption of benzonitrile at the Au(111) electrode surface. We combine electrochemical methods with in situ infrared spectroscopy to describe (i) the energetics of molecular adsorption at the gold electrode surface, (ii) the character of the interaction of the adsorbed molecule with the metal substrate, and (iii) the influence of the electric field on the orientation of the adsorbed molecule. In the last section we describe surface aggregation phenomena. We apply AFM and STM to determine the structure of hemimicelles formed at the Au(111) electrode surface by adsorbed molecules of sodium dodecyl sulfate and we discuss the potential-controlled transformation of these hemimicelles into a condensed monolayer.Key words: surface electrochemistry, electrosorption, surface aggregation, gold electrodes, molecular adsorption, ionic adsorption.

Evidence for relaxed and high-quality growth of GaN on SiC(0001)

By using polarization-dependent x-ray absorption spectroscopy at the Ga edge, we study the growth of GaN on SiC(0001) in the thickness range 0.7–150 nm. We find that the growth is always relaxed (i.e., nonpseudomorphic) even for the thinnest epilayers, i.e., below the expected critical thickness. No evidence is found for a mixed Ga/Si interface plane, while a C/N mixed interface plane cannot be ruled out. The results are discussed with reference to the electronic structure of the SiC/GaN heterojunction and in particular to band offsets and strain-induced piezoelectric polarization.

Role of apical holes for superconductivity in p-type cuprates: NEXAFS results

From the early days of high-temperature superconductivity on, the question of a possible role of apical sites in p-type superconductors has been controversely discussed in the community. Several recent observations made by us, using polarization-dependent O1s and Cu2p near-edge X-ray absorption spectroscopy on single crystals of infinite-layer compounds, of Ca-doped T ' phase compounds, and of Y1-xCaxBa2Cu3Oy, are all consistent with the conjecture that there is significant influence of the apical site on high-T-c superconductivity. Most notably, the Ca-dependent hole counts for the latter family show that superconductivity is absent if there are no holes on the apical site. This is independent of the hole count in the CuO2 planes.

Preparation and Electronic Properties of YBa2Cu3Ox Films with Controlled Oxygen Stoichiometries

We describe a novel technique capable of controlling the oxygen content of YBa2Cu3Ox (YBCO) films in a precise and reversible manner. The temperature dependence of resistivity and the distinct two-plateau behavior in critical temperature Tco versus oxygen content plot of these films are consistent with those observed in the bulk and single crystals of YBCO. The O 1s and Cu 2p absorption spectra of these films were measured by polarization-dependent X-ray absorption spectroscopy (XAS). The intensity variations of the pre-edge peaks as a function of oxygen content are discussed. We also used these films to systematically study the electron-phonon coupling strength and the position of Fermi level by using a femtosecond pump-probe technique. A clear sign-reversal of the transient reflectivity, which was consistently explained by the thermomodulation model, was observed. Both of these optical measurements support the idea that the electronic structure of YBCO cuprates is based on the charge transfer model with hybridization between the Cu and O sites.

Unoccupied electronic structure ofSr2CuO2Cl2andBa2Cu3O4Cl2: Experiment and theory

The unoccupied electronic structure of the layered cuprates ${\mathrm{Sr}}_{2}{\mathrm{CuO}}_{2}{\mathrm{Cl}}_{2}$ and ${\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{4}{\mathrm{Cl}}_{2}$ has been studied using polarization-dependent x-ray absorption spectroscopy at the O $1s$ and Cu ${2p}_{3/2}$ edges and band-structure calculations within the local-density approximation. In contrast to almost all high-temperature superconductors, there are no oxygen atoms outside the Cu-O planes in these oxychlorides. Our results represent therefore direct experimental information regarding the oxygen-derived unoccupied electronic structure of undoped Cu-O planes in square-planar coordination. The O 1$s$ and Cu ${2p}_{3/2}$ x-ray absorption spectra of ${\mathrm{Sr}}_{2}{\mathrm{CuO}}_{2}{\mathrm{Cl}}_{2}$ and ${\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{4}{\mathrm{Cl}}_{2}$ are quite similar with an essentially two-dimensional upper Hubbard band and extra oxygen and copper related absorption fine structure at higher energies. From its polarization dependence and a comparison to the results of the band-structure calculations, it is concluded that this extra fine structure is related to transitions into O $2p$ and Cu ${3d}_{{3z}^{2}\ensuremath{-}{r}^{2}}$ orbitals which have become partly unoccupied due to hybridization with Sr $4d/\mathrm{B}\mathrm{a}$ $5d/\mathrm{C}\mathrm{u}$ ${4p}_{z}$ and Cu $4s$ orbitals, respectively. Differences between these two compounds are the larger width of the upper Hubbard band and the higher relative Cu ${3d}_{{3z}^{2}\ensuremath{-}{r}^{2}}$ hole occupation above the upper Hubbard band in ${\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{4}{\mathrm{Cl}}_{2}$. These differences can be related to the extra ${\mathrm{Cu}}_{B}$ atoms in the ${\mathrm{Cu}}_{3}{\mathrm{O}}_{4}$ planes of ${\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{4}{\mathrm{Cl}}_{2}$ with respect to the ${\mathrm{CuO}}_{2}$ planes of ${\mathrm{Sr}}_{2}{\mathrm{CuO}}_{2}{\mathrm{Cl}}_{2}$.

X-ray absorption spectroscopy of Pr x Y1?x Ba2Cu3O7?y single crystals

Substituting Y in orthorhombic (Y,RE)Ba2Cu3O7 by any rare-earth element RE has generally little effect on the superconducting properties. For RE = Pr, however, superconductivity is completely suppressed. To elucidate this effect we have studied the unoccupied electronic structure of PrxY1−xBa2Cu3O7−y (x = 0.0,0.4, 0.8) by polarization-dependent O1s x-ray absorption spectroscopy on detwinned single crystals. Along with the comparison of undoped (y ≈ 0.9) to the doped materials (y ≈ 0.1), this allows a test of the current theoretical explanations for the suppression of superconductivity. While we can rule out models involving hole filling or charge transfer from planes to chains our data is consistent with approaches based on Pr4f−02pπ hybridization.