Structure of Co-Doped Alq3 thin films investigated by grazing incidence X-ray absorption fine structure and Fourier transform infrared spectroscopy

The structural properties of Co-doped tris(8-hydroxyquinoline)aluminum (Alq(3)) have been studied by grazing incidence X-ray absorption fine structure (GIXAFS) and Fourier transform infrared spectroscopy (FTIR). GIXAFS analysis suggests that there are multivalent Co-Alq(3) complexes and the doped Co atoms tend to locate at the attraction center with respect to N and O atoms and bond with them. The FTIR spectra indicate that the Co atoms interact with the meridional (mer) isomer of Alq(3) rather than forming inorganic compounds.     

First combined total reflection X-ray fluorescence and grazing incidence X-ray absorption spectroscopy characterization of aeolian dust archived in Antarctica and Alpine deep ice cores

Aeolian mineral dust archived in polar and mid latitude ice cores represents a precious proxy for assessing environmental and climatic variations at different timescales. In this respect, the identification of dust mineralogy plays a key role. In this work we performed the first preliminary X-ray absorption spectroscopy (XAS) experiments on mineral dust particles extracted from Antarctic and from Alpine firn cores using grazing incidence geometry at the Fe K-edge. A dedicated high vacuum experimental chamber was set up for normal-incidence and total-reflection X-Ray Fluorescence and Absorption Spectroscopy analyses on minor amounts of mineral materials at the Stanford Synchrotron Radiation Laboratory. Results show that this experimental technique and protocol allows recognizing iron inclusion mineral fraction on insoluble dust in the 1–10 µg range.     

Study of surface oxidation of iron foils by grazing incidence electron-induced X-ray emission spectroscopy

Surface oxidation of metallic iron was studied by grazing incidence electron-induced X-ray emission spectroscopy. We have shown the feasibility of the quantitative chemical-state analysis in the nanometer depth range on the basis of the results by Monte Carlo simulations in the grazing incidence setup. It is also found that the self-absorption effect is not negligible in the nanometer depth range.     

Combining imaging ellipsometry and grazing incidence small angle X-ray scattering for in situ characterization of polymer nanostructures

A combination of microbeam grazing incidence small angle X-ray scattering (μGISAXS) and imaging ellipsometry is introduced as a new versatile tool for the characterization of nanostructures. μGISAXS provides a local lateral and depth-sensitive structural characterization, and imaging ellipsometry adds the position-sensitive determination of the three-dimensional morphology in terms of thickness, roughness, refractive index, and extinction coefficient. Together μGISAXS and imaging ellipsometry enable a complete characterization of structure and morphology. On the basis of an example of buildup of nanostructures from monodisperse colloidal polystyrene nanospheres on a rough solid support, the scope of this new combination is demonstrated. Roughness is introduced by a dewetting structure of a diblock copolymer film with one block being compatible with the colloidal nanoparticles and one block being incompatible. To demonstrate the potential for kinetic investigations, μGISAXS and imaging ellipsometry are applied to probe the drying process of an aqueous dispersion of nanospheres on such a type of rough substrate.     

Analytical applications of absorption spectroelectrochemistry at grazing incidence

Analytical applications of spectroelectrochemistry are limited by the short path-length, in the absorbing medium, that can be produced with most light-beam/electrode configurations. This disadvantage is overcome for grazing incidence. A cell fitted with glassy-carbon electrodes and used in a conventional spectrophotometer is described and applied to model systems illustrating the use of (a) homogeneous redox reactions, (b) homogeneous redox reaction followed by chemical reaction and (c) electro-deposition followed by stripping into a reagent solution. This last technique is the spectroelectrochemical analogue of anodic-stripping voltammetry.     

Variable incidence angle X-ray absorption fine structure spectroscopy: a zirconia film study

Variable incidence angle X-ray absorption fine structure (VIAXAFS) spectroscopy offers a non-destructive ability to investigate film nano-structures. This technique was applied, spanning sample-beam angles from a grazing to normal incidence on a film obtained by zirconia sputtering on flat sample of stainless steel. X-ray absorption fine structure analysis on the Zr K edge identified chemical, defects and fractal structures through the film depth. VIAXAFS revealed occurrence of zirconium monoxide fractions at the surface a reduced state of zirconium oxide vs. the zirconium dioxide bulk. The discussion underlines that the technique may quantify the profile of various sub-layers, nano-pores, dislocations, vacancies or defect features.     

Review on grazing incidence X-ray spectrometry and reflectometry

Grazing incidence X-ray techniques are now widely used for surface and thin film analysis. The present article overviews the recent advancement since 1993 of the grazing incidence X-ray spectrometry and reflectometry in both theoretical and experimental aspects. Every current topic related to the total reflection X-ray fluorescence spectrometry (TXRF) is described in detail through the introduction of numerous published works on the application in the various fields of the science and industrial technologies. Recent rapid growth in diffuse scattering at grazing incidence as well as in specular reflection is another important scope. The combined measurements of different grazing incidence X-ray techniques might be a future trend for realizing further advanced analysis of the surface and interfaces of materials.     

Grazing incidence X-ray photoemission spectroscopy of SiO2 on Si

We have applied grazing incidence X-ray photoemission spectroscopy to the determination of the thickness of SiO₂ layers on Si, as well as surface carbon that is present. The measurements take advantage of the different optical constants of the layers. X-rays incident on the surface at grazing angle undergo total external reflection, where the fields in each layer are subject to highly non-linear changes as a function of incidence angle. X-ray photoemission excited by these fields gives information on atomic species, chemical state, and layer thickness. Simultaneous fits are made to the photoemission spectra in each layer. The method is illustrated for a thermally grown oxide layer and a native oxide on Si.     

X-ray absorption spectroscopy of transition-metal trichalcogenides

The sulfur K and metal L_III absorption spectra of transition-metal trichalcogenides (TMTC's) were measured. The matrix element effect plays an important role in these spectra. It was considered that the structures up to 5 eV above the absorption edge reflect the chalcogen antibonding band, the metal nonbonding d_z² band, and the metal d bands, and that the higher energy structures are derived from the metal s and p bands. The chalcogen antibonding band arises from chalcogen pairing and the metal d, s, and p bands are the mixture bands with chalcogen p orbitals. Evidence that shows that the lowest conduction band of the group IV TMTC's is the chalcogen antibonding band is presented. The overlap of the metal d and metal s bands is promoted by increasing the atomic number of chalcogen atoms.     

X-ray absorption spectroscopy of selenate reductase

The metal sites of selenate reductase from Thauera selenatis have been characterized by Mo, Se, and Fe K-edge X-ray absorption spectroscopy. The Mo site of the oxidized enzyme has 3 to 4 sulfur ligands at 2.33 A from two molybdopterin cofactors, one Mo=O group at 1.68 A and one Mo-O with an intermediate bond length of 1.81 A. The reduced enzyme has a des-oxo active site, again with about four Mo-S ligands (at 2.32 A) and possibly one oxygen ligand at 2.22 A. The enzyme was found to contain Se in a reduced form (probably organic) although the sequence does not indicate the presence of selenocysteine. The Se is coordinated to both a metal (probably Fe) and a lighter scatterer such as carbon.     

X-ray absorption spectroscopy of nanostructured polyanilines

In this paper, X-ray absorption near edge spectroscopy at the nitrogen K edge (N K XANES) data of polyaniline (PANI) and its derivatives were revisited and expanded. The N K XANES spectra of PANI nanocomposites and PANI nanofibers were also investigated. The analysis of N K XANES spectra were done by the deconvolution of bands and the 1s → π* and 1s → σ* transitions were assigned by a correlation with the N K XANES spectra of smaller organic compounds. The “free” forms of PANI were dominated by bands from 397.7 eV to 399.1 eV attributed to imine and radical cation nitrogen atoms, respectively. Nitrogen bonded to phenazine-like rings can also be seen, mainly for PANI prepared at pH higher than 3.0. The spectra of nanocomposites show sharper bands than the “free” polymers as well as new bands at 398.8 eV and 405–406 eV. These new bands were assigned to phenazine-like rings and azo bonds in the structure of the polymers (polyaniline, polybenzidine, and poly(p-phenylediamine)) within the galleries of the montmorillonite clay. PANI nanofibers doped with HCl or HClO₄ show bands related to phenazine-like rings and/or dication segments of PANI, indicating that these segments are important in the formation of PANI nanofibers.     

X-ray absorption spectroscopy measurements of liquid water

Recent studies, based on X-ray absorption spectroscopy (XAS) and X-ray Raman scattering (XRS), have shown that the hydrogen bond network in liquid water consists mainly of water molecules with only two strong hydrogen bonds. Since this result is controversial, it is important to demonstrate the reliability of the experimental data, which is the purpose of this paper. Here we compare X-ray absorption spectra of liquid water recorded with five very different techniques sensitive to the local environment of the absorbing molecule. Overall, the spectra obtained with photon detection show a very close similarity and even the observable minor differences can be understood. The comparison demonstrates that XAS and XRS can indeed be applied reliably to study the local bonding of the water molecule and thus to reveal the hydrogen bond situation in bulk water.     

X-ray absorption and X-ray photoelectron spectroscopy of a rhodium colloid

Transition metal colloids are potential precursors of heterogeneous catalysts with application to selective chemical reactions. Sample preparation techniques are described. Experimental details are given of the characterization of these often air-sensitive particles by X-ray photoelectron and X-ray absorption spectroscopy. First results obtained with both techniques for a Rh-colloid show that the metal is mainly present in the zerovalent chemical state. But the spectra indicate further chemical states of Rh which can be assigned to the outermost metal atoms of the colloid interacting with organic ligands or to the educt Rh-halides.     

X-ray absorption and X-ray photoelectron spectroscopy of a rhodium colloid

Transition metal colloids are potential precursors of heterogeneous catalysts with application to selective chemical reactions. Sample preparation techniques are described. Experimental details are given of the characterization of these often air-sensitive particles by X-ray photoelectron and X-ray absorption spectroscopy. First results obtained with both techniques for a Rh-colloid show that the metal is mainly present in the zerovalent chemical state. But the spectra indicate further chemical states of Rh which can be assigned to the outermost metal atoms of the colloid interacting with organic ligands or to the educt Rh-halides.     

Thin film structure of tetraceno[2,3-b]thiophene characterized by grazing incidence X-ray scattering and near-edge X-ray absorption fine structure analysis

Understanding the structure-property relationship for organic semiconductors is crucial in rational molecular design and organic thin film process control. Charge carrier transport in organic field-effect transistors predominantly occurs in a few semiconductor layers close to the interface in contact with the dielectric layer, and the transport properties depend sensitively on the precise molecular packing. Therefore, a better understanding of the impact of molecular packing and thin film morphology in the first few monolayers above the dielectric layer on charge transport is needed to improve the transistor performance. In this Article, we show that the detailed molecular packing in thin organic semiconductor films can be solved through a combination of grazing incidence X-ray diffraction (GIXD), near-edge X-ray absorption spectra fine structure (NEXAFS) spectroscopy, energy minimization packing calculations, and structure refinement of the diffraction data. We solve the thin film structure for 2 and 20 nm thick films of tetraceno[2,3-b]thiophene and detect only a single phase for these thicknesses. The GIXD yields accurate unit cell dimensions, while the precise molecular arrangement in the unit cell was found from the energy minimization and structure refinement; the NEXAFS yields a consistent molecular tilt. For the 20 nm film, the unit cell is triclinic with a = 5.96 A, b = 7.71 A, c = 15.16 A, alpha = 97.30 degrees, beta = 95.63 degrees, gamma = 90 degrees; there are two molecules per unit cell with herringbone packing (49-59 degree angle) and tilted about 7 degrees from the substrate normal. The thin film structure is significantly different from the bulk single-crystal structure, indicating the importance of characterizing thin film to correlate with thin film device performance. The results are compared to the corresponding data for the chemically similar and widely used pentacene. Possible effects of the observed thin film structure and morphology on charge carrier mobility are discussed.     

Applicability of a cut-off reflector for grazing incidence X-ray fluorescence analysis

The applicability of a cut-off reflector, instead of the commonly used multilayer reflector, for grazing incidence X-ray fluorescence (GI-XRF) analysis is demonstrated. Owing to the precise angular adjustment possible in the total reflection X-ray fluorescence (TXRF) spectrometer developed in house, it is possible to adjust the cut-off reflector so as to pass all X-ray energies up to Cu-K_α, eliminating Cu-K_β and higher X-ray energies emitted from a Cu target X-ray generator. The advantage of this technique is that one gets a higher flux of Cu-K_α radiation (>98%) compared to 80–90% from a good quality multilayer optics. Moreover, the same cut-off reflector, used at different grazing angles, serves the purpose for different primary beam energies. The suitability of such an arrangement for GI-XRF analysis for surface characterization has been demonstrated by analyzing a 50 ng aqueous residue of Fe on top of a float glass substrate. The GI-XRF results thus obtained are compared with those obtained using a multilayer monochromator in the primary beam as well as with theoretical calculations.     

X-ray absorption spectroscopy of rare earth intermediate oxides

The M_IV–V and L_III absorption spectra (between 850 and 7500 eV) of intermediate rare earth oxides (Pr₇O₁₂ and Pr₉O₁₆) were studied. These oxides required careful preparation and handling in order to assure their composition.The spectra are characterized by multiplet features that are interpreted as having contributions from both trivalent and tetravalent sites. In the M_IV–V spectra the appearance of distinct multiplet lines and additional weaker features demonstrate clearly the increasing ratio of the tetravalent sites as the oxygen content increases from Pr₇O₁₂ to PrO₂. Similar behavior has been observed for the CeO_x and TbO_x systems. These observations show that in these oxides, the trivalent and tetravalent sites are inequivalent and that the evidence of valence transition is seen in the appearance of the complex spectral features originating from the tetravalent sites.     

Analysis of crystalline phases in airborne particulates by grazing incidence X-ray diffractometry

Amounts of crystalline phases of airborne particulates collected on a silicon wafer (10 x 10 mm) using a high-volume air sampler were analyzed by grazing incidence X-ray diffractometry (GIXD). Airborne particulates were classified into five size ranges (1.09-10.5 microm) with a cascade impactor attached on the sampling ports of the high-volume air sampler. GIXD was used throughout this analysis to obtain better sensitivity for small amounts of airborne particulates on the silicon wafer. Calibration standards on the silicon wafer for the diffractometric determination were prepared by the suspension droplet method of the crystalline standards dispersed in cyclohexane. Analytical lines were (020) for gypsum, (101) for quartz, (104) for calcite, (200) for halite, and (110) for sal ammoniac. The sample and the calibrating standards were heated at 350 degrees C for 2 h to avoid mutual interference with gypsum (041 and 221) when calcite and halite were determined. The GIXD method enables us to determine 0.23-13.2 microg of gypsum, quartz, calcite, halite and sal ammoniac in 0.110-0.233 mg of airborne particulates on the silicon wafer.     

Phase transition of alkylsilane monolayers studied by temperature-dependent grazing incidence X-ray diffraction

The phase transition of organosilane monolayers on Si-wafer substrate surfaces prepared from octadecyltrichlorosilane (OTS) or docosyltrichlorosilane (DOTS) was investigated on the basis of grazing incidence X-ray diffraction (GIXD) at various temperatures. The OTS monolayer was prepared by a chemisorption method. The DOTS monolayer was prepared by a water-cast method (DOTS). The GIXD measurement clarified that the OTS monolayer also changed from hexagonal phase to amorphous state above a melting point of otadecyl groups. The GIXD measurements also clarified that the molecular aggregation state of the DOTS monolayer changes from an anisotropic phase to an isotropic phase with an increase in temperature. An estimated linear thermal expansion coefficient of the lattice lengths of a and b of the DOTS monolayer in the rectangular crystalline state assigned a similar value to those of bulk polyethylene with an orthorhombic crystalline lattice. The setting angle of the ab plane of the rectangular DOTS monolayer also showed similar behavior to that of the ab plane of bulk polyethylene.     

A study of cadmium sulfide nanocrystalline films by grazing incidence X-ray diffraction

Thin cadmium sulfide films were prepared on a monocrystalline-crystal silicon substrate by chemical deposition from aqueous solutions. Grazing incidence X-ray diffraction revealed that the cadmium sulfide films are comprised of nanocrystal particles, with 80% of the particles having a size of 5 ± 1 nm. Some nanocrystals have a wurtzite structure, while others, a sphalerite one. The presence of cubic phase in the films is indicative of a nonequilibrium state of the nanocrystalline films. Thirty minutes after the onset of the formation of cadmium sulfide, the size and crystal structure of the constituent particles of the film become independent of the deposition time—only the film thickness increases. In addition, the initial stage of the formation of the cadmium sulfide film is accompanied by the deposition of cadmium hydroxide Cd(OH)₂.