Structural, magnetic and electronic structure studies of Mn doped TiO2 thin films

We report structural, magnetic and electronic structure study of Mn doped TiO₂ thin films grown using pulsed laser deposition method. The films were characterized using X-ray diffraction (XRD), dc magnetization, X-ray magnetic circular dichroism (XMCD) and near edge X-ray absorption fine structure (NEXAFS) spectroscopy measurements. XRD results indicate that films exhibit single phase nature with rutile structure and exclude the secondary phase related to Mn metal cluster or any oxide phase of Mn. Magnetization studies reveal that both the films (3% and 5% Mn doped TiO₂) exhibit room temperature ferromagnetism and saturation magnetization increases with increase in concentration of Mn doping. The spectral features of XMCD at Mn L_3,2 edge show that Mn²⁺ ions contribute to the ferromagnetism. NEXAFS spectra measured at O K edge show a strong hybridization between Mn, Ti 3d and O 2p orbitals. NEXAFS spectra measured at Mn and Ti L_3,2 edge show that Mn exist in +2 valence state, whereas, Ti is in +4 state in Mn doped TiO₂ films.     

X‐ray spectromicroscopy in soil and environmental sciences

X‐ray microscopy is capable of imaging particles in the nanometer size range directly with sub‐micrometer spatial resolution and can be combined with high spectral resolution for spectromicroscopy studies. Two types of microscopes are common in X‐ray microscopy: the transmission X‐ray microscope and the scanning transmission X‐ray microscope; their set‐ups are explained in this paper. While the former takes high‐resolution images from an object with exposure times of seconds or faster, the latter is very well suited as an analytical instrument for spectromicroscopy. The morphology of clusters or particles from soil and sediment samples has been visualized using a transmission X‐ray microscope. Images are shown from a cryo‐tomography experiment based on X‐ray microscopy images to obtain information about the three‐dimensional structure of clusters of humic substances. The analysis of a stack of images taken with a scanning transmission X‐ray microscope to combine morphology and chemistry within a soil sample is shown. X‐ray fluorescence is a method ideally applicable to the study of elemental distributions and binding states of elements even on a trace level using X‐ray energies above 1 keV.     

Er -Li ion exchange in lithium niobate crystals: an EXAFS study

Lithium niobate crystals doped with erbium by the ion exchange process were studied by means of extended X-ray absorption fine structure spectroscopy and secondary ion mass spectrometry. The study of the local atomic order allowed the determination of the erbium environment in the crystal matrix and the investigation on the role of preparation conditions. Received 21 November 2002 Published online 1st April 2003 RID="a" ID="a"e-mail: sada@padova.infm.it     

The molecular orientation of DNA bases on H-passivated Si(1 1 1) surfaces investigated by means of near edge X-ray absorption fine structure spectroscopy

Layers of the DNA bases adenine, cytosine, and guanine were deposited onto hydrogen passivated Si(1 1 1) surfaces. The average tilt angles of these molecules with respect to the substrate surface were determined by the angular dependence of the Near Edge X-ray Absorption Fine Structure (NEXAFS) of the carbon K-edge. The interpretation of the NEXAFS spectra was assisted by a semi-empirical approach to the calculation of the π^∗-transition region which employs density functional theory calculations and core level photoemission data.     

Formation of argentic clusters and small Ag nanoparticles in soda-lime silicate glass

The formation of argentic clusters and very small Ag nanoparticles of 0.5 to 2 nm size in commercial soda-lime glass silver-doped by Ag/Na ion exchange in a mixed nitrate melt has been studied by electron microscopy and EXAFS. Particles formation was induced already during the ion exchange procedure, or by subsequent ion irradiation with 1.5 MeV He⁺ or 3 MeV Au⁺. The presence of nanoparticles was also macroscopically revealed by their surface plasmon resonance. The structural characterization indicates that specific configurations of silver oxide-like structures, so-called argentic clusters, are involved in the initial stage of nanoparticles formation.     

Structural characterisation of Ni clusters in AlN via X-ray absorption, X-ray diffraction and transmission electron microscopy

Ni ions were implanted in bulk AlN with the goal to form embedded metallic clusters. Combining several characterisation techniques such as X-ray absorption spectroscopy, X-ray diffraction and high resolution transmission electron microscopy, we determined the lattice parameter of the Ni clusters that display a fcc crystalline structure. The average size increases when the ion fluence is increased or after a thermal treatment. Thanks to moiré fringes observed by high resolution transmission electron microscopy and to satellite peaks seen on the diffraction patterns, we concluded that the annealed Ni clusters orientate their (002) planes on the (101) of AlN. Moreover, the satellite positions allowed us to calculate Ni cluster average diameters, that are in agreement with average sizes deduced by X-ray absorption spectroscopy. Received 25 August 1999 and Received in final form 8 February 2000     

Using X-PEEM to study biomaterials: Protein and peptide adsorption to a polystyrene–poly(methyl methacrylate)-b-polyacrylic acid blend

Recent synchrotron-based soft X-ray photoemission electron microscopy (X-PEEM) studies of protein and peptide interaction with phase segregated and patterned polymer surfaces in the context of optimization of candidate biomaterials are reviewed and a study of a new system is reported. X-PEEM and atomic force microscopy (AFM) were used to investigate the morphology of a phase-segregated thin film of a polystyrene/poly(methyl methacrylate)-b-polyacrylic acid (PS/PMMA-PAA) blend, and its interactions with negatively charged human serum albumin (HSA) and positively charged SUB-6 (a cationic antimicrobial peptide, RWWKIWVIRWWR-NH₂) at several pHs. At neutral pH, where the polymer surface is partially negatively charged, HSA and SUB-6 peptide showed contrasting adsorption behavior which is interpreted in terms of differences in their electrostatic interactions with the polymer surface.     

Probing the adsorption structure of a multifunctional organic molecule: a NIXSW and NEXAFS study of 3-chlorothiophene on Cu(111)

The adsorption structure of 3-chlorothiophene on Cu(111) has been investigated using a combination of normal incidence X-ray standing wavefield absorption (NIXSW) and near edge X-ray absorption fine structure (NEXAFS) spectroscopy. For coverages up to saturation of the chemisorbed layer, the 3-chlorothiophene bonds through the S atom. The S atom adsorbs in an atop site with a Cu–S distance identical, within experimental error, to that observed for thiophene on the same substrate. From a combination of NEXAFS and NIXSW, thiophene was found to adsorb with the aromatic ring flat lying. From NIXSW measurements the S–Cl axis was found to be inclined by 12±2° from the surface. Whilst NEXAFS data suggested an orientation of 23±8° for the aromatic ring of 3-chlorothiophene. The Cl atom interacted only weakly with the substrate, with a Cl–surface distance longer than the Cu–Cl van der Waal separation.     

Species formed at cuprite fracture surfaces; observation of O 1s surface core level shift

Surfaces of mineral cuprite prepared by fracture under UHV have been characterised by synchrotron XPS and near-edge X-ray absorption spectroscopy before and after exposure to ambient air. Before exposure of the cuprite, the Cu 2p photoelectron and Cu L_2,3-edge absorption spectra were consistent with Cu^I with very little d⁹ character. Surface-enhanced O 1s spectra from the unexposed mineral revealed a surface species, with binding energy 0.95 ± 0.05 eV below the principal cuprous oxide peak, assigned to under-coordinated oxygen. A second surface species, with binding energy about 1 eV higher than the principal peak, was assigned to either hydroxyl derived from chemisorbed water vapour or surface oxygen dimers produced by restructuring of the cuprite fracture surface. The width of the principal O 1s peak was 0.66 ± 0.02 eV. The observed Cu L₃- and O K-edge absorption spectra were in good agreement with those simulated for the cuprite structure. After exposure of the fracture surface to ambient air, the low binding energy O 1s surface species was barely discernible, the original high binding energy O 1s surface species remained of comparable intensity, new intensity appeared at an even higher (∼1.9 eV) binding energy, and the Cu L_2,3-edge spectrum indicated the presence of Cu^II, consistent with the formation of a thin surface layer of Cu(OH)₂.