Soft x-ray spectroscopies in liquids and at solid-liquid interface at BACH beamline at Elettra.

The beamline for advanced dichroism of the Istituto Officina dei Materiali-Consiglio Nazionale delle Ricerche, operating at the Elettra synchrotron in Trieste (Italy), works in the extreme ultraviolet-soft x-ray photon energy range with selectable light polarization, high energy resolution, brilliance, and time resolution. The beamline offers a multi-technique approach for the investigation of the electronic, chemical, structural, magnetic, and dynamical properties of materials. Recently, one of the three end stations has been dedicated to experiments based on electron transfer processes at the solid/liquid interfaces and during photocatalytic or electrochemical reactions. Suitable cells to perform soft x-ray spectroscopy in the presence of liquids and reagent gases at ambient pressure were developed. Here, we present two types of static cells working in transmission or in fluorescence yield and an electrochemical flow cell that allows us to carry out cyclic voltammetry in situ and electrodeposition on a working electrode and to study chemical reactions under operando conditions. Examples of x-ray absorption spectroscopy measurements performed under ambient conditions and during electrochemical experiments in liquids are presented.

The effect of long-range order on intermolecular interactions in organic semiconductors: zinc octaethyl porphyrin molecular thin film model systems.

In order to optimize the performance of devices based on porphyrin thin films it is of great importance to gain a physical understanding of the various factors which affect their charge transport and light-harvesting properties. In this work, we have employed a multi-technique approach to study vacuum deposited zinc octaethyl porphyrin (ZnOEP) thin films with different degrees of long-range order as model systems. An asymmetrical stretching of the skeletal carbon atoms of the porphyrin conformer has been observed and attributed to ordered molecular stacking and intermolecular interactions. For ordered films, a detailed fitting analysis of the X-ray absorption near edge structure (XANES) using the MXAN code establishes a symmetry reduction in the molecular conformer involving the skeletal carbon atoms of the porphyrin ring; this highlights the consequences of increased π-π stacking of ZnOEP molecules adopting the triclinic structure. The observed asymmetrical stretching of the π conjugation network of the porphyrin structure can have significant implications for charge transport and light harvesting, significantly influencing the performance of porphyrin based devices.

Local adsorption structure and bonding of porphine on Cu(111) before and after self-metalation.

We have experimentally determined the lateral registry and geometric structure of free-base porphine (2H-P) and copper-metalated porphine (Cu-P) adsorbed on Cu(111), by means of energy-scanned photoelectron diffraction (PhD), and compared the experimental results to density functional theory (DFT) calculations that included van der Waals corrections within the Tkatchenko-Scheffler approach. Both 2H-P and Cu-P adsorb with their center above a surface bridge site. Consistency is obtained between the experimental and DFT-predicted structural models, with a characteristic change in the corrugation of the four N atoms of the molecule's macrocycle following metalation. Interestingly, comparison with previously published data for cobalt porphine adsorbed on the same surface evidences a distinct increase in the average height of the N atoms above the surface through the series 2H-P, Cu-P, and cobalt porphine. Such an increase strikingly anti-correlates the DFT-predicted adsorption strength, with 2H-P having the smallest adsorption height despite the weakest calculated adsorption energy. In addition, our findings suggest that for these macrocyclic compounds, substrate-to-molecule charge transfer and adsorption strength may not be univocally correlated.

Peculiar covalent bonding of C60/6H-SiC(0 0 0 1)-(3 × 3) probed by photoelectron spectroscopy.

High resolution photoemission with synchrotron radiation was used to study the interface formation of a thin layer of C60 on 6H-SiC(0 0 0 1)-(3 × 3), characterized by protruding Si-tetramers. The results show that C60 is chemisorbed by orbital hybridization between the highest-occupied molecular orbital (HOMO) and the p z orbital of Si adatom at the apex of the tetramers. The covalent nature of the bonding was inferred from core level as well as valence band spectra. The Si 2p spectra reveal that a large fraction (at least 45%) of the Si adatoms remain unbound despite the reactive character of the associated dangling bonds. This is consistent with a model in which each C60 is attached to the substrate through a single covalent C60-Si bond. A binding energy shift of the core levels associated with sub-surface Si or C atoms indicates a decrease of the SiC band bending caused by a charge transfer from the C60 molecules to the substrate via the formation of donor-like interface states.

Graphene nanobubbles on TiO2 for in-operando electron spectroscopy of liquid-phase chemistry.

X-Ray Photoelectron Spectroscopy (XPS) and X-Ray Absorption Spectroscopy (XAS) provide unique knowledge on the electronic structure and chemical properties of materials. Unfortunately this information is scarce when investigating solid/liquid interfaces and chemical or photochemical reactions under ambient conditions because of the short electron inelastic mean free path (IMFP) that requires a vacuum environment, which poses serious limitation on the application of XPS and XAS to samples present in the atmosphere or in the presence of a solvent. One promising approach is the use of graphene (Gr) windows transparent to both photons and electrons. This paper proposes an innovative system based on sealed Gr nanobubbles (GNBs) on a titanium dioxide TiO2 (100) rutile single crystal filled with the solution of interest during the fabrication stage. The GNBs were successfully employed to follow in-operando the thermal-induced reduction of FeCl3 to FeCl2 in aqueous solution. The electronic states of chlorine, iron and oxygen were obtained through a combination of electron spectroscopy methods (XPS and XAS) in different phases of the process. The interaction of various components in solution with solid surfaces constituting the cell was obtained, also highlighting the formation of a covalent C-Cl bond in the Gr structure. For the easiness of GNB fabrication and straightforward extension to a large variety of solutions, we envisage a broad application of the proposed approach to investigate in detail electronic mechanisms that regulate liquid/solid electron transfer in catalytic and energy conversion related applications.

The nature of the Fe-graphene interface at the nanometer level.

The emerging fields of graphene-based magnetic and spintronic devices require a deep understanding of the interface between graphene and ferromagnetic metals. This paper reports a detailed investigation at the nanometer level of the Fe-graphene interface carried out by angle-resolved photoemission, high-resolution photoemission from core levels, near edge X-ray absorption fine structure, scanning tunnelling microscopy and spin polarized density functional theory calculations. Quasi-free-standing graphene was grown on Pt(111), and the iron film was either deposited atop or intercalated beneath graphene. Calculations and experimental results show that iron strongly modifies the graphene band structure and lifts its π band spin degeneracy.

Tuning the magneto-optical response of iron oxide nanocrystals in Au- and Ag-based plasmonic media.

We investigated the magneto-optical response of chemically synthesized iron oxide magnetic nanocrystals, optically coupled with ordered planar arrays of plasmonic nanoparticles. We compare the signals from two classes of systems, featuring either Au or Ag as the plasmonic counterpart. The localized surface plasmon resonance of the Ag and Au nanoparticles arrays were superimposed or detuned, respectively, with respect to the dominant magneto-optical transitions of the magnetic material. Under resonance, a significant enhancement of the magneto-optical signal was observed. In both cases, we could separate the purely plasmonic and the magnetic contributions in the magneto-optical spectrum of the optically coupled composite based on their different magnetic-field dependence.

Surface symmetry-breaking and strain effects on orbital occupancy in transition metal perovskite epitaxial films.

The electron occupancy of 3d-orbitals determines the properties of transition metal oxides. This can be achieved, for example, through thin-film heterostructure engineering of ABO(3) oxides, enabling emerging properties at interfaces. Interestingly, epitaxial strain may break the degeneracy of 3d-e(g) and t(2g) orbitals, thus favoring a particular orbital filling with consequences for functional properties. Here we disclose the effects of symmetry breaking at free surfaces of ABO(3) perovskite epitaxial films and show that it can be combined with substrate-induced epitaxial strain to tailor at will the electron occupancy of in-plane and out-of-plane surface electronic orbitals. We use X-ray linear dichroism to monitor the relative contributions of surface, strain and atomic terminations to the occupancy of 3z(2)-r(2) and x(2)-y(2) orbitals in La(2/3)Sr(1/3)MnO(3) films. These findings open the possibility of an active tuning of surface electronic and magnetic properties as well as chemical properties (catalytic reactivity, wettability and so on).

Time-resolved soft x-ray absorption setup using multi-bunch operation modes at synchrotrons.

Here, we report on a novel experimental apparatus for performing time-resolved soft x-ray absorption spectroscopy in the sub-ns time scale using non-hybrid multi-bunch mode synchrotron radiation. The present setup is based on a variable repetition rate Ti:sapphire laser (pump pulse) synchronized with the ~500 MHz x-ray synchrotron radiation bunches and on a detection system that discriminates and singles out the significant x-ray photon pulses by means of a custom made photon counting unit. The whole setup has been validated by measuring the time evolution of the L(3) absorption edge during the melting and the solidification of a Ge single crystal irradiated by an intense ultrafast laser pulse. These results pave the way for performing synchrotron time-resolved experiments in the sub-ns time domain with variable repetition rate exploiting the full flux of the synchrotron radiation.

Evidence for strong itinerant spin fluctuations in the normal state of CeFeAsO0.89F0.11 iron-oxypnictide superconductors.

The electronic structure in the normal state of CeFeAsO0.89F0.11 oxypnictide superconductors has been investigated with x-ray absorption and photoemission spectroscopy. All of the data exhibit signatures of Fe d-electron itinerancy. Exchange multiplets appearing in the Fe 3s core level indicate the presence of itinerant spin fluctuations. These findings suggest that the underlying physics and the origin of superconductivity in these materials are likely to be quite different from those of the cuprate high-temperature superconductors. These materials provide opportunities for elucidating the role of magnetic fluctuations in high-temperature superconductivity.

Electronic structure of Mn-doped ZnO by x-ray emission and absorption spectroscopy.

We report an investigation of Mn-doped ZnO pellets with diluted Mn concentration by soft-x-ray emission and absorption spectroscopy. We have compared the electronic structure of two samples with different Mn concentration and different magnetic properties at room temperature: ferromagnetism in one case (Zn(0.98)Mn(0.02)O) and no magnetic order in the other (Zn(0.96)Mn(0.04)O). The results show that most of the Mn ions of the ferromagnetic sample are in the divalent state. For the nonmagnetic sample, a larger contribution of higher oxidation Mn states is present, which can be correlated to the suppressed ferromagnetism. The presence of oxygen atoms bonded to Mn ions and hybridized Mn 3d-O 2p states has been detected in both compounds. The partial density of states in the valence band has been measured with x-ray emission spectroscopy and the Mn 3d states have been found inside the bandgap of ZnO.

Electronic band structure of Gd: a consistent description.

The dispersion of the Gd (5d6s)-valence bands has been investigated by means of spin- and angle-resolved photoemission. The spin analysis of various spectral features shows that their weak dispersion and unusual broadening is due to the photoelectron lifetime rather than to correlation induced band narrowing as previously proposed. These results resolve a long-standing discrepancy between theoretical and experimental descriptions of the rare earth band structure.

Temperature-dependent fermi gap opening in the c(6x4)-C60/Ag(100) two-dimensional superstructure.

High-resolution angle-integrated photoemission of one monolayer of C (60) chemisorbed on Ag(100) shows the reversible opening of a gap at the Fermi level at temperatures 25< or =T< or =300 K. The gap reaches a maximum value of approximately 10 meV at T< or =70 K. This finding is the first evidence of an electronic phase transition in C60 monolayers and has implications on the ongoing debate about surface superconductivity in C60-based bulk materials.

Erythrothrombocytapheresis and plasmathrombocytapheresis with storage in T-sol of platelets collected by the new Amicus cell separator.

The Amicus cell separator is the latest apparatus introduced into the international market for high-yield, low WBC contamination and short-procedure time thrombocytapheresis. In its original configuration the apparatus collects platelets for subsequent resuspension in plasma and no collection of PRBC can be carried out along with thrombocytapheresis. In this paper we present the results of plasma-thrombocytapheresis and erythro-thrombocytapheresis after the adaptation of the Amicus to the collection and storage of platelets in a non-plasma medium and the concurrent collection of PRBC. From our study it is concluded that PRBC collection doesn't modify the quality of the platelet product obtained from random donors (platelets pre-count 263x10e3/microliter) in terms of yield which is 4.6x10e11 platelets, WBC contamination (1.3x10e5) or procedure time (63 +/- 26 min.). The quality of the platelet products is satisfactory too, as measured by aggregation induced by collagen and ristocetin or by the substantial stability of membrane glycoproteins (CD62-63-51-36-42B). The concentrate had an average content of 58.8 g of hemoglobin per bag, a final volume of 376 +/- 13 ml, (after the addition of 100 ml of SAG-M) and a normal mechanic and osmotic fragility.