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ABSTRACT: A careful choice of the surface coverage of iron phthalocyanine (FePc) on Ag (110) around the single monolayer allows us to drive with high precision both the long-range supramolecular arrangement and the local adsorption geometry of FePc molecules on the given surface. We show that this opens up the possibility of sharply switching the catalytic activity of FePc in the oxygen reduction reaction and contextual surface oxidation in a reproducible way. A comprehensive and detailed picture built on diverse experimental evidence from scanning tunnelling microscopy, X-ray photoelectron spectroscopy and X-ray absorption spectroscopy, coupled with density functional theory calculations, sheds new light on the nature of the catalytically active molecule-surface coordination and on the boundary conditions for its occurrence. The results are of relevance for the improvement of the catalytic efficiency of metallo-macrocycles as viable substitutes for platinum in the cathodic compartment of low-temperature fuel cells.
Nature Material 10/2012; 11(11):970-7. · 32.84 Impact Factor
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The Journal of chemical physics 08/2012; 137(7):079902. · 3.09 Impact Factor
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ABSTRACT: We demonstrate the possibility to achieve the doping of eumelanin thin films through K(+) incorporation during the electrodeposition of the film. K-doping changes the optical properties of the eumelanin thin films, reducing the energy gap from 1.0 to 0.6 eV, with possible implications for the photophysical properties. We have identified the doping-related occupied and unoccupied electronic states and their spectral weight using resonant photoemission spectroscopy (ResPES) and x-ray absorption at the C and N K-edges (near edge x-ray absorption fine spectroscopy, NEXAFS). All data are consistently interpreted by ab initio calculations of the electronic structure within the frame of the macrocycle model developed for the eumelanin protomolecule. Our analysis puts in evidence the intercalation of K with one specific oligomer (a tetramer composed of one indolequinone and 3 hydroquinone monomers) in correspondence of the nitrogen macrocycle. The predicted variation of the tetramer spacing is also in agreement with the recent x-ray diffraction experiments. The charge donation from K to N and C atoms gives rise to new electronic states at the top of the valence band and in NEXAFS resonances of the unoccupied orbitals. The saturation of the tetramer macrocycles leaves an excess of K that bind to N and C atoms in alternative configurations, as witnessed by the occurrence of additional spectral features in the carbon-related ResPES measurements.
The Journal of chemical physics 05/2012; 136(20):204703. · 3.09 Impact Factor
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ABSTRACT: Resonant photoemission from the valence band of a (√3 × √3)R30° reconstructed Mn:Ge(111) metallic interface has been carefully analyzed with the aim to track the transition from resonant Raman to normal Auger emission. The transition energy has been compared with the Mn 2p binding energy, as well as with the Mn L(3) absorption edge energy. Close similarities emerge with respect to the case of elemental Mn thin films, suggesting that the excitation dynamics is dominated by the electronic properties of Mn 3d states, in spite of the bonding with Ge atoms. The switching from the resonant Raman Auger (RRAS) to the normal Auger regime is found about 2 eV below the Mn L(3) absorption edge. A change of the lineshape due to the transition from an overall N - 1 electron final state (RRAS channel) to an N - 2 electron final state (normal Auger channel) is evidenced by the analysis of the experimental data, which also allowed the ratio to be tracked between charge delocalization and core-hole time scales as the photon energy is tuned across the Mn L(3) edge.
Journal of Physics Condensed Matter 05/2012; 24(23):235502. · 2.55 Impact Factor
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ABSTRACT: The gap state that appears upon reduction of TiO2 plays a key role in many of titania's interesting properties but its origin and spatial localization have remained unclear. In the present work, the TiO2(110) surface is reduced in a chemically controlled way by sodium adsorption. By means of resonant photoelectron diffraction, excess electrons are shown to be distributed mainly on subsurface Ti sites strikingly similar to the defective TiO2(110) surface, while any significant contribution from interstitial Ti ions is discarded. In agreement with first principles calculations, these findings demonstrate that the distribution of the band gap charge is an intrinsic property of TiO2(110), independent of the way excess electrons are produced.
Physical Review Letters 03/2012; 108(12):126803. · 7.37 Impact Factor
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M Cantoni,
R Bertacco,
A Brambilla,
M Finazzi,
L Duo,
F Ciccacci,
A Verdini, L Floreano,
A Morgante,
M Passoni,
C S Casari,
Li A Bassi
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ABSTRACT: The Fe growth on ZnSe(001) takes place via the initial formation of superparamagnetic nano-islands that subsequently coalesce, giving rise to a continuous film for a nominal thickness of 8 Fe monolayers. For a very low Fe coverage (2 Fe monolayers), we show that the surface barrier potential (i.e. the barrier potential seen by electrons incident on the surface), measured by absorbed current spectroscopy, attains very large values (6.9 eV at room temperature) and dramatically changes as a function of temperature, with an increase of∼1.5 eV from room temperature down to 130 K, largely exceeding similar changes observed in both thin films and nanoparticles. This phenomenon disappears as the thickness increases and is fully reversible with temperature. Nonequilibrium phenomena due to the experimental conditions are present, but are not able to explain the observed data. Inverse photoemission, core level photoemission, x-ray photoemission diffraction, and scanning tunneling microscopy are employed in order to find temperature-dependent properties of the Fe islands: while only minor changes as a function of temperature are present in the electronic band structure, the Fe crystal structure, and the morphology of the islands, a noticeable temperature dependence of the Se segregation through the Fe islands is found.
PHYSICAL REVIEW. B, CONDENSED MATTER AND MATERIALS PHYSICS. 01/2012; 85:155456-1-155456-8.
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AIP ADVANCES. 01/2011; 1:032147-.
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ABSTRACT: In-situ metalation of porphyrin molecules in
ultrahigh vacuum (UHV) is of great interest for the characterization
of pure species in a controlled environment. Here, we
report the characterization of the electronic states and the
molecules' geometrical adaptation during the formation of pure
2H-5,10,15,20-tetraphenylporphyrin (2H-TPP) and Fe- tetraphenylporphyrin
(Fe-TPP) layers on Ag(111) single crystal.
Core level absorption spectra indicate the flat conformation of
the monolayer suggesting an adatom hopping instead of a
surface mediated dopant diffusion for the metalation process. Photoemission points out that the interaction between Fe dz-states and Ag bands increases the monolayer metallic character already induced by the charge transfer from the substrate.
The Journal of Physical Chemistry C 01/2011; 115:4155. · 4.80 Impact Factor
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ABSTRACT: We present the formation, under ultrahigh vacuum conditions, of
a three-dimensional organic architecture on a Au(111) surface based on the
NH2−COOH interaction. The surface is first functionalized with a selfassembled
monolayer (SAM) of an amine-terminated molecule (cysteamine,
CA); then, a layer of benzoic acid (BA) is grown on top. We characterized this
self-assembled structure by means of X-ray photoemission and absorption
spectroscopy. The formation of a hydrogen bond between the two molecular
species anchors the BA molecules to the CA. The structure is homogeneous in
terms of its morphology and chemical properties. We also show that the
structure (molecular orientation) of the BA SAM formed on the CA SAM is
different from that of the BA SAM on the bare Au surface. The chemical
recognition and molecular ordering nature of the BA−CA self-assembly makes it
a promising candidate for the bottom-up parallel fabrication of hierarchically
assembled nanodevices starting from functionalized building blocks.
Journal of Physical Chemistry Letters 01/2011; 2:3124. · 6.21 Impact Factor
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M. Fanetti,
A. Calzolari,
P. Vilmercati,
C. Castellarin-Cudia,
P. Borghetti,
G. Di Santo, L. Floreano,
Verdini,
A. Cossaro,
I. Vobornik,
E. Annese,
F. Bondino,
S. Fabris,
A. Goldoni
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ABSTRACT: We present a combined experimental and theoretical study of the ultrathin film of Co-octaethylporphyrin
(Co-OEP) molecules deposited on the Ag(110) surface. The morphological and electronic properties of this heterogeneous metallorganic interface were studied by means of scanning tunneling microscopy (STM), near-edge X-ray adsorption fine structure (NEXAFS) spectroscopy, ultraviolet photoemission
spectroscopy (UPS), and density functional theory calculations (DFT). The long-range self-ordered single layer of Co-OEP was obtained by thermal desorption of the molecular multilayer. The
single-layer molecules were arranged in a noncommensurate rectangular lattice aligned with the substrate high-symmetry directions. The combination of experimental techniques and numerical simulations indicated that in this configuration each molecularmacrocycle is tilted with respect to the metal surface of about 15�. The strong molecular interaction with the substrate leads to the electron transfer fromthemetal substrate to the molecule. The direct interaction with the substrate involvesmostly the Co metal center, whichmodifies the valence state with respect to the free Co-OEP molecules due to the hybridization between Co states and Ag bands.
The Journal of Physical Chemistry C 01/2011; 115(23):11560. · 4.80 Impact Factor
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A. Cossaro,
M. Dell'Angela,
A. Verdini,
M. Puppin,
G. Kladnik,
M. Coreno,
M. de Simone,
A. Kivimaki,
D. Cvetko,
M. Canepa, L. Floreano
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ABSTRACT: The in situ formation of prototypes of complex hybrid organic structures gives the opportunity to characterize their morphologic and spectroscopic properties, a mandatory step toward the development of more reliable and efficient organic devices. In this context, a proper functionalization of the substrate’s surface is required for the linkage of small molecules or proteins. We report here how thin films of cysteamine have been successfully assembled on Au(111) by evaporation in ultra high vacuum conditions. The molecular orientation has been investigated by means of X-ray absorption spectroscopy. Both monolayer and multilayered films of cysteamine have been characterized by means of high-resolution photoemission spectroscopy. A compact, homogeneous self-assembled monolayer has been obtained, which provides the functionalization of the surface with a -NH2 linker. The investigation on the multilayer film revealed the presence of a zwitterionic cysteamine moiety.
The Journal of Physical Chemistry C 01/2010; 114:15011. · 4.80 Impact Factor
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Physical Review B 01/2010; 81:-. · 3.69 Impact Factor
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Applied Physics Letters 01/2010; 92:-. · 3.84 Impact Factor
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L. Sangaletti,
P. Borghetti,
P. Ghosh,
S. Pagliara,
P. Vilmercati,
C. Castellarin-Cudia, L. Floreano,
A. Cossaro,
A. Verdini,
R. Gebauer,
A. Goldoni
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ABSTRACT: The electronic structure of eumelanin thin films has been investigated by means of x-ray absorption and photoemission spectroscopies. The main features of the experimental data are interpreted on the basis of density-functional calculations for the isolated monomers participating to the eumelanin macromolecule. In order to single out the polymerization effects, we followed a bottom-up scaling approach to establish the minimum supramolecular level of organization that can provide a consistent spectroscopical picture of an altogether complex and highly disordered system. A tetramer macrocycle, made by three hydroquinones and one indolequinone, is found to reproduce the observed polymerization effects at the N K edge, while preserving the experimental spectral weight among the different monomers. This tetramer is different from that predicted for the synthesis from isolated monomers, providing an experimental evidence of the role of the reaction path on the stabilization of macrocycles in condensed-phase eumelanin.
Physical Review B 11/2009; 80(17). · 3.69 Impact Factor
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ABSTRACT: The electronic properties of Cu-phthalocyanine (CuPc) molecules flat lying along the channels of the Au(110) reconstructed surface have been investigated by means of ultraviolet and x-ray photoelectron spectroscopy. The ordered chains give rise to a highly ordered single-layer structure with a (5x3) symmetry. Although from the core-level analysis not any significant charge transfer between the molecules and the underlying Au surface is observed, the valence band photoemission data bring to light CuPc-induced features localized at the interface. In particular, energy versus momentum dispersion of an interface state reveals a bandwidth of about 90 meV along the enlarged Au channels, where the CuPc chains are formed, with a defined fivefold symmetry well fitting the CuPc intermolecular distance.
The Journal of chemical physics 11/2009; 131(17):174710. · 3.09 Impact Factor
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A Cossaro,
R Mazzarello,
R Rousseau,
L Casalis,
A Verdini,
A Kohlmeyer, L Floreano,
S Scandolo,
A Morgante,
M L Klein,
G Scoles
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ABSTRACT: The structure of self-assembled monolayers (SAMs) of long-chain alkyl sulfides on gold(111) has been resolved by density functional theory-based molecular dynamics simulations and grazing incidence x-ray diffraction for hexanethiol and methylthiol. The analysis of molecular dynamics trajectories and the relative energies of possible SAM structures suggest a competition between SAM ordering, driven by the lateral van der Waals interaction between alkyl chains, and disordering of interfacial Au atoms, driven by the sulfur-gold interaction. We found that the sulfur atoms of the molecules bind at two distinct surface sites, and that the first gold surface layer contains gold atom vacancies (which are partially redistributed over different sites) as well as gold adatoms that are laterally bound to two sulfur atoms.
Science 09/2008; 321(5891):943-6. · 31.20 Impact Factor
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P Krüger,
S Bourgeois,
B Domenichini,
H Magnan,
D Chandesris,
P Le Fèvre,
A M Flank,
J Jupille, L Floreano,
A Cossaro,
A Verdini,
A Morgante
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ABSTRACT: The charge distribution of the defect states at the reduced TiO(2)(110) surface is studied via a new method, the resonant photoelectron diffraction. The diffraction pattern from the defect state, excited at the Ti-2p-3d resonance, is analyzed in the forward scattering approach and on the basis of multiple scattering calculations. The defect charge is found to be shared by several surface and subsurface Ti sites with the dominant contribution on a specific subsurface site in agreement with density functional theory calculations.
Physical Review Letters 03/2008; 100(5):055501. · 7.37 Impact Factor
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Physical Review B 01/2008; 77:-. · 3.69 Impact Factor
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ABSTRACT: We have investigated the controversy surrounding the (sqrt[3] x sqrt[3]) R30 degrees structure of self-assembled monolayers of methylthiolate on Au(111) by first principles molecular dynamics simulations, energy and angle resolved photoelectron diffraction, and grazing incidence x-ray diffraction. Our simulations find a dynamic equilibrium between bridge site adsorption and a novel structure where 2 CH3S radicals are bound to an Au adatom that has been lifted from the gold substrate. As a result, the interface is characterized by a large atomic roughness with both adatoms and vacancies. This result is confirmed by extensive photoelectron and grazing incidence x-ray diffraction measurements.
Physical Review Letters 02/2007; 98(1):016102. · 7.37 Impact Factor
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M Pedio,
F Borgatti,
A Giglia,
N Mahne,
S Nannarone,
S Giovannini,
C Cepek,
E Magnano,
G Bertoni,
E Spiller,
M Sancrotti,
L Giovanelli, L Floreano,
R Gotter,
A Morgante
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ABSTRACT: We report a near-edge x-ray absorption spectroscopy (NEXAFS) study of the C K-edge of C60 molecules interacting with Si(100) and Si(111) surfaces. Annealing the C60/Si systems at 1050 K induces fullerene fragmentation and leads to SiC nucleation. Tuning the probing depth and studying polarization dependence, the NEXAFS results shed light on the evolution of the molecular states after thermal treatment, up to the fullerene fragmentation and SiC formation. The NEXAFS spectra of a single layer of C60 deposited at 670 K onto both surfaces show a strong C60-substrate interaction. A polarization dependence of the spectra can be detected; this behaviour is similar to the results obtained on C60 covalently chemisorbed on Al surfaces. We find, for both substrates, that fullerene fragmentation starts at the C-Si interfaces, involving the C atoms lying close to the Si substrate, implying an increase of the number of C-Si bonds at the fragmentation stage.
Physica Scripta 11/2006; 2005(T115):695. · 1.20 Impact Factor
