On-surface products from de-fluorination of C60F48 on Ag(111): C60, C60Fx and silver fluoride formation.

By employing diverse surface sensitive synchrotron radiation spectroscopies we demonstrate that the fluorine content of initial C60F48 deposited at room temperature on Ag(111) varies with molecular coverage. At the very early stages of deposition, C60F48 fully de-fluorinates and transforms into C60. Strong indications of silver fluoride formation are provided. The chemical footprint of fluorinated fullerenes emerges at relatively low molecular coverage indicating that the degree of fullerene de-fluorination decreases (from total to partial de-fluorination) as molecules are deposited. Full de-fluorination stops well before the substrate surface is completely covered by fullerenes. At the molecular level, the fluorine loss observed by spectroscopic techniques is supported by scanning tunneling microscopy imaging. Both molecules and metal surface are importantly involved in the process.

Ubiquitous deprotonation of terephthalic acid in the self-assembled phases on Cu(100).

We performed an exhaustive study of terephthalic acid (TPA) self-assembly on a Cu(100) surface, where first-layer molecules display two sequential phase transitions in the 200-400 K temperature range, corresponding to different stages of molecular deprotonation. We followed the chemical and structural changes by means of high-resolution X-ray photoelectron spectroscopy (XPS) and variable-temperature scanning tunneling microscopy (STM), which were interpreted on the basis of density functional theory (DFT) calculations and photoemission simulations. In order to reveal the spectroscopic contributions of the molecules in different states of deprotonation, we modified the substrate reactivity by deposition of a small amount of Sn, which hampers the deprotonation reaction. We found that the characteristic molecular ribbons of the TPA/Cu(100) α-phase at a low temperature contain a significant fraction of partially deprotonated molecules, in contrast to the expectation of a fully protonated phase, where the self-assembly was claimed to be simply driven by the intermolecular double hydrogen bonds [OHO]. On the basis of our simulations, we propose a model where the carboxylate groups of the partially deprotonated molecules form single hydrogen bonds with the carboxylic groups of the fully protonated molecules. Using real time XPS, we also monitored the kinetics of the deprotonation reaction. We show that the network of mixed single and double hydrogen bonds inhibits further deprotonation up to ∼270 K, whereas the isolated molecules display a much lower deprotonation barrier.

The electronic properties of three popular high spin complexes [TM(acac)3, TM = Cr, Mn, and Fe] revisited: an experimental and theoretical study.

The occupied and unoccupied electronic structures of three high spin TM(acac)3 (TM = Cr, Mn, and Fe) complexes (I, II, and III, respectively) were studied by revisiting their literature vapour-phase He(i) and, when available, He(ii) photoemission (PE) spectra and by means of original near-edge X-ray absorption fine structure (NEXAFS) spectroscopic data recorded at the O K-edge (OK-edge) and TM L2,3-edges (TML2,3-edges). The assignments of the vapour-phase He(i)/He(ii) PE spectra were guided by the results of spin-unrestricted non-relativistic Slater transition state calculations, while the OK-edge and TML2,3-edge spectroscopic pieces of evidence were analysed by exploiting the results of spin-unrestricted scalar-relativistic time-dependent density functional theory (DFT) and DFT/ROCIS calculations, respectively. Although the actual symmetry (D3, in the absence of any Jahn-Teller distortion) of the title molecules allowed an extensive mixing between TM t2g-like and eg-like atomic orbitals, the use of the Nalewajski-Mrozek TM-O bond multiplicity index combined with a thorough analysis of the ground state (GS) outcomes allowed the assessment of the TM-O bond weakening associated with the progressive TM 3d-based eg-like orbital filling. The experimental information provided by OK-edge spectra was rather poor; nevertheless, the combined use of symmetry, orbitals and spectra allowed us (i) to rationalise minor differences characterizing spectral features along the series, (ii) to quantify the contribution provided by the ligand-to-metal-charge-transfer (LMCT) excitations to the different spectral features, and (iii) to recognize the t2g-/eg-like nature of the TM 3d-based orbitals involved in LMCT transitions. As far as the TML2,3-edge spectra and the DFT/ROCIS results were concerned, the lowest lying I,IIL3 spectral features included states having either the GS spin multiplicity (S(I) = 3/2, S(II) = 2) or, at higher excitation energies (EEs), states with ΔS = ±1. In contrast to that, only states with ΔS = 0, -1 significantly contributed to the IIIL3 spectral pattern. Along the whole series, the L3 higher EE side was systematically characterized by states involving TM2p → π4 MLCT excitations; as such, coupled-single excitations with ΔS = 0 were involved in I and II, while single MLCT TM2p → π4 transitions with ΔS = -1 were involved in III.

Tuning the catalytic activity of Ag(110)-supported Fe phthalocyanine in the oxygen reduction reaction.

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.

Binding geometry of hydrogen-bonded chain motif in self-assembled gratings and layers on Ag(111).

Upon adsorption on the (111) facet of Ag, 4-[trans-2-(pyrid-4-yl-vinyl)] benzoic acid (PVBA) self-assembles into a highly ordered, chiral twin chain structure at submonolayer coverages with domains that can extend for micrometers in one dimension. Using polarization-dependent measurements of C and N K-shell excitations in near-edge X-ray absorption fine structure (NEXAFS) spectra, we determine the binding geometry of single PVBA molecules within this unique ensemble for both low and high coverage regimes. At submonolayer coverage, the molecule is twisted to facilitate the formation of hydrogen bonds. The gas-phase planarity is gradually recovered as the coverage is increased, with complete planarity coinciding with loss of order in the overlayer. Thermal treatment of the PVBA film results in deprotonation of the carboxyl tail of the molecule, but despite the suppression of the stabilizing hydrogen-bonds, the overlayer remains ordered.

Functional K-doping of eumelanin thin films: Density functional theory and soft x-ray spectroscopy experiments in the frame of the macrocyclic protomolecule model [corrected].

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.

Tracking the excitation dynamics in the Mn:Ge(111) metallic interface by resonant electron spectroscopy.

Resonant photoemission from the valence band of a (√3 × âˆš3)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.

Intrinsic nature of the excess electron distribution at the TiO2(110) surface.

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.

Electronic states of CuPc chains on the Au(110) surface.

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.

X-ray diffraction and computation yield the structure of alkanethiols on gold(111).

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.

Defect states at the TiO2(110) surface probed by resonant photoelectron diffraction.

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.

Structure of a CH3S monolayer on Au(111) solved by the interplay between molecular dynamics calculations and diffraction measurements.

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.

Electronic properties of a pure and sodium-doped C(70) single layer adsorbed on Al polycrystalline surface.

Core level and valence band photoemission measurements combined with near edge x-ray absorption fine structure measurements were performed on a single C(70) layer adsorbed on polycrystalline Al (1 ML-C(70)/Al) (ML-monolayer), pure and doped with sodium atoms. The data obtained from the pure ML chemisorbed on Al surface show a semiconducting behavior of the system, which is characterized by a covalent bond between the adsorbate and the substrate. The same data show also that the C(70) molecules tend to orient themselves with the C(5v) axis perpendicular to the surface in analogy to what observed for 1 ML-C(70)/Cu(111). By doping the sample with sodium atoms a charge transfer from the alkali atoms to the lowest unoccupied molecular orbital (LUMO) of the C(70) molecules takes place, as underlined by the gradual increasing intensity of the C(70) LUMO peak as a function of doping. Nevertheless, no metallic phases are observed for any doping step.

Selectivity of auger decays to the local surface environment.

The line shape of the Auger decay of adatoms is studied by a joint theoretical and experimental effort, the former within a DFT framework, and the latter with synchrotron radiation measurements. We investigate the KL(2,3)V Auger deexcitation of Na on Al(111), a system with different adsorption geometries. In particular, we study the (sqrt[3]xsqrt[3])R30 degrees phase at 1/3 ML (monolayer) and the more complex (2 x 2) structure at 1/2 ML coverage. From the comparison between theory and experiment, we unambiguously determine features that allow for the determination of the adsorption environment from the adatom Auger spectrum.

Temperature driven reversible breakdown of pseudomorphism in ultrathin Fe/Cu3Au films.

We observe that ultrathin Fe/Cu(3)Au(001) films in the 6-13 A thickness range, beyond the thickness of pseudomorphism breakdown at room temperature, exhibit a temperature dependent structural phase transition in the range T(c) approximately 345-380 K. In the high temperature state the Fe film becomes pseudomorphic, while breakdown of pseudomorphism reversibly occurs as the system is cooled below the transition temperature. The difference between substrate and overlayer thermal expansion coefficient is highlighted as the driving force for the observed transition.