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Surface modification of reactive oxide substrates to obtain a less strongly interacting template for dye adsorption may be a way to enhance performance in dye-sensitized solar cells. In this work, we have investigated the electronic and structural properties of 4,4′-bipyridine (bipy) as modifier adsorbed on the TiO2(110) surface. The modified surface is then coated with iron phthalocyanine (FePc) and the properties of this heterostructure are investigated with synchrotron based photoelectron spectroscopy, x-ray absorption spectroscopy, and scanning tunneling microscopy. We find that a saturated monolayer consisting of standing bipy molecules with one nitrogen atom pointing outward is formed on the oxide surface. FePc adsorb in molecular chains along the [001] direction on top of bipy and ordered in a tilted arrangement with adjacent molecules partially overlapping. Already from the first layer, the electronic properties of FePc resemble those of multilayer films. FePc alone is oxidized on the TiO2(110) surface, but preadsorbed bipy prevents this reaction. The energy level lineup at the interface is clarified. [ABSTRACT FROM AUTHOR]

We have performed core level photoelectron spectroscopy on a W(110) single crystal with femtosecond XUV pulses from the free-electron laser at Hamburg (FLASH). We demonstrate experimentally and through theoretical modelling that for a suitable range of photon fluences per pulse, time-resolved photoemission experiments on solid surfaces are possible. Using FLASH pulses in combination with a synchronized optical laser, we have performed femtosecond time-resolved core-level photoelectron spectroscopy and observed sideband formation on the W 4f lines indicating a cross correlation between femtosecond optical and XUV pulses. [ABSTRACT FROM AUTHOR]

Photon-induced band bending changes and subsequent relaxation at the buried insulator/semiconductor interface of BaF2/Si(001) have been investigated. This is achieved by creating free charge carriers through absorption of a visible pump laser pulse (λ=532 nm, E=2.33 eV) in the silicon, followed by time resolved photoelectron spectroscopy using vacuum-ultraviolet synchrotron radiation (E=18 eV) as a probe of the BaF2 valence band (VB). Since the excitation takes place in the semiconductor and the probe photoemission signal originates from the BaF2, the relaxation at the interface has been probed. [ABSTRACT FROM AUTHOR]

X-ray spectroscopy studies of potassium intercalated metal-free phthalocyanine multilayers adsorbed on Al(110) have been undertaken. Photoelectron spectroscopy measurements show the presence of several charge states of the molecules upon K intercalation, due to a charge transfer from the alkali. In addition, the comparison of valence band photoemission spectra with the density functional theory calculations of the density of states of the H2Pc- anion indicates a filling of the formerly lowest unoccupied molecular orbital by charge transfer from the alkali. This is further confirmed by x-ray absorption spectroscopy (XAS) studies, which show a decreased density of unoccupied states. XAS measurements in different experimental geometries reveal that the molecules in the pristine film are standing upright on the surface or are only slightly tilted away from the surface normal but upon K intercalation, the molecular orientation is changed in that the tilt angle of the molecules increases. [ABSTRACT FROM AUTHOR]

Abstract: Charge transfer pathways and charge transfer times in molecular films and in adsorbate layers depend both on the details of the electronic structure as well as on the degree of the initial localization of the propagating excited electronic state. For C6F6 molecular adsorbate films on the Cu(111) surface we determined the interplay between excited state localization and charge transfer pathways. In particular we selectively prepared a free-particle-like LUMO band excitation and compared it to a molecularly localized core-excited C1s→ π ⁎ C6F6 LUMO state using time-resolved two-photon photoemission (tr-2PPE) and core–hole–clock (CHC) spectroscopy, respectively. For the molecularly localized core-excited C1s→ π ⁎ C6F6 LUMO state, we separate the intramolecular dynamics from the charge transfer dynamics to the metal substrate by taking the intramolecular dynamics of the free C6F6 molecule into account. Our analysis yields a generally applicable description of charge transfer within molecular adsorbates and to the substrate. [Copyright &y& Elsevier]

Abstract: Thin films of titanyl phthalocyanine (TiOPc) have been adsorbed on InSb(111) (3×3) and InSb(100) c(8×2) surfaces and studied with respect to their electronic structure using photoemission (PES), density functional theory (DFT) and scanning tunneling microscopy (STM). The interface chemical interaction is weak in both cases; no adsorbate induced surface band bending is observed and the energy level alignment across the interface is determined by the original position of the substrate Fermi level and the charge neutrality level of the molecule. Room temperature adsorption results in disordered films on both surfaces. The behaviors after annealing are different; on InSb(100) well-ordered molecular chains form along and on top of the In-rows, whereas on (111) no long range order is observed. The disorder leads to intermolecular interactions between the titanyl group and neighboring benzene rings leading to a split of TiOPc HOMO (highest occupied molecular orbital) by as much as 0.8eV. [Copyright &y& Elsevier]

Abstract: The orientation of hexafluorobenzene (C6F6) on the Cu(111) surface has been determined for different coverages with the help of near edge X-ray absorption fine structure (NEXAFS) spectroscopy and X-ray photoelectron spectroscopy (XPS). The adsorption geometry and the bonding mode of C6F6 differ significantly in comparison to its hydrocarbon analog C6H6. C6F6 is found to adsorb on Cu(111) with the ring plane parallel to the surface for coverages below 10ML. Next to the distinct multilayer, bilayer and monolayer phases we also present evidence of sub-monolayer (i.e., 1/2ML) coverage with different electronic structure. These findings are explained in a phenomenological model based on fluorine’s property as a σ-acceptor and a π-donor and the resulting bond polarization within the molecule, which is stabilized by image-potential screening within the substrate. [Copyright &y& Elsevier]