Search

While the demonstrated power conversion eff i ciency of organic photovoltaics (OPVs) now exceeds 10%, new design rules are required to tailor interfaces at the molecular level for optimal exciton dissociation and charge transport in higher effi ciency devices. We show that molecular shape-complementarity between donors and acceptors can drive performance in OPV devices. Using core hole clock (CHC) X-ray spectroscopy and density functional theory (DFT), we compare the electronic coupling, assembly, and charge transfer rates at the interface between C 60 acceptors and fl at- or contorted-hexabenzocorone (HBC) donors. The HBC donors have similar optoelectronic properties but differ in molecular contortion and shape matching to the fullerene acceptors. We show that shape-complementarity drives self-assembly of an intermixed morphology with a donor/acceptor (D/A) ball-and-socket interface, which enables faster electron transfer from HBC to C 60 . The supramolecular assembly and faster electron transfer rates in the shape complementary heterojunction lead to a larger active volume and enhanced exciton dissociation rate. This work provides fundamental mechanistic insights on the improved effi ciency of organic photovoltaic devices that incorporate these concave/convex D/A materials.

The Fe62Ni20Cr18 valence band was studied by scanning the photon energy across the 2p(3/2) core-level threshold of each element of the alloy. A resonant enhancement of the weak 3d-like features was observed. In pure transition metals, similar valence band resonances are explained by a radiation-less Raman de-excitation emission, which is active at threshold and degenerate with the two-hole satellite of direct photoemission. Present structures are associated to satellite features occurring in Fe62Ni20Cr18, and their intensities and binding energies are compared to those of the pure metal components. The alloy satellite resonant behaviour reveals some peculiar modi. cations of: a) the crossover between the radiationless Raman scattering and the Auger emission regimes; b) the ratio of the relative intensities of the main and satellite peaks. We mainly assign these differences to the hetero-nuclear bonds in the alloy.

The atomic-like behavior of the photoionization cross-section of core levels in solids is remarkably displayed in the observation of linear magnetic dichroism in the angular distribution of photoelectrons. Structure-related effects are clearly visible as modulations of the dichroism signal, induced by photoelectron diffraction, a fact that can be exploited to obtain information from magnetic surfaces. A systematic investigation of such effects has been carried out for Fe 2p and 3p core levels on the ALOISA beamline at ELETTRA. We present results for Fe(0 0 1) with a reference calibration for amorphous RCP-Fe grown on Fe30Ni50B20, which provides the atomic-like standard, free from diffraction effects.

The temperature-dependent behavior of the Rh(111)-(2\ifmmode\times\else\texttimes\fi{}2)-1O phase was investigated by He-atom scattering (HAS) and low-energy electron diffraction. The adsorption system undergoes an order-disorder phase transition at ${\mathrm{T}}_{\mathrm{c}}$=280\ifmmode\pm\else\textpm\fi{}5 K, with critical exponents found to be consistent with the four-state Potts model. Beyond the phase transition the HAS specular peak intensity exhibits a strong and reversible increase. This finding points toward a reduction of the surface charge-density corrugation induced by the phase transition itself. Around 160 K, hydrogen adsorbed on the Rh(111)-(2x2)-1O surface reacts with oxygen to form water, and drives the overlayer in an out-of-equilibrium condition which is characterized by a dramatic domain-wall proliferation.

We have investigated the structure of nitrogen-hydrogen complexes in ${\text{GaAs}}_{1\ensuremath{-}y}{\text{N}}_{y}$ and ${\text{In}}_{x}{\text{Ga}}_{1\ensuremath{-}x}{\text{As}}_{1\ensuremath{-}y}{\text{N}}_{y}$ dilute nitride alloys by performing x-ray absorption near-edge structure spectroscopy (XANES). We simulated the spectra based on first-principles calculations of the most recent defective structures proposed in the literature for hydrogenated materials. The comparison between the experimental data and simulations allows us to clarify that the core of the defect is a complex with ${\text{C}}_{2v}$ structure in the neutral charge state, in agreement with the expansion of the lattice parameter measured by x-ray diffraction. Our results are compatible with the presence of H satellites bound to neighboring Ga atoms but not with complexes involving more than two H atoms bound to the same N. Nevertheless, we were not able to determine uniquely the number of H satellites, which may depend on growth conditions. Strain related to the epitaxial growth has a very little effect on the XANES spectra.

Charge transfer processes between donor-acceptor complexes and metallic electrodes are at the heart of novel organic optoelectronic devices such as solar cells. Here, a combined approach of surface-sensitive microscopy, synchrotron radiation spectroscopy, and state-of-the-art ab initio calculations is used to demonstrate the delicate balance that exists between intermolecular and molecule-substrate interactions, hybridization, and charge transfer in model donor-acceptor assemblies at metal-organic interfaces. It is shown that charge transfer and chemical properties of interfaces based on single component layers cannot be naively extrapolated to binary donor-acceptor assemblies. In particular, studying the self-assembly of supramolecular nanostructures on Cu(111), composed of iuorinated copperphthalocyanines (F16CuPc) and diindenoperylene (DIP), it is found that, in reference to the associated single component layers, the donor (DIP) decouples electronically from the metal surface, while the acceptor (F16CuPc) suffers strong hybridization with the substrate © 2009 WILEY-VCH Verlag GmbH & Co. KGaA., This work was supported by the European Community through the Integrated Infrastructure Initiative ‘‘European Light Sources Activities – Synchrotrons and Free Electron Lasers’’. The work was further supported through the Spanish Ministerio de Educacio´n y Ciencia (MAT2007-63083, FIS2007-65702-C02-01), ‘‘Grupos Consolidados UPV/EHU del Gobierno Vasco’’ (IT-319-07, IT-257-07), CSIC, and the European Community throughe-I3 ETSF project (Contract Number 211956), and NANO-ERA CHEMISTRY. We acknowledge support by the ‘‘Red Espanola de Supercomputacion’’ and SGIker ARINA(UPV/EHU). J. M. G.-L. acknowledges funding Spanish MEC through Juan de la Cierva program.

The study of the early stage of Mn growth on GaAs(001)-c(4 × 4) surface has been performed by in situ Scanning Tunneling Microscopy and X-ray Photoelectron Spectroscopy. Starting from GaAs(001) grown by molecular beam epitaxy, the surface was investigated after Mn deposition and after low temperature annealing at about 250 ∘C. The aim of this work is to understand the mechanism of Mn–As interaction and the behavior of Mn on the GaAs(001) substrate. The results demonstrate the high reactivity and mobility of Mn with the formation of compounds such as Mn subarsenide (MnAsx), MnAs (confirmed by STM results) and, probably, GaMnAs.

The angular distribution of the intensity in photoemission experiments is affected by electron diffraction patterns and by a smoothly varying ISO contribution originated by both intrumental details and physical properties of the samples. The origin of the various contributions to the ISO component has been identified since many years. Nonetheless in this work we present original developement of the ED analysis, which arises from the evolution of instrumental performance, in terms of analyzers positioning and angular resolution, as well as collimation and size of X-ray beams in third generation synchrotron sources. The analytical treatement of the instrumental factors is presented in detail for the end station of the ALOISA beamline (Trieste Synchrotron), where a wide variety of scattering geometries is available for ED experiments. We present here the basic formulae and their application to experimental data taken on the Fe/Cu3Au(001) system in order to highlight the role of the various parameters included in the distribution function. A specific model for the surface illumination has been developed as well as the overlayer thickness and surface roughness have been considered., RevTex, nine pages with five eps figures; to be published in J. Electron Spectrosc. Relat. Phenom

The effects of photoelectron diffraction on the magnetic linear dichroism in the angular distribution (MLDAD) signal have been measured for experiments on crystalline iron and cobalt. Experimental photoemission data of Fe3p and Co 3p core levels have been obtained in chiral geometry from bulk Fe(100) and from Co/Fe) (100) and Fe/Co/Fe( 100) epitaxial interfaces. A prominent forward-scattering peak is observed at normal emission and. correspondingly, a severe reduction of the Fe and Co magnetic dichroism asymmetry. The comparison between full multiple-scattering calculations and experimental results provides a rationale for understanding the role of photoelectron diffraction in MLDAD experiments. In this connection we show that both surface and hulk atoms contribute to the measured dichroic signal. but the line shape of bulk contribution to the spectra can be retrieved.

NiO ultrathin films have been grown on Ag(001) by Ni deposition in an O-2 atmosphere. The thickness range 5-50 NIL has been investigated. X-ray photoelectron spectroscopy has been used to study the stoichiometric composition and chemical purity of the oxide films. We found completely oxidized stoichiometric NiO films. Their contamination has been found to be limited to the topmost layers. Photoelectron diffraction has given information concerning the local crystal structure of the films. The film atomic geometry has been found to be the same independent of thickness in the 0-50 ML range. The films have the expected (001) rock-salt structure with the same in plane orientation as the Ag(001) substrate. Specular X-ray reflectivity has allowed a very accurate thickness evaluation and has given information on the width of the density gradients at the film-substrate and vacuum-film interfaces, found to be of the order of a few atomic layers.

The UHV deposition of alpha-hexathiophene (T6) oligomers on the 1x2 reconstructed Au(110) surface has been studied in situ. The evolution of the surface morphology during the deposition is followed by the He atom scattering method. A proliferation of Au monoatomic steps is observed during deposition, indicating a strong interaction between Au substrate and T6 molecules. The build-up of a well-ordered single monolayer is obtained with a rectangular unit cell. Its dimensions indicate that T6 molecules lie on the surface, aligned along the rows of close-packed Au atoms.

A helium-atom scattering experiment is reported on the evolution of vacancy islands produced by low-energy ion bombardment on InSb (110). It is seen that island growth occurs through dynamical scaling during the sputtering process, in agreement with the scaling hypothesis that is the basis of nucleation and growth models. After interruption of the sputtering, the spatial distribution of steps on the surface (strictly related to the island size and distance) gradually modifies during the self-organizing surface recovery. In this case the surface reduces its free energy by decreasing the step density; the corresponding increase of the terrace width occurs in the absence of free vacancy supersaturation and the process is diffusion limited. This change of the growth mechanism causes the breaking of dynamical scaling.

The electron density of the (1×2)Pt(110) surface is determined by He reflectivity measurements and compared to the electron densities previously reported for (1×2)Au(110) and (1×2)Rh(110). With respect to the superposition of free atomic electron densities, significant charge depletion and enrichment are observed, respectively, on top of the surface atoms and in the threefold sites of the (111) microfacets.

He beam scattering measurements of the erosion of the InSb(110) surface by low-energy Ar+ bombardment are reported. Layer-by-layer erosion is observed for surface temperatures above 510 K and is found to proceed by nucleation of vacancy islands, island growth, and coalescence. The average island distance is measured at different stages of the erosion process and it is found to evolve in good agreement with the rate equations derived from models of island growth by atomic deposition. A general increase of the average terrace width is also observed with increasing number of removed layers. The activation barrier for intralayer diffusion of InSb dimer vacancies is found to be Ed=1.14±0.06 eV.

We show how the anisotropy of the (1 x 2) reconstructed Au(110) surface can be effectively used to achieve spontaneous ordered planar growth of rodlike conjugated oligomers uniaxially oriented on the substrate. The growth of the first few monolayers of alpha-sexithienyl (T6) films on the (1 x 2) reconstructed Au(110) surface in ultrahigh vacuum has been investigated by He atom scattering (HAS), scanning tunneling microscopy (STM), and low-energy electron diffraction (LEED). T6 molecules lie flat on the substrate forming long chains along the [110] direction of the Au rows. The interchain spacing is coverage dependent and uniaxially commensurate with the substrate along [001]. Three ordered structures with 12, 16, and 20 Angstrom periods are formed during the deposition of the first two T6 monolayers. This T6 overlayer acts as a template for the growth of at least five monolayers which are still planar and orientationally ordered.