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The on-surface synthesis of boroxine-containing molecules can be a convenient method of introducing specific functionalities. Here, we show the validity of a previously described synthesis protocol on the Au (111) surface by applying it to a different molecular precursor. We study in detail the assembly of the precursor, highlighting possible intermediate stages of the condensation process. We combine scanning tunneling microscopy and X-ray spectroscopies to fully characterize both the morphology and the electronic properties of the system. DFT calculations are presented to assign the main electronic transitions originating the B K-edge absorption spectrum. The study paves the way to a facile strategy for functionalizing a surface with molecules of tailored sizes and compositions.

The incorporation of singlet fission (SF) chromophores in solar cells is expected to bring significant increases in the power conversion efficiency thanks to multiexciton generation. However, efficient charge generation in the device is determined by the energy level alignment (ELA) between the active materials, which should favor exciton transport and separation under illumination. By combining ultraviolet photoemission spectroscopy and optical differential reflectance measurements, we determine the ELA in a prototypical SF heterojunction between pentacene (Pc) and perylene-tetracarboxylic-diimide (PTCDI) grown on Ag(111). Time-resolved X-ray photoelectron spectroscopy on such a system reveals light-induced modifications of the ELA, by measuring the transient shift of the core level photoemission lines we observe an accumulation of long-lived holes in the PTCDI within the first hundred picoseconds after the optical pump.

The creation of stable molecular monolayers on metallic surfaces is a fundamental challenge of surface chemistry. N-Heterocyclic carbenes (NHCs) were recently shown to form self-assembled monolayers that are significantly more stable than the traditional thiols on Au system. Here we theoretically and experimentally demonstrate that the smallest cyclic carbene, cyclopropenylidene, binds even more strongly than NHCs to Au surfaces without altering the surface structure. We deposit bis(diisopropylamino)cyclopropenylidene (BAC) on Au(111) using the molecular adduct BAC-CO2 as a precursor and determine the structure, geometry, and behavior of the surface-bound molecules through high-resolution X-ray photoelectron spectroscopy, atomic force microscopy, and scanning tunneling microscopy. Our experiments are supported by density functional theory calculations of the molecular binding energy of BAC on Au(111) and its electronic structure. Our work is the first demonstration of surface modification with a stable carbene other than NHC; more broadly, it drives further exploration of various carbenes on metal surfaces.

Crown ethers, assembled into a regular 2D array via a chemical guesthost recognition process, have been successfully employed to trap sodium atoms on a surface, under ultra- high vacuum conditions.

The thermal treatment of a monolayer of 1-naphthylmethylamine (NMA) on Ni(1 1 1) surface reveals a complex chemistry promoted by the high surface reactivity. Both the amino-terminated functional group and the naphthalene body of NMA undergo a progressive dehydrogenation, which eventually leads to the coalescence of the molecules. As a result, 2D structures of increasing size are synthesized on the surface, characterized by a graphene-like lattice in which nitrogen atoms are mainly incorporated as pyridinic substitutional defects. In this paper, we show the evidence of the formation of extended nitrogen-doped graphene (N-GR) domains at temperatures as low as 300 °C, with band dispersion measurements that reveal p-type doping.

The synthesis and preliminary characterization of a boron-based 2D framework are presented. The peculiar electronic and morphological properties of this compound, together with its facile formation process, enable it to be used as a novel smart material for the design of electronic devices.

Charge transport properties of a vertically stacked organic heterojunction based on the amino-carboxylic (A-C) hydrogen bond coupling scheme are investigated by means of X-ray resonant photoemission and the core-hole clock method. We demonstrate that hydrogen bonding in molecular bilayers of benzoic acid/cysteamine (BA/CA) with an A-C coupling scheme opens a site selective pathway for ultrafast charge transport through the junction. Whereas charge transport from single BA layer directly coupled to the Au(111) is very fast and it is mediated by the phenyl group, the interposition of an anchoring layer of CA selectively hinders the delocalization of electrons from the BA phenyl group but opens a fast charge delocalization route through the BA orbitals close to the A-C bond. This evidences that hydrogen bonding established upon A-C recognition can be exploited to spatially/orbitally manipulate the charge transport properties of heteromolecular junctions.

Charge transfer rates at metal/organic interfaces affect the efficiencies of devices for organic based electronics and photovoltaics. A quantitative study of electron transfer rates, which take place on the femtosecond timescale, is often difficult, especially since in most systems the molecular adsorption geometry is unknown. Here, we use X-ray resonant photoemission spectroscopy to measure ultrafast charge transfer rates across pyridine/Au(111) interfaces while also controlling the molecular orientation on the metal. We demonstrate that a bi-directional charge transfer across the molecule/metal interface is enabled upon creation of a core-exciton on the molecule with a rate that has a strong dependence on the molecular adsorption angle. Through density functional theory calculations, we show that the alignment of molecular levels relative to the metal Fermi level is dramatically altered when a core-hole is created on the molecule, allowing the lowest unoccupied molecular orbital to fall partially below the metal Fermi level. We also calculate charge transfer rates as a function of molecular adsorption geometry and find a trend that agrees with the experiment. These findings thus give insight into the charge transfer dynamics of a photo-excited molecule on a metal surface.

Primary amines can interact with neighbor molecules or with a metal substrate via weak bonds involving the electron lone pair of their amino functional group. Near edge X-ray absorption spectra (NEXAFS) on the N 1s edge show that the structure of the empty molecular orbitals localized on the nitrogen atom is very sensitive to these interactions. Here we investigate the origin of these changes by means of theoretical calculations. NEXAFS spectra are simulated for the 1,4-benzenediamine (BDA) molecule in its free, crystalline, and monolayer on Au(111) forms. We identify the electronic states which are affected by these amino-based interactions. In the case of the molecular layer grown on the gold substrate, we show how the results of the calculations can be used to identify intermolecular interactions in fluencing adsorption geometries in molecular monolayers. (Figure Presented).

A combined theoretical and experimental study of the assembly of triphenylboroxines on Au(111) reveals the charge delocalization properties of the interface., We performed a combined experimental and theoretical study of the assembly of phenylboronic acid on the Au(111) surface, which is found to lead to the formation of triphenylboroxines by spontaneous condensation of trimers of molecules. The interface between the boroxine group and the gold surface has been characterized in terms of its electronic properties, revealing the existence of an ultra-fast charge delocalization channel in the proximity of the oxygen atoms of the heterocyclic group. More specifically, the DFT calculations show the presence of an unoccupied electronic state localized on both the oxygen atoms of the adsorbed triphenylboroxine and the gold atoms of the topmost layer. By means of resonant Auger electron spectroscopy, we demonstrate that this interface state represents an ultra-fast charge delocalization channel. Boroxine groups are among the most widely adopted building blocks in the synthesis of covalent organic frameworks on surfaces. Our findings indicate that such systems, typically employed as templates for the growth of organic films, can also act as active interlayers that provide an efficient electronic transport channel bridging the inorganic substrate and organic overlayer.

A number of studies have investigated the properties of monomeric and double-decker phthalocyanines (Pcs) adsorbed on metal surfaces, in view of applications in spintronics devices. In a combined experimental and theoretical study, we consider here a different member of the Pcs family, the (RuPc)(2) dimer, whose structure is characterized by two paired up magnetic centers embedded in a double-decker architecture. For (RuPc)(2) on Ag(111), we show that this architecture works as a preserving cage by shielding the Ru-Ru pair from a direct interaction with the surface atoms. In fact, while noticeable surface-to-molecule charge transfer occurs with the ensuing quenching of the molecular magnetic moment, such phenomena occur here in the absence of a direct Ru-Ag coupling or structural rearrangement, at variance with other Pcs and thanks to the above shielding effect. These unique properties of the (RuPc)(2) architecture are expected to permit an easy control of the surface-to-molecule charge-transfer process as well as of the molecular magnetic properties, thus making the (RuPc)(2) dimer a significant paradigm for innovative "cage" structures as well as a promising candidate for applications in spintronics nano or single-molecule devices.

We present a combined study of the adsorption and ordering of the l-tyrosine amino acid on the close-packed Ag(111) noble-metal surface in ultrahigh vacuum by means of low-temperature scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. On this substrate the biomolecules self-assemble at temperatures exceeding 320 K into linear structures primarily following specific crystallographic directions and evolve with larger molecular coverage into two-dimensional nanoribbons which are commensurate with the underlying atomic lattice. Our high resolution topographical STM data reveal noncovalent molecular dimerization within the highly ordered one-dimensional nanostructures, which recalls the geometrical pattern already seen in the l-methionine/Ag(111) system and supports a universal bonding scheme for amino acids on smooth and unreactive metal surfaces. The molecules desorb for temperatures above 350 K, indicating a relatively weak interaction between the molecules and the substrate. XPS measurements reveal a zwitterionic adsorption, whereas NEXAFS experiments show a tilted adsorption configuration of the phenol moiety. This enables the interdigitation between aromatic side chains of adjacent molecules via parallel-displaced pi-pi interactions which, together with the hydrogen-bonding capability of the hydroxyl functionality, presumably mediates the emergence of the self-assembled supramolecular nanoribbons.

Advanced molecular electronic components remain vital for the next generation of miniaturized integrated circuits. Thus, much research effort has been devoted to the discovery of lossless molecular wires, for which the charge transport rate or conductivity is not attenuated with length in the tunneling regime. Herein, we report the synthesis and electrochemical interrogation of DNA-like molecular wires. We determine that the rate of electron transfer through these constructs is independent of their length and propose a plausible mechanism to explain our findings. The reported approach holds relevance for the development of high-performance molecular electronic components and the fundamental study of charge transport phenomena in organic semiconductors.

The electronic properties of a C70 single layer chemisorbed on Cu(1 1 1) surface, in which the charge state has been modified by Na doping, were studied via synchrotron radiation photoemission and absorption spectroscopy. A two-dimensional metallic phase is observed in the whole range of charge states investigated (from 1 up to 3 electrons/C70). Moreover, all the results suggest that the orientation of the molecules, with the C5v axis perpendicular to the surface, is not affected by Na doping.

The C 60 molecules are known to bind covalently to the Pt(1 1 1) surface. We performed a structural study of one C 60 layer on Pt(1 1 1) by using X-ray photoelectron diffraction (XPD). Our results show that the molecules are bonded to the metal surface with their six-carbon rings facing the Pt(1 1 1) surface and with the CC bonds perpendicularly oriented with respect to the direction of the close-packed atom rows. Unlike to what previously found for Cu(1 1 1) and Al(1 1 1), the measured diffraction pattern shows a three-fold symmetry indicating that only one orientation is taken by the chemisorbed molecules. This can be explained by a stronger interaction of the C 60 molecules with the second layer Pt atoms.

An amino-carboxylic motif is identified as a novel synthon in the formation of 2D hetero-organic architectures at surfaces. The well-defined interacting scheme we describe herein represents an ideal prototypical system for further investigation of the interaction at surfaces of the two functional groups. This journal is

The Fe LMM Auger and Auger vacancy satellite spectra of ultrathin Fe films grown on Cu(100) have been measured using photon excitation energies above and below the L 2 ionisation edge, and in coincidence with the Fe 2p 3/2 and Fe 2p 1/2 photoelectrons. A comparison of the relative intensities of the satellites and main Auger transitions indicates that the M 4,5 holes of L 3 M 4,5 double hole states are localised for longer than the L 3 lifetime for the 0.3 and 10 ML coverages but have a lifetime comparable to that of L 3 holes for the 1 ML coverage. The results for the 1 ML coverage are supported by the results of the coincidence experiments. The reasons for the variation in the lifetime of M 4,5 holes are discussed in terms of changes in the number of Fe–Fe neighbours and the density of states at the Fermi energy in these systems.

The ratio of the two components of the L 23 M 23 V Auger transition is currently used to determine the stoichiometry of titanium oxides. The reduction of Ti manifests itself by a relative increase (decrease) in intensity of the Auger component lying at high (low) kinetic energy, which is assigned to an intraatomic (interatomic) transition involving Ti 3d (O 2p) valence electrons. The LMV Auger transition is herein revisited via its resonant behaviour at the Ti L 23 absorption edge in TiO 2 . Resonances of the high energy component occurs exclusively at the L 2 edge, while resonances of the other component are seen at the L 3 edge, but also at the L 2 edge. The latter being attributed to a L 2 L 3 V Coster–Kronig transition, the low- and high-lying LMV are identified as L 3 MV (including those following CK) and L 2 MV transitions, respectively, at variance with the previous assignment.

We studied the formation of graphene nanoribbons (GNRs) via the self-assembly of 10,10?-dibromo-9,9?-bianthryl precursor molecules on gold surfaces with different synchrotron spectroscopies. Through X-ray photoemission spectroscopy core-level shifts, we followed each step of the synthetic process, and could show that the Br-C bonds of the precursors cleave at temperatures as low as 100 °C on both Au(111) and Au(110). We established that the resulting radicals bind to Au, forming Au-C and Au-Br bonds. We show that the polymerization of the precursors follows Br desorption from Au, suggesting that the presence of halogens is the limiting factor in this step. Finally, with angle-resolved ultraviolet photoemission spectroscopy and density functional theory we show that the GNR/Au interaction results in an upshift of the Shockley surface state of Au(111) by [similar]0.14 eV, together with an increased electron effective mass.

We study the formation of covalent goldcarbon bonds in benzyltrimethylstannane (C10H16Sn) deposited on Au in ultra-high-vacuum conditions. Through X-ray photoemission spectroscopy and X-ray absorption measurements, we find that the molecule fragments at the Snbenzyl bond when exposed to Au surfaces at temperatures as low as -110 degrees C. The resulting benzyl species is stabilized by the presence of Au(111) but only forms covalent AuC bonds on more reactive Au surfaces like Au(110). We also present spectroscopic proof for the existence of an electronic gateway state localized on the AuC bond that is responsible for its unique electronic properties. Finally, we use DFT-based nudged elastic band calculations to elucidate the crucial role played by the under-coordinated Au surface in the formation of AuC bonds.

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.

Charge transfer through noncovalent interactions is crucial to a variety of chemical phenomena. These interactions are often weak and nonspecific and can coexist, making it difficult to isolate the transfer efficiency of one type of bond versus another. Here, we show how core-hole clock spectroscopy can be used to measure charge transfer through noncovalent interactions. We study the model system 1,4-benzenediamine molecules bound on an Au surface through an Au–N donor–acceptor bond as these are known to provide a pathway for electronic conduction in molecular devices. We study different phases of the molecule/Au system and map charge delocalization times from carbon and nitrogen sites on the molecule. We show that charge delocalization across Au–N donor–acceptor bond occurs in less than 500 as. Furthermore, the Au–N bond also enhances delocalization times from neighboring carbon sites, demonstrating that fast charge transfer across a metal–organic interface does not require a covalently bonded system.

Using He atom scattering, layer-by-layer erosion of InSb(110) by low energy ion bombardment has been observed to proceed via nucleation of vacancy islands, island growth and coalescence. The mechanism is in full agreement with theoretical models developed for crystal growth, with diffusing adatoms substituted by diffusing vacancies. In particular it has been observed that coalescence of the vacancy islands sets in after the removal of small fractions of a monolayer and that the average terrace width increases with increasing number of sputtered monolayers.

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.

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.

Metal-dependent conformations: a change in the adaptation of tetraphenylporphyrins (TPPs) on Ag(111) was observed in the presence of a metal ion in the macrocycle. Upon annealing at T>575 K, 2H-TPP molecules increase the overlap of the phenyl π orbitals with the substrate, thus reducing the distance. The presence of Co creates a strong bond between Co dz(2) and the Ag sp states, leaving the porphyrin macrocycle at a larger distance to the surface.

The exoemission in the reaction of O2 with Cs surfaces is found to depend strongly on the translational energy of the impinging O2 molecules. Seeded beam experiments show that in the initial oxidation state exoemisson increases with increasing O2 velocity. Above 0.5 eV of kinetic energy the increase is found to be proportional to exp (−v∗/v) with a value v∗ of 1.6 × 104 m s .

We have investigated the Pt(110) surface by He atom scattering. The (1 x 2) deconstruction transition at 1087 K is shown to fall in 2D Ising universality class with antiphase domain walls proliferating predominantly along the [001] direction. The surface roughening is observed at 1160 ± 10 K through a power law broadening of the anti-phase diffraction peak along the [001] direction. The two distinct phase transition confine the temperature range in which the surface is disordered but still bound to a common height level.

We present a combined multimethod experimental and theoretical study of the geometric and electronic properties of Co-tetraphenyl-porphyrin ?Co-TPP? molecules adsorbed on a Ag?111? surface. Scanning tunnel- ing microscopy ?STM? topographs reveal that Co-TPP forms highly regular arrays with a square unit cell. Hereby, the Co-TPP molecules do not occupy a unique adsorption site on the Ag?111? atomic lattice. The central Co atom of the Co-TPP is found to reside predominantly above fcc and hcp hollow sites of the substrate, as determined from the photoelectron diffraction patterns. A strong adsorption-induced deformation of Co-TPP involving a saddle-shaped macrocycle is evidenced by high-resolution STM images and quantified by near-edge x-ray absorption fine-structure measurements. By scanning tunneling spectroscopy we resolved discrete molecular electronic states and mapped the pertaining spatial charge-density distribution. Specifically, we discuss the interaction of orbitals originating from the Co-metal center with the porphyrin macrocycle and show that the varying adsorption sites induce a modulation in the Co-TPP lowest unoccupied molecular orbital. These findings are corroborated by density-functional-theory calculations.

Using photoemission spectroscopy, we determine the relationship between electronic energy level alignment at a metal-molecule interface and single-molecule junction transport data. We measure the position of the highest occupied molecular orbital (HOMO) relative to the Au metal Fermi level for three 1,4-benzenediamine derivatives on Au(111) and Au(110) with ultraviolet and resonant x-ray photoemission spectroscopy. We compare these results to scanning tunnelling microscope based break-junction measurements of single molecule conductance and to first-principles calculations. We find that the energy difference between the HOMO and Fermi level for the three molecules adsorbed on Au(111) correlate well with changes in conductance, and agree well with quasiparticle energies computed from first-principles calculations incorporating self-energy corrections. On the Au(110) which present Au atoms with lower-coordination, critical in break-junction conductance measurements, we see that the HOMO level shifts further from the Fermi level. These results provide the first direct comparison of spectroscopic energy level alignment measurements with single molecule junction transport data.

In molecular devices, the importance of interfaces cannot be neglected as they determine charge injection and charge flow and, therefore, the device performance. Herein we report on the interaction of one single layer of Zn-tetraphenyl-porphyrin with Ag(110) and Si(111). Photoemission, near-edge X-ray absorption, and resonant photoemission are used to study the bonding nature, the adsorption geometry as well as the dynamics of electron transfer between the molecules and the metal or semiconductor surfaces. Molecule-substrate charge transfer is driven by the overlap with the molecular pi orbitals. In particular, the coupling of the phenyl legs with the substrate and the relative excited charge injection are dramatically different for the two surfaces considered.

One approach developed to improve the performance of bottom contact source/drain electrodes is to treat the contacts with thiols before deposition of the semiconductor. There is evidence indicating that improvement is due to both morphological effects and improved work function matching. Especially suggestive evidence shows that thiols that increase the effective work function of the contacts (e.g., fluorinated thiols) yield better device performance than work function decreasing thiols (e.g., alkane thiols). Here, we compare two technologically relevant thiol treatments, an alkane thiol (1-hexadecanethiol), and a fluorinated thiol (pentafluorobenzenethiol), in pentacene organic field effect transistors. Using in situ semiconductor deposition, x-ray photoemission, and x-ray absorption spectroscopy, we are able to directly observe the interaction between the semiconductor and the thiol-treated gold layers. Our spectroscopic analysis suggests that there is not a site-specific chemical reaction between the pentacene and the thiol molecules. A homogeneous standing-up pentacene orientation was observed in both treated substrates, consistent with the morphological improvement expected from thiol treatment in both samples. Our study shows that both the highest occupied molecular orbital--Fermi level offset and $\text{C}\text{ }1s$ binding energy are shifted in the two thiol systems, which can be explained by varied dipole direction within the two thiols, causing a change in surface potential. The additional improvement of the electrical performance in the pentafluorobenzenethiol case is originated by a reduced hole injection barrier that is also associated with an increase in the density of states in the lowest unoccupied molecular orbital.

The structure of RuO 2 (110) and the mechanism for catalytic carbon monoxide oxidation on this surface were studied by low-energy electron diffraction, scanning tunneling microscopy, and density-functional calculations. The RuO 2 (110) surface exposes bridging oxygen atoms and ruthenium atoms not capped by oxygen. The latter act as coordinatively unsaturated sites—a hypothesis introduced long ago to account for the catalytic activity of oxide surfaces—onto which carbon monoxide can chemisorb and from where it can react with neighboring lattice-oxygen to carbon dioxide. Under steady-state conditions, the consumed lattice-oxygen is continuously restored by oxygen uptake from the gas phase. The results provide atomic-scale verification of a general mechanism originally proposed by Mars and van Krevelen in 1954 and are likely to be of general relevance for the mechanism of catalytic reactions at oxide surfaces.

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 application of the resonant photoemission spectroscopy (RPES) to various organic molecule based systems is reviewed. The chemical specificity and the possibility to conduct experiments in the energy domain that provides a time scale for charge dynamics, make the RPES a powerful tool to study organic heterojunctions and in particular to probe the charge transfer processes at organic interfaces. We briefly discuss the models used in RPES data analysis to extract the time scale of the excited charge delocalisation and the spatial correlation of core, valence occupied and unoccupied molecular states. As an example we report on 3,4,9,10-perylene tetracarboxylic acid dianhydride (PTCDA) on (1 × 2) Au(1 1 0) surface where organic layer metallicity is directly evidenced in RPES experiments. A particular attention is dedicated to bio-mimetic model molecules whose electronic structure at interfaces is the fundamental key for the design of real devices. In the last section we consider recent experiments that could open the way to new fields of applications regarding biological molecules and single molecule systems where RPES could elucidate the link between the quantum and the meso-scopic properties of such systems.

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 self-assembly of the amino acid L-methionine on Cu(111) was investigated under ultrahigh vacuum (UHV) conditions by scanning tunneling microscopy (STM), helium atom scattering (HAS) and X-ray photoelectron spectroscopy (XPS). The system is strongly influenced by the Substrate reactivity and the deposition temperature. The STM and HAS structural analysis yields that, for temperatures below 273 K, the biomolecules assemble in strings oriented with an angle of - 10 degrees with respect to the < 110 > axes of the substrate. For temperatures above 283 K, a regular and ordered one-dimensional (1D) phase arises following an angle of + 10 degrees with respect to the same directions. High resolution STM data of this ordered 1D arrangement evidence molecular dimerization and dimer alignment into ordered chains which are commensurate with the Cu(111) atomic lattice. XPS measurements reveal that the high temperature ordered phase consists of an exclusively anionic ensemble with a deprotonated carboxylic group and a neutral amino group, while the low temperature phase is heterogeneously composed of both zwitterionic and anionic species, depending on whether the molecules are immobilized in clusters of dimers on the free terraces or at the low-coordinated adsorption sites of the Substrate step-edges. These combined results evidence a structural transformation of the supramolecular assembly which is triggered by a thermally activated process involving the underlying Cu(111) substrate and which carries the intrinsic chiral signature of the adsorbed molecular units.

The peculiar electrochemical and photophysical properties of porphyrin and fullerene molecules make them promising candidates for the construction of two- and three-dimensional organic-based materials. An important question is how pristine fullerene and porphyrin will organize when deposited on surfaces via in vacuum molecular beam evaporation. Here we show that codeposition of C(70) and Zn-tetraphenyl-porphyrin (ZnTPP) induces the self-assembly of electron-rich flat aromatic molecules at the curved surface of C(70), thus enhancing the chromophore interaction and forming a supramolecular multilayer donor-acceptor structure. While the ground-state electronic spectra almost reflect a simple summation of ZnTPP and C(70) components, the excited-state electrons at the porphyrin macrocycle can rapidly delocalize to the fullerene. The excited charge transfer time scale is faster than 1-2 fs, as shown by resonant photoemission for the core-excited charges.

A single layer (SL) of pentacene molecules deposited on the Cu(119) surface and on an organic self-assembled monolayer (SAM) has been investigated by near-edge x-ray absorption fine structure (NEXAFS) at the C $K$ edge, and by photoemission at the $\mathrm{C}\phantom{\rule{0.2em}{0ex}}1s$ core level. The lowest unoccupied molecular orbitals (LUMOs) of the pentacene SL on the SAM are basically unaffected with respect to that of pentacene in the gas phase, indicating a weak interaction of pentacene with the SAM, while a strong redistribution of the LUMO-related final states is observed when the molecules are deposited on the Cu(119) substrate, sign of an electronic mixing between molecular and metal electronic states, in agreement with recent theoretical predictions. The strong dichroic response of the NEXAFS signal indicates upstanding pentacene molecules oriented 16\ifmmode^\circ\else\textdegree\fi{} off normal for the organic-organic heterostructure. The rehybridization of pentacene orbitals with Cu states at the pentacene/Cu interface hinders an accurate determination of the molecule orientation, which is, however, compatible with molecules lying flat on the Cu(119) terraces.

We studied the molecular orientation of pentacene monolayer phases on the Au(110) surface by means of near-edge X-ray absorption spectroscopy at the carbon K-shell and scanning tunneling microscopy. The highest coverage phase, displaying a (6 x 8) symmetry, is found to be formed by two types of differently oriented molecules mimicking regular arrays of nanorails. Flat-lying molecules, aligned side-by-side with the long molecular axis along the [001] direction, form long crosstie chains extending in the [1 (1) over bar0] direction. In between the adjacent flat chains, additional molecules, tilted by 90 degrees around their molecular axis, line up head-to-tail into rails extending along [1

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.

The engineering of complex architectures from functional molecules on surfaces provides new pathways to control matter at the nanoscale. In this article, we present a combined study addressing the self-assembly of the amino acid l -methionine on Ag(111). Scanning tunneling microscopy data reveal spontaneous ordering in extended molecular chains oriented along high-symmetry substrate directions. At intermediate coverages, regular biomolecular gratings evolve whose periodicity can be tuned at the nanometer scale by varying the methionine surface concentration. Their characteristics and stability were confirmed by helium atomic scattering. X-ray photoemission spectroscopy and high-resolution scanning tunneling microscopy data reveal that the l -methionine chaining is mediated by zwitterionic coupling, accounting for both lateral links and molecular dimerization. This methionine molecular recognition scheme is reminiscent of sheet structures in amino acid crystals and was corroborated by molecular mechanics calculations. Our findings suggest that zwitterionic assembly of amino acids represents a general construction motif to achieve biomolecular nanoarchitectures on surfaces.

We studied the growth of pentacene (C22H14) on the Au(110) surface by means of He atom scattering and Synchrotron X-ray photoemission. We found that two-dimensional commensurate growth only occurs in the monolayer range for a substrate temperature, T(s), higher than approximately 370 K. Larger amounts of deposited molecules forms three-dimensional uncorrelated clusters on the wetting layer. The desorption of second layer molecules occurs at T(s) > or = 420 K. The highest coverage ordered phase displays a (6 x 8) symmetry and corresponds to the saturation coverage at T(s) = 420 K. The (3 x 6) symmetry phase, recently reported for a multilayer planar film [Ph. Guaino, et al. Appl. Phys. Lett. 2004, 85, 2777], is only found at a coverage slightly lower than the (6 x 8) one. The (3 x 6) phase corresponds to the saturation coverage of the first layer at T(s) = 470 K.

We have studied the vapor-phase deposition of L-cysteine on the Au(110) surface by means of synchrotron-based techniques. Relying on a comparison with previous X-ray photoemission analysis, we have assigned the fine structure of the C K-shell X-ray absorption spectra to the nonequivalent carbon bonds within the molecule. In particular, the C1s --sigma* transition, where the sigma* state is mainly localized on the C-S bond, is shifted well below the ionization threshold, at approximately -5 eV from the characteristic pi* transition line related to carboxylic group. From the polarization dependence of the absorption spectra in the monolayer coverage range, the molecules are found to lay flat on the surface with both the C-S bond and the carboxylic group almost parallel to the surface. We performed in situ complementary surface X-ray diffraction, SXRD, measurements to probe the rearrangement of the Au atoms beneath the L-cysteine molecules. Since the early stage of deposition, L-cysteine domains are formed which display an intermediate fourfold symmetry along [001]. The self-assembly of molecules into paired rows, extending along the [1(-)10] direction, is fully compatible with our observations, as has been reported for the case of D-cysteine molecules grown on Au(110) [Kühnle, A. et al. Phys. Rev. Lett. 2004, 93, 086101.]

Here we demonstrate that a molecular dye film composed by a single layer Of C-70 and a single layer of Zn-tetraphenyl-porphyrin (ZnTPP) adsorbed atop can be grown on large substrates using molecular beam epitaxy. The electrostatic interactions between the constituent molecules drive the formation of this ordered donor-acceptor layer. The overlapping between the pi* orbitals of ZnTPP and C-70 allows the delocalisation of the excited electrons from the excited molecule on a timescale of similar to 6 fs. Generally, the ZnTPP/C-70 film, acting as a p-n junction, delocalise more efficiently the excited electrons as compared to pure ZnTPP films on a timescale that competes effectively against loss processes, suggesting its application for a more efficient solar energy conversion in organic-based devices.

The electronic properties and the molecular orientation of Zn-tetraphenyl-porphyrin films deposited on Si(111) have been investigated using synchrotron radiation. For the first time we have revealed and assigned the fine structures in the electronic spectra related to the HOMOs and LUMOs states. This is particularly important in order to understand the orbital interactions, the bond formation and the evolution of the electronic properties with oxidation or reduction of the porphyrins in supramolecular donor-acceptor complexes used in photovoltaic devices., text 11 pages, 4 figures submitted for publication

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.

We report on a high-resolution X-ray photoemission spectroscopy study on molecular-thick layers Of (L)-cysteine deposited under ultrahigh vacuum conditions on Au(110). The analysis of core level shifts allowed us to distinguish unambiguously the states of the first-layer molecules from those of molecules belonging to the second layer. The first-layer molecules strongly interact with the metal through their sulfur headgroup. The multipeaked structure of the N 1s, O 1s, and C 1s core levels is interpreted in terms of different molecular moieties. The neutral acidic fraction (HSCH2CH(NH2)COOH) is abundant at low coverage likely associated with isolated molecules or dimers. The zwitterionic phase (HSCH2CH(NH3+)COO-) is largely dominant as the coverage approaches the monolayer limit and is related to the formation of ordered self-assembled molecular structures indicated by electron diffraction patterns. The occurrence of a small amount of cationic molecules (HSCH2CH(NH3+)COOH) is also discussed. The second-layer molecules mainly display zwitterionic character and are weakly adsorbed. Mild annealing up to 100 degrees C leads to the desorption of the second-layer molecules leaving electronic states of the first layer unaltered.

Advances in materials science have produced a wide array of new solid-state systems with tunable properties and previously unattainable combinations of phenomena that hold the promise of entirely new approaches to technological applications. Invariably, these new materials are increasingly complex and include a large number of constituents in a variety of chemical states. Entirely new theoretical and experimental approaches are needed to gain the insights necessary for intelligent engineering of these materials. In the past 20 years, a steadily increasing number of electron-electron coincidence experiments on atoms and molecules have demonstrated the capability of investigating complicated systems with sensitivity and specificity well beyond the limits imposed by conventional electron spectroscopies. Over the past decade or so, Auger-photoelectron coincidence spectroscopy (APECS) has emerged as a powerful technique for obtaining detailed information about complex materials systems. Moreover, the recent advent of angle-resolved (AR)-APECS has introduced a new level of discrimination in studying the distribution of electrons photoemitted from complex systems. In this review, we describe the basic ideas behind APECS and discuss a study of the SiO(2) system as an example of the unique information this technique can provide. We then introduce the concept of AR-APECS, explain its novel state and angular momentum selectivity that can be used to disentangle information about complex systems that is hidden to conventional spectroscopies. Examples of AR-APECS measurements from Cu, Ge, and graphite that exemplify the capabilities of this technique are presented. (C) 2004 Elsevier B.V. All rights reserved.

International audience; The ability to determine the nature and the occurrence of defects is a central need of ceramic surface chemistry. In titanium oxides, the Ti-LMV Auger decays line shape is very sensitive to the titanium degree of oxidation, and has long been empirically used as a qualitative probe of the stoichiometry. In the present work, resonant Auger and resonant valence-band measurements at the Ti-L2,3 edges in TiO2, TiO2–x and metallic titanium provide a clear evidence that the evolutions of the Ti-LMV Auger line shape are due to drastic changes in the valence-band profile and in the probability of L2L3V Coster-Kronig decay processes when a fraction of titanium ions is reduced. Both are shown to be consequences of the filling of Ti-3d states near the Fermi level on defects sites. These conclusions are confirmed by Auger-photoelectron coincidence experiments.