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We use synchrotron radiation-induced core level photoemission spectroscopy to investigate the influence of vacancies, produced by ion bombardment, on monolayer graphene/Ni(111) exposed to CO at pressures ranging from ultra-high vacuum up to near ambient (5.6 mbar) conditions.

When imaged by scanning tunneling microscopy (STM) at constant current under nontopographic conditions, nanosized MgO islands grown on Ag(100) may show a fractal dark internal structure and/or appear embedded in the surface. On the contrary, when imaged under topographic conditions, the same islands appear as nearly flat plateaus. Islands showing different contrast and a different contrast dependence on sample bias have been often reported in the literature, but a thorough explanation is still missing. Here we demonstrate, on the basis of of experimental data and computational results, that such structure is due to the accumulation of oxygen at the MgO islands-substrate interface. Interestingly, interfacial oxygen concentrates at the center of the islands.

Due to its biocompatibility, TiO2 is a relevant material for the study of bio-interfaces. Its electronic and chemical properties are influenced by defects, which mainly consist of oxygen vacancies or adsorbed OH groups and which affect, consequently, also the interaction with biological molecules. Here we report on an x-ray photoemission spectroscopy and near edge adsorption fine structure study of glutamic acid (Glu) adsorption on the rutile TiO2(110) surface, either clean or partially hydroxylated. We show that Glu anchors to the surface through a carboxylate group and that the final adsorption state is influenced by the presence of hydroxyl groups on the surface prior to Glu deposition. Indeed, molecules adsorb both in the anionic and in the zwitterionic form, the former species being favored on the hydroxylated substrate.

In this paper we performed the deposition and self-assembly of a Pd-cyclometallated compound on Ag(1 1 0) surface for the first time. The system is investigated from the morphological and chemical point of view by scanning tunneling microscopy and x-ray photoemission spectroscopy, respectively, and the results are validated by ab-initio calculations. Our combined experimental and theoretical study aims at elucidating the atomistic details of the chemical steps following Pd cyclometallate deposition on the metallic substrate. To do that, we analyze the electronic and chemical properties of the species present on the surface at the end of the preparation process at room temperature and at 150 degrees C. We observe an unexpected complex chemistry: on one side, the organometallic molecules are found to dissociate into fragments, forming a well-ordered metal-carbon network; on the other side, Pd atoms become buried in the bulk of the metal substrate following metal exchange with surface Ag atoms. The details of this mechanistic study reveal the active role played by the metal substrate in promoting the chemistry of the deposited Pd cyclometallates and could open new perspectives for the application of this class of materials in heterogeneous catalysis.

The Pd cyclometallated complex [(5-bromo-2-phenylpyridine)Pd(μ-Cl)]2 is deposited on Ag(1 1 0) at room temperature by sublimation in ultra-high vacuum. The thermal evolution of the system is followed by scanning tunnelling microscopy and X-ray photoemission spectroscopy, while the initial and final configurations are validated by ab-initio calculations. We observe the surface induced dissociation of the molecule and the occurrence of a cross coupling reaction between the two organic fragments, leading to the surface assisted synthesis of diphenyl-bipyridine molecules. Such a process, occurring with low probability at RT, is thermally activated and competes with desorption. At variance with most cross-coupling reactions at surfaces reported in literature, in this case the reactants come from the dissociation of the same compound so that only one precursor is employed, leading to a simplified preparation protocol. The Br and Cl atoms dissociated from the molecule bind to the surface and promote an extended surface reconstruction upon annealing, which was not observed previously upon deposition of halogenated aromatic compounds.

Ni nanoclusters up to 30 angstrom in diameter are grown by Ni deposition on ultrathin MgO/Ag(100) films at different temperatures and characterized by combining low-temperature scanning tunneling microscopy with photo-emission and vibrational spectroscopies. At 200 K, both small NixOyD aggregates and 2D Ni nanoparticles of average size close to 12 angstrom form. The latter have a metallic nature and efficiently catalyze CO dissociation at 200 K. When Ni is deposited at 300 K, only larger 3D Ni clusters are observed.

Due to its biocompatibility, TiO

The current understanding of the dynamics of gas-surface interactions is that all of the energy lost in the collision is transferred to vibrations of the target. Electronic excitations were shown to play a marginal role except for cases in which the impinging particles have energies of several electronvolts. Here we show that this picture does not hold for metal surfaces supporting acoustic surface plasmons. Such loss, dressed with a vibronic structure, is shown to make up a prominent energy transfer route down to the terahertz region for Ne atoms scattering off Cu(111) and is expected to dominate for most metals. This mechanism determines, e.g., the drag force acting on telecommunication satellites, which are typically gold-plated to reduce overheating by sunshine. The electronic excitations can be unambiguously discerned from the vibrational ones under mild hyperthermal impact conditions.

This chapter provides selected examples of experimental investigations addressing the role of rotational and vibrational energy in adsorption at surfaces. Such information can generally be obtained in different ways characterized by different degrees of state selectivity. The first part of the chapter deals with the effect of rotational energy, while the second considers the effect of vibrational energy. The former investigations compared the behavior of rotationally hot and cold molecular beams by measuring the sticking probability on well-defined surfaces under controlled, ultrahigh-vacuum conditions. When detection of the rotational state of gas-phase molecules became feasible, a large amount of information was obtained indirectly: researchers first determined experimentally the rotational state of desorbing (or scattered) molecules and then used microreversibility (otherwise called detailed balance) arguments to extract information on the effect of the rotational states on the sticking probability. In recent years, methods to prepare molecules in partially and, in some cases, even fully defined rotational quantum states were developed. The central part of the section highlights recent results obtained using electrostatic and finally magnetic hexapoles. The first section of the chapter ends by reviewing results obtained by exploiting the velocity dependence of the degree of alignment of molecules seeded with inert carriers in a supersonic molecular beam. A brief selection of resultsobtained with laser methods concludes this section. Laser methods are on the contrary indispensable for the preparation of vibrationally excited molecules, which is the topic of the second section of the chapter. The relative roles of kinetic and vibrational energy are discussed, and the concepts of entrance and late barrier in the potential energy surface are introduced. Pioneering experiments showing greater specificity in surface reactions are briefly reviewed.

Highly hydrophobic aluminium surfaces fabricated by chemical etching are investigated by Contact Angle Goniometry, Scanning Electron Microscopy and X-ray Photoelectron Spectroscopy in order to correlate the wettability of our samples with their morphology at the sub-micrometer scale and with the chemical composition of the very first surface layers. We find that the etched aluminium surfaces have binary structures with nanoscale block-like convexities and hollows, which provide more space for air trapping. We also demonstrate that both hierarchical micro/nanostructures and surface composition endow these surfaces with excellent hydrophobic properties. XPS analysis shows indeed that the contact angle anti-correlates with the amount of metallic aluminium present at the surface, but also confirms the essential role of the adsorption of airborne carbon compounds. The hydrophobic behaviour depends therefore on the combined effects of surface morphology and surface chemistry.

Adsorption energy is a fundamental thermodynamic quantity in the description of gas–surface interactions. In general, it depends not only on the ad-species and on the chemical nature of the substrate but also on the surface coverage and the density and nature of the surface defects. It can also be significantly affected by the presence of coadsorbates and by temperature when the latter determines a different arrangement of species on the substrate or even the formation of different moieties at the surface. On the one hand, the measure of the heat of adsorption by the isosteric method can be used only in selected cases, where adsorption occurs reversibly and a reliable control of the coverage over a large enough range of temperatures is experimentally accessible. On the other hand, temperature-programmed desorption, which until now has been the most widely used technique, is not applicable when adsorption occurs irreversibly, and care is needed to properly analyze the experimental data, especially when the heat of adsorption is strongly coverage dependent. Single-crystal calorimetry (SCAC) provides a means of overcoming most such limitations. Since it is impossible to cover all the results obtained by this technique in a single review, a rich bibliography is provided and only selected results (in the opinion of the authors) are summarized here. The first section addresses the results obtained by the infrared calorimeter of the Cambridge group, which mostly deals with the coverage dependence of the heat of adsorption of different gases on single-crystal (both low and high Miller index) surfaces. The second section deals with the results obtained by the pyroelectric calorimeter by the Washington group, which cover both the adsorption of large organic molecules and of nongaseous species. The impressive results obtained to date have drawn other research groups into the field, and new designs have appeared that enable, for example, the study of the adsorption of gas molecules at the surface of nanoparticles and at oxide surfaces. The last section of this chapter is devoted to such results.

Graphene nanoribbons (GNRs) are at the frontier of research on graphene materials since the 1D quantum confinement of electrons allows for the opening of an energy gap. GNRs of uniform and well-defined size and shape can be grown using the bottom-up approach, i.e. by surface assisted polymerization of aromatic hydrocarbons. Since the electronic properties of the nanostructures depend on their width and on their edge states, by careful choice of the precursor molecule it is possible to design GNRs with tailored properties. A key issue for their application in nanoelectronics is their stability under operative conditions. Here, we characterize pristine and oxygen-exposed 1.0 nm wide GNRs with a well-defined mixed edge-site sequence (two zig-zag and one armchair) synthesized on Ag(110) from 1,6-dibromo-pyrene precursors. The energy gap and the presence of quantum confined states are investigated by scanning tunneling spectroscopy. The effect of oxygen exposure under ultra-high vacuum conditions is inferred from scanning tunneling microscopy images and photoemission spectra. Our results demonstrate that oxygen exposure deeply affects the overall system by interacting both with the nanoribbons and with the substrate; this factor must be considered for supported GNRs under operative conditions.

We investigated the interaction of CO with a Ni(1 1 1) sample partially covered with carbide by Near Ambient Pressure (PCO ~ 2 mbar) X-Ray Photoemission Spectroscopy. Above 500 K we observe the formation of strongly and weakly interacting graphene in comparable amount and CO intercalation underneath the latter moiety. Above 600 K, physisorbed CO2 forms, a process which we ascribe to the onset of the Boudouard reaction under the graphene cover.

The chemical reactivity of single layers of supported graphene (G) is affected by the nature of the underlying substrate: in particular CO chemisorption occurs on G/Ni(111), while graphene on Cu is inert. Here, we demonstrate experimentally that doping of the G layer with nitrogen atoms further increases the reactivity of the G/Ni(111) system towards CO. The doped layer is obtained by sputtering pristine G/Ni(111) with N2+ ions. For an ∼11% dopant concentration, an additional electron energy loss at 238 meV appears in the HREEL spectra besides the loss around 256 meV present also on pristine G/Ni(111). The new feature corresponds to a CO species with a higher desorption temperature and, consequently, a higher adsorption energy than the one forming on pristine G/Ni(111). At low coverage, the adsorption energy is estimated to be ∼ 0.85 eV/molecule.

Recently it was demonstrated that atomic oxygen can cause the extraction of substrate atoms off metal surfaces thus generating chemically different active sites. For Ag(110) this process occurs when ${\mathrm{O}}_{2}$ is dosed at 175 K leading, at low coverage, to the formation of single Ag vacancies. Vacancy creation proceeds thereby via the formation of O-Ag-O complexes, which involve a local reconstruction of the surface and ignite the disruption of the Ag substrate. In this paper, we report on the details of such processes and on the isolated structures formed by the O adatoms in the limit of very low coverage. We employ scanning tunneling microscopy and density functional theory to unravel the complex structures of O/Ag(110) which are transiently present under specific reaction conditions. A variety of features such as isolated gray dots, sombreros, shallow gray and white structures oriented along [001] and $[1\overline{1}0]$, gray stripes, and lozenges were identified and assigned to O adatoms in different configurations. The zigzag chains interact strongly with the STM tip and are easily disrupted, giving rise to highly mobile, sombrero-shaped, isolated O adatoms also far away from the scanned area, i.e., from the current injection spot. Around 200 K, not only Ag vacancies, which are mobile with anisotropic migration, can merge together into rather complex features, but also the mobile Ag atoms are trapped by O adatoms, thus facilitating the formation of an oxygen-decorated Ag chain along [001] which ultimately induces the well-known added-row reconstruction.

In this article, we review the latest experimental results on the influence of surface defect in the interaction of different kinds of molecules with metal surfaces, using stepped surfaces as a model system. We will discuss both the adsorption of “simple” molecules of catalytic interest, such as H 2 , O 2 , CO, and NO at metal surfaces, and the organic–inorganic interface. The latter is represented, on the one side, by polyaromatic molecules, relevant for nanoelectronics, and on the other side, by amino acids as representative of biological molecules. The substrates are chosen among “nonreactive” noble metals as well as among “reactive” transition metals.

We report the study of Ni nanoclusters deposited on MgO/Ag(100) ultrathin films (one monolayer) at T = 200 K. We show by STM analysis and DFT calculations that in the limit of low Ni coverage the formation of nanoclusters of four to six atoms occurs and that these aggregates are flat rather than 3D, as expected for Ni tetramers, pentamers, or hexamers. Both the shape of the clusters and the interatomic distance between neighboring Ni atoms are indicative that the nanoparticles do not consist of pure metal atoms but that a NiyOx structure has formed thanks to the availability of atomic oxygen accumulated at the MgO/Ag interface, with Ni clusters acting as oxygen pumps. Besides being of relevance in view of the use of metal nanoclusters in catalysis and other applications, this finding gives a further proof of the peculiar behavior of ultrathin oxide films.

Graphene is usually considered a chemically inert material. Theoretical studies of CO adsorption on free-standing graphene predict quite low adsorption energies (

International audience; What is the driving force behind the self-assembly of molecules on a surface? Why can different organizations of an assembly coexist under the same conditions? The coexistence of two experimentally observed complex phases of glutamic acid on Ag(100) is discussed using a detailed atomistic thermodynamics approach based on periodic DFT electronic energies. The interplay between bond formation and loss of degrees of freedom, corresponding to the enthalpy-entropy balance, is used to explain in detail and quantitatively the subtle equilibrium between the two differently organized self-assemblies.

Free standing graphene is chemically inert but, as recently demonstrated, CO chemisorption occurs at lowcrystal temperature on the single layer grown by ethene dehydrogenation on Ni(111). Such layer is inhomogeneous since different phases coexist, the relative abundance of which depends on the growth conditions. Here we show by X ray photoemission and high resolution electron energy loss spectroscopies that the attained CO coverage depends strongly on the relative weight of the different phases as well as on the concentration of carbon in the Ni subsurface region. Our data show that the chemical reactivity is hampered by the carbon content in the substrate. The correlation between the amount of adsorbed CO and the weight of the different graphene phases indicates that the top-fcc configuration is the most reactive. Published by AIP Publishing.

This chapter will provide an overview of the properties of low-dimensional plasmons, discussing particularly characteristic examples. We will start with two-dimensional sheet plasmons (Sect. 19.1), concentrating on the plasmonic properties of the system most widely investigated in the recent past, graphene. Further emphasis will be given to low-dimensional plasmons coupled to other electron gases, which leads to linearization in the form of acoustic surface plasmons, but also to crossover of dimensionality, depending on plasmonic wavelengths. Finally we turn to quasi-one-dimensional systems and their corresponding plasmons, and try at the end to solve the puzzle of broad loss peaks but still fairly large plasmonic lifetimes.

International audience; (S)-Glutamic acid adsorbed on Ag(100) organizes in different self-assembled structures depending on surface temperature [Smerieri, M.; Vattuone, L.; Kravchuk, T.; Costa, D.; Savio, L. (S)-Glutamic Acid on Ag(100): Self-Assembly in the Nonzwitterionic Form. Langmuir 2011, 27, 2393-2404]. In particular, two of these structures, referred to as ``square'' and ``flower'' geometries, are found to coexist on the surface upon deposition at T = 350 K. The former assembly was fully resolved at the atomic level in the work of Smerieri et al. [Smerieri, M.; Vattuone, L.; Costa, D.; Tielens, F.; Savio, L. Self-Assembly of (S)-Glutamic Acid on Ag(100): A Combined LT-STM and Ab Initio Investigation. Langmuir 2010, 26, 7208-7215], in which we proved that the driving force for adsorption is the van der Waals interactions between the molecules and the Ag surface, that is, that molecules are in a physisorbed state. In this paper, we complete our work by presenting the characterization of the ``flower'' structure. In contrast to the case of the ``square'' assembly, a strong chemical bond between glutamic acid radicals and the surface is at the basis of the geometry. Whereas the chemisorbed central GLU tetramer interacts strongly with the surface, the physisorbed surrounding GLU molecules conserve some degree of freedom in the layer which counterbalances the weak adsorption energy. The ``flower'' and the ``square'' assemblies have similar dispersion energy and H-bond interaction energy; as a consequence of the different chemical state of the GLU molecules, however, such contributions have a very different relative weight in the stabilization of the two structures.

We investigate CO adsorption at single vacancies of graphene supported on Ni(111) and polycrystalline Cu. The borders of the vacancies are chemically inert but, on the reactive Ni(111) substrate, CO intercalation occurs. Adsorbed CO dissociates at 380 K, leading to carbide formation and mending of the vacancies, thus preventing their effectiveness in sensor applications.

Thin oxide films have physical and chemical properties which may be significantly different from those of the corresponding bulk materials. For their complete characterization the information on the lattice dynamics which can be retrieved by vibrational spectroscopy is mandatory. Here we show that the number of observed phonon modes and their frequencies can indeed provide relevant information about stoichiometry, structure and thickness of the film.

66-3 FIELD Section Title:Surface Chemistry and Colloids Dipartimento di Fisica, Universita di Genova, Genoa, Italy. FIELD URL: written in English; Self-assembly of org. mols. at metal surfaces is of greatest importance in nanoscience; in fact, it opens new perspectives in the field of mol. electronics and in the study of biocompatible materials. Combining an exptl. low-temp. scanning tunneling microscopy investigation with ab initio calcns., we succeeded to describe in detail (S)-glutamic acid adsorption on Ag(100) at T = 350 K. We find that (S)-glutamic acid organizes in a squared structure and, at variance with the majority of cases reported in literature, it adsorbs in the neutral form, 4.6 Å above the surface plane. The interaction with the poorly reactive Ag substrate is only due to weak van der Waals forces, while H-bonds between carboxyl groups and the formation of a OCOH-OCOH-OCOH-OCOH cycle at the vertex of the squares are the main responsible for the self-assembly. [on SciFinder(R)]

International audience; As recently established (J. Am. Chem. Soc. 2008, 130, 12552), ethene adsorption on Cu(410) occurs both molecularly and dissociatively, the latter resulting in carbon contamination of the Surface. Here we report on the coverage-dependent dynamics of C(2)H(4) adsorption on clean and carbon-contaminated Cu(410). For the bare surface, the initial sticking probability has a very weak dependence on kinetic energy and is almost independent of angle of incidence. Molecular adsorption is in both cases precursor-mediated and nonactivated. Ethene dissociation takes place during adsorption as well as upon annealing. Both paths proceed via a molecular precursor. The former is translationally activated, while the latter depends strongly on the heating rate. The presence of preadsorbed carbon, resulting from previous uptakes, affects both the sticking probability and the attained saturation coverage. The latter quantity is shown to be a sensitive probe of carbon precoverage. A scheme of the complicated potential energy surface of this system is derived and discussed.

One of the main goals of surface science is the understanding of the elementary steps occurring in catalytic reactions in the heterogeneous phase, in order to identify promoters and rate limiting factors at the atomic scale with the ultimate scope of designing more efficient catalysts. To reduce the complexity of the problem and focus attention on individual steps of the reaction of interest, most experiments have been performed so far under controlled, ultra-high vacuum conditions and on single crystal surfaces cut along low Miller index planes. On the other hand, catalytic reactions occurring industrially for the massive production of everyday life chemicals are far away from these well-defined conditions. Reactors work at high temperature and pressure, while the catalysts consist usually of supported powders exhibiting different atomic terminations and a high concentration of low coordinated sites (steps, kinks, vacancies etc.). The structural difference between the ordered samples used in surface science and the real catalysts is known as structure gap and, in the understanding of catalytic processes, it can be as relevant as the more widely invoked pressure gap, related to the difference in pressure between chemical reactors and ultra-high vacuum apparatuses. Although the importance of defect sites and the relevance of the structure gap have been evident for decades, the systematic study of defected surfaces began only recently, after a reasonable understanding of the simpler systems was reached. The most promising approach to this topic is the use of single crystal surfaces cut along high Miller index planes, i.e. stepped surfaces showing a high density of one majority low coordination site which mimics a defect. This approach allows a shortcut between the need for ordered substrates and controlled conditions and the availability of particular atomic configurations, a condition only partially mitigated with the advent of scanning probe microscopy with nanoscale resolution. Of course, only one defect-type at a time can be studied in this way. The present report summarizes the knowledge achieved so far for the gas–surface interaction in presence of well-defined defects and for simple reactions at such sites.

Many reactions like the ethylene epoxidation which work readily in industrial reactors can not be reproduced under controlled laboratory conditions. This failure is ascribed to so-called “pressure” and “structure” gaps. The latter is possibly determined by an active role of defects, since surface science experiments are performed on nearly perfect surfaces, quite unlike those of active catalytic powders. One of the mostly investigated, but so far unresolved systems, is O/Ag which shows a unique selectivity for the partial oxidation of ethylene. Subsurface oxygen was claimed to play a pivotal role in activating oxygen adatoms, but the mechanism of O incorporation remained mysterious. The study of O2 adsorption at stepped surfaces and by supersonic molecular beams allowed for considerable progresses, showing that open steps favour dissociation and constitute a gateway through which subsurface sites can be accessed.

Collisional alignment in the supersonic expansion of oxygen seeded in lighter carriers allows to produce rotationally aligned beams which were used to study the sticking probability, S, and the reactivity, \(P_{{co_{2} }}\), of helicoptering and cartwheeling O2 in the lowest rotational state impinging normally onto Pd(100). Molecules with different rotational alignment are thereby chosen by mechanically selecting either the slow or the fast speed tail of the beam velocity distribution. Here we show for O2 reacting with CO pre-covered Pd(100) that even a moderate increase in the nozzle temperature, TN, with respect to room temperature implies significant changes in both S and \(P_{{co_{2} }}\). Since TN affects mainly the rotational state distribution in the formed beam, we conclude that molecules in rotational states with K ≥ 3 behave very differently from those with K = 1. Data are presented for both small and intermediate CO pre-coverage.

We report on the experimental evidence of the role of rotational alignment of the gas-phase molecules in the interaction of propylene with Ag(001). Molecular alignment has been controlled by a velocity selection of the impinging molecules, flying in a supersonic seeded molecular beam. The experimental findings indicate that at low surface coverage the sticking probability is independent of molecular alignment, while when coverage exceeds few percent of a monolayer, molecules impinging rotating parallel to the surface (helicopter-like configuration) achieve a higher chance to be trapped than those which impinge rotating perpendicularly (cartwheels). The sudden appearance of a large stereodynamic effect suggests that the adsorption proceeds via a mobile precursor state and is tentatively correlated with a change in the configuration of the added propylene molecules, which adsorb tilted rather than flat at the surface.

It is shown that at high O 2 - coverage on Ag(1 0 0) photo dissociation by soft X-rays generates oxygen atoms of covalent and of oxidic nature. The former species is obtained also by thermal dissociation and is the only observed product when irradiating at low O 2 - coverage. The latter one is produced only photochemically and assigned to an Ag oxide. The fragments produced in the photochemical process end up eventually in identical or in different sites, depending on the presence of other O 2 - molecules in the immediate neighbourhood.

The interest in thin and ultrathin oxide films is increasing rapidly due to the new properties and the many possible technological applications of such materials. In this frame the interaction with...

International audience; This comment clarifies two issues related to the (001) surface reconstructions of cubic SiC, namely: (i) The failure of the bridge-bond model for H atoms interacting with the 3C-SiC(001) 3 × 2 reconstruction to explain all the experimental data based on different techniques, while a recent model has reconciled theory and experimental results. This model has not been discussed or even mentioned in the review by Pollmann et al.; and (ii) In their review, two models of the Si-terminated c(4 × 2) 3C-SiC(001) surface reconstruction are presented as equally probable. This is clearly not the case and the reasons are explained in this comment.

We report on a combined high resolution electron energy loss spectroscopy and x-rays photoelectron spectroscopy investigation of the interaction at room temperature of H 2 O with thin and ultra-thin MgO films grown on Ag(100). We find a strongly enhanced dissociation probability in the monolayer and sub-monolayer regime, indicative of an active role of the Ag substrate in the dissociation process. The active sites are suggested to be oxygen atoms with low coordination at the border of monolayer MgO islands.

We show that the interaction of C2H4 with Ag(4 1 0) is dominated by steering of the incoming molecules towards the steps, which are the only sites active for chemisorption on this surface. The measured sticking probability, S, is almost independent of the angle of impact. Molecular dynamics simulation shows that this is the signature of the steering, which leads to loss of memory of the incidence angle at low energies. 2003 Elsevier B.V. All rights reserved.

The role of defects in catalytic reactions, especially those involving low rates and high degrees of selectivity, has been debated ever since the very early days of surface science. However, most studies on gas–surface interaction and chemisorption performed under controlled laboratory conditions have dealt so far with nearly perfect low-Miller-index surfaces, which are rather unlike the active powders employed as catalysts in industrial reactors. The failure in reproducing some chemical reactions, which occur readily under industrial conditions, was therefore often ascribed to the so-called structure gap separating the surface science approach from the real industrial conditions. Overcoming this limit without losing control over the experiment at the nanoscopic scale is therefore an issue of pivotal importance. It can be attacked by studying adsorption on either single-crystal surfaces damaged by ion bombardment or on surfaces aligned along high-Miller-index planes. In this paper we shall discuss O2 interaction with Ag(410), a vicinal surface of Ag(100) characterized by open (110)-like steps and by (100) terraces. The use of a supersonic molecular beam to dose O2 allowed us to gain information on the interaction of the gasphase molecules with steps and terraces separately, by selecting the angle of incidence and the impact energy. The open steps turned out to be active sites for dissociation, while flat Ag(100) planes are unreactive. For molecular adsorption a reduction in the activation barrier is observed at the steps, while the Ag(100) nanoterraces are much less reactive than wide (100) planes. The main results are confirmed by the preliminary investigation of O2/Ag(210).

The mechanism of oxygen incorporation in Ag is still poorly known. As recently demonstrated [Phys. Rev. B 63 (2001) R1404], oxygen adatoms removal by CO oxidation leaves the Ag(0 0 1) surface in a modified state in which oxygen segregation from the bulk and the formation of a surface oxide phase can be induced by further CO exposure. Here we show that the same channels, forming during CO oxidation and linking surface and subsurface sites, allow also for the migration of oxygen adatoms into the subsurface region. When dosing O2 on an Ag(1 0 0) surface, on which oxygen had been previously removed by CO oxidation, we observe indeed the formation of the same surface oxide phase produced by oxygen segregation. A characterisation of this phase by X-rays photoemission spectroscopy and high resolution electron energy loss spectroscopy is given, from which we deduce that it extends several layers deep into the volume. 2002 Elsevier Science B.V. All rights reserved.

The ability to engineer nearly perfect ultrathin oxide layers, up to the limit of monolayer thickness, is a key issue for nanotechnological applications. Here we face the difficult and important case of ultrathin MgO films on Ag(100), for which no extended and well-ordered layers could thus far be produced in the monolayer limit. We demonstrate that their final morphology depends not only on the usual growth parameters (crystal temperature, metal flux, and oxygen partial pressure), but also on aftergrowth treatments controlling so far neglected thermodynamics constraints. We thus succeed in tuning the shape of the oxide films from irregular, nanometer-sized, monolayer-thick islands to slightly larger, perfectly squared, bilayer islands, to extended monolayers limited apparently only by substrate steps.

In spite of the relevance of ultrathin MgO films for the study of model systems as well as for technological applications, great difficulties have been found so far in the growth of extended, well-ordered, ultrathin films. Combining scanning tunneling microscopy, X-ray photoemission spectroscopy, and high-resolution electron energy loss spectroscopy experiments with ab initio calculations, we demonstrate here that the structure of sub-monolayer MgO films grown on Ag(100) by reactive deposition is strongly affected not only by the growth conditions but also by after-growth treatments. The latter ones allow one to quench the thermodynamically most stable configuration at the deposition temperature or let the system evolve toward the low-temperature equilibrium state. Moreover, we give experimental and theoretical evidence of the accumulation of oxygen atoms at the MgO/Ag interface at the highest deposition temperature, which reduces the stress of the oxide film favoring the formation of extended terraces. The result is the possibility to tune the morphology of the films from small islands with corrugated borders, to perfectly square islands of larger size, to MgO terraces several tens of nanometers wide.

Understanding acoustic surface plasmons (ASPs) in the presence of nanosized gratings is necessary for the development of future devices that couple light with ASPs. We show here by experiment and theory that two ASPs exist on Au(788), a vicinal surface with an ordered array of monoatomic steps. The ASPs propagate across the steps as long as their wavelength exceeds the terrace width, thereafter becoming localized. Our investigation identifies, for the first time, ASPs coupled with intersubband transitions involving multiple surface-state subbands. © 2014 American Physical Society. DFG