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A concerted theoretical and experimental investigation of the silver/anatase hybrid nanocomposite, a very promising material for advanced sensing applications, is presented. We measure its exceptional electrochemical virtues in terms of current densities and reproducibility, providing their explanation at the atomic-scale level and demonstrating how and why silver acts as a positive electrode. Using periodic plane-wave DFT calculations, we estimate the overall amount of electron transfer toward the semiconductor side of the interface at equilibrium Suitably designed (photo)electrochemical experiments strictly agree, both qualitatively and quantitatively, with the theoretical charge transfer estimates. The unique permanent charge separation occurring in the device is possible because of the favorable synergy of Ag and TiO2, which exploits in a favorable band alignment, while the electron-hole recombination rate and carrier mobility decrease when electrons cross the metal semiconductor interface. Finally, the hybrid material is proven to be extremely robust against aging, showing complete regeneration, even after 1 year.

Metal oxides are versatile substrates for the design of a wide range of SAM-based organic-inorganic materials among which ZnO nanostructures modified with phosphonic SAM are promising semiconducting systems for applications in technological fields such as biosensing, photonics, and field-effect transistors (FET). Despite previous studies reported on various successful grafting approaches, issues regarding preferred anchoring modes of phosphonic acids and the role of a second reactive group (i.e., a carboxylic group) are still a matter of controversial interpretations. This paper reports on an experimental and theoretical study on the functionalization of ZnO nanorods with monofunctional alkylphosphonic and bifunctional carboxyalkylphosphonic acids. X-ray photoelectron and infrared spectroscopies have been combined with DFT modeling to explain and understand the interactions that drive the surface anchoring of phosphonic acids on ZnO surface. It was found that both monofunctional and bifunctional acids anchor on ZnO through a multidentate bonding which involves both P=O and P-O moieties of the phosphonic group. Moreover, anchored bifunctional acids bend to the surface, promoting a further interaction between surface hydroxyl groups and carboxylic terminations. This secondary interaction can be limited by increasing the surface density of the anchored molecules.

The relative stability of nine different well defined CeO2 surfaces has been studied by periodic density functional calculations using GGA + U and B3LYP exchange-correlation functional. Both methods consistently predict that CeO2(111) is the most stable surface and also provide a consistent picture of the most stable surfaces which indeed are in agreement with previous studies based on empirical interatomic potentials. The facility of ceria surfaces to undergo a redox process has been investigated by forcing spin-polarized solutions, which lead to the occupancy of Ce 4f orbitals. These calculations provide evidence that surfaces with low-coordinated Ce cations are likely to be reduced more easily than regular low-index Miller surfaces. Fil: Branda, Maria Marta. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Bahía Blanca. Instituto de Física del Sur. Universidad Nacional del Sur. Departamento de Física. Instituto de Física del Sur; Argentina Fil: Ferullo, Ricardo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Bahía Blanca. Instituto de Química del Sur. Universidad Nacional del Sur. Departamento de Química. Instituto de Química del Sur; Argentina Fil: Causá, Mauro. Università degli Studi di Napoli Federico II; Italia Fil: Illas, Francesc. Universidad de Barcelona; España

Surface hydroxy groups are known to play a significant role in the photocatalytic behavior of TiO2. The molecular or dissociative character of H2O adsorption on titania is strongly affected by the local surface structure, but dissociation, producing −OH, should prevail on surface sites with a high degree of coordinative unsaturation. Here we report an IR investigation of hydroxy groups on low coordination sites of TiO2 P25 (Degussa), a commercial titania powder considered a staple in the field of heterogeneous photocatalysis. The presence of coordinatively defective surface terminations well agrees with the roughness of the border of TiO2 particles observed by high resolution transmission electron microscopy. Furthermore, IR spectra of adsorbed CO were used to probe the Lewis acidity of surface Ti4+ ions exposed after the removal of water molecules and hydroxy groups. Evidence for two additional types of surface cationic sites with respect those reported in the literature was found.

Biomaterials-associated-infections (BAI) are serious complications in modern medicine. Although non-adhesive coatings, like polymer-brush coatings, have been shown to prevent bacterial adhesion, they do not support cell growth. Bi-functional coatings are supposed to prevent biofilm formation while supporting tissue integration. Here, bacterial and cellular responses to poly(ethylene glycol) (PEG) brush-coatings on titanium oxide presenting the integrin-active peptide RGD (arginine-glycine-aspartic acid) (bioactive "PEG-RGD") were compared to monofunctional PEG brush-coatings (biopassive "PEG") and bare titanium oxide (TiO(2)) surfaces under flow. Staphylococcus epidermidis ATCC 35983 was deposited on the surfaces under a shear rate of 11 s(-1) for 2 h followed by seeding of U2OS osteoblasts. Subsequently, both S. epidermidis and U2OS cells were grown simultaneously on the surfaces for 48 h under low shear (0.14 s(-1)). After 2 h, staphylococcal adhesion was reduced to 3.6 +/- 1.8 x 10(3) and 6.0 +/- 3.9 x 10(3) cm(-2) on PEG and PEG-RGD coatings respectively, compared to 1.3 +/- 0.4 x 10(5) cm(-2) for the TiO(2) surface. When allowed to grow for 48 h, biofilms formed on all surfaces. However, biofilms detached from the PEG and PEG-RGD coatings when exposed to an elevated shear (5.6 s(-1)). U2OS cells neither adhered nor spread on PEG brush-coatings, regardless of the presence of biofilm. In contrast, in the presence of biofilm, U2OS cells adhered and spread on PEG-RGD coatings with a significantly higher surface coverage than on bare TiO(2). The detachment of biofilm and the high cell surface coverage revealed the potential significance of PEG-RGD coatings in the context of the "race for the surface" between bacteria and mammalian cells.

Biomaterials-associated-infections (BAI) are serious complications in modern medicine. Although non-adhesive coatings, like polymer-brush coatings, have been shown to prevent bacterial adhesion, they do not support cell growth. Bi-functional coatings are supposed to prevent biofilm formation while supporting tissue integration. Here, bacterial and cellular responses to poly(ethylene glycol) (PEG) brush-coatings on titanium oxide presenting the integrin-active peptide RGD (arginine-glycine-aspartic acid) (bioactive "PEG-RGD") were compared to monofunctional PEG brush-coatings (biopassive "PEG") and bare titanium oxide (TiO(2)) surfaces under flow. Staphylococcus epidermidis ATCC 35983 was deposited on the surfaces under a shear rate of 11 s(-1) for 2 h followed by seeding of U2OS osteoblasts. Subsequently, both S. epidermidis and U2OS cells were grown simultaneously on the surfaces for 48 h under low shear (0.14 s(-1)). After 2 h, staphylococcal adhesion was reduced to 3.6 +/- 1.8 x 10(3) and 6.0 +/- 3.9 x 10(3) cm(-2) on PEG and PEG-RGD coatings respectively, compared to 1.3 +/- 0.4 x 10(5) cm(-2) for the TiO(2) surface. When allowed to grow for 48 h, biofilms formed on all surfaces. However, biofilms detached from the PEG and PEG-RGD coatings when exposed to an elevated shear (5.6 s(-1)). U2OS cells neither adhered nor spread on PEG brush-coatings, regardless of the presence of biofilm. In contrast, in the presence of biofilm, U2OS cells adhered and spread on PEG-RGD coatings with a significantly higher surface coverage than on bare TiO(2). The detachment of biofilm and the high cell surface coverage revealed the potential significance of PEG-RGD coatings in the context of the "race for the surface" between bacteria and mammalian cells.

On the basis of experimental evidence and DFT calculations, we propose a simple yet viable way to stabilize and chemically activate gold nanoclusters on MgO. First the MgO surface is functionalized by creation of trapped electrons, (H+)(e-) centers (exposure to atomic H or to H2 under UV light, deposition of low amounts of alkali metals on partially hydroxylated surfaces, etc.); the second step consists in the self-aggregation of gold clusters deposited from the gas phase. The calculations show that the (H+)(e-) centers act both as nucleation and activation sites. The process can lead to thermally stable gold cluster anions whose catalytic activity is enhanced by the presence of an excess electron. © 2008 American Chemical Society.

The adsorption of small amounts of alkali metal atoms (Li, Na, K, Rb, and Cs) on the surface of MgO powders and thin films has been studied by means of EPR spectroscopy and DFT calculations. From a comparison of the measured and computed g values and hyperfine coupling constants (hfccs), a tentative assignment of the preferred adsorption sites is proposed. All atoms bind preferentially to surface oxide anions, but the location of these anions differs as a function of the deposition temperature and alkali metal. Lithium forms relatively strong bonds with MgO and can be stabilized at low temperatures on terrace sites. Potassium interacts very weakly with MgO and is stabilized only at specific sites, such as at reverse corners where it can interact simultaneously with three surface oxygen atoms (rubidium and cesium presumably behave in the same way). Sodium forms bonds of intermediate strength and could, in principle, populate more than a single site when deposited at room temperature. In all cases, large deviations of the hfccs from the gas-phase values are observed. These reductions in the hfccs are due to polarization effects and are not connected to ionization of the alkali metal, which would lead to the formation of an adsorbed cation and a trapped electron. In this respect, hydrogen atoms behave completely differently. Under similar conditions, they form (H(+))(e(-)) pairs. The reasons for this different behavior are discussed.

This paper deals with the investigation of a stepwise surface priming strategy based on zirconium phosphate-phosphonate chemistry (ZP priming), which was applied for the first time to nanostructured, micrometer-thick titanium dioxide electrodes. Through the extensive use of secondary ion mass spectrometry(SIMS) depth profiling technique, it was possible to monitor the composition of the titania samples after every single reaction step of the priming. In particular, the impregnation of the surface with zirconium was found to be the determining step of the whole treatment. SIMS data allowed for the identification of the diffusion mechanism that governs the uptake of zirconium from an aqueous Zr(IV)-containing solution. Furthermore, from the depth profiling results, it was possible to optimize the priming experimental conditions in order to achieve the desired uniform surface modification along the entire nanostructured TiO2 layer. ZP-priming of nanostructured titania electrodes could be exploited within the field of dye-sensitized devices for solar energy conversion, by providing an anchoring platform for stable binding, at the oxide surface, of photoactive molecular systems bearing phosphonic moieties.

Metal–oxide interfaces play a fundamental role in determining the functional properties of artificial layered heterostructures, which are at the root of present and future technological applications. Magnetic exchange and magnetoelectric coupling, spin filtering, metal passivation, catalytic activity of oxide-supported nano-particles are just few examples of physical and chemical processes arising at metal–oxide hybrid systems, readily exploited in working devices. These phenomena are strictly correlated with the chemical and structural characteristics of the metal–oxide interfacial region, making a thorough understanding of the atomistic mechanisms responsible of its formation a prerequisite in order to tailor the device properties. The steep compositional gradient established upon formation of metal–oxide heterostructures drives strong chemical interactions at the interface, making the metal–oxide boundary region a complex system to treat, both from an experimental and a theoretical point of view. However, once properly mastered, interfacial chemical interactions offer a further degree of freedom for tuning the material properties. The goal of the present review is to provide a summary of the latest achievements in the understanding of metal/oxide and oxide/metal layered systems characterized by reactive interfaces. The influence of the interface composition on the structural, electronic and magnetic properties will be highlighted. Particular emphasis will be devoted to the discussion of ultra-thin epitaxial oxides stabilized on highly oxidizable metals, which have been rarely exploited as oxide supports as compared to the much more widespread noble and quasi noble metallic substrates. In this frame, an extensive discussion is devoted to the microscopic characterization of interfaces between epitaxial metal oxides and the Fe(001) substrate, regarded from the one hand as a prototypical ferromagnetic material and from the other hand as a highly oxidizable metal.

The vibrational properties of adsorbed carbonyls on heterogeneous Pd/oxide systems (SiO2−Al2O3, Al2O3, and MgO) are compared and discussed. On the basis of the surface science literature, the complex IR spectra of carbonyls formed on Pd particles are interpreted in terms of what occurs on well-defined Pd(111) and Pd(100) faces and on edge/corner defects, typical of metal nanoparticles, thus bridging the gap between single crystals and high surface area systems. Furthermore, the influence of the electronic properties of the support (i.e., surface acidity/basicity) in determining the spectral features of carbonyls formed on Pd particles is clearly demonstrated.

Metal-support interactions are frequently invoked to explain the enhanced catalytic activity of metal nanoparticles dispersed over reducible metal oxide supports, yet the atomic-scale mechanisms are rarely known. In this report, scanning tunneling microscopy was used to study a Pt1-6/Fe3O4 model catalyst exposed to CO, H-2, O-2, and mixtures thereof at 550 K. CO extracts lattice oxygen atoms at the cluster perimeter to form CO2, creating large holes in the metal oxide surface. H-2 and O-2 dissociate on the metal clusters and spill over onto the support. The former creates surface hydroxy groups, which react with the support, ultimately leading to the desorption of water, while oxygen atoms react with Fe from the bulk to create new Fe3O4(001) islands. The presence of the Pt is crucial because it catalyzes reactions that already occur on the bare iron oxide surface, but only at higher temperatures.

To improve cardiovascular implant success, metal-based stents are designated to modulate endothelial cells adhesion and migration in order to prevent restenosis and late thrombosis diseases. Biomimetic coatings with extra-cellular matrix adhesive biomolecules onto stents surfaces are a strategy to recover a healthy endothelium. However, the appropriate bioactive sequences to selective promote growth of endothelium and the biomolecules surface immobilization strategy remains to be elucidated. In this study, biofunctionalization of cobalt chromium, CoCr, alloy surfaces with elastin-like recombinamers, ELR, genetically modified with an REDV sequence, was performed to enhance metal surfaces endothelialization. Moreover, physical adsorption and covalent bonding were used as biomolecules binding strategies onto CoCr alloy. Surfaces were activated with plasma and etched with sodium hydroxide previous to silanization with 3-chloropropyltriethoxysilane and functionalized with the ELR. CoCr alloy surfaces were successfully biofunctionalized and the use of an ELR with an REDV sequence, allows conferring bioactivity to the biomaterials surface, demonstrating a higher cell adhesion and spreading of HUVEC cells on the different CoCr surfaces. This effect is emphasized as increases the amount of immobilized biomolecules and directly related to the immobilization technique, covalent bonding, and the increase of surface charge electronegativity. Our strategy of REDV elastin-like recombinamers immobilization onto CoCr alloy surfaces via covalent bonding through organosilanes provides a bioactive surface that promotes endothelial cell adhesion and spreading. (C) 2015 Elsevier B.V. All rights reserved.

Arg, Lys and Asp amino acids are known to play a critical role in the adhesion of the RKLPDA engineered peptide on the (101) surface of the titania anatase phase. To understand their contribution to peptide adhesion, we have considered the relevant charge states due to protonation (Arg and Lys) or deprotonation (Asp) occurring in neutral water solution, and studied their adsorption on the (101) anatase TiO2 surface by ab initio total energy calculations based on density functional theory. The adsorption configurations on the hydrated surface are compared to those on the dry surface considering also the presence of the hydration shell around amino acid side-chains. This study explains how water molecules mediate the adsorption of charged amino acids showing that protonated amino acids are chemically adsorbed much more strongly than de-protonated Asp. Moreover it is shown that the polar screening of the hydration shell reduces the adsorption energy of the protonated amino acids to a small extent, thus evidencing that both Arg and Lys strongly adhere on the (101) anatase TiO2 surface in neutral water solution and that they play a major role in the adhesion of the RKLPDA peptide.

The electronic properties of a single Ru atom and a Ru10 cluster adsorbed on stoichiometric and reduced anatase, a-TiO2(101), and tetragonal zirconia, t-ZrO2(101), surfaces have been determined with density functional theory calculations with Hubbard corrections (DFT+U). The main purpose of the work is to better understand the role of dispersed metals on the surface of titania and zirconia catalysts in conversion of biomass to biofuels. On the stoichiometric surfaces, the metal adsorption does not imply major charge transfers. The situation is different on the reduced surfaces where electron transfer occurs from the oxide to the metal; this effect is more pronounced on zirconia than on titania. On both surfaces, the presence of the Ru nanoparticle favors the removal of O from the surface layers. This can result in the occurrence of O reverse spillover, with displacement of an O atom from a lattice position of the stoichiometric surfaces to a specific adsorption site of Ru10. This process is thermodynamically accessible for both TiO2 and ZrO2 surfaces showing that the metal deposition can result in an easier reduction of the oxide support due to this effect more than to a direct electron transfer from the metal.

We present molecular dynamics simulation results on the processes happening upon deposition of oxygen ions on the NiO(0 0 1) surface. We found that in presence of local excess of charge, the deposited O2- ions combine with surface Ni ions to form ad-molecules that subsequently escape from the surface. In the case of electrically neutral system we found that the oxygen adatoms form chains of NiO molecules that appear to be stable, although diffusing on the surface, leading to a more active substrate. These findings explain experimental results referring to catalytic probes used in reactive plasma diagnostics. (C) 2002 Elsevier Science B.V. All rights reserved. Physica B-Condensed Matter

Nickel vapor-deposited on the SrTiO3(110) surface was studied using scanning tunneling microscopy, photoemission spectroscopy (PES), and density functional theory calculations. This surface forms a (4 x 1) reconstruction, composed of a 2-D titania structure with periodic six- and ten-membered nanopores. Anchored at these nanopores, Ni single adatoms are stabilized at room temperature. PES measurements show that the Ni adatoms create an in-gap state located at 1.9 eV below the conduction band minimum and induce an upward band bending. Both experimental and theoretical results suggest that Ni adatoms are positively charged. Our study produces well-dispersed singleadatom arrays on a well-characterized oxide support, providing a model system to investigate single-adatom catalytic and magnetic properties. Práce se zabývá stabilizací jednotlivých atomů Ni na dvourozměrné porézní vrstvě TiO2 na povrchu SrTiO3(110). Atomární struktura povrchu je měřena pomocí STM.

Discoveries of room-temperature ferromagnetism (RTFM) in semiconductors hold great promise in future spintronics technologies. Unfortunately, this ferromagnetism remains poorly understood and the debate concerning the nature, carrier-mediated versus defect-mediated, of this ferromagnetism in semiconducting oxides is still open. Here, by using X-ray absorption (XAS) and X-ray magnetic circular dichroism (XMCD), it is demonstrated that the oxygen ions have a ferromagnetic response in different ZnO-based compounds showing RTFM behavior: ZnO nanoparticles capped with organic molecules and ZnO/ZnS heterostructures. These results demonstrate the intrinsic occurrence of RTFM in these systems, and point out that it is not related to the metallic cation but it relays on the conduction band of the semiconductor., This work was partially supported by Spanish MAT2011–27573-C04–04, MAT2010–16022, CSD2009–00013, MAT2011–27470-C02–02, MAT2010– 09346-E, the Madrid Government project NANOBIOMAGNET S2009/MAT-1726, and by the Aragón DGA NETOSHIMA grant. The synchrotron radiation experiments were performed at SPring-8 (Proposal Numbers: 2011B3873, 2011B1700 and 2011B0024) and at APS (Proposal No. GUP-10283). Use of the Advanced Photon Source was supported by the U. S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02–06CH11357. The authors acknowledge to J. Freeland for his assistance during the SR experiments at APS. C.G. acknowledges the Ministerio de Educación y Ciencia of Spain for a PhD. Grant. M.A.L.-M. acknowledges to CSIC for a JAE-Doc contract within the Junta para la Ampliación de Estudios program. N.C. acknowledges the financial support of the FSE-MEC, Ramón y Cajal program (ref. RYC-2007–01715). E.C. acknowledges the Ministerio de Educación of Spain for a postdoctoral grant in foreign research centres.

We have investigated the termination and the surface reconstruction of Fe3O4 prepared by oxygen-assisted molecular beam epitaxy on MgO(0 0 1). The epitaxial Fe3O4 layer consists of two structural domains as a result of the fact that its lattice parameter is twice that of MgO. The domains with a lateral size of about 150 nm were observed to be slightly rotated (by an angle of about 1°) around the normal [0 0 1] axis. The surface was investigated with low-energy electron diffraction (LEED) and with low-energy ion scattering (LEIS). We performed 3-D calculations of the ion trajectories in the computer simulations of LEIS, using the program match. The azimuthal scans of scattered and recoiled particles measured in LEIS were compared with computer simulations for many trial structures, including those published in the recent literature. Good agreement was found for a relaxed Fe terminated surface in which every second Fe atom is missing. The topmost layer has an inward relaxation of about 40%. This model explains the symmetry seen in LEED, and it fulfills the criterion of charge neutrality of this polar surface.

Preliminary results of low-energy ion scattering (LEIS) and low-energy electron diffraction (LEED) studies of (100) are presented. Thin epitaxial (100) films were grown by O-assisted molecular beam epitaxy (MBE) of Fe on a MgO(100) substrate. LEED measurements revealed a ()R45 reconstruction pattern as has already been reported in the literature. The surface structure on an atomic scale was studied by LEIS. The sample surface was bombarded with 3 or 6 keV Ar ions in a grazing-angle geometry, and azimuthal scans of the yields of the recoiled Fe and O atoms emitted at scattering angles of 37 and 53 were measured. The scans were analyzed by performing computer simulations of the scattering and recoiling of atoms for a number of trial models. We confirm the presence of two mutually perpendicular structural domains at the surface of the films grown. Bulk contains Fe atoms in tetrahedral and octahedral positions. The surface was found to be terminated by tetrahedral Fe atoms. Further details of the atomic arrangement and surface relaxation were studied.

We report on the layer thickness dependence of Ni 2p core-level line shapes of epitaxially grown, in a layer-by-layer fashion, NiO on a single-crystal MgO (100) substrate. The results demonstrate the sensitivity of the core-level line shape to the nearest as well as next-nearest-neighbor coordination number. The results are consistent with a recent theoretical study of nonlocal screening effects. \textcopyright{} 1996 The American Physical Society.

Magnetit, Fe3O4, ist ein faszinierendes Material aufgrund seiner elektronischen, magneti- schen, und katalytischen Eigenschaften. Fe3O4 kristallisiert in der inversen Spinnellstruktur. In der (001)-Richtung wird der Kristall durch ein alternierendes Stapeln einer A-Schicht (tetraedrische FeA Atome) und einer B-Schicht (oktaedrische FeB und O-Atome) gebildet. Diese Arbeit untersucht die (001) Oberfl��che B-Schicht.Die B-Schicht bildet eine c(2x2) Rekonstruktion. Dazu werden Paare von benachbarten FeB und O-Ionen leicht seitlich verschoben, sodass sich wellenf��rmige Reihen mit zwei in��quivalenten Pl��tzen bilden. Diese werden hier als schmale und breite Pl��tze bezeichnet. Die h��ufigsten Defekte auf dieser Oberfl��che sind Sauerstoff-Fehlstellen (VO), an denen Wassermolek��le bei Raumtemperatur (RT) dissoziieren und Paare von OH Gruppen an der Oberfl��che bilden. Ein weiterer h��ufiger Defekt sind anti-phasen Dom��nengrenzen (APDG). APDG bilden sich auf der Oberfl��che, da die c(2x2) Rekonstruktion zwei Dom��nen in Bezug auf die Volumsstruktur besitzt. Treffen zwei in��quivalente Dom��nen aufeinander, bildet sich eine APDG. Simulationen der Rastertunnelmikroskopbilder (RTM) reproduzieren das Aussehen der APDG der experimentellen Daten und zeigen, dass sie bevorzugte Adsorp- tionspl��tze f��r Hydroxyl-Gruppen sind.Wasser adsorbiert dissoziativ auch in Abwesenheit von VO bis zu einer S��ttigungsbedeckkung von 0,66 ML (1 ML =1, 42 �� 10e14 Atome/cm2 ). Nach der Adsorption von Wasser auf Fe3O4(001) schaut die Oberfl��che in RTM gleich aus wie nach der Adsorption von atomaren H; es gibt keine Hinweise ��ber den Verbleib Wasser-Hydroxyl (OH)-gruppen. Das Fehlen von OH wird durch niederenergetische He+ Ionenstreuung und der Verwendung von H18O best��tigt. Tempern der hydrierten Oberfl��che f��hrt zu Abstraktion der Gitter O-Atome und der Desorption von Wasser.Bei RT und niedriger Bedeckung adsorbiert Fe als einzelne Adatome, bei h��herer Bedeckung findet man Adatome und Cluster. Schwaches Tempern der Probe f��hrt zu einer Oberfl��che, auf der Fe Monomere (FeA) und oktaedrischen Fe-Paare (Fe-OP) in den schmalen Pl��tzen der Oberfl��chenrekonstrution sitzen. Zweifach koordinierte FeA und f��nffach koordinierte Fe-OP bilden sich bei 423 K, w��hrend ausschliesslich Fe-OP bei 573 K beobachtet wurden. Bei h��herer Fe Bedeckung weist die Oberfl��che eine FeO-Struktur auf, bei der alle Oktaeder- l��cken mit Fe-Kationen gef��llt sind. R��ntgen-Photoelektronenspektroskopie zeigt, dass alle Fe-reichen Oberfl��chen gro��teils Fe 2+ enthalten.Die c(2x2) rekonstruierte B-Schicht ist eine relativ robust Vorlage f��r Adsorption. Bedeckungen unter eine Monolage und bei RT, von H, Fe, Au, und Pd adsorbieren als einzige Aadatome an den schmalen Pl��tzen der Oberfl��che und bilden so geordnete Strukturen. Au-Atome zeigen eine bemerkenswerte Stabilit��t, sowohl chemisch in bis zu 10-4 mbar CO, als auch thermisch bis zu einer Probentemperatur von 673 K. Bei h��heren Temperaturen wurden drei-dimensionale Au-Cluster unterschiedlicher Gro��e beobachtet. XPS-Messungen stehen im Einklang mit einer positiven Ladung der einzelnen Au-Adatome.Bei Pd Adatomen induziert CO Mobilit��t durch Bildung eines Pd-CO Komplexes. Wenn sich mindestens drei Pd-CO auf der Oberfl��che treffen, erfolgt homogene Keimbildung zu einem Cluster. Diese Cluster wachsen durch Migration und Koaleszenz. OH-Gruppen an der Oberfl��che haben den gegenteiligen Effekt zu CO: Immobilisierung der Pd-CO Gruppe und Stabilisierung eines Pd Adatoms auf der Oberfl��che., Magnetite, Fe3O4, is a fascinating material for its electronic, magnetic, and catalytic properties. Fe3O4 crystallizes in the inverse spinel structure. In the (001) direction, the crystal is composed of an alternating stack of A layers (containing tetrahedral FeA atoms), and B layers (octahedral FeB and O atoms). This thesis investigates the (001) B layer surface.The B layer surface forms a c(2x2) reconstruction. Here, pairs of FeB and neighboring O ions are slightly displaced laterally, forming undulating rows with two inequivalent FeA bulk continuation sites. We term these narrow and wide sites, and a preference for adsorbates to occupy the narrow site is a common theme in this work.The most common defects on this surface are oxygen vacancies (VO's).These readily dissociate water molecules at room temperature (RT), forming pairs of surface OH groups. Another common defect are anti-phase domain boundaries (APDBs). APDBs are formed on the surface because the c(2x2) reconstruction has two equivalent domains with respect to the bulk structure. Where two inequivalent domains meet, an APDB is formed.Simulated scanning tunneling microscopy (STM) images reproduce the appearance of the APDBs in the experimental data and reveal that they are preferential adsorption sites for hydroxyl groups.Water adsorbs dissociatively also in the absence of VO 's, up to a saturation coverage of 0.66 ML of surface OH (1 ML =1.42 �� 10e14atoms/cm2). However, STM images of the water-exposed Fe3O4(001) surface appear identical to those obtained following deposition of atomic H, with no evidence for the water hydroxyl (OH) species. The absence of the OH is confirmed by low energy He+ ion scattering utilizing H18O. Annealing of the hydrogenated surface leads to 2 the abstraction of lattice O atoms and desorption of water.At RT, Fe adsorbs as single adatoms for low coverage, and as adatoms and clusters for higher coverage. Mild annealing of the sample leads to a surface containing Fe monomers (FeA) and Fe octahedral pairs (Fe-OP) at narrow sites. Two-fold coordinated FeA and 5-fold coordinated Fe-OP form at 423 K, while only Fe-OP's are observed at 573 K. For higher Fe coverage the sur- face exhibits an FeO-like structure with all surface octahedral sites filled with Fe cations. X-ray photoelectron spectroscopy indicates that all Fe-rich surfaces contain mostly Fe2+.The c(2x2) reconstructed B layer surface acts as a rather robust adsorption-template. At fractional monolayer coverages and at RT, H, Fe, Au, and Pd adsorb as single adatoms at the narrow sites of the surface unit cell, forming ordered structures on the surface.Supported Au atoms show a remarkable stability, both chemically in up to 10-4 mbar of CO, and thermally up to 673 K. At higher annealing temperatures, different sizes of three-dimensional Au clusters are observed. XPS measurements are consistent with a positive charge state of the single adatoms.For Pd adatoms, CO induces mobility via formation of a Pd-CO complex.When at least three Pd-CO meet on the surface, homogeneous nucleation of a cluster takes place. Clusters grow through cluster migration and coalescence. Surface OH groups have the opposite effect to CO, immobilizing the Pd-CO group and stabilizing an Pd adatom on the surface.

InAlAs has the potential to be used as a barrier layer in buried channel quantum well field effect transistor devices due to favorable lattice-matching and carrier confinement properties with InGaAs. Field effect device structures of this nature may also require a high-k oxide deposited on the InAlAs surface to reduce leakage current. This study investigates the impact of surface preparations and atomic layer deposition of HfO2 on these surfaces using x-ray photoelectron spectroscopy to analyse the chemical interactions taking place, as well as the electrical performance of associated capacitor devices. A large concentration of As related surface features is observed at the InAlAs surface, and is attributed to a large D-it response in electrical measurements. (C) 2013 AIP Publishing LLC.

Tris(phenylthio)benzene molecules have been synthesized in order to explore their ability to trap single Au adatoms on an Au(111) surface. The resulting metal–organic complexes have been characterized with low-temperature scanning tunneling microscopy and infrared reflection absorption spectroscopy; possible structure models have been derived from density functional calculations. Upon room temperature deposition, the thiophenyl derivatives form dimer structures, comprising two molecules and six Au adatoms. Below 100 K, isolated molecules are found as well that have trapped up to six Au atoms. On the basis of the experimental results and calculated formation energies of the complexes, we discuss potential applications of the thioethers for the redispersion of metals on a catalyst surface. First experiments performed on Au particle ensembles prepared on alumina thin films suggest that the molecular ligands are indeed able to change the distribution of gold on the oxide surface.

As a model system for photochemistry on oxide surfaces, the generation of atomic oxygen by photodissociation of N2O adsorbed on thin MgO films which are grown on Ag(001) has been studied under UHV conditions. The reactivity of the generated atomic oxygen was examined by studying the oxidation of CO. If MgO with adsorbed N2O is irradiated with photons (E = 5.0 eV), TPD spectra show a N2O depletion and generation of N2 (desorbing ≈ 55 K) and atomic oxygen (desorbing recombinatively at ≈ 550 K). An analysis of the photon dose dependence suggests two N2O dissociation pathways on the surface with reaction cross sections of about 10^−19 cm^2: The irradiation with E = 5.0 eV leads to an electron-hole-pair excitation at the edges of the MgO film. Excited electrons are trapped at certain defect sites which lead to localized electrons and localized holes at low coordinated sites. These localized charges can cause a dissociation of nearby adsorbed N2O. The presence of the electron and the hole lead to singly charged atomic oxygen. EPR spectroscopy done in cooperation with the department of Hajo Freund confirmed that the generated atomic oxygen is correlated with a free spin. A site-selective chemistry can be applied with different photon energies. If a photon energy of E = 6.4 eV is used, the reaction cross sections increase to 10^−17 cm^2 which is explained by the increased probability to create a trapped electron: The photoexcitation of MgO films at terrace sites leads to a higher amount of generated electron-hole pairs compared to the photoexcitation at lower coordinated sites. The reaction yield could also be controlled by varying the MgO film preparation: Due to a larger surface roughness for thicker MgO films (which led to more electron traps), UV-induced atomic oxygen generation scales with the MgO film thickness. Irradiation (E = 5.0 eV) of the coadsorbate consisting of CO and atomic oxygen leads to a formation of CO2., Als Modellsystem für Photochemie auf Oxiden wurde in dieser Arbeit die Bildung von atomaren Sauerstoff durch Photodissoziation von N2O-Adsorbaten auf dünnen MgO-Filmen, präpariert auf Ag(001), unter UHV-Bedingungen untersucht. Die Reaktivität des atomaren Sauerstoffs wurde anhand von CO Oxidation überprüft. Nach UV-Bestrahlung zeigen thermische Desorptionsspektren eine Abnahme der ursprünglichen N2O Bedeckung und eine Bildung von N2 (Desorption bei ≈ 55K) und atomaren Sauerstoff (rekombinative Desorption bei ≈ 550 K). Die Photonenabhängigkeit der N2O-Dissoziation läßt sich mit einem Mechanismus bestehend aus zwei Reaktionspfaden beschreiben, deren Querschnitte jeweils ≈ 10^−19 cm^2 sind: UV-Bestrahlung (E = 5.0 eV) führt zu einer Elektronen-Loch- Paar-Anregung an Kanten des MgO-Films. Angeregte Elektronen werden an bestimmten Defekten eingefangen und führen zu lokalisierten Elektronen und Löchern, wodurch in der Nähe adsorbiertes N2O dissoziiert. Durch diese Ladungen entsteht einfach geladener atomarer Sauerstoff, dessen freier Spin durch eine EPR-Studie in Kooperation mit der Abteilung von Hajo Freund nachgewiesen wurde. Oberflächenselektive Chemie kann durch unterschiedliche Photonenenergien realisiert werden. N2O-Dissoziation mit einer Photonenenergie von E = 6.4 eV erhöht die Reaktionsquerschnitte auf 10^−17 cm^2. Die erhöhte Wahrscheinlichkeit, lokalisierte Ladungen zu generieren ist eine Erklärung hierfür: Die Photonenanregung an Terrassen von MgO-Filmen führt zu einer größeren Anzahl an generierten Elektron-Loch-Paaren als die an geringer koordinierten Oberflächenstellen. Auch durch Variation der MgO-Präparation kann die Reaktionsausbeute kontrolliert werden: Da dickere Filme eine rauhere Oberfläche haben und damit mehr Elektronenfallen aufweisen, wächst die UV- induzierte Generation von atomaren Sauerstoff mit der MgO-Filmdicke. Bestrahlung eines Koadsorbats bestehend aus vorher generiertem atomaren Sauerstoff und CO führt zur Bildung von CO2 und bestätigt die Reaktivität des atomaren Sauerstoffs.

International audience; 2D double-quantum H-1-H-1 NMR unambiguously shows that the "isolated" Si-OH surface silanols of dehydroxylated SBA-15 are converted upon treatment with ammonia into single silylamine surface site Si-NH2. The "gem" di-silanols (= Si(OH)(2)) remain intact. Treatment using HMDS produces (= Si(OSiMe3)(2)) but leaves Si-NH2 untouched. The resulting surface is hydrophobic and stable.

We discuss unreported transitions of oxidized GaAs surfaces between (super) hydrophilic and hydrophobic states when stored in ambient conditions. Contact angles higher than 90 degrees and high adhesive force were observed for several air-aged epitaxial samples grown under different conditions as well as on epi-ready wafers. Regardless of the morphologies of the surface, superhydrophilicity of oxygen-plasma treated samples was observed, an effect disappearing with storage time. Reproducible hydrophobicity was likewise observed, as expected, after standard HCl surface etching. The relation between surface oxides and hydrophobic/hydrophilic behavior is discussed. (C) 2011 American Institute of Physics. [doi:10.1063/1.3619797]

Zn K edge and O K edge x-ray absorption near-edge structure (XANES) spectra of ZnO surfaces are calculated. The difference between theoretical XANES for ZnO surfaces and ZnO bulk is then compared to the earlier observed differences between experimental XANES for ZnO nanostructures and ZnO bulk as taken from the literature. It follows from our calculations that the differences between the experimental XANES of bulk ZnO and nanocrystalline ZnO is not due to the enhanced role of the surfaces in nanostructures. Rather, the difference in XANES has to reflect differences in the local geometry around the photoabsorbing sites. The dependence of XANES of ZnO surfaces on the polarization of the incoming radiation is also investigated theoretically and found to be similar as in the bulk.

International audience; Here we show that the stoichiometry and, consequently, the chemical activity towards hydroxylation of MgO(100) films grown by reactive deposition on Ag(100) strongly depends on the O 2 partial pressure during film growth. Oxygen-deficient films undergo dramatic relative oxygen uptake either by exposure to a partial pressure of water vapor or by aging in vacuum for a sufficiently long time. Conversely, on stoichiometric monolayer MgO islands, photoemission analysis of the O 1s level and scanning tunneling microscopy images are consistent with the prediction that dissociative adsorption of water occurs only at the borders of the islands.

1 - Article; Biointegration of titanium implants in the body is controlled by their surface properties. Improving surface properties by coating with a bioactive polymer is a promising approach to improve the biological performance of titanium implants. To optimize the grafting processes, it is important to fully understand the composition and structure of the modified surfaces. The main focus of this study is to provide a detailed, multitechnique characterization of a bioactive poly(sodium styrene sulfonate) (pNaSS) thin film grafted from titanium surfaces via a two-step procedure. Thin titanium films (similar to 50 nm thick with an average surface roughness of 0.9 +/- 0.2 nm) prepared by evaporation onto silicon wafers were used as smooth model substrates. X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS) showed that the titanium film was covered with a TiO(2) layer that was at least 10 nm thick and contained hydroxyl groups present at the outermost surface. These hydroxyl groups were first modified with a 3-methacryloxypropyltrimethoxysilane (MPS) cross-linker. XPS and ToF-SIMS showed that a monolayer of the MPS molecules was successfully attached onto the titanium surfaces. The pNaSS film was grafted from the MPS-modified titanium through atom transfer radical polymerization. Again, XPS and ToF-SIMS were used to verify that the pNaSS molecules were successfully grafted onto the modified surfaces. Atomic force microscopy analysis showed that the film was smooth and uniformly covered the surface. Fourier transform infrared spectroscopy indicated that an ordered array of grafted NaSS molecules were present on the titanium surfaces. Sum frequency generation vibration spectroscopy and near edge X-ray absorption fine structure spectroscopy illustrated that the NaSS molecules were grafted onto the titanium surface with a substantial degree of orientational order in the styrene rings.

A strategy for creating a general-purposes surface functionalization platform is reported, based on direct attachment of phosphate groups onto hydroxylated surfaces and subsequent formation of a terpyridine-based monolayer. Such a platform is suitable for the construction, onto technologically relevant oxide surfaces, of single- and multilayer structures of interest in technological applications. In particular, the paper describes the successful attachment of 4-(2,2':6',2''-terpyridine-4-yl)benzenephosphonic acid (1, PPTP) onto a SiO(2) surface previously functionalized by means of Zr-phosphate groups. Two alternative anchoring strategies of the PPTP were explored: (i) a direct one-step way, implying no protection of terpyridinic functionality, and (ii) a three-step way, implying protection and successive deprotection of this group. It was found that, in the first case, the PPTP ligand anchoring to the Zr-containing phosphate layer takes place by means of terpyridinic group. At variance of this, in the second case, due to the protection of the terpyridinic functionality, the anchoring process takes place through the phosphonic group, making the terpyridinic moiety available for further reactions, i.e., multilayer constructs. X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS) were used to study the functionalized surfaces, providing information on coverage, chemical structure, and stoichiometry of the various functionalized layers and, among the others, clear evidence of the PPTP linkage and orientation.

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.

We report on first-principles density functional calculations about the adsorption of water molecules on the nonpolar ZnO(1010) surface, as a function of the molecular coverage. Our results allow us to unravel the reaction mechanisms that drive the partial dissociation of water molecules at saturation coverage: Although not a favored event, concurrent adsorption/dissociation may occur as a compromise between steric repulsion and covalent and hydrogen bond formation with both the substrate and the impinging molecules. The scenario is altered by the presence of defects. We discuss the role of the most common point and extended defects at the outermost layer: these systems exhibit remarkably different electronic properties leading to peculiar and unexpected characteristics for the wet defective surface. Enhanced reactivity of edged nanostructures is predicted, while the catalytic role of oxygen vacancies is questioned. The effects of metallicity induced by hydrogen adsorption on the interaction with water are finally analyzed. We suggest also experimental probes to identify the various adsorption geometries and fully characterize the water vapor/oxide interface.

The functionalization of optically transparent substrates is of importance, for example, in the field of biosensing. In this article, a new method for modification of silica surfaces is presented that is based on a photochemical reaction of terminal alkenes with the surface. This yields highly hydrophobic surfaces, which are thermally stable up to at least 400 degrees C. The formed monolayer provides chemical passivation of the underlying surface, according to studies showing successful blocking of platinum atomic layer deposition (ALD). The reaction is photochemically initiated, requiring light with a wavelength below 275 nm. X-ray photoelectron spectroscopy and infrared spectroscopy studies show that the alkenes initially bind to the surface hydroxyl groups in Markovnikov fashion. At prolonged reaction times (5 h), however, oligomerization occurs, resulting in layer growth normal to the surface. The photochemical nature of the reaction enables the use of photolithography as a tool to constructively pattern silica surfaces. Atomic force microscopy shows that the features of the photomask are well transferred. The newly developed method can complement existing patterning methods on silica that are based on soft lithography.

The purpose of this comment is to provide additional insight into the reliability of microelectromechanical capacitive switches (MEMSs) investigated by Molinero and Casta er [Appl. Phys. Lett. 92, 043502 (2008)]. We show that the presence or absence of ambient humidity determines whether the shift in the capacitance-voltage (C-V) curve of oxide-based MEMS occurs as a result of voltage stress. In humid air, negative and positive shifts in the C-V curve are observed after negative and positive bias stress. In dry air no such shifts in the C-V curve are seen. These shifts are similar to those reported on oxide-based switches by Molinero and Castaner [Appl. Phys. Lett. 92, 043502 (2008)] where they show shifts occurring in room ambient pressure, but not in vacuum. This indicates that not only air pressure but also air humidity can be responsible for shifts in MEMS. (C) 2009 American Institute of Physics. (doi: 10.1063/1.3255008)