10 research outputs found

    Temperature-Driven Reversible Rippling and Bonding of a Graphene Superlattice

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    In order to unravel the complex interplay between substrate interactions and film configuration, we investigate and characterize graphene on a support with non-three-fold symmetry, the square Ir(100). Below 500 Ā°C, distinct physisorbed and chemisorbed graphene phases coexist on the surface, respectively characterized by flat and buckled morphology. They organize into alternating domains that extend on mesoscopic lengths, relieving the strain due to the different thermal expansion of film and substrate. The chemisorbed phase exhibits exceptionally large one-dimensional ripples with regular nanometer periodicity and can be reversibly transformed into physisorbed graphene in a temperature-controlled process that involves surprisingly few Cā€“Ir bonds. The formation and rupture of these bonds, rather than ripples or strain, are found to profoundly alter the local electronic structure, changing graphene behavior from semimetal to metallic type. The exploitation of such subtle interfacial changes opens new possibilities for tuning the properties of this unique material

    NH<sub>3</sub>ā€“NO Coadsorption System on Pt(111). II. Intermolecular Interaction

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    Coadsorption of ammonia and nitric oxide on the (111) surface of platinum causes the mutual stabilization of the two adsorbed species, arranged in an ordered 2 Ɨ 2 mixed layer. Furthermore, their interaction leads also to stable, isolated triangular units, which we observe on the surface after annealing to 345 K. Having provided in the preceding article (10.1021/jp406068y) a detailed structural description of the NH<sub>3</sub>ā€“NO mixed layer, we focus here on the stabilizing intermolecular interactions. By combining scanning tunneling microscopy (STM) experiments and density functional theory (DFT) calculations, we identify the isolated triangular units as formed by one NH<sub>3</sub> and three NO molecules, and we characterize them in terms of structure, energetics, and charge rearrangement. Eventually, we investigate the nature of the chemical bond between the coadsorbed NH<sub>3</sub> and NO both in the mixed layer and in the isolated triangular units, pointing out the essential role of the surface mediation in inducing attractive dipoleā€“dipole interactions and the presence of hydrogen bonds

    Experimental and Theoretical Investigation of the Restructuring Process Induced by CO at Near Ambient Pressure: Pt Nanoclusters on Graphene/Ir(111)

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    The adsorption of CO on Pt nanoclusters grown in a regular array on a template provided by the graphene/Ir(111) MoireĢ was investigated by means of infrared-visible sum frequency generation vibronic spectroscopy, scanning tunneling microscopy, X-ray photoelectron spectroscopy from ultrahigh vacuum to near-ambient pressure, and <i>ab initio</i> simulations. Both terminally and bridge bonded CO species populate nonequivalent sites of the clusters, spanning from first to second-layer terraces to borders and edges, depending on the particle size and morphology and on the adsorption conditions. By combining experimental information and the results of the simulations, we observe a significant restructuring of the clusters. Additionally, above room temperature and at 0.1 mbar, Pt clusters catalyze the spillover of CO to the underlying graphene/Ir(111) interface

    Tailoring Bimetallic Alloy Surface Properties by Kinetic Control of Self-Diffusion Processes at the Nanoscale

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    Achieving control of the nanoscale structure of binary alloys is of paramount importance for the design of novel materials with specific properties, leading to, for example, improved reaction rates and selectivity in catalysis, tailored magnetic behavior in electronics, and controlled growth of nanostructured materials such as graphene. By means of a combined experimental and theoretical approach, we show that the complex self-diffusion mechanisms determining these key properties can be mostly defined by kinetic rather than energetic effects. We explain how in the Niā€“Cu system nanoscale control of self-diffusion and segregation processes close to the surface can be achieved by finely tuning the relative concentration of the alloy constituents. This allows tailoring the material functionality and provides a clear explanation of previously observed effects involved, for example, in the growth of graphene films and in the catalytic reduction of carbon dioxide

    Local Electronic Structure and Density of Edge and Facet Atoms at Rh Nanoclusters Self-Assembled on a Graphene Template

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    The chemical and physical properties of nanoclusters largely depend on their sizes and shapes. This is partly due to finite size effects influencing the local electronic structure of the nanocluster atoms which are located on the nanofacets and on their edges. Here we present a thorough study on graphene-supported Rh nanocluster assemblies and their geometry-dependent electronic structure obtained by combining high-energy resolution core level photoelectron spectroscopy, scanning tunneling microscopy, and density functional theory. We demonstrate the possibility to finely control the morphology and the degree of structural order of Rh clusters grown in register with the template surface of graphene/Ir(111). By comparing measured and calculated core electron binding energies, we identify edge, facet, and bulk atoms of the nanoclusters. We describe how small interatomic distance changes occur while varying the nanocluster size, substantially modifying the properties of surface atoms. The properties of under-coordinated Rh atoms are discussed in view of their importance in heterogeneous catalysis and magnetism

    <i>In Situ</i> Observations of the Atomistic Mechanisms of Ni Catalyzed Low Temperature Graphene Growth

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    The key atomistic mechanisms of graphene formation on Ni for technologically relevant hydrocarbon exposures below 600 Ā°C are directly revealed <i>via</i> complementary <i>in situ</i> scanning tunneling microscopy and X-ray photoelectron spectroscopy. For clean Ni(111) below 500 Ā°C, two different surface carbide (Ni<sub>2</sub>C) conversion mechanisms are dominant which both yield epitaxial graphene, whereas above 500 Ā°C, graphene predominantly grows directly on Ni(111) <i>via</i> replacement mechanisms leading to embedded epitaxial and/or rotated graphene domains. Upon cooling, additional carbon structures form exclusively underneath rotated graphene domains. The dominant graphene growth mechanism also critically depends on the near-surface carbon concentration and hence is intimately linked to the full history of the catalyst and all possible sources of contamination. The detailed XPS fingerprinting of these processes allows a direct link to high pressure XPS measurements of a wide range of growth conditions, including polycrystalline Ni catalysts and recipes commonly used in industrial reactors for graphene and carbon nanotube CVD. This enables an unambiguous and consistent interpretation of prior literature and an assessment of how the quality/structure of as-grown carbon nanostructures relates to the growth modes

    <i>In Situ</i> Observations of the Atomistic Mechanisms of Ni Catalyzed Low Temperature Graphene Growth

    No full text
    The key atomistic mechanisms of graphene formation on Ni for technologically relevant hydrocarbon exposures below 600 Ā°C are directly revealed <i>via</i> complementary <i>in situ</i> scanning tunneling microscopy and X-ray photoelectron spectroscopy. For clean Ni(111) below 500 Ā°C, two different surface carbide (Ni<sub>2</sub>C) conversion mechanisms are dominant which both yield epitaxial graphene, whereas above 500 Ā°C, graphene predominantly grows directly on Ni(111) <i>via</i> replacement mechanisms leading to embedded epitaxial and/or rotated graphene domains. Upon cooling, additional carbon structures form exclusively underneath rotated graphene domains. The dominant graphene growth mechanism also critically depends on the near-surface carbon concentration and hence is intimately linked to the full history of the catalyst and all possible sources of contamination. The detailed XPS fingerprinting of these processes allows a direct link to high pressure XPS measurements of a wide range of growth conditions, including polycrystalline Ni catalysts and recipes commonly used in industrial reactors for graphene and carbon nanotube CVD. This enables an unambiguous and consistent interpretation of prior literature and an assessment of how the quality/structure of as-grown carbon nanostructures relates to the growth modes

    <i>In Situ</i> Observations of the Atomistic Mechanisms of Ni Catalyzed Low Temperature Graphene Growth

    No full text
    The key atomistic mechanisms of graphene formation on Ni for technologically relevant hydrocarbon exposures below 600 Ā°C are directly revealed <i>via</i> complementary <i>in situ</i> scanning tunneling microscopy and X-ray photoelectron spectroscopy. For clean Ni(111) below 500 Ā°C, two different surface carbide (Ni<sub>2</sub>C) conversion mechanisms are dominant which both yield epitaxial graphene, whereas above 500 Ā°C, graphene predominantly grows directly on Ni(111) <i>via</i> replacement mechanisms leading to embedded epitaxial and/or rotated graphene domains. Upon cooling, additional carbon structures form exclusively underneath rotated graphene domains. The dominant graphene growth mechanism also critically depends on the near-surface carbon concentration and hence is intimately linked to the full history of the catalyst and all possible sources of contamination. The detailed XPS fingerprinting of these processes allows a direct link to high pressure XPS measurements of a wide range of growth conditions, including polycrystalline Ni catalysts and recipes commonly used in industrial reactors for graphene and carbon nanotube CVD. This enables an unambiguous and consistent interpretation of prior literature and an assessment of how the quality/structure of as-grown carbon nanostructures relates to the growth modes

    Chemistry of the Methylamine Termination at a Gold Surface: From Autorecognition to Condensation

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    13The self-assembly of the naphthylmethylamine molecules (NMA) on the Au(111) surface is investigated by a combined experimental and theoretical approach. Three well-defined phases are observed upon different thermal treatments at the monolayer stage. The role played by the methylamine termination is evidenced in both the moleculeā€“molecule and moleculeā€“substrate interactions. The autorecognition process of the amino groups is identified as the driving factor for the formation of a complex hydrogen bonding scheme in small molecular clusters, possibly acting also as a precursor of a denitrogenation condensation process induced by thermal annealing.reservedmixedDri, Carlo; Fronzoni, Giovanna; Balducci, Gabriele; Furlan, Sara; Stener, Mauro; Feng, Zhijing; Comelli, Giovanni; Castellarin-Cudia, Carla; Cvetko, Dean; Kladnik, Gregor; Verdini, Alberto; Floreano, Luca; Cossaro, AlbanoDri, Carlo; Fronzoni, Giovanna; Balducci, Gabriele; Furlan, Sara; Stener, Mauro; Feng, Zhijing; Comelli, Giovanni; Castellarin Cudia, Carla; Cvetko, Dean; Kladnik, Gregor; Verdini, Alberto; Floreano, Luca; Cossaro, Alban
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