9 research outputs found

    Atomic Oxygen on Graphite: Chemical Characterization and Thermal Reduction

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    The chemisorption of O atoms on graphite and the thermal reduction of the oxidized surface were studied by means of high energy resolution photoelectron spectroscopy with synchrotron radiation. The C 1s and O 1s core levels and the valence band spectra were used to identify the different oxidizing surface species and to evaluate the extension of the sp<sup>2</sup> conjugation as a function of oxidation time and annealing temperature. We found that epoxy groups are the dominant species only at the low oxidation stage, and ethers and semiquinones form as oxidation proceeds. The evolution of the ether/epoxy ratio with increasing oxygen coverage provides evidence for the occurrence of C–C bond unzipping. Epoxy groups are the functionalities with the lowest thermal stability and start to desorb around 370 K, strongly affecting the desorption temperature of other functional groups. The ratio between ethers and epoxy groups determines the balance between epoxy–epoxy and epoxy–ether reactions, the latter promoting the removal of C atoms from the C backbone. Adsorbate spectroscopy during thermal annealing definitely proves the catalytic effect of the basal plane oxygen atoms on the desorption reactions

    Dual Path Mechanism in the Thermal Reduction of Graphene Oxide

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    Graphene is easily produced by thermally reducing graphene oxide. However, defect formation in the C network during deoxygenation compromises the charge carrier mobility in the reduced material. Understanding the mechanisms of the thermal reactions is essential for defining alternative routes able to limit the density of defects generated by carbon evolution. Here, we identify a dual path mechanism in the thermal reduction of graphene oxide driven by the oxygen coverage: at low surface density, the O atoms adsorbed as epoxy groups evolve as O<sub>2</sub> leaving the C network unmodified. At higher coverage, the formation of other O-containing species opens competing reaction channels, which consume the C backbone. We combined spectroscopic tools and ab initio calculations to probe the species residing on the surface and those released in the gas phase during heating and to identify reaction pathways and rate-limiting steps. Our results illuminate the current puzzling scenario of the low temperature gasification of graphene oxide

    Epitaxial Growth of a Single-Domain Hexagonal Boron Nitride Monolayer

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    We investigate the structure of epitaxially grown hexagonal boron nitride (<i>h</i>-BN) on Ir(111) by chemical vapor deposition of borazine. Using photoelectron diffraction spectroscopy, we unambiguously show that a single-domain <i>h</i>-BN monolayer can be synthesized by a cyclic dose of high-purity borazine onto the metal substrate at room temperature followed by annealing at <i>T</i> = 1270 K, this method giving rise to a diffraction pattern with 3-fold symmetry. In contrast, high-temperature borazine deposition (<i>T</i> = 1070 K) results in a <i>h</i>-BN monolayer formed by domains with opposite orientation and characterized by a 6-fold symmetric diffraction pattern. We identify the thermal energy and the binding energy difference between fcc and hcp seeds as key parameters in controlling the alignment of the growing <i>h</i>-BN clusters during the first stage of the growth, and we further propose structural models for the <i>h</i>-BN monolayer on the Ir(111) surface

    Graphene-Induced Substrate Decoupling and Ideal Doping of a Self-Assembled Iron-phthalocyanine Single Layer

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    Iron-phthalocyanine molecules self-assemble on the moiré pattern of graphene/Ir(111) as a flat and weakly interacting layer, as determined by core-level photoemission and absorption spectroscopy. The graphene buffer layer decouples the FePc two-dimensional structure from the underlying metal; the electronic structure of the FePc molecular macrocycles is preserved; and the Fe-L<sub>2,3</sub> edges present narrower and slightly modified resonances at the FePc single-layer coverage with respect to a thin film. The FePc layer induces a slight electron doping to the Ir-supported graphene resulting in the Dirac cone position expected for an ideal free-standing-like graphene layer with the standard Fermi velocity

    Bis(triisopropylsilylethynyl)pentacene/Au(111) Interface: Coupling, Molecular Orientation, and Thermal Stability

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    The assembly and the orientation of functionalized pentacene at the interface with inorganics strongly influence both the electric contact and the charge transport in organic electronic devices. In this study electronic spectroscopies and theoretical modeling are combined to investigate the properties of the bis­(triisopropylsilylethynyl)­pentacene (TIPS-Pc)/Au(111) interface as a function of the molecular coverage to compare the molecular state in the gas phase and in the adsorbed phase and to determine the thermal stability of TIPS-Pc in contact with gold. Our results show that in the free molecule only the acene atoms directly bonded to the ligands are affected by the functionalization. Adsorption on Au(111) leads to a weak coupling which causes only modest binding energy shifts in the TIPS-Pc and substrate core level spectra. In the first monolayer the acene plane form an angle of 33 ± 2° with the Au(111) surface at variance with the vertical geometry reported for thicker solution-processed or evaporated films, whereas the presence of configurational disorder was observed in the multilayer. The thermal annealing of the TIPS-Pc/Au(111) interface reveals the ligand desorption at ∼470 K, which leaves the backbone of the decomposed molecule flat-lying on the metal surface as in the case of the unmodified pentacene. The weak interaction with the metal substrate causes the molecular dissociation to occur 60 K below the thermal decomposition taking place in thick drop-cast films

    Self-Assembly of Graphene Nanoblisters Sealed to a Bare Metal Surface

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    The possibility to intercalate noble gas atoms below epitaxial graphene monolayers coupled with the instability at high temperature of graphene on the surface of certain metals has been exploited to produce Ar-filled graphene nanosized blisters evenly distributed on the bare Ni(111) surface. We have followed in real time the self-assembling of the nanoblisters during the thermal annealing of the Gr/Ni(111) interface loaded with Ar and characterized their morphology and structure at the atomic scale. The nanoblisters contain Ar aggregates compressed at high pressure arranged below the graphene monolayer skin that is decoupled from the Ni substrate and sealed only at the periphery through stable C–Ni bonds. Their in-plane truncated triangular shapes are driven by the crystallographic directions of the Ni surface. The nonuniform strain revealed along the blister profile is explained by the inhomogeneous expansion of the flexible graphene lattice that adjusts to envelop the Ar atom stacks

    Controlling Hydrogenation of Graphene on Ir(111)

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    Combined fast X-ray photoelectron spectroscopy and density functional theory calculations reveal the presence of two types of hydrogen adsorbate structures at the graphene/Ir(111) interface, namely, graphane-like islands and hydrogen dimer structures. While the former give rise to a periodic pattern, dimers tend to destroy the periodicity. Our data reveal distinctive growth rates and stability of both types of structures, thereby allowing one to obtain well-defined patterns of hydrogen clusters. The ability to control and manipulate the formation and size of hydrogen structures on graphene facilitates tailoring of its properties for a wide range of applications by means of covalent functionalization

    Transfer-Free Electrical Insulation of Epitaxial Graphene from its Metal Substrate

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    High-quality, large-area epitaxial graphene can be grown on metal surfaces, but its transport properties cannot be exploited because the electrical conduction is dominated by the substrate. Here we insulate epitaxial graphene on Ru(0001) by a stepwise intercalation of silicon and oxygen, and the eventual formation of a SiO<sub>2</sub> layer between the graphene and the metal. We follow the reaction steps by X-ray photoemission spectroscopy and demonstrate the electrical insulation using a nanoscale multipoint probe technique

    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
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