Alessandro Baraldi

Bandgap opening in graphene induced by patterned hydrogen adsorption

Graphene, a single layer of graphite, has recently attracted considerable attention owing to its remarkable electronic and structural properties and its possible applications in many emerging areas such as graphene-based electronic devices. The charge carriers in graphene behave like massless Dirac fermions, and graphene shows ballistic charge transport, turning it into an ideal material for circuit fabrication. However, graphene lacks a bandgap around the Fermi level, which is the defining concept for semiconductor materials and essential for controlling the conductivity by electronic means. Theory predicts that a tunable bandgap may be engineered by periodic modulations of the graphene lattice, but experimental evidence for this is so far lacking. Here, we demonstrate the existence of a bandgap opening in graphene, induced by the patterned adsorption of atomic hydrogen onto the Moiré superlattice positions of graphene grown on an Ir(111) substrate.

Dual Path Mechanism in the Thermal Reduction of Graphene Oxide

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(2) 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.

Carbon Dioxide Hydrogenation on Ni(110)

We demonstrate that the key step for the reaction of CO 2 with hydrogen on Ni(110) is a change of the activated molecule coordination to the metal surface. At 90 K, CO 2 is negatively charged and chemically bonded via the carbon atom. When the temperature is increased and H approaches, the H-CO 2 complex flips and binds to the surface through the two oxygen atoms, while H binds to the carbon atom, thus yielding formate. We provide the atomic-level description of this process by means of conventional ultrahigh vacuum surface science techniques combined with density functional theory calculations and corroborated by high pressure reactivity tests. Knowledge about the details of the mechanisms involved in this reaction can yield a deeper comprehension of heterogeneous catalytic organic synthesis processes involving carbon dioxide as a reactant. We show why on Ni the CO 2 hydrogenation barrier is remarkably smaller than that on the common Cu metal-based catalyst. Our results provide a possible interpretation of the observed high catalytic activity of NiCu alloys.

Metal-organic coordination interactions in Fe-Terephthalic acid networks on Cu(100)

Metal-organic coordination interactions are prime candidates for the formation of self-assembled, nanometer-scale periodic networks with room-temperature structural stability. We present X-ray photoelectron spectroscopy measurements of such networks at the Cu(100) surface which provide clear evidence for genuine metal-organic coordination. This is evident as binding energy shifts in the O 1s and Fe 3p photoelectron peaks, corresponding to O and Fe atoms involved in the coordination. Our results provide the first clear evidence for charge-transfer coordination in metal-organic networks at surfaces and demonstrate a well-defined oxidation state for the coordinated Fe ions.

Real-time X-ray photoelectron spectroscopy of surface reactions

Abstract The experimental determination of the composition and structure of gas–solid surface interface at different stages of surface reactions is a crucial point in elucidating the reaction mechanism. This requires a quantitative surface sensitive technique, providing information on how the substrate surface, adsorbed species and their bonding configuration evolve at the time scale of the surface processes. The present review illustrates how the high performance levels achieved in X-ray photoelectron spectroscopy at the third generation synchrotron facilities, in particular the reduced data acquisition time down to a second range, have made possible studies of surface processes in real time. The article summarizes the wealth of knowledge that has been gained using representative examples of adsorption systems, where the relation between adsorption–desorption rate, adsorbate coverage, bonding configuration and interconversion between adsorption sites was established, and simple reaction systems, where the effects of the substrate structure and of the changes in the adsorbate layer under non-linear reaction conditions were probed.

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

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(2) 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.

Surface core level shifts of clean and oxygen covered Ru(0001)

We present the results of high resolution core level photoelectron spectroscopy employed to investigate the electronic structure of clean and oxygen covered Ir(111) surface. Ir 4f7/2 core level spectra are shown to be very sensitive to the local atomic environment. For the clean surface we detected two distinct components shifted by 550meV, originated by surface and bulk atoms. The larger Gaussian width of the bulk component is explained as due to experimentally unresolved subsurface components. In order to determine the relevance of the phonon contribution we examined the thermal behaviour of the core level lineshape using the Hedin-Rosengren theory. From the phonon- induced spectral broadening we found the Debye temperature of bulk and surface atoms to be 298 and 181K, respectively, which confirms the softening of the vibrational modes at the surface. Oxygen adsorption leads to the appearance of new surface core level components at 200meV and +230meV, which are interpreted as due to first-layer Ir atoms differently coordinated with oxygen. The coverage dependence of these components demonstrates that the oxygen saturation corresponds to 0.38ML, in good agreement with recent density functional theory calculations.

Controlling Hydrogenation of Graphene on Ir(111)

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.

STM and SPA-LEED studies of O-induced structures on Rh(100) surfaces

Abstract Oxygen adsorption on the Rh(100) surface has been studied by spot profile analysis low energy electron diffraction (SPA-LEED) and scanning tunnelling microscopy (STM). Our results show that oxygen can be reliably dosed onto the Rh(100) surface to produce distinct p(2 × 2) and c(2 × 2) phases. Additionally, the symmetry of the phase formed at saturation coverage is identified as (2 × 2)p4g rather than (2 × 2)p2gg as had been proposed previously. STM images of both the c(2 × 2) and (2 × 2)p4g phases show that the O atoms sit in alternate four-fold hollow sites. It is proposed that the difference between these two structures is a result of overall oxygen coverage. In the c(2 × 2) case we speculate that surface strain is relieved by the formation of islands, but that for the saturation coverage (2 × 2)p4g structure surface strain is reduced by the O atom burrowing into the surface, causing the characteristic clock reconstruction of the substrate Rh atoms.

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

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.