S. Günther

Graphene on Ru(0001): A 25 x 25 Supercell

The structure of a single layer of graphene on Ru(0001) has be en studied using surface x-ray diffraction. A surprising superstructure has been determined, whereby 2 5×25 graphene unit cells lie on 23 ×23 unit cells of Ru. Each supercell contains 2 × crystallographically inequivalent subcells caused by co rrugation. Strong intensity oscillations in the superstructure rods d emonstrate that the Ru substrate is also significantly corrugated down to several monolayers, and that the bonding between graphene and Ru is strong and cannot be caused by van der Waals bonds. Charge transfer from the Ru s ubstrate to the graphene expands and weakens the C–C bonds, which helps accommodate the in-plane tensile stress. The elucidation of this superstructure provides important information in the pote ntial application of graphene as a template for nanocluster arrays.

Photoelectron microscopy and applications in surface and materials science

We review the recent achievements of photoelectron microscopy (PEM), which is a rapidly developing technique that is significantly advancing the frontiers of surface and materials science. The operation principles of scanning photoelectron microscopes (SPEM), using different photon optic systems to obtain a micro-probe of sub-micrometer dimensions, and of the full-field imaging microscope, using electrostatic lenses for magnification of the irradiated sample area, are presented. The contrast mechanisms, based on photon absorption and photon-induced electron emission, are described and the expected development in the photon and electron optics and detection systems are discussed. Particular attention is paid to the present state-of-art performance of the microscopes collecting photoelectrons (PEs), which carry specific information about the lateral variations in the chemical, magnetic and electronic properties of the material under investigation. Selected results, obtained recently with instruments installed at synchrotron light facilities, are used to illustrate the potential of PEM in characterising micro-phases and dynamic processes with a lateral resolution better than 100 nm.

Graphene oxide windows for in situ environmental cell photoelectron spectroscopy

The performance of new materials and devices often depends on processes taking place at the interface between an active solid element and the environment (such as air, water or other fluids). Understanding and controlling such interfacial processes require surface-specific spectroscopic information acquired under real-world operating conditions, which can be challenging because standard approaches such as X-ray photoelectron spectroscopy generally require high-vacuum conditions. The state-of-the-art approach to this problem relies on unique and expensive apparatus including electron analysers coupled with sophisticated differentially pumped lenses. Here, we develop a simple environmental cell with graphene oxide windows that are transparent to low-energy electrons (down to 400 eV), and demonstrate the feasibility of X-ray photoelectron spectroscopy measurements on model samples such as gold nanoparticles and aqueous salt solution placed on the back side of a window. These proof-of-principle results show the potential of using graphene oxide, graphene and other emerging ultrathin membrane windows for the fabrication of low-cost, single-use environmental cells compatible with commercial X-ray and Auger microprobes as well as scanning or transmission electron microscopes.

Comparison of electronic structure and template function of single-layer graphene and a hexagonal boron nitride nanomesh on Ru(0001)

Thomas Brugger, Sebastian Günther, Bin Wang, Hugo Dil, 4 Marie-Laure Bocquet, 2 Jürg Osterwalder, Joost Wintterlin, and Thomas Greber ∗ Physik-Institut, Universität Zürich, Winterthurerstrasse 190, CH-8057 Zürich, Switzerland Department Chemie, Ludwig-Maximilian Universität, Butenandtstrasse 5-13, D-81377 München, Germany Université de Lyon, Laboratoire de Chimie, École Normale Supérieure de Lyon, CNRS, France Swiss Light Source, Paul Scherrer Institute, CH-5232 Villigen, Switzerland (Dated: July 29, 2008)

ESCA Microscopy at ELETTRA: what it is like to perform spectromicroscopy experiments on a third generation synchrotron radiation source

Abstract We present ESCA Microscopy, the first X-ray microscopy beamline operating on ELETTRA, the third generation synchrotron radiation source in Trieste, Italy. ESCA Microscopy is an advanced user facility open to the international scientific community; its operation is based on the use of a Fresnel zone plate to demagnify to submicrometre dimensions the photon beam emitted by an undulator in the 200–1000 eV energy range. ESCA Microscopy was designed as a scanning photoemission microscope especially suited for surface analysis; it also operates in transmission mode and should find many applications in materials science, chemistry and physics. We describe here the beamline experimental setup and present some recent results obtained during the first months of operation. While so doing, we will outline certain spectroscopic aspects and point out some operational problems that are typical of scanning photoemission microscopy, a technique which should find in advanced sources such as ELETTRA the right conditions to achieve maturity.

Periodicity, work function and reactivity of graphene on Ru(0001) from first principles

Using density functional theory (DFT), a detailed analysis of the energetic and electronic properties of monolayer graphene on Ru(0001) was performed. Firstly, we demonstrate that the corrugated (12×12)C/(11×11)Ru structure is an absolute minimum among smaller and larger commensurabilities. Secondly, we show that the reduced work function originates from the charge depletion in the graphene skeleton states at bonded carbon atoms, in contrast to the assumed n-doping of π states. We further propose that the higher-lying regions of the graphene layer form an array of electronically disconnected nanographene islands. Finally, we explore the adsorption properties of this template with respect to metals by comparing the adsorption of Au and Pt single atoms. A 1.1 eV (0.7 eV) preference is found for Au (Pt) atop adsorption in the low region with 0.2–0.3 eV selectivity for the (hcp, top) locations, in agreement with recent experiments on Pt clusters.

Homoepitaxial growth on Ni(100) and its modification by a preadsorbed oxygen adlayer

Abstract Homoepitaxial growth of thin Ni films and the influence of a O c(2 × 2) adlayer on the growth behavior were investigated by scanning tunneling microscopy. At 300 K and 0.3 monolayers/min flux rate Ni film growth on the clean surface proceeds via homogeneous nucleation, lateral growth, and coalescence of two-dimensional, monolayered Ni islands, exhibiting an almost perfect layer-by-layer growth behavior. The relatively high density of 2D Ni islands of 1.8 × 10 12 cm −2 indicates a low mobility of the Ni adatoms. The oxygen adlayer, which floats on the surface in its initial c(2 × 2) structure, has little effect on the growth mode and the actual growth process. Its influence manifests in the dynamics of 2D island growth, where the island edges are reoriented from along [011] on the clean surface to along [001] on the oxygen covered surface. This observation and the simultaneous change from square to pronounced rectangular island shapes are explained in a simple picture based on the registry relation between O c(2 × 2) adlayers on two subsequent Ni layers. A tentative model for the oxygen transport to the surface is proposed involving a simple displacement of O adatoms, during Ni condensation at the island edges.

Nucleation and growth of thin metal films on clean and modified metal substrates studied by scanning tunneling microscopy

The nucleation and growth of several metal/metal thin film systems have been studied and compared using scanning tunneling microscopy (STM). Homogeneous and heterogeneous nucleation have been observed and the effect of surface inhomogeneities on the nucleation behavior has been studied. Systematic measurements of the film morphology as a function of coverage and annealing temperature have allowed different two‐ and three‐dimensional growth mechanisms to be studied. Both kinetic and thermodynamic factors are found to determine these growth processes and in some cases could be separately identified. In addition, the effect of chemically modifying the substrate by oxygen preadsorption has been studied and compared to the clean substrate systems. A variety of changes in the growth behavior were found to occur, which can be understood in terms of changes in both the kinetics and thermodynamics due to the oxygen adlayer.

Catalytic ammonia oxidation on platinum: mechanism and catalyst restructuring at high and low pressure

Catalytic ammonia oxidation over platinum has been studied experimentally from UHV up to atmospheric pressure with polycrystalline Pt and with the Pt single crystal orientations (533), (443), (865), and (100). Density functional theory (DFT) calculations explored the reaction pathways on Pt(111) and Pt(211). It was shown, both in theory and experimentally, that ammonia is activated by adsorbed oxygen, i.e. by O(ad) or by OH(ad). In situ XPS up to 1 mbar showed the existence of NH(x)(x= 0,1,2,3) intermediates on Pt(533). Based on a mechanism of ammonia activation via the interaction with O(ad)/OH(ad) a detailed and a simplified mathematical model were formulated which reproduced the experimental data semiquantitatively. From transient experiments in vacuum performed in a transient analysis of products (TAP) reactor it was concluded that N(2)O is formed by recombination of two NO(ad) species and by a reaction between NO(ad) and NH(x,ad)(x= 0,1,2) fragments. Reaction-induced morphological changes were studied with polycrystalline Pt in the mbar range and with stepped Pt single crystals as model systems in the range 10(-5)-10(-1) mbar.

Artefact formation in scanning photoelectron emission microscopy

The fast developing analytical technique, synchrotron radiation scanning photoemission spectromicroscopy has opened the opportunity for probing surface processes and composition of materials on a submicron spatial scale. Here we describe some artefact and unforeseen phenomena that can occur when a photon flux with high intensity is focused onto a microspot. Using as examples selected data obtained recently with the scanning photoemission microscope built at the ultrabright synchrotron source ELETTRA we illustrate the possible effects of surface morphology and undesired processes, such as photon-assisted carbon deposition, heat dissipation, charging and photon-induced reduction of the sample. All these events can cause severe changes in the chemical maps and photoelectron spectra and provide misleading results. The physical nature of the artefacts are outlined and discussed, as well as the possibilities to reduce their influence or to use them for quantification of some photon-induced phenomena becoming important in spectromicroscopy experiments carried out at the third generation synchrotron sources.