Thomas Greber

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.

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)

Surface Trapping of Atoms and Molecules with Dipole Rings

The trapping of single molecules on surfaces without the formation of strong covalent bonds is a prerequisite for molecular recognition and the exploitation of molecular function. On nanopatterned surfaces, molecules may be selectively trapped and addressed. In a boron nitride nanomesh formed on Rh(111), the pattern consisted of holes 2 nanometers in diameter on a hexagonal superlattice, separated by about 3 nanometers. The trapping was further investigated with density functional theory and the photoemission of adsorbed xenon, where the holes were identified as regions of low work function. The analysis showed that the trapping potential was localized at the rims of the holes.

An endohedral single-molecule magnet with long relaxation times: DySc2N@C80.

The magnetism of DySc(2)N@C(80) endofullerene was studied with X-ray magnetic circular dichroism (XMCD) and a magnetometer with a superconducting quantum interference device (SQUID) down to temperatures of 2 K and in fields up to 7 T. XMCD shows hysteresis of the 4f spin and orbital moment in Dy(III) ions. SQUID magnetometry indicates hysteresis below 6 K, while thermal and nonthermal relaxation is observed. Dilution of DySc(2)N@C(80) samples with C(60) increases the zero-field 4f electron relaxation time at 2 K to several hours.

Supramolecular Assemblies Formed on an Epitaxial Graphene Superstructure

The seminal work of Novoselov et al. has stimulated great interest in the controllable growth of epitaxial graphene monolayers. While initial research was focussed on the use of SiC wafers, the promise of transition metals as substrates has also been demonstrated and both approaches are scalable to large-area production. 12] The growth of graphene on transition metals such as Ru, Rh and Ir leads to a moir!-like superstructure, 10,12,13] similar to that observed for BN monolayers. Here we show that such a superstructure can be used to control the organization of extended supramolecular nanostructures. The formation of two-dimensional supramolecular arrays has received increasing attention over recent years primarily due to potential applications in nanostructure fabrication as well as fundamental interest in self-assembly processes. Such studies can be highly dependent on the nature of the substrate used, and the interplay between surface and adsorbed supramolecular structure is a topic of significant conjecture. Until now metallic surfaces or highly oriented pyrolytic graphite (HOPG) have typically been the surfaces of choice for such studies. Our results demonstrate that graphene is compatible with, and can strongly influence molecular selfassembly. We have studied the adsorption of perylene tetracarboxylic diimide (PTCDI) and related derivatives on a graphene monolayer grown on a Rh(111) heteroepitaxial thin film (Figure 1). In particular, we show that a near-commensur-

Charge-transfer induced particle emission in gas surface reactions

Abstract Highly exothermic gas surface reactions may lead to the emission of electrons, ions and photons. These particles stem from the non-adiabatic de-excitation of a system in which charge is transferred between the surface and the adsorbate. The main concepts for chemisorptive particle emission are reviewed, as is its application for the recording of reaction kinetics and finally the information that it can provide on the dynamics of charge-transfer reactions at surfaces is examined.

Formation of one-dimensional self-assembled silicon nanoribbons on Au(110)-(2 × 1)

We report results on the self-assembly of silicon nanoribbons (NRs) on the (2 × 1) reconstructed Au(110) surface under ultra-high vacuum conditions. Upon adsorption of 0.2 monolayer (ML) of silicon, the (2 × 1) reconstruction of Au(110) is replaced by an ordered surface alloy. Above this coverage, a new superstructure is revealed by low energy electron diffraction (LEED), which becomes sharper at 0.3 Si ML. This superstructure corresponds to Si nanoribbons all oriented along the [1¯10] direction as revealed by LEED and scanning tunneling microscopy (STM). STM and high-resolution photoemission spectroscopy indicate that the nanoribbons are flat and predominantly 1.6 nm wide. In addition, the silicon atoms show signatures of two chemical environments corresponding to the edge and center of the ribbons.

Spin-polarized Fermi surface mapping

Abstract The magnetic and electronic properties of itinerant ferromagnets and their interplay have been studied in the last few years by spin resolved electron spectroscopy on one hand and by high-resolution angle-resolved photoemission experiments on the other. We discuss how the two approaches can be combined in a high resolution electron spectrometer with spin resolution for angle-scanned Fermi surface mapping experiments. We have built this new instrument, which allows an advance into a deeper understanding of magnetic thin film or multilayer systems, where band structures become intricately dense in momentum space and where the magnetization direction can change from layer to layer. Spin-resolution is thus required to arrive at a correct assignment of spectral features. A fully three-dimensional polarimeter makes the instrument ‘complete’ in the sense that all properties of the photoelectron are measured. First experiments on Ni(111) conclusively confirm previous band and spin assignments at the Fermi level and demonstrate the correct functioning of the apparatus.

A photoelectron spectrometer for k-space mapping above the Fermi level

The setup of an electron spectrometer for angle-resolved photoemission is described. A sample goniometer offers the opportunity for angle scanned photoemission over 2π solid angle above the surface. A monochromatized high flux He discharge photon source is exploited to measure thermally populated electronic states above the Fermi level EF. At energies greater than EF+5kBT the signal from a constant density of states declines below the photoelectron background caused by photons with higher energies than He Iα (21.2 eV). For He IIα (40.8 eV) the residual photoelectron background is lower and photoemission up to 6kBT above EF can be performed. Data showing two cuts through the Fermi surface of silver are presented. Furthermore the dispersion of the Shockley surface state on Ag (111) above the Fermi energy is quantified.

Energy distribution curves of ultrafast laser-induced field emission and their implications for electron dynamics

Hirofumi Yanagisawa, Matthias Hengsberger, Dominik Leuenberger, Martin Klöckner, Christian Hafner, Thomas Greber, and Jürg Osterwalder Physik Institut, Universität Zürich, Winterthurerstrasse 190, CH-8057 Zürich, Switzerland Laboratory for Electromagnetic Fields and Microwave Electronics, ETH Zürich, Gloriastrasse 35, CH-8092 Zürich, Switzerland Present address: Department of Physics, ETH Zürich, Wolfgang-Pauli-Strasse 16, CH-8093 Zürich, Swizerland (Dated: January 15, 2013)