In-plane orientation effects on the electronic structure, stability and Raman scattering of monolayer graphene on Ir(111)

We employ angle-resolved photoemission spectroscopy (ARPES) to investigate the electronic structures of two rotational variants of epitaxial, single-layer graphene on Ir(111). As grown, the more-abundant R0 variant is nearly charge-neutral, with strong hybridization between graphene and Ir bands near the Fermi level. The graphene Fermi surface and its replicas exactly coincide with Van Hove singularities in the Ir Fermi surface. Sublattice symmetry breaking introduces a small gap-inducing potential at the Dirac crossing, which is revealed by n-doping the graphene using K atoms. The energy gaps between main and replica bands (originating from the moir\'e interference pattern between graphene and Ir lattices) is shown to be non-uniform along the mini- zone boundary due to hybridization with Ir bands. An electronically mediated interaction is proposed to account for the stability of the R0 variant. The variant rotated 30{\deg} in-plane, R30, is p-doped as grown and K doping reveals no band gap at the Dirac crossing. No replica bands are found in ARPES measurements. Raman spectra from the R30 variant exhibit the characteristic phonon modes of graphene, while R0 spectra are featureless. These results show that the film/substrate interaction changes from chemisorption (R0) to physisorption (R30) with in-plane orientation. Finally, graphene-covered Ir has a work function lower than the clean substrate but higher than graphite.Comment: Manuscript plus 7 figure

Three-Fold Diffraction Symmetry in Epitaxial Graphene and the SiC Substrate

The crystallographic symmetries and spatial distribution of stacking domains in graphene films on SiC have been studied by low energy electron diffraction (LEED) and dark field imaging in a low energy electron microscope (LEEM). We find that the graphene diffraction spots from 2 and 3 atomic layers of graphene have 3-fold symmetry consistent with AB (Bernal) stacking of the layers. On the contrary, graphene diffraction spots from the buffer layer and monolayer graphene have apparent 6-fold symmetry, although the 3-fold nature of the satellite spots indicates a more complex periodicity in the graphene sheets.Comment: An addendum has been added for the arXiv version only, including one figure with five panels. Published paper can be found at http://link.aps.org/doi/10.1103/PhysRevB.80.24140

Determining the structure of Ru(0001) from low-energy electron diffraction of a single terrace

While a perfect hcp (0001) surface has three-fold symmetry, the diffraction patterns commonly obtained are six-fold symmetric. This apparent change in symmetry occurs because on a stepped surface, the atomic layers on adjacent terraces are rotated by 180 degrees. Here we use a Low-Energy Electron Microscope to acquire the three-fold diffraction pattern from a single hcp Ru terrace and measure the intensity-vs-energy curves for several diffracted beams. By means of multiple scattering calculations fitted to the experimental data with a Pendry R-factor of 0.077, we find that the surface is contracted by 3.5(+-0.9) at 456 K.Comment: 10 pages, 4 figures. Corrected some typos, added more details. Accepted for publication in Surface Science (Letters

Labyrinthine Island Growth during Pd/Ru(0001) Heteroepitaxy

Using low energy electron microscopy we observe that Pd deposited on Ru only attaches to small sections of the atomic step edges surrounding Pd islands. This causes a novel epitaxial growth mode in which islands advance in a snakelike motion, giving rise to labyrinthine patterns. Based on density functional theory together with scanning tunneling microscopy and low energy electron microscopy we propose that this growth mode is caused by a surface alloy forming around growing islands. This alloy gradually reduces step attachment rates, resulting in an instability that favors adatom attachment at fast advancing step sections

Highly Enhanced Concentration and Stability of Reactive Ce^3+ on Doped CeO_2 Surface Revealed In Operando

Trivalent cerium ions in CeO_2 are the key active species in a wide range of catalytic and electro-catalytic reactions. We employed ambient pressure X-ray photoelectron spectroscopy and electrochemical impedance spectroscopy to quantify simultaneously the concentration of the reactive Ce^3+ species on the surface and in the bulk of Sm-doped CeO_2(100) in hundreds of millitorr of H2–H2O gas mixtures. Under relatively oxidizing conditions, when the bulk cerium is almost entirely in the 4+ oxidation state, the surface concentration of the reduced Ce^3+ species can be over 180 times the bulk concentration. Furthermore, in stark contrast to the bulk, the surface’s 3+ oxidation state is also highly stable, with concentration almost independent of temperature and oxygen partial pressure. Our thermodynamic measurements reveal that the difference between the bulk and surface partial molar entropies plays a key role in this stabilization. The high concentration and stability of reactive surface Ce^3+ over wide ranges of temperature and oxygen partial pressure may be responsible for the high activity of doped ceria in many pollution-control and energy-conversion reactions, under conditions at which Ce^3+ is not abundant in the bulk

A high-efficiency spin-resolved phototemission spectrometer combining time-of-flight spectroscopy with exchange-scattering polarimetry

We describe a spin-resolved electron spectrometer capable of uniquely efficient and high energy resolution measurements. Spin analysis is obtained through polarimetry based on low-energy exchange scattering from a ferromagnetic thin-film target. This approach can achieve a similar analyzing power (Sherman function) as state-of-the-art Mott scattering polarimeters, but with as much as 100 times improved efficiency due to increased reflectivity. Performance is further enhanced by integrating the polarimeter into a time-of-flight (TOF) based energy analysis scheme with a precise and flexible electrostatic lens system. The parallel acquisition of a range of electron kinetic energies afforded by the TOF approach results in an order of magnitude (or more) increase in efficiency compared to hemispherical analyzers. The lens system additionally features a 90{\deg} bandpass filter, which by removing unwanted parts of the photoelectron distribution allows the TOF technique to be performed at low electron drift energy and high energy resolution within a wide range of experimental parameters. The spectrometer is ideally suited for high-resolution spin- and angle-resolved photoemission spectroscopy (spin-ARPES), and initial results are shown. The TOF approach makes the spectrometer especially ideal for time-resolved spin-ARPES experiments.Comment: 16 pages, 11 figure

Imaging Spin Reorientation Transitions in Consecutive Atomic Co layers

By means of spin-polarized low-energy electron microscopy (SPLEEM) we show that the magnetic easy-axis of one to three atomic-layer thick cobalt films on ruthenium crystals changes its orientation twice during deposition: one-monolayer and three-monolayer thick films are magnetized in-plane, while two-monolayer films are magnetized out-of-plane, with a Curie temperature well above room temperature. Fully-relativistic calculations based on the Screened Korringa-Kohn-Rostoker (SKKR) method demonstrate that only for two-monolayer cobalt films the interplay between strain, surface and interface effects leads to perpendicular magnetization.Comment: 5 pages, 4 figures. Presented at the 2005 ECOSS conference in Berlin, and at the 2005 Fall meeting of the MRS. Accepted for publication at Phys. Rev. Lett., after minor change

Structure and magnetism in ultrathin iron oxides characterized by low energy electron microscopy

We have grown epitaxial films a few atomic layers thick of iron oxides on ruthenium. We characterize the growth by low energy electron microscopy. Using selected area diffraction and intensity vs. voltage spectroscopy, we detect two distinct phases which are assigned to wustite and magnetite. Spin polarized low energy electron microscopy shows magnetic domain patterns in the magnetite phase at room temperature.Comment: 21 pages, 10 figures, for J. Phys Cond Matt special LEEM/PEEM issue in honor of E. Baue

Self-doping effects in epitaxially grown graphene

Self-doping in graphene has been studied by examining single-layer epitaxially grown graphene samples with differing characteristic lateral terrace widths. Low energy electron microscopy was used to gain real-space information about the graphene surface morphology, which was compared with data obtained by angle-resolved photoemission spectroscopy to study the effect of the monolayer graphene terrace width on the low energy dispersions. By altering the graphene terrace width, we report significant changes in the electronic structure and quasiparticle relaxation time of the material, in addition to a terrace width-dependent doping effect.Comment: Published in Applied Physics Letters 93, 243119 (2008