K. F. McCarty

Factors influencing graphene growth on metal surfaces

Graphene forms from a relatively dense, tightly bound C-adatom gas when elemental C is deposited on or segregates to the Ru(0001) surface. Nonlinearity of the graphene growth rate with C-adatom density suggests that growth proceeds by addition of C atom clusters to the graphene edge. The generality of this picture has now been studied by use of low-energy electron microscopy (LEEM) to observe graphene formation when Ru(0001) and Ir(111) surfaces are exposed to ethylene. The finding that graphene growth velocities and nucleation rates on Ru have precisely the same dependence on adatom concentration as for elemental C deposition implies that hydrocarbon decomposition only affects graphene growth through the rate of adatom formation. For ethylene, that rate decreases with increasing adatom concentration and graphene coverage. Initially, graphene growth on Ir(111) is like that on Ru: the growth velocity is the same nonlinear function of adatom concentration (albeit with much smaller equilibrium adatom concentrations, as we explain with DFT calculations of adatom formation energies). In the later stages of growth, graphene crystals that are rotated relative to the initial nuclei nucleate and grow. The rotated nuclei grow much faster. This difference suggests firstly, that the edge-orientation of the graphene sheets relative to the substrate plays an important role in the growth mechanism, and secondly, that attachment of the clusters to the graphene is the slowest step in cluster addition, rather than formation of clusters on the terraces.

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-versus-voltage spectroscopy, we detect two distinct phases which are assigned as wüstite and magnetite. Spin-polarized low energy electron microscopy reveals magnetic domain patterns in the magnetite phase at room temperature.

Structure of ultrathin Pd films determined by low-energy electron microscopy and diffraction

Palladium (Pd) films have been grown and characterized in situ by low-energy electron diffraction (LEED) and microscopy in two different regimes: ultrathin films 2?6 monolayers (ML) thick on Ru(0001), and ~20?ML thick films on both Ru(0001) and W(110). The thinner films are grown at elevated temperature (750?K) and are lattice matched to the Ru(0001) substrate. The thicker films, deposited at room temperature and annealed to 880?K, have a relaxed in-plane lattice spacing. All the films present an fcc stacking sequence as determined by LEED intensity versus energy analysis. In all the films, there is hardly any expansion in the surface-layer interlayer spacing. Two types of twin-related stacking sequences of the Pd layers are found on each substrate. On W(110) the two fcc twin types can occur on a single substrate terrace. On Ru(0001) each substrate terrace has a single twin type and the twin boundaries replicate the substrate steps.

Initial stages of FeO growth on Ru(0001)

We study how FeO wüstite films on Ru(0001) grow by oxygen-assisted molecular beam epitaxy at elevated temperatures (800–900 K). The nucleation and growth of FeO islands are observed in real time by low-energy electron microscopy (LEEM). When the growth is performed in an oxygen pressure of 10(−6) Torr, the islands are of bilayer thickness (Fe–O–Fe–O). In contrast, under a pressure of 10(−8) Torr, the islands are a single FeO layer thick. We propose that the film thickness is controlled by the concentration of oxygen adsorbed on the Ru. More specifically, when monolayer growth increases the adsorbed oxygen concentration above a limiting value, its growth is suppressed. Increasing the temperature at a fixed oxygen pressure decreases the density of FeO islands. However, the nucleation density is not a monotonic function of oxygen pressure.

Three-fold diffraction symmetry in epitaxial graphene and the SiC substrate

Three-Fold Diffraction Symmetry in Epitaxial Graphene and the SiC Substrate D.A. Siegel, 1, 2 S.Y. Zhou, 1, 2 F. El Gabaly, 3 A. K. Schmid, 2 K. F. McCarty, 3 and A. Lanzara 1, 2 Department of Physics, University of California, Berkeley, CA 94720, USA Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA Sandia National Laboratories, Livermore, California 94551, USA (Dated: November 15, 2009) The crystallographic symmetries and spatial distribution of stacking domains in graphene films on 6H-SiC(0001) 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 or rhombohe- dral) 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. The past few years have witnessed a growing need to identify new methods for the synthesis of graphene films for both basic research and industrial applications. Of all the methods explored so far, substrate growth methods seem the most promising due to the ease and reliability of growth of large-scale films 1–3 . However, the presence of a substrate can often impose non-bulk-like structures on overlayer films that alter their properties; one well- known example is the case of thin magnetic films 4 . In graphene, the choice of substrate can have a major im- pact on the properties of the film as well. For example, substrates that break the A-B sublattice symmetry (or equivalently the 6-fold rotational symmetry) of graphene, such as hexagonal boron nitride or AB-stacked bilayer graphene, result in the opening of a bandgap and the de- struction of the Dirac behavior of the quasiparticles in graphene 5–10 . On the other hand, substrates where this symmetry is preserved, such as graphene grown on the C face of SiC (which possesses azimuthal rotations between layers) can retain this Dirac behavior even for multilayer films 3,11–13 . Therefore, understanding how graphene grows on top of a substrate is fundamental to engineering new graphene sheets with controlled properties. Here we will focus on epitaxial graphene grown on SiC(0001), one of the most studied graphene systems because of its potential for industrial application due to the pres- ence of a bandgap in measured as well as calculated spectra 9,10 . The mechanism behind this gap opening is still under debate, so understanding the precise structure of the graphene/buffer layer system remains an impor- tant issue 14,15 . One way to answer questions about the structure of epitaxial graphene might be through low energy elec- tron diffraction (LEED), which is a more direct probe of the crystal symmetry than STM 14–16 provided that the diffraction can be performed with a spatial resolu- tion that is smaller than the structural domains of the crystal. For example, LEED from a single Ru(0001) ter- race has the 3-fold symmetry of the hcp layer stacking, while LEED from a region containing multiple terraces has an averaged 6-fold symmetry 17 . Similar measure- FIG. 1: (Color online) (a) A LEED image (43eV) of an epitaxially-grown graphene sample. The region of intensity to the left of center is due to secondary electrons. (b) A car- toon of the LEED image in (a) that shows the relevant sets of diffraction spots more clearly. In both panels, two SiC spots are marked with stars, two graphite spots are marked with triangles, two 6 × 6 spots are marked with squares, and the (0,0) spot is marked with a circle. ments can be performed with greater spatial resolution by using dark-field low-energy electron microscopy (LEEM) imaging. Dark-field LEEM images are real-space images derived from higher order diffraction spots. This differs from bright field LEEM, where the images are obtained from the specularly reflected beam, the (0,0) diffraction spot. Thus, dark field LEEM can be viewed as a tool comparable to LEED, where the dark field LEEM im- age is a map of the intensity of a single LEED spot as a function of sample position. Combining several such im- ages obtained on inequivalent diffraction spots, one can determine direct evidence of asymmetries in the LEED diffraction peaks as a function of position in the LEEM image. Here we characterize the crystallographic structure of graphene/SiC films and the spatial distribution of stack- ing domains by high resolution dark-field LEEM imaging. We find that the 6-fold symmetry is broken for the 1x1 SiC LEED spots, and for the 1x1 graphite LEED spots of multilayer (≥2 graphene layers in addition to the buffer layer) graphene. On the contrary, the apparent 6-fold symmetry of the graphite LEED spots is preserved in the buffer layer and single-layer graphene, showing that the stacking between these two layers differs from that

Nanoscale periodicity in stripe-forming systems at high temperature: Au/W(110).

We observe using low-energy electron microscopy the self-assembly of monolayer-thick stripes of Au on W(110) near the transition temperature between stripes and the nonpatterned (homogeneous) phase. We demonstrate that the amplitude of this Au-stripe phase decreases with increasing temperature and vanishes at the order-disorder transition (ODT). The wavelength varies much more slowly with temperature and coverage than theories of stress-domain patterns with sharp boundaries would predict, and maintains a finite value of about 100 nm at the ODT. We argue that such nanometer-scale stripes should often appear near the ODT.

In situ characterization of the formation of a mixed conducting phase on the surface of yttria-stabilized zirconia near Pt electrodes

The electrochemical reactions of solid oxide fuel cells occur in the region where gas-phase species, electrode, and electrolyte coincide. When the electrode is an ionic insulator and the electrolyte is an electronic insulator, this triple phase boundary is assumed to have atomic dimensions. Here we use photoemission electron microscopy to show that the reduced surface of the electrolyte yttriastabilized zirconia (YSZ) undergoes a metal-insulator transition near Pt negative electrodes. YSZs electron conducting region functions as an extended triple phase boundary that can be many microns in size, depending on oxygen pressure, temperature, applied voltage, and time.

Measuring the magnetization of three monolayer thick Co islands and films by x-ray dichroism

Co islands and films are characterized by x-ray magnetic circular dichroism photoemission electron microscopy. The spatial resolution capabilities of the technique together with atomic growth control permit obtaining perfectly flat triangular islands with a given thickness (3 ML), very close to an abrupt spin-reorientation transition. The magnetic domain configurations are found to depend on island size: while small islands can be magnetized in a single-domain state, larger islands show more complex patterns. Furthermore, the magnetization pattern of the larger islands presents a common chirality. By means of dichroic spectromicroscopy at the

Enhanced Self-Diffusion on Cu(111) by Trace Amounts of S: Chemical-Reaction-Limited Kinetics

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

absorption edges, an experimental estimate of the ratio of the spin and orbital magnetic moment for three monolayer thick films is obtained.