Elena Starodub

Orientation-dependent work function of graphene on Pd(111)

Selected-area diffraction establishes that at least six different in-plane orientations of monolayer graphene on Pd(111) can form during graphene growth. From the intensities of low-energy electron microscopy images as a function of incident electron energy, we find that the work functions of the different rotational domains vary by up to 0.15 eV. Density functional theory calculations show that these significant variations result from orientation-dependent charge transfer from Pd to graphene. These findings suggest that graphene electronics will require precise control over the relative orientation of the graphene and metal contacts.

Growth from Below: Graphene Bilayers on Ir(111)

We elucidate how graphene bilayers form on Ir(111). Low-energy electron diffraction (LEED) reveals that the two graphene layers are not always rotationally aligned. Monitoring this misalignment during growth shows that second-layer islands nucleate between the existing layer and the substrate. This mechanism occurs both when C segregates from the Ir and when elemental C is deposited from above. Low-energy electron microscopy (LEEM) and angle-resolved photoemission spectroscopy (ARPES) show that second-layer nucleation occurs preferentially under the first-layer rotational variants that are more weakly bound to the substrate. New-layer nucleation tends to occur inhomogeneously at substrate defects. Thus new-layer nucleation should be rapid on substrates that weakly bind graphene, making growth unstable toward mound formation initiated at substrate defects. In contrast, stronger binding permits layer-by-layer growth, as for Ru(0001). ARPES shows that bilayer graphene has two slightly p-doped π-bands. The work function of bilayer graphene is dominated by the orientation of the bottom layer.

Extraordinary epitaxial alignment of graphene islands on Au(111)

Pristine, single-crystalline graphene displays a unique collection of remarkable electronic properties that arise from its two-dimensional, honeycomb structure. Using in situ low-energy electron microscopy, we show that when deposited on the (111) surface of Au carbon forms such a structure. The resulting monolayer, epitaxial film is formed by the coalescence of dendritic graphene islands that nucleate at a high density. Over 95% of these islands can be identically aligned with respect to each other and to the Au substrate. Remarkably, the dominant island orientation is not the better lattice-matched 30° rotated orientation but instead one in which the graphene [01] and Au [011] in-plane directions are parallel. The epitaxial graphene film is only weakly coupled to the Au surface, which maintains its reconstruction under the slightly p-type doped graphene. The linear electronic dispersion characteristic of free-standing graphene is retained regardless of orientation. That a weakly interacting, non-lattice matched substrate is able to lock graphene into a particular orientation is surprising. This ability, however, makes Au(111) a promising substrate for the growth of single crystalline graphene films.

Graphene Growth by Metal Etching on Ru (0001)

Low-energy electron microscopy (LEEM) reveals a new mode of graphene growth on Ru(0001) in which Ru atoms from a step edge are injected under a growing graphene sheet. The injected atoms can form under-graphene islands, or incorporate into the topmost Ru layer, thereby increasing its density and forming dislocation networks. Density functional calculations imply that Ru islands nucleated between the graphene layer and the substrate are energetically stable; scanning tunneling microscopy (STM) reveals that dislocation networks exist near step edges.

Insight into Magnetite’s Redox Catalysis from Observing Surface Morphology during Oxidation

We study how the (100) surface of magnetite undergoes oxidation by monitoring its morphology during exposure to oxygen at ~650 °C. Low-energy electron microscopy reveals that magnetites surface steps advance continuously. This growth of Fe3O4 crystal occurs by the formation of bulk Fe vacancies. Using Raman spectroscopy, we identify the sinks for these vacancies, inclusions of α-Fe2O3 (hematite). Since the surface remains magnetite during oxidation, it continues to dissociate oxygen readily. At steady state, over one-quarter of impinging oxygen molecules undergo dissociative adsorption and eventual incorporation into magnetite. From the independence of growth rate on local step density, we deduce that the first step of oxidation, dissociative oxygen adsorption, occurs uniformly over magnetites terraces, not preferentially at its surface steps. Since we directly observe new magnetite forming when it incorporates oxygen, we suggest that catalytic redox cycles on magnetite involve growing and etching crystal.

Growth structure and work function of bilayer graphene on Pd(111)

Using in situ low-energy electron microscopy and density functional theory, we studied the growth structure and work function of bilayer graphene on Pd(111). Low-energy electron diffraction analysis established that the two graphene layers have multiple rotational orientations relative to each other and the substrate plane. We observedheterogeneous nucleationandsimultaneousgrowthofmultiple,facetedlayerspriortothecompletionof secondlayer.Weproposethatthefacetedshapesareduetothezigzag-terminatededgesboundinggraphenelayers growing under the larger overlying layers. We also found that the work functions of bilayer graphene domains are higher than those of monolayer graphene, and depend sensitively on the orientations of both layers with respect to the substrate. Based on first-principles simulations, we attribute this behavior to oppositely oriented electrostatic dipoles at the graphene/Pd and graphene/graphene interfaces, the strengths of which depend on the orientations of the two graphene layers.

Viable thermionic emission from graphene-covered metals

Thermionic emission from monolayer graphene grown on representative transition metals, Ir and Ru, is characterized by low-energy electron microscopy. Work functions were determined from the temperature dependence of the emission current and from the electron energy spectrum of emitted electrons. The high-temperature work function of the strongly interacting system graphene/Ru(0001) is sufficiently low, 3.3 ± 0.1 eV, to have technological potential for large-area emitters that are spatially uniform, efficient, and chemically inert. The thermionic work functions of the less strongly interacting system graphene/Ir(111) are over 1 eV larger and vary substantially (0.4 eV) between graphene orientations rotated by 30°.

Order-disorder phase transition on the (100) surface of magnetite

Using low-energy electron diffraction, we show that the room-temperature

Oxidation of Graphene on Metals

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

reconstruction of Fe