We use low-energy electron microscopy (LEEM), low-energy electron diffraction (LEED) and scanning tunneling microscopy (STM) to study different orientations of single-layer graphene sheets on Ir(111). The most-abundant orientation has previously been characterized in the literature. Using selective-area LEED we find three other variants, which are rotated 14°, 18.5° and 30° with respect to the most common variant. The "30°-rotated structure is also studied by STM. We propose that all 4 variants are moiré structures that can be classified using simple geometric rules involving periodic and quasi-periodic structural motifs. In addition, LEEM reveals that linear defects form in the graphene sheets during cooling from the synthesis temperature. STM shows that these defects are ridges, suggesting that the graphene sheets delaminate locally as the Ir substrate contracts.

Scanning tunneling topography of long-unexplained "square root of 37" and "square root of 39" periodic wetting arrangements of water molecules on Pt(111) reveals triangular depressions embedded in a hexagonal H2O-molecule lattice. Remarkably, the hexagons are rotated 30° relative to the "classic bilayer" model of water-metal adsorption. With support from density functional theory energetics and image simulation, we assign the depressions to clusters of flat-lying water molecules. 5- and 7-member rings of H2O molecules separate these clusters from surrounding "H-down" molecules.

Ice films as many as 30 molecular layers thick can be imaged with scanning tunneling microscopy ͑STM͒ when negative sample biases Ͻ−6͑Ϯ1͒ V and subpicoamp tunneling currents are used. We observe that water deposited onto Pt͑111͒ below 120 K forms amorphous films, whereas metastable cubic ice appears between 120 and 150 K. To determine the mechanisms of ice growth, we investigate the thickness-dependent film morphology. Cubic ice emerges from screw dislocations in the crystalline ice film that are caused by the mismatch in the atomic Pt-step height and the ice-bilayer separation.

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] inplane 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 6

We have observed the formation of graphene on SiC by Si sublimation in an Ar atmosphere using lowenergy electron microscopy, scanning tunneling microcopy, and atomic force microscopy. This work reveals unanticipated growth mechanisms, which depend strongly on the initial surface morphology. Carbon diffusion governs the spatial relationship between SiC decomposition and graphene growth. Isolated bilayer SiC steps generate narrow ribbons of graphene by a distinctive cooperative process, whereas triple bilayer steps allow large graphene sheets to grow by step flow. We demonstrate how graphene quality can be improved by controlling the initial surface morphology to avoid the instabilities inherent in diffusion-limited growth.The unique electronic properties of graphene have stimulated the development of synthesis routes for improved film quality. 1,2 Graphene films form readily on SiC surfaces: sublimation of Si at elevated temperature leaves behind a high concentration of carbon atoms, which assemble ͑"graphitize"͒ into graphene layers. 3,4 Recent approaches to higher-quality films involve heating in argon at atmospheric pressure 5,6 or supplying excess Si. 7 These new approaches lead to significant improvement in the domain size and electronic properties compared to vacuum graphitization, 8 and call for comprehensive understanding of the kinetic pathways underlying these improvements. The difficulty of studying this system is exacerbated by the relatively high process temperature ͑Ͼ1150°C͒, where standard real-time characterization tools are hard to apply. The large number of coexisting intermediate steps and the incompletely characterized surface structures add to the difficulty. Furthermore, the general problem of how new phases form as a surface is depleted of one chemical component during sublimation is not well understood, despite its crucial importance to hightemperature materials processing. Graphene formation on SiC differs from normal epitaxial growth because the constituent atoms are supplied from the substrate itself and are not distributed homogeneously across the surface during the growth. Thus, new fundamental issues, such as how and where carbon atoms are created, and how far they have to diffuse on the surface to form graphene layers, need to be addressed. [9][10][11][12] Here, we report on the initial stages of the first-layer graphene formation on 6H-SiC͑0001͒. 13 Our work establishes the fundamental role of surface diffusion and the importance of the surface morphology in the emergence of a new phase where one or more components are subliming. Our approach is to start with the ͑6 ͱ 3 ϫ 6 ͱ 3͒R30°carbonrich termination ͑"buffer layer"͒ 14-18 made through Arassisted graphitization. Because this procedure yields large step-free areas of the buffer layer, the morphology of the surface as graphene grows can be clearly determined. We find that the surface near the growing graphene selforganizes into arrowlike patterns because Si sublimation and graphene growth are spatially connected. We propose that ...