Search

The ability of metal surfaces to dissociate hydrogen molecules is key to many ways that metals react to their environment. Often, the barrier to dissociation is linked to the formation of surface hydrogen adatoms. Here, we show that hydrogen can be more strongly bound to water-covered surfaces in the form of hydronium ions than as adatoms. Density functional theory reveals that the hydronium binding is proportional to the surface electronic work function. For the case of Pt(111), a particularly high work function surface, the proton affinity of adsorbed water films can be 0.4 eV larger than that of the bare metal surface. This binding is large enough to make the water films susceptible to the formation of hydroxyl and hydronium ion pairs. We present evidence from scanning tunneling microscopy for the existence of hydronium ions in water films on Pt(111). This new insight into the stability of hydronium-containing water layers provides a basis for more realistic models of the chemical reactivity of water films on metals.

We developed a method for examining ice formation on solid substrates exposed to cloud-like atmospheres. Our experimental approach couples video-rate optical microscopy of ice formation with high-resolution atomic-force microscopy (AFM) of the initial mineral surface. We demonstrate how colocating stitched AFM images with video microscopy can be used to relate the likelihood of ice formation to nanoscale properties of a mineral substrate, e.g., the abundance of surface steps of a certain height. We also discuss the potential of this setup for future iterative investigations of the properties of ice nucleation sites on materials.

The selection of austenitic stainless steels for hydrogen service is challenging since there are few intrinsic metrics that relate alloy composition to hydrogen degradation. One such metric, presented here, is intrinsic stacking fault energy (SFE). This work reviews the exiting literature to use estimated intrinsic SFE values, calculated with a sub-regular solution thermodynamic model, to compare the retention of tensile ductility of γ-austenitic stainless steels in the presence of hydrogen. The goal is to demonstrate SFE as a metric to screen γ-austenitic stainless steels that use diverse alloying strategies for hydrogen compatibility. A transition in the tensile reduction of area of both 300-series and manganese-stabilized stainless steels is observed at a calculated stacking fault energy of approximately 39 mJ m−2, below which pronounced hydrogen degradation on tensile ductility is observed. Calculated intrinsic stacking fault energy is demonstrated as a high-throughput screening metric for a diverse range of austenitic stainless steel compositions with regard to hydrogen compatibility.

We study the structure of the threading edge dislocations, or ``elbows,'' which are an essential component of the well-known herringbone reconstruction of the (111) surface of Au. Previous work had shown that these dislocations can be stabilized by long-range elastic relaxations into the bulk. However, the validity of the harmonic spring model that had been used to estimate the energies of the dislocations is uncertain. To enable a more refined model of the dislocation energetics, we have imaged the atomic structure of these dislocations using scanning tunneling microscopy. We find that the harmonic spring model does not adequately reproduce the observed structure. We are able to reproduce the structure, however, with a two-dimensional Frenkel-Kontorova (FK) model that uses a pairwise Morse potential to describe the interactions between the top layer Au atoms on a rigid substrate. The parameters of the potential were obtained by fitting the energy of uniaxially compressed phases, or ``stripes'', computed with density functional theory, as a function of surface Au density. Within this model, the formation of the threading dislocations remains unfavorable. However, the large forces on the substrate atoms near the threading-dislocation cores, render the assumption of a completely rigid substrate questionable. Indeed, if the FK parameters are modified to account for the relaxation of just one more atomic layer, threading dislocations can, in principle, become favorable, even without bulk elastic relaxations. Additional evidence for a small elbow energy is that our computed change in the Au(111) surface stress tensor caused by the $(\sqrt{3}\ifmmode\times\else\texttimes\fi{}22)$ reconstruction is considerably smaller than previous estimates.

Ice in the atmosphere affects Earth's radiative properties and initiates most precipitation. Growing ice often requires a solid surface, either to catalyze freezing of supercooled cloud droplets or to serve as a substrate for ice deposited from water vapor. There is evidence that this surface is typically provided by airborne mineral dust; but how chemistry, structure and morphology interrelate to determine the ice-nucleating ability of mineral surfaces remains elusive. Here, we combine optical microscopy with atomic force microscopy to explore the mechanisms of initial ice growth on alkali feldspar, a mineral proposed to dominate ice nucleation in Earth's atmosphere. When cold air becomes supersaturated with respect to water, we discovered that ice rapidly spreads along steps of a feldspar surface. By measuring how ice propagation depends on surface-step height we establish a scenario where supercooled liquid water condenses at steps without having to overcome a nucleation barrier, and subsequently freezes quickly. Our results imply that steps, which are common even on macroscopically flat feldspar surfaces, can accelerate water condensation followed by freezing, thus promoting glaciation and dehydration of mixed-phase clouds.

HKUST-1 or Cu3BTC2 (BTC = 1,3,5-benzenetricarboxylate) is a prototypical metal–organic framework (MOF) that holds a privileged position among MOFs for device applications, as it can be deposited as...

The earliest stages of soot formation in flames are believed to involve the formation of small, nanoscale clusters of polycyclic aromatic hydrocarbon molecules. The structure of these clusters is still highly uncertain, however, impeding the construction of quantitative models of soot inception and growth. To provide insight into the structure of incipient soot, we produced nanoclusters of hydrocarbon molecules by annealing coronene films deposited on Pt(111), and examined them with scanning tunneling microcopy. We find that clusters containing ∼20–100 molecules, are disordered agglomerations of stacks that are ∼5–6 molecules tall. These structures are quite distinct from crystalline coronene, but bear a striking resemblance to recently proposed models for the equilibrium structure of similarly-sized clusters that are assumed to initiate soot formation. In contrast to mature soot, the surfaces of these clusters contain very few molecules with graphitic planes oriented parallel to the surface.

The passivation of polycrystalline nickel surfaces against hydrogen uptake by oxygen is investigated experimentally with low energy ion scattering (LEIS), direct recoil spectroscopy (DRS), and thermal desorption spectroscopy (TDS). These techniques are highly sensitive to surface hydrogen, allowing the change in hydrogen adsorption in response to varying amounts of oxygen exposure to be measured. The chemical composition of a nickel surface during a mixed oxygen and hydrogen exposure was characterized with LEIS and DRS, while the uptake and activation energies of hydrogen on a nickel surface with preadsorbed oxygen were quantified with TDS. By and large, these measurements of how the oxygen and hydrogen surface coverage varied in response to oxygen exposure were found to be consistent with predictions of a simple site-blocking model. This finding suggests that, despite the complexities that arise due to polycrystallinity, the oxygen-induced passivation of a polycrystalline nickel surface against hydrogen uptake can be approximated by a simple site-blocking model.

We have used scanning tunneling microscopy and density functional theory calculations to study molecular layers of coronene on Cu(111). The structure and stability of these layers is determined by the balance between coronene-substrate and coronene-coronene interactions. Here, we characterize this balance by measuring the maximum coverage before coronene dewets the substrate to form three-dimensional islands. We find that coronene molecules lie parallel to the substrate at the maximum coverage, in contrast to previous observations of tilted coronene on metal surfaces. We attribute this previously reported tilt to a metastability caused by an activation barrier to nucleate three-dimensional islands.

HKUST-1 or Cu

Hydrogen gas accelerates fatigue crack growth and reduces fracture toughness in ferritic structural materials such as pipelines and pressure vessels. The extent to which the crack growth rates are accelerated depends upon environment, mechanical loading conditions, and material. In this work, the effects of loading conditions and environment, specifically oxygen impurities, are examined on an X100 pipeline steel in high pressure hydrogen gas. Fatigue crack growth rates were measured in a gas mixture consisting of nominally 100 ppm O2 in a balance of H2 gas to evaluate the effects of pressure and load ratio (R-ratio) on the manifestation of hydrogen-accelerated fatigue crack growth (HA-FCG). Tests were performed at 21 MPa, 2.1 MPa, and 1.4 MPa and at load ratios of 0.5 and 0.1. The onset of HA-FCG was observed to be dependent on both absolute pressure and load ratio and it will be shown that a critical combination can result in complete mitigation of HA-FCG over the stress intensity factor range (ΔK) examined. Tests were predominantly performed at 10 Hz; however, a single test was performed at 1 Hz which exhibited negligible HA-FCG compared to a test at 10 Hz which did exhibit HA-FCG. Rising load fracture toughness tests were conducted via constant displacement rates to generate J-R curves in both pure H2 and 100 ppm O2 mixed gas. At similar absolute pressures, fracture toughness was measured to be greater in the 100 ppm O2 mixed gas compared to the pure H2. Hydrogen-assisted fracture was completely alleviated at pressures below 2.1 MPa in the 100 ppm O2 mixed gas, in which fracture toughness values were consistent with tests in air.

Using scanning probe microscopy and temperature programed desorption we examined the interaction between water and two common clathrate-forming gases, methane and isobutane, at low temperature and low pressure. Water co-deposited with up to 10–1 mbar methane or 10–5 mbar isobutane at 140 K onto a Pt(111) substrate yielded pure crystalline ice, i.e., the exposure to up to ~ 107 gas molecules for each deposited water molecule did not have any detectable effect on the growing films. Exposing metastable, less than 2 molecular layers thick, water films to 10–5 mbar methane does not alter their morphology, suggesting that the presence of the Pt(111) surface is not a strong driver for hydrate formation. This weak water–gas interaction at low pressures is supported by our thermal desorption measurements from amorphous solid water and crystalline ice where 1 ML of methane desorbs near ~ 43 K and isobutane desorbs near ~ 100 K. As a result, similar desorption temperatures were observed for desorption from amorphous solid water.

We have used low-energy electron microscopy (LEEM) and diffraction (LEED) to examine the significance of lattice orientation in graphene growth on Cu(0 0 1). Individual graphene domains undergo anisotropic growth on the Cu surface, and develop into lens shapes with their long axes roughly aligned with Cu〈1 0 0〉 in-plane directions. The long axis of a lens-shaped domain is only rarely oriented along a C〈1 1〉 direction, suggesting that carbon attachment at “zigzag” graphene island edges is unfavorable. A kink-mediated adatom attachment process is consistent with the behavior observed here and reported in the literature. The details of the ridged moire pattern formed by the superposition of the graphene lattice on the (0 0 1) Cu surface also evolve with the graphene lattice orientation, and are predicted well by a simple geometric model. Managing the kink-mediated growth mode of graphene on Cu(0 0 1) will be necessary for the continued improvement of this graphene synthesis technique.

Due to challenges of ecological restoration of post-mining areas to pre-operational conditions in aspect of water quality and quantity, development and assessment of ecological based minimal landscape discharge is necessary. In order to determine it, the basic nature and human factors and their relations within a catchment area had to be described and the essential parameters defined. The hydro ecological relations in the landscape are described by means of the numerous measurement methods and interpretation algorithms. The further aim was these procedures and the interpretation thresholds concerning the Minimum Required Landscape Discharge to recognize and optimally adapt for application.

From our daily life we are familiar with hexagonal ice, but at very low temperature ice can exist in a different structure––that of cubic ice. Seeking to unravel the enigmatic relationship between these two low-pressure phases, we examined their formation on a Pt(111) substrate at low temperatures with scanning tunneling microscopy and atomic force microscopy. After completion of the one-molecule-thick wetting layer, 3D clusters of hexagonal ice grow via layer nucleation. The coalescence of these clusters creates a rich scenario of domain-boundary and screw-dislocation formation. We discovered that during subsequent growth, domain boundaries are replaced by growth spirals around screw dislocations, and that the nature of these spirals determines whether ice adopts the cubic or the hexagonal structure. Initially, most of these spirals are single, i.e., they host a screw dislocation with a Burgers vector connecting neighboring molecular planes, and produce cubic ice. Films thicker than ∼20 nm, however, are dominated by double spirals. Their abundance is surprising because they require a Burgers vector spanning two molecular-layer spacings, distorting the crystal lattice to a larger extent. We propose that these double spirals grow at the expense of the initially more common single spirals for an energetic reason: they produce hexagonal ice.

We developed a method to predict the height corrugations of moir\'e structures resulting from varying azimuthal orientations of graphene on hexagonal boron nitride substrates. Our model accounts for the flexural rigidity of graphene and assumes a sinusoidal corrugation of the preferred height of C atoms above the substrate. Using four parameters derived from density functional theory (DFT), the model computes corrugations of incommensurate moir\'e structures currently inaccessible to DFT with an estimated accuracy on the order of 0.01 \AA{}. We predict that azimuthally aligned graphene domains are energetically favored over rotated ones.

Die Herstellung der okologischen Durchgangigkeit war Auflage der Genehmigungsbehorde fur den Weiterbetrieb der Kraftwerke Hollenstein und Pulling am Schwarzen Regen im Bayerischen Wald. Der Betreiber, die Kraftwerk am Hollenstein AG, entwickelte fur die Talsperre Hollenstein eine neuartige Druckkammerfischschleuse mit energetischer Nutzung. Dadurch brachte man Okologie und Okonomie in Einklang. Diese Fisch aufstiegs anlage eignet sich fur Talsperren und Wehrkraftwerke mit groser Fallhohe und schwankenden Oberwasserpegel wie z. B. bei Hochwasserruckhaltebecken. Die Ergebnisse des Mo ni to ring werden in einem gesonderten Artikel in diesem Heft der Fachzeitschrift Was ser Wirtschaft vorgestellt.

Ice plays an important role in many naturally occurring phenomena. For example, most rainfall in temperate climates is triggered by the nucleation of ice around μm-sized particles in atmospheric clouds. Another reason to be interested in ice is that thin supported ice films provide excellent model systems for studying the interaction of water with solid surfaces. Despite the importance of water-solid interactions for catalysis, corrosion, water purification and fuel cells, even the basics of this interaction are poorly understood. In fact, the best theoretical models often fail to predict even the simplest phenomena. A fundamental question, for example, is whether water/ice wets a given substrate, i.e., whether the substrate is covered up by a uniformly thick water/ice film. (In the case of non-wetting an ice film would break up into separate three-dimensional (3D) crystallites of varying height, exposing the substrate.)

Gas-handling components for high-pressure gaseous hydrogen (such as in the fuel system of fuel cell electric vehicles) are manufactured almost exclusively from austenitic stainless steels. Relatively few studies, however, have evaluated the fatigue life of this class of steels in hydrogen environments, especially at low temperature. Low temperature is important for two reasons: (1) austenitic stainless steels show an apparent minimum in tensile ductility at temperature near 220K when exposed to hydrogen environments; and (2) the service temperature range for the automotive industry is generally consider to be 233K to 358K (−40°C to +85°C). While the temperature of maximum hydrogen embrittlement from tensile tests is very near the minimum of the service temperature range, it remains unclear if the same trend applies to fatigue life properties. In this paper, we evaluate the effect of hydrogen on fatigue life of strain-hardened Type 316L. The tested alloy features a relatively high nickel content of 12 wt% and high yield strength of 590 MPa. Additionally, reduction of cost and weight of hydrogen-handling components is necessary to enhance the competitiveness of fuel cell vehicle technologies. Cost reductions can be achieved by considering alloys with lower nickel content, while higher strength materials enable lower weight. Simple estimates of cost and weight reductions that can be realized are discussed.Copyright © 2015 by ASME

We have studied the dislocation structures that occur in films of Ag, Au, and Ag0.5Au0.5 alloy on a Ru(0 0 0 1) substrate. Monolayer (ML) films form herringbone phases while films two or more layers thick contain triangular patterns of dislocations. We use scanning tunneling microscopy (STM) and low-energy electron diffraction (LEED) to determine how the film composition affects the structure and periodicity of these ordered structures. One layer of Ag forms two different herringbone phases depending on the exact Ag coverage and temperature. Low-energy electron microscopy (LEEM) establishes that a reversible, first-order phase transition occurs between these two phases at a certain temperature. We critically compare our 1 ML Ag structures to conflicting results from an X-ray scattering study [H. Zajonz et al., Phys. Rev. B 67 (2003) 155417]. Unlike Ag, the herringbone phases of Au and AgAu alloy are independent of the exact film coverage. For two layer films in all three systems, none of the dislocations in the triangular networks thread into the second film layer. In all three systems, the in-plane atomic spacing of the second film layer is nearly the same as in the bulk. Film composition does, however, affect the details of the two layer structures. Ag and Au films form interconnected networks of dislocations, which we refer to as “trigons.” In 2 ML AgAu alloy, the dislocations form a different triangular network that shares features of both trigon and moire structures. Yet another well-ordered structure, with square symmetry, forms at the boundaries of translational trigon domains in 2 ML Ag films but not in Au films.

Using low-energy electron microscopy, we show that de-wetting of Ag and Cu films on Ru(0 0 0 1) occurs by 3-D islands migrating across step edges in the “downhill” direction. We have observed islands thicken by more than 50 atomic layers in this manner. The island migration allows 3-D growth to occur in a way that avoids the nucleation barrier associated with forming 2-D islands on top of 3-D islands. Indeed, without substrate steps this nucleation barrier is not surmounted, and no 3-D islands are formed in the films studied.

The recognition of geological and thermal conditions of the rock mass in the case of designing a vertical borehole heat exchanger as the ground source for heat pump installations is a key issue affecting the efficiency of the heating/cooling system operation. This is especially important for large-sized buildings with a high demand for thermal power, which affects into the size of the ground source installation. The aim of the article is to indicate the difference in the obtained results concerning thermal calculations at the design stage of the brine/water heat pump installation with the vertical heat exchanger, in relation to the theoretical values of the rock mass thermal conductivity and the real (measured) values obtained during the thermal response test (TRT). For this purpose, calculations of thermal efficiency from one meter of the current rock mass were made, with particular emphasis on the change in the value of the thermal conductivity coefficient in the tested drilling profiles. Correspondingly, heat pump coefficients of performance (COP) were calculated, which allowed to analyze the influence of the over/undersizing phenomenon of the ground source on the technical parameters of the heat pump's operation and the economic effect of the investment.

Graphene films grown by vapour deposition tend to be polycrystalline due to the nucleation and growth of islands with different in-plane orientations. Here, using low-energy electron microscopy, we find that micron-sized graphene islands on Ir(111) rotate to a preferred orientation during thermal annealing. We observe three alignment mechanisms: the simultaneous growth of aligned domains and dissolution of rotated domains, that is, ‘ripening'; domain boundary motion within islands; and continuous lattice rotation of entire domains. By measuring the relative growth velocity of domains during ripening, we estimate that the driving force for alignment is on the order of 0.1 meV per C atom and increases with rotation angle. A simple model of the orientation-dependent energy associated with the moiré corrugation of the graphene sheet due to local variations in the graphene–substrate interaction reproduces the results. This work suggests new strategies for improving the van der Waals epitaxy of 2D materials., Understanding the factors that control island orientation in the growth of two-dimensional materials is likely to improve their quality. Here, using low-energy electron microscopy, the authors show that post-nucleation annealing of 2D materials can improve rotational order.

We examined the growth and stability of ice layers on Ni(111) up to ∼7 molecular layers (ML) thick using scanning tunneling microscopy. At low coverage, films were comprised of ∼1 nm wide two-dimensional (2D) clusters. Only above ∼0.5 ML did patches of continuous 2D layers emerge, coexisting with the clusters until the first ML was complete. The structure of the continuous layer is clearly different from that of the 2D clusters. Subsequently, a second molecular layer grew on top of the first. 3D crystallites started to form only after this 2nd ML was complete. 2D clusters re-appeared when thicker films were partially evaporated, implying that these clusters represent the equilibrium configuration at low coverage. Binding energies and image simulations computed with density functional theory suggest that the 2D clusters are partially dissociated and surrounded by H adatoms. The complete 2D layer contains only intact water molecules because of the lack of favorable binding sites for H atoms. We propose molecular structures for the 2D layer that are composed of the same pentagon-heptagon binding motif and water density observed on Pt(111). The similarity of the water structures on Pt and Ni suggests a general prescription for generating low-energy configurations on close-packed metal substrates.

We applied Scanning Probe Microscopy and Density Functional Theory (DFT) to discover the basics of how adsorbates wet insulating substrates, addressing a key question in geochemistry. To allow experiments on insulating samples we added Atomic Force Microscopy (AFM) capability to our existing UHV Scanning Tunneling Microscope (STM). This was accomplished by integrating and debugging a commercial qPlus AFM upgrade. Examining up-to-40-nm-thick water films grown in vacuum we found that the exact nature of the growth spirals forming around dislocations determines what structure of ice, cubic or hexagonal, is formed at low temperature. DFT revealed that wetting of mica is controlled by how exactly a water layer wraps around (hydrates) the K+ ions that protrude from the mica surface. DFT also sheds light on the experimentally observed extreme sensitivity of the mica surface to preparation conditions: K atoms can easily be rinsed off by water flowing past the mica surface.

We report a new mechanism of self-organization that can lead to robust surface ordering. We have quantitatively analyzed the thermal motion of holes created by sulfur atoms in a silver monolayer on a ruthenium surface, which we observed in real time with scanning tunneling microscopy. We find that the stability of the array of holes is determined by the arrangement and structure of misfit dislocations in the film.

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^{\circ} 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.

We use scanning tunneling microscopy to study the structure of graphene islands on Au(111) grown by deposition of elemental carbon at 950 \ifmmode^\circ\else\textdegree\fi{}C. Consistent with low-energy electron microscopic observations, we find that the graphene islands have dendritic shapes. The islands tend to cover depressed regions of the Au surface, suggesting that Au is displaced as the graphene grows. If small tunneling currents are used, it is possible to image simultaneously the graphene/Au moir\'e and the Au herringbone reconstruction, which forms underneath the graphene on cooling from the growth temperature. The delicate herringbone structure and its periodicity remain unchanged from the bare Au surface. Using a Frenkel--Kontorova model, we deduce that this striking observation is consistent with an attraction between graphene and Au of less than 13 meV per C atom. Raman spectroscopy supports this weak interaction. However, at the tunneling currents necessary for atomic-resolution imaging of graphene, the Au reconstruction is altered, implying influential tip--sample interactions and a mobile Au surface beneath the graphene.

High-resolution scanning tunneling microscopy (STM) reveals that the first layer of water on Ru(0001) and also on Pd(111) consists of hexagonal molecular domains of two types, rotated by ${30}^{\ensuremath{\circ}}$ relative to one another. Pentagon and heptagon clusters bridge the two types of hexagons. One of the orientations is in registry with the substrate. Its molecules lie flat and their O atoms form strong bonds to the metal atoms lying directly below. In the other domain the molecules have dangling H bonds. They are weakly bound to the substrate and lie correspondingly higher. This bonding motif, though nonperiodic, is of similar nature to the periodic wetting structure recently reported on Pt(111), and very different from the conventional ``ice-like'' bilayer. First-principles density functional theory (DFT) simulations of the STM images support these conclusions.

Two interpretations have been proposed of dark triangles observed in scanning tunneling microscopy (STM) images of the best bound, √37×√37-R25.3°, and √39×√39-R16.1° periodic water monolayers on Pt(111). In one, a "Y"-shaped tetramer of water molecules is removed, leaving a vacancy island behind; the other assumes the Y is replaced by a hexagon of H(2)O molecules, amounting to a di-interstitial. Consistent only with the di-interstitial model are thermal desorption and CO coadsorption data, STM line scans, and total energies obtained from density functional theory calculations.

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.

We have observed the formation of graphene on SiC by Si sublimation in an Ar atmosphere using low-energy 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.

Kleinwasserkraftanlagen konnen uber verschiedene Parameter klassifiziert werden. Die gebrauchlichste Unterscheidung wird nach der Leistung getroffen. Der Anteil dieser Anlagen an der Gesamtstromerzeugung aus Wasserkraft in Deutschland liegt mit 10 % in einem noch geringen ausbaufahigen Bereich. Die dezentrale Energieerzeugung in Entwicklungslandern wird auch unter der Masgabe des Klimaschutzes in den nachsten Jahren betrachtlich zu nehmen.

38 pags., 26 figs., We study how single-crystal chromium films of uniform thickness on W(110) substrates are converted to arrays of three-dimensional (3D) Cr islands during annealing. We use low-energy electron microscopy (LEEM) to directly observe a kinetic pathway that produces trenches that expose the wetting layer. Adjacent film steps move simultaneously uphill and downhill relative to the staircase of atomic steps on the substrate. This step motion thickens the film regions where steps advance. Where film steps retract, the film thins, eventually exposing the stable wetting layer. Since our analysis shows that thick Cr films have a lattice constant close to bulk Cr, we propose that surface and interface stress provide a possible driving force for the observed morphological instability. Atomistic simulations and analytic elastic models show that surface and interface stress can cause a dependence of film energy on thickness that leads to an instability to simultaneous thinning and thickening. We observe that de-wetting is also initiated at bunches of substrate steps in two other systems, Ag/W(110) and Ag/Ru(0001). We additionally describe how Cr films are converted into patterns of unidirectional stripes as the trenches that expose the wetting layer lengthen along the W[001] direction. Finally, we observe how 3D Cr islands. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft., This work was supported by the Director, Office of Science, Office of Basic Energy Sciences, Materials Sciences and Engineering Division, of the US Department of Energy under Contracts No. DE-AC04-94AL8500 and DE-AC02- 05CH11231, through funding from the Comunidad Autónoma de Madrid and the CSIC through Project No. CCG07-CSIC/MAT-2030, and by the Spanish Ministry of Science and Innovation through Project No. MAT2006-13149-C02-02.

To determine the role of surface diffusion on the morphology of ice surfaces we track the evolution with STM of 2D ice-island arrays on the basal surface of ice films on Pt(111) between 115 and 135 K. In contrast with previous measurements at higher temperatures, we find that the evolution is dominated by surface diffusion. The extracted surface self-diffusion coefficient has an activation energy of 0.4+/-0.1 eV, much less than the value previously measured for bulk diffusion.

Die Energieressourcen, die weltweit zur Verfugung stehen, sind begrenzt und knapp. Deshalb wird standig nach alternativen Moglichkeiten gesucht. Das Prinzip der Nachhaltigkeit ist dabei anzuwenden. Besonders nachhaltig sind die erneuerbaren Energietrager wie Sonne, Wind und Wasser.

Ice films as many as 30 molecular layers thick can be imaged with scanning tunneling microscopy (STM) when negative sample biases $l\ensuremath{-}6(\ifmmode\pm\else\textpm\fi{}1)\text{ }\text{V}$ and subpicoamp tunneling currents are used. We observe that water deposited onto Pt(111) 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.

Quantifying dewetting phenomena at the microscopic level is the key to deciphering how a balance between kinetic and equilibrium effects determines ice-film morphology on Pt(111). Overcoming the difficulty of imaging nominally insulating ice multilayers with scanning tunneling microscopy allowed us to track the dewetting process. The results show that the rate at which new layers nucleate, and not surface diffusion, determines how fast individual crystallite shapes equilibrate. Applying nucleation theory to measured growth rates versus crystallite size, we obtain new bounds on the energetics both of step formation on ice and of the Pt-ice interface.

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.

Deposition of S onto a monolayer of $\mathrm{Ag}/\mathrm{Ru}(0001)$ transforms the herringbone pattern of the clean Ag film into a strikingly regular array of 2D-vacancy islands [K. Pohl et al., Nature (London) 397, 238 (1999)]. Time-resolved scanning tunneling microscopy reveals that this nanometer-scale restructuring occurs by a cooperative mechanism involving the sequential formation of triangular regions with fcc and hcp stacking. Using a 2D Frenkel-Kontorova model, we can simulate the creation of these triangular building blocks via basic dislocation motions and reactions.