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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.

We study where and how hematite (α-Fe2O3) nucleates and grows during the oxidation of magnetite(100) single crystals. Hematite inclusions grow along 〈110〉 directions of the magnetite (Fe3O4), leading to a biaxial array of hematite slabs in an electrically conducting matrix of magnetite. The slab arrays form in both bulk single crystals and thin films of magnetite. Atomic force microscopy reveals that the surface growth of magnetite that accompanies hematite formation is faster adjacent to the hematite slabs. In situ X-ray photoelectron and X-ray absorption spectroscopies at 600 °C in an oxygen environment reveal that the conversion of the Fe2+ in magnetite to Fe3+ in hematite occurs without the formation of the metastable phase maghemite (γ-Fe2O3). We offer an explanation of why Fe 3O4(100) oxidizes faster than Fe3O 4(111). © 2014 American Chemical Society.

We have used low energy electron microscopy to study the energetics of steps on the (001) surface of heavily boron doped Si. When the surface is cooled below 980{degree}C, the S{sub B} step restructures dramatically, assuming a sawtooth profile consisting of triangular facets. The apex angle of the facets is strongly temperature dependent. By quantitative analysis of thermal step fluctuations and 2D island shapes we show that the free energy of isolated S{sub A} steps decreases sharply as the temperature is lowered. Using the Wulff construction, we demonstrate that the observed faceting is a direct consequence of the vanishing S{sub A} step free energy. {copyright} {ital 1997} {ital The American Physical Society}

We have used scanning tunneling microscopy to study the Ostwald ripening of 2D islands of Cu grown on Cu(001). By considering the time dependence of the sizes of individual islands we have characterized the mechanisms for the ripening. Our result is unexpected for a simple metal surface: The flow of atoms from one island to another is limited by attachment-detachment kinetics at the island edges. To explain this result, we propose that the transport of atoms between islands occurs by vacancy, rather than by adatom, diffusion. {copyright} {ital 1997} {ital The American Physical Society}

We have used low energy electron microscopy to study the time dependence of atomic height islands and holes at the extrema of a fabricated biperiodic grating on Si(001). From the pairwise disappearance of these islands and holes as the grating thermally smooths we conclude that steps on Si(001) are remarkably permeable, i.e., adatoms can cross steps without becoming incorporated into them. Comparison of the time dependence of the island areas with model calculations shows that the adatom incorporation probability is actually much smaller than the crossing probability.

Using low-energy electron microscopy, we have studied the critical fluctuations associated with the disordering of the $(3\ifmmode\times\else\texttimes\fi{}1)$ reconstruction on Si(113). We not only observe and quantify the increased spatial correlations near ${T}_{c}$, but also the associated slowing down of the relaxation of long wavelength critical fluctuations. The dependence of the slowing down on correlation length is consistent with theories for phase transitions with nonconserved order parameters. In addition, we show that steps limit the size of the correlation length, as is often conjectured.

We have used low-energy electron microscopy to study atomic exchange between the bulk and the (111) surface of Pt above 1100 K. We find that the area of two-dimensional surface islands increases when the temperature is raised and decreases when the temperature is lowered, suggesting that thermal bulk vacancies are created at the surface. By measuring the temperature dependence of the change in area of the islands, we determine a Pt bulk vacancy formation energy of 1.54±0.07 eV, consistent with previous measurements and calculations.

8 pag, 6 figs. -- Supporting Information is available at Publisher's web, 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 magnetite's 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 magnetite's 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. © 2013 American Chemical Society., Our research was supported by the Office of Basic Energy Sciences, Division of Materials and Engineering Sciences, U.S. Department of Energy under contract no. DE-AC04-94AL85000 and the Spanish Ministry of Science and Innovation through project no. MAT2009-14578-C03-01.

et al., Using low-energy electron diffraction, we show that the room-temperature (2×2)R45â̂̃ reconstruction of Fe3O4(100) reversibly disorders at ∼450â̂̃C. Short-range order persists above the transition, suggesting that the transition is second order and Ising-like. We interpret the transition in terms of a model in which subsurface Fe3+ is replaced by Fe2+ as the temperature is raised. This model reproduces the structure of antiphase boundaries previously observed with scanning tunneling microscopy, as well as the continuous nature of the transition. To account for the observed transition temperature, the energy cost of each charge rearrangement is 82 meV. © 2013 American Physical Society., This research was supported by the Office of Basic Energy Sciences, Division of Materials and Engineering Sciences, U. S. Department of Energy under Contract No. DE-AC04-94AL85000 and by the goverment of Spain through Project No. MAT2009-14578-C03-01/03 and MAT2012-38045-C04-01/04. LV and IB thanks the Spanish CSIC for support through a JAE-Doc contract.

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.

Low-energy ion beam techniques such as direct recoil spectroscopy (DRS) are well-suited for directly detecting adsorbed hydrogen. However, with this approach it has previously only been possible to detect the hydrogen configuration on single-crystal surfaces for a narrow range of conditions (e.g., when the adsorbate atoms reside close to the surface). In this study we investigate the experimental and modeling tools needed to extend DRS to a much wider range of adsorption geometries. At grazing incidence, we show how adsorbed hydrogen affects ion focusing along open surface channels, thereby revealing fine details about the binding geometry even for adsorbates residing high above the substrate. Under such conditions, the scattering process is characterized by ions interacting with many atoms simultaneously. Therefore interpreting these types of measurements properly requires realistic molecular dynamics (MD) simulations with accurate scattering potentials at large distances. As an illustration of the progress that can be made, we consider how hydrogen is recoiled from W(100)$+$H(ads) during exposure to a low-energy (1 keV) Ne${}^{+}$ analysis beam at grazing incidence. The closest approach distance of the ions is $g$1 $\AA{}$, making hydrogen located high above the substrate readily accessible. By mapping the hydrogen recoils over different crystallographic directions using DRS, we identify focusing mechanisms that provide information on the adsorbed hydrogen configuration. When W(100) is continuously dosed with H${}_{2}$(g) to maintain a saturation coverage, we observe enhanced recoil signals along the $\ensuremath{\langle}$100$\ensuremath{\rangle}$ and $\ensuremath{\langle}$110$\ensuremath{\rangle}$ azimuths consistent with hydrogen residing in bridge sites. The reproduction of the complex experimental scattering spectra by our MD modeling techniques offers a pathway to locate hydrogen well within its vibrational envelope.

We present the results of kinetic Monte Carlo simulations of homoepitaxial growth on a patterned substrate in the presence of an extra barrier to a diffusing adatom crossing of steps from above (Ehrlich-Schwoebel barrier) on topographically patterned surfaces. Our results indicate that over a wide range of Ehrlich-Schwoebel barrier heights, incident atom fluxes, and temperatures that multilayer islands or ``growth mounds'' grow in arrangements which are directed by the topographical pattern. Our simulations indicate that a series of arrangements should form as the temperature is changed due to a competition between the temperature-dependent mound size and the pattern period. We compare these predictions with experimental observation of directed mound assembly on nanopatterned GaAs(001).

Thermionic emission from monolayer graphene grown on representative transition metals, Ir and Ru, is characterized by low-energy electron microscopy (LEEM). 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 \pm 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{\deg}., Comment: Published in Applied Physics Letters

Using low-energy electron microscopy, we determine that self-diffusion of the Cu(001) surface is slowed by the presence of a $c$($2\ifmmode\times\else\texttimes\fi{}2$)-Pd buried surface alloy. We probe surface diffusion using Cu-adatom island-ripening measurements. On alloyed surfaces, the island decay rate decreases monotonically as the Pd concentration is increased up to $\ensuremath{\sim}$0.5 monolayer (ML), where the $2\ifmmode\times\else\texttimes\fi{}2$ buried alloy is Pd saturated. We propose that the Pd slows island ripening by inhibiting the diffusion of surface vacancies across terraces. For dilute alloys ($\ensuremath{\lesssim}$0.2-ML Pd), this conclusion is supported by density-functional theory calculations, which show that surface vacancies migrate more slowly owing to an attraction to isolated buried Pd atoms. The results illustrate a fundamental mechanism by which even a dilute alloy thin-film coating may act to inhibit surface-diffusion-mediated processes, such as electromigration.

We use low-energy electron microscopy to investigate how graphene grows on Cu(111). Graphene islands first nucleate at substrate defects such as step bunches and impurities. A considerable fraction of these islands can be rotationally misaligned with the substrate, generating grain boundaries upon interisland impingement. New rotational boundaries are also generated as graphene grows across substrate step bunches. Thus, rougher substrates lead to higher degrees of mosaicity than do flatter substrates. Increasing the growth temperature improves crystallographic alignment. We demonstrate that graphene growth on Cu(111) is surface diffusion limited by comparing simulations of the time evolution of island shapes with experiments. Islands are dendritic with distinct lobes, but unlike the polycrystalline, four-lobed islands observed on (100)-textured Cu foils, each island can be a single crystal. Thus, epitaxial graphene on smooth, clean Cu(111) has fewer structural defects than it does on Cu(100)., Article revised following reviewer comments

We report a simple and general procedure to create arrays of atomically flat terraces on single crystal surfaces. Facets of three-dimensional (3D) metal islands formed after hetero-epitaxial growth are often flat and, through annealing or growth at elevated temperature, the formation of rather large (micron-scale) atomically flat-top facets can be promoted. We find that the step-free nature of top facets on such islands can be transferred to the substrate surface through room-temperature ion-sputter etching, followed by an annealing step. We use low-energy electron microscopy (LEEM) and Auger electron spectroscopy (AES) for in situ monitoring of the process steps while fabricating arrays of step-free surface regions on W(110), Ru(0001), Cu(100), and Fe(100) single crystals.

We have developed a method to grow large, self-assembled, defect-free arrays of vacancy or adatom stripes on atomically flat, boron-doped Si(001)-(2\ifmmode\times\else\texttimes\fi{}1). The subnanometer-high stripes form between \ensuremath{\sim}870 and 990 \ifmmode^\circ\else\textdegree\fi{}C, with a spacing that depends on temperature. Si deposition is used to prevent sublimation-induced defect formation and to allow time for ordering via surface diffusion. Ordering mechanisms, observed in real time by low-energy electron microscopy, include island nucleation and growth, longitudinal splitting, and coarsening. At formation temperatures, the arrays are only stable when the area fractions of vacancy (${\ensuremath{\theta}}_{\mathrm{v}}$) or adatom stripes (${\ensuremath{\theta}}_{\mathrm{a}}$) are \ensuremath{\sim}\textonehalf{}, consistent with stress domain theory predictions. At room temperature, arrays are preserved indefinitely and are a potential template for nanowire growth.

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 configuration and motion of surface steps on the aperiodic icosahedral ($i$-) Al-Pd-Mn quasicrystal using low-energy electron microscopy and scanning tunneling microscopy. As the quasicrystal is cooled from high temperature, bulk vacancies migrate to the surface causing the surface to be etched. Surprisingly, this etching occurs by two types of steps with different heights moving in different directions with different velocities. The steady-state surface morphology is a uniformly spaced rhomboidal step network. This network requires that the layer stacking near the surface deviates from the bulk quasicrystal stacking.

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.

We use low-energy electron microscopy to investigate how graphene is removed from Ru(0001) and Ir(111) by reaction with oxygen. We find two mechanisms on Ru(0001). At short times, oxygen reacts with carbon monomers on the surrounding Ru surface, decreasing their concentration below the equilibrium value. This undersaturation causes a flux of carbon from graphene to the monomer gas. In this initial mechanism, graphene is etched at a rate that is given precisely by the same non-linear dependence on carbon monomer concentration that governs growth. Thus, during both growth and etching, carbon attaches and detaches to graphene as clusters of several carbon atoms. At later times, etching accelerates. We present evidence that this process involves intercalated oxygen, which destabilizes graphene. On Ir, this mechanism creates observable holes. It also occurs mostly quickly near wrinkles in the graphene islands, depends on the orientation of the graphene with respect to the Ir substrate, and, in contrast to the first mechanism, can increase the density of carbon monomers. We also observe that both layers of bilayer graphene islands on Ir etch together, not sequentially., Comment: 15 pages, 10 figures. Manuscript revised to improve discussion, following referee comments. Accepted for publication in Journal of Physical Chemistry C, Feb. 11, 2010

Low-energy electron microscopy (LEEM) reveals a new mode of graphene growth on Ru(0001) in which Ru atoms are etched from a step edge and injected under a growing graphene sheet. Based on density functional calculations, we propose a model wherein injected Ru atoms form metastable islands under the graphene. Scanning tunneling microscopy (STM) reveals that dislocation networks exist near step edges, consistent with some of the injected atoms being incorporated into the topmost Ru layer, thereby increasing its density., Comment: 20 pages. 9 figures

We discuss two types of step‐height doubling transitions on vicinal surfaces. In one type, exemplified by Ge(111)‐[121], a phase transition can occur since the symmetry between alternate terraces is broken at low temperatures. There is evidence in both experiment and in Monte Carlo simulations that this transition can have Ising character. In the second type, exemplified by Si(001)‐[110], one subset of terraces is always favored, so that no symmetry is broken. If an actual phase transition occurs (rather than a smooth crossover), it is expected to be first order, implying a coexistence region in a temperature‐misorientation phase diagram. Finally, the coalescence of steps into steps of height more than two atomic layers is briefly considered.

The manner in which phase transformations occur in solids determines important structural and physical properties of many materials. The main problem in characterizing the kinetic processes that occur during phase transformations is the difficulty of observing directly, in real time, the growth of one phase at the expense of another. Here we use low-energy electron microscopy to study the real-time kinetics of a phase transformation confined to the silicon (111) surface. We show that the transformation is governed by the rate at which material is exchanged between the first layer of the crystal and the surface. In bulk phase transformations, the dynamics are usually governed either by the rate of diffusion of material to the phase boundaries or by the structural rearrangement of atoms at the phase boundary. The kinetic process that we have identified here has no bulk analogue and leads to domain dynamics that are qualitatively different from those expected for bulk systems.

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 first, that the edge-orientation of the graphene sheets relative to the substrate plays an important role in the growth mechanism, and second, that attachment of the clusters to the graphene is the slowest step in cluster addition, rather than formation of clusters on the terraces.

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.

High resolution low‐energy electron diffraction, scanning tunneling microscopy (STM), and transmission electron microscopy (TEM) have been used to study the temperature dependence of a vicinal (stepped) Si(111) surface with a polar angle of 6° from (111) along an azimuth rotated about 10° away from the high symmetry [112] direction. At the (1×1) to (7×7) reconstructive transition, the kinks at the step edges precipitate into a phase with high step density and increased rotation from the [112] direction, leaving behind a [112] oriented phase with triple‐layer height steps and (7×7) reconstruction. The inclination and azimuth of the orientation of the kinked phase change continuously with temperature. When the surface is cooled to room temperature, STM images confirm a coexistence between the [112] oriented step‐tripling phase and the kinked phase. In the step‐tripling phase, the steps are very straight (kinks are rare) and oriented in the [112] direction. There is a small fraction of single‐layer...

Scanning tunneling microscope (STM) and high resolution low‐energy electron diffraction studies of slowly cooled Si(111) surfaces misoriented by 1.3° towards the [112] direction show the existence of a preferred displacement of the (7×7) reconstruction across single‐layer steps. STM results show that this correlation is quite strong: it occurred in 42 of the 46 steps observed. The correlation leads to a quantization of the terrace widths to units of the (7×7) unit cell. Less extensive results on the 6° misoriented surface suggest that the (7×7) reconstruction is also correlated across triple‐layer steps.

By observing with low-energy electron microscopy whether individual alumina islands grow or shrink for different substrate temperatures and ${\mathrm{O}}_{2}$ pressures, we determine the stability of thin oxide layers on the NiAl(110) surface. At each temperature, a well-defined ${\mathrm{O}}_{2}$ pressure exists where islands do not change in size. Yet we conclude that the oxide cannot be in thermodynamic equilibrium with ${\mathrm{O}}_{2}$ gas and NiAl bulk, because the ${\mathrm{O}}_{2}$ pressures needed to attain this state are 20 orders of magnitude higher than expected. We discuss what kinetic processes can lead to the observed steady state, where the ${\mathrm{O}}_{2}$ pressure needed for stability differs greatly from thermodynamic predictions.

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.

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.

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.

Using low-energy electron microscopy (LEEM), we have discovered a novel phase separation mechanism for Pb on Ge(111). When the low Pb coverage ($1\ifmmode\times\else\texttimes\fi{}1$) phase coexists with the high coverage $\ensuremath{\beta}$ phase, the surface consists of $\ensuremath{\sim}100\text{ }\text{ }\mathrm{nm}$ sized domains that spontaneously switch from one phase to the other. We argue this striking mechanism occurs because nanometer-scale domains can have density fluctuations comparable to the density difference between the two phases.

We find that the topography and composition of a reactive surface can evolve during epitaxy via motion of point and line defects within the material. We observe the response of a NiAl surface to an Al atom flux with low-energy electron microscopy. Initially, new NiAl layers grow as Al atoms exchange with bulk Ni atoms. When the surface is critically enriched in Al, condensation occurs at dislocations. They dissociate, move linearly, and leave tracks of altered composition and new atomic steps. We show how these dynamics depend on the identity and quantity of point defects near the surface.

Scanning tunneling microscopy shows that a nanopattern forms as the $\mathrm{Pt}(110)\mathrm{\text{\ensuremath{-}}}(1\ifmmode\times\else\texttimes\fi{}2)$ surface is exposed to oxygen at room temperature or above. The nanopattern consists of $[1\overline{1}0]$ oriented O-induced stripes assembling into a ($11\ifmmode\times\else\texttimes\fi{}2$) superstructure at high O coverage. The stripes form because the O adsorption energy increases by expanding the Pt lattice along the ridges of the surface as compared to the bulk. From interplay with density functional theory calculations, we show that the O-induced nanoscale periodicity is caused by short-ranged elastic relaxations confined to the surface.

Using low-energy electron microscopy, we have observed a reversible transition in the shape of Pb adatom and vacancy islands on Cu(111). With increasing temperature, circular islands become elongated in one direction. In previous work we have shown that surface stress domain patterns are observed in this system with a characteristic feature size which decreases with increasing temperature. We show that the island shape transition occurs when the ratio of the island size to this characteristic feature size reaches a particular value. The observed critical ratio matches the value expected from stress domains.

The density of two-dimensional Ag adatom gases on W(110) is determined by monitoring local electron reflectivity using low energy electron microscopy (LEEM). This method of adatom concentration measurement can detect changes in adatom density at least as small as 10$^{-3}$ ML for a $\mu$m size region of the surface. Using this technique at high temperatures, we measure the sublimation rates of Ag adatoms on W(110). At lower temperatures, where Ag adatoms condense into monolayer islands, we determine the temperature dependence of the density of adatoms coexisting with this condensed phase and compare it with previous estimates., Comment: Presented at the ECOSS 23 Conference (Berlin 2005)

We track individual twin boundaries in Ag films on Ru(0001) using low-energy electron microscopy. The twin boundaries, which separate film regions whose close-packed planes are stacked differently, move readily during film growth but relatively little during annealing. The growth-driven motion of twin boundaries occurs as film steps advance across the surface--as a new atomic Ag layer reaches an fcc twin boundary, the advancing step edge carries along the boundary. This coupling of the microstructural defect (twin boundary) and the surface step during growth can produce film regions over 10 microm wide that are twin free.

We find that small temperature changes cause steps on the NiAl(110) surface to move. We show that this step motion occurs because mass is transferred between the bulk and the surface as the concentration of bulk thermal defects (i.e., vacancies) changes with temperature. Since the change in an island's area with a temperature change is found to scale strictly with the island's step length, the thermally generated defects are created (annihilated) very near the surface steps. To quantify the bulk/surface exchange, we oscillate the sample temperature and measure the amplitude and phase lag of the system response, i.e., the change in an island's area normalized to its perimeter. Using a one-dimensional model of defect diffusion through the bulk in a direction perpendicular to the surface, we determine the migration and formation energies of the bulk thermal defects. During surface smoothing, we show that there is no flow of material between islands on the same terrace and that all islands in a stack shrink at the same rate. We conclude that smoothing occurs by mass transport through the bulk of the crystal rather than via surface diffusion. Based on the measured relative sizes of the activation energies for island decay, defectmore » migration, and defect formation, we show that attachment/detachment at the steps is the rate-limiting step in smoothing.« less

The length scale of stress domain patterns formed at solid surfaces is usually calculated using isotropic elasticity theory. Because this length depends exponentially on elastic constants, deviations between isotropic and anisotropic elasticity can lead to large errors. Another inaccuracy of isotropic elasticity theory is that it neglects the dependence of elastic relaxations on stripe orientation. To remove these inaccuracies, we calculate the energy of striped domain patterns using anisotropic elasticity theory for an extensive set of surfaces encountered in experimental studies of self-assembly. We present experimental and theoretical evidence that elastic anisotropy is large enough to determine the stripe orientation when Pb is deposited on Cu(111). Our analytical and numerical results should be useful for analysis of a broad range of experimental systems.

Pb deposition on Cu(111) causes the surface to self-assemble into periodically arranged domains of a Pb-rich phase and a Pb-poor phase. Using low-energy electron microscopy (LEEM) we provide evidence that the observed temperature-dependent periodicity of these self-assembled domain patterns is the result of changing domain-boundary free energy. We determine the free energy of boundaries at different temperatures from a capillary wave analysis of the thermal fluctuations of the boundaries and find that it varies from 22 meV/nm at 600 K to 8 meV/nm at 650 K. Combining this result with previous measurements of the surface stress difference between the two phases we find that the theory of surface-stress-induced domain formation can quantitatively account for the observed periodicities.

We find that less than 0.01 monolayer of S can enhance surface self-diffusion on Cu(111) by several orders of magnitude. The measured dependence of two-dimensional island decay rates on S coverage (theta(S)) is consistent with the proposal that Cu3S3 clusters are responsible for the enhancement. Unexpectedly, the decay and ripening are diffusion limited with very low and very high theta(S) but not for intermediate theta(S). To explain this result we propose that surface mass transport in the intermediate region is limited by the rate of reaction to form Cu3S3 clusters on the terraces.

persists to at least 9 ML of Ag on Ru(0001) and presumably applies for the bulk heterophase Ag� 111� =Ru� 0001� interface. Our STM observations show that this interface reconstruction effectively relieves the 7% misfit strain between the Ag and Ru bulk lattices. We use atomistic modeling to determine the arrangement of atoms at the interface, thereby revealing the dislocation structure associated with this interface reconstruction. Finally, we consider why this type of reconstruction occurs at the Ag� 111� =Ru� 0001� interface, but not the Cu� 111� =Ru� 0001� interface. Our observations demonstrate that the structure of heterophase interfaces can be very complex and that we are far from a complete understanding of the issues determining even equilibrium metal-metal interface structures.

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.