The role of H-H interactions and impurities on the structure and energetics of H/Pd(111).

Understanding hydrogen incorporation into palladium requires detailed knowledge of surface and subsurface structure and atomic interactions as surface hydrogen is being embedded. Using density functional theory (DFT), we examine the energies of hydrogen layers of varying coverage adsorbed on Pd(111). We find that H-H and H-Pd interactions promote the formation of the well-known 3×3 phases but also favor an unreported (3 × 3) phase at high H coverages for which we present experimental evidence. We relate the stability of isolated H vacancies of the (3 × 3) phase to the need of H2 molecules to access bare Pd before they can dissociate. Following higher hydrogen dosage, we observe initial steps of hydride formation, starting with small clusters of subsurface hydrogen. The interaction between H and Pd is complicated by the persistent presence of carbon at the surface. X-ray photoelectron spectroscopy experiments show that trace amounts of carbon, emerging from the Pd bulk despite many surface cleaning cycles, become mobile enough to repopulate the C-depleted surface at temperatures above 200 K. When exposed to hydrogen, these surface carbon atoms react to form benzene, as evidenced by scanning tunneling microscopy observations interpreted with DFT.

Orientation-dependent growth mechanisms of graphene islands on Ir(111).

Using low-energy electron microscopy, we find that the mechanisms of graphene growth on Ir(111) depend sensitively on island orientation with respect to Ir. In the temperature range of 750-900 °C, we observe that growing rotated islands are more faceted than islands aligned with the substrate. Further, the growth velocity of rotated islands depends not only on the C adatom supersaturation but also on the geometry of the island edge. We deduce that the growth of rotated islands is kink-nucleation-limited, whereas aligned islands are kink-advancement-limited. These different growth mechanisms are attributed to differences in the graphene edge binding strength to the substrate.

Interpretation of high-resolution images of the best-bound wetting layers on Pt(111).

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.

Pentagons and heptagons in the first water layer on Pt(111).

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.

Observation of surface self-diffusion on ice.

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.

Preparing arrays of large atomically flat regions on single crystal substrates.

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.

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.

Labyrinthine island growth during Pd/Ru(0001) heteroepitaxy.

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.

Spontaneous domain switching during phase separation of Pb on Ge(111).

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 x 1) phase coexists with the high coverage beta phase, the surface consists of approximately 100 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.

Evolution of a reactive surface via subsurface defect dynamics.

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.

Role of surface elastic relaxations in an O-induced nanopattern on Pt(110)-(1 x 2).

Scanning tunneling microscopy shows that a nanopattern forms as the Pt(110)-(1 x 2) surface is exposed to oxygen at room temperature or above. The nanopattern consists of [11[over]0] oriented O-induced stripes assembling into a (11 x 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.

How Pb-overlayer islands move fast enough to self-assemble on Pb-Cu surface alloys.

Low-energy electron microscopy reveals that two-dimensional, approximately 50 000 atom, Pb-overlayer and vacancy islands both have diffusion coefficients of 25.6+/-0.8 nm2/sec at 400 degrees C on Pb-Cu surface alloys. This high mobility, key to self-assembly in this system, results from the fast transport of Pb atoms on the surface alloy and of Cu through the Pb overlayer. A high Pb vacancy concentration, predicted by ab initio calculations, facilitates the latter.

Reversible shape transition of pb islands on Cu(111).

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.

Twin boundaries can be moved by step edges during film growth.

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.

Enhanced self-diffusion on Cu(111) by trace amounts of s: chemical-reaction-limited kinetics.

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.

Strain relief through heterophase interface reconstruction: Ag(111)/Ru(0001).

We report an experimental (scanning tunneling microscopy) and theoretical (embedded atom method) study of a heterophase interface reconstruction between Ag(111) and Ru(0001). Despite the large 7% mismatch, the second layer of Ag from the Ru exhibits a hexagonal structure with Ag bulk spacing, providing a close match to bulk Ag. The first layer of Ag (next to Ru) is reconstructed in a highly symmetrical and regular structure containing monolayer long threading dislocations. We argue that this structure may generally occur to relieve strain in a certain class of heterophase interfaces.

Self-assembly via adsorbate-driven dislocation reactions.

Deposition of S onto a monolayer of Ag/Ru(0001) transforms the herringbone pattern of the clean Ag film into a strikingly regular array of 2D-vacancy islands [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.

Thermal motion and energetics of self-assembled domain structures: Pb on Cu(111).

Low energy electron microscope measurements of the thermal motion of 50-200 nm diameter Pb islands on Cu(111) are used to establish the nature and determine the strength of interactions that give rise to self-assembly in this two-dimensional, two-phase system. The results show that self-assembled patterns arise from a temperature-independent surface stress difference of approximately 1.2 N/m between the two phases. With increasing Pb coverage, the domain patterns evolve in a manner consistent with models based on dipolar repulsions caused by elastic interactions due to a surface stress difference.

Role of bulk thermal defects in the reconstruction dynamics of the TiO2(110) surface.

We use low-energy electron microscopy to show that changing the temperature of oxygen-deficient, rutile-structure crystals causes steps on the (110) surfaces to move. This motion occurs because the concentration of bulk oxygen vacancies changes with temperature, requiring that material be added to or subtracted from the surface. During cooling below a bulk-stoichiometry-dependent temperature, the surface reconstructs into a 1x2 structure in the regions surface steps have swept through, showing that the structural and compositional changes needed to form the 1x2 phase are facilitated by the surface-to-bulk mass flow.

Linking surface stress to surface structure: measurement of atomic strain in a surface alloy using scanning tunneling microscopy.

Annealed submonolayer CoAg/Ru(0001) films form an alloy with a structure that contains droplets of Ag surrounded by Co [G. E. Thayer, V. Ozolins, A. K. Schmid, N. C. Bartelt, M. Asta, J. J. Hoyt, S. Chiang, and R. Q. Hwang, Phys. Rev. Lett. 86, 660 (2001)]. To understand how surface stress contributes to the formation of this structure, we use scanning tunneling microscopy to extract atomic displacements at the boundaries between regions of Co and Ag. Comparing our measurements to Frenkel-Kontorova model calculations, we show how stress due to lattice mismatch contributes to the formation of the alloy droplet structure. In particular, we quantitatively evaluate how competing strain and chemical energy contributions determine surface structure.