Atomic structures on faceted W(111) surfaces induced by ultrathin films of Pd

The faceting of Pd/W(111) surfaces has been studied using a Scanning tunneling microscope (STM). Three-sided pyramidal facets having {211} faces with dimensions ranging from 3 to 15 nm can be induced by ultrathin Pd films (≥ 1 monolayer), upon annealing to 700 K or higher. From atomic-resolution STM-images of these surfaces, we obtain direct confirmation of the {211} structure on individual facets of the 3-sided pyramids. In addition, the atomic structure of the facet edges indicates that edge energy may play a role in faceting. When the as-deposited coverage of Pd is greater than the critical value ( 1 monolayer) for inducing faceting, the extra Pd atoms diffuse to form 3-dimensional clusters, some with discernible crystalline structures, upon annealing.     

Thermal faceting of (110) and (111) surfaces of MgO

Using LEED, we have observed the thermal faceting of MgO (110) and (111) surfaces into sets of (100) faces. Some faceting occurs under ion bombardment at room temperature, and annealing at 900–1400 K by means of electron bombardment produces complete faceting, with facets up to 1 μm across. These observations are consistent with theories of the stability of ionic crystal surfaces.     

Self-organized nanoscale pattern formation on vicinal Si(111) surfaces via a two-stage faceting transition

We demonstrate a self-organized pattern formation on vicinal Si(111) surfaces that are miscut toward the [2;11] direction. All the patterns, consisting of a periodic array of alternating (7 x 7) reconstructed terraces and step-bunched facets, have the same periodicity and facet structure, independent of the miscut angle, while the width of the facets increases linearly with miscut angle. We attribute such unique pattern formation to a surface faceting transition that involves two transition stages: the first stage forms a stress-domain structure defining the universal periodicity; the second stage forms the low-energy facets controlling the facet width.     

Theoretical study of oxygen chemisorption on (111) and (100) silicon surfaces

The chemisorption of atomic oxygen on (111) and (100) silicon surfaces has been studied by the MNDO method using a cluster approach. The results show that, for both surfaces, chemisorption occurs preferentially on bridge positions, but chemisorption on top positions can play a significant role especially for the (111) surface.     

CO adsorption studies on pure and Ni-covered Cu(111) surfaces

The adsorption of CO on pure and Ni-covered Cu(111) surfaces has been studied by means of LEED, TDS, UPS and work function measurements during adsorption and desorption. Different Ni-coverages between 0.1 and 2 monolayers were obtained by Ni-evaporation controlled by a quartz micro balance and by AES. Near room temperature Ni grows in a layer-by-layer mode on Cu(111). The island structure of the surfaces with submonolayer Ni-coverages is clearly demonstrated by TDS und LEED results obtained after CO adsorption. As with surfaces of bulk Cu-Ni alloys CO adsorption on Cu(111) with submonolayer Ni-coverage is dominated by a site effect. Cu-, Ni-, and mixed adsorption sites can be distinguished. The CO induced work function changes for Ni- and Cu-site adsorption show the same sign as observed with the pure metals. Mixed site adsorption has only a minor influence on the work function. A “ligand effect” observed only for the Ni-site adsorption, and only at small Ni-coverages is discussed in detail. Studies on the adsorption kinetics reveal that the Cu-sites may serve as precursor sites for Ni-site adsorption. Detailed UPS studies demonstrate that the CO-induced emission maxima observed on Cu surfaces with submonolayer Ni-coverages can be interpreted as a superposition of the respective adsorption features observed with the pure metals, roughly separated by their work function difference.     

Selective formation of ultrathin PbSe on Ag(111)

Two-dimensional (2D) semiconductors, such as lead selenide (PbSe), locate at the key position of next-generation devices. However, the ultrathin PbSe is still rarely reported experimentally, particularly on metal substrates. Here, we report the ultrathin PbSe synthesized via sequential molecular beam epitaxy on Ag(111). The scanning tunneling microscopy is used to resolve the atomic structure and confirms the selective formation of ultrathin PbSe through the reaction between Ag₅Se₂ and Pb, as further evidenced by the theoretical calculation. It is also found that the increased accumulation of Pb leads to the improved quality of PbSe with larger and more uniform films. The detailed analysis demonstrates the bilayer structure of synthesized PbSe, which could be deemed to achieve the 2D limit. The differential conductance spectrum reveals a metallic feature of the PbSe film, indicating a certain interaction between PbSe and Ag(111). Moreover, the moiré pattern originated from the lattice mismatch between PbSe and Ag(111) is observed, and this moiré system provides the opportunity for studying physics under periodical modulation and for device applications. Our work illustrates a pathway to selectively synthesize ultrathin PbSe on metal surfaces and suggests a 2D experimental platform to explore PbSe-based opto-electronic and thermoelectric phenomena.     

Computer simulations of electron stimulated desorption patterns from oxygen on W(100) and (111) surfaces

Computer simulations of Electron-Stimulated-Desorption lon-Angular-Distribution patterns (ESDIAD) from oxygen covered W(100) and (111) surfaces were performed, using classcial dynamical formulas for the calculation of the O₊ ion trajectories. A model for the reconstruction of the O covered W(100) surface in the temperature range of 700–900 K is presented. The simulated ESD patterns have been brought into agreement with experimental results from literature by the proper choice: first of the repulsive atomic potentials acting on the ions, second of the rms amplitudes of the O atoms and third of the constants in the formula for Auger neutralization. The angular widths of the ESD spots were fitted by introducing bending vibrations of the chemisorbed O atoms and also by distributing the directions of the chemical bonds in limited angular areas. Assuming Franck-Condon type transitions and neglecting intermediate states, the final ion energies led to distances between the starting positions of the ions and the neighboring W atoms in the range of the known chemical bond lengths. The repulsive atomic potentials, obtained from Hartree-Fock-Slater self-consistent field calculations for W and O atoms in different electronic states and with different electronic charges, were compared with the potentials giving best agreement with the experimental ESDIAD results. In this way, qualitative conclusions concerning the electronic charge at the surfaces were derived.     

Magnetic properties of ultrathin Co films on Si (111)

Ultrathin cobalt films on clean (7×7) and Au covered Si (111) substrates were prepared by molecular beam epitaxy. The structure was studied by using scanning tunnelling microscopy and low energy electron diffraction. Magnetic properties were determined with the magneto-optic Kerr effect. It was found that Co nucleates in grains that prefer to grow along the bunched step edges of the Si substrate ([112?] direction), which induces a strong in-plane uniaxial anisotropy. By introducing Au buffer layers, the magnetic characteristics were improved by preventing the silicide reaction between Si and Co. Moreover, the tendency for step decoration disappears gradually results in the in-plane uniaxial anisotropy reduction.     

Strain analysis of epitaxial ultrathin films on Pt(111)

X-ray photoelectron diffraction (XPD) measurements have demonstrated that Ni ultrathin films on Pt(111) exhibit a single domain fcc stacking pseudomorphic to the substrate, with a consequent trigonal distortion of the Ni unit cell in order to accommodate the in-plane expansion of the Ni lattice parameter of about 11% with respect to its bulk value. We show that the amount of interlayer contraction and the strain energy resulting form the trigonal distortion are very well predicted by a strain analysis in the framework of simple linear elasticity theory. Strain analysis also allows to discuss in some detail the controversial case of Co growth on the same substrate. Finally, we discuss the dependence of the strain energy of the overlayer on the substrate crystal orientation and its effects on chemisorption experiments performed on heteroepitaxial monolayers (HML).     

Simulation of adsorbate-induced faceting on curved surfaces

A simple solid-on-solid model, proposed earlier to describe overlayer-induced faceting of bcc(1 1 1) surface, is applied to faceting of curved surfaces covered by an adsorbate monolayer. Surfaces studied in this paper are formed by a part of sphere around the [1 1 1] pole. Results of Monte Carlo simulations show that the morphology of a faceted surface depends on the annealing temperature. At an initial stage the surface around the [1 1 1] pole consists of 3-sided pyramids and step-like facets, then step-like facets dominate and their number decreases with temperature, finally a single big pyramid is formed. It is shown that there is a reversible phase transition at which a faceted surface transforms to an almost spherical one. It is found that the temperature of this phase transition is an increasing function of the surface curvature. Simulation results show that measurements of high temperature properties performed directly and after fast cooling down to a low temperature lead to different results.     

Nature of versatile chemisorption on TiC(111) and TiN(111) surfaces

Density-functional calculations on the polar TiX(111) (X = C, N) surfaces show (i) for clean surfaces, strong Ti3d-derived surface resonances (SR’s) at the Fermi level and X2p-derived SR’s deep in the upper valence band and (ii) for adatoms in periods 1-3, pyramidic trends in atomic adsorption energies, peaking at oxygen (9 eV). A concerted-coupling model, where adatom states couple to both kinds of SR’s in a concerted way, describes the adsorption. The chemisorption versatility and the general nature of the model indicate ramifications and predictive abilities in, e.g., growth and catalysis.     

Adatom ascending at step edges and faceting on fcc metal (110) surfaces

Using first-principles total-energy calculations, we show that an adatom can easily climb up at monatomic-layer-high steps on several representative fcc metal (110) surfaces via a place exchange mechanism. Inclusion of such novel adatom ascending processes in kinetic Monte Carlo simulations of Al(110) homoepitaxy as a prototypical model system can lead to the existence of an intriguing faceting instability, whose dynamical evolution and kinetic nature are explored in comparison with experimental observations.     

Electric-dipole layer on Au(111) surfaces

We measured the barrier height (BH) a UHV scanning tunneling microscope on Au(111) surfaces with Au, Pt, and carbon nanotube tips. The 222?{^[¯] }3^[¯]\sqrt\Box3\Box]] reconstruction was observed with all the tips, and the current-voltage relation reflected the density of states of the tips. The BH measured on the reconstructed Au(111) surface using a modulation method showed a bias-polarity dependence as 30%, at low currents (<100 pA) irrespective of the tip material, while on unreconstructed Au surfaces, BH values were independent of the bias polarity, suggesting a dipole layer originating from the reconstructed Au(111) surface.