Reflectance difference spectroscopy study of Ag growth on W(1 1 0)

We report a reflectance difference spectroscopy (RDS) investigation of the epitaxial growth of Ag on the W(1 1 0) surface. Monitoring the growth in real time, the RDS signal at 4.6 eV shows an oscillatory behavior corresponding to the layer-by-layer growth of the first three monolayers. The oscillations are attributed to the variation of the optical anisotropy contributed by the W(1 1 0) substrate and the Ag film. By analyzing the spectral evolution during growth, characteristic optical-electronic fingerprints can be deduced for each added atomic layer. In particular, the binding energy of d-like quantum well states has been used as an indicator for the number of Ag atomic layers and, hence, as a sensitive probe of the Ag thin film growth.     

Effects of electronic confinement and substrate on the low-temperature growth of Pb islands on Si(1 0 0)-2 × 1 surfaces

The growth of Pb films on the Si(1 0 0)-2 × 1 surface has been investigated at low temperature using scanning tunneling microscopy. Although the orientation of the substrate is (1 0 0), flat-top Pb islands with (1 1 1) surface can be observed. The island thickness is confined within four to nine atomic layers at low coverage. Among these islands, those with a thickness of six layers are most abundant. Quantum-well states in Pb(1 1 1) islands of different thickness are acquired by scanning tunneling spectroscopy. They are found to be identical to those taken on the Pb(1 1 1) islands grown on the Si(1 1 1)7 × 7 surface. Besides Pb(1 1 1) islands, two additional types of Pb islands are formed: rectangular flat-top Pb(1 0 0) islands and rectangular three-dimensional (3D) Pb islands, and both their orientations rotate by 90° from a terrace to the adjacent one. This phenomenon implies that the structures of Pb(1 0 0) and 3D islands are influenced by the Si(1 0 0)-2 × 1 substrate.     

Photoelectron spectroscopy study of Pb/Ag(1 1 1) in the submonolayer range

The vacuum deposition of Pb onto Ag(1 1 1) gives rise to two different surface structures depending on coverage and deposition temperature. At room temperature (RT), low energy electron diffraction (LEED) reveals a sharp reconstruction completed at 1/3 Pb monolayer (ML). Beyond, a close-packed Pb(1 1 1) incommensurate overlayer develops. At low temperature (LT, ∼100 K) the incommensurate structure is directly observed whatever the coverage, corresponding to the growth of close-packed two-dimensional Pb(1 1 1) islands. Synchrotron radiation Pb 5d core-level spectra clearly demonstrate that in each surface structure all Pb atoms have essentially a unique, but different, environment. This reflects the surface alloy formation between the two immiscible metals in the reconstruction and a clear signature of the de-alloying process at RT beyond 1/3 ML coverage.     

The growth and stability of Fe layers on the Mo(1 1 1) surface

The initial growth and the stability of Fe layers on the Mo(1 1 1) surface was studied with Auger electron spectroscopy, low energy electron diffraction, scanning tunneling microscopy and thermal desorption spectroscopy. At room temperature at least the first two monolayers grow layer-by-layer. The first layer is stable up to about 1200 K. Excess Fe starts to agglomerate at about 400 K and forms with increasing temperature thick flat-top islands which start to sublime at a somewhat below 1200 K. A strong decrease of the adsorption energy with coverage was found in the first monolayer. No {2 1 1} or { 1 1 0} micro-faceting could be seen at any coverage upon annealing.     

In situ STM study of nanosized Ru and Os islands spontaneously deposited on Pt(1 1 1) and Au(1 1 1) electrodes

We provide an overview of structure and reactivity of selected bimetallic single crystal electrodes obtained by the method of spontaneous deposition. The surfaces that are described and compared are the following: Au(1 1 1)/Ru, Pt(1 1 1)/Ru and Pt(1 1 1)/Os. Detailed morphological information is presented and the significance of this work in current and further study of nanoisland covered surfaces in the catalytic and spectroscopic perspective is highlighted. All surfaces were investigated by in situ STM and by electroanalytical techniques. The results confirm our previous data that nanosized Ru islands are formed with specific and distinctive structural features, and that the Ru growth pattern is different for Au(1 1 1) and Pt(1 1 1). For Au(1 1 1), Ru is preferentially deposited on steps, while a random and relatively sparse distribution of Ru islands is observed on terraces. In contrast, for Ru deposited on Pt(1 1 1), a homogeneous deposition over all the Pt(1 1 1) surface was found. Os is also deposited homogeneously, and at a much higher rate than Ru, and even within a single deposition it forms a large proportion of multilayer islands. On Au(1 1 1), the Ru islands on both steps and terraces reach the saturation coverage within a short deposition time, and the Ru islands grow to multilayer heights and assume hexagonal shapes. On Pt(1 1 1), the Ru saturation coverage is reached relatively fast, but when a single deposition is applied, Ru nanoislands of mainly monoatomic height are formed, with the Ru coverage not exceeding 0.2 ML. For Ru deposits on Pt(1 1 1), we demonstrate that larger and multilayer islands obtained in two consecutive depositions can be reduced in size--both in height and width--by oxidizing the Ru islands and then by reducing them back to a metallic state. A clear increase in the Ru island dispersion is then obtained. However, methanol oxidation chronoamperometry shows that the surface with such a higher dispersion is less active to methanol oxidation than the initial surface. A preliminary interpretation of this effect is provided. Finally, we studied CO stripping reaction on Pt(1 1 1)/Ru, Au(1 1 1)/Ru and on Pt(1 1 1)/Os. We relate CO oxidation differences observed between Pt(1 1 1)/Ru and Pt(1 1 1)/Os to the difference in the oxophilicity of the two admetals. In turn, the difference in the CO stripping reaction on Pt(1 1 1)/Ru and Au(1 1 1)/Ru with respect to the Ru islands is linked to the effect of the substrate on the bond strength and/or adlayer structure of CO and OH_ads species.     

Morphology of Ni ultrathin films on Mo(1 1 0) and W(1 0 0) studied by LEED and STM

The morphology of ultrathin Ni films on Mo(1 1 0) and W(1 0 0) has been studied by low-energy electron diffraction and scanning tunneling microscopy. Ni films grow pseudomorphically on Mo(1 1 0) at 300 K for a coverage of 0.15 ML. A (8 × 1) structure is found at 0.4 ML, which develops into a (7 × 1) structure by 0.8 ML. The film undergoes a structural change to fcc Ni(1 1 1) at 6 ML. The growth mode switches from layer-by-layer to Stranski-Krastanov between 4 ML and 6 ML. Annealing at around 850 K results in alloying of submonolayer films with the substrate, while for higher coverages the Ni agglomerates into nanowedge islands. Ni films grow pseudomorphically on W(1 0 0) up to a coverage of around 2 ML at 300 K, above which there is a structural change from bcc to hcp Ni with the epitaxial relationship . This is accompanied by the formation of orthogonal domains of uniaxial strain-relieving dislocations from the third layer of the film. For coverages up to 1 ML the growth proceeds by formation of two-dimensional islands, but shifts to three-dimensional growth by 2 ML with rectangular islands aligned along the 〈0 1 1〉 substrate directions. Annealing at around 550 K results in agglomeration of Ni into larger islands and increasing film roughness.     

Interaction of Ag and O on W(1 1 0) studied by scanning tunneling microscopy

The structural changes of Ag films on W(1 1 0) upon coadsorption of oxygen have been studied by scanning tunneling microscopy. The exposure of one monolayer Ag to oxygen leads to a phase separation into an Ag bilayer and patches of O-covered W(1 1 0). The effective Ag island thickness increases linearly with oxygen exposure. For Ag submonolayer-islands the onset of the bilayer formation is delayed, the induction period increases with the available free W area. We conclude that the steps of the transport process are (1) dissociation of oxygen on W and on the Ag islands, (2) site exchange of atomic oxygen with Ag atoms predominantly at the island edges - while on W(1 1 0) the oxygen is immobile, (3) diffusion of the displaced Ag atoms to the island edges where they are incorporated into the monolayer and (4) initiation of Ag bilayer formation, once the W(1 1 0) is saturated with O. This indicates an unexpected activity of the Ag monolayer on W(1 1 0) towards oxygen dissociation. In case of a reversed deposition sequence, where submonolayer quantities of Ag are adsorbed on an oxygen-precovered W(1 1 0) surface, growth of Ag clusters is observed. The distribution of cluster size and cluster height depends critically on the spatial order within the predeposited oxygen overlayer - it is obvious that the oxygen overlayer on the W surface acts as a structured template for preferential Ag nucleation.     

Ultrathin Fe layers on Ag (1 0 0) surface

Iron layers (0.15-10 ML thick) deposited on Ag (1 0 0) substrates were investigated by conversion electron Mössbauer spectrometry over a broad temperature range. The layers were characterized by scanning tunneling microscopy. Different forms of the layers, depending on their thickness, were observed. Minimum roughness of the layers were found at 0.15 and 10 ML thickness values. The Mössbauer spectra showed systematic thickness dependence. At low thickness values, broad doublets were observed, while above 6 ML, magnetic split spectra appeared at room temperature. At low temperatures, magnetically split spectra appeared with parameter values characteristic of Fe-Ag and Fe-Fe atomic interactions. The hyperfine split spectra indicated magnetic anisotropy and an enhanced saturation hyperfine magnetic field of ?40 T. The latter value is the highest ever measured for iron in thin layers.     

Growth of (√3 × √3)-Ag and (1 1 1) oriented Ag islands on Ge/Si(1 1 1) surfaces

We have studied the growth of Ag on Ge/Si(1 1 1) substrates. The Ge/Si(1 1 1) substrates were prepared by depositing one monolayer (ML) of Ge on Si(1 1 1)-(7 × 7) surfaces. Following Ge deposition the reflection high energy electron diffraction (RHEED) pattern changed to a (1 × 1) pattern. Ge as well as Ag deposition was carried out at 550 °C. Ag deposition on Ge/Si(1 1 1) substrates up to 10 ML has shown a prominent (√3 × √3)-R30° RHEED pattern along with a streak structure from Ag(1 1 1) surface. Scanning electron microscopy (SEM) shows the formation of Ag islands along with a large fraction of open area, which presumably has the Ag-induced (√3 × √3)-R30° structure on the Ge/Si(1 1 1) surface. X-ray diffraction (XRD) experiments show the presence of only (1 1 1) peak of Ag indicating epitaxial growth of Ag on Ge/Si(1 1 1) surfaces. The possibility of growing a strain-tuned (tensile to compressive) Ag(1 1 1) layer on Ge/Si(1 1 1) substrates is discussed.     

Growth of Si nanostructures on Ag(0 0 1)

The first stages of the growth of silicon on Ag(0 0 1) at moderate temperatures start by the formation of a p(3 × 3) superstructure, which continuously evolves with increasing coverage toward a more complex superstructure. In this paper, the atomic arrangement of the p(3 × 3) and of the “complex” superstructure has been investigated using scanning tunnelling microscopy, surface X-ray diffraction and low energy electron diffraction. The atomic model retained for the p(3 × 3) reconstruction consists in four silicon atoms (tetramers) adsorbed near hollow and bridge sites of the top most Ag(0 0 1) surface layer. For higher coverages, i.e., when the “complex” superstructure starts to develop, the silicon overlayer forms periodic stripes, most probably bi-layers, with a graphitic like structure.     

Au island growth on a Si(1 1 1) vicinal surface

Au island nucleation and growth on a Si(1 1 1) 7 × 7 vicinal surface was studied by means of scanning tunneling microscopy. The surface was prepared to have a regular array of step bunches. Growth temperature and Au coverage were varied in the 255-430 °C substrate temperature range and from 1 to 7 monolayers, respectively. Two kinds of islands are observed on the surface: Au-Si reconstructed islands on the terraces and three-dimensional (3D) islands along the step bunches. Focusing on the latter, the dependence of island density, size and position on substrate temperature and on Au coverage is investigated. At 340 °C and above, hemispherical 3D islands nucleate systematically on the step edges.     

Growth of oriented crystalline Ag nanoislands on air-exposed Si(0 0 1) surfaces

Growth of Ag islands under ultrahigh vacuum condition on air-exposed Si(0 0 1)-(2 × 1) surfaces has been investigated by in-situ reflection high energy electron diffraction (RHEED). A thin oxide is formed on Si via exposure of the clean Si(0 0 1)-(2 × 1) surface to air. Deposition of Ag on this oxidized surface was carried out at different substrate temperatures. Deposition at room temperature leads to the growth of randomly oriented Ag islands while well-oriented Ag islands, with (0 0 1)_Ag||(0 0 1)_Si, [1 1 0]_Ag||[1 1 0]_Si, have been found to grow at substrate temperatures of ≥350 °C in spite of the presence of the oxide layer between Ag islands and Si. The RHEED patterns show similarities with the case of Ag deposition on H-passivated Si(0 0 1) surfaces.     

Electronic properties of Au nano-particles supported on stoichiometric and reduced TiO2(1 1 0) substrates

Growth modes and electronic properties were analyzed for Au nano-particles grown on stoichiometric and reduced TiO₂(1 1 0) substrates by medium energy ion scattering (MEIS) and photoelectron spectroscopy(PES) using synchrotron-radiation-light. Initially, two-dimensional islands (2D) with a height of one and two atomic layers grow and higher coverage increases the islands height to form three-dimensional (3D) islands for the stoichiometric TiO₂(1 1 0) substrate. In contrast, 3D islands start to grow from initial stage with a small Au coverage (?0.1 ML, 1 ML = 1.39 × 10¹⁵  atoms/cm²: Au(1 1 1)) probably due to O-vacancies acting as a nucleation site. Above 0.7 ML, all the islands become 3D ones taking a shape of a partial sphere and the Au clusters change to metal for both substrates. We observed the Au 4f and Ti 3p core level shifts together with the valence band spectra. The Ti 3p peak for the O-deficient surface shifts to higher binding energy by 0.25 ± 0.05 eV compared to that for the stoichiometric surface, indicating downward band bending by an electron charge transfer from an O-vacancy induced surface state band to n-type TiO₂ substrate. Higher binding energy shifts of Au 4f peaks observed for both substrates reveal an electron charge transfer from Au to TiO₂ substrates. The work functions of Au nano-particles supported on the stoichiometric and reduced TiO₂ substrates were also determined as a function of Au coverage and explained clearly by the above surface and interface dipoles.     

Nanowedge island formation on Mo(1 1 0)

The deposition of ultrathin Fe films on the Mo(1 1 0) surface at elevated temperatures results in the formation of distinctive nanowedge islands. The model of island formation presented in this work is based on both experiment and DFT calculations of Fe adatom hopping barriers. Also, a number of classical molecular dynamics simulations were carried out to illustrate fragments of the model. The islands are formed during a transition from a nanostripe morphology at around 2 ML coverage through a Bales-Zangwill type instability. Islands nucleate when the meandering step fronts are sufficiently roughened to produce a substantial overlap between adjacent steps. The islands propagate along the substrate [0 0 1] direction due to anisotropic diffusion/capture processes along the island edges. It was found that the substrate steps limit adatom diffusion and provide heterogeneous nucleation sites, resulting in a higher density of islands on a vicinal surface. As the islands can be several layers thick at their thinnest end, we propose that adatoms entering the islands undertake a so-called “vertical climb” along the sides of the island. This is facilitated by the presence of mismatch-induced dislocations that thread to the sides of the islands and produce local maxima of compressive strain. Dislocation lines also trigger initial nucleation on the surface with 2-3 ML Fe coverage. The sides of the nanowedge islands typically form along low-index crystallographic directions but can also form along dislocation lines or the substrate miscut direction.     

Growth and thermal evolution of submonolayer Pt films on Ru(0 0 0 1) studied by STM

The growth of submonolayer Pt on Ru(0 0 0 1) has been studied with scanning tunneling microscopy. We focus on the island evolution depending on Pt coverage θ_Pt, growth temperature T_G and post-growth annealing temperature T_A. Dendritic trigonal Pt islands with atomically rough borders are observed at room temperature and moderate deposition rates of about 5 × 10⁻⁴ ML/s. Two types of orientation, rotated by 180° and strongly influenced by minute amounts of oxygen are observed which is ascribed to nucleation starting at either hcp or fcc hollow sites. The preference for fcc sites changes to hcp in the presence of about one percent of oxygen. At lower growth temperatures Pt islands show a more fractal shape. Generally, atomically rough island borders smooth down at elevated growth temperatures higher than 300 K, or equivalent annealing temperatures. Dendritic Pt islands, for example, transform into compact, almost hexagonal islands, indicating similar step energies of A- and B-type of steps. Depending on the Pt coverage the thermal evolution differs somewhat: While regular islands on Ru(0 0 0 1) are formed at low coverages, vacancy islands are observed close to completion of the Pt layer.     

Formation of ordered arrays of Ag nanowires and nanodots on Si(5 5 7) surface

The ordered arrays of Ag nanowires and nanodots have been grown in ultra-high vacuum on the Si(5 5 7) surface containing regular steps of three bilayer height. Formation of Ag nanostructures have been studied by scanning tunneling microscopy, low energy electron diffraction and Auger electron spectroscopy at room temperature. It was shown that a sample exposure in the vacuum before Ag growth affects the shape of the forming Ag islands. This effect is caused by oxygen adsorption on the silicon surface from the residual atmosphere in the vacuum chamber. When Ag is deposited on the clean silicon surface the islands, overlapping several (1 1 1) neighboring terraces, form. The arrays of silver nanowires elongated along steps and silver nanodots, arranged in lines parallel to the steps, can be formed on the Si(5 5 7) surface depending on the amount of adsorbed oxygen.     

Self-assembled germanium nano-clusters on silver(1 1 0)

The adsorption of germanium on Ag(1 1 0) has been investigated by scanning tunnelling microscopy (STM), as well as surface X-ray diffraction (SXRD). At 0.5 germanium monolayer (ML) coverage, Low Energy Electron Diffraction (LEED) patterns reveals a sharp c(4 × 2) superstructure. Based on STM images and SXRD measurements, we present an atomic model of the surface structure with Ge atoms forming tetramer nano-clusters perfectly assembled in a two-dimensional array over the silver top layer. The adsorption of the germanium atoms induces a weak perturbation of the Ag surface. Upon comparison with results obtained on the (1 1 1) and (1 0 0) faces, we stress the role played by the relative interactions between silver and germanium on the observed surface structures.     

Fabrication of rubrene nanowires on vicinal (0 0 0 1) sapphire surfaces

Morphology of high-vacuum deposited rubrene thin films on the annealed (0 0 0 1) vicinal sapphire surfaces was studied by atomic force microscopy in non-contact mode. Atomic force microscopy images of rubrene thin films indicate that a regular array of steps on the sapphire surface acts as a template for the growth of the arrays of rubrene nanosize wires. To further demonstrate that morphological features of a substrate are crucial in determining the morphology of rubrene layers we have grown rubrene on the sapphire surfaces that were characterized by the terrace-and-step morphology with islands. We have found preferential nucleation of rubrene molecules at the intersection between a terrace and a step, as well as around the islands located on terraces.     

Growth and thermal stability of ultra-thin Ag and Au layers on Mo(1 1 1) surface

The influence of temperature on the growth process of ultra-thin Ag and Au layers on the Mo(1 1 1) surface was investigated. At 300 K growth of the Stranski-Krastanov type was found for Ag; for Au growth of the monolayer plus simultaneous multilayers type was found, where a base layer is one physical layer. The first three geometrical adsorbed layers for Ag are thermally stable. For annealed Au layers triangle features with base side length from 15 to 35 Å were formed for θ < 6 monolayer (ML), and for θ > 6 ML part of the Au formed a flat adlayer with Au atoms grouped in equilateral triangles with side length 7 Å. The presence of Au layers does not cause faceting, layers are not smooth which could be caused by the fact that Au does not wets the substrate. For Ag thick layers reversible wetting/non-wetting transition was observed at 600 K. Ag layers on Mo(1 1 1) surface did not lead to faceting.