Surface and subsurface oxide formation on Ni(1 0 0) and Ni(1 1 1)

The oxidation of Ni(1 0 0) and Ni(1 1 1) at elevated temperatures and large oxygen exposures, typical of the methods used in the preparation of NiO(1 0 0) films for surface studies, has been investigated by medium energy ion scattering (MEIS) using 100 keV H⁺ incident ions. Oxide film growth proceeds significantly faster on Ni(1 1 1) than on Ni(1 0 0), but on both surfaces oxide penetration occurs to depths significantly greater than 100 Å with total exposures of 1200 and 6000 L respectively. The metal/oxide interface is extremely rough, with metallic Ni extending to the surface, even for much thicker oxide films on Ni(1 1 1). On Ni(1 1 1), NiO growth occurs with the (1 0 0) face parallel to the Ni(1 1 1) surface and the close-packed 〈1 1 0〉 directions parallel. On Ni(1 0 0) the MEIS blocking curves cannot be reconciled with a single orientation of NiO(1 0 0) (with the 〈1 1 0〉 directions parallel) on the surface, but is consistent with the substantial orientational disorder (including tilt) previously identified by spot-profile analysis LEED.     

Ultrathin nickel oxide films grown on Ag(0 0 1): a study by XPS, LEIS and LEED intensity analysis

The structure of a nickel oxide film 2 ML thick has been investigated by LEED intensity analysis. The NiO film was prepared by evaporating Ni in presence of O₂ at a pressure in the 10⁻⁶ mbar range. The growth of the oxide film was followed by XPS, LEIS and LEED. In the early stages of deposition, the film shows a (2 × 1) superstructure in LEED. After deposition of 2 ML of NiO, a sharp (1 × 1) LEED pattern is observed. The intensity versus electron energy curves of the LEED spots were measured for this NiO(1 × 1) film and analysed by means of the tensor LEED method. A good level of agreement of the experimental LEED intensities with those calculated for a pseudomorphic NiO(0 0 1) film was obtained. We found that oxygen atoms at the oxide-substrate interface are on-top silver atoms. The interlayer distance in the oxide does not differ significantly from that in bulk NiO(0 0 1), within the accuracy of the analysis. An outward displacement (0.05 ± 0.05 Å) of oxygen atoms with respect to nickel atoms was found at the oxide film surface. The interlayer distance at the silver-nickel oxide interface is 2.43 ± 0.05 Å.     

Scanning tunneling microscopy and spectroscopy of Mo clusters grown on TiO2(1 1 0)

Molybdenum was deposited in two steps (3 eq. ML and 1 eq. ML) on the light blue rutile TiO₂(1 1 0) (1 × 1) surface at room temperature, each Mo deposition cycle being followed by an annealing up to 950-1000 K. This procedure was found to lead to formation of separated clusters having a size in very wide range (1-20 nm). Scanning tunneling microscopy showed a dependence of the cluster morphology as a function of the size. The scanning tunneling spectra of Mo clusters was studied as a function of cluster dimensions and discussed in comparison with photoelectron spectroscopy results previously obtained for homogeneous Mo films. The dI/dV curves do not display the valence band structure of deposited material, which could be explained by the Schottky barrier formation.     

Structure and reactivity of the hydrated hematite (0 0 0 1) surface

The structure of the hydroxylated hematite (0 0 0 1) surface was investigated using crystal truncation rod diffraction and density functional theory. The combined experimental and theoretical results suggest that the surface is dominated by two hydroxyl moieties—hydroxyls that are singly coordinated and doubly coordinated with Fe. The results are consistent with the formation of distinct domains of these surface species; one corresponding to the hydroxylation of the surface Fe-cation predicted to be most stable under UHV conditions, and the second a complete removal of this surface Fe species leaving the hydroxylated oxygen layer. Furthermore, our results indicate that the hydroxylated hematite surface structures are significantly more stable than their dehydroxylated counterparts at high water partial pressures, and this transition in stability occurs at water pressures orders of magnitude below the same transition for α-alumina. These results explain the observed differences in reactivity of hematite and alumina (0 0 0 1) surfaces with respect to water and binding of aqueous metal cations.     

Structure analysis of stoichiometric LiNbO3(0 0 0 1) surfaces using low-energy neutral scattering spectroscopy

The atomic structure of LiNbO₃(0 0 0 1) surface was investigated by low-energy neutral scattering spectroscopy (LENS). Poled stoichiometric LiNbO₃ (SLN) samples were prepared for the measurements. The LENS was developed for surface structure and composition analysis particularly of highly insulating materials and was successfully applied to the structure analysis of the SLN(0 0 0 1) surface. The polar angle dependences of intensity of scattered He⁰ from the poled SLN surfaces indicate obvious differences between the negatively and the positively charged surfaces. It is suggested that O atoms cover the surfaces, and the first metal layers underneath the O layer consist of Li and Nb for negatively and positively charged surfaces, respectively, parallel to the applied electric field.     

Growth of epitaxial thin Pd(1 1 1) films on Pt(1 1 1) and oxygen-terminated FeO(1 1 1) surfaces

Thin Pd films (1-10 monolayers, ML) were deposited at 35 K on a Pt(1 1 1) single crystal and on an oxygen-terminated FeO(1 1 1) monolayer supported on Pt(1 1 1). Low energy electron diffraction, Auger electron spectroscopy, and Kr and CO temperature programmed desorption techniques were used to investigate the annealing induced changes in the film surface morphology. For growth on Pt(1 1 1), the films order upon annealing to 500 K and form epitaxial Pd(1 1 1). Further annealing above 900 K results in Pd diffusion into the Pt(1 1 1) bulk and Pt-Pd alloy formation. Chemisorption of CO shows that even the first ordered monolayer of Pd on Pt(1 1 1) has adsorption properties identical to bulk Pd(1 1 1). Similar experiments conducted on FeO(1 1 1) indicate that 500 K annealing of a 10 ML thick Pd deposit also yields ordered Pd(1 1 1). In contrast, annealing of 1 and 3 ML thick Pd films did not result in formation of continuous Pd(1 1 1). We speculate that for these thinner films Pd diffuses underneath the FeO(1 1 1).     

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.     

Bimodal growth of manganese silicide on Si(1 0 0)

Two different growth modes of manganese silicide are observed on Si(1 0 0) with scanning tunneling microscopy. 1.0 and 1.5 monolayer Mn are deposited at room temperature on the Si(1 0 0)-(2 × 1) substrate. The as-grown Mn film is unstructured. Annealing temperatures between room temperature and 450 °C lead to small unstructured clusters of Mn or Mn_xSi_y. Upon annealing at 450 °C and 480 °C, Mn reacts chemically with the Si substrate and forms silicide islands. The dimer rows of the substrate become visible again. Two distinct island shapes are found and identified as MnSi and Mn₅Si₃.     

A multi-scale Monte Carlo study of oxide structures on the Cu(1 0 0) surface

We present a multi-scale Monte Carlo study of the oxidation of the Cu(1 0 0) surface based on the Bortz-Kalos-Lebowitz model with the equilibrium energetics obtained from ab initio calculations. The radial and island size distribution functions are examined and Cu-O structures are analyzed at different temperatures and coverages. We concentrate on the coverages of 0.3 monolayer O or less, with variable sub-monolayer coverages of Cu. The results show that even though the ab initio calculations yield a higher barrier for O than for Cu adatom diffusion on Cu(1 0 0), the stability of Cu structures causes the O adatoms to be more mobile on the Cu(1 0 0) surface than the Cu adatoms. We are able to reproduce the c(2 × 2)-O domains seen in the experiments. However, we give an alternative explanation based on the repulsive interactions of O that, on one hand, cause the local ordering and, on the other hand, prohibits large well-ordered domains. We also give interpretation on the formation of the R45°-O reconstruction of Cu(1 0 0) above the O coverages of 0.3 monolayer based on the ab initio energetics.     

STM observation of initial growth of Sn atoms on Ge(0 0 1) surface

We have studied initial growth of Sn atoms on Ge(0 0 1) surfaces at room temperature and 80 K by scanning tunneling microscopy. For Sn deposition onto the Ge(0 0 1) substrate at room temperature, the Sn atoms form two kinds of one-dimensional structures composed of ad-dimers with different alignment, in the 〈3 1 0〉 and the 〈1 1 0〉 directions, and epitaxial structures. For Sn deposition onto the substrate at 80 K, the population of the dimer chains aligning in the 〈3 1 0〉 direction increases. The diffusion barrier of the Sn adatom on the substrate kinetically determines the population of the dimer chain. We propose that the diffusion barrier height depends on surface strain induced by the adatom. The two kinds of dimer chains appearing on the Ge(0 0 1) and Si(0 0 1) surfaces with adatoms of the group-IV elements are systematically interpreted in terms of the surface strain.     

First-principles study of indium on silicon (1 0 0)—the structure, defects and interdiffusion

The atomic structures of indium (In) on silicon (Si) (1 0 0)-(2 × 1) surface are investigated by the local density approximation using first-principles pseudopotentials. Total energy optimizations show that the energetically favored structure is the parallel ad-dimer model. The adsorption energy of In on ideal Si(1 0 0)-(1 × 1) surface is significantly higher than that on reconstructed Si(1 0 0)-(2 × 1) surface, suggesting that In adsorption does not break the Si-Si dimer bond of the substrate. When Si surface contains single dimer vacancy defects, In chain will be interrupted, leading to disconnected In nanowires. Displacive adsorption of In on Si(1 0 0) is also considered, and the calculation suggests that interdiffusion of In into Si substrate will not be favorable under equilibrium conditions.     

Epitaxial growth of ultra-thin NiO films on Cu(1 1 1)

The very first stages of the growth of NiO on Cu(1 1 1) is examined on a microscopic scale. The paper focuses on the morphological and structural characterization of nanostructures formed in the 0-1 Å thickness range. Ultra-thin NiO films, obtained through evaporation of a Ni rod under an oxygen atmosphere were grown at 550 K. In the early stages of the growth the oxide film morphology shows 10-30 nm large, monolayer high, islands with a partial incorporation of metallic Ni in the first Cu(1 1 1) surface plane. The first layer is formed by an epitaxial atomic layer exhibiting a STM contrast similar to the one observed on adsorbed oxygen on Cu(1 1 0). A NiO cluster nucleation and coalescence mechanism is proposed in order to explain the formation of the second NiO layer. A α-Ni₂O₃ hexagonal phase, or a structural distortion of the NiO(1 1 1)()R30° structure could both explain the complex LEED patterns.     

Hydrogen adsorption and diffusion on Pd(1 1 1)

The adsorption, diffusion and ordering of hydrogen on Pd(1 1 1) was studied by scanning tunneling microscopy in the temperature range of 37-90 K. At low coverage isolated hydrogen atoms were observed. They formed √3×√3-1H islands as the coverage increased. Above 1/3 monolayer (ML) coverage areas of a new phase with √3×√3-2H structure were formed, with both structures coexisting between 1/3 and 2/3 ML. Finally a 1 × 1 structure was formed after high exposures of hydrogen above 50 K, with a coverage close to 1 ML. Atomically resolved images reveal that H binds to fcc hollow sites.     

Formation of linear structures of Sr and Gd films on the Mo(1 1 2) and W(1 1 2) surfaces

At low coverages, alkaline earth and rare earth layers adsorbed on furrowed transition metal surfaces--such as W(1 1 2) and Mo(1 1 2)--tend to form commensurate linear structures built of monoatomic chains directed across the furrows. Presented Monte Carlo simulations provide insight into parameters of the long-range indirect interaction that, in competition with a dipole-dipole interaction, leads to formation of the linear structures. The most impressive result of the simulations is the revealed repulsive effective interaction between adjacent atoms in linear Sr chains on W(1 1 2) surface. In this case, formation of the linear chains is accomplished due to pronounced minima in the potential of indirect interaction along the surface furrows. This result predicts that a single Sr chain cannot exist on the W(1 1 2) surface.     

Theoretical investigation of the Au/Si(1 1 1)-(5 × 2) surface structure

The atomic structure of the Au/Si(1 1 1)-(5 × 2) surface has been studied by density-functional theory calculations. Two structure models, proposed experimentally by Marks et al. and Hasegawa et al., have been examined on an equal ground. In our total-energy calculations, both models are found to be locally stable and energetically comparable. In our electronic-structure analyses, however, both models fail to reproduce the key features of angle-resolved photoemission spectra and scanning-tunneling-microscopy images, indicating that the considered models need to be modified. Suggestions for the modification are given based on the present calculations.     

A photoelectron diffraction investigation of vanadyl phthalocyanine on Au(1 1 1)

Scanned-energy mode photoelectron diffraction using the O 1s and V 2p emission perpendicular to the surface has been used to investigate the orientation and internal conformation of vanadyl phthalocyanine (VOPc) adsorbed on Au(1 1 1). The results confirm earlier indications from scanning tunnelling microscopy that the VO vanadyl bond points out of, and not into, the surface. The VO bondlength is 1.60 ± 0.04 Å, not significantly different from its value in bulk crystalline VOPc. However, the V atom in the adsorbed molecule is almost coplanar with the surrounding N atoms and is thus pulled down into the approximately planar region defined by the N and C atoms by 0.52 (+0.14/−0.10) Å, relative to its location in crystalline VOPc. This change must be attributed to the bonding interaction between the molecule and the underlying metal surface.     

Eu- and Yb-induced reconstructions on a vicinal Si(1 0 0) surface

Early stages of rare-earth metal (Yb and Eu) growth on a vicinal, single-domain Si(1 0 0)2 × 1 surface have been studied in the coverage range of 0.1-0.3 monolayer (ML) by low energy electron diffraction, scanning tunneling microscopy, and synchrotron radiation photoemission spectroscopy. We show that Yb induces the 2 × 3 periodicity in the whole range of coverage studied. The 2 × 3 reconstruction coexists with the local 3 × 2/4 × 2 structure at about 0.2 ML of Yb. In contrast, Eu forms the 3 × 2 periodicity at 0.1-0.2 ML, whereas this structure is converted into the 2 × 3 phase at about 0.3 ML. The atomic arrangement and electronic properties of these reconstructions and the adsorbate-mediated modification of surface morphology are investigated.     

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.     

Morphological evidence for surface pre-melting on Si(1 1 1)

We present what we believe to be the first morphological evidence for the occurrence of surface pre-melting on the Si(1 1 1) surface. Our results complement the extensive previous evidence from diffraction and ion scattering techniques for the presence of pre-melted (liquid-like) layers on Si(1 1 1) below the bulk melting temperature and also suggest how atomic steps are involved in the initiation of such layers. Our results are based on atomic force microscopy studies of morphologies that are preserved when surfaces are annealed in a range of high temperatures and then rapidly cooled to room temperature for observation. A unique feature of the experiments is the use of specially prepared atomically flat or very low step density surfaces; this allows us to see how the liquid-like morphologies are associated with the steps and also allows the high temperature structures to survive the cooling process without being absorbed into the steps which normally would exist on a surface vicinal to (1 1 1). Quenched-in structures ascribed to pre-melting also act as sinks for diffusing ‘excess’ adatoms generated by the (1 × 1) to (7 × 7) transition and this leads to the formation of dendritic islands.