Sulfur-induced mobilization of Au surface atoms on Au(1 1 1) studied by real-time STM

The interaction of sulfur with gold surfaces has attracted considerable interest due to numerous technological applications such as the formation of self-assembled monolayers and as a chemical sensor. Here, we report on the interaction of sulfur with Au(1 1 1) at two different temperatures (300 K and 420 K) studied by real-time scanning tunnelling microscopy, low energy electron diffraction and Auger electron spectroscopy. In the low coverage regime (<0.1 ML), S adsorption lifts the herringbone reconstruction of the clean Au(1 1 1) surface indicating a lateral expansion of the surface layer. An ordered (√3 × √3)R30° sulfur adlayer develops as the coverage reaches ∼0.3 ML. At higher S coverages (>0.3 ML) gold surface atoms are removed from regular terrace sites and incorporated into a growing gold sulfide phase. At 300 K this process leads to the formation of a rough pit and mound surface morphology. This gold sulfide exhibits short-range order and an incommensurate, long-range ordered AuS phase develops upon annealing at 450-525 K. In contrast, formation of an ordered AuS phase via rapid step-retraction rather than etch pit formation is observed during S-interaction with Au(1 1 1) surfaces at 420 K. Our results shed new light on the S-Au(1 1 1) interaction.     

Initial stages of oxidation of Cu(1 1 1)

Several surface analysis techniques were combined to study the initial stages of oxidation of Cu(1 1 1) surfaces exposed to O₂ at low pressure (<5 × 10⁻⁶ mbar) and room temperature. Scanning tunneling microscopy (STM) results show that the reactivity is governed by the restructuring of the Cu(1 1 1) surface. On the terraces, oxygen dissociative adsorption leads to the formation of isolated O adatoms and clusters weakly bound to the surface. The O adatoms are located in the fcc threefold hollow sites of the unrestructured terraces. Friedel oscillations with an amplitude lower than 5 pm have been measured around the adatoms. At step edges, surface restructuring is initiated and leads to the nucleation and growth of a two-dimensional disordered layer of oxide precursor. The electronic structure of this oxide layer is characterised by a band gap measured by scanning tunneling spectroscopy to be ∼1.5 eV wide. The growth of the oxide islands progresses by consumption of the upper metal terraces to form triangular indents. The extraction of the Cu atoms at this interface generates a preferential orientation of the interface along the close-packed directions of the metal. A second growth front corresponds to the step edges of the oxide islands and progresses above the lower metal terraces. This is where the excess Cu atoms extracted at the first growth front are incorporated. STM shows that the growing disordered oxide layer consists of units of hexagonal structure with a first nearest neighbour distance characteristic of a relaxed Cu-Cu distance (∼0.3 nm), consistent with local Cu₂O(1 1 1)-like elements. Exposure at 300 °C is necessary to form an ordered two-dimensional layer of oxide precursor. It forms the so-called “29” superstructure assigned to a periodic distorted Cu₂O(1 1 1)-like structure.     

Surface segregation of aluminum atoms on Cu-9 at.% Al(1 1 1) studied by Auger electron spectroscopy and low energy electron diffraction

Auger electron spectroscopy (AES) and low energy electron diffraction (LEED) were applied to investigate the segregation of aluminum atoms on a Cu-9 at.% Al(1 1 1) surface. We observed that the Al concentration in the top layer ranged between about 9 and 36 at.% after the sample we used was annealed at different temperatures. The phenomenon of Al atoms segregating on the surfaces was explained well by considering the diffusion length of Al atoms in bulk Cu. LEED measurements showed that R30° structures grew as the concentration of Al atoms increased. The segregation phenomena on surfaces resulted in a stable two-dimensional Cu₆₇Al₃₃ alloy phase in the top layer.     

The CuGaSe2(0 0 1) surface: A (4 × 1) reconstruction

We report on the formation of a stable (4 × 1) reconstruction of the chalcopyrite CuGaSe₂(0 0 1) surface. Using Ar⁺ ion-bombardment and annealing of epitaxial CuGaSe₂ films grown on GaAs(0 0 1) substrates it was possible to obtain flat, well-ordered surfaces showing a clear (4 × 1) reconstruction. The cleanliness and structure were analyzed in situ by AES and LEED. AES data suggest a slight Se-enrichment and Cu-depletion upon surface preparation. Our results demonstrate that (0 0 1) surfaces of the Cu-III-VI₂(0 0 1) material can show stable, unfacetted surfaces.     

Epitaxial growth of cobalt clusters on the bare and the oxide film grown on NiAl(1 0 0) surfaces

The structure of the nano-sized cobalt clusters on bare NiAl(1 0 0) and an oxidized NiAl(1 0 0) surfaces have been investigated by AES, LEED and RHEED. The deposition of Co onto bare NiAl(1 0 0) at room temperature led to small crystalline Co grains and surface asperities of substrate. The latter is likely induced by replacement of surface Al, Ni atoms by Co deposit. At 800 K Co particles aggregate to form clusters, but incorporation of Co into bulk NiAl(1 0 0) could occur upon annealing at 900 K. On the other hand, pure face-centered cubic (fcc) phase of Co crystallites of ≈1 nm in diameter with inclusion of smaller-sized particles (D < 1 nm) are observed on Θ-Al₂O₃ after Co deposition at room temperature. After annealing the Co nano-clusters grow larger at expense of small particles (D ≈ 3 nm), where the [1 1 0] and [−1 1 0] axis of the Co(0 0 1) facets are parallel to the [1 0 0] and [0 1 0] directions of (0 0 1)oxide, respectively. The in-plane lattice constant of Co clusters is ca. 4% larger than that of bulk Co, yielding less strain at the Co/oxide interface. A 15° ± 10% random orientation of the normal to (0 0 1) facet of Co clusters with respect to (0 0 1)oxide surface was deduced from the “arc”-shape reflection spots in RHEED. These results suggest that both orientation and phase of Co clusters are strongly affected by the nature and structure of oxide surface.     

The structure of the surface compound CrN formed by cosegregation on a Fe–15%Cr–N(100) single crystal surface

Cosegregation is known to cause the formation of two-dimensional chemical compounds (surface compounds) which can be epitaxed to substrate surfaces of a suitable structure. In the present work the cosegregation-induced formation of the CrN surface compound on nitrided Fe–15%Cr–N(100) single crystal surfaces was studied by means of Auger electron spectroscopy and low-energy electron diffraction. Intensity versus energy spectra (I(E)) were measured and analysed fully dynamically to investigate the structural details of the CrN surface compound. It is found that nitrogen is segregated to the surface forming the sample's top layer and substantial amounts of chromium are cosegregated with nitrogen. Nitrogen atoms reside in four-fold symmetric hollow sites about 0.1 Å above the metallic substrate. There is a huge relative expansion of the distance between the first and second metal atom layers (Δd₁₂/d₀≈26%), while the distances between deeper layers are almost bulk-like. The small distance between the nitrogen and the top metal layer as well as the huge layer expansion Δd₁₂/d₀ are in agreement with results found for N/Cr(100).     

Growth and structure of vanadium oxide on anatase (1 0 1) terraces

The interaction of vanadium oxide with epitaxial anatase films exposing (1 0 1) terraces was characterized. The TiO₂ films were grown on vicinal LaAlO₃ (1 1 0) substrates by oxygen plasma-assisted molecular beam epitaxy (OPA-MBE); reflection high energy and low energy electron diffraction (RHEED and LEED) indicated that the films exposed (1 0 1) terraces of the anatase TiO₂ polymorph. When a vanadium oxide monolayer was deposited onto the anatase surface by OPA-MBE at 725 K, only (1 × 1) RHEED and LEED patterns were observed. The V X-ray photoelectron spectroscopy (XPS) peak intensities indicated that the monolayer wetted the anatase surface and so the diffraction patterns were attributed to an epitaxial vanadia layer. Analysis of the vanadium oxide monolayer by X-ray and ultraviolet photoelectron spectroscopies revealed that the V was predominantly 5+. When the vanadia coverage was increased at 725 K, Auger electron spectra showed only very slow attenuation of the anatase Ti peaks while spots began to develop in RHEED patterns recorded along the LaAlO₃ direction; both indicative of 3-D cluster formation. In the orthogonal direction, the RHEED patterns showed unusual diagonal streaks. Meanwhile, the (1 × 1) LEED pattern persisted even after 30 nm of vanadia was deposited. This was attributed to gaps between the 3-D clusters exposing the epitaxial monolayer. Core level XPS spectra of the 3-D clusters revealed a broad V 2p_3/2 peak that was centered at the position expected for V⁴⁺ but could be deconvoluted into three peaks corresponding to V³⁺, V⁴⁺, and V⁵⁺. It is shown that crystallographic shear that accommodates such variations in the oxygen content of V oxides can lead to the diagonal streaks in RHEED patterns recorded along the LaAlO₃ [0 0 1] direction even as the pattern in the orthogonal direction shows sharp transmission spots. The results show that vanadia growth on anatase (1 0 1) proceeds through the Stranski-Krastanov mode with a strong vanadia-titania interaction stabilizing a dispersed vanadia monolayer. The results are compared with previous data for vanadia growth on anatase (0 0 1) where smooth, epitaxial VO₂ films grow ad infinitum.     

RHEED study of the (1 1 0) cleavage surface of CdTe:Cr single crystals

The structure of (1 1 0) plane of Cr-doped CdTe single crystals has been studied by reflection high energy electron diffraction and scanning electron microscopy. Diffraction patterns consist of diffraction spots and Kikuchi lines. However, for very small incident angle, the Debye rings are observed. The constant lattice attributed to these rings is 0.8% less then for pure CdTe. These anomalous properties of the near surface layer are likely to occur due to the concentration of Cr atoms creating compressive surface strains or the effect of crystal cleavage.     

Surface structures of K on Mo(1 1 0) surface investigated by RHEED

Phase transitions of K on Mo(1 1 0) have been studied by RHEED technique. As Ba and Cs structures on the bcc(1 1 0) surface, surface structures of K were hexagonal from RT to 250 °C for θ > 0.9 ML. The hexagonal structure successively expanded from α to γ structure with Nishiyama-Wassermamm (N-W) orientation relationship. The nearest neighbor spacing in the α structure at RT was 4.50 Å, which is very closed to the atomic distance of K in metal, and stretched to 5.14 Å in the γ structure at T = 200 °C. At temperatures greater than T = 250 °C, the γ structure oriented in N-W and Kurdjumov-Sachs (K-S) relationships at the same time and stayed up to the temperature of 450 °C. These two orientations of γ structure also appeared in all temperature range for 0.4 < θ < 0.9 ML.     

Structure and composition of the NiPd(1 1 0) surface

The NiPd(1 1 0) alloy surface was studied using low energy electron diffraction to measure the structure and composition of the first three atomic layers. The surface layer is highly enriched in Pd and has a significantly buckled structure. The second layer is also buckled, with displacements even larger than the surface layer. The second layer also exhibits intralayer segregation (chemical ordering), with alternate close-packed rows of atoms being Ni enriched and Pd enriched. The third layer has a structure and composition close to that of the bulk alloy. These results are compared with results for the other low index faces of NiPd, the extensive literature on NiPt alloy surfaces, and the growing body of theoretical literature for NiPd alloy surfaces.     

The epitaxial growth of Pd electrodeposition on Au(1 0 0) studied by LEED and RHEED

The epitaxial growth of Pd adlayers electrochemically deposited onto Au(1 0 0) has been studied by LEED, RHEED and AES. For the first 6 ML, the Pd deposits grow pseudomorphically on Au(1 0 0) with a lateral expansion of 4.5% with respect to bulk Pd. The strain in the expanded commensurate (1 × 1) Pd layers on Au(1 0 0) begins to be relieved at the Pd coverage between 6 and 9 ML range by formation of a compressed Pd film with respect to Au(1 0 0) surface and the compression increases continuously with thickness. At ca. 20 ML Pd the lattice constant of the film approaches to the bulk Pd and three-dimensional Pd islands develop since around 30 ML coverage. No superstructure due to the Pd-Au surface alloy can be found for coverages from monolayer up to 30 ML Pd on Au(1 0 0). A c(2 × 2) phase has been observed on the Pd-deposited Au(1 0 0) electrodes, which is ascribed to an ordered Cl adlayers adsorbed on Pd adlayers rather than a Pd-Au surface alloy.     

Structure and electronic properties of Fe nanostructures on MgO(0 0 1)

We present and discuss X-ray absorption spectroscopy and resonant photoemission measurements on Fe nanostructures self-assembled on MgO(0 0 1). For Fe coverages below 1 ML equivalent we measured an increase of the Fe L₂₃ branching ratio and changes in the splitting, widths and relative intensities of the different final states in the L₃M₂₃M₂₃ resonant Auger peak. Scanning tunnelling microscopy indicates that the average lateral dimensions of the self-aggregated structures decrease with decreasing Fe amount, from 12 nm at 15 ML nominal Fe amount to 5 nm at 2 ML Fe. This observation allows to interpret the observed changes in the 3d band electronic properties in terms of the evolution of the Fe local atomic coordination from a bulk-like situation to a configuration where low dimensionality effects are significant.     

Variety of iron silicides grown on Si(0 0 1) surfaces by solid phase epitaxy: Schematic phase diagram

We systematically studied the formation of various iron-silicide phases, grown on Si(0 0 1) surfaces by solid phase epitaxy, with scanning tunneling microscopy, low-energy electron diffraction and reflection high-energy electron diffraction. We found and studied the phases of c(2 × 2) islands, rectangle-like islands, elongated islands, layered islands, dome-like islands, eddy and cracked structures, and small clusters. A schematic phase diagram of these phases is successfully summarized against iron coverage at room temperature and subsequent annealing temperature.     

Well-ordered (1 0 0) InAs surfaces using wet chemical treatments

Atomic ordering of HCl-isopropanol (HCl-iPA) treated and vacuum annealed (1 0 0) InAs surfaces was studied by scanning tunneling microscopy (STM), low-energy electron diffraction (LEED), and reflectance anisotropy spectroscopy (RAS). On the as-treated surface, a diffused (1 × 1) pattern is observed, which successively evolves to the β₂(2 × 4)/c(2 × 8) and (4 × 2)/c(8 × 2) ones after annealing to 330 °C and 410 °C, respectively. At the intermediate temperature of 370 °C, an ₂(2 × 4)/(4 × 2) mixed reconstruction is observed. Reflectance anisotropy spectra are compared with those of the corresponding reconstructions observed after As-decapping and found to be quite similar. Therefore we conclude that high-quality (1 0 0) InAs surfaces can be obtained by wet chemical treatment in an easy, inexpensive and practical way.     

Epitaxial growth of MnO/Ag(0 0 1) films

Among the 3d transition metal monoxides, MnO plays a particular role due to the high spin ground state of the 3d⁵ configuration of Mn²⁺. Therefore, MnO could be an excellent model system to investigate many kinds of spin dependent or magnetic interactions between electrons in the surface of an antiferromagnetic solid. But in practice, most experimental investigations on crystalline MnO(0 0 1) surfaces suffer from the bad structural quality of the surface of cleaved bulk crystals. In this paper, we report on the epitaxial growth of MnO/Ag(0 0 1) surfaces, which is proved to be a method to produce crystalline MnO(0 0 1) surfaces of high structural order.     

Alloy formation of Co/Ni/Pt(1 1 1)

The initial growth and alloy formation of ultrathin Co films deposited on 1 ML Ni/Pt(1 1 1) were investigated by Auger electron spectroscopy (AES), low energy electron diffraction (LEED), and ultraviolet photoelectron spectroscopy (UPS). A sequence of samples of d_Co Co/1 ML Ni/Pt(1 1 1) (d_Co = 1, 2, and 3 ML) were prepared at room temperature, and then heated up to investigate the diffusion process. The Co and Ni atoms intermix at lower annealing temperature, and Co-Ni intermixing layer diffuses into the Pt substrate to form Ni-Co-Pt alloys at higher annealing temperature. The diffusion temperatures are Co coverage dependent. The evolution of UPS with annealing temperatures also shows the formation of surface alloys. Some interesting LEED patterns of 1 ML Co/1 ML Ni/Pt(1 1 1) show the formation of ordered alloys at different annealing temperature ranges. Further studies in the Curie temperature and concentration analysis, show that the ordered alloys corresponding to different LEED patterns are Ni_xCo_1−xPt and Ni_xCo_1−xPt₃. The relationship between the interface structure and magnetic properties was investigated.     

Anisotropic kinetics on growing Ge(0 0 1) surfaces

Reflection high-energy electron diffraction (RHEED), reflectance difference spectroscopy (RDS), and scanning tunneling microscopy (STM) have been used to study the anisotropic kinetics on the growing Ge(0 0 1) surface. While switching of dimer direction in alternate (2 × 1)/(1 × 2) layers causes the bilayer-period oscillations in RD response, RHEED oscillations are governed by variations in surface step densities. We show that the RHEED oscillations are strongly affected by the growth front morphology: when the growth front becomes distributed over several layers, the transition from bilayer- to monolayer-period occurs in RHEED oscillations.     

Oxidation and thermal reduction of the Cu(1 0 0) surface as studied using positron annihilation induced Auger electron spectroscopy (PAES)

Changes in the surface of an oxidized Cu(1 0 0) single crystal resulting from vacuum annealing have been investigated using positron annihilation induced Auger electron spectroscopy (PAES). PAES measurements show a large increase in the intensity of the annihilation induced Cu M_2,3VV Auger peak as the sample is subjected to a series of isochronal anneals in vacuum up to annealing temperature 300 °C. The intensity then decreases monotonically as the annealing temperature is increased to ∼600 °C. Experimental probabilities of annihilation of surface-trapped positrons with Cu 3p and O 1s core-level electrons are estimated from the measured intensities of the positron annihilation induced Cu M_2,3VV and O KLL Auger transitions. Experimental PAES results are analyzed by performing calculations of positron surface states and annihilation probabilities of surface-trapped positrons with relevant core electrons taking into account the charge redistribution at the surface, surface reconstructions, and electron-positron correlations effects. The effects of oxygen adsorption on localization of positron surface state wave function and annihilation characteristics are also analyzed. Possible explanation is proposed for the observed behavior of the intensity of positron annihilation induced Cu M_2,3VV and O KLL Auger peaks and probabilities of annihilation of surface-trapped positrons with Cu 3p and O 1s core-level electrons with changes of the annealing temperature.     

Oxygen-induced nano-faceting of Ir(2 1 0)

The adsorption of oxygen and the nanometer-scale faceting induced by oxygen have been studied on Ir(2 1 0). Oxygen is found to chemisorb dissociatively on Ir(2 1 0) at room temperature. The molecular desorption process is complex, as revealed by a detailed kinetic analysis of desorption spectra. Pyramid-shaped facets with {3 1 1} and (1 1 0) orientations are formed on the oxygen-covered Ir(2 1 0) surface when annealed to T?600 K. The surface remains faceted for substrate temperatures T<850 K. For T>850 K, the substrate structure reverts to the oxygen-covered (2 1 0) planar state and does so reversibly, provided that oxygen is not lost due to desorption or via chemical reactions upon which the planar (2 1 0) structure remains. A clean faceted surface was prepared through the use of low temperature surface cleaning methods: using CO oxidation, or reaction of H₂ to form H₂O, oxygen can be removed from the surface while preserving (“freezing”) the faceted structure. The resulting clean faceted surface remains stable for T<600 K. For temperatures above this value, the surface irreversibly relaxes to the planar state.     

Reactivity to sulphur of clean and pre-oxidised Cu(1 1 1) surfaces

The reactivity of clean and pre-oxidised Cu(1 1 1) surfaces exposed to sulphur (H₂S) has been studied at room temperature by Auger electron spectroscopy, low energy electron diffraction and scanning tunneling microscopy. On the clean surface, the sulphur-saturated surface structure is dominated by the or so-called “zigzag” superstructure. It is shown that a single orientation domain is favoured by the slight misorientation (∼2°) of the surface with respect to the (1 1 1) plane. Scanning tunneling microscopy measurements also revealed two minority structures. Pre-oxidation was performed by exposure to 1.5 × 10⁴ L of O₂ at 300 °C. Under exposure to H₂S (1 × 10⁻⁷ mbar) at room temperature, the oxygen is totally substituted by sulphur. Once initiated, sulphur adsorption seems to propagate to cover the whole surface on the O-covered surface faster than on the clean Cu(1 1 1). At saturation by adsorbed sulphur, the surface is completely covered by the superstructure of highest coverage. This enhanced uptake of sulphur is assigned to the surface reconstruction of the copper surface induced by the pre-oxidation, causing a stronger reactivity of the Cu atoms released by the decomposition of the oxide.