Systematic studies on surface modifications by ARUPS on Shockley-type surface states

The quasi two-dimensional surface state on noble metal (1 1 1)-surfaces can be used as a sensitive probe for different surface modifications, adsorption processes, and interactions between adsorbate and substrate. Already one monolayer of physisorbed Xe on Au(1 1 1) is responsible for a characteristic shift of the Shockley state towards the Fermi level and the surface state experiences an increase in spin-orbit splitting of up to 35%. In contrast to the physisorption process of rare gases, a sub-monolayer coverage of an alkali metal, e.g., Na on Au(1 1 1), has the opposite effect on the Shockley state, i.e. an increase in binding energy, until it reaches the bottom of the L-gap and vanishes into the bulk states. Additionally, we studied the intermetallic system Ag/Au(1 1 1) which differs substantially from the other systems because of the similarity in the electronic structure between substrate and overlayer.     

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.     

Correlating STM contrast and atomic-scale structure by chemical modification: Vacancy dislocation loops on FeO/Pt(1 1 1)

By chemically modifying the FeO(1 1 1) thin film on Pt(1 1 1), we show that it is possible to unambiguously correlate its STM morphology with its underlying structure without recourse to STM simulations. Partial reduction of the oxide surface leads to the formation of triangularly-shaped oxygen vacancy dislocation loops at specific sites in the moiré structure of the film. Their presence allows unambiguous identification of the high-symmetry domains of the moiré structure, whose differing chemical properties govern the templating effect on adsorbed metal atoms, clusters and molecules.     

Structure and reactivity of Pd doped Ag surfaces

We have performed first principles calculations for clean and Pd doped Ag(1 1 1) and Ag(1 0 0) surfaces, with and without adsorbed O and CO. Our results for the structure of the Pd doped Ag surfaces indicate that Pd atoms are located lower than the surrounding Ag surface atoms. We find that O atoms adsorbed on Pd doped Ag(1 1 1) reside at the fcc hollow sites, the site next to Pd being slightly favored. Moreover, we provide results for O and CO co-adsorption on the clean and Pd doped Ag(1 1 1) surfaces, indicating that Pd can act as an electronic promoter for the CO oxidation reaction.     

Herringbone and triangular patterns of dislocations in Ag, Au, and AgAu alloy films on Ru(0 0 0 1)

We have studied the dislocation structures that occur in films of Ag, Au, and Ag_0.5Au_0.5 alloy on a Ru(0 0 0 1) substrate. Monolayer (ML) films form herringbone phases while films two or more layers thick contain triangular patterns of dislocations. We use scanning tunneling microscopy (STM) and low-energy electron diffraction (LEED) to determine how the film composition affects the structure and periodicity of these ordered structures. One layer of Ag forms two different herringbone phases depending on the exact Ag coverage and temperature. Low-energy electron microscopy (LEEM) establishes that a reversible, first-order phase transition occurs between these two phases at a certain temperature. We critically compare our 1 ML Ag structures to conflicting results from an X-ray scattering study [H. Zajonz et al., Phys. Rev. B 67 (2003) 155417]. Unlike Ag, the herringbone phases of Au and AgAu alloy are independent of the exact film coverage. For two layer films in all three systems, none of the dislocations in the triangular networks thread into the second film layer. In all three systems, the in-plane atomic spacing of the second film layer is nearly the same as in the bulk. Film composition does, however, affect the details of the two layer structures. Ag and Au films form interconnected networks of dislocations, which we refer to as “trigons.” In 2 ML AgAu alloy, the dislocations form a different triangular network that shares features of both trigon and moiré structures. Yet another well-ordered structure, with square symmetry, forms at the boundaries of translational trigon domains in 2 ML Ag films but not in Au films.     

Photoemission study of glycine adsorption on Cu/Au(1 1 1) interfaces

The adsorption of glycine on Au(1 1 1) pre-deposited with different amounts of Cu was investigated with both conventional X-ray photoelectron spectroscopy (XPS) and synchrotron-based photoemission. In the Cu submonolayer range, glycine physically adsorbs on the Cu/Au(1 1 1) surfaces in its zwitterionic form and completely desorbs at 350 K. The C 1s, O 1s and N 1s core level binding energies monotonically increase with Cu coverage. This indicates that, in the Cu submonolayer range, the admetal is alloyed with Au rather than forming overlayers on the Au(1 1 1) substrate, consistent with our recent experimental and theoretical results [X. Zhao, P. Liu, J. Hrbek, J.A. Rodriguez, M. Pérez, Surf. Sci. 592 (2005) 25]. Upon increasing the amount of deposited Cu over 1 ML, part of the glycine overlayer transforms from the zwitterionic form to the anionic form (NH₂CH₂COO⁻) and adsorbs chemically on the Cu/Au(1 1 1) surface with the N 1s binding energy shifted by −2.3 eV. When the amount of deposited Cu is at 3.0 or 6.0 ML, the intensity of the N 1s chemisorption peak increases with aging time at 300 K. It indicates that glycine adsorption induces Cu segregation from the subsurface region onto the top layer of the substrate. Judging from the initial N 1s peak intensities, it is concluded that 64% and 36% of the top layer are still occupied by Au atoms before glycine adsorption even when the amounts of deposited Cu are 3.0 and 6.0 ML, respectively. On the Au(1 1 1) surface pre-dosed with 6.0 ML of Cu, part of the chemisorbed glycine will desorb and part will decompose upon heating to 450-500 K. In addition, about 20% of the glycine exists in the neutral form when the glycine overlayer was dosed on Cu/Au(1 1 1) held at 100 K.     

Formation, stability and CO adsorption properties of PdAg/Pd(1 1 1) surface alloys

The formation, stability and CO adsorption properties of PdAg/Pd(1 1 1) surface alloys were investigated by X-ray photoelectron spectroscopy (XPS) and by adsorption of CO probe molecules, which was characterized by temperature-programmed desorption (TPD) and high resolution electron energy loss spectroscopy (HREELS). The PdAg/Pd(1 1 1) surface alloys were prepared by annealing (partly) Ag film covered Pd(1 1 1) surfaces, where the Ag films were deposited at room temperature. Surface alloy formation leads to a modification of the electronic properties, evidenced by core-level shifts (CLSs) of both the Pd(3d) and Ag(3d) signal, with the extent of the CLSs depending on both initial Ag coverage and annealing temperature. The role of Ag pre-coverage and annealing temperature on surface alloy formation is elucidated. For a monolayer Ag covered Pd(1 1 1) surface, surface alloy formation starts at ∼450 K, and the resulting surface alloy is stable upon annealing at temperatures between 600 and 800 K. CO TPD and HREELS measurements demonstrate that at 120 K CO is exclusively adsorbed on Pd surface atoms/Pd sites of the bimetallic surfaces, and that the CO adsorption behavior is dominated by geometric ensemble effects, with adsorption on threefold hollow Pd₃ sites being more stable than on Pd₂ bridge sites and finally Pd₁ a-top sites.     

Broadening mechanisms for the adsorbate-induced resonance in the Na/Cu(1 1 1) and Cs/Cu(1 1 1) systems

Adsorption of alkali atoms on the (1 1 1) and (1 0 0) noble metal surfaces has been shown recently to induce long-lived resonances located inside the surface projected band gap. However, the width of these resonances, as it appears in two-photon photo-emission experiments, is much larger than the inverse of their lifetime. We report on a theoretical study of some broadening mechanisms of these resonance lines in the Na/Cu(1 1 1) and Cs/Cu(1 1 1) systems at low coverage, including the homogeneous natural line broadening and the inhomogeneous statistical broadenings due to the distribution of adsorption heights associated to the quantal vibration of the alkali adsorbate and to the lateral disorder of alkali adsorption on the surface. The inhomogeneous mechanisms are shown to induce a very large broadening of the resonance line, in quantitative agreement with experimental results. The most important broadening effect appears to be the effect of the distribution of alkali adsorption heights.     

Surface alloying of immiscible metals induced by surface state shift

By using first principles calculations, it is found that the noble metal atoms Ag, Au and Cu would like to occupy the vacancy sites of the W(0 0 1) or Mo(0 0 1) surface to form the substitutional surface alloys, despite the fact that they do not like to form alloy in the bulk. The electronic local function (ELF) for these substitutional surface alloys shows that there is no obvious chemical bonding between the noble metals and W or Mo. The analysis of electronic structures lets us conclude that the surface alloying of immiscible metals may originate from the surface state shift of W (or Mo) induced by changes of the electronic environment of surface W (or Mo) when surface W (or Mo) atoms are alternatively replaced by Ag (Au or Cu).     

Lateral and vertical manipulations of single atoms on the Ag(1 1 1) surface with the copper single-atom and trimer-apex tips

We study the lateral and vertical manipulations of single Ag and Cu atoms on the Ag(1 1 1) surface with the Cu single-atom and trimer-apex tips using molecular statics simulations. The reliability of the lateral manipulation with the Cu single-atom tip is investigated, and compared with that for the Ag tips. We find that overall the manipulation reliability (MR) increases with the decreasing tip height, and in a wide tip-height range the MR is better than those for both the Ag single-atom and trimer-apex tips. This is due to the stronger attractive force of the Cu tip and its better stability against the interactions with the Ag surface. With the Cu trimer-apex tip, the single Ag and Cu adatoms can be picked up from the flat Ag(1 1 1) surface, and moreover a reversible vertical manipulation of single Ag atoms on the stepped Ag(1 1 1) surface is possible, suggesting a method to modify two-dimensional Ag nanostructures on the Ag(1 1 1) surface with the Cu trimer-apex tip.     

The adsorption and stability of sulfur containing amino acids on Cu{5 3 1}

The adsorption of l-cysteine and l-methionine amino acids on a chiral Cu{5 3 1} surface was investigated with high resolution X-ray photoelectron spectroscopy (XPS) and carbon K-edge near edge X-ray absorption fine structure (NEXAFS) Spectroscopy using synchrotron radiation. XPS shows that at 300 K l-cysteine adsorbs through two oxygen, a nitrogen and a sulfur atom, in a four point ‘quadrangular footprint’, whereas l-methionine adsorbs through only two oxygen and a nitrogen atom in a ‘triangular footprint’. NEXAFS was used to clarify the adsorption geometry of both molecules, which suggests a binding orientation to the top layer and second layer atoms in two different orientations associated with adsorption sites on {1 1 0} and {3 1 1} microfacets of the Cu{5 3 1} surface.     

Electron and lattice structure of ultra thin Ag films on Si(1 1 1) and Si(0 0 1)

We studied the low temperature (T ? 130 K) growth of Ag on Si(0 0 1) and Si(1 1 1) flat surfaces prepared by Si homo epitaxy with the aim to achieve thin metallic films. The band structure and morphology of the Ag overlayers have been investigated by means of XPS, UPS, LEED, STM and STS. Surprisingly a (√3 × √3)R30° LEED structure for Ag films has been observed after deposition of 2-6 ML Ag onto a Si(1 1 1)(√3 × √3)R30°Ag surface at low temperatures. XPS investigations showed that these films are solid, and UPS measurements indicate that they are metallic. However, after closer STM studies we found that these films consists of sharp Ag islands and (√3 × √3)R30°Ag flat terraces in between. On Si(0 0 1) the low-temperature deposition yields an epitaxial growth of Ag on clean Si(0 0 1)-2 × 1 with a twinned Ag(1 1 1) structure at coverage’s as low as 10 ML. Furthermore the conductivity of few monolayer Ag films on Si(1 0 0) surfaces has been studied as a function of temperature (40-300 K).     

Electrical conductance at initial stage in epitaxial growth of Pb, Ag, Au and In on modified Si(1 1 1) surface

The electronic properties of thin metallic films of Pb, Ag, Au and In atoms deposited at 105 K on well defined metallic surface, i.e. Si(1 1 1)-(6 × 6)Au surface with 10 ML of annealed Pb, were investigated using four-point probe method in UHV condition. The structure of the substrate and deposited metals were monitored by the RHEED system. The electrical conductance, measured during the deposition of In and Pb atoms, shows the local minimum for the coverage equals about 0.3 ML whereas for Au and Ag atoms the conductance decreases during the first monolayer growth. For Au atoms the local maximum in the conductance was observed for the coverage about 0.55 ML, which can be connected with localized states. To describe theoretically the conductance behavior the tight-binding Hamiltonian and equation of motion for the Green’s function were used and good qualitative agreement was obtained.     

Structural investigation of the c(√2 × 5√2)R45° surface alloy formed by Ti deposition on Cu(0 0 1)

The alloying process of Ti deposited on Cu(0 0 1) was studied by means of XPS, LEIS, XPD and LEED intensity analysis. With the sample held at 570 K, a linear decrease of the Cu LEIS signal as a function of the amount of Ti deposited is observed in the early stages of deposition until a constant value is reached. At the onset of the plateau a c(√2 × 5√2)R45° LEED pattern starts to be visible. XPD and LEED intensity measurements were performed for the c(√2 × 5√2)R45° phase prepared depositing ca. 1.5 monolayer of Ti. The angle-scanned XPD curves measured for the phase c(√2 × 5√2)R45° reveal that Ti atoms substitute Cu atoms in the fcc lattice of the substrate. The polar XPD curves show that at least the first four layers of the substrate are involved in the alloying process. We found that the (3 1 0) plane of the Cu₄Ti alloy (D1_a type-structure) fits, without significant contraction or expansion of the lattice parameters, the c(√2 × 5√2)R45° structure. The intensity versus energy curves of the diffracted beams were calculated on the basis of this structural model using the tensor LEED method. The results of the LEED intensity analysis provide a further evidence of the formation of a slab of Cu₄Ti(3 1 0) layers.     

Alloy formation in the Au{1 1 1}/Ni system - An investigation with scanning tunnelling microscopy and medium energy ion scattering

Using the techniques of scanning tunnelling microscopy (STM) and medium energy ion scattering (MEIS), we examine the growth and annealing behaviour of ultrathin Ni films on Au{1 1 1} at 300 K. As has been shown previously, submonolayer growth of Ni on Au{1 1 1} is strongly influenced by the presence of the herringbone reconstruction with two-dimensional clusters nucleating at herringbone elbows. Second layer growth commences prior to the completion of the monolayer. After multiple layers have been deposited, the surface morphology retains a similar cluster-like appearance. Annealing produces surfaces exhibiting long range Moiré structures and, at higher temperature, triangular misfit dislocations. We use MEIS to examine the composition and structure of these surface alloy phases and conclude that in each case, they consist of an essentially pure Au surface layer on a bimetallic second layer.     

Study on the adsorption of fluorescein on Ag(1 1 0) substrate

The adsorption of fluorescein on the Ag(1 1 0) surface has been investigated by the first-principles pseudopotential method. Various adsorption geometries have been calculated and the energetically most favorable structure of fluorescein/Ag(1 1 0) was identified. The fluorescein molecule, in most favorable structure, is on hollow site, and the adsorption energy is 2.34 eV. Here the adsorption sites refer to the positions at the first layer of the substrate where the middle carbon atom of the fluorescein molecule is located. The bonding strength of the fluorescein molecule to the Ag substrate is site selective, being determined by electron transfer to the oxygen atoms of the molecule and local electrostatic attraction between the oxygen atoms and the silver atoms.     

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.     

Structural analysis of Si(1 1 1)-√21 × √21-Ag surface by reflection high-energy positron diffraction

The atomic structure of Si(1 1 1)-√21 × √21-Ag surface, which is formed by the adsorption of small amount of Ag atoms on the Si(1 1 1)-√3 × √3-Ag surface, was determined by using reflection high-energy positron diffraction. The rocking curve measured from the Si(1 1 1)-√21 × √21-Ag surface was analyzed by means of the intensity calculations based on the dynamical diffraction theory. The adatom height of the extra Ag atoms from the underlying Ag layer was determined to be 0.53 Å with a coverage of 0.14 ML, which corresponds to three atoms in the √21 × √21 unit cell. From the pattern analyses, the most appropriate adsorption sites of the extra Ag atoms were proposed.     

Surface structures formed by individual adsorption and coadsorption of Mn and Bi on Cu(0 0 1), studied by LEED

We have studied the individual adsorption of Mn and Bi, and their coadsorption on Cu(0 0 1) by low-energy electron diffraction (LEED). For Mn, we have determined the c(2 × 2) structure formed at 300 K, whose structure had been determined by several methods. We reconfirmed by a tensor LEED analysis that it is a substitutional structure and that a previously reported large corrugation (0.30 Å) between substitutional Mn and remaining surface Cu atoms coincides perfectly with the present value. In the individual adsorption of Bi, we have found a c(4 × 2) structure, which is formed by cooling below ∼250 K a surface prepared by Bi deposition of ∼0.25 ML coverage at 300 K where streaky half-order LEED spots appear. The c(4 × 2) structure has been determined by the tensor LEED analysis at 130 K and it is a substitutional structure. In the coadsorption, we found a c(6 × 4) structure, which has been determined by the tensor LEED analysis. It is very similar to the previously determined structure of the c(6 × 4) formed by coadsorption of Mg and Bi, and embedded MnBi₄ clusters are arranged in the top Cu layer instead of MgBi₄. Large lateral displacements of Bi atoms in the c(6 × 4)-(Mn + Bi) suggest that the Mn atoms undergo the size-enhancement caused by their large magnetic moment.