ML Au Research Articles

The influence of the interlayer exchange coupling on the magnetism of an Fe(0 0 1) monolayer

We investigated changes in the magnetic properties of a single Fe monolayer on Au(0 0 1) induced by the interlayer exchange coupling (IEC) to FeAu monoatomic superlattices. The grazing incidence nuclear resonant scattering of X-rays (GI-NRS) combined with the 57Fe probe layer concept allowed us to selectively monitor the local structure and magnetism of the Fe(0 0 1) monolayer in a coupled state. The dependence of the monolayer hyperfine parameters on temperature and spacer thickness was determined from the fitted GI-NRS time spectra, collected for the selected thickness of the stepped Au spacer. The influence of the coupling on the hyperfine magnetic field B HF was negligible at temperatures much lower than T C of the uncoupled Fe monolayer ( T C = 210 K as checked by MOKE). The analysis of the time-spectra accumulated at 200 K showed a non-monotonous dependence of the average B HF as a function of the spacer thickness, related to IEC oscillations. The maximum influence of IEC on B HF (13 T) was found for the 4 ML Au spacer.

Au-Induced Nanostructuring of Vicinal Si Surfaces

We have investigated the Au-induced nanofaceting of vicinal silicon surfaces tilted from [111] towards [112̅]. Using samples miscut 3.8°, and 8°, from [111] respectively we have used scanning tunnelling microscopy (STM) to measure the surface morphology as a function of Au coverage (0.04 to 0.44 ML). As expected, Au adsorption produces dramatic changes in surface morphology on both samples. On the 3.8° sample we find that as little as 0.04 ML of Au is sufficient to remove the faceting present on the clean surface. As more Au is deposited 1-dimensional chain structures nucleate at step edges. These chain structures eventually grow to form (775)-Au facets. At 0.17 ML we observe a surface with Si(111)7 × 7 terraces and (775)-Au nanofacets. With more Au, the (111) terraces transform from a 7 × 7 to a 5 × 2 reconstruction and at 0.4 ML the sample consists of Si(111)5 × 2-Au terraces separated by (775)-Au facets. The morphology of the 8° sample also depends critically on Au coverage. Below 0.32 ML all 8° surfaces include (775)-Au nanofacets. Above 0.32 ML, the (775)-Au facet is no longer stable and Au is accommodated on the surface via the formation of higher angle facets with smaller chain spacing. In both samples, the persistence of the (775)-Au facet reinforces the idea that it represents a low energy facet on these Au modified vicinal surfaces.

Reactivity of Molecularly Chemisorbed Oxygen on a Au/TiO2 Model Catalyst

We present results of an investigation into the reactivity of molecularly chemisorbed oxygen with CO on a Au/TiO2 model catalyst at 77 K. We previously discovered that exposing the model catalyst sample to a radio-frequency-generated plasma jet of oxygen results in co-population of both atomically and molecularly chemisorbed oxygen species on the sample. We tested the reactivity of the molecularly chemisorbed oxygen by comparing the CO2 produced from a sample populated with both species to the CO2 produced from a sample that has been cleared of molecularly chemisorbed oxygen employing collision-induced desorption. Samples that are populated with both species consistently result in greater CO2 produced than samples with only atomic oxygen. We interpret this result to indicate that molecularly chemisorbed oxygen on the sample can directly participate in the CO oxidation reaction. The reactivity of molecularly chemisorbed oxygen has been investigated for five different gold coverages (0.5, 0.75, 1, 1.25, and 2 ML), and we observe that there is a greater fractional difference in the CO2 produced (difference between sample populated with both molecularly and atomically adsorbed oxygen and sample populated solely with atomically adsorbed oxygen) for the 1 ML Au coverage than for the other coverages for equivalent oxygen plasma-jet exposures. However, it is not possible to unambiguously conclude that this observation is directly related to a particle size effect on the chemistry since the absolute O(2,a) and O(a) content on the various surfaces is different for all the coverages studied because of the plasma-jet technique that we employed for populating the surfaces with oxygen. Unfortunately, this precludes a direct comparison of the reactivity of molecular oxygen in the carbon monoxide oxidation reaction as a function of gold coverage and hence particle size.

Growth and crystalline structure of Au layers on Ni(1 1 1)

We present the results of investigations concerning the growth and crystalline structure of Au layers deposited on the Ni(1 1 1) surface at RT obtained with the use of Auger electron spectroscopy (AES) and directional elastic peak electron spectroscopy (DEPES). For clean Ni(1 1 1) experimental DEPES profiles exhibit intensity maxima corresponding to the close packed rows of atoms. Theoretical DEPES scans were obtained with the use of single scattering cluster (SSC) calculations by means of phase shifts, inelastic mean free paths and scattering cross-sections. A good correspondence between experimental and theoretical scans was obtained for the [ 2 1 ¯ 1 ¯ ] and [ 1 1 ¯ 0 ] azimuths of the Ni(1 1 1) surface. Auger peak kinetics recorded during continuous Au deposition suggests the formation of a flat Au layer till 0.54 ML and then the simultaneous growth of a few layers. Very low intensity reflections and considerable background intensities were found in the LEED patterns, which indicate a weak long-range order in the adsorbate. DEPES profiles obtained for 8 ML of Au on Ni(1 1 1) reflect a short-range order in the adsorbate. Intensity maxima observed in DEPES profiles are characteristic of two mutually rotated Au(1 1 1) domains. Domain populations were found by an R-factor analysis giving n ¯ 1 = 49 ± 2 % for rotated and n ¯ 2 = 51 ± 2 % for unrotated domains.

Film growth arising from the deposition of Au onto ani-Al–Pd–Mn quasicrystal: a medium energy ion scattering study

The room temperature deposition of 7 ML of Au onto the fivefold symmetric surface oficosahedral Al–Pd–Mn leads to the formation of a several monolayers thick Au–Al alloyfilm. An AlAu film with 1:1 stoichiometry is formed, which shows no evidenceof ordered structure, being either amorphous or polycrystalline. Annealing to325 °C causes more Al to diffuse into the film, producingAl2Au but still with no indication of structure. Experiments using 0.5 ML of pre-deposited Indemonstrated a surfactant effect as the In ‘floated’ on the surface during growth andproduced a reduction in film roughness. However, contrary to previous findings the film wasstill either amorphous or polycrystalline, with no evidence of quasi-crystalline or aperiodicstructure. Experiments were also conducted using smaller doses of Au to look for theformation of an epitaxial layer and, if formed, determine the registry with the substrate.However, no change in the Pd blocking curves for the surface could be seen, suggesting thatthe Au does not adsorb in well defined sites. This result is not surprising whenconsidering that even for these low doses Al is drawn into the film, changing thecomposition and probably the structure of the topmost layers of the substrate,so that the potential adsorption sites on the clean surface may no longer exist.

Scanning tunneling microscopy study of Au growth on Ge(0 0 1): Bulk migration, self-organization, and clustering

Gold growth on Ge(0 0 1) was studied as a function of coverage and growth temperature using scanning tunneling microscopy (STM). Initial Au deposition at 475 K resulted only in two-dimensional Ge islands and numerous vacancies on the surface. The Ge islands were attributed to Ge atoms ejected onto the surface by Au moving sub-surface. At higher temperatures the vacancies ordered into (1 + 2 + 1) dimer vacancy complexes, and step structures not seen on clean Ge surfaces were observed. When more Au was deposited between 475 and 800 K, alternating bright and dim chains appeared on the surface that were attributed to Au–Au and Au–Ge dimer rows, respectively. An increase in the density of double-height steps was observed as the chains formed suggesting that the Ge atoms incorporated into the mixed dimer rows were extracted from step edges. Increasing the Au coverage further caused the chains to organize into well-ordered (4 × 2) domains. After depositing 1.5 ML of Au at 675 K the surface was completely covered by the chains. Similar chains have been observed during Pt growth on Ge but not during Ag growth, supporting the idea that the 5d metals favor the formation of one-dimensional chains. After the surface was covered with chains, additional Au aggregated into three-dimensional clusters. The smaller of these clusters exhibited a rectangular shape consistent with the formation of Au(1 1 0) clusters. The [1 1 0]-orientation was confirmed through atomic resolution imaging of the tops of larger clusters. Larger clusters, however, adopted a different shape—an asymmetric octagon—and had their top surfaces tilted with respect to the Ge(0 0 1) surface. The tilt was attributed to stress relief due to the lattice mismatch, which broke the equivalence between the surface energies of inclined facets on opposite sides of the cluster, accounting for the asymmetric shape. Annealing the Au-covered surfaces to 1000 K caused all of the Au to move into the bulk leaving a defective Ge surface.

The Interaction of Coinage Metal Clusters with the MgO(100) Surface.

The results of a systematic study of the interaction of small coinage metal clusters (Mn, n = 1-3) and extended deposition (one and two MLs; ML = monolayer) with the regular and locally defected (Fs center and divacancy) neutral MgO(100) surface are presented. The calculations have been performed at the DFT level employing plane waves as a basis set and using a gradient-corrected exchange-correlation functional (PW91). The adhesion energy along the group follows a trend that can be rationalized in terms of the strength and "stickiness" of the metallic bond, electrostatic polarization effects, and chemical interactions. Coinage metal dimers and trimers are absorbed on the regular surface in an upright position with little modification with respect to the gas-phase structure and can easily diffuse from site to site (in the case of trimers, also because of their fluxional character). In the case of extended deposition, the adhesion energy increases when passing from one to two MLs because of a "metal-on-top" stabilization mechanism. Neutral localized defects on the surface such as the Fs center (generated by a missing O atom) and the double vacancy (generated by a missing MgO dimer) act as strong trapping centers for small clusters and remarkably increase the adhesion of metal slabs to the surface in the case of extended deposition. At variance with the Fs center, the double vacancy induces a strong structural and energetic modification of the surrounding oxide lattice, varying also as a function of the metal deposition. A peculiar structural rearrangement consisting of the segregation of the metal slab into "islands" on the surface is observed in the case of one ML Cu and (to a lesser extent) Au.

Agglomeration, support effects, and CO adsorption on Au/TiO 2(1 1 0) prepared by ion beam deposition

The agglomeration behavior of 0.05 ML of Au deposited as Au + at 1 eV impact energy on rutile TiO 2(1 1 0) was investigated by a combination of ion scattering, X-ray photoelectron spectroscopy (XPS), CO adsorption, and CO temperature-programmed desorption (TPD). Samples were studied over the temperature range between 115 K and 800 K, and on both near-stoichiometric UHV-annealed TiO 2, and TiO 2 with a high density of oxygen vacancies created by He + bombardment. At low temperatures, Au is atomically dispersed, shifting into complexes with oxygen vacancies for T anneal around room temperature, and finally agglomerating into small clusters for T anneal ⩾ 450 K. CO adsorbed on dispersed atoms results in nearly 1 eV shift in the Au 4f binding energy to higher energy, and CO desorption peaking at 280 K. CO adsorbs more weakly on Au–vacancy complexes and on Au in clusters, as indicated by both TPD and XPS. Comparisons with experiments depositing thermal Au, and implications for size-selected CO oxidation on Au n /TiO 2 are discussed.

Surface structure of Au/InSb(0 0 1) system investigated with scanning force microscopy

Dynamic force microscopy and Kelvin probe force microscopy (KPFM) have been used to study 0.2 ML Au deposited on clean c(8×2) InSb(0 0 1) surface. Rectangular islands of typical size of 9 nm across have been observed. Upon annealing at 650 K these islands preserve their initial shape. KPFM has shown that the islands are made of material chemically different from that of the surrounding substrate surface. However, the change of LEED pattern to c(4×4) strongly suggests conversion of substrate surface between the islands from In-rich to Sb-rich probably due to alloying of surface indium with gold in the islands.

Ammonia synthesis on Au modified Fe(1 1 1) and Ag and Cu modified Fe(1 0 0) surfaces

In order to investigate any influence of steps and possible positive effects of making surface alloys the ammonia synthesis has been investigated over Au modified Fe(1 1 1) and Ag and Cu modified Fe(1 0 0) single crystals in the temperature range 603–773 K, using a system combining ultra-high vacuum and a high-pressure cell. Ammonia was synthesized from a stoichiometric (N 2:3H 2) gas mixture at a pressure of 2 bar. By deposition of small amounts of Au, the ammonia production activity of the Fe(1 1 1) surface can be enhanced. More important, for the gold modified surface, the reaction order in ammonia pressure is less negative. The activity goes through a maximum as the Au coverage increases, and then to zero as the coverage approaches 1 ML. At 713 K and 400 ppm NH 3, a maximum enhancement of 50% is observed at 0.04 ML Au. Based on features in the TPD spectra, the enhancement is suggested to be due to lifting of the nitrogen induced reconstruction of the Fe(1 1 1). On the Fe(1 0 0) surface strong effects were expected from modification with Cu and Ag since the in general more reactive atomic steps always present on such surfaces would be blocked. However, only minor and modest negative effects were found. The Cu and Ag were found to be inefficient step blockers as they were forced away from the surface by the formation of the much more stable surface nitrogen.

Synthesis and applications of poly(acryl p-aminobenzenesulfonamideamidine- p-aminobenzenesulfonylamide) chelating fiber for pre-concentrating and separating trace Bi(III), Hg(III), Au(III) and Pd(IV) from solution samples.

Poly(acryl p-aminobenzenesulfonamideamidine- p-aminobenzenesulfonylamide) chelating fiber containing "S", "N", and "O" elements was synthesized from polyacrylonitrile fiber and p-aminobenzene sulfonamide and used to enrich and separate trace Bi(III), Hg(III), Au(III), and Pd(IV) ions from wastewater and ore sample solution. The enrichment acidity, flow rate, elution conditions, reuse, interference ions, saturated adsorption capacity, constant of adsorption rate, analytical accuracy, and actual samples on chelating fiber were investigated by means of inductively coupled plasma optical emission spectrometry (ICP-OES) with satisfactory results. Solutions of 100 ng mL(-1) of Bi(III), Hg(III), Au(III), and Pd(IV) ions can be enriched quantitatively by this chelating fiber at a rate of 1.0 mL min(-1) at pH 4 and desorbed quantitatively with 20 mL of 0.25 M HCl and 2% CS(NH(2))(2) solution at 50 degrees C (with recovery >/=97%). When the chelating fiber was reused for 20 times, the recoveries of the analyzed ions enriched by the fiber were still over 95% (except for Hg(III)). One thousand-fold excesses of Mn(2+), Ca(2+), Zn(2+), Mg(2+), Fe(3+), Cu(2+), Ni(2+), Al(3+), and Ba(2+) ions and thousands-fold excesses of Na(+ )and K(+) cause little interference in the pre-concentration and determination of the analyzed ions. The saturated adsorption capacity of Bi(III), Hg(III), Au(III), and Pd(IV) was 4.850 x 10(-4), 3.235 x 10(-4), 2.807 x 10(-4), and 3.386 x 10(-4) mol g(-1), respectively. The constants of adsorption rate were 0.409 min(-1) for Bi, 0.122 min(-1) for Hg, 0.039 min(-1) for Au, and 0.080 min(-1 )for Pd. The relative standard deviations (RSDs) for the enrichment and determination of 10 ng mL(-1) Bi(III), Hg(III), Au(III), and Pd(IV) were lower than 2.3%. The results obtained for these ions in actual samples by this method were basically in agreement with the given values with average errors of less than 1.0%. FT-IR spectra shows that the existence of -SO(2)-Ar, -H(2)N-Ar, O=C-NH-, HN=C-NH-, and -HN-SO(2) functional groups are verified in the chelating fiber. From the FT-IR spectroscopy, we can see that Hg(III), Au(III), and Pd(IV) are mainly combined with nitrogen and sulfur (or oxygen), and Bi(III) is mainly combined with nitrogen (or oxygen) of the groups to form a chelating complex.

Metal epitaxy depending on the growth temperature during deposition

Time of flight impact-collision ion scattering (TOF-ICISS) has been used to investigate the heteroepitaxial growth of 3 ML of Au or Ag on Ni(1 1 1), and Ag on Cu(1 1 1) over a wide temperature range from 170 through 680 K. We found that two different types of epitaxial growth exist: M(1 1 1)[ 1 1 2 ̄ ]/Sub(1 1 1)[ 1 1 2 ̄ ] (normal mode) and M(1 1 1)[ 1 1 2 ]//Sub(1 1 1))[ 1 1 2 ̄ ] (reverse mode) (M represents Au or Ag, and Sub represents Ni or Cu, respectively). Moreover the relative amounts of these two growth modes show an observed oscillatory dependence on the growth temperature during metal deposition. It is our belief that this is previously unobserved phenomena. It is sufficiently general to encompass metal-on-metal systems.

A medium energy ion scattering study of metal-on-metal epitaxy and surfactant-mediated growth for the Au on Cu(111) system

The growth of Au on Cu(111) single crystal substrate has been investigated both with and without Sb surfactant. Room temperature deposition of approximately 10 ml of Au onto Cu(111) gave a film with twinned fcc structure, a reasonable degree of crystallinity and clear indications of inter-diffusion at the interface. Annealing this layer to progressively higher temperatures encouraged the diffusion of the Au into the substrate, thus reducing the strain arising from the lattice mismatch between Au and Cu. For a 400°C anneal the Au had almost entirely dissolved into the Cu substrate, such that no apparent twinning could be observed. The use of pre-deposited Sb as a surfactant in the growth of the Au layer was investigated for two levels, 0.2 and 0.5 ml, since above 0.33 ml Sb induces a stacking fault into the top most layer of Cu(111). For the lower coverage, similar results were observed in terms of the degree of crystalline order, but there was some indication that the twinning effect was partially suppressed. This effect was not observed for the higher Sb coverage. The compositional information as a function of depth indicated that for both coverages, the Sb remained at the surface. However, it was also evident that for the lower Sb coverage somewhat greater inter-diffusion between the Au and Cu occurred and it is possible that it is this effect that is actually responsible for the apparent reduction in twinning. These results indicate little benefit in the use of Sb as a surfactant for the epitaxial growth of Au on Cu(111).

Magnetic Phase Transition of [Fe/Au] Multilayers with an Ultra-thin Fe Layer

The magnetic phase transition of [m ML Fe/12.5 ML Au] 20 (m=0.7, 0.9, 1.1) multilayers was investigated. The quality of samples was well defined by RHEED observation and the 4 circle X-ray diffraction method. Perpendicular anisotropy was observed for samples thinner than 2ML Fe. The Curie temperature TC and a critical exponent β were precisely determined by the new method to evaluate the spontaneous magnetization from the measured magnetization as a function of temperature. The Curie temperature TC decreased with decreasing Fe thickness. The critical exponent β for an ultra-thin Fe layer of around 1 ML is similar to that for the Heisenberg model. The spacer layer of Au plays an important role in making the system's three-dimensional ferromagnetism.

Suppression of Nitrogen Oxide Dissociation by Gold on Pt(335)

Prototype NOx sensors for use in automotive exhaust can contain Au/Pt electrodes that dissociate O2 for removal by electrochemical pumping, but these electrodes should not dissociate NO. In these sensors, the NO diffuses to a second chamber where it is then detected. To understand the process that occurs at the Au/Pt electrode we have used temperature-programmed desorption to study the dissociation of NO and O2 on Pt(335) and on partially Au covered Pt(335). With a gold coverage of 0.15 ML, the dissociation probability of NO is decreased by a factor of 5 relative to bare Pt. Increasing Au coverage to 0.3 ML decreases NO dissociation to an undetectable level. At 0.3 ML Au, the saturation coverage of atomic O is reduced by only 20%. The variation of NO dissociation with Au coverage occurs in the range where all of the Au atoms are at steps, but Au is more effective at decreasing NO dissociation than can be explained by a simple site-blocking model. The effect of adsorbed O on NO dissociation was also invest...

Oxidation of CO on Gold-Covered Pt(335)

The authors have studied the adsorption and reaction of oxygen and CO on a stepped Pt surface with varying amounts of Au, using temperature-programmed desorption and reaction (TPD and TPR), low-energy electron diffraction (LEED), high-resolution electron energy loss spectroscopy, and steady-state reaction measurements. When the surface is fully covered with Au it is inert to oxygen adsorption and to CO oxidation, and supports only a single weakly bound CO adsorption state. The surface covered with 0.7 ML Au, however, exhibits properties different from either bare Pt or bare Au. Their TPD and LEED results suggest the coexistence of completely Au-covered regions and regions with Au on the step edges but not on the terraces. Dissociative oxygen adsorption is reduced by 90%, and the remaining oxygen is confined to Pt sites near the Au/Pt boundaries. The Au-covered regions support weakly bound CO adsorption states with desorption temperatures of 120, 190, and 240 K. CO in these states can diffuse rapidly and react efficiently with adsorbed atomic oxygen at temperatures as low as 150 K. In low-temperature TPR experiments the reaction is limited by the availability of adsorbed oxygen under almost all conditions. Under steady-state conditions, however, it is limited by themore » availability of CO even at low temperatures and CO partial pressures up to 10{sup {minus}6} Torr. Adding CO partial pressure does not inhibit the reaction. Consequently, adsorbed CO does not completely block all the sites at which oxygen dissociates on this surface, unlike on bare platinum.« less

Nucleation and growth of Au overlayers on Pt(100)-hex-R0.7° studied by STM and photoelectron spectroscopy

The nucleation and growth of Au overlayers on the hexagonally reconstructed Pt(100) surface have been studied by scanning tunneling microscopy (STM) and photoelectron spectroscopy. Adsorption of Au atoms on Pt(100)-hex-R0.7° induces a local lifting of the surface reconstruction of Pt, in units of reconstruction rows. The anisotropy of the Pt reconstruction leads to anisotropy in Au surface diffusion, as well as in growth of the Au overlayer. Au forms rectangular (1×1) islands of monolayer height, with the long axis of the islands being parallel to the rows of the Pt(100) hexagonal reconstruction. Nucleation occurs at both flat terraces and step edges, preferably at step edges intersecting the reconstruction rows. Islands also nucleate at structural defects in the surface. The STM images show that Pt atoms are incorporated into the first monolayer Au islands, contributing to about 20% of the island area. Growth of the second monolayer islands starts well before completion of the first monolayer. The first and second monolayers grow simultaneously until the first monolayer is nearly completed, without any nucleation of third monolayer islands. Further growth proceeds in a quasi-layer-by-layer mode. After deposition of more than 4 ML of Au, a (1×7) surface reconstruction is seen to emerge. High-resolution photoelectron spectroscopy measurements show contributions to the Au 4f 7/2 core level spectrum from Au atoms in four different local environments in the surface: Au atoms at the Pt interface at (83.32±0.02) eV binding energy, surface Au atoms with (1×1) symmetry at (83.50±0.02) eV binding energy, surface Au atoms with a (1×7) symmetry at (83.63±0.02) eV and bulk-like Au atoms at (83.87±0.02) eV.

Au adsorption induced faceting and phase transitions of facet planes on the Si[110] zone studied by UHV-REM

Au was deposited on a cylindrical surface made in a Si (1 1 ̄ 0) wafer and Au adsorption induced facet formation on the [1 1 ̄ 0] zone was studied by ultra-high vacuum reflection electron microscopy. We studied structural changes of the surfaces with orientations between the (110) and the (113) planes through the (111) plane. It was found that four high index surfaces, (441), (553), (335) and (5 5 11), in addition to low index surfaces, (110), (111) and (113), showed images with flat surfaces and with well defined reflection high energy electron diffraction patterns after deposition of 0.5 ML Au. Such high index surfaces have not been observed as flat surfaces on a clean cylindrical Si surface. They are considered to be facet planes stabilized by Au adsorption. So-called hill and valley structures were seen between some pairs of the Au adsorption induced facet planes and the (110), (111) and (113) planes. The facet planes have reversible structural phase transition at temperatures between 740 and 820°C, which depend on the facet planes. It should also be noted that the formation of the facet planes depends on the substrate temperature (in other words annealing temperature after deposition of Au) and on the amount of Au deposit. The adsorption induced facet formation is a kind of self-organization of surface morphology.

Au onW(001):Change in structure and orientation

Ultraviolet photoemission spectroscopy (UPS) measurements during the deposition of Au on form$\mathrm{W}(001)$ at 150 K show that over 30 ML of Au have to be deposited before the electronic structure of the deposit stabilizes. This large film thickness before a stationary growth situation is reached is a result of the large misfit between various orientations of Au with the $\mathrm{W}(001)$ surface. The orientation at characteristic thicknesses is analyzed with a combination of reflection high-energy electron diffraction, x-ray photoelectron diffraction (XPD), and UPS. The first 3.5 ML form a pseudomorphic body-centered-tetragonal layer that is thermally stable at least up to 900 K. The surface layer of the pseudomorphic triple layer shows a hexagonal reconstruction and consists of 1.5 ML of atoms. After this initial range, the film continues to grow in the bcc (001) orientation up to 10 ML at both 150 and 300 K. Beyond this coverage the orientation changes and a fcc (001)-oriented film is observed for coverages above 23 ML. At 150 K, a (111) orientation appears between 10 and 23 ML, which can be obtained in a pure and perfect form by annealing at 600 K. The transition to fcc (001) shifts to higher coverages with increasing deposition rate.

Effect of a Composition Discontinuity on the Evolution of a Bimetal Interface Studied by Photoemission Microscopy: Au Patch Deposited on a Ag/Si(111) Surface

The evolution of a 2 ML Au rectangular patch deposited on a [Formula: see text] surface containing three-dimensional Ag islands was studied by scanning photoemission microscopy. The surface composition and electronic structure of the Au patch and of the transition region between the patch and the Au-free Ag/Si area were characterized on a submicron scale at room temperature and after annealing at various temperatures up to 993 K. The spatial expansion of the Au patch due to Au surface diffusion and the chemical state of Ag and Au in the different interface zones were examined by chemical mapping and by analysis of the Ag 3d and Au 4f core level and valence band spectra, taken at selected spots of the surface, From the analysis of the Au 4f core level line shape, we identified a chemical state for Au at the forefront of the Au migration on the [Formula: see text] surface and assigned it to an adsorption site on top of Si trimers. This was the dominating Au species for low (< 0.2 ML) Au coverages, Following the evolution of the Au patch upon annealing we discovered substantial differences in its composition and structural changes compared to a continuous 2 ML Au film deposited on a [Formula: see text] surface. The important role of the Ag and Au surface diffusion in the evolution of an interface containing compositional discontinuities was discussed.