G. Rovida

Chemisorption of oxygen on the silver (111) surface

Abstract The interaction of oxygen with the (111) surface of a silver single crystal is studied, mainly in the pressure range from 10 −3 up to 1 torr and at temperatures from room up to 500°C. The experimental techniques employed were LEED, secondary electron spectroscopy, work function variation measurements, and desorption kinetics. Exposure to the high pressures was made with a sample isolation valve. The experimental procedures are examined in detail and critically discussed. The results obtained with the different techniques allow a correlation with many studies of other authors. The LEED technique indicates that in the range of pressures and temperatures examined, a surface superstructure is stable, having a unit mesh with sides four times greater than that of the silver (111) plane. The presence of this surface phase seems to be related to oxygen adsorbed in the dissociated form. On this assumption, an interpretation of the structure is proposed, which is based on a coincidence lattice formed by a (111) plane of Ag 2 O on the (111) plane of the metal. This interpretation is also in agreement with the thermodynamic data.

Chlorine monolayers on the low-index faces of silver☆

Abstract Chlorine chemisorption layers were obtained on the (100), (110) and (111) faces of silver single crystals by means of the reaction with dichloroethane. The structure and stability of the surface phases was studied mainly by LEED and Auger spectroscopy. On the (100) face, a c(2 × 2) superstructure was found at all coverages, which was interpreted as a reconstructed layer with a structure similar to that of the AgCl (100) planes. On the other two faces, different superstructures were observed, depending on the coverage. At low coverages, (2 × 1) and (√3 × √3)-R 30° superstructures were found on the (110) and (111) faces, respectively, which seem to be better interpretable as reconstructed layers. At maximum coverages, c(4 × 2) and (3 × 3) superstructures were observed respectively on the two faces. These phases were interpreted as mixed layers with a packing of the silver and chlorine atoms similar to that of the AgCl (111) planes. The proposed models are in agreement with the different thermal stabilities of the layers. The models of the maximum-coverage structures correspond to about the same chlorine amount on the three faces, in agreement with the Auger results. The structures are also discussed in relation to the known epitactic relations for the growth of AgCl on silver.

Initial stages of oxidation of the Ni3Al alloy: structure and composition of the aluminum oxide overlayer studied by XPS, LEIS and LEED

The oxidation of the Ni3Al alloy was studied by XPS, LEED and LEIS. At pressures of oxygen under approximately 5 × 10−5 Pa and at temperatures higher than approximately 700 K, a layer of Al2O3 grows on the surface forming islands of uniform thickness of the order of 5 A. The structure of these islands is related to that of γ-alumina.

LEED, AES and thermal desorption study of iodine chemisorption on the silver (100), (111) and (110) faces

Abstract The chemisorption of iodine vapor on the three low index faces of silver single crystals has been studied by combined LEED, AES and thermal desorption mass spectrometry. By exposure to gaseous iodine, only a c(2 × 2) phase is found on Ag(100), while on Ag(111) and Ag(110) several structures are observed depending on the iodine coverage. On all the three faces, the saturation coverage is about the same. Further iodine uptake leads to the formation of a thin layer of epitactic silver iodide, which, upon heating, undergoes structural changes before desorption.

Surface alloying at the SnPt(111) interface: a study by x-ray photoelectron diffraction

Abstract The formation of surface alloys obtained by annealing ultrathin films of Sn deposited on the Pt(111) surface was investigated by low energy electron diffraction and X-ray photoelectron diffraction. The Pt(111)(2 × 2)-Sn and Pt(111)(√3 × √3)R30°-Sn form after deposition of amounts of Sn in the range from 0.2 to 1 ML and subsequent annealing at 1000 K are found by XPD to be single atomic layer surface alloys. Depositing 4–5 ML of Sn and annealing at 400–600 K produces an ordered phase exhibiting a (2 × 2) LEED pattern. This phase is a multilayer surface alloy and it has the same structure as the bulk Pt 3 Sn alloy.

LEED crystallographic investigation of ultrathin films formed by deposition of Sn on the Pt(111) surface

Abstract Deposition of Sn on the Pt(111) surface followed by annealing at 1000 K leads to the formation of ordered phases showing (2 × 2 and ( 3 × 3 ) R 30° LEED patterns, depending on the surface coverage of Sn. Both these phases were studied by LEED dynamical analysis. The best agreement between experimental and calculated I–V curves was obtained by means of models based on the formation of mixed Pt-Sn layers on the surface where Pt and Sn atoms are nearly coplanar with a slight upward buckling of Sn atoms. The structures of these phases are similar to those already observed for the Pt3Sn(111) surface.

Study of the reconstructed (001) surface of the Pt80Co20 alloy

The structure of the clean, (001) oriented single crystal surface of a platinum alloy containing 20 % at of cobalt (‘Pt 80 Co 20 ’) was studied. He + Low Energy Ion Scattering Spectroscopy data indicated that the outermost surface plane of the alloy is composed of virtually pure platinum. Low Energy Electron Diffraction showed a complex pattern which can be interpreted as a c(58 × 22) coincidence unit mesh. A model of the surface can be proposed in terms of a reconstructed surface layer which forms a compact ‘quasi-hexagonal’ plane. The structure and bond lengths in the outermost plane of Pt 80 Co 20 (001) are nearly identical to those of the reconstructed Pt(001) surface.

Epitaxy and alloying at the CoPt(111) interface: a study by X-ray photoelectron diffraction

Abstract Cobalt was deposited on the clean Pt(111) surface at room temperature by thermal evaporation. The overlayer growth mechanism and structure was examined by X-ray photoelectron diffraction, combined with X-ray photoelectron spectroscopy and low energy He+ scattering. During the initial stages of growth cobalt forms a uniform layer of a single atomic layer thickness. Further growth results in the formation of an epitaxial fcc cobalt multilayer. Annealing at moderate temperature causes the formation of a fcc CoPt alloy of variable thickness.

Initial stages of oxidation of the Ni3Al alloy: a study by X-ray photoelectron spectroscopy and low energy He+ scattering

Abstract The oxidation of the Ni 3 Al alloy at low oxygen pressure was studied by low energy He + scattering and X-ray photoelectron spectroscopy. For exposures up to approximately 10 L of oxygen at 900 K the formation of A1 2 O 3 was observed. From a simulation of the relation of XPS and LEISS data for different mechanisms of growth of the oxide overlayer, we show that a linear relation of the LEISS signal to the XPS signal is expected if the layer grows forming islands of uniform thickness. The experimental results indicate that the thickness of the A1 2 O 3 islands is indeed uniform up to the completion of the monolayer. The thickness of the layer was found to be approximately 5 A from an analysis of the XPS results.

Measurements of oxygen adsorption on Si(111) surfaces by LEED

Abstract The fractional coverage as a function of exposure to oxygen was measured from the intensities of various diffraction beams, and the condensation coefficient was derived at room temperature and above (up to 600 °C). An interpretation of the adsorption kinetics is proposed. No influence of the electron beam, of the ion gauge, or of the surface irregularity upon the adsorption rate was found. The results are compared with those obtained on well-defined Si(111) surfaces by other authors.