Marco Torrini

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.

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.

LEED structural analysis of the (001) surface of the ordered fcc Pt3Ti alloy

Abstract The surface structure of the (001) plane of the ordered fcc Pt 3 Ti alloy was studied by dynamical LEED analysis. The model that gives the best theory-experiment agreement corresponds to bulk truncation, with the outermost plane being pure platinum. The results also indicate a contraction of the first interplanar distance with respect to the bulk value and a slight upward buckling of Ti atoms in the second outermost plane.

The growth mechanism and structure of ultrathin cobalt films deposited on the Pd(111) surface

Abstract Ultrathin films of cobalt were deposited by thermal evaporation on the Pd(111) surface at room temperature. The combination of low energy ion scattering (LEIS) and X-ray photoelectron spectroscopy (XPS) shows that cobalt grows forming multi-atomic layer islands. X-ray photoelectron diffraction (XPD) and low energy electron diffraction (LEED) indicate that for the first 1–2 atomic layers the cobalt islands adopt an fcc structure with the same in-plane parameter as the substrate. For thicker films, cobalt maintains the fcc stacking sequence with a gradual relaxation to the lattice parameter of bulk Co.

Structure of the ZnO(0001) surface studied by X-ray photoelectron diffraction

Abstract The general abilty of photoelectron diffraction (XPD) to investigate the surface structure of oxides is shown. The actual termination of a ZnO single crystal is determined, oriented along the basal plane, distinguishing between the two possible (0001) and (000 1 ) polar surfaces. The collected XPD curves have been compared with theoretical calculations. For all the transitions examined, the features of the XPD azimuthal curves were found to repeat for a period of 60°, that is, the surface has an apparent sixfold symmetry which has been interpreted as being due to the presence of domains, separated by biatomic steps, equally distributed on the surface and rotated 60° with respect to each other. The existence of two surface domains rotated 60° was taken into account by averaging calculations for the two orientations. We tested two possible structural models, corresponding to the (0001) and to the (000 1 ) bulk termination structures, comparing the XPD azimuthal curves of the Zn 2p 3 2 , Zn 3p and O 1s transitions.

Alloying at the CoPt(111) interface: a study by crystallographic low energy electron diffraction

After depositing a single atomic layer of cobalt on the Pt(111) surface, annealing at 600 K leads to the formation of a surface alloy of stable composition and structure. This phase was examined by crystallographic low energy electron diffraction finding that of the two topmost layers the first layer contains 20 at% of cobalt and the second one 80 at% of cobalt. No significant amounts of cobalt could be detected below the second atomic layer from the surface.

Relaxation and atomic vibrations at the Ni(110) surface

Abstract The mean-square displacements in three directions for the atoms of the Ni(110) surface are calculated by means of a simplified treatment, using a Morse potential function. Taking into account a surface relaxation, the theoretical results are in good agreement with the experimental data.

Spinel formation at the CrZnO(0001̄) interface: a structural study by X-ray photoelectron diffraction

The formation of a spinel phase was observed after chromium deposition on a ZnO single crystal substrate and subsequent annealing. The structure of the spinel and its epitaxial relationship with the substrate were investigated by using X-ray photoelectron diffraction. Experimental data are discussed on the basis of a single scattering cluster-spherical wave model calculation. The presence of octahedrally coordinated Zn was ruled out and the deviation from stoichiometry is attributed to amorphous ZnO patches in the spinel matrix.

Test of structural models for the (4×4) phase formed by oxygen adsorption on the Pt3Sn(1 1 1) surface

Exposure of the Pt3Sn(1 1 1) surface to oxygen at a pressure in the 10 � 6 mbar range at 800 K leads to the formation of a chemisorbed phase with a (4 � 4) periodicity. In previous works we investigated the structure of the (4 � 4) phase by means of scanning tunneling microscopy (STM) and XPD. STM images show that the (4 � 4) periodicity results from an array of protrusions and holes. XPD data suggest that oxygen adsorption induces the reconstruction of the substrate with the formation of a Sn–O overlayer. In the present work, several structural models compatible with the STM and XPD results were tested by tensor LEED analysis. The best agreement with the experimental I–V curves was obtained for a model similar to that proposed to interpret the STM images of the Ag(1 1 1)(4 � 4)–O phase. According to this structure, the protrusions observed in STM would correspond to tin atoms occupying on-top positions. 2002 Elsevier Science B.V. All rights reserved.

Epitaxy and structure of the chloride phase formed by reaction of chlorine with Cu(100). A study by X-ray photoelectron diffraction

Abstract Chloride layers of thickness of several atomic layers were prepared by low temperature reaction of Cl 2 with a clean Cu(001) surface. The structure of the chloride layer was examined by X-ray photoelectron diffraction (XPD). The results of the comparison of experimental XPD data and calculations carried out using the single scattering cluster-spherical wave (SSC-SW) model show that the chloride grows as epitaxial fcc, zincblende CuCl, with the (111) plane parallel to the substrate surface. The results also indicate that the (111) “chlorine” termination is the actual termination of the chloride phase, out of the two possible terminations of the polar zincblende structure.