E. Zanazzi

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.

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.

Structure of Rh&[;110&];(1x2) and Rh&[;110&]; (2x2)p2mg-O surfaces

Abstract The structure of the oxygen-induced Rh&[;110&]; (2x2 )p2mg-O phase has been determined using low-energy electron diffraction intensity-voltage (LEED I – V ) analysis, and consists of a missing row reconstructed substrate with oxygen atoms in a zigzag arrangement. The oxygen atoms occupy the upper threefold sites on the &[;111&]; microfacets, and the nearest-neighbour bonding geometry is the same as for the (2x1) oxygen-induced low-temperature phase. In spite of the similar geometries, the reactivities of these structures for CO oxidation are very different. When the oxygen is removed from the (2x2)p2mg surface at low temperature, the substrate retains the (1x2) missing row reconstruction. LEED analysis of the (1X2) surface indicates that the first interlayer contraction is 11%, which is lower than the value determined previously by He scattering.

LEED structure analysis of A √3 × √3 -30° overlayer of iodine on Ag{111}

Abstract New preparation methods, new LEED intensity data and a new analysis on the classic much studied adsorbed system Ag{111}√3 × √3-30°-I suggest that this system can have several structural arrangements: in fact, we found a somewhat different structure than that found in 1973 by Forstmann, Berndt and Buttner. We were unable to produce a stable structure satisfactory for LEED analysis by the method used in the 1973 study. The earlier model with I in the three-fold symmetrical hollows with no Ag atom below does not fit our experiments satisfactorily, but models in which I atoms occupy both types of three-fold hollows fit better.

Test of structural models for Cu(001)-(√2 × 2√2) R45°-O by LEED intensity analysis

The Cu(001)-(√2 × 2√2)R45°-O structure was investigated by LEED intensity analysis. Several structural models were considered in the calculations. The models proposed on the basis of HREELS results can be ruled out. It was confirmed that the missing row model proposed by Zeng et al gives the best agreement with the experimental data. The structural parameters for this model were refined considering a buckling of the first and second copper layers.

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.

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.

LEED analysis of the Co{001}c(2 × 2)O structure

Abstract A LEED intensity analysis of the c(2 × 2) structure obtained upon adsorption of oxygen gas on Co {00l} is reported. Three structural models have been tested on the basis of a total of 13 LEED spectra for three angles of incidence and one azimuth. The correct model has oxygen atoms chemisorbed in the four-fold symmetrical hollows formed by four adjacent substrate atoms. Atomic arrangement and metal-oxygen distances are analogous to those found in Ni{001}c(2 × 2)O.