L. Ruan

Chemisorption of H, O, and S on Ni(110): general trends

Abstract The interaction of adsorbates such as H, O, and S with a Ni(110) surface has been studied with Scanning Tunneling Microscopy. The results are discussed with special emphasis on similarities and differences between the various adsorbate-induced structures. General trends in the experimental observations can be understood in the framework of the effective-medium theory.

A scanning tunneling microscopy study of the interaction of S with the Cu(111) surface

The interaction of sulfur with the Cu(111) surface has been studied with scanning tunneling microscopy in conjunction with low-energy electron diffraction. Two different phases have been observed. At low exposures, ∽ 20 langmuir of H2S at room temperature, a previously unknown commensurate phase, which is present in six equivalent domain orientations, has been identified as |41−14| in matrix notation. At higher exposures, a phase transition to a (√7 X √7)R19° structure takes place. During formation, Cu atoms are consumed from step edges. This indicates that the observed phases are not simple S overlayer structures, but involve a reconstruction of the Cu surface to a sulphide overlayer. By measuring the step area consumed as a function of exposure, the number of Cu atoms in the unit cells of the two phases has been determined. The concentration of the S atoms in the (7 × 7)R19° phase has been measured with Rutherford backscattering.

The decomposition of ammonia on an oxygen-precovered Ni(110) surface studied by scanning tunneling microscopy

Abstract The decomposition of ammonia on a Ni(110) surface with preadsorbed oxygen has been investigated in ultra-high vacuum at room temperature using scanning tunneling microscopy (STM). We propose a reaction model in which the high reactivity observed at low O coverage is ascribed to a direct interaction between the NH 3 molecules and the terminating atoms of the short, mobile -Ni-O- added rows which are observed on the surface under these conditions. This model is consistent with the observation that the surface becomes inert at high O coverage. We believe that the present reaction model can also explain results from some other experiments in which preadsorbed oxygen has been found to act as a promoter for dissociation of H-containing species, such as for NH 3 on Cu(110) and H 2 O on Ni(110).

STM investigations of adsorption induced phases on metal surfaces

Abstract The interaction of small molecules such as H 2 and O 2 with Cu and Ni surfaces has been investigated with scanning tunneling microscopy. The results are reviewed with special emphasis on the observed similarities between the different adsorption-induced reconstructions and on the wide variety of formation mechanisms. New results for the adsorption of S on Cu surfaces are presented. On Cu(111) we have determined a previously unknown phase with a large unit cell. On the Cu(110) surface, an uncommon mass-transport sequence has been observed, and a model for one of the S adsorption-induced structures (the p(5 × 2) phase) is discussed.

Direct observations of changes in surface structures by scanning tunneling microscopy

Recently, we have studied the interaction of reactive adsorbates H, C, O, and S with Ni and Cu surfaces by scanning tunneling microscopy (STM). In this paper, we briefly illustrate and discuss how such studies provide significant insight into the understanding of dynamic surface processes such as adsorbate-induced restructuring, surface reactions, and adsorbate-adsorbate interactions. The STM results demonstrate that there is a strong coupling between the chemisorption/reaction process and the distortion of the metal surface.

A scanning tunneling microscopy investigation of the Ni(110)-p(4 × 1)S phase

Abstract Three different methods have been applied to produce the p(4 × 1)S structure on Ni(110): A simple annealing of the p(3 × 2)S phase, exposure to H2S at elevated temperature, and, finally, formation via reaction of H2S with Ni(110)-p(2 × 1)O. The resulting phases are characterized, and the observation of completely new p(7 × 1) and p(11 × 1) structures is reported. A p(4 × 1)S model is presented which is consistent with the STM results.

An STM study of the interaction of benzene with an oxygen-precovered Ni(110) surface

Abstract The interaction of benzene (C 6 H 6 ) with a Ni(110) surface with preadsorbed oxygen has been investigated in ultra-high vacuum at room temperature (RT) using scanning tunneling microscopy (STM). At an initial O coverage of ∼ 0.30 monolayers, the benzene induces a compression of the NiOue5f8 added rows from a (3 × 1) to a (2 × 1) structure, and the C 6 H 6 molecules decorate the troughs which have been created by the compression. Annealing to ∼ 400 K induces H scission which is accompanied by the disappearance of C 6 H 6 molecules in the STM images. By further heating, the surface is observed to undergo a complicated structural transformation in which the oxygen is finally removed by recombinative CO desorption, and after heating to ∼ 600 K, the surface is clean, apart from areas which are covered by the carbidic p(4 × 5)C phase. No signs of a direct reaction between C 6 H 6 and O have been observed at RT, even at a lower initial O coverages.

Scanning Tunneling Microscopy Investigations of Adsorption Induced Surface Reconstructions

Using selected examples of reconstructions induced by oxygen or sulphur adsorption on copper surfaces it is shown how Scanning Tunneling Microscopy can reveal the formation mechanisms of new structures, determine the unit cells for complicated phases and investigate the level of structural perfection. Some dynamic aspects including the possible influence of the tunneling tip are discussed as well.