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Reactivity of Pd and Sn adsorbates on plasma and thermally oxidized SnO2(110)
A563 Surface Science 257 (1991) 63-69 North-Holland
63
Strong lateral interactions in the activated state for surface diffusion: CO/Ru(O01) V.P. Zhdanov Institute of Catalysis, Novosibirsk 630090, USSR Received 28 January 1991; accepted for publication 22 May 1991 The lattice-gas model is used to analyze diffusion of particles adsorbed on the vertex sites of a triangular lattice. The chemical diffusion coefficient is calculated in the Bethe-Peierls approximation employing a seven-site "colony" and taking into account lateral interactions between nearest-neighbour nonactivated particles and also lateral interactions of the activated complexes with their environment. General results are used to interpret the experimental data for CO diffusion on Ru(001). In this case, lateral interactions in the activated state are shown to be very strong ( = 4-6 kcal/mol).
Surface Science 257 (1991) 70-78 North-Holland
Reactivity of Pd and Sn adsorbates on plasma and thermally oxidized Sn02(llO) R. Cavicchi and S. Semancik Process Measurements Division, National Institute of Standards and Technology, Gaithersburg, MD 20899, USA Received 26 October 1990; accepted for publication 14 May 1991 The surface accumulation layer that forms as a result of oxygen vacancies near the surface of an oxide semiconductor can be used as an extremely sensitive probe of phenomena occurring at the surface. By incorporating a UHV-compatible, four-point conductance measurement in our surface analysis apparatus, we have investigated the formation of metal-semiconductor interfaces on SnO2(lI0) with varying oxygen stoichiometry. For only a 0.1 monolayer equivalent (ML) coverage of Sn deposited onto a high-oxygen-content surface prepared by oxygen plasma treatment, we observe a 500-fold increase in conductance. Our results suggest that at this coverage, Sn reacts with the excess chemisorbed oxygen and also creates point defects by abstracting oxygen from the lattice. In contrast, Pd reacts only with chemisorbed species, as indicated by a smaller conductance change and by differences in the size-effect shifted core levels observed at the same coverage for thermal and plasma oxidized surfaces. Oxygen plasma treatment of an 8 ML Pd film is shown to be an effective room temperature oxidation procedure and is used as a basis of comparison for studies of the reaction of submonolayer coverages of Pd on plasma and thermally oxidized surfaces. The coverage dependence of the surface conductance for Pd on thermal and plasma oxidized surfaces exhibits a peak near 1 ML, possibly indicating the onset of Schottky barrier formation: barriers underneath the metal islands deplete the surface of carriers; or the formation of scattering centers. At higher coverage, inter-island conductance dominates and the conductance rises. Surfaces prepared with decreasing oxygen content (thermal, vacuum annealed, ion-sputtered) show increased apparent alloying of Pd to surface Sn.
Surface Science 257 (1991) 79-85 North-Holland
79
Surprising effect of deposition temperature on Pt/Pd(110) reconstruction W.-Y. Leung, P.J. Schmitz 1, H.C. Kang 2 and P.A. Thiel * Department of Chemistry and Ames Laboratory, Iowa State Unioersity, Ames, 1,4 50011, USA Received 5 March 1991; accepted for publication 8 May 1991 Pt films deposited on Pd(ll0) can exhibit (1 x 2) and (1 x 3) superstructures w.hen they are annealed above room temperature. At a coverage of 3 monolayers, the film "remembers" the conditions of its deposition below room temperature. When the film is deposited at temperatures below 200 K, annealing above room temperature produces a bright and stable (1 × 2). When the deposition
63
Strong lateral interactions in the activated state for surface diffusion: CO/Ru(O01) V.P. Zhdanov Institute of Catalysis, Novosibirsk 630090, USSR Received 28 January 1991; accepted for publication 22 May 1991 The lattice-gas model is used to analyze diffusion of particles adsorbed on the vertex sites of a triangular lattice. The chemical diffusion coefficient is calculated in the Bethe-Peierls approximation employing a seven-site "colony" and taking into account lateral interactions between nearest-neighbour nonactivated particles and also lateral interactions of the activated complexes with their environment. General results are used to interpret the experimental data for CO diffusion on Ru(001). In this case, lateral interactions in the activated state are shown to be very strong ( = 4-6 kcal/mol).
Surface Science 257 (1991) 70-78 North-Holland
Reactivity of Pd and Sn adsorbates on plasma and thermally oxidized Sn02(llO) R. Cavicchi and S. Semancik Process Measurements Division, National Institute of Standards and Technology, Gaithersburg, MD 20899, USA Received 26 October 1990; accepted for publication 14 May 1991 The surface accumulation layer that forms as a result of oxygen vacancies near the surface of an oxide semiconductor can be used as an extremely sensitive probe of phenomena occurring at the surface. By incorporating a UHV-compatible, four-point conductance measurement in our surface analysis apparatus, we have investigated the formation of metal-semiconductor interfaces on SnO2(lI0) with varying oxygen stoichiometry. For only a 0.1 monolayer equivalent (ML) coverage of Sn deposited onto a high-oxygen-content surface prepared by oxygen plasma treatment, we observe a 500-fold increase in conductance. Our results suggest that at this coverage, Sn reacts with the excess chemisorbed oxygen and also creates point defects by abstracting oxygen from the lattice. In contrast, Pd reacts only with chemisorbed species, as indicated by a smaller conductance change and by differences in the size-effect shifted core levels observed at the same coverage for thermal and plasma oxidized surfaces. Oxygen plasma treatment of an 8 ML Pd film is shown to be an effective room temperature oxidation procedure and is used as a basis of comparison for studies of the reaction of submonolayer coverages of Pd on plasma and thermally oxidized surfaces. The coverage dependence of the surface conductance for Pd on thermal and plasma oxidized surfaces exhibits a peak near 1 ML, possibly indicating the onset of Schottky barrier formation: barriers underneath the metal islands deplete the surface of carriers; or the formation of scattering centers. At higher coverage, inter-island conductance dominates and the conductance rises. Surfaces prepared with decreasing oxygen content (thermal, vacuum annealed, ion-sputtered) show increased apparent alloying of Pd to surface Sn.
Surface Science 257 (1991) 79-85 North-Holland
79
Surprising effect of deposition temperature on Pt/Pd(110) reconstruction W.-Y. Leung, P.J. Schmitz 1, H.C. Kang 2 and P.A. Thiel * Department of Chemistry and Ames Laboratory, Iowa State Unioersity, Ames, 1,4 50011, USA Received 5 March 1991; accepted for publication 8 May 1991 Pt films deposited on Pd(ll0) can exhibit (1 x 2) and (1 x 3) superstructures w.hen they are annealed above room temperature. At a coverage of 3 monolayers, the film "remembers" the conditions of its deposition below room temperature. When the film is deposited at temperatures below 200 K, annealing above room temperature produces a bright and stable (1 × 2). When the deposition