Quasi-one-dimensionally ordered chains of silver and cobalt clusters are grown on the R͑15 ϫ 12͒-C/ W͑110͒ template and investigated by scanning-tunneling microscopy. Both Ag and Co nucleate at the same area within the large template unit cell. We attribute this area to the carbon-poor part of the unit cell. Clusters exhibit a narrow size distribution, peaking at cluster sizes of seven to eight atoms. The strong preference of this cluster size is attributed primarily to the size of favorable adsorption areas in the R͑15 ϫ 12͒ unit cell. For cobalt atoms the adsorption strength seems to be more homogeneous across the unit cell than for Ag, allowing also growth of small clusters on less favorable regions of the unit cell at low temperatures.
Silver nanoclusters arranged in quasi-one-dimensional chains with a nearest-neighbor cluster-cluster distance of 1.4 nm were prepared on the R͑15ϫ 12͒-C/ W͑110͒ surface. The silver cluster chains form local thermodynamic equilibrium structures. Interactions between neighboring clusters are addressed by investigating the length distributions of silver cluster chains with scanning tunneling microscopy. Comparison with theoretical expectations derived from a one-dimensional Ising model yields evidence for a slightly repulsive interaction energy of about 20-30 meV.
Noble metal nanostructures of Au, Ag and Cu were prepared on two types of carbon-modified W(110) surfaces—R(15 × 12) and R(15 × 3)—and investigated by means of scanning tunneling microscopy. For all deposited metals qualitatively the same behaviour is observed: On the R(15 × 12)-template always isotropic clusters are formed. In contrast, on the R(15 × 3)-substrate the anisotropy of the nanostructures can be tuned from clusters at low temperatures via thin nanowires to thicker nanobars at high deposition temperatures. At intermediate temperatures on the R(15 × 3) the anisotropic Au nanowires arrange themselves into straight lines along domain boundaries induced by deposition of the Au metal. Similarities and differences to Au nanostructures as recently reported by Varykhalov et al. [A. Varykhalov, O. Rader, W. Gudat. Physical Review B 77, 035412 (2008).] are discussed.
Highlights► Real-time in situ monitoring of W(1 1 0) surface cleaning. ► Real-time in situ monitoring of carburazation kinetics on W(1 1 0) by RDS. ► Phase transformation R(15 × 3)-C/W(1 1 0)–R(15 × 12)-C/W(1 1 0) studied by RDS, AES and LEED.
Adsorption and coadsorption of carbon monoxide and oxygen on different types of Au clusters on R(15 × 3)C/W(110) and R(15 × 12)C/W(110), respectively, are studied with respect to the catalytic behavior for oxidation of CO as well as of surface carbon. Carburization of the W(110) surface results in a weakening of the adsorption bond for molecularly adsorbed CO. Dissociation of carbon monoxide, which occurs on W(110), is reduced on the low-carbon coverage R(15 × 12) surface and completely suppressed on the carbon-saturated R(15 × 3) phase. Deposition of gold results in a blocking of adsorption sites for molecularly adsorbed CO and reopening of the dissociation channel. Probably the latter is associated with the existence of double-layer gold clusters and islands. At room temperature the gold clusters on both carburized templates are stable in CO atmosphere as shown by in-situ STM measurements. In contrast, exposure to oxygen alters the clusters on the R(15 × 12) surface, implying dissociation of oxygen not only on the substrate but also on or in immediate vicinity of the gold clusters. On the Au-free carburized templates oxygen adsorbs dissociatively and is released as CO at temperatures beyond 800 K due to reaction with carbon atoms from the templates. Deposition of gold enhances the desorption rate of the formed CO at the low-temperature end of the recombinative CO desorption range, indicating a promoting effect of gold for oxidation of surface carbon. In contrast, low-temperature CO oxidation catalyzed by the deposited Au clusters is not observed. Two reasons could be identified: (1) weakly bound CO with desorption temperatures between 100 and 200 K (as reported for other related systems) is not observed, and (2) oxygen atoms are bonded too strongly to the templates.
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