Abstract:We report on a combined scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), and density functional theory (DFT) study on the surface-assisted assembly of the hexaiodosubstituted macrocycle cyclohexa-m-phenylene (CHP) toward covalently bonded polyphenylene networks on Cu(111), Au(111), and Ag(111) surfaces. STM and XPS indicate room temperature dehalogenation of CHP on either surface, leading to surface-stabilized CHP radicals (CHPRs) and coadsorbed iodine. Subsequent covalent intermolecular bond formation between CHPRs is thermally activated and is found to proceed at different temperatures on the three coinage metals. The resulting polyphenylene networks differ significantly in morphology on the three substrates: On Cu, the networks are dominated by "open" branched structures, on the Au surface a mixture of branched and small domains of compact network clusters are observed, and highly ordered and dense polyphenylene networks form on the Ag surface. Ab initio DFT calculations allow one to elucidate the diffusion and coupling mechanisms of CHPRs on the Cu(111) and Ag(111) surfaces. On Cu, the energy barrier for diffusion is significantly higher than the one for covalent intermolecular bond formation, whereas on Ag the reverse relation holds. By using a Monte Carlo simulation, we show that different balances between diffusion and intermolecular coupling determine the observed branched and compact polyphenylene networks on the Cu and Ag surface, respectively, demonstrating that the choice of the substrate plays a crucial role in the formation of two-dimensional polymers.

We demonstrate, by surface-assisted coupling of specifically designed molecular building blocks, the fabrication of regular two-dimensional polyphenylene networks with single-atom wide pores and sub-nanometer periodicity.

Corannulene (COR) buckybowls were proposed as near ideal hosts for fullerene C60, but direct complexation of C60 and COR has remained a challenge in supramolecular chemistry. We report the formation of surface-supported COR-C60 host-guest complexes by deposition of C60 onto a COR lattice on Cu(110). Variable-temperature scanning tunneling microscopy studies reveal two distinctly different states of C60 on the COR host lattice, with different binding energies and bowl-ball separations. The transition from a weakly bound precursor state to a strongly bound host-guest complex is found to be thermally activated. Simple model calculations show that this bistability originates from a subtle interplay between homo- and heteromolecular interactions.

The catalytic activity of deposited Pt 7 clusters has been studied as a function of the reduction state of the TiO 2 (110)-(1 × 1) support for the CO oxidation reaction. While a slightly reduced support gives rise to a high catalytic activity of the adparticles, a strongly reduced one quenches the CO oxidation. This quenching is due to thermally activated diffusion of Ti 3+ interstitials from the bulk to the surface where they deplete the oxygen adsorbed onto the clusters by the formation of TiO x (x ≃ 2) structures. This reaction is more rapid than the CO oxidation. The present results are of general relevance to heterogeneous catalysis on TiO 2 -supported metal clusters and for reactions involving oxygen as intermediate.

No abstract

A vicinal Au(11 12 12) surface, naturally patterned into a rectangular superlattice, has been used as a template to prepare C60 nanostructures with long-range order and uniform size. At a coverage of 0.1 monolayer and at room temperature, a two-dimensional long-range ordered superlattice of molecular nanochains is achieved, which perfectly replicates the periodicity of the template surface. The fullerene nanochains are found to be located exclusively on the face-centered cubic stacking domains at the lower step edges. Our experiments demonstrate that highly periodic molecular nanochains can be fabricated through a site-selective anchoring method.

We present a method for the quantitative determination of the surface potential landscape of nanostructured surfaces based on the local analysis of the lowest field emission resonances by scanning tunneling spectroscopy. The method has a lateral resolution of approximately 1 nm and is applied to elucidate the site-specific adsorption properties of the strain relief pattern formed by two monolayers of Ag on Pt(111). For the example of C60 fullerenes, we show that the surface potential difference of up to 0.35 eV is responsible for the site-selective immobilization on the strain relief pattern.

We determined the CO oxidation rates for sizeselected Pt n (n ∈ {3,7,10}) clusters deposited onto TiO 2 (110). In addition, we investigated the cluster morphologies and their mean sizes before and after the reaction. While the clusters are fairly stable upon annealing in ultrahigh vacuum up to 600 K, increasing the temperature while adsorbing either one of the two reactants leads to ripening already from 430 K on. This coarsening is even more pronounced when both reactants are dosed simultaneously, i.e., running the CO oxidation reaction. Since the ripening depends on the size initially deposited, there is nevertheless a size effect; the catalytic activity decreases monotonically with increasing initial cluster size. ■ INTRODUCTIONIt is well established that small metal clusters have physical and chemical properties distinctively different from those of the bulk. While the properties change rather smoothly in the socalled scalable size regime, each additional atom can substantially change the properties of interest in the nonscalable size range. 1 Metal clusters of well-defined size, shape, and composition, supporting on metal oxide surfaces, provide model catalysts that offer a fundamental understanding of the processes taking place in real catalysts via structure−reactivity correlation at the molecular level. 2 This has been shown for the smallest clusters in numerous examples, three decades ago in the famous photographic experiment by Fayet et al., 3 and more recently in the work of Heiz et al. for metal clusters on MgO(100) 4−9 and of Anderson et al., 10,11 Watanabe et al.,12 and our group 13 for metal clusters on rutile TiO 2 (110). In all these examples, size-selected cluster deposition was employed in order to prepare monodispersed supported clusters which are difficult to obtain by conventional preparation methods. We note that experiments with a finite cluster size distribution also make it possible to study size effects in the scalable regime by varying the mean cluster size. 14 However, to firmly associate a catalytic property with a certain size, one has to probe the cluster morphology not only before but also after the reaction. This information is essentially lacking in most of the experiments cited above.It is fairly well accepted that metal catalysts deactivate, in particular under reaction conditions, as a consequence of morphological changes of the supported clusters, and that this change depends on the cluster size and cluster−support interaction. 2,15−24 This argument is even more important when the smallest entities, with only a handful of atoms, are under investigation. Accordingly, not only in model catalysts with well-defined initial conditions but also in real catalysts, this stability under the reaction conditions sets a clear limitation to the minimum cluster size that can be reasonably employed.Here we present a systematic study of the initial sizedependent reactivity and of the thermal as well as chemical stability of very small Pt clusters on rutile TiO 2 (110)-(1×1) in ...