Metal clusters, deposition

A new experimental setup has recently been designed to study the chemical properties of size-selected metal clusters deposited on oxide substrates [210,211], Pd clusters have been produced by a laser evaporation source, ionized, then guided by ion optics through differentially pumped vacuum chambers and size-selected by a quadrupole mass spectrometer [210-212], The monodispersed clusters have been deposited with low kinetic energy (0,l-2eV) onto an MgO thin-film surface. The clusters-assembled materials obtained in this way exhibit peculiar activity and selectivity in the polymerization of acetylene to form benzene and aliphatic hydrocarbons [224], Figure 6 shows the temperature-programmed reaction (TPR) spectra for the cyclotrimerization of acetylene on supported Pd (1 30)... 

Electron spectroscopy, particularly photoemission, has been used successfully to study the electronic structure of small metal clusters deposited on well-characterized model systems [3-5,8,16,20,34-42,66,71,72,76-86]. The core-level binding energy... 

The utility of planar-supported model catalysts, consisting of metal clusters deposited on metal oxide thin films, as models for high-surface-area industrial... 

In this chapter, we focus on recent experimental and theoretical studies of chemical and catalytic properties of gas-phase metal clusters and metal clusters deposited onto support materials with sizes in the nonscalable regime. 

Very little is known theoretically about the metal-oxide interface although a rather limited number of first principles theoretical studies have dealt with the general problem of metal-ceramic interaction. The aim of this section is to discuss some fundamental aspects of the bonding of metal clusters deposited on the surface of simple binary oxides, in particular MgO and AI2O3. 

We have been interested in investigating the size-dependent electronic structure and reactivity of metal clusters deposited on solid substrates. Thus, we have shown that when the cluster size is small (SI nm), an energy gap opens up. Bimetallic clusters show additive effects due to alloying and cluster size in their electronic properties. Small metal clusters of Cu, Ni and Pd show enhanced chemical reactivity with respect to CO and other molecules. Metal clusters and colloids, especially those with protective ligands, have been reviewed in relation to nanomaterials. We have recently developed methods of preparing nanoparticles of various metals as well as nanocrystalline arrays of thiolized nanoparticles of Au, Ag and Pt. In Fig. 16, we show the TEM image of thiol-derivatized Au... 

In this presentation, different theoretical methods for circumventing these problems shall be discussed. They shall be illustrated through applications on various types of clusters. These include isolated metal clusters with one or two types of atoms, metal clusters deposited on a surface, nanostructured HAIO, semiconductor nanoparticles, and metaUocarbohedrenes. Special emphasis is put on the construction of descriptors that can be used in identifying general trends. 

Regnlar arrays of platinnm were achieved by chemical reduction of a platinnm salt that had been deposited onto the S-layer of Sporosarcina ureae [132]. This S-layer exhibits sqnare lattice symmetry with a lattice constant of 13.2 nm. Transmission electron microscopy revealed the formation of well-separated metal clusters with an average diameter of 1.9 nm. Seven clnster sites per nnit cell were observed. UV-VIS spectrometry was nsed to study the growth kinetics of the clnsters. 

Metal clusters on supports are typically synthesized from organometallic precursors and often from metal carbonyls, as follows (1) The precursor metal cluster may be deposited onto a support surface from solution or (2) a mononuclear metal complex may react with the support to form an adsorbed metal complex that is treated to convert it into an adsorbed metal carbonyl cluster or (3) a mononuclear metal complex precursor may react with the support in a single reaction to form a metal carbonyl cluster bonded to the support. In a subsequent synthesis step, metal carbonyl clusters on a support may be treated to remove the carbonyl ligands, because these occupy bonding positions that limit the catalytic activity. 

As pointed out above, an STM tip can be used to nucleate and grow single clusters. In this type of experiment, cluster deposition on a STM tip is achieved when it is retracted about 10 to 20 run from the substrate surface. Under these conditions, where the feedback loop is disabled, absence of mechanical contact between the tip and the substrate in ensured. Then a positive potential pulse is applied to the tip, the metal deposited on it is dissolved, and it diffuses toward the substrate surface, where a nucleus develops and grows to yield a cluster, typically 20 nm wide. 

Matsushita, M Hayakawa, Y. and Sawada, Y. (1985) Fractal structure and cluster statistics of zinc-metal trees deposited on a line electrode. Phys. Rev. A, 32, 3814-3816. 

Palladium clusters deposited on amorphous carbon have been studied by XPS and UPS [28] and both techniques show broadening of the d-band peak as cluster size increases. The d-threshold shifts towards Ep as cluster size increases. In UPS studies the d-emission of the single atom has its peak at 3.0 eV below Ep, whereas the d-threshold is 2eV below Ep. Palladium clusters evaporated onto Si02 have been studied by UPS [38]. At large coverages of the Pd metal evaporated (> 10 atoms/cm ), a high emission intensity at Ep excited with photons of 21.2 eV (He(I)) or 40.8 eV (He(II)) as excitation source, is observed. This feature is characteristic in the spectra from bulk Pd samples. At the lowest metal coverage (3 x 10 atoms/cm ),... 

All these results indicate that one is just at the beginning of understanding the function of catalysts being deposited on a semiconductor. There is still quite a confusion in many papers published in this field. Therefore the catalytic properties depend so much on the procedure of deposition . It seems to be rather difficult to produce a catalyst for 02-formation, as shown by results obtained with Ti02 (see e.g.) . Rather recently new concepts for the synthesis of new catalysts have been developed applicable for multielectron transfer reactions. Examples are transition metal cluster compounds such as M04 2RU1 gSeg and di- and trinuclear Ru-complexes . 

Supported model catalysts are frequently prepared by thermally evaporating metal atoms onto a planar oxide surface in UHV. The morphology and growth of supported metal clusters depend on a number of factors such as substrate morphology, the deposition rate, and the surface temperature. For a controlled synthesis of supported model catalysts, it is necessary to monitor the growth kinetics of supported metal... 

Most of the work on nanostructuring electrode surfaces, which can be found in the literature, deals with the deposition of small metal clusters at predetermined positions. Over the years, we have developed a technique that is based on the jump-to-contact between tip and substrate [89] (Figure 5.15h) and that allows the formation of metal clusters in quick succession and without destroying the single crystallinity of the substrate. The principle behind this method is sketched in Figure 5.19 [90, 92] By applying an electrode potential to the STM tip that is slightly... 

Why do we believe that a Cu monolayer is inserted between SAM and gold substrate The 2D-deposit grows and dissolves extremely slowly. Another indication is that the 2D deposit is very stable and shows no displacement by the scanning tip. Cu clusters on top of an alkanethiol-SAM would be only weakly bound and should be easily pushed away by the tip at higher tunnel currents, very much like metal clusters on a hydrogen-terminated Si(lll) surface, which for that very reason are difficult to image by STM (or AFM [122]). And finally, the cyclic voltammograms (Fig. 33) point to the formation of a buried monolayer . 

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