Cluster deposition

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. 

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 ),... 

Figure 35. Work function and energy gap as a function of the height of gold clusters deposited on single crystal of anatase Ti02 [103].

Figure 34. Scanning tunnel microscopy and local barrier height for gold clusters deposited on a single mtile Ti02 substrate [103].

Gold clusters deposited on the activated carbon had the smallest average diameter of 1.7nm, while gold on graphite and decolorizing carbon had average particle sizes of 2.8 and 6.8 nm respectively. 

Figure 3.9. Transient C02 formation rates on Pd30 (a) and Pd8 (b) mass-selected clusters deposited on a MgO(lOO) film at different reaction temperatures [74]. In these experiments CO was dosed from the gas background while NO was dosed via a pulsed nozzle molecular beam source. The turnover frequencies (TOFs) calculated from the experiments displayed in (a) and (b) are displayed in the last panel (c). C02 formation starts at lower temperatures but reaches lower maximum rates on the larger cluster. (Figure provided by Professor Heiz and reproduced with permission from Elsevier, Copyright 2005).

Figure 10.31 STM images of Ag, Ag-Au, and Au clusters deposited on Ti02. (A) Imaged under UHV conditions.

In addition, the rate of Oz reduction, forming 02 by electron, is of importance in preventing carrier recombination during photocatalytic processes utilizing semiconductor particles. 02 formation may be the slowest step in the reaction sequence for the oxidation of organic molecules by OH radicals or directly by positive holes. Cluster deposition of noble metals such as Pt, Pd, and Ag on semiconductor surfaces has been demonstrated to accelerate their formation because the noble metal clusters of appropriate loading or size can effectively trap the photoinduced electrons [200]. Therefore, the addition of a noble metal to a semiconductor is considered as an effective method of semiconductor surface modification to improve the separation efficiency of photoinduced electron and hole pairs. 

A wide range of fhese materials has been investigated for fuel cell use, usually as supports for PfRu particles for DMFC testing (presumably due to the ease of experimenfafion). The fheoretically inerf surfaces of CNTs pose some difficulties for mefal cluster deposition because no sites exist for deposition or stabilization. Therefore, clusters fend to be deposited onto defecf and amorphous portions of samples (see Figure 1.18). 

We also compare our results to XPS data on bulk gold and on the smaller centered clustercompound Au 11L7X3 (with X = Cl or I) [74]. A comparison is also made to XPS results obtained on bare gold clusters deposited on poorly conducting substrates [75, 76]. This gives still more support to the idea of the metallic bonding of the Aujj clusters. 

Vandamme N, Snauwaert J, Janssens E, Vandeweert E, Lievens P, Van Haesendonck C (2004) Visualization of gold clusters deposited on a dithiol self-assembled monolayer by tapping mode atomic force microscopy. Surf Sci 558 57-64... 

Waddill GD, Vitomirov IM, Aldao CM, Weaver JH (1989) Cluster deposition on GaAs (110) formation of abrupt, defect-free interfaces. Phys Rev Lett 62(13) 1568... 

Plate 15. Thermal conversion of Sb4 clusters on Si(lOO). (a) Sb clusters deposited on Si(OOl) surface at room temperature show three distinct types of precursors as well as the final-state clusters of two dimers, (b) After annealing at 410 K for 20 minutes, almost all precursors are converted to the final-state clusters. See Mo (1992) for details. Original images courtesy of Y. W. Mo. 

All these results show that Cd(OH)2 colloids do adsorb on a substrate (either under conditions where Cd(OH)2 is present in solution or, according to the studies of Rieke and Bentjen and Ortega-Borges and Lincot [48], even when it is not present in solution but under solution conditions close to solid hydroxide formation). The induction period when no deposition is seen in the hydroxide-cluster deposition therefore is understood to mean that a fast and nongrowing Cd(OH)2 adsorption has occurred, which is too fast and/or too httle to measure by the experimental methods used to make the kinetic curves, and that only when the hydroxide starts to convert into the chalcogenide, by reaction of the slowly formed chalcogenide ion with the hydroxide, does real film formation proceed. 

Deposition by a pure ion-by-ion mechanism should also solve this problem, since no hydroxide is involved. However, in this case we encounter the problem of the high K p of ZnS compared to CdS, which again means that more sulphide is needed. For thiourea, this means a higher pH, which again means that strong com-plexation is needed to prevent Zn(OH)2 formation, by reducing the free [Cd ]. However, this will also reduce the rate of ZnS deposition. While there are many examples in the literature of cluster deposition of ZnS, there does not even seem to be one unambiguous case of ion-by-ion deposition of this semiconductor. 

In common with CdSe deposition from selenosulphate baths, cluster deposition of PbSe normally resulted in specular films, while ion-by-ion films were initially highly scattering as thin films but eventually (usually) became specularly reflecting with increase in thickness. As for CdSe, the development of specularity with thickness of ion-by-ion films could be explained by filling in of voids between the large crystals. 

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