Clusters, on substrate

The dynamics and growth of small metal clusters on substrates is of course important for realistic catalysts, but is also relevant in other areas. One recent example is the work of Wu et al., who examined Cu atoms and small Cu clusters (up to 4 atoms) on WN(001) as a model for Cu agglomeration in barrier materials for semiconductor devices. A noteworthy aspect of this work is that it explicitly considered the pathways for dilfusion and growth of small clusters. 

Figure 7. Schematic diagram showing the proposed nucleation mechanism diamond nuclei form on a DLC interlayer. (I) Formation of carbon clusters on substrate surface and change in bonding structure from sp to sp. (II) Conversion of sp sp bonding.

Recently a technique for the preparation of catalyst particles with a narrow size distribution was developed [8], yielding colloidal metal clusters stabilized by a shell of surfactants. By adsorbing these clusters on substrate surfaces, model electrodes for dispersed electrocatalysts can be prepared [9]1 Figure 2 compares two samples prepared from different colloidal solutions of such clusters adsorbed on a gold surface. It is evident that both samples differ significantly wifii respect to their mesoscopic structure. 

Wippermann developed a galvanic deposition of noble metals on carbon for usage in fuel cells [3]. Noble metal cluster such as platinum ion deposits on carbon particles which is in the contact interface of conducting carbon and electrolyte due to delivery of electrons via contact interface. Schindler developed the deposition of metal cluster on various metals and semiconductor substrate [3]. By cathodic accumulation of metal atoms on a conducting STM tip due to anodic dissolution, local enrichment can be achieved which allows localized cathodic deposition. Kolb applies localized deposition on the metal cluster on substrate by direct transferring of atoms from the STM tip [4]. 

An alternative type of tip-induced nanostructuring has recently been proposed. In this method, a single-crystal surface covered by an underpotential-deposited mono-layer is scanned at a close tip-substrate distance in a certain surface area. This appears to lead to the incorporation of UPD atoms into the substrate lattice, yielding a localized alloy. This procedure works for Cu clusters on Pt(l 11), Pt(lOO), Au(l 11), and for some other systems, but a model for this type of nanostructuring has not been available until now. (Xiao et al., 2003). 

The second procedure is different from the previous one in several aspects. First, the metallic substrate employed is Au, which does not show a remarkable dissolution under the experimental conditions chosen, so that no faradaic processes are involved at either the substrate or the tip. Second, the tip is polarized negatively with respect to the surface. Third, the potential bias between the tip and the substrate must be extremely small (e.g., -2 mV) otherwise, no nanocavity formation is observed. Fourth, the potential of the substrate must be in a region where reconstruction of the Au(lll) surface occurs. Thus, when the bias potential is stepped from a significant positive value (typically, 200 mV) to a small negative value and kept there for a period of several seconds, individual pits of about 40 nm result, with a depth of two to four atomic layers. According to the authors, this nanostructuring procedure is initiated by an important electronic (but not mechanical) contact between tip and substrate. As a consequence of this interaction, and stimulated by an enhanced local reconstruction of the surface, some Au atoms are mobilized from the Au surface to the tip, where they are adhered. When the tip is pulled out of the surface, a pit with a mound beside it is left on the surface. The formation of the connecting neck between the tip and surface is similar to the TILMD technique described above but with a different hnal result a hole instead of a cluster on the surface (Chi et al., 2000). 

Detailed studies about metal deposition from the gas phase onto SAMs have been published [108-110], The central question for the system substrate/SAM/deposit there (as well as in electrochemistry) is the exact location of the deposited metal On top of the SAM or underneath Three clearly different situations are easily foreseen (Fig. 31). (1) Metal on top of the SAM. Depending on a strong or weak chemical interaction between metal and SAM (e.g., functional end group of the SAM), the deposit will spread out on top of the SAM or it will cluster on the SAM. (2) Metal penetrating the SAM (e.g., at defects in the SAM) and connecting to the metal substrate underneath the SAM. This configuration is often pictured as a mushroom, with a thin connective neck and a large, bulky head. (3) Deposited metal is inserted be-... 

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 . 

The way in which cluster expansion occurs is not understood. One suggestion is that radical species such as Fe(CO)4 (triplet) or Co(CO)4 are produced and that attack of these radicals on substrates leads to polyhedral expansion ... 

Figure 13.17 Role of clusters in substrate binding—in aconitase the cluster geometry shifts from 4- to 6-coordination on substrate binding. The coordinating iron atom abstracts the hydroxide anion during dehydration. (From Imlay, 2006. Reproduced with permission of Blackwell Publishing Ltd.)...

A standard technique for the comparison of XPS results on bare gold clusters supported on substrates [74,75] is to relate surface coverage during deposition to average diameter of the particles formed by surface diffusion after deposition. 

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