Cu clusters

Massobrio C, Pasquarello A and Corso A D 1998 Structural and electronic properties of small Cu clusters using generalized-gradient approximations within density functional theory J. Chem. Phys. 109 6626... 

The following two pictures (Figure 6.2-8a and b) were acquired at h-500 mV and at -I-450 mV vs. Cu/Cu and show that at h-450 mV vs. Cu/Cu monolayer high Cu clusters nucleate at the steps between different Au terraces. Thus, the pair of shoulders in the cyclic voltammogram is correlated with this surface process. 

Many of these points are well illustrated by Cu2, which has become a benchmark for theoretical calculations owing to its relative simplicity and the availability of accurate experimental data. The theoretical spectroscopic constants are quite poor unless the 3d electrons are correlated, even though both Cu atoms nominally have a 3d °4s occupation. In fact, quantitative agreement with experiment is achieved only if both the 3d and 4s electrons are correlated, both higher excitations and relativistic effects are included, and large one-particle basis sets, including several sets of polarization functions, are used (24,25). This level of treatment is difficult to apply even to Cua, let alone larger Cu clusters. 

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). 

Figure 5.20 Two examples for the nanodecoration of a Au(l 1 1) electrode by tip-generated Cu clusters. Electrolyte 0.05 M H2S04 + ImM CuS04. (Reproduced with permission from Refs [90, 93].)...

If metal deposition is fast (as in the case of Cu in sulfuric acid solution), cluster generation can be performed at kHz rates. Obtaining an array of 10 000 Cu clusters on Au(l 11) takes a couple of minutes [15]. Typical parameters are 10-20 ms pulses at a rate of 50-80 Hz. 

The high stability of the metal clusters allows one to hold the sample potential slightly positive of the Nernst potential, typically at +10 mV versus Cu/Cu2 + in the case of copper. Thus, normal electrodeposition onto the sample directly from solution is prevented, whereas the tip-generated Cu clusters remain on the surface [96]. 

Fig. 19. X-t scan of an STM, monitoring the growth of an individual Cu cluster on Au(l 11) at —0.18 V vs. Cu/Cu++ over a period of about 2 min. Formation of 14 monolayers are seen. Electrolyte 0.5 M H2S04 + 5 mM CuS04 [43],...

As mentioned before, the stripe pattern deteriorates slowly with increasing number of Cu layers, but it remains visible for a long time. Eventually Cu clusters emerge with normal fee structure. In Fig. 24 an STM image of Au(100) is shown, the surface of which is covered by a nominally thick Cu overlayer. On top of the wavy Cu phase, clusters with regular bulk structure have been formed. A similar situation is depicted in Fig. 25 for Cu on Ag(100), where a large Cu crystallite with a flat... 

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 12 RP fragments cap alternately the Cu4 faces of the Cu24 polyhedron, resulting in fivefold-coordinated phosphorus atoms. This structure resembles that of the recently described [Cu24(NPh)i4]4 anionic cluster (40). The Cu-P and Si-P distances are unremarkable. The construction principle of parallel Cu layers to form a metal-like package has also been observed for other Cu clusters (41). The main reason for the different structures of Cu2PR and Li2PR clusters is the covalent character of the Cu-P bond, with the additional involvement of favorable Cu-Cu interactions. The latter are probably due to relativistic d10-d10 interactions (dispersion-type of interaction) (42, 43). 

Chemical reduction of Cu -loaded G4-OH dendrimers (G4-OH/Cu +) with excess NaBH4 results in intradendrimer Cu clusters (Fig. 3). Evidence for this comes from the immediate change in solution color from blue to golden brown the absorbance bands originally present at 605 nm and 300 nm disappear and are replaced with a monotonically increasing spectrum of nearly exponential slope towards shorter wavelengths (Fig. 11). 

This behavior results from the appearance of a new interband transition corresponding to formation of intradendrimer Cu clusters. The measured onset of this transitions at 590 nm agrees with the reported value [121], and the nearly exponential shape is characteristic of a band-like electronic structure, strongly suggesting that the reduced Cu does not exist as isolated atoms, but rather as clusters [122]. The presence of metal clusters is also supported by loss of signal in the EPR spectrum [123] following reduction of the dendrimer Cu + composite. 

The absence of an absorption peak arising from Mie plasmon resonance (around 570 nm) [124] indicates that the Cu clusters are smaller than the Mie-onset particle diameter of about 4 nm [124-126]. Plasmon resonance cannot be detected for very small metal clusters because the peak is flattened due to the large imaginary dielectric constant of such materials [122]. 

Transmission electron microscopy (TEM) results also indicate the presence of intradendrimer Cu clusters after reduction. Micrographs of Cu clusters within G4-OH reveal particles having a diameter less than 1.8 nm [127], much smaller than the 4.5 nm diameter of G4-OH [115,128]. 

The ability to prepare well-defined intradendrimer metal nanoclusters depends strongly on the chemical composition of the dendrimer. Spectroscopic results, such as those shown in Fig. 7, indicate that when G4-NH2, rather than the hydroxyl-terminated dendrimers just described, is used as the template a maximum of 36 Cu + ions are sorbed most of these bind to the terminal primary amine groups. Reduction of a solution containing 0.6 mmol/1 CUSO4 and 0.05 mmol/1 G4-NH2 results in a clearly observable plasmon resonance band at 570 nm (Fig. 11) [122,124,125] which indicates that the Cu clusters prepared in this way are larger than 4 nm in diameter. This larger size is a consequence of ag-... 

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