Single cluster properties

The first results obtained on single ligand-stabilized metal clusters have been reported by van Kempen et al They performed scanning tunneling spectroscopy at 4.2 K on a Pt309(phen)36O20 cluster deposited from a droplet of an aqueous solution of the clusters on a flat gold substrate (Fig. 4). 

The I U) characteristics exhibit charging effects, which indicates that the ligands are electrical insulators, as desired. They act as tunnel barriers between the cluster and the substrate. The experimentally observed charging energy varies from 50 to 500 meV 140 meV would be expected from the classical formula for Eq of a spherical particle with a dielectric constant Cr = 10 for the surrounding ligands (see 

To avoid this problem Chi et al. reported STS on AU55 monolayers prepared on various substrates (Fig. 5) by means of a two-step self-assembly (SA) process and by a combined Langmuir-Blodgett/SA-process. The spectroscopy gives clear evidence of Coulomb blockade at room temperature originating from the double barrier at the ligand-stabilized cluster as the central electrode. 

The capacitance of the cluster/substrate-junction was calculated to be 3.9 x 10 F. This value is in agreement with the value of the cluster capacitance previously determined by temperature-dependent impedance measurements. These results are, furthermore, in good agreement with capacitance data obtained from self-assembled gold nanoparticles on a dithiol-modified Au surface, reported by Andres and coworkers.Here tunneling spectroscopy has been performed on 1.8 nm Au particles which were grown in the gas phase and a cluster-substrate capacitance of 1.7 X 10 F was obtained. Thus, the small capacitance enables the observation of Coulomb blockade phenomena at room temperature. 

---

The next step in the direction of a deeper understanding of nanostructured materials depends on being able to isolate the individual structurally determined cluster units from the crystal lattice and then determine the physical properties of the single clusters in question. This long-term objective has been partially achieved in the gas phase investigation of a structurally determined Gai9R6 cluster [R = C(SiMe3)3] in an FT mass spectrometer (cf. Section 2.3.4.2.5, Ga clusters)... 

Clusters of Ag atoms have metal-like properties at 55 atoms, but they are not metals. The gap between HOMO and LUMO being 0.2 eV is still 10 times larger than kT expected for a bulk metal. The work function is calculated to be 1 eV larger than that of the bulk, and the cohesive energy is only about one-third that expected for bulk. The electrons behave like free electrons. The trend in each of these quantities with size is in the direction from single atom properties toward bulk metal properties. Ousters of Pd have characteristics that differ from bulk metal properties in much the same way as do Ag clusters. One exception is the calculated number of unoccupied d states per atom of small clusters, which is very close to bulk values of 0.6. 

Conclusion We have used several, partially new methods to study single particle properties in a mixture of metal clusters. We are now moving to more refractory metals where we expect to see rigid structure at small particle size and a large range of chemical properties depending strongly on size. 

In Sect. 1.2.1 of the present chapter, we describe the most important cluster sources successfully used today. Section 1.2.2 introduces experimental techniques for mass-selecting single cluster sizes from the distribution generated by the cluster sources. In the gas phase as well as for clusters on surfaces, the densities are extremely low, thus only highly sensitive methods can be used for the characterization of the chemical and catalytic properties of the model systems. Some of the most commonly used techniques employed in gas-phase experiments are presented and discussed in Sect. 1.2.3, surface analysis techniques for cluster studies are presented in Sect. 1.2.4. 

According to the clustering over the whole property space the evaluation by nematode tests after single clustering will be compared with those of chemicals and aquatic ecotoxicological tests too. However, here the sample set includes additional samples from River Oder due to relatively few samples from River Elbe (nematodes tests started in 2000 only). 

Classification of lake sediments by means of factor analysis and hierarchical clustering has been reported by Hopke et. al. C1103- For a set of 79 sediment samples, 32 characteristic properties were determined (e.g. concentration of 15 elements, percent organic matter, criteria of the particle size distribution, water depth). Cluster analysis detected a single cluster for samples from the centre of the lake and three different clusters for samples from near the shore. 

---