TY  - THES
T1  - Isolated bimetallic clusters and clusters on surfaces
A1  - Hristova,Elisaveta
Y1  - 2009/05/19
N2  - In the past years significant interest in clusters has been developed due to their fundamental importance in both basic and applied science. This increasing interest is amply justified by the unique properties of clusters and by the promise these systems hold as components of optical, magnetic, and electronic sensors and devices. Especially alloy clusters are of increasing interest as the electric, magnetic and catalytic properties of a monometallic cluster can be improved by adding a second component. In order to optimize the materials properties for a given application, it is of paramount importance to have an accurate understanding of the relation between composition/cluster size on the one side and property on the other. Here, computer simulations represent a useful method for predictions of cluster properties and confirming experimental data.
In this work we have performed global optimization on the structures of Ni-Cu, K-Cs and Rb-Cs bimetallic clusters. For NinCum bimetallic clusters with N=n+m up to 20 atoms, N=23 and 38 atoms we have demonstrated that most of the bimetallic cluster structures have geometries similar to those of pure Ni clusters. In contrast to the bulk, the ground state structures of Ni-Cu clusters do not experience a smooth transition between the structures of pure copper and pure nickel clusters as the number of Ni atoms changes. Furthermore, an icosahedron, a double icosahedron, and a triple icosahedron with one, two, and three Ni atoms at the centre, respectively, are especially stable (magic). In addition, it is found that Ni atoms occupy mainly high-coordination inner (core) sites, while Cu atoms show a tendency to occupy lower-coordination sites at the cluster surface. 
For K-Cs and Rb-Cs clusters we have found that the introduction of K and Rb substitutions in a Cs cluster for the size range N=34-50 results in new structures, different from those of the pure elements. These are highly symmetric and belong to the same structural family. The last fact leads to a more regular cluster growth in the case of the bimetallic clusters.
Another part of this thesis deals with deposition and global optimization of clusters on surfaces. An understanding of the cluster-surface interaction is 
important for the development of suitable materials, e.g. thin films, and it plays an important role in nucleation processes and crystal growth. In this thesis we simulate the experimental conditions of the Low Energy Cluster Beam experiment to study the influence of the atom type and the impact energy on the structural and energetic properties of the products of deposition of Ni13 and Cu13 clusters on Ni(111) and Cu(111) surfaces. It is shown that the shape of the nickel clusters deposited on a Cu(111) surface remains well kept, while the copper clusters impacting a Ni(111) surface collapse forming double and triple layered products. In the case of AgN clusters with N=2-20 adsorbed on Ag(111) and Ni(111) surfaces, the lowest-energy structures are determined.
We have found that from N=18 upwards a reversal of the magic numbers for the Ag/Ni(111) system compared to the Ag/Ag(111) system takes place, which is due to the predominance of the adatom-substrate interactions compared to the adatom-adatom interactions.
Finally, due to the large size mismatch it is energetically unfavorable for Ag to form pseudomorphic monolayer structures on Ni(111) and there is considerable strain produced at the interface.
The effect of this strain will give rise to disordered and elongated structures of the adsorbed Ag clusters.
KW  - Cluster,  Molekulardynamik
KW  - Globale Optimierung
CY  - Saarbrücken
PB  - Universitäts- und Landesbibliothek
AD  - Postfach 151141, 66041 Saarbrücken
UR  - http://scidok.sulb.uni-saarland.de/volltexte/2009/2149
ER  -