Gold cluster formation on C60 surfaces observed with scanning tunneling microscopy:: Au-cluster beads and self-organized structures

The growth mechanism of Au-clusters on fullerene layers has been investigated by scanning tunneling microscopy in ultrahigh vacuum at room temperature. The fullerene layers, which serve as substrates, are formed on a graphite surface and exhibit the typical combination of round and fractal shapes, and small sections of the original graphite substrate are exposed. The immobile Au-clusters are concentrated on the C-60 terminated surface section, and the original fullerene island structures are preserved. A preferential nucleation of Au-clusters is observed at the C-60-graphite edges while the C-60-C-60 edges remains undecorated. These Au-clusters are placed directly on the edge and shared by the graphite and fullerene layer. They form bead-like structures, which densely populate this edge, while the first layer C-60 islands are clearly depleted of Au-clusters. A roughness analysis of the fullerene surface indicates the presence of Au atoms (or very small clusters), which are embedded in the fullerene surface, and likely situated in the troughs in between the large molecules. These Au atoms are highly mobile and cannot be individually resolved at room temperature. The analysis of the spatial and size distributions of Au-clusters provides the basis for the development of a qualitative model, which describes the relevant surface processes in the Au-fullerene system. The simultaneous deposition of Au and fullerene on graphite leads to the formation of highly organized structures, in which Au-clusters are embedded in a ring of fullerene molecules with a constant width of about 4 nm. The mechanism for the formation of these structures is highly speculative at present and further experiments will be pursued in the near future. A comprehensive analysis of the Au-C-60 system is presented, which contributes to the advancement in our understanding of the metal-fullerene interaction and furthers the development of composite materials of interest in the synthesis of solar cells and metal contacts to organic materials. (C) 2007 Elsevier B.V. All rights reserved.