A variable temperature scanning tunneling microscopy study of organizing C60 molecules on nanostructured gold surface

Zhang, Xin (2010). A variable temperature scanning tunneling microscopy study of organizing C60 molecules on nanostructured gold surface. University of Birmingham. Ph.D.

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Abstract

The nucleation and growth of C $_{60}$ molecular nanostructures on the Au (111) surface are studied under various conditions using variable temperature UHV STM. At room temperature, the C $_{60}$ /Au interface exhibits rich features observed in the STM images, for instance the lifting of the reconstruction and the appearance of dim molecules in the close-packed C $_{60}$ overlayer. Furthermore, the structural changes at the interface result in different bonding configurations of the adsorbed C $_{60}$ molecules, of which the strong bonding configuration can retain individual C $_{60}$ molecules on the elbow sites even at room temperature. This further leads to C $_{60}$ island nucleation at unexpected location. C $_{60}$ molecules deposited at reduced temperatures, 47 K and 180 K, respectively, do not induce atomic rearrangement of the Au (111) surface. The C $_{60}$ molecules are bonded to the intact reconstructed surface. However, the nucleation site and the island growth mode are still determined by the reconstruction pattern of the substrate. By increasing the C $_{60}$ coverage step-by-step and comparing with the growth scenario observed at the two temperatures, the details of the intermolecular and molecule-substrate interactions are revealed. Temperature dependent processes for the C $_{60}$ /Au (111) system are studied by gradually raising the sample temperature from 47 K to room temperature. Diffusion is activated at ~160 K leading to ripening of the C $_{60}$ molecular structures. At higher temperatures, when a large number of C $_{60}$ molecules are released from the elbow sites, C $_{60}$ clusters with a magic number of seven are formed. These clusters only diffuse within the FCC regions of the surface with their diffusing direction confined by the reconstruction ridges (the DLs). The ripening process is also studied at room temperature over a long timescale. A combination of Ostwald ripening and Smoluchowski ripening process is observed.