Superstructure formation of large organic adsorbates on a metal surface: A systematic approach using oligothiophenes on Ag(111)

A series of “end-capped” oligothiophenes ECnT with different chain lengths n $(n=3\mathrm{}–6)$ were vapor deposited onto the Ag(111) surface. The adsorption and structural ordering was investigated by thermal desorption spectroscopy, low-energy electron diffraction, and scanning tunneling microscopy. For all molecules we observe highly ordered monolayers with flat-lying molecules and long-range ordered domains of several hundred Å diameter. For the two smaller oligomers (EC3T and EC4T), two phases—a commensurate (relaxed) and an incommensurate (compressed)—are found, whereas the two longer molecules (EC5T and EC6T) form only incommensurate phases. The six unit cells exhibit significantly different symmetries, containing 1, 2, or 4 molecules. The formation of the geometric structures is discussed under the aspect of molecular size, molecular symmetry, and interplay of the molecule-substrate and molecule-molecule interactions. The last aspect was also investigated by concomitant force-field calculations.