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 $(n=3--6)$ 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 {} 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.