Superstructure formation of large organic adsorbates on a metal surface: Asystematic 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(lll) surface. The adsorption an d 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 a nd long-range ordered domains of several hundred Angstrom diameter. For the two smaller oligomers (EC3T and EC4T), two phases-a commensurate (relaxed) and an incommensurate (compressed)-are found, whereas the two longer molec ules (EC5T and EC6T) form only incommensurate phases. The six unit cells ex hibit 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. [S0163-1829(98)02143-2].