Influence of alkyl chain substitution on sexithienyl-metal interface morphology and energetics

The interface between Ag(111) and vacuum sublimated alpha,omega-dihexylsexithienyl (DH6T) was investigated using ultraviolet photoelectron spectroscopy and atomic force microscopy. While the monolayer of DH6T is lying flat on the metal surface, we found that already in the second molecular layer the molecules are almost standing upright. This abrupt change in molecular orientation lowered the hole injection barrier (Delta(h)) of DH6T/Ag by 0.5 eV between monolayer and multilayer. Delta(h) for DH6T multilayers was even lowered by 0.8 eV compared to unsubstituted sexithienyl multilayers. The reduction of Delta(h) is attributed to the electronic decoupling of molecules in the first from those in the second layer via the hexyl chains.