Molecular chains and carpets of sexithiophenes onAu(111)

Abstract

The two organic molecular materials $\ensuremath{\alpha}$-sexithiophene (6T) and $\ensuremath{\alpha},\ensuremath{\omega}$-dihexylsexithiophene (DH6T) adsorbed on $\mathrm{Au}(111)$ in the (sub)monolayer range were investigated by scanning tunneling microscopy (STM) in order to explore the effect of alkyl substitution on the self-assembly at surfaces. Metal substrate step edges are identified as preferred nucleation sites for 6T, while stable nucleus formation for DH6T occurs at kinks of the $\mathrm{Au}(111)$ herringbone reconstruction. At low coverage, 6T forms continuous chains of single-molecular width along Au step edges, involving molecular conformation changes by rotations around C-C bonds of neighboring thiophene units. In contrast, DH6T exhibits no ordered structures in the submonolayer range. At monolayer coverage, substantially different structures were observed for the two molecules. 6T forms rows of molecules with parallel long molecular axes, whereas DH6T forms lines along these axes, where the conjugated cores are embedded in a matrix of hexyl chains. Because of different preferred nucleation sites, 6T forms a continuous molecular carpet on extended $\mathrm{Au}(111)$ terraces, whereas DH6T resembles a patchworklike carpet as domain boundaries are induced by the $\mathrm{Au}(111)$ herringbone surface structure, leading to reduced domain sizes. Alkylation of 6T thus drastically changes the adsorption behavior and the resulting layer structure on the Au surface. These results should be valuable for developing new directed self-assembly schemes on prepatterned surfaces.