Island shape anisotropy in organic thin film growth induced by ion-beam irradiated rippled surfaces

Controlling thin film growth of conjugated molecules is a key factor in organic electronics. Here, we report on the growth of the organic semiconductor para-hexaphenyl (6P) on ion bombarded, rippled $TiO_{2}$(110) surfaces. These surfaces represent a one-dimensionally patterned substrate with alternating descending and ascending step trains with typical step distances below 1 nm. A clear island shape anisotropy and a discretization of the island width according to the ripple wavelength have been observed which are addressed to anisotropic detachment of molecules differently bound to the island rim at ascending and descending steps. By changing the average ripple length from $\sim$ 11 nm to $\sim$ 60 nm, the islands' length-to-width ratio could be tuned between $\sim$ 1.5 : 1 and $\sim$ 4.5 : 1. Further, strong diffusion anisotropy along and perpendicular to the ripples was found to be responsible for an increasing island density with decreasing ripple length