Influence of TiO₂(110) surface roughness on growth and stability of thin organic films

We have investigated the growth and stability of molecular ultra-thin films, consisting of rod-like semiconducting para-hexaphenyl (6P) molecules vapor deposited on ion beam modified TiO₂(110) surfaces. The ion bombarded TiO₂(110) surfaces served as growth templates exhibiting nm-scale anisotropic ripple patterns with controllable parameters, like ripple depth and length. In turn, by varying the ripple depth one can tailor the average local slope angle and the local step density/terrace width of the stepped surface. Here, we distinguish three types of substrates: shallow, medium, and deep rippled surfaces. On these substrates, 6P sub-monolayer deposition was carried out in ultra-high vacuum by organic molecular beam evaporation (OMBE) at room temperature leading to the formation of islands consisting of upright standing 6P molecules, which could be imaged by scanning electron microscopy and atomic force microscopy (AFM). It has been found that the local slope and terrace width of the TiO₂ template strongly influences the stability of OMBE deposited 6P islands formed on the differently rippled substrates. This effect is demonstrated by means of tapping mode AFM, where an oscillating tip was used as a probe for testing the stability of the organic structures. We conclude that by increasing the local slope of the TiO₂(110) surface the bonding strength between the nearest neighbor standing molecules is weakened due to the presence of vertical displacement in the molecular layer in correspondence to the TiO₂ atomic step height.