Commensurate growth of densely packed PTCDI islands on the Rutile TiO2(110) surface

We have studied the deposition of perylenetetracarboxylic-diimide, PTCDI, on the rutile (1 x 1)TiO2(110) surface. At variance with other polyaromatic hydrocarbons, like acenes, PTCDI displays a significant interaction with this dielectric substrate. At moderate substrate temperature (similar to 400 K), first layer molecules aggregate into two-dimensional islands corresponding to a (1 x 5) commensurate phase. According to our surface diffraction, STM, and NEXAFS studies, the substrate accommodates one PTCDI molecule per unit cell, atop each oxygen row. Because of steric repulsion, molecules lie on their long edge, tilted by similar to 35 degrees with respect to the surface. This constraint determines a strong pi-pi coupling between adjacent molecules, resulting into a geometry similar to that reported for acenes on (1 x 1)-TiO2(110), but quite uncommon for perylenes.