Nucleation and Growth of Perfluoropentacene on Self-Assembled Monolayers: Significant Changes in Island Density and Shape with Surface Termination

We have examined the nucleation and growth of perfluoropentacene (PFP) on SiO2 and on a variety of surfaces possessing different terminating self-assembled-monolayers (SAMs) using in situ synchrotron X-ray scattering and ex situ atomic force microscopy. The SAMs ranged from very low surface energy hydrophobic surfaces (perfluorooctyltrichlorosilane) to higher surface energy hydrophilic surfaces (3-methacryloxypropyltrichlorosilane). From real time X-ray scattering, we find that the growth of PFP, while crystalline, becomes very three-dimensional after completion of the first 1−2 monolayers, independent of the substrate surface termination. Concerning growth in the submonolayer regime, we find that nucleation is homogeneous and that the absolute density of islands depends strongly on the surface termination, while the relative change of the island density with increasing growth rate is essentially independent of the underlying SAM. From the latter, we find that a critical island size of ∼2−3 molecules can describe all of the data. On the other hand, the dependence of the island density on termination implicates a significant change in the diffusivity of PFP with the identity of the SAM. The shape of the islands also depends on the surface termination but somewhat unexpectedlythe islands are most compact and faceted on surfaces where the diffusivity of isolated PFP molecules is the smallest. This result highlights the difference in transport mechanisms concerning diffusion across the substrate and that around the periphery of an island in molecular systems. Finally, on all surfaces, the films formed in the multilayer regime are similar and are described by rough, highly anisotropic features, perhaps dominated by a single low index face.