Paper SS2-TuM9
Manipulating Island Density and Shape in Organic Thin Film Growth: the Nucleation of Perfluoropentacene on Self-Assembled Monlayers

Tuesday, October 19, 2010, 10:40 am, Room Santa Ana

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 (AFM). The SAMs ranged from very low surface energy hydrophobic surfaces (perfluorooctyltrichlorosilane, FOTS), to higher surface energy hydrophilic surfaces (3-methacryloxypropyltrichlorosilane, MAOPTS). From real time x-ray scattering we find that the growth of PFP, while crystalline, becomes very 3D 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 ~ 3 molecules can describe all 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, with values differing by over 2 orders of magnitude. The shape of the islands also depends on the surface termination, but somewhat unexpectedly—the islands are most compact and facetted on surfaces where the diffusivity of isolated PFP molecules is the smallest. The shapes of the islands on these surfaces can be interpreted by arguments based on equilibrium, where polygonal islands expose low energy facets. These results demonstrate the sensitivity of the initial stages of nucleation to the nature and identity of the underlying substrate.