Initial stages of a para-hexaphenyl film growth on amorphous mica

Para-hexaphenyl (6P) molecules were deposited in the submonolayer range on a sputter- modified muscovite mica(001) surface by physical vapor deposition under ultrahigh vacuum conditions. The ultrathin films were investigated in situ by thermal desorption spectroscopy and ex situ by atomic force microscopy. On the sputter-modified amorphous mica surface the 6P molecules form two-dimensional islands of standing molecules. From the island size and capture zone distribution, as well as from the island density as a function of deposition rate and substrate temperature, all relevant parameters, characterizing the nucleation process, could be deduced. The critical island size was determined to be two and/or three molecules. The preexponential factor for surface diffusion of the monomers was determined to be 2 \ifmmode\times\else\texttimes\fi{} 10${}^{17}$ ${\mathrm{s}}^{\ensuremath{-}1}$. The binding energy of a critical nucleus with $i$ $=$ 3 was found to be 1.5 eV, using the calculated value of 0.02 eV for the diffusion energy barrier. This is in good agreement with our calculations of the binding energy for a critical cluster consisting of three lying molecules. Furthermore, the calculations show that the critical clusters are lying flat on the surface; they transform into islands of standing molecules only after the incorporation of further molecules. The shape of the islands is slightly dendritic at low surface temperature and becomes more dendritic with increasing temperature. This unusual behavior, which is the opposite of the frequently observed temperature dependence of metal islands, is also discussed in some detail.