Structure and optical properties of ultrathin p-phenylene oligomer films on dielectrics

A systematic investigation has been performed in order to determine the unit cells of ultrathin p-nP films grown under surface science conditions on dielectrics. We have employed low energy electron diffraction (LEED) as well as polarization dependent linear absorption spectroscopy on films deposited onto alkali halide (NaCl, KCl, NaF, LiF, KBr) and mica(0001) single crystals at various substrate temperatures. Surface unit cells of the films are determined as a function of chain length n (n=3-6) and deposition parameters. Linear optical spectroscopy reveals a strong dichroism and allows us to distinguish between laying and standing molecules on the substrate. In contrast to the alkali halides, we observe on mica at elevated surface temperatures the growth of single crystalline 'needles', the orientation of which is controlled by the presence of surface dipoles on the cleaved mica surface.