Codeposited pentacene:perfluoropentacene grown on SiO2: A microstructural study by transmission electron microscopy

The structure and morphology of codeposited pentacene (PEN, C22H14) and perfluoropentacene (PFP, C22F14) grown with mixing ratios of [2:1], [1.1] and [1:2] on SiO2 substrates were investigated by means of scanning electron microscopy (SEM) and transmission electron microscopy (TEM) techniques. Electron diffraction experiments showed the presence of a mixed phase consisting of PEN and PFP with similar lattice parameters for all samples. The electron diffraction pattern of the mixed phase exhibits a rather similar symmetry as the one of pure PEN in [001] orientation. This suggests that the crystalline structure of the mixed phase and of pure PEN should be very similar. However, the lattice spacing of the mixed phase is slightly larger in some directions than the one of pure PEN. SEM, scanning TEM (STEM) and dark field imaging reveal a change in morphology for different PEN:PFP mixing ratios during growth. In the case of PFP excess, PFP fibers on a background layer formed by the mixed phase are observed. The equimolecular mixture of PEN and PFP exhibits a structure consisting of fiber-like structures. With excess of PEN a grainy structure with contributions of pure PEN and of the mixed phase is found with a grain size in the range of (10−60) nm.