Structural studies of parasexiphenyl thin films: Importance of the deposition parameters

Parasexiphenyl was synthesized by electroreduction of monobromoterphenyl in the presence of a Ni catalyst and purified by vacuum sublimation and condensation in a temperature gradient. Thin films can be processed by vacuum evaporation under controlled deposition conditions,which play a major role. A high substrate temperature (470 K), a very low deposition rate (10(-2) nm s(-1)) and a film thickness around 350 nm allow one to obtain the best crystallinity. Two textured crystalline phases are found, differing mainly by the c parameter length, possibly due to a slightly different tilt angle of the molecule along the c axis of the monoclinic unit cell. A preferred orientation shows up under these experimental conditions, the molecular axis being perpendicular to the substrate surface. Consequences of the texture and of the molecular packing in the unit cells on the vibrational and electronic absorption spectra are presented.