Roughness scaling properties of sexithienyl thin films are investigated by scanning force microscopy as a function of the substrate deposition temperature. The correlation length of the surface fluctuations follows the behavior of the average domain size, increasing exponentially with temperature from submicron to micron scales. Self-affinity is exhibited on 3 orders of magnitude of the spatial frequencies when the morphology changes from grain aggregates to lamellae. The decay with temperature of the roughness scaling exponent α from 1 to 0.7 suggests a transition from diffusion-limited growth to a strong adsorption regime.

We have investigated the chemical nature and the electronic structure of the interface between a low work function metal, aluminum, and a conjugated polymer semiconductor, polythiophene. We have studied the initial stages of the interface formation by depositing the metal onto the surface of a polymer film. Charge transfer processes between the metal and the polymer are analyzed using core-level x-ray photoelectron spectroscopy (XPS); the evolution upon metallization of the valence electronic levels directly related to the polymer electronic structure is followed with ultraviolet photoelectron spectroscopy (UPS). With these techniques, we investigate the deposition of aluminum on two polythiophene systems (i) the alkyl-substituted poly-3-octylthiophene and (ii) the α-sexithiophene oligomer. The experimental data are compared to the results of a recent quantum chemical study on model systems consisting of thiophene oligomers (up to sexithiophene) interacting with a few Al atoms. The interaction of polythiophene with Al atoms is found to modify dramatically the structure of the conjugated backbone, as strong carbon–aluminum bonds are formed in the α positions of the thiophene rings. A large charge transfer takes place from the Al atoms to the polymer chain, and the upper π levels of the polymer are strongly affected. The metallization is contrasted to the doping of conjugated polymers with alkali metals.

We present a quantitative investigation by atomic-force microscopy of the growth of an organic thin film made of sexithienyl evaporated on mica in high vacuum. Different forms of aggregation in sexithienyl thin films are induced upon a systematic increase in the temperature of the substrate during the evaporation. For temperatures up to 150 °C, the films consist of tightly packed grains whose size increases with temperature according to an Arrhenius behavior with ∼0.36 eV activation energy. The χ-square grain-size distribution suggests that growth rate is controlled by diffusion. This mechanism is supported by a simple model calculation of the van der Waals interaction between a single T6 molecule and a (100) T6 surface, that yields energy barriers to translation and rotation of the molecule equal to 0.3 and 0.5 eV, respectively. At 200 °C the film undergoes a morphological change to a lamellar structure and extended microcrystalline structures appear at higher temperatures. The presence of preferential directions suggests that orientational ordering is induced by the mica substrate. The possibility of obtaining ordered aggregates by a suitable choice of substrate and temperature during evaporation has relevance towards the realization of molecular devices with anisotropic properties.