Highly ordered growth of ?-quaterthiophene films by seeded supersonic molecular beam deposition: A morphological study

It is shown that morphology and structure of films of oligothiophenes can be controlled well via supersonic molecular beam epitaxy (SuMBE). The supersonic expansion of oligomers seeded in inert gases makes it possible to tune beam parameters such as kinetic and internal energy, momentum and flux in a range that is shown to play a critical role in the morphology of the surface of the films. By simply varying the seeding in the source, we change the initial state of the oligomers in the beam. In this way, very different morphologies are obtained, ranging from a dendritic-like, typical of a disordered growth, to ordered layered structures. In this case the terraces observed were characterized by widths on a micron scale and height equal to the molecular length. This last morphology, studied by Atomic Force Microscopy in the tapping mode, is consistent with X-ray diffraction data and the optical response of the same films. These properties are maintained up to unprecedented film thicknesses (>=500 nm).