Growth by supersonic molecular-beam epitaxy of oligothiophene films with controlled properties

Supersonic molecular beam epitaxy can be fruitfully used to grow films of organic materials, controlling their degree of ordering and their morphologies. This is due to the fine control on the initial state of the compounds in the beam. Very sharp and 'tuneable' energy distributions in the beam can be achieved so that the growth is not governed by processes at thermodynamic equilibrium but by the kinetics. This allows us, in some cases, to 'force' the growth in the desired directions instead of leaving this only to the interplay between the weak interaction forces and to the equilibrium thermodynamics to drive the growth. Comparing the optical response, morphology and structure of films grown under different degrees of supersonicity of the beam we show that one can control the growth in details. The grain size and density of defects can be drastically changed by appropriately tuning the beam's parameters. X-ray diffraction and pole figure analysis of the films are reported and discussed.