Among the growth techniques employed for preparing the active layer of organic semiconductors, ultra-high vacuum
ones have permitted to achieve the best results in terms of control of thickness of the layer, purity, and for the possibility
to modulate its morphological and structural properties by employing different growth conditions. Organic molecular
beam epitaxy (OMBE) represents a natural choice for the fabrication of high quality field effect transistors especially
when small conjugated oligomers are used as the active material. In spite of the advantages of this technique, the
intrinsic properties of molecular materials (high diffusivity, weak interactions with most substrates, strong tendency to
desorb, etc.) hinder the possibility to achieve a stable layer-by-layer growth, which is on the basis of the technological
development of inorganic devices. Here, we show how the problem can be addressed and solved: by using properly
prepared organic single crystals as substrate, a stable layer-by-layer epitaxial growth of organic semiconductors with the
OMBE technique is demonstrated, exploiting solely the weak van der Waals interactions acting between substrate and
overlayer. With this method, organic heterostructures of oligothiophenes and oligocenes are shown to be easily grown
with a control at the monolayer level, as demonstrated through the discussion of a detailed morphological and optical
characterisation. These results represent the starting point for the development of a technology based on all-organic
nanostructures.
The complex dielectric tensor of quaterthiophene crystals is reported as deduced from ellipsometry measurements on single crystals and compared with that calculated on the basis of the microscopic theory. The internal reflectivity at the interface between the crystal and air has been derived, enabling discussion of the effect of self-waveguiding of the emission on the basis of the components of the material dielectric tensor.
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