Growth and charaterisation of all-organic heterostructures

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.