Organic epitaxy

Summary

This paper presents a critical discussion of epitaxy when applied to organic molecular materials. Indeed, the possible growth of ordered layers, with known and controlled structural relationships with the substrates, is fundamental both for a deep comprehension of the material properties themselves and for future device applications. This is well documented for inorganics, but it is valid also for organic materials, whose interest for organic molecular electronics is increasing more and more. Nonetheless, a complete and fully satisfactory description of organic epitaxy is still lacking. The commonly accepted criteria for identifying epitaxial growth of overlayers on solid substrates were elaborated for inorganic materials and therefore match with the characteristics of these materials (atom-based materials, strong interactions and chemical bonds, lattice parameters of few Å). When dealing with organics, the same criteria reveal serious limits, as indeed could be expected, given the peculiarities of these materials, in particular the rather weak intermolecular interactions holding together the solid. After reviewing the basic properties of molecular solids, the attempts to widen the known concept of inorganic epitaxy to organics are reported. Then, the techniques used to detect and study organic epitaxy are illustrated and the discussion of particularly relevant case studies proposed, which permits to demonstrate how additional criteria related to the intermolecular interactions at the film/substrate interface and to the molecular arrangement at the surface can better fit with the behavior of organic materials.