Organic-organic epitaxy is a successful strategy for growing highly oriented and crystalline heterojunctions of organic semiconductors. Within this class of materials, crystalline rubrene is especially promising because of its outstanding hole mobility. Here, the (200) surface of rubrene single crystals was used as substrate for growing thin films of another organic semiconductor: R-quaterthiophene. The film growth proceeds via a 3D mechanism, with the formation of a fewmonolayer-thick crystalline islands oriented along two main directions, as deduced by the optical reflection of the R-quaterthiophene/rubrene heterostructure measured over macroscopic regions. An atomic-scale analysis of the film surface performed with scanning force microscopy revealed a complete textural order achieved through a line-on-line epitaxial relation. With the help of empirical force-field calculations of the heteroepitaxial interface, this orienting propensity is demonstrated to be strictly related to the peculiar corrugation of the rubrene crystal surface, showing marked furrows along two orthogonal crystallographic directions.

Stable rubrene derivatives displaying the same crystal packing features as orthorhombic rubrene are synthesized and their solid state properties studied.

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The transverse component of the friction forces acting on the tip of an atomic force microscope scanning on the surface of an organic crystal was monitored as a function of the scan direction. The relation between friction and the crystallographic system is disclosed, revealing that the symmetry of the friction phenomenon is dictated by the direction of the prominent corrugations of the crystal surface. It is also illustrated that molecular-resolution images can be collected through the monitoring of the motion of the tip in a transverse direction with respect to the scan direction.

Rubrene (RUB) single crystal displaying the orthorhombic polymorph structure is one of the most promising organic semiconducting material in terms of charge carrier mobility and exciton diffusion length. In view of the development of RUB-based devices where structural disorder in the active components would reduce performances, RUB has to be integrated in the form of crystalline thin film either as a single active component or as a part of multilayer heterojunctions. Here, we show how to obtain highly crystalline and oriented RUB thin film heterostructures by growing RUB on top of another organic semiconductor thin film used as templating layer, thus taking advantage of organic epitaxy. A detailed analysis of the heteroepitaxial interface in terms of adhesion energy is presented with a detailed discussion of the epitaxial relationship between RUB overlayer and the layer underneath and of the driving forces leading to organic epitaxy with RUB.

Great effort is being devoted to the fabrication of electronic devices based on rubrene thin films, because its outstanding charge transport properties make it one of the most promising organic semiconducting materials. Nonetheless, charge transport is strongly affected by the degree of crystallinity and degradation by photo-oxidation of rubrene. In the present work, in order to understand the dynamics of oxidation of rubrene when in the crystalline thin-film form, a combination of scanning probe techniques, such as Kelvin probe, phase contrast, and surface morphology atomic force microscopy, is used to study the oxidation process under ambient conditions of rubrene crystalline ultrathin films grown by organic molecular beam epitaxy on tetracene substrates. These films have a thickness of one or two molecular layers and consist of separated, island-like, epitaxial domains, whose orientation is determined by organic epitaxy. Theoretical calculations of the rubrene peroxide molecular dipole and structural data of rubrene and of purposely grown rubrene peroxide crystals, are exploited to determine the oxidation dynamics of such thin films and its connection with morphological, structural, and dielectric properties of the films. We demonstrate the formation of a native crystalline rubrene peroxide layer on top of the pristine rubrene crystalline domains.

Organic-organic heteroepitaxy can represent the winning technique for growing crystalline and oriented heterostructures of organic semiconductors. However, a sound physical interpretation of mechanisms that control epitaxy is still missing for these low symmetry molecular systems, generally not obeying the usual lattice matching rules for inorganic systems. We discuss here a couple of paradigmatic examples of organicorganic heteroepitaxy suggesting a possible physical rationale for the formation of the heterostructure interfaces as it arises from experimental characterization and computer modeling with atom-atom potential simulations.

Epitaxial thin films of Pt(II) octaethyl porphyrin were grown on the cleavage surface of graphite and potassium hydrogen phthalate. The structure of the films was characterized by high resolution atomic force microscopy, showing that the two substrates template the growth of two layered phases with dramatic structural differences, strongly influencing π-stacking of adjacent molecules. The impact of these characteristics on the electronic structure of the film phases is illustrated by a thorough optical investigation.