Organic-organic hetero-epitaxy of semiconductor crystals: α-quaterthiophene on rubrene

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: α-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 α-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