Van Der Waals Bound Organic Semiconductor/2d-Material Hybrid Heterosystems: Intrinsic Epitaxial Alignment Of Perfluoropentacene Films On Transition Metal Dichalcogenides

Hybrid systems consisting of transition metal dichalcogenides (TMDCs) and organic semiconductors (OSCs) are promising candidates for future optoelectronic device architectures, as they combine the outstanding charge transport properties of TMDCs with the high photoabsorption cross-section and the ability to tailor the energy levels of OSCs through synthetic means. Since crystalline organic layers have very anisotropic optical absorption and emission characteristics, their azimuthal alignment is an important parameter for the efficiency of the optical coupling in such hybrid systems. The lack of dangling bonds makes surfaces of 2D materials quite inert and smooth and therefore allows an unrestricted growth of organic adlayers without the requirement of relaxation or commensurability at the interface, which is an important prerequisite for van der Waals (vdW) epitaxy. Here, we have studied the formation and azimuthal alignment of crystalline adlayers of the OSC perfluoropentacene (PFP) on the basal plane of MoSe2, WSe2, MoS2, and MoTe2 single crystals representing the most ideal TMDC surfaces, which are further compared with corresponding films on graphene and hBN. Combining specular X-ray reflectivity with polarization resolved optical reflection measurements allowed the azimuthal alignment of crystalline PFP domains to be precisely analyzed, which revealed characteristic twist angles between adlayer and substrate lattices for the various 2D materials. This orientational order can be rationalized as an on-line coincidence and is well reproduced by model calculations that are based on the scheme of projection of real-space adlayer lattice points onto the substrate unit cell. The extreme sensitivity of the resulting rotational alignment of epitaxial adlayers on the smallest changes of the lattice parameters is demonstrated by distinctly different twist angles for PFP films that were grown at an elevated temperature and thus show a slight thermal expansion. The presently introduced and validated model to describe the epitaxial alignment in weakly bound crystalline adlayers can also be applied to other vdW bound heterosystems and can be useful to elucidate the peculiarities of vdW epitaxy.