Van der Waals Bound Organic Semiconductor/2D-Material Hybrid Heterosystems: Intrinsic Epitaxial Alignment of Perfluoropentacene Films on Transition Metal Dichalcogenides

Abstract

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.

Perfluorinated Acenes: Crystalline Phases, Polymorph-Selective Growth, and Optoelectronic Properties

“…In previous work it was demonstrated that the growth of PFP films on graphene leads to a new π-stacked polymorph (PSP) where all molecules are flat lying and adopt a slip-stacked molecular packing . Later this interface stabilized phase was also found upon PFP growth on hexagonal basal planes of other weakly interacting substrates, like graphite, hexagonal boron nitride (hBN), or transition metal dichalcogenides (TMDCs). ,,, This raises the question of whether such an interface stabilized phase exists also for smaller perfluoro­acenes. Therefore, we have studied the film growth of PFT and PFA on graphene, which were characterized by means of XRD and NEXAFS.…”

Searching for New Polymorphs by Epitaxial Growth

“…Rearrangements of the molecules frequently occur at the interface so that deviations from the known crystallographic packing of the molecules appear. Successful explanations of organic epitaxy are based on lattice mismatch calculations − and ledge-directed or angle-directed crystallization. , A classification of epitaxial types, including both rigid and flexible lattices, was recently published …”

Leveraging Molecular Properties to Tailor Mixed-Dimensional Heterostructures beyond Energy Level Alignment

“…While it is tempting to assume that all large aromatic systems will lie π-face-on, the difference in orientation between H 2 Pc and CuPc on MoS 2 indicates that additional factors need to be considered. While some work on this topic has been performed, , many questions remain unanswered. One difficulty in studying organic adlayer orientation and morphology is the common presence of surface contaminants.…”