Terrylene on monolayer WS2: coverage-dependent molecular re-orientation and interfacial electronic energy levels

The electronic, optical, and functional properties of van der Waals heterostructures comprising organic and two-dimensional inorganic semiconductors depend on the structure of the molecular assembly at and near the interface. Despite the rising interest in such heterostructures, very little is known about the, potentially complex, interplay between the structure and resulting (opto)electronic properties. Herein, we demonstrate with photoemission spectroscopy and scanning tunneling microscopy experiments a coverage dependence of the molecular assembly of terrylene deposited onto monolayer WS2 (with sapphire serving as the substrate) and show how this impacts interfacial electronic properties. Up to monolayer coverage, terrylene molecules adapt a flat-lying orientation, which changes to an inclined orientation for higher coverages. This re-orientation is accompanied with a reduction in terrylene ionization energy by over 400 meV and an accordingly larger energy level offset of frontier energy levels of the two semiconductors and shift of the highest occupied molecular orbital energy level away from the WS2 valence band. This can, for instance, reduce the charge-separation efficiency of the heterostructure with molecular multilayer coverage compared to that with only monolayer coverage. Furthermore, the modification of monolayer WS2 excitonic features through molecular film deposition was evaluated using optical spectroscopy, yielding effective dielectric constants for a series of Rydberg excitons and exciton binding energies for bare and terrylene-covered monolayer WS2 supported by sapphire. Altogether, these findings allow a comprehensive and detailed understanding of the (opto)electronic properties of this prototypical van der Waals heterostructure.