Electronic coupling and photoluminescence anisotropy in van-der-Waals-stacks of tungsten disulphide with molecular single crystals†

Abstract

Heterostructures comprised of organic semiconductors and transition metal dichalcogenides are promising candidates for device applications. In order to promote efficient charge or exciton transfer, the arrangement of the molecules at the heterointerface is crucial. However, to complement theoretical efforts, there is presently a lack of model systems to comprehensively study electronic coupling phenomena. Here, we explore heterostructures of tungsten disulphide with monocrystalline thin flakes of the organic compound pyrenemethylammonium chloride. The ionic character of the organic compound enables controlled fabrication of heterostructures via exfoliation of single crystals, thereby minimizing structural disorder in the molecular compound. In order to explore the optical properties and the photoexcitation dynamics related to the heterointerface, we employ time and polarization resolved photoluminescence spectroscopy. Evidence for electronic coupling is provided by three experimental findings: first, a faster decay of the luminescence of the organic compound in the region of the heterostructure indicates exciton or charge transfer, which is also surprisingly efficient for the structurally relaxed localized excimer state. Second, we find that the organic compound introduces a polarization anisotropy into the emission of tungsten disulphide, potentially due to the lowering of symmetries in the band structure. Third, we identify a weakly emissive feature at an energy around 1.7 eV, which could be related to an interfacial state. Overall, ionic molecular systems could serve as a versatile platform for combined experimental and theoretical studies of the interactions at the heterointerface.