Controlling Exciton Dynamics at Organic/2D Interfaces with Nanoscale Molecular Patterns

Recently, the moiré pattern formed at 2D transition metal dichalcogenide (TMD) heterostructures has been used to control the dynamics of interlayer excitons (IX) at the TMD/TMD interface. At organic/TMD interfaces, similar periodic potentials can be created by controlling the nanoscale molecular pattern on top of the 2D layer. In this work, two molecules, PTCDI and PTCDA, which possess similar properties but form different molecular patterns on a MoS₂ surface, are investigated. Using photoemission spectroscopy, a large HOMO energy level splitting is observed in PTCDI on MoS₂, but not in PTCDA on MoS₂. Density functional theory calculation shows that the splitting originates from the different molecular patterns. The energy splitting causes very different IX’s dynamics for the two heterostructures, which are studied by time-resolved photoemission spectroscopy. For PTCDI/MoS₂, the electron within the IX is spatially localized near the interface. On the other hand, for PTCDA/MoS₂, the electron within the IX is delocalized across the molecular film. Because of the spatial localization, the IX at PTCDI/MoS₂ shows an order of magnitude stronger photoluminescence intensity as compared to the IX at PTCDA/MoS₂. On the other hand, the IX at PTCDA/MoS₂ is more likely to dissociate into free carriers because of the electron delocalization within the IX. This work is supported by NSF grant DMR-2109979.