Dual-Mode Exciton Coupling in Epitaxially Registered Organic–Inorganic 2D Heterocrystals

Two-dimensional (2D) heterocrystals comprising molecules and semiconductors can serve as an ideal platform for studying interfacial excitons and for future optoelectronic applications, yet the energy and charge flow across these atomically sharp interfaces remain unclear. In this work, we investigated PTCDA/MoS2 as a prototypical 2D organic–inorganic heterostructure and revealed dual-mode exciton coupling between the constituent crystals. Monolayer-resolved PTCDA molecular crystals were grown on monolayer MoS2 via physical vapor assembly, and their crystallographic details, including the stacking angle, were determined by electron diffraction. Upon the formation of the heterostructures, PTCDA’s photoluminescence was nearly completely quenched by efficient interfacial excited-state decay, whereas that of MoS2 increased markedly with PTCDA thickness. Using differential reflectance and photoluminescence excitation (PLE) spectroscopy, we found that the enhancement arises from two distinct contributions. One contribution, observed without direct PTCDA excitation, is consistent with ground-state charge transfer injecting holes into MoS2, which suppresses negative trion formation and enhances the radiative recombination of neutral excitons. The other, correlated with PTCDA absorption in the PLE response, supports resonance energy transfer, which diverts excitation energy from PTCDA to MoS2. Our findings help reconcile previously proposed mechanisms and show how molecular and MoS2 excitonic responses evolve with molecular thickness in a structurally defined heterocrystal.