Ultrathin transition metal dichalcogenide (TMD) heterostructures can be constructed via van der Waals interaction, including MoS2/WSe2. They show unique phenomena including interlayer excitons (IX), a bound state of electron and hole separately residing in the opposite layers. The IX in TMDs have extraordinary characteristics appropriate for various spintronic and excitonic devices. Despite the significant potential, however, the effects of electrostatic interactions that govern IX behaviors are not fully understood. This work investigates the mechanisms by inserting an ultrathin organic layer (&lt; 2 nm) between MoS2/WSe2 heterobilayers. The organic layer governs the electrostatic interaction of IX in two distinct pathways. Firstly, the molecular system reduces the dielectric constant in MoS2/WSe2, leading to an enhanced dielectric screening. Simultaneously, the inserted organic layer increases the separation distance of charges in IX. Therefore, the IX in MoS2/organic/WSe2 will have a weaker exciton binding energy than those in TMD heterolayers. We estimated the binding energy reduction to be ~40 meV via hydrogen-like-particle and dielectric-slab models. The weakened electrostatic interaction was confirmed by observing the photoluminescence (PL) emission characteristics. The organic-layer-embedded TMD heterostructure demonstrated a blue-shifted PL emission of IX, compared to MoS2/WSe2 heterobilayer. This is consistent with our predictions on the changes of the IX binding energy. Secondly, we explored two distinct interlayer charge transfer mechanisms such as tunneling and band-assisted transfer. A layer of organic molecules, forming a type-II junction, facilitated photoexcited electron transfer by providing an energetically favorable, intermediate energy level. The hybrid heterostructure thus had a greater population of IX, enhancing the repulsive dipolar interaction among IX. This resulted in a further blue-shifted PL emission compared to the samples that rely on tunneling mechanisms for IX formation. Our findings will provide deeper insights on IX which may be useful for future optoelectronics.

Interlayer excitons in atomically thin van der Waals heterostructures: Electrostatic interaction dependent behaviors in organic layer inserted heterohybrids

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