Superacid treatment of dielectric-encapsulated monolayer molybdenum disulfide for enhanced photoluminescence.

With a direct bandgap and strong photoluminescence (PL) emission, monolayer molybdenum disulfide (1L MoS2) is a promising material for next-generation optoelectronic technologies. However, the intrinsically low PL quantum yield (PLQY) and the requirement for dielectric integration present significant challenges in the realisation of practical MoS2-based devices. Chemical treatment with the superacid bis(trifluoromethanesulfonyl)amide (TFSA) is known to enhance dramatically the PL emission of 1L MoS2 through defect passivation and p-type doping. Conversely, atomic layer deposition (ALD) of a dielectric material can introduce defect states and induce n-type doping, thus degrading the MoS2 optical performance. Despite the competing effects of these two processing routes, the combined impact of superacid treatment and ALD dielectric growth on 1L MoS2 has not been thoroughly studied. In this work, we reveal the instability of the TFSA-enhanced MoS2 PL intensity during subsequent ALD of aluminium oxide (Al2O3), due to thermal exposure and precursor interaction. In contrast, we demonstrate that TFSA treatment of thermal Al2O3-encapsulated 1L MoS2 can successfully enhance the PL intensity by up to two orders of magnitude, even for dielectric films of up to ∼96% surface coverage. This enhancement is suppressed only when the Al2O3 layer becomes sufficiently thick (>∼40 nm) and approaches complete coalescence, preventing access of TFSA-derived species to the underlying MoS2. This work establishes an effective encapsulation/passivation strategy for enhancing the optical performance of dielectric-covered 1L MoS2, offering a practical pathway for integrating chemical passivation into device-relevant architectures.