Stable photoluminescence enhancement in monolayer molybdenum disulfide by superacid treatment

Monolayer molybdenum disulfide (1L MoS₂) has emerged as a promising material for next-generation optoelectronic technologies, owing to its direct bandgap and resulting photoluminescence (PL) emission in the visible spectral range. Despite its favorable PL character, the emission efficiency from as-grown 1L MoS₂ is often poor, with the PL quantum yield (PLQY) reported to be as low as <1%. Treatment with superacid bis(trifluoromethanesulfonyl)amide (TFSA) has been widely reported to enhance the 1L MoS₂PL intensity by up to 2 orders of magnitude, capable of achieving near-unity PLQY (≥95%). However, the long-term stability of the elevated 1L MoS₂ PL emission under ambient conditions, which is a critical factor in the performance of practical optoelectronic devices, is not well-understood. In this work, we study the evolution of the PL character of TFSA-treated 1L MoS₂ over 120 days of ambient storage. We reveal that the initial TFSA treatment leads to the formation of a nanoscale superacidic layer on the 1L MoS₂ surface, yielding an order-of-magnitude increase in PL intensity, coupled with a blueshift and narrowing of the PL emission. We attribute these modifications to a synergistic effect of defect passivation and charge doping. By combining atomic force microscopy with spatially resolved Raman/PL spectroscopy, we demonstrate that the superacid nanolayer morphology and enhanced PL emission are preserved under ambient conditions for at least 120 days. This work establishes the long-term ambient stability of TFSA-enhanced 1L MoS₂, highlighting superacid treatment as a viable route for realizing stable atomically thin optoelectronic devices.