Photoactivation of Strong Photoluminescence in Superacid-Treated Monolayer Molybdenum Disulfide

To advance the development of atomically thin optoelectronics using two-dimensional (2D) materials, engineering strong luminescence with a physicochemical basis is crucial. Semiconducting monolayer transition-metal dichalcogenides (TMDCs) are candidates for this, but their quantum yield (QY) is known to be poor. Recently, a molecular superacid treatment of bis(trifluoromethane)sulfonimide (TFSI) generated unambiguously bright monolayer TMDCs and a high QY. However, this method is highly dependent on the processing conditions and therefore has not been generalized. Here, we shed light on environmental factors to activate the photoluminescence (PL) intensity of the TFSI-treated monolayer MoS₂, with a factor of more than 2 orders of magnitude greater than the original by photoactivation. The method is useful for both mechanically exfoliated and chemically deposited samples. The existence of photoirradiation larger than the band gap demonstrates enhancement of the PL of MoS₂; on the other hand, activation by thermal annealing, as demonstrated in the previous report, is less effective for enhancing the PL intensity. The photoactivated monolayer MoS₂ shows a long lifetime of ∼1.35 ns, more than a 30-fold improvement over the original as exfoliated. The consistent realization of the bright monolayer MoS₂ reveals that air exposure is an essential factor in the process. TFSI treatment in a N₂ environment was not effective for achieving a strong PL, even after the photoactivation. These findings can serve as a basis for engineering the bright atomically thin materials for 2D optoelectronics.