Solid-State Ionomer-Interlayered Bulk Monolayer MoS₂ Membranes with Thickness-Scalable Bright Luminescence

Monolayer MoS2 is attractive for future photonics and optoelectronics for its direct-gap excitons with ultrahigh binding energies. However, the atomically thin structure is prone to nonradiative defects, challenging to scale for large-area production, and inherently limited in optical cross-section, all hindering technological integration. Stacking monolayers to increase the optical cross-section results in multilayers with diminished photoluminescence activity. Electrochemical molecular intercalation of multilayer MoS2 weakens interlayer coupling, creating a bulk monolayer material that retains monolayer characteristics within a multilayered "bulk" architecture. However, the electrochemical process often severely degrades the optical quality and is difficult to implement for scalable thin film production. Here, we report a luminescent monolayer MoS2 colloidal ink for scalable solution processing of large-area bulk monolayer MoS2 membranes with thickness-scalable bright luminescence. Using perfluorinated sulfonic-acid ionomer (Nafion) as a multifunctional ligand, we achieve a stable dispersion of monolayer MoS2 ink that can be readily processed into large-area stretchable membranes with diverse form factors. The Nafion interlayers decouple interlayer interactions while effectively passivating nonradiative defects, preserving direct-gap monolayer characteristics in thickness-scalable MoS2/Nafion membranes. This leads to unprecedented enhancements in both linear and nonlinear excitonic transitions, showing over 100-fold brighter excitonic photoluminescence and nearly 1000 times stronger second harmonic generation. These remarkable properties remain robust against intense laser irradiation, diverse environmental conditions, and mechanical deformations. Our studies overcome the longstanding challenge of poor optical quality in solution-processed monolayer MoS2 ink materials, establishing a large-area, thickness-scalable platform for excitonic devices across diverse technological domains, including photonics, optoelectronics, solution-processable bioelectronics, and electrochemical sensing.