Dynamic Charge Redistribution as the Key Mechanism for NO 2 Detection in MoS 2 Revealed by In Operando Scanning Photoelectron Microscopy

: Molybdenum disulfide (MoS2) represents a promising candidate for low-power chemoresistive nitrogen dioxide (NO2) sensing, and its potential for technological applications has generated considerable research interest. However, the sensing mechanism remains poorly understood, particularly regarding the type of MoS2-NO2 interaction and if morphology, defectivity, and thickness influence charge redistribution mechanisms. Therefore, we investigated MoS2 thin films grown by ionized jet deposition (IJD), a technique suitable for large-area and industry-compatible fabrication, alongside exfoliated monolayer MoS2 chemoresistors to elucidate the microscopic origin of NO2-induced resistance changes. Using controlled gas sensing combined with in operando micro-focused X-ray photoelectron spectroscopy and scanning photoemission microscopy, we directly correlated resistance modulation with electronic structure variations, which is further supported by density functional theory calculations. Both atomically thin MoS2 monolayers and nanostructured defect-rich IJD-MoS2 thin films showed Mo 3d and S 2p core-level shifts to higher binding energies during NO2 exposure, explained by local charge redistribution at the surface and modified core-level screening rather than net charge-transfer doping (i.e., transfer of free charge carriers into delocalized bands) and the associated band bending. In addition, IJD-MoS2 thin films exhibited a transient Mo 3d component attributable to weak Mo-O bonding at defect sites, rather than from permanent oxidation. The additional component and core-level shifts were reversible upon NO2 removal, evidencing a dynamic effect on the electronic structure. These results provide the first in operando insight into technologically scalable MoS2 chemoresistors and establish design principles for MoS2-based NO2 sensing applications.