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

High Resolution Image Download MS PowerPoint Slide Molybdenum disulfide (MoS 2 ) represents a promising candidate for low-power chemoresistive nitrogen dioxide (NO 2 ) sensing, and its potential for technological applications has generated considerable research interest. However, the sensing mechanism remains poorly understood, particularly regarding the type of MoS 2 −NO 2 interaction and if morphology, defectivity, and thickness influence charge redistribution mechanisms. Therefore, we investigated MoS 2 thin films grown by ionized jet deposition (IJD), a technique suitable for large-area and industry-compatible fabrication, alongside exfoliated monolayer MoS 2 chemoresistors to elucidate the microscopic origin of NO 2 -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 MoS 2 monolayers and nanostructured defect-rich IJD-MoS 2 thin films showed Mo 3d and S 2p core-level shifts to higher binding energies during NO 2 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-MoS 2 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 NO 2 removal, evidencing a dynamic effect on the electronic structure. These results provide the first in operando insight into technologically scalable MoS 2 chemoresistors and establish design principles for MoS 2 -based NO 2 sensing applications.