Response to NO2 and other gases of resistive chemically exfoliated MoS2-based gas sensors

Abstract We report on the fabrication, the morphological, structural, and chemical characterization, and the study of the electrical response to NO 2 and other gases of resistive type gas sensors based on liquid chemically exfoliated (in N-methyl pyrrolidone, NMP) MoS 2 flakes annealed in air either at 150 °C or at 250 °C. The active material has been analyzed by scanning electron microscopy (SEM), and micro Raman and X-ray core level photoemission spectroscopies. SEM shows that MoS 2 exfoliated flakes are interconnected between electrodes of the sensing device to form percolation paths. Raman spectroscopy of the flakes before annealing demonstrates that the flakes are constituted by crystalline MoS 2 , while, annealing at 250 °C, does not introduce a detectable bulk contamination in the expected form of MoO 3 . The sensor obtained by thermal annealing in air at 150 °C exhibits a peculiar p -type response under exposure to NO 2 . In line with core level spectroscopy evidences, this behavior is potentially ascribed to nitrogen substitutional doping of S vacancies in the MoS 2 surface (nitrogen atoms being likely provided by the intercalated NMP). Thermal annealing the MoS 2 flakes in air at 250 °C irreversibly sets an n -type behavior of the gas sensing device, with a NO 2 detection limit of 20 ppb. This behavior is assigned, in line with core level spectroscopy data, to a significant presence of S vacancies in the MoS 2 annealed flakes and to the surface co-existence of MoO 3 arising from the partial oxidation of the flakes surface. Both p- and n -type sensors have been demonstrated to be sensitive also to relative humidity. The n -type sensor shows good electrical response under H 2 exposure.