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

We report on the fabrication, the morphological, structural, and chemical characterization, and the study of the electrical response to NO2 and other gases of resistive type gas sensors based on liquid chemically exfoliated (in N-methyl pyrrolidone, NMP) MoS2 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 MoS2 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 MoS2, while, annealing at 250 °C, does not introduce a detectable bulk contamination in the expected form of MoO3. The sensor obtained by thermal annealing in air at 150 °C exhibits a peculiar p-type response under exposure to NO2. In line with core level spectroscopy evidences, this behavior is potentially ascribed to nitrogen substitutional doping of S vacancies in the MoS2 surface (nitrogen atoms being likely provided by the intercalated NMP). Thermal annealing the MoS2 flakes in air at 250 °C irreversibly sets an n-type behavior of the gas sensing device, with a NO2 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 MoS2 annealed flakes and to the surface co-existence of MoO3 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 H2 exposure.