  Gas sensing mechanism of H2S, NH3, NO2 and NO toxic gases on transition metal
dichalcogenides based Janus MoSSe monolayers are investigated using the density
functional theory. The pristine and defect included MoSSe layers are considered
as a host material for adsorption study. Three types of defects (i) molybdenum
vacancy, (ii) selenium vacancy, and (iii) sulfur/selenium vacancy are studied
to understand their impact on electronic properties and sensing of these gas
molecules. The formation energy is computed to predict the stability of these
defects and noticed that selenium vacancy is the most stable among other
defects. The adsorption of gas molecules is evaluated in terms of adsorption
energy, vertical height, charge difference density, Bader charge analysis,
electronic and magnetic properties. The maximum adsorption energy for H2S, NH3,
NO2 and NO molecules on pristine Janus MoSSe monolayer are ~ -0.156eV,
-0.203eV, -0.252eV, and -0.117eV, respectively. Selenium and sulfur/selenium
defects significantly improve the sensing of the gas molecules. NO2 gas
molecule dissociates and forms oxygen doped NO adsorption in selenium and
sulfur/selenium defect included MoSSe Janus monolayer. The adsorption energy
values are ~ -3.360eV and -3.404eV for Se and S/Se defects included MoSSe
layer, respectively. Further, the adsorption of NO2 molecule induced about
1/mu/B magnetic moment. In contrast, NO molecule showed chemisorption on the
surface of the selenium and sulfur/selenium defect included Janus MoSSe
monolayers, whereas H2S and NH3 molecules showed physisorption with their
adsorption energies in the range of -0.146 to -0.238 eV and -0.140 to -0.281
eV, respectively. The adsorption of H2S, NH3, NO2 and NO molecule on the
pristine and defected monolayers suggest that selenium and sulfur/selenium
vacancy defects are more prominent for NO2 and NO gas molecule adsorption.
