Interaction between H₂O, N₂, CO, NO, NO₂ and N₂O molecules and a defective WSe₂ monolayer

In this study, the interaction between gas molecules, including H₂O, N₂, CO, NO, NO₂ and N₂O, and a WSe₂ monolayer containing an Se vacancy (denoted as V_Se) has been theoretically studied. Theoretical results show that H₂O and N₂ molecules are highly prone to be physisorbed on the V_Se surface. The presence of the Se vacancy can significantly enhance the sensing ability of the WSe₂ monolayer toward H₂O and N₂ molecules. In contrast, CO and NO molecules highly prefer to be molecularly chemisorbed on the V_Se surface with the non-oxygen atom occupying the Se vacancy site. Furthermore, the exposed O atoms of the molecularly chemisorbed CO or NO can react with additional CO or NO molecules, to produce C-doped or N-doped WSe₂ monolayers. The calculated energies suggest that the filling of the CO or NO molecule and the removal of the exposed O atom are both energetically and dynamically favorable. Electronic structure calculations show that the WSe₂ monolayers are p-doped by the CO and NO molecules, as well as the C and N atoms. However, only the NO molecule and N atom doped WSe₂ monolayers exhibit significantly improved electronic structures compared with V_Se. The NO₂ and N₂O molecules will dissociate directly to form an O-doped WSe₂ monolayer, for which the defect levels due to the Se vacancy can be completely removed. The calculated energies suggest that although the dissociation processes for NO₂ and N₂O molecules are highly exothermic, the N₂O dissociation may need to operate at an elevated temperature compared with room temperature, due to its large energy barrier of ∼1 eV.