Interaction between HO, N, CO, NO, NO and NO molecules and a defective WSe monolayer.

In this study, the interaction between gas molecules, including HO, N, CO, NO, NO and NO, and a WSe monolayer containing an Se vacancy (denoted as V) has been theoretically studied. Theoretical results show that HO and N molecules are highly prone to be physisorbed on the V surface. The presence of the Se vacancy can significantly enhance the sensing ability of the WSe monolayer toward HO and N molecules. In contrast, CO and NO molecules highly prefer to be molecularly chemisorbed on the V 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 WSe 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 WSe 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 WSe monolayers exhibit significantly improved electronic structures compared with V. The NO and NO molecules will dissociate directly to form an O-doped WSe 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 NO and NO molecules are highly exothermic, the NO dissociation may need to operate at an elevated temperature compared with room temperature, due to its large energy barrier of ∼1 eV.