Interaction between the small gas molecules and the defective WSe2 monolayer

In this study, the interaction between the gas molecules, including H2O, N2, CO, NO, NO2 and N2O, with the WSe2 monolayer containing a Se vacancy (denoted as VSe) is theoretically studied. It is found that H2O and N2 molecules are highly prone to be physisorbed on the VSe surface. The presence of the Se vacancy should significantly enhance the sensing ability of the WSe2 monolayer toward the H2O and N2 molecules. In contrast, CO and NO molecules highly prefer to be molecularly chemisorbed on the VSe surface with the non-oxygen atom occupying the Se site. Further, the exposed O atoms of the molecularly chemisorbed CO or NO can be reacted with the additional CO or NO molecule, to produce the C-doped or N-doped WSe2 monolayer. Our 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. The calculated energies suggest that the filling of the CO or NO molecule and the removal of the exposed O atom by the additional CO or NO molecule are both energetically and dynamically favorable. However, only the NO molecule and N atom doped WSe2 monolayers exhibit significantly improved electronic structures compared with the defective WSe2 monolayer. The NO2 and N2O molecules will dissociate directly to form a O-doped WSe2 with a physisorbed NO or N2. After desorbing the physisorbed NO or N2, the O-doped WSe2 monolayer can be obtained, 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 endothermic, the N2O dissociation may need to operate at an elevated temperature compared with the room temperature, due to its large energy barrier of ~ 1 eV.