  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.
