It is shown

that the MoS2 sheet is considerably polarized upon the adsorption of gas molecules, and electrostatic interaction plays a role in the attractive interaction. The polarization in the H2O, NH3, NO, and NO2 cases are stronger than that in the O2 and CO cases, giving rise to a larger interaction energy. It explains why the former gives larger adsorption energies (-234, -250, -211, and -276 meV for H2O, NH3, NO, and NO2, respectively) than the latter (-116 and -128 meV for O2 and CO, respectively) mentioned above. Figure 3 Charge density difference plots. Charge density difference plots for (a) O2, (b) H2O, (c) NH3, (d) NO, (e) NO2, and (f) CO interacting with monolayer MoS2. The red (green) distribution corresponds to charge accumulation (depletion). Selleck AMN-107 The isosurface is taken as 5 × 10-4 e/Å3. The direction and value of charge transfer are also denoted.

We examine the electronic properties of monolayer MoS2 adsorbed with gas molecules. The band structure before adsorption is presented in Figure 4a. It is found that the pristine monolayer MoS2 is a semiconductor with a direct band gap of 1.86 eV at K point, which is in good agreement with reported works [37–39]. The band structures for both valence bands and conduction bands of monolayer MoS2 are not significantly altered when H2O, NH3, and CO are adsorbed, and the gap values remain around 1.86 eV (not shown here). The situation is similar in the cases of O2, NO, and NO2 except the flat impurity states in the gap of the host monolayer induced C646 cost by these adsorbates. While O2 introduces two close-lying down-spin states 0.519 oxyclozanide and 0.526 eV above the Fermi level (EF) in the band gap, NO2 introduces an unoccupied down-spin state 0.31 eV above EF, as given in Figure 4c. Three impurity states emerge inside the band gap upon the adsorption of NO, namely, one occupied up-spin state 0.12 eV below EF, one unoccupied up-spin state 0.11 eV above EF, and one unoccupied down-spin state close to the conduction band edge with an energy separation of 0.064 eV between them (see Figure

4b). The adsorption of O2, NO, and NO2 on the MoS2 surface, on the other hand, creates magnetic moments of 2.0, 1.0, and 1.0 μ B per supercell, respectively. Figure 4 Band structures. Band structures of (a) pristine, (b) NO-adsorbed, and (c) NO2-adsorbed monolayer MoS2. The black (red) line corresponds to the up-spin (down-spin) bands, whereas the dashed green line denotes the Fermi level. As the charge transfer between the adsorbed molecule and monolayer MoS2 plays a crucial role in determining the performance of the MoS2 sensor, it may be sensitive to the applied electric field, similar to the case of graphene [40]. For brevity, NO and NO2 adsorbed monolayers are chosen as the representative systems.