# First-Principles Study for Gas Sensing of Defective SnSe\({}_{2}\) Monolayers
## 3 Results
### Gases Adsorbed on Different SnSe\({}_{2}\) Monolayers
#### 3.2.1 No\({}_{2}\) Adsorption
Furthermore, the O-doped SnSe\({}_{2}\) monolayer adsorbs the NO\({}_{2}\) molecule and would induce a little increase in the adsorption energy but decrease the Bader charge by about \(-\)0.32 eV and \(-\)0.145e, respectively, as shown in Table 1.  
When NO\({}_{2}\) is adsorbed on the N-doped SnSe\({}_{2}\) monolayer, the value of h between NO\({}_{2}\) and the N-doped SnSe\({}_{2}\) monolayer is 0.72 A. This indicates that the strong interaction comes from the N-N atoms interaction between NO\({}_{2}\) and the N dopant. The \(\rm E_{ad}=-\)2.98 eV of NO\({}_{2}\) adsorbed on the N-doped SnSe\({}_{2}\) monolayer is greater than on the Se-vacancy SnSe\({}_{2}\) monolayer. \(\rm\Delta Q_{b}=-\)0.368e of NO\({}_{2}\) adsorbed on the N-doped SnSe\({}_{2}\) monolayer is smaller than on the Se-vacancy SnSe\({}_{2}\), but greater than the pristine and O-doped SnSe\({}_{2}\). The relatively high \(\rm E_{ad}\) and \(\rm\Delta Q_{b}\) demonstrate strong interaction between NO\({}_{2}\) and the N-doped SnSe\({}_{2}\) monolayer. The DOS of NO\({}_{2}\) adsorbed on different SnSe\({}_{2}\) monolayers is shown in Figure 4. The shadowed area is the total DOS. The blue and orange lines are the PDOS of N and O, respectively. The \(\rm E_{F}\) is denoted by a dashed line. DOS of NO\({}_{2}\) adsorption on different SnSe\({}_{2}\) monolayers, as shown in Figure 4a-d, are quite different to Figure 2a-d without NO\({}_{2}\) adsorption cases. This indicates that all the SnSe\({}_{2}\) monolayers could induce obvious difference of electronic structure before/after NO\({}_{2}\) adsorption. When NO\({}_{2}\) is adsorbed on the pristine SnSe\({}_{2}\), the total DOS presents a trap state across \(\rm E_{F}\) in the energy range about \(-\)0.04 eV to \(-\)0.02 eV, as shown in Figure 4a. A large amount of PDOS induced by N and O atoms of gas molecule is located in the aforementioned trap state, which induces the flat band and trap electron on it as mentioned in previous work [8]. The pattern of DOS in Figure 4a is different from DOS of the pristine SnSe\({}_{2}\) monolayer without adsorption as shown in Figure 2a. For NO\({}_{2}\) adsorption on the Se-vacancy SnSe\({}_{2}\) monolayer, the total DOS presents a bandwidth near the \(\rm E_{F}\) with an energy range of \(-\)0.19 eV to \(-\)0.12 eV, as shown in Figure 4b. A considerable amount of PDOS contributed by N and O atoms of the gas molecule is located the aforementioned bandwidth. Therefore, the pattern of DOS in Figure 4b is quite different from DOS of the Se-vacancy SnSe\({}_{2}\) without adsorption as shown in Figure 2b. When NO\({}_{2}\) is adsorbed on the O-doped SnSe\({}_{2}\) monolayer, the total DOS presents a trap state across \(\rm E_{F}\) in the energy range about \(-\)0.06 eV to \(-\)0.03 eV, as shown in Figure 4c. A large amount of PDOS induced by N and O atoms of the gas molecule is located in the aforementioned trap state, which corresponds to a flat band. The trap state would trap electrons on it and decrease the carrier mobility. The DOS of NO\({}_{2}\) adsorption the O-doped SnSe\({}_{2}\) monolayer is very similar with the pristine SnSe\({}_{2}\) monolayer, but different from the O-doped SnSe\({}_{2}\) monolayer without adsorption as shown in Figure 2c. For NO\({}_{2}\) adsorption on the N-doped SnSe\({}_{2}\) monolayer, the total DOS shown in Figure 4d demonstrates a pattern for a semiconductor with an energy gap of about 0.86 eV. The PDOS introduced by N and O atoms of gas molecule below \(\rm E_{F}\) and above 0.86 eV marked in blue and orange lines as shown in Figure 4d, which is quite different from Figure 4a-c. It is worth noting that the peak of PDOS of the N atom near the \(\rm E_{F}\) shifts to \(-\)0.20 eV when NO\({}_{2}\) is adsorbed as shown in Figure 4d. It indicatesthat PDOS of the N atoms near \(\mathrm{E_{F}}\) in Figure 2d are moved to below \(\mathrm{E_{F}}\) because the strong interaction occurred after NO\({}_{2}\) adsorption.