# 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  
Figure 3 shows the optimized structures of NO\({}_{2}\) adsorbed on SnSe\({}_{2}\) monolayers, including top views as shown in Figure 3a-d and side views, as shown in Figure 3e-h. When NO\({}_{2}\) is adsorbed on the pristine SnSe\({}_{2}\) monolayer, the site of NO\({}_{2}\) adsorption is near the Se atom, the d\({}_{\text{O-Se}}\) is 2.97 A and the orientation is N-top, as shown in Figure 3a,e. When NO\({}_{2}\) is adsorbed on the Se-vacancy SnSe\({}_{2}\) monolayer, the site is near the Sn atom, the d\({}_{\text{O-Sn}}\) is 2.28 A and the orientation is N-bottom, as shown in Figure 3b,f. The optimized structure of NO\({}_{2}\) on the O-doped SnSe\({}_{2}\) monolayer is upon the site of the O atom, the d\({}_{\text{O-O}}\) is 3.05 A and the orientation is N-top, as shown in Figure 3c,g. The optimized structure of NO\({}_{2}\) on the N-doped SnSe\({}_{2}\) monolayer is located on the N site and the d\({}_{\text{N-N}}\) is 1.38 A with the orientation N-bottom, as shown in Figure 3d,h. Moreover, the vertical distances h between NO\({}_{2}\) and the defective SnSe\({}_{2}\) monolayers are 2.45 A, \(-\)0.08 A, 1.75 A, 0.72 A, respectively. The negative value of h means the gas molecule is sucked into the SnSe\({}_{2}\) monolayer. In conclusion, the values of h between NO\({}_{2}\) and the defective SnSe\({}_{2}\) monolayer are shorter than NO\({}_{2}\) adsorbed on the pristine SnSe\({}_{2}\) monolayer. When comparing with orientation and the h, the structures of NO\({}_{2}\) adsorbed on the SnSe\({}_{2}\) monolayers could divide into two groups. First, the NO\({}_{2}\) adsorption on the pristine and O-doped SnSe\({}_{2}\) monolayer are both N-top orientation. Second, the NO\({}_{2}\) adsorption on the Se-vacancy and N-doped SnSe\({}_{2}\) monolayer are both with N-bottom orientation. The values of h are smaller for NO\({}_{2}\) adsorption on the Se-vacancy and N-doped SnSe\({}_{2}\) monolayer than those on the pristine and O-doped SnSe\({}_{2}\) monolayer. Figure 3: The most stable configurations of different SnSe\({}_{2}\) monolayers adsorbing NO\({}_{2}\) molecule. The top view of (**a**) pristine, (**b**) Se vacancy, (**c**) O-doped and (**d**) N-doped SnSe\({}_{2}\) monolayer; the side view of (**e**) pristine, (**f**) Se vacancy, (**g**) O-doped and (**h**) N-doped SnSe\({}_{2}\) monolayer, respectively. The Sn, Se, O, and N atom are purple, green, red and grey, respectively. The value of \(\rm E_{ad}\) and \(\rm\Delta Q_{b}\) of NO\({}_{2}\) adsorbed on the pristine SnSe\({}_{2}\) monolayer are \(-\)0.29 eV and \(-\)0.164e, respectively [8]. Furthermore, the \(\rm E_{ad}\) of NO\({}_{2}\) adsorbed on the Se-vacancy and doped SnSe\({}_{2}\) monolayer are greater than NO\({}_{2}\) adsorbed on the pristine SnSe\({}_{2}\) monolayer, as shown in Table 1.  
The value of h between NO\({}_{2}\) and the Se-vacancy SnSe\({}_{2}\) monolayer is \(-\)0.08 A. This induces a large adsorption energy and Bader charge because the NO\({}_{2}\) molecule is sucked into the vacancy site. The \(\rm E_{ad}\) and \(\rm\Delta Q_{b}\) are up to \(-\)1.84 eV and \(-\)0.926e, respectively, as listed in Table 1. The value of \(\rm E_{ad}\) is greater than the pristine and the O-doped SnSe\({}_{2}\) monolayer but smaller than the N-doped SnSe\({}_{2}\) system. The value of \(\rm\Delta Q_{b}=-\)0.926e of NO\({}_{2}\) adsorbed on the Se-vacancy SnSe\({}_{2}\) monolayer is the maximum of that on the SnSe\({}_{2}\) monolayers. This result indicates that NO\({}_{2}\) molecule would strongly interact with the Se-vacancy SnSe\({}_{2}\) monolayer. As mentioned, the structure of NO\({}_{2}\) adsorbed on the O-doped SnSe\({}_{2}\) monolayer is similar with on the pristine SnSe\({}_{2}\) monolayer despite of the O atom replacing the Se atom. In the detail, the value of h between NO\({}_{2}\) and the O-doped SnSe\({}_{2}\) monolayer is 1.75 A, which is also a little smaller than that on the pristine SnSe\({}_{2}\) monolayer.