# First-Principles Study for Gas Sensing of Defective SnSe\({}_{2}\) Monolayers
## 3 Results
### Gases Adsorbed on Different SnSe\({}_{2}\) Monolayers
#### 3.2.2 Nh\({}_{3}\)  
Figure 5 shows the optimized structures of NH\({}_{3}\) adsorbed on SnSe\({}_{2}\) monolayers, including top views as shown in Figure 5a-d and side views as shown in Figure 5e-h. When NH\({}_{3}\) is adsorbed on the pristine SnSe\({}_{2}\) monolayer, the site is positioned above the Sn atom, the d\({}_{\text{N-Se}}\) is 3.35 A and the orientation is N-bottom as shown in Figure 5a,e. When NH\({}_{3}\) is adsorbed on the Se-vacancy SnSe\({}_{2}\) monolayer, the site is positioned above the site of the single vacancy, the d\({}_{\text{H-Se}}\) is 2.90 A and the orientation is N-bottom, as shown in the Figure 5b,f. The most stable configuration of NH\({}_{3}\) adsorbed on the O-doped SnSe\({}_{2}\) monolayer is positioned at the O site, the d\({}_{\text{H-O}}\) is 3.06 A and the orientation is N-top as shown in Figure 5c,g. The optimized structure of NH\({}_{3}\) adsorbed on the N-doped SnSe\({}_{2}\) monolayer is positioned on the N site, the d\({}_{\text{N-N}}\) is 2.46 A and the orientation is N-bottom as shown in Figure 5d,h. It is worth noting that the orientation of NH\({}_{3}\) adsorbed on O-doped SnSe\({}_{2}\) monolayer is different from the other SnSe\({}_{2}\) monolayers, which is N-top orientation but others are N-bottom. The vertical distances h between NH\({}_{3}\) and the different SnSe\({}_{2}\) monolayers are 2.47 A, 0.45 A, 1.93 A, 1.45 A as shown in Figure 5e-h, respectively. The values of h of NH\({}_{3}\) adsorbed on the Se-vacancy and doped SnSe\({}_{2}\) monolayer are smaller than NH\({}_{3}\) adsorbed on the pristine SnSe\({}_{2}\) monolayer. Figure 4: DOS of NO\({}_{2}\) adsorption on the (**a**) pristine, (**b**) Se-vacancy, (**c**) O-doped and (**d**) N-doped SnSe\({}_{2}\) monolayer. The shadowed area is the total DOS. The blue and orange lines are the PDOS of N and O, respectively. The \(\mathrm{E_{F}}\) is denoted by a dashed line and shift to zero. When NH\({}_{3}\) is adsorbed on the pristine SnSe\({}_{2}\) monolayer, the value of h, E\({}_{\rm ad}\) and \(\Delta\)Q\({}_{\rm b}\) are 2.47 A, \(-\)0.18 eV and 0.028e [8], respectively. Compared to the value of E\({}_{\rm ad}\) and h in the Table 1, the E\({}_{\rm ad}\) of NH\({}_{3}\) adsorption almost follows a positive correlation to h, except for NH\({}_{3}\) adsorption on the O-doped SnSe\({}_{2}\) monolayer. When NH\({}_{3}\) is adsorbed on the Se-vacancy SnSe\({}_{2}\) monolayer, the value of h = 0.45 A is the minimum among NH\({}_{3}\) adsorption on SnSe\({}_{2}\) monolayers. Furthermore, the value of E\({}_{\rm ad}\) = \(-\)0.82 eV of NH\({}_{3}\) on the Se-vacancy SnSe\({}_{2}\) monolayer reaches the maximum compared with other cases. However, the charge transfer amount \(\Delta\)Q\({}_{\rm b}\) = 0.016e of NH\({}_{3}\) adsorbed on the Se-vacancy SnSe\({}_{2}\) monolayer is smaller than gas molecule on the pristine SnSe\({}_{2}\) (0.028e). For NH\({}_{3}\) adsorption on the O-doped SnSe\({}_{2}\) monolayer, the value of h = 1.93 A between NH\({}_{3}\) and the O-doped SnSe\({}_{2}\) monolayer is smaller than that on the pristine SnSe\({}_{2}\) monolayer. The values of E\({}_{\rm ad}\) and \(\Delta\)Q\({}_{\rm b}\) of NH\({}_{3}\) on the O-doped SnSe\({}_{2}\) monolayer are \(-\)0.13 eV and 0.000e, which are both the minimum among the NH\({}_{3}\) adsorption. This indicates that the ability of NH\({}_{3}\) adsorption of the O-doped SnSe\({}_{2}\) monolayer is weaker than the pristine SnSe\({}_{2}\) monolayer. For NH\({}_{3}\) adsorption on the N-doped SnS\({}_{2}\) monolayer, the value of h = 1.45 A between NH\({}_{3}\) and the O-doped SnSe\({}_{2}\) monolayer is smaller than that on the pristine SnSe\({}_{2}\) monolayer. E\({}_{\rm ad}\) also has a greater value \(-\)0.36 eV than that on the pristine SnSe\({}_{2}\) monolayer, and \(\Delta\)Q\({}_{\rm b}\) of NH\({}_{3}\) on the N-doped SnSe\({}_{2}\) monolayer has a maximum value 0.215e. This shows an enhancement of NH\({}_{3}\) adsorption on the N-doped SnSe\({}_{2}\) monolayer.