# 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 6 shows DOS of NH\({}_{3}\) adsorption on different SnSe\({}_{2}\) monolayers. The blue, orange, cyan lines are the PDOS of N, O, H, respectively. The DOS of NH\({}_{3}\) adsorption on the pristine SnSe\({}_{2}\) monolayers demonstrates a pattern of a semiconductor with energy gap as shown in Figure 6a. However, the DOS of NH\({}_{3}\) adsorption on the defective SnSe\({}_{2}\) monolayers presents a flat band as shown in Figure 6b-d. When NH\({}_{3}\) adsorbed on the pristine SnSe\({}_{2}\) monolayer, the total DOS shown in Figure 6a demonstrates a pattern for a semiconductor with an energy gap about 0.81 eV. The PDOS introduced by N and H atoms of gas molecules below E\({}_{\rm F}\) and above 0.81 eV is marked in blue and cyan lines as shown in Figure 6a. There is no obviously change of the electronic structure of the pristine SnSe\({}_{2}\) as shown in Figure 2a, which is consistent with our previous work [8]. Figure 5: The most stable configurations of different SnSe\({}_{2}\) monolayers adsorbing NH\({}_{3}\) 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, N and H atoms are purple, green, red, grey and cyan, respectively. When NH\({}_{3}\) adsorbed on the Se-vacancy SnSe\({}_{2}\) monolayer, the total DOS presents a bandwidth in energy range of \(-0.17\) eV to E\({}_{\text{F}}\) as shown in Figure 6b. The DOS pattern of Figure 6b is a semiconductor with energy gap about 0.27 eV. Only a small amount of PDOS contributed by N and H atoms of gas molecule is located the aforementioned bandwidth. This is so that NH\({}_{3}\) adsorption does not induce obvious difference on the DOS of the Se-vacancy SnSe\({}_{2}\) without adsorption as shown in Figure 2b. For the O-doped SnSe\({}_{2}\) monolayer with NH\({}_{3}\) adsorption, the total DOS presents a flat band below E\({}_{\text{F}}\) as shown in Figure 6c. A large amount of PDOS contributed by N and H atoms of gas molecules is located in the aforementioned flat band denoted by blue and cyan lines in Figure 6c. Because the flat band is located below E\({}_{\text{F}}\), the DOS pattern of Figure 6c is a semiconductor with energy gap about 0.70 eV. Therefore, the NH\({}_{3}\) adsorption does not change the pattern of DOS of the O-doped SnSe\({}_{2}\) monolayer as shown in Figure 2c. For the N-doped SnSe\({}_{2}\) monolayer with NH\({}_{3}\) adsorption, the total DOS presents a trap state across E\({}_{\text{F}}\) in the energy range about \(-0.10\)eV to \(-0.03\)eV as shown in Figure 6d. A large amount of PDOS induced by N and H atoms of gas molecules is located in the aforementioned trap state. The trap state responds to a flat band which would decrease the mobility of the SnSe\({}_{2}\) systems after gas adsorption. The pattern of DOS in Figure 6d is different from that of the N-doped SnSe\({}_{2}\) monolayer without adsorption as shown in Figure 2d. Comparing the DOS of NH\({}_{3}\) adsorption on different SnSe\({}_{2}\) monolayers as shown in Figure 6a-d with the SnSe\({}_{2}\) monolayers without adsorption as shown in Figure 2a-d, only the N-doped SnSe\({}_{2}\) monolayer could induce obvious differences of electronic structure before/after NH\({}_{3}\) adsorption. Figure 6: DOS of NH\({}_{3}\) 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, orange, cyan lines are the PDOS of N, O, H, respectively. The E\({}_{\text{F}}\) is denoted by a dashed line and shift to zero.