# First-Principles Study for Gas Sensing of Defective SnSe\({}_{2}\) Monolayers
## 3 Results
### Gases Adsorbed on Different SnSe\({}_{2}\) Monolayers  
In this section, we discuss the most stable configurations of NO\({}_{2}\) and NH\({}_{3}\) molecules adsorbed on different SnSe\({}_{2}\) monolayer systems, whichare listed in Table 1.  
The adsorption energy (E\({}_{\text{ad}}\)) of gas molecules on SnSe\({}_{2}\) monolayers is defined as  
\[\text{E}_{\text{ad}}=\text{E}_{\text{Gas + SnSe2}}-\text{E}_{\text{SnSe2}}-\text{E}_{\text{Gas}}, \tag{1}\]  
where E\({}_{\text{Gas + SnSe2}}\) is the total energy of the gas molecules adsorbed on SnSe\({}_{2}\) monolayers, E\({}_{\text{SnSe2}}\) is the total energy of the SnSe\({}_{2}\) monolayer and E\({}_{\text{Gas}}\) is the energy of isolated gas molecule. A negative E\({}_{\text{ad}}\) value means that gas molecules on the SnSe\({}_{2}\) monolayer is energetically favorable. In our previous work [8], the adsorption energy of NO\({}_{2}\) adsorbed on the pristine SnSe\({}_{2}\) monolayer is higher than that of NH\({}_{3}\) adsorption in theoretical prediction. In experimental demonstrations, the pristine SnSe\({}_{2}\) monolayer can detect NO\({}_{2}\) in lower concentrations than NH\({}_{3}\). Although the value of adsorption energy is not direct and not the only relation to the sensitivity, both of them could coincide in terms of theoretical calculation and experimental measurements. Generally, the E\({}_{\text{ad}}\) values of NO\({}_{2}\) on SnSe\({}_{2}\) monolayer systems (\(-0.29\) to \(-2.98\) eV) are larger than NH\({}_{3}\) (\(-0.13\) to \(-0.82\) eV). This indicates that the sensitivity of NO\({}_{2}\) is higher than NH\({}_{3}\) on SnSe\({}_{2}\) monolayers. Moreover, the distance between NO\({}_{2}\) and SnSe\({}_{2}\) monolayers is smaller than the distance between NH\({}_{3}\) and SnSe\({}_{2}\) monolayer. The Bader charge population analysis [21] is summarized in Table 1 where negative \(\Delta\)Q\({}_{\text{b}}\) indicates electron charge transfer from SnSe\({}_{2}\) to a gas molecule and a positive \(\Delta\)Q\({}_{\text{b}}\) shows charge transfer from a gas molecule to SnSe\({}_{2}\) monolayers. Our calculation shows that the NO\({}_{2}\) is an electron charge acceptor, \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Gas** & \begin{tabular}{c} **SnSe\({}_{2}\)** \\ **System** \\ \end{tabular} & \begin{tabular}{c} **Gas** \\ **Orientation** \\ \end{tabular} & \
\begin{tabular}{c} **E\({}_{\text{ad}}\) (eV)** \\ \end{tabular} &
\begin{tabular}{c} **d\({}_{\text{atom-atom}}\) (Å)** \\ \end{tabular} & **h (Å)** & \(\Delta\)**Q\({}_{\text{b}}\)** & **Source** \\ \hline \multirow{4}{*}{NO\({}_{2}\)} & Pristine & N-top & \(-0.29\) & \(2.97\) & \(2.45\) & \(-0.164\)e & Reference [8] \\  & Se vacancy & N-bottom & \(-1.84\) & \(2.28\) & \(-0.08\) & \(-0.926\)e & This work \\  & O-doped & N-top & \(-0.32\) & \(3.05\) & \(1.75\) & \(-0.145\)e & This work \\  & N-doped & N-bottom & \(-2.98\) & \(1.38\) & \(0.72\) & \(-0.368\)e & This work \\ \hline \multirow{4}{*}{NH\({}_{3}\)} & Pristine & N-bottom & \(-0.18\) & \(3.35\) & \(2.47\) & \(0.028\)e & Reference [8] \\  & Se vacancy & N-bottom & \(-0.82\) & \(2.90\) & \(0.45\) & \(0.016\)e & This work \\ \cline{1-1}  & O-doped & N-top & \(-0.13\) & \(3.06\) & \(1.93\) & \(0.000\)e & This work \\ \cline{1-1}  & N-doped & N-bottom & \(-0.36\) & \(2.46\) & \(1.45\) & \(0.215\)e & This work \\ \hline \hline \end{tabular}
\end{table} 
Table 1: The most stable structural configurations of different gas molecules adsorbed on SnSe\({}_{2}\) monolayer systems. E\({}_{\text{ad}}\), d\({}_{\text{atom-atom}}\), h and \(\Delta\)Q\({}_{\text{b}}\) of molecules adsorbed on the pristine and defective SnSe\({}_{2}\) monolayers. Figure 2: The density of states (DOS) of 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 partial density of state (PDOS) of N and O atoms, respectively. The E\({}_{\text{F}}\) is denoted by a dashed line and shift to zero.