# First-Principles Study for Gas Sensing of Defective SnSe\({}_{2}\) Monolayers
## 1 Introduction  
Recently, two-dimensional transition metal dichalcogenide (2D-TMD) materials have gained great attention due to their unique structural and electrical properties. Since graphene was introduced into the research, other families of 2D materials with layered structures are also fast emerging for some better applications. 2D-TMD materials with a narrow tunable band gap and replaceable cation and anion [1; 2; 3] are more advantageous than the pristine graphene which lacks band gap. SnSe\({}_{2}\), a IV-VI semiconductor, has been widely studied for optoelectronic and thermoelectric applications [4; 5]. For example, SnSe\({}_{2}\) is used as a high-performance photodetector that shows relatively fast photoresponse at room temperature with a high photo-to-dark ratio [4]. 2D-TMDs materials have been applied as gas sensors due to their large surface-area-to-bulk ratio. The layered SnX\({}_{2}\) (X = 5, Se) nanosheets show a significant sensitivity to individual molecules, such as NO\({}_{2}\) and NH\({}_{3}\)[6; 3; 7; 8]. The SnX\({}_{2}\) (X = S, Se)-based gas sensors show a good response to NO\({}_{2}\) at room temperature [6; 7; 8]. Furthermore, the SnSe\({}_{2}\) monolayer shows a higher sensitivity for NO\({}_{2}\) molecule adsorption than SnS\({}_{2}\) in our related work [7; 8]. Also, the charge transfers and the flat band contributed by gas adsorption induces the conductivity difference of the SnSe\({}_{2}\) monolayer reported in our previous study [8]. More recently, the doped SnSe\({}_{2}\) nanosheets have been widely studied because of its interesting electronic and optoelectronic properties, [9; 10; 11; 12; 13]. [PERSON] et al. [9] systematically studied n-type/p-type and isoelectronic doping cases on SnSe\({}_{2}\) nanosheets based on density functional theory (DFT). [PERSON] et al. [9] suggest that P and As are not promising candidates for p-type doping because those atoms contribute trap states near the Fermi level (E\({}_{\text{F}}\)). Although the N atom is a promising candidate for p-type doping which induces states near the valence band maximum (VBM), it is difficult to achievethe N-doped SnSe\({}_{2}\) in reality. For O-, S-, Te-doped SnSe\({}_{2}\), the density of states (DOS) of all doped SnSe\({}_{2}\) are similar to that of pristine SnSe\({}_{2}\) monolayer [9]. On the other hand, n-type doping of F, Cl and Br are highly recommended, especially Br, since the states near both the VBM and the conduction band minimum (CBM) result in a high carrier density and conductivity [10; 11; 12]. For the gas sensor, the SnSe\({}_{2}\) monolayer demonstrates a high sensitivity and charge transfer to NO\({}_{2}\)[8]. The defective graphene would enhance sensitivity for gas sensing which has been theoretically and experimentally reported [14; 15]. [PERSON] et al. [14] theoretically reveal that the defective graphene has stronger interaction with CO, NO and NO\({}_{2}\) than the pristine graphene. Also, the B-doped graphene gives the tightest binding with NH\({}_{3}\). [PERSON] et al. [15] demonstrate the defect-engineered graphene oxide chemical sensors, which exhibit ultrahigh sensitivity for NO\({}_{2}\) and NH\({}_{3}\) from the experimental data. However, there is no related study regarding the gas-sensing properties of the doped SnSe\({}_{2}\) nanosheets. In this work, on the basis of DFT, we investigate the gas detection properties for NO\({}_{2}\) and NH\({}_{3}\) adsorbed on the defective SnSe\({}_{2}\) monolayer by substitution of the Se site with a single vacancy, O or N atoms. To the best of our knowledge, the gas sensors of the defective SnSe\({}_{2}\) monolayers are investigated for the first time. In order to understand the sensing mechanism, we report adsorption energy, charge transfer, DOS and structural parameters of gas molecules adsorption on defective SnSe\({}_{2}\) monolayers. We also discuss and compare the gas-sensing parameters of defective SnSe\({}_{2}\) monolayers with the pristine SnSe\({}_{2}\) monolayer.