First-Principles Study for Gas Sensing of Defective SnSe 2 Monolayers

: We report the interaction between gas molecules (NO 2 and NH 3 ) and the SnSe 2 monolayers with vacancy and dopants (O and N) for potential applications as gas sensors. Compared with the gas molecular adsorbed on pristine SnSe 2 monolayer, the Se-vacancy SnSe 2 monolayer obviously enhances sensitivity to NO 2 adsorption. The O-doped SnSe 2 monolayer shows similar sensitivity to the pristine SnSe 2 monolayer when adsorbing NO 2 molecule. However, only the N-doped SnSe 2 monolayer represents a visible enhancement for NO 2 and NH 3 adsorption. This work reveals that the selectivity and sensitivity of SnSe 2 -based gas sensors could be improved by introducing the vacancy or dopants.


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. SnSe2, a IV-VI semiconductor, has been widely studied for optoelectronic and thermoelectric applications [4,5]. For example, SnSe2 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 SnX2 (X = S, Se) nanosheets show a significant sensitivity to individual molecules, such as NO2 and NH3 [3,[6][7][8]. The SnX2 (X = S, Se)-based gas sensors show a good response to NO2 at room temperature [6][7][8]. Furthermore, the SnSe2 monolayer shows a higher sensitivity for NO2 molecule adsorption than SnS2 in our related work [7,8]. Also, the charge transfers and the flat band contributed by gas adsorption induces the conductivity difference of the SnSe2 monolayer reported in our previous study [8].
More recently, the doped SnSe2 nanosheets have been widely studied because of its interesting electronic and optoelectronic properties, [9][10][11][12][13]. Huang et al. [9] systematically studied n-type/p-type and isoelectronic doping cases on SnSe2 nanosheets based on density functional theory (DFT). Huang 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 (EF). Although the N atom is a promising candidate for ptype doping which induces states near the valence band maximum (VBM), it is difficult to achieve the N-doped SnSe2 in reality. For O-, S-, Te-doped SnSe2, the density of states (DOS) of all doped SnSe2 are similar to that of pristine SnSe2 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 SnSe2 monolayer demonstrates a high sensitivity and charge transfer to NO2 [8].
The defective graphene would enhance sensitivity for gas sensing which has been theoretically and experimentally reported [14,15]. Zhang et al. [14] theoretically reveal that the defective graphene has stronger interaction with CO, NO and NO2 than the pristine graphene. Also, the B-doped graphene gives the tightest binding with NH3. Lee et al. [15] demonstrate the defect-engineered graphene oxide chemical sensors, which exhibit ultrahigh sensitivity for NO2 and NH3 from the experimental data. However, there is no related study regarding the gas-sensing properties of the doped SnSe2 nanosheets.
In this work, on the basis of DFT, we investigate the gas detection properties for NO2 and NH3 adsorbed on the defective SnSe2 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 SnSe2 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 SnSe2 monolayers. We also discuss and compare the gas-sensing parameters of defective SnSe2 monolayers with the pristine SnSe2 monolayer. We find that the vacancy and doped SnSe2 monolayer can enhance the selectivity and sensitivity of gas sensing.

Method
The SnSe2 monolayer structure is based on the experimental lattice parameters of bulk SnSe2 [16]. The initial lattice constants of SnSe2 monolayer are a = b = 3.81 Å and thickness of vacuum is set about 16 Å. All calculated structures contain the fully lattice constants and atom positions optimization. After structure optimization, the lattice constants of the SnSe2 monolayer are a = b = 3.87 Å and the energy gap is 0.78 eV. The Visualization for Electronic and Structural Analysis (VESTA) software is a 3D visualization program for structural models [17]. We use VESTA to show the crystal structure and the defective single layer in this work. The calculation was implemented in the Vienna Ab initio Simulation Package (VASP) and performed by the projector augmented wave (PAW) method with the Perdew-Burke-Ernzerhofer (PBE) generalized gradient approximation (GGA) [18,19]. The SnSe2 monolayer is constructed with a 3 × 3 × 1 supercell in order to perform gas molecules adsorption calculation. The energy cutoff for a plane-wave basis was set up to 400 eV within the 12 × 12 × 1 Monkhorst-Pack k-point grid for all study cases. The energy convergence threshold and force convergence criteria were set to 10 −5 eV per unit cell and 0.01 eV Å −1 .
We also discussed the vacancy and doped SnSe2 monolayers that adsorb NO2 and NH3 gas molecules. First of all, we put the NO2 and NH3 molecules on 3 Å above the Se site or the dopants. We calculated two orientations of N atom, the N atom of gas molecules toward and backward the defective SnSe2 monolayers, named N-bottom and N-top, respectively. In order to understand the defect effect of SnSe2 monolayers adsorbing gas molecule, a Se atom was substituted by a vacancy or a dopant atom (N and O). In this paper, the structures of the SnSe2 monolayers with gas adsorption were relaxed. The initial configurations for gases adsorption on the defective SnSe2 monolayers are illustrated in Figures S1 and S2 of the supplementary material.

SnSe2 Monolayers
The pristine 1T phase SnSe2 monolayer is a hexagonal crystal structure as shown in Figure 1a. A Sn atom is sandwiched between two Se atoms which form a Se-Sn-Se arrangement with ABC stacking as shown in Figure 1e. The defective SnSe2 monolayer with Se vacancy in the center of supercell is represented in Figure 1b and 1f. Figure 1c and 1g are the defective SnSe2 monolayer in which a Se atom is substituted by O atom. Furthermore, Figure 1d and 1h are the defective SnSe2 monolayer in which a Se atom is substituted by a N atom. The O and N dopants are sucked into the vacancy site as shown in Figure 1g and 1h. Figure 2 is the DOS with or without defects on the SnSe2 monolayer. The pristine SnSe2 monolayer has an indirect band gap of 0.78 eV as shown in Figure 2a, which is consistent with the previous reported value [8,20]. In the Se-vacancy SnSe2 monolayer, there are occupied states very near EF as shown in Figure 2b. However, the DOS are very different between the O-doped and the N-doped SnSe2 monolayer. The DOS of the O-doped SnSe2 monolayer is similar to the pristine SnSe2 monolayer with the energy gap 0.84 eV because the states contributed from impurities are far from EF, as shown in Figure 2c. The resultant impurity state from the doped O atom is near the band edge, above 0.8 eV and below −0.1 eV denoted by the orange line in Figure 2c. For the N-doped SnSe2 monolayer, the partial density of state (PDOS) induced by the N dopant is near VBM denoted by the blue line in Figure 2d. The DOS of the SnSe2 monolayer with N and O dopant also is consistent with the result of Huang et al. [9].

Gases Adsorbed on Different SnSe2 Monolayers
In this section, we discuss the most stable configurations of NO2 and NH3 molecules adsorbed on different SnSe2 monolayer systems, whichare listed in Table 1.
The adsorption energy (Ead) of gas molecules on SnSe2 monolayers is defined as where EGas + SnSe2 is the total energy of the gas molecules adsorbed on SnSe2 monolayers, ESnSe2 is the total energy of the SnSe2 monolayer and EGas is the energy of isolated gas molecule. A negative Ead value means that gas molecules on the SnSe2 monolayer is energetically favorable. In our previous work [8], the adsorption energy of NO2 adsorbed on the pristine SnSe2 monolayer is higher than that of NH3 adsorption in theoretical prediction. In experimental demonstrations, the pristine SnSe2 monolayer can detect NO2 in lower concentrations than NH3. 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 Ead values of NO2 on SnSe2 monolayer systems (−0.29 to −2.98 eV) are larger than NH3 (−0.13 to −0.82 eV). This indicates that the sensitivity of NO2 is higher than NH3 on SnSe2 monolayers. Moreover, the distance between NO2 and SnSe2 monolayers is smaller than the distance between NH3 and SnSe2 monolayer. The Bader charge population analysis [21] is summarized in Table 1 where negative ΔQb indicates electron charge transfer from SnSe2 to a gas molecule and a positive ΔQb shows charge transfer from a gas molecule to SnSe2 monolayers. Our calculation shows that the NO2 is an electron charge acceptor, whereas NH3 is an electron charge donor in all of the study cases. Moreover, the absolute values of ΔQb for NO2 adsorbed on SnSe2 monolayers are greater than that for NH3 adsorbed on SnSe2 monolayers.
In Table 1, we use the two parameters, datom-atom and h, to describe the position of gas molecules adsorbed on the SnSe2 monolayers. The datom-atom means the shortest distance between the lowest atom of a gas molecule and the highest atom of SnSe2 monolayer. The h indicates the vertical distance between them. We mark datom-atom and h in the structure of gas adsorbed on the SnSe2 monolayers to compare the positions of gas adsorption in different SnSe2 monolayers. This would be shown in following figures. Furthermore, we also discuss the h, Ead and ΔQb to analyze the gas-sensing mechanism for defective SnSe2 monolayers as shown in Table 1.
In the following sections, the detail will be discussed about adsorption energy, charge transfer, DOS and structural parameters of gas molecules adsorption on defective SnSe2 monolayers.

Gas SnSe2 System Gas Orientation Ead (eV) datom-atom (Å) h (Å)
ΔQb  The value of Ead and ΔQb of NO2 adsorbed on the pristine SnSe2 monolayer are −0.29 eV and −0.164e, respectively [8]. Furthermore, the Ead of NO2 adsorbed on the Se-vacancy and doped SnSe2 monolayer are greater than NO2 adsorbed on the pristine SnSe2 monolayer, as shown in Table 1.
The value of h between NO2 and the Se-vacancy SnSe2 monolayer is −0.08Å. This induces a large adsorption energy and Bader charge because the NO2 molecule is sucked into the vacancy site. The Ead and ΔQb are up to −1.84 eV and −0.926e, respectively, as listed in Table 1. The value of Ead is greater than the pristine and the O-doped SnSe2 monolayer but smaller than the N-doped SnSe2 system. The value of ΔQb = −0.926e of NO2 adsorbed on the Se-vacancy SnSe2 monolayer is the maximum of that on the SnSe2 monolayers. This result indicates that NO2 molecule would strongly interact with the Se-vacancy SnSe2 monolayer.
As mentioned, the structure of NO2 adsorbed on the O-doped SnSe2 monolayer is similar with on the pristine SnSe2 monolayer despite of the O atom replacing the Se atom. In the detail, the value of h between NO2 and the O-doped SnSe2 monolayer is 1.75 Å, which is also a little smaller than that on the pristine SnSe2 monolayer. Furthermore, the O-doped SnSe2 monolayer adsorbs the NO2 molecule and would induce a little increase in the adsorption energy but decrease the Bader charge by about −0.32 eV and −0.145e, respectively, as shown in Table 1.
When NO2 is adsorbed on the N-doped SnSe2 monolayer, the value of h between NO2 and the N-doped SnSe2 monolayer is 0.72 Å. This indicates that the strong interaction comes from the N-N atoms interaction between NO2 and the N dopant. The Ead = −2.98 eV of NO2 adsorbed on the Ndoped SnSe2 monolayer is greater than on the Se-vacancy SnSe2 monolayer. ΔQb = −0.368e of NO2 adsorbed on the N-doped SnSe2 monolayer is smaller than on the Se-vacancy SnSe2, but greater than the pristine and O-doped SnSe2. The relatively high Ead and ΔQb demonstrate strong interaction between NO2 and the N-doped SnSe2 monolayer. When NO2 is adsorbed on the pristine SnSe2, the total DOS presents a trap state across EF in the energy range about −0.04 eV to -0.02 eV, as shown in Figure 4a. A large amount of PDOS induced by N and O atoms of gas molecule is located in the aforementioned trap state, which induces the flat band and trap electron on it as mentioned in previous work [8]. The pattern of DOS in Figure 4a is different from DOS of the pristine SnSe2 monolayer without adsorption as shown in Figure 2a.
For NO2 adsorption on the Se-vacancy SnSe2 monolayer, the total DOS presents a bandwidth near the EF with an energy range of −0.19 eV to -0.12 eV, as shown in Figure 4b. A considerable amount of PDOS contributed by N and O atoms of the gas molecule is located the aforementioned bandwidth. Therefore, the pattern of DOS in Figure 4b is quite different from DOS of the Se-vacancy SnSe2 without adsorption as shown in Figure 2b.
When NO2 is adsorbed on the O-doped SnSe2 monolayer, the total DOS presents a trap state across EF in the energy range about −0.06 eV to -0.03 eV, as shown in Figure 4c. A large amount of PDOS induced by N and O atoms of the gas molecule is located in the aforementioned trap state, which corresponds to a flat band. The trap state would trap electrons on it and decrease the carrier mobility. The DOS of NO2 adsorption the O-doped SnSe2 monolayer is very similar with the pristine SnSe2 monolayer, but different from the O-doped SnSe2 monolayer without adsorption as shown in Figure 2c.
For NO2 adsorption on the N-doped SnSe2 monolayer, the total DOS shown in Figure 4d   When NH3 is adsorbed on the pristine SnSe2 monolayer, the value of h, Ead and ΔQb are 2.47 Å, −0.18 eV and 0.028e [8], respectively. Compared to the value of Ead and h in the Table 1, the Ead of NH3 adsorption almost follows a positive correlation to h, except for NH3 adsorption on the O-doped SnSe2 monolayer.

total N O
When NH3 is adsorbed on the Se-vacancy SnSe2 monolayer, the value of h = 0.45 Å is the minimum among NH3 adsorption on SnSe2 monolayers. Furthermore, the value of Ead = −0.82 eV of NH3 on the Se-vacancy SnSe2 monolayer reaches the maximum compared with other cases. However, the charge transfer amount ΔQb = 0.016e of NH3 adsorbed on the Se-vacancy SnSe2 monolayer is smaller than gas molecule on the pristine SnSe2 (0.028e).
For NH3 adsorption on the O-doped SnSe2 monolayer, the value of h = 1.93 Å between NH3 and the O-doped SnSe2 monolayer is smaller than that on the pristine SnSe2 monolayer. The values of Ead and ΔQb of NH3 on the O-doped SnSe2 monolayer are −0.13 eV and 0.000e, which are both the minimum among the NH3 adsorption. This indicates that the ability of NH3 adsorption of the Odoped SnSe2 monolayer is weaker than the pristine SnSe2 monolayer.
For NH3 adsorption on the N-doped SnS2 monolayer, the value of h = 1.45 Å between NH3 and the O-doped SnSe2 monolayer is smaller than that on the pristine SnSe2 monolayer. Ead also has a greater value −0.36 eV than that on the pristine SnSe2 monolayer, and ΔQb of NH3 on the N-doped SnSe2 monolayer has a maximum value 0.215e. This shows an enhancement of NH3 adsorption on the N-doped SnSe2 monolayer.  When NH3 adsorbed on the pristine SnSe2 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 EF 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 SnSe2 as shown in Figure 2a, which is consistent with our previous work [8].
When NH3 adsorbed on the Se-vacancy SnSe2 monolayer, the total DOS presents a bandwidth in energy range of −0.17 eV to EF 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 NH3 adsorption does not induce obvious difference on the DOS of the Se-vacancy SnSe2 without adsorption as shown in Figure 2b.
For the O-doped SnSe2 monolayer with NH3 adsorption, the total DOS presents a flat band below EF 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 EF, the DOS pattern of Figure 6c is a semiconductor with energy gap about 0.70 eV. Therefore, the NH3 adsorption does not change the pattern of DOS of the O-doped SnSe2 monolayer as shown in Figure 2c.
For the N-doped SnSe2 monolayer with NH3 adsorption, the total DOS presents a trap state across EF in the energy range about −0.10eV to -0.03eV 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 SnSe2 systems after gas adsorption. The pattern of DOS in Figure 6d is different from that of the N-doped SnSe2 monolayer without adsorption as shown in Figure 2d.
Comparing the DOS of NH3 adsorption on different SnSe2 monolayers as shown in Figure 6a-d with the SnSe2 monolayers without adsorption as shown in Figure 2a-d, only the N-doped SnSe2 monolayer could induce obvious differences of electronic structure before/after NH3 adsorption.

Discussion
We discuss the gas-sensing parameters, including Ead and ΔQb, compared to gas molecules adsorbed on the pristine SnSe2 monolayer and have demonostrated the high selectivity and sensitivity of the defective and doped SnSe2 monolayer for gas-sensor candidates. The alteration of electronic structure is reflected by the change in electrical conductance of the SnSe2 monolayers. We also discuss the DOS of gas adsorptions in the SnSe2 monolayers to illustrate the change in the electrical conductance due to alteration of the electronic structure.

Gas Molecules on the Se-Vacancy SnSe2 Monolayer
When NO2 adsorbed on the Se-vacancy SnSe2 monolayer, the values of Ead = −1.84 eV and ΔQb = −0.926e were the maximum among that of the SnSe2 monolayers. This reveals strong interaction between NO2 molecule and the Se-vacancy SnSe2 monolayer. For the Se-vacancy SnSe2 monolayer, NO2 adsorption induces wide-ranging trap states crossover EF to decrease the carrier mobility as represented by the pattern of DOS in Figure 4b.
However, the NH3 molecule is adsorbed on the Se-vacancy SnSe2 monolayer with higher Ead = −0.82 eV and lower ΔQb = 0.016e compared to the pristine SnSe2 monolayer (Ead = −0.18 eV, ΔQb = 0.028e [8]). NH3 adsorption on the Se-vacancy SnSe2 monolayer contributes DOS below EF, as shown in Figure 6b. However, the charge transfer amount of NH3 on the Se-vacancy SnSe2 monolayer is smaller than that of NH3 on the pristine SnSe2 monolayer. Therefore, we cannot be sure of the conductivity difference of the Se-vacancy SnSe2 monolayer with/without NH3 adsorption.
The Se-vacancy SnSe2 monolayer shows excellent sensitivity for NO2 molecules, whereas the Sevacancy SnSe2 monolayer cannot show obvious conductivity difference for NH3 adsorption. In brief, the Se-vacancy SnSe2 monolayer shows enhancement only for NO2.

Gas Molecules on the O-Doped SnSe2 Monolayer
When NO2 adsorbed on the O-doped SnSe2 monolayer, the values of Ead = −0.32 eV and ΔQb = −0.145e are close to the molecule on the pristine SnSe2 monolayer (Ead = −0.29 eV, ΔQb = −0.164e [8]). The DOS of the O-doped SnSe2 monolayer is similar to that of the pristine SnSe2 monolayer, so that the similar appearance of DOS occurs in NO2 adsorption on the pristine and O-doped SnSe2 monolayer as shown in Figure 4a and Figure 4c. The DOS of NO2 on the O-doped SnSe2 monolayer induces trap state crossover EF and decreases the carrier mobility. The electronic structure altered by NO2 adsorbed on the O-doped SnSe2 monolayer would contribute a change of electrical conductance, just like NO2 adsorption on the pristine SnSe2 monolayer [8].
On the other hand, the value of Ead = −0.13 eV and ΔQb = 0.000e for NH3 on the O-doped SnSe2 monolayer are both the minimum among the NH3 adsorption. The DOS of NH3 on the O-doped SnSe2 monolayer is below the EF, which results in similar electrical properties with the O-doped SnSe2 monolayer without gas adsorption. This means that there is no obvious change of electrical conductance.
In conclusion, gas molecules on the O-doped SnSe2 monolayer shows high sensitivity for NO2 adsorption and even with a weaker detection for NH3, compared to gases on the pristine SnSe2. This result indicates that the O-doped SnSe2 monolayer has better selectivity to these two gases in comparison with pristine SnSe2.

Gas Molecules on the N-Doped SnSe2 Monolayer
When NO2 adsorbed on the N-doped SnSe2 monolayer, Ead = −2.98 eV is the maximum among the NO2 adsorption and ΔQb = −0.368e has relatively high value. It indicates that the strong interaction occurred between NO2 molecule and the N-doped SnSe2 monolayer. Moreover, the DOS of the Ndoped SnSe2 monolayer before/after NO2 adsorption are totally different, as shown in Figure 2d and Figure 4d. Compared to the DOS of the N-doped SnSe2 monolayer before/after NO2 adsorption, the DOS crossover EF of the N-doped SnSe2 moves to below the EF.
When NH3 is adsorbed on the N-doped SnSe2 monolayer, Ead = −0.36 eV also has a greater value than that on the pristine SnSe2 monolayer and ΔQb = 0.215e has a maximum value. In the DOS of NH3 adsorbed on the N-doped SnSe2 monolayer, the trap state is induced to decrease the carrier mobility. This implies that there would be an obvious change of electrical conductance. The N-doped atom is high sensitivity gas sensor for NO2 and NH3 shown as in our DFT calculation.
When setting gas molecules on the N-doped SnSe2, it reveals obvious enhancement for both NO2 and NH3 adsorption.

Conclusions
In summary, the adsorption of NO2 and NH3 on the Se-vacancy, O-doped and N-doped SnSe2 monolayer are investigated and compared to the pristine SnSe2 monolayer. Due to the high adsorption energy and large charge transfer of gas adsorption on the Se-vacancy SnSe2 monolayer, the Se-vacancy SnSe2 monolayer shows a better sensitivity only to NO2. However, the sensitivity of NH3 adsorbed on the Se-vacancy SnSe2 monolayer has higher adsorption energy but lower charge transfer amount than the pristine SnSe2 monolayer. Furthermore, the O-doped SnSe2 monolayer has similar interaction with NO2 with the pristine SnSe2 monolayer, but weaker interaction with NH3 than the pristine SnSe2 monolayer. This indicates that the O-doped SnSe2 monolayer has similar sensitivity to the pristine SnSe2 monolayer and better selectivity than the pristine SnSe2 monolayer. The N-doped SnSe2 strongly interacts both with NO2 and NH3 and shows obvious sensing enhancement for those two gases. In brief, the vacancy and doped SnSe2 monolayers can enhance the selectivity and sensitivity of gas sensing for NO2 and NH3 molecules. This work demonstrates the potential of the SnSe2-based gas sensors by introducing defects and dopants in the SnSe2 monolayer.