High Sensitivity and Selectivity of AsP Sensor in Detecting SF6 Decomposition Gases

The sensing properties of monolayer arsenic phosphorus (AsP) for the adsorption of SF6, H2O, O2, and SF6 decomposition gases (SO2 and H2S) are theoretically investigated by the first-principle calculations. We calculate the adsorption energy, equilibrium distance, Mulliken charge transfer, and electron localization function (ELF) to explore whether AsP is suitable for detecting SF6 decomposition gases. By comparing the adsorption performance of SF6, H2O, O2, and H2S gases, we have revealed that the SO2 gas molecules could form stable chemisorption with AsP monolayer. The results demonstrate that AsP is highly sensitive and selective to SO2 gas molecules with robust adsorption energy and apparent charge transfer. Furthermore, the current-voltage (I–V) curves reveal that only the adsorption of SO2 can largely modify the resistance of AsP. Our results show that gas sensors based on AsP monolayer could be better than that of black phosphorene (BP) to diagnose the state of online gas-insulated switchgear (GIS).


Results
The most stable configurations of the different gas molecules adsorption on AsP monolayer are illustrated in Fig. 1, and the corresponding E a , d 0 and Q are listed in Table 1. The positive sign of Q means charge transfer from monolayer AsP to the adsorbates. As listed in Table 1, the equilibrium distance of SF 6 , SO 2 , H 2 S, H 2 O, and O 2 on the AsP monolayer (3.09, 2.59, 3.07, 2.51, and 2.80 Å, respectively) are larger than P-F (1.75 Å), P-S (2.14 Å), P-H (1.43 Å), and P-O (1.74 Å) bonds 17 . The E a of the most energetically favorable structures for SF 6 , SO 2 , H 2 S, H 2 O, and O 2 molecules adsorbed on AsP are −0.480, −1.031, −0.069, −0.433, and −0.342 eV, respectively. Clearly, the adsorption energy of H 2 S on AsP is significantly smaller than the others, indicating that the AsP monolayer is not suitable for sensing this molecule. The E a value of SO 2 adsorption on AsP monolayer is also larger than that of SO 2 adsorption on BP (−0.748 eV) 18 , indicating that a higher level sensitivity for SO 2 detection with AsP than that with BP.
Charge transfer is another important factor to estimate the sensitivity of gas sensors. To further explore the adsorption properties between gas molecules and the AsP monolayer, the electron difference densities (EDD) are shown in Fig. 2. The Mulliken charge transfer results for SF 6 /H 2 S-AsP systems show that the charge is depleted   To further obtain insight into the charge redistribution of the adsorption system, we plot the electron localization function (ELF) slices in Fig. 3. There is no obvious electron localization overlap between SF 6 , H 2 S, H 2 O, and O 2 gas molecules and the AsP monolayer, which means the physisorption feature for these molecules adsorbed on AsP monolayer. The ELF of the SF 6 -, H 2 S-, H 2 O-, and O 2 -AsP systems do not have electron sharing area between gas molecules and the AsP monolayer, and thus the AsP monolayer is not sensitive to these molecules. For the SO 2 -AsP system, the electrons are slightly shared between SO 2 molecule and AsP monolayer, revealing that the surface charge of the AsP monolayer is largely redistributed after SO 2 adsorption. This is consistent with the result of the Mulliken charge transfer. For SO 2 adsorption, it would be more reasonable to treat it as chemisorption due to the large binding energy, electron transfer and also slightly overlapped electron distribution as shown in Fig. 3(b). To further explore the adsorption mechanisms of SO 2 molecules adsorbed on AsP monolayer, we plot the total electronic densities of states (DOS) and projected density of states (PDOS) in Fig. 4. Obviously, the main  electronic level contributions of SO 2 to the total system localize between −4 and −1.3 eV in the valence band, 1 and 1.3 eV in the conduction band, which is away from the Fermi level. The electrons are slightly shared between AsP monolayer and SO 2 molecule, which reveals the intensity of the interaction between the SO 2 molecule and the AsP monolayer. These findings imply that the strong adsorption of SO 2 on AsP monolayer is mostly due to the electron Coulomb interaction between the lonely paired electrons of SO 2 and the empty orbital of P atom, without hybridization 19 . Thus, we can deduce that gas sensors based on AsP are sensitive and selective to SO 2 gas in the background of the SF 6 decomposition gases.
To further verify the validity of our work, we calculate the I-V response of AsP sensor before and after the gas adsorption, as shown in Fig. 5(a). Armchair direction is chosen for transport calculation because the mobility along the armchair direction is significantly larger than the zigzag direction. There is no current (about 0.1 nA) passing through the devices when the bias voltage is smaller than 1.0 V due to the existence of band gap of pure AsP. When bias over 1.0 V, the current starts to increase dramatically. However, for the SO 2 adsorption, with the increase of the bias voltage from 1.2 to 2.2 V, the current is clearly smaller than other cases. The reduction of current indicates the resistance of AsP is increased after the SO 2 adsorption, which can be easily measured in the experiment. It should be emphasized that the increased resistance is caused by the larger charge transfer between the AsP monolayer and SO 2 molecule. To gain deeper insight into the resistance change of AsP caused by the different adsorbates, we plot the current ratios before and after adsorption of gas in Fig. 5(b). It can be found that the current ratios for SO 2 adsorption are significantly lower than that for H 2 S and SF 6 adsorption. The value of the current reduction is about 21.3% for SO 2 adsorption under a bias of 2.2 V, while the current reductions are 2.9% and 0.8% for H 2 S and SF 6 adsorption respectively under the same bias. The current reduction ratio for SO 2 adsorption is about seven times as that for H 2 S and SF 6 adsorption, which can be easily distinguished by the magnification. The current is slightly enhanced after the H 2 O and O 2 adsorption under a bias of 2.2 V. The resistance in AsP monolayer is highly selective and sensitive to SO 2 in SF 6 decomposition gases, which further demonstrates that it can be an excellent sensing material for online GIS diagnosis.

Discussion
It was reported that BP could also be used for SO 2 gas detection in SF 6 decomposition gases 11 . However, the maximum current reduction is about 7% for SO 2 adsorption, and not more than 1.5% for H 2 S and SF 6 adsorption. For comparison purposes, we estimate the sensor response (S) with the formula: where ΔR is the resistance change after SO 2 adsorption and R is the prior resistance of AsP monolayer. It should be emphasized that it is different to compare S directly in experiments because the sensitivities of 2D materials are affected by many factors, such as their thicknesses, detecting areas and so on. Nonetheless, our AsP sensor shows better response to SO 2 than BP in the theoretical calculations, indicating that AsP monolayer could do better than BP in fabricating high sensitivity SO 2 sensors for application in online GIS diagnosis, at least comparable to BP.

Methods
The first principles calculations based on density functional theory (DFT) are performed using the Atomistix Tool Kit (ATK) codes at room temperature (T = 300 K) 20 . The generalized gradient approximation (GGA) with Perdew-Burke-Ernzerhof (PBE) exchange-correlation potential is adopted 21 . Fritz Haber Institute (FHI) pseudopotential using Troullier-Martins scheme with a double-ζ basis set is employed 22 . Spin polarization is only included during the calculations of the adsorption of O 2 because it is a paramagnetic molecule. We use the Grimme's DFT-D2 dispersion correction approach for van der Waals (vdW) corrections thanks for its higher  23 . The vacuum region is set to more than 15 Å to avoid the effect of interaction springing from the adjacent layers. A well conserved Monkhorst-Pack 8 × 8 × 1 k-point mesh is adopted for geometry optimization and electronic properties calculations with a density mesh cutoff energy of 300 Ry. Previously reported optimized lattice constants for monolayer AsP (a = 3.5 Å and b = 4.65 Å) are considered in this work 14 . We take a 3 × 3 supercell of monolayer AsP. The current-voltage (I-V) characteristics are calculated by using the nonequilibrium Green's function (NEGF) method 24 . The k-point sampling is set to 5 × 1 × 100, and the mesh cutoff is set to 200 Ry for I-V calculation. The current of two-probe systems are calculated by the Landauer-Bütiker formula: is the electron transport coefficient calculated from the Green's functions, f and u are the Fermi-Dirac distribution function and the electrochemical potential, respectively. The subscripts "r" and "l" represent the right and left electrode. To find the most stable configurations, we consider four different sites for each gas adsorbed on monolayer AsP, which are set up at the top of upper As/P atom, the middle of As-P bond, and the center of the puckered hexagon. The moderate distance (2.5 Å) between a single molecule and monolayer AsP layer is adopted for each initial adsorption case. On the basis of the above settings, all the configurations are fully optimized and relaxed until the force and stress tolerance are mitigated to less than 0.05 eV/Å and 0.001 eV/Å 3 , respectively. To study the interactions between monolayer AsP and targeted gas molecules, the adsorption energy (E a ), the Mulliken charge transfer (Q) and the adsorption distance (d 0 ) are systematically calculated. The adsorption energy is defined as: where E gas , E AsP , and E total are the total energy of gas molecule, AsP monolayer, and gas molecule-AsP system, respectively. The adsorption distance is defined as the equilibrium's nearest atoms between AsP monolayer and gas molecules.

Conclusion
In conclusion, we have investigated the sensing properties of AsP monolayer for two main SF 6 decomposition gas molecules (SO 2 , H 2 S) and three background gas molecules (SF 6 , H 2 O, and O 2 ) adsorption by using the first-principles calculations. The results demonstrate that SF 6 , H 2 S, H 2 O, and O 2 gas molecules show physical adsorption on AsP monolayer, while AsP monolayer strongly adsorb SO 2 molecules via robust chemical bonds. It is found that the E a and Q values of SO 2 molecule adsorbed on AsP monolayer are obviously larger than the others, which may allow it as a desirable gas sensor for detecting SO 2 . The I-V curves demonstrate that the resistance of AsP monolayer is only largely affected by SO 2 adsorption, indicating that the gas sensors based on AsP are highly sensitive and selective to SO 2 . Therefore, we can deduce that AsP is a promising candidate for high sensitivity and selectivity SO 2 sensing applications in online GIS diagnosis for SF 6 decomposition gases.