Defect-Engineering of 2D Dichalcogenide VSe2 to Enhance Ammonia Sensing: Acumens from DFT Calculations

Opportune sensing of ammonia (NH3) gas is industrially important for avoiding hazards. With the advent of nanostructured 2D materials, it is felt vital to miniaturize the detector architecture so as to attain more and more efficacy with simultaneous cost reduction. Adaptation of layered transition metal dichalcogenide as the host may be a potential answer to such challenges. The current study presents a theoretical in-depth analysis regarding improvement in efficient detection of NH3 using layered vanadium di-selenide (VSe2) with the introduction of point defects. The poor affinity between VSe2 and NH3 forbids the use of the former in the nano-sensing device’s fabrications. The adsorption and electronic properties of VSe2 nanomaterials can be tuned with defect induction, which would modulate the sensing properties. The introduction of Se vacancy to pristine VSe2 was found to cause about an eight-fold increase (from −012 eV to −0.97 eV) in adsorption energy. A charge transfer from the N 2p orbital of NH3 to the V 3d orbital of VSe2 has been observed to cause appreciable NH3 detection by VSe2. In addition to that, the stability of the best-defected system has been confirmed through molecular dynamics simulation, and the possibility of repeated usability has been analyzed for calculating recovery time. Our theoretical results clearly indicate that Se-vacant layered VSe2 can be an efficient NH3 sensor if practically produced in the future. The presented results will thus potentially be useful for experimentalists in designing and developing VSe2-based NH3 sensors.


Introduction
With the development of technology, the requirement for gas sensors in the fields of industry, agriculture, medicine, air-quality monitoring, etc., has been amplified [1,2]. For instance, gases such as carbon monoxide, nitrogen oxide, nitrogen dioxide, ammonia, etc. are harmful to living beings and can trigger serious health issues [3,4]. To eliminate such hazardous gases from the environment, lucrative sensors with good stability, sensitivity, and selectivity are desirable. In the past, metal oxides such as ZnO, SnO 2 , and so on were explored as efficient sensors having good sensitivity and selectivity towards the sensing of harmful gases [5]. Although metal oxides are cheaper and need low fabrication costs, their elevated operating temperature restricts their use in sensing devices [6]. Following this, various types of sensing materials have been reported in the past. Among all the reported sensing materials, chemi-resistors are recommended as promising sensitive and selective sensors [7,8]. For instance, Oudenhoven et al. reported a thin layer of ionic liquid [BMIM][NTf 2 ] as the electrolyte, capable of sensing NH 3 even at a level of 1 ppm [9]. On the other hand, Amirjani et al. reported a calorimetric sensor for detecting NH 3 by The modality of detection of NH3 with VSe2 nanosheets has thus been theoretically studied in the present work. The effect of defect-engineered nanosheets has also been considered in this work by introducing V-defected as well as Se-defected layered VSe2 nanomaterials. Using first-principles calculations, the change in the electronic and magnetic properties of defected VSe2 monolayers has been compared with the pristine material. The sensing capabilities of pristine and defected VSe2 monolayers have also been assessed in terms of adsorption energy values, electronic, magnetic, and charge transfer properties with the NH3 molecule.

Computational Methods
The density functional theory (DFT) computations were accomplished by means of the Projector Augmented Wave (PAW) principles as implemented in the Vienna ab initio Simulation Package (VASP) [74][75][76][77]. In the simulations, generalized gradient approximation (GGA) was used for exchange-correlation functions [78]. During the computations, the convergence criteria for Hellman-Feynman forces were kept at 0.01 eV/Å alongside the plane wave cut-off energy of 600 eV. The long-range interactions may impact the sensing properties of the material. Hence, long-range interactions were taken care of with Grimme's DFT-D3 functional [79,80]. The Γ-centered K-points grid of 6 × 6 × 1 was used for the integration of the first Brillouin zone [81]. A vacuum of 20 Å was introduced in the z-direction to avoid the interactions between the layers in the Z direction. The thermal stability of the VSe2 monolayer adsorbed with NH3 was computed with the help of abinitio molecular dynamics simulations (AIMD). The AIMD simulations were carried out in the NVT ensemble using the Nosé-Hoover thermostat to determine the thermal stability of VSe2 + NH3 and VSe2(Sev) + NH3 systems at 400 K. The simulations were carried out for a total time of 5 ps with a time step of 1 fs. The modality of detection of NH 3 with VSe 2 nanosheets has thus been theoretically studied in the present work. The effect of defect-engineered nanosheets has also been considered in this work by introducing V-defected as well as Se-defected layered VSe 2 nanomaterials. Using first-principles calculations, the change in the electronic and magnetic properties of defected VSe 2 monolayers has been compared with the pristine material. The sensing capabilities of pristine and defected VSe 2 monolayers have also been assessed in terms of adsorption energy values, electronic, magnetic, and charge transfer properties with the NH 3 molecule.

Computational Methods
The density functional theory (DFT) computations were accomplished by means of the Projector Augmented Wave (PAW) principles as implemented in the Vienna ab initio Simulation Package (VASP) [74][75][76][77]. In the simulations, generalized gradient approximation (GGA) was used for exchange-correlation functions [78]. During the computations, the convergence criteria for Hellman-Feynman forces were kept at 0.01 eV/Å alongside the plane wave cut-off energy of 600 eV. The long-range interactions may impact the sensing properties of the material. Hence, long-range interactions were taken care of with Grimme's DFT-D3 functional [79,80]. The Γ-centered K-points grid of 6 × 6 × 1 was used for the integration of the first Brillouin zone [81]. A vacuum of 20 Å was introduced in the zdirection to avoid the interactions between the layers in the Z direction. The thermal stability of the VSe 2 monolayer adsorbed with NH 3 was computed with the help of abinitio molecular dynamics simulations (AIMD). The AIMD simulations were carried out in the NVT ensemble using the Nosé-Hoover thermostat to determine the thermal stability of VSe 2 + NH 3 and VSe 2 (Se v ) + NH3 systems at 400 K. The simulations were carried out for a total time of 5 ps with a time step of 1 fs.

Structural Analysis of Pristine and Defected VSe 2
The 4 × 4 × 1 supercell of VSe 2 was used to mimic the two-dimensional monolayer in this work. The geometry-relaxed structure of pristine VSe 2 is shown in Figure 2a. In this structure, the metal atom layer is embedded between the selenium atom layers. Using the optimized structure of pristine VSe 2 , V-defected VSe 2 was constructed by removing a single V-metal atom from the monolayer [ Figure 2b]. Similarly, the Se-defected layer was modeled by eliminating a Se-atom from the monolayer [ Figure 2c]. The V and Se-defected monolayers are described as VSe 2 (V v ) and VSe 2 (Se v ), distinctly. The optimized structures of VSe 2 , VSe 2 (V v ), and VSe 2 (Se v ) are used for the further adsorption of the NH 3 molecule at various possible positions, as mentioned below.

Structural Analysis of Pristine and Defected VSe2
The 4 × 4 × 1 supercell of VSe2 was used to mimic the two-dimensional monolayer in this work. The geometry-relaxed structure of pristine VSe2 is shown in Figure 2a. In this structure, the metal atom layer is embedded between the selenium atom layers. Using the optimized structure of pristine VSe2, V-defected VSe2 was constructed by removing a single V-metal atom from the monolayer [ Figure 2b]. Similarly, the Se-defected layer was modeled by eliminating a Se-atom from the monolayer [ Figure 2c]. The V and Se-defected monolayers are described as VSe2(Vv) and VSe2(Sev), distinctly. The optimized structures of VSe2, VSe2(Vv), and VSe2(Sev) are used for the further adsorption of the NH3 molecule at various possible positions, as mentioned below.

Adsorption of NH3 on VSe2, VSe2(Vv), and VSe2(Sev)
To understand the NH3 sensing of pure and defected VSe2, the NH3 molecule was placed at various possible sites, 2 Å above the VSe2, VSe2(Vv), and VSe2(Sev) monolayers. The structurally relaxed geometries upon NH3 introduction on VSe2, VSe2(Vv), and VSe2(Sev) monolayers are depicted in Figure 3. The stability of the NH3 adsorbed complexes is assessed in terms of adsorption energy values both with and without van der Waals (VdW) interactions. 3.2. Adsorption of NH 3 on VSe 2 , VSe 2 (V v ), and VSe 2 (Se v ) To understand the NH 3 sensing of pure and defected VSe 2 , the NH 3 molecule was placed at various possible sites, 2 Å above the VSe 2 , VSe 2 (V v ), and VSe 2 (Se v ) monolayers. The structurally relaxed geometries upon NH 3 introduction on VSe 2 , VSe 2 (V v ), and VSe 2 (Se v ) monolayers are depicted in Figure 3. The stability of the NH 3 adsorbed complexes is assessed in terms of adsorption energy values both with and without van der Waals (VdW) interactions.
The adsorption energy is computed using the following equation: In this equation, E (complex) is the energy of the NH 3 adsorbed VSe 2 /VSe 2 (V v )/VSe 2 (Se v ) systems. The E (monolayer) represents the energy of the VSe 2 or VSe 2 (V v ) or VSe 2 (Se v ) systems. The last term E (NH3) represents the energy of the isolated ammonia gas molecule. The adsorption energy is computed using the following equation: In this equation, E(complex) is the energy of the NH3 adsorbed VSe2/VSe2(Vv)/VSe2(Sev) systems. The E(monolayer) represents the energy of the VSe2 or VSe2(Vv) or VSe2(Sev) systems. The last term E(NH3) represents the energy of the isolated ammonia gas molecule.
The adsorption energy values are shown in Table 1. It can be observed from Table 1 that the NH3 molecule is weakly bound to the pure VSe2. Or, in other words, the NH3 shows weak affinity towards the VSe2 monolayer, specifying that pure material is not much suitable for sensing purposes. The result shown in Table 1 for the VSe2 + NH3 system corresponds to the adsorption energy of 0.124 eV for the case when the N atom of NH3 has been placed upright the V atom of VSe2. The same practice has been repeated for the other three possible sites, i.e., Se atom, V-Se bond, and center of a hexagonal ring consisting of V and Se atoms, and all four obtained adsorption energy values are shown in Table  S1. As can be seen, the adsorption energy for the arrangement corresponding to the "above V" case is the least (though positive without VdW incorporation); further, all calculations are based on that arrangement. However, VSe2(Vv) and VSe2(Sev) monolayers show stronger affinity towards NH3 with adsorption energy values of −0.22 and −0.66 eV, respectively. The present studies also determined the influence of long-range interactions by computing the adsorption energy values with DFT-D3 functional to consider van der Waal interaction. It can be observed from Table 1 that the adsorption energy values Relaxed structures of (a) NH 3 (N atom directly placed above V atom) on pristine VSe 2 , (b) NH 3 (N atom directly placed above V atom) on VSe 2 deficient with V atom, and (c) NH 3 (N atom directly placed above V atom) on VSe 2 deficient with Se atom. The adsorption energy values are shown in Table 1. It can be observed from Table 1 that the NH 3 molecule is weakly bound to the pure VSe 2 . Or, in other words, the NH 3 shows weak affinity towards the VSe 2 monolayer, specifying that pure material is not much suitable for sensing purposes. The result shown in Table 1 for the VSe 2 + NH 3 system corresponds to the adsorption energy of 0.124 eV for the case when the N atom of NH 3 has been placed upright the V atom of VSe 2 . The same practice has been repeated for the other three possible sites, i.e., Se atom, V-Se bond, and center of a hexagonal ring consisting of V and Se atoms, and all four obtained adsorption energy values are shown in Table S1. As can be seen, the adsorption energy for the arrangement corresponding to the "above V" case is the least (though positive without VdW incorporation); further, all calculations are based on that arrangement. However, VSe 2 (V v ) and VSe 2 (Se v ) monolayers show stronger affinity towards NH 3 with adsorption energy values of −0.22 and −0.66 eV, respectively. The present studies also determined the influence of long-range interactions by computing the adsorption energy values with DFT-D3 functional to consider van der Waal interaction. It can be observed from Table 1 that the adsorption energy values improve with the inclusion of VdW interactions. The values reported in Table 1 suggest that the VSe 2 (Sev) + NH 3 forms the most stable complex due to higher adsorption energy values. The bond lengths between NH 3 and the adsorbent are also measured and are given in Table 1. In the case of VSe 2 (Sev) + NH 3 , the distance between the vanadium atom of the monolayer and the N Biosensors 2023, 13, 257 6 of 14 atom of NH 3 is reduced as compared to the VSe 2 + NH 3 complex. This supports stronger adsorption interactions between VSe 2 (Se v ) and the NH 3 molecule. As the VSe 2 (Se v ) + NH 3 forms the most stable complex, the change in the electronic properties of pure and Se-defected monolayers with the adsorption of NH 3 molecule is studied in this work and has been comparatively discussed further. Table 1. Adsorption energies for the adsorption of NH 3 on VSe 2 , VSe 2 (V v ), and VSe 2 (Se v ) systems with and without VdW functional. The bond lengths between the atoms of adsorbate and adsorbent are given in Å units.

System
Adsorption Energy (eV) Bond Length (Å) To study the effect of a further increase in defect density, a VSe 2 structure deficient with two Se atoms has been relaxed and again optimized with the insertion of an NH 3 molecule. (Figure 4). The resultant adsorption energy values (−1.33 and −1.58 eV with VdW), as shown in Table 1, indicate stronger adsorption. Such observation is promising to conclude that doubling the Se vacancy population is beneficial for better NH 3 detection.
that the VSe2(Sev) + NH3 forms the most stable complex due to higher adsorption energy values. The bond lengths between NH3 and the adsorbent are also measured and are given in Table 1. In the case of VSe2(Sev) + NH3, the distance between the vanadium atom of the monolayer and the N atom of NH3 is reduced as compared to the VSe2 + NH3 complex. This supports stronger adsorption interactions between VSe2(Sev) and the NH3 molecule. As the VSe2(Sev) + NH3 forms the most stable complex, the change in the electronic properties of pure and Se-defected monolayers with the adsorption of NH3 molecule is studied in this work and has been comparatively discussed further. To study the effect of a further increase in defect density, a VSe2 structure deficient with two Se atoms has been relaxed and again optimized with the insertion of an NH3 molecule. (Figure 4). The resultant adsorption energy values (−1.33 and −1.58 eV with VdW), as shown in Table 1, indicate stronger adsorption. Such observation is promising to conclude that doubling the Se vacancy population is beneficial for better NH3 detection.

Total Density of States (TDOS) Plots
In order to get insights regarding charge transfer and the interaction mechanism of NH 3 with pristine and defected VSe 2 , we have presented total and partial density of states analyses. The TDOS plot of a pure VSe 2 monolayer is specified in Figure 3a. To determine the magnetic behavior, spin-up and spin-down states are plotted. It is observed from the figure that the pure material is magnetic due to the asymmetry in spin states. The existence of the density of states at the fermi level implies the metallic behavior of the materials, consistent with earlier findings [64,65]. The total density of states enhanced by the adsorption of the NH 3 molecule on VSe 2 is shown in Figure 5a. In the case of the VSe 2 (Se v ) system, an enhancement in TDOS is observed below the Fermi level, as depicted in Figure 5b. The enhancement in the density of states occurs due to the unbound V-atom bonds after the removal of the Se atom from the monolayer. The change in the density of states with the adsorption of NH 3 supports the orbital interactions. The density of states is also enhanced at the fermi level with the adsorption of NH 3 on the VSe 2 (Se v ) system. figure that the pure material is magnetic due to the asymmetry in spin states. The existence of the density of states at the fermi level implies the metallic behavior of the materials, consistent with earlier findings [64,65]. The total density of states enhanced by the adsorption of the NH3 molecule on VSe2 is shown in Figure 5a. In the case of the VSe2(Sev) system, an enhancement in TDOS is observed below the Fermi level, as depicted in Figure 5b. The enhancement in the density of states occurs due to the unbound V-atom bonds after the removal of the Se atom from the monolayer. The change in the density of states with the adsorption of NH3 supports the orbital interactions. The density of states is also enhanced at the fermi level with the adsorption of NH3 on the VSe2(Sev) system.

Partial Density of States (PDOS) Plots
To investigate the orbital interactions, the spin-polarized partial density of states (PDOS) is analyzed. The spin-polarized partial density of states (PDOS) for N-2p and H-1s orbitals in NH3 and VSe2(Sev) + NH3 were computed and are shown in Figure 6a. In the case of the NH3 molecule, the partial density of states for N-2p and H-1s orbitals is spotted in the valence band. These partial densities of states disappeared (or were reduced) with the adsorption of NH3 on the VSe2(Sev) monolayer. Further, the spin-polarized partial density of states (PDOS) of V-3d orbitals for VSe2 + Sev and VSe2(Sev) + NH3 were computed and are shown in Figure 6b. On comparing the PDOS of V-3d orbitals of VSe2(Sev) and VSe2(Sev) + NH3 systems, it can be observed that the densities of states are enhanced in the

Partial Density of States (PDOS) Plots
To investigate the orbital interactions, the spin-polarized partial density of states (PDOS) is analyzed. The spin-polarized partial density of states (PDOS) for N-2p and H-1s orbitals in NH 3 and VSe 2 (Se v ) + NH 3 were computed and are shown in Figure 6a. In the case of the NH 3 molecule, the partial density of states for N-2p and H-1s orbitals is spotted in the valence band. These partial densities of states disappeared (or were reduced) with the adsorption of NH 3 on the VSe 2 (Se v ) monolayer. Further, the spin-polarized partial density of states (PDOS) of V-3d orbitals for VSe 2 + Se v and VSe 2 (Se v ) + NH 3 were computed and are shown in Figure 6b. On comparing the PDOS of V-3d orbitals of VSe 2 (Se v ) and VSe 2 (Se v ) + NH 3 systems, it can be observed that the densities of states are enhanced in the latter with the adsorption of the NH 3 molecule. This suggests that the monolayer is acting as an electron acceptor, whereas NH 3 is acting as an electron donor. So, we can say that there is a charge transfer from NH 3 to VSe 2 (Se v ) due to the adsorption of NH 3 .
The total density of states and partial density of states plots have shown that the electronic properties of the VSe 2 monolayer can be tuned with the defect induction, which impacts the adsorption properties.
latter with the adsorption of the NH3 molecule. This suggests that the monolayer is actin as an electron acceptor, whereas NH3 is acting as an electron donor. So, we can say th there is a charge transfer from NH3 to VSe2(Sev) due to the adsorption of NH3.
The total density of states and partial density of states plots have shown that the ele tronic properties of the VSe2 monolayer can be tuned with the defect induction, whic impacts the adsorption properties.

Charge Transfer Analysis
The interactions between the analyte and host were determined in terms of Bad charge analysis [82]. The VSe2(Sev) monolayer shows a net gain of 0.009e of charge due adsorption of the NH3 molecule whereas, the NH3 molecule shows a net loss of 0.009e charge, suggesting that the monolayer acts as an electron acceptor. The Bader charge ana ysis is in accordance with the partial density of states (PDOS) plots ( Figure 6). The abov observation is consistent with the opinion of earlier researchers regarding ammonia sen ing in terms of charge transfer course. (Table 2) [37,[83][84][85][86]. Additionally, a charge densi difference plot has been shown in Figure 7. It is performed with the relation: For all three systems, the ISO values are around 0.04e, wherein red regions deno regions of charge loss and green or blue regions denote charge gain. In all three system a charge loss region is noted around the N atom of the NH3 molecule, while a charge ga region is noted over the VSe2 surface with a Se vacancy.

Charge Transfer Analysis
The interactions between the analyte and host were determined in terms of Bader charge analysis [82]. The VSe 2 (Se v ) monolayer shows a net gain of 0.009e of charge due to adsorption of the NH 3 molecule whereas, the NH 3 molecule shows a net loss of 0.009e of charge, suggesting that the monolayer acts as an electron acceptor. The Bader charge analysis is in accordance with the partial density of states (PDOS) plots ( Figure 6). The above observation is consistent with the opinion of earlier researchers regarding ammonia sensing in terms of charge transfer course. (Table 2) [37,[83][84][85][86]. Additionally, a charge density difference plot has been shown in Figure 7. It is performed with the relation: For all three systems, the ISO values are around 0.04e, wherein red regions denote regions of charge loss and green or blue regions denote charge gain. In all three systems, a charge loss region is noted around the N atom of the NH 3 molecule, while a charge gain region is noted over the VSe 2 surface with a Se vacancy.

Thermal Stability from Molecular Dynamics Simulations
A nanosensor should be stable at higher temperatures for its efficient performance. Moreover, the gas molecules adsorbed on it should remain intact in the system until the sensing procedure is completed. As pristine VSe2 is a synthesized material, it is thermally stable at room temperature. So, we have investigated the thermal stability of VSe2 + NH3 and VSe2(Sev) + NH3 systems. The ab initio molecular dynamics simulations were carried out to investigate the thermal stability of the considered material at higher temperatures. The snapshots of equilibrated VSe2 + NH3 and VSe2(Sev) + NH3 systems after 5 ps at 400 K are shown in Figure 8. The bond length fluctuations (between N of NH3 and V of VSe2) with the temperature are plotted in Figure 9. We can notice that for pristine VSe2, the NH3 molecule goes away from the system starting with a temperature of 108 K. It seems that the NH3 molecule desorbs from the system once the temperature is increased, with desorption starting around 108 K. This is because NH3 is bonded very weakly on pristine VSe2 and goes out of the system at higher temperatures. So NH3 desorbs from the system below room temperature for pristine VSe2. So, pristine VSe2 is not suitable for NH3 sensing due to weaker interactions and low adsorption energy. But for VSe2(Sev) + NH3 system, the bond length

Thermal Stability from Molecular Dynamics Simulations
A nanosensor should be stable at higher temperatures for its efficient performance. Moreover, the gas molecules adsorbed on it should remain intact in the system until the sensing procedure is completed. As pristine VSe 2 is a synthesized material, it is thermally stable at room temperature. So, we have investigated the thermal stability of VSe 2 + NH 3 and VSe 2 (Se v ) + NH 3 systems. The ab initio molecular dynamics simulations were carried out to investigate the thermal stability of the considered material at higher temperatures. The snapshots of equilibrated VSe 2 + NH 3 and VSe 2 (Se v ) + NH 3 systems after 5 ps at 400 K are shown in Figure 8.

Thermal Stability from Molecular Dynamics Simulations
A nanosensor should be stable at higher temperatures for its efficient performance Moreover, the gas molecules adsorbed on it should remain intact in the system until th sensing procedure is completed. As pristine VSe2 is a synthesized material, it is thermall stable at room temperature. So, we have investigated the thermal stability of VSe2 + NH and VSe2(Sev) + NH3 systems. The ab initio molecular dynamics simulations were carrie out to investigate the thermal stability of the considered material at higher temperatures The snapshots of equilibrated VSe2 + NH3 and VSe2(Sev) + NH3 systems after 5 ps at 400 K are shown in Figure 8. The bond length fluctuations (between N of NH3 and V of VSe2) with the temperatur are plotted in Figure 9. We can notice that for pristine VSe2, the NH3 molecule goes awa from the system starting with a temperature of 108 K. It seems that the NH3 molecul desorbs from the system once the temperature is increased, with desorption startin around 108 K. This is because NH3 is bonded very weakly on pristine VSe2 and goes ou of the system at higher temperatures. So NH3 desorbs from the system below room tem perature for pristine VSe2. So, pristine VSe2 is not suitable for NH3 sensing due to weake interactions and low adsorption energy. But for VSe2(Sev) + NH3 system, the bond lengt The bond length fluctuations (between N of NH 3 and V of VSe 2 ) with the temperature are plotted in Figure 9. We can notice that for pristine VSe 2 , the NH 3 molecule goes away from the system starting with a temperature of 108 K. It seems that the NH 3 molecule desorbs from the system once the temperature is increased, with desorption starting around 108 K. This is because NH 3 is bonded very weakly on pristine VSe 2 and goes out of the system at higher temperatures. So NH 3 desorbs from the system below room temperature for pristine VSe 2 . So, pristine VSe 2 is not suitable for NH 3 sensing due to weaker interactions and low adsorption energy. But for VSe 2 (Se v ) + NH 3 system, the bond length fluctuations are not much. It is around 10% of the mean value, suggesting that adsorbed NH 3 remains intact at 300 K and even up to 400 K on the sensing material. This is due to the fact that the adsorption energy of NH 3 on defected VSe 2 has increased from −0.12 eV for the pristine system to −0.97 eV for the VSe 2 (Se v ) system. Strong adsorption energy is due to charge transfer from NH 3 to defected VSe 2 . So, the defected VSe 2 is promising for NH 3 sensing.
fluctuations are not much. It is around 10% of the mean value, suggesting that adsorbed NH3 remains intact at 300 K and even up to 400 K on the sensing material. This is due to the fact that the adsorption energy of NH3 on defected VSe2 has increased from −0.12 eV for the pristine system to −0.97 eV for the VSe2(Sev) system. Strong adsorption energy is due to charge transfer from NH3 to defected VSe2. So, the defected VSe2 is promising for NH3 sensing.

Recovery time (τ)
The reversible sensors could be used repeatedly and hence, are economically convenient for utilization in industrial sectors [65]. The recovery time analysis helps to determine the extent to which a sensor can be used reversibly. The recovery time determines the time required for an analyte to desorb from the host surface. It can be computed using the following equation [65]: In the equation, the ν denotes the frequency factor or the reciprocal of the pre-exponential factor of the Arrhenius equation [87]. The terms Eads, k, and T denote the adsorption energy, Boltzmann constant, and temperature, respectively. Using this equation, the recovery time for VSe2 + NH3 and VSe2(Sev) + NH3 systems were computed at 300 K and 500 K for visible yellow light and UV light. The τ values are shown in Table 3. The tabulated values show that at 300 K under UV radiation, VSe2(Sev) + NH3 system promises a convenient recovery time (~2 s). This suggests that VSe2(Sev) + NH3 system can act as a reusable sensor. Apart from the above, response time is also considered a very important parameter for determining the sensitivity of any gas detector. When the gas is initially applied, it

Recovery time (τ)
The reversible sensors could be used repeatedly and hence, are economically convenient for utilization in industrial sectors [65]. The recovery time analysis helps to determine the extent to which a sensor can be used reversibly. The recovery time determines the time required for an analyte to desorb from the host surface. It can be computed using the following equation [65]: In the equation, the ν denotes the frequency factor or the reciprocal of the preexponential factor of the Arrhenius equation [87]. The terms E ads , k, and T denote the adsorption energy, Boltzmann constant, and temperature, respectively.
Using this equation, the recovery time for VSe 2 + NH 3 and VSe 2 (Se v ) + NH 3 systems were computed at 300 K and 500 K for visible yellow light and UV light. The τ values are shown in Table 3. The tabulated values show that at 300 K under UV radiation, VSe 2 (Se v ) + NH 3 system promises a convenient recovery time (~2 s). This suggests that VSe 2 (Se v ) + NH 3 system can act as a reusable sensor. Apart from the above, response time is also considered a very important parameter for determining the sensitivity of any gas detector. When the gas is initially applied, it takes a few seconds for the sensor output current to attain steady-state conditions [88]. The response time of the sensor is commonly specified by the T 90 or T 50 time. T 90 is the time for the sensor's response current to reach 90% of its steady-state value. Similarly, the T 50 metric is the time required for the sensor to reach 50% of its steady-state value [88]. Future progress in this work can consist of determining the response time for VSe 2 to detect NH 3 .
In spite of promising results, improvements in 2D VSe 2 are needed to attain better sensitivity, selectivity, and stability. Specifically, there is scope for improvement in recovery time owing to the slow gas desorption process to enable it suitable for usage at room temperature. Currently, this kind of resource seems to be substandard in terms of sensing presentations when contrasted with metal oxide nanostructures; however, their performance is on par with that of pristine graphene. The technology available as of now to physically fabricate planer structures is still not industrially budget-friendly, so more technological advancement is necessary.

Conclusions
The structural, electronic, and sensing properties of pure and defected VSe 2 monolayers have been investigated with density functional theory calculations. The energetic stability of VSe 2 (Se v ) + NH 3 and VSe 2 (V v ) + NH 3 monolayers is studied as adsorption energy values. The VSe 2 (Se v ) binds strongly with the adsorbed NH 3 molecule compared to the pure nanomaterial. With the introduction of Se vacancy, the adsorption energy increases from −0.12 eV in the pristine case to −0.97 eV for VSe 2 (Se v ). Charge transfer from NH 3 to defected VSe 2 is responsible for stronger adsorption. It has been observed that NH 3 acts as a charge donor and the host, i.e., VSe 2 , as a charge acceptor to cause the adsorption to be effective. The thermal stability of the VSe 2 (Se v ) + NH 3 system was investigated by performing ab initio molecular dynamics simulations at 300 K and 400 K and the system was found to be structurally stable even at higher temperatures. The recovery time analysis suggests that the VSe 2 (Se v ) monolayer can act as a reusable nanosensor. The present studies show that the sensing properties of the VSe 2 monolayer can be significantly improved with the introduction of Se-defects in the lattice structure. Or, in other words, tuning structural and electronic properties through the introduction of Se vacancy aids in enhancing the sensing properties of the VSe 2 monolayer for NH 3 adsorption. The obtained results will be potentially helpful for experimentalists to design defect-engineered TMD-based novel gas sensors.
Supplementary Materials: The following supporting information can be downloaded at: https:// www.mdpi.com/article/10.3390/bios13020257/s1, Table S1: Adsorption energies for the adsorption of NH3 on VSe2 at different sites.

Data Availability Statement:
The data presented in this study are available on reasonable request to the corresponding author.