FACTA UNIVERSITATIS Series: Electronics and Energetics Vol. 35, No 1, March 2022, pp. 43-59 https://doi.org/10.2298/FUEE2201043C © 2022 by University of Niš, Serbia | Creative Commons License: CC BY-NC-ND Original scientific paper FIRST PRINCIPLE INSIGHT INTO Co-DOPED MoS2 FOR SENSING NH3 AND CH4 * Bibek Chettri1, Abinash Thapa2, Sanat Kumar Das1, Pronita Chettri1, Bikash Sharma2 1Department of Physics, Sikkim Manipal Institute of Technology, Majitar, Sikkim, India 2Department of Electronics and Communication Engineering, Sikkim Manipal Institute of Technology, Majitar, Sikkim, India Abstract. In this work we present the atomistic computational study of the adsorption properties of Co doped MoS2 adsorbed ammonia (NH3) and methane (CH4). The adsorption distance, adsorption energy (Ead), charge transfer (Qt), bandgap, Density of States (DOS), Projected Density of States (PDOS), transport properties, sensitivity and recovery time have been reported. The diffusion property of the system was calculated using Nudge Elastic Band (NEB) method. The calculated results depict that after suitable doping of Co on MoS2 monolayer decreases the resistivity of the system and makes it more suitable for application as a sensor. After adsorbing NH3 and CH4, Co doped MoS2 bandgap, DOS and PDOS become more enhanced. The adsorption energy calculated for NH3 and CH4 adsorbed Co doped MoS2 are -0.9 eV and -1.4 eV. The reaction is exothermic and spontaneous. The I-V curve for Co doped MoS2 for CH4 and NH3 adsorption shows a linear increase in current up to 1.4 V and 2 V, respectively, then a rapid decline in current after increasing a few volts. The Co doped MoS2 based sensor has a better relative resistance state, indicating that it can be employed as a sensor. The sensitivity for CH4 and NH3 were 124 % and 360.5 %, respectively, at 2 V. With a recovery time of 0.01s, the NH3 system is the fastest. In a high-temperature condition/environment, the Co doped MoS2 monolayer has the potential to adsorb NH3 and CH4 gas molecules. According to NEB, CH4 gas molecules on Co doped MoS2 has the lowest energy barrier as compared to NH3 gas molecules. Our results indicate that adsorbing NH3 and CH4 molecules in the interlayer is an effective method for producing Co doped MoS2 monolayers for use as spintronics sensor materials. Key words: Density Functional Theory, gas sensor, adsorption energy, TMD. Received September 1, 2021; received in revised form November 17, 2021 Corresponding author: Bikash Sharma Sikkim Manipal Institute of Technology, Sikkim, India E-mail: ju.bikash@gmail.com * An earlier version of this paper was presented at the 4th International conference on 2021 Devices for Integrated Circuit (DevIC 2021), May 19-20, 2021, in Kalyani, West Bengal, India [1]. B. CHETTRI, A. THAPA, S. K. DAS, P. CHETTRI, B. SHARMA 44 1. INTRODUCTION NH3 and CH4 are the common gases which are used for industrial and agricultural purposes [1]. NH3 and CH4 are colourless and tasteless gases that are difficult to identify, and they make people suffocate when its concentration is high in the air [2][3][4]. CH4 reduces the level of oxygen, resulting in headaches, dizziness, increased rate of heartbeat and causing breathlessness in human beings [5][6][7]. Therefore, good and sensitive gas sensors for the detection of hazardous gases such as ammonia (NH3) and methane (CH4), is critical for both industrial and civilian purposes [8][9][10]. Hence, the demand for gas sensors with high sensitivity, low power consumption and short recovery time has increased [11][12]. In recent years, two-dimensional (2D) materials such as Transition Metal Dichalcogenides (TMDs) have gained immense attention. 2D MoS2, n-type semiconductor [13] with a bandgap of 1.3-1.8 eV [13][14][15] has been one of the most promising materials for the application of gas sensors due to its superior sensitivity [16][17]. MoS2 monolayer has a large surface-to-volume ratio[18], tunable electrical properties [19][20] and magnetic properties [21][22]. Much research has been focused on making MoS2 more prominent by suitable doping [23][24]. Xianxian et al. investigated the adsorption behaviour of Rh-doped MoS2 monolayer towards SO2, SOF2 and SO2F2 and found the improved performance towards the adsorption of gas molecules as compared to pristine MoS2 [25]. Guochao et al. verified the excellent sensitivity property of Au doped MoS2 for sensing C2H6 and C2H2 [26]. Likewise, Zhen et al. confirmed the center of MoS2 as the best possible site for doping Fe, Co, Ni and Cu [27]. The doping and codoping on MoS2 confirmed the high sensitivity of NO and NO2 gas by Ehab et al. [28]. Not only in MoS2 but doping has significantly increased the material properties of other nanomaterials [29]. Chettri et al. explored the changes in the electronic and magnetic properties of h-BN after suitable doping [30]. Y. Wang et al. used DFT to determine that Fe doped MoS2 could be used as a spintronic gas sensor to detect NO gas molecules [31]. Additionally, using DFT, Y.-H. Zhang et al. discovered that transition metal-doped MoS2 can be used as a spintronic gas sensor to detect CO gas molecules [32]. The gas sensing properties of Ptn doped WSe2 nanosheet to SF6 breakdown products were explored by Linga Xu et al., who discovered that incorporating a Pt atom greatly increases the sensing properties of WSe2 nanosheet [33]. The overall results show the novelty of MoS2 after suitable doping. Transition Metal (TM) doped MoS2 are more prominent since the interaction between TM and MoS2 is strong [1] and provides numerous free electrons [34][1]. Due to the strong orbital hybridization between the atoms and gas molecules, increased sensitivity is observed [35]. In this paper, we explored the adsorption properties of a Co doped monolayer (hereafter referred to as Co-MoS2). The adsorption distance, binding energy, adsorption energy and charge transfer were studied to investigate the most stable configuration of doping of Co and adsorption of gas molecules on MoS2 monolayer. Further, the bandgap, Density of States (DOS) and Projected Density of States (PDOS) were studied to understand the electronic properties. In the end, the I-V characteristics followed by sensitivity and recovery time was calculated to understand the property of the gas sensor. 45 First Principle Insight into Co-doped MoS2 for Sensing NH3 and CH4 2. METHODOLOGY Density functional theory [36] computation was carried out in QuantumATK [37]. For the exchange-correlation term, the Perdew-Burke-Ernzerh Generalised Gradient Approximation (GGA-PBE) function was used [38][39] with Troullier-Martins type pseudopotential [40]. The non-conserving Double-Zeta Polarized (DZP) was used as the basis set [41] with a 5×5×1 Monkhorst-pack k-point grid [42]. The Density of States was computed with a higher k-point of 15×15×1 [32]. To understand the magnetic characteristics of the system, we used spin- polarized computations for all calculations [31]. The geometry was relaxed with limited memory Broyden-Fletcher-Goldfarb-Shanno (LBFGS) algorithm [43] with a minimum force of 0.05 eV/Å [43]. The considered cut-off energy for all the calculations was 100 Ha. The inclusion of dispersion correction is described by Grimme’s DFT-D2 method. Pulay mixer algorithm was implemented to control the self-consistent iteration with a tolerance of 0.0002 Ry and 100 maximum steps [44]. The 4×4×1 supercell of MoS2 monolayer with 15 Å vacuum space along z-direction was considered for the calculation with 32 S atoms and 16 Mo atoms. The calculated lattice constant of MoS2 is 3.8 Å, which satisfies the theoretical value [45][46]. The MoS2 monolayer was fully relaxed with Stillinger-Weber (SW) potentials [47]. The binding energy of Co-MoS2 is calculated using [48], 2 2b MoS Co Co MoS E E E E − = + − Here, EMoS2, ECo and ECo−MoS2 represents the total energy of pristine MoS2, isolated Co and Co-MoS2 monolayer respectively. The adsorption energy of NH3 and CH4 gas molecules on the Co-MoS2 is calculated using[48], 2 2ads Co MoS gas Co MoS gas E E E E − − − = + − Here, ECo−MoS2, Egas and ECo−MoS2−gas represents the total energy of Co-MoS2 monolayer, isolated gas molecules and gas molecules adsorbed in Co-MoS2 monolayer respectively. The total charge transfer Qt was obtained using the Mulliken method [49]. The Qt is calculated by [49], ( ) ( )t absorbed gas isolated gas Q Q Q= − Qabsorbed(gas) and Qisolated(gas) is the carried charge of gas molecules before and after gas adsorption respectively. We used a two-probe configuration with the left electrode, right electrode, and central region to investigate the system's transport properties [50]. The Non Equilibrium Green’s Function (NEGF) approach, as implemented in QuantumATK [51], was used to compute the transport properties of the considered structure. The device supercell was sampled using a 2 D Fast Fourier Transform (FFT2D) Poisson solver with 1×1×150 k-points for the device simulation [41]. On the electrode faces, the Dirichlet boundary condition is used, whereas, on all other faces, the boundary condition is set to periodic. For all device computations, the average Fermi level is used as the energy zero parameter in the Krylov self-energy calculator. The transmission is derived from the device's extended green's function as follows: † ( ) [ ] L R T E Tr G G=   B. CHETTRI, A. THAPA, S. K. DAS, P. CHETTRI, B. SHARMA 46 The greens’ function is given by [52][53] 1 ( ) [ ( ) ( )] L R G E ES H E E − = − − − The current voltage characteristics are now derived using the following equation by integrating the transmission function across a suitable voltage [54]. / 2 0 / 2 ( ) ( , ) f a f a E ev a a E ev I V G T E V dE + − =  The sensitivity of the Co-MoS2 to absorb urea and methanol was analyzed, obtained from the equation [55] 0 0 [( ) / ] 100%S R R R= −  where R0 and R represent the resistance of Co-MoS2 without and with gas adsorption respectively. In addition, to study the property of gas sensors we estimated the recovery time. The better property of the gas sensor is predicted by a faster recovery time [50]. The recovery time is calculated using the following formula [56]: 1 a B E k T A e − = where A is the apparent factor which is equal to 1012 s-1, kB is the Boltzmann constant (8.62 ×10-5 eV/K. Ea is the absolute value of adsorption energy and T is the temperature [57]. The diffusion coefficient is calculated using the equation below [58][59]. 𝐷 = 1 6𝑁 lim 𝑡→∞ 𝑑 𝑑𝑡 ∑〈[𝑟𝑖(𝑡) − 𝑟0(𝑡 2)]〉 𝑁 𝑖=1 Where 𝑟𝑖(𝑡) denotes the position of atom i at time of t, N is the number of diffusion atoms in the system, 𝑟0(𝑡) is the initial position of atom i. 3. RESULT AND DISCUSSION The structure of NH3 and CH4 is shown in Fig.1 (a) and (b) respectively. The central N atom in NH3 molecules bonds with three H atoms. The bond length of N with three H atoms is 1 Å with a bond angle of 108.03º. The central C atom bonds with four H atoms in CH4 molecules with bond length and bond angle of 1.09Å and 109.47º respectively. As per the Mulliken population analysis, the N and H atoms have a positive charge of 4.941e and 1.019e respectively in NH3. The C and H atoms have the positive charge of 3.776e and 1.056e respectively in CH4 molecules as calculated by the Mulliken population analysis. Table 1 summarizes the respective values. 47 First Principle Insight into Co-doped MoS2 for Sensing NH3 and CH4 The most stable structure of the Co-doped monolayer was obtained by calculating the binding energy, charge transfer and distance between the atoms for three positions where Co atom was kept on the top of S atom (ST), top of Mo atom (MoT) and above the hexagonal ring (hollow) of MoS2. The calculated parameters are shown in Table 1. The calculated binding energy for the hollow site is 3.97 eV and the total charge transfer is -0.003e. The distance between S and Mo atoms is 1.41 Å and 3.54 Å respectively. The binding energy of the MoT site was calculated to be highest, i.e., 4.54 Å and the lowest was for the ST site i.e., -4.3 Å. The charge transfer of MoT and ST is 0.021e and 0.163e. For the hollow site, the Co loses electrons on the hollow site after doping, whereas Co gets electrons in MoT and ST site after doping. From here we can conclude that the MoT site has strong binding energy and a shorter distance between S-Co and Mo-Co atoms. The binding is relatively strong, and MoT is the most favorable position for the Co atom doped on the MoS2 monolayer. The most stable structure Co on top of Mo atom (MoT) is shown in Fig. 2 (b) and (c). Fig. 2 The structure of (a) top view, (b) side view of MoS2, (c) top view and (d) side view of Co- MoS2 Fig. 1 The structure of (a) NH3 and (b) CH4 Table 1 Parameters for NH3 and CH4 Molecules Bond Distance Bond Angle Qt(e) CH4 C-H: 1.09Å C-H: 109.47º C: 3.776 H: 1.056 NH3 N-H: 1 Å N-H: 108.03º N: 4.941 H: 1.019 B. CHETTRI, A. THAPA, S. K. DAS, P. CHETTRI, B. SHARMA 48 Table 2 Parameters for Co-MoS2 Site Eb(eV) Qt(e) ds(Å) dMo(Å) ST -4.3 0.163 1.41 3.54 MoT 4.5 0.021 1.81 2.58 Hollow 3.9 -0.003 2.26 3.09 Furtheron, to investigate more about the effect of Co on MoS2, we calculated the electronic properties like bandgap, the Density of States (DOS) and Projected Density of States (PDOS) of pristine MoS2 monolayer and Co-MoS2 monolayer. The optimized structure of the MoS2 monolayer is shown in Fig. 2 (a) and (b). The bond length between Mo and S atom is 2.42 Å and the bond angle of S-Mo-S is 81.63º, which is close to the previous study [60]. The calculated band gap of pristine MoS2 is 1.68 eV and after substitution of Co atom, bandgap reduces to 0.2 eV. The bandgap value calculated for monolayer MoS2 is consistent with earlier literature [25][61][20]. The reduction in bandgap signifies the improvement in the conduction property of the material. It depicts the less energy required for the transition of electrons between the valence band and conduction band. The band structure of pristine MoS2 and Co-MoS2 is shown in Fig. 3 (a) and (b) respectively. The DOS graph of pristine MoS2 is shown in Fig. 5 (a). The black and red lines in this diagram represent spin up and down, respectively. The DOS distance between the valence band and conduction band separation is much similar to the band structure of Fig. 3 The calculated Bandgap of (a) Pristine MoS2, (b) Co-MoS2, (c) Co-MoS2 adsorbed CH4 and (d) Co-MoS2 adsorbed NH3 49 First Principle Insight into Co-doped MoS2 for Sensing NH3 and CH4 pristine MoS2. The DOS of pure MoS2 is symmetric, with spin up and spin down mirroring each other. This result indicates that MoS2 in its purest form is not magnetic. To better understand the electrical property, we looked at the PDOS of pristine MoS2, as seen in Fig. 6(a). The p orbitals of S atoms dominate the upper and lower parts of the valence band. Similarly, Mo atoms d orbitals dominate the higher and lower parts of the conduction band. In both the upper and lower sides of the conduction band and valence band, there is a significant overlap of the p orbital of the S atom with the d orbital of the Mo atoms near the Fermi level. The strong hybridization of both atoms was shown by this overlap. Fig. 5 (b) and 6 (b) show the DOS and PDOS graphs for Co-MoS2, respectively. The Co-d orbital contributes to the introduction of impurity states around the Fermi level, narrowing the energy bandgap to 0.2 eV. Because of the antisymmetric spin-up and spin-down states, the computed PDOS shows that the system has now become magnetic. The hybridization around the Fermi level of the conduction band is due to the d orbitals of Mo atoms and the d orbitals of Co atoms, according to the PDOS. Similarly, p orbitals of S atoms and d orbitals of Co atoms cause hybridization near the Fermi level of the valence band. Both the lower and upper sides of the conduction and valence bands show hybridization. 3.1. Adsorption Property of CH4 and NH3 on Co-MoS2 To investigate the most stable position for the adsorption of CH4 molecules on Co- MoS2, we used three possible sites. Table 3 summarizes the adsorption energy, charge transfer, and adsorption distance calculations. The C atom was placed close to the Co atom (C-Co), the H atom was placed close to the Co atom (H-Co), and both the H and C atoms were placed close to the Co atom (H-C-Co). The adsorption energy for H-Co was calculated to be -0.2 eV, and the distance between H-Co was calculated to be 1.37. The adsorption energy for C-Co was determined to be 0.1 eV. For the H-C-Co position, higher adsorption energy of -1.4 eV was calculated. It adsorbs gas molecules at 1.75 for C-Co atoms and 1.57 for H-Co atoms. Because of the position of CH4 that was kept above Co- MoS2, there is a slight increase in adsorption distance when compared to the other two positions. Furthermore, the charge transfer for all three positions was calculated using Mulliken analysis. The total charge transfer Qt was negative for all three positions, indicating that CH4 molecules act as an electron donor, transferring an electron to Co- MoS2. C-Co, H-Co, and H-C-Co locations have Qt values of -0.116e, -0.17e, and -0.076e, respectively. As a result of the aforesaid results, we determined that the H-C-Co site is the most stable for CH4 molecule adsorption on a Co-MoS2 monolayer. Furthermore, we investigated the H-C-Co site adsorption ability for CH4 molecule adsorption on Co-MoS2. Fig. 4 (c) and 4 (d) depicts the most stable position. At energies of -0.17 eV, -0.18 eV, - 0.19 eV, -0.2 eV, and -0.3 eV, a strong peak in the valence band, which is more populated than the conduction band, can be seen. In order to better understand the role of spin density at the Fermi level, we show the PDOS of CH4 in Co-MoS2 in Fig. 6. (c). The s, p, and d orbitals of the H, C, and Co, atoms have high peaks around the Fermi level of the upper and lower valence band, respectively. At the lower side of the conduction band, substantial hybridization of the s, p, and d orbitals of H, C, and Co atoms can be seen, indicating a large contribution to spin polarization from the CH4 molecule. Our findings imply that adsorbing CH4 molecules in the interlayer is a good way to create a Co-MoS2 monolayer as a spintronics sensor material. When MoS2 is doped with Co, it becomes a spintronics- based CH4 sensor. B. CHETTRI, A. THAPA, S. K. DAS, P. CHETTRI, B. SHARMA 50 Fig. 4 The structure of (a) top view, (b) side view of adsorbed NH3, (c) top view and (d) side view of adsorbed CH4 in Co-MoS2 Table 3 Parameters for adsorption of NH3 on Co-MoS2 Site Ead(eV) Qt(e) distance(Å) H-Co -0.9 -0.173 H-Co: 1.23 N-Co -0.8 -0.436 N-Co: 1.28 H-N-Co -0.1 -0.114 H-Co: 1.55; N-Co: 1.89 Table 4 Parameters for adsorption of CH4 on Co-MoS2 Site Ead(eV) Qt(e) distance(Å) C-Co 0.1 -0.116 C-Co: 1.95 H-Co -0.2 -0.17 H-Co: 1.37 H-C-Co -1.4 -0.076 C-Co: 1.75; H-Co: 1.57 51 First Principle Insight into Co-doped MoS2 for Sensing NH3 and CH4 Fig. 5 Density of States of (a) Pristine MoS2, (b) Co-MoS2, (c) adsorbed NH3 and (d) adsorbed CH4 on Co-MoS2 We adopted three types of models to investigate the most stable site for the adsorption of NH3 molecules on a Co-MoS2 monolayer. The H atom was placed near the Co atom (H-Co), the N atom was placed near the Co atom (N-Co) and N and H atoms were placed near the Co atom (H-N-Co). The calculated parameters of the aforesaid site are presented in Table 3. The lowest adsorption energy was calculated for the H-No-Co site of -0.1 eV followed by the N- Co site of -0.8 eV. The highest adsorption energy was obtained for the H-Co site of -0.9 eV. When adsorption energy is negative, the adsorption process is exothermic. The adsorption distance of the H-N-Co site is 1.55 Å for H-Co and 1.89 Å for the N-Co bond, respectively. There was observed some reduction in adsorption distance of N-Co i.e., 1.28 Å. This might be affected by the alignment of NH3 molecules kept near Co-MoS2. For the H-Co site, the adsorption distance was reduced to 1.23 Å indicating the shortest adsorption distance among all the sites. The shorter adsorption distance signifies the adsorption between gas molecules and the Co-MoS2 monolayer has a strong interaction. In addition, the total charge transfer Qt obtained by Mulliken analysis was found to be negative for all the sites. The negative Qt indicates that NH3 acts as an electron donor and transfers an electron to Co-MoS2. The corresponding total charge transfer of H-Co, N-Co and H-N-Co sites are -0.173e, -0.436e and -0.114e respectively. Due to the shorter adsorption distance, strong adsorption energy and B. CHETTRI, A. THAPA, S. K. DAS, P. CHETTRI, B. SHARMA 52 negative Qt, the H-Co site is considered as one of the most stable sites for adsorption of NH3 on Co-MoS2. Furthermore, we calculated the bandgap, DOS and PDOS to understand the electronic property of the H-Co side. The most stable site is shown in Fig. 4 (a) and (b). The DOS for Co-MoS2 adsorbed NH3 is shown in Fig. 5 (d). In contrast to the cases where NH3 was not adsorbed with Co-MoS2, we discovered that when NH3 is adsorbed with Co-MoS2, a few new states near the Fermi level appear. Furthermore, the magnetic metal property with the spin channel in crossing the Fermi level with a bandgap of 0 eV is shown by the spin up and spin down. These conditions could be caused by the presence of the NH3 molecule. The PDOS graph in Fig. 6 (d) shows the effects of NH3 gas adsorption on Co-MoS2. The primary peaks of NH3 in Co-MoS2 are formed by p orbitals of N atoms and are positioned at -0.3 eV and 0 eV, as seen in Fig. 6 (d). H atoms orbitals produce states with energies of 4.9 eV, which is far from the Fermi level. Although the contributions of the p orbitals are close to the Fermi level, their peaks are much weaker than those of the s orbitals. The d orbitals of Co atoms also produce some impurities around the bottom side of the Fermi level. As seen in PDOS, the d and p orbitals of the Co and N atoms play a key role in enhancing the conductivity of the NH3 adsorbed system. Our findings suggest that adsorbing NH3 molecules in the interlayer is a promising technique to make a Co-MoS2 monolayer that can be used as a spintronics sensor. MoS2 becomes a spintronics-based NH3 sensor when it is doped with Co. Fig. 6 Projected Density of States of (a) Pristine MoS2, (b) Co-MoS2, (c) adsorbed NH3 and (d) adsorbed CH4 on Co-MoS2 53 First Principle Insight into Co-doped MoS2 for Sensing NH3 and CH4 3.2. Transport Property of CH4 and NH3 on Co-MoS2 The I-V characteristic curve aids in determining the sensing device resistance status. Fig.7 (a) and 7 (b) show the device supercells that we used in our calculations (b). Fig. 8 depicts the Co-MoS2 sensor I-V characteristic curve. Currents in Co-MoS2 increase linearly up to 5.8 µA when a bias voltage of 1.4 V is applied. There is a linear degradation in the current as the bias voltage is increased further. Similarly, the current value increases linearly with the bias voltage in the CH4 and NH3 configurations. The greatest current value, 9.2 µA, is achieved in the NH3 configuration with a bias voltage of 2 V, as seen in the graph. In addition, with a 1.4 V applied bias voltage, a value of 5.8 µA is achieved in the CH4 configuration. After that, it starts to decrease for both configurations and approaches the present minimum value. Table 5 shows the Co-MoS2 based sensor resistance condition at 2 V. Table 5 shows that the Co-MoS2 without the detecting gas has a high resistance state of 921 Ωk at 2 V. The variance of resistance in the NH3 and CH4 configurations at 2 V, i.e., 411 Ωk and 200 Ωk, is lower. Fig. 7 Device Supercell of (a) NH3 and (b) CH4 on Co-MoS2 Fig. 8 I-V plot for Co-MoS2 monolayer for adsorption NH3 and CH4 B. CHETTRI, A. THAPA, S. K. DAS, P. CHETTRI, B. SHARMA 54 Table 5 The Co-MoS2 based sensor resistance state at 2V Device Voltage (V) Resistance (kΩ) Co-MoS2 2 921 Co-MoS2-CH4 2 411 Co-MoS2-NH3 2 200 3.3. Sensitivity, Recovery Time and Diffusion Barrier of CH4 and NH3 on Co-MoS2 It is a well-known fact that a good sensor must have excellent selectivity to detect specific gas molecules. We also computed the sensitivity of CH4 and NH3 configurations for this purpose. To acquire a better understanding of the Co-MoS2 monolayer sensitivity to targeted molecules at 2 V, we investigated it. The CH4 adsorption sensitivity of Co-MoS2 was 124 %. The sensitivity of Co-MoS2 to NH3 adsorption has also been calculated to be 360.5 %. Table 6 Bandgap, sensitivity, and recovery time of CH4 and NH3 on Co-MoS2 Configuration Bandgap Sensitivity Recovery Time Co-MoS2-CH4 -0.9 124% 1.4×10 8 s at 350 K 4.7×105 s at 400k 4.7×103 s at 450 K Co-MoS2-NH3 -0.8 360.5% 9 s at 350 K 0.2 s at 400k 0.1 s at 450 K Aside from that, the recovery time of methanol and urea on Co-MoS2 is examined because reusability is an important indicator for gas sensors. The desorption time for the NH3 arrangement is 9 s at 350 K and 0.2 sec at 400 K. At 450 K, the fastest recovery time was calculated to be 0.01 s for NH3 molecules. At 450 K, the fastest recovery time for CH4 molecules is 4.7×103 sec. At 400 K and 350 K, the recovery times were 4.3×105 sec and 1.4×108 sec, respectively. Because the CH4 system has the maximum adsorption energy, the recovery rate is low. According to the computed value of recovery time, as the temperature rises, the recovery time decreases. Hence, NH3 and CH4 gas molecules adsorbed Co-MoS2 monolayer is highly suitable for the application to monitor such gases in the furnace of industry. The gas molecules diffusion characteristics in CH4 and NH3 on Co-MoS2 are crucial for evaluating response performance, a quick diffusion of gas molecules in sensing material will result in a fast response and short recovery time of the gas sensor. As a result, the energy barriers of gases are calculated using the nudged elastic band (NEB) method in QuantumATK. The gas molecules diffusion characteristics in Co-MoS2 are crucial for evaluating response performance, a quick diffusion of gas molecules in sensing material will result in a fast response and short recovery time of the gas sensor [62][63][64]. As a result, the energy barriers of gases are calculated using the nudged elastic band (NEB) method in QuantumATK. For each of the nine diffusion images, the energy barrier of CH4 and NH3 gas molecules on Co-MoS2 is computed. The initial path in our NEB calculation is image 1, and the final path is image 9. The image dependent pair potential approach with a 55 First Principle Insight into Co-doped MoS2 for Sensing NH3 and CH4 maximum distance of 1 Ǻ was employed to develop the NEB image. The energy barrier for all the images is listed in Table 7. The diffusion barriers for CH4 throughout the pathways vary from 0.01 eV to 1.69 eV, which is much lower than the NH3 barrier ranges. The diffusion barrier for NH3 varies from 0.89 eV to 5.15 eV along the paths. It means that CH4 gas molecules diffuse considerably more easily than NH3 gas molecules in Co-MoS2. Furthermore, the diffusions of all gases in the Co-MoS2 monolayer are not isotropic, image 1 to 9 for CH4 gas molecules and image 1 to 2 for NH3 gas molecules correspond to the lowest diffusion barrier, which is due to the inherent lack of electronic and structural symmetry [65]. Table 7 Diffusion barrier of CH4 and NH3 on Co-MoS2 Diffusion Image Diffusion Barriers (eV) CH4 NH3 1 to 2 0.44 0.89 1 to 3 1.05 2.05 1 to 4 1.15 2.68 1 to 5 1.69 2.57 1 to 6 1.47 1.89 1 to 7 0.9 0.96 1 to 8 0.2 0.99 1 to 9 0.01 5.15 Fig. 9 Diffusion barrier of (a) CH4 and (b) NH3 on Co-MoS2 4. CONCLUSION Using the DFT method we investigated the adsorption distance, adsorption energy, charge transfer, bandgap, DOS, PDOS, transport property, sensitivity and recovery time of the NH3 and CH4 adsorbed Co-MoS2 monolayer. The top of the Mo atom was calculated to be the stable position for the doping of the Co atom in MoS2. The top of the Mo atom site has the highest binding energy of 4.5 eV. The doping of the Co atom in MoS2 drastically reduces the bandgap to 1.19 eV from 1.68 eV. This suggests the conduction property of MoS2 is enhanced. We found that NH3 and CH4 system has the shorter B. CHETTRI, A. THAPA, S. K. DAS, P. CHETTRI, B. SHARMA 56 adsorption distance. The adsorption energy of NH3 and CH4 systems are -0.9 eV and -1.4 eV. From the charge transfer, NH3 and CH4 molecules act as electron donors and Co-MoS2 as an electron acceptor. After the Co atom was substituted in the MoS2 monolayer, the magnetic property was detected. Our device has a linear increase in the current until 1.4 V and 2 V for CH4 and NH3 configurations, respectively, and shows variation in resistance, according to the I-V characteristics computed by NEGF. Furthermore, the Co-MoS2 monolayer shows exceptional sensitivity for adsorbing CH4 and NH3 molecules, with the sensitivity of 124 % and 360.5 %, respectively. The recovery time suggests that NH3 and CH4 systems are suitable for high-temperature applications. The fastest recovery time was obtained for NH3 with 0.01 s. According to the computed energy barrier, CH4 gas molecules diffuse more easily in Co-MoS2. Therefore, the Co-MoS2 monolayer is suitable for the adsorption of NH3 and CH4 gas molecules and holds a high application in industrial purposes. As a result, the Co-MoS2 monolayer appears to be a potential candidate for use as a spintronic sensor to detect NH3 and CH4 molecules. Acknowledgement. This work was supported by All India Council for Technical Education (AICTE) Govt. of India under Research Promotion Scheme for North-East Region (RPS-NER) vide ref.: File No. 8-139/RIFD/RPS-NER/Policy-1/2018-19. REFERENCES [1] P. Karki, B. Chettri, A. Thapa, P. Chettri and B. 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