English

• 0.   Introduction

  Gaseous pollutants, such as Cl₂ and CO, pose a potential threat to human health and the environment. Therefore, it is critical to develop efficient and accurate gas detection methods to ensure that our society and environment are protected. Cl₂ is a toxic gas with strong irritating and corrosive properties. Inadvertent exposure to Cl₂ in industrial production, laboratories, and daily life can cause serious harm to human health. Therefore, it is important to monitor the concentration of Cl₂ in real time and take timely action to prevent accidents. CO is another common toxic gas mainly emitted from combustion processes, such as vehicle exhaust and home heating appliances. CO is a colorless, odorless gas, but inhalation at high concentrations may lead to carbon monoxide poisoning. Therefore, carbon monoxide monitoring is a critical part of ensuring indoor and outdoor air quality.

  Graphene as the first two-dimensional (2D) material has received extensive attention from scientists for its excellent properties^[1]. Graphene is characterized by a high surface area ratio, high thermal conductivity, and high electron mobility, giving it a large sensing area, and therefore can be used to improve the response speed of semiconductor gas sensors^[2]. After the success of graphene, many 2D materials such as hexagonal boron nitride (h-BN)^[3], silicene^[4], and transition metal dihalides (TMDs)^[5-7] have also received widespread attention, and they can also be used for gas sensors^[8-9], photocatalysis^[10], solar cells^[11-12], and spintronic devices^[13-14]. However, these materials are limited by some drawbacks, such as the zero bandgap of graphene, the excessively wide bandgap of h-BN, and the relatively low electron mobility of TMDs. Therefore, there is a need to find a semiconductor material with a non-zero wide bandgap and high electron mobility as a gas sensor. AI-Balushi et al. successfully synthesized 2D gallium nitride, also known as graphene-like GaN as a wide-bandwidth indirect bandgap semiconductor material with high electron mobility by migration‑enhanced encapsulation growth technique^[15]. In recent years, the study of changing the properties of materials through doping has been increasingly favored^[16-20]. Zhou et al. achieved a significant increase in the adsorption of SO₂ and NO₂ by introducing vacancy defects into graphene^[21]. Cui et al. achieved an increase in the gas sensing ability by introducing vacancy defects into g-GaN^[22]. The adsorption capacity of g-GaN was improved not only by introducing vacancy defects but also by doping. Chen et al. adsorbed H₂S, NH₃, and SO₂ by single doping of g-GaN with Fe and Mn atoms and found that doping of transition metal atoms could improve the adsorption capacity of g-GaN on gas molecules^[23]. Roohi et al. achieved efficient adsorption of SO₂ and NO₂ by doping g-GaN with Fe, Ni, and Zn atoms^[24]. Therefore, it is a very effective method to improve the sensing ability of gallium nitride on gas molecules by doping.

  1.   Calculation and models

  The first-principles calculations based on density functional theory (DFT) are implemented in the Vienna ab initio simulation package (VASP)^[25-29]. The projector augmented wave (PAW)^[30] potentials were used to describe the electron-ion interactions. For the exchange-correlation potential, we adopted the PBE functional with generalized gradient approximation (GGA)^[31], while the electron-ion interaction was described by projected augmented wave potentials. The DFT-D3^[32] method of Grimme was used to describe the weak dispersion forces. We chosed 500 eV as a kinetic energy cutoff for the plane wave basis sets. The crystal structure of g-GaN consists of a 4×4×1 supercell. All the atoms in the unit cells were relaxed until the force in each atom declined below 0.1 eV·nm^-1, and the threshold energy convergence was set to 1×10^-6 eV. The Brillouin zone integration was performed within Gamma centered Monkhorst-Pack scheme^[33] using 10×10×1 k-points. The hexagonal unit cell or supercell was considered in the xy plane, whereas a vacuum layer of more than 1.8 nm was adopted in the z direction to exclude any interactions with neighboring cells. The initial adsorption distance selected in this paper was 0.25 nm. The adsorption energy (E_ad) calculated for gas molecules adsorbed on top of intrinsic and transition metal atom doped g-GaN determines the most positively favorable adsorption sites in the geometry of the adsorption system. The adsorption energy is defined as:

  $ E_{\mathrm{ad}}=E_{\mathrm{g} \text {-GaN }+\mathrm{gas}}-E_{\mathrm{g} \text {-GaN }}-E_{\mathrm{gas}} $

  (1)

  Where E_g-GaN+gas, E_g-GaN, and E_gas are the adsorption system, g-GaN monolayer, and gas molecular energies, respectively. A positive value of adsorption energy implies that the adsorption process requires the absorption of heat to be realized, indicating that it is unfavorable for adsorption. Conversely, a negative value means that the adsorption process is exothermic, which is favorable for adsorption^{[23, 34-36]}. In addition, the charge transfer between the g-GaN substrate and the gas molecules was calculated using Bader charge analysis, if its value is positive this means that the charge is transferred from the g-GaN substrate to the gas molecules^[37]. It is calculated as the gain or loss of electrons from the nucleus of the gas molecule before and after adsorption.

  2.   Results and discussion

  2.1   Gas adsorption on g-GaN

  2.1.1   Stability and geometric structure

  In previous papers, phonon calculations show that the g-GaN is dynamically stable, and the calculated cohesive energy is -7.85 eV^[38]. The lattice constant of g-GaN was calculated to be 0.358 nm, the bond length of Ga—N was 0.185 nm, and the bond angles of N—Ga—N and Ga—N—Ga were 120°, following previous literature^{[22, 39]}. In this paper, to obtain the most stable adsorption configuration between gas molecules and g-GaN, the two possible initial adsorption sites of gas molecules, namely the top of the Ga atom and the top of the N atom, were considered. For Cl₂, the two molecular orientations were parallel or perpendicular to the substrate. For CO gas molecules, four molecular orientations were tested at each adsorption site, namely when the molecular plane was parallel to the substrate, C atoms were at the top of Ga or N atoms, and when the molecular plane was perpendicular to the substrate, C or O atoms were closer to the substrate.

  As shown in Table 1, We calculated the adsorption energies, adsorption distances, adsorption distance change value, magnetic moments, and transferred charges for Cl₂ and CO adsorption at different adsorption sites of intrinsic g-GaN. The adsorption distance was the perpendicular distance between the atom of a gas molecule close to the substrate and its closest atom in a straight line. A negative value for the change in adsorption distance indicates a smaller adsorption distance. It can be observed that most of them were positive, indicating that the intrinsic g-GaN is not strong for the adsorption of the two gas molecules. It is found that the most stable adsorption site for gas molecule Cl₂ adsorption on g-GaN is the top of the Ga atom, the plane of the gas molecule is parallel to the substrate, and the adsorption energy is 0.340 eV. As shown in Fig. 1a, after structural optimization, the Cl₂ molecule was decomposed into two Cl atoms, and the adsorption distance was reduced to 0.181 9 nm. As the adsorption energy is 0.340 eV, the energy ranges from 0.1-1.0 eV (9.6-96.5 kJ·mol^-1) and is positive indicating that this adsorption process requires heat absorption, demonstrating that the system is unstable physisorption. For the CO gas molecule, the most stable adsorption site is the gas molecule parallel to the base and the O atom is located on top of the N atom, and its adsorption energy is 1.083 eV. As shown in Fig. 1b the two atoms of the CO molecule were not in the same plane, and the C atom was closer to the base, and the adsorption distance increased to 0.302 nm. It is worth noting that the positive adsorption energy, the large adsorption distance, and the small transferred charge indicate that the interaction between the gas molecules and the substrate is weak. By calculating the data of adsorption energy and adsorption distance of gas molecules adsorbed at different sites, it can be inferred that the intrinsic g-GaN adsorption capacity for both gas molecules is weak. A comparison of the adsorption systems reveals that the larger the adsorption distance, the larger the adsorption energy, the smaller the transferred charge, and the more unstable the system. It was found that the systems were still non-magnetic after the adsorption of gas molecules.

  Table 1

  

  Table 1.  E_ad, adsorption height (d), adsorption distance change value (a), magnetic moments (M_tot), charge of transfer (C), and band gap (E_g) of gas molecules adsorbed on g-GaN systems

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  Adsorption style	Site	Ead / eV	d / nm	a / nm	Mtot / μB	C / e	Eg / eV
  GaN					0		2.182
  Cl2	parallel	0.340	0.182	-0.068	0	0.703	1.805
  	Ga top-vertical	0.587	0.396	0.146	0	0.143
  	N top-parallel	0.698	0.244	-0.006	0	0.306
  CO	Ga top-parallel	1.690	0.372	0.122	0	0.108
  	Ga top-vertical-C	1.106	0.297	0.047	0	0.015
  	Ga top-vertical-O	1.134	0.320	0.070	0	0.014
  	N top-parallel	1.083	0.302	0.052	0	0.027	2.209
  	N top-vertical-C	1.676	0.363	0.113	0	0.020
  	N top-vertical-O	1.153	0.350	0.100	0	0.010

  Figure 1

  

  Figure 1.  Top and side views of the most stable adsorption configurations of (a) Cl₂ and (b) CO adsorbed on g-GaN

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  2.1.2   Electronic properties

  To better understand the effect of gas molecule adsorption on the electronic properties of g-GaN, the band structure of the system was investigated. As shown in Fig. 2, the band structure of g-GaN with the two adsorption systems was plotted. The results show that the spin-up and spin-down channels were symmetric, indicating that the adsorption system is nonmagnetic. Compared with the g-GaN band structure (Fig. 2a), the Cl₂ adsorption system was found to have a significantly higher number of conduction bands around 3.5 eV and a significantly higher number of valence bands between -1 and -2 eV. And its band gap decreased to 1.805 eV (Fig. 2b). For the CO adsorption system, compared to the intrinsic g-GaN, the number of energy bands increased between 2-3 eV with little change in valence bands, and the band gap increased to 2.209 eV (Fig. 2c). The small change in the band gap due to adsorption induced by gas molecules results in a change of conductivity, which involves the sensitivity of the electrical signal in gas sensing. Therefore, intrinsic g-GaN is expected to be used in suitable devices for Cl₂ and CO gas detection.

  Figure 2

  

  Figure 2.  Band structures of (a) g-GaN, (b) Cl₂ adsorbed on g-GaN, (c) CO adsorbed on g-GaN based on the most stable configuration

  Blue lines represent spin-up, and red lines represent spin-down; Fermi level was shifted to zero.

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  Fig. 3 shows the total density of states (DOS) and the partial density of states (PDOS) plots of g-GaN for adsorbed and unadsorbed gas molecules. From the DOS of Cl₂ adsorbed on g-GaN plotted in Fig. 3a, it can be seen that an unoccupied DOS peak appeared at 1.7 eV, which is mainly contributed by the 3p state of the Cl atom and the 2p state of the nearest N atom at the adsorption site, and appears to overlap significantly. Therefore, there is orbital hybridization between the Cl atom and the N atom at the adsorption site. After Bader charge analysis, it is found that Cl₂ acts as an acceptor to accept electrons in each adsorption system. As shown in Fig. 3b, the DOS of CO adsorption on g-GaN can be seen as an unoccupied DOS peak at 2.7 eV, which can be seen by the PDOS to be mainly contributed by the 2p state of the O atom and the 2p state of the nearest N atom at the adsorption site, with an obvious overlap, indicating the existence of orbital hybridization between the O atom and the N atom at the adsorption site. In addition to this, the total DOS of the CO adsorption system and intrinsic g-GaN could be seen to be not significantly different near the Fermi energy level. And since there is no orbital hybridization between the C atom of the CO gas molecule and the substrate, and there is little charge transfer between the gas molecule and the substrate and the adsorption distance, it follows that the CO adsorption on g-GaN is physisorbed.

  Figure 3

  

  Figure 3.  Total DOS and PDOS of (a) Cl₂ and (b) CO adsorbed on g-GaN based on the most stable configuration

  Fermi level was set to zero energy and indicated by the black dashed line.

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  The charge density difference (CDD) can clearly show the interaction and charge transfer between gas molecules and g-GaN. Fig. 4 shows the differential charge density maps of two gas molecules adsorbed on g-GaN for stable adsorption configurations, calculated as Eq.2.

  $ \Delta \rho=\rho_{\text {total }}-\rho_{\mathrm{g}_\mathrm{-GaN}}-\rho_{\mathrm{gas}} $

  (2)

  Figure 4

  

  Figure 4.  Top and side views of the CDD of (a) Cl₂ and (b) CO adsorbed on g-GaN based on the most stable configuration

  Isosurface value was taken at 3 e•nm^-3 for a, and 0.1 e•nm^-3 for b; Yellow and cyan regions correspond to charge accumulation and depletion, respectively.

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  where ρ_total, ρ_g-GaN, and ρ_gas are the charge densities of the gas adsorbed on intrinsic g-GaN, intrinsic g-GaN, and gas molecules, respectively. Fig. 4a shows the CDD diagram of Cl₂ adsorption on g-GaN, from which it was clear that the charge accumulation mainly occurred on the gas molecules, and the charge dissipation mainly appeared on the g-GaN of the substrate. Fig. 4b shows the CDD of CO adsorption on g-GaN, and it can be seen that both CO gas molecules and the nearest N and Ga atoms at the adsorption site showed charge accumulation and loss, and the charge transfer by Bader charge analysis was only 0.027e. The charge accumulation and dissipation for both adsorption systems are consistent with the calculated charge transfer directions.

  2.2   Adsorption of gas molecules on TM-doped g-GaN

  2.2.1   Stability and geometric structure

  To improve the sensitivity of g-GaN to gas molecules, we investigated the effect of TM (Fe and Co) doping on the adsorption of gas molecules on g-GaN. Gas molecules were initially placed on top of TM atoms using different molecular orientations. The experimentally calculated data for each doped adsorption system are shown in Table 2. Calculated results show that the adsorption energy by doping Fe and Co was significantly smaller and became negative. As shown in Fig. 5a and 5b, for the Fe-doped g-GaN adsorbed gas molecule, after structural optimization, it was found that Cl₂ decomposed into two Cl atoms and bonded to Fe and Ga atoms, respectively. The adsorption energy was -5.522 eV and the adsorption distance was 0.218 nm. Compared with the adsorption of Cl₂ via noble metal doped SnS₂ done by the previous work, the adsorption system in this paper is more stable and the doped element is a non-precious metal, which is more cost- effective^[40]. The C atom in the CO gas molecule bonded to the Fe atom and the CO molecule became slightly tilted from the initial perpendicular to the substrate after structural optimization. Due to the bonding, the bonded Fe and Ga atoms in the substrate were bulged. Its adsorption energy was -3.574 eV, which was smaller than that of CO on C-doped g-GaN, and the adsorption distance was also smaller^[41]. Calculations show that the Fe doping greatly improves the adsorption capacity of g-GaN for Cl₂ and CO, and the adsorption process becomes exothermic instead of absorbing heat, which makes the adsorption structure stable, and the charge transferred from g-GaN to the gas molecules becomes more and the adsorption distance decreases.

  Table 2

  

  Table 2.  E_ad, M_tot, C, d, a, and E_g of gas molecules adsorbed on TM (Fe and Co) doped g-GaN systems

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  Adsorption style	Site	Ead / eV	d / nm	a / nm	Mtot / μB	C / e	Eg / eV
  Ga15N16Fe					5		0.707
  Ga15N16Co					4		1.477
  Cl2	Fe top-parallel	-5.522	0.218	0.032	3	1.092	0.439
  	Fe top-vertical	-4.241	0.235	-0.015	3	-0.157
  	Co top-parallel	-3.025	0.226	-0.024	4	1.121	0.420
  	Co top--vertical	-1.813	0.230	-0.020	4	0.614
  CO	Fe top-parallel	-3.396	0.326	0.076	5	0.031
  	Fe top-vertical-C	-3.574	0.177	-0.073	3	0.208	0.239
  	Fe top-vertical-O	-3.355	0.303	0.053	5	0.015
  	Co top-parallel	-1.079	0.357	0.107	4	0.088
  	Co top-vertical-C	-1.836	0.169	-0.081	2	0.171	0.420
  	Co top-vertical-O	-1.508	0.345	0.095	4	0.013

  Figure 5

  

  Figure 5.  Top and side views of the most stable adsorption configurations of (a, c) Cl₂ and (b, d) CO adsorbed on (a, b) Fe- and (c, d) Co-doped g-GaN

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  As shown in Fig. 5c and 5d, for Co-doped g-GaN adsorbed gas molecules, the stable adsorption structure was obtained by structure optimization, and it was found that the same as the Fe-doped adsorption system, the Cl₂ decomposed into two Cl atoms and bonded to the substrate Co and Ga atoms, respectively, but unlike the Fe-doped system one of the Cl atoms was bonded to the substrate′s two Ga atoms, which was accompanied by bonding atom. The bulge of the bonded atoms was accompanied by a bulge. For CO adsorption, as in the Fe-doped system, the C atoms bonded with the dopant atoms Co and bulge, but the gas molecules were still oriented in the same way as in the initial state, i.e., perpendicular to the substrate. Its adsorption energy was -1.836 eV, which was smaller than that of CO on C-doped g-GaN, and the adsorption distance was also smaller, indicating that this adsorption system is more stable. This is a smaller and more stable adsorption energy for CO adsorption compared to Cui et al. who adsorbed CO through the introduction of a single vacancy defect to g-GaN^[22]. By comparing the previous experimental results, it can be concluded that it is feasible to improve the adsorption capacity of g-GaN by doping in this paper.

  Comparison of the doped adsorption systems revealed that the larger the adsorption distance, the larger the adsorption energy, the smaller the corresponding transferred charge, and the more unstable the system. We found that for the most stable adsorption configuration after the adsorption of gas molecules, the magnetic moments are mostly reduced. In summary, the introduction of TM-doped atoms greatly improves the adsorption capacity of g-GaN for Cl₂ and CO gas molecules compared to the intrinsic g-GaN adsorption of gas molecules.

  2.2.2   Electronic properties and magnetic properties

  Based on the most stable TM-doped g-GaN adsorption configuration, the band structures of TM-doped g-GaN and gas molecules adsorbed in TM-doped g-GaN were calculated as shown in Fig. 6. The energy bands of TM-doped g-GaN and gas molecules adsorbed in the TM-doped g-GaN system can be seen in the figure, and the spin-up and spin-down channels were asymmetric, implying that the spin-simplex state is broken, suggesting the presence of magnetism. Calculations show that the gas molecules are nonmagnetic semiconductors when adsorbed and not adsorbed on intrinsic g-GaN. Therefore, we can conclude that the magnetic moment of the adsorbed system originates from the TM dopant. The magnetic moments of the Fe- and Co-doped doped systems were calculated to be 5μ_B and 4μ_B per unit cell, respectively, whereas it was found that the magnetic moments of the adsorbed systems decreased after Cl₂ and CO adsorbed on the two doped systems except for Cl₂ adsorbed on the Co-doped system whose magnetic moment was still 4μ_B per unit cell. This suggests that the gas molecules in the three adsorbed systems have stronger interactions with the substrate and that the magnetic moments of the doped systems may be partially quenched by the adsorbed gas molecules. As shown in Fig. 7, the spin state density plots of Cl₂ and CO adsorbed on Fe- and Co-doped g-GaN, it can be seen that in the four adsorption systems, the magnetism is mainly generated by the doped Fe and Co atoms in the substrate as well as their nearest neighboring N atoms and that except for the small amount of magnetism when Cl₂ adsorb on the Co-doped system which is introduced by one of the Cl atoms, the gas molecules in the other systems are involved in the magnetism introduction, consistent with the reduction of the magnetic moment above. As shown in Fig. 6, a comparison of the energy band diagrams of the adsorbed and unadsorbed gas molecule systems revealed that the band gap increased significantly near the Fermi energy level, which also led to a decrease in the band gap of the system after gas adsorption. As shown in Table 2, the bandgap of the gas adsorbed on the doped system was significantly smaller than that of the doped system with unadsorbed gas molecules, which indirectly indicates the strong interaction between the gas molecules and the substrate.

  Figure 6

  

  Figure 6.  Band structures of (a) Fe-doped g-GaN, (b) Cl₂ and (c) CO adsorbed on Fe-doped g-GaN, (d) Co-doped g-GaN, (e) Cl₂ and (f) CO adsorbed on Co-doped g-GaN

  Blue lines represent spin-up, and red lines represent spin-down; Fermi level was set to zero and indicated by the black dashed line.

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  Figure 7

  

  Figure 7.  Top and side views of the spin densities for (a, c) Cl₂, (b, d) CO adsorbed on (a, b) Fe-doped g-GaN and (c, d) Co-doped g-GaN

  Isosurface value was taken at 10 e•nm^-3; Red equivalence plane corresponds to the one with the most spin states.

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  Next, the total DOS of TM-doped g-GaN with and without adsorption of gas molecules, as well as the PDOS of the gas molecules, TM atoms, and their nearest neighboring N atoms are shown in Fig. 8. As shown in Fig. 8a, the total DOS of Fe-doped g-GaN and Fe-doped g-GaN with adsorbed Cl₂ had a nonzero band gap and spin polarization near the Fermi energy level, indicating that both the adsorbed and unadsorbed Cl₂ systems are magnetic semiconductors. The PDOS analysis revealed that the adsorbed Cl₂ exhibited spin peaks situated at -0.3 and 0.4 eV near the Fermi energy level, which were mainly contributed by Fe atoms′ 3d state. The 3d state of Fe atoms is strongly hybridized with the 3p state of Cl atoms, which enhances the interaction between Fe atoms and Cl atoms and improves the adsorption capacity of g-GaN for Cl₂. As shown in Fig. 8b for the total DOS plots of CO adsorbed and unadsorbed on Fe-doped g-GaN, it can be seen that similar to the case of Cl₂ adsorption, Fe-doped g-GaN remains as a magnetic semiconductor after adsorption of CO, and considering a small number of overlapping states appearing near the Fermi energy levels of -0.1 and 0.1 eV, it suggests that the introduction of dopant enhances the interactions between the CO and the substrate. These results also indicate that doping Fe enhances the adsorption capacity of g-GaN for both gas molecules, Cl₂ and CO. As shown in Fig. 8c, the total DOS plots of Cl₂ adsorbed and unadsorbed on Co-doped g-GaN, it can be seen that both adsorbed and unadsorbed were magnetic semiconductors. The PDOS analysis shows spin peaks at -0.3, 0.2, and 0.4 eV neared the Fermi energy level, and the 3d state of Co atoms was strongly hybridized with the 3p state of Cl atoms, suggesting that the Cl₂ interacts strongly with the substrate. As shown in Fig. 8d, the total DOS of the Co-doped system with adsorbed and unadsorbed CO gas molecules can be seen as magnetic semiconductors as the Cl₂ adsorbed system, with the difference that the spin peak appeared only in the spin-up channel at 0.7 eV. By PDOS analysis, only a small amount of 2p states of C and O were weakly hybridized with 3d of Co atoms near the Fermi energy level, indicating that Co doping can enhance the sensitivity of g-GaN to Cl₂ and CO.

  Figure 8

  

  Figure 8.  Total DOS and PDOS of (a, c) Cl₂, (b, d) CO adsorbed on (a, b) Fe-doped g-GaN and (c, d) Co-doped g-GaN based on the most stable configuration

  Fermi level was set to zero energy and indicated by the black dashed lines.

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  The CDD images of two gas molecules adsorbed on TM-doped g-GaN are shown in Fig 9. The CDD images of Cl₂ adsorbed on Fe- and Co-doped g-GaN as shown in Fig. 9a and 9c indicate that the charge accumulation occurs mainly on the gas molecules and the electron loss occurs mainly on the doped substrate. Bader′s charge analysis shows that the gas molecule Cl₂ acts as an electron acceptor during adsorption, and the charge transfer between the TM-doped g-GaN and Cl₂ molecules was very large at 1.092e and 1.121e, respectively. Therefore, it can be seen that Fe- and Co-doped g-GaN can improve the adsorption capacity for Cl₂. As shown in Fig. 9b and 9d, the CDD images of CO adsorption on Fe- and Co-doped g-GaN, it can be seen that, unlike the adsorption of Cl₂ gas molecules, the electron accumulation mainly occurs between the gas molecules and the g-GaN substrate, and the electron loss mainly occurs on the substrate. The charge transfer is the same as that of Cl₂ gas molecules, both gas molecules get electrons, but the size of the transferred charge is significantly smaller than that of the Cl₂ gas adsorption system, which is 0.208e and 0.171e. Considering that the adsorption energies of the four adsorption systems are relatively small, the adsorption distances are small, and the charge transfer is obvious, it can be concluded that the adsorption of Cl₂ and CO on the Fe- and Co-doped g-GaN is chemisorption.

  Figure 9

  

  Figure 9.  Top and side views of the charge density difference for (a, c) Cl₂, (b, d) CO adsorbed on (a, b) Fe-doped g-GaN and (c, d) Co-doped g-GaN

  Isosurface value was taken as 3 e•nm^-3 for (a) and (c), 1 e•nm^-3 for (b) and (d); Yellow and cyan regions correspond to charge accumulation and depletion, respectively.

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  3.   Conclusions

  Gas sensors are widely used in many fields, and Ⅲ-Ⅴ monolayers have a potential application background in gas sensors due to their unique properties. Based on DFT, we have systematically investigated the adsorption properties of gas molecules (Cl₂ and CO) on intrinsic g-GaN and doped g-GaN (Fe and Co mono-doped). Based on the calculation results were analyzed in terms of adsorption energy, Bader charge transfer, band structure, and density of electronic states. It is found that the doping of Fe and Co significantly alters the corresponding electronic and gas adsorption properties. The adsorption energies of Cl₂ and CO adsorbed on doped g-GaN were much smaller than those adsorbed on intrinsic g-GaN. Cl₂ can even be decomposed by adsorption on doped g-GaN. The charge density difference, Bader charge transfer, band structure, and electronic DOS indicate that there are strong interactions between both Cl₂ and CO gas molecules and Fe- and Co-doped g-GaN, reflecting that the adsorption processes are all chemisorption. In summary, TM-doped g-GaN is expected to be a candidate material for effective adsorption or filtration of Cl₂ and CO.

  
  
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  Figure 1  Top and side views of the most stable adsorption configurations of (a) Cl₂ and (b) CO adsorbed on g-GaN

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  Figure 2  Band structures of (a) g-GaN, (b) Cl₂ adsorbed on g-GaN, (c) CO adsorbed on g-GaN based on the most stable configuration

  Blue lines represent spin-up, and red lines represent spin-down; Fermi level was shifted to zero.

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  Figure 3  Total DOS and PDOS of (a) Cl₂ and (b) CO adsorbed on g-GaN based on the most stable configuration

  Fermi level was set to zero energy and indicated by the black dashed line.

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  Figure 4  Top and side views of the CDD of (a) Cl₂ and (b) CO adsorbed on g-GaN based on the most stable configuration

  Isosurface value was taken at 3 e•nm^-3 for a, and 0.1 e•nm^-3 for b; Yellow and cyan regions correspond to charge accumulation and depletion, respectively.

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  Figure 5  Top and side views of the most stable adsorption configurations of (a, c) Cl₂ and (b, d) CO adsorbed on (a, b) Fe- and (c, d) Co-doped g-GaN

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  Figure 6  Band structures of (a) Fe-doped g-GaN, (b) Cl₂ and (c) CO adsorbed on Fe-doped g-GaN, (d) Co-doped g-GaN, (e) Cl₂ and (f) CO adsorbed on Co-doped g-GaN

  Blue lines represent spin-up, and red lines represent spin-down; Fermi level was set to zero and indicated by the black dashed line.

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  Figure 7  Top and side views of the spin densities for (a, c) Cl₂, (b, d) CO adsorbed on (a, b) Fe-doped g-GaN and (c, d) Co-doped g-GaN

  Isosurface value was taken at 10 e•nm^-3; Red equivalence plane corresponds to the one with the most spin states.

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  Figure 8  Total DOS and PDOS of (a, c) Cl₂, (b, d) CO adsorbed on (a, b) Fe-doped g-GaN and (c, d) Co-doped g-GaN based on the most stable configuration

  Fermi level was set to zero energy and indicated by the black dashed lines.

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  Figure 9  Top and side views of the charge density difference for (a, c) Cl₂, (b, d) CO adsorbed on (a, b) Fe-doped g-GaN and (c, d) Co-doped g-GaN

  Isosurface value was taken as 3 e•nm^-3 for (a) and (c), 1 e•nm^-3 for (b) and (d); Yellow and cyan regions correspond to charge accumulation and depletion, respectively.

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  Table 1.  E_ad, adsorption height (d), adsorption distance change value (a), magnetic moments (M_tot), charge of transfer (C), and band gap (E_g) of gas molecules adsorbed on g-GaN systems

  Adsorption style	Site	Ead / eV	d / nm	a / nm	Mtot / μB	C / e	Eg / eV
  GaN					0		2.182
  Cl2	parallel	0.340	0.182	-0.068	0	0.703	1.805
  	Ga top-vertical	0.587	0.396	0.146	0	0.143
  	N top-parallel	0.698	0.244	-0.006	0	0.306
  CO	Ga top-parallel	1.690	0.372	0.122	0	0.108
  	Ga top-vertical-C	1.106	0.297	0.047	0	0.015
  	Ga top-vertical-O	1.134	0.320	0.070	0	0.014
  	N top-parallel	1.083	0.302	0.052	0	0.027	2.209
  	N top-vertical-C	1.676	0.363	0.113	0	0.020
  	N top-vertical-O	1.153	0.350	0.100	0	0.010

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  Table 2.  E_ad, M_tot, C, d, a, and E_g of gas molecules adsorbed on TM (Fe and Co) doped g-GaN systems

  Adsorption style	Site	Ead / eV	d / nm	a / nm	Mtot / μB	C / e	Eg / eV
  Ga15N16Fe					5		0.707
  Ga15N16Co					4		1.477
  Cl2	Fe top-parallel	-5.522	0.218	0.032	3	1.092	0.439
  	Fe top-vertical	-4.241	0.235	-0.015	3	-0.157
  	Co top-parallel	-3.025	0.226	-0.024	4	1.121	0.420
  	Co top--vertical	-1.813	0.230	-0.020	4	0.614
  CO	Fe top-parallel	-3.396	0.326	0.076	5	0.031
  	Fe top-vertical-C	-3.574	0.177	-0.073	3	0.208	0.239
  	Fe top-vertical-O	-3.355	0.303	0.053	5	0.015
  	Co top-parallel	-1.079	0.357	0.107	4	0.088
  	Co top-vertical-C	-1.836	0.169	-0.081	2	0.171	0.420
  	Co top-vertical-O	-1.508	0.345	0.095	4	0.013

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