Efficient N2- and O2-Sensing Properties of PtSe2 With Proper Intrinsic Defects

Developing efficient N2 and O2 gas sensors is of great importance to our daily life and industrial technology. In this work, first-principles calculations are performed to study the N2 and O2 gas-sensing properties of pure and defected PtSe2. It is found that both N2 and O2 adsorb weakly on pure PtSe2, and adsorption of the molecules induces negligible changes in the electrical and optical properties. Whereas the Pt@Se anti-site defect significantly improves the N2 adsorption capacity of PtSe2 and induces notable changes in the electrical property. Similar results are also observed for the Pt and Se vacancies and Pt@Se anti-site defects when examining O2 adsorption. In addition, notable changes in the optical absorption spectra of the PtSe2 with Pt@Se defect are induced upon N2 adsorption, which also occurs for PtSe2 with Pt and Se vacancies and Pt@Se anti-site defects upon O2 adsorption. These results demonstrate that PtSe2 with the corresponding defects can be both excellent electrical and optical sensors for detecting N2 and O2 gases. Our work offers a new avenue for preparing efficient gas sensors.


INTRODUCTION
Oxygen is not only essential to the lives of humans and animals but also the key to the combustiondependent processes such as power generation, chemical compound production, and heating. Controlling the air-to-fuel ratio during the combustion process at the critical point of excess oxygen is beneficial for improving the combustion efficiency, product generation, and safe combustion (Shuk and Jantz, 2015;Zhang et al., 2017). Because the presence of O 2 corrodes gas storage and transportation systems, monitoring O 2 in biomethane is also a necessary part (Urriza-Arsuaga et al., 2019). In the medical and food processing and waste management industries, sometimes it is also necessary to measure the oxygen content (Hong et al., 2018;Wang et al., 2019). Therefore, an efficient sensor for detecting O 2 molecules plays an important role in modern technology. On the other hand, biogas is believed to be a promising substitute for natural gas due to its high methane content. However, the presence of impurity gas like N 2 leads to a lower heating value. In addition, fuel dilution with N 2 gas is generally used to reduce heat radiation, which is one of the main factors limiting the efficiency of gas turbines and internal combustion engines. Therefore, in order to meet the quality specifications, it is also necessary to detect and control N 2 gas (Yi et al., 2013).
The generally used O 2 and N 2 gas sensors can be classified as electrical sensors and optical sensors according to the sensing principles: First, each kind of sensor requires efficient adsorption of the targeting gas molecules on the sensing material. For an electrical sensor, prominent charge transfer between the gas molecules and sensing material or charge trapping upon molecule adsorption converts the adsorption of gas molecules into electrical signal for detection. For an optical sensor, the adsorption of gas molecules notably alters the optical absorption spectrum of the sensing material.
On the other hand, the intrinsic defects, which can significantly affect the chemical, electrical, optical, and magnetic properties of PtSe 2 , are also extensively investigated. For example, Junfeng Gao et. al. studied the atomic structures and thermodynamic stability of vacancy defects. The study of Husong Zheng et. al. shows that the intrinsic Pt vacancy, Se vacancy, and Se@Pt anti-site defects can widely exist in ultrathin layered PtSe 2 (Zheng et al., 2019); Ahmet Avsar et. al. found that Pt vacancy is responsible for the layer-dependent magnetism of PtSe 2 (Avsar et al., 2019). In 2020, Jun Ge et. al. also reported the existence of magnetic moments induced by Pt vacancy defects in PtSe 2 flakes (Ge et al., 2020).
Considering the ubiquity and easy introduction of intrinsic defects in PtSe 2 , in this work, we explore the N 2 and O 2 gassensing properties of both pure PtSe 2 and PtSe 2 with intrinsic defects, including Pt and Se vacancy defects (hereafter denoted as Pt-v and Se-v), Pt@Se and Se@Pt anti-site defects, and Pt and Se interstitial defects (hereafter denoted as Pt-inter and Se-inter), by first-principles calculations. It is found that PtSe 2 with the Pt@Se anti-site defect has strong N 2 adsorption capacity and exhibits significant change in the electrical properties upon N 2 adsorption. Similar results are also observed for PtSe 2 with Pt-v, Se-v, and Pt@ Se defects when examining O 2 adsorption. In addition, notable changes in the optical absorption spectra of the PtSe 2 with Pt@Se defect are induced upon N 2 adsorption, which also occurs for PtSe 2 with Pt-v, Se-v, and Pt@Se upon O 2 adsorption. These results demonstrate that PtSe 2 with the corresponding defects can sensitively detect N 2 and O 2 molecules.

COMPUTATIONAL METHODS
The first-principles calculations are conducted using the Vienna Ab initio Simulation Package (VASP) (Kresse and Furthmüller, 1996). A cutoff energy of 400 eV is used for plane wave expansion, and the accuracy for self-consistent iteration is set to 10 -5 eV. 4 × 4 supercells of pure PtSe 2 are used for modeling the defected and molecules adsorbed on PtSe 2 , and the Brillouin zones for them are sampled with 3 × 3 × 1 gamma-centered k-points (Monkhorst and Pack, 1976). A vacuum layer larger than 30 Å is used for separating the atoms from their periodic images. For geometric optimization, the generalized gradient approximation (GGA) functional of Perdew-Burke-Ernzerhof is used (Perdew et al., 1996), and the atomic structures are fully relaxed until the residual forces on each atom are smaller than 0.02 eV/Å. To describe the interaction between the molecule and surface, DFT-D2 correction is used in the calculation. The more accurate HSE06 functional is used for calculating the electronic structures of pure and defected PtSe 2 (Heyd et al., 2003). In order to quantify the electron charge redistribution between the adsorbed gas molecule and PtSe 2 , the Bader charge is analyzed based on the method of Henkelman (Henkelman et al., 2006;Sanville et al., 2007).

RESULTS AND DISCUSSION
Adsorption Structures of Gas Molecules on Pure and Defected PtSe 2 Adsorption of gas molecules on PtSe 2 is an important parameter determining its gas-sensing properties. To establish the most stable adsorption structures of gas molecules on the monolayer, we first set many different configurations of gas molecules on the basal plane of PtSe 2 , which are then geometrically optimized. In this article, the Pt-v (Se-v) defect is formed by removing one Pt (Se) atom from a 4 × 4 supercell of pure PtSe 2 ; the Pt@Se (Se@Pt) defect is formed by substituting Pt (Se) for Se (Pt), and the Ptinter and Se-inter defects are formed by inserting Pt and Se atoms into the pure PtSe 2 . The structures with the largest adsorption energies are regarded as the most possible ones. The adsorption energy is defined as follows: , the absorption energy Ea (in eV), the change in the molecular bond length Δ (in Å) upon adsorption, and the values of Bader charges on molecules for the various adsorption structures.
where E molecule is the energy of an isolated gas molecule, E monolayer is the energy of pure and defected PtSe 2 , and E total is that of the molecule adsorbed system. The obtained most possible structures of nitrogen and oxygen adsorbed on pure and defected PtSe 2 are shown in Figure 1. Figure 1A shows the adsorption structures of N 2 on pure PtSe 2 and PtSe 2 with six kinds of intrinsic defects. Figure 1 shows the adsorption structures of O 2 on pure PtSe 2 and PtSe 2 with the six kinds of intrinsic defects. In Table 1, we list the detailed adsorption energy, related bond lengths, and Bader charges on molecules for the adsorption structures shown in Figure 1. It is necessary to point out that the reason why the Pt-v defects in Figures 1A,B are different is that the atomic structures around the Pt-v defect change significantly upon adsorption of N 2 , whereas the changes in atomic structures are minor upon adsorption of O 2 .
To describe the bonding length of the molecules on the PtSe 2 surface, we define the adsorption distance L as the closest distance between the atoms of gas molecules and the surface atoms. As shown in Figure 1; Table 1, it is observed that the N 2 molecule bonds with the surface Se atom of pure PtSe 2 and the adsorption distance and absorption energy is, respectively, 3.47 Å and 0.08 eV, which is similar to the vdW interaction length between Se and N atoms. This indicates that N 2 adsorbs on the surface by very weak vdW force. For the PtSe 2 with Pt-v defect, N 2 still bonds with the surface Se atom and the absorption energy becomes about 1.83 eV, which is much larger than that on pure PtSe 2 . This is because the atomic structures around the Pt-v defect change significantly upon adsorption of N 2 , which releases a significant amount of energy as will be discussed in later section. However, the adsorption distance is as long as 3.68 Å. For PtSe 2 with Pt@Se anti-site defect, N 2 adsorbs on the surface by forming N-Pt bond, and the bond length and absorption energy are, respectively, 1.98 Å and 0.61 eV, which is within the sum of atomic radii of N and Pt (2.33 Å), indicating chemical interaction between them. For PtSe 2 with Se-v, Se@Pt, Pt-inter, and Se-inter defects, the N 2 molecule still bonds with the surface Se atom and the adsorption energies are only slightly larger than those on pure PtSe 2 , and the adsorption distances are between 3.5 and 3.6 Å. Therefore, the interaction between these defected structures and N 2 molecule is of the vdW nature.
As shown in Figure 1B; Table 1, O 2 bonds with the surface Se atoms of pure PtSe 2 , and the corresponding adsorption distance and adsorption energy are, respectively, 2.85 Å and 0.51 eV, which is larger than the lengths of any chemical bonds between O and Se. This indicates that O 2 adsorbs on the surface mainly by vdW force. For PtSe 2 with Pt-v defect, the adsorption distance becomes 1.86 Å, which is similar to the sum of covalent radii of O and Se (1.89 Å), and the absorption energy is as large as 2.36 eV. This indicates that O 2 is chemically bonded to the surface. For PtSe 2 with Se-v and Pt@Se defects, the O 2 molecule is bonded to the surface by forming one or more Pt-O chemical bonds, and the adsorption energies are, respectively, 3.20 and 1.98 eV, and the bond lengths are, respectively, 2.05 and 1.92 Å, which is smaller than the sum of atomic radii of O and Pt (2.25 Å). Notably, as shown in Figure 1B, the O 2 molecule is deeply embedded in the vacancy sites of PtSe 2 with Pt-v and Se-v defects, indicating strong adsorption of O 2 on the surfaces. For PtSe 2 with Se@Pt, Pt-inter, and Se-inter defects, O 2 still bonds with the surface Se atom, and the adsorption energies are only slightly larger than those on the pure surface and the adsorption distances are between 2.60 and 2.91 Å. Therefore, the interaction between these defect structures and O 2 molecules is of vdW nature.
From the above results, it is noted that O 2 and N 2 molecules only weakly adsorb on the pure PtSe 2 , while all the intrinsic defects enhance more or less the interaction between the gas molecules and PtSe 2 . In particular, the Pt@Se anti-site defect transforms the initially weak vdW interaction into a strong chemical interaction between the molecules and PtSe 2 , and the Pt-v and Se-v defects also result in strong chemical interactions between O 2 and PtSe 2 . In addition, O 2 adsorbs more strongly than N 2 on both the pure and defected PtSe 2 . These results are also supported by the changes in the molecular bond lengths and the Bader charges on the molecules, as listed in Table 1.

Charge Transfer and Electronic Structures of Gas Molecules on Pure and Defected PtSe 2
Prominent charge transfer between the gas molecules and PtSe 2 upon molecule adsorption is a fundamental prerequisite for transforming the existence of a gas molecule into electrical signal during gas-sensing application. To investigate the charge transfer between them, the Bader charge on molecules and charge density difference (CDD) for the molecule adsorbed on pure and defected PtSe 2 are calculated and shown in Figure 2. The CCD is calculated according to the following equation: where ρ total , ρ monolayer , and ρ molecule are the charge densities of the molecule-adsorbed system, pure or defected PtSe 2 without molecule adsorption, and the isolated gas molecule, respectively. From Figure 2, it is noted that there is always charge transfer from PtSe 2 to N 2 and O 2 upon molecule adsorption, except that the charge transfer between N 2 and pure PtSe 2 is very weak, with a Bader charge of 0.02 e. For PtSe 2 with Pt-v, Se-v, Se@Pt, Pt-inter, and Se-inter defects, the charge transfer between N 2 and them is almost the same as that between N 2 and pure PtSe 2 , whereas the Pt@Se anti-site defect significantly promotes charge transfer from PtSe 2 to N 2 , with a Bader charge of 0.14 e. Moreover, the results of CDD show that significant charge redistribution around the Pt@Se anti-site defect also occurs upon adsorption of N 2 , suggesting additional charge trapping effect of the defect. For O 2 on pure PtSe 2 ( Figure 2B), the amount of charge transfer is relatively large (about 0.20 e), and the introduction of Pt-v, Se-v, and Pt@Se defects further increases the amount of charge transfer, with Bader charges being up to 0.89 e, 0.80 e, and 0.44 e, respectively. For PtSe 2 with Se@Pt, Pt-inter, and Se-inter defects, the charge transfer is only slightly larger than that on pure PtSe 2 , with Bader charges between 0.20 e and 0.29 e. Similarly, the results of CDD in Figure 2 show that significant charge redistribution around the various intrinsic defects occurs upon adsorption of O 2 , suggesting additional charge trapping effects of the defects. In order to understand the adsorption structures, charge transfer between molecules and PtSe 2 , and the charge trapping effects of the intrinsic defects shown above, we investigate the density of states (DOS) for pure and defected PtSe 2 with the adsorption of gas molecules, the DOS of isolated N 2 and O 2 molecules, and the DOS of pure and defected PtSe 2 . As shown in Figure 3, the DOS of isolated N 2 molecules shows that the 2π bonding orbital just lies under the Fermi level and has a lower height than 5σ orbital near −1 eV. The DOS of N 2 basically still retains the characteristics of isolated N 2 after adsorption on pure PtSe 2 and PtSe 2 with Pt-v, Se-v, Se@Pt, Pt-inter, and Se-inter defects, which is consistent with the weak interaction between N 2 and pure and the defected PtSe 2 , whereas the DOS of N 2 changes significantly upon the adsorption on PtSe 2 with the Pt@Se anti-site defect. For example, the antibonding orbital of N 2 near 4 eV splits into three peaks with lower height, and the relative values of the two bonding orbitals near −7.5 eV reverse, and they are in resonance with electronic states from PtSe 2 , indicating strong chemical interaction between N 2 and PtSe 2 . In addition, because N 2 bonds with the PtSe 2 surface by forming Pt-N chemical bonds, the large electronegativity of N results in notable electron gain from Pt, which leads to the large value of Bader charge on N 2 . On the other hand, as shown in Figure 4, the electronic states near the valence band maximum (VBM) and conduction band minimum (CBM) are almost unchanged upon adsorption of N 2 on pure PtSe 2 and PtSe 2 with Se-v, Se@Pt, Pt-inter, and Se-inter defects, whereas small gap states near both CBM and VBM appear upon adsorption of N 2 on the PtSe 2 with Pt@Se defect, which may additionally trap electrons and holes. These are consistent with the results of CDD shown above. For the PtSe 2 with Pt-v defect, the atomic structures around the defect significantly change upon adsorption of N 2 , which introduces many gap states, as will be discussed in the following section.
As shown in Figure 3B, the DOS of isolated O 2 shows five peaks lower than the Fermi level, and the 2π* antibonding orbital is the highest occupied orbital. The DOS of O 2 hybridizes notably and to different extent with the DOS of PtSe 2 upon adsorption on pure PtSe 2 and defected PtSe 2 , and the hybridization is especially significant on PtSe 2 with Pt-v, Se-v, and Pt@Se defects. These are consistent with the strong and different interactions between O 2 and pure and defected PtSe 2 . Moreover, because O 2 bonds with the PtSe 2 surface by forming Pt (Se)-O vdW or chemical bonds, the large electronegativity of O results in notable electron gain, which leads to the large values of Bader charges on O 2 . On the other hand, as shown in Figure 4B, new electronic states near the VBM and CBM are introduced upon adsorption of O 2 on pure PtSe 2 and all the defected PtSe 2 , which may additionally trap electrons and holes. Therefore, significant redistribution of charge density around the defects occurs, as shown in the CDD of Figure 2B.
From the results above, it is noted that for N 2 , it is mainly the Pt@Se anti-site defect that can notably enhance the charge transfer between the gas molecules and PtSe 2 and charge trapping states, while the other defects show negligible effects. For O 2 , all the defects, except Se-inter, enhance the charge transfer between the molecule and PtSe 2 , and the effects of Ptv, Se-v, and Pt@Se defects are the most significant. In addition,

OPTICAL ABSORPTION PROPERTIES OF PURE AND DEFECTED PTSE2 WITH MOLECULE ADSORPTION
The intrinsic defects and molecule adsorption not only affect the electronic structure and electrical properties of PtSe 2 but may also affect its optical absorption properties. In order to study how they affect the optical properties of PtSe 2 , we calculated the optical absorption coefficients of pure and defected PtSe 2 absorbed with N 2 and O 2 . The specific calculation procedure is the same as one of our previous works Yong et al., 2020;Jian et al., 2021). Figure 5 shows the optical absorption coefficients for polarization of E field along the in-plane x direction of pure and defected PtSe 2 with and without the adsorption of N 2 and O 2 . Note that the results for the polarization of E field along the in-plane y direction are almost the same as those along the in-plane x direction. As shown in Figure 5, for adsorption of N 2 , the optical absorption coefficients of pure PtSe 2 and PtSe 2 with Se-v, Se@Pt, Pt-inter, and Se-inter defects are basically the same as those of the structures without N 2 adsorption. For the PtSe 2 with Pt-v defect, there are two absorption peaks around 0.33 and 0.72 eV, whereas the adsorption of N 2 eliminates these two absorption peaks and introduces a new and prominent absorption peak around 1.07 eV. For the PtSe 2 with Pt@Se defect, the optical absorption edges are slightly extended to lower energy. On the other hand, for adsorption of O 2 , the optical absorption coefficients of pure PtSe 2 and PtSe 2 with Pt-inter and Se-inter defects are roughly the same as those of the corresponding structures without adsorption. For the PtSe 2 with Pt-v defect, the initial two absorption peaks around 0.33 and 0.72 eV are weakened and the absorption valley at 1.3 eV is filled upon the adsorption of O 2 . For the PtSe 2 with Se-v defect, the notable absorption peak around 1.0 eV is quenched, and the optical absorption edge blue shifts significantly upon O 2 adsorption. For PtSe 2 with Pt@Se and Se@Pt defects, the optical absorption edges, mainly red shift slightly upon O 2 adsorption. The notable changes in the absorption coefficients of defected PtSe 2 upon N 2 and O 2 adsorption further verify the significant interactions between them and suggest that the characteristic changes in the optical spectra may be utilized for making high-performance and sensitive optical N 2 and O 2 gas detectors. Figure 6A shows the initial structures of the PtSe 2 with Pt-v defect, which is formed by removing one of the Pt atoms in the supercell model and first-principles optimization. Upon N 2 adsorption, the surrounding atomic structures of the Pt-v vacancy site change significantly, and these changes are retained when removing the adsorbed N 2 molecule. As shown in Figure 6B, the upper Se atom closest to the defect site moves to the original position of Pt vacancy and bonds with the surrounding Se atoms. To characterize the differences in the properties of the two structures, the electronic structure and optical properties of them are calculated and shown in Figures 6C,D. As can be seen, the introduced gap states and the DOS near the valence band edge are very different for the two structures. The initial Pt-v structure introduces both occupied and unoccupied gap states near the valence band edge, whereas the new Pt-v structure introduces occupied gap states near the valence band edge and unoccupied gap states near the conduction band edge, thus exhibiting very different electrical properties. Because of this, the optical absorptions of them also show different characteristics. As shown in Figure 6D, the gray area shows that there are two absorption peaks around 0.33 and 0.72 eV, resulting from the gap states near the Fermi level, and there is an absorption valley at 1.3 eV. For the new Pt-v structure, there is mainly a characteristic absorption peak at 1.1 eV, resulting from the transition between gap states. The different optical absorption properties of the two structures may be used to differentiate the specific atomic structures of the Pt-v defect.

SPECIAL RESULTS OF PT-V DEFECT INTRODUCED BY N 2 ADSORPTION
The recent studies have shown that in few-layer PtSe 2 flakes, the Pt vacancy defect on the surface and inside can produce localized magnetic moments. The versatile properties of Pt-v and easy tunability of Pt-v with the adsorption of N 2 revealed here may be used to understand and tune the magnetic properties of PtSe 2 . Frontiers in Chemistry | www.frontiersin.org May 2021 | Volume 9 | Article 676438

CONCLUSION
In conclusion, we have studied the N 2 and O 2 gas-sensing properties of monolayer PtSe 2 by characterizing the geometric structures, charge transfer, electronic structures, and the optical absorption of pure and defected PtSe 2 with and without the adsorption of N 2 and O 2 molecules. It is found that both N 2 and O 2 adsorb weakly on pure PtSe 2 , whereas the Pt@Se anti-site defect significantly improves the N 2 adsorption capacity of PtSe 2 by converting the initial weak vdW interaction on pure PtSe 2 into strong chemical interaction. Moreover, the defect not only promotes charge transfer from PtSe 2 to N 2 but also introduces charge trapping states around the defect, which leads to a significant change in the electrical properties of the structure. Similar results are also observed for the Pt-v, Se-v, and Pt@Se defects when examining O 2 adsorption. In addition, a notable change in the optical absorption spectra of the PtSe 2 with Pt@Se defect is induced upon N 2 adsorption, and this also occurs for PtSe 2 with Pt-v, Se-v, and Pt@Se upon O 2 adsorption. Therefore, PtSe 2 with the corresponding defects are promising materials for preparing sensitive electrical and optical sensors for detecting N 2 and O 2 molecules. Our work demonstrates the important role of intrinsic defects in improving and extending the sensing performance of PtSe 2 , which may be generalized to other materials. Surprisingly, it is also found that significant changes in the atomic structures around Pt vacancy defect are induced upon adsorption of N 2 , which results in very different electronic and optical properties. The versatile properties of Pt vacancy and easy tunability with N 2 molecules revealed here may have potential application for understanding and tuning the recently reported magnetic properties of PtSe 2 .

DATA AVAILABILITY STATEMENT
The original contributions presented in the study are included in the article/Supplementary Material, and further inquiries can be directed to the corresponding author.