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Understanding the adsorption behavior of small molecule in MoS2 device based on first-principles calculations

Although layered MoS2 has been proposed as a potential candidate for gas detection devices due to high surface-to-volume ratio, high sensitivity, and selectivity, the adsorption behavior of small molecules is still ambiguous. Here, we performed the first-principles calculations to investigate the adsorption behavior of small molecules on layered MoS2 surface, and the effects of defects and environment are considered. Our results reveal that NO and NO2 can be chemically adsorbed on defective monolayer MoS2, which is attributed to the forming of covalent bonds. And the forming of covalent bonds can lead to an increase in adsorption energies. Whereas, gas molecules can only be physically adsorbed on perfect MoS2. Meanwhile, as compared with adsorption behavior of NH3, NO, and NO2 on clean MoS2, the environmental gases (CO2, N2, and H2O) may result in an increase in adsorption strength of NH3, NO, and NO2 on MoS2 surface.


Introduction
Detection of industrial gas and environmental pollution is critical for public health. Thus, it is important to find new materials that have superior characteristics in gas sensing. Two-dimensional (2D) materials have inspired impressive passion in scientific community, their mechanical, electrical and optical properties are superior and strikingly different from their layered bulk counterparts. More important, because of the high surface-tovolume ratio and splendid sensing capability, 2D materials, such as graphene, two-dimensional (2D) transition metal dichalcogenides (TMDs), etc., are acting as a role for real-time detections of industrial processes and environment. MoS 2 is a typical TMDs material, that has been studied extensively as a candidate of various electronic applications [1][2][3][4]. For MoS 2 from bulk to atomic monolayer, a transition of bandgap from 1.2 to 1.8 eV will occur, crossing from indirect to direct one [5]. Due to the unique electrical properties, layered MoS 2 has spurred intense research interests towards the development of nano-electronic devices by utilizing these novel performances, such as field-effect transistors (FETs), photodetectors, and light-emitting diode (LEDs) [6][7][8][9], etc. Especially, layered MoS 2 have been regarded as a candidate of gas detection devices [10,11]. For example, covalent functionalization was demonstrated experimentally and theoretically, which suggests that covalent adsorption can be achieved in the layered MoS 2 [12]. Currently, plenty of reports have proved the high sensing capability of pristine MoS 2 flakes as highly efficient gas sensors [13,14], the adsorption behavior of small molecules is still ambiguous for layered MoS 2 under different effects, such as defect effect, doping and environment effect, etc.
In general, perfect layered MoS 2 exhibits excellent electrical properties as electrical devices (FETs), such as short-channel effect, better electrostatic control, relative higher mobility, and so on [15,16]. However, in the applications of FETs, the existence of the intrinsic defect, such as sulfur and molybdenum vacancies, would greatly change the electric properties [17,18]. Generally speaking, because chalcogen atoms are easy to volatilize, S vacancies are common in mechanical exfoliated and chemical vapor deposition MoS 2 sheets [19], and new states below the conduction band edge caused by S vacancy leading to localization and undesirable electrons doping [20]. The localization and undesirable electrons doping could significantly influence the adsorption behavior of MoS 2 . Besides the intrinsic defect, the environmental effect is another important factor. It is well known that 2D materials are sensitive to the environment due to the inherent properties, which have been regarded as the impediment in stable and mass production of devices [21][22][23]. Therefore, such susceptibility to the environment may also ultimately affect the adsorption behavior of layered MoS 2 .
To clearly understand the adsorption characteristics of the layered MoS 2 , and then better design and fabricate gas sensor devices, an accurate prediction and determination of adsorption behavior for 2D semiconductor sensor devices is of fundamental importance especially for technological applications. Currently, first-principles calculations have become a core calculation method in materials, chemistry, biology, and other research fields due to their unique accuracy and the lack of empirical parameters. Based on the first-principles calculations, plenty of new materials with superior characteristics are predicted and discovered [24,25]. In this work, we performed the first-principles calculations to systematically investigate the adsorption behavior of layered MoS 2 with several kinds of absorbed small molecules, as well as the electronic properties. More important, the defect and environmental effect on the adsorbed small molecules have been discussed in detail. According to the results, some superior small-molecule gas sensors based on monolayer MoS 2 are expected to be designed and fabricated.

Theoretical method
The small molecule gas sensor is usually based on the diode or FETs. Here, the diode is selected as the prototype device. Figure 1 shows a structure of gas sensors based on layered MoS 2 . The first-principles calculations were utilized based on density functional theory (DFT) [26] with the CASTEP code [27]. The generalized gradient approximation (GGA) functional of Perdew, Burke and Ernzerhof (PBE) was used to treat the exchange and correlation potentials [28]. The van der Waals interactions were corrected using Tkatchenko-Scheffler method [29]. All the atomic positions and lattice parameters were optimized with the maximum Hellmann-Feynman forces of 0.01 eV/Å, which leads to obtaining relaxed structures. A 4×4×2 Monkhorst-Pack k-point grid was employed and the plane wave energy cutoff was set to 450 eV [30], self-consistent field (SCF) tolerance was converged to 5×10 -7 eV/atom, which confirms a reasonable calculated result [31]. We adopted a 5×5×1 supercell containing 75 atoms (25 Mo and 50 s atoms). The distance between the neighboring interface was set to be 25 Å to minimize the interaction between layers [32]. Figures 1(b)-(d) displays the configurations for subsequent calculations. Here, the lattice parameters and Mo-S bonds in the MoS 2 monolayer were firstly optimized with the values of 3.166 Å and 2.403 Å, respectively, in good agreement with the reported values (lattice parameter: 3.16 Å, Mo-S bond length: 2.41 Å) [33]. The band gap calculated is about 1.721 eV, which is in good line with the previous experimental and theoretical data [34].
Adsorption energy of gas molecule on pristine and defective MoS 2 was calculated using [35].

MoS gas MoS gas
is the total energy of the MoS2/gas system, E , MoS 2 and E gas are the energies of layered MoS 2 and the gas molecule, respectively.
In terms of a similar method, the adsorption energy of gas molecule on the layered MoS 2 in different environments can be determined as, is the total energy of the system, E gas2 is the energy of the isolated small molecule, E MoS gas1 2 + is the total energy of MoS 2 with environmental gas. The charge transfer between gas molecules and MoS 2 defined as [36].    [34,37]. As compared with the pristine sample with the larger energy gap, the energy gaps of layered MoS 2 with different small molecules are almost similar, except for N-based small molecules (especially NO and NO 2 ) which have been remarkably decreased. To explain the phenomenon, we have calculated the density of states (DOS). Figure 3 shows the DOS and partial DOS (PDOS) of pristine monolayer MoS 2 and MoS 2 with adsorbed N-based small molecules, respectively. As compared with DOS of pristine MoS 2 , the DOS of MoS 2 with adsorbed small molecules (NO and NO 2 ) overall move to lower energy levels. More important, a new peak has appeared near the Fermi level. Based on the PDOS, one can find that the new peak derives from the p state of NO (NO 2 ) molecule. Therefore, the decrease of bandgap for N-based small molecules adsorbed on monolayer MoS 2 is attributed to the introduction of p state from NO (NO 2 ) molecule. Then, by using equations (1) and (3), we have calculated the adsorption energies and charge transfer between MoS 2 and small molecules, as presented in table 1. It is found that perfect layered MoS 2 exhibits a strong sensitivity to N-based gas molecules, especially for NO and NO 2 . However, it is worth noting that, lower adsorption energies mean that the elementary molecule (such as H 2 and N 2 ) are weakly adsorbed on perfect MoS 2 . The calculated adsorption energies and charge transferred are consistent with the reported results, at which for NH 3 3 . In order to uncover the different adsorption behavior for the defective MoS 2 with Mo vacancy and S vacancy, we have further analyzed the electron localization function (ELF), as shown in figure 4. From the ELF, we deem that for NO and NO 2 small molecules adsorbed on defective MoS 2 , the different adsorption behaviors are attributed to that whether or not bonds can be formed between the small molecules and MoS 2 . In terms of our calculated results, O-S bond has formed between NO 2 and MoS 2 with Mo vacancy, as well as N-S bond between NO and MoS 2 . Similarly, N-Mo bond has formed between NO and MoS 2 with S vacancy. Thus, the significant increase of adsorption stability of the defective MoS 2 originates in the formation of the chemical bond. As a result, in the layered MoS 2 , covalent adsorption of N-based functional groups has been achieved.

Small molecule adsorption on defective
To further understand the adsorption behavior, we also utilized the charge density difference (Δρ) to analyze the charge transfer. Figure 5 shows the plane-averaged Δρ and side view of charge density difference plots. Our results show that for NO 2 adsorbed on defective MoS 2 , Δρ near MoS 2 interface region displays a negative charge value, which suggests that the charge will transfer from NO 2 to MoS 2 . However, Δρ for defective MoS 2 with different defects is notably different. For example, Δρ of NO 2 adsorbed on MoS 2 with Mo vacancy is about −6.9×10 -4 e/Å, which is twice larger than that of NO 2 adsorbed on MoS 2 with S vacancy (−3.2×10 -4 e/Å). Otherwise, based on equation (3), the value of the transfer charge has been obtained, that is, the charge transfer from NO 2 to MoS 2 is −0.32 and −0.12 e for NO 2 on Mo vacancy-MoS 2 and S vacancy-MoS 2 , respectively. As for the adsorption of NO on defective MoS 2 , Δρ near the MoS 2 interface shows a positive charge value, which suggests that the charge will transfer from MoS 2 to NO. That is, Δρ of NO on Mo vacancy-MoS 2 is about 4.24×10 -4 e/Å, while Δρ of NO on S vacancy-MoS 2 is about 9.72×10 -4 e/Å. The charge transfer from MoS 2 to NO is −0.11 and −0.64 e for the adsorption of NO on Mo vacancy-MoS 2 and S vacancy-MoS 2 , respectively. Thus, these results indicate that the stronger the charge transfer is, the larger the adsorption energy is. In general, as two materials forming a covalent bond, Δρ will change significantly. Therefore, the larger change of Δρ, accompanying with the charge transfer, confirms the formation of O-S bond, N-S bond and N-Mo bond, respectively.

Small molecule adsorption on MoS 2 in different environments
Since layered MoS 2 are sensitive to the environment due to the inherent properties, understanding the adsorption behavior of layered MoS 2 in different environments can provide crucial input in the development of sensing technology based on layered MoS 2 . Here, we focus on the adsorption of N-based small molecules in different environments. Table 3 shows the adsorption energies of NH 3 , NO 2 and NO molecules adsorbed on the layered MoS 2 in CO 2 , N 2, and H 2 O environments, respectively. The calculated results display that the adsorption stability of N-based small molecules adsorbed on the layered MoS 2 will increase under the environmental influence, as compared with that on perfect layered MoS 2 . However, the adsorption behavior in different environments is much weaker than that on defective MoS 2 , which implies that the adsorption of small molecules in different environments belongs to physical absorption. ELF analysis also displays that chemical bonds are not formed between the N-based small molecules and MoS 2 . To clearly understand the environmental effect on the adsorption behavior, we also calculated the Δρ. Figure 6 shows the calculated plane-averaged Δρ and side view of plots. In figure 6, one can see that Δρ near the MoS 2 interface region is positive for NH 3 and NO adsorbed on MoS 2 in different environments. The results indicate that NH 3 and NO act as charge donors, at which the charge will transfer from small molecules to MoS 2 . Whereas, NO 2 molecule adsorbed on MoS 2 in different environments acts as charge acceptors, at which the charge will transfer from MoS 2 to NO 2 .   . Plane-averaged differential charge density (Δρ) and charge density difference plots for the adsorption of (a) NH 3 , (b) NO 2, and (c) NOon MoS 2 with CO 2 , N 2, and H 2 O environments. Inset: Side view of charge density difference plots. In which, the charge accumulation is represented in red and charge depletion is in blue, respectively.

Small molecule adsorption mechanism on MoS 2
It is well known that the adsorption behavior of small molecules mainly includes physical and chemical adsorptions. Here, the forming of covalent bonds between gas small molecules and monolayer MoS 2 means chemical adsorption, otherwise it is physical adsorption [39]. According to the description above, one can tell that physical and chemical adsorptions coexist for small molecules adsorbed on the layered MoS 2 . For small molecules adsorbed on perfect MoS 2 , it is mainly embodied in physical absorption. For this adsorption behavior, the outer-shell electrons between small molecules and MoS 2 are going to overlap, which induces a small amount of charge transfer among them. For small molecules adsorbed on defective MoS 2 , it is mainly manifested in chemical absorption, at which a covalent chemical bond may be formed between small molecules and MoS 2 . At the same time, a great deal of charge transfer will occur among them. For small molecules adsorbed on MoS 2 in different environments, it is also embodied in physical absorption. However, due to the influence of environment gas, except the outer-shell electrons are going to overlap, a larger amount of charge transfer will occur among them, as compared with that on perfect MoS 2 . Figure 7 shows the schematic diagram of the adsorption behavior of small molecules adsorbed on MoS 2 .

Conclusions
To understand the adsorption behavior of small molecules in MoS 2 device, the adsorption of small molecules on MoS 2 under different conditions has been investigated based on the first-principles calculations. We demonstrate that, as compared with small molecules adsorbed on perfect MoS 2 , adsorption behaviors on defective MoS 2 and MoS 2 with environmental gases have remarkably changed. On perfect MoS 2 , gas molecules are all physically adsorbed, whereas, NO ad NO 2 can be chemically adsorbed on Mo vacancy-MoS 2 , at which O-S and N-S covalent bonds are formed between NO and MoS 2 , NO 2 and MoS 2 , respectively. Meanwhile, NO can also be chemically adsorbed on S vacancy-MoS 2, owing to the N-Mo covalent bond between NO and S vacancy-MoS 2 . The forming of covalent bonds can be demonstrated by the electron localization function. Adsorption energies of gas molecules on perfect MoS 2 , MoS 2 with gas environment, and defective MoS 2 increase gradually. As a result, the environmental gas can improve the adsorption stability to a certain extent, and defects can remarkably enhance the stability but extend the recovery time of sensor.